The mechanism of HCOOH decomposition on Pd(111) surface leading to the formation of CO 2 and CO has been systematically investigated to identify the preference of CO 2 or CO as the dominant product. Here, we present the main results obtained from periodic, self-consistent density functional theory calculations. Four possible pathways of HCOOH decomposition, initiated by the activation of the O− H, C−H, and C−O bonds of HCOOH, as well as the activation of simultaneous C−H and C−O bonds of HCOOH, have been proposed and discussed. Then, the effects of coadsorbed H 2 O and its coverage on the decomposition of HCOOH have been also considered. Our results show that CO 2 is preferentially formed as the dominant product of HCOOH decomposition on Pd(111) surface via a dual-path mechanism, which involves both the carboxyl (trans-COOH) and formate (bi-HCOO) intermediates, along with alternative bond-breaking possible steps in those intermediates. The dehydrogenation of HCOOH on Pd surface is a vital process for CO 2 formation. Further, the coadsorbed H 2 O and its coverage play an important role in the decomposition of HCOOH, and the preferred catalytic pathway of CO 2 formation is qualitatively dependent on surface H 2 O coverage. Therefore, our results would at the microscopic level provide insights into the mechanism, energetics, and possible reactive intermediates of HCOOH decomposition regarding the preference of CO 2 formation as the dominant product for the catalytic reactions involving HCOOH and for a direct HCOOH fuel cell on Pd system.

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Catalytic hydrogenation of CO2 to methanol is a promising way to recycle and utilize CO2. In this study, the elementary steps leading to HCOO and CO formation have been explored to identify hydroxylation effect of the oxide support on the selectivity in CO2 hydrogenation on Cu/γ-Al2O3 catalyst by the density functional theory (DFT) slab calculations. Two models: Cu4 cluster supported on the dry γ-Al2O3(110) surface, D(Cu4), and on the hydroxylated γ-Al2O3(110) surface, H(Cu4), have been used to model Cu/γ-Al2O3. On D(Cu4), the formation of HCOO is preferred kinetically. On H(Cu4), HCOO formation is still kinetically favorable. These results indicate that the hydroxylation of γ-Al2O3 support cannot alter the pathway of CO2 hydrogenation forming the dominate product HCOO, and ultimately, the selectivity of CO2 hydrogenation for HCOO formation on Cu/γ-Al2O3 is higher, which supports the experimental fact that Al2O3-supported Cu catalyst is widely used to synthesize methanol by CO2 hydrogenation.

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The effect of Co 2 C crystal facets on the selectivity of C 2 species (C 2 oxygenates and hydrocarbons) in Fischer−Tropsch synthesis (FTS) reaction was investigated using density functional theory calculations, and the selectivity comparisons among five exposed Co-termination ( 101), ( 011), ( 010), (110), and (111) crystal facets are examined to shed light on the essential relationship between FTS selectivity and the structure of Co 2 C crystal facets. The results show that the C−C bond of C 2 species prefers to be formed instead of C 1 species CH 4 over Co 2 C catalysts in the FTS reaction, and the selectivity of C 2 species and the dominant existence form of key CH x intermediates are sensitive to the crystal facet of Co 2 C catalysts, which are closely associated with Co 2 C crystal facet's electronic and structural properties. The electronic and structural properties of different Co 2 C crystal facets show that the high selectivity of C 2 oxygenates over the ( 011) and ( 111) facets are attributed to the upshift of their surface d-band centers, as well as the presence of the step B 5 -type active unit with five Co atoms consisted of much denser surface active sites. However, both ( 101) and ( 010) facets exhibit high selectivity toward C 2 hydrocarbons, and the (110) facet presents high selectivity toward the formation of CH 4 . Thus, regulating the exposed crystal facets of Co 2 C catalyst can control the selectivity of desirable C 2 species. This work provides evidence at a molecular level to support that the sensitivity of Co 2 C crystal facet is a cause to affect the selectivity of desired products in the FTS reaction.