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

Characterizing catalyst stability by identifying the predominant mechanisms, timescales and driving forces of catalyst reconstruction under relevant reaction conditions is necessary for the design and commercialization of new catalysts. Here, in this paper, we study Rh/TiO₂ catalysts under CO₂ hydrogenation conditions (773 K, 75% H₂, 25% CO₂) at high 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 catalyst structure. This is a prototypical catalyst for CO₂ hydrogenation where Rh structure and Rh-TiO₂ interactions have been proposed to explain reactivity, selectivity (between CO and CH₄ formation) and catalyst stability. The influence of the initial Rh structure (varying from Rh single atoms to Rh nanoparticles), support stability, regeneration and pretreatment(s), and the chemical potential(s) of the reaction environment on reaction selectivity and catalyst stability were explored. The product selectivity between CO and CH₄ was determined to be dependent on the relative fraction of Rh single atoms and Rh nanoparticle-TiO₂ interfacial sites under reaction conditions, each exhibiting distinct stability under prolonged time on stream. Surprisingly, Rh single atoms exhibited stability for the duration of 90 h reactivity measurements, even at high Rh density (≥ 1.8 Rh atoms/nm²) on the support, while Rh nanoparticles sintered under reaction conditions. As a result, all catalysts exhibited increasing selectivity to CO with increasing time on stream (> 10 h). We conclude the distribution of Rh structures evolved over time under reaction conditions through three distinct reconstruction mechanisms (Rh particle fragmentation, Ostwald ripening, and particle migration and coalescence) that occurred on varying . Authors: Schroeder, Emily K. [Univ. of California, Santa Barbara, CA (United States)]; Hong, Seunghwa [Univ. of California, Santa Barbara, CA (United States)]; Chen, Xiaobo [Brookhaven National Laboratory (BNL), Upton, NY (United States). Center for Functional Nanomaterials (CFN)]; Hoffman, Adam S. [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States). Stanford Synchrotron Radiation Lightsource (SSRL)]; Chen, Zhihengyu [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States). Stanford Synchrotron Radiation Lightsource (SSRL)]. DOE Contract: AC02-76SF00515; SC0012704. Subjects: 25 ENERGY STORAGE; CO2 hydrogenation; catalyst deactivation; interface; reverse water gas shift; single atom catalyst; sintering