Kinetic Origins of CO2 Methanation Activity of ZrO2-Based Catalysts with Rh or Ru Nanoparticles

Abstract Low-temperature CO2 hydrogenation to methane has the potential to form the basis of power-to-gas technologies, which could help close the carbon cycle and address environmental concerns. Catalysts with supported metals that can activate hydrogen are typically used for this reaction but do not yet meet the requirements of large-scale applications. Their tailored design remains challenging due to the ambiguity about the role of support and supported metal and how they interact with each other to ensure industrially relevant performance. To contribute to closing this knowledge gap, we prepared ZrO2- and YZrOx-supported catalysts possessing 1 nm Ru or Rh nanoparticles at a low metal loading of about 0.1 wt %, which was chosen for economic reasons. The best-performing Ru/ZrO2 catalyst outperformed various previously developed Ru- or/and Rh-containing catalysts in terms of metal-related activity despite operating at around 90% equilibrium conversion. To gain a detailed understanding of the kinetics and mechanism of CO2 methanation, we performed a comprehensive study combining steady-state isotopic transient kinetic analysis (SSITKA), temporal analysis of products (TAP), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The SSITKA method enabled us to determine the concentration and the lifetime of the surface intermediates of gas-phase CH4. The concentration was found (i) to be significantly higher than that of surface atoms of Rh or Ru and (ii) to increase with increasing reaction temperature. Thus, support plays a pivotal active role in the reaction kinetics and mechanism. Moreover, a direct correlation was found between the lifetime and the strength of CO2 adsorption as determined by TAP experiments. This finding suggests that the adsorptive catalyst property is an important activity-governing factor. This property can be controlled by the degree of support reduction; the higher the reduction degree, the longer the lifetime. The reduction degree depends on the kind of supported metal and dopant for ZrO2. In situ DRIFTS studies confirmed that CO2 methanation proceeds on the support via the formate (HCOO–) and methoxy (OCH3) intermediates, ultimately yielding gas-phase CH4. The synergistic action of oxygen vacancies and weak OH– basic centers was suggested to facilitate CO2 adsorption and its rapid conversion into active HCOO– species, thereby enabling the fast formation of reaction intermediates. The coexistence of oxygen vacancies and weak OH– groups on the surface of ZrO2-based supports may provide a faster pathway for CH4 formation. Therefrom, the presented approach and the obtained knowledge may be used for the purposeful design of catalysts not only for CO2 methanation but also for CO2 hydrogenation to higher hydrocarbons to hinder the formation of CH4.