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1562652 
Journal Article 
Pressure and Materials Effects on the Selectivity of RuO2 in NH3 Oxidation 
Perez-Ramirez, J; Lopez, N; Kondratenko, EV 
2010 
Yes 
Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 
114 
39 
16660-16668 
WOS:000282209800075 
The pressure and materials gaps in heterogeneous catalysis
often complicate the extrapolation of results from surface science experiments over single
crystals to real catalysis at elevated pressures and polycrystalline samples. Previous ammonia
oxidation studies reported ca. 100% NO selectivity and the absence of N2O on RuO2(110) in
ultrahigh vacuum (UHV) at 530 K, p(NH3) = 10(-7) mbar, and O-2/NH3 = 20 (Wang, Y.; Jacobi, K.;
Schone, W.-D.; Ertl, G. J. Phys. Chem. 13 2005, 109, 7883). Differently, our steady-state and
transient experiments over polycrystalline RuO2 at ambient pressure reveal that N-2 is the
predominant product. The NO selectivity was as low as 6% at O-2/NH3 = 2 and reached a maximum of
65% at the highest temperature (773 K) and effective oxygen-to-ammonia ratio of 140, whereas the
maximum N2O selectivity was 25% at 100% NH3 conversion. Density functional theory simulations of
the competing paths leading to NO, N2O, and N-2 over RuO2(110) and RuO2(101) at different
coverages by O- and N-containing species provided insights into the selectivity differences
between the extreme operation regimes. Comparison between the (101) and (110) facets reveals that
the materials effect is not likely to explain the different product distribution. Instead. the
pressure effect (8 orders of magnitude higher at ambient pressure than in UHV) does. Whereas NO
is formed by the direct reaction of coadsorbed N and O atoms, N-2 can be formed through two
different routes: direct N N recombination or N2O decomposition. The second path is only likely
at high pressures because it implies more diffusion steps of surface species, which are highly
unlikely at low coverage. Thus, the main pressure effect is to facilitate alternative routes for
N-2 formation.