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Pressure and Materials Effects on the Selectivity of RuO2 in NH3 Oxidation Javier Pe´rez-Ramı´rez,*,† Nu´ria Lo´pez,*,‡ and Evgenii V. Kondratenko§ Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, HCI E 125, Wolfgang-Pauli-Strasse 10, CH-8093, Zurich, Switzerland, Institute of Chemical Research of Catalonia (ICIQ), AV. Paı¨sos Catalans 16, 43007, Tarragona, Spain, and Leibniz Institute for Catalysis at the UniVersity of Rostock, Albert-Einstein-Strasse 29a, D-18059, Rostock, Germany ReceiVed: July 5, 2010; ReVised Manuscript ReceiVed: August 22, 2010
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 O2/NH3 ) 20 (Wang, Y.; Jacobi, K.; Schone, W.-D.; Ertl, G. J. Phys. Chem. B 2005, 109, 7883). Differently, our steady-state and transient experiments over polycrystalline RuO2 at ambient pressure reveal that N2 is the predominant product. The NO selectivity was as low as 6% at O2/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 N2 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, N2 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 N2 formation. 1. Introduction The oxidation of ammonia to nitric oxide, nitrous oxide, and nitrogen prominently exemplifies the importance of selectivity in heterogeneous catalysis: (i) NH3 oxidation to NO at high temperatures (>973 K) over noble metal alloys or oxides is a key step in the production of nitric acid by the Ostwald process.1-3 (ii) NH3 oxidation to N2O at intermediate temperatures (∼623 K) over alumina-supported bimetallic Mn-Bi catalysts4 attracts increasing attention because nitrous oxide is a unique monooxygen donor for selective oxidation of hydrocarbons.5 (iii) NH3 oxidation to N2 at low temperatures (1.5 eV for RuO2(101)) compared with the formation of N2 and NcusO (1.1-0.58 eV). In the diffusion-controlled regime, the only possible reactions are Ncus + Ncus and Ncus + Ocus. In addition, once formed NcusO can evolve by rotation to neighboring sites and desorb from
the NOcus configuration. In a high-coverage situation characteristic of ambient-pressure operation, the influence of surface diffusion is less important and Ncus + NcusO configurations in neighboring sites can occur. The latter species leads to N2 formation via N2O decomposition because the competing NcusO desorption is hindered. In fact, it is known that supported RuO2 catalysts are highly active for N2O decomposition.37 The high de-N2O activity of bulk RuO2 was verified in our study using feed mixtures containing N2O and N2O + O2 (Figure 10). The N2O conversion in the temperature range of 523-773 K (window used in ammonia oxidation tests) was 100% at 623 K or higher temperature, even in excess O2. 5. Conclusions The pressure and materials gaps in heterogeneous catalysis are a big obstacle to bridge results from surface science experiments over single crystals with “real catalysis” at high pressure and polycrystalline samples. We have unraveled the reasons behind the markedly distinct selectivity character of RuO2 in NH3 oxidation at ambient pressure (this work) and under UHV conditions in earlier UHV experiments by Wang et al.15 Although the UHV experiments over RuO2(110) yielded a NO selectivity approaching 100% at O2/NH3 ) 20, our ambient-pressure experiments over polycrystalline RuO2 did not exceed 65% at O2/NH3 ) 140. To understand this behavior, DFT was employed to calculate the competing reaction paths
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on two representative RuO2 surfaces, (110) and (101). Whereas NO is formed by the direct reaction of coadsorbed N and O atoms, N2 can be formed through two different routes: direct N + N recombination or decomposition of N2O. This second path is only open at high pressures because then sizable coverages are present. Regarding the materials effect, the barriers for product formation and evolution to the gas phase are smaller on the (101) surface when compared with those of the (110). However, the energy profiles are parallel and thus cannot account for the selectivity differences observed. A quantitative comparison to the experiments would require a detailed evaluation of the coverages through highly dedicated kinetic Monte Carlo (kMC) approaches. Acknowledgment. Amol P. Amrute and Vera Go¨lden are acknowledged for experimental input. The Spanish MICINN (CTQ2009-07553/BQU and Consolider-Ingenio 2010 Grant CSD2006-0003) is acknowledged for financial support and the BSC-RES for allocation of computational resources. Supporting Information Available: Binding energies of intermediates as a function of the coverage for the (110) surface. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Sadykov, V. A.; Isupova, L. A.; Zolotarskii, I. A.; Bobrova, L. N.; Noskov, A. S.; Parmon, V. N.; Brushtein, E. A.; Telyatnikova, T. V.; Chernyshev, V. I.; Lunin, V. V. Appl. Catal., A 2000, 204, 59. (2) Pe´rez-Ramı´rez, J.; Kondratenko, E. V.; Kondratenko, V. A.; Baerns, M. J. Catal. 2004, 227, 90. (3) Pe´rez-Ramı´rez, J.; Vigeland, B. Angew. Chem., Int. Ed. 2005, 44, 1112. (4) Ivanova, A. S.; Slavinskaya, E. M.; Mokrinskii, V. V.; Polukhina, I. A.; Tsybulya, S. V.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Rogov, V. A.; Zaikovskii, V. I.; Noskov, A. S. J. Catal. 2004, 221, 2134. (5) Parmon, V. N.; Panov, G. I.; Uriarte, A.; Noskov, A. S. Catal. Today 2005, 100, 115. (6) Li, Y.; Armor, J. N. Appl. Catal., B 1997, 13, 131. (7) Gang, L.; Anderson, B. G.; van Grondelle, J.; van Santen, R. A.; van Gennip, W. J. H.; Niemantsverdriet, J. W.; Kooyman, P. J.; Knoester, A.; Brongersma, H. H. J. Catal. 2002, 206, 60. (8) Golodets, G. I. Stud. Surf. Sci. Catal. 1983, 15, 312–364. (9) Schriber, T. J.; Parravano, G. Chem. Eng. Sci. 1967, 22, 1067. (10) Cui, X.; Zhou, J.; Ye, Z.; Chen, H.; Li, L.; Ruan, M.; Shi, J. J. Catal. 2010, 270, 310.
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