Cobalt speciation in cobalt oxide-apatite materials: structure–properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion
Identifieur interne : 000653 ( Istex/Corpus ); précédent : 000652; suivant : 000654Cobalt speciation in cobalt oxide-apatite materials: structure–properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion
Auteurs : Kaoutar El Kabouss ; Mohamed Kacimi ; Mahfoud Ziyad ; Souad Ammar ; Alain Ensuque ; Jean-Yves Piquemal ; François Bozon-VerdurazSource :
- Journal of Materials Chemistry [ 0959-9428 ] ; 2006.
Abstract
Impregnation of calcium hydroxyapatite by a solution of Co(ii) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are isolated six-coordinated Co2+ ions. For Co content ≥0.4 wt%, the presence of tetrahedral Co2+ and octahedral Co3+ species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered CoxOy entities. For Co content ≥1.7 wt%, Co3O4 nanocrystals are generated, as evidenced by XRD and magnetic measurements. In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co2+ ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co2+ ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.
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DOI: 10.1039/b602514e
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<front><div type="abstract">Impregnation of calcium hydroxyapatite by a solution of Co(ii) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are isolated six-coordinated Co2+ ions. For Co content ≥0.4 wt%, the presence of tetrahedral Co2+ and octahedral Co3+ species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered CoxOy entities. For Co content ≥1.7 wt%, Co3O4 nanocrystals are generated, as evidenced by XRD and magnetic measurements. In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co2+ ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co2+ ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.</div>
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<notesStmt><note>Isolated Co2+ ions exchanged with Ca2+ ions of calcium hydroxyapatite are resistant to oxidation and show much better performance in dehydrogenation than cooperating cobalt sites found in Co3O4. [b602514e-ga.tif]</note>
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<title type="display">Journal of Materials Chemistry</title>
<sercode>JM</sercode>
<publisher><orgname><nameelt>The Royal Society of Chemistry</nameelt>
</orgname>
</publisher>
<issn type="print">0959-9428</issn>
<issn type="online">1364-5501</issn>
<coden>JMACEP</coden>
<cpyrt>This journal is © The Royal Society of Chemistry</cpyrt>
</journalref>
<volumeref><link></link>
</volumeref>
<issueref><link>25</link>
</issueref>
<pubfront><fpage></fpage>
<lpage></lpage>
<no-of-pages></no-of-pages>
<date><year>2006</year>
<month>Unassigned</month>
<day>Unassigned</day>
</date>
</pubfront>
</published>
<art-front><titlegrp><title>Cobalt speciation in cobalt oxide-apatite materials: structure–properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion</title>
</titlegrp>
<authgrp><author aff="affa affb"><person><persname><fname>Kaoutar</fname>
<surname>El Kabouss</surname>
</persname>
</person>
</author>
<author aff="affa"><person><persname><fname>Mohamed</fname>
<surname>Kacimi</surname>
</persname>
</person>
</author>
<author aff="affa"><person><persname><fname>Mahfoud</fname>
<surname>Ziyad</surname>
</persname>
</person>
</author>
<author aff="affb"><person><persname><fname>Souad</fname>
<surname>Ammar</surname>
</persname>
</person>
</author>
<author aff="affb"><person><persname><fname>Alain</fname>
<surname>Ensuque</surname>
</persname>
</person>
</author>
<author aff="affb"><person><persname><fname>Jean-Yves</fname>
<surname>Piquemal</surname>
</persname>
</person>
</author>
<author aff="affb" role="corres"><person><persname><fname>François</fname>
<surname>Bozon-Verduraz</surname>
</persname>
</person>
</author>
<aff id="affa"><org><orgname><nameelt>Laboratoire de Physico-Chimie des Matériaux et Catalyse</nameelt>
<nameelt>Faculté des Sciences</nameelt>
<nameelt>Département de Chimie</nameelt>
</orgname>
</org>
<address><city>Rabat</city>
<country>Morocco</country>
</address>
</aff>
<aff id="affb"><org><orgname><nameelt>Groupe de Chimie des Matériaux Divisés et Catalyse</nameelt>
<nameelt>ITODYS</nameelt>
<nameelt>UMR-CNRS 7086</nameelt>
<nameelt>Université Paris 7-Denis Diderot</nameelt>
<nameelt>case 7090</nameelt>
</orgname>
</org>
<address><addrelt>2, place Jussieu</addrelt>
<city>75251 Paris cedex 05</city>
<country>France</country>
</address>
</aff>
</authgrp>
<art-toc-entry><ictext>Isolated Co<sup>2+</sup>
ions exchanged with Ca<sup>2+</sup>
ions of calcium hydroxyapatite are resistant to oxidation and show much better performance in dehydrogenation than cooperating cobalt sites found in Co<inf>3</inf>
O<inf>4</inf>
.</ictext>
<icgraphic xsrc="b602514e-ga.tif" id="ga"></icgraphic>
</art-toc-entry>
<abstract><p>Impregnation of calcium hydroxyapatite by a solution of Co(<scp>ii</scp>
) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are <it>isolated</it>
six-coordinated Co<sup>2+</sup>
ions. For Co content ≥0.4 wt%, the presence of <it>tetrahedral Co<sup>2+</sup>
and octahedral Co<sup>3+</sup>
</it>
species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
entities. For Co content ≥1.7 wt%, Co<inf>3</inf>
O<inf>4</inf>
nanocrystals are generated, as evidenced by XRD and magnetic measurements.</p>
<p>In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co<sup>2+</sup>
ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co<sup>2+</sup>
ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.</p>
</abstract>
</art-front>
<art-body><section><no>1.</no>
<title>Introduction</title>
<p>Cobalt oxide materials have a wide range of applications such as magnetic materials,<citref idrefs="cit1 cit2">1,2</citref>
gas sensors,<citref idrefs="cit3 cit4">3,4</citref>
anode materials for rechargeable Li-ion batteries,<citref idrefs="cit4 cit5 cit6 cit7">4–7</citref>
high temperature solar selective absorbers<citref idrefs="cit8">8</citref>
and electrochromism.<citref idrefs="cit9">9</citref>
These studies differ by the morphology of the material used (powders, films, nanocrystals), the nature of the precursor and the preparation method involved. In the field of heterogeneous catalysis, cobalt oxide species have sometimes been used as a <it>support</it>
, <it>e.g.</it>
in gold catalysts,<citref idrefs="cit10">10</citref>
but most often as an <it>active phase</it>
deposited on a support. Concerning oxidation catalysis, studies have appeared concerning the total oxidation of hydrocarbons.<citref idrefs="cit11 cit12">11,12</citref>
As to the oxidative dehydrogenation of ethane (ODE) which represents a key alternative for ethylene production, numerous studies have concerned vanadium- or niobium-based catalysts<citref idrefs="cit13 cit14 cit15 cit16 cit17 cit18 cit19 cit20">13–20</citref>
but some of these studies were rather devoted to the production of oxygenates.<citref idrefs="cit13">13</citref>
In opposition, cobalt oxide catalysts have rarely been used.<citref idrefs="cit21 cit22 cit23">21–23</citref>
As evidenced by several authors, the acid–base properties of the solid are of prime importance in the reaction mechanism of ODE<citref idrefs="cit16 cit17 cit18 cit21 cit24">16–18,21,24</citref>
and the conversion of butan-2-ol to butenes or to butanone is recognized as a valuable test of these properties.<citref idrefs="cit25 cit26 cit27 cit28 cit29 cit30">25–30</citref>
For an oxide, the strength of the acid or basic sites depends on the ionic character of the metal–oxygen bond. Hydroxyapatites such as Ca<inf>10</inf>
(PO<inf>4</inf>
)<inf>6</inf>
(OH)<inf>2</inf>
, in short CaHap, present a marked flexibility favouring the modulation of acid–base features and the tailoring of transition metal sites. In a previous paper,<citref idrefs="cit23">23</citref>
we have shown that a limited amount of calcium ions can be exchanged by Co<sup>2+</sup>
ions, which remain <it>isolated</it>
and are not oxidized in air at 823 K. Hence the cobalt–apatite system represents an appropriate model catalyst to investigate the influence of acid–base properties and the nature of redox centers (speciation), especially the role of <it>isolated</it>
or <it>cooperative</it>
cobalt sites in the oxidative dehydrogenation of ethane.</p>
<p>The present work involves (i) increasing the Co content further by using the impregnation method; (ii) the <it>conjoint</it>
use of X-ray diffraction, magnetic and spectroscopic (UV–Visible–NIR, XPS) studies to characterize the nature and the size of cobalt species; (iii) the evaluation of catalytic performances in the oxidative dehydrogenation of ethane and in butan-2-ol conversion.</p>
</section>
<section><no>2.</no>
<title>Experimental</title>
<subsect1><no>2.1.</no>
<title>Preparation of materials</title>
<p>Calcium hydroxyapatite, CaHap, was synthesized according to a procedure already published.<citref idrefs="cit23">23</citref>
Cobalt–hydroxyapatite catalysts, in short Co/CaHap, were prepared by suspending 1 g of CaHap in 25 mL of an aqueous solution of Co(NO<inf>3</inf>
)<inf>2</inf>
of known concentration (pH = 6). The mixture was heated at reflux under stirring during 1 hour, then evaporated to dryness and calcined in air at 823 K during 12 hours. The series is labelled Co(<it>z</it>
)/CaHap) where <it>z</it>
indicates the Co content in wt% (<it>cf.</it>
<tableref idrefs="tab1">Table 1</tableref>
).</p>
<table-entry id="tab1"><title>Chemical analysis and BET surface area of Co(<it>z</it>
)/CaHap samples</title>
<table><tgroup cols="4"><colspec colname="1" colwidth="7.38pi"></colspec>
<colspec colname="2" colwidth="9.70pi"></colspec>
<colspec colname="3" colwidth="7.38pi"></colspec>
<colspec colname="4" colwidth="7.38pi"></colspec>
<thead><row><entry colname="1" morerows="1">Sample</entry>
<entry colname="2" morerows="1">Surface area/m<sup>2</sup>
g<sup>−1</sup>
</entry>
<entry namest="3" nameend="4">Co (wt%)</entry>
</row>
<row><entry colname="3">Theoretical</entry>
<entry colname="4">Measured</entry>
</row>
</thead>
<tbody><row><entry>CaHap</entry>
<entry align="char" char=".">51</entry>
<entry align="char" char=".">0</entry>
<entry align="char" char=".">0</entry>
</row>
<row><entry>Co(0.2)/CaHap</entry>
<entry align="char" char=".">48</entry>
<entry align="char" char=".">0.20</entry>
<entry align="char" char=".">0.20</entry>
</row>
<row><entry>Co(0.4)/CaHap</entry>
<entry align="char" char=".">46</entry>
<entry align="char" char=".">0.40</entry>
<entry align="char" char=".">0.42</entry>
</row>
<row><entry>Co(0.9)/CaHap</entry>
<entry align="char" char=".">52</entry>
<entry align="char" char=".">0.90</entry>
<entry align="char" char=".">0.86</entry>
</row>
<row><entry>Co(1.3)/CaHap</entry>
<entry align="char" char=".">44</entry>
<entry align="char" char=".">1.30</entry>
<entry align="char" char=".">1.26</entry>
</row>
<row><entry>Co(1.7)/CaHap</entry>
<entry align="char" char=".">48</entry>
<entry align="char" char=".">1.70</entry>
<entry align="char" char=".">1.66</entry>
</row>
<row><entry>Co(2.3)/CaHap</entry>
<entry align="char" char=".">51</entry>
<entry align="char" char=".">2.30</entry>
<entry align="char" char=".">2.31</entry>
</row>
<row><entry>Co(3.4)/CaHap</entry>
<entry align="char" char=".">48</entry>
<entry align="char" char=".">3.40</entry>
<entry align="char" char=".">3.30</entry>
</row>
<row><entry>Co(5)/CaHap</entry>
<entry align="char" char=".">45</entry>
<entry align="char" char=".">5.00</entry>
<entry align="char" char=".">5.00</entry>
</row>
<row><entry>Co(14)/CaHap</entry>
<entry align="char" char=".">40</entry>
<entry align="char" char=".">14.0</entry>
<entry align="char" char=".">14.0</entry>
</row>
</tbody>
</tgroup>
</table>
</table-entry>
</subsect1>
<subsect1><no>2.2.</no>
<title>Characterization techniques</title>
<p>Chemical analyses were carried out at the CNRS Central Service of Analysis (Vernaison) by inductive coupling plasma-atomic emission spectroscopy (ICP-AES).</p>
<p>X-Ray diffraction patterns were obtained with a Panalytical X'Pert Pro diffractometer equipped with an X'celerator detector, in the 10°–110° (2<it>θ</it>
) range using Co Kα radiation (<it>λ</it>
= 1.7889 Å). The data were collected at room temperature with a step of 0.017 degrees (2<it>θ</it>
) and a time by step equal to 25 s.</p>
<p>The specific surface areas of the samples evacuated at 573 K were measured using nitrogen adsorption at 77 K with a Micromeritics Gemini 2360 apparatus.</p>
<p>FTIR transmission spectra were recorded on a Bruker Equinox spectrometer between 4000 and 400 cm<sup>−1</sup>
using self-supporting and KBr disks.</p>
<p>Diffuse reflectance spectra were recorded at room temperature between 190 and 2500 nm on a Varian Cary 5E spectrometer equipped with a double monochromator and an integrating sphere coated with polytetrafluoroethylene (PTFE). PTFE was used as a reference.</p>
<p>DC magnetic susceptibility was measured on a SQUID magnetometer in the 5–300 K range under a magnetic field of 200 Oe. The data, corrected from the diamagnetic contribution of apatite, are presented per mole of cobalt.</p>
<p>X-Ray photoelectron spectra were recorded on (i) a SSX-100 spectrometer (Surface Science Laboratory) equipped with a monochromated Al Kα source (1486.6 eV, 20 W) under about 10<sup>−7</sup>
Pa; (ii) a VG Scientific ESCALAB 250 system equipped with a micro-focused, monochromated Al Kα X-ray source (1486.6 eV, 650 µm spot size, 200 W) and a magnetic immersion lens which focuses electrons emitted from the sample over a cone of up to ±45° into the main lens column of the equipment. The magnetic immersion lens permits enhancement of the sensitivity by about one order of magnitude. The spectra were acquired in the constant analyzer energy mode, with a pass energy of 150 and 40 eV for the survey and the narrow regions, respectively. In addition, ultimate spectral resolution was achieved for the C1s and Si2p regions by setting the pass energy at 10 eV. The spectra were digitized, summed, smoothed and reconstructed using Gauss–Lorentz components. The measurements were carried out on powdered samples dispersed on an indium plaque, using the C1s peak at 285 eV as a reference. The surface composition of the samples was estimated from the XPS peak areas corrected by the difference in cross sections according to Scofield data.<citref idrefs="cit31">31</citref>
</p>
<p>Transmission electron microscopy (TEM) observations were carried out with a Jeol CXII microscope operating at 100 kV. The sample powder was dispersed in ethanol, one drop of the suspension was deposited on the carbon membrane of the microscope grid and the solvent was evaporated at room temperature. High resolution transmission electron microscopy (HRTEM) was performed with a Jeol JEM 2010F UHR operating at 200 kV. Local Energy Dispersive X-ray analyses were conducted on the Jeol JEM 2010F UHR using a PGT IMIX PC system with an analysis spot of 25 nm.</p>
</subsect1>
<subsect1><no>2.3.</no>
<title>Catalytic measurements</title>
<p>Prior to the reaction, the catalysts were sieved to a grain size ranging from 125 to 180 µm, then set into the reactor between two quartz wool plugs and treated by an air stream (for butan-2-ol conversion) or a nitrogen stream (for ODE) at 723 K during 2 hours, then cooled or heated to the reaction temperature.</p>
<subsect2><no>2.3.1.</no>
<title>Butan-2-ol conversion</title>
<p>Butan-2-ol conversion was studied between 373 K and 493 K in a U-shaped continuous flow microreactor operated at atmospheric pressure. An O<inf>2</inf>
–N<inf>2</inf>
mixture (<it>P</it>
<inf>O<inf>2</inf>
</inf>
/<it>P</it>
<inf>N<inf>2</inf>
</inf>
= 1/9) was led through a saturator containing butan-2-ol at constant temperature (283 K, <it>P</it>
<inf>butan-2-ol</inf>
= 840 Pa). The total flow rate of the feed mixture was maintained at 60 cm<sup>3</sup>
min<sup>−1</sup>
. The reaction mixture was analyzed by chromatography on a 4 m (1/8 in) stainless-steel column containing Carbowax 1500 (15%) on Chromosorb PAW (60/80 mesh).</p>
</subsect2>
<subsect2><no>2.3.2.</no>
<title>Oxidative dehydrogenation of ethane</title>
<p>The oxidative dehydrogenation of ethane (ODE) was performed in the 723 K–823 K temperature range in a quartz U-shaped fixed-bed microreactor operated at atmospheric pressure. The feed mixture was composed of ethane (6 vol%), O<inf>2</inf>
(3 vol%) and N<inf>2</inf>
(91 vol%) and the total flow rate was maintained at 60 cm<sup>3</sup>
min<sup>−1</sup>
. Analyses of the effluent gases were performed using two online chromatographs, the first for hydrocarbons separation on a Porapak Q column, equipped with a FID, and the second one equipped with a silica gel column and a TCD for the oxygenated products. Under the selected experimental conditions, only ethylene and CO<inf><it>x</it>
</inf>
were detected.</p>
<p>Pure CaHap exhibits a weak activity (ethane conversion = 6%) only at 823 K.</p>
</subsect2>
</subsect1>
</section>
<section><no>3.</no>
<title>Characterization of the catalysts</title>
<subsect1><no>3.1.</no>
<title>Chemical analysis, surface area and FTIR spectroscopy</title>
<p>The measured Co wt contents are close to the theoretical ones and the specific surface areas do not depend significantly on the cobalt loading (<tableref idrefs="tab1">Table 1</tableref>
) except for <it>z</it>
=14 where some fragility of the support has been detected by TEM (see below Section 3.3).</p>
<p>The support CaHap is white but the samples Co(<it>z</it>
)/CaHap are violet for <it>z</it>
< 0.9, grey for 0.9 ≤ <it>z</it>
≤ 1.7 and black for <it>z</it>
≥ 2.3.</p>
<p>The FTIR spectra show only the bands of the support whatever the cobalt content.</p>
</subsect1>
<subsect1><no>3.2.</no>
<title>XRD patterns</title>
<p>The XRD patterns of the Co(<it>z</it>
)/CaHap after calcination at 823 K are presented below (<figref idrefs="fig1">Fig. 1</figref>
). For samples containing less than 1.7 wt% Co, the patterns are identical to that of CaHap. The (311) peak of Co<inf>3</inf>
O<inf>4</inf>
is detected for <it>z</it>
≥ 1.7. The mean crystallite size determined by the Scherrer method varies from 25 nm (<it>z</it>
= 14 and <it>z</it>
= 3.4) to 15 nm (<it>z</it>
= 2.3 and <it>z</it>
= 1.7).</p>
<figure xsrc="b602514e-f1.tif" id="fig1"><title>XRD patterns of the Co(<it>z</it>
)/CaHap samples (* = (311) peak of Co<inf>3</inf>
O<inf>4</inf>
).</title>
</figure>
</subsect1>
<subsect1><no>3.3.</no>
<title>Transmission electron microscopy</title>
<p>Three samples with various cobalt contents (2.3, 3.4 and 14.0 wt%) have been examined.</p>
<p>(i) For <it>z</it>
= 2.3, apatite fringes are clearly evidenced. The high contrast areas have been identified by EDX as cobalt-rich species. In some zones, where the apatite fringes are detected below the cobalt zone (<figref idrefs="fig2">Fig. 2a</figref>
), these species may be associated to badly crystallized and very thin cobalt–oxygen Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
entities. In other zones, a lot of numerous small particles are detected with a size distribution ranging from 1.5 to 4.5 nm (<figref idrefs="fig2">Fig. 2b</figref>
) together with larger particles in the 10–35 nm range (not shown). These particles may be identified to Co<inf>3</inf>
O<inf>4</inf>
nanocrystals evidenced by XRD, with a mean crystal size of 15 nm deduced from XRD line broadening.</p>
<figure xsrc="b602514e-f2.tif" id="fig2"><title>(a) TEM picture for <it>z</it>
= 2.3. (b) Particle size distribution.</title>
</figure>
<p>(ii) For <it>z</it>
= 3.4 (picture not shown), well crystallized areas and reticular planes are observed on some support particles. The morphology of the CaHap particles is well defined but not homogeneous. Aside from spherical particles, with a diameter between 20 and 46 nm, platelets with a main axis between 35 and 50 nm are observed. The cobalt-rich particles are present as small spheres in the 4–18 nm range and as larger ensembles in the 20–40 nm range. No badly crystallized zones are detected.</p>
<p>(iii) For <it>z</it>
= 14 (picture not presented), the sample becomes amorphous under the electron beam. However, some support areas keep a good crystallinity with reticular planes detected. The spatial repartition of the Co<inf>3</inf>
O<inf>4</inf>
spherical particles is not uniform and their size varies from 4 to 29 nm.</p>
</subsect1>
<subsect1><no>3.4.</no>
<title>UV–Visible–NIR DRS</title>
<subsect2><no>3.4.1.</no>
<title>Literature survey</title>
<p>Difficulties are encountered in the interpretation of the optical spectrum of cobalt species in oxide matrices. The tetrahedral and octahedral high spin (HS) Co<sup>2+</sup>
and octahedral low spin (LS) Co<sup>3+</sup>
species, indeed, are often treated as individual non-interacting sites. But when these entities are close enough, they form clusters or three-dimensional ensembles and they must be considered as interacting components. This is the case of Co<inf>3</inf>
O<inf>4</inf>
, which forms a normal spinel lattice composed of tetrahedral (HS) Co<sup>2+</sup>
and octahedral (LS) Co<sup>3+</sup>
. In this case, metal to metal charge transfers (MMCT) [t<inf>2</inf>
(Co<sup>2+</sup>
) → t<inf>2g</inf>
(Co<sup>3+</sup>
)] are expected. But interactions may also occur with metal cations of the host oxide matrix when these cations may change their oxidation state (<it>e.g.</it>
in TiO<inf>2</inf>
). In these <it>redox active matrices</it>
, metal (guest)–metal (host) charge transfers occur and interpretations are not straightforward. Hence, model compounds of non-interacting cobalt species should then be found in <it>redox inactive oxide matrices</it>
, such as Al<inf>2</inf>
O<inf>3</inf>
, SiO<inf>2</inf>
, MgO, glasses and ZnO (at least in non-reducing conditions). In isolated (non-interacting) cobalt ions, d–d transitions in tetrahedral and octahedral (HS) Co<sup>2+</sup>
are so intricate in the visible range (<tableref idrefs="tab2">Table 2</tableref>
, <citref idrefs="cit32 cit33 cit34 cit35 cit36 cit37 cit38 cit39" position="baseline">ref. 32–39</citref>
) that the discrimination must involve examination of the NIR range.<citref idrefs="cit40">40</citref>
In addition, six-coordinated <it>D</it>
<inf>3h</inf>
symmetry has been encountered in (HS) Co<sup>2+</sup>
.<citref idrefs="cit23">23</citref>
(LS) Co<sup>3+</sup>
species prefer an octahedral environment and only two d–d transitions are generally detected.<citref idrefs="cit41">41</citref>
On the other hand, ligand to metal charge transfer transitions (LMCT) are involved. To summarize, the spectral features of isolated <it>T</it>
<inf>d</inf>
and <it>O</it>
<inf>h</inf>
cobalt species may be depicted as follows (<tableref idrefs="tab2">Table 2</tableref>
):</p>
<table-entry id="tab2"><title>Attribution of d–d transitions of cobalt in oxide matrices (wavelengths in nm)</title>
<table><tgroup cols="8"><colspec colname="1" colwidth="10.60pi"></colspec>
<colspec colname="2" colwidth="5.59pi"></colspec>
<colspec colname="3" colwidth="13.39pi" align="left"></colspec>
<colspec colname="4" colwidth="5.42pi" align="char"></colspec>
<colspec colname="5" colwidth="5.55pi"></colspec>
<colspec colname="6" colwidth="4.56pi" align="char"></colspec>
<colspec colname="7" colwidth="5.42pi" align="char"></colspec>
<colspec colname="8" colwidth="4.56pi" align="char"></colspec>
<thead><row><entry colname="1" morerows="1"> </entry>
<entry namest="2" nameend="3">Co<sup>2+</sup>
(<it>T</it>
<inf>d</inf>
)</entry>
<entry namest="4" nameend="6" align="left">Co<sup>2+</sup>
(<it>O</it>
<inf>h</inf>
)</entry>
<entry namest="7" nameend="8" align="left">Co<sup>3+</sup>
(<it>O</it>
<inf>h</inf>
)</entry>
</row>
<row><entry colname="2"><it>ν</it>
<inf>2</inf>
<fnoteref idrefs="tab2fna"></fnoteref>
</entry>
<entry colname="3"><it>ν</it>
<inf>3</inf>
</entry>
<entry colname="4" align="left"><it>ν</it>
<inf>1</inf>
</entry>
<entry colname="5"><it>ν</it>
<inf>2</inf>
</entry>
<entry colname="6" align="left"><it>ν</it>
<inf>3</inf>
</entry>
<entry colname="7" align="left"><it>ν</it>
<inf>1</inf>
</entry>
<entry colname="8" align="left"><it>ν</it>
<inf>2</inf>
</entry>
</row>
</thead>
<tfoot><row><entry namest="1" nameend="8"><footnote id="tab2fna"><it>ν</it>
<inf>1</inf>
is located in the IR range and rarely identified.</footnote>
</entry>
</row>
</tfoot>
<tbody><row><entry>Co/ZnO<citref idrefs="cit32">32</citref>
</entry>
<entry>1612, 1470, 1315</entry>
<entry align="char" char=".">650, 600, 566</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
</row>
<row><entry>Co/ZnO<citref idrefs="cit33">33</citref>
</entry>
<entry>1630, 1390, 1310</entry>
<entry align="char" char=".">650, 620, 570</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
</row>
<row><entry>Co/APO molecular sieves<citref idrefs="cit34">34</citref>
</entry>
<entry>Not recorded</entry>
<entry align="char" char=".">626, 580, 540</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
</row>
<row><entry>Co/MgO<citref idrefs="cit35">35</citref>
</entry>
<entry>—</entry>
<entry>—</entry>
<entry>1176</entry>
<entry>581</entry>
<entry>526</entry>
<entry>—</entry>
<entry>—</entry>
</row>
<row><entry>Co/Al<inf>2</inf>
O<inf>3</inf>
<citref idrefs="cit36">36</citref>
</entry>
<entry>1560, 1350, 1250</entry>
<entry align="char" char=".">625, 590, 550</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>700</entry>
<entry>450</entry>
</row>
<row><entry>Co/SiO<inf>2</inf>
<citref idrefs="cit37">37</citref>
</entry>
<entry>Not recorded</entry>
<entry align="char" char=".">580, 650</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>710</entry>
<entry>410</entry>
</row>
<row><entry>ZnCo<inf>2</inf>
O<inf>4</inf>
<citref idrefs="cit38 cit39">38,39</citref>
</entry>
<entry>—</entry>
<entry align="char" char=".">—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>667</entry>
<entry>400</entry>
</row>
<row><entry>CoAl<inf>2</inf>
O<inf>4</inf>
<citref idrefs="cit36">36</citref>
</entry>
<entry>1370</entry>
<entry align="char" char=".">662, 588 552</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
<entry>—</entry>
</row>
<row><entry>CoO<citref idrefs="cit33">33</citref>
</entry>
<entry>—</entry>
<entry>—</entry>
<entry>1280</entry>
<entry>606 (sh), 770</entry>
<entry>550</entry>
<entry>—</entry>
<entry>—</entry>
</row>
</tbody>
</tgroup>
</table>
</table-entry>
<p>(i) <it>localized (intrasite) spin-allowed d–d transitions</it>
: for (HS) Co<sup>2+</sup>
(<it>O</it>
<inf>h</inf>
), three transitions are expected, <it>ν</it>
<inf>2</inf>
being very close to <it>ν</it>
<inf>3</inf>
, but symmetry lowering (due in particular to the Jahn–Teller effect) may induce additional transitions; for Co<sup>2+</sup>
(<it>T</it>
<inf>d</inf>
), only two transitions are generally observed (among three expected) because the low energy transition (<it>ν</it>
<inf>1</inf>
) appears in the IR range; for (LS) Co<sup>3+</sup>
(<it>O</it>
<inf>h</inf>
), two spin allowed transitions are recorded in the visible range;</p>
<p>(ii) <it>ligand to metal charge transfer</it>
(LMCT) <it>transitions</it>
: two types are involved: [p(O<sup>2−</sup>
) → t<inf>2</inf>
(Co<sup>2+</sup>
) and p(O<sup>2−</sup>
) → t<inf>2g</inf>
(Co<sup>3+</sup>
)]. Bands in the 350–820 nm range have been ascribed to these transitions but there is no agreement in the literature concerning their position.<citref idrefs="cit7 cit9 cit34 cit38 cit39 cit42 cit43 cit44 cit45 cit46 cit47">7,9,34,38,39,42–47</citref>
</p>
<p>For interacting cobalt ions encountered in Co<inf>3</inf>
O<inf>4</inf>
, metal–metal charge transfers (MMCT) are expected; they are believed to occur either near 750 nm<citref idrefs="cit7">7</citref>
or around 1300 nm<citref idrefs="cit9 cit38 cit42 cit43 cit47">9,38,42,43,47</citref>
somewhat overlapping the d–d transitions.</p>
</subsect2>
<subsect2><no>3.4.2.</no>
<title>Results and discussion</title>
<p>The spectrum of CaHap (not shown) contains mainly a band located at 296 nm, attributed to O<sup>2−</sup>
→ Ca<sup>2+</sup>
charge transfer, and several bands in the near infra-red due to (i) <it>ν</it>
<inf>(OH)</inf>
overtones of <it>ν</it>
<inf>(OH)</inf>
of surface hydroxyl groups (1386 and 1432 nm) and (ii) combinations of <it>ν</it>
<inf>(OH)</inf>
and <it>δ</it>
<inf>(OH)</inf>
(1940 and 2220 nm).<citref idrefs="cit23">23</citref>
</p>
<p>The UV–visible–NIR spectra of Co(<it>z</it>
)/CaHap samples are displayed in <figref idrefs="fig3">Fig. 3</figref>
on the Schuster–Kubelka–Munk (SKM) scale. The wavelengths and the attributions of bands are presented in <tableref idrefs="tab3">Table 3</tableref>
.</p>
<figure xsrc="b602514e-f3.tif" id="fig3"><title>Diffuse reflectance spectra of Co(<it>z</it>
)/Hap samples: 1) <it>z</it>
= 0.2; 2) <it>z</it>
= 0.4; 3) <it>z</it>
= 0.9; 4) <it>z</it>
= 1.3; 5) <it>z</it>
= 1.7; 6) <it>z</it>
= 2.3; 7) <it>z</it>
= 3.4 and 8) <it>z</it>
= 14.</title>
</figure>
<table-entry id="tab3"><title>Wavelength (nm) and attribution of UV–visible–NIR d–d transitions of Co/CaHap samples</title>
<table><tgroup cols="8"><colspec colname="1" colwidth="5.12pi"></colspec>
<colspec colname="2" colwidth="5.78pi"></colspec>
<colspec colname="3" colwidth="5.78pi"></colspec>
<colspec colname="4" colwidth="6.33pi"></colspec>
<colspec colname="5" colwidth="6.45pi"></colspec>
<colspec colname="6" colwidth="6.33pi"></colspec>
<colspec colname="7" colwidth="5.22pi"></colspec>
<colspec colname="8" colwidth="5.22pi"></colspec>
<thead><row><entry colname="1" morerows="1">Co(<it>z</it>
)/Hap</entry>
<entry namest="2" nameend="3">Co<sup>2+</sup>
(<it>T</it>
<inf>d</inf>
)</entry>
<entry namest="4" nameend="6">Co<sup>2+</sup>
(<it>O</it>
<inf>h</inf>
)</entry>
<entry namest="7" nameend="8">Co<sup>3+</sup>
(<it>O</it>
<inf>h</inf>
)</entry>
</row>
<row><entry colname="2"><it>ν</it>
<inf>2</inf>
<sup>4</sup>
A<inf>2</inf>
→<sup>4</sup>
T<inf>1</inf>
(F)</entry>
<entry colname="3"><it>ν</it>
<inf>3</inf>
<sup>4</sup>
A<inf>2</inf>
→<sup>4</sup>
T<inf>1</inf>
(P)</entry>
<entry colname="4"><it>ν</it>
<inf>1</inf>
<sup>4</sup>
T<inf>1g</inf>
→<sup>4</sup>
T<inf>2g</inf>
(F)</entry>
<entry colname="5"><it>ν</it>
<inf>2</inf>
<sup>4</sup>
T<inf>1g</inf>
→<sup>4</sup>
A<inf>2g</inf>
(F)</entry>
<entry colname="6"><it>ν</it>
<inf>3</inf>
<sup>4</sup>
T<inf>1g</inf>
→<sup>4</sup>
T<inf>1g</inf>
(P)</entry>
<entry colname="7"><it>ν</it>
<inf>1</inf>
<sup>1</sup>
A<inf>1g</inf>
→<sup>1</sup>
T<inf>1g</inf>
</entry>
<entry colname="8"><it>ν</it>
<inf>2</inf>
<sup>1</sup>
A<inf>1g</inf>
→<sup>1</sup>
T<inf>2g</inf>
</entry>
</row>
</thead>
<tfoot><row><entry namest="1" nameend="8"><footnote id="tab3fna">Contribution of LMCT and MMCT transitions.</footnote>
</entry>
</row>
</tfoot>
<tbody><row><entry>0.2 ≤ <it>z</it>
≤ 0.4</entry>
<entry>—</entry>
<entry align="char">—</entry>
<entry>1200–1500</entry>
<entry>570, 640</entry>
<entry>520</entry>
<entry align="left" char=".">740–750</entry>
<entry align="left" char=".">360–380</entry>
</row>
<row><entry colname="1">0.9 ≤ <it>z</it>
≤ 2.3</entry>
<entry colname="2">1200–1500</entry>
<entry colname="3" align="char">630</entry>
<entry colname="4">1200–1500</entry>
<entry namest="5" nameend="6" morerows="1">masked by d–d transitions of Co<sup>3+</sup>
(Oh) and CT transitions</entry>
<entry colname="7" align="left" char=".">725<fnoteref idrefs="tab3fna"></fnoteref>
</entry>
<entry colname="8" align="left" char=".">430–495<fnoteref idrefs="tab3fna"></fnoteref>
</entry>
</row>
<row><entry colname="1"><it>z</it>
≥ 3.4</entry>
<entry colname="2">1200–1500</entry>
<entry colname="3" align="char">630</entry>
<entry colname="4">1200–1500</entry>
<entry colname="7" align="left" char=".">720<fnoteref idrefs="tab3fna"></fnoteref>
</entry>
<entry colname="8" align="left" char=".">440, 475<fnoteref idrefs="tab3fna"></fnoteref>
</entry>
</row>
</tbody>
</tgroup>
</table>
</table-entry>
<p>For low Co contents, (<it>z</it>
= 0.2 and <it>z</it>
= 0.4), the spectra (magnified in <figref idrefs="fig4">Fig. 4</figref>
) present: (i) a band at 360 nm moving to 380 nm upon increasing Co loading, (ii) a massif of three bands and shoulders at 520 nm, 570 nm and 640 nm, (iii) a shoulder at 750 nm moving to 740 nm, (iv) a broad and weak band between 1200 and 1500 nm. The latter broad band and the massif of three bands are assigned to six-coordinated Co<sup>2+</sup>
ions introduced in CaHap by ion-exchange.<citref idrefs="cit23">23</citref>
For comparison, the spectra of exchanged samples containing 0.19 and 0.38 wt% Co are presented in <figref idrefs="fig4">Fig. 4</figref>
. In agreement with previous results,<citref idrefs="cit22 cit23 cit41">22,23,41</citref>
the band at 520 nm may be ascribed to the <it>ν</it>
<inf>3</inf>
transition [<sup>4</sup>
T<inf>1g</inf>
(F) → <sup>4</sup>
T<inf>1g</inf>
(P)] and the bands at 570 and 640 nm to the <it>ν</it>
<inf>2</inf>
transition [<sup>4</sup>
T<inf>1g</inf>
(F) → <sup>4</sup>
A<inf>2g</inf>
(F)]. The splitting of this band originates from spin–orbit coupling or the Jahn–Teller effect.<citref idrefs="cit36 cit41">36,41</citref>
The broad band appearing between 1200 and 1500 nm is attributed, because of its low intensity, to the <it>ν</it>
<inf>1</inf>
transition [<sup>4</sup>
T<inf>1g</inf>
(F) → <sup>4</sup>
T<inf>2g</inf>
(F)] rather than to the <it>ν</it>
<inf>2</inf>
transition [<sup>4</sup>
A<inf>2</inf>
(F) → <sup>4</sup>
T<inf>1</inf>
(F)] of Co<sup>2+</sup>
in tetrahedral symmetry.<citref idrefs="cit41">41</citref>
Contribution of the <it>ν</it>
′<inf>1</inf>
transition [4A′<sup>2</sup>
(F) → <sup>4</sup>
E′(F)] of <it>D</it>
<inf>3h</inf>
surroundings (probably six-coordinated) may also be envisaged.<citref idrefs="cit23">23</citref>
These results suggest that at low content, Co<sup>2+</sup>
ions are exchanged with Ca<sup>2+</sup>
ions of the apatite matrix during the short impregnation time. However, bands <it>A</it>
(360–380 nm) and <it>B</it>
(740–750 nm) are not observed on the spectra of exchanged Co<sup>2+</sup>
. These bands may be assigned to the <it>ν</it>
<inf>1</inf>
[<sup>1</sup>
A<inf>1g</inf>
→ <sup>1</sup>
T<inf>g</inf>
] and <it>ν</it>
<inf>2</inf>
[<sup>1</sup>
A<inf>1g</inf>
→ <sup>1</sup>
T<inf>2g</inf>
] transitions in the octahedral low spin (LS) Co<sup>3+</sup>
ion.<citref idrefs="cit36 cit37 cit39 cit40 cit41">36,37,39–41</citref>
It is relevant to note that, upon increasing the Co content, <it>A</it>
moves towards lower energies (higher wavelengths) while <it>B</it>
is shifted towards higher energies (lower wavelengths).</p>
<figure xsrc="b602514e-f4.tif" id="fig4"><title>Comparison of impregnated (spectra in bold) and exchanged samples for low Co contents.</title>
</figure>
<p>Upon increasing the Co content further to 0.9 wt%, the spectrum suffers significant changes, much more important than those observed for the exchanged samples (<figref idrefs="fig5">Fig. 5</figref>
). It shows: (i) twin peaks at 430–495 nm, (ii) a large shoulder near 725 nm with a very small peak near 630 nm, (iii) a marked intensity increase of the broad and intense band between 1200 and 1500 nm. This intensity increase is a signature of the tetrahedral Co<sup>2+</sup>
species (<it>ν</it>
<inf>2</inf>
), the <it>ν</it>
<inf>1</inf>
transition appearing at 630 nm. The bands at 430–495 nm and 725 nm may be ascribed to d–d transitions in Co<sup>3+</sup>
(<it>O</it>
<inf>h</inf>
) entities (see <tableref idrefs="tab2 tab3">Table 2 and 3</tableref>
) with a contribution of LMCT [p(O<sup>2−</sup>
) → t<inf>2</inf>
(Co<sup>2+</sup>
) and p(O<sup>2−</sup>
) → t<inf>2g</inf>
(Co<sup>3+</sup>
)] and MMCT [t<inf>2</inf>
(Co<sup>2+</sup>
) → t<inf>2g</inf>
(Co<sup>3+</sup>
)].<citref idrefs="cit7">7</citref>
These spectral features indicate the formation of Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters composed of ensembles of octahedral Co<sup>3+</sup>
and tetrahedral Co<sup>2+</sup>
, precursors of Co<inf>3</inf>
O<inf>4</inf>
nanocrystals.</p>
<figure xsrc="b602514e-f5.tif" id="fig5"><title>Comparison of impregnated (spectrum in bold) and exchanged samples for intermediate Co contents.</title>
</figure>
<p>At higher Co loadings (<it>z</it>
≥ 1.7), the intensity ratio of the most intense peaks (430–495 nm and 725 nm) increases and finally reaches unity. Comparison with the spectrum of a mechanical mixture of Co<inf>3</inf>
O<inf>4</inf>
and CaHap (not presented) shows that Co<inf>3</inf>
O<inf>4</inf>
is present, as already evidenced by XRD. Simultaneously, the color moves from grey to black.</p>
<p>It may be concluded that, according to the cobalt content, the cobalt ions are present as (i) isolated Co<sup>2+</sup>
, (ii) Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters, (iii) Co<inf>3</inf>
O<inf>4</inf>
nanocrystals.</p>
</subsect2>
</subsect1>
<subsect1><no>3.5.</no>
<title>Magnetic measurements</title>
<p>Except for the cobalt-richest sample (<it>z</it>
= 14), the magnetic susceptibility <it>χ</it>
of this series decreases as a hyperbolic function of temperature, typical of paramagnetic systems (not shown). For the Co(14)/CaHap sample, the <it>χ</it>
(<it>T</it>
) curve presents an inflexion point at low temperature (<figref idrefs="fig6">Fig. 6</figref>
). This suggests contributions of both antiferromagnetic and paramagnetic functions. After correction of the paramagnetic contribution (<figref idrefs="fig6">Fig. 6</figref>
), the <it>χ</it>
(<it>T</it>
) curve of this sample is practically superposed onto that of the Co<inf>3</inf>
O<inf>4</inf>
reference compound, well known to show an antiferromagnetic 3D order below <it>T</it>
<inf>N</inf>
= 40 K.<citref idrefs="cit39">39</citref>
In fact, for this sample, <it>χ</it>
(<it>T</it>
) presents a maximum at 40 K and decreases to zero upon further cooling. Hence, impregnation of CaHap by concentrated Co<sup>2+</sup>
solutions (14 wt% cobalt) leads to the formation of Co<inf>3</inf>
O<inf>4</inf>
particles, in agreement with XRD data.</p>
<figure xsrc="b602514e-f6.tif" id="fig6"><title><it>χ</it>
(<it>T</it>
) curves of a Co(14)/CaHap sample and of Co<inf>3</inf>
O<inf>4</inf>
.</title>
</figure>
<p>However, the same sample also contains isolated paramagnetic Co<sup>2+</sup>
ions. These ions are probably exchanged with Ca<sup>2+</sup>
ions of CaHap during the short impregnation time (1 hour); this interpretation is supported by (i) the UV–visible–NIR spectra of low-loaded cobalt samples, similar to the spectra of samples prepared by ion exchange (see Section 3.4),<citref idrefs="cit23">23</citref>
(ii) the paramagnetic character of cobalt-poor (≤1.7 wt%) samples, observed at low temperature on the <it>χ</it>
(<it>T</it>
) curves. For these samples, the <it>χ</it>
<sup>−1</sup>
function decreases linearly with the temperature (<figref idrefs="fig7">Fig. 7</figref>
) in the 300 K–20 K range. This behaviour is typical of the Curie–Weiss law. However, for <it>z</it>
= 0.2, the agreement with the Curie law is poor because of a large orbital contribution, which attests the predominance of octahedral sites.</p>
<figure xsrc="b602514e-f7.tif" id="fig7"><title>Variation of <it>χ</it>
<sup>−1</sup>
<it>versus</it>
temperature for cobalt-poor samples and for Co<inf>3</inf>
O<inf>4</inf>
.</title>
</figure>
<p>The values of <it>θ</it>
<inf>p</inf>
(paramagnetic Curie temperature) and <it>µ</it>
<inf>eff</inf>
(effective magnetic moment in Bohr magnetons) are collected in <tableref idrefs="tab4">Table 4</tableref>
. <it>θ</it>
<inf>P</inf>
is near zero for cobalt-poor samples (<it>z</it>
≤ 1.7), which confirms their paramagnetism. For the other samples, when <it>z</it>
increases, <it>θ</it>
<inf>P</inf>
decreases down to −140 K for <it>z</it>
= 14, a value close to that of Co<inf>3</inf>
O<inf>4</inf>
(−150 K). This evolution of <it>θ</it>
<inf>P</inf>
with the cobalt content suggests the existence of increasing antiferromagnetic correlations between Co<sup>2+</sup>
cations until a 3D antiferromagnetic order is established for <it>z</it>
= 14. On the other hand, the <it>µ</it>
<inf>eff</inf>
values decrease when <it>z</it>
increases, which may arise from a change of oxidation number and/or a modification of the site symmetry of Co<sup>2+</sup>
. As shown in <tableref idrefs="tab4">Table 4</tableref>
, indeed, <it>µ</it>
<inf>eff</inf>
decreases from 6.16 <it>µ</it>
<inf>B</inf>
(for <it>z</it>
= 0.2) to 2.84 <it>µ</it>
<inf>B</inf>
(for <it>z</it>
= 14). In fact, the <it>µ</it>
<inf>eff</inf>
values reported in the literature for Co<sup>2+</sup>
ions in purely octahedral symmetry, <it>e.g.</it>
in cobalt(<scp>ii</scp>
) hydroxide β-Co(OH)<inf>2</inf>
, are about 5.2 <it>µ</it>
<inf>B</inf>
.<citref idrefs="cit48">48</citref>
These values are high because of the strong spin–orbit coupling of Co<sup>2+</sup>
ions in that symmetry (fundamental state <sup>4</sup>
T<inf>2g</inf>
). However, the fundamental term is <sup>4</sup>
F<inf>9/2</inf>
for a free Co<sup>2+</sup>
ion (3 d<sup>7</sup>
); when the orbital contribution is fully taken into account, <it>µ</it>
<inf>eff</inf>
amounts to 6.63 <it>µ</it>
<inf>B</inf>
.<citref idrefs="cit49">49</citref>
Such high values have been reported for octahedral Co<sup>2+</sup>
in aluminosilicate glasses containing 40 mol% Co.<citref idrefs="cit50">50</citref>
In purely tetrahedral symmetry, the <it>µ</it>
<inf>eff</inf>
values of Co<sup>2+</sup>
ions (fundamental state <sup>4</sup>
A<inf>2</inf>
) vary between 3.87 and 4.28 <it>µ</it>
<inf>B</inf>
(no significant orbital contribution) as in CoAl<inf>2</inf>
O<inf>4</inf>
<citref idrefs="cit51">51</citref>
or Zn<inf>1−<it>x</it>
</inf>
Co<inf><it>x</it>
</inf>
O.<citref idrefs="cit33">33</citref>
On the other hand, the <it>µ</it>
<inf>eff</inf>
value of LS Co<sup>3+</sup>
ions (d<sup>6</sup>
) is zero. Hence, for the cobalt-poorest sample Co(0.2)/CaHap, the Co<sup>2+</sup>
ions are located in an octahedral oxygen environment resulting from an exchange with Ca<sup>2+</sup>
ions. For cobalt-richer samples, the µ<inf>eff</inf>
values are all below the spin-only value (3.87 <it>µ</it>
<inf>B</inf>
) and decrease from 3.44 to 2.84 <it>µ</it>
<inf>B</inf>
. This means that a part of the Co ions are present as HS tetrahedral Co<sup>2+</sup>
(<it>µ</it>
<inf>eff</inf>
≈ 4.0 <it>µ</it>
<inf>B</inf>
) and as LS Co<sup>3+</sup>
(<it>µ</it>
<inf>eff</inf>
= 0), which suggests the formation of first Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
species and then spinel oxide Co<inf>3</inf>
O<inf>4</inf>
. For the cobalt-richest sample Co(14)/CaHap, the <it>µ</it>
<inf>eff</inf>
value is very close to that of Co<inf>3</inf>
O<inf>4</inf>
. It has been previously shown<citref idrefs="cit23">23</citref>
that Co<sup>2+</sup>
ions exchanged with Ca<sup>2+</sup>
ions of CaHap are not oxidized upon calcination at 823 K because of strong support interaction. The formation of Co<sup>3+</sup>
ions for Co contents ≥0.4 wt% indicates that the interaction of the Co<sup>2+</sup>
precursor with the apatite support is weak upon impregnation, <it>i.e.</it>
the role of the apatite is less significant.</p>
<table-entry id="tab4"><title>Main magnetic features of Co(<it>z</it>
)/CaHap samples and of Co<inf>3</inf>
O<inf>4</inf>
</title>
<table><tgroup cols="5"><colspec colname="1" colwidth="7.37pi"></colspec>
<colspec colname="2" colwidth="7.17pi"></colspec>
<colspec colname="3" colwidth="7.17pi"></colspec>
<colspec colname="4" colwidth="7.19pi"></colspec>
<colspec colname="5" colwidth="9.97pi"></colspec>
<thead><row><entry>Sample</entry>
<entry>Co%</entry>
<entry><it>µ</it>
<inf>eff</inf>
/<it>µ</it>
<inf>B</inf>
</entry>
<entry><it>θ</it>
<inf>p</inf>
/K</entry>
<entry>Behaviour</entry>
</row>
</thead>
<tfoot><row><entry namest="1" nameend="5"><footnote id="tab4fna">Value measured on the <it>χ</it>
<sup>−1</sup>
(<it>T</it>
) curve corrected for the paramagnetic contribution</footnote>
</entry>
</row>
</tfoot>
<tbody><row><entry>Co(0.2)/CaHap</entry>
<entry align="char" char=".">0.2</entry>
<entry align="char" char=".">6.16</entry>
<entry align="char" char=".">6</entry>
<entry>Paramagnetic (with orbital contribution)</entry>
</row>
<row><entry>Co(0.9)/CaHap</entry>
<entry align="char" char=".">0.9</entry>
<entry align="char" char=".">3.44</entry>
<entry align="char" char=".">9</entry>
<entry>Paramagnetic (without orbital contribution)</entry>
</row>
<row><entry>Co(1.3)/CaHap</entry>
<entry align="char" char=".">1.3</entry>
<entry align="char" char=".">3.33</entry>
<entry align="char" char=".">6</entry>
<entry>Paramagnetic (without orbital contribution)</entry>
</row>
<row><entry>Co(1.7)/CaHap</entry>
<entry align="char" char=".">1.7</entry>
<entry align="char" char=".">3.44</entry>
<entry align="char" char=".">−7</entry>
<entry>Paramagnetic, with occurrence at low temperature (<100 K) of weak antiferromagnetic correlations at short distances, including contributions of small quantities of Co<inf>3</inf>
O<inf>4</inf>
</entry>
</row>
<row><entry>Co(3.4)/CaHap</entry>
<entry align="char" char=".">3.4</entry>
<entry align="char" char=".">3.26</entry>
<entry align="char" char=".">−49</entry>
<entry>Paramagnetic with stronger antiferromagnetic correlations at short distances ascribed to larger amounts of Co<inf>3</inf>
O<inf>4</inf>
</entry>
</row>
<row><entry>Co(14)/CaHap</entry>
<entry align="char" char=".">14.0</entry>
<entry align="char" char=".">2.84</entry>
<entry align="char" char=".">−140<fnoteref idrefs="tab4fna"></fnoteref>
</entry>
<entry>Antiferromagnetic (<it>T</it>
<inf>N</inf>
= 33 K) with a weak paramagnetic contribution recorded at low <it>T</it>
</entry>
</row>
<row><entry>Co<inf>3</inf>
O<inf>4</inf>
</entry>
<entry align="char" char=".">73.4</entry>
<entry align="char" char=".">2.88</entry>
<entry align="char" char=".">−150</entry>
<entry>Antiferromagnetic (<it>T</it>
<inf>N</inf>
= 33 K)<citref idrefs="cit1 cit2">1,2</citref>
</entry>
</row>
</tbody>
</tgroup>
</table>
</table-entry>
<p>The <it>χ</it>
<it>T</it>
= f(<it>T</it>
) curves for various cobalt loadings (<figref idrefs="fig8">Fig. 8</figref>
) also give pertinent information not apparent from <figref idrefs="fig7">Fig. 7</figref>
. For <it>z</it>
= 1.7, <it>χT</it>
seems independent of temperature only above 100 K. Below 100 K, <it>χT</it>
decreases rapidly to zero, suggesting the occurrence of short distance antiferromagnetic correlations between paramagnetic Co<sup>2+</sup>
ions. For <it>z</it>
= 3.4, <it>χT</it>
decreases slowly from ambient temperature down to 100 K, then more rapidly to zero upon cooling, which also confirms the antiferromagnetic correlations.</p>
<figure xsrc="b602514e-f8.tif" id="fig8"><title>Variation of <it>χT</it>
<it>versus</it>
temperature for some Co(<it>z</it>
)/Hap samples and Co<inf>3</inf>
O<inf>4</inf>
.</title>
</figure>
<p>However, the absence of a maximum on the magnified <it>χ</it>
<inf>M</inf>
= f(<it>T</it>
) curve (not presented) calls for the absence of a long distance order. As they appear at high temperature these correlations become stronger but the occurrence of a three-dimensional order is still scarce. This result may be accounted for by the formation of Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters, suggested by TEM observations (see above Section 3.3), before the appearance of Co<inf>3</inf>
O<inf>4</inf>
nanocrystals detected by XRD for <it>z</it>
≥ 1.7. Finally, for <it>z</it>
= 14, the variation of <it>χT</it>
is nearly superimposed on that of Co<inf>3</inf>
O<inf>4</inf>
(<figref idrefs="fig8">Fig. 8</figref>
), confirming the predominance of this oxide on the support surface.</p>
<p>To sum up, the magnetic study performed on this series was based on (i) the examination of <it>χ</it>
(<it>T</it>
), <it>χ</it>
<sup>−1</sup>
(<it>T</it>
) and <it>χT</it>
(<it>T</it>
) curves, (ii) the determination of <it>θ</it>
<inf>p</inf>
and <it>µ</it>
<inf>eff</inf>
values upon varying the cobalt content. It shows that the impregnation method progressively induces the formation of surface Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters and Co<inf>3</inf>
O<inf>4</inf>
. However, a paramagnetic contribution is still observed on all samples, because some isolated Co<sup>2+</sup>
ions are exchanged with Ca<sup>2+</sup>
ions during the impregnation stage.</p>
</subsect1>
<subsect1><no>3.6</no>
<title>X-Ray photoelectron spectroscopy (XPS)</title>
<p>The Co2p<inf>3/2</inf>
binding energies (BE), the surface atomic ratio (Co/Ca)<inf>XPS</inf>
, the bulk atomic ratio, the spin–orbit coupling energy Δ<it>E</it>
(Co2p<inf>1/2</inf>
− Co2p<inf>3/2</inf>
) and the S/M (satellite peak/main peak) ratios are presented in <tableref idrefs="tab5">Table 5</tableref>
.</p>
<table-entry id="tab5"><title>XPS data of Co(<it>z</it>
)/CaHap samples and reference compounds</title>
<table><tgroup cols="6"><colspec colname="1" colwidth="6.15pi"></colspec>
<colspec colname="2" colwidth="6.15pi"></colspec>
<colspec colname="3" colwidth="6.15pi"></colspec>
<colspec colname="4" colwidth="6.15pi"></colspec>
<colspec colname="5" colwidth="6.15pi"></colspec>
<colspec colname="6" colwidth="6.15pi"></colspec>
<thead><row><entry><it>z</it>
</entry>
<entry>BE (Co 2p<inf>3/2</inf>
)/eV</entry>
<entry>(Co/Ca)<inf>XPS</inf>
(at%)</entry>
<entry>(Co/Ca)<inf>bulk</inf>
(at%)</entry>
<entry>Δ<it>E</it>
/eV</entry>
<entry>S/M</entry>
</row>
</thead>
<tbody><row><entry>1.3</entry>
<entry align="char" char=".">781.6</entry>
<entry align="char" char=".">10.6</entry>
<entry align="char" char=".">2.25</entry>
<entry align="char" char=".">15.6</entry>
<entry align="char" char=".">0.67</entry>
</row>
<row><entry>2.3</entry>
<entry align="char" char=".">781.6</entry>
<entry align="char" char=".">9.82</entry>
<entry align="char" char=".">4.01</entry>
<entry align="char" char=".">15.4</entry>
<entry align="char" char=".">0.65</entry>
</row>
<row><entry>3.4</entry>
<entry align="char" char=".">781.6</entry>
<entry align="char" char=".">10.4</entry>
<entry align="char" char=".">6.00</entry>
<entry align="char" char=".">15.3</entry>
<entry align="char" char=".">0.34</entry>
</row>
<row><entry>14</entry>
<entry align="char" char=".">780.5</entry>
<entry align="char" char=".">35</entry>
<entry align="char" char=".">27.8</entry>
<entry align="char" char=".">15.2</entry>
<entry align="char" char=".">0.30</entry>
</row>
<row><entry>Co(OH)<inf>2</inf>
</entry>
<entry align="char" char=".">781.3</entry>
<entry align="char" char=".">—</entry>
<entry align="char" char=".">—</entry>
<entry align="char" char=".">16.0</entry>
<entry align="char" char=".">0.71</entry>
</row>
<row><entry>Co<inf>3</inf>
O<inf>4</inf>
</entry>
<entry align="char" char=".">780.4</entry>
<entry align="char" char=".">—</entry>
<entry align="char" char=".">—</entry>
<entry align="char" char=".">15.1</entry>
<entry align="char" char=".">0.37</entry>
</row>
</tbody>
</tgroup>
</table>
</table-entry>
<p>It appears that: (i) the surface Co/Ca atomic ratios (measured by par XPS) are higher than the bulk ones, which shows a surface enrichment in cobalt; (ii) the Co2p<inf>3/2</inf>
binding energy decreases when the cobalt loading increases, which expresses the formation of Co<inf>3</inf>
O<inf>4</inf>
, already evidenced by other methods; (iii) Δ<it>E</it>
(Co2p<inf>1/2</inf>
− Co2p<inf>3/2</inf>
) and the S/M ratio decrease when the cobalt content increases. By comparison with previous data,<citref idrefs="cit38 cit52 cit53">38,52,53</citref>
it may be concluded that the Co<sup>3+</sup>
concentration increases at the expense of Co<sup>2+</sup>
with the formation of Co<inf>3</inf>
O<inf>4</inf>
, in accordance with the magnetic data.</p>
<p>The scheme below summarizes the species appearing upon increasing the Co content.<ugraphic xsrc="b602514e-u1.tif" id="ugr1" display="displayed"></ugraphic>
</p>
</subsect1>
</section>
<section><no>4.</no>
<title>Catalytic studies</title>
<subsect1><no>4.1.</no>
<title>Butan-2-ol conversion</title>
<p>In the absence of oxygen in the feed mixture, butan-2-ol conversion is very low (about 1.4%) even at 513 K, and both butenes and butanone are detected in equivalent proportions. This indicates that CaHap presents acidic and basic properties but the low conversion restricts the interest of this test.</p>
<p>On the other hand, in the presence of oxygen, the conversion is already significant at 393 K and leads almost exclusively to butan-2-one with only a negligible amount of butenes (less than 0.5%). Although the introduction of oxygen favours dehydrogenation, butan-2-ol conversion in the presence of oxygen allows a more significant comparison of samples with different Co content than in the absence of oxygen. <figref idrefs="fig9">Fig. 9</figref>
reports the butan-2-one yield at stationary state <it>versus</it>
the cobalt content of the samples at different temperatures. It can be observed that the butan-2-one yield is maximum for <it>z</it>
= 0.9; the performance of the other samples is significantly lower and is even exceeded by pure hydroxyapatite at 493 K.</p>
<figure xsrc="b602514e-f9.tif" id="fig9"><title>Butan-2-one yield <it>versus</it>
temperature for different cobalt loadings.</title>
</figure>
<p>The butan-2-one yield weighted by the cobalt content (<figref idrefs="fig10">Fig. 10</figref>
) sharply decreases when the cobalt content increases from <it>z</it>
= 0.2 to <it>z</it>
= 1.3, but remains almost constant and independent of temperature between <it>z</it>
= 1.3 and <it>z</it>
= 2.3. Similar discontinuities are observed for the same cobalt contents when studying the apparent activation energy and the pre-exponential factor <it>A</it>
(curves not shown). Hence, the cobalt-poorest catalysts (<it>z</it>
= 0.2 and <it>z</it>
= 0.9) do not behave as the richer ones. As seen above, the Co<sup>2+</sup>
ions in the Co(0.2)/Hap sample are located in an octahedral oxygen environment resulting from an exchange with Ca<sup>2+</sup>
ions during the short preparation time. On the other hand, the Co(0.9)/Hap sample also contains tetrahedral Co<sup>2+</sup>
and octahedral Co<sup>3+</sup>
species. When the metal content increases further, Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters and Co<inf>3</inf>
O<inf>4</inf>
are formed during the calcination stage but these entities seem inactive in the conversion of butan-2-ol in this temperature range. Hence the mechanism of conversion of butan-2-ol to butenes should mainly involve Co<sup>2+</sup>
ions and neutral oxygen vacancies V<inf>o</inf>
containing two electrons, symbolized by <ugraphic xsrc="b602514e-u2.tif" id="ugr2" display="inline"></ugraphic>
, as proposed below: <ugraphic xsrc="b602514e-u3.tif" id="ugr3" display="displayed"></ugraphic>
</p>
<figure xsrc="b602514e-f10.tif" id="fig10"><title>Butan-2-one yield weighted by the cobalt content at various temperatures.</title>
</figure>
</subsect1>
<subsect1><no>4.2</no>
<title>Oxidative dehydrogenation of ethane</title>
<p><figref idrefs="fig11 fig12">Fig. 11 and 12</figref>
show the global conversion of ethane and the ethylene yield at different temperatures <it>versus</it>
the cobalt content. The conversion of ethane increases with the cobalt content until it reaches a maximum which shifts from about <it>z</it>
= 1.7 to <it>z</it>
= 0.9 when the temperature increases from 723 K to 823 K.</p>
<figure xsrc="b602514e-f11.tif" id="fig11"><title>Ethane conversion <it>versus</it>
cobalt content at different temperatures.</title>
</figure>
<figure xsrc="b602514e-f12.tif" id="fig12"><title>Ethylene yield <it>versus</it>
cobalt content at different temperatures.</title>
</figure>
<p>The ethylene yield exhibits the same trend with a maximum shifting from <it>z</it>
≈ 2 to <it>z</it>
= 0.9. The presence of these maximums can be explained by the decrease of activity of the cobalt sites. The specific dehydrogenation activity of the Co<sup>2+</sup>
sites is high only when isolated Co<sup>2+</sup>
ions are predominant. Upon increasing the Co content (<it>z</it>
≥ 0.9), less active cobalt ions with different symmetry (tetrahedral Co<sup>2+</sup>
) and oxidation degree (Co<sup>3+</sup>
) become predominant, with concomitant lowering of the basic properties. It is also relevant to notice the discontinuity in the apparent activation energy of ethylene production (<figref idrefs="fig13">Fig. 13</figref>
) for <it>z</it>
= 0.9. This explanation is in agreement with the higher activity of the samples prepared by exchange, which contain only isolated Co<sup>2+</sup>
ions,<citref idrefs="cit23">23</citref>
as evidenced in Section 4.3.</p>
<figure xsrc="b602514e-f13.tif" id="fig13"><title>Apparent activation energy <it>E</it>
<inf>a</inf>
of ethylene production <it>versus</it>
cobalt content.</title>
</figure>
<p>Hence, at low cobalt content, the most efficient sites for ethylene formation may be identified as isolated Co<sup>2+</sup>
–O–Ca<sup>2+</sup>
entities. Adsorbed ethane may lose hydrogens by interaction with bridging oxygens, which may generate ethylene. Upon increasing the Co content, the nature of the sites turns to Co<sup>2+</sup>
–O–Co<sup>3+</sup>
ensembles with lower oxygen basicity.</p>
<p>A critical Co content corresponds to the formation of Co<inf>3</inf>
O<inf>4</inf>
crystals at the surface of the samples for <it>z</it>
≥ 1.7. It has been shown elsewhere that pure Co<inf>3</inf>
O<inf>4</inf>
gives a very low ethylene yield (<it>ca</it>
4% at 823 K).<citref idrefs="cit22">22</citref>
</p>
<p>On the other hand, a valuable comparison of the catalysts requires the study of the selectivity at the same conversion. For each temperature, the global conversion of ethane has been varied by changing the mass of the catalyst while the contact time was kept constant by the addition of silica. Between 723 K and 823 K, the catalysts for which 0.9 ≤ <it>z</it>
≤ 3.4 are the most selective (see the data at 823 K in <figref idrefs="fig14">Fig. 14</figref>
).</p>
<figure xsrc="b602514e-f14.tif" id="fig14"><title>Ethylene selectivity <it>versus</it>
ethane conversion at 823 K.</title>
</figure>
</subsect1>
<subsect1><no>4.3</no>
<title>Comparison with exchanged samples</title>
<p>It has been previously shown that on exchanged samples<citref idrefs="cit23">23</citref>
the ethylene yield reaches a maximum when the cobalt content increases from 0 to 1.35 wt%, the limit of the exchange capacity. This was explained by the compensation of the intrinsic dehydrogenation activity of cobalt by the decrease of basicity of apatite induced by the replacement of Ca<sup>2+</sup>
by Co<sup>2+</sup>
(decrease of basicity of bridging oxygens). It is now relevant to compare the catalytic performances of these exchanged samples and those of impregnated samples in both butan-2-ol conversion and ethane dehydrogenation.</p>
<p><it>Butan-2-ol conversion</it>
(<figref idrefs="fig15">Fig. 15</figref>
)</p>
<figure xsrc="b602514e-f15.tif" id="fig15"><title>Comparison of butanone yields measured on exchanged and impregnated samples.</title>
</figure>
<p>For all samples, the butanone yield depends on the cobalt content. However, it varies strongly with temperature for the exchanged samples whereas it is much less temperature-sensitive for the impregnated samples. Comparison with the performance of pure CaHap (<it>z</it>
= 0) suggests that impregnation (above 0.9 wt% Co) levels off the basicity of the catalysts and the efficiency of the cobalt sites.</p>
<p><it>Ethane dehydrogenation</it>
(<figref idrefs="fig16">Fig. 16</figref>
)</p>
<figure xsrc="b602514e-f16.tif" id="fig16"><title>Comparison of ethylene yields measured on exchanged and impregnated samples.</title>
</figure>
<p>For low cobalt contents (<it>z</it>
≤ 1.35) at 723 K, the catalysts exhibit the same performances whatever the preparation method. This equality is still observed at 773 K for lower cobalt contents (<it>z</it>
≤ 0.9) but practically disappears at 823 K. At this temperature, the maximum yield observed (for <it>z</it>
= 0.9) is larger for the exchanged samples (21.3%) than for the impregnated ones (16.6%). As these performances are obtained when only Co<sup>2+</sup>
ions are present, the Co<sup>3+</sup>
/Co<sup>2+</sup>
couple should not be implied. The mechanism proposed involves the contribution of these sites and of oxygen vacancies:<eqntext>C<inf>2</inf>
H<inf>6</inf>
+ O<inf>o</inf>
→ C<inf>2</inf>
H<inf>4</inf>
+ H<inf>2</inf>
O + V<inf>o</inf>
</eqntext>
<eqntext>½O<inf>2</inf>
+ V<inf>o</inf>
→ O<inf>o</inf>
</eqntext>
where O<inf>o</inf>
is a lattice oxygen ion and V<inf>o</inf>
a neutral oxygen vacancy (containing two electrons), as proposed above for the butan-2-ol conversion (Section 4.1).</p>
</subsect1>
</section>
<section><no>5.</no>
<title>Conclusion</title>
<p>Controlling the preparation procedures (exchange, impregnation) and using the specific properties of the apatite support allow the tailoring of supported cobalt species. We have shown that a small amount of cobalt is exchanged with calcium even during the short impregnation time. The resulting species are isolated, paramagnetic Co<sup>2+</sup>
ions, which resist oxidation up to 823 K. This evidences the strong basic effect of the hydroxyapatite carrier. However, in these conditions, the exchange capacity is low. Deposing higher amounts of cobalt leads to species in weak interaction with the support, easily oxidized to Co<sup>3+</sup>
and prone to generate first Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters, undetectable by XRD but exhibiting short-distance antiferromagnetic order, and finally antiferromagnetic Co<inf>3</inf>
O<inf>4</inf>
nanocrystals.</p>
<p>Structure–activity relationships may be drawn from the comparison of the catalytic properties of the samples prepared by exchange and by impregnation. The most efficient sites for ethylene formation from ethane (21% yield at 823 K) and butanone production from butan-2-ol (70% yield at 453 K) may be identified to isolated Co<sup>2+</sup>
–O–Ca<sup>2+</sup>
entities. We propose that, in these entities, the bridging oxygen is engaged in the redox process, without oxidation of Co<sup>2+</sup>
into Co<sup>3+</sup>
. In a recent work on Co–AlPO<inf>4</inf>
and V–Co–Al–PO<inf>4</inf>
catalysts,<citref idrefs="cit21">21</citref>
the role of cobalt in the selectivity to ethylene has been assigned to the acidic properties generated by the substitution of Al<sup>3+</sup>
by Co<sup>2+</sup>
, with ethylene yields at 873 K of around 12% for Co–AlPO<inf>4</inf>
and 20% for V–Co–Al–PO<inf>4</inf>
. It has been claimed that acidic materials are preferred for ODE in the case of vanadium catalysts.<citref idrefs="cit54">54</citref>
In the case of cobalt catalysts, such a conclusion does not seem to apply. The role of cobalt is to favour the reactivity of the bridging oxygen (formation/replenishing of the vacancies) whereas calcium ions brings the basicity, promoting hydrogen abstraction. This effect strongly depends on the cobalt content. Increasing the cobalt content above 1 wt% is detrimental to catalytic performance because (i) the basicity is lower and (ii) the Co<sup>2+</sup>
–O–Co<sup>3+</sup>
ensembles (Co<inf><it>x</it>
</inf>
O<inf><it>y</it>
</inf>
clusters, Co<inf>3</inf>
O<inf>4</inf>
nanocrystals) are less active than isolated Co<sup>2+</sup>
ions.</p>
</section>
</art-body>
<art-back><ack><p>The authors are indebted to the French Ministère des Affaires Étrangères for the financial support (Action intégrée 183/MA/99), to Ms. Carole Connan for XPS analysis and to Mr. Frédéric Herbst for TEM measurements.</p>
</ack>
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<mods version="3.6"><titleInfo><title>Cobalt speciation in cobalt oxide-apatite materials: structure–properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion</title>
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<titleInfo type="alternative" contentType="CDATA"><title>Cobalt speciation in cobalt oxide-apatite materials: structure–properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion</title>
</titleInfo>
<name type="personal"><namePart type="given">Kaoutar</namePart>
<namePart type="family">El Kabouss</namePart>
<affiliation>Laboratoire de Physico-Chimie des Matériaux et Catalyse, Faculté des Sciences, Département de Chimie, Rabat, Morocco</affiliation>
<affiliation>Groupe de Chimie des Matériaux Divisés et Catalyse, ITODYS, UMR-CNRS 7086, Université Paris 7-Denis Diderot, case 7090, 2, place Jussieu, 75251 Paris cedex 05, France</affiliation>
</name>
<name type="personal"><namePart type="given">Mohamed</namePart>
<namePart type="family">Kacimi</namePart>
<affiliation>Laboratoire de Physico-Chimie des Matériaux et Catalyse, Faculté des Sciences, Département de Chimie, Rabat, Morocco</affiliation>
</name>
<name type="personal"><namePart type="given">Mahfoud</namePart>
<namePart type="family">Ziyad</namePart>
<affiliation>Laboratoire de Physico-Chimie des Matériaux et Catalyse, Faculté des Sciences, Département de Chimie, Rabat, Morocco</affiliation>
</name>
<name type="personal"><namePart type="given">Souad</namePart>
<namePart type="family">Ammar</namePart>
<affiliation>Groupe de Chimie des Matériaux Divisés et Catalyse, ITODYS, UMR-CNRS 7086, Université Paris 7-Denis Diderot, case 7090, 2, place Jussieu, 75251 Paris cedex 05, France</affiliation>
</name>
<name type="personal"><namePart type="given">Alain</namePart>
<namePart type="family">Ensuque</namePart>
<affiliation>Groupe de Chimie des Matériaux Divisés et Catalyse, ITODYS, UMR-CNRS 7086, Université Paris 7-Denis Diderot, case 7090, 2, place Jussieu, 75251 Paris cedex 05, France</affiliation>
</name>
<name type="personal"><namePart type="given">Jean-Yves</namePart>
<namePart type="family">Piquemal</namePart>
<affiliation>Groupe de Chimie des Matériaux Divisés et Catalyse, ITODYS, UMR-CNRS 7086, Université Paris 7-Denis Diderot, case 7090, 2, place Jussieu, 75251 Paris cedex 05, France</affiliation>
</name>
<name type="personal"><namePart type="given">François</namePart>
<namePart type="family">Bozon-Verduraz</namePart>
<affiliation>Groupe de Chimie des Matériaux Divisés et Catalyse, ITODYS, UMR-CNRS 7086, Université Paris 7-Denis Diderot, case 7090, 2, place Jussieu, 75251 Paris cedex 05, France</affiliation>
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<abstract>Impregnation of calcium hydroxyapatite by a solution of Co(ii) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are isolated six-coordinated Co2+ ions. For Co content ≥0.4 wt%, the presence of tetrahedral Co2+ and octahedral Co3+ species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered CoxOy entities. For Co content ≥1.7 wt%, Co3O4 nanocrystals are generated, as evidenced by XRD and magnetic measurements. In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co2+ ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co2+ ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.</abstract>
<note>Isolated Co2+ ions exchanged with Ca2+ ions of calcium hydroxyapatite are resistant to oxidation and show much better performance in dehydrogenation than cooperating cobalt sites found in Co3O4. [b602514e-ga.tif]</note>
<relatedItem type="host"><titleInfo><title>Journal of Materials Chemistry</title>
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<originInfo><publisher>The Royal Society of Chemistry.</publisher>
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<identifier type="ISSN">0959-9428</identifier>
<identifier type="eISSN">1364-5501</identifier>
<identifier type="coden">JMACEP</identifier>
<identifier type="RSC sercode">JM</identifier>
<part><date>2006</date>
<detail type="volume"><caption>vol.</caption>
<number>16</number>
</detail>
<detail type="issue"><caption>no.</caption>
<number>25</number>
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<extent unit="pages"><start>2453</start>
<end>2463</end>
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<identifier type="DOI">10.1039/b602514e</identifier>
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<accessCondition type="use and reproduction" contentType="copyright">This journal is © The Royal Society of Chemistry</accessCondition>
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