Density Functional Theory Investigation of the Role of Cocatalytic

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Kinetics, Catalysis, and Reaction Engineering

Density Functional Theory Investigation of the Role of Cocatalytic Water in Methane Steam Reforming over Anatase TiO (101) 2

Alec Hook, Timothy P. Nuber, and Fuat E. Celik Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00944 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Industrial & Engineering Chemistry Research

Density Functional Theory Investigation of the Role of Cocatalytic Water in Methane Steam Reforming over Anatase TiO2 (101) Alec Hook,1 Timothy P. Nuber,2 Fuat E. Celik1,* 1

Department of Chemical and Biochemical Engineering

2

Department of Mechanical and Aerospace Engineering Rutgers, The State University of New Jersey 98 Brett Road, Piscataway, NJ 08854, USA

* Corresponding author: Tel. +1 848 445 5558 E-mail address: [email protected] (Fuat E. Celik)

1 ACS Paragon Plus Environment

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Abstract The methane steam reforming (MSR) mechanism on the anatase TiO2 (101) surface has been investigated using periodic density functional theory. Reaction energies for several pathways were calculated and pathways involving high energy intermediates were eliminated from consideration. The remaining pathways all involved formaldehyde and formyl as intermediates. Activation energy barriers were calculated for these, revealing that C-H activation in methane and elementary dehydrogenation reactions in general involved very high energy transition states when calculated on clean surfaces. When water was coadsorbed on reaction surfaces as chemisorbed OH and H, hydrogen transfer reactions were facilitated by the proximity between hydrogen donor and acceptor and the strong affinity of hydroxyl for hydrogen. As the product of such hydrogen transfer reactions was physisorbed water, water activation completes the catalytic cycle and the role of water can be termed cocatalytic. Cocatalytic water lowered the most difficult dehydrogenation barriers by 2-3 eV, including reducing the methane activation barrier from 4.96 eV to 2.95 eV, making activity on the TiO2 surface relevant at high MSR reaction temperatures. Due to the high barrier for CHO dehydrogenation, the water-gas shift reaction is expected to make CO2 the preferred product, rather than CO, during MSR over metalfree TiO2.


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1. Introduction Hydrogen gas has potential as a clean-burning energy carrier that produces only water as a byproduct of combustion. Hydrogen fuel, when used in a fuel cell for stationary or transportation power, can provide higher thermodynamic efficiency than conventional internal combustion. However, elemental hydrogen is not found on earth, and must be chemically separated from one of its compounds such as water or methane. Commercially, methane steam reforming (MSR) is used to convert fossil methane to hydrogen gas and carbon monoxide (as syngas) at high temperature, providing over half of global hydrogen production.1-3 The reaction is endergonic and endothermic, requiring high energy input at high temperature.4-6 Together with MSR, the water-gas shift (WGS) reaction takes places, where carbon monoxide reacts with water to form carbon dioxide and hydrogen. CH4 + H2O → CO + 3 H2

CO + H2O → CO2 + H2

Many transition metal catalysts have been investigated for the methane steam reforming reaction, including Rh, Ru, Ni, Ir, Pt and Pd. The steam reforming mechanism on metals is suggested to proceed via successive dehydrogenation of CH4 all the way to atomic carbon before undergoing oxidation. The mechanism of MSR has been investigated both experimentally and computationally using density functional theory (DFT). The initial C-H activation in methane is often found to be the rate-determining step at higher temperatures,7-12 while at lower temperature the oxidation of carbon to CO was thought to be limiting.13 More recent computational modeling has considered a wider range of possible pathways, involving several oxygenated intermediates formed by reaction of CHx fragments with surface O and OH groups, revealing that atomic carbon may not necessarily be involved in the reaction pathway. On Pt(111), hydroxymethylene and formyl where found to be energetically favorable intermediates between carbon and carbon



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monoxide,14 and hydroxymethylidyne played a similar role on Pd(111) and Pd(100).15 Reactive oxygen species on palladium oxides have been implicated in the formation of methoxyl on PdO(111) surfaces,16,17 and calculations over PdO(100)18 and PdO(101)19 have evoked formaldehyde as an intermediate due to the large barriers for CHx dehydrogenation. Formyl was suggested in the mechanism of methane oxidation over both PdO(100) and CeO2(111) as a low energy intermediate and precursor to CO, rather than atomic carbon.20 Typically, alumina is used as the support for metal nanoparticles in MSR, but several advantages have been suggested for using TiO2 as a support instead, including lower coke formation, wider range of feasible steam to carbon ratios, and lower reaction temperature for supported Ni particles.21-23 The Ni/TiO2 system has been investigated for reforming of alcohols as well, including methanol,24 ethanol,25 and glycerol,26 where the interaction of the metal with the support has been implicated in the determining catalyst stability. The application of TiO2 and other semiconductors in methane steam reforming is potentially interesting as a photochemical alternative to the high-temperature thermocatalytic route in current industrial practice. Such a process, if it could lower the high-temperature requirement of MSR, could potentially double the atom efficiency by reducing or eliminating the need for fuel burn for heat generation, reducing carbon dioxide emissions and improving resource sufficiency. The Yoshida group has been investigating supported precious metals such as Pt and Rh on TiO2,27 titanates,28,29 tantalates,30,31, and gallium oxide,32 for photocatalytic MSR under mild conditions. In the reaction of water and methane over Pt/TiO2, CO2 was the primary carbonaceous product, with trace amounts of CO and ethane observed, and strong evidence for the formation of surface formates. Recent experimental work in our own group has found that


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metal-free anatase is active for MSR at 973 K under photocatalytic (UV) and thermocatalytic (dark) reaction conditions, yielding H2 and CO2 selectively. The WGS reaction has also been investigated both experimentally and computationally over supported metal nanoparticles, 33-41 where reducible metal oxides such as TiO2 have been implicated in the dissociation of water.42-44 The structure and behavior of water molecules on TiO2 surfaces has been probed with DFT methods to give insight into binding geometries and energies.45-48 In our previous work,49 we investigated WGS mechanisms on the (101) surface of anatase TiO2 with density functional theory calculations. The associative pathway with carboxyl as the intermediate was favored on the clean TiO2 surface. At high water coverage, adsorbed water molecules would take a cocatalytic role in hydrogen transfer steps by acting as a hydrogen acceptor or donor, reducing the activation energy barriers of such steps. This meant that the associative formate pathway was competitive with the carboxyl pathway when the cocatalytic role of water was taken into consideration. Formation of OH radicals was predicted to be restricted to photocatalytic reaction conditions due to the large formation energies, consistent with experimental reports.50-52 In the present work, DFT is applied to the MSR reaction on anatase TiO2 (101). A series of potential reaction intermediates formed from dehydrogenation, O addition, and OH addition reactions with methane and its derivatives is considered. Using binding energies and structures for these intermediates, potential energy surfaces for multiple reaction pathways are generated. The most energetically unfavorable pathways are excluded from further consideration. The remaining pathways all involve formaldehyde dehydrogenation to formyl as an intermediate, via either methoxyl or methylene intermediates. Activation energy barriers for these remaining pathways are calculated and compared. The most favorable pathways involve the formation of



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formaldehyde and formyl as surface intermediates. Most C-H cleavage reaction steps possess high activation energy barriers. In this context, dissociatively adsorbed water can assist in dehydrogenation steps by acting as a hydrogen acceptor, similar to the effect seen in WGS. This cocatalytic effect of chemisorbed water lowered these barriers by up to 3 eV. The water-assisted dehydrogenation mechanisms are therefore essential in the description of the activity of anatase (101) for MSR, and help interpret the metal-free activity found in high-temperature MSR experiments. 2. Methods Periodic DFT calculations were performed using the VASP code53,54 within the generalized gradient approximation (GGA-PBE)55 using projector-augmented wave (PAW)56,57 potentials. Corrections to the on-site Coulomb interactions have been accounted for through the Hubbard term (DFT+U).58 Dudarev’s approach59 was applied to increase the accuracy of the band gap and reaction kinetics with a Ueff = 4 for titanium atoms. This Ueff was chosen to align both the band gap and lattice parameters with experimental values as well as prevent electron delocalization.60–62 The single-electron wave functions were expanded using plane waves with an energy cutoff of 400 eV. All oxide slabs were based on the (101) surface of anatase TiO2, and modeled by a (2 × 2) surface unit cell with four (101) layers for a total of 32 Ti atoms and 64 O atoms. The lattice constants used for anatase TiO2 were a = 3.784 Å and c = 9.515 Å.63 A vacuum layer of 24 Å was used to separate periodic images of the slab in the z direction (normal to the surface), and a dipole correction was applied and the electrostatic potential was adjusted to ensure that interaction between the surface slab and its periodic images was negligible.64 The Brillouin zone was sampled using a (2 × 2 × 1) Gamma-centered Monkhorst-Pack k-point mesh65 following a


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convergence test for adsorbate binding energies with respect to sampling mesh size. All four layers were allowed to relax in all calculations. Binding energies converged with respect to number of crystallographic layers at slab thicknesses of four (101) layers. Calculated differences between four layers (32 Ti atoms) and five layers (40 Ti atoms) were 0.7 eV. Each of these reactions’ transition states were high in energy due in part to the large distance between active sites on the anatase (101) surface. Undercoordinated oxygen atoms, which were the preferred binding sites for many adsorbates including hydrogen atoms, are 3.8 Å apart. Any adsorbates that bind to these sites through the carbon atom must place carbon and hydrogen on these two distant sites in the final state, or else place the hydrogen in an energetically unfavorable binding site as in Figure 6. The transition state must therefore span a large physical distance without any stabilization from nearby surface atoms. In the case of hydrogen transfer between formyl and carbon monoxide, the hydrogen atom in this transition state was found to have a gas-like character, with no interaction with the surface.49 Another way to describe this effect is that when both products from the C-H bond scission prefer the same site and site density is low, the competition for binding sites leads to long migration distances for the smaller daughter fragment – the H atom – and consequently large barriers. Not all dehydrogenation reactions were equally difficult. C-H bond scission in formaldehyde was only 1.62 eV. The reaction energy of only 0.10 eV was also significantly less



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than with the previous four examples. This was because CH2O and CHO are the only adsorbates where both the reactant and the product interact with the surface via C and O atoms, and as such the transition state is somewhat stabilized relative to other C-H bond scission reactions. The O-H bond cleavage barrier in CH2OH was facile in contrast to all C-H cleavage reactions, with a barrier of only 0.48 eV. This is readily explained by the preferred binding structure of hydroxymethyl in Figure 4. The oxygen atom is trivalent in this structure, with the dissociating hydrogen atom dissociate pointing towards the hydrogen acceptor site, and an undercoordinated surface oxygen atom only 1.82 Å away. Unlike hydrogen atom transfer reactions including all the dehydrogenation reactions above, O and OH addition reactions are not expected to benefit from cocatalytic water, and use water as a reactant instead. This means that the barriers for methyl oxidation to methoxyl, methylene oxidation to formaldehyde, and methyl hydroxylation to hydroxymethyl are unchanged in pathways where cocatalytic water is considered. All three of these reactions had small reaction energies of +0.11 eV, -0.02 eV, and -0.21 eV respectively, and used chemisorbed water as a reactant. In methyl and methylene oxidation reactions, the OH group in chemisorbed water acts as the oxidant, transferring an O atom to the CHx species and transferring H to a nearby lattice oxygen atom. The barriers for these reactions were similar, 2.06 eV and 2.00 eV respectively. These large barriers arise due to the concerted cleavage of the O-H bond while forming the C-O bond and a new O-H bond in the transition states. Attempts to find step-wise mechanisms were unsuccessful. Taking methylene oxidation as an example, the hydrogen atom from chemisorbed water transferred to the surface oxygen atom already bonded to one H atom. The product of this elementary step is formaldehyde and a water molecule formed from a lattice oxygen atom,


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leaving an oxygen vacancy. This reaction therefore pays the same >1 eV energy penalty for forming the vacancy by placing two H atoms on the same oxygen atom. This energy is then recovered as the two hydrogen atoms diffuse away from each other on the surface, restoring the oxygen atom to its lattice position. Therefore, the overall 2.00 eV barrier for this step includes +1.18 eV to form the formaldehyde/water/VO state and the overall reaction energy of -0.02 eV includes the -1.20 eV recovered by healing the vacancy. The reaction of water with oxygen vacancies is discussed in more detail in reference (49). Table 3. Reaction energies and activation energy barriers in eV for elementary steps in MSR. Reaction ∆Ε Ea (2) CH4 CH3 + H +0.97 4.96 (2’) a CH4 + H2O(c) CH3 + H+ H2O(p) +0.75 2.95 (4) c CH3 + H2O(c) CH3O + 2H +0.11 2.06 (5) CH3 CH2 + H +0.94 4.55 (5’) a CH3 + H2O(c) CH2 + H+ H2O(p) +0.72 1.68 (8) d CH3O + H CH2O + 2H +0.81 4.33 (8’) a,d CH3O + H + H2O(c) CH2O + 2H + H2O(p) +0.60 1.69 (9) CH2OH CH2O + H +0.19 0.48 (9’) a CH2OH + H2O(c) CH2O + H + H2O(p) -0.03 4.3 eV barriers for many dehydrogenation steps, especially the initial methane activation barrier to form surface methyl (4.96 eV), essentially making all MSR pathways equally unlikely. However, the addition of chemisorbed water to the anatase surface revealed the cocatalytic role of water in reducing the barriers of difficult dehydrogenation steps. The ability of the OH group to act as a hydrogen acceptor and form physisorbed water stabilized the most difficult dehydrogenation transition states by >2 eV. This led to much more feasible apparent activation energy barriers of 2.2-2.6 eV relative to gaseous methane and steam as reactants. Though differences were small, the methoxyl pathway was slightly preferred over the methylene pathways to form formaldehyde. Even elementary reactions without hydrogen transfer benefitted from the presence of chemisorbed water. Direct oxidation and hydroxyl mechanisms using chemisorbed water as a reactant were thermodynamically favored over reactions involving removing lattice oxygen to form oxygen vacancies. These results further support the proposed active role of adsorbed water in catalyzing reactions on TiO2 surfaces. The cocatalytic activity of water has the potential to dramatically alter potential energy landscapes on anatase surfaces, changing the expected surface species and


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their concentrations under reaction conditions. In particular, elementary reactions of methane, methanol, and formaldehyde derivatives that previously had prohibitively large kinetic barriers on the clean anatase surface appear to become feasible in the presence of adsorbed water.

Supporting Information Table S1: Initial, final, and transition state structures for elementary steps.

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Density Functional Theory Investigation of the Role of Cocatalytic

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