Preparation of Pd/SiO2 Catalysts for Methanol Synthesis. II. Thermal Decomposition of [Pd(NH3)4]2+/SiO2

A. L. BONIVARDI; M. A. BALTANÁS

THERMOCHIMICA ACTA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1991 vol. 191 p. 63 - 94

0040-6031

As a part of an experimental program aimed at the systematic analysis of the controlled deposition of Pd/SiO, on well characterized macroporous and microporous silica gels (Davison G-59, 254 m2 g-? and G-03, 558 m2 g-?) the nature of the surface species upon drying and thermal decomposition of the tetrammin epalladium complex (TPSiO) obtained by ion exchange (IE) of palladium acetate in aqueous ammonium hydroxide (Pd loading: 0.5-ll%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 = 393 K) leaves a stable diamminepalladium complex (DPBO) 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-? (N,) and 2-32 K mir-? (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 NH, 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 N, is used, but in air several signals indicate that a sequence of transformations of the Pd occurs. Ultradispersed Pd? is the final product on both catalyst types when N, is the decomposing atmosphere, whereas either a mixture of Pd? +[(SiO),12- Pd?+ (on the microporous G-03) or pure [(SiO),12- Pd2+ (on the macroporous G-59) are the final products when air is employed.