(555b) N2O Dissociation on Co3O4 Based Catalysts: Reducibility of Co3O4 and its Catalytic Consequences

Authors: 
Hong, Y. - Presenter, University of California, Berkeley
Getsoian, A., University of California - Berkeley
Lambert, C., Ford Research Lab
Iglesia, E., University of California at Berkeley

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

*iglesia@berkeley.edu

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 powdersor
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