Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Density Functional Theory Investigation of the Role of Cocatalytic Water in the Water Gas Shift Reaction over Anatase TiO2 (101) Alec Hook and Fuat E. Celik* Department of Chemical and Biochemical Engineering Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States S Supporting Information *
ABSTRACT: The water gas shift reaction (WGS) mechanism on the anatase TiO2 (101) surface has been investigated using periodic density functional theory. Reaction energies and activation energy barriers for direct CO oxidation and associative pathways with carboxyl and formate intermediates were calculated and compared. Unassisted by water, the formation of formate was infeasible due to the difficulty in transferring hydrogen from the anatase surface to hydrogenate CO, while the carboxyl mechanism had the lowest apparent activation energy barrier among the three. Coadsorbed water species were found to be effective hydrogen transfer cocatalysts, with molecularly physisorbed water acting as a hydrogen donor and dissociatively chemisorbed water acting as a hydrogen acceptor. Using water as the hydrogen donor, CO hydrogenation became competitive with the carboxyl mechanism, indicating that the reactivity of water could change the relative competitiveness of different WGS pathways. These results suggest that adsorbed water concentration plays an important role in determining adsorbate speciation and concentration on the anatase surface during WGS.
1. INTRODUCTION Clean energy production and conversion are among the biggest challenges of the 21st century. These include effective utilization of energy resources, as well as clean end-use applications that limit emissions of CO2 and other pollutants. Hydrogen gas is a promising clean-burning energy carrier as water is the only byproduct of its consumption. Steam reforming of methane is the primary method for industrial hydrogen production today, producing over 50% of the world’s hydrogen,1−3 and coproducing carbon monoxide. Chemical synthesis with hydrogen such as Fischer−Tropsch synthesis utilizes the water gas shift reaction (WGS) reaction to adjust the critical ratio of hydrogen gas to carbon monoxide.4 Primary fuel uses of hydrogen gas such as fuel cells heavily depend on the WGS reaction to increase hydrogen yield and remove carbon monoxide from the hydrogen stream, as many hydrogen fuel cell catalysts are easily poisoned by carbon monoxide.5
An important operating variable in the WGS reaction is the water/carbon monoxide feed ratio, as it greatly impacts both conversion and the extent of coke formation. Higher water content in the feed increases conversion and decreases coking.25 The effect of water concentration on surface chemistry is complex. For industrial Cu/ZnO catalysts, nonempirical rate laws are available26,27 for low water/carbon monoxide ratios, but these elementary rate laws did not fit behavior at higher water/CO ratios, and empirical power-law equations are needed for accurate rate prediction.28 One reason for the large concentration effect is likely due to the site competition between water and WGS intermediates.29 The surface water concentration therefore seems to be able to affect the dominant WGS mechanism. Several WGS reaction pathways have been proposed including a simple redox reaction,30 and pathways involving carboxyl (CO2H) or formate (HCOO) intermediates.31,32 On Pt33 and Cu,34 carboxyl appears to be the key WGS intermediate. For both of these pathways, however, the activation and chemisorption of water has been shown to be kinetically important.35 The common TiO2 polymorphs are anatase and rutile. Anatase is preferred in many catalytic applications due to smaller particle sizes leading to larger specific surface areas. For photocatalytic purposes, it is tempting to suggest that rutile, the polymorph with the smaller band gap energy at 3.03 eV,36
CO + H 2O ↔ CO2 + H 2
Many catalysts have been studied for the WGS reaction, most of them consisting of metal particles supported on metal oxides.6−14 Typically the role of the metal is to bind carbon monoxide,6,15,16 while the reducible metal oxide, such as titanium dioxide or cerium oxide, binds and disassociates the water.17−19 Effective catalysts rely on cooperativity between the metal and metal oxide to catalyze the reaction.20,21 Pt-based catalysts have attracted interest due to high CO conversion14 compared to other transition metals, and there is evidence of an advantageous metal−support interaction with Pt, especially with TiO2.22−24 © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
February April 15, April 26, April 26,
2, 2018 2018 2018 2018 DOI: 10.1021/acs.iecr.8b00532 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
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.61 A vacuum layer of 24 Å was used to separate periodic images of the slab in the z direction (normal to the surface), 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.62 The Brillouin zone was sampled using a (2 × 2 × 1) Gammacentered Monkhorst−Pack k-point mesh63 following a convergence test for adsorbate binding energies with respect to sampling mesh size. All four layers were allowed to relax in all calculations. Binding energies and oxygen vacancy formation 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
