ARTICLE pubs.acs.org/JPCC
Mechanisms for Selective Catalytic Oxidation of Ammonia over Vanadium Oxides Ru-Ming Yuan, Gang Fu, Xin Xu,* and Hui-Lin Wan* State Key Laboratory for Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Department of Chemistry, Xiamen University, Xiamen, 361005, China ABSTRACT: Selective catalytic oxidation (SCO) of ammonia to a nitrogen molecule is an important process involved in many applications such as removing NH3 slip in selective catalytic reduction (SCR) of NOx, reducing the NH3 concentration from biomass-derived fuels, etc. Here we perform density functional theory calculations in conjunction with cluster models to investigate the SCO mechanisms on V2O5 surfaces. Our calculations show that, at the initial stage, NH3 can be activated by transferring an electron to the metal oxide surfaces, giving rise to an NH3+ intermediate. We disclose that the subsequent pathways are strongly dependent on the availability of the gaseous species. When oxygen is limited or absent, N2H4 can be produced from NH3+ reacting with a second NH3 or from two activated intermediates (e.g., NH2 + NH2 or ONH2 + NH2), and oxidation of N2H4 into N2 by VdO is viable. On the other hand, when oxygen is abundant, NH3+ will react with O2 to make a NH3+ 3 3 3 O2 complex. Such a species will quickly decompose into NO, which switches on the selective catalytic reduction (SCR) reaction, eventually leading to the formation of N2. We propose that the combination of an efficient Ostwald reaction catalyst for NH3 to NO transformation with a capable SCR reaction catalyst for NO reduction by NH3 to N2 can lead to a good candidate catalyst for SCO at the high O2/NH3 ratio condition.
1. INTRODUCTION Ammonia is a colorless toxic gas with a pungent, suffocating odor. Although in wide use, ammonia is caustic to human tissues. If its concentration exceeds 50100 ppm, ammonia will cause serious damage to the eyes, throat, and nose.1 Moreover, ammonia emitted from livestock as well as industrial processes has already made a significant contribution to the acidification of the environment via its oxidized form NOx.2 Nowadays, a variety of technologies, including thermal and catalytic oxidation processes, physical separation, scrubbing, and biofiltration, have been applied to abate ammonia from waste gas.1 Among them, selective catalytic oxidation (SCO) of ammonia to nitrogen molecule is believed to be one of the most efficient and promising technologies.3 It is a subject of broad interest, closely related to a wide range of applications, including removing NH3 slip in selective catalytic reduction (SCR) of NOx, reducing the NH3 concentration from biomassderived fuels, etc.35 In the course of SCO, molecular oxygen is used as oxidizing agent and the main reaction proceeds in the following way 4NH3 þ 3O2 f 2N2 þ 6H2 O
ð1Þ
However, besides the desired products, other nitrogen-containing species, such as N2O and NO, can be formed simultaneously via reactions 2 and 3 2NH3 þ 2O2 f N2 O þ 3H2 O
ð2Þ
4NH3 þ 5O2 f 4NO þ 6H2 O
ð3Þ
r 2011 American Chemical Society
Reactions 13 are all of practical importance. Notably, reaction 3 is the basis of industrial manufacture of nitric acid (the Ostwald process6). From the viewpoint of thermodynamics, all these reactions are strongly exothermic. Thus, how to harness these oxidation processes on the road to the desired direction depends critically on our knowledge of the reaction mechanisms. On the other hand, ammonia oxidation is an undesired side reaction in company with SCR. In a typical SCR, the ratio of NO to NH3 is 1:1 as represented in reaction 4 4NO þ 4NH3 þ O2 f 4N2 þ 6H2 O
ð4Þ
When the oxidation of ammonia (see reactions 13) is also initiated, the NO conversion that follows reaction 4 will be significantly decreased. Hence, a big issue in SCR is to minimize the activity of ammonia oxidation.79 Currently, the catalytic technologies for reactions 3 and 4 are well established and have been used world widely. However, the SCO process is a new technology and still under development. In their earlier work, Il’chenko et al.3,10,11 have compared the catalytic performance of a large number of catalysts including metals and metal oxides and concluded that acidic metal oxides, such as V2O5, MoO3, and WO3, exhibit excellent N2 selectivity at low temperature (i.e., near 100% at 230 °C). Interestingly, these oxides are just the key components in the industrial SCR catalyst Received: July 2, 2011 Revised: September 23, 2011 Published: September 26, 2011 21218
dx.doi.org/10.1021/jp206265p | J. Phys. Chem. C 2011, 115, 21218–21229
The Journal of Physical Chemistry C V2O5W(Mo)O3/TiO2. Indeed, many research papers on the SCO process have been reported in connection with SCR.7 Despite of numerous mechanistic studies concerning the SCO reaction, two key questions remain elusive: (i) how NH3 is activated; (ii) how NN bond in N2 is formed. Although the reaction should start with activation of the NH bond in NH3, various mechanisms, as well as plenty of intermediates, have been proposed in the literature. On the basis of isotopic labeling study, Janssen et al.12,13 suggested that ammonia could dissociatively adsorb on two neighboring VdO sites, giving rise to an aminooxy species ONH2. This proposal was challenged by Ozkan et al.,14,15 who claimed that ONH2 was too active, which should better serve as a precursor for oxidation of NH3 to NO. Instead, they proposed an ONH3 species that was responsible for the formation of N2 and N2O. By means of FTIR, Ramis and Busca et al.1620 systematically examined the adsorption and transformation of ammonia over a diversity of oxide catalysts. Their studies led them to conclude that ammonia was activated on a Lewis acid site to give an amide species (NH2) on central metal and a hydrogen atom on the lattice oxygen. Such a picture was supported by semiempirical MSINDO calculations of Jug et al.21 and density functional theory (DFT) slab model calculations of Vittadini et al.22 on model systems of V2O5/TiO2. On the other hand, Topsøe et al.2325 supposed a kinetic role for an NH3+ species, which was assumed to be produced by transferring an H atom from NH4+ to V5+dO, where NH4+ was generated by NH3 adsorption on a Brønstend acid site. Kobayashi et al.26 performed CASSCF calculations to show that NH3+ species can indeed be stabilized by adsorption on V2O5 surfaces. Nevertheless, previous work of Ramis and Busca et al. suggested that Brønsted acidity was not directly involved nor required in the SCO reaction.17 For the NN bond formation, there exist two different opinions. The first one may be called a direct route where two active NHx (x = 02) intermediates, either on lattice oxygen or on metal, combine to make a dinitrogen species. By means of XPS, Janssen et al.27 proposed that there existed a OHN NHO species on MoO3/SiO2, which was derived from oxidative coupling of two O-NH2 species. By using FTIR, Ramis and Busca et al.,1620 on the other hand, suggested that two V-NH2 species could dimerize into a N2H4 species, which was further oxidized into N2. On the basis of HREELS, Jacobi et al.28 found that NH2 is not stable over RuO2(110) surfaces, which would undergo dehydrogenation to produce adsorbed N, followed by coupling with another N atom to make N2. The other route for NN bond formation is often referred as internal or in situ SCR (iSCR) where NH3 is first oxidized into NO, which can then be reduced by NHx intermediates or unreacted NH3 to N2. Experimentally, both NH2 and NO intermediates have been identified by Matyshak et al.2932 during ammonia oxidation. In addition, Curtin et al.33 found that the amount of NO detected in the product stream increased with the decrease of the percent conversion of NH3 or the contact time over a CuO/Al2O3 catalyst. This indicated that NO was the primary product and the reduction to N2 was achieved by SCR. Similarly, on a Fe zeolite system, results of Yang et al.34 and Akah et al.35,36 supported the iSCR mechanism. Low temperature SCO catalysts (
