Catalytic role of gold in gold-based catalysts: A density functional theory study on the CO oxidation on gold

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 (O-2-->20(ad)) 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) O-2 can adsorb on the interface of Au/TiO2 with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O-2 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+O-2-->CO2+O, and CO+O-->CO2; and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

On the need to use steady-state or operando techniques to investigate reaction mechanisms: An in situ DRIFTS and SSITKA-based study example

The nature of the surface species formed at the surface of 2 wt.% Pt/CeO2 catalyst during the forward water-gas-shift (WGS, CO + H2O -> CO2 + H-2) and the reverse reaction (RWGS) were essentially identical. More, the surface concentration of formate, carbonate and carbonyl species was similar in each case. The presence of well-resolved IR bands allowed an unequivocal relative quantitative analysis of each species, avoiding the use of the carboxylate stretching region (1600-1200 cm(-1)). However, the quantitative analysis in the case of an isotopic study was complicated due to the overlapping of the various isotope bands, yet this problem could be overcome by integrating the high-wavenumber part of the bands. The reactivity of the surface species formed under RWGS conditions was followed under two different gaseous streams. Firstly, the reactivity of these intermediates were followed under an inert gas (i.e., At), in which case carbonates were essentially stable and less reactive than formates. Secondly, the reactivity of the same surface species was followed when switching to the corresponding C-13-labelled feed (i.e., (CO2)-C-13 + H-2), in which case carbonates were exchanged significantly faster than formates. While carbonates species have been reported as reaction intermediate under reaction conditions, the increased stability or surface poisoning by these carbonates in the absence of reaction mixture was highlighted. Ultimately, this work re-emphasises the need to use steady-state conditions if the true operando reactivity of the adsorbates and structure of the solid are to be determined. (c) 2005 Elsevier B.V. All rights reserved.

An array-based study of reactivity under solvent-free mechanochemical conditions-insights and trends

An array-based approach is put forward to obtain insight into reactivity under mechanochemical solvent-free conditions. We describe a survey of sixty potential reactions between twelve metal salts MX2 {(M = Cu, X-2 = (OAc)(2), (HCO2)(2), (F3CCO2)(2), (acac)(2), (F(6)acac)(2), (NO3)(2), SO4; M = Ni, X-2 = (OAc)(2), (NO3)(2), SO4; M = Zn, X-2 (OAc)(2), (NO3)(2)} and five bridging organic ligands {isonicotinic acid (HINA), 1,4-benzenedicarboxylic acid (H2BDC), acetylenedicarboxylic acid (H(2)ADC), 1,3,5-benzenetricarboxylic acid (H3BTC), 4,4'-bipyridyl (BIPY). Reaction conditions involved a ball mill, applied for 15 min at 30 Hz, without external heating. When examined by XRPD, forty of the combinations gave detectable reactions, thirty-eight with crystalline products. Of these, twenty-nine reactions were quantitative (consuming all of at least one reactant). Comparison of XRPD patterns with patterns simulated from single crystal X-ray diffraction data in the Cambridge Structural Database allowed structural identification of six products. Of particular interest are the microporous framework materials [Cu(INA)(2)] and [Cu-3(BTC)(2)] (HKUST-1) obtained by reaction of the corresponding carboxylic acids with copper acetate. Other non-porous polymers with 3-dimensional connectivity, [Ni(ADC)(H2O)(4)], or 1-dimensional connectivity, [Cu(acac)(2)(BIPY)] and [Cu(F6acac)(BIPY)] were also obtained. Reaction between zinc acetate and H2ADC gave a new product which had not previously been characterised by single-crystal X-ray crystallography, but whose XRPD pattern suggests that it is isostructural with the known nickel polymer [Ni(ADC)(H2O)(4)]. Two further isostructural nickel and zinc products were obtained in reactions between HINA and nickel nitrate and zinc nitrate. Trends observed within the array are discussed. Copper acetate and copper formate were the most effective starting materials for reaction with carboxylic acids, potentially related to the basicity of their anions and the solvating effects of the formic and acetic acid byproducts. Amongst the ligands there was a general negative corelation between melting point and reactivity. The issue of pore templating in microporous phases and the generation of new structures is also discussed in relation to the Cu(INA)(2), Cu-3(BTC)(2) and nickel nitrate-BIPY systems. Overall, the study suggests that mechanochemical reactivity between metal salts and organic ligands under solvent free conditions is remarkably general. Use of array-based approaches as demonstrated here is advocated a useful way to reveal underlying trends in reactivity under solvent free mechanochemical conditions and to highlight particular cases for more detailed study.