Comprehensive theoretical analysis of the influence of surface alloying by zinc on the catalytic performance of Cu(1 1 0) for the production of methanol from CO2 selective hydrogenation: Part 1 – Thermochemical aspects

Abstract

Open copper facets such as Cu(1 1 0) were recently reported to most likely act as an active center of commercial Cu/ZnO/Al2O3 methanol synthesis catalysts. Surprisingly, unlike other surfaces of the metal component, the Cu(1 1 0) facet suffers from severe deactivation by deposited reduced zinc adatoms under catalyst working conditions. Elucidating the catalytic behavior of the novel reactive site and the Zn poisoning effect in detail is highly desirable, however, very challenging. As the first paper of a series devoted to tackle these intriguing issues, we have performed density functional calculations and atomistic thermodynamic modeling to examine the mechanism of CO2 hydrogenation on a CuZn(1 1 0) surface alloy and, as a reference, its parent Cu(1 1 0) substrate. Based on the obtained formation free energies, guest Zn atoms were predicted to be enriched on the uppermost layer of CuZn(1 1 0), preferentially leading to Cu-based single-atom alloys. According to the computations of the authors, the methanol production on the two systems may proceed via formate (HCOO*) rather than both the CO* and carboxyl (HOCO*) species, laying pivotal foundations for future kinetic analysis. The structural insensitivity of the production process is traced back to slight improvement of the stability of most reaction intermediates by partial surface alloying. More in detail, there exist several thermodynamically viable intermediate-mediated strategies that convert bidentate HCOO* to another precursor formaldehyde (H2CO*), that is, bi-HCOO* → dioxymethylene (H2COO*) → hydroxymethoxy (H2COOH*) → H2CO*, bi-HCOO* → formic acid (HCOOH*) → H2COOH* → H2CO*, and bi-HCOO* → HCOOH* → formyl (HCO*) → H2CO*. Under the assistance of the semiempirical Brönsted–Evans–Polanyi (BEP) principle and previous published works, comparison of reaction free-energy data for key elemecntary steps indicates that on CuZn(1 1 0) and Cu(1 1 0), the rate-limting step of methanol synthesis is H3CO* hydrogeantion. Moreover, the Zn-contaminated surface renders the H3CO* + H* reaction thermodynamically and kinetically less favorable than on the latter one, which thus inhibits CO2 hydrogenation to H3COH. In summary, our finding is applicable in explaining the catalytic performance variation when Cu(1 1 0) is alloyed with Zn atoms.