(679i) Theoretical Study of the Direct Transformation of CO2 and CH4 into Surface CH3coo Species on Single-Atom Alloys

Direct reactions of CO₂ with CH₄ are attractive because two potent greenhouse compounds are utilized at once. We present a systematic density functional theory (DFT)-based theoretical study on determining what controls the efficacy of a series of catalysts for catalyzing the formation of acetate as a way to bypass thermodynamic penalties associated with such reactions. The well-known dry reforming reaction (CO₂ + CH₄ --> 2CO + 2H₂) has a strongly positive ΔGº_rxn (+173 kJ/mol). CO₂ + CH₄ --> CH₃COOH is another possibility that has a smaller but still positive ΔG°_rxn(+71 kJ/mol). Instead, we propose to form the acetate group as an intermediate and couple it with suitable oxygenates to form various organic esters. While there is an increasing number of studies in the literature describing attempts to synthesize acetic acid from CO₂ and CH₄, the direct transformation of organic acetates from CO₂ and CH₄ has received little attention.

Periodic DFT calculations were carried out using the Vienna ab initio simulation package (VASP). The projector augmented wave (PAW) method was used to represent the potentials due to nuclei and the core electrons. The exchange-correlation functional was the generalized gradient approximation with Perdew-Burke-Ernzerhof, known as GGA-PBE. The wave function at each k-point was expanded with a plane wave basis set and a kinetic energy cutoff up to 400 eV. All structures were optimized until the force component on every atom was less than 0.03 eV/Å.

First, we investigate C-H scission and CH₃+CO₂ coupling steps on 3d metals - Co(0001), Ni(111), Cu(111); 4d metals - Ru(0001), Rh(111), Pd(111); 5d metals - Ir(111), Pt(111), Au(111). Herein, CH₄ easily activates on most of the assigned transition metals. On the other hand, E_a for coupling between CH₃ and CO₂ is notably higher on most of the metals and the highest on Pt and Au. High E_a of CH₃+CO₂ step on transition metals represents a bottleneck for the acetate formation. Then, we develop transition states (TS) scaling relationship to predict the optimal reactivity of the catalysts based on the descriptors. The TS scaling relationship demonstrates that metal catalysts with strong elemental C binding energy are more favorable for CH₄ activation from a kinetic and thermodynamic standpoint. Similarly, elemental O binding energy has a similar influence on the C-C coupling reaction on metal catalysts. Thus, metal catalysts capable of co-activating CH₄ and CO₂ at a low barrier should have a higher affinity for elemental C and O. Therefore, we propose a rational guideline for developing a new form of bi-metallic alloys, single-atom alloys (SAAs), and their efficacy in stabilizing the TS structures based on our findings. To identify alloys that perform better at activating the C-C coupling between CH₃ and CO₂, we then look at Ru-based SAAs, i.e., Re/Ru(0001), Ti/Ru(0001), Nb/Ru(0001), by doping more oxyphillic dopants. The calculated E_a exhibits the enhanced catalytic role for the C-C coupling step on Ti/Ru(0001), Nb/Ru(0001) than pure host metal, Ru(0001). The metal dopants (Ti, Nb) act as the active site for lowering the E_a value of the rate-determining step, C-C coupling for synthesizing CH₃COO species by stabilizing the TS structure. On the other hand, Re/Ru(0001) shows comparable reactivity to the host metal Ru(0001) as Re is no longer act as an active site. This is supported by the structural stability analysis results - segregation (ΔEseg / ΔEseg(CH₃)) and aggregation energy (ΔEagg). We expect our approach to stimulate further research to transform CH₄ and CO₂ directly and provide insights into the rational design of SAAs for the practical conversion of greenhouse gases on an industrial scale.