Thermochemistry and micro-kinetic analysis of methanol synthesis on ZnO (0 0 0 1)
Graphical abstract
Highlights
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Most stable surface phase depends on carbon chemical potential.
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Formaldehyde or methoxy hydrogenation is rate-limiting step.
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Carbon dioxide poisoning of ZnO catalysts is due to formate coverage.
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Micro-kinetic model predicts high temperature and pressure activity.
Introduction
The catalytic synthesis of methanol from synthesis gas is one of the most important industrial chemical processes with a worldwide production capacity of around 40 million tons per year [1]. Currently, methanol is used primarily as a starting material or solvent for chemical synthesis, although increasingly, it is employed as a fuel [1], [2], [3]. The diversity of compounds that can be synthesized from methanol along with its promise as an energy carrier makes the development of more efficient and sustainable production schemes a priority for the chemical industry of the future [3].
Methanol was originally produced from synthesis gas using a zinc oxide (ZnO) catalyst discovered in the 1920s. The catalyst required high temperatures (600–700 K) and pressures (200–300 bar) to operate industrially [1], [2], [4]. The high pressure methanol process was rendered obsolete in the 1960s with the development of a lower pressure process based on a Cu–ZnO catalyst [2], [4], [5]. The active phase of this Cu–ZnO system has been the subject of much debate [1], [5], [6], [7], [8], [9], [10], though several investigations have shown strong evidence that the active site is on Cu and that the reaction is structure sensitive [10], [11], [12], [13].
A detailed understanding of the methanol synthesis reaction is hindered by several fundamental obstacles. The industrial process is carried out at very high pressures which are required to drive the reaction thermodynamics. This presents a challenge in extrapolation of the results of ultra-high vacuum (UHV) studies to relevant conditions. In addition, the elementary steps in methanol synthesis form a complex network due to the possibility of hydrogenating carbon monoxide (CO) or carbon dioxide (CO2), along with the water–gas shift reaction, which couples the two pathways together. This makes it challenging to elucidate the reaction mechanism, although convincing evidence has been shown for CO hydrogenation on ZnO [14] and for CO2 hydrogenation on Cu–ZnO [1], [5], [15]. Furthermore, the industrial Cu–ZnO catalyst exhibits a highly heterogeneous morphology and the activity is dependent on preparation procedures [5], [10]. For this reason, it can be difficult to glean understanding from the simplified model systems commonly used in experiment and theory.
In this work, we seek to make progress toward understanding methanol synthesis by investigating the reaction on the ZnO catalyst. The thermochemistry of oxygen and CO adsorption on several ZnO faces is examined, confirming the polar surfaces as the most likely active facets. The interaction of methanol synthesis intermediates with the Zn-terminated ZnO (0 0 0 1) surface is examined in detail using density functional theory (DFT). These results are used to construct a surface phase diagram as a function of CO, H2O and H2 chemical potentials. The phase diagram indicates that surface terminations including formate and methoxy are highly stable even at low CO pressures, revealing the importance of considering a carbon reservoir. The energetics are also used in a mean-field micro-kinetic model which includes CO and CO2 hydrogenation pathways as well as the water–gas shift reaction. This model is employed in order to examine the temperature and pressure dependence of the methanol synthesis rate on ZnO. The results indicate that this model is capable of describing the experimental trends in the operating conditions of the ZnO catalyst and suggest site poisoning by formate (HCOO) as the reason that high temperatures, high total pressures, and low CO2 concentrations are required.
Section snippets
Density functional theory
All electronic structure calculations are carried out using the grid-based projector augmented wave code GPAW [16]. Exchange–correlation energies are treated using both the RPBE [17] and BEEF-vdW [18] functionals. All calculations are preformed on a finite-difference grid, and the Brillouin zone is sampled by a Monkhorst–Pack k-point mesh [19]. Several surface facets and system sizes are investigated, and the values of the grid spacing and k-point density required for convergence varied; all
ZnO bulk and surface facets
Bulk ZnO is most stable in a hexagonal wurtzite structure. The lattice constants are determined via minimization of the energy using a downhill simplex algorithm [36]. The values are found to be a = 3.33 Å, c = 5.32 Å (a = 3.35 Å, c = 5.32 Å) for the BEEF-vdW (RPBE) functionals. These are within 3% of the experimental values (a = 3.25 Å, c = 5.21 Å) [37]. The enthalpy of formation (converted to the standard NIST references via experimental values) of ZnO is computed to be −3.42 eV (−sss3.06 eV) with BEEF-vdW
Conclusions
The binding energies of CO and oxygen are used to probe the reactivity of various ZnO surface facets. It is shown that the non-polar and surfaces interact very weakly with adsorbed oxygen, while the polar zinc-terminated (0 0 0 1) surface is considerably more reactive. Interestingly the CO binding energy is comparable for both surfaces despite the large difference in oxygen reactivity. Since methoxy and most intermediates in the CO2 hydrogenation pathway bind through oxygen, the
Acknowledgements
A.J.M. was supported by the National Science Foundation in cooperation with the Danish National Research Foundation through the Nordic Research Opportunity and Graduate Research Fellowship Program (Grant No. DGE-1147470). Computational resources were provided by Haldor Topsøe A/S (A.J.M., J.S., P.G.M.), the Danish Center for Scientific Computing (A.J.M., J.R., I.C.) and the U.S. Department of Energy, Office of Basic Energy Sciences (A.J.M., F.S., J.K.N.).
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