First-principles Study On NH₃and NO Adsorption And Dissociation On Ir Sufrace

Due to its great importance in solving the energy crisis as well as inpollution control, the surface chemistry of ammonia dissociation andoxidation, and the reduction of nitrogen oxides （NO_x） have been extensivelywell studied. Among transition metal catalysts used for these reactions,Ir-based catalysts have attracted special attention and already been used as apotential contender to replace the currently used catalysts, due to their highlow-temperature activity and selectivity. Since the limitations of experimentalmethods, the mechanism of these reactions on Ir surfaces have not beenintensively studied, and the key factors of activity and selectivity have notbeen attained. In the present study, the adsorption and dissociation of NH₃andNO on several iridium single surfaces were studied on the basis of periodicdensity functional theory and microkinetic modeling, and some information ofthermodynamics and kinetics were obtained. The present study provides adeep insight into the trends of activity and selectivity of different iridiumsurfaces on defferent temperature and ratios of NH₃/O₂. The main results wereproposed as follows:1. The mechanism of ammonia decomposition and oxidation on Ir（110）were studied. The results indicate that NH₃dissociation is more favorable thandesorption at atop site, while at top site NH₃desorption and dissociation arecompetitive. On the other hand, when O or OH is coadsorbed, the NH₃dehydrogenation is slightly inhibited and mainly via hydrogen abstractionreaction rather than thermal decomposition, while it is reversed for NH₂dehydrogenation. The former mechanism is favored for O assisted NHdehydrogenation, while it changed to latter one for OH. On clean Ir（110）, N+NHâ†'N₂+H pathway is the major N₂formation pathway and N+N is alsoinvolved but less competitive, while N+N becomes the predominant one andis enhanced on O-predosed Ir（110）. NO formation occurs only at highertemperature when N₂is desorbed from the surface. The microkinetic analysisfurther confirms that the dominant product is N₂at low temperature whilebecomes NO as temperature increases, and the temperature of NO formation decreases when O₂partial pressure increases.2. The mechanism of ammonia decomposition and oxidation on Ir（100）and Ir（111） surfaces were studied. It is shown that ammonia dissociation anddesorption on Ir（100） are competitive, while it doesn’t decomposition onIr（111）. In both cases, the reaction of NH₃dehydrogenation were promoted bythe predosed oxygen or hydroxyl, and the temperature of NH₃desorption werealso increased. The predosed oxygen or hydroxyl promotes NH₃and NHdehydrogenation steps, while NH₂dehydrogenation is slightly inhibitedrelative to clean Ir（100）. In both cases, the hydrogen transfer from NHxspecies to predosed O or OH is favored over thermal decomposition of NHx.Furthermore, the predosed O exhibits higher activity on NH₃and NHdehydrogenation steps than OH, while the case is reversed for NH₂. On cleanIr（100）, N+N pathway is the major N₂formation pathway when TPDexperiment starts from200K, and N+NH is also involved but lesscompetitive; however, three pathways N+N, N+NH and NH+NH are allpossible with respect to TPD experiment starting from410K. On O-andOH-predosed Ir（100）, N+N pathway is the predominant pathway and isenhanced by the predosed O or OH. The microkinetic analysis furtherconfirms that N₂is the resulting product at different temperatures and ratios ofNH₃/O₂, and the formation of NO is unfavorable. With respect to Ir（111）surface, the hydrogen abstraction reaction is the major NHxdehydrogenationmechanism, while the thermal decomposition is less competitive. In addition,the predosed OH exhibits higher activity on NH₃and NH₂dehydrogenationsteps than O, while the case is reversed for NH. The high selectivity towardN₂of Ir（100） and Ir（111） surfaces is mainly due to the high NO dissociationactivity.3. The mechanism of NO adsorption and dissociation on clean andCO-predosed Ir（100） and Ir（111） surfaces, and the effect of predosed O onIr（100） surface were studied. The results show that NO prefers the bridge siteson clean and O-predosed Ir（100） surface, and top site is less favorable. Twodissociation pathways for the adsorbed NO on bridge and top site are located:One is a direct decomposition of NO and the other is diffusion of NO from theinitial state to the hollow site followed by dissociation into N and O atoms.The latter pathway is more favorable than the former one due to the lower energy barrier and is the primary pathway for NO dissociation. Based on theDFT results, microkinetic analysis suggests that the recombination of two Nadatoms on the di-bridge sites is the predominant pathway for N₂formation,whereas the formation of N₂O or NO₂is unlikely to occur during NO reduction.The predosed O atoms inhibit the dissociation of NO at the bridge and top sitesin different degrees, leading to the competition of NO dissociation on bothsites. N₂is still the predominant product, and N₂O and NO₂are unlikely to beproduced on O-predosed Ir（100）. Compared with clean Ir（100）, the predosed Oatoms on the surface have almost no impact on the formation of N₂at low N/Ocoverage, while promote the N₂formation at high N/O coverage. With respectto Ir（111） surface, top, three fold Hcp and Fcc are all possible sites for NOadsorption, and the three fold Hcp and Fcc sites adsorbed NO dissociate, whiletop site one doesn’t dissociate. N NO N₂O N₂Ois the predominantN₂formation pathway, while theN N N₂pathway is also involved butless competitive. The predosed CO can react with N or O atom forming NCOand CO₂. Since the very low reaction barrier of, the coverage of Natoms is really low, which indicates that CO₂is the resulting product, whereasthe formation of NCO is less competitive. There are larger numbers of N and Oatoms on Ir （111） surface due to the larger reaction barrier of N₂formation,resulting the reaction of NCO and CO₂formation are competitive.Based on the calculated results obtained above, iridium possesses highactivity and selectivity on the reactions of ammonia decomposition andoxidation, and NO reduction, well consistent with experimental observation.The present study provide a deep insight into the nature of adsorbates andiridium metal surface as well as further understanding the catalytic process,and may be helpful in the search of better and much effective catalyst.