In Situ FT-IR Study on the Effect of Cobalt Precursors on CO Adsorption Behavior

Cobalt-rhenium based catalysts were prepared by coimpregnation from two different cobalt precursors: cobalt nitrate [CoRe(N)] and cobalt acetate [CoRe(A)]. They were characterized by H-2-TPR, ICP, XRD, DRIFTS, and activity/selectivity in CO hydrogenation. The results showed that precursors have a significant effect on the cluster size, dispersion, and CO adsorption/CO hydrogenation activities. XRD showed no bulk crystallinity for the CoRe(A) catalyst, whereas peaks corresponding to a Co3O4 phase were found for the CoRe(N) catalyst. TPR results suggested greater cobalt-rhenium contact for the CoRe(A) catalyst, with Re facilitating reduction of cobalt oxide by hydrogen spillover. Activity/selectivity studies showed that the CoRe(N) catalyst is more active for CO hydrogenation with high selectivity toward hydrocarbons, while the CoRe(A) catalyst has far higher selectivity to oxygenates (but considerably lower overall activity). DRIFTS studies at 25 degrees C for CO reacting with CoRe(N) showed lower frequency carbonyl bands (2057 and 1942 cm(-1)), whereas CoRe(A) had CO bands at much higher frequencies (2183-2175, 2125, and 2074 cm-1). The carbonyl bands in the 2183-2175 cm(-1) region are assigned to Co(II)/Co(III)-CO from the presence of nonreduced Co3O4 on the surface of the CoRe(A) catalyst. DRIFTS studies under CO hydrogenation conditions are also presented. Lower wavenumber IR bands seen between 1990 and 1920 cm(-1) for CoRe(N) are tentatively assigned to bridging CO's on the cobalt and terminal carbonyls on Re(0) clusters. Only higher frequency CO's are observed for CoRe(A) corresponding to less electron-rich cobalt centers, linear CO coordination, and oxygenate production. The presence of nanoparticle catalysts and highly dispersed Re on the CoRe(A) catalyst is proposed to be key factors in the high oxygenate selectivity. CO is weakly adsorbed on these sites facilitating the M-CO bond dissociation and increasing the CO insertion probability leading to the oxygenate formation.