Computational Study of C-C Coupling Reactions on Heterogeneous Catalysts

The utilization of carbon dioxide (CO2) in chemical production has attracted global research interest. Reacting CO2 with methane (CH4) removes these greenhouse gases from the atmosphere and turns both compounds into building blocks for organic compound synthesis. A commonly explored pathway involves dry reforming of methane (DRM), which reacts CH4 and CO2 to form syngas, a mixture of H2 and CO. Syngas is a widely used feedstock for synthesizing chemicals ranging from methanol to fuels via the Fischer-Tropsch (FT) process. However, DRM has a large positive ΔGº, which requires the reaction to be carried out at a high temperature (1000~1200 K) to achieve a favorable equilibrium constant. Therefore, DRM requires significant energy input, which contributes to instead of reducing greenhouse gas production. This research is focused on alternative routes for reacting CO2 with CH4 to avoid high energetic penalties. The fundamental premise is the formation of acetate (CH3COO) species on catalyst surfaces as a key intermediate toward producing organic compounds such as vinyl acetate, alkyl acetates, acetic anhydride, and cellulose acetate, all of which are important industrial chemicals. We perform computational modeling based on first-principles density functional theory (DFT) calculations to generate atomic-level insights that guide the design of heterogeneous catalysts for catalyzing methane carboxylation by CO2 (MCC) to form CH3COO. We investigate three types of catalytic materials, including ceria, pure metals, and alloys, to identify the factors that determine the effectiveness of these materials. Based on the insights, we propose single-atom alloys (SAAs) by doping a late d-metal (Ni and Cu) with a small amount of a more reactive metal (Zr, Hf, and Co). The catalytic advantage of the SAAs stems from stabilizing the CHx-CO2 coupling step at the more oxophilic dopant site while the host metal activates CH4. We also examine the functionalization of Cu by phenyl phosphonic acid (PPA) molecules to protect the metal surface from oxidation

Elucidating the Mechanism of Ambient-Temperature Aldol Condensation of Acetaldehyde on Ceria

Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations, we conclusively demonstrate that acetaldehyde (AcH) undergoes aldol condensation when flown over ceria octahedral nanoparticles, and the reaction is desorption-limited at ambient temperature. -Crotonaldehyde (CrH) is the predominant product whose coverage builds up on the catalyst with time on stream. The proposed mechanism on CeO(111) proceeds via AcH enolization (i.e., α C-H bond scission), C-C coupling, and further enolization and dehydroxylation of the aldol adduct, 3-hydroxybutanal, to yield -CrH. The mechanism with its DFT-calculated parameters is consistent with reactivity at ambient temperature and with the kinetic behavior of the aldol condensation of AcH reported on other oxides. The slightly less stable -CrH can be produced by the same mechanism depending on how the enolate and AcH are positioned with respect to each other in C-C coupling. All vibrational modes in DRIFTS are identified with AcH or -CrH, except for a feature at 1620 cm that is more intense relative to the other bands on the partially reduced ceria sample than on the oxidized sample. It is identified to be the C=C stretch mode of CHCHOHCHCHO adsorbed on an oxygen vacancy. It constitutes a deep energy minimum, rendering oxygen vacancies an inactive site for CrH formation under given conditions