and ZnO(0001) - American Chemical Society

Apr 7, 2009 - Matthew P. Hyman,† Vannesa M. Lebarbier,‡ Yong Wang,‡ Abhaya K. Datye,§ and. John M. Vohs*,†. Department of Chemical and ...
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J. Phys. Chem. C 2009, 113, 7251–7259

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A Comparison of the Reactivity of Pd Supported on ZnO(101j0) and ZnO(0001) Matthew P. Hyman,† Vannesa M. Lebarbier,‡ Yong Wang,‡ Abhaya K. Datye,§ and John M. Vohs*,† Department of Chemical and Biomolecular Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, Pacific Northwest National Laboratory, Richland, Washington 99354, and Department of Chemical and Nuclear Engineering, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: March 18, 2009

The dependence of ZnO surface structure on Pd/ZnO-catalyzed methanol decomposition was investigated by using model catalysts consisting of Pd films and particles on ZnO(101j0) and ZnO(0001) single crystals. XPS studies showed that vapor-deposited Pd grows two dimensionally at 300 K and agglomerates into particles upon heating. Temperature-programmed desorption (TPD) experiments showed that CO adsorption was weaker on Pd/ZnO(0001) relative to Pd/ZnO(101j0) and that PdZn alloy formation was more facile on the ZnO(0001) compared to ZnO(101j0). Large differences in the amount of CO produced during methanol TPD on the Pd/ZnO(0001) and Pd/ZnO(101j0) samples were also observed and attributed to the presence of highly active sites at the Pd-ZnO(0001) interface. Comparisons to high surface area Pd/ZnO catalysts indicate that similar structural effects may also influence their reactivity. Introduction Renewable hydrogen production is of interest as an alternative to fossil fuels for environmental, economic, and security concerns.1 Alcohols are attractive hydrogen carriers and methanol steam reforming is known to produce nearly CO-free hydrogen when catalyzed by Cu/ZnO;2 however, the catalyst is prone to sintering and is pyrophoric once reduced, thus complicating its use in many distributed power applications.3 These drawbacks have resulted in recent attention being focused on Pd/ZnO as an alternative catalyst for the steam reforming of methanol.4-11 When supported by ZnO, Pd achieves high methanol conversion and selectivity to CO2 and H2. The ZnO support is critical to obtaining high selectivity and Iwasa et al. found the rate of hydrogen production of ZnO supported Pd to exceed that of unsupported Pd by more than four times.7 Additionally, the selectivity of unsupported Pd for steam reforming of methanol to produce CO2 and H2 (rather than CO and H2) was found to be 0.1% for the same conditions that yielded 97% selectivity on Pd/ZnO. Studies of high surface area catalysts indicate that the highest selectivity is achieved when a stoichiometric PdZn alloy is formed,7,12 suggesting that the reaction takes place on sites that contain both Pd and Zn atoms. Studies of model single crystal catalysts, however, are not completely consistent with this conclusion. For example, Jeroro and Vohs studied twodimensional PdZn alloys formed on a Pd(111) surface and found that Zn concentrations above 25% were all but inactive toward methanol dehydrogenation.13 The role of ZnO in altering the selectivity of the Pd/ZnO catalyst is also not well understood. Previous studies show that the formation of the PdZn alloy is facile on Pd/ZnO under methanol steam reforming conditions9 and is required for the emergence of the steam reforming reaction pathway that produces CO2 and H2.8,12 Additionally, * To whom correspondence should be addressed. E-mail: vohs@ seas.upenn.edu. † University of Pennsylvania. ‡ Pacific Northwest National Laboratory. § University of New Mexico.

Iwasa et al. found that adding Zn to Pd on other supports (e.g., Al2O3, MgO, CeO2) also increased both the activity and selectivity.8 While the ZnO support still produced the most active and selective catalyst, it is noteworthy that PdZn supported on activated carbon exhibited reactivity nearly identical to that of Pd/ZnO. This result would appear to suggest the primary role of the ZnO support to be the source of the Zn, although under methanol steam reforming conditions some of the Zn is likely to be oxidized, so the presence of ZnO may still be important.14 To provide insight into the role of the ZnO support in obtaining the high selectivity for methanol steam reforming, we have previously studied the structure and reactivity of model catalysts consisting of monolayer and multilayer films of Pd supported on ZnO(0001).15,16 In these studies it was shown that vapor-deposited Pd follows a two-dimensional (2D) island growth mode at 300 K with 2D metal islands formed up to a coverage of ∼0.7 monolayers (ML) at which point growth on top of the metal islands commenced. The metal islands were metastable, however, and agglomerated into particles upon heating. Methanol TPD studies obtained with these model catalysts showed a complex dependency on the structure of the Pd film and also provided evidence for PdZn alloy formation for samples annealed at 800 K and above (these TPD results are described in more detail below). It is well-known that reactions of Brønsted acids, such as alcohols, on ZnO are highly structure sensitive and exhibit a strong dependence on the exposed crystal plane, with reactions on ZnO(0001) significantly more facile than those on ZnO(101j0) and ZnO(0001j).17-21 On the basis of TPD and STM studies it has been proposed that the active sites for dissociative adsorption of Brønsted acids on both ZnO(101j0) and ZnO(0001) consist of cation-anion site pairs at step edges in which each of the ions has one coordination vacancy.17,20-22 Using STM Diebold et al. have shown that vacuum-annealed ZnO(0001) surfaces contain a high density of triangular pits whose edges consist of 2.6 Å high, oxygen-terminated, single-layer steps.22,23 The O2ions at these step edges help stabilize the polar surface. The

10.1021/jp809934f CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Figure 1. Representations of (a) ZnO(101j0) and (b) ZnO(0001) each with one step edge (oxygen, large circles; zinc, small circles).

nonpolar ZnO(101j0) surface does not require charge compensation and does not undergo significant reconstruction. Vacuumannealed ZnO(101j0) surfaces consist of large, mostly defect free terraces, separated by either single- or double-layer steps (3 and 5.4 Å in height),22,23 and exhibit very low reactivity toward Brønsted acids including methanol. Schematic diagrams of the polar ZnO(0001) and nonpolar ZnO(101j0) surfaces, each showing one step edge, are displayed in Figure 1. Studies of the reactivity of metals supported on ZnO have also shown crystal plane dependencies.24,25 For example, Roberts and Gorte observed differences in the bonding of CO to Pt films supported on ZnO(0001) and ZnO(0001j).25 Recent studies of methanol steam reforming on high surface area Pd/ZnO samples also indicate that the morphology of the ZnO support has a significant effect on reactivity.26 These previous studies suggest that the structure of the ZnO support may also influence the reactivity of Pd/ZnO methanol steam reforming catalysts and serve to motivate the present study where we have used temperature-programmed desorption (TPD) to compare the adsorption and reaction of CO and methanol on Pd/ZnO(0001) and Pd/ZnO(101j0). The results of this study demonstrate a clear dependence of the exposed ZnO crystal plane on Pd-catalyzed methanol decomposition. This paper is organized as follows. X-ray photoelectron spectroscopy (XPS) and CO TPD results that provide insight into the morphology of the Pd layer as a function of temperature and PdZn alloy formation are initially presented. TPD results that demonstrate differences in the decomposition of methanol on Pd/ZnO(101j0) and Pd/ZnO(0001) are discussed, followed by the presentation of a proposed model that accounts for these differences. Finally, the results for the model catalysts are compared to methanol steam reforming data obtained from high surface area Pd/ZnO catalysts. Experimental Methods XPS (X-ray source: VG Microtech; electron energy analyzer: Leybold-Heraeus) and TPD experiments (quadrupole mass spectrometer: UTI) were performed in a single ultrahigh vacuum chamber with a background pressure below 5 × 10-10 Torr. The ZnO(101j0) and ZnO(0001) single crystals (MTI Corporation) used in this study were approximately 1 cm × 1 cm × 1 mm in size and attached to the sample manipulator with tantalum foil mounts. A chromel-alumel thermocouple was attached to each crystal with a ceramic adhesive (Ultratemp 516, Aremco).

Hyman et al. Prior to mounting, the ZnO single crystals were polished until optically smooth by using diamond pastes with grit size down to 0.25 µm. In the UHV chamber, samples were cleaned by sputtering with 2 kV Ar+ ions followed by annealing at 800 K. This process was repeated until the sample was determined to be clean with XPS. Pd deposition was achieved by using an evaporative Pd source that consisted of a resistively heated 0.2 mm diameter W wire wrapped with a 0.127 mm diameter Pd wire (Alfa Aesar, 99.9% purity). The sample temperature was maintained at approximately 300 K during deposition. The amount of Pd deposited was measured by using a quartz crystal film thickness monitor (Maxtek). Pd growth rates varied somewhat, but did not exceed 0.5 Å/s. Pd coverage is reported in monolayers (ML) assuming a surface density of Pd(111), i.e.,1.53 × 1015 atoms/cm2. The energy scale for the XP spectra was referenced to the Zn(2p3/2) peak of the substrate, which was assumed to be at 1021.7 eV. During TPD experiments, the crystal was placed in front of a 5 mm aperture on a glass bulb covering the quadrupole mass spectrometer and was heated at a rate of 3 deg/s. CO (Matheson, 99.99% purity) and CH3OH (Sigma-Aldrich, 99.99% purity) were dosed through a leak valve that was attached on the vacuum side to a 6 mm diameter stainless steel tube. The sample was positioned in front of this tube during dosing. In all TPD experiments the sample was dosed with either 5 L of CO or 10 L of CH3OH. These dosages take into count the enhancement factor of the dosing tube, which was estimated to be a factor of approximately 100 relative to that determined from the overall chamber pressure. For both CO and CH3OH these doses were found to be sufficient to saturate the Pd surface. The liquid CH3OH was purified by using multiple freeze-pumpthaw cycles prior to use. For comparison purposes, the rates of methanol steam reforming on two different high surface area Pd/ZnO catalysts which exhibit significantly different ZnO structures were also measured. The preparation and characterization of these catalysts will be described in detail elsewhere.27 The catalysts were prepared by impregnating ZnO powders with palladium(II) acetate salt (Aldrich, 99.9%) dissolved in acetone via incipient wetness technique. ZnO supports were prepared by precipitation from a solution consisting of Zn(NO3)2 dissolved in a mixture of benzyl alcohol and 1-pentanol followed by calcination in air. For one of the ZnO supports, denoted ZnO-A, polyvinylpyrrolidone was added to the precursor solution prior to the addition of Zn(NO3)2. ZnO powders prepared without polyvinylpyrrolidone in the precursor solution are denoted ZnO-B. The ZnO-A sample consists of nanorods with a high aspect ratio, while the ZnO-B support has a plate-like morphology. The Pd loadings were 3.5% and 5% for the Pd/ZnO-A and Pd/ZnO-B, respectively. After impregnation, the samples were dried for 8 h at 110 °C and calcined at 350 °C under air for 3 h. Methanol steam reforming activity was evaluated using a 4 mm i.d. quartz tube reactor loaded with approximately 200 mg of catalyst. Prior to activity tests, the catalyst was reduced in situ under 10%H2/N2 at 623 K for 2 h. Experiments were run at gas hourly space velocity (GHSV) ) 51 430 h-1 and 523 K with a water-to-methanol molar ratio of 1.8:1 in the feed. GC analysis was used to measure the relative yields of the various reaction products. Results and Discussion XPS. Before presenting the data for the reaction of methanol on Pd supported on ZnO(101j0) and ZnO(0001) it is important to understand the structure of the metal layer. We have

Reactivity of Pd Supported on ZnO(101j0) and ZnO(0001)

Figure 2. Zn(2p3/2)/Pd(3d5/2) peak intensity ratio for freshly deposited Pd on ZnO(101j0) as a function of Pd coverage.

Figure 3. Pd(3d5/2) binding energy (circles) and Zn(2p3/2)/Pd(3d5/2) peak intensity ratios (squares) as a function of annealing temperature.

previously used XPS, HREELS, and LEED to characterize in detail the growth of Pd films on the ZnO(0001) surface.16 As noted above, in that study it was shown that a vapor-deposited Pd film at 300 K follows a 2D island growth mode28 where 2D metal islands are formed up to a critical island size at which point layer-by-layer growth on top of the metal islands becomes dominant. For Pd/ZnO(0001) the critical island size was estimated to be approximately 0.7 ML. A similar growth mode has also been reported for Pt on this surface.29 The 2D Pd films on ZnO(0001) were found to be metastable and agglomerated into 3D particles upon heating above 500 K. XPS results for Pd/ZnO(0001) also provided evidence for incorporation of Zn into the Pd layer upon annealing under vacuum at temperatures in excess of 500 K. In the present study, XPS and CO TPD were used to characterize the electronic and physical structure of vapordeposited Pd layers on ZnO(101j0) as a function of the sample pretreatment conditions. The XPS results are presented in Figures 2 and 3. Figure 2 displays the ratio of the intensity of the Zn(2p3/2) photoelectron peak to that of the Pd(3d5/2) peak as a function of the amount of Pd deposited. The Pd(3d) and Zn(2p) spectra can be found in the Supporting Information. While additional data would be required to unambiguously determine the growth mode of the Pd film, it is noteworthy that the ratio decreases sharply going from 0.5 to 1 ML, and is nearly zero by 3 ML. This indicates that Pd initially covers most of the surface and is consistent with the 2D island growth mode that has been proposed for Pd on ZnO(0001).28 For purposes

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7253 of this work, the most important feature of Figure 2 is that Pd deposition does not appear to form 3D clusters at 300 K. For the freshly deposited 0.5 and 1 ML Pd films on ZnO(101j0) the Pd(3d5/2) photoelectron peak was centered at 335.1 eV. This peak shifted down by 0.2 to 334.9 eV for the 3 ML Pd film. For all Pd coverages the Pd(3d3/2) peak was located 5.3 eV higher in energy than the corresponding Pd(3d5/2) peak. The peak positions for the 3 ML Pd film are consistent with those reported in the literature for bulk Pd.30 Interpreting the core level shifts of the 0.5 and 1 ML Pd films is difficult since they may be due to initial and final state effects, and/or electronic interactions with the ZnO surface. It has been reported that the surface core levels for Pd(111), Pd(110), and Pd(100) shift -0.28, -0.55, and -0.44 eV, respectively, relative to the bulk core states.31 In contrast, a decrease in final state screening that may result from decreasing film thickness would be expected to increase the binding energies. Thus, the observed increase in 3d binding energy with decreasing Pd coverage is consistent with a final state effect, rather than a surface effect. Electronic interactions between the Pd and the ZnO(101j0) surface could also cause the observed small shift in the core levels. Such effects have been reported for Pt an ZnO where Schottky barrier formation has been observed for Pt/ZnO(0001).24 Figure 3 displays the Pd(3d5/2) binding energy and the Zn(2p)/ Pd(3d) peak area ratio for the 1 ML Pd samples as a function of the annealing temperature. In this set of experiments the sample was annealed for 30 min at the indicated temperature and then allowed to cool to room temperature at which point the XPS spectrum was collected. The Zn(2p)/Pd(3d) ratio was found to increase from 12 to 18 upon annealing to 500 K suggesting that this treatment caused some agglomeration of the film. The ratio remained unchanged, however, after annealing at 600 K, indicating that the Pd structure is relatively stable between 400 and 600 K. Much more significant increases in the Zn(2p)/Pd(3d) ratio were observed upon annealing above 600 K with the value increasing to 36 after annealing at 850 K, demonstrating that the initial film-like Pd structure undergoes agglomeration to form three-dimensional clusters in this temperature range. This result is similar to that reported previously for Pd films supported on ZnO(0001).28 As shown in the figure, the increases in the Zn(2p)/Pd(3d) ratios upon heating above 600 K were accompanied by decreases in the binding energies of the Pd(3d5/2) peak. As noted above, this peak was located at 335.1 eV for the as-deposited 1 ML Pd film. The peak remained at this position for annealing temperatures up to 600 K and then decreased upon annealing to higher temperatures. After annealing at 850 K the peak shifted to 334.5 eV. This shift is consistent with an increase in final state screening due to the increasing particle size.32 Thus, these changes are again consistent with agglomeration of the Pd film to form three-dimensional clusters. It is also noteworthy that the XPS spectra provided no indication of PdZn alloy formation. This result is in contrast to what we observed previously for Pd/ZnO(0001)16 where distinct shoulders were observed on the high binding energy sides of the Pd(3d) peaks upon annealing at 700 K and above. Alloying of Pd with Zn is known to cause a decrease in the Pd(3d) binding energies33 and these shoulders were therefore attributed to alloy formation. The absence of these features in the XPS spectra of the Pd/ZnO(101j0) samples annealed at high temperature indicates that alloy formation is less facile on the ZnO(101j0) surface compared to the ZnO(0001) surface. Incorporation of a small amount of Zn in the Pd deposits on the ZnO(101j0) surface

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Figure 4. CO TPD spectra for various coverages of Pd/ZnO(101j0).

cannot be ruled out, however, since even significant concentrations of Zn in Pd produce only modest binding energy shifts.33 CO TPD. CO TPD was also used to characterize the structure and electronic properties of the supported Pd layer. Since CO has a sticking coefficient of nearly one on Pd at room temperature and does not adsorb on the ZnO surface above 250 K,34 the amount of CO adsorbed provides a measure of the Pd surface area. The bonding of CO to Pd is also sensitive to the local structure and electronic properties of the adsorption site,35-41 which is reflected in the desorption temperature during TPD. With this in mind, CO TPD experiments were performed to gain further insight into the structural dynamics of the Pd surface. Figure 4 displays CO desorption spectra obtained from freshly deposited 0.5, 1, and 3 ML Pd films when dosed with 5 L of CO at 300 K. The CO desorption curves from the 1 and 3 ML Pd surfaces each contain a peak centered at 465 K that is somewhat skewed toward lower temperatures. This peak temperature and shape is similar to that reported for CO desorption from low-index Pd single-crystal surfaces.38,41-43 For example, on Pd(111) the CO desorption peak is centered near 470 K (the peak location increases somewhat with decreasing coverage) and has been attributed to CO adsorbed in 3-fold hollow sites.38,42 On Pd(110) and Pd(100) which do not have 3-fold sites, CO desorption is also peaked near 470 K and corresponds to CO adsorbed on bridge sites.38,43 All three surfaces exhibit low-temperature shoulders on the primary CO desorption peak resulting from CO adsorbed on lower coordination sites. One would expect there to be a high concentration of lower coordination sites on the 0.5 ML Pd relative to the 1 and 3 ML films since more of the Pd atoms will be at the perimeter of metal islands. This may explain why the CO desorption peak from the 0.5 ML Pd film is centered at a slightly lower temperature of 455 K. The similarity in the CO desorption spectra from the supported Pd films and the low-index planes of Pd suggests that there are no strong interactions between the Pd and the ZnO(101j0) support that alter the electron density on the metal. Thus, the shifts in the Pd(3d) binding energies for the films relative to bulk Pd are more likely to be due to final state effects rather than Pd-ZnO interactions. As shown in Figure 5, annealing the 1 ML Pd surface to a series of higher temperatures induced marked changes in the CO desorption spectra from its initial similarity to that obtained from low-index planes of Pd. In this series of experiments the sample was annealed for 30 min at the indicated temperatures before the CO TPD data were collected. The ending temperature in each TPD run was selected as to not exceed the annealing

Figure 5. CO TPD spectra from 1 ML Pd/ZnO(101j0) for selected heat treatments.

temperature. This was done to limit changes in Pd morphology during the TPD run. In the CO desorption curve obtained after annealing at 500 K a low-temperature peak emerges at 370 K, while the high-temperature peak shifts to 445 K. Progressively annealing at higher temperatures caused measurable but less dramatic changes in the shape of the CO desorption peaks, including a decrease in the overall peak area, which is consistent with the XPS results and can be attributed to agglomeration of the film into particles. After annealing at 700 K the hightemperature CO peak shifted further to lower temperature and was centered at 435 K. The high-temperature CO peak also became more prominent after annealing at 800 K. TPD studies of CO on supported Pd particles and stepped Pd single crystals indicate that low-temperature (300-400 K) CO desorption on Pd is characteristic of CO bonded to undercoordinated step edges and other nonterrace sites.36,37,39,44-47 Thus, the initial effect of annealing appears to be the formation of step edges on the mostly planar as-deposited Pd film. The most significant change in the CO peak area occurs between 600 and 700 K, consistent with the XPS data indicating that significant agglomeration of the film starts to occur near this temperature. Note, however, that the relative proportion of the low- and high-temperature CO desorption peaks is much affected by annealing at only 500 K. The decrease in the intensity of the low-temperature peak when the sample was annealed at 800 K suggests a decrease in the proportion of under-coordinated step edges and kink sites, and an increase in terrace sites. This may be due to the reemergence of low-index terraces as the Pd particle size increases. Figure 6 illustrates possible morphological changes in a Pd layer upon heat treatment. This figure is not intended to necessarily reflect the actual structure of the Pd layer in the current study, but rather to demonstrate how agglomeration of a Pd film into particles could produce CO adsorption sites consistent with those suggested by the TPD results. As shown in the figure the initial film-like structure (structure A) is likely to be comprised mostly of terrace sites. Certainly some boundary and other lower coordination sites will be present; nevertheless, the CO TPD results indicate that the surface is dominated by low-index Pd planes with a minimum of step edges. Annealing to intermediate temperatures results in the formation of a more threedimensional structure that still exhibits terraces, depicted by structure B. A high concentration of terrace sites remains, although the proportion of under-coordinated step edges in-

Reactivity of Pd Supported on ZnO(101j0) and ZnO(0001)

Figure 6. Illustration of Pd particle morphology changes due to annealing.

creases significantly. Annealing to higher temperatures leads to formation of particles (structure C) that contain (111) and (100) facets. Small particles have a high concentration of edge sites that give rise to the low-temperature features in the CO TPD spectra. The relative concentration of these sites decreases with increasing particle size causing a decrease in the amount of CO desorption at low temperature for samples annealed at high temperatures. While the changes in the morphology of the Pd deposits with increasing annealing temperature can account for most of the features in the TPD results in Figure 5, the location of the hightemperature desorption feature in the annealed samples, 435-445 K, is somewhat unusual. In nearly all previous CO TPD studies of Pd single crystals and Pd particles on oxide supports the highest temperature CO desorption feature has been centered at temperatures between 450 and 500 K,37-46,48,49 which is significantly higher than that obtained in the present study for the annealed Pd/ZnO(101j0) samples. As will be discussed below this difference may be due to the incorporation of a small amount of Zn into the Pd upon annealing. To provide insight into whether the local atomic structure of the ZnO surface affects its interaction with the supported Pd, the CO TPD results for 1 ML Pd/ZnO(101j0) were compared to a similar set of data from a 1 ML Pd/ZnO(0001) sample (see Figure 7). As noted above, we have previously shown that vapor-deposited Pd films on ZnO(0001) have structural characteristics similar to those reported here for Pd/ZnO(101j0)16 and follow a 2D island growth mode at 300 K and agglomerate upon heating above ∼450 K. The bottom spectra in Figure 7a, which corresponds to freshly deposited Pd layers, shows that CO adsorbs more weakly on Pd/ZnO(0001) compared to Pd/ ZnO(101j0) as evidenced by a 20 K lower peak maximum. The plot of the peak areas versus annealing temperature in Figure 7b shows that the peak areas for the two surfaces are nearly

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7255 identical, however, which is consistent with the as-deposited films having similar structures on the two surfaces. We have previously attributed the unusually low CO desorption temperature for Pd/ZnO(0001) to electronic interactions between Pd and the ZnO(0001) surface.15 The similarity of the CO desorption temperature from the 2D Pd film on ZnO(101j0) to that for the low-index planes of Pd suggests a weaker electronic interaction at this interface. Samples annealed at 500 K produced similar CO TPD spectra. The downward temperature shift was observed for both substrates and the offset in the peak temperatures remained. After annealing at 650 K, the CO TPD peak shapes were different for the two samples with more desorption occurring at low temperatures (325 to 375 K) for Pd/ZnO(0001) compared to Pd/ZnO(101j0). Furthermore, the desorption peaks on the 650 K annealed Pd/ZnO(0001) surface were shifted 30 K to lower temperatures relative to those from Pd/ZnO(101j0). The XPS characterization studies indicate that on both ZnO crystal planes significant agglomeration of the Pd films into particles starts to occur near 650 K. While these changes in the morphology of the Pd layer may be at least partially responsible for the differences in the CO desorption peak shapes, they do not entirely explain the differences between the samples annealed at 650 K. The predominance of the low-temperature CO desorption states for the Pd/ZnO(0001) sample is unusual and not observed for Pd particles on other supports.37,49 Previous studies of the interaction of CO with PdZn alloy surfaces suggest that decoration of the surface of the Pd particles on ZnO(0001) with Zn may play a role here. For example Jeroro et al.42 have shown that incorporation of less than 0.1 ML of Zn into a Pd(111) surface significantly destabilizes adsorbed CO and changes the preferred bonding site from 3-fold or bridged to atop but has little affect on the amount of CO that adsorbs. The CO desorption peak shapes were similar again after annealing at 800 K, with the high-temperature peak being the most intense for both the Pd/ZnO(0001) and Pd/ZnO(101j0) samples. As noted above for Pd/ZnO(101j0), the temperature of the primary CO desorption peak, 435 K, is unusually low. This is even more so the case for Pd/ZnO(0001), where the peak is centered at 420 K. We again believe that these lowpeak temperatures result from incorporation of some Zn from the support into the Pd particles. In the case of the Pd/ZnO(0001) surface, XPS results provide evidence for this for samples annealed at 700 K and above.16 The fact that the peak shift is less for the Pd/ZnO(101j0) sample indicates that less Zn has become incorporated into the Pd particles on this surface. This conclusion is also consistent with the XPS results obtained in the present study, which did not provide evidence for PdZn alloy formation. Together the CO and XPS results, therefore, indicate that incorporation of Zn into the supported Pd particles upon annealing in vacuum is more facile on ZnO(0001) relative to ZnO(101j0). Furthermore, the large decrease in CO peak area with annealing temperature observed on both surfaces is mostly a result of agglomeration, not PdZn alloy formation. Methanol TPD. Methanol TPD experiments were performed for 1 ML Pd coverage on both the ZnO(101j0) and ZnO(0001) supports with use of a 10 L dose of methanol which was found to be sufficient to saturate the metal surface. On Pd(111), methanol desorbs with a broad peak centered at 160 K, while PdZn surface alloys on Pd(111) exhibit multiple methanol desorption peaks dependent on the Zn concentration.13 However, in all cases methanol completely desorbs below 200 K. Since methanol was dosed at 300 K in these experiments, no methanol was detected in the desorption spectra. The only carbonaceous

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Figure 7. (a) CO TPD spectra from 1 ML Pd deposited on ZnO(101j0) (solid line) and ZnO(0001) (dotted line) for selected heat treatments. (b) Corresponding peak areas for all spectra shown in part a plus additional spectra (ZnO(101j0), filled circles; ZnO(0001), hollow squares).

Figure 8. CO product spectra from methanol TPD on 1 ML Pd/ZnO(101j0) for selected heat treatments.

product detected in all experiments was CO, consistent with methanol decomposition on Pd, which dehydrogenates methanol below room temperature. Although H2 is produced in the methanol dehydrogenation reaction, it desorbs from Pd and PdZn at ∼300 K,50 the temperature at which methanol was dosed. Hence, no H2 was detected during the TPD experiments. It is also important to note that the sticking coefficient of methanol on the ZnO surfaces is very low at 300 K and products indicative of the reaction of methanol on ZnO were not observed for the conditions used in this study. (Note that in our previous study of Pd/ZnO(0001)15 there is an error in the reported methanol doses. An enhancement factor due to a dosing needle was not accounted for and the actual doses are a factor of ∼200 greater than those reported.) Figure 8 displays the CO desorption spectra resulting from methanol decomposition on the Pd/ZnO(101j0) sample. The experiments were performed in a similar manner to those for CO TPD with the exception that the sample was initially heated to 500 K to remove CO accumulated during Pd deposition. As

stated previously, CO was the only decomposition product detected from the methanol-dosed samples. The bottom CO desorption spectrum in Figure 8 corresponds to a sample with a freshly deposited Pd layer and contains a peak centered at 465 K. This CO peak was skewed somewhat toward lower temperatures but was sharper than the CO desorption peaks from the CO-dosed Pd/ZnO(101j0) surface (see Figure 5). The position of the CO peak decreased with increasing annealing temperature to a low value of 445 K, although the peak shape remained nearly constant. On the basis of a comparison to the CO TPD results for the 1 ML Pd/ZnO(101j0) sample annealed at 500 K, the amount of CO produced during methanol decomposition corresponds to ∼20% of the saturation CO coverage. Since the saturation amount of CO decreases with annealing temperature while the amount of CO produced via methanol decomposition remains nearly constant (Figures 5 and 7), the CO yield from methanol decomposition as a function of the saturation CO coverage increases with annealing temperature up to ∼35% at 800 K. These TPD results are similar to those reported previously for bulk Pd where methanol decomposes to CO and hydrogen below room temperature resulting in a desorption-limited CO peak centered near 490 K on Pd(111)51 and Pd(100),52 and 455 K on Pd(110).53 On Pd(111) methoxide species prefer to bond in the 3-fold hollow sites resulting in a saturation coverage of approximately 0.27 ML.13 Saturation CO coverage has been determined to be between 0.5 and 0.67 ML at 300 K on Pd(111),35,54 which means that methanol decomposition on Pd(111) yields between 40% and 54% of the saturation CO coverage. On other low-index Pd single crystals, methanol also presumably produces CO at significantly less than saturation coverage. Therefore, the low yields observed for methanol dehydrogenation on 1 ML Pd/ZnO(101j0) are consistent with yields on low-index planes of Pd and suggest that the nonterrace sites may be ill suited to decompose methanol. The lack of low-temperature CO desorption and a relatively constant peak area for Pd/ZnO(101j0) can be contrasted with the methanol TPD data for Pd/ZnO(0001) previously published by our group.15 For Pd coverages of 0.75 ML, and to a lesser extent 1.5 ML on ZnO(0001), annealing the sample increases the CO peak area during methanol TPD by a large factor. For

Reactivity of Pd Supported on ZnO(101j0) and ZnO(0001)

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Figure 9. (a) CO product spectra from methanol TPD on 1 ML Pd deposited on ZnO(101j0) (solid line) and ZnO(0001) (dotted line) for selected heat treatments. (b) Corresponding peak areas for all spectra shown in 9a plus additional spectra (ZnO(101j0), filled circles; ZnO(0001), hollow squares).

these conditions, CO desorption begins at 300 K on Pd/ ZnO(0001), resulting in a very broad peak. Since these previous experiments were performed with a much higher methanol dose that resulted in the formation of methoxide species on exposed portions of the ZnO(0001) support, a new set of methanol TPD experiments for 1 ML Pd/ZnO(0001) were collected by using a 10 L dose to allow for a more direct comparison to the data for Pd/Zn(101j0). Figure 9a displays CO desorption spectra for both surfaces following 500, 650, 700, and 800 K heat treatments. The spectra following the 500 K anneal are similar, each containing a single CO desorption peak at high temperature. Consistent with the CO TPD results, the CO peak resulting from methanol decomposition on Pd/ZnO(0001) is centered 17 K lower in temperature than that for Pd/ZnO(101j0). Since the CO and methanol TPD experiments were performed with different Pd films, this result serves to demonstrate the reproducibility of the CO TPD data. Similar results were obtained after annealing at 650 K with the CO peaks broadening somewhat and decreasing in temperature by 28 K on Pd/ZnO(0001) and 22 K on Pd/ZnO(101j0) relative to their respective values when annealed at 500 K. Annealing at 700 K elicited much starker differences between the two samples. Consistent with our previous study, CO desorption is observed beginning at 300 K on Pd/ZnO(0001) and exhibits a peak centered 33 K lower than that on Pd/ZnO(101j0). Annealing at 800 K yields similar results, although the Pd/ZnO(0001) peak center shifts to 423 K, 24 K lower than on Pd/ZnO(101j0). As noted previously, this peak shift may be the result of greater PdZn alloying on Pd/ ZnO(0001) in relation to Pd/ZnO(101j0). Figure 9b displays the amount of CO produced for methanol TPD on both samples as a function of annealing temperature. While the quantity of CO produced on Pd/ZnO(101j0) remains constant, that produced by Pd/ZnO(0001) increases approximately 3-fold as the annealing temperature was increased from 500 to 700 K. As a percentage of the CO saturation coverage, the CO yield from methanol increases from 10% at 500 K to above 80% at 700 and 800 K. The latter is significantly higher than that observed for Pd(111).13 Incorporation of some Zn into the Pd is a possible explanation for the large differences in the CO yield during methanol TPD.

As noted above, the CO desorption peak shifts that occur upon annealing indicate a higher degree of PdZn alloy formation for Pd/ZnO(0001) compared to Pd/ZnO(101j0). The fact that the Pd/ZnO(0001) sample annealed at 650 K exhibits a large CO peak shift, indicating partial alloy formation, but no significant increase in the CO yield, however, argues against alloy formation being responsible. The results obtained in our previous study of methanol dehydrogenation on Zn/Pd(111) also argue against PdZn alloy formation being the cause of the increased CO yield.13 Rather the opposite effect was observed in that study with the CO yield during TPD following a saturation dose of methanol decreasing slightly as the Zn coverage was increased to 0.1 ML, after which it decreased sharply. Changes in the structure of the Pd deposits on the ZnO(0001) surface upon annealing are another possible explanation for the large differences in the CO yield during methanol TPD. For example, it is possible that the Pd particles formed upon annealing at 700 K contain a high concentration of step, edge, or defect sites that are more active for methanol dehydrogenation compared to the higher coordination sites present on the Pd film at lower temperatures. On the basis of this explanation, however, one might expect to observe a similar effect on Pd/ZnO(101j0), since Pd particles were also formed on this surface upon annealing. Results obtained in our previous study for a 0.5 ML Pd/ZnO(0001) sample suggest another structural effect that may be important.15 For this sample an unusually high yield of CO was obtained following a saturation exposure of methanol for freshly deposited Pd that had not been heated prior to the TPD experiment in order to maintain the 2D structure of the metal (see Figure 4 of ref 15). The amount of CO produced then decreased as the sample was annealed to a series of progressively higher temperatures. This result and the trends observed in the present study leads us to propose that changes in the concentration of sites at the Pd-ZnO(0001) interface are the likely cause for the changes in CO yield during methanol TPD on Pd/ ZnO(0001). Previous studies have also concluded that sites at the metal-ZnO(0001) interface play a role in the reactions of methanol on Pt/ZnO(0001)29 and CO2 on Cu/ZnO(0001).55 If such sites have high activity for methanol dehydrogenation this reaction could occur during methanol dosing with the CO

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Figure 10. Comparison of CO spectra resulting from CO TPD (solid) and methanol TPD (dotted) experiments on 1 ML Pd/ZnO annealed at 800 K. Labels identify the substrate crystal plane.

product diffusing to other sites on the Pd islands or particles. This would result in a CO yield that is limited by the saturation coverage of CO rather than that of methoxide. As shown in Figure 10, which compares the CO desorption spectra from 800 K annealed Pd/ZnO(101j0) and Pd/ZnO(0001) samples that were dosed with saturation amounts (for the metal) of CO and methanol, this appears to indeed be the case. Note that the CO desorption peaks are nearly the same size for Pd/ZnO(0001), while for Pd/(101j0) the CO peak from the methanol-dosed sample is much smaller than that from the CO-dosed sample. For island growth of the vapor-deposited Pd film at 300 K, the concentration of Pd-ZnO interfacial sites will increase with Pd coverage up to the point where individual islands start to coalescence. At such point the concentration of interfacial sites will start to decrease. Thus, it is likely that there would be a high concentration of such sites for a freshly deposited Pd 0.5 ML Pd film, and their concentration would decrease when the film agglomerates into particles. At high Pd coverages (e.g., 1 ML) there would be a lower concentration of interfacial sites for the as-deposited film, since at this point the film covers most of the surface. Annealing this surface to induce agglomeration of the Pd film into particles would result in an increase in the concentration of interfacial sites. This scenario is consistent with the trends shown in Figure 9 for the 1 ML Pd/ZnO(0001) sample. While more insight into the evolution of the structure of the Pd film upon heating and the resulting particles is needed to further substantiate this proposed mechanism involving interfacial sites, the data strongly suggest that such sites are important. Furthermore, the stark differences in the methanol TPD results obtained for the Pd/ZnO(0001) and Pd/ZnO(101j0) samples demonstrates that the structure of the surface of the underlying ZnO support also has a dramatic effect on reactivity. Indeed if one accepts that highly active interfacial sites are important on Pd/ZnO(0001), the data suggest that such sites are absent on Pd/ZnO(101j0). This result is perhaps not that surprising when one examines the structures of the two ZnO surfaces. As discussed in the introduction, the STM studies of Diebold et al.22,23 show that vacuum-annealed ZnO(101j0) surfaces have low roughness and a structure consistent with that expected for an ideal termination of the bulk. This relatively

defect free surface exhibits low reactivity toward most adsorbates including alcohols.19 In contrast, the vacuum-annealed ZnO(0001) surface contains a high concentration of oxygenterminated step edges forming triangular pits that stabilize the electrostatically unstable ideally cleaved surface.22 The step edges have high reactivity toward alcohols and have been proposed as the active sites for methanol oxidation on this surface.17 Since the ZnO(0001) step edges are also likely to be nucleation sites for the Pd films and particles, it is possible that sites at the Pd-ZnO step edge interface play a role in the dehydrogenation of methanol adsorbed on the Pd. The CO TPD results obtained in this study for Pd/ZnO(0001) also provide additional insight into the origin of a feature observed in our previous study of this system. In our previous TPD study of the reaction of methanol on Pd/ZnO(0001) CO2 peaks were observed at both 525 and 575 K.15 The latter results from the decomposition of formate species formed by the reaction of methoxide groups on exposed portions of the ZnO surface. This peak was not observed in the present study (see Figure 9) because a much lower methanol dose was used to limit the formation of methoxide species on the ZnO. The CO2 peak at 525 K was attributed to either reaction of CO adsorbed on the Pd (or PdZn alloy) with oxygen supplied by the ZnO support or to reaction of an intermediate (methoxide or formate) species adsorbed at the Pd-ZnO interface. The lack of this peak in the TPD data for CO-dosed Pd/ZnO(0001) provides support for the latter explanation. Comparison to High Surface Area Catalysts. Finally, to allow connections to be made between the model and high surface area catalysts, two Pd/ZnO high surface area catalysts with dramatically different support morphologies were evaluated in methanol steam reforming. As described in the Experimental Methods section, the ZnO-A support consists of nanorods with high aspect ratio, whereas the ZnO-B support has a plate-like geometry. More detail on the atomic level structural differences between the ZnO-A and ZnO-B samples will be described elsewhere,27 but it is likely that these two samples have different exposed ZnO crystal planes. Table 1 summarizes the characterization data (BET surface areas and PdZn particle sizes determined from XRD) as well as the methanol steam reforming activity and selectivity data at 523 K for both catalysts. Note that the catalysts have similar BET surface areas and two different Pd loadings were used to produce catalysts with a similar PdZn particle size. The latter was done to allow the effects of the ZnO support structure on reactivity to be studied since our previous study10 showed that methanol steam reforming on PdZn is structure sensitive. Under the conditions studied, methanol conversion was ∼8% on both catalysts; however, Pd/ ZnO-A exhibited a significantly higher selectivity to CO2 (96.4%) than that of Pd/ZnO-B (85.7%). High CO2 selectivities have been previously attributed to the high extent of PdZn alloy formation.4,5,9,12 Thus, it appears that ZnO-A may predominantly expose crystal planes that allow for more facile PdZn alloy formation than those exposed on ZnO-B. While more structural information is needed for the powder catalysts to make a direct comparison to the model single-crystal catalysts, there are some

TABLE 1: Summary of Reactivity Measurements and Characterization Data for Pd/ZnO Catalystsa PdZn particle size (nm) catalyst

BET surface (m2/g)

XRD

TEM

Pd loading (%)

CH3OH conversion (%)

CO2 selectivity (%)

Pd/ZnO-A Pd/ZnO-B

41.5 42.0

11.5 10.4

6.5 6

3.5 5.0

8.0 8.2

96.4 85.7

a

Catalyst mass ) 194 mg; GHSV ) 51 430 h-1; T ) 523 K; feed water to methanol molar ratio ) 1.8:1.

Reactivity of Pd Supported on ZnO(101j0) and ZnO(0001) similarities in the results obtained for both types of samples which suggest that the observed differences in the ease of alloy formation on Pd/ZnO(0001) and Pd/ZnO(101j0) may be important in the high surface area analogues. Conclusions In this work, the adsorption and reaction of CO and methanol on Pd/ZnO(101j0) and Pd/ZnO(0001) was studied by using a combination of TPD and XPS to characterize the surface properties. Vapor-deposited Pd films at 300 K on both ZnO surfaces are initially two-dimensional with large terraces resembling low-index Pd surfaces. XPS and CO TPD results indicate that annealing the Pd/ZnO samples at moderate temperatures (500-650 K) induces morphological changes in the Pd films, producing particles with a high concentration of low-coordination sites such as step edges. Annealing at higher temperatures (700-800 K) produces larger particles with lowindex facets. CO TPD results also demonstrate that CO bonds more weakly on monolayer Pd films on ZnO(0001) compared to ZnO(101j0) suggesting that there is a stronger electronic interaction at the Pd-ZnO(0001) interface. The CO desorption peak was observed to shift to lower temperatures on annealed samples which is consistent with the incorporation of some Zn into the Pd layer The magnitude of the shift was higher for Pd/ZnO(0001) than for Pd/ZnO(101j0) suggesting that PdZn alloy formation is more facile on ZnO(0001) compared to ZnO(101j0). Large differences were observed in the CO yield during TPD from Pd/ZnO(0001) and Pd/ZnO(101j0) samples that were exposed to methanol. For Pd/ZnO(101j0), the CO coverage on the Pd produced by dehydrogenation of a saturation dose of methanol was similar to that obtained for Pd single crystals. In contrast, the CO coverage produced by dehydrogenation of a saturation coverage of methanol on Pd/ZnO(0001) increased dramatically as the sample was annealed to higher temperatures reaching values that far exceed those obtained on Pd singlecrystal surfaces. We have attributed this structure sensitivity to synergism between Pd and the ZnO(0001) substrate and the presence of sites at the Pd-ZnO(0001) interface that are highly active for methanol dehydrogenation. Comparisons to the reactivity of high surface area Pd/ZnO catalysts suggest that the observed differences in the reactivity of Pd/ZnO(0001) and Pd/ZnO(101j0) may play a role in determining the activity and selectivity of high surface area Pd/ZnO methanol steam reforming catalysts. Acknowledgment. We gratefully acknowledge funding for this work provided by the U.S. Department of Energy (grant nos. DE-FG02-04ER15605 (M.P.H., J.M.V.) and DE-FG0205ER15712 (V.M.L., Y.W., A.K.D.)). Supporting Information Available: Pd(3d) and Zn(2p) spectra used to generate Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. ReV. 2006, 106, 4044. (2) Velu, S.; Suzuki, K.; Osaki, T. Chem. Commun. 1999, 2341. (3) Trimm, D. L.; Onsan, Z. I. Catal. ReV. 2001, 43, 31. (4) Iwasa, N.; Kudo, S.; Takahashi, H.; Masuda, S.; Takezawa, N. Catal. Lett. 1993, 19, 211.

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