Structural Evolution and Stability of Rh/TiO₂ Catalysts under CO₂ Hydrogenation Conditions: Influence of the Initial Rh Structure

Abstract

Characterizing catalyst stability by identifying the predominant mechanisms, time scales, and driving forces of catalyst reconstruction under relevant reaction conditions is necessary for the design and commercialization of emerging catalysts. Here, we study Rh/TiO 2 catalysts under CO 2 hydrogenation conditions (773 K, 75% H 2, 25% CO 2 ) at 10–50% initial CO 2 conversion and utilize reactivity studies along with ex situ and in situ spectroscopy and microscopy to characterize changes in catalyst activity and structure as a function of time on stream and the initial Rh domain structure. Rh/TiO 2 is a prototypical catalyst for CO 2 hydrogenation where Rh structure and Rh–TiO 2 interactions have been proposed to explain reactivity, selectivity (between CO and CH 4 formation), and catalyst stability. Here, the influences of the initial Rh structure (varying from Rh single atoms to Rh nanoparticles), support reconstruction, regeneration and pretreatment, and the composition of the reaction environment on reaction selectivity and catalyst stability were explored. Product selectivity between CO and CH 4 was determined to be dependent on the relative fraction of Rh single atoms and Rh nanoparticle-TiO 2 interfacial sites under the reaction conditions, each exhibiting distinct stability under the explored reaction conditions. Surprisingly, Rh single atom active sites were stable for the duration of 90 h reactivity measurements, even at high Rh surface density (≥1.8 Rh atoms/nm 2 ) on the support, while Rh nanoparticles sintered. All catalysts exhibited increasing selectivity to CO with time on stream (>10 h). We conclude that the distribution of Rh structures evolved under reaction conditions through three distinct mechanisms (Rh particle fragmentation, Ostwald ripening, and particle migration and coalescence) that occurred on varying time scales and that changes in catalyst reactivity on the ∼90 h time scale were primarily controlled by the distribution and density of initial Rh structures.