Direct Methane-to-Methanol Conversion: Insight from First-Principles

The Journal of Physical Chemistry C ..... Better insight can be gained by analyzing the geometry of the transition states for the three processes unde...
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J. Phys. Chem. C 2007, 111, 17015-17019

17015

Direct Methane-to-Methanol Conversion: Insight from First-Principles Calculations Guido Fratesi,* Paola Gava, and Stefano de Gironcoli INFM-DEMOCRITOS and SISSA, Via Beirut 2, 34014 Trieste, Italy ReceiVed: May 29, 2007; In Final Form: August 31, 2007

A simple direct mechanism for methane-to-methanol conversion has been investigated by first principles on a series of oxygen-precovered transition-metal surfaces. Energy barriers and reaction paths have been determined for three competing elementary processes by the nudged elastic band algorithm. Indicators of reactivity toward each elementary step have been identified, providing significant insight into a rational search for a suitable catalyst. The effect of chemical environment, local geometry, strain, and coadsorption have been addressed, and general guidelines have been identified. On the basis of this analysis, we suggest that upon suitable conditions O-dosed Ag surfaces could display considerable reactivity toward direct methane-to-methanol conversion.

1. Introduction Direct low-temperature conversion of methane (CH4) to methanol (CH3OH)

1 CH4 + O2 f CH3OH + 1.36 eV 2

(1)

is one of the “dream” reactions of modern catalysis. Methanol is a versatile chemical used as a raw material in the synthesis of other substances, as fuel in thermal combustion or in fuel cell applications, and it has been proposed as a possible hydrogen carrier. Actually, a methanol-based economy has even been proposed1 as an alternative to future hydrogen economy because of the simpler infrastructure that it would require. Commercial production of methanol has been operating for nearly a century via an indirect two-step process: production of syngas (a CO and H2 mixture) by steam reforming of methane and the methanol synthesis from syngas.2-4 The process involves two high-temperature steps and is not economical at the large scale required for significant use of methanol as an alternative to oil. A low-temperature direct conversion of methane to methanol would be highly desirable and would allow exploiting large resources of natural gas found at oil extraction sites and presently reinjected because of difficulties in methane transportation and storage. Much effort has been devoted in the past few decades to the study of possible catalytic processes in the homogeneous phase (gas or liquid) with limited success.5-11 Heterogeneous catalysis has been studied to a much lesser extent. In our recent theoretical analysis12 of methane-to-methanol conversion on a reactive transition-metal surface [Rh(111)], we have shown that the ratelimiting step in methanol synthesis is likely to be the formation of C-O bonds, in agreement with similar results obtained by others on Pd(111)13 and Pt(111)14,15 substrates. Furthermore, the deprotonation reaction of CH4 is actually undesired because it easily leads to complete deprotonation and poisoning of the substrate by coking.16-19 On the basis of these observations, it is important to focus the study on the reaction of methane with * Current address: Dipartimento di Scienza dei Materiali, Universita` degli Studi di Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy. E-mail: [email protected].

an oxygen-precovered transition-metal surface, before C-H bond activation. In this work, we study theoretically methane-to-methanol conversion in a large number of transition-metal systems taking into consideration a simple reaction mechanism where CH4 reacts directly with some preadsorbed oxygen to form methanol in a single elementary step

(a) CH4 + O(a) f CH3OH(a) Competing reactions that methane can undergo are deprotonation with consequent adsorption of atomic H and CH3 radical on the substrate

(b) CH4 f CH3(a) + H(a) and the CH4 deprotonation assisted by adsorbed oxygen

(c) CH4 + O(a) f CH3(a) + OH(a) These three elementary reaction steps have been studied systematically on the (111) surfaces of three transition-metal elements across a row in the periodic table (Rh, Pd, and Ag) and the three noble metals (Cu, Ag, and Au). 2. Computational Details All energetics presented in this paper have been obtained by ab initio simulations in the framework of density-functional theory.20,21 The Perdew-Burke-Ernzerhof22 generalized gradient approximation has been adopted for the exchange and correlation functional. Three-layer, periodically repeated slabs have been chosen as model for the surfaces. The bottom two layers were kept fixed at the bulk-truncated positions, while the upper one was allowed to relax. Molecules were placed on this side of the slab and relaxed together with the upper layer until the forces on the atoms were less than 0.02 eV/Å, even thought adsorption energies were well converged already using a threshold on forces of 0.05 eV/Å. A 2 × 2 surface unit cell has been adopted for the calculations. The plane-wave ultrasoft pseudopotential method23 was used as implemented in the PWscf code of the Quantum-ESPRESSO distribution.24 Converged results were obtained with a cutoff of 27 Ry for expanding wavefunctions and 216 Ry for the electron density and the

10.1021/jp074134b CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007

17016 J. Phys. Chem. C, Vol. 111, No. 45, 2007

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Figure 2. Transition states for reactions occurring at the Ag(111) surface: (a) CH4 + O f CH3OH; (b) CH4 f CH3 + H; (c) CH4 + O f CH3 + OH. Figure 1. Activation energies, in eV, for the three elementary reactions (a-c) described in the text. Left panel: dependence on 4d-level filling. Right panel: variation along the noble-metal group.

effective potential. The surface Brillouin zone was sampled with a 4 × 4 equispaced mesh of points25 not containing Γ h . Electronic states were occupied according to first-order Methfessel-Paxton weights26 and a smearing parameter of 0.03 Ry. We computed transition-state (TS) geometries and activation energies by using the climbing-image nudged elastic band method (NEB).27,28 Reaction paths were optimized until the norm of the forces at the transition state was less than 0.05 eV/Å. The possibility of a noncompensated spin population was considered. However, strong hybridization with the surface generally occurs and no magnetic structure was found, apart for the TS of reaction (a). The latter is characterized by a half-filled pz orbital in sp2hybridized CH3; neglecting the magnetization of this structure would have resulted in overestimating the activation energy on the substrates considered by 0.3 eV. 3. Results Our theoretical activation energies for the five considered substrates are reported in Figure 1. As expected from general arguments,29 the activation energy for the very well studied methane dehydrogenation16-19,30-32 E(b) act, increases significantly with d-band filling moving from the more-reactive Rh (0.73 eV), to Pd (0.89 eV), to the less-reactive Ag (2.29 eV) substrate (reaction b and Figure 1, left panel). It is instead less obvious why the activation energy for H transfer from CH4 to O, reaction c, is rather constant (E(c) act ) 1.10, 1.21, and 1.06 eV on Rh, Pd, and Ag) and especially why the energy barrier for direct methanol formation via process a decreases on lessreactive substrates (E(a) act ) 2.06, 1.67, and 1.19 eV on Rh, Pd, and Ag, respectively). In particular, on Ag(111) E(a) act is significantly smaller than E(b) and only slighlty larger than E(c) act act. One could naively hope to extend these trends, so as to make the methanol formation the preferred process, by moving further rightwards in the periodic table but the d shell is already filled in Ag and considering Cd does not help. Moreover, the search for the optimal catalyst toward process a cannot be reduced simply to the search for the least-reactive material, as can be inferred from the nonmonotonic trends along the noble-metal group (Figure 1, right panel). Better insight can be gained by analyzing the geometry of the transition states for the three processes under study. We show them in Figure 2a-c in the case of silver substrate, and in Table 1 we compare the corresponding interatomic distances to those in metastable structures. These geometries result to be very similar for the different substrates, thus allowing us to identify simple relations between the activation energies of a given reaction and some easily computable combination of adsorption energies. These “indicators” encode in a simple way the understanding gained on the mechanism of the reaction steps and are powerful

TABLE 1: Relevant Interatomic Distances at the Transition States on Ag(111) Depicted in Figure 2, as Compared to Equilibrium Values for Species Adsorbed or in the Gas Phase

C-H C-O O-H C-Ag O-Ag

a CH4 + O(a) f CH3OH(a) 2.50 Å vs 1.10 Å in CH4 2.44 Å vs 1.44 Å in CH3OH/Ag(111) 0.99 Å vs 0.98 Å in OH/Ag(111) 3.95 Å vs 2.20 Å in CH3/Ag(111) 2.35 Å vs 2.31 Å in OH/Ag(111)

C-H C-Ag H-Ag

b CH4 f CH3(a) + H(a) 1.91 Å vs 1.10 Å in H4 2.34 Å vs 2.20 Å in CH3/Ag(111) 1.78 Å vs 1.67 Å in H/Ag(111) (top)

C-H O-H C-Ag O-Ag

c CH4 + O(a) f CH3(a) + OH(a) 1.41 Å vs 1.10 Å in CH4 1.25 Å vs 0.98 Å in OH/Ag(111) 2.48 Å vs 2.20 Å in CH3/Ag(111) 2.16 Å vs 2.14 Å in O/Ag(111)

tools in the rational search for a better catalyst: not only do they allow us to screen a large set of systems giving a good estimate of the activation energies with modest computational effort (as compared to the full evaluation of the reaction barrier by NEB) but they also clearly indicate which parameters (i.e., adsorption energies) are more important to be optimized. Let us examine individually the three reactions considered and determine for each of them a suitable indicator. We label in the following by EX the potential energy of species X in the most-stable adsorption configuration (more-negative numbers mean stronger bonds). The TS of reaction a closely resembles an adsorbed OH and a CH3 loosely bound to it, see Figure 2a, and Table 1: on Ag(111), the C-H bond is elongated by 1.40 Å, while the O-H distance is only 0.01 Å larger than that in OH. Therefore, one can expect a lower E(a) act for substrates that favor OH binding with respect to O (the initial state of the reaction). Consistent with the previous discussion, this happens usually on less-reactive materials because O forms a stronger bond than OH. We therefore suggest that the difference in binding energies I (a) ) EOH - EO could be used as indicator to estimate E(a) act. We indeed found a good linear relation among the two quantities calculated for the (111) surfaces of Rh, Pd, Ag, Cu, and Au, as reported in Figure 3a. The solid line shown there has unit slope by assumption and intercept fitted to the data:

E(a) act ) EOH - EO + 0.02 ( 0.08 eV

(2)

Reaction b is of “late-TS” kind, which means that the atomic configuration at TS is similar to the final dissociated state: the C-H bond is elongated by 0.81 Å, and the H-Ag distance is only 0.11 Å larger than that for H adsorbed in the top site (we do not compare to H adsorbed in the most-stable hollow site

Methane-to-Methanol Conversion

J. Phys. Chem. C, Vol. 111, No. 45, 2007 17017 TABLE 2: Adsorption Energies of Molecules and Radicals in the Most-Stable Adsorption Site on (111) Surfaces of Selected Transition Metals and Energy Barriers for the Three Reactions Investigated (All Values in eV)

Figure 3. Activation energies of reactions a-c plotted against the indicators proposed in the text. Solid lines indicate the relations given in eqs 2, 3, and 5. The scale of the graphs is the same to show the different variations observed for the three reactions. For completeness, in panel c the relation given by eq 4 is also reported as dashed line.

EO EOH EH ECH3 ECH3OH (a) Eact (b) Eact (c) Eact

Rh

Pd

Ag

Cu

Ag

Au

-4.96 -2.88 -2.80 -1.78 -0.38 2.06 0.73 1.10

-4.20 -2.48 -2.79 -1.66 -0.29 1.67 0.89 1.21

-3.63 -2.57 -2.15 -0.91 -0.16 1.19 2.29 1.06

-4.83 -3.11 -2.51 -1.37 -0.20 1.84 1.72 1.31

-3.63 -2.57 -2.15 -0.91 -0.16 1.19 2.29 1.06

-3.20 -1.80 -2.24 -1.21 -0.15 1.34 1.93 1.23

because the reaction takes place mostly on top of a single substrate atom), see Figure 2b and Table 1. Consequently, it has been proposed30 that the activation energy for this reaction should be correlated to the adsorption energy of isolated CH3 and H, and thus more difficult on later transition metals. Our calculations confirm this analysis and the sum of adsorption energies I (b) ) ECH3 + EH can then be used to estimate E(b) act, as can be seen from Figure 3b and the following fit:

The analysis above enables us to extract the conditions to be satisfied in the search for a suitable catalyst for direct methaneto-methanol conversion; that is, a substrate or reaction site for which methanol formation via process a is the favored one. First, we notice that the analysis of process b can now be discarded because the conditions for a low activation energy for reactions a and b are mutually exclusive. Then, combining eqs 2 and 5, we obtain the condition for which a is the preferred process in terms of the indicator I (a):

E(b) act ) ECH3 + EH + 5.40 ( 0.10 eV

I (a) ) EOH - EO < 1.16 ( 0.17 eV

(3)

As for reaction c, inspection of Figure 2c suggests that this reaction is not of late-TS type nor it is of early-TS type (TS similar to the IS) but somehow intermediate among the two situations. Indeed, one observes that C-H and O-H bonds are similarly stretched with respect to equilibrium CH4 and OH [on Ag(111), respectively, by 0.31 and 0.27 Å]. This makes the definition of an indicator less straightforward than that in the two previous cases. Let us examine the two limiting cases in some detail. Assuming reaction c is late-TS type, one could expect that a good indicator for the activation energy would be the adsorption-energy difference of products and reactants: I (c) ) ECH3 + EOH - EO. A linear fit through the data would then give

E(c) act ) ECH3 + EOH - EO + 0.97 ( 0.12 eV

(4)

reported in Figure 3c as dashed line. The quantity I (c) is very insensitive to the substrate details, varying in a range of only 0.3 eV across the considered substrates, to be compared with the 1-2 eV variation of the other indicators on the same systems. This can be understood qualitatively by the fact that during the process the strong O-metal bond is replaced by two weaker OH- and CH3-metal bonds and cancellation occurs. We expect this feature to be rather general. Equation 4 would then indicate that E(c) act is only weakly sensitive to substrate details. Assuming instead that reaction c is early-TS type, E(c) act would again be almost independent of the substrate, owing to compensations between the similar influence of the substrate on transition and initial states. In conclusion, one expects an almost constant E(c) act in both limiting cases. It is then very reasonable to adopt the average of our calculated energy barriers as a simple estimate for the activation energy of this process

E(c) act ) 1.18 ( 0.09 eV

(5)

reported in Figure 3c as a solid line. Our DFT estimates for the adsorption energies used in the determination of eqs 2, 3, and 5 are reported in Table 2, together with activation barriers.

(6)

This condition, together with the identification of the relevant indicator, is the main result of the present work. 4. Discussion According to eq 6, candidate catalysts for methane-tomethanol direct conversion through process a can therefore be identified as systems having value of I (a) appreciably smaller than a threshold of ∼1 eV. Methanol formation could in principle still be possible after reaction c, directly by CH3 and OH recombination or by morecomplex multistep reactions. However, CH3 can undergo competing reactions such as dehydrogenation to CH2, either on the clean surface or assisted by O or by OH group, or combine with O to form CH3O. Our calculations suggest that on Ag(111) the latter process could be the preferred one, with an activation energy (0.63 eV) much lower than the one for methanol synthesis (1.33 eV). Methoxy, which would then be available on the surface, is one of the key intermediates in methanol decomposition to formaldehyde.33 Therefore, the occurrence of process c could be undesirable because it would reduce selectivity toward CH3OH in favor of CH2O production or complete combustion. The synthesis via CH3-OH recombination is expected to be even less likely on more-reactive substrates, where the activation energy will be generally higher (1.80 eV on [Rh(111)]12) and where CH3 dehydrogenation will also be possible. Let us now understand which factors can influence the value of I (a). A few model Ag structures will be considered for this purpose. Silver is chosen because Ag(111) is the surface with the lowest value of the indicator (I (a) ) 1.06 eV) among the five substrates studied. A first factor that has often been indicated34-36 as having a strong impact on catalytic activity is the availability of undercoordinated reaction sites. Undercoordination is of particular interest here because it has been reported to enhance the activation of methane and to inhibit further methyl dehydrogenation.31,37 A single silver adatom on the Ag(111) surface is then chosen as representative of the limit of extreme undercoordination. Our previous calculations for Rh adatoms on

17018 J. Phys. Chem. C, Vol. 111, No. 45, 2007 Rh(111),12 as compared to clean Rh(111), showed a decrease of I (a) by 0.49 eV and a corresponding decrease of E(a) act by 0.42 eV. No considerable change in I (a) is, however, observed for Ag adatoms on Ag(111) (1.04 eV) with respect to clean Ag(111), showing that undercoordination alone is not sufficient to modify I (a) on silver. Strain in the substrate layer can affect its adsorption properties and reactivity significantly.38,39 This effect has also been investigated here. Upon in-plane expansion/compression of the surface, adsorption of O and OH is indeed stabilized/destabilized by about 0.05 eV per percentual variation in the surface lattice parameter; the variation, however, is very similar for the two species, so there is no net effect on the value of I (a). Coadsorption of competing or promoting species is another classical way to modify surface reactivity. In the present case, the adsorption energy of oxygen on Ag can be modified simply by the presence of additional adsorbed oxygen, that is, by increasing O coverage. In addition to direct O-O electrostatic repulsion, the presence of additional oxygen reduces the electronic charge available on the surface for bonding an O atom. Thus, the adsorption energy of O decreases dramatically when the oxygen concentration on the surface is increased, as already studied in detail for Ag(111).40 The effect on the adsorption energy of OH is smaller because OH bonding involves a smaller charge transfer. Therefore, an increase in O coverage can be effectively used to tune the value of I (a). We investigated this effect by adsorbing additional O atoms in surface hollow sites, corresponding to a total coverage of 0.5 and 1.0 ML. We emphasize here that these structures, with such a large oxygen surface coverage, have not been observed experimentally, but they are useful in the present context because they allow us to explore very small values of I (a). The computed values for the indicator are indeed well below the threshold of 1 eV: I (a) ) 0.62 eV for 0.5 ML oxygen coverage and as low as I (a) ) 0.04 eV in the case of 1.0 ML of oxygen adsorbed on-surface on Ag(111). Full calculation of the activation energies for reactions a and c gives, for the 0.5 ML oxygen coverage (c) structure, E(a) act ) 0.60 eV and Eact ) 0.88 eV, confirming the predictive power of eq 2 and showing that at high enough onsurface oxygen coverage direct conversion would become the preferred process. As for the extreme case of 1.0 ML oxygen coverage, the calculated activation energy for reaction a is E(a) act ) 0.19 eV, again in good agreement with the value predicted on the basis of eq 2. At such a high O coverage, reaction c is not even possible because the substrate lacks a free adsorption site for the methyl group. The results of our calculations are collected in Figure 4 where it can be seen that trends for the activation energy, deduced from substrates whose value of I (a) are in the range of 1-2 eV, can be extrapolated successfully down to I (a) ≈ 0, for which reaction a would be almost barrierless. According to our analysis, high oxygen surface coverage on a weakly reactive substrate like Ag is a system where methaneto-methanol conversion via process a would be favored with respect to competing reactions and where our analysis could be tested by controlled experiments. The O/Ag(111) model configurations studied here are, however, very simple and unrealistic while the O/Ag system is in reality extremely complex,41-45 and a full characterization has not yet been achieved, despite intensive experimental and theoretical efforts motivated by its importance in heterogeneous catalysis for selective oxidation, such as ethylene epoxidation and formaldehyde synthesis from methanol. At high oxygen pressure, in particular, migration of oxygen atoms to subsurface sites is

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Figure 4. Activation energies for (a) methanol formation (circles) and (c) hydrogen transfer to oxygen (squares) as a function of the indicator I (a) ) EOH - EO. Solid lines indicate the estimate of activation energies on the basis of I (a) via eqs 2 and 5, as evaluated from the data for Rh, Pd, Ag, Cu, and Au(111) (marked with full symbols). The corresponding standard deviation is also shown as an error bar. Empty symbols refer to values of I (a) and corresponding Eact for the other systems discussed in the text and not used in the fits.

Figure 5. O/Ag(210) structure considered in the text. In each panel, dotted lines indicate step edges. The arrows point to oxygen atoms in different adsorption sites: (a) at the step-edge, (b) at a subsurface octahedral site, and (c) at an hollow site on (100) nanofacets. The unit cell chosen for the calculations is marked by thin solid lines in panel a.

likely to occur, possibly with the formation of oxide-like structures. Even before that happens, already at a coverage of 0.375 ML the Ag(111) surface undergoes a p(4 × 4) reconstruction whose structure has been for a long time controversial46 and it seems to have been microscopically understood only very recently by studies that invalidate all previous interpretations.47,48 Our calculations on this newly proposed structure show values of the indicator higher than our threshold, I (a) ) 1.25-1.30 eV for the two species of oxygen atoms involved in the surface reconstruction. The characterization of oxygen species in the differently oriented Ag(210) surface is instead rather wellestablished in the literature.45,49-51 Three different oxygen adsorption sites have been identified at high coverage by surface vibrational spectroscopy:49,51 subsurface octahedral, step-edge (Ag-O chain), and hollow site on (100) nanofacets (see Figure 5). Our calculations for this surface indicate the last one as the less-stable one, in agreement with the observation that it is the first to desorb.49 The corresponding value of I (a) is 0.85 eV, (c) thus suggesting E(a) act < Eact for this surface. This has indeed been confirmed by the direct evaluation of the activation energies, yielding E(a) act ) 0.78 eV, in agreement with eq 2, and E(c) act ) 1.36 eV. This result further supports our conclusion that silver surfaces, under particular conditions of high oxygen

Methane-to-Methanol Conversion coverage, can be interesting systems for investigating the feasibility of direct methane-to-methanol synthesis via process a. Besides the synthesis of the desired chemicals, a good catalyst should also allow for their desorption before further reactions occur. In the present case, methanol decomposition should be prevented. From this point of view, large oxygen coverages would have the counter effect to ease hydrogen extraction,52 otherwise difficult on weakly reactive substrates. As an example, methanol is quite stable on clean Ag(111) (dehydrogenation and decomposition to CH3 and OH are endothermic by 0.81 and 0.99 eV, respectively), but on the O-dosed surface oxygenassisted dehydrogenation to CH3O and OH is possible, with an energy barrier of only 0.21 eV. Subsequent O-assisted dehydrogenation of methoxy to formaldehyde is almost barrierless and exothermic by 1.44 eV. This contributes to understand why it has not yet been possible to find a suitable metal catalyst for the partial oxidation of methane to methanol. The increase of oxygen coverage is, however, only one of the possible ways to tune the indicator proposed for the synthesis of methanol via reaction a. The analysis given in this paper may guide the search for other possibilities to optimize the synthesis process, which should then be tested against the decomposition one. Coadsorption of other strongly electronegative species could be a valid candidate. 5. Conclusions The relevant parameters influencing the reactivity for processes a-c have been identified from the analysis of several transition-metal substrates. In particular, the gained insight allows us to identify the indicator I (a) ) EOH - EO that rules catalyst selectivity for direct conversion of methane to methanol: for I (a) lower than ∼1 eV, process a is predicted to be favored with respect to processes b and c. Not all factors influencing adsorption energies are effective in tuning I (a). We demonstrated that increasing oxygen coverage can reduce the indicator significantly and favor the formation of methanol with respect to competing processes. This conclusion was drawn by the study of simplified model substrates, but is also supported by our theoretical results for a realistic Ag(210) structure. Novel catalysts for the direct methane-to-methanol conversion could then be searched taking into consideration the following guidelines: (i) oxygen has to adsorb dissociatively at catalyst’s surface but (ii) the substrate should not be too reactive, in order to avoid C-H bond cleavage, and (iii) weakly bound oxygen atoms, compared to OH adsorption, should be present at the surface. We hope that the results presented in this work may stimulate further experimental and theoretical research on methane-tomethanol direct conversion via heterogeneous catalysis. Acknowledgment. We thank Nicola Bonini, Anton Kokalj, Stefano Baroni, Renzo Rosei, and Carlo Sbraccia for stimulating discussions. This work has been supported in part by the Italian MIUR through PRIN. Calculations were performed at the CINECA computing center, also thanks to INFM computing grants. References and Notes (1) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44, 2636. (2) Sheldon, R. A. Chemicals from Synthesis Gas: Catalytic Reactions of CO and H2; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1983. (3) Gesser, H. D.; Hunter, N. R. In Methane ConVersion by OxidatiVe Processes; Wolf, E. E., Ed.; Van Nostrand Reinhold: New York, 1992; Chapter 12, pp 403-425.

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