Significance of Surface Formate Coverage on the Reaction Kinetics of Methanol Synthesis from CO₂ Hydrogenation over Cu

Abstract

The hydrogenation of CO 2 to methanol over copper-based catalysts has attracted considerable attention recently. Among all the proposed reaction mechanisms, a large number of experimental and theoretical studies have focused on the one that includes a HCOO intermediate due to the fact that high coverages of formate over catalyst surfaces were observed experimentally. To systematically understand the influence of formate species coverage on the reaction kinetics of methanol synthesis, the energetics of the CO 2 hydrogenation pathway over clean and one-or two-formate preadsorbed Cu(211) are obtained using density functional theory calculations, and these energetics are further employed for microkinetic modeling. We find that the adsorption energies of the intermediates and transition states involved in the reaction pathway are changed in the presence of spectating formate species, and consequently, the potential energy diagrams are varied. Microkinetic analysis shows that the turnover frequencies (TOFs) over different formate preadsorbed surfaces vary under the same reaction condition. In particular, the reaction rates obtained over clean Cu(211) are generally the lowest, while those over one-or twoformate preadsorbed surfaces depend on the reaction temperatures and pressures. Meanwhile, we find that only when the formate coverage effect is considered, some of the TOFs obtained from microkinetic modeling are in fair agreement with previous experimental results under similar conditions. After the degree of rate control analysis, it is found that the combination of HCOO and HCOOH hydrogenation steps can be treated as the "effective rate-determining step", which can be written as HCOO* + 2H* → H 2 COOH* + 2*. Therefore, the formation of methanol is mainly controlled by the surface coverage of formate and hydrogen at the steady state, as well as the free energy barriers of the effective rate-determining step, i.e., effective free energy barriers.

Major Advances and Challenges in Heterogeneous Catalysis for Environmental Applications: A Review

“…The mechanism of this process was researched by Wu and Yang [185], who found that the combination of formate and formic acid hydrogenation steps can be treated as the "effective rate-determining step". Therefore, the formation of methanol is mainly controlled by the surface coverage of formate and hydrogen at a steady state, as well as by the effective free energy barriers.…”

Unravelling the mechanisms of CO2 hydrogenation to methanol on Cu-based catalysts using first-principles multiscale modelling and experiments

“…For catalytic testing, a GHSV of 6000 h −1 for a mixture of CO 2 and H 2 (CO 2 : H 2 = 1 : 3) was used. The reactor pressure (20,30,40,50 bar for different runs) and temperature (200, 220, 240, 260, 280, 300°C for different runs) were kept constant for each run.…”

“…This is in line with recent studies which show that formation of methanol is mainly controlled by the surface coverage of formate and hydrogen at the steady state. 50 To show the behaviour of the active catalyst in more detail, we plot in Fig. 10 (see also the ESI † for the animated video) a Fig.…”

Multiscale modelling of CO2 reduction to methanol over industrial Cu/ZnO/Al2O3 heterogeneous catalyst: Linking ab initio surface reaction kinetics with reactor fluid dynamics

“…kinetic Monte Carlo -KMC) and macro scale (e.g. computational fluid dynamics -CFD) is gaining importance, particularly when considering the engineering and intensification of unconventional feedstock processing, as well as the design of emerging catalysis routes (Hagman et al, 2018;Kattel et al, 2017;Maestri and Cuoci, 2013;Posada-Borbón et al, 2018;Wu and Yang, 2017). In order to do so, it is mandatory that we understand the reacting system on the atomic scale.…”