Tuning Surface Chemistry Of CeO₂ Catalysts With Oxygen Vacancy

Catalysis plays an important role in modern chemical industry,energy and environment.Fundamental understanding of surface chemistry of solid catalysts at the atomic and molecular level,i.e.,structure-performance relationship and catalytic reaction mechanism,is of great importance for the structural design of efficient catalysts.Ceria is widely used as solid catalysts,and oxygen vacancies are an important structure factor affecting the catalytic property.In this thesis,effects of oxygen vacancies on surface chemistry of CeO2 catalysts have been comprehensively studied.The main results are summarized as the following:(1)Combined(quasi)in situ spectroscopic techniques are used to study H2-CeO2 interaction.H2 adsorption at room temperature results in partial oxidation of CeO-x with surface oxygen vacancies,demonstrating the occurrence of H2 homolytic cleavage to H-species that represents a novel H2 activation mode on oxide surfaces;meanwhile,the resulting H-species facilely diffuse into the bulk of CeO2-x.As the temperature increases,H2 adsorption tends to form more stable hydroxyl groups on CeO2-x,leading to partial surface reduction.At temperatures where CeO2 reduction by H2 occurs with the formation of additional oxygen vacancies,stable H-species forms at the bulk oxygen vacancies of CeO2-x.(2)H2 adsorption sensitively depending on the oxygen vacancy structures of CeO2-x is used to probe oxygen vacancy structures of various CeO2-x nanocrystals with different types of predominantly-exposed facets.Upon H2 adsorption at room temperature,the bulk oxidation extent follows an order of CeO2-x nanorods exposing the {111} facets>CeO2-x nanocubes exposing the {100} facets>>CeO2-x nanorods exposing the {110} facets.This demonstrates that oxygen vacancies preferentially locate in the bulk of CeO2-x nanorods exposing the {111} facets and CeO2-x nanocubes exposing the {100} facets but on the surface of CeO2-x nanorods exposing the {110}facets.(3)Combined(quasi)in situ spectroscopic techniques are used to study oxygen vacancy effect and reactivity of various hydrogen species for CeO-catalyzed C2H2 semihydrogenation reaction.Under the reaction condition,the CeO surface is almost fully hydroxylated.C2H2 only molecularly adsorb at the Ce sites of hydrogenated CeO surface and hydrogenates with the protons of OH groups to selectively produce C2H4.The surface oxygen vacancy and the resulting H-species at the surface oxygen vacancy on hydroxylated CeO2 surface facilitate the C2H2 semihydrogenation reaction via reduction of reaction barrier,however,the H-species is capable of mediate the dissociate adsorption of C2H2 into C2H species,which facilely undergoes a pathway with sequential hydrogenation reactions to produce C2H6 via surface intermediates of CCH2,CCH3,CHCH3 and CH2CH3,decreasing the C2H4 selectivity of C2H2 hydrogenation reaction.(4)A novel approach of "Synthesis in Glovebox" in which the creation of surface oxygen vacancies on CeO2 and the subsequent catalyst synthesis are both carried out in a glovebox with O2 concentration lower than 0.1 ppm in order to stabilize surface oxygen vacancies is applied to synthesize Pd/CeO2 catalysts.Surface oxygen vacancies were created on CeO2 by H2-reduction treatment at various temperatures,and Pd(NO3)2 aqueous solutions were then impregnated onto the reduced supports.It was found that Pd cations preferentially interact with surface oxygen vacancies and some get reduced by surface oxygen vacancies during the impregnation process.A subsequent calcination of Pd(NO3)2/reduced-CeO2 precursors leads to Pd/CeO2 catalysts with uniform atomically-dispersed Pd-O-Ce surface superstructure that can be further utilized to acquire CeO2-supported metallic Pd and CeO2-supported Pd(II)catalysts with enhanced dispersion,activity and stability respectively in CO oxidation and C3H8 combustion reaction.