Energy dispersive extended X-ray absorption fine structure, mass spectrometric, and diffuse reflectance infrared studies of the interaction of Al2O3-supported Rh-I(CO)(2)Cl species with NO and re-formation under CO

Newton, Mark A., Burnaby, Daryl G., Dent, Andrew J., Diaz-Moreno, Sofia, Evans, John, Fiddy, Steven G., Neisius, Thomas and Turin, Sandra (2002) Energy dispersive extended X-ray absorption fine structure, mass spectrometric, and diffuse reflectance infrared studies of the interaction of Al2O3-supported Rh-I(CO)(2)Cl species with NO and re-formation under CO. The Journal of Physical Chemistry B, 106 (16), 4214-4222. (doi:10.1021/jp013749k).

Abstract

The interaction between supported Rh-1(CO)(2)Cl species, prepared by metallo-organic chemical vapor deposition (MOCVD) of [Rh(CO)(2)Cl](2) to hydroxylated gamma-Al2O3, and NO has been investigated using time-resolved, energy dispersive extended X-ray absorption fine structure (EDE)/mass spectrometry (MS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). MOCVD of [Rh-I(CO)(2)Cl](2) leads to the formation of a Rh-I(CO)(2)Cl{O-Al} adlayer which, when fresh, reacts with NO to form a majority {Al-O}(2)RhCl(NO-) species at room temperature via a two-step mechanism involving an {Al-O}Rh(NO)(2)Cl species. The application of DRIFTS allows a direct association of the bent RhNO bonding in the {Al-O}(2)RhCl(NO)(-) with "high-wavenumber" Rh(NO-) species displaying (nu)(NO) at ca. 1750 cm(-1) often observed in supported Rh systems. DRIFTS investigations on analogous Rh-1(CO)(2)Cl/TiO2 systems show the same reactivity toward NO, with a bent nitrosyl being formed rather than the more commonly dominant linear Rh(NO+) species. DRIFTS also indicates that a second reaction is possible. This becomes increasingly significant for Rh(CO)(2)Cl{O-Al} samples exposed to air for ca. 2-3 days and results in the Al-O}Rh-I(CO)(2)Cl species reacting with NO to form a new species displaying adsorptions at 2150-2110 and 1750-1700 cm-1. Once formed, this latter species reacts no further at room temperature under NO. The DRIFTS spectrum of this species is interpreted as being due to {AI-O}Rh(CO)(NO)Cl species existing in cis and trans configurations: the isomer with the carbonyl group trans to the Cl ligand being the prefer-red form at room temperature. The reconversion of the Rh(NO-) species under CO shows complex temperature dependence. The consumption of the R-h(NO-) shows only a weak temperature dependence in terms of EDE, but the observed evolution of NO, shows a strong temperature dependence. The combination of EDE and MS indicates rapid formation of an intermediate species, most likely {Al-O}Rh(CO)(NO)Cl, which at room temperature converts to the geminal dicarbonyl species slowly. The possible origins of this behavior, and the parameters determining the formation of "linear" and/ or "bent" rhodium nitrosyls in support Rh systems are discussed.

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