Morphology-Dependent Surface Physical Chemistry And Catalytic Properties Of Cu₂O Nanocrystals

Fundamental understanding of the relationship between surface structure and catalytic performance of catalyst nanoparticles is of great importance for the rational design and development of efficient catalysts. The main approach is to emply materials with well-defined surface structure as model catalysts to facilitate the correlation of observed catalytic performance with surface structure. Traditional model catalysts are limited to single crystals and single crystal thin films. The recent progress in controlled synthesis techniques has realized the successful synthesis of nanocrystals with structures that provid novel model catalysts for the investigation of structure-catalytic performance relationship of powder catalysts.Based on above research concepts, in the present thesis, we have successfully synthesized uniform Cu2O nancrystals with different morphologies including cubic Cu2O nanocrystals exposing{100} crystal planes, octahedral Cu2O nanocrystals exposing{111} crystal planes, dodecahedral Cu2O nanocrystals exposing{110} crystal planes,{100} truncated octahedral Cu2O nanocrystals exposing{100} and {111} crystal planes, and{110} truncated octahedral Cu2O nanocrystals exposing {110} and{111} crystal planes, and employed these uniform CU2O nanocrystals as model oxide catalysts to systematically study their crystal plane-dependent surface physical chemistry and catalytic performance:(1) The etching behaviors of CU2O nanocrystals in aqueous acetic acid and ammonia solution have been investigated. The Cu(I)-O bond length of ctystal plane exposed on Cu2O nanocrystals determines their stability in which the shorter the Cu(I)-O bond length is ({100}<{111}<{110}), more stable the crystal plane is ({100}>{111}>{110}). The nucleation and growth of Cu(OH)2and CuO nanostructures formed during the course of etching in aqueous ammonia solution depend on the the surface stoichiometry (Cu:O ratio) of crystal plane exposed on CibO nanocrystals.(2) We have successfully developed the method of controlled oxidation to remove surfactants adsorbed on Cu2O nanocrystals introduced during the synthesis without changing their surface and bulk structures, which enables us to study the influence of adsorbed surfactants on the reducibility and catalytic performance of Cu2O nanocrystals. The influence of adsorbed surfactants on the reducibility of Cu2O nanocrystals depends on both the type of surfactant and the type of reducing agents. PVP surfactant can promote the reduction of Cu2O nanocrystals by H2and the catalytic activity of Cu2O nanocrystals in CO oxidation, and the analysis of apparent reaction kinetic data demonstrates that PVP surfactant enhances the pre-exponential factor of CO oxidation catalyzed by Cu2O nanocrystals.(3) We have investigated the catalytic performances of surfactant-free Cu2O nanocrystals in catalyzing the propylene oxidation with molecular oxygen. The morphology and thus exposed crystal planes of Cu2O nanocrystals determine not only the catalytic activity but also the catalytic selectivity. Octahedral Cu2O nanocrystals exposing{111} crystal planes with coordination-unsaturated Cu(Ⅰ) ions are most active for propylene conversion and most selective for acrolein formation; Dodecahedral Cu2O nanocrystals exposing{110} crystal planes are most selective for propylene epoxide formation; Cubic Cu2O nanocrystals exposing{100} crystal planes are most selective for CO2formation. Various surface reaction intermediates were observed to form on Cu2O nanocrystals with different morphologies during the reaction by in-situ DRIFTS technique.In addition, we have also developed Au colloids-catalyzed decomposition of CuH nanoparticles to prepare Cu2O-Au nanocomposites with novel morphologies and investigated their photocatalytic performance. We have also found that PVP capping agent on the surface of Cu2O-Au nanocomposites can assist the low-temperature reduction of Cu2O to form AuCu alloys.