Insights into Catalytic Oxidation at the Au/TiO2 Dual Perimeter Sites
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- 作者：Isabel X. Green ; Wenjie Tang ; Matthew Neurock ; John T. Yates ; Jr.
- 刊名：Accounts of Chemical Research
- 出版年：2014
- 出版时间：March 18, 2014
- 年：2014
- 卷：47
- 期：3
- 页码：805-815
- 全文大小：828K
- 年卷期：v.47,no.3(March 18, 2014)
- ISSN：1520-4898
文摘
Gold (Au) nanoparticles supported on reducible oxides such as TiO2 demonstrate exceptional catalytic activity for a wide range of gas phase oxidation reactions such as CO oxidation, olefin epoxidation, and water gas shift catalysis. Scientists have recently shifted their hypotheses on the origin of the reactivity of these materials from the unique electronic properties and under-coordinated Au sites on nanometer-sized particles to bifunctional sites at the Au鈥搒upport interface.
In this Account, we summarize our recent experimental and theoretical results to provide insights into the active sites and pathways that control oxidation over Au/TiO2 catalysts. We provide transmission IR spectroscopic data that show the direct involvement of the Au鈥揟i4+ dual perimeter sites, and density functional theory results that connect the electronic properties at these sites to their reactivity and to plausible reaction mechanisms. We also show the importance of interfacial Au鈥揟i4+ sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-蟽 interactions with interfacial Au鈥揟i4+ sites. This results in apparent activation energies for O2 activation of 0.16鈥?.60 eV thus allowing these materials to operate over a wide range of temperatures (110鈥?20 K) and offering the ability also to control H鈥揌, C鈥揌, and C鈥揙 bond scission. At low temperatures (100鈥?30 K), adsorbed O2 directly reacts with co-adsorbed CO or H2.
In addition, we observe the specific consumption of CO adsorbed on TiO2. The more strongly held CO/Au species do not react at 120 K due to high diffusion barriers that prevent them from reaching active interfacial sites. At higher temperatures, O2 directly dissociates to form active oxygen adatoms (O*) on Au and TiO2. These readily react with bound hydrocarbon intermediates via base-catalyzed nucleophilic attack on unsaturated C鈺怬 and C鈺怌 bonds or via activation of weakly acidic C鈥揌 or O鈥揌 bonds. We demonstrate that when the active Au鈥揟i4+ sites are pre-occupied by O*, the low temperature CO oxidation rate is reduced by a factor 22. We observe similar site blocking for H2 oxidation by O2, where the reaction at 210 K is quenched by ice formation. At higher temperatures (400鈥?20 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C鈥揌 bonds and assist in C鈥揙 bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2鈺怌鈺怌鈺怬).
The consecutive C鈥揌 bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C鈥揙 bond activation occurs at the Au鈥揟i4+ dual perimeter sites. There is a bound-intermediate transfer from the TiO2 support to the Au sites during the course of reaction as the reactant (CH3COO/TiO2) and the product (Au2鈺怌鈺怌鈺怬) are bound to different sites. We demonstrate that IR spectroscopy is a powerful tool to follow surface catalytic reactions and provide kinetic information, while theory provides atomic scale insights into the mechanisms and the active sites that control catalytic oxidation.
In this Account, we summarize our recent experimental and theoretical results to provide insights into the active sites and pathways that control oxidation over Au/TiO2 catalysts. We provide transmission IR spectroscopic data that show the direct involvement of the Au鈥揟i4+ dual perimeter sites, and density functional theory results that connect the electronic properties at these sites to their reactivity and to plausible reaction mechanisms. We also show the importance of interfacial Au鈥揟i4+ sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-蟽 interactions with interfacial Au鈥揟i4+ sites. This results in apparent activation energies for O2 activation of 0.16鈥?.60 eV thus allowing these materials to operate over a wide range of temperatures (110鈥?20 K) and offering the ability also to control H鈥揌, C鈥揌, and C鈥揙 bond scission. At low temperatures (100鈥?30 K), adsorbed O2 directly reacts with co-adsorbed CO or H2.
In addition, we observe the specific consumption of CO adsorbed on TiO2. The more strongly held CO/Au species do not react at 120 K due to high diffusion barriers that prevent them from reaching active interfacial sites. At higher temperatures, O2 directly dissociates to form active oxygen adatoms (O*) on Au and TiO2. These readily react with bound hydrocarbon intermediates via base-catalyzed nucleophilic attack on unsaturated C鈺怬 and C鈺怌 bonds or via activation of weakly acidic C鈥揌 or O鈥揌 bonds. We demonstrate that when the active Au鈥揟i4+ sites are pre-occupied by O*, the low temperature CO oxidation rate is reduced by a factor 22. We observe similar site blocking for H2 oxidation by O2, where the reaction at 210 K is quenched by ice formation. At higher temperatures (400鈥?20 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C鈥揌 bonds and assist in C鈥揙 bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2鈺怌鈺怌鈺怬).
The consecutive C鈥揌 bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C鈥揙 bond activation occurs at the Au鈥揟i4+ dual perimeter sites. There is a bound-intermediate transfer from the TiO2 support to the Au sites during the course of reaction as the reactant (CH3COO/TiO2) and the product (Au2鈺怌鈺怌鈺怬) are bound to different sites. We demonstrate that IR spectroscopy is a powerful tool to follow surface catalytic reactions and provide kinetic information, while theory provides atomic scale insights into the mechanisms and the active sites that control catalytic oxidation.