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The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons

Identifieur interne : 000222 ( Istex/Corpus ); précédent : 000221; suivant : 000223

The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons

Auteurs : V V Bhat ; C I Contescu ; N C Gallego

Source :

Nanotechnology [ 0957-4484 ] ; 2009.

RBID : ISTEX:15BA219EAA5CDEAAEEBD3D9A427116EA6A41392F

Abstract

This paper reports on differences in stability of Pd hydride phases in palladium particleswith various degrees of contact with microporous carbon supports. A sample containing Pdembedded in activated carbon fibre (2wt Pd) was compared with commercial Pdnanoparticles deposited on microporous activated carbon (3wt Pd) and with support-freenanocrystalline palladium. The morphology of the materials was characterized by electronmicroscopy, and the phase transformations were analysed over a large range of hydrogenpartial pressures (0.00310 bar) and at several temperatures using in situ x-ray diffraction.The results were verified with volumetric hydrogen uptake measurements. Resultsindicate that higher degrees of Pdcarbon contacts for Pd particles embeddedin a microporous carbon matrix induce efficient pumping of hydrogen out of-PdHx. It was also found that thermal cleaning of carbon surface groups prior toexposure to hydrogen further enhances the hydrogen pumping power of themicroporous carbon support. In brief, this study highlights that the stability of-PdHx phase supported on carbon depends on the degree of contact between Pd and carbon andon the nature of the carbon surface.

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https://api.istex.fr/document/15BA219EAA5CDEAAEEBD3D9A427116EA6A41392F/fulltext/pdf

DOI: 10.1088/0957-4484/20/20/204011

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<author><name sortKey="Bhat, V V" sort="Bhat, V V" uniqKey="Bhat V" first="V V" last="Bhat">V V Bhat</name>
<affiliation><mods:affiliation>Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, MS-6087, Oak Ridge, TN 37831, USA</mods:affiliation>
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<affiliation><mods:affiliation>E-mail:bhatvv@ornl.gov</mods:affiliation>
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<author><name sortKey="Contescu, C I" sort="Contescu, C I" uniqKey="Contescu C" first="C I" last="Contescu">C I Contescu</name>
<affiliation><mods:affiliation>Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, MS-6087, Oak Ridge, TN 37831, USA</mods:affiliation>
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<front><div type="abstract">This paper reports on differences in stability of Pd hydride phases in palladium particleswith various degrees of contact with microporous carbon supports. A sample containing Pdembedded in activated carbon fibre (2wt Pd) was compared with commercial Pdnanoparticles deposited on microporous activated carbon (3wt Pd) and with support-freenanocrystalline palladium. The morphology of the materials was characterized by electronmicroscopy, and the phase transformations were analysed over a large range of hydrogenpartial pressures (0.00310 bar) and at several temperatures using in situ x-ray diffraction.The results were verified with volumetric hydrogen uptake measurements. Resultsindicate that higher degrees of Pdcarbon contacts for Pd particles embeddedin a microporous carbon matrix induce efficient pumping of hydrogen out of-PdHx. It was also found that thermal cleaning of carbon surface groups prior toexposure to hydrogen further enhances the hydrogen pumping power of themicroporous carbon support. In brief, this study highlights that the stability of-PdHx phase supported on carbon depends on the degree of contact between Pd and carbon andon the nature of the carbon surface.</div>
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<abstract>This paper reports on differences in stability of Pd hydride phases in palladium particleswith various degrees of contact with microporous carbon supports. A sample containing Pdembedded in activated carbon fibre (2wt Pd) was compared with commercial Pdnanoparticles deposited on microporous activated carbon (3wt Pd) and with support-freenanocrystalline palladium. The morphology of the materials was characterized by electronmicroscopy, and the phase transformations were analysed over a large range of hydrogenpartial pressures (0.00310 bar) and at several temperatures using in situ x-ray diffraction.The results were verified with volumetric hydrogen uptake measurements. Resultsindicate that higher degrees of Pdcarbon contacts for Pd particles embeddedin a microporous carbon matrix induce efficient pumping of hydrogen out of-PdHx. It was also found that thermal cleaning of carbon surface groups prior toexposure to hydrogen further enhances the hydrogen pumping power of themicroporous carbon support. In brief, this study highlights that the stability of-PdHx phase supported on carbon depends on the degree of contact between Pd and carbon andon the nature of the carbon surface.</abstract>
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<header><title-group><title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containing
 activated carbons</title>
<short-title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containing
 activated carbons
 </short-title>
<ej-title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containing
 activated carbons
 </ej-title>
</title-group>
<author-group><author address="nano294638ad1" email="nano294638ea1"><first-names>V V</first-names>
<second-name>Bhat</second-name>
</author>
<author address="nano294638ad1" email="nano294638ea2"><first-names>C I</first-names>
<second-name>Contescu</second-name>
</author>
<author address="nano294638ad1" alt-address="nano294638ad2" email="nano294638ea3"><first-names>N C</first-names>
<second-name>Gallego</second-name>
</author>
<short-author-list>V V Bhat <italic>et al</italic>
 </short-author-list>
</author-group>
<address-group><address id="nano294638ad1" showid="no"><orgname>Materials Science and Technology Division, Oak Ridge National Laboratory</orgname>, PO Box
 2008, MS-6087, Oak Ridge, TN 37831,
 <country>USA</country>
</address>
<address id="nano294638ad2" alt="yes">Author to whom any correspondence should be addressed</address>
<e-address id="nano294638ea1"><email mailto="bhatvv@ornl.gov">bhatvv@ornl.gov</email>
</e-address>
<e-address id="nano294638ea2"><email mailto="contescuci@ornl.gov">contescuci@ornl.gov</email>
</e-address>
<e-address id="nano294638ea3"><email mailto="gallegonc@ornl.gov">gallegonc@ornl.gov</email>
</e-address>
</address-group>
<history received="3 October 2008" finalform="17 November 2008" online="23 April 2009"></history>
<abstract-group><abstract><heading>Abstract</heading>
<p indent="no">This paper reports on differences in stability of Pd hydride phases in palladium particles
 with various degrees of contact with microporous carbon supports. A sample containing Pd
 embedded in activated carbon fibre (2 wt% Pd) was compared with commercial Pd
 nanoparticles deposited on microporous activated carbon (3 wt% Pd) and with support-free
 nanocrystalline palladium. The morphology of the materials was characterized by electron
 microscopy, and the phase transformations were analysed over a large range of hydrogen
 partial pressures (0.003–10 bar) and at several temperatures using <italic>in situ</italic> x-ray diffraction.
 The results were verified with volumetric hydrogen uptake measurements. Results
 indicate that higher degrees of Pd–carbon contacts for Pd particles embedded
 in a microporous carbon matrix induce efficient ‘pumping’ of hydrogen out of
 β-
 PdH<sub><italic>x</italic>
</sub>. It was also found that thermal cleaning of carbon surface groups prior to
 exposure to hydrogen further enhances the hydrogen pumping power of the
 microporous carbon support. In brief, this study highlights that the stability of
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase supported on carbon depends on the degree of contact between Pd and carbon and
 on the nature of the carbon surface. </p>
</abstract>
</abstract-group>
</header>
<body refstyle="numeric"><sec-level1 id="nano294638s1" label="1"><heading>Introduction</heading>
<p indent="no">Efficient storage of hydrogen in solid state remains a key issue for development of a
 hydrogen-based economy. The use of microporous adsorbents, in particular activated
 carbons and metal organic frameworks, comes close to some of DOE target levels, but only
 at cryogenic temperatures. This is because hydrogen interacts very weakly with solid
 adsorbents when adsorption is caused only by dispersion forces. Enhancing the levels of
 adsorption beyond the limits of physisorption-only materials has been lately a
 target of intense research. It has been proposed that addition of small amounts of
 transition metals to microporous adsorbents can enhance hydrogen storage by
 either one, or both, of the following mechanisms: (1) ligand binding of molecular
 H<sub>2</sub> to
 form so-called Kubas complexes [<cite linkend="nano294638bib1">1</cite>
, <cite linkend="nano294638bib2">2</cite>], or (2) by dissociative chemisorption followed
 by surface migration and storage of hydrogen at remote sites, otherwise inaccessible to molecular
 H<sub>2</sub> 
 (the spillover mechanism) [<cite linkend="nano294638bib3">3</cite>]. The latter mechanism was invoked to explain
 experimental reports [<cite linkend="nano294638bib3" range="nano294638bib3,nano294638bib4,nano294638bib5">3–5</cite>]
 of excess hydrogen uptakes, larger than what could normally be attributed to
 chemisorption on metal catalyst particles (Pt, Pd, Ni) and/or formation of bulk hydrides
 (Pd).</p>
<p>Although accepted as a valid step in the mechanism of many catalytic processes [<cite linkend="nano294638bib6">6</cite>
, <cite linkend="nano294638bib7">7</cite>], the spillover mechanism has been recognized only recently in the hydrogen
 storage literature. Particular attention was given to metals known to catalytically activate
 hydrogen reactions, and in particular, to Pt [<cite linkend="nano294638bib5">5</cite>
] and Ni [<cite linkend="nano294638bib8">8</cite>]. Both metals
 can adsorb hydrogen dissociatively, thus acting as a source of H atoms that may spill over
 the metal–support interface. A more interesting metal is Pd, which not only chemisorbs
 hydrogen (in atomic form), but also dissolves it to form two bulk hydride phases, known as
 α-
 PdH<sub><italic>x</italic>
</sub> 
 (<italic>x</italic>∼0.02)
 and β-PdH<sub><italic>x</italic>
</sub>
 (<italic>x</italic>∼0.67). The
 relationship between phase composition, hydrogen pressure, and temperature has been thoroughly studied
 for the Pd–H<sub>2</sub> 
 system [<cite linkend="nano294638bib9">9</cite>
, <cite linkend="nano294638bib10">10</cite>] with Pd in bulk form, powder, sponge, or dispersed as
 nanoparticles. However, the role of palladium hydrides in catalytic reactions and in
 metal-enhanced hydrogen storage is still an active research field [<cite linkend="nano294638bib11" range="nano294638bib11,nano294638bib12,nano294638bib13">11–13</cite>]. For example, it is not clear
 why the two hydride phases have different catalytic activity and selectivity for a
 range of catalytic reactions [<cite linkend="nano294638bib14">14</cite>
]. It was reported (but not confirmed [<cite linkend="nano294638bib15">15</cite>]) that Pd particles supported on various carbon materials may differ in
 chemisorption properties from Pd supported on alumina or silica in catalysts [<cite linkend="nano294638bib16">16</cite>]. It was also reported that the enhancement of hydrogen adsorption
 capacity observed for Pd supported on carbon does not increase significantly with
 the hydrogen pressure [<cite linkend="nano294638bib17">17</cite>], and is not reproducible in subsequent
 adsorption–desorption cycles [<cite linkend="nano294638bib18">18</cite>
]. Moreover it was reported that the Pd–H<sub>2</sub> 
 phase equilibrium for supported Pd depends on the size of Pd clusters or nanoparticles [<cite linkend="nano294638bib19">19</cite>
] or the nature of the support [<cite linkend="nano294638bib20">20</cite>
, <cite linkend="nano294638bib21">21</cite>], whereas such
 differences were not observed for self-standing support-free polycrystalline and
 nanocrystalline Pd [<cite linkend="nano294638bib22">22</cite>]. These unsolved questions demand a greater
 understanding of the influence of physical and chemical natures of both catalyst and
 support and their interactions on spillover.</p>
<p>An essential step in the spillover mechanism is the interfacial transfer of hydrogen species
 from catalyst to support sites [<cite linkend="nano294638bib13">13</cite>
]. A stochastic analysis of spillover by Jain <italic>et al</italic>
 [<cite linkend="nano294638bib23">23</cite>] demonstrated that spillover may induce ‘leaking’ of hydrogen species
 from catalyst sites when binding to the support is energetically favourable. The ratio
 between the rate of interfacial hydrogen transfer and that of hydrogen desorption from the
 support determines the equilibrium hydrogen coverage on support. This highlights
 the need of enhancing the effective mass transfer from catalyst to support in
 order to achieve appreciable hydrogen concentrations on the support and thereby
 to meet the DOE targets. An intimate contact between catalyst and support is
 essential for such transfer. Building on an earlier approach from Boudart’s group [<cite linkend="nano294638bib24">24</cite>
], Yang and co-workers [<cite linkend="nano294638bib4">4</cite>
, <cite linkend="nano294638bib25">25</cite>] proposed a method for
 enhancing the rate of interfacial transfer through developing intimate contacts, such as
 ‘chemical bridges’. However, the question remains how much improvement can be
 achieved through spillover alone. Based on Monte Carlo calculations, Jain <italic>et al</italic>
 [<cite linkend="nano294638bib23">23</cite>] reported that systems where direct hydrogen adsorption to
 the support is not possible are unlikely to achieve DOE targets for hydrogen
 capacity.</p>
<p>In this context, it is interesting to examine whether systems where direct hydrogen
 adsorption to the support is favourable (i.e. on microporous adsorbents with the proper
 range of pore sizes), combined with an efficient hydrogen spillover source, may lead to
 enhanced adsorption levels. Examples of significant adsorption enhancement caused by
 physically mixing Pd/carbon catalysts and high capacity activated carbons have been
 reported [<cite linkend="nano294638bib3" range="nano294638bib3,nano294638bib4,nano294638bib5">3–5</cite>]. When Pd
 was introduced before carbonization in the carbon precursor, the hydrogen capacity
 exceeded the combined capacity calculated for adsorption on support plus formation
 of Pd hydride [<cite linkend="nano294638bib26">26</cite>
]. Several reports [<cite linkend="nano294638bib17">17</cite>
, <cite linkend="nano294638bib18">18</cite>
, <cite linkend="nano294638bib26" range="nano294638bib26,nano294638bib27,nano294638bib28">26–28</cite>] indicate that the enhanced
 hydrogen adsorption in Pd-modified adsorbents is the highest in the pressure
 and temperature range corresponding to formation and decomposition of Pd
 hydride. There is a clear need to investigate the synergetic interactions between
 Pd-catalyst and carbon support. This paper reports how such synergetic interactions
 depend on the degree of Pd–carbon contact and the surface properties of carbon
 support.</p>
</sec-level1>
<sec-level1 id="nano294638s2" label="2"><heading>Experimental details</heading>
<p indent="no">In this study, two Pd-containing carbon samples were compared to pure
 Pd. The first one, activated carbon fibres containing Pd (Pd-ACF), was
 prepared by mixing isotropic petroleum pitch with 1 wt% Pd equivalent of
 Pd(acac)<sub>2</sub> salt and
 melt-spinning the mixture into fibres. After oxidative-stabilization the fibres were carbonized in nitrogen (1000 °C) and physically
 activated in CO<sub>2</sub> 
 at 875 °C 
 (to 40% burn-off) to obtain Pd particles embedded in the carbon matrix. The
 resultant metal content in Pd-ACF is 2 wt% Pd. Further details on the synthesis and
 microstructural properties of Pd-ACF are presented elsewhere [<cite linkend="nano294638bib29">29</cite>]. The
 second sample is a commercial Pd-catalyst that contained 3 wt% Pd deposited on
 porous carbon [<cite linkend="nano294638bib30">30</cite>
, <cite linkend="nano294638bib31">31</cite>] and was acquired from Acros Organics.
 The amount of carbon support in both Pd-ACF and Pd-catalyst samples
 were nearly equal, 98 and 97 wt% respectively. Additionally a support-free
 Pd powder (Pd-black; 99.9% pure), from Sigma-Aldrich, with particle size
 ∼10 nm, was used as a reference. Table <tabref linkend="nano294638tab1">1</tabref>
 summarizes the physical properties of all these samples (<italic>S</italic>
<sub>BET</sub> = BET surface
 area; <italic>V</italic>
<sub>μ<italic>p</italic>
(DR−N2)</sub> = micropores 
 volume from N<sub>2</sub> 
 adsorption by Dubinin–Radushkevich method;
 <italic>V</italic>
<sub>μ<italic>p</italic>
(NLDFT−CO2)</sub> = micropores volume
 from CO<sub>2</sub> 
 adsorption by DFT method).</p>
<table id="nano294638tab1" width="42pc"><caption id="tc1" label="Table 1"><p indent="no">Physical properties of samples used in this study.</p>
</caption>
<tgroup cols="6"><colspec colnum="1" colname="col1" align="left"></colspec>
<colspec colnum="2" colname="col2" align="left"></colspec>
<colspec colnum="3" colname="col3" align="left"></colspec>
<colspec colnum="4" colname="col4" align="left"></colspec>
<colspec colnum="5" colname="col5" align="left"></colspec>
<colspec colnum="6" colname="col6" align="left"></colspec>
<thead><row><entry>Sample</entry>
<entry>Pd content</entry>
<entry><italic>S</italic>
<sub>BET</sub> 
 (m<sup>2</sup>
 g<sup>−1</sup>
)</entry>
<entry><italic>V</italic>
<sub>μ<italic>p</italic>
(DR−N2)</sub> 
 (cm<sup>3</sup>
 g<sup>−1</sup>
)</entry>
<entry><italic>V</italic>
<sub>μ<italic>p</italic>
(NLDFT−CO2)</sub> 
 (<10 Å)(cm<sup>3</sup>
 g<sup>−1</sup>
)</entry>
<entry><italic>V</italic>
<sub>μ<italic>p</italic>
(NLDFT−CO2)</sub> 
 (<8 Å)<sup>a</sup> 
 (cm<sup>3</sup>
 g<sup>−1</sup>
)</entry>
</row>
</thead>
<tbody><row><entry>Pd-ACF</entry>
<entry>2</entry>
<entry>1880</entry>
<entry>0.88</entry>
<entry>0.51</entry>
<entry>0.40</entry>
</row>
<row><entry>Pd-catalyst</entry>
<entry>3</entry>
<entry>1380</entry>
<entry>0.57</entry>
<entry>0.25</entry>
<entry>0.20</entry>
</row>
<row><entry>Pd-black</entry>
<entry>99.9</entry>
<entry>50</entry>
<entry>n/a</entry>
<entry>n/a</entry>
<entry>n/a</entry>
</row>
</tbody>
<tfoot><sup>a</sup> 
 Modelling studies have shown that the optimal micropore width for physisorption of
 H<sub>2</sub> 
 at near ambient temperatures and moderate pressures is
 <8 Å [<cite linkend="nano294638bib32">32</cite>
, <cite linkend="nano294638bib33">33</cite>
].</tfoot>
</tgroup>
</table>
<p>The morphology of all samples was characterized using SEM and STEM.
 Hydrogen sorption was measured by the volumetric technique using 99.999%
 pure hydrogen gas (Matheson Tri-Gas) on a Quantachrome Autosorb 1C.
 Adsorption isotherms were measured at four temperatures (25, 40, 60 and
 80 °C) over a pressure
 range from 10<sup>−3</sup>
 mbar to 1 bar. Thermal pretreatments of all the samples prior to adsorption were done <italic>in
 situ</italic>, avoiding exposure to atmosphere. Temperature-programmed desorption (TPD) studies
 of surface chemical groups on Pd-ACF were performed at temperatures up to
 1000 °C (heating
 rate 5 °C min<sup>−1</sup>) using Quantachrome Autosorb-1C system fitted with
 mass-spectrometer (Pfeiffer Prisma QME200). The carrier gas was He (30 ml min<sup>−1</sup>) and the
 sample size was ∼50 mg.</p>
<p><italic>In situ</italic> high pressure x-ray diffraction (XRD) studies were performed with the XRK 900
 Anton Paar reactor chamber attached to PANalytical X’Pert Pro diffractometer. The
 Paar chamber allows operation to pressures up to 10 bar and temperatures up to
 900 °C. The samples were pretreated in the XRK900 cell at different temperatures (ambient, 300 or
 900 °C) in vacuum before recording XRD. After pretreatment the cell was pressurized to 10 bar
 H<sub>2</sub> to ensure that all Pd
 was converted to β-
 PdH<sub><italic>x</italic>
</sub>. After this, the hydrogen
 partial pressure (<italic>P</italic>
<sub>H2</sub>) was decreased step-wise and diffraction patterns were recorded at each step in order to monitor the transition
 from β-
 PdH<sub><italic>x</italic>
</sub> to
 α-
 PdH<sub><italic>x</italic>
</sub> /Pd.
 Multiple diffraction patterns were recorded at each step to ensure that the sample reached equilibrium.
 Lowering of <italic>P</italic>
<sub>H2</sub> 
 was done by either simply decreasing the cell pressure or by diluting hydrogen
 with a known amount of helium. For instance, the cell containing 2 bar
 H<sub>2</sub> 
 was pressurized to 10 bar with He and allowed to equilibrate for 15 min.
 Then, the total cell pressure was decreased to 2.5 bar to achieve 0.5 bar
 <italic>P</italic>
<sub>H2</sub>. Accuracy
 of the calculated hydrogen partial pressure was measured by determining the equilibrium pressure for
 β/α transition in pure Pd
 and was found to be ± 2 mbar. XRD patterns were refined using the Rietveld technique. Individual phase
 concentrations were deduced by performing multiphase refinement. The crystallite
 size and lattice parameters were determined based on peak width and position,
 respectively.</p>
</sec-level1>
<sec-level1 id="nano294638s3" label="3"><heading>Results</heading>
<sec-level2 id="nano294638s3.1" label="3.1"><heading>Morphological characterization</heading>
<p indent="no">SEM and STEM images of Pd-ACF samples along with Pd-catalyst and Pd-black reference
 samples are shown in figure <figref linkend="nano294638fig1">1</figref>. SEM micrograph of Pd-ACF shows long carbon fibres of
 ∼15 µm 
 in diameter. Bright particles on the carbon fibres were confirmed to be Pd using EDX and
 back scattering images. Figures <figref linkend="nano294638fig1">1</figref> (d) and (e) represent SE and Z-contrast images of Pd-ACF from STEM
 characterization. Both images were simultaneously recorded at the same location. SE image
 shows one large Pd particle on carbon surface, while the Z-contrast image reveals a
 large number of Pd particles embedded in carbon matrix. The particles range
 from sub-nanometre to 25 nm in size. SEM and STEM studies combined show
 that most of Pd particles are less than 100 nm in size and embedded in carbon
 matrix, and a few large particles are distributed on the outside surface of carbon
 fibre.
 <figure id="nano294638fig1" parts="single" width="page" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463801.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463801.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc1" type="figure" label="Figure 1"><p indent="no">SEM micrographs of Pd-ACF (a), Pd-catalyst (b) and Pd-black (c) and their corresponding
 schematic representation of Pd distribution. SE (d) and Z-contrast images (e) of
 Pd-ACF were recorded simultaneously from the same location. The Z-contrast image
 reveals the presence of numerous Pd nanoparticles embedded in the carbon matrix.</p>
</caption>
</figure>
</p>
<p>The carbon support particles of Pd-catalyst are irregularly shaped. No Pd particles could
 be observed at either low or high magnification. A back scattering image and
 EDX-mapping confirmed the presence of Pd particles uniformly deposited on carbon
 surface.</p>
<p>Pd-black shows cauliflower-like agglomerates, several micrometres in size, made out of
 smaller Pd particles. The average particle size within the agglomerate was estimated to be
 ∼10 nm, based on measured
 BET surface area (∼50 m<sup>2</sup>
 g<sup>−1</sup>
) and assuming spherical-shaped particles.</p>
<p>The observed distribution of Pd particles and the nature of their contact with carbon
 support in all the samples are schematically represented in the inset of corresponding SEM
 (figure <figref linkend="nano294638fig1">1</figref>). Pd in Pd-ACF has highest degree of Pd–carbon contact since most of
 the particles are embedded in the carbon matrix. In case of Pd-catalyst, because Pd is
 deposited on the carbon surface, the degree of Pd–carbon contact is lesser than Pd-ACF.
 Pd-black is completely free of contact with carbon support.
 </p>
</sec-level2>
<sec-level2 id="nano294638s3.2" label="3.2"><heading>Effect of degree of Pd–carbon contact on phase transition in palladium hydride</heading>
<p indent="no">To evaluate the effect of the degree of contact between Pd particles and the carbon matrix
 (embedded in versus deposited on), systematic studies of the Pd and Pd hydride phases
 were carried out using <italic>in situ</italic>
 x-ray diffraction. Figure <figref linkend="nano294638fig2">2</figref> shows diffraction patterns recorded at room temperature
 for all samples as a function of partial pressure of hydrogen (<italic>P</italic>
<sub>H2</sub>). The figure highlights the (111) diffraction peaks of
 α and
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phases. In Pd-catalyst both peaks are broad, indicative of the presence of very small crystallites,
 in agreement with morphological characterization results. In contrast, the (111) peaks of
 α and
 β phases
 in Pd-black and Pd-ACF are sharper, corresponding to larger crystallites. The average crystallite sizes
 of β-PdH<sub><italic>x</italic>
</sub>, calculated based on peak width using Scherrer formula, are: 7 nm for Pd-catalyst, 18 nm
 for Pd-black and 45 nm for Pd-ACF.</p>
<p>The lattice parameter of Pd in all the three samples was determined after out-gassing at
 300 °C.
 The lattice parameters of Pd-ACF and Pd-catalyst were 3.8921 and 3.8934 Å compared to 3.8929 Å
 in Pd-black. The absence of any significant change in the lattice parameter (more than error
 ± 0.005 Å) discards
 the possibility of carbon insertion into Pd lattice during high temperature treatment to form palladium
 carbide (Pd<sub>1−<italic>x</italic>
</sub>
C<sub><italic>x</italic>
</sub> 
 (<italic>x</italic>
<0.1)), as reported elsewhere [<cite linkend="nano294638bib29">29</cite>
, <cite linkend="nano294638bib34">34</cite>
, <cite linkend="nano294638bib35">35</cite>
].</p>
<p>Figure <figref linkend="nano294638fig2">2</figref> also shows that on lowering
 <italic>P</italic>
<sub>H2</sub>,
 the β(111) 
 peak of all samples shifts to higher angles without changing its intensity. This
 corresponds to shrinking of the lattice on removal of H atoms from interstitial sites in
 β-
 PdH<sub><italic>x</italic>
</sub>. In addition, for Pd-black a decrease in intensity of
 β(111) and appearance
 of α(111) peak are
 noted when <italic>P</italic>
<sub>H2</sub> 
 drops to ∼20 mbar. Both phases show equal intensity peaks at
 ∼18 mbar, which represents the equilibrium pressure for
 β to
 α transition
 (<italic>P</italic>
<sub>β/α</sub>)
 during H<sub>2</sub> 
 desorption from this sample. The value matches exactly the
 <italic>P</italic>
<sub>β/α</sub> 
 reported in the literature for the desorption branch of
 β/α 
 transition [<cite linkend="nano294638bib36">36</cite>]. Complete transformation to
 α phase
 occurs at ∼15 mbar.
 <figure id="nano294638fig2" parts="single" width="page" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463802.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463802.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc2" type="figure" label="Figure 2"><p indent="no">Room temperature XRD patterns of Pd-ACF (a), Pd-catalyst (b) and Pd-black (c) as a function
 of <italic>P</italic>
<sub>H2</sub>. The peaks correspond to (111) diffraction of both
 α and
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phases. The horizontal black lines mark the equilibrium transition pressure from
 β to
 α phase
 (<italic>P</italic>
<sub>β/α</sub>
).</p>
</caption>
</figure>
</p>
<p>A similar trend in β 
 to α 
 transition is observed for both Pd-catalyst and Pd-ACF. However the
 <italic>P</italic>
<sub>β/α</sub> 
 values are higher than that of the reference pure support-free Pd
 sample: 22 mbar for Pd-catalyst and 30 mbar for Pd-ACF. The increase of
 <italic>P</italic>
<sub>β/α</sub> indicates
 that the β-hydride phase in carbon-supported palladium is less stable than in the pure Pd sample.
 This observation confirms that the support plays an important role in destabilizing
 β 
 phase.</p>
<p>Similar XRD patterns were recorded at four different temperatures as a function of
 <italic>P</italic>
<sub>H2</sub> 
 during desorption of hydrogen. The diffraction patterns were refined using the Rietveld
 method to determine the concentration fraction of the two hydride phases, their average
 crystallite size and the lattice parameters. Figure <figref linkend="nano294638fig3">3</figref> compares the dependence of phase concentrations of
 α and
 β hydrides versus
 <italic>P</italic>
<sub>H2</sub> for all samples
 at 25, 40, 60 and 80 °C. All samples were pretreated in vacuum at
 300 °C 
 prior to exposure to hydrogen. The effect of temperature on
 <italic>P</italic>
<sub>β/α</sub> 
 is typical for endothermic decomposition of the H-rich hydride:
 <italic>P</italic>
<sub>β/α</sub> 
 increases with increasing the temperature. However,
 <italic>P</italic>
<sub>β/α</sub> 
 values at a given temperature are different between samples, and increase with increasing
 degree of Pd–carbon contact (figure <figref linkend="nano294638fig1">1</figref>). For example, at
 60 °C the
 <italic>P</italic>
<sub>β/α</sub> 
 of Pd-black, Pd-catalyst, and Pd-ACF are 110, 150 and 180 mbar. The increase of
 <italic>P</italic>
<sub>β/α</sub> 
 with the degree of Pd–carbon contact suggests that the presence
 of the carbon support has a distinct effect in destabilization of the
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase.
 <figure id="nano294638fig3" parts="single" width="column" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463803.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463803.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc3" type="figure" label="Figure 3"><p indent="no">Phase transition from β 
 (solid symbols) to α 
 (hollow symbols) Pd hydride in (a) Pd-ACF; (b) Pd-catalyst; and
 (c) Pd-black at different temperatures as a function of decreasing
 <italic>P</italic>
<sub>H2</sub>. Only pure
 β phase exists at high
 hydrogen pressure. As <italic>P</italic>
<sub>H2</sub> 
 decreases, α 
 phase develops gradually on the expense of
 β phase. The pressure
 where α and
 β phases reach equal
 concentrations is <italic>P</italic>
<sub>β/α</sub>. Further
 decrease in the <italic>P</italic>
<sub>H2</sub> leads to
 the formation of pure α 
 phase.</p>
</caption>
</figure>
</p>
<p>Figure <figref linkend="nano294638fig4">4</figref> compares the variations in average crystallite size (ACS) with
 <italic>P</italic>
<sub>H2</sub> during the
 β/α transition (on
 decrease of <italic>P</italic>
<sub>H2</sub>) and the corresponding changes in concentration of
 α and
 β phases. It is observed that on
 lowering <italic>P</italic>
<sub>H2</sub>, the concentration
 of α phase grows at the
 expense of the β phase.
 For Pd-ACF, the ACS of β 
 phase (ACS-β) is ∼45 nm before transition. Once the transition sets in,
 α-phase particles
 with just ∼ 
 25 nm ACS are formed; at the same time ACS-
 β starts
 to increase. Recalling that ACS is the statistical average of Pd crystallite sizes, the results indicate that
 smaller β 
 crystallites undergo transition first (at higher
 <italic>P</italic>
<sub>H2</sub>). Further
 lowering of <italic>P</italic>
<sub>H2</sub> 
 induces phase changes in crystallites of intermediate size, as inferred from the apparent increase in ACS of
 both α and
 β phases. ACS-
 β increases up to
 ∼70 nm with decreasing
 concentration of β 
 phase and ACS-α 
 reaches ∼40 nm after the completion of transition. Similar results were also observed for other Pd-ACF
 samples (not presented here) with different Pd crystallite size distribution.
 <figure id="nano294638fig4" parts="single" width="page" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463804.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463804.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc4" type="figure" label="Figure 4"><p indent="no">Variation in the concentration of average crystallite size (top row) during
 β to
 α 
 transition and corresponding concentration of individual phases of Pd-ACF (a),
 Pd-catalyst (b) and Pd-black (c) at room temperature with decreasing
 <italic>P</italic>
<sub>H2</sub>
.</p>
</caption>
</figure>
</p>
<p>ACS-β 
 of Pd-black and Pd-catalyst are 18 and 7 nm respectively. These materials have very
 narrow crystallite size distributions compared to Pd-ACF. Because of that, the
 β/α transitions
 are sharp, unlike in Pd-ACF. Similar to Pd-ACF, both Pd-black and Pd-catalyst show the trend of ACS-
 β increase during
 transition. Also ACS-α 
 is small when the α 
 phase is first formed.
 </p>
</sec-level2>
<sec-level2 id="nano294638s3.3" label="3.3"><heading>Effect of pretreatment on surface properties and
 <italic>H</italic>
<sub>2</sub> 
 uptake</heading>
<p indent="no">It is expected that the carbon support in Pd-ACF has a rich inventory of oxygen-containing
 surface chemical groups that remained after physical activation and cooling in
 CO<sub>2</sub>. These surface groups decompose on heat treatment and can be identified by
 temperature-programmed desorption (TPD). Acidic groups (carboxylic acids, acid
 anhydrides, lactones, phenols) decompose at lower temperatures and generate
 CO<sub>2</sub> and
 H<sub>2</sub>O, while basic groups (chromenes, pyrones, quinones) decompose to CO at
 higher temperatures [<cite linkend="nano294638bib37">37</cite>]. Therefore it is possible to qualitatively
 characterize the chemical nature of carbon surface groups by monitoring
 CO<sub>2</sub>,
 H<sub>2</sub>O 
 and CO evolution as a function of temperature. Figure <figref linkend="nano294638fig5">5</figref> shows variation of mass peaks for
 CO<sub>2</sub>
<sup>+</sup>,
 CO<sup>+</sup> and
 H<sub>2</sub>
O<sup>+</sup> 
 recorded during TPD characterization of Pd-ACF sample, starting from three
 different initial conditions: (a) Pd-ACF sample as obtained after activation in
 CO<sub>2</sub>; (b) after pretreatment
 in He at 300 °C; and (c) after
 pretreatment in He at 1000 °C.</p>
<p>The TPD of untreated Pd-ACF (figure <figref linkend="nano294638fig5">5</figref>(a)) shows that desorption of
 H<sub>2</sub>O and
 CO<sub>2</sub> initiates at around
 100 °C and attains the
 maximum rate at ∼300 °C. At higher temperatures, the release rate of
 H<sub>2</sub>O and
 CO<sub>2</sub> slowly decreases
 and ends at ∼800 °C. On the other hand, notable release of CO starts at about
 300 °C and increases
 until 900 °C. Further heat treatment decreases CO concentration. Holding the sample at
 1000 °C 
 for 2 h removes all CO and reduces its concentration to the background level.
 <figure id="nano294638fig5" parts="single" width="column" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463805.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463805.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc5" type="figure" label="Figure 5"><p indent="no">TPD of Pd-ACF (a) as obtained after activation in
 CO<sub>2</sub>; (b) after heat treating
 at 300 °C; and (c) after
 heat treating at 1000 °C.</p>
</caption>
</figure>
</p>
<p>In figure <figref linkend="nano294638fig5">5</figref>(b) the TPD trace of Pd-ACF pretreated at
 300 °C shows no
 desorption before 300 °C 
 and follows the typical trend of Pd-ACF above
 300 °C. The diminishing
 of a major part of CO<sub>2</sub> 
 and H<sub>2</sub>O 
 signals confirms the removal of most acidic groups during pretreatment at
 300 °C,
 while the presence of CO indicates that basic groups are retained. A pretreatment of Pd-ACF at
 1000 °C 
 completely cleans the carbon surface of functional groups (figure <figref linkend="nano294638fig5">5</figref>
(c)).</p>
<p>Based on the information obtained on the surface chemical groups on Pd-ACF, the
 hydrogen uptake capacity of Pd-ACF was evaluated after pretreatment at 120, 300 and
 1000 °C. Measurements at each temperature were done without exposing the sample
 to ambient atmospheric conditions. Here only the isotherm measured at
 80 °C 
 will be presented (figure <figref linkend="nano294638fig6">6</figref>(a)). All isotherms show a similar trend: a small uptake was measured consistently
 when H<sub>2</sub> 
 was first admitted (at 3 mbar) over a sample in vacuum (10<sup>−3</sup> mbar). The
 amount adsorbed (∼0.002 wt% or [H]/[Pd] ∼0.1) corresponds to chemisorption of hydrogen on Pd and formation of
 α-
 PdH<sub><italic>x</italic>
</sub>. At
 80 °C 
 direct adsorption of hydrogen on the carbon support is negligible. A sudden increase in
 H<sub>2</sub> 
 uptake between 300 and 500 mbar is caused by transition from
 α-
 PdH<sub><italic>x</italic>
</sub> to
 β-
 PdH<sub><italic>x</italic>
</sub>. After complete
 transformation to β 
 phase occurred, the isotherms continued with a slight positive slope up to 1 bar, the maximum
 pressure of these measurements. Using gravimetric measurements, it was confirmed that enhanced
 H<sub>2</sub> 
 uptake (compared to Pd-free ACF at equal activation level) continues to higher pressure
 (20 bar), although with a lower slope [<cite linkend="nano294638bib26">26</cite>].
 <figure id="nano294638fig6" parts="single" width="page" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463806.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463806.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc6" type="figure" label="Figure 6"><p indent="no">(a) Hydrogen uptake properties of Pd-ACF pretreated at
 different temperatures. All the measurements were done at
 80 °C as a function
 of increasing H<sub>2</sub> 
 pressure. The right side ordinate represents [H]/[Pd] calculated for 2 wt% Pd in Pd-ACF.
 (b) Effect of cycling on hydrogen uptake capacity of Pd-ACF after pretreatment at
 300 °C. The isotherms were
 recorded at 80 °C with 900
 min evacuation at 25 °C 
 between the cycles.</p>
</caption>
</figure>
</p>
<p>In a separate test, H<sub>2</sub> adsorption
 isotherm was measured at 80 °C 
 for Pd-ACF evacuated at 300 °C. Two more adsorption cycles were done after evacuation at room temperature, but without
 any heat treatment. The results (figure <figref linkend="nano294638fig6">6</figref>(b)) show clear differences between the first and the subsequent isotherms. The
 <italic>P</italic>
<sub>α/β</sub> 
 in second and third cycle (320 mbar) is lower than
 <italic>P</italic>
<sub>α/β</sub>∼ 360 mbar just after
 pretreatment at 300 °C. The fact that <italic>P</italic>
<sub>α/β</sub> 
 is sensitive to the state of carbon surface after pretreatment and cycling contradicts the
 explanation proposed by Suleiman <italic>et al</italic>
 [<cite linkend="nano294638bib21">21</cite>] according to which the phase
 transition shifts to higher pressure because of mechanical stress exerted by a hard matrix
 on embedded Pd particles.</p>
<p>The observations on α/β 
 transition during H<sub>2</sub> 
 adsorption (figure <figref linkend="nano294638fig6">6</figref>(a)) were corroborated with information on
 β/α transition
 upon H<sub>2</sub> 
 desorption (figure <figref linkend="nano294638fig7">7</figref>
). Before contacting Pd-ACF with 10 bar of hydrogen, the samples were evacuated <italic>in situ</italic> at
 25 °C, or at elevated
 temperatures (300 and 900 °C). Thus the state of the surface before contacting with
 H<sub>2</sub> 
 corresponds to that evidenced by TPD tests discussed above. All XRD patterns were recorded at
 80 °C. Figure <figref linkend="nano294638fig7">7</figref> shows variation in the concentration of
 α and
 β phases as a function
 of decreasing <italic>P</italic>
<sub>H2</sub>. A
 pure β phase exists
 at 10 bar when H<sub>2</sub> 
 is introduced on Pd-ACF evacuated at either
 25 °C or
 300 °C. In contrast, Pd-ACF
 pretreated at 900 °C shows
 the presence of ∼3<italic>% </italic>
α-PdH<sub><italic>x</italic>
</sub> 
 phase even under 10 bar of hydrogen. In addition, the
 β/α 
 transition occurs at higher pressure on the sample pretreated at
 900 °C 
 (<italic>P</italic>
<sub>β/α</sub>∼400 mbar) than on the samples pretreated at 25 and
 300 °C (275 and
 300 mbar). The result suggests that a cleaner carbon surface (stripped from surface groups) destabilizes
 β-
 PdH<sub><italic>x</italic>
</sub> phase (shown by the
 increase in <italic>P</italic>
<sub>β/α</sub>), so that
 small PdH<sub><italic>x</italic>
</sub> crystallites
 do not convert into β 
 hydride phase even under 10 bar of hydrogen.
 <figure id="nano294638fig7" parts="single" width="column" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463807.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463807.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc7" type="figure" label="Figure 7"><p indent="no">Phase concentrations of β 
 (solid symbols) and α 
 (hollow symbols) Pd hydride phases in Pd-ACF samples pretreated at 25, 300 and
 900 °C. The phase concentrations were calculated based on XRD recorded at
 80 °C as a function
 of decreasing <italic>P</italic>
<sub>H2</sub>
.</p>
</caption>
</figure>
</p>
</sec-level2>
</sec-level1>
<sec-level1 id="nano294638s4" label="4"><heading>Discussion</heading>
<p indent="no">Reviewing the results for all three materials, it appears that
 <italic>P</italic>
<sub>β/α</sub> (on desorption)
 and <italic>P</italic>
<sub>α/β</sub> 
 (on absorption) have systematic variations between the samples and, for the same sample,
 also depend on the pretreatment conditions. When characterized in comparable conditions,
 <italic>P</italic>
<sub>β/α</sub> increases in the
 order Pd-black< 
 Pd-catalyst< 
 Pd-ACF; this trend was observed at several temperatures (figures <figref linkend="nano294638fig2">2</figref>
 and <figref linkend="nano294638fig3">3</figref>). When Pd-ACF was compared after pretreatment at different
 temperatures, it was found that equilibrium transition pressures during absorption
 <italic>P</italic>
<sub>α/β</sub> 
 (figure <figref linkend="nano294638fig6">6</figref>(a)) and desorption
 <italic>P</italic>
<sub>β/α</sub> 
 (figure <figref linkend="nano294638fig7">7</figref>) also increase with pretreatment temperature. Moreover, on Pd-ACF pretreated at
 900 °C conversion
 to H-rich β-
 PdH<sub><italic>x</italic>
</sub> is not complete
 at 80 °C even under
 10 bar of H<sub>2</sub> 
 (figure <figref linkend="nano294638fig7">7</figref>). Apart from the effect of degree of Pd–carbon
 contact and pretreatment temperature, cycling also has an influence on
 <italic>P</italic>
<sub>α/β</sub>.
 The <italic>P</italic>
<sub>α/β</sub> 
 of freshly out-gassed Pd-ACF shifts to lower pressures after the first absorption–desorption
 cycle (figure <figref linkend="nano294638fig6">6</figref>
(b)).</p>
<p>The upward shift of <italic>P</italic>
<sub>β/α</sub> 
 and <italic>P</italic>
<sub>α/β</sub> 
 observed both on absorption and desorption is tantamount with a destabilization of the H-rich
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase for Pd in presence of carbon in comparison with the nanocrystalline
 Pd-black used as a reference. The results show that two factors might
 be responsible for this destabilization. On one hand, it appears that
 <italic>P</italic>
<sub>β/α</sub> 
 increases with the degree of contacts between Pd particles and the carbon support, as
 schematically shown in figure <figref linkend="nano294638fig1">1</figref> (insets). At any given temperature, the support-free Pd-black has the lowest
 <italic>P</italic>
<sub>β/α</sub>, while Pd-ACF with palladium embedded in the carbon matrix has the highest
 <italic>P</italic>
<sub>β/α</sub>;
 the <italic>P</italic>
<sub>β/α</sub> 
 of Pd-catalyst, with palladium deposited on carbon surface is between the two. On the
 other hand, the results on Pd-ACF show that the chemical state of the carbon
 surface is another important factor that strongly influences the stability of the
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase. It was shown above that pretreating Pd-ACF at high temperatures before contacting with
 H<sub>2</sub> 
 removes oxygen-containing groups from carbon surface (figure <figref linkend="nano294638fig5">5</figref>) and shifts the transition pressure
 <italic>P</italic>
<sub>β/α</sub> 
 to higher levels (figures <figref linkend="nano294638fig6">6</figref>
(a) and <figref linkend="nano294638fig7">7</figref>). Both effects show that
 β-
 PdH<sub><italic>x</italic>
</sub> is less stable (is
 formed at higher H<sub>2</sub> 
 pressure) on a carbon surface denuded of all surface chemical groups.</p>
<p>The effect of support on the transition between
 α and
 β 
 hydride phases can be quantified by calculating enthalpy changes using van’t Hoff plot
 (figure <figref linkend="nano294638fig8">8</figref>
). The inset in figure <figref linkend="nano294638fig8">8</figref>
(a) shows that Δ<italic>H</italic>
<sub>α/β</sub> 
 values on absorption are ∼35.5 ± 1 kJ mol<sup>−1</sup> 
 for all pretreated Pd-ACF samples, matching exactly
 Δ<italic>H</italic>
<sub>α/β</sub> 
 reported for bulk Pd [<cite linkend="nano294638bib9">9</cite>]. On desorption,
 Δ<italic>H</italic>
<sub>β/α</sub> decreases in the series
 Pd-black> Pd-catalyst
 > Pd-ACF. Pretreatment
 of Pd-ACF at 900 °C 
 further decreases Δ<italic>H</italic>
<sub>β/α</sub> to
 a level that is close to Δ<italic>H</italic>
<sub>α/β</sub>
. Analysis of absorption–desorption hysteresis in bulk Pd [<cite linkend="nano294638bib10">10</cite>
] and nanocrystalline Pd [<cite linkend="nano294638bib38">38</cite>] shows that
 Δ<italic>H</italic>
<sub>β/α</sub> on desorption is
 always larger than Δ<italic>H</italic>
<sub>α/β</sub> 
 on absorption. The difference can be attributed to the higher stability of chemisorbed hydrogen (∼95 kJ mol<sup>−1</sup>
) [<cite linkend="nano294638bib9">9</cite>
, <cite linkend="nano294638bib39">39</cite>] which is difficult to desorb from Pd surface, as represented in
 figure <figref linkend="nano294638fig9">9</figref>.
 <figure id="nano294638fig8" parts="single" width="page" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463808.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463808.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc8" type="figure" label="Figure 8"><p indent="no">van’t Hoff’s plot of phase transitions on Pd-ACF compared to other samples and different
 pretreatment conditions during absorption (a) and desorption (b). The insets represent
 corresponding enthalpy changes.</p>
</caption>
</figure>
<figure id="nano294638fig9" parts="single" width="column" position="float" printstyle="normal" orientation="port"><graphic><graphic-file version="print" format="EPS" scale="100" filename="images/9463809.eps"></graphic-file>
<graphic-file version="ej" format="JPEG" filename="images/9463809.jpg"></graphic-file>
</graphic>
<caption id="nano294638fc9" type="figure" label="Figure 9"><p indent="no">Schematic energy diagram for hydrogen absorption and desorption
 into Pd lattice that shows the reason for the difference between
 Δ<italic>H</italic>
<sub>β/α</sub> and
 Δ<italic>H</italic>
<sub>α/β</sub>. It is hypothesized that in the presence of carbon support, the chemisorbed
 hydrogen spills over from Pd surface to H-trapping sites on carbon. With
 increasing Pd–carbon contact and number of H-trapping sites, the effectiveness of
 hydrogen transfer (‘pumping power of support’) increases and thereby destabilizes
 β-PdHx. In the figure, energy of hydrogen dissociation (<italic>E</italic>
<sub>DIS</sub>
), physisorption (<italic>E</italic>
<sub>PH</sub>), chemisorption
 (<italic>E</italic>
<sub>CH</sub>), subsurface
 states (<italic>E</italic>
<sub>SS</sub>) and
 diffusion (<italic>E</italic>
<sub>DIF</sub>) are
 shown along with Δ<italic>H</italic>
<sub>α/β</sub> 
 and Δ<italic>H</italic>
<sub>β/α</sub>
. The figure is adapted after [<cite linkend="nano294638bib39">39</cite>
].</p>
</caption>
</figure>
</p>
<p>There are contradictory literature reports on the effect of support and particle size on
 H<sub>2</sub> 
 absorption/desorption isotherms for small Pd particles. Mütschele and
 Kirchheim [<cite linkend="nano294638bib22">22</cite>] compared hydrogen uptake of support-free
 nanocrystalline Pd (8–12 nm particles) and bulk polycrystalline Pd (20 µm 
 grains). They concluded that the crystallite size of Pd does not have any effect on
 <italic>P</italic>
<sub>β/α</sub>
. Zütttel <italic>et al</italic>
 [<cite linkend="nano294638bib19">19</cite>
] and Pundt <italic>et al</italic>
 [<cite linkend="nano294638bib39">39</cite>] reported
 that decreasing the crystallite size lowers the miscibility gap between
 α and
 β 
 phases, but they did not invalidate the conclusion of Mütschele and Kirchheim [<cite linkend="nano294638bib22">22</cite>].
 Other reports deviate from this conclusion. Wunder <italic>et al</italic>
 [<cite linkend="nano294638bib16">16</cite>] proposed that
 smaller Pd crystallites on inorganic supports and carbon black might have higher
 <italic>P</italic>
<sub>α/β</sub> 
 than bulk Pd. On the other hand, a recent report by Kuji <italic>et al</italic>
 [<cite linkend="nano294638bib38">38</cite>] claims that smaller crystallites have lower
 <italic>P</italic>
<sub>α/β</sub>
, in contradiction with Mütschele <italic>et al</italic>
 [<cite linkend="nano294638bib22">22</cite>
] and Wunder <italic>et al</italic> 
 [<cite linkend="nano294638bib16">16</cite>
]. Suleiman <italic>et al</italic>
 [<cite linkend="nano294638bib21">21</cite>] proposed that upwards shifts of
 <italic>P</italic>
<sub>α/β</sub> 
 for Pd particles stabilized on hard (polymer) supports is caused by accumulation of elastic
 strain during lattice expansion at the metal/matrix interface.</p>
<p>The disagreement in the literature invites further analysis. It is worth noting
 that all earlier reports are based on comparison between samples of different
 nature and origin. The differences attributed to crystallite size might have had a
 different cause. In the present study the focus is on a single sample, Pd-ACF,
 which shows a broad distribution of particle sizes. XRD analysis was used to
 determine the size of crystallites that undergo transition first, and simultaneously
 measure phase concentration, crystallite size and lattice parameters during
 β to
 α 
 transition. Such a complex analysis is not possible through commonly used volumetric and
 gravimetric techniques. The present study demonstrates that small crystallites of
 β hydride in Pd-ACF
 transform into α 
 hydride at higher pressure than large crystallites, in contrast with the result by Kuji <italic>et al</italic> 
 [<cite linkend="nano294638bib38">38</cite>
] and apparently in agreement with Wunder <italic>et al</italic>
 [<cite linkend="nano294638bib16">16</cite>].
 However, one more factor needs consideration before concluding. Morphological
 analysis of Pd-ACF shows that the smallest Pd particles (from sub-nanometre to
 <100 nm) are embedded in the carbon support, while very large crystallites (100–1000 nm) are
 distributed on the support surface. The former have higher degree of contact with the
 carbon support and are by far more numerous. Hence it is difficult to establish
 whether the embedded nature or the size of crystallites cause the increase of
 <italic>P</italic>
<sub>β/α</sub> 
 observed with Pd-ACF. The variation of ACS in Pd-black and Pd-catalyst during
 transition (figure <figref linkend="nano294638fig4">4</figref>) cannot be used as an argument in support of the crystallite size effect on
 <italic>P</italic>
<sub>β/α</sub>
 [<cite linkend="nano294638bib16">16</cite>
] because the observed change is within error limits.</p>
<p>At the same time, comparing the trend for
 <italic>P</italic>
<sub>β/α</sub> 
 for three different palladium samples, it appears that the stability of
 β-
 PdH<sub><italic>x</italic>
</sub> phase is
 primarily not dependent on average crystallite sizes. Pd-catalyst has the smallest Pd crystallite size, but
 its <italic>P</italic>
<sub>β/α</sub> is
 intermediate between Pd-black and Pd-ACF. Similarly, even though Pd-ACF has the largest crystallites,
 its <italic>P</italic>
<sub>β/α</sub> 
 is the highest compared to other samples. Based on the lack of correlation, the effect of
 crystallite size should be ruled out as the main cause contributing to destabilization of
 β 
 hydride. All indications from the present study are that the extent of Pd–carbon contact and the
 properties of the support (presence of microporosity in the appropriate size for physisorption of
 H<sub>2</sub> or
 absence of oxygen-containing surface groups) play the primary determining role in destabilization of
 β 
 hydride phase.</p>
<p>The possibility of facile transfer of H atoms from the catalyst surface to carbon
 has been theoretically predicted in a recent study for the Pt-carbon system [<cite linkend="nano294638bib12">12</cite>
, <cite linkend="nano294638bib13">13</cite>]. Hydrogen atoms may subside on graphenes in mobile,
 weakly bonded forms (physisorbed) or in states strongly bonded to carbon atoms
 (chemisorbed). Extending the conclusion of this study to the Pd–carbon system, it
 can be argued that higher degree of Pd–carbon contacts increase the effective
 transfer of spilt over hydrogen to the trapping sites on support and help destabilize
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase. Removal of oxygenated surface functions from carbon increases the concentration of
 hydrogen trapping sites by further adding to this effect.</p>
<p>This hypothesis can be further substantiated by the fact that
 <italic>P</italic>
<sub>α/β</sub> of Pd-ACF
 pretreated at 300 °C 
 shifts to lower pressures (by about 40 mbar) in the subsequent cycles (figure <figref linkend="nano294638fig6">6</figref>(b)). After the first cycle, it is expected that spilt over hydrogen occupies most
 of the hydrogen trapping sites, like micropores and will saturate dangling bonds on carbon
 atoms by forming C–H bonds. The formation of C–H bonds under similar conditions was
 recently confirmed by inelastic neutron scattering studies [<cite linkend="nano294638bib40">40</cite>]. Such blocking
 of trapping sites decreases the pumping power of the support and consequently shifts
 <italic>P</italic>
<sub>α/β</sub> 
 to lower pressure.</p>
<p>The destabilization of β-PdH<sub><italic>x</italic>
</sub> 
 in carbon-supported samples may be regarded as the result of ‘pumping’ of H atoms out of the
 β 
 phase in the presence of high surface area carbons. The interfacial transfer is the first step
 of the hydrogen spillover process. The driving force causing ‘leaking’ of H atoms from
 the Pd-catalyst is the presence of either carbon surface atoms with unpaired
 electrons (‘dangling bonds’) or micropores of appropriate width having higher
 adsorption potential for hydrogen [<cite linkend="nano294638bib33">33</cite>]. The results show that the
 ‘pumping power’ of the carbon support increases with the degree of Pd–carbon
 contacts and the increase in microporosity. Pd-ACF has both higher degree of
 Pd–carbon contacts than Pd-catalyst and higher volumes of narrow micropores (<8 Å) where the adsorption
 potential of H<sub>2</sub> 
 is the highest [<cite linkend="nano294638bib33">33</cite>]. The increase in the degree of Pd–carbon contact is
 expected to lower the energy barrier for diffusion and enhance the rate of interfacial
 hydrogen transfer as discussed by various authors [<cite linkend="nano294638bib7">7</cite>] and schematically
 represented in figure <figref linkend="nano294638fig9">9</figref>. The decrease in
 Δ<italic>H</italic>
<sub>β/α</sub> 
 with increasing degree of Pd–carbon contact could be the result of improved
 interfacial hydrogen transfer. Removing chemically bonded oxygen from carbon
 surface further enhances H leaking from the hydride phase and decreases
 Δ<italic>H</italic>
<sub>β/α</sub>
. Takagi <italic>et al</italic>
 [<cite linkend="nano294638bib41">41</cite>] reported that stripping of surface oxides enhances the hydrogen
 uptake in Pd supported on activated carbon fibres. They explained the enhancement by
 removal of steric hindrances at the entrance of micropores with appropriate size. An alternate
 explanation is that cleaning of oxygenated surface groups from carbon support increases
 the density of unsaturated carbon atoms. All these factors isolated or in cooperation,
 increase the density of H-trapping sites and contribute to enhancing the transfer rate of
 H atoms from Pd hydride to carbon support, thereby further destabilizing the
 β 
 phase.</p>
</sec-level1>
<sec-level1 id="nano294638s5" label="5"><heading>Conclusion</heading>
<p indent="no">Hydrogen spillover from Pd to the support in carbon-supported Pd catalysts is extensively
 studied as a mechanism to improve the catalytic activity of Pd and also to enhance the
 hydrogen storage capacity of carbon. In this study, the influence of nature of carbon support
 and the degree of Pd–carbon contacts are investigated by monitoring the transition between
 β-
 PdH<sub><italic>x</italic>
</sub> and
 α-
 PdH<sub><italic>x</italic>
</sub> 
 in pure Pd and two Pd–carbon materials with different degrees of Pd–carbon contact.
 Using high pressure XRD, the structural parameters, crystallite sizes and phase
 concentrations of hydride phases were monitored during transition. The study
 was complemented with morphology characterization and hydrogen adsorption
 measurements.</p>
<p>The results show that the equilibrium pressure of the
 β/α 
 transition (<italic>P</italic>
<sub>β/α</sub>) 
 increases with the degree of the Pd–carbon contact in the order Pd-black
 < Pd-catalyst
 < Pd-ACF.
 This trend, which is observed at several temperatures, suggests that the carbon support destabilizes
 the β-PdH<sub><italic>x</italic>
</sub> 
 phase. In other words, higher degree of Pd–carbon contacts of Pd particles embedded in
 the microporous carbon matrix induce efficient ‘pumping’ of hydrogen out of
 β-
 PdH<sub><italic>x</italic>
</sub>. Variations
 in both <italic>P</italic>
<sub>β/α</sub> 
 and <italic>P</italic>
<sub>α/β</sub> 
 induced by thermal cleaning of the surface and cycling between absorption and desorption
 on Pd-ACF suggest that removing oxygen-containing surface groups further enhances the
 hydrogen pumping power of the microporous carbon. Facile transfer of H atoms from H-rich
 β-
 PdH<sub><italic>x</italic>
</sub> 
 phase, which was theoretically predicted in a similar system, is driven by presence of
 efficient H-trapping sites on carbon (unsaturated carbon atoms, micropores with high
 adsorption potential). In brief, this study highlights the importance of higher degree of
 Pd–carbon contacts and of the state of the carbon surface as factors contributing to
 enhanced hydrogen spillover.</p>
</sec-level1>
<acknowledgment><heading>Acknowledgments</heading>
<p indent="no">This research is supported by the Division of Materials Science and Engineering, US
 Department of Energy, under contract DE-AC05-00OR22725 with UT Battelle, LLC. A
 portion of this research was conducted at ORNL’s Center for Nanophase Materials
 Sciences, which is sponsored by Scientific User Facilities Division, Office of Basic Energy
 Sciences, US Department of Energy. VVB acknowledges the appointment under ORNL
 Postdoctoral Research Associates Programme administered jointly by ORISE and ORNL.
 The authors acknowledge help from Drs Andrew Edward Payzant and Adam Rondinone of
 Center for Nanophase Materials Sciences with XRD studies, and contributions of
 Professor Dan D Edie and Mr Halil Tekinalp with synthesis of ACF and Pd-ACF
 materials.
 </p>
</acknowledgment>
</body>
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<mods version="3.6"><titleInfo><title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons</title>
</titleInfo>
<titleInfo type="abbreviated"><title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons</title>
</titleInfo>
<titleInfo type="alternative"><title>The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containing activated carbons</title>
</titleInfo>
<name type="personal"><namePart type="given">V V</namePart>
<namePart type="family">Bhat</namePart>
<affiliation>Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, MS-6087, Oak Ridge, TN 37831, USA</affiliation>
<affiliation>E-mail:bhatvv@ornl.gov</affiliation>
<role><roleTerm type="text">author</roleTerm>
</role>
</name>
<name type="personal"><namePart type="given">C I</namePart>
<namePart type="family">Contescu</namePart>
<affiliation>Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, MS-6087, Oak Ridge, TN 37831, USA</affiliation>
<affiliation>E-mail:contescuci@ornl.gov</affiliation>
<role><roleTerm type="text">author</roleTerm>
</role>
</name>
<name type="personal"><namePart type="given">N C</namePart>
<namePart type="family">Gallego</namePart>
<affiliation>Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, MS-6087, Oak Ridge, TN 37831, USA</affiliation>
<affiliation>Author to whom any correspondence should be addressed</affiliation>
<affiliation>E-mail:gallegonc@ornl.gov</affiliation>
<role><roleTerm type="text">author</roleTerm>
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<typeOfResource>text</typeOfResource>
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<dateIssued encoding="w3cdtf">2009</dateIssued>
<copyrightDate encoding="w3cdtf">2009</copyrightDate>
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<language><languageTerm type="code" authority="iso639-2b">eng</languageTerm>
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<note type="production">Printed in the UK</note>
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<abstract>This paper reports on differences in stability of Pd hydride phases in palladium particleswith various degrees of contact with microporous carbon supports. A sample containing Pdembedded in activated carbon fibre (2wt Pd) was compared with commercial Pdnanoparticles deposited on microporous activated carbon (3wt Pd) and with support-freenanocrystalline palladium. The morphology of the materials was characterized by electronmicroscopy, and the phase transformations were analysed over a large range of hydrogenpartial pressures (0.00310 bar) and at several temperatures using in situ x-ray diffraction.The results were verified with volumetric hydrogen uptake measurements. Resultsindicate that higher degrees of Pdcarbon contacts for Pd particles embeddedin a microporous carbon matrix induce efficient pumping of hydrogen out of-PdHx. It was also found that thermal cleaning of carbon surface groups prior toexposure to hydrogen further enhances the hydrogen pumping power of themicroporous carbon support. In brief, this study highlights that the stability of-PdHx phase supported on carbon depends on the degree of contact between Pd and carbon andon the nature of the carbon surface.</abstract>
<relatedItem type="host"><titleInfo><title>Nanotechnology</title>
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<titleInfo type="abbreviated"><title>Nanotechnology</title>
</titleInfo>
<identifier type="ISSN">0957-4484</identifier>
<identifier type="eISSN">1361-6528</identifier>
<identifier type="publisherID">Nano</identifier>
<identifier type="CODEN">NNOTER</identifier>
<identifier type="URL">stacks.iop.org/Nano</identifier>
<part><date>2009</date>
<detail type="volume"><caption>vol.</caption>
<number>20</number>
</detail>
<detail type="issue"><caption>no.</caption>
<number>20</number>
</detail>
<extent unit="pages"><start>1</start>
<end>10</end>
<total>10</total>
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<identifier type="istex">15BA219EAA5CDEAAEEBD3D9A427116EA6A41392F</identifier>
<identifier type="DOI">10.1088/0957-4484/20/20/204011</identifier>
<identifier type="PII">S0957-4484(09)94638-4</identifier>
<identifier type="articleID">294638</identifier>
<identifier type="articleNumber">204011</identifier>
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The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons

The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containingactivated carbons

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