Essential Role of Water in the Autocatalysis Behavior of Methanol Synthesis from CO₂ Hydrogenation on Cu: A Combined DFT and Microkinetic Modeling Study

Abstract

Water is able to promote many chemical reactions in an autocatalysis manner, and the essential role that water plays in the system is still worth discussing. In the process of methanol synthesis from CO2 hydrogenation on Cu, whether the promoting species is molecular water or water derived O/OH is controversial. To systematically understand the influence of the presence of O/OH on the reaction kinetics of CO2 hydrogenation to methanol, we here carry out density functional theory calculations to obtain the energetics over O/OH preadsorbed Cu(211) and further use them for microkinetic modeling in order to calculate the formation rate of methanol. The calculation results show that the free energy barriers of CO2 activation by molecular water through both HCOO and COOH routes are higher than those by the hydrogen atom on clean and OH or O preadsorbed Cu(211). The subsequent microkinetic modeling indicates that the formation rate of methanol over Cu(211) is improved in the presence of O/OH. Detailed analyses on the coverage and degree of rate control of surface species reveal that the presence of O/OH on the catalyst surface will destabilize the spectating formate and lower the energies of rate-controlling transition states. The formate coverage effect is further included in the microkinetic modeling, and we find that the reaction rate is further increased at lower temperatures. Our current work provides evidence that the surface adsorbed O and OH are able to promote the formation of methanol from CO2 hydrogenation and, more importantly, highlights the fact that the activity of methanol formation is sensitive to the surface adsorbates.

Surface Chemistry of Carbon Dioxide on Copper Model Catalysts Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy

“…A series of control experiments on Cu(997) indicates that the CO 2 hydrogenation to formate is promoted by the presence of water. This is consistent with recent studies on the effect of water in the CO 2 activation [82,83].…”

Surface reaction kinetics of the methanol synthesis and the water gas shift reaction on Cu/ZnO/Al2O3

“…The (211) facets were chosen, as it was found through experiments and theoretical calculations that they are the most active surfaces for the methanol synthesis. 18,22,24,30,31 Since surface defects scale linearly with the overall observed activity, the reactivity of other facets (e.g. 111) can be neglected.…”

“…In eqn (21) and (22) there is no term multiplying T•ln(T) for ΔG 0 m,DFT , like there is in eqn (18) and (19) for ΔG 0 m,3p . Each ΔG 0 m,DFT needs to be modified with the addition of parameter β multiplying T•ln(T), so that the equations are able to match.…”

“…Microkinetic modeling takes into account the chemistry behind the process, 17 being useful for better understanding of the system, for process optimization and for catalyst development. Based on data derived from first principles density functional theory (DFT) during the last decade, detailed microkinetic models have been proposed for methanol synthesis, [18][19][20][21][22][23] in which different surface reaction paths are considered and all reactions are potentially rate limiting. Grabow and Mavrikakis 18 proposed a mechanism for CO/CO 2 hydrogenation and the WGSR on Cu (111).…”

“…Park et al 21 proposed a mechanism on Cu (211), in which adsorption and activation energies were taken from DFTderived data, and pre-exponential factors were fitted to experimental data. Xu et al 22 proposed a mechanism on Cu (211) and compared three situations: clean surface, preadsorbed O*, and preadsorbed OH*. The conclusion was that the preadsorbed species create faster reaction paths for the hydrogenation of formate.…”