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

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