Competing Mechanisms in CO Hydrogenation over Co-MnO_x Catalysts

Abstract

We study the hydrogenation of CO under ambient pressure conditions over a Co-MnO x model catalyst using chemical transient kinetics (CTK) under calibrated molecular flow conditions. Alkanes and alkenes are shown to form with markedly differing kinetics. Quantitation of the data allows accumulating carbon and oxygen coverages to be determined at any instant of the “buildup” transients. Anderson–Schulz–Flory (ASF) chain lengthening probabilities are evaluated while approaching the steady-state of the reaction. A linear dependence of these probabilities on the transient CO gas pressure provides evidence for a CO insertion mechanism being in operation under high-coverage conditions. A detailed kinetic analysis of reactant/product formation and scavenging is in agreement with this conclusion. However, for coverages below the monolayer limit, fast CO dissociation, probably hydrogen-assisted and promoted by Mn2+, also enables significant CH x –CH y coupling to occur. Evidence was obtained from high resolution transmission electron microscopy (HRTEM) that a phase transition from Co to Co2C was triggered under atmospheric pressure conditions for the Co-MnO x catalyst.

Promoting the Cleavage of C–O Bonds at the Interface between a Metal Oxide Cluster and a Co(0001) Support

“…Once again, a similar band was previously observed in CO hydrogenation studies and tentatively attributed to the O-C bond in adsorbed alkoxy. 6 The occurrence of such species in the present study seems to be unlikely, though. Instead, one may speculate about a model-specific adsorption mode in which CO binds to two O 2-of the The experimental measurement of FTIR seem to support this conclusion, despite an obvious overlap of low-frequency modes in adsorbed CO with the possible occurrence of more complex adsorbate species like formate/carboxylate and carbonate.…”

“…As a result of these efforts, metal-oxide supports with distinct metal valence fluxionality have been identified to shift the product selectivity from paraffins to olefins and oxygenates. For example, studies with Co/MnO x catalysts, possibly alkali-promoted, have demonstrated their capacity to produce olefins [1][2][3][4][5][6][7] or alcohols/aldehydes. [7][8][9][10] Interestingly, such Co/MnO x catalysts undergo a significant chemical reconstruction involving a Co to Co 2 C phase transition during CO hydrogenation, as simultaneously demonstrated by Zhong et al 1 and Xiang 7 and further investigated by others.…”

Superior Fischer-Tropsch performance of uniform cobalt nanoparticles deposited into mesoporous SiC

“…Literature frequently associates CO 2 formation with the occurrence of the water gas shift reaction. On the other hand, according to mechanistic studies in our laboratory, CO 2 has to be regarded a signature species resulting from the occurrence of oxygencontaining (carboxyl-type) surface species [41]. It is not possible, at present, to relate the increased CO 2 formation over Co/Dav to either process.…”

Recent Progress in CO Hydrogenation over Bimetallic Catalysts for Higher Alcohol Synthesis

“…The mechanism of CO insertion over copper-based FT catalysts is shown in Scheme 2. [37] In a recent study, Athariboroujeny et al [38] investigated the competing mechanisms in CO hydrogenation over cobalt-based catalysts between CO insertion and CÀ C coupling. Their findings led to a general conclusion that the CO insertion mechanism seems to be the common process.…”