Propane Combustion Over Pt Catalysts And Co-Cr-O Mixed Oxides Catalysts

Alkanes are very stable volatile organic compounds and their combustion is challenging.Catalytic combustion of propane as a model reaction is of importance and value to investigate the performance of the catalysts.In this work,several series of catalysts(WO_x modified Pt/BN,Pt/Ce_xZr_1-xO₂ and Co-Cr-O mixed oxides catalysts)were prepared and tested for propane combustion.Combined with activity tests anddetailed characterizations,essential factors influencing the performance and reaction mechanisms were investigated.The main contents of the work are as follows:1.The addition of WO₃ in the Pt/BN catalysts significantly promoted their performance for propane combustion.The highest activity was obtained on a catalyst with a Pt content of 1 wt.%and a W content of 7 wt.%?1Pt-7W/BN?(a rate of 363.4?mol g_Pt^-11 s^-11 and a turnover frequency of 0.92 s^-11 at 220 ?),which were 7-fold and45-fold as high as those on the 1Pt/BN(a rate 50.3?mol g_Pt^-11 s^-11 and a turnover frequency of 0.021 s^-11 at 220 ?),respectively.The W-promoted catalysts were stable under dry and wet reaction conditions,which is promising in practical applications.Based on detailed kinetics and in-situ DRIFTS results,the Pt-WO₃ interfacial sites serve as new active sites which are responsible for the improved activity.The facile reaction between propane adsorbed on Pt atom and surface hydroxyl groups on adjacent WO_x accelerates the cleavage of the C-H bond,which is the kinetically essential step in this reaction system.2.Three supports(ZrO₂,CeO₂ and Ce_0.7Zr_0.3O₂)were prepared using a precipitation method,and supported Pt catalysts were prepared using an impregnation method.These catalysts were tested for total oxidation of propane and the effects of water vapor on the catalytic behaviors were investigated.For the catalysts calcined at500 ?,the activities follow an order of Pt/ZrO₂-500>Pt/CeO₂-500> Pt/Ce_0.7Zr_0.3O₂-500.For those calcined at 900 ?,the activities followed an order of Pt/ZrO₂-900>Pt/Ce_0.7Zr_0.3O₂-900>Pt/CeO₂-900.When 2.5 vol.%water vapor was added in the feed stream,the activities of the Pt/ZrO₂ catalysts were suppressed(with the T₅₀0 increased by 10–25 ?);the activity of the Pt/CeO₂-500 was suppressed(with the T₅₀0 increased by 10 ?)while that of the Pt/CeO₂-900 was improved(with the T₅₀decreased by 25 ?);the activities of the Pt/Ce_0.7Zr_0.3O₂ catalysts were improved(with the T₅₀0 decreased by 5–25 ?).It is concluded that the presence of metallic Pt species is beneficial to the improvement of catalytic activity.The Pt/Ce0.7Zr0.3O₂-500 catalyst contained exclusively oxidized surface Pt species(Pt²⁺)while the Pt/Ce_0.7Zr_0.3O₂-900catalyst contained some metallic surface Pt species,which explains the improved activity observed on the Pt/Ce_0.7Zr_0.3O₂-900.3.A series of Co-Cr-O mixed oxides with different Co/Cr molar ratios were synthesized and tested for total oxidation of propane.The reaction behaviors are closely related to the structural features of the mixed oxide.The catalyst with a Co/Cr molar ratio of 1/2?1Co₂Cr?and a spinel crystalline structure had the best activity?a reaction rate of 1.38?mol g-1 s-1 at 250 ??,which was attributed to the synergistic roles of high surface acidity and good low temperature reducibility of this catalyst,as evidenced by ammonia temperature–programmed desorption of ammonia and temperature–programmed reduction of hydrogen,as well as the temperature-programmed surface reaction of propane.Detailed kinetic study on some catalysts revealed that the Langmuir-Hinshelwood mechanism was applied to describe the reaction routes on the 1Co₂Cr,while the Eley-Rideal mechanism was applied to the2Co1Cr and 1Co5Cr catalysts.Finally,in-situ diffuse reflectance infrared spectroscopic investigation revealed the presence of polydentate carbonate species on the 1Co₂Cr,which accumulated on the surface at low temperature?<250 ??but decomposed at higher temperature.