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

Abstract

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 CO 2 methanation on Ni/γ-Al 2 O 3 catalysts. Two surface adsorbate species, adsorbed carbonyl (*CO ads ) and adsorbed formate (*HCOO ads ), are detected. The band intensity of *HCOO ads decreases along the catalyst bed with the CO 2 concentration in the gas phase, identifying surface formate as a kinetically relevant intermediate. This finding is in line with an associative mechanism where CO 2 adsorbs on γ-Al 2 O 3 forming carbonate or bicarbonate, being rapidly hydrogenated to formate. Formate reduction is the rate-determining step, with all subsequent hydrogenation steps to CH 4 being fast. The band intensity of *CO ads does not change, irrespective of position in the catalyst bed. This invariance of *CO ads can be interpreted in two ways. *CO ads could be a spectator species that is present at the catalyst surface but not involved in any kinetically relevant reaction channel. Alternatively, *CO ads could be formed by rapid dissociative adsorption of CO 2 at the surface of the Ni nanoparticles with a high adsorption equilibrium constant, leading to an almost constant *CO ads coverage within the investigated CO 2 conversion range. If the rate-determining step in the reaction sequence to CH 4 would then occur after the formation of *CO ads, e.g., *CO ads → *C ads + *O ads or *CO ads + *H ads → *HCO ads, an almost constant *CO ads signal would result as well.

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