Measuring Adsorbate Profiles in Heterogeneous Catalytic Reactors by Iso-Potential Operando DRIFTS Applied to CO2 Methanation on Ni

The development and improvement of catalytic processes require a detailed understanding of catalyst dynamics, reaction mechanisms, and structure–activity relationships inside catalytic reactors, from the laboratory to production scale. This paper presents the methodology of iso-potential operando DRIFTS for measuring the profiles of surface adsorbates inside catalytic reactors. Iso-potential operando spectroscopy (IPOS) in general and iso-potential operando DRIFTS in particular separate the functionality "catalytic reactor" and "spectroscopic cell" from each other. The catalytic reactor is equipped with a mechanism of spatial sampling and spatial temperature measurement. A small fraction of the reaction mixture is sampled locally in the reactor and transferred continuously into a spectroscopic cell containing a very small amount of the same catalyst as in the reactor. The temperature is set to the same value as is locally measured in the reactor. In this way, the catalyst in the spectroscopic cell is exposed to the same chemical potential as that locally in the catalytic reactor. It is hypothesized that it takes on the same structure, the same surface adsorbates, and shows the same reactivity. IPO DRIFTS is applied to CO2 methanation on Ni/γ-Al2O3 catalysts. Two surface adsorbate species, adsorbed carbonyl (*COads) and adsorbed formate (*HCOOads), are detected. The band intensity of *HCOOads decreases along the catalyst bed with the CO2 concentration in the gas phase, identifying surface formate as a kinetically relevant intermediate. This finding is in line with an associative mechanism where CO2 adsorbs on γ-Al2O3 forming carbonate or bicarbonate, being rapidly hydrogenated to formate. Formate reduction is the rate-determining step, with all subsequent hydrogenation steps to CH4 being fast. The band intensity of *COads does not change, irrespective of position in the catalyst bed. This invariance of *COads can be interpreted in two ways. *COads could be a spectator species that is present at the catalyst surface but not involved in any kinetically relevant reaction channel. Alternatively, *COads could be formed by rapid dissociative adsorption of CO2 at the surface of the Ni nanoparticles with a high adsorption equilibrium constant, leading to an almost constant *COads coverage within the investigated CO2 conversion range. If the rate-determining step in the reaction sequence to CH4 would then occur after the formation of *COads, e.g., *COads → *Cads + *Oads or *COads + *Hads → *HCOads, an almost constant *COads signal would result as well.