Al2O3 Catalyst

Article pubs.acs.org/JPCC

Methanol Steam Reforming over Indium-Promoted Pt/Al2O3 Catalyst: Nature of the Active Surface Roland L. Barbosa,† Vasiliki Papaefthimiou,† Yeuk T. Law,† Detre Teschner,‡ Michael Hav̈ ecker,‡,¶ Axel Knop-Gericke,‡ Ralf Zapf,§ Gunther Kolb,§ Robert Schlögl,‡ and Spyridon Zafeiratos*,† †

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 Strasbourg, 25, rue Becquerel, F 67087 Strasbourg Cedex 2, France ‡ Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany § Institut für Mikrotech Mainz GmbH IMM, D-55129 Mainz, Germany ¶ Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: The surface state of the Pt/In2O3/Al2O3 catalyst coated onto a microchannel stainless steel reactor was investigated under working conditions using synchrotron-based ambient pressure photoelectron (APPES) and X-ray absorption near-edge structure (XANES) spectroscopies, combined with online mass spectrometry. The surface of the fresh catalyst consists of metallic Pt, In2O3, and Al2O3. Reduction under 0.2 mbar of H2 at 250 °C leads to surface enhancement of Pt and partial reduction of In2O3, while Al2O3 remains unchanged. Reoxidation in O2 atmosphere stimulates surface segregation of In2O3 over Pt, accompanied by partial oxidation of Pt to PtOx. Based on these results a dynamic, gas-phasedependent surface state is demonstrated. Under methanol steam reforming conditions, the surface state rapidly adapts under the reaction stream regardless of the pretreatment. However, correlation of gas phase with spectroscopic results under working conditions pointed out the beneficial effect of surface indium to reduce the CO selectivity. Finally, evidence of a distorted symmetry of Al sites on Pt/In2O3/Al2O3 catalyst compared to that of γ-Al2O3 is given. The findings obtained in the present study are of fundamental significance in understanding the relation between the surface state and the catalytic performance of a functional methanol reforming catalyst.

1. INTRODUCTION

Efficient low-temperature methanol reformers can be integrated with high-temperature polymer electrolyte fuel cells in a single solid system, called internal reforming alcohol HT-PEM fuel cell (IRAFC).5 This configuration allows the production of H2 in situ by utilizing directly the waste heat of the electrochemical process in the anode to cover the energy demands of the endothermic steam reforming reaction. To be able to obtain optimum efficiency of methanol reforming with negligible side reactions, highly active catalysts are needed that can provide the desired fast kinetics at low temperatures.4 Noble metal catalysts, and in particular Pt/In2O3/Al2O3 catalysts, have been found to be promising in this aspect, as they have shown 10 times higher activity compared to the commonly used and investigated Cu-based catalysts.6 Pt catalyzes predominantly methanol decomposition, converting methanol to CO and H2.1 CO, however, is an atmospheric pollutant and a poison for the fuel cell. With steam, a secondary reaction of water-gas shift (WGS) converts some of the CO to

The application of hydrogen as a clean energy carrier has been to the forefront in the quest of alternative energy sources. The use of hydrogen in fuel cell systems offers potentially higher efficiency compared to the conventional combustion engines and higher energy density compared to conventional batteries. In addition, when produced from renewable sources, it has almost zero emissions of carbon oxides and harmful substances such as nitrogen and sulfur oxides.1−4 Despite these beneficial properties, the widespread commercialization has been thwarted with problems including the distribution and storage of pure H2 as well as the high cost of that ideal fuel.1,2 A practical solution to this barrier is the use of liquids rich in hydrogen as a source of hydrogen via on-board reformers. One of the most popular alternative fuels for this purpose is methanol. It has a high H/C ratio, is liquid at atmospheric pressure, has low boiling point, is miscible with water, does not suffer from sulfur contamination, is readily metabolized by ambient organisms, and can be converted to hydrogen at relatively lower temperatures (150−350 °C) compared to other alcohols (>500 °C).1 © 2013 American Chemical Society

Received: September 21, 2012 Revised: March 7, 2013 Published: March 7, 2013 6143

dx.doi.org/10.1021/jp309401q | J. Phys. Chem. C 2013, 117, 6143−6150

The Journal of Physical Chemistry C

Article

coated with Pt/In2O3/Al2O3 (Supporting Information, Figure S1b). The specific surface area of the catalyst was 97 m2/g. The characteristic scanning and transmission electron microscopy images of the catalyst are given as Supporting Information, Figure S2 and Figure S3, respectively. The average Pt particle size of the fresh catalyst deduced by transmission electron microscopy (TEM) is around 2 nm, while scanning electron microscopy (SEM) images after reaction indicate large sintered grains decorated with smaller particles. X-ray diffraction (XRD) patterns of fresh and spent catalysts (Supporting Information, Figure S4,top) indicate crystallization under reaction conditions. However, peaks which could definitely ascribe to Pt−In alloy formation were difficult to resolve. To overcome this drawback and test if the preparation method gives mixed Pt−In alloys or not, the Pt−In solution was impregnated on SiO2 support using identical synthesis procedure. This time, the XRD pattern (Supporting Information, Figure S4,bottom), had more intense diffraction peaks than before and shows all the characteristic diffraction angles (according to the literature data 65-2989) of the PtIn2 phase. From these results, it is indicated that the majority of Pt and In are in close contact with each other and do not arrange as individual particles over the alumina support. In situ ambient pressure photoelectron (APPES) and X-ray absorption near-edge structure (XANES) spectroscopies combined with time-resolved mass spectrometry were carried out at ISISS beamline at BESSY synchrotron facility in Helmholtz Zentrum Berlin, in a setup described elsewhere.18 The sample was heated at a rate of 20 °C/min by an IR laser (CW, 808 nm) from the rear and the temperature was measured by a K-type thermocouple attached onto the surface. A total pressure of 0.2 mbar on the reaction cell was used for reducing (H2), oxidizing (O2), and methanol steam reforming conditions (CH3OH:H2O = 1:2). The gas-phase composition was monitored through a differentially pumped quadrupole mass spectrometer (QMS) attached to the reaction cell. The decrease of CH3OH (m/e = 31) QMS intensity was used to evaluate CH3OH conversion under reaction conditions. Relative product selectivities are estimated by the increase of the H2 (m/e = 2), CO (m/e = 28), and CO2 (m/e = 44) QMS intensities induced by the catalytic reaction. A correction of the ion current signals of m/e = 28 due to CH3OH fragment (20% of m/e = 31) was also taken into account. It should be noted, however, that QMS results presented in this study are used to illustrate the functional state of the catalysts during spectroscopic characterization and qualitatively correlate the surface state with the catalytic performance. A complete characterization of the reaction mechanism is not attempted since the conditions utilized in this study (low pressure and use of the spectrometer as a catalytic reactor) are optimized to facilitate the spectroscopic characterization but not catalytic tests. Hence, catalytic results after various pretreatments are used on a comparable basis and not to quantify the catalyst performance in relation to other reforming catalysts. Consequently, QMS signals are not calibrated to the sensitivity factor of each gas and only the comparison of the signals between various conditions and not absolute values are considered here. The Pt 4f, In 3d, Al 2p, C 1s, and O 1s photoelectron peaks were recorded using suitably selected excitation photon energies in order to be able to perform depth profiling of the atomic compositions under reaction conditions. The XPS source energies of 855, 655, 450, and 255 eV were used for Pt

CO2. Moreover, pure In2O3 itself has been found to be more than 95% selective toward CO2 in steam reforming of methanol (SRM) but 100 times less active as compared to the Pd/In2O3 catalysts measured at 493 K.7,8 γ-Al2O3 is typically used to support steam reforming catalysts because of the high surface area and the favored metal−support interactions.9 Pretreatment of the catalysts under reducing conditions is used to activate metal/oxide catalysts like Pd−ZnO or Pt− In2O3 prior to steam reforming reaction.10 It is believed that, upon reduction, the support is reduced forming a surface alloy with the metal (e.g., Pd−Zn or Pt−In). This effect can be reversed via oxidative treatments. Pt/In2O3/Al2O3 catalysts have been shown to be even more active than the Pd-based counterparts in methanol reforming reaction.6 The addition of In2O3 to the noble metal catalyst modifies the catalytic properties of the metal in such a way that enhances CO2 selectivity.11 This can be explicated in terms of geometric and electronic effects; the former suggests a decrease in the number of Pt atoms in the ensemble constituting the active sites in the formation of CO or CO precursors. The latter proposes a reduction in the adsorbed CO bond strength on the surface metal atoms due to a change in the electronic properties of Pt by formation of Pt−In bimetallic phases.12−15 For the Pd/ In2O3 system, the high CO2 selectivity in methanol steam reforming was ascribed to the modification of the electronic properties of the material with the presence of PdIn alloy,16 while under certain conditions, encapsulation of Pd by In2O3 suppresses the catalytic activity.11 For the hydrogenolysis reaction, the catalytic enhancement of Pt with the addition of In promoter has been mainly attributed to geometric effects, with any electronic effects playing a minor role.12 Finally, for alkane dehydrogenation, the reaction dependence of the activity and selectivity on the bulk In/Pt ratio was observed.17 In this work, we report on an in situ investigation of Pt/ In2O3/Al2O3 catalyst under reducing, oxidizing, and methanol steam reforming conditions. The catalyst is coated over a microchannel reactor like the actual catalyst prepared for IRAFC reformers, while the chosen conditions (temperature and reaction mixture) are simulating the working reaction environment. It was found that the surface of the catalyst is dynamic and promptly responds to the chemical potential of the reaction environment. Therefore, regardless of the pretreatment, the surface reorders rapidly to equilibrate with the gas environment under reaction conditions. Correlation of the spectroscopic results with gas-phase analysis indicates that relatively higher In surface concentration is beneficial in increasing CO2 selectivity.

2. EXPERIMENTAL SECTION The catalysts was prepared according to a procedure described elsewhere.6 In particular, initially γ-Al2O3 wash-coated onto the microchannels stainless steel sheets (Supporting Information, Figure S1a) followed by drying at room temperature and calcination at 600 °C in air. The alumina coating was then impregnated in a single step using an impregnating solution that contained both Pt and In precursors (H2PtCl6·6H2O and In(NO3)3·xH2O, respectively, ALFA AESAR) and subsequently dried and calcined in air at 350 °C. The loading of Pt and In2O3 has been previously studied and found to have an optimum selectivity for CO2 with low CO concentration at 10% Pt and 12% In (atom %).6 Therefore, this target composition was studied here. The experiments were performed using (1 × 1) cm2 specimens cut from microstructured stainless steel sheet 6144



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Figure 1. Deconvoluted Pt 4f, Al 2p (a) and In 3d (b) core-level spectra of Pt/In2O3/Al2O3 during exposure of the sample at 0.2 mbar of H2 (top) and subsequently of O2 (bottom) gas environments at 250 °C. The X-ray energies used were 1025 eV for In 3d spectra and 655 eV for Pt 4f and Al 2p.

reduced to In metal as evidenced by the In 3d spectrum shown in Figure 1b, with two components at 443.5 eV related to metallic In and at 444.5 eV associated with In2O3.15 With about 60% among surface indium constituents, In2O3 is the dominant In species at the surface. In oxidizing conditions, more than two components are observed for Pt 4f7/2 peak as shown in Figure 1a. This includes the metallic Pt component at 71.3 eV and an apparent PtOx contribution (around 50%) centered at 72.6 eV.22 The binding energy of the monovalent Pt is similar to that under H2 atmosphere, which may indicate similarities in the interaction of Pt with the Al2O3 support and In additive in both conditions. The In 3d core-level spectrum shown in Figure 1b reveals a single In 3d5/2 peak at 444.4 eV with 7.55 eV spin−orbit splitting between the 3d components, characteristic of fully oxidized In2O3.15 Indium oxide is easily reducible and this has been the property of the oxide that is regarded to be responsible for its high reactivity.7 For methanol steam reforming on Pd, the reducibility of In2O3 allows the formation of bimetallic Pd−In, believed to be responsible for high CO2 selectivity16 and involved in the mechanism of H2 formation over the Pd−In2O3 system.8 It was recently shown that, in the case of Pt−In alloy formation, the binding energies of the Pt 4f and In 3d peaks are almost identical to those of the pure metallic values (apparent shifts less than 0.1 eV).23 The presence of In 3d peak at 444.5 eV excludes the possibility that such alloy exists in oxidative atmosphere. However, in reducing atmosphere, where both Pt 4f and In 3d peaks exhibit metallic-like components, APPES results can neither exclude nor confirm the formation of Pt−In surface alloy. Platinum oxides have conflicting reports in terms of their effect when appearing in catalysts. Positive effects include an enhancement of CO oxidation24 and their action as initiator catalysts in the partial oxidation of propanol.25 Yet, the presence of PtOx species has also been linked to catalyst deactivation.26 In our case, the PtOx components are not stable under reducing conditions as they revert back to the metallic state upon reintroduction of H2 at 250 °C (the Pt 4f spectrum

4f and Al 2p core levels, and 1225, 1020, 825, and 625 eV for In 3d, respectively. For C 1s and O 1s, 865 and 1110 eV were utilized, respectively. The selection of the source energy ensures for the similarity of the photoelectrons kinetic energies for the constituent elements. The elemental atomic concentrations were obtained by dividing the core-level peak areas by the incident photon flux and the atomic photoionization cross sections corresponding to the X-ray energies used.19 The peak profiles as well as the full width at half-maximum (fwhm) of the components used for the deconvolution of the In 3d and Pt 4f spectra were determined by reference peaks of oxidized (In2O3) and reduced (In and Pt) samples. In particular, the line shape for In 3d and Al 2p peaks was Gaussian/Lorentzian function, while for the Pt 4f peak a mixed Gaussian/Lorentzian with high binding energy asymmetry was used. The relative ratio and binding energy shift of the Pt 4f and In 3d spin orbit components, as well as their fwhm, were fixed during fitting, while the peak position was allowed to vary within reasonable range (±0,15 eV). For the removal of the background the Shirley method was used. XANES spectra at the Al K-edge were recorded in the total electron yield mode enhanced by additional electrons created by ionization of the gas phase above the sample. The sample taken as a reference of γ-Al2O3, was the Al2O3 coating used as the catalyst support after submitted to pretreatment identical to that of the catalyst (wash-coated onto the microchannels stainless steel reactor, followed by drying at room temperature and calcination at 600 °C in air).

3. RESULTS AND DISCUSSION 3.1. Surface Characterization under Reducing and Oxidizing Conditions. Figure 1a shows Pt 4f and Al 2p corelevel peaks under H2 (top) and O2 (bottom) environments at 250 °C. The Pt 4f spectrum features two components at binding energies (BE) of 71.3 eV (Pt 4f7/2) and 74.6 eV (Pt 4f5/2) assigned to metallic Pt according to previous studies using the same instrument,20,21 and 74.1 eV (Al 2p peak) ascribed to Al2O3. At the same conditions, In2O3 was partially 6145



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within the Al2O3 and In2O3 matrix with indium oxide dominating the surface. At reductive conditions, the surface Pt becomes metallic and In2O3 is partially reduced to In metal while the Al2O3 oxidation state remains unaffected. Pt tends to segregate to the surface causing the enhancement of the Pt metal signal while indium content becomes attenuated. This restructuring behavior under H2 is correlated with the surface free energy of the metals.27 Under O2 atmosphere, the formation of In2O3 (ΔHf = −221.27 kcal/mol)28 is energetically more stable than that of Pt oxides (ΔHf from −45 kcal/ mol for PtO2 to −43 kcal/mol for PtO),29 leading to the segregation and preferential oxidation of In at the surface30 as evidenced by the calculated atomic concentrations in Figure 2. Compared to indium, aluminum has a more negative enthalpy of oxide formation (ΔHf = −395 kcal/mol).31 This can rationalize the stability of the Al2O3 phase under the pressure and temperature conditions studied here. 3.2. Surface Characterization under Methanol Steam Reforming Conditions. Surface characterization under methanol steam reforming conditions was performed over catalysts pretreated (activated) under either 0.2 mbar of O2 followed by 0.2 mbar of H2 at 250 °C (H2 pretreatment) or 0.2 mbar of O2 at 250 °C (O2 pretreatment). The aim was to study the effect of the initial surface composition on the reaction performance. In parallel to the spectroscopic characterization, the gas phase above the sample was monitored by on line mass spectrometry. Figure 3 summarizes the changes in the principal signals as a function of time for catalysts subjected both in oxidative and

acquired was similar to the one in H2 ambient shown in Figure 1a). Figure 2 exhibits the calculated atomic concentration of Pt, Al, and In as a function of the photoelectron kinetic energies

Figure 2. Comparison of the atomic concentrations of the surface components of Pt/In2O3/Al2O3 catalyst under 0.2 mbar of H2 and O2 gas environments at different analysis depths. The calculation of the atomic components was obtained from the XPS spectra of Pt 4f, Al 2p, and In 3d using different photon energies. On the upper x-axis the estimated information depth is given. Error bars represent the estimated systematic error in the atomic concentration based on the uncertainties of the atomic photoionization cross section and of the model used in the calculations. Lines serve as guides to the eye.

under reducing and oxidizing conditions. Given that the inelastic mean free path (IMFP) increases as the photoelectron energy is increased, lower photoelectron energy signifies higher surface sensitivity.20 The atomic concentrations show similar trends regardless of the gas environment. The Pt atomic concentration gradually increases with increasing photoelectron kinetic energy. This is rationalized by higher concentration of Pt at deeper subsurface levels. Under H2 environment the Pt atomic concentration is systematically higher compared to that in O2, implying surface enhancement of the Pt (metallic) in H2. The Al atomic concentration also increases considerably as deeper layers are probed, while In shows the opposite trend. This indicates enrichment of In at the first few atomic layers (about 2 nm). The surface amount of In (oxide) is enhanced in oxidative environments (while metallic Pt in reducing), since the In atomic concentration is systematically higher in O2 compared to H2. Previous studies have shown that indium oxide tends to encapsulate metallic components of the catalyst.11 Under oxidative treatments the formation of In2O3 shell around Pd particles was demonstrated for Pd/In2O3.11 In our case, systematically higher In atomic concentration in O2 compared to H2 environment can certainly imply encapsulation phenomena of indium oxides over Pt and Al2O3. We should note here that observations of Figure 2 can be only justified by the interplay between In and Pt on the surface of mixed Pt−In particles, while individual Pt and In particles, if any, should have only minor influence (see Supporting Information, paragraph S5 for more details). The above-presented results indicate a dynamic surface state dependent on the gas environment. Platinum seems to be

Figure 3. Mass spectrometry data recorded during methanol steam reforming (0.2 mbar, CH3OH:H2O = 1:2) of Pt/In2O3/Al2O3 catalyst pretreated in H2 (solid line) and O2 (scatter line). The temperature profile is shown at the right axis. Dash region represents the time period of acquisition of APPES spectra.

reducing pretreatments. As anticipated, the results give evidence for the sample activity as this is demonstrated by the increased signals of the reaction products H2, CO, and CO2 (m/e = 2, 28, 44, respectively) and the consumption of CH3OH (m/e = 31) and H2O (m/e = 18) as the temperature rises. Comparison of the mass signals indicates some qualitative differences on the catalytic performance in relation to the pretreatment. In particular, pretreatment in O2 leads to slightly 6146



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Figure 4. Deconvoluted (a) Pt 4f and Al 2p and (b) In 3d core-level spectra of Pt/In2O3/Al2O3 under 0.2 mbar of methanol and water (CH3OH:H2O = 1:2) obtained at 250 °C using photon energy of 655 eV for Pt and Al and 1025 eV for In. (c) Comparison of the atomic concentrations of the surface components of Pt/In2O3/Al2O3 catalyst at different depths for samples pretreated in H2 and O2. The calculation of the atomic components was obtained from the XPS spectra of Pt 3d, Al 2p, and In 3d using different photon energies. Error bars represent the estimated systematic error in the atomic concentration based on the uncertainties of the atomic photoionization cross section and of the model used in the calculations.

state of the differently pretreated catalysts; in the O2-treated sample, In is fully oxidized and there is a considerable amount of PtOx. Since H2 production and methanol conversion is practically the same for both samples in this temperature region, the CO and CO2 signal deviation can be related (at least to some extent) to In2O3 and PtOx reduction by CH3OH upon annealing, as supported by the steady-state photoemission results presented below. Subsequently, the state of the catalysts under steady-state conditions (at 250 °C) is considered. Regardless of the pretreatment, the core-level spectra of Pt, Al, and In under steady-state conditions were strikingly similar (Supporting Information, Figure S6), so only results after H2 pretreatment are analyzed in Figure 4, a and b. Under reaction conditions, Pt is in zero valence state, Al2O3 is unchanged, and In2O3 is partly reduced. Compared to previous In 3d spectra recorded in pure H2 in which In2O3 component is about 60%, the In2O3 component under reaction conditions is around 40%, which manifests the higher reducing potential of the reaction mixture. Figure 4c shows the atomic concentrations of the components of the sample under reforming conditions in various information depths. The pattern is consistent with the atomic distribution of the sample under H2 and O2 shown in Figure 2 but the absolute atomic concentrations are quite different. Under reaction conditions, the surface atomic concentration of Pt is almost double in all analysis depths compared to that in H2 while, to less extent, the same is true for the Al atomic concentration. On the other hand, In atomic concentration is systematically lower under reaction conditions. The chemical state of the surface elements (Supporting Information, Figure S6), as well as the surface amount of Pt, do not depend on the pretreatment, as they readily transform under the reaction stream. However, reductive pretreatment

less active (less CH3OH and H2O consumption) catalyst while less CO2 and relatively more H2 and CO are produced. The main reactions involved in the conversion of methanol into hydrogen, are the following: CH3OH → CO + 2H 2

decomposition (DOM)

(1)

CH3OH + H 2O → CO2 + 3H 2 steam reforming (SRM) CO + H 2O → CO2 + H 2

(2)

water gas shift (WGS) (3)

Over noble metal catalysts, lower CO selectivity has been related to weaker support acidity7 and to surface alloy formation due to reduction of the support.8 In general, acidic supports, like Al2O3, have been blamed to promote the DOM and suppress the WGS reaction, in contrast to basic supports like In2O3.7,32 In addition, as has been shown in detail for the Pd−Zn system, the intrinsic characteristic of group VIII metals to promote DOM reaction suppresses upon alloying (e.g., with Zn or In), leading to lower CO production.8 In our case the relatively higher CO selectivity measured in the O2-pretreated sample implies that oxidative treatments favor methanol decomposition and/or suppress WGS reaction. The ignition temperature of the reaction (around 130 °C as shown in Figure 3) is independent of the pretreatment as is manifested by methanol conversion along with CO2 and H2 production. However, for the H2-pretreated sample, CO production is only observed after 160 °C, indicating that, up to that temperature, reaction 1 is suppressed. In the case of the preoxidized sample, relatively higher CO and CO2 production rates are observed at this region (130−160 °C). This deviation can be rationalized in terms of the different initial oxidation 6147



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induces systematically higher In and lower Al concentration at all depths studied (up to 4 nm), indicating a direct link between the two catalyst elements. Kinetic limitations which inhibit the system from reaching the equilibrium can account for the difference in the surface concentration between O2 and H2 pretreated Pt−In/Al2O catalysts. As has been shown before, various metastable surface compositions can often be maintained under the same chemical potential, depending on the initial state.33 Since the oxidation state and surface amount of Pt are practically independent of the surface pretreatment, we propose that the observed differences on the CO selectivity under reforming conditions are related to the interplay between In and Al surface sites. Apparently, indium species exists in two valence states (oxidized and reduced) and is likely that each indium state affects the surface reactivity in a different way. Indium oxide has a less pronounced Lewis acid character than that of aluminum oxide; see ref 34 and references therein. Therefore, increasing surface Al2O3 amount may be related to the increase of the overall catalyst acidity, and vice versa for indium oxide. Weaker acidity of the oxide support suppresses DOM reaction, while in parallel enhances WGS and therefore decreases CO production.6,35 This observation can rationalize the relatively lower CO selectivity found for our sample after H2 treatment, since in that case less Al and more In oxide is observed. Besides, higher surface concentrations of metallic indium might influence the CO selectivity by expanding the Pt−In interaction (alloy formation) which also suppresses CO production. However, as mentioned above, it is very difficult to obtain solid experimental evidence to answer the question if under reaction conditions Pt and In form an intermetallic surface compound or a two-phase Pt−In mixture. Unfortunately, fast reoxidation of indium prevents any post mortem analysis using supplementary techniques. Since the Al 2p photoemission peak is not so informative of the Al state, Al K-edge XANES spectra were used to probe the structural coordination changes under methanol steam reforming conditions. It is well established that the Al K-edge spectral features are very sensitive to the local bonding environment of Al ions.36 Figure 5 shows the Al K-edge spectra recorded under reforming conditions from H2 and O2 pretreated samples as well as the reference γ-Al2O3, used in the wash-coating for catalyst preparation. The bottom plot shows the reference spectrum that features peaks around 1567.7 and 1571 eV, typical for distorted octahedral (Oh) Al sites, and a shoulder at ca. 1566 eV assigned to tetrahedrally coordinated (Td) Al sites on γ-Al2O3.9,37,38 The Al K-edge peaks of the differently pretreated Pt/In2O3/ Al2O3 samples are identical, supporting the negligible influence of the sample preconditioning to its reaction state. However, when compared to the reference γ-Al2O3 spectrum, the intensity ratio of the Td to Oh related peaks is largely different. Variations in the Al K-edge spectrum can be rationalized by modifications in the coordination of surface Al ions on the Pt/ In2O3/Al2O3 catalyst compared to γ-Al2O3, as for example transformation of 4-fold to 5-fold bonded Al ions.38 In addition, even if the coordination of Al ions remains similar to that of γAl2O3, theoretical investigations have shown that the spectral features critically depend on structural details, such as the Al− O bond length and bond angle, as well as the chemical environment; see refs 38 and 39 and references therein. This result suggests that Al2O3 is not only functioning as a high

Figure 5. XANES Al K-edge spectra of Pt/In2O3/Al2O3 under reaction conditions (0.2 mbar, CH3OH:H2O = 1:2, 250 °C) from H2 and O2pretreated samples. Also shown is Al K-edge spectrum of a reference γAl2O3 sample.

surface area support for the Pt/In2O3 catalyst, but actively participates to the catalyst’s configuration. It is interesting to note that, while the nominal Pt loading is 10% (atom %), its surface atomic concentration calculated by photoemission results at the outer 2 nm of the catalyst does not exceed 5% (see Figure 4c). It should be kept in mind that, although the temperature and the reaction mixture correspond to these used for practical application of the catalysts, the overall pressure in our study is still 3 orders of magnitude less. However, our results clearly manifested that there is a direct correlation between the In oxidation state and its concentration on the surface (surface segregation). This correlation, represented in Figure 6a, is linear, indicating that upon complete In reduction (for example, as expected for highly reducing conditions like higher H2 pressure and temperature), In surface concentration will drop, while that of Pt will increase, pointing out surface segregation of Pt. It should be noted that In oxidation state is closely related to the oxygen chemical potential of the gas phase. Metallic and oxidized indium species are not homogeneously mixed, but probably form a layer structure. This is shown by depth-dependent measurements of the In 3d peak (Figure 6b and Supporting Information, Figure S7) indicating that the signal of oxidized In is enhanced as deeper layers are probed. This is evidence of reduced/metallic In at the outermost layers while oxidized In is mainly located in deeper layers. Although, as mentioned, neither In 3d nor Pt 4f peak produces intense chemical shifts upon alloying, the presence of metallic-like In species on the outermost surface layers enhances the possibility of alloying with surface Pt traces. Overall, the presented research gives a detailed picture of the active surface state of Pt−In/Al2O3 catalyst, which is essential for the design of reforming catalysts with enhanced performance. Apart from the particular aspects related to the development of better catalytic materials, maybe of equal importance is the awareness that the surface state of a catalyst is not static but dynamic, and is readily transformed under the reaction mixture. The acknowledgment of this fact, which is a 6148



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before (fresh) and after (used) methanol steam reforming reaction; (5) Qualitative explanation of why the differences in the depth profile results are justified only by mixed and not by individual Pt−In particles model; (6) Pt 4f and In 3d APPES spectra of Pt/In2O3/Al2O3 powder catalyst under methanol steam reforming reaction conditions; (7) In 3d APPES spectra of Pt/In2O3/Al2O3 catalyst under methanol steam reforming reaction conditions recorded with various photon energies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of the project “Development of an Internal Reforming Alcohol High Temperature PEM Fuel Cell Stack (IRAFC)” supported by the seventh Framework Program under Grant No. 245202. We acknowledge the Helmholtz Zentrum Berlin electron storage ring BESSYII- for provision of synchrotron radiation at ISISS beamline. We thank Prof. E. Savinova and Dr. F. Garin for the fruitful discussions. R.B. gratefully acknowledges financial support from EU FP7/20072013 DEMMEA collaborative project under g.a. No 245156/ 10.

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Figure 6. (a) Calculated In and Pt atomic concentrations using photoelectron kinetic energy of 180 eV (information depth around 2 nm) as a function of the oxidation degree of In, recorded at 250 °C under various gas-phase environments. (b) Metallic to oxidized In intensity ratio in various analysis depths obtained by deconvolution of the In 3d peak. Lines serve as guides to the eye.

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common ground in almost all previous in situ surface characterization studies of catalysts, may act as the stimuli for the reconsideration and redefinition of our present understanding on how catalysts work.

4. CONCLUSIONS The surface state of Pt/In2O3/Al2O3 catalyst coated onto a microchannel reactor was investigated under reducing, oxidizing, and methanol reforming conditions. The oxidation state and composition of the surface elements are dynamic and adapt within a few minutes to the gas-phase environment. Under the examined methanol reforming conditions, the active surface consists of Pt in the metallic state, partly reduced In2O3, and fully oxidized Al2O3. Relatively higher surface concentration of indium is associated with lower CO selectivity. This result is rationalized by the modification of the support acidity by indium oxide and/or by the formation of Pt−In surface alloy. Evidence for the modification of Al2O3 coordination on the Pt/In2O3/Al2O3 catalysts compared to that of pure γ-Al2O3 support was also found.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

(1) SEM image of the reactor; (2) SEM image of the spent Pt/ In2O3/Al2O3 catalyst; (3) TEM image of fresh Pt/In2O3/Al2O3 catalyst; (4) XRD patterns of thePt/In2O3/Al2O3 catalyst 6149



Article

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Al2O3 Catalyst

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