(143c) The Proximity Effects and the Role of Molecular Shuttles in Water-Gas Shift and Methanol Synthesis
WGS turnover rates, defined as the rates per exposed Cu sites, remained the same (523 K; 3.3 kPa CO; 10 kPa H2O) when Cu/ZnO/Al2O3 catalysts were physically mixed with the same ZnO/Al2O3 oxide to form pellets. The rates, in contrast, monotonically decreased with the extent of dilution with SiO2. Such trends implicate that the separation of the two functions with SiO2 causes the concentration gradient of the reactive intermediate (e.g., HCOOH) within the pellet, leading to lower WGS rates when they were limited by the metal function that dehydrogenates HCOOH to form CO2 and H2. More detailed assessments of WGS reactions via bifunctional catalysis require in-depth inquiries into each function that forms and uses reactive HCOOH intermediates and the transport of HCOOH between the two functions.
HCOOH formation elementary steps from CO and H2O on oxides and the magnitude of kinetic and thermodynamic parameters are assessed from reverse HCOOH dehydration on model oxides, anatase and rutile TiO2 (TiO2(a), TiO2(r)), at temperatures and reactant (and HCOOH) pressures relevant to WGS and methanol synthesis; the forward reaction is difficult to be assessed due to the detection limits of HCOOH. HCOOH dehydration rates on TiO2(a) and TiO2(r) increased monotonically with HCOOH pressure (0.1-0.5 kPa; 533 K), indicating the presence of HCOOH-derived species as minor surface species. The rates were independent of CO and CO2 pressures, while inhibited by H2O due to its competitive adsorption on the active Ti5c-O2c centers. The weak kinetic isotopic effects of HCOOD and DCOOD implicate that the rates are limited by the HCOOH adsorption elementary step, which was confirmed from density functional theory calculations. The measured free energy barriers were similar for TiO2(a) and TiO2(r) catalysts (ÎG533KÇ = 132 vs. 130 kJ mol-1) despite of their very different DFT-derived HCOOH binding energies (to form bidentate formate, ÎG533K = -101 vs -148 kJ mol-1). These trends result from the kinetically-relevant transition states that occur very early in the adsorption reaction coordinate and thus do not sense the stability of the adsorption product, bidentate formates. Such mechanistic interpretations, in turn, give the rate equation and parameters for reverse CO hydration reaction to form HCOOH, which are used to derive the prevalent HCOOH concentrations generated on TiO2 oxide during WGS and methanol synthesis.
HCOOH formed on oxides can transport to the proximate metal function (Cu) to form WGS products (CO2 and H2). HCOOH dehydrogenation rates on Cu increased monotonically with HCOOH pressures (<0.5 kPa; 503 K), suggestive of the unoccupied Cu sites as the most abundant surface species. The rates decreased with increasing CO pressure (0-10 kPa; 503 K) due to its competitive adsorption on active Cu sites but were unaffected by H2O (0-3 kPa; 503 K). At low acid coverages, the reaction is mediated by carboxylate intermediates, instead of bidentate formates, consistent with the insensitiveness of rates to H2 pressure (0-30 kPa; 0.1 kPa HCOOH; 503 K). The turnover rates are limited by the HCOOH adsorption step that forms carboxylate, inferred by the strong kinetic isotopic effects with DCOOH but not with HCOOD.
These mechanistic interpretations of HCOOH dehydration and dehydrogenation reactions on oxide and metal functions are combined with the transport model to explain the WGS kinetics on catalytic systems with different inter-function distances and at the kinetic regime, limited by either oxide or metal function. The results of this work uncover the origin of proximity effects in WGS and methanol synthesis, the requirement imposed by a short-living reactive intermediate. In more general prospective, this work provides the analytic framework that can be used to understand many other cascade reactions via reactive molecular carriers that are ubiquitous in heterogeneous catalysis.
The authors acknowledge the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC05-76RL0-1830) for financial support. This work was accomplished by using computational resources at the Environmental Molecular Science Laboratory (EMSL), the National Energy Research Scientific Computing Center (NERSC), and the Extreme Science and Engineering Discovery Environment (XSEDE).
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