TiO₂-Morphology-Dependent Catalytic Surface Chemistry Of Au/TiO₂ Catalysts

There are a lot of structural factors influencing catalytic performance of solid catalysts,particularly metal oxide-supported metal catalysts.Au/TiO₂ catalysts are a representative of metal oxide-supported metal catalysts and have been extensively studied as catalysts for catalyze low-temperature CO oxidation and propylene epoxidation with H2 and O₂.However,fundamental understandings of both reactions are still of debates,and great efforts are also devoted to rational structural design of efficient Au/TiO₂ catalysts.Currently,the rapid development in nanotechnology has enriched the structure-controlled synthesis methods of metal/oxide catalysts and the characterization techniques of metal/oxide catalysts are becoming comprehensive,both of which are benefial for fundamental studies of heterogeneous catalysis.Based on the above considerations,Au/TiO₂ catalysts are chosen as the target system in this doctoral dissertation.The morphology of anatase TiO₂ support,Au-TiO₂ interaction,Au particle size and defects of the Au/TiO₂ catalysts have been comprehensively tuned,their effects on the catalytic performance of Au/TiO₂ catalysts in CO oxidation reaction and gas phase propylene epoxidation have been studied,and the structure-activity relationship and reaction mechanism have been successfully understood.The main results are summarized as the following:1.Au/TiO₂ catalysts were prepared employing TiO₂ nanocrystals with predominantly exposed {001},{100},and {101} facets as supports and observed to exhibit facet-dependent dual-perimeter-sites catalysis in low-temperature CO oxidation.Adsorbed CO?a?molecules remain stable on TiO₂ surfaces up to 223 K.Dual-perimeter-sites catalysis in low-temperature CO oxidation occurs in Au/TiO₂{100}and Au/TiO₂{101} catalysts with catalytically active Au-TiO₂ perimeter sites exhibit large coverage gradients between CO?a?adsorbed at Ti and Au sites that facilitate the migration of CO molecules from TiO₂ to the Au-TiO₂ perimeter sites,but seldom within Au/TiO₂{001}catalyst with catalytically inactive Au-TiO₂ perimeter sites that exhibits few coverage gradients between CO molecules adsorbed at Ti and Au sites.These results provide novel insights in the structure-activity of Au/TiO₂ catalysts for low-temperature CO oxidation.2.Au/TiO₂{001},Au/TiO₂{100} and Au/{101} catalysts with different Au loadings were prepared and characetrizaed.The nucleation,growth and agglomeration of Au particles on TiO₂ nanocrystals were observed to be related to the morphology-dependent defect structure of TiO₂ and the Au-TiO₂ interaction of the catalyst.When the Au loading is 0.2%to 1%,the Au particles are mainly homogeneously nucleate and grow on these three TiO₂ nanocrystals.When the loading of Au is increased to 2%and 5%,the Au particles are mainly agglomerated on the TiO₂{001}nanocrystals,and the Au particles are mainly nucleated and grown on the TiO₂{100} nanocrystals,and on the TiO₂{101} nanocrystals,the Au particles have both nucleation and growth and slight agglomeration.The electronic effect of Au particles on CO adsorption and oxidation was observed.Fine Au nanoparticles with electronic structure deviating from bulk Au exhibit a weaker ability to adsorb CO,activate the surface lattice oxygen at the Au-TiO₂ interface and a low intrinsic activity in catalyzing the low-temperature CO oxidation reaction.3.Hydrogenated TiO₂ nanocrystals were prepared and used as supports to prepare supported Au catalysts,and facet-dependent defect structures and catalytic activity in low-temperature CO oxidation were observed.Hydrogenation treatment decreases the particle sizes of Au nanoparticles supported on TiO₂{001}nanocrystals,but does not affect the particle sizes of Au nanoparticles supported on TiO₂{100} and TiO₂{101}nanocrystals.For CO oxidation at RT and above,Au/H-TiO₂{001} is less active than Au/TiO₂{001},while Au/H-TiO₂{100} and Au/H-TiO₂{101} are more active than Au/TiO₂{100}and Au/TiO₂{101} respectively.For CO oxidation between 153 and 223 K,Au/H-TiO₂ catalysts are all active than corresponding Au/TiO₂ catalysts.4.Catalytic performance of various Au/TiO₂ and Au/H-TiO₂ catalysts in propylene epoxidation with H2 and O₂ were studied,and highly active catalysts were fabricated by optimizing the preparation conditions.Under a typical reaction condition,a C3H6 conversion as high as 5.4%,a PO selectivity of 96%and a PO yield as high as 137.4 mmolPO/h/gAu were acquired.Comprehensive characterization results demonstrate the intimately-contacted Au-Ti4c interface with Au-TiO₂ charge transfer as the active site.