Arany katalizátorokon lejátszódó heterogén katalitikus folyamatok IR és XPS vizsgálata = IR and XPS studies of heterogeneous catalytic reactions on gold catalysts

Reszletes vizsgalatokat vegeztunk az NO + CO kolcsonhatas bemutatasara Pd/SiO2 katalizatoron. Ezt a programot az indokolta, hogy Goodman es munkatarsai (J. Phys. Chem. B 2003, 107, 2759) Pd egykristalyon uj elnyelesi savot eszleltek az NO + CO reakcioban 2258 cm-1-nel , amelyet a Pd-NCO rezgeshez rendeltek. Ez a feltetelezes teljesen ellentetben all korabbi kutatasaink eredmenyeivel, amely sze- rint a Pd-NCO komplex ilyen magas hőmersekleten mar egyaltalan nem stabilis. 2250-2260 cm-1-nel megjelent elnyelesi savokat csak a HNCO adszorpciojakor eszleltuk. Mindebből arra kovetkeztettunk, hogy a Goodmanek altal talalt elnyelesi sav semmikep- pen sem tartozhat a Pd-NCO-hoz. Eredmenyeinket a J. Phys. Chem.B 2004,108 14178- 14180 folyoiratban kozoltuk. Goodman es munkatarsai valaszukban ( Reply to " Com- ment on Combined in situ and infrared kinetic study of the catalytic CO + NO reaction on Pd(III) at pressures up to 240 mbar " Christian Hess, Emrah Ozensoy, and D. Wayne Go- odman ) modositottak korabbi elkepzelesuket es a 2258 cm-1es savot a palladiumbol ki- diffundalo hidrogen es az NCO kozotti kapcsolatbol keletkező HNCO kepződesenek tulajdonitottak. | It was stated long ago that gold is a catalitically inactive metal because of its 5 d10 configu- ration. Recent publications have, however revealed that gold has a surprisingly high cata- litic activity when it is dispersed over metal oxides, particularly in the oxidation of CO at low temperatures and in the NO + CO reaction. In this project the adsorption, dissociation and reaction of NO with CO has been investigated on supported Au catalysts. The main method was FT IR spectroscopy. Although very limited dissociation of NO was observed on reduced Au catalyst, catalytic tests showed that all the samples used effectively catalyse the NO+CO reaction above 573 K. By means of FTIR spectroscopy several new absorption bands have been detected on the Au samples, the positions of new bands depended on the nature of the support. It was 2212 cm-1 for Au/TiO2 2220-2230 cm-1 for Au/MgO 2256 cm-1 for Au/Al2O3 and 2305 cm-1 for Au/SiO2. These bands were attributed to the asymmetric stretch of NCO attached to the oxides. This idea was strengthened by the results obtained following HNCO adsorption on supports alone which gave the same absorption bands. It was demonstrated and assumed that NCO species is formed on Au crystallites, and then migrates from the Au onto the acceptor sites of support where it is accumulated and stabilized. The Au-NCO species was characterized by an absorption band at 2185-2195 cm-1 produced by the low temperature adsorption of HNCO on Au/SiO2.