CO(2) Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO(2) at Elevated Pressures in Gas Phase Media

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO(2) hydrogenation at 20 bar and a H(2):CO(2) ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO(2) conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO(2) hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO(2) adsorption, weakens the C–H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H(2)-TPR/TPR-MS and H(2)-TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H(2) reduction showed no evidence of a promoting effect involving electron charge transfer.