Steam Reforming of Methanol with Sm2O3âCeO2-Supported

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Ind. Eng. Chem. Res. 2009, 48, 8364–8372

Steam Reforming of Methanol with Sm2O3-CeO2-Supported Palladium Catalysts: Influence of the Thermal Treatments of Catalyst and Support Luisa M. Go´mez-Sainero,*,† Richard T. Baker,‡ Arturo J. Vizcaıńo,§ Stephen M. Francis,‡ Jose´ A. Calles,§ Ian S. Metcalfe,| and Juan J. Rodriguez† A´rea de Ingenierıá Quı´mica, Facultad de Ciencias, UniVersidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain, School of Chemistry, UniVersity of St. Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K., Department of Chemical and EnVironmental Technology, ESCET, Rey Juan Carlos UniVersity, c/ Tulipań s/n, 28933 Mo´stoles, Spain, and School of Chemical Engineering and AdVanced Materials, UniVersity of Newcastle upon Tyne, Newcastle NE1 7RU, U.K.

The design of new anode materials to promote the reforming of methanol is important for the development of intermediate temperature direct methanol solid oxide fuel cells. A previous study showed Pd/CeO2-Sm2O3 to be a good candidate. Here, the influence of the calcination pretreatment of the support and the reduction temperature of the catalyst on the steam reforming of methanol are investigated, as they can become key factors in the performance of the catalyst. Conversion, H2 yield, and TOF were considerably higher when the support was calcined at 800 °C (Pd/CS-800) instead of 1000 °C (Pd/CS-1000). Characterization results suggest a stronger interaction of Pd particles with the support in Pd/CS-1000, which hinders its accessibility to the gas atmosphere, and a less homogeneous distribution of Pd particles. In both cases, the activity increases on increasing the reduction temperature from 400 to 500 °C. In addition, these catalysts were highly resistant to deactivation. 1. Introduction Solid oxide fuel cells (SOFCs) are a very attractive option for electrical power generation in stationary, mobile, and portable applications. The use of H2 as the feed has been reported to lead to a very good performance of SOFCs in terms of power density and durability. Nevertheless, the direct use of alcohols like methanol would have several benefits, in systems known as direct internal reforming SOFCs. In these, methanol is converted at the anode producing hydrogen and carbon monoxide, which are electrochemically consumed by the fuel cell, so generating electrical power. The use of methanol offers several advantages over other fuels. It is a liquid fuel so it is easy to transport, handle, and store, it has high energy density, and it can be obtained from renewable sources. Direct methanol fuel cells are more convenient as they avoid the need for an external reformer, which would raise the overall cost of the system. Moreover, in direct reforming, the hydrogen consumption by the electrochemical reaction would directly promote the conversion of the alcohol at the anode. On the other hand, SOFCs are usually operated at high temperatures (900-1000 °C). These temperatures lead to materials constraints, a high cost of manufacture, and problems of long-term stability. Lowering the temperature below about 500-600 °C would make the manufacture of SOFCs much more cost-effective.1 Furthermore, the energy input, at startup, would be considerably reduced, which is a requirement for electric motor-vehicle applications.2 However, ionic conductivity is decreased when reducing the operating temperature. This can be addressed by making the electrolyte layer as thin as possible and by introducing ionic conductivity into the anode material to favor the electrochemical reaction. * To whom correspondence should be addressed. Tel.: +34 914976939. Fax: +34 914973516. E-mail: [email protected]. † Universidad Autońoma de Madrid. ‡ University of St Andrews. § Rey Juan Carlos University. | University of Newcastle upon Tyne.

Several papers on direct methanol SOFCs have been published in recent years.3-7 When the commonly used SOFC anode, Ni-YSZ, was used with methanol, good efficiencies were obtained3 at an operating temperature of 1000 °C. However, the extent of deactivation of the catalyst was considerably higher as compared to the use of hydrogen as feed. The authors attributed this deactivation to coke deposition because Ni is known to promote cracking reactions. When decreasing the operating temperature of SOFCs, several authors5,8,9 found a decrease in cell potential, which was associated with slower methanol decomposition and/or reforming kinetics. Sasaki et al.5 observed a decrease in cell potential at 800 °C and below. This could be related to an increase in the amount of unreacted methanol as the operating temperature of the cell decreased. They concluded that it was necessary to design new electrocatalysts to improve the catalytic activity for the decomposition and/or reforming reactions, to increase the concentration of H2 obtained. CeO2- and Cu-based anodes have also been studied.4,6 Good initial performances were obtained when operating at low temperatures, but rapid and irreversible deactivation was observed for Cu-CGO, Cu-CeO2, and Cu-CeO2-YSZ anodes, due to delamination of the electrode in the first case and to Cu coarsening in the others. Moreover, while copper catalysts were found to be effective for methanol transformation, they were poor electrocatalysts for H2 oxidation. As stated in a previous study,10 ceria-samaria mixtures have significant potential for application as SOFC anodes. They are mixed ionic and electronic conductors at high temperatures. However, below about 700 °C, they are predominantly ionconducting materials. Their use has been proposed to extend the active reaction zone at the anode as the operating temperature of the cell is decreased.11 In this work, it was found that Ni-CeO2-Sm2O3 cermets gave rise to a higher open-circuit potential than did Ni-YSZ. The Ni-CeO2-Sm2O3 anodes that gave rise to the highest H2 concentration in the reaction mixture and correspondingly to the best selectivity for methanol reform-

10.1021/ie900630z CCC: $40.75  2009 American Chemical Society Published on Web 08/13/2009

Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

ing were found to give rise to the highest open-circuit potential. Good performance with CeO2-Sm2O3 anodes was also obtained by other authors.12-14 Furthermore, the addition of rare earths, in particular Sm2O3, to catalysts has also been found to improve their performance in the reforming of hydrocarbons.15-22 This behavior was ascribed to the basicity of Sm2O3, which was thought to enhance the adsorption of reactants, and to the contribution of spillover phenomena. With a view to their application as anodes in DMIT-SOFCs, the behavior of a 2 wt % Pd/(CeO2)0.8(Sm2O3)0.2 catalyst in the reforming of methanol was investigated in a previous study.10 The composition of the catalyst was selected with the aim of promoting the catalytic function of the anode and to confer suitable ionic and electronic conductivity. Very good results in terms of H2 production were obtained. As the calcination pretreatment of the support may be a key factor in the final conductivity and catalytic properties of these materials, and reduction temperature can significantly affect catalytic activity, the present work focuses on the effect of these parameters on the physicochemical and catalytic properties of the catalyst. 2. Experimental Section 2.1. Catalyst Preparation. The (CeO2)0.8(Sm2O3)0.2 (CS) support was prepared by ball-milling CeO2 and Sm2O3 for 24 h. 99.9% purity CeO2 (Aldrich) and Sm2O3 (Acros Organics) were used. A fraction of the support material was calcined by heating in air to 800 °C over 5 h and maintaining the material at this temperature for 2 h, while the other was calcined by heating to 1000 °C in 10 h and maintaining this temperature for 5 h. The calcination temperatures were chosen to promote solid state reaction. Two 2 wt % loaded Pd catalysts were prepared by incipient wetness impregnation of the supports calcined at 800 and 1000 °C (named Pd/CS-800 and Pd/CS-1000, respectively). Aqueous solutions of H2PdCl4 (pH < 1) of appropriate concentration were used. After impregnation, the catalyst precursors were dried overnight at room temperature, then at 120 °C for 2 h and calcined at 400 °C for a further 2 h. The heating rate was 100 °C h-1. The activation of the catalysts was carried out by reduction using a 20% H2/He stream at a flow rate of 50 cm3 min-1 (STP). 2.2. Catalyst Characterization. Specific surface areas (SSAs) of the catalysts and the supports were determined by the BET technique, by obtaining N2 adsorption-desorption isotherms at 77 K. The isotherms were recorded using a Gemini 2360 surface area analyzer. The samples were previously outgassed for 3 h at 300 °C and a residual pressure of 10-3 Torr. The X-ray diffraction patterns of the catalysts and supports were obtained in a D-5000 Siemens diffractometer, equipped with a SiLi Kevex detector, which eliminates the fluorescence of the sample. The powdered sample was scanned using Cu KR monochromatic radiation (λ ) 0.15406 nm). A scanning range of 2θ ) 10-70° and scan step size of 0.020° with 5 s collection time were used. Temperature programmed reduction (TPR) experiments were performed in a flow system using a Micromeritics TPD/TPR 2900 model equipped with a thermal conductivity detector (TCD). The sample (60 mg) was cooled to -20 °C under a flow of argon. A 5%H2/Ar mixture was passed through the sample at a flow rate of 40 cm3 min-1 (STP), and TPR profiles were obtained by heating the sample from subambient temperature to 700 °C at a heating rate of 10 °C min-1. The effluent gas was passed through a cold trap to remove the water produced. The H2 content of the dried gas was obtained using the TCD and was recorded as a function of temperature using

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computer software. The TPR experiments were started at subambient temperature to ensure the detection of the reduction of Pd, which typically occurs at around ambient temperature. CO chemisorption using the volumetric technique was performed using a Micromeritics ASAP 2010C instrument to determine the Pd dispersion of the catalysts as described in a previous study.10 X-ray photoelectron spectroscopy (XPS) analyses were carried out in a VG Sigma Probe multichannel spectrometer using Al KR radiation (1486.6 eV). The instrument was equipped with a flood gun to reduce sample charging. The samples were mixed with about 20 wt % graphite powder, to provide an internal standard, and pressed into pellets for use in the spectrometer. The full scan spectra were recorded at a pass energy of 80 eV with 0.5 eV steps. The partial spectra were recorded at a pass energy of 20 eV with 0.1 eV steps. All spectra were recorded with a dwell time of 40 ms. Atomic ratios of the elements were calculated from the relative peak areas of the respective core level lines using Wagner sensitivity factors.23 2.3. Catalytic Activity Measurements. Methanol steam reforming tests were carried out in a Microactivity-Pro unit (PID Eng&Tech. S.L.). The equipment consists of a stainless steel tubular reactor (9.2 mm i.d., L ) 300 mm) located inside an electric oven. The temperature in the catalytic bed was measured by means of a thermocouple. A six-port VICI valve allowed the gas flow to be directed into the reactor or to bypass it. These components were located inside a stainless steel hot box, equipped with an air convector, to vaporize liquid reactants, preheat the carrier gas, and prevent condensation of volatile products in the ducts. The mixture of liquid reactants (methanol and water) was fed by means of a GILSON 307 piston pump. At the reactor outlet, a thermoelectric unit was used to separate condensable vapors. The gaseous product stream from the reactor was analyzed online using a Varian CP-3380 gas chromatograph equipped with a heated sampling valve, two columns (6 m Hayesep Q and 1 m Molecular Sieve 13X), and a thermal conductivity detector. Helium was used as both the carrier gas and the reference gas. Catalyst (100 mg) was placed on a quartz wool plug inside the tube reactor and reduced under a flow of 20 vol % H2/N2 (50 cm3/min, STP) at the desired temperature (400 or 500 °C) for 1 h. The heating rate was 3 °C/min. After this catalyst activation step, the reaction temperature was set to 400 °C, and the catalytic test was performed isothermally at atmospheric pressure. The liquid water/methanol mixture (1.2 molar ratio) was fed at 2 cm3/h, vaporized at 160 °C, and further diluted by a flow of N2 (50 cm3/min, STP). Reaction parameters were calculated as follows: XMeOH )

FMeOH,in - FMeOH,out FMeOH,in

RH2 )

TOF )

FH2,out FMeOH,in

rMeOH · 106.4 mPd · D

where XMeOH is the methanol conversion, FMeOH,in and FMeOH,out are the methanol molar flow rates at the inlet and at the outlet of the reactor, respectively, RH2 is the yield to hydrogen, FH2,out is the hydrogen molar flow rate at the outlet of the reactor, TOF is the turnover frequency, mPd is the palladium loading of the

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Table 1. BET Surface Area of Supports and Catalysts sample SBET (m2 g-1)

CS-800a 9.7

Pd/CS-800b 18.4

CS-1000a 4.4

Pd/CS-1000b 10.6

a Of the corresponding metal oxide supports, after calcination. the impregnated catalyst precursors, after calcination at 400 °C.

b

Of

Figure 2. XRD patterns of (a) CS-1000 support and (b) Pd/CS-1000 catalyst after calcination.

Figure 1. XRD patterns of (a) CS-800 support and (b) Pd/CS-800 catalyst after calcination.

catalyst, the value 106.4 is the atomic mass of Pd, and D is the Pd dispersion. 3. Results and Discussion 3.1. Catalyst Characterization. The BET surface areas of the CeO2-Sm2O3 supports, after calcination at 800 and 1000 °C, and those of the corresponding catalysts, after Pd impregnation and calcination at 400 °C, are presented in Table 1. The support calcined at the higher temperature showed lower SSA. In both cases, the surface area of the catalysts was higher than those of the corresponding supports. In a previous study,10 for the catalyst prepared from the support calcined at 800 °C, this behavior was attributed to partial dissolution of the Sm2O3 by the acid medium used in the preparation of the catalyst. Very low pH (below 1) solutions were employed in the impregnation step. During the drying and calcination steps, the dissolved material would be reprecipitated as smaller particles, so accounting for the increase in surface area. A similar susceptibility to partial dissolution was found for the support calcined at the higher temperature. In fact, the relative increase in SSA is even higher than for the sample calcined at 800 °C. The XRD patterns of the supports and the catalysts are displayed in Figures 1 and 2. Similar patterns were obtained for the samples calcined at the two different temperatures. In both cases, the starting support materials used in the preparation of the Pd catalysts were highly crystalline. The diffraction lines corresponding to CeO2 and Sm2O3 are clearly observed. The CeO2 appears to be phase-pure with cubic crystallography (space group Fm3m), and with diffraction lines at Bragg angles of 28.6°, 33.1°, 47.5°, 56.4°, 59.2°, and 69.4°. The Sm2O3 appears to be a mixture of two known phases. The pattern for this support exhibits the most intense diffraction lines at 28.3°, 32.7°, 47°, and 55.8°, corresponding to Sm2O3 with cubic structure and space group IA3. However, the appearance of well-defined diffraction lines characteristic of Sm2O3 in the monoclinic system (space group C2/m), such as those at 27.8°, 29.2°, 29.9°,

30.8°, 31.3°, and 32.0°, provides evidence for the existence of a mixture of these two phases. The existence of discernible diffraction lines of Sm2O3 in both supports indicates that, despite the increase in the calcination temperature, significant amounts of free Sm2O3 were present. Similar results were obtained by other authors for samples of similar composition.21,24 Nevertheless, the presence of new phases in the supports cannot be discounted, considering that the diffraction pattern for mixed oxides of Sm and Ce, SmxCe1-xO2-δ (with x ) 0.1-0.4), is reported25 to belong to the same crystallographic space group and to have unit cell dimensions very similar to those of CeO2. Figures 1 and 2 compare the XRD patterns of the supported Pd catalysts (after the calcination step) with those of the bare metal oxide supports. No diffraction lines that could be assigned to metallic Pd (2θ ) 40.3°, 68.3°) are observed in the pattern of either catalyst. This could indicate that the supported Pd phase was well dispersed on the CS supports. In both catalyst samples, the effect that the Pd impregnation process had on the diffraction patterns of the supports was remarkable. For the catalysts, the intensity of the diffraction lines corresponding to Sm2O3 was significantly decreased as compared to the patterns of the supports alone. It can be concluded that the crystallinity of CeO2 was not modified by the impregnation process, whereas the crystallinity of the Sm2O3 seems to have decreased significantly after impregnation, leading to a semiamorphous phase (this was confirmed for Pd/ CS-800 catalyst by HR-TEM26). It appears to take place in both catalysts and is even a little more pronounced in the Pd/CS1000 catalyst. This is in agreement with the results obtained from the textural analysis and, therefore, confirms the process of dissolution and reprecipitation of Sm2O3, which takes place during the catalysts preparation, as explained above. On the other hand, it seems to confirm the existence of similar amounts of free Sm2O3 in both supports. Under the conditions used here, a more severe calcination pretreatment does not appear to enhance solid state reaction between the single oxide components, CeO2 and Sm2O3. Figure 3 shows the TPR profiles of the catalysts with the CS support as compared to those with the single oxide supports, Pd/CeO2 and Pd/Sm2O3. H2 consumption was not detected in any case at temperatures below 360 K, nor was a negative peak observed at around 353 K, which would correspond to the decomposition of bulk Pd hydride (β-PdH). This hydride can


Figure 3. TPR profiles of supported palladium catalysts: (a) Pd/Sm2O3; (b) Pd/CS-800; (c) Pd/CS-1000; and (d) Pd/CeO2. Table 2. Pd Dispersion (D) and Estimated Mean Particle Size (dm) in Supported Pd Catalysts after Reduction with H2 at 400 °C catalyst

D (%)

dm (nm)

Pd/CS-800 Pd/CS-1000

20.5 15.7

5.1 6.7

be formed by the absorption of hydrogen into metallic Pd.27 Therefore, no reduction of Pd below 360 K occurred in any of the samples. TPR spectra for the Pd/CS-800, for the Pd/CS1000, and for Pd/Sm2O3 showed two regions of Pd reduction: a narrow reduction peak at low temperature and a broad reduction feature at much higher temperature. For the two Pd/ CS catalysts, the low temperature peak is at slightly higher temperature (∼380 K) than the low temperature peak for Pd/ CeO2 (∼360 K) but matches closely with the corresponding low temperature peak for Pd/Sm2O3. All of these low temperature peaks can be assigned to reduction of Pd present at the surface of the support. In addition, the similarity in the positions of these peaks for the two Pd/CS catalysts and the Pd/Sm2O3 indicates that this surface Pd is mainly supported on Sm2O3 rather than on CeO2. The broad high temperature features observed for the two Pd/CS catalysts are unlikely to be explained by the existence of very large Pd particles in view of the reasonably high Pd dispersion values obtained (Table 2). They can, however, be explained by the structure of the catalyst surface layers and by the distribution of Pd within these. In contrast to the SSA and XRD results, where both of these catalysts exhibited similar trends, some differences were observed between their TPR profiles. In the higher temperature region, a clear secondary peak is observed for Pd/CS-1000 at around 630 K, which appears to correspond only to a small shoulder in Pd/CS-800. Moreover, the maximum of the main reduction peak occurs at a higher temperature for Pd/CS-1000 (∼763 K) than for Pd/CS-800 (∼733 K). The high temperature feature is also broader and the area is slightly smaller for the Pd/CS-1000. This indicates a broader range of strength of interaction between the Pd and the support in Pd/CS-1000 than in Pd/CS-800 and also that these

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interactions are stronger on average for the former. As was stated in our previous study,10 these high temperature features correspond to the reduction of the Pd, which was strongly associated with a Sm-containing layer formed as a consequence of the impregnation method used in the preparation of the catalyst. In the proposed structure,10 Pd was widely dispersed within this Sm-containing layer and interacted strongly with it. This layer in turn decorated the surface of the CeO2 particles. This phenomenon appeared to have occurred to the same extent when the catalyst support was calcined at 800 and at 1000 °C. However, it appears that, because of the lower SSA, this layer was thicker in the Pd/CS-1000 than in the Pd/CS-800 sample, causing the Pd within it to be “buried” over a larger depth range and to be more masked on average, and therefore less accessible to the gas atmosphere, in Pd/CS-1000. This would explain the main differences between the high temperature features for these two catalysts. This explanation is consistent with a recent HRTEM study of the Pd/CS-800 catalyst.26 The significant difference in size of the secondary peaks in the broad reduction feature agrees with the XPS results reported below: a considerably higher proportion of Pd was observed in the outer region of the Smcontaining phase in the Pd/CS-1000 catalyst than in Pd/CS800. However, this cannot be attributed to Pd on the surface because the low temperature reduction peaks are of similar area in the two Pd/CS catalysts. Therefore, this result must relate to near-surface Pd. Such a relatively high concentration of nearsurface Pd would explain the large secondary peak in the TPR spectrum of the Pd/CS-1000. The high concentration of near-surface Pd itself could be tentatively attributed to greater segregation of Pd toward the surface because of the higher content in Pd of the surface layer per unit surface area of CeO2. Table 2 shows the Pd dispersion and mean Pd particle size data determined from CO chemisorption measurements. As might be expected because of its lower surface area, Pd/CS1000 had a lower Pd dispersion (15.7%) than did Pd/CS-800 (20.5%). The mean Pd particle sizes estimated from the dispersion results are 5.1 and 6.7 nm for Pd/CS-800 and Pd/ CS-1000, respectively. These values fall into the range of those reported in the literature for CeO2- and Sm2O3-supported catalysts.17,28 The difference in Pd particle size between the two catalysts is not considered to be large enough to justify the differences seen in the reduction behavior. This supports the explanation given above in the sense that these differences are also related to a stronger interaction between the Pd and the support. In our previous work,10 surface enrichment in Sm2O3 was observed for the Pd/CS-800 catalyst by means of XPS. As some authors observed surface enrichment in Sm2O3 in a CS support11,24 after the mixed hydroxides were calcined at high temperature, in the present work the surface of both catalysts and supports has been analyzed to elucidate if enrichment occurs mainly after the impregnation process or after the calcination pretreatment of the support. Core level spectra including O 1s, Pd 3d, Sm 3d, and Ce 3d were recorded for the supports and the calcined Pd/CS catalysts. C 1s was used as the internal reference. The Ce 3d regions of the binding energy spectrum for the supports and the catalysts are displayed in Figure 4, while the respective Sm 3d regions are shown in Figure 5. The Pd 3d regions of the binding energy spectrum for the catalysts can be seen in Figure 6. Information about the chemical state of the elements and their relative concentration on the surface of the samples was obtained. When the oxidation states of Ce, Sm, and Pd were analyzed (Figures 4-6), no differences between the samples were found, which confirms that the main interac-

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Figure 4. Ce 3d core level XPS spectra of catalysts and support: (a) Pd/ CS-800; (b) Pd/CS-1000; (c) CS-800; and (d) CS-1000.

tion between the metallic phase and the support was not electronic in nature. The Ce 3d transition bands at binding energies of 882.7, 888.8, and 898.5 eV in all of the samples (Figure 4) correspond to Ce 3d5/2 transitions of Ce4+ species.29 The Sm 3d5/2 core level spectra (Figure 5) were deconvoluted to give a major band, which is centered at a binding energy of 1083 eV for the CS-1000 support and the PdCS-800 catalyst. This is characteristic of electron transitions in Sm2O3. In the case of CS-800 and PdCS-1000 samples, this band was shifted slightly to lower and higher binding energies, respectively, but similar well-defined peaks were observed, and the distance between the 3d5/2 and 3d3/2 bands was maintained. The shoulder or broad foot observed on the low binding energy side of these features is probably a consequence of the strong charge-transfer effect of unpaired 4f electrons in Sm2O3.30 A decrease in the proportion of Ce in the outer surface of the catalysts when compared to the supports is clearly observed in the spectra (Figure 4). In agreement with this, an increase of the proportion of Sm is also observed (Figure 5). Integration of the areas under the curves showed that Sm enrichment at the outer sample surface in the CS-1000 support occurred to some

Figure 5. Sm 3d core level XPS spectra of catalysts and supports: (a) Pd/ CS-800; (b) Pd/CS-1000; (c) CS-800; and (d) CS-1000.

extent. As discussed previously,10 if CeO2 and Sm2O3 were uniformly distributed in the support, the same Sm/Ce atomic ratio would be expected at the surface as in the bulk. In this case, the atomic ratio measured by XPS would be similar to that calculated from the overall molar ratio of CeO2 and Sm2O3 in the catalyst. XPS values higher than those predicted by the nominal molar ratio would indicate a higher proportion of Smcontaining material on the outer surface of the catalyst grains, while values below those predicted would suggest location of the Sm mainly inside the catalysts grains. In any case, a different atomic ratio of Sm/Ce in the bulk and at the external surface of the catalyst would indicate segregation of Ce- and Sm-containing phases. For CS-800 and CS-1000, respectively, the numbers of Sm atoms at the surface are 1.1 and 3.0 times those expected for a homogeneous distribution. Thus, Sm2O3 surface enrichment observed in our previous work with the Pd/CS-800 catalyst must take place almost entirely during the impregnation process. The support calcined at the higher temperature did exhibit a certain surface enrichment in Sm2O3. This suggests that some migration


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Figure 7. Evolution of methanol conversion with time on stream: (9) Pd/ CS-800 catalyst reduced at 400 °C; (2) Pd/CS-800 catalyst reduced at 500 °C; (b) Pd/CS-1000 catalyst reduced a 400 °C; and ([) Pd/CS-1000 catalyst reduced at 500 °C.

Figure 6. Pd 3d core level XPS spectra of catalysts after reduction at 400 °C: (a) Pd/CS-800; and (b) Pd/CS-1000.

of Sm2O3 to the surface of the grains took place during the calcination treatment at high temperature. This Sm2O3-rich surface phase was associated by some authors11 with grain boundaries in the CeO2 or with reactive ceria surface sites, which could initiate the sintering process. Nevertheless, surface enrichment in Sm is much more marked in Pd/CS catalysts than in the bare supports (Figures 4 and 5), and considerably higher in the Pd/CS-1000 catalyst where the Sm/Ce ratio is more than twice that seen in Pd/CS-800. The analysis of the profiles corresponding to the Pd 3d spectra (Figure 6) indicates that, in the reduced catalysts, Pd appears to be predominantly in the metallic state with a band centered at 335.0 eV. The oxidation state of Pd is the same in both catalysts; therefore, the broadening of the TPR reduction peak at high temperature (see Figure 3) cannot be assigned to a difference in the electronic properties of the metal species. The stronger interaction between the Pd particles and the support in the Pd/CS-1000 catalyst must therefore be geometric in nature. The analysis of Pd/Ce surface atomic ratios indicates that the ratios of surface Pd atoms to surface Ce atoms were considerably higher than those expected if these elements were uniformly distributed in the catalyst samples. Therefore, enrichment of both Sm and Pd at the catalyst surface is suggested, which once more supports the three-component configuration proposed before.10 The Pd/Sm ratio was 5 times higher in the Pd/CS-1000 catalyst, which is indicative of a less homogeneous distribution of both phases in this catalyst. 3.2. Catalytic Activity. Figure 7 shows the evolution of methanol conversion with time on stream, for the different catalysts. Carbon balances were maintained, within 5%, over the time on stream. Blank experiments were performed with the support materials only, and no methanol conversion was observed. The Pd/CS-800 catalyst showed a considerably higher methanol conversion than did the Pd/CS-1000 catalyst regardless of the catalyst reduction temperature. Only minor deactivation is observed with time on stream for the Pd/CS-800 catalyst.

Figure 8. Evolution of H2 yield with time on stream: (9) Pd/CS-800 catalyst reduced at 400 °C; (2) Pd/CS-800 catalyst reduced at 500 °C; (b) Pd/CS1000 catalyst reduced at 400 °C; and ([) Pd/CS-1000 catalyst reduced at 500 °C. Table 3. Catalytic Activity Results for Supported Pd Catalysts after 95 min on Streama

catalyst Pd/CS-800 Pd/CS-1000

catalyst methanol reduction conversion temperature (°C) (%) 400 500 400 500

79.5 87.6 35.0 50.8

H2 yield (%) 156.6 171.6 69.2 84.3

CO/CO2 TOF molar ratio (h-1) 17.5 26.7 26.8 32.7

6488 3727

a Experimental conditions: pressure, 1 atm; reaction temperature, 400 °C; catalyst mass, 0.1 g; N2 flow rate, 50 cm3 min-1; H2O/CH3OH feed, 2 cm3 min-1; H2O/CH3OH molar ratio, 1.

The loss of activity appeared to be more significant for Pd/CS1000. It is important to note that in both cases, reduction at the higher temperature led to an increase in methanol conversion. Similar trends were observed for the hydrogen yield (Figure 8). H2, CO, and CO2 were the main reaction products, and traces of methane were detected in some cases. The catalytic activity results are compared in Table 3 after 95 min of operation. Methanol conversion values of 79.5% and 87.6%, and hydrogen yields of 156.6% and 171.6%, were obtained with Pd/CS-800 at reduction temperatures of 400 and 500 °C, respectively. The activity of Pd/CS-1000 was significantly lower with conversion values of 35.0% and 50.8%, and H2 yields of 69.2% and 84.3%, at reduction temperatures of 400 and 500 °C, respectively. Nevertheless, high values of CO selectivity were obtained in all cases. This is in agreement with the results reported by other authors. Methanol decomposition, rather than methanol reforming, appears to be predominantly accepted in the literature31,32

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as the main reaction, when noble metals are used. Moreover, CO selectivity was found to be highly dependent on the electronic properties of the metal species involved. Elemental Pd was found to promote the decomposition reaction in contrast to other compositions, such as Pd alloys and Pd-Zn alloys in particular,32-36 which were found to promote the reforming reaction. The CO2 detected would be produced either by the direct reforming of methanol occurring to some extent or by the subsequent transformation of CO via the water gas-shift reaction. The catalyst prepared from the support calcined at 1000 °C exhibits even higher levels of CO selectivity than did the Pd/CS-800 sample. As stated before, when a high CO selectivity is attained, as in this work, the decomposition reaction is accepted to be predominant for group VIII metals.31,32 Takezawa et al. proposed decomposition to take place through η2(C,O)-HCHO species formed on group VIII metals, which rapidly decompose because of the strong back-donation of electrons from the metals into the πCO* antibonding orbital of the aldehyde.37-40 The CO2 produced would come from the water gas-shift reaction coupled with the methanol decomposition (reaction 1). H2O

CH3OH ⇒ CO + 2H2 ⇒ CO2 + 3H2

(1)

Another possibility is that the reaction proceeds through the combination of the reforming reaction and the subsequent reverse water gas-shift reaction 2. The same authors propose the following pathway for the steam reforming of methanol to proceed over copper-based catalysts. -H2

H2O (or HCOO-)

CH3OH 98 HCHO 98

(2)

HCOOH (or HCOO-) f CO2+ H2 The mechanism involves the reaction of the intermediate, formaldehyde, which is probably initiated by nucleophilic addition of H2O (or HO-). CO2 would be subsequently transformed into CO through the reverse water gas-shift reaction. Further studies are necessary, but in our case, no traces of formaldehyde were detected, which may suggest that the reaction proceeds via the decomposition mechanism. The differences in CO selectivity between the two catalysts, together with the much higher activity of Pd/CS-800, mean that Pd/CS-800 is considerably more efficient for H2 production. 3.3. Influence of the Physicochemical Properties of the Catalysts on their Catalytic Properties. The low methanol conversion observed for the Pd/CS-1000 catalyst as compared to that of Pd/CS-800 can be partly attributed to the difference in Pd dispersion (Table 2) between the two catalysts. Nevertheless, it is not the only cause. When methanol conversion per exposed Pd atom is calculated (see turnover frequency (TOF) data in Table 3 for some of the catalysts), it can be seen that about 40% less methanol molecules are converted per surface Pd atom in the Pd/CS-1000 catalyst than in the Pd/CS-800 catalyst. Therefore, the activity of the surface Pd atoms is enhanced in the CS-800 supported catalyst, indicating that the reaction is structure sensitive and that the extent of methanol conversion cannot be attributed exclusively to the extent of Pd dispersion. In both catalysts, Pd is associated with Sm2O3, and both components were deposited on the surface of CeO2 particles. However, in the case of Pd/CS-1000, thicker layers of Sm2O3 appear to cover the outer surface of the CeO2 particles, thus

making the Pd phase less accessible to the gas atmosphere and leading to a decrease in the activity. Moreover, segregation of a greater proportion of Pd to the outer catalyst surface was found to occur in the Pd/CS-1000 catalyst, leading to a less homogeneous distribution of Pd particles within the Sm2O3 support. The lower Pd dispersion (Table 2) and the less favorable distribution of Pd in the internal pores of the Sm2O3 particles greatly affect the performance of the Pd/CS-1000 catalyst as deduced from the activity results (Table 3). As stated above, the higher activity of the Pd/CS-800 catalyst cannot be attributed only to its higher number of accessible Pd atoms. The activity per exposed atom of Pd (TOF) in Pd/CS800 was found to be nearly double that for Pd/CS-1000. As was verified in previous work,10 the nature of the material supporting the Pd particles appears to play a significant role in the performance of the catalysts in the methanol conversion. The high performance of Pd/CS-800 catalyst was ascribed to the basicity of Sm2O3, which is able to provide active centers for the adsorption of carbon-containing reactants, either directly from the gas phase or from the Pd particles via a spillover mechanism. Promotion of CH4 reforming and other reactions by the addition of Sm2O3 to catalysts or their use as the support has been reported previously.15-22 These authors explained the results by referring to the basicity of Sm2O3 and to the contribution of spillover phenomena. The promotion of the activity was ascribed to the ability of methane to adsorb onto the basic metal oxide forming methyl radicals,38-40 and, moreover, to the possibility of the spillover of partially dissociated CHx species from the noble metal to the basic sites on the support. The contribution of spillover phenomena is confirmed in the present work. The better distribution of Pd in the Sm2O3 phase seems to favor the spillover of adsorbed species in Pd/CS-800 when compared to Pd/CS-1000, because there are no differences in the nature of the support material. The adsorption of methanol and/or a number of different intermediates, which all present a certain degree of acidic character, appears to be enhanced by the basic nature of the support and by surface diffusion of the reactants between welldispersed and distributed metallic and active basic centers. In consequence, when Sm2O3 is added to the catalyst, instead of using CeO2 alone, the activity and H2 production are increased in absolute terms as well as on a per exposed Pd atom basis.10 However, the temperature used to calcine the support is a key factor in the evolution of these benefits. A synergistic effect was observed when the support was calcined at 800 °C, whereas, when the calcination temperature of the support was increased to 1000 °C, a less favorable distribution of Pd particles within the Sm2O3 phase resulted, and this hindered the mobility of the reactants between the metallic and active basic centers, leading to a decrease of the TOF value. In addition, a poorer dispersion was obtained, which leads to lower total activity and lower H2 production for this catalyst. When increasing the reduction temperature, emergence of Pd particles from the Pd-Sm mixed phase appears to be favored, making them more accessible to the gas atmosphere and thus leading to a higher activity. As the masking of Pd particles with Sm2O3 layers appears to be more severe in Pd/CS-1000 catalyst, the effect of the reduction temperature is more pronounced, leading to a significant increase in the activity of this catalyst. 4. Conclusions The calcination pretreatment of the support had a significant influence on the physicochemical and catalytic properties of the 2 wt % Pd/CeO2-Sm2O3 (80/20 mol ratio) catalysts studied in


this work. The support calcined at 1000 °C showed a lower BET surface area than that calcined at 800 °C. For both supports, an increase in the surface area of the catalyst was observed upon addition of the Pd function. Similar phases were found in both supports, one of these being free Sm2O3. In both cases, the starting support materials used in the preparation of the Pd catalysts were highly crystalline, while the crystallinity of Sm2O3 decreased strongly after Pd impregnation. Surface analysis of supports and catalysts showed surface enrichment in Sm2O3 in both catalysts, occurring mainly after the impregnation process. These results confirm the dissolution of Sm2O3 during the impregnation process and its reprecipitation with Pd to form a mixed phase on the surface of the CeO2. However, the concentration of Sm2O3 in the outer surface was greater in Pd/ CS-1000. The thicker layer of Sm2O3 in Pd/CS-1000 led to a stronger interaction with the Pd particles, making them even less accessible to the gas atmosphere. In addition, Pd appears to be less homogeneously distributed within the Sm2O3. A poorer Pd dispersion and a greater concentration of Pd in the outer surface of Sm2O3 were found by means of CO chemisorption and XPS, respectively. Consequently, the total activity and TOF of Pd/CS-1000 were considerably lower than those of Pd/CS800. The less homogeneous distribution of Pd seemed to hinder the spillover of reactants from the metal particles to the basic active centers on the support. A calcination temperature of 800 °C is recommended for the support. The reduction temperature has a significant influence on the activity of the catalysts. Increasing the reduction temperature may facilitate the emergence of Pd particles from the semiamorphous Sm-Pd mixed phase, making them more accessible to the gas atmosphere and thus increasing the total activity. Finally, only minor deactivation of the catalysts was observed with time on stream, especially for Pd/CS-800 catalyst. Acknowledgment Part of this work was carried out at the School of Chemical Engineering of the University of Edinburgh under a “Marie Curie” Research Training Fellowship, contract number ERK5CT-1999-50003. L.M.G.-S. gratefully acknowledges the European Commission for financial support and Edinburgh University for hosting her Fellowship. Supporting Information Available: Carbon balances for catalytic activity experiments. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Steele, B. C. H., Ed. Ceramic Oxygen Ion Conductors and Their Technological Applications; Ceram. Proc., Institute of Materials: London, 1997. (2) Yamamoto, O. Solid oxide fuel cells: fundamental aspects and prospects. Electrochim. Acta 2000, 45, 2423. (3) Laosiripojana, N.; Assabumrungat, S. Catalytic steam reforming of methane, methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC. J. Power Sources 2007, 163, 943. (4) Brett, D. J. L.; Atkinson, A.; Cumming, D.; Ramı´rez-Cabrera, E.; Rudkin, R.; Brandon, N. P. Methanol as a direct fuel in intermediate temperature (500-600 °C) solid oxide fuel cells with copper based anodes. Chem. Eng. Sci. 2005, 60, 5649. (5) Sasaki, K.; Watanabe, K.; Teraoka, Y. Direct-alcohol SOFCs: Current-voltage characteristics and fuel gas compositions. J. Electrochem. Soc. 2004, 151, A965. (6) Kim, T.; Ahn, J. K.; Vohs, M.; Gorte, R. J. Deactivation of ceriabased SOFC anodes in methanol. J. Power Sources 2007, 164, 42. (7) Feng, C. B.; Wang, Y.; Zhu, B. Catalysts and performances for direct methanol low-temperature (300 to 600 °C) solid oxide fuel cells. Electrochem. Solid-State Lett. 2006, 9, A80.

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ReceiVed for reView April 20, 2009 ReVised manuscript receiVed August 1, 2009 Accepted August 1, 2009 IE900630Z

Steam Reforming of Methanol with Sm2O3âCeO2-Supported

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