Catalytic Oxidation Of CO/toluene By Pt-based Supported Catalysts And Its Reaction Mechanism

Nowadays,the characteristics of air pollution in China have changed from local and single air pollution to regional and compound air pollution.The continuously increased volatile organic compounds?VOCs?are one of the important precursors for the formation of ozone?O₃?and fine particulate matter(PM_2.5),leading to frequent pollution events of high concentration of near-surface ozone and secondary organic aerosols.Carbon monoxide?CO?is flammable,toxic,chemically stable and can exist in the atmosphere for a long time.Therefore,it is necessary to simultaneously control the emission of VOCs and CO to reduce their pollution to the atmosphere.Catalytic combustion technology is one of the most effective technologies for the treatment of VOCs and CO,and the development of efficient and economical catalysts is the core of this technology.The Pt-based supported catalysts have showed higher activity in both VOCs and CO oxidation reactions,but its cost is very high,resources are limited,and CO poisoning is easy to occur on the surface of catalysts.Therefore,it is necessary to improve the low temperature catalytic performance and the utilization rate of precious metals of Pt-based catalysts,effectively reducing their operating costs.The methods on adding the second non-precious metal?Co,Ni,Cu,etc.?active component and regulating the strong interaction between precious metal and metal oxide support can not only improve the activity of catalyst,but also reduce the cost of catalyst.Moreover,the interaction between noble metal and support can be regulated by loading noble metal on different supports to stabilize noble metal particles and improve catalytic performance.Therefore,a series of bimetallic Pt-M?M=Ni,Co and Cu?nano-alloy catalysts and Pt-based?Pt-Al₂O₃,Pt-Co₃O₄ and Pt-Ce O₂?catalysts were prepared,and the structure-activity relationship between the structures of catalysts and the catalytic performances of CO and toluene co-oxidation was discussed.The reaction mechanism of CO and toluene oxidation over Pt-based catalysts was studied by in situ DRIFTS spectroscopy.The main contents and conclusions are as follows:?1?The cheap transition metal element M were introduced into the precious metal Pt to form the nanoalloys,and these nanoalloys were successfully loaded on the Al₂O₃ support to obtained the bimetal Pt-M?M=Ni,Co and Cu?/Al₂O₃ catalysts.And the effect of the introduction of cheap transition metals on the structures of Pt-based catalysts and the catalytic activities of CO or toluene oxidation was investigated.It was found that the Pt-Co/Al₂O₃catalyst showed the best catalytic activity in the single CO/toluene oxidation.Moreover,the bimetallic Pt-M/Al₂O₃ catalysts showed the same catalytic performance(T₉₉/CO=160?)for the single CO oxidation,and their catalytic activities were higher than those of Pt/Al₂O₃ catalyst.When CO and toluene were co-oxidized,the catalytic activity of all Pt-based catalysts decreased,but the Pt-Co/Al₂O₃catalyst still showed well catalytic activity.In addition,the Pt-Co/Al₂O₃catalyst had good stability and moisture resistance.Based on the characterization analyses,it was found that the surface of Pt-Co/Al₂O₃catalyst had higher dispersion of Pt particles and TOF value,as well as a higher concentration of surface oxygen vacancy,good lattice oxygen migration/activation rate and lower temperature reducibility,resulting in the best catalytic activity for CO and toluene oxidation.?2?Pt nanoparticles with the size of 3.3-3.7nm were synthesized by ethylene glycol reduction,and the Pt-Al₂O₃?Pt-Co₃O₄ and Pt-Ce O₂ catalysts were obtained after Pt loaded on different supports.Based on the catalytic activities,it was found that the catalytic performance of Pt-Ce O₂ catalyst was the best for CO and toluene oxidation under different reaction conditions,especially for CO and toluene co-oxidation.Among them,the Pt-Ce O₂ catalyst with double active sites?noble metal sites and oxygen-deficient sites?can reduce the mutual inhibition between CO and toluene molecules,while the Pt-Al₂O₃ catalyst did not have such above characteristic,resulting in a significant reduction in catalytic performance when CO and toluene were co-oxidized.In addition,the Pt-Ce O₂ catalyst showed the excellent stability and well moisture resistance,and the presence of water vapor can promote the CO or toluene oxidation over the Pt-Ce O₂ catalyst,especially the CO and toluene co-oxidation.The results showed that the Pt-Ce O₂ catalyst was a double active site catalyst,which had a strong metal-support interactions?Pt-O-Ce?between the surface oxygen and adjacent Pt species.Therefore,the Pt-Ce O₂ catalyst can not only induce the formation of oxygen vacancy,but also improve the dispersion of Pt nanoparticles and the low temperature oxidation reduction performance,thus improve the catalytic activity of catalyst.When CO and toluene were co-existed,the Pt-Ce O₂ catalyst can also effectively reduce the inhibition of CO and toluene competitive adsorption.?3?According to the analyses of in situ DRIFTS spectra,the reaction pathway of CO oxidation followed that CO was preferentially adsorbed on the surface of Pt-based catalyst,and then quickly converted into various carbonates and bicarbonates species,finally completely decomposed into CO₂.The catalytic oxidation pathway of toluene was followed as:toluene was rapidly adsorbed,activated,and transferred to form benzyl radical species due to the C-H bond fracture of the methyl group,and then oxidized to key intermediates?benzaldehyde,benzoic acids,formates species,etc.?,and finally was completely oxidized into CO₂ and H₂O.In addition,under the condition of mixed oxidation of CO and toluene,the competitive adsorption of CO and toluene at the same active site slowed down the oxidation reaction rate,but the oxidation reaction pathways of CO and toluene may be independent from each other.The results showed that the strong metal-support interactions of Pt-Ce O₂ catalyst with double active sites could accelerate the formation of various key intermediates,as well as the activation and migration of oxygen species,thus effectively alleviating the mutual inhibition of CO and toluene.