Temperature-programmed Surface Reaction Study Of CO Oxidation Over Au-based Catalysts At Low Temperature:An Insight Into Nature Of The Reaction Process

The removal of CO is of great scientific significance and potential application in air purification,gas sensing,fuel cell,and car exhaust systems for pollution control.The supported Au-based catalyst is one of the most effective methods to remove the trace amount of CO at low temperature.Many works have involved in the mechanism of CO over the Au nanoparticles supported on metal-oxides and got many consensus.However,the behaviors of the reactants,reaction intermediate and products in the reaction process of CO oxidation are ultra-complex.Therefore,the debate on the reaction mechanism as well as the nature of the reaction process in CO oxidation(especially in the presence of H2)over Au-based catalysts at low-temperature has continued for more than two decades.In this paper,a modified deposition-precipitation method was used to prepare the Au and bimetallic Au-Ag catalysts supported on TiO2.The properties of as-prepared catalysts were characterized by XRD,N2 adsorption-desorption,UV-Vis,XPS,TEM,TPR and EPR.To describe and explore the behaviors of the reactants,reaction intermediates and products in the reaction process of CO oxidation,the temperature-programmed surface reaction(TPSR)method coupled with a mass spectrometry(MS)was employed to test the steady-process at a special temperature(30?80?)of oxidizing CO over Au/TiO2.Moreover,the chemisorption behaviors of CO,O2 and H2 were tested by TPD and FTIR techniques.Finally,based on the obtained results,the reaction mechanism as well as the nature of the reaction process in CO oxidation was preliminarily discussed in detail.The research work obtained some results as following:(?)In the O2-poor stream(CO stream containing trace amount of H2O,CO,H2 and O2),the CO oxidation was a multi-reaction process over Au/TiO2 containing several reactions which has mainly two pathways.On the one hand,the adsorbed CO at Au sites could be directly oxidized by H2O,including that:(i)CO adsorbed at TiO2 sites reacted with the adsorbed H2O to form Ti-COOH species below 80 ?,which is desorbed into CO2 and H2O at 80 ?.(ii)CO adsorbed at Au sites reacted with the adsorbed H2O to form CO2.(iii)CO adsorbed at Au sites reacted with H2O to form H2 and CO2 by the water-gas shift reaction.On the other hand,the adsorbed CO at Au sites could be oxidized by the physiorbed O2 molecular,which followed as(iv)CO adsorbed at Au site reacted with O2 to form CO2.(?)In the presence of H2,it would make different influences on CO oxidation over Au/TiO2 with that over TiO2 samples.It was found that the H2 pre-adsorption could keep CO oxidation without H2O consumption over Au/TiO2,but suppressed CO oxidation over TiO2.The chemisorption testing showed that the H2 adsorption at Au/TiO2 could benefit to the formation of Ti-bonded hydroxyl species(Ti4+-OH),while the H2 adsorption at TiO2 would consume the Ti-bonded hydroxyl species and form the bridge hydroxyl species(Ti4+-OH-Ti4+).These results indicate that only the Ti-bonded hydroxyl species(not all kinds of hydroxyl species)could act as the active species of oxidizing CO.Furthermore,it is suggested that the dissociative H adsorbed at Au sites could activate the lattice oxygen of TiO2 to form the active Ti-bonded hydroxyl species,which exhibited a strong reducibility than the H directly adsorbed at TiO2.(?)In the O2 stream,the CO oxidation was given priority to CO with O2,which could be further confirmed by the CO oxidation behavior over bimetallic Au-Ag/TiO2 catalyst.It was found that the bimetallic Au-Ag/TiO2 catalyst exhibited a higher CO conversion as compared with the monometallic Au/TiO2 or Ag/TiO2 sample at room temperature.Based on the results of CO-adsorbed DRIFTS and EPR over Au/TiO2 and Au-Ag/TiO2 samples,we suggested that the introduction of Ag into Au would weaken the interaction between Au and TiO2 sites,which resulted in the changes in the structure and properties of Au,Ag and TiO2 sites,and the subsequent promoting effect on the adsorptions and activations of both CO and O2 on Au-Ag/TiO2.That's to say,the CO oxidation was given priority to CO with O2 in O2 stream,which was consistent with the reaction process over Au/TiO2 tested by a TPSR.The innovations of this study could be mainly concluded as follows:(1)Designing an improved TPSR method to describe the behavior and role of the reactants over the Au-based catalyst at a special temperature(30?80 ?);(2)Decelerating the reaction rate by a special method(i.e.decreasing the content of oxidants down to a very trace amount or introduce the reactants step by step)in a reasonable range,which is favored for describing the behavior of oxidizing CO over catalysts in a relative steady processes;(3)This TPSR testing may be available to study the process of other reactions,which maybe provides a new approach to investigate the reaction mechanism as well as the nature of the reaction process.