Single crystal and high area titania supported rhodium: the interaction of supported Rh(CO) 2 with NO

Abstract

Model Rh/TiO 2 catalysts have been prepared by metal organic chemical vapour deposition (MOCVD) of [Rh(CO) 2Cl] 2 to TiO 2(1 1 0) and high area (Degussa P25) powder TiO 2 samples. The rhodium geminal dicarbonyl species (Rh(CO) 2) is produced on each surface, and the reaction of the supported Rh(CO) 2 with NO has been studied using FT-RAIRS, transmission FT-IR and XPS. TiO 2(1 1 0)–Rh(CO) 2 is converted by exposure to NO at 300 K solely to a highly dispersed Rh(NO) + species with ν(NO) observed as a transmission band in FT-RAIRS at 1920 cm −1. This species is thermally more stable than the geminal dicarbonyl species, and XPS measurements indicate that the NO is removed without the formation of adsorbed nitrogen residues by 600 K. Re-exposure to CO results in the complete regeneration of TiO 2(1 1 0)–Rh(CO) 2 from TiO 2(1 1 0)–Rh(NO) +. TiO 2(P25)–Rh(CO) 2 is present in a variety of surface environments, with broad bands observed at the same frequencies as for TiO 2(1 1 0)–Rh(CO) 2 ( ν sym(CO)=2110 cm −1 and ν asym(CO)=2030 cm −1). Exposure of TiO 2(P25)–Rh(CO) 2 to NO at 300 K results in the formation of TiO 2(P25)–Rh(NO) + with ν(NO) at 1920 cm −1, and TiO 2(P25)–Rh(CO)(NO) with ν(NO) at 1750 cm −1 and ν(CO) at 2110 cm −1. TiO 2(P25)–Rh 0 clusters, formed through the thermal decomposition of TiO 2(P25)–Rh(CO) 2 at various temperatures, react with NO to produce additional surface nitrosyl species. On a surface heated to 380 K where TiO 2(P25)–Rh(CO) 2 decarbonylation has only just taken place, reaction with NO at 300 K results in the formation of the same species as those produced through the reaction of TiO 2(P25)–Rh(CO) 2 directly NO, i.e. (TiO 2(P25)–Rh(NO) +) and TiO 2(P25)–Rh(CO)(NO). Re-exposure of this surface to CO results in the complete reconversion of the dispersed nitrosyl to TiO 2(P25)–Rh(CO) 2. When larger clusters are formed on a surface by heating to 650 K, reaction with NO leads initially to the adsorption of linear and bridged bound NO on TiO 2(P25)–Rh 0 with respective ν(NO) bands observed in the IR at 1818 and 1680 cm −1. Further exposure of NO, however, results in the complete disruption of the TiO 2(P25)–Rh 0 clusters. This is evidenced by the disappearance of the bridging and linear bands and the appearance of a strong band associated with ν(NO) of TiO 2(P25)–Rh(NO) + at 1920 cm −1, and bands at 1745 and 1550 cm −1 assigned to dispersed TiO 2(P25)–Rh(NO) − and TiO 2(P25)–Rh(NO 2) −/(NO 3) −. The latter species we suggest are stabilised by surface defects such as oxygen vacancies which may have been formed during the clustering of the Rh 0. After thermal treatment to temperatures of 800 K, where encapsulation of the TiO 2(P25)–Rh 0 clusters should occur, we see a strong suppression of NO adsorption on the clusters. Nevertheless disruption of clusters still takes place with the formation of TiO 2(P25)–Rh(NO) − even under these (SMSI) conditions.