PREPARATION OF PD SIO2 CATALYSTS FOR METHANOL SYNTHESIS .2. THERMAL-DECOMPOSITION OF [PD(NH3)4]2+ SIO2

As a part of an experimental program aimed at the systematic analysis of the controlled deposition of Pd/SiO2 on well characterized macroporous and microporous silica gels (Davison G-59, 254 m2 g-1 and G-03, 558 m2 g-1) the nature of the surface species upon drying and thermal decomposition of the tetramminepalladium complex (TPSiO) obtained by ion exchange (IE) of palladium acetate in aqueous ammonium hydroxide (Pd loading: 0.5-11%Pd w/w), has been systematically followed by the combined use of diffuse reflectance spectroscopy (DRS), differential scanning calorimetry (DSC), thermogravimetric analysis (TG), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). DRS indicates that drying in air (T almost-equal-to 393 K) leaves a stable diamminepalladium complex (DPSiO) on the surfaces. Decomposition of DPSiO upon calcination in inert and oxidizing atmospheres was studied from 308 to 773 K, with heating rates of 4-64 K min-1 (N2) and 2-32 K min-1 (air). Two decomposition zones are identified with DSC and TG. (1) In the low-temperature region (308-473 K) endothermal signals which correspond to a liberation of NH3 around 359 K (G-59) or 371 K (G-03) were observed; (2) the high-temperature region (473-773 K) only shows endothermal peaks when N2 is used, but in air several signals indicate that a sequence of transformations of the Pd occurs. Ultradispersed Pd0 is the final product on both catalyst types when N2 is the decomposing atmosphere, whereas either a mixture of Pd0 + [(SiO)2]2-Pd2+ (on the microporous G-03) or pure [(SiO)2]2-Pd2+ (on the macroporous G-59) are the final products when air is employed.