Surface and Structural Features of Pt/Pd-Loaded Mesoporous Silica-Delaminated Zirconium Phosphate Systems

Abstract

Mixed ZrPO4·O2P(OH)2·2H2O−SiO2 (ZrPSi-x) mesoporous materials containing variable amounts of SiO2 (x = 53.4−90.8 wt % SiO2) were prepared by the sol−gel method. The original γ-zirconium phosphate (γ-ZrP) material was exfoliated in a water−acetone mixture before mixing with the desired amounts of tetrapropylammonium hydroxide and tetraethyl silicate solution. The textural properties of these calcined solids were evaluated from the nitrogen adsorption−desorption isotherms. The values of the BET area increased with SiO2 loading and reached a maximum value for a SiO2 loading of 79.0%, after which they decreased strikingly at higher loadings. However, pore size distributions were quite similar (3.4−4.5 nm) and narrow in all the samples. Structural characterization of these materials by X-ray diffraction revealed that the positions of the peaks are virtually the same for all the substrates, thus confirming that the γ-ZrP layers remain unaltered during the exfoliation−flocculation and silica-deposition processes. Upon further impregnation of these materials with platinum and palladium salts, nanoparticles of Pt and Pd developed on the support surface. Pt/Pd−ZrPSi-x catalysts were characterized using chemical analysis, nitrogen adsorption−desorption isotherms at 77 K, transmission electron microscopy (TEM), temperature-programmed desorption of NH3 (TPD), X-ray diffraction, FTIR spectroscopy of adsorbed CO, volumetric CO chemisorption, and X-ray photoelectron spectroscopy measurements. The sample Pt/Pd−ZrPSi-64 exhibited only one class of small (5.2 nm determined by CO chemisorption) metal particles, its specific area was rather high (335 m2/g), and it exhibited acidic layers regularly distributed on the silica matrix and a fairly homogeneous metal dispersion. The other samples have metal phases deposited on the external surface, mostly on the SiO2 phase. For the Pt/Pd−ZrPSi-64 sample, the FTIR spectra of CO chemisorbed on the metal phases and photoelectron spectra of Pt 4f and Pd 3d core levels preclude the involvement of electronic effects in the supported Pt and Pd nanoparticles, suggesting that the Pd and Pt particles are separated on the support surface. Observation of the simultaneous hydrogenation of toluene and naphthalene in the presence of small amounts of an S-containing compound (dibenzothiophene) confirmed the impact of these structures on reactivity. It was observed that toluene hydrogenation, which is in turn a demanding hydrogenation reaction, is strongly favored in the Pt/Pd−ZrPSi-64 sample, in which small metal particles developed in the close neighborhood of acid sites located on the support. An explanation of the synergetic effect involving H2 dissociation on the dispersed metal particles followed by migration of H-atoms via spillover until the acid sites where toluene molecules are adsorbed is proposed.