Isabel Xiaoye Green

Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst

The low-temperature oxidation of carbon monoxide proceeds initially with oxygen molecules that bridge titanium and gold sites. The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO2) catalysts is that the reaction occurs on metal sites at the Au/TiO2 interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO2 during CO oxidation. Infrared-kinetic measurements indicate that O-O bond scission is activated by the formation of a CO-O2 complex at dual Ti-Au sites at the Au/TiO2 interface. Density functional theory calculations, which provide the activation barriers for the formation and bond scission of the CO-O2 complex, confirm this model as well as the measured apparent activation energy of 0.16 electron volt. The observation of sequential delivery and reaction of CO first from TiO2 sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.

Insights into Catalytic Oxidation at the Au/TiO2 Dual Perimeter Sites

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-support 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-Ti(4+) 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-Ti(4+) sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-σ interactions with interfacial Au-Ti(4+) sites. This results in apparent activation energies for O2 activation of 0.16-0.60 eV thus allowing these materials to operate over a wide range of temperatures (110-420 K) and offering the ability also to control H-H, C-H, and C-O bond scission. At low temperatures (100-130 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═O and C═C bonds or via activation of weakly acidic C-H or O-H bonds. We demonstrate that when the active Au-Ti(4+) 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 (400-420 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C-H bonds and assist in C-O bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2═C═C═O). The consecutive C-H bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C-O bond activation occurs at the Au-Ti(4+) 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═C═C═O) 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.

Low-Temperature Catalytic H2 Oxidation over Au Nanoparticle/TiO2 Dual Perimeter Sites†

The catalytic oxidation of H2 is of great interest due to its role in H2O2 synthesis, catalytic oxidation of hydrocarbons, and the selective removal of CO from hydrogen streams as well as for its simplicity which makes it ideal for fundamental bond making and breaking studies. This is especially true for supported Au nanoparticles which were found to show unusually high activity by Haruta et al. Previous explanations for this activity have invoked quantum size effects for Au particles in the 2 nm range, electronic effects in thin films of Au, and enhancement of the fraction of perimeter sites. Recent experiments on inverse TiO2/Au catalysts have suggested that the enhanced catalytic activity may be due to active sites at the TiO2/Au interface rather than a quantum size effect. 6] The presence of H2 gas in CO + O2 reaction streams is known to produce enhanced catalytic activity for CO oxidation over Au/TiO2 catalysts. The promotional effect originates from the addition of H2 and has been ascribed to H2 s ability to regenerate the catalyst by reducing the hydrocarbon accumulation or by its reaction with O2 to form hydroperoxy (OOH*) intermediates which readily oxidize CO. Previous theoretical studies have provided unique insights for this reaction, but have only focused on the role of Au. To our best knowledge, there are no reported theoretical studies on the H2 + O2 reaction that have considered the influence or involvement of the TiO2 perimeter sites at the Au–TiO2 interface. Herein, we use kinetic analyses together with in situ infrared spectroscopic studies and density functional theory (DFT) calculations to examine the activity of the Au sites as well as the Au and TiO2 perimeter sites at the Au–TiO2 interface and elucidate a plausible reaction mechanism. (We define a perimeter site as a Au or TiO2 site at the external boundary between Au and TiO2 surfaces. A dual perimeter site involves a Au perimeter site and a TiO2 perimeter site that operate together during the catalytic reaction.) High-vacuum transmission IR experiments were used to follow H2O production on Au/TiO2 powder synthesized by the deposition–precipitation method (see Supporting Information, sections I, II and Figure S1 for further details). The average Au particle size is approximately 3 nm, determined by transmission electron microscopy (Figure S2). It has been reported that atomic H dissolved in TiO2 [10] may be detected by the IR background upward shift, which is caused by trapped electrons from H in the conduction band

Inhibition at Perimeter Sites of Au/TiO2 Oxidation Catalyst by Reactant Oxygen

TiO(2)-supported gold nanoparticles exhibit surprising catalytic activity for oxidation reactions compared to noble bulk gold which is inactive. The catalytic activity is localized at the perimeter of the Au nanoparticles where Au atoms are atomically adjacent to the TiO(2) support. At these dual-catalytic sites an oxygen molecule is efficiently activated through chemical bonding to both Au and Ti(4+) sites. A significant inhibition by a factor of 22 in the CO oxidation reaction rate is observed at 120 K when the Au is preoxidized, caused by the oxygen-induced positive charge produced on the perimeter Au atoms. Theoretical calculations indicate that induced positive charge occurs in the Au atoms which are adjacent to chemisorbed oxygen atoms, almost doubling the activation energy for CO oxidation at the dual-catalytic sites in agreement with experiments. This is an example of self-inhibition in catalysis by a reactant species.

Localized Partial Oxidation of Acetic Acid at the Dual Perimeter Sites of the Au/TiO2 Catalyst—Formation of Gold Ketenylidene

Chemisorbed acetate species derived from the adsorption of acetic acid have been oxidized on a nano-Au/TiO(2) (∼3 nm diameter Au) catalyst at 400 K in the presence of O(2)(g). It was found that partial oxidation occurs to produce gold ketenylidene species, Au(2)═C═C═O. The reactive acetate intermediates are bound at the TiO(2) perimeter sites of the supported Au/TiO(2) catalyst. The ketenylidene species is identified by its measured characteristic stretching frequency ν(CO) = 2040 cm(-1) and by (13)C and (18)O isotopic substitution comparing to calculated frequencies found from density functional theory. The involvement of dual catalytic Ti(4+) and Au perimeter sites is postulated on the basis of the absence of reaction on a similar nano-Au/SiO(2) catalyst. This observation excludes low coordination number Au sites as being active alone in the reaction. Upon raising the temperature to 473 K, the production of CO(2) and H(2)O is observed as both acetate and ketenylidene species are further oxidized by O(2)(g). The results show that partial oxidation of adsorbed acetate to adsorbed ketenylidyne can be cleanly carried out over Au/TiO(2) catalysts by control of temperature.