Scrutinizing the mechanism of CO2 hydrogenation over Ni, CO and bimetallic NiCo surfaces: Isotopic measurements, operando-FTIR experiments and kinetics modelling

Abstract

The mechanistic understanding of the CO2 hydrogenation reaction is essential for the rational design of active and selective catalysts for this process, nevertheless, there are still important controversies related to the structural requirements, the sequence of elementary steps and the reaction intermediate involved in the formation of CH4 and CO(g). Hence, a detailed mechanistic study was performed on SiO2-supported mono- and bimetallic NiCo catalysts by combining kinetic, spectroscopic and isotopic measurements under methanation conditions, to explain the effect of catalysts composition on the CO2 hydrogenation rate and selectivity toward CO(g) and CH4. It was observed an anti-synergistic effect for the CO2 hydrogenation turnover rate on the NiCo bimetallic catalysts, which is attributed to the inhibition of the CH4 formation pathway on the bimetallic surfaces; on the other hand, the CO(g) formation turnover rate values resulted close to the weighted average values and increases linearly with the cobalt content in the catalysts. The results suggest that the two reaction products are formed through parallel routes with different rate-determining steps, involving two types of active sites where carbonyl species adsorb differently: strongly adsorbed species (CO*) lead to CH4 formation via the H-assisted CO bond dissociation while weakly adsorbed (CO⊕) desorbs to produce CO(g). This was consistent with the inverse H/D kinetic isotopic effect (KIE) observed for methane formation and the KIE values close to unity for CO(g) formation over all catalysts. Operando-infrared measurements suggested that weakly and strongly adsorbed carbonyl species are in quasi-equilibrium at the catalyst surface. It was proposed a sequence of elementary steps for CO2 methanation reaction on Ni, Co and NiCo catalysts, from which a Langmuir-Hinshelwood models for CO(g) and CH4 formation rates were derived. These models properly represent the kinetic data for products formation rates, and content physiochemically-consistent parameters, which point out that the CH4 formation from CO2 hydrogenation can be boosted over a catalytic surface that strongly adsorbs CO, hampers its transformation into CO⊕ and shows a high hydrogenation rate of carbonyl species.