In-Situ FT-IRAS Study of the Hydrogenation of CO on Ru(001): Potassium-Promoted Synthesis of Formate

Article abstract of DOI:10.1021/j100139a031

Utilizing in-situ Fourier transform infrared reflection absorption spectroscopy, we have characterized potassium-promoted Ru(001) surfaces under CO hydrogenation conditions at elevated temperature and pressure (1-50 Torr).Interaction of the <*>3x<*>3-R30 deg-K-Ru(001) surface with CO at 300 K resulted in the formation of carbonate, which was hydrogenated to formate via the reaction K2CO3 + CO + H2 ->/<- 2KHCO2.The steady-state ratio of formate and carbonate coverage under reaction conditions was found to depend on the CO:H2 pressure ratio of formate and carbonate being present in excess hydrogen.Time-resolved FT-IRAS determined the initial rate of formate synthesis from carbonate to (1.7 +/- 0.8) * 10^-3 molecules site^-1 s^-1 at 500 K.Isotope transient measurements resulted in a comparable synthesis rate under equilibrium conditions and hence demonstrate the reactivity of the formate.The isotope transient data and the formate to carbonate conversion in excess hydrogen are consistent with two possible mechanisms: (i) Decomposition of the formate to carbonate, i.e., the reverse of the formate synthesis reaction, or (ii) further hydrogenation of the formate to methanol or methane.Characteristic vibrational features show that both formate and carbonate are directly bound to the potassium.This compound formation leads to a contraction of the potassium layer and to island formation.The formate observed under reaction conditions was shown to be more stable than model compounds produced under UHV conditions.This enhanced stability under reaction conditions is attributed to two factors: (i) The bond formation between potassium and the formate and (ii) physical site blocking due to coadsorbed CO.The results demonstrate a dual promoter mechanism of the potassium in the CO hydrogenation reaction over Ru(001)-(i) promotion of CO dissociation resulting in the formation of carbonate and (ii) direct participation of the potassium in the synthesis of formate via compound formation.