Kinetics of CO oxidation over Co₃O₄/γ‐Al₂O₃. Part III: Mechanism

Abstract

Interpretations of a variety of transient experiments are combined with information from the literature to reveal the roles played by the Co304 catalyst surface and adsorbed species. The reaction path depends on the oxidation state of the catalyst. A rather coherent picture of oxidation-reduction catalysis by a metal oxide is obtained. DEEPAK SCOPEThe mechanism of oxidation-reduction reactions on metal oxide catalysts is not well understood. In particular, very little coherence exists in the information available on the very active catalyst for CO oxidation, C084. It is believed that the trivalent cobalt ion in the spinel structure is responsible for CO oxidation.However, the roles of lattice oxygen and various adsorbed species are not at all clear. Steady state and unsteady state kinetic studies were reported in the previous papers (Parts I and 11). The dynamic studies provided a wealth of phenomenological insights that can be used to postulate a mechanism for this catalytic reaction. The postulated mechanism can then be checked for consistency with the rate equation from the steady state studies and with information from the fragmented literature. The mechanism thus postulated and evaluated may promote practical exploitation of this catalytic reaction, provide insight into the general matter of metal oxide catalysis, and suggest productive avenues for future research. CONCLUSIONS AND SIGNIFICANCEThe phenomenological conclusions from the reactor dynamics studies (Part 11) and specific chemical information from the literature provided the basis for the postulation of a mechanism for the oxidation of CO over Co304/y-A1&3. The reaction path depends in a number of ways on the oxidation state of the catalyst. From the mechanism a rate equation can be derived for the circumstances of the steady state experiments. Excellent agreement is found between this rate equation and the one obtained in Part I. This provides partial validation of the postulated mechanism. The mechanistic insights also suggest the origins of the two kinds of catalyst deactivation contemplated in Part I. Finally, a rather coherent picture of oxidation-reduction catalysis by a metal oxide is obtained.

Environment-Dependent Catalytic Performance and Phase Stability of Co3O4in the Preferential Oxidation of Carbon Monoxide StudiedIn Situ

“…The concurrent chemical phase analysis using PXRD shows that the starting Co 3 O 4 phase remains stable at all tested temperatures (Figure a). Assuming an MvK mechanism for CO oxidation over Co 3 O 4 , the presence of excess O 2 is likely the reason for the stabilization of Co 3 O 4 . ,− Furthermore, the in situ PXRD results in Figure S5 show that Co 3 O 4 reduces in a 1:99 CO/N 2 gas mixture, that is, in the absence of 1% O 2 .…”

“…The full consumption of O 2 from 200 °C, due to both CO and H 2 oxidation, leaves the catalyst susceptible to reduction, which explains the onset formation of CoO at the same temperature. Assuming a MvK mechanism for CO oxidation over Co 3 O 4 , ,− this mechanism may no longer be at play after the depletion of O 2 and the formation of CoO, which possibly causes the CO oxidation activity to drop.…”

Impact of Nanoparticle–Support Interactions in Co3O4/Al2O3 Catalysts for the Preferential Oxidation of Carbon Monoxide

“…The weak NPSI in CAT 1 are thought to be advantageous in that a higher amount (or surface area) of the active Co 3 O 4 sites is made available to the gas reactants instead of strongly interacting with the support. Also, according to most literature, CO oxidation over Co 3 O 4 is believed to proceed via the Mars–van Krevelen (MvK) mechanism, − ,, which requires the Co 3 O 4 surface to be redox active. Therefore, the high activity of CAT 1 can also be attributed to the facile reducibility of the (surface) oxide phase.…”

Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

“…Based on the H 2 -TPR profiles of the four catalysts (Figure 3), the Fe-based oxide catalysts required higher temperatures to reduce than the Co-based ones since they exhibited smaller crystallite sizes. Assuming the Mars-van Krevelen (MvK) mechanism for CO oxidation, which requires the catalyst surface to be easily reduced (and re-oxidised) [45][46][47][48], it is possible that the low CO oxidation activity of the Fe-based oxides was caused by their less reducible nature. This has also been proposed for other less reducible catalysts that displayed low CO oxidation activity during CO-PrOx [11,16,49].…”