Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation (original) (raw)

Catalytic Role of Gold in Gold-Based Catalysts: A Density Functional Theory Study on the CO Oxidation on Gold

Journal of the American Chemical Society, 2002

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO2 has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O 2 dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O 2 dissociation on Au (O2f2Oad) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O 2 on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O2 can adsorb on the interface of Au/TiO2 with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O2 dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O2fCO2+O, and CO+OfCO2; and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

Surface oxygen vacancies in gold based catalysts for CO oxidation

Experimental catalytic activity measurements, diffuse reflectance infrared Fourier spectroscopy, and density functional theory calculations are used to investigate the role and dynamics of surface oxygen vacancies in CO oxidation with O 2 catalyzed by Au nanoparticles supported on a Y-doped TiO 2 catalyst. Catalytic activity measurements show that the CO conversion is improved in a second cycle of reaction if the reactive flow is composed by CO and O 2 (and inert) while if water is present in the flow, the catalyst shows a similar behaviour in two successive cycles. DRIFTS-MS studies indicate the occurrence of two simultaneous phenomena during the first cycle in dry conditions: the surface is dehydroxylated and a band at 2194 cm À1 increases (proportionally to the number of surface oxygen vacancies). Theoretical calculations were conducted in order to explain these observations. On one hand, the calculations show that there is a competition between gold nanoparticles and OH to occupy the surface oxygen vacancies and that the adsorption energy of gold on these sites increases as the surface is being dehydroxylated. On the other hand, these results evidence that a strong electronic transfer from the surface to the O 2 molecule is produced after its adsorption on the Au/TiO 2 perimeter interface (activation step), leaving the gold particle in a high oxidation state. This explains the appearance of a band at a wavenumber unusually high for the CO adsorbed on oxidized gold particles (2194 cm À1) when O 2 is present in the reactive flow. These simultaneous phenomena indicate that a gold redispersion on the surface occurs under reactive flow in dry conditions generating small gold particles which are very active at low temperature. This fact is notably favoured by the presence of surface oxygen vacancies that improve the surface dynamics. The obtained results suggest that the reaction mechanism proceeds through the formation of a peroxo-like complex formed after the electronic transfer from the surface to the gas molecule.

Gold catalysts: A new insight into the molecular adsorption and CO oxidation

Chemical Engineering Journal, 2009

The molecular adsorption and CO oxidation on a gold-deposited TiO 2 catalyst were investigated by means of molecular dynamics simulation. The results indicate that the molecules (i.e., O 2 , CO, and H 2 O) are selectively adsorbed on the specific locations such as gold particle, gold-support perimeter interface, and support surface. The adsorption and dissociation of H 2 O molecules at the perimeter interface enhance the supply of oxygen, thus promoting the oxidation of CO on the Au/TiO 2 catalyst. However, the presence of Cl − ions could significantly impede CO oxidation due to their competition with O 2 , CO, and H 2 O for the adsorption sites. A reaction mechanism of CO oxidation is postulated on this basis. The findings are useful in developing a comprehensive picture about CO oxidation on gold-deposited TiO 2 and in the design of new gold catalysts with high catalytic activity.

Activation of CO, O 2 and H 2 on gold-based catalysts

Applied Catalysis A-general, 2005

Au/CeO x /Al 2 O 3 is highly active for CO oxidation at low temperatures and full conversion is already achieved around 60 8C. From experimental results, it is concluded that ceria can act as oxygen supplier probably via Mars and van Krevelen mechanism.

Geometric and electronic structure of Au on Au/CeO 2 catalysts during the CO oxidation: Deactivation by reaction induced particle growth

Changes of the geometric and electronic structure of gold on Au/CeO 2 catalysts induced by different pre-treatments (oxidative and reductive) and by the CO oxidation reaction at 80°C were followed by operando XANES / EXAFS measurements. The results showed that i) oxidative pre-treatment (O 2 ) leads to larger Au nanoparticles than reductive pre-treatment (CO), that ii) Au is predominantly metallic during CO oxidation, irrespective of the preceding pre-treatment, and that iii) there is a reaction induced Au particle growth. Correlations with the activity of the respective catalysts and its temporal evolution give insights into the origin of deactivation of these catalysts under reaction conditions, in particular on reaction induced changes in the Au particle size.

Selective oxidation of CO over model gold-based catalysts in the presence of H

Journal of Catalysis, 2005

The model catalysts Au/Al 2 O 3 , Au/ZrO 2 , and Au/TiO 2 were produced by laser vaporization of a metallic gold rod followed by deposition of the formed clusters onto the support powders. This technique allows to obtain a narrow size distribution of highly dispersed gold particles on the support and, most importantly, similar sizes whatever the support. This makes it possible to accurately study of the influence of the support identity on the catalytic reaction. A detailed investigation of the preferential oxidation of CO in the presence of H 2 was undertaken. Catalytic performances in the PROX reaction were compared with those in the oxidation of CO in the temperature range of 25-420 • C. A boost in the conversion of CO was observed in the presence of H 2 at low temperature; the extent of this boost is dependent on the support identity. Hence the reactivity order found for CO oxidation (Au/Al 2 O 3 Au/ZrO 2 < Au/TiO 2 ) was changed. In fact, in the presence of H 2 , the reaction rates for the oxidation of CO become rather similar on all three systems.  2005 Elsevier Inc. All rights reserved.

A computational study of catalysis by gold in applications of CO oxidation

2010

Au based catalysts have been extensively studied since Masatake Haruta in Japan discovered that small Au nanoparticles supported on transition metal oxides are exceptionally active catalysts for oxidation reactions at low temperature. However, what makes gold, being inert in the bulk form, active is still a big question. Related key challenges to understand are the particle size effect, the role of supports and the nature of the active site. The reactivity of different Au surfaces (given below) and reaction mechanisms for CO oxidation, water gas shift reaction (WGSR) and Preferential oxidation of CO (PrOx) have been studied throughout this thesis. Density Functional Theory has been used to calculate the energetical, geometrical and vibrational properties of the adsorbates as well as minimum energy path for the reactions on the different surfaces. To explore the nature of active sites, decrease in the coordination of gold atoms and co-operative effects between metal and support have ...