High CO2 Selectivity of ZnO Powder Catalysts for Methanol Steam

Article pubs.acs.org/JPCC

High CO2 Selectivity of ZnO Powder Catalysts for Methanol Steam Reforming Barr Halevi,† Sen Lin,‡ Aaron Roy,† He Zhang,§ Ese Jeroro,∥ John Vohs,∥ Yong Wang,§ Hua Guo,⊥ and Abhaya K. Datye*,† †

Department of Chemical & Nuclear Engineering and Center for Microengineered Materials, MSC01 1120, University of New Mexico, Albuquerque, New Mexico 87131-0001, United States ‡ Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis−State Key Laboratory Breeding Base, Fuzhou University, Fuzhou 350002, China § Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States, and The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164-2710, United States ∥ Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ⊥ Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131-0001, United States ABSTRACT: To investigate the contribution of the ZnO supports in PdZn/ZnO catalysts used for methanol steam reforming (MSR), we have investigated methanol steam reforming on two different ZnO powder samples. The powder samples included a commercial powder with both polar and nonpolar surfaces exposed, and a plate-like sample preferentially exposing the polar ZnO(001)/ (001̅) surfaces. ZnO was found to be active and selective for MSR, with the polar surfaces being most active, but the activation energy for reaction (∼100 kJ/mol) was much larger than on unsupported PdZn(∼50 kJ/mol) or PdZn/ZnO. The experimental findings are supported by DFT calculations on key elementary steps and reaction intermediates on the ZnO(001) surface. We conclude that the contribution of the ZnO support to the MSR activity PdZn/ZnO catalyst system may not be very significant.

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INTRODUCTION ZnO is a major component in a number of catalysts including Cu/ZnO/Al2O3 that is used for methanol synthesis and more recently Pd/ZnO which is a selective and stable catalyst for methanol steam re-forming (MSR).1 Although the PdZn alloy is thought to be responsible for much of the observed MSR reactivity of Pd/ZnO, there are indications that the ZnO support also affects reactivity.2−4 The most active, selective, and stable PdZn/ZnO catalysts are those made on highly crystalline ZnO with well-defined crystalline facets.3 It is possible that the exposed facets indirectly influence reactivity through differences in the ease of formation of the active PdZn intermetallic from Pd particles supported on ZnO. However, it is also possible that ZnO is itself active for MSR and that different facets exhibit differing reactivities. Further examination of the MSR activity of well-defined ZnO powders is necessary to better understand the stability, and energetics, of MSR activity on ZnO. The behavior of ZnO single crystals has been studied by several groups who found that the dominant low index planes of ZnO undergo significant reconstruction due to heating and exposure to adsorbates. Heating of ZnO to 400 °C+ leads to reconstruction in the form of regular single-unit-cell high © 2013 American Chemical Society

islands that are triangular on the (001) plane and rectangular on the (001̅) and (100) planes.5−7 The islands on the (001) plane occur in greater density and all three faces of the triangular islands are symmetric with (100) termination. The rectangular islands on the (001) and (100) faces are composed of (001), (001̅), and two (110) faces. The island growth is thought to occur so as to reduce the charge imbalance between the polar (001) and (001̅) surfaces. Upon heating to higher temperatures, the islands transform to terraces with similar structures. Exposure of the polar ZnO(001) to atmospheric pressure of hydrogen leads to formation of hydroxyls, and Schottky-type defects.8 Hydroxyls are similarly formed on the (100) face, but Schottky defects are not formed, suggesting that charge compensation may help drive Schottky defect formation. Water (H2O) adsorbs dissociatively on ZnO(001), at low coverages (∼1 langmuir) near oxygen-terminated steps on the surfaces with hydroxyls binding to Zn cations.9 As the water Received: September 10, 2012 Revised: January 30, 2013 Published: March 4, 2013 6493

dx.doi.org/10.1021/jp308976u | J. Phys. Chem. C 2013, 117, 6493−6503

The Journal of Physical Chemistry C

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experimental data, several key steps in the MSR reaction pathway are investigated on ZnO(001) using DFT in this work. The facet-dependent surface chemistry for ZnO single crystals suggests that steady state MSR activity on ZnO may also depend on the nature of exposed surface facets. The literature on polycrystalline ZnO reactivity is less extensive than single crystal work, despite the obvious industrial applications. What has been published is, however, consistent with the single crystal results, once extrapolated to higher pressures. Water adsorbs dissociatively on sites similar to those of the ZnO polar (001) and (001̅) surfaces while adsorbing molecularly on sites found on nonpolar surfaces such as ZnO(100).25 CO2 adsorbs on different ZnO powders forming primarily bidentate carbonates on the Zn+/O− cation/anion pairs which are known to cover the (100) surface and (100) /(001) steps and desorbs at approximately 280 °C. Weakly bound unidentate hydroxylcarbonates are also reported, but only close to room temperature. The adsorbed CO2 interacts strongly with the ZnO surface and exchanges O, as evident by isotopic labeling experiments.26 CO adsorbs more weakly than CO2 on ZnO,27 and can hydrogenate to formyl28 or oxidize by the surface to form CO2.29 The relative breadth of literature for methanol-related decomposition species on ZnO model systems coupled with the more limited published body of knowledge for ZnO polycrystalline powders suggests that ZnO should be active for MSR. However, the available literature for ZnO suggests a wide range of different possible active sites for ZnO polycrystalline samples, so that different ZnO samples may have very different performance for MSR. Therefore, in this work, we measured the MSR activity of ZnO powders made so that they preferentially expose different facets, under industrially relevant flow reaction conditions. We found that MSR kinetics and selectivities on the ZnO powders are linked to the different facets of the ZnO powders and DFT derived energetics help identify the likely reaction pathway and active site characteristics.

coverage increases, the ZnO surface reconstructs to form large irregular terraces, which break down to form even more irregular surfaces at 20+ langmuir exposures.9−11 This trend is consistent with the hypothesis that charge-balance is the driving force for ZnO polar surface reconstruction. Extrapolating this trend to atmospheric pressures suggests that atmospheric pressure water will completely disorder ZnO(001). On the nonpolar (100) surface, water adsorbs both dissociatively and nondissociatively, with dynamic exchange between the two states.11−13 And indeed, water does adsorb strongly on ZnO at atmospheric pressures. Microcalorimetric adsorption studies of water on ZnO powders show an initial heat of dissociative adsorption for water of 140 kJ/mol for low coverages, decreasing to 46 kJ/mol at saturation levels, with ∼9 kJ/mol added due to interaction with the neighboring adsorbed water.14,15 Methanol (CH3OH) adsorbs dissociatively on ZnO16 with methoxyl species binding to defect-associated Zn cations. The adsorbed methoxyl can dissociate to highly mobile methyl groups that can diffuse readily across the surface. Adsorption of methanol also eliminates surface island reconstruction, demonstrating the dynamic nature of the ZnO surfaces. In addition to these observed interactions between adsorbates and the surface structure, there also appears to be significant interaction between adsorbates and the surface during reactions because Zn is seen to evaporate from ZnO at 400 °C+ during the decomposition of alcohols under temperature programmed desorption in a vacuum.17 For methanol decomposition, it is known that the reactivity depends on the type of ZnO surface. Cheng et al.,16 for example, showed that the polar Znterminated ZnO(001) surface and stepped nonpolar ZnO surfaces have higher activity than the nonpolar flat ZnO(100) surface or O-terminated ZnO(001̅),18 with formaldehyde being formed only over the polar ZnO(001) surface. Observed activity for methanol decomposition on the different ZnO facets is therefore correlated with the density of defects associated with the step and regular island surface reconstructions observed for the facets. Thus methanol reactivity on ZnO is most probably linked to defect density, with the ZnO(001)Zn showing much greater reactivity than other surfaces. Theoretical studies of ZnO and its surface reactivity are challenging, particularly on the polar faces.19 Although the methanol synthesis reaction on ZnO has been studied using density functional theory (DFT),20 no complete study of MSR on ZnO has been reported, except for the initial steps, namely the adsorption and dissociation of H2O and CH3OH, on ZnO(001).21 On Cu and PdZn surfaces, it is known that the MSR process is initiated by dissociative adsorption of both methanol and water, forming respectively methoxyl (CH3O*) and hydroxyl (OH*) species on the surface.22 Methoxyl dehydrogenates to formaldehyde (CH2O*) and the formaldehyde intermediate quickly reacts with a hydroxyl, leading to formate (HCOO*) species, which decomposes to produce CO2 + H2 products. The rate limiting step in this reaction is the dehydrogenation of methoxyl to the formaldehyde intermediate. The selectivity of MSR on Cu and PdZn surfaces toward CO2 was explained by recent DFT studies,23,24 which suggested that the CH2O* + OH* reaction competes effectively with other reaction/desorption channels of the formaldehyde intermediate because of its relatively low barrier and large exothermicity. Methanol decomposition and water adsorption experiments on ZnO single crystal model catalysts suggest that a similar pathway for MSR on ZnO exists. To help interpreting

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EXPERIMENTAL SECTION Catalyst Synthesis. The commercial ZnO powder used was 99.98% ZnO from Aldrich. ZnO plates were prepared by a modification of a previously reported PVP directed crystallization method.30 Two g of PVP (Sigma-Aldrich; M = 40 000) was dissolved in 200 mL of pentanol, and then 8 mL of 0.15 M aqueous NaOH solution and 6 mL of 0.1 M ethanol solution of Zn(NO3)2·6H2O(Aldrich, 99.99%+) were sequentially added. The reaction mixture was kept at 90 °C for 1 h, washed with ethanol and distilled water several times, then calcined at 350 °C for 3 h. Thermogravimetric Analysis (TGA). TGA was performed using a TA Instruments model SDTQ600 flowing gas microbalance equipped with a Pfeiffer differentially pumped quadrupole mass spectrometer (QMS). QMS spectra were adjusted for overlapping features and m/z 64 and 66 were assigned to Zn in accordance with Zn isotopic abundance. Experiments were conducted under atmospheric pressure using 50 sccm of zero air, CP grade N2, and 7% H2/N2. Samples were loaded into a small alumina boat, the chamber flushed for 30 min, and samples were then heated at 25 °C/min. The contribution of the alumina boat was subtracted by duplicate runs without the ZnO sample. Brunauer−Emmett−Teller (BET) Surface Area. BET surface area was determined using a Micromeritics Gemini 6494



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system after samples were outgassed in flowing Helium overnight at 120 °C. X-ray Powder Diffraction. (XRD) patterns were recorded using a Scintag Pad V diffractometer with DataScan 4 software (from MDI, Inc.) for system automation and data collection. Cu Kα radiation (40 kV, 35 mA) was used with a Bicron Scintillation detector (with a pyrolytic graphite curved crystal monochromator). Data sets were analyzed with Jade 9.5 Software (from MDI, Inc.) using the ICDD (International Center for Diffraction Data) PDF2 database (rev. 2004) for phase identification. Rietveldt analysis was performed using the software package GSAS,31 employing an anisotropic microstrain model in the peak profile function.32 Scanning and High-Resolution Transmission Electron Microscopy (STEM and HRTEM). STEM and HRTEM were performed on a JEOL 2010F FASTEM field emission gun scanning transmission electron microscope equipped with EDS. The probe size for analytical work was 1.0 nm, and accelerating voltage was 200 kV. Scanning Electron Microscopy (SEM). SEM was performed on a Hitachi S-5200, with a resolution of 0.5 nm at 30 kV and 1.7 nm at 1 kV, EDS was carried out at 20 kV using a PGT system. Microreactor Catalytic Activity Tests. All catalysts were pelletized and sieved to 106−260 μm, and then 50 mg, or as much as was available, of ZnO sieved powder was loaded in a 1.7 mm i.d. reactor tube with a packed catalyst bed depth of ∼40 mm and ∼5 mg quartz wool plug on either end of the catalyst bed. Reactivity studies were performed by placing the reactor tube in a temperature programmable convection furnace capable of heating at 30 °C/min. The product gases were analyzed using a series/bypass configured mol-sieve/ Poropak Q column in a Varian CP-3800 GC equipped with TCD detector. The reactor feed system used MKS mass flow controllers and a high pressure pump-fed vaporizer system for introducing liquid reactants such as water and methanol. A heating loop consisting of a coiled tube helped ensure that the feed and reactor were isothermal. Prior to activity tests, the catalyst was oxidized in situ for 60 min using 50 sccm 2%O2/ He at 250 °C. Samples were tested for MSR activity at 250− 400 °C until steady state performance was achieved. The samples were then subjected to an accelerated aging treatment in situ using 5% H2/He flowing at 50 sccm at 400 °C for 12 h. These samples were then retested for MSR activity. These samples are referred to as “aged”. Methanol steam re-forming activity was evaluated using 0.003 mL/min of premixed water/ methanol mixture (molar ratio of 1.1:1) with a carrier gas stream of 77.5 sccm preheated argon through a vaporizer operating at 100 °C and introduced to the reactor. Because only CO and CO2 were produced under MSR on ZnO, selectivity to CO2 was defined as S = [CO2]/([CO2] + [CO]). High Vacuum Temperature Programmed Desorption (TPD). TPD experiments were conducted in a diffusionpumped system equipped with a VGQ quadrupole mass spectrometer, a temperature controlled radiatively heated sample basket, a cold-cathode gauge for pressure measurement, and a leak valve for introduction of probe gases. Approximately 20 mg of the powder sample was loaded in the basket, pumped down for 4 h, and then heated to 350 °C in vacuum 2 h to remove adsorbed gases. Samples were exposed to 2 Torr probe gas for 30 min and then pumped down for 4 h. Samples were then heated at 10 K/min up to 427 °C and desorption products were monitored by a quadruple mass spectrometer. Over-

lapping fragmentation patterns were corrected to determine the evolution of products. Density Functional Theory Calculations. All plane-wave DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP).33−35 Energies and geometries were calculated using the generalized gradient approximation (GGA) of the Perdew−Wang 91 (PW91) exchange− correlation functional.36 For valence electrons a plane-wave basis set was employed with a cutoff of 400 eV and the ionic cores were described with the projector augmented-wave (PAW) method.37,38 A 3 × 3 × 1 Monkhorst−Pack k-point grid was adopted to sample the Brillouin zone.39 The Fermi level was smeared using the Methfessel−Paxton method with a width of 0.1 eV.40 The optimization of the bulk hexagonal wurtzite ZnO crystal yielded lattice parameters of a = b = 3.281 Å and c = 5.256 Å, in good agreement with previous work.19 We focus here on the ZnO(001) surface because it is well established that the reactivity of ZnO arises from this zinc terminated polar facet,16,18 an observation confirmed by the current study. Following our earlier work,21 the model of the ZnO(001) surface consists of five double layers for a total of 20 oxygens and 20 zinc ions within a 2 × 3 unit cell. The unit cell is larger than that used in our earlier work21 to accommodate the pertinent species involved in reactions. The two top layers were fully relaxed during the calculations. The large number of atoms in the unit cell is necessary because of the polar nature of the surface, but it makes the calculations extremely demanding. In addition, the leading errors due to the artificial dipole generated by the slab model were corrected using the methods introduced in the literature.41,42 A vacuum space of 18 Å was used between the slabs. The adsorption energy was calculated as follows: Ead = E(adsorbate+surface) − E(free molecule) − E(free surface). Transition states were calculated using the climbing image nudged elastic band (CI-NEB) method43,44 with energy (10−4 eV) and force (0.05 eV/Å) convergence criteria. Stationary and transition points were confirmed by normal-mode analysis using a finite differencing method with an atomic displacement of 0.02 Å. The normal frequencies were used to compute vibrational zero-point energy (ZPE) corrections. All reported energies are ZPE corrected with the exception of Tables 3 and 4.

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RESULTS AND DISCUSSION Thermogravimetric Analysis. The results of TGA experiments on the commercial Aldrich ZnO conducted in 50 sccm flowing air, N2, and 7% H2/N2 at a constant heating rate of 25 °C/min are presented in Figure 1. A 1% weight loss occurs at 275 °C under all gases, accompanied by small heat intakes and a CO2 (m/z = 44) desorption product observed by the quadrupole mass spectrometer (QMS), suggesting oxidation of surface carbonaceous adsorbates. In all of the gases a gradual ∼0.001% drop in weight is observed between 280 and 480 °C, likely due to some surface adsorbate oxidation as well as the expected associated Zn emission.16,17 Under flowing air the sample is stable up to 800 °C, exhibiting a ∼0.001 wt % gain that may be due to oxidation of substoichiometric Zn. Under N2 the ZnO is stable up to 600 °C, losing 2 wt % by 800 °C. When ZnO is exposed to flowing 7% H2, we find that sample weight is lost beginning at 500 °C, with 6 wt % lost by 800 °C. Further, holding ZnO at 500 °C under flowing H2 leads to a loss of ∼0.12 wt % per h. Thus it is 6495



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separated and only sinter slightly after the aging treatment. Table 1 lists the crystallite dimensions derived from XRD line broadening, average diameters, and calculated surface areas based on the average diameter. We report experimentally measured BET surface areas for the fresh samples and calculated surface areas for the aged samples. The BET surface areas for both Aldrich and plates samples are similar and remain largely unchanged after aging. The diffraction peak broadening is related to the actual size of the crystallites in a direction normal to the diffracting planes; therefore, unequal diffraction peak widths are indicative of anisotropic broadening which can be attributed to the differences in the shape of these crystallites. Changes in peak widths are therefore indicative of changes in particle shape after reaction testing. The change in crystallite dimensions as determined from anisotropic peak broadening after use and in situ reduction is summarized in Table 1. XRD line broadening indicates that the Aldrich ZnO sample shows a slight preferential exposure of the prismatic (100) surfaces as evident from the larger thickness in the [001] direction. After reaction, the Aldrich sample lost this preferential exposure of the prismatic surfaces as evident from nearly equal width in the three directions. The plates showed an initial preferential exposure of the (001) surfaces as evident from the smaller thickness in the [001] direction. The width of these plates perpendicular to the (100) surfaces was initially too large to measure by diffraction (due to the line broadening being comparable to the instrumental broadening). After reaction, the plates lost some of this preferential exposure because the dimensions in the three directions were now closer to each other. There was no significant change in the surface areas of the powders after aging, the major change is a rounding of the particles and a loss of some of the initial preferential exposure of polar ZnO surfaces in the plates and the prism faces for the Aldrich ZnO. Scanning Electron Microscopy. SEM imaging of ZnO samples, Figure 3A−D, shows that the accelerated aging procedure serves to round facet edges, sinter crystallites together, and increase the sphericity. The Aldrich sample initially shows faceted crystallites, with crystallites sizes in the 30−60 nm range, consistent with XRD results. The crystallites appear to sinter after aging resulting in larger agglomerates in the 100−400 nm diameter range. However, there was no significant increase in the XRD derived crystallite dimensions, suggesting these agglomerates must contain grain boundaries retaining the original crystallite dimensions. The plates initially display a hexagonal structure with sharply faceted edges and a width to thickness ratio greater than 5 width, confirming the preferential exposure of the basal planes, as confirmed by XRD (shorter thickness in the [001] direction). There are many instances of 2−3 plates stacked along their tops and bottoms. After aging, the edges of the plates appear more rounded and the 2−3 individual plate stacks appear to be sintered together, but without an apparent increase in crystallite size as seen also with the Aldrich sample. A representative schematic summarizing the morphology and exposed surfaces of the two ZnO samples is presented in Figure 4. Temperature Programmed Desorption. TPD spectra for saturation exposure of CH3OH on 0.04 g of Aldrich ZnO are shown in Figure 5. These results are generally similar to those obtained in previous studies of CH3OH adsorption on ZnO(001) single crystals45 and other reported polycrystalline samples.46 In these previous reports, the reaction of CH3OH on the ZnO(001) surface was observed to proceed via dissociative

Figure 1. Thermogravimetric analysis performed under flowing () air, (--) N2, and (···) 7% H2/N2. Weight percent is indicated on the left axis, and heat flow on the right axis.

clear that reducing ZnO at 500 °C leads to loss of material. The changes in morphology seen by SEM suggest that reduction of ZnO and volatilization of Zn contributes to sintering and restructuring of the ZnO support at temperatures over 500 °C, and to a certain extent perhaps even as low as 300 °C. The rate of Zn evaporation from ZnO under flowing H2 was too low to monitor by QMS below 800 °C, at which temperature increases in intensity of the QMS signal m/z =64 and 66 confirm that Zn is evaporating from the sample. QMS sensitivity was not sufficiently high to determine if the evaporating Zn was accompanied by oxygen. X-ray Diffraction. XRD patterns for the as-prepared and aged samples are presented in Figure 2, showing only ZnO

Figure 2. XRD patterns for the samples with inset showing an expanded view of (100), (002), and (101) peaks for the as-prepared and aged samples after MSR reactivity tests.

diffraction peaks for all samples. The inset with magnified (100), (002), and (101) peaks shows the changes in peak widths for samples before and after reactivity testing. ZnO is hexagonal, for simplicity we have used the three digit notation for planes (hkl) instead of (hkil) where i = −(h + k). Because it was not possible to measure BET surface area directly on the samples after reactivity testing (due to the small sample size used for reactivity tests), the surface area for aged samples was calculated using the surface area for spherical particles, SA = 6/ (ρD) where ρ is density for ZnO (5.6 g/cm3) and D is the average crystallite diameter. The surface area calculated for spherical particles was multiplied by an adjustment factor to account for surface roughness and nonsphericity. This adjustment factor was calculated using the ratio of measured to calculated surface areas for the as-prepared samples. SEM images, Figure 3, show that powder crystallites are well 6496



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Figure 3. SEM micrographs of ZnO powders: (A) Aldrich; (B) Aldrich aged; (C) plates; (D) plates aged.

Table 1. XRD Summary Particle Diameter Perpendicular to the Listed Planes, nm As Prepared Plates Aldrich aged Plates Aldrich

Calculated SA, m2/gr

Measured BET, m2/gr 6.3 6.6 estimated 6.9 7.5

(100)

(002)

(101)

Average D, nm

100 67.6

76 75.7

100 73.1

92.6 72.2

11.6 14.8

70.7 58.6

61.7 56.4

62.4 57.5

65.0 57.5

16.5 18.6

Figure 5. TPD spectra obtained after the adsorption of CH3OH on ZnO powder showing the CH2O desorption at 300 °C and CO, CO2, H2O, and H2 desorption at 350 °C.

Figure 4. Schematic of representative ZnO Morphology based on XRD and SEM.

adsorption to produce adsorbed methoxyl and hydroxyl species. The methoxyl either decomposes at ∼300 °C to produce formaldehyde and hydrogen or is oxidized by oxygen from the ZnO lattice to yield an adsorbed formate (HCOO). The desorption of formaldehyde at temperatures lower than CO precludes CO hydrogenation from being the path to formaldehyde and instead confirms that formaldehyde is produced through methanol dehydrogenation. The formate decomposes through dehydration or dehydrogenation pathways at higher temperatures to produce CO, CO2, H2, and H2O. This decomposition pathway is confirmed in Figure 4 where a CH2O desorption peak from methoxyl decomposition was observed at 300 °C and CO, CO2, H2, and H2O desorption products consistent with formate decomposition were seen at 350 °C. These TPD spectra also show that the dehydrogen-

ation of adsorbed formate to produce CO2 is similar in scale to dehydration to make CO. It is likely that the parallel desorption products of CO/H2/ H2O are due to the dehydrogenation of formate to make CO and H2. Because water is known to remain adsorbed on ZnO to 500 °C,47 the desorbing water is likely a product of hydrogenation of surface hydroxyls during the surface oxidation of CO, with a Redhead-analysis-derived apparent activation energy (Ea) of 178 kJ/mol. The CO2 product is likely due to oxidation of CO, as CO has been found to adsorb on ZnO48 and oxidize to CO2 at 150 °C or higher with a rate that is first order in CO, zero order in oxygen, with Ea ∼ 104 kJ/mol.49 Redhead analysis yields apparent an activation energy of 210 kJ/mol for the dehydrogenation of formate to CO and H2. 6497



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Microreactor Studies. The ZnO powders tested were found to be catalytically active for MSR with selectivities to CO2 greater than 99% for many of the samples. The ZnO samples were not active for the water−gas shift reaction or its reverse under pressures, temperatures, and residence times of the MSR tests. Samples were stable over several temperature ramps from 300 to 400 °C, as illustrated by the two repeat ramps for the Aldrich sample in Figure 6. The ZnO powders

Figure 7. Summary of specific activity at 350 °C and energy of activation (Ea) for samples as prepared (AP) and aged.

The Aldrich sample had near 100% selectivity to CO2 before and after the aging (Figure 8). The aging significantly decreased

Figure 6. Arrhenius plot for ZnO samples: (●) plates; (○) plates aged; (■) Aldrich; (□) Aldrich repeated; (◇) Aldrich aged. Trend lines indicating Arrhenius slope of aged samples are indicated for (--) plates and (―) Aldrich. Error bars are similar in size to the data markers and are explicitly shown for the Aldrich sample only.

were then subjected to an accelerated aging test, 12 h reduction at 400 °C in flowing 5% hydrogen, based on the expected production of H2 at high conversions and temperatures. The aging procedure led to a decrease in observed energies of activation and improved specific activity of the powders even while it led to the significant crystallite restructuring as illustrated in Figures 2 and 3. The improved catalytic activity after aging together with the restructuring of the crystallites indicates the possibility of the formation of thermally activated surface defects, which have been shown to correlate with increased ZnO activity.50,51 The MSR conversion of asprepared and the aged samples and trend lines for the aged samples are displayed in the Arrhenius plot in Figure 6, with the derived trend lines, Ea, selectivities to CO2, and specific activity at 350 °C tabulated in Table 2 and illustrated graphically in Figure 7. Selectivities were maintained to high temperature even though conversions approached 70% at the highest temperatures.

Figure 8. Temperature dependence of selectivity and specific activity for aged plates and Aldrich ZnO samples.

the energy of activation for the Aldrich sample from 200 to 140 kJ/mol. Both activation energy and specific activity on the plates remained nearly constant after the accelerated aging treatment, going from 93 to 98 kJ/mol and 0.53 to 0.56 μmol s−1 m−2, matching the upper end of activation energies reported for Cu/ZnO/Al2O3 catalysts.52 The accelerated aging, known to lead to ZnO restructuring as confirmed by Figure 1, served to greatly enhance selectivity to CO2 on the ZnO plates. Although the plates initially had selectivity increasing from 69 to 95% over the 300−400 °C range, the selectivity changed to a uniform 95% after the aging. Thus it appears that the restructuring due to aging of the ZnO plates in H2 also serves to greatly enhance the selectivity to CO2 under MSR. The changes in the activation energy and specific activity at 350 °C due to the aging are plotted in Figure 7. After aging, the energy of activation decreased drastically on the Aldrich samples. The activity seen for ZnO plates also suggests a strong correlation between MSR activity and selectivity and the faceting of the ZnO powder. Although the plate and Aldrich samples have similar surface areas, the respective specific activities are 0.55 and 0.34 μmol·s−1·m−2. Thus the plates with preferential exposure of polar surface shows a 50% enhancement in specific activity compare to the Aldrich samples which contain a lower proportion of exposed polar surfaces. The reactivity trends for ZnO therefore show that ZnO is active for MSR, is stable under accelerated aging tests and can be highly selective to CO2. Specific activity values range from 0.26 to 0.55 μmol·s−1·m−2 at 350 °C, selectivities after aging range from 94% to near 100% CO2, and energies of activation

Table 2. Summary of Reactivity at 350 °C Conversion, Specific Activity, and Selectivity to CO2a

AP Plates Aldrich aged Plates Aldrich

Conversion, %

Activity, μmol/gcat

Activity, μmol/s/m2

Selectivity, % CO2

Ea, kJ/mol

0.14 0.09

3.33 1.71

0.53 0.26

69 100

93 196

0.16 0.13

3.86 2.50

0.56 0.34

94 100

98 140

a

Also listed is the activation energy. The sample surface area is from Table 1. 6498



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Table 3. Adsorption Energies and Geometries (Side and Top Views) of Pertinent Species in MSR on ZnO(001)

discussed in our previous work,21 they are not included here. From Table 3, it is clear that both hydroxyl and methoxyl species chemisorb on the ZnO(001) surface at fcc hollow sites. As expected, the oxygen atom in these species maximizes its interaction with surface zinc ions by forming three equivalent O−Zn bonds. On the other hand, the hydrogen atom adsorbs at a top site on Zn. The most stable conformation for formaldehyde is also an fcc hollow site, with the oxygen coordinated by two zinc ions and the carbon interacting with another zinc cation. After dehydrogenation of formaldehyde, the resulting formyl forms a bridging configuration, with the carbon and oxygen atoms bound to adjacent zinc cations. Finally, the product of the CH2O* + OH* condensation reaction, namely CH2OOH*, has a bidentate adsorption configuration with both of its oxygen atoms bonded to surface zinc ions. We performed NEB calculations for the three most likely key reaction steps in the MSR reaction pathway. The initial states (IS), transition states (TS), and final states (FS) of the reactions are depicted in Table 4 with ZPE corrected values listed in parentheses. The first reaction is the dehydrogenation of CH3O* (CH3O* → CH2O* + H*), which is the proposed rate-limiting step. The barrier was found to be 1.73 (1.43) eV and the reaction is endothermic by 0.93 (0.77) eV, where the values in parentheses are ZPE corrected. The barriers for initial

range from 98 to 140 kJ/mol. The activity of a commercial ZnO powder for MSR was also recently reported by Penner et al., where ZnO was found to be selective to CO2 for MSR with an Ea of ∼130 kJ/mol.53 These authors suggest that the ZnO might assist the reaction of supported PdZn metal particles by converting the formaldehyde that may be formed on the metal particles. However, when we compare the specific reactivity and activation barriers, we find that ZnO is orders of magnitude less active than PdZn alloys, which show specific activities of 2.2 μmol·s−1·m−2 at 250 °C.54 We also see a role of surface facets because the highest specific activity and lowest activation energy is associated with the plate samples which have the highest relative exposure of the (001) surface. Lastly, we observe that selectivity to CO2 on the plates is improved by aging in flowing H2, which is also known to cause surface restructuring and defect formation, suggesting that the active sites for MSR on ZnO are defects formed on the polar surfaces of ZnO. DFT Calculations. To help understand the enhanced specific reactivity on the ZnO polar surfaces, we performed DFT calculations for several key reaction steps in the proposed mechanism. First, the calculated adsorption energies and geometries of several pertinent species involved in MSR on ZnO(001) are listed in Table 3. Because the initial adsorption and decomposition of methanol and water on ZnO have been 6499



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Table 4. Activation Energies, Exothermicities (eV) and Geometries for Key Reaction Steps in MSR

formaldehyde and hydroxyl. Indeed, NEB calculations for the CH2O* + OH* → CH2OOH** reaction find that this barrier is 0.62 (0.61) eV with an exothermicity of −0.11 (−0.01) eV. These values can be compared with +0.11 and −0.46 eV on Cu(111),23 and +0.16 eV and −0.34 eV on PdZn(111).24 Importantly, the barrier for the condensation reaction is lower than the desorption energy of formaldehyde (1.62 eV) and the barrier for its dehydrogenation (0.69 eV). It is clear that this reaction step is quite facile, as those on metal surfaces, and it leads eventually to the formation of the observed CO2 + H2 product. The lower barrier for this step on the metal surfaces makes it unlikely that the ZnO could play a role in the supported metal catalysts. The calculated barrier for the rate-limiting step in MSR on ZnO of 1.43 eV (138 kJ/mol) closely matches the 100−140 kJ/ mol observed energy of activation for MSR on ZnO powders. Further, our DFT calculations suggest that the selectivity of MSR toward CO or CO2 is determined by the relative barrier heights for elementary steps involving the formaldehyde intermediate, which can undergo desorption, dehydrogenation,56,57 or reaction with an adsorbed hydroxyl species. The calculated barriers in this study reveal that the reaction between CH2O* and OH*, which eventually leads to CO2 formation, is

dissociative adsorption of methanol (0.39 eV) and water (0.42 eV) on the same surface21 are significantly less than that, as are the formaldehyde dehydrogenation and formaldehyde-hydroxyl condensation reactions, which will be described shortly. The overall barrier for MSR on ZnO is thus taken to be that for the dehydrogenation of methoxyl to formaldehyde. This barrier is significantly higher than those calculated for the same process on Cu(111) (1.16 eV) and PdZn(111) (1.17 eV),55 suggesting that MSR on ZnO is less facile, consistent with the experimental evidence presented here for ZnO and in previous publication for PdZn.54 The dehydrogenation of formaldehyde (CH2O* → CHO** + H*) was also investigated. This step, which leads to formyl and eventually to the CO* product, has a barrier of 0.86 (0.69) eV and endothermicity of 0.65 (0.50) eV. The formyl product has a bidentate coordination with two surface zinc ions, whereas the hydrogen is located at a top site. The barrier for this reaction on ZnO (001) is similar to those on Cu (0.66 eV) and PdZn (0.65 eV).55 The relatively low barrier for this pathway suggests that CO production on ZnO is possible, as was observed in the methanol decomposition TPD experiments on ZnO powders. The high selectivity to CO2 observed for ZnO is attributed to the condensation reaction between 6500



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preferred to that of CH2O* dehydrogenation, but of course only if hydroxyl is available. An alternative pathway involves the reaction between formaldehyde and methoxyl,22 but our previous studies on Cu(111) and PdZn(111) have demonstrated that this methyl formate pathway is of minor importance.58,59 The observed microreactor MSR selectivities, where mostly CO2 is produced but some CO is as well, can therefore be explained by the availability of water. On active sites where water is adjacent to methoxyl, as occurs readily on ZnO(001),25 the reaction produces CO2, whereas isolated methoxyl sites, for example, on the ZnO(100) where water does not dissociate readily,25 lead to CO formation. These DFT calculations therefore serve to explain the ability of ZnO to catalyze MSR and indicate that facile activation of water is a key factor in the good selectivity to CO2 observed for ZnO.

Article

AUTHOR INFORMATION

Corresponding Author

*Tel: 1-505-277-0477. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding for this work provided by the U.S. Department of Energy for grant DE-FG02-05ER15712 and partial funding from grant DE-FG02-08ER46530. The theoretical work was funded by Science & Technology Development Foundation of Fuzhou University (2012-XY-7 to SL), Natural Science Foundation of Fujian Province, China (2012J05022 to SL), a New Direction grant from the Petroleum Research Fund administered by the American Chemical Society (48797-ND6 to HG), and the National Science Foundation (CHE-0910828 to HG). A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We thank Boris Kiefer for materials science theory discussions and insight and Eric Petersen for the XRD data. We thank Fernando Garzon of Los Alamos National laboratory for access to the micro-XRF.

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CONCLUSIONS Faceted ZnO powders have been shown to be beneficial for MSR on PdZn/ZnO.3 To understand the contribution of the differently faceted ZnO supports, we measured methanol steam re-forming on ZnO powder samples of differing morphology and compared the experimental results with DFT calculations of the most likely elementary steps and reaction intermediates. One of the samples was a high purity commercial ZnO powder having both polar and nonpolar surfaces exposed whose reactivity was compared to a sample we synthesized in the lab that shows plate-like morphology exposing predominantly the polar ZnO(001)/(001̅) surfaces. TPD studies showed that methanol decomposes to produce formaldehyde, then CO and CO2. Microreactor MSR studies show that ZnO samples can be highly selective to CO2 under MSR with specific activity reaching 0.55 μmol·s−1·m−2 at 350 °C. Although ZnO shows good selectivity under MSR, it is orders of magnitude less active than PdZn alloys that show specific activities of 2.2 μmol·s−1·m−2 at 250 °C. Further, the plate sample with preferential exposure of the polar surfaces shows a 50% enhancement in specific activity compared to the commercial samples that contain a lower proportion of exposed polar surfaces. Thus the highest specific activity and lowest activation energy is associated with the plate samples that have the highest relative exposure of the polar ZnO surfaces. Selectivity to CO2 on the plates is improved by thermal reduction, which is also known to cause surface restructuring and defect formation, suggesting that the active sites for MSR on ZnO are defects formed on the polar surfaces of ZnO. As proposed for single crystals, the activity of polycrystalline ZnO for MSR is linked to active sites that are more prevalent on the polar ZnO surfaces. DFT calculations show that elementary step reaction barriers are lower for formaldehyde/hydroxyl adsorbate condensation, which leads to CO2 production. At the same time, the decomposition pathway to CO is more costly but possible when hydroxyls are not present. Therefore, the polar surfaces that are known to dissociately adsorb water facilitate the formaldehyde/hydroxyl condensation reaction, which leads to desired CO2 production under MSR, rather than CO. ZnO is therefore found to be active and selective for MSR, linking the high density of active sites to the polar surfaces of ZnO, but the 100 kJ/mol activation energy is much larger than the 50 kJ/mol activation energy on PdZn or PdZn/ZnO. Thus the contribution of ZnO to the reactivity of the PdZn/ZnO catalyst system can be discounted, much like has been previously shown for the Cu/ZnO system as well.60

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High CO2 Selectivity of ZnO Powder Catalysts for Methanol Steam

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