Oxidation Reactions On Noble Metal Model Catalysts

Mechanic studies of noble metals(Au,Pt)-catalyzed oxidation reactions are of great importance.For examples,roles of water in low-temperature CO oxidation are still disputed,and few works have been reported for adsorption and activation of O2 on Au surfaces in Au-catalyzed oxidation reactions.In this thesis,employing a Au(997)stepped single crystal surface consisting of(111)terraces and(111)steps respectively with ninefold-and sevenfold-coordinated Au atoms and a Pt(111)single crystal surface as model catalysts,reactions mechanisms of oxidation reactions with O2 on Au surfaces and water-involved CO oxidation mechanism on Pt surface have been comprehensively studied by means of thermal desorption spectroscopy,X-ray photoelectron spectroscopy and polarization-modulated reflection-absorption infrared spectroscopy.The main results are as follows:1)Molecularly-adsorbed O2(a)molecules at the step sites of Au(997)are successfully prepared via a near-ambient pressure O2 exposure at low temperatures,and their interaction with co-adsorbed water and reactivity toward co-adsorbed CO(a)have been studied.Co-adsorbed O2(a)and H2O(a)interact via H-bonding at low-temperatures to form {O2·(H2O)n} complex that partiallydecompose into{(OOH)m(OH)z(H2O)n} species and further into O(a)adatoms upon heating.During surface reactions between co-adsorbed CO(a)and O2(a),three CO2 production channels are observed,in the turn of formation temperatures,to be CO(a)+O2(a)reaction,water-involved CO oxidation,and CO(a)+O(a)reaction;meanwhile,water-mediated oxygen exchange reaction between co-adsorbed CO(a)and O2(a)also occurs.2)Oxidation of methanol with moleculalr-adsorbed O2(a)on Au(997)is studied,and different oxidative coupling mechanisms of methanol to methyl formate by O2(a)and O(a)are revealed.CH3OH(a)undergoes step dehydrogenation reactions with O2(a)at low temperatures to produce CH3O(a)and CH2O2(a),and CH2O2(a),CH3O(a)and O2(a)undergo oxidative coupling reaction to produce methyl formate(HCOOCH3);CH3OH(a)undergoes step dehydrogenation reactions with O(a)at high temperatures to produce CH3O(a)and CH2O2(a),then CH2O2(a)decomposes into HCHO(a)and O(a),and then HCHO(a),CH3O(a)and O(a)undergo oxidative coupling reaction to produce methyl formate(HCOOCH3).3)Oxidation of propylene with molecular-adsorbed O2(a)on Au(997)is studied,and site-dependent propylene oxidation behaviors are demonstrated.Co-adsorbed C3H6(a)and O2(a)at the(111)step sites undergo facile combustion reactions with to produce CO2,CO and H2O at low temperatures while co-adsorbed C3H6(a)at the(111)terrace sites and O2(a)at the(111)step sites undergo combustion and partial oxidation reactions to respectively produce CO2/CO and acrolein at low temperatures;C3H6(a)at the(111)step sites undergoes partial oxidation reactions with O(a)to produce acrolein at high temperatures.The selectivity of propylene oxidation reactions depends on the coverage of oxygen species,in which the combustion reaction is preferred at high oxygen coverages and the partial oxidation reaction is preferred at low oxygen coverages.4)Influences of water on CO oxidation on Pt(111)are studied,and a double-edged sword effect of water is revealed.Co-adsorbed O2(a)and H2O(a)interact via H-bonding at low temperatures to form {O2(a)·(H2O)m.complex that can enhance the probability of O2(a)dissociation into O(a)upon heating.However,at large coverages of O2(a)and H20(a),H2O(a)can partially displace O2(a)to form {O2(g)·(H2O)n}complex,which decreases the probability of O2(a)dissociation into O(a)upon heating.During CO oxidation,co-adsorbed H2O(a)with appropriate amounts can promote CO2 formation and open a novel low-temperature CO2 production pathway via the{O2(a)·(H2O)m} complex while co-adsorbed H2O(a)with large coverages weakens the CO2 production due to the formation of O2(g)·(H2O)n} complex.The above results deepen fundamental understandings of reaction mechanisms and structure-activity relation of noble metals-catalyzed oxidation reactions.