(691d) Observation and Elucidation of the Bimetallic Structure of MoO3-Promoted Rh Catalysts for Syngas Conversion to Alcohols Using in-Situ xas

The development of high-pressure in-situ capabilities for catalyst characterization at the Stanford Synchrotron Radiation Lightsource has been used to gain insight into the structure-activity relationship of silica-supported MoO₃-promoted Rh catalysts for syngas conversion to higher alcohols. Rh shows promise to convert syngas (H₂ and CO) into higher oxygenates but catalytic performance is hindered due to competing side reactions. The addition of promotors has been shown to alter the activity and/or selectivity to higher oxygenates with little understanding as to what is the structure of the promoted catalyst or why the promotor changes catalytic performance.

Atomic layer deposition was used to selectively deposit MoO₃ onto silica-supported Rh nanoparticles to understand the role of the promotor in altering catalyst performance. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at the Rh and Mo K-edges were collected on the catalyst under i) as prepared conditions, ii) during reduction in H₂ at 250°C, and iii) under reaction conditions (250°C, 20 bar, flow of 2:1 H₂:CO). The as prepared catalyst was characterized as a MoO₃ over layer on Rh₂O₃ particles. TPR XANES showed that the Mo in the presence of Rh was able to reduce at lower temperatures than conventional MoO₃-only catalysts. The reduced catalyst was characterized as Mo substituted into the surface of the fcc lattice of the Rh nanoparticle. Under reaction conditions the Mo XAFS changed to a slightly oxidized Mo species, still in the fcc lattice, while the Rh stayed constant indicating that the Mo was on the Rh particle surface. DFT modeling complemented the XAS results showing that a Mo-OH species can be substituted into the Rh surface, and its presence should stabilize oxygen containing intermediates shifting selectivity from hydrocarbons to methanol and longer carbon chain oxygenates.