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Title: Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES

Abstract

X-ray absorption spectroscopy (XAS) was utilized to investigate both the Au particle size on several supports composed by silica and titanium oxide, and the coordination of TiO2 in the support phase. Particularly, we wanted to utilize the technique to probe mixing in the support phase, by using different synthetic methods such as by functionalizing silica or by ALD (atomic layer deposition) techniques as prepared in our laboratories, and the growth and stability of Au nanoparticles deposited on these supports. The study using cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2 resulted in TiO2 being dispersed in the SiO2 matrix; however, a second phase starts forming as the TiO2 content increases as indicated by the EXAFS Ti-O shell shift in position and increase of coordination number. On the supports prepared by cosynthesis, Au particles were smaller and more stable. The study using the surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2 produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. Although undercoordinated, the presence of the Ti-Ti shells indicate that the titania species are not molecularly dispersed onmore » the SiO2 surface as hypothesized, but there is indeed a cross-linking of the titania moieties.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
960110
Report Number(s):
BNL-83096-2009-JA
Journal ID: ISSN 1932-7447; TRN: US201016%%1254
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry C; Journal Volume: 111; Journal Issue: 46
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTROSCOPY; ATOMS; CATALYSTS; COORDINATION NUMBER; CROSS-LINKING; DEPOSITION; OXIDES; PARTICLE SIZE; SILICA; STABILITY; TITANIUM OXIDES; SILICON OXIDES; national synchrotron light source

Citation Formats

Schwartz,V., Mullins, D., Yan, W., Zhu, H., Dai, S., and Overbury, S. Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES. United States: N. p., 2007. Web. doi:10.1021/jp074426c.
Schwartz,V., Mullins, D., Yan, W., Zhu, H., Dai, S., & Overbury, S. Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES. United States. doi:10.1021/jp074426c.
Schwartz,V., Mullins, D., Yan, W., Zhu, H., Dai, S., and Overbury, S. Mon . "Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES". United States. doi:10.1021/jp074426c.
@article{osti_960110,
title = {Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES},
author = {Schwartz,V. and Mullins, D. and Yan, W. and Zhu, H. and Dai, S. and Overbury, S.},
abstractNote = {X-ray absorption spectroscopy (XAS) was utilized to investigate both the Au particle size on several supports composed by silica and titanium oxide, and the coordination of TiO2 in the support phase. Particularly, we wanted to utilize the technique to probe mixing in the support phase, by using different synthetic methods such as by functionalizing silica or by ALD (atomic layer deposition) techniques as prepared in our laboratories, and the growth and stability of Au nanoparticles deposited on these supports. The study using cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2 resulted in TiO2 being dispersed in the SiO2 matrix; however, a second phase starts forming as the TiO2 content increases as indicated by the EXAFS Ti-O shell shift in position and increase of coordination number. On the supports prepared by cosynthesis, Au particles were smaller and more stable. The study using the surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2 produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. Although undercoordinated, the presence of the Ti-Ti shells indicate that the titania species are not molecularly dispersed on the SiO2 surface as hypothesized, but there is indeed a cross-linking of the titania moieties.},
doi = {10.1021/jp074426c},
journal = {Journal of Physical Chemistry C},
number = 46,
volume = 111,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • X-ray absorption spectroscopy (XAS) was utilized to investigate both the Au particle size on several supports composed by silica and titanium oxide, and the coordination of TiO2 in the support phase. Particularly, we wanted to utilize the technique to probe mixing in the support phase, by using different synthetic methods such as by functionalizing silica or by ALD (atomic layer deposition) techniques as prepared in our laboratories, and the growth and stability of Au nanoparticles deposited on these supports. The study using cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2 resulted in TiO2 being dispersed in the SiO2 matrix;more » however, a second phase starts forming as the TiO2 content increases as indicated by the EXAFS Ti-O shell shift in position and increase of coordination number. On the supports prepared by cosynthesis, Au particles were smaller and more stable. The study using the surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2 produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. Although undercoordinated, the presence of the Ti-Ti shells indicate that the titania species are not molecularly dispersed on the SiO2 surface as hypothesized, but there is indeed a cross-linking of the titania moieties.« less
  • The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomicmore » structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hagg carbide and Fe{sub 3}O{sub 4}. After 10 h of CO/He treatment at 290 C, the main component observed was Hagg carbide. A model containing Hagg and {var_epsilon}-carbides, and Fe{sub 3}O{sub 4}, was also investigated. However, the r-factor was not significantly impacted by including {var_epsilon}-carbide in the fitting, and the resulting contribution of {var_epsilon}-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.« less
  • Low temperature (308 K) hydrogenation of acetone was used as a probe reaction to investigate the support effect on the hydrogenation activity of Pt/Ni bimetallic catalysts supported on TiO{sub 2}, SiO{sub 2} and {gamma}-Al{sub 2}O{sub 3}. The oxide supports significantly affected the catalytic properties of Pt/Ni catalysts, in which Pt/Ni/SiO2 bimetallic catalysts exhibited significantly higher activity than the other two bimetallic catalysts. TEM measurements revealed that the three supported Pt/Ni bimetallic catalysts have similar particle size distribution, while CO chemisorption measurements showed very different chemisorption capacity. Extended X-Ray absorption fine structure (EXAFS) measurements of the Pt L{sub III}-edge indicated thatmore » Pt atoms were fully reduced and the Pt-Ni bimetallic bonds were formed on all three catalysts. The extent of Pt-Ni bond formation followed the trend of SiO{sub 2} > {gamma}-Al{sub 2}O{sub 3} > TiO{sub 2}, which correlated very well with the hydrogenation activity.« less
  • Low temperature (308 K) hydrogenation of acetone was used as a probe reaction to investigate the support effect on the hydrogenation activity of Pt/Ni bimetallic catalysts supported on TiO{sub 2}, SiO{sub 2} and γ-Al{sub 2}O{sub 3}. The oxide supports significantly affected the catalytic properties of Pt/Ni catalysts, in which Pt/Ni/SiO{sub 2} bimetallic catalysts exhibited significantly higher activity than the other two bimetallic catalysts. TEM measurements revealed that the three supported Pt/Ni bimetallic catalysts have similar particle size distribution, while CO chemisorption measurements showed very different chemisorption capacity. Extended X-Ray absorption fine structure (EXAFS) measurements of the Pt LIII-edge indicated thatmore » Pt atoms were fully reduced and the Pt–Ni bimetallic bonds were formed on all three catalysts. The extent of Pt–Ni bond formation followed the trend of SiO{sub 2} > γ-Al{sub 2}O{sub 3} > TiO{sub 2}, which correlated very well with the hydrogenation activity.« less