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Title: Structure-activity correlations on Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} thin film model catalysts after oxidation and reduction

Abstract

A study of the effect of different supports on the stability and catalytic activity of Rh nanoparticles is only meaningful if the metal particles are perfectly identical in the catalysts to be compared. The authors have applied epitaxial thin film deposition to produce homogeneous distributions of well-faceted Rh nanocrystals, divided the sample in two, and subsequently supported the metal particles by Al{sub 2}O{sub 3} and TiO{sub 2}. Three corresponding pairs of Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} model catalysts with mean particle sizes of 7.8, 13.3, and 16.7 nm were prepared and activated by different oxidation-reduction treatments at temperatures up to 723 K. The size, morphology, and structure of the metal particles and of the support and the changes upon activation were determined by transmission electron microscopy. Wetting of the support and coalescence of Rh particles were observed to occur upon high-temperature reduction of large (>10 nm) and closely-spaced Rh particles, around 623 K on titania and around 723 K on alumina. Catalysts with smaller Rh particles did not show such pronounced changes. The rate of ring opening of methylcyclobutane at 373 K was measured on the three pairs of Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} model catalysts as amore » function of the reduction temperature after preoxidation at 723 K. Rh/alumina catalysts exhibited maximum activity after reduction at 523 K due to the formation of low-coordinated sites as shown by electron microscopy. The activity of Rh/titania peaked after reduction at 373 K and decreased almost exponentially with T{sub red} up to 673 K. In the case of Rh/titania, the changes in particle size and microstructure as observed in the electron microscope can only account for part of the kinetic results. The rapid activity decrease in titania-supported Rh particles with increasing T{sub red} is paralleled to an increase in the number of oxygen vacancies and in low-valent Ti cations, on the titania surface. The latter may affect the electric field at the metal-support boundary and thereby reduce the catalytic activity for hydrocarbon reactions. In addition, the decoration of Rh particles by migrating Ti suboxide will account for the diminished activity after reduction at higher temperature.« less

Authors:
; ; ;  [1]
  1. Leopold-Franzens-Univ. Innsbruck (Austria). Inst. of Physical Chemistry
Publication Date:
OSTI Identifier:
687735
Resource Type:
Journal Article
Journal Name:
Journal of Catalysis
Additional Journal Information:
Journal Volume: 186; Journal Issue: 1; Other Information: PBD: 15 Aug 1999
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 40 CHEMISTRY; RHODIUM; ALUMINIUM OXIDES; TITANIUM OXIDES; CATALYSTS; CATALYST SUPPORTS; CONFIGURATION INTERACTION; CATALYTIC EFFECTS; PARTICLE SIZE; MICROSTRUCTURE

Citation Formats

Rupprechter, G., Seeber, G., Goller, H., and Hayek, K. Structure-activity correlations on Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} thin film model catalysts after oxidation and reduction. United States: N. p., 1999. Web. doi:10.1006/jcat.1999.2555.
Rupprechter, G., Seeber, G., Goller, H., & Hayek, K. Structure-activity correlations on Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} thin film model catalysts after oxidation and reduction. United States. doi:10.1006/jcat.1999.2555.
Rupprechter, G., Seeber, G., Goller, H., and Hayek, K. Sun . "Structure-activity correlations on Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} thin film model catalysts after oxidation and reduction". United States. doi:10.1006/jcat.1999.2555.
@article{osti_687735,
title = {Structure-activity correlations on Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} thin film model catalysts after oxidation and reduction},
author = {Rupprechter, G. and Seeber, G. and Goller, H. and Hayek, K.},
abstractNote = {A study of the effect of different supports on the stability and catalytic activity of Rh nanoparticles is only meaningful if the metal particles are perfectly identical in the catalysts to be compared. The authors have applied epitaxial thin film deposition to produce homogeneous distributions of well-faceted Rh nanocrystals, divided the sample in two, and subsequently supported the metal particles by Al{sub 2}O{sub 3} and TiO{sub 2}. Three corresponding pairs of Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} model catalysts with mean particle sizes of 7.8, 13.3, and 16.7 nm were prepared and activated by different oxidation-reduction treatments at temperatures up to 723 K. The size, morphology, and structure of the metal particles and of the support and the changes upon activation were determined by transmission electron microscopy. Wetting of the support and coalescence of Rh particles were observed to occur upon high-temperature reduction of large (>10 nm) and closely-spaced Rh particles, around 623 K on titania and around 723 K on alumina. Catalysts with smaller Rh particles did not show such pronounced changes. The rate of ring opening of methylcyclobutane at 373 K was measured on the three pairs of Rh/Al{sub 2}O{sub 3} and Rh/TiO{sub 2} model catalysts as a function of the reduction temperature after preoxidation at 723 K. Rh/alumina catalysts exhibited maximum activity after reduction at 523 K due to the formation of low-coordinated sites as shown by electron microscopy. The activity of Rh/titania peaked after reduction at 373 K and decreased almost exponentially with T{sub red} up to 673 K. In the case of Rh/titania, the changes in particle size and microstructure as observed in the electron microscope can only account for part of the kinetic results. The rapid activity decrease in titania-supported Rh particles with increasing T{sub red} is paralleled to an increase in the number of oxygen vacancies and in low-valent Ti cations, on the titania surface. The latter may affect the electric field at the metal-support boundary and thereby reduce the catalytic activity for hydrocarbon reactions. In addition, the decoration of Rh particles by migrating Ti suboxide will account for the diminished activity after reduction at higher temperature.},
doi = {10.1006/jcat.1999.2555},
journal = {Journal of Catalysis},
number = 1,
volume = 186,
place = {United States},
year = {1999},
month = {8}
}