Pathways of Methanol Steam Reforming on PdZn and Comparison

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Pathways of Methanol Steam Reforming on PdZn and Comparison with Cu Sen Lin,†,‡ Daiqian Xie,*,† and Hua Guo*,§ †

Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou, 350002, China § Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States ABSTRACT: PdZn alloy has been shown to catalyze methanol steam reforming (MSR), producing hydrogen gas and carbon dioxide with high selectivity. Despite many studies, the mechanism for MSR on this catalyst is still not completely understood. In this work, several possible pathways of MSR are explored using a plane-wave density functional theory. The focus is placed on the reaction network starting from a facile reaction between adsorbed formaldehyde and hydroxyl species, produced from the decomposition of methanol and water, respectively. These pathways were found to have barriers lower than the rate-limiting step, namely, the dehydrogenation of methoxyl, and they involve species that have been detected in various experiments. Interestingly, the reaction pathways share many similarities with the MSR process on copper, which is the traditional catalyst for MSR.

I. INTRODUCTION Despite their high-energy efficiency and environmental friendliness, polymer electrolyte membrane fuel cells (PEMFCs) have yet to find wide usage. A major roadblock has been the difficulties associated with the storage and transportation of the hydrogen fuel. To circumvent this problem, it has been suggested that methanol can serve as a hydrogen carrier,1 which generates on demand the hydrogen fuel to onboard PEMFCs. A leading candidate for in situ hydrogen generation is the methanol steam reforming (MSR) process2
5 CH3 OH þ H2 O f 3H2 þ CO2

ΔH 0 ¼ 49:6 kJ=mol

The MSR-based solution is attractive in several aspects. First, the liquid nature of methanol at room temperature allows leveraging the existing infrastructure in storage and dispense of transportation fuels such as gasoline and diesel. Second, the industrial scale production of methanol from other feedstocks, such as natural gas, oil, and even biomass, is well-established. Finally, methanol is a relatively clean fuel, with essentially no sulfur, a large H/C ratio, and biodegradable. MSR is a well-established catalytic process, and the traditional catalyst is copper-dispersed on oxide support.4 The copper-based catalysts are efficient and highly selective toward CO2 over CO,3
5 which is important for fuel cell applications because the CO byproduct poisons the anode. However, these catalysts do not have sufficient thermal stability, largely due to metal sintering at the operating temperatures.3,4 As a result, there has been strong desire in finding new and more stable catalysts for MSR. Current interest in this direction has been focused on the Pd/ZnO catalyst, first discovered by Iwasa and coworkers.6 The new MSR catalyst r 2011 American Chemical Society

has been shown to have a similar selectivity toward CO2 and with much better thermal stability. To improve further the efficiency of MSR catalysts, it is important to understand the catalytic mechanism, which is the focal point of this work. Since its initial discovery, much work has been performed to identify the active phase of the Pd/ZnO catalyst. There is now overwhelming evidence that the PdZn alloy, formed by heating Pd/ZnO under reductive conditions, is responsible for the observed MSR process on the catalyst.7
15 Interestingly, PdZn alloy has a similar valence density of states to that of Cu, as shown by density functional theory (DFT)16 and X-ray photoelectron spectroscopy (XPS) valence band spectra.17,18 Therefore, it is very tempting to compare the catalytic mechanisms of the two MSR catalysts. The mechanism of Cu-catalyzed MSR is not fully understood either, although recent DFT studies have unraveled many key aspects of the reaction network. MSR is initiated by O
H bond cleavage of both methanol and water, which generates surface methoxyl (CH3O*) and hydroxyl (OH*), respectively. It is wellknown experimentally that the rate-limiting step for the MSR process is the dehydrogenation of surface methoxyl.19
23 Indeed, this process has been found to have a high barrier on Cu(111), based on recent DFT calculations.24
29 It was also known that formaldehyde (CH2O*) is an important intermediate,30 which may be the key for the selectivity. Unlike Pd, which has a low barrier for the direct dehydrogenation of formaldehyde to CO,25,29,31 Cu has a much higher barrier for the same process.24,25,28,31 Instead Received: July 10, 2011 Revised: August 25, 2011 Published: September 18, 2011 20583

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The Journal of Physical Chemistry C of dehydrogenation, formaldehyde could instead react with hydroxyl on Cu.8 Indeed, recent DFT studies have found that the aforementioned reaction is rather facile, with a low barrier and large exothermicity.29,32 It can thus compete effectively with the desorption and dehydrogenation of formaldehyde. More importantly, the product of the reaction, CH2COOH*, has been shown theoretically to lead eventually to CO2 via the formic acid (CHOOH*), formate (CHOO**), or dioxomethylene (CH2O2**) intermediates, with barriers lower than the rate-limiting step.32 This so-called formate mechanism is consistent with a recent diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) study,22 which has detected spectroscopic signatures of methoxyl, hydroxyl, and formate. On PdZn, much work has also been done.18 Experimentally, the kinetics of the Pd/ZnO catalyst has recently been investigated.33,34 It was shown that the rate constant depends strongly on the partial pressure of methanol but only weakly on the partial pressure of water.33 The measured activation energy of 94.8 kJ/mol33 is quite similar to that on the copper-based catalyst (103 kJ/mol).21 Mechanistic studies have also been reported, and the characteristics of the reaction are similar to that on the copper catalyst. For example, Iwasa and Takezawa found that the steam reforming of formaldehyde yielded the same product as MSR on the Pd/ZnO catalyst, suggesting a key role of the formaldehyde intermediate.11 Ranganathan et al. have identified surface formate as a key intermediate, and they also indicated that formaldehyde and formic acid might participate in the MSR reaction.35 Rameshan and coworkers suggested that the selectivity of the PdZn catalyst is controlled by the subsurface structure.36,37 These authors found evidence that formaldehyde is the key in selectivity toward CO2. Specifically, the production of CO facilitated by dehydrogenation of formaldehyde is suppressed on multilayer PdZn alloys. In the mean time, these alloys seem to promote the production of CO2, which is presumably the result of the reaction of formaldehyde with hydroxyl. Recent advances in making homogeneous PdZn particles are expected to provide a more realistic model for further kinetic and mechanistic studies of the MSR catalysis.38 However, it is clear that experimental data alone are not sufficient to provide a complete picture of the catalysis. Theoretical studies, particularly those based on plane-wave density functional theory,39 are often complementary and might provide a more detailed picture of the catalysis. DFT studies have already shown that the dehydrogenation of methoxyl also has a high barrier on PdZn,25,40
42 although the barriers for the initial decomposition steps are quite significant.41,42 However, there have been relatively few investigations into the subsequent steps leading to the CO2 product. Like Cu, the dehydrogenation of formaldehyde on PdZn is much more difficult than on Pd.25,31 Indeed, it has recently been shown experimentally that the addition of Zn on Pd(111) severely curtails the production of CO in methanol decomposition.43 It is thus very tempting to propose that MSR is catalyzed on PdZn in a similar way as on Cu,8 namely, via the reaction between the formaldehyde intermediate and surface hydroxyl species. To answer this question, we report here plane-wave DFT studies of two pathways initiated by the reaction between formaldehyde and hydroxyl on PdZn(111) and compare the energetics with those on Cu(111). This publication is organized as follows. The computational method is outlined in Section II. The next two sections (Sections III and IV) present and discuss the results in the context of MSR. The final section (Section V) concludes.

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II. THEORY All DFT calculations were carried out by using the Vienna ab initio simulation package (VASP)44
46 with the gradient-corrected PW91 exchange-correction functional.47 The ionic cores were described with the projector augmented-wave (PAW) method, and for valence electrons a plane-wave basis set was employed.48,49 The energy cutoff was set to 400 eV, and the Brillouin zone was sampled using a 3  3  1 Monkhorst-Pack k-point grid,50 which was tested to be sufficiently accurate for all calculations. The Fermi level was smeared using the Methfessel
Paxton method with a width of 0.1 eV.51 In this work, we use the homogeneous 1:1 PdZn alloy as the model for the MSR catalyst and focus on the (111) face, which is one of the most stable faces of the alloy.16 After optimizing the bulk crystal, the lattice parameter was found to be a = b = 4.139 Å, c = 3.378 Å, in good agreement with the previously reported values.16 A slab model for the PdZn(111) surface consists of four layers of a 4  4 unit cell, with all surface atoms fixed at their bulk positions. This unit cell is substantially larger than that used in the Cu MSR studies, resulting in much higher computational costs. Our tests further indicated that relaxing the first layer will not significantly change adsorption energies. A vacuum space of 14 Å was used between the slabs to separate the substrate and its repeatable images in the z direction. The adsorption energy was calculated as follows: Eads = E(adsorbate + surface)
E(free molecule)
E(free surface). The climbing image nudged elastic band (CI-NEB) method52,53 was used to determine the reaction profile with the energy (10
4 eV) and force (0.05 eV/Å) convergence criteria. Stationary points were confirmed by normal-mode analysis using a displacement of 0.02 Å and an energy convergence criterion of 10
6 eV, and the vibrational frequencies were used to compute zero-point energy (ZPE) corrections. III. RESULTS III-A. Adsorption. We focus here on species pertinent to the latter steps in MSR, as many other species involved in the initial steps have been thoroughly studied on PdZn.25,41,42 In Table 1, the adsorption energies and geometries of the preferred adsorption configurations for several key species are listed. Our results are in quite good agreement with those reported in the literature.25,31,42 It can be seen from the Table that closed-shell species, such as H2O*, CO2*, CH2O*, and CHOOH*, are weakly adsorbed on the PdZn(111) surface, and the relatively small adsorption energy is dominated by van der Waals interactions. It should be noted that DFT is known to give poor description of dispersion forces, so these binding energies should not be considered to be quantitatively accurate. H2O*, CO2*, and CH2O* all lay parallel to the surface, but CHOOH* is perpendicularly adsorbed. These adsorption patterns are very similar to those on Cu(111).29,32 However, the existence of two metal sites on the PdZn surface provides some heterogeneity for the adsorbates. For these species, the oxygen moiety seems to have a slight preference on the zinc site. The other unsaturated species in Table 1 typically bind to the surface much more strongly. As previously observed, there is some preference for the oxygen moiety to maximize its interaction with Zn sites, whereas the carbon and hydrogen moieties prefer to stick to Pd sites. The H* species, for example, adsorbs at a Pd2Zn hollow site with an adsorption energy of
2.49 eV. 20584

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Table 1. Preferred Adsorption Site and Binding Energy for Various Pertinent Species on PdZn(111)a

a

Entries in the parentheses are the ZPE-corrected values. Atoms are color labeled: Pd(dark blue), Zn(light blue), O(red), C(black), and H(grey).

The OH* species adsorbs at a PdZn2 hollow site with an adsorption energy of
2.97 eV. These results are consistent with our previous studies on molecular adsorption on PdZn surfaces with higher accuracy.42 Similar to Cu(111),29,32 the key formate species has two possible adsorption configurations on PdZn(111). The unidentate configuration (CHOO*), which has a smaller binding energy, features an oxygen at a PdZn2 site. The bidentate form (CHOO**) has the two oxygens atop of Pd and Zn. Their adsorption energies of
2.03 and
2.54 eV can be compared with that on Cu(111), namely,
2.14 and
2.63 eV.32 (Interestingly, the CHOO species can also adsorb in a bidentate fashion on two neighboring Zn sites but with a smaller adsorption energy.) The strongest adsorbate studied in this work is dioxomethylene (CH2OO**), which adsorbs in a bidentate fashion with the oxygens at Pd
Zn bridge sites. Finally, an intermediate species from the reaction between CH2O* and OH*, namely, CH2OOH*, adsorbs with its carbonyl oxygen at a PdZn2 hollow site. The adsorption energies for the last two species (
3.75 and
1.92 eV) are also close to those on Cu(111), namely,
3.98 and
2.08 eV.32 The parallel between PdZn(111) and Cu(111) in adsorption of various species has been previously noted by R€osch and coworkers,18,25,31 and our results provide further support to their observations. III-B. Reactions. As discussed in Section I, we will focus on the reaction network initiated by the reaction between adsorbed formaldehyde and hydroxyl species. To compare with MSR catalyzed by copper, we focus on pathways B and C identified

in our previous work on Cu(111).32 Pathway A was deemed not viable because of higher barriers. In addition, some high barrier reactions identified in our previous work on Cu(111) were not investigated, primarily due to the substantially higher computational costs for the PdZn system. Specifically, we have performed NEB calculations for the following elementary steps CH2 O þ OH f CH2 OOH þ 

ðR1Þ

CH2 OOH þ  f CHOOH þ H

ðR2Þ

CHOOH þ OH þ  f CHOO þ H2 O

ðR3Þ

CH2 OOH þ OH þ  f CH2 OO þ H2 O

ðR4Þ

CH2 OO þ  f CHOO þ H

ðR5Þ

CHOO f CHOO þ 

ðR6Þ

CHOO þ  f CO2  þ H

ðR7Þ

Their energetics, including the activation energy (Ea) and reaction energy (ΔE), are listed in Table 2, and the geometries of the initial states (ISs), transition states (TSs), and final state (FSs) are depicted in Figures 1
6. For comparison, the energetics of the corresponding reactions on Cu(111) are included. 20585

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Table 2. Calculated Activation and Reaction Energies (eV) for Several Elementary Reactions on PdZn(111) Studied in This Worka step no.

elementary reaction

activation

exothermicity

energy Ea

ΔE

1 CH2O* + OH* f CH2OOH*

0.17 (0.16)
0.50 (
0.34)

2 CH2OOH*f CHOOH* + H*

0.59 (0.58)
0.35 (
0.53)

3 CHOOH* + OH*f CHOO** + H2O*

0.06 (0.05)
0.61 (
0.57)

4 CH2OOH* + OH* f CH2OO** + H2O* 0.36 (0.32)
0.05 (
0.05) 0.64 (0.58)
0.88 (
1.03) 5 CH2OO** f CHOO** + H*

a

6 CHOO** f CHOO*

0.54 (0.50)

7 CHOO* f CO2* + H*

0.37 (0.22)
0.48 (
0.59)

0.55 (0.51)

Entries in the parentheses are the ZPE-corrected values. Figure 3. Same as Figure 1 for R3.

Figure 1. Energetics (blue) and geometries of various species involved in Reaction R1 on PdZn(111). The results on Cu(111) are given in red for comparison.

Figure 2. Same as Figure 1 for R2.

As shown in Figure 1, the exothermic reaction between CH2O* and OH* has a very small barrier (0.16 eV), similar to the corresponding process on Cu(111).29,32 The CH2O* species which is weakly bound to the PdZn(111) surface, approaches OH* locating on a PdZn2 hollow site, yielding the product CH2OOH*. This process is a key reaction for selectivity, as shown in Figure 7, because it diverts the reaction flux from the dehydrogenation of CH2O* and subsequently CHO, which eventually leads to the CO product. This dehydrogenation reaction has a much higher barrier on PdZn (Ea = 0.66 eV) and Cu (Ea = 0.65 eV) but is relatively easy on Pd (Ea = 0.22 eV).25,31 This explains why

CO is the major product of MSR on Pd whereas CO2 is the major product of MSR on PdZn and Cu. Perhaps more importantly, R1 is thermodynamically and kinetically favored, which allows it to compete effectively with the desorption of formaldehyde. As we discussed in our previous work,32 this is essential for the completion of the MSR reaction. With the reaction network starting out with R1, there are two possible reaction pathways, as shown in Figure 7. The first one involves the dehydrogenation of the CH2OOH intermediate, leading to formic acid (CHOOH*). As shown in Figure 2, this reaction R2 has a moderate barrier (0.58 eV), which is lower than the corresponding barrier (0.90 eV) on Cu(111).32 In addition, it is ∼0.6 eV more exothermic on PdZn. In the transition state, the length of the breaking C
H bond is 1.49 Å. After dissociation, the H atom moves to its preferred Pd2Zn hollow site. The formic acid can then be converted to formate by reacting with OH*, as shown in Figure 3. In the initial state of R3, these two species coadsorb nearby. The abstraction of the hydroxyl H of CHOOH* by OH* produces the bidentate formate CHOO** and H2O* locating on a Zn top site with one H forming a hydrogen bond with one O atom of the formate. This reaction R3 is extremely facile, with essentially no barrier and a large exothermicity, again in similar fashion to the corresponding reaction on Cu(111).32 This is important as the formic acid is weakly adsorbed, and it is likely to desorb if it does not readily react. The bidentate formate (CHOO**) is converted to unidentate CHOO* at a PdZn2 hollow site through O in R6 before it produces the final product (CO2*) via dehydrogenation R7. Interestingly, in the transition state of R7, the length of the cleaving C
H bond is 1.54 Å, which is the same as that on Cu(111).32 After dissociation, the H adsorbs at a Pd2Zn hollow site, whereas the carbon dioxide is loosely bound on PdZn(111). The reactions R6 and R7 should probably be considered as a single step because the unidentate formate is not particularly stable, as shown in Figure 6. As a matter of fact, a recent study by Grabow and Mavrikakis54 found no stable CHOO* on Cu(111), whereas our previous work indicates that it is stabilized by only 0.04 eV on the same surface.32 The combined barrier for R6 + R7 is 0.72 eV, which can be compared with that on Cu(111) of 0.96 eV.32 This step has the highest barrier among the reactions reported here but is still lower than the rate-limiting dehydrogenation of CH3O* on PdZn(111), which is 0.89 eV.25 The proposed reaction network for MSR on PdZn is depicted in Figure 7. The other pathway branches out with R4, in which the hydroxyl H of CH2OOH* is abstracted by a nearby OH* species. The bidentate dioxomethylene CH2OO** was found to 20586

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Figure 4. Same as Figure 1 for R4.

Figure 5. Same as Figure 1 for R5.

Figure 6. Same as Figure 1 for R6 and R7.

hydrogen bond with H2O*. This reaction is almost thermoneutral, with a somewhat higher barrier (0.32 eV) than that on Cu(111),32 as shown in Figure 4. The further dehydrogenation of CH2OO** R5 yields formate in its bidentate form (CHOO**), as shown in Figure 5, with a moderate barrier and large exothermicity. The exothermicity is 0.2 eV larger on PdZn than on Cu. The CHOO** product adsorbs at its preferred PdZn bridge sites. This reaction profile is nearly the same as its counterpart on Cu(111).32 The subsequent steps are identical to the first pathway.

IV. DISCUSSION In this work, we have for the first time proposed a complete microscopic reaction mechanism for MSR on PdZn, as depicted

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Figure 7. Proposed reaction network for MSR on PdZn. The reaction barriers/exothermicities are also given in black for values reported in this work and gray for those reported in the literature. The CO and CO2 channels are colored coded as red and green, respectively.

in Figure 7. The adsorption pattern and reaction profiles discussed above and reported in recent literature suggest that MSR on Cu and PdZn catalysts are quite similar. In particular, both are initiated by the O
H bond cleavage of adsorbed methanol (CH3OH*) and limited by the dehydrogenation of methoxyl (CH3O*).25 Both proceed with the formaldehyde intermediate, which reacts swiftly with hydroxyl produced by decomposition of H2O*. As we discussed previously32 and here, this low barrier step is essential to avoid the pathway leading to the CO product. Interestingly, this step is much less favorable on Pd(111) (Ea = 0.58 eV),29 which is presumably responsible for its inability to produce CO2. Two subsequent pathways are examined. The first involves formic acid (CHOOH*), whereas the second involves dioxomethylene (CH2OO**), but both lead to formate (CHOO**). The intermediacy of formaldehyde and formate in the MSR catalyzed by copper and PdZn catalysts is supported by experimental evidence.11,35
37 However, it is still difficult to determine which of the two pathways is preferred on PdZn because the highest barriers in both pathways are about the same. A more quantitative analysis of the kinetics will have to wait for future kinetic Monte Carlo studies. The rate-limiting step of MSR on PdZn(111), namely, the dehydrogenation of methoxyl (CH3O*), has a barrier of 0.89 eV, which is lower than that on Cu(111), which is 1.16 eV.25 These theoretical results can be compared with experimental values of 0.98 (PdZn)33 and 1.07 eV (Cu),21 respectively. It is apparent that there are many other factors, such as morphology of the catalyst and involvement of defect sites, that can influence the activation energy, but the current level of theory
experiment agreement is quite reasonable. The energetics of various reactions studies here also offer insight into the detectability of several intermediate species on the catalyst surface during MSR. The formate species possesses a relatively high reaction barrier (0.72 eV) to the CO2* + H* product as well as higher barriers for reactions back to CH2OO** (1.61 eV) and to CHOOH* (0.62 eV). In addition, its binding energy of
2.54 eV also prevents it from desorption under MSR conditions. As a result, the formate species may accumulate during MSR and should be relatively easy to detect as a surface species. Formic acid (CHOOH*) reacts with OH* swiftly once it is formed R3, and its population is expected to be small and difficult to detect. The dioxomethylene (CH2OO**) should also be detectable given its high binding energy and sizable barriers toward formate (CHOO**) and CH2OOH*. 20587

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The Journal of Physical Chemistry C Admittedly, the studies reported in this work are by no means exhaustive. There may very well be other reaction pathways. For example, there have been proposals that MSR involves methyl formate (CHOOCH3).19,20,30 Although not discussed here, our recent investigation on Cu(111) has clearly suggested that the methyl formate pathway plays only a minor role in MSR on copper-based catalysts.55 Given the similarities in the formate pathways revealed here, we do not expect qualitatively different conclusions on PdZn surfaces.

V. CONCLUSIONS Using PdZn(111) as the model, we have theoretically examined several elementary reactions relevant to MSR on PdZn catalysts using a plane-wave DFT method. Combining with DFT studies already done on the same PdZn(111) surface, we conclude that the reaction network on PdZn(111) is very similar to that on Cu(111). Both catalytic processes are limited by the dehydrogenation of methoxyl (CH3O*), and both involves a facile reaction between formaldehyde (CH2O*) and hydroxyl (OH*) on the metal surfaces, which eventually leads to the formation of a surface formate species (CHOO**) before its dehydrogenation to the final CO2 product. The CO channel is effectively blocked off by a relatively high barrier for the CH2O* dehydrogenation. These results highlight the importance of the formaldehyde intermediate serving as a branching point for different product channels. Despite the fact that none of the reactions related to this intermediate is rate-limiting, it plays a central role in the selectivity of the catalysts. The similarities between the two catalysts have been suspected before and are now confirmed. Importantly, these proposed reaction pathways are consistent with existing experimental evidence on the reaction mechanism, lending support for our mechanistic model. They also provide the basis for future kinetic simulations of MSR processes, which will generate measurable attributes such as overall rate constant and transient population of intermediate species. The insights gained by studying the reaction networks on these two surfaces may shed valuable light on designing future catalysts for the MSR process. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (D.X.); [email protected] (H.G.).

’ ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (20725312 and 91021010 to D.X.), by Chinese Ministry of Science and Technology (2007CB815201 to D.X.), by a New Direction grant from the Petroleum Research Fund administered by the American Chemical Society (48797-ND6 to H.G.), and by U.S. National Science Foundation (CHE-0910828 to H.G.). H.G. also thanks Prof. Abhaya Datye for many stimulating discussions. ’ REFERENCES (1) Olah, G. A. Catal. Lett. 2004, 93, 1. (2) Brown, L. F. Int. J. Hydrogen Energy 2001, 26, 381. (3) Trimm, D. L.; Onsan, Z. I. Catal. Today 2001, 43, 31. (4) Palo, D. R.; Dagle, R. A.; Holladay, J. D. Chem. Rev. 2007, 107, 3992.

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dx.doi.org/10.1021/jp206511q |J. Phys. Chem. C 2011, 115, 20583–20589