The direct conversion of methane to methanol is a desirable alternative to the current multistage indirect synthesis of methanol via syngas. Recently, there has been attention on atomically dispersed catalysts in metal oxide/zeolite supports for the aqueous phase partial oxidation of methane, using liquid hydrogen peroxide as the oxidant.
Atomically dispersed Rh on t-ZrO2 and CeO2 nanowires (NWs) can convert methane to methanol using H2O2 as the oxidant and also form other products like CH3OOH and CO2. Here the objectives of this investigation are 1) to establish the active site characteristics of the 0.3 wt% of Rh on ZrO2 and 0.29 wt% of Rh on CeO2 NWs, satisfying all the experimental catalyst characterization data, 2) to identify an appropriate value of the Hubbard U correction for use with the Zr 4d states, and 3) to unravel the reaction mechanism and pathways for the formation of CH3OH and CH3OOH during methane oxidation on Rh/t-ZrO2 with hydrogen peroxide as the oxidant. This investigation employed computational IR spectroscopy using CO as the probe molecule to identify the active site characteristics of supported Rh SAs on t-ZrO2 and CeO2 NWs. t-Rh0.03Zr0.97O2(200) and Rh0.02Ce0.98O2(110) surfaces show the formation of the geminal di-carbonyl with the computed vibrational frequencies closely matching those from the DRIFTS experiments in the literature. However, the widely studied t-Rh0.03Zr0.97O2(101) and Rh0.03Ce0.97O2(111) surfaces in the literature don’t form geminal di-carbonyl on the surface. Investigations reinforce the need to carefully assess the characteristic active site motifs in single-atom catalysts formed in metal oxides. Additionally, CH4 and H2O probe molecules were utilized to estimate the Hubbard U for use with t-ZrO2, and Hubbard-U on Zr was obtained to be 2.5 eV. The mechanistic analysis of methane partial oxidation on t-Rh0.02Zr0.98O2(200) with U=2.5 eV on Zr revealed the critical role of CO in the formation of CH3OOH, which is one of the key products observed during the partial oxidation reaction.
Although most of the catalytic systems investigated in the literature use water as the solvent, a study on Fe/ZSM-5 catalyst showed that the use of a mixture of sulfolane and water as solvent enhanced methanol selectivity compared to water. Classical MD simulations of the reactants and the products in these solvents showed a substantial difference in transport properties, compared to water. The observed higher selectivity of methanol on the Fe/ZSM-5 catalyst in sulfolane and its aqueous mixtures could potentially be because of a combination of the effects on transport of the reactants and products, and their involvement in the elementary reactions. The current studies would be extended to predict the roles of solvent sulfolane in altering reaction mechanisms and energetics on t-Rh/ZrO2 as the catalytic system.