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.
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.
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