Active Oxygen on a Au/TiO<sub>2</sub> Catalyst: Formation, Stability, and CO Oxidation Activity

Widmann, Dipl. Chem. Daniel; Behm, R. Jürgen

doi:10.1002/anie.201102062

Cited by 351 publications

(380 citation statements)

References 28 publications

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“…Additionally, for reaction at 400˝C, the total amounts of active oxygen for CO oxidation and for H 2 oxidation are also almost identical (12.7ˆ10 18 O atoms¨g cat´1 and 12.6ˆ10 18 O atoms¨g cat´1 , respectively; see Table 2). The higher amount of O act species available and, accordingly, a higher amount of TiO 2 surface lattice oxygen removal at 400˝C is in agreement with findings in previous studies, where this was explained by the migration of TiO 2 surface lattice oxygen to Au-TiO 2 perimeter sites at elevated temperatures [27]. This enables the removal also of O act species which were originally located further away from the Au-TiO 2 interface perimeter.…”

Section: Active Oxygen Removal By Co and Hsupporting

confidence: 92%

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

2016

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Aiming at an improved mechanistic understanding of the preferential oxidation of CO on supported Au catalysts, we have investigated the competition between CO and H 2 for stable, active oxygen (O act ) species on a Au/TiO 2 catalyst during the simultaneous exposure to CO and H 2 with various CO/H 2 ratios at 80˝C and 400˝C by quantitative temporal analysis of products (TAP) reactor measurements. It is demonstrated that, at both higher and lower temperature, the maximum amount of active oxygen removal is (i) independent of the CO/H 2 ratio and (ii) identical to the amount of active oxygen removal by CO or H 2 alone. Hence, under preferential CO oxidation (PROX) reaction conditions, in the simultaneous presence of CO and H 2 , CO and H 2 compete for the same active oxygen species. In addition, also the dependency of the selectivity towards CO oxidation on the CO/H 2 ratio was evaluated from these measurements. Consequences of these findings on the mechanistic understanding of the PROX reaction on Au/TiO 2 will be discussed.

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Section: Active Oxygen Removal By Co and Hsupporting

confidence: 92%

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

2016

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“…Previous studies have correlated carbonates formation with catalytic activity, pretreatment conditions [23,73], presence of moisture in the gas [10,16], and reaction temperature [74]. Carbonates have also been shown to be decomposed in the presence of water [20], in the presence of H 2 [75], and during CO oxidation reaction at high temperatures [25,63].…”

Section: Loss Of Active Sites and Carbonates Buildupmentioning

confidence: 99%

“…A variety of oxygenated species have also been suggested to play a role in the catalysis, including support hydroxyls [11][12][13][14][15], water [10,[16][17][18][19][20], and carbonates [21,22]. Additionally, O 2 activation, which is generally considered to be the key catalytic step, is not well understood [23][24][25][26] nor are the roles of perimeter sites around Au particles [27,28] or the causes of deactivation [29][30][31].…”

Section: Introductionmentioning

confidence: 99%

CO oxidation over Au/TiO2 catalyst: Pretreatment effects, catalyst deactivation, and carbonates production

Lopez

Powell

Panthi

et al. 2013

Journal of Catalysis

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a b s t r a c tA commercially available Au/TiO 2 catalyst was subjected to a variety of thermal treatments in order to understand how variations in catalyst pretreatment procedures might affect CO oxidation catalysis. Catalytic activity was found to be inversely correlated to the temperature of the pretreatment. Infrared spectroscopy of adsorbed CO experiments, followed by a Temkin analysis of the data, indicated that the thermal treatments caused essentially no changes to the electronics of the Au particles; this, and a series of catalysis control experiments, and previous transmission electron microscopy (TEM) studies ruled out particle growth as a contributing factor to the activity loss. Fourier transform infrared (FTIR) spectroscopy showed that pretreating the catalyst results in water desorption from the surface, but the observable water loss was similar for all the treatments and could not be correlated with catalytic activity. A Michaelis-Menten kinetic treatment indicated that the main reason for deactivation is a loss in the number of active sites with little changes in their intrinsic activity. In situ FTIR experiments during CO oxidation showed extensive buildup of carbonate-like surface species when the pretreated catalysts were contacted with the feed gas. A semi-quantitative infrared spectroscopy method was developed for comparing the amount of carbonates present on each catalyst; results from these experiments showed a strong correlation between the steady-state catalytic activity and amount of surface carbonates generated during the initial moments of catalysis. Further, this experimental protocol was used to show that the carbonates reside on the titania support rather than on the Au, as there was no evidence that they poison Au-CO binding sites. The role of the carbonates in the reaction scheme, their potential role in catalyst deactivation, and the role of surface hydroxyls and water are discussed.

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“…The size of the gold particles substantially affects the catalytic activity, suggesting the key importance of metal/support interactions on a nanometer scale [4]. Reactions, and in particular CO oxidation, are believed to occur at specific active Au sites at the Au/TiO 2 interface [5][6][7][8]. Although much is known regarding Au/TiO 2 catalytic activity in the presence of a gas phase, the complexity increases steeply when solvent is included.…”

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confidence: 99%

On the Impact of Solvation on a Au/TiO₂Nanocatalyst in Contact with Water

Camellone

Marx

2013

J. Phys. Chem. Lett.

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Water, the ubiquitous solvent, is also prominent in forming liquid-solid interfaces with catalytically active surfaces, in particular with promoted oxides. We study the complex interface of a gold nanocatalyst, pinned by an F-center on titania support, and water. The ab initio simulations uncover the microscopic details of solvent-induced charge rearrangements at the metal particle. Water is found to stabilize charge states differently from the gas phase as a result of structure-specific charge transfer from/to the solvent, thus altering surface reactivity. The metal cluster is shown to feature both "cationic" and "anionic" solvation, depending on fluctuation and polarization effects in the liquid, which creates novel active sites. These observations open up an avenue toward "solvent engineering" in liquid-phase heterogeneous catalysis.Highly dispersed gold nanoparticles supported on oxides have been shown to catalyze a number of important reactions, including low-temperature CO oxidation and the water gas shift reaction [1,2]. Reducible oxides, in particular titania (TiO 2 ), are ideal catalytic supports [3]. The size of the gold particles substantially affects the catalytic activity, suggesting the key importance of metal/support interactions on a nanometer scale [4]. Reactions, and in particular CO oxidation, are believed to occur at specific active Au sites at the Au/TiO 2 interface [5][6][7][8]. Although much is known regarding Au/TiO 2 catalytic activity in the presence of a gas phase, the complexity increases steeply when solvent is included.Liquid-solid interfaces as such are relevant to many industrial applications of great significance, such as (photo-)catalysis, solar cells, gas sensors, or biocompatible devices. In heterogeneous catalysis, it has been shown that the presence of water increases the observed rate of CO oxidation [9,10]. The degree of rate enhancement depends on the type of support used. In particular for the case of Au/TiO 2 catalysts it has been shown that their activity at about 3000 ppm H 2 O is so high that full conversion of CO is reached [10]. Thus, the Au/TiO 2 surface displays a pronounced catalytic activity toward the water-gas-shift (WGS) reaction. This fundamental reaction represents a key reaction to produce extra H 2 fuel from steam reforming, which is reversible and exothermic, according to the following reaction: CO + H 2 O ⇌ CO 2 + H 2 . The so-called carboxyl and redox mechanisms have been proposed for the WGS reaction on metal/oxide surfaces; in the former mechanism the CO species reacts with terminal hydroxyl groups, whereas in the latter CO reacts with an O atom from OH dissociation or from the oxide support. In both mechanism proposed the starting point is a H 2 O molecule that is initially adsorbed on the metallic cluster with its oxygen atom being attached to a metal atom.Moreover, even large (as opposed to nanoscale) gold particles, which are usually catalytically inert, show considerable oxidation activity at aqueous conditions [11,12]. Because of its high diel...

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Active Oxygen on a Au/TiO₂ Catalyst: Formation, Stability, and CO Oxidation Activity

Cited by 351 publications

References 28 publications

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

CO oxidation over Au/TiO2 catalyst: Pretreatment effects, catalyst deactivation, and carbonates production

On the Impact of Solvation on a Au/TiO₂Nanocatalyst in Contact with Water

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Active Oxygen on a Au/TiO2 Catalyst: Formation, Stability, and CO Oxidation Activity

Cited by 351 publications

References 28 publications

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

Competition of CO and H2 for Active Oxygen Species during the Preferential CO Oxidation (PROX) on Au/TiO2 Catalysts

CO oxidation over Au/TiO2 catalyst: Pretreatment effects, catalyst deactivation, and carbonates production

On the Impact of Solvation on a Au/TiO2Nanocatalyst in Contact with Water

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Active Oxygen on a Au/TiO₂ Catalyst: Formation, Stability, and CO Oxidation Activity

On the Impact of Solvation on a Au/TiO₂Nanocatalyst in Contact with Water