Catalytically active structures of SiO₂-supported Au nanoparticles in low-temperature CO oxidation

Abstract

Various Au/SiO 2 catalysts have been prepared by the deposition-precipitation method followed by calcination in air or reduction in H 2 . The structures of supported Au nanoparticles were characterized in detail by XRD, TEM, XPS, in situ XANES and operando DRIFTS of CO chemisorption, and their catalytic activity in CO oxidation was evaluated. Calcined in air, the gold precursor decomposes into Au(I) species at low temperatures and further to Au(0) at elevated temperatures, forming supported Au nanoparticles mostly larger than 4.5 nm. Reduced in H 2 , the gold precursor can be facilely reduced to Au(0) at low temperatures, forming supported Au nanoparticles with different size distributions depending on the reduction temperature. Supported Au nanoparticles around 3-4.5 nm with both abundant low-coordinated Au atoms and bulk Au-like electronic structure effectively chemisorb CO and catalyze CO oxidation at room temperature (RT). Larger supported Au nanoparticles with bulk Au-like electronic structure but few low-coordinated Au atoms do not chemisorb CO and catalyze CO oxidation at RT, and finer supported Au nanoparticles with abundant low-coordinated Au atoms but bulk Auunlike electronic structure also do not chemisorb CO and catalyze CO oxidation at RT. These results provide solid and comprehensive experimental evidence that supported Au nanoparticles with both abundant low-coordinated Au atoms and bulk Au-like electronic structure are the catalytic active structures for catalyzing CO oxidation at RT without the involvement of oxide supports. The density of low-coordinated Au atoms increases with the decrease of their size, but their electronic structure eventually deviates from bulk Au-like electronic structure; therefore, the catalytic activity of SiO 2supported Au nanoparticles in low-temperature CO oxidation inevitably exhibits a volcano-shaped dependence on their size with the optimum size between 3 and 4.5 nm.

Size-Dependent Reaction Pathways of Low-Temperature CO Oxidation on Au/CeO2 Catalysts

“…4,33,34 We previously observed that the Au 4f binding energy of Au particles below 2 nm supported on SiO 2 was higher than those of larger Au particles and bulk Au, 9 but Au particles below 2 nm supported on CeO 2 in 0.18%-Au/CeO 2 catalyst exhibit the same Au 4f binding energy as those larger Au particles in 0.97%-and 5.7%-Au/CeO 2 catalysts. The BET specific surface areas of 0.18%-, 0.97%-and 5.7%-Au/CeO 2 catalysts are 79.2, 71.6 and 75.2 m 2 /g, respectively.…”

“…4,5 The decrease in the size of supported Au particles increases the Au-support perimeter interface length and thus enhances the catalytic activity of supported Au catalysts following the periphery reaction mechanism. [6][7][8][9] The decrease in the size of Au particles can also change their electronic structure, including the quantum size effect, and subsequently the catalytic activity. [6][7][8][9] The decrease in the size of Au particles can also change their electronic structure, including the quantum size effect, and subsequently the catalytic activity.…”

“…5 The size of supported Au particles can affect their geometric structure/morphology and subsequently the number of low-coordinated Au atoms generally with enhanced catalytic activity. 5,9 However, it has been recognized that CO oxidation catalyzed by supported Au catalysts follows various reaction pathways involving different active sites and corresponding surface species/intermediates, 10 involving CeO 2 surfaces upon CO adsorption on Au/CeO 2 catalysts 15,16,27 and the facile oxidation of CO(a) adsorbed on Au surfaces by O 2 16,26,27 whereas TAP results demonstrated 4 the participation of carbonate, bicarbonate and formate species involving CeO 2 surfaces in CO oxidation. 5,9 However, it has been recognized that CO oxidation catalyzed by supported Au catalysts follows various reaction pathways involving different active sites and corresponding surface species/intermediates, 10 involving CeO 2 surfaces upon CO adsorption on Au/CeO 2 catalysts 15,16,27 and the facile oxidation of CO(a) adsorbed on Au surfaces by O 2 16,26,27 whereas TAP results demonstrated 4 the participation of carbonate, bicarbonate and formate species involving CeO 2 surfaces in CO oxidation.…”

CO Oxidation Activities of Ni and Pd-TiO2@SiO2Core-Shell Nanostructures

“…[4][5][6] The use of silica (SiO 2 ) spheres as an oxide support in the development of catalysts has attracted a great deal of attention in recent years. [7][8][9][10][11][12][13][14][15][16][17][18] For example, Yan et al prepared Co 3 O 4 nanoparticles in SiO 2 nanocapsules and found a 100% CO conversion rate at 150°C, 7 which was much better than that observed for other Co 3 O 4 nanostructures such as nanowires with exposed reactive {111} planes. Ni@SiO 2 , Co@SiO 2 and Fe@SiO 2 yolk-shell structures were prepared by Park et al and shown to be good for steam reforming of methane with high temperature stability and recyclability.…”

“…[13][14][15][16][17][18] In tests of the use of Ag/SiO 2 and Au/SiO 2 catalysts for CO oxidation, size and thermal pretreatment were shown to be important factors.…”