Adsorption And Reaction Behaviors Of Probe Molecules On Au(997)Surface:a Density Functional Theory Study

The defect and step sites of catalysts have relatively low atom coordination numbers,which can significantly improves the adsorption and activation of adsorbates.Therefore,it is of great significance in catalysis.In this thesis,based on the systematic study of oxygen adsorption on Au?997?with the terrace and step sites by density functional theory calculation,the quantitative correlation for different oxygen species with coverage and Au coordination number are established in theory.Besides,probe molecules?CO_x,NO_x,H₂O and HCOOH,etc?adsorption and reaction behaviors at the different areas and sites on Au?997?are systematically studied.The effects of coordination number of Au nanoparticles and different oxygen species are investigated at the molecular level,which provides an important theoretical foundation for the development of gold catalysts with high efficiency at low temperature.By the detailed calculations of oxygen adatoms on Au?997?,three kinds of oxygen species are identified according to the oxygen coverage and Au coordination number.The only O adatom near step site exists if its coverage is lower than 1/18 ML;while O adatoms near step and at terrace sites coexist in the range of 1/18 to 2/9 ML.If oxygen coverage is more than 2/9 ML,oxygen islands forms,which are in good agreement with the experimental TDS spectrum.NO and CO physisorbs on Au?997?;CO₂ hardly adsorbs on Au?997?surface;while chemical adsorption of NO₂ occurs.Based on NO and CO oxidation with three different oxygen species,their oxidation activities decreases as the following order:oxygen island,O adatoms at terrace and near step sites.Based on the theoretical investigation of H₂O adsorption and reaction behaviors at the different areas and sites on Au?997?with and without pre-adsorbed O?a?species,the single H₂O prefers to weakly physisorb on the top site of Au?997?;while the interactions will increase when the second water molecule adsorbs on surface due to the hydrogen bond formation among the adsorbed H₂O molecules.When it dissociates on Au?997?,OH?a?and H?a?species prefer to occupy the bridge and fcc hollow sites,respectively.In addition,H₂O and its dissociated species are all inclined to adsorb on the step area of Au with 7-coordination number.Moreover,the adsorption energy of H₂O on Au?997?with the pre-adsorbed O?a?is higher than that on the clean surface,and H₂O is prone to be activated on O?a?-covered Au?997?surface.According to the transition state calculation,the reaction barriers of H₂O reacting with 0 adatom at step and terrace sites are 0.78 and 0.46 eV,respectively.According to the computational results of HCOOH,HCOO and COOH adsorption and reaction behaviors at the different areas and sites on Au?997?,HCOOH perpendicularly adsorbs at the top surface site with the carbonyl oxygen and the hydroxyl hydrogen atoms binding to the Au atoms.HCOO adsorbs on Au?997?to form a bidendate species;while COOH adsorbs through its carbon and the hydroxyl oxygen atoms interacting with surface.Their adsorption behaviors also are dependent on the Au coordination number,which the adsorption structures of HCOOH,HCOO and COOH on the step sites with low-coordination number are more stable than those on the terrace surface.The formic acid decomposes through the formate?HCOO?intermediates.The introduction of O?a?and OH?a?species on Au?997?stabilizes the adsorption of HCOOH and its dissociated intermediates.Furthermore,it accelerates their dissociation.