N2O Dissociation on Co3O4 Based Catalysts: Reducibility of Co3O4 and Its Catalytic Consequences
Yongchun Hong1, Andrew Getsoian2, Christine Lambert2 and Enrique Iglesia1*
1 University of California at Berkeley, Berkeley, CA 94720
2 Ford Research and Advanced Engineering, Dearborn, MI 48121
N2O is a greenhouse gas with up to 300-fold higher global warming potential than CO2 and it is also one of the major ozone depleting chemicals. [1] The abatement of N2O emissions by its catalytic decomposition on metal oxides such as Co3O4 forms N2 and O2, but the latter, a major component in effluent streams (> 10%) inhibits reaction rates. [2] N2O-CO reaction forms N2 and CO2, but requires conditions that balance the residual concentrations of N2O and CO. Co3O4 based catalysts are among the most promising candidates for redox catalysis enabled by Co2+/Co3+ interconversion, including NO oxidation [3] and N2O decomposition [4]. Our study addresses the mechanistic details of N2O decomposition and N2O-CO reactions on Co3O4 catalysts, including bulk Co3O4 powders and Co3O4 domains dispersed on insulators (γ-Al2O3, SiO2, ZrO2) and semiconductors (TiO2, CeO2-ZrO2, CeO2). The results show that reducibility of Co3O4 domains affects the reactivity and the nature of the N2O dissociation pathways for both N2O decomposition and N2O-CO reaction by controlling the number of reduced centers that prevail during catalysis and the ratio of the steady-state concentrations of surface oxygen atoms and O-vacancies ([O*]/[*]). Co3O4 domains dispersed on semiconducting materials are more reducible than bulk Co3O4 or Co3O4 domains dispersed on insulators; therefore, they exhibit much higher reactivities for both N2O decomposition and N2O-CO reaction.
The turnover rates for N2O decomposition and N2O-CO reaction differ markedly for each Co3O4 based catalysts. All catalysts show significant N2O-CO turnover rates (> 0.001 s-1) below 430 K, while such turnover rates require temperatures above 600 K for N2O decomposition. These differences reflect the ability of the CO reductant to form vacancies more effectively than via O2 evolution during N2O decomposition, thus leading to much higher [*]/[O*] during catalysis and to a much higher concentration of bound N2O* for a given N2O pressure. For both reactions, however, turnover rates were similar for Co3O4 powders and Co3O4 domains dispersed on insulators, while semiconducting supports, such as CeO2, lead to Co3O4 domains with higher reactivity. These trends reflect interfacial heterojunctions between Co3O4 and semiconductor, for which band bending allows electron delocalization and a decrease in the energy of the lowest unoccupied molecular orbital (LUMO) in Co3O4 domains; such lower energies improve the thermodynamics of reduction of Co3O4, thus increasing the density of vacant sites during steady-state catalysis.
N2O decomposition on Co3O4 based catalysts showed first order kinetics on N2O and strong inhibition by O2 and rates were accurately described by Langmuir models of surface reactivity. Such treatments show that N2O dissociates in the kinetically-relevant step via N2O* reactions with O* (route I), and the O* species formed desorb as O2 in a quasi-equilibrated step, with O* as the most abundant surface intermediate (MASI). The ratio of N2O decomposition turnover rates on dispersed Co3O4 domains on each support to those on Co3O4 powders did not depend on O2 pressure, indicating that differences in reactivity among the catalysts are independent with the [O*]/[*], presumably because vacancy sites on all Co3O4 catalysts have similar oxygen binding energy and therefore the reactivity of all catalysts are inhibited by O2 to the same extend; and differences therefore only reflect the differences in the number of reducible sites among catalysts.
N2O-CO turnover rates on all Co3O4 based catalysts were proportional to N2O pressure and independent of CO pressure for [CO]/[N2O] reactant ratios above unity, but decreased monotonically as [CO]/[N2O] ratios decrease below unity. The addition of O2 to CO/N2O mixtures did not influence N2O-CO turnover rates at 400 K, because O2 dissociation does not occur at detectable rates, thus preventing the capping of vacant sites by O* or the required activation of O2 for CO oxidation catalysis. N2O-CO turnover rates are accurately described by Langmuir models in which N2O* reacts with a vicinal * in the kinetically-relevant step (route II), with the relative coverages of * and O* set by the kinetic coupling between the formation of O* via N2O* reactions with * and the removal of O* via its reactions with CO*. The coverages of oxygen vacancy ([*]) during N2O-CO reaction on Co3O4 catalyst were measured using N2O titration: [*] increased monotonically with increasing [CO]/[N2O] ratio and approaching to constants (0.07 and 0.3, at 403 K and 483 K, respectively), which are the portions of reducible sites on the surface.
The findings suggest that the reducibility of Co3O4 domains, reflected by the number of oxygen vacancies and the [O*]/[*] ratio, governs the reactivity and reaction pathway for N2O dissociation: oxygen vacancies can be generated more effectively by CO reduction than by O2 evolution and therefore Co3O4 domains showed higher reactivity in N2O-CO reaction than in N2O decomposition; Co3O4 domains with semiconducting supports are more reducible than bulk Co3O4 powders or Co3O4 domains dispersed on insulators, leading to more vacancy site available for catalysis and consequently higher reactivity in both reactions; scavenging of oxygen vacancy (*) by dissociative adsorption of O2 leads to higher [O*]/[*] ratios in N2O decomposition and therefore route I is favored; while scavenging of surface oxygen species (O*) by CO leads to lower [O*]/[*] ratios in N2O-CO reaction and therefore route II is favored. The findings also suggest that N2O-CO reaction on Co3O4 based catalysts represents more promising approach for N2O abatement than N2O decomposition, because it can be operated under stoichiometric CO/N2O ratio at temperature (403 K) that is close to cold start conditions and is not inhibited by O2, while N2O decomposition requires much higher temperatures and is inhibited by O2. The findings shed some lights on the understanding of the catalytic consequences of metal oxide reducibility in catalysis mediated by oxygen vacancy, such as NO oxidation and N2O decomposition, and provide guidance for the development of better N2O abatement strategy.
The authors acknowledge financial support from Ford Motor Company.
References
1. Ravishankara, A. R., Daniel, J. S., Portmann, R. W., Science 326, 123 (2009).
2. Lambert, C., Dobson, D., Gierczak, C., Guo G., Ura, J., Warner, J. Int. J. Powertrains., 3, 4 (2014).
3. Weiss, B.M., Artioli, N., Iglesia, E., ChemCatChem, 4, 1937 (2012).
4. Kapteijn, F., Rodriguez-Mirasol, J., Moulijn, J. A., Appl. Catal. B, 9, 25 (1996) ADDIN EN.REFLIST
See more of this Group/Topical: Catalysis and Reaction Engineering Division