Surface and Structural Features of Pt/Pd-Loaded Mesoporous Silica-Delaminated Zirconium Phosphate Systems
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- 作者：B. Pawelec ; S. Murcia-Mascaró ; s ; and J. L. G. Fierro
- 刊名：Langmuir
- 出版年：2002
- 出版时间：October 15, 2002
- 年：2002
- 卷：18
- 期：21
- 页码：7953 - 7963
- 全文大小：364K
- 年卷期：v.18,no.21(October 15, 2002)
- ISSN：1520-5827
文摘
Mixed ZrPOB>4·O2P(OH)2·2H2O-SiO2 (ZrPSi-x) mesoporous materials containing variable amounts ofSiO2 (x = 53.4-90.8 wt % SiO2) were prepared by the sol-gel method. The original 
-zirconium phosphate(-ZrP) material was exfoliated in a water-acetone mixture before mixing with the desired amounts oftetrapropylammonium hydroxide and tetraethyl silicate solution. The textural properties of these calcinedsolids were evaluated from the nitrogen adsorption-desorption isotherms. The values of the BET areaincreased with SiO2 loading and reached a maximum value for a SiO2 loading of 79.0%, after which theydecreased 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 revealedthat the positions of the peaks are virtually the same for all the substrates, thus confirming that the -ZrPlayers remain unaltered during the exfoliation-flocculation and silica-deposition processes. Upon furtherimpregnation of these materials with platinum and palladium salts, nanoparticles of Pt and Pd developedon the support surface. Pt/Pd-ZrPSi-x catalysts were characterized using chemical analysis, nitrogenadsorption-desorption isotherms at 77 K, transmission electron microscopy (TEM), temperature-programmed desorption of NH3 (TPD), X-ray diffraction, FTIR spectroscopy of adsorbed CO, volumetricCO chemisorption, and X-ray photoelectron spectroscopy measurements. The sample Pt/Pd-ZrPSi-64exhibited only one class of small (5.2 nm determined by CO chemisorption) metal particles, its specific areawas rather high (335 m2/g), and it exhibited acidic layers regularly distributed on the silica matrix anda fairly homogeneous metal dispersion. The other samples have metal phases deposited on the externalsurface, mostly on the SiO2 phase. For the Pt/Pd-ZrPSi-64 sample, the FTIR spectra of CO chemisorbedon the metal phases and photoelectron spectra of Pt 4f and Pd 3d core levels preclude the involvementof electronic effects in the supported Pt and Pd nanoparticles, suggesting that the Pd and Pt particles areseparated on the support surface. Observation of the simultaneous hydrogenation of toluene and naphthalenein the presence of small amounts of an S-containing compound (dibenzothiophene) confirmed the impactof these structures on reactivity. It was observed that toluene hydrogenation, which is in turn a demandinghydrogenation reaction, is strongly favored in the Pt/Pd-ZrPSi-64 sample, in which small metal particlesdeveloped in the close neighborhood of acid sites located on the support. An explanation of the synergeticeffect involving H2 dissociation on the dispersed metal particles followed by migration of H-atoms viaspillover until the acid sites where toluene molecules are adsorbed is proposed.
-zirconium phosphate(-ZrP) material was exfoliated in a water-acetone mixture before mixing with the desired amounts oftetrapropylammonium hydroxide and tetraethyl silicate solution. The textural properties of these calcinedsolids were evaluated from the nitrogen adsorption-desorption isotherms. The values of the BET areaincreased with SiO2 loading and reached a maximum value for a SiO2 loading of 79.0%, after which theydecreased 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 revealedthat the positions of the peaks are virtually the same for all the substrates, thus confirming that the -ZrPlayers remain unaltered during the exfoliation-flocculation and silica-deposition processes. Upon furtherimpregnation of these materials with platinum and palladium salts, nanoparticles of Pt and Pd developedon the support surface. Pt/Pd-ZrPSi-x catalysts were characterized using chemical analysis, nitrogenadsorption-desorption isotherms at 77 K, transmission electron microscopy (TEM), temperature-programmed desorption of NH3 (TPD), X-ray diffraction, FTIR spectroscopy of adsorbed CO, volumetricCO chemisorption, and X-ray photoelectron spectroscopy measurements. The sample Pt/Pd-ZrPSi-64exhibited only one class of small (5.2 nm determined by CO chemisorption) metal particles, its specific areawas rather high (335 m2/g), and it exhibited acidic layers regularly distributed on the silica matrix anda fairly homogeneous metal dispersion. The other samples have metal phases deposited on the externalsurface, mostly on the SiO2 phase. For the Pt/Pd-ZrPSi-64 sample, the FTIR spectra of CO chemisorbedon the metal phases and photoelectron spectra of Pt 4f and Pd 3d core levels preclude the involvementof electronic effects in the supported Pt and Pd nanoparticles, suggesting that the Pd and Pt particles areseparated on the support surface. Observation of the simultaneous hydrogenation of toluene and naphthalenein the presence of small amounts of an S-containing compound (dibenzothiophene) confirmed the impactof these structures on reactivity. It was observed that toluene hydrogenation, which is in turn a demandinghydrogenation reaction, is strongly favored in the Pt/Pd-ZrPSi-64 sample, in which small metal particlesdeveloped in the close neighborhood of acid sites located on the support. An explanation of the synergeticeffect involving H2 dissociation on the dispersed metal particles followed by migration of H-atoms viaspillover until the acid sites where toluene molecules are adsorbed is proposed.