Modifications Over Pd/Al₂O₃-TiO₂ Catalysts And Their Effects On The Performance Of Methanol Deep Oxidation

Volatile organic compounds?VOCs?are usually recognized as the major contributors to air pollutions and harmful to the environment and human health.With the more stringent emission standards,the reduction of VOCs is becoming more and more important.Catalytic oxidation is one of the most ideal and effective technique for the elimination in which the VOCs can be oxidized into carbon dioxide and water.The core task of catalytic oxidation technique is the choice of catalysts.Therefore,the catalysts which possess high performance are indispensable,particular at low temperatures.Supported noble-metal catalysts are widely used in the catalytic oxidation of VOCs because they are durable and efficient,especially Pd and Pt catalysts.Although the Pt is considered to be the most active precious metal elements for hydrocarbon combustion reaction,there also exists deficiency in practical applications.In some studies,Pt was proved lower activity at a relatively low temperature,tolerance to moisture and lower resistance to thermal sintering in an oxidizing environment than palladium.Some researches stated that the addition of a secondary noble metal could further increase the catalytic performance of the noble metal based catalysts,so the Pd-Pt catalysts attracted more attention.The Pd-Pt catalysts were always used in the catalytic oxidation of methane and toluene.Furthermore,their reactivity was much higher than single Pd and Pt catalysts.The double noble-metal like Pd-Ag catalysts were also used in methanol oxidation reaction,but the performance at low temperatures was not ideal.However,the Pd-Pt metals were common in methanol electro-oxidation and rarely present in catalytic oxidation.Therefore,the 1.0%Pd/Al₂O₃-TiO₂?95%?sample which was active for ethanol oxidation in our previous work was chosen as the basic catalyst.We aim to obtain Pd-Pt catalysts which possess high activity at low temperatures in methanol oxidation reaction through the changes of Pd contents and the addition of Pt and CeO₂.We studied the effects of Pd contents,Pt addition and Ce position on the physicochemical properties and reactivity of catalysts.Based on the above work,we got an initial understanding about the mechanism of methanol oxidation and the research contents were displayed as following:?1?The Pd-Pt/CAT?CeO₂-Al₂O₃-TiO₂?catalysts which possessed different Pd contents were prepared by impregnation method and used in methanol oxidation reaction.The results showed that the performance of methanol total oxidation were influenced by Pd contents.The 0.8%Pd-Pt/CAT catalyst perfomed better in methanol conversion and selective production of CO₂.The active components dispersed well at low content and the aggregated phenomena appeared when the Pd content reached 1.0wt.%.The higher adsorbed oxygen content and higher-valence TiO_x over 0.8%Pd-Pt/CAT catalyst were more conducive to the higher activity.The larger proportion of Ti⁴⁺was prone to“attack”water to produce-OH and then formed the Pt-OH_ads or Pd-OH_ads to react with CO_ads.?2?We found that the addition of Pt significantly enhanced the catalytic performance of Pd based catalysts by comparing the activity test results of Pd and Pd-Pt catalysts.The results revealed that Pd-Pt catalysts possessed more abundant adsorbed oxygen?O_??,Ti⁴⁺and higher-valence TiO_x than that of Pd catalysts.Additionally,the existence of Pd-Pt-Ti O_x might be conducive to the transmission of intermediate or O_?,which further accelerated the rate of oxidation reaction.Based on the characterization conclusions and experimental results,a possible reaction pathway was also proposed about how O_?took part in methanol oxidation through the transition between different oxides with high and low valence.?3?The activity otherness between catalysts with different Ce position proved that the Ce adding position did influence the reactivity.The results demonstrated that the different Ce position changed the pore shape and diameter.Catalysts that Ce was directly introduced into support possessed more abundant adsorbed oxygen and the more Ti³⁺made the more gaseous oxygen take part in the catalyst surface reaction.The Al-O-Ti structures facilitated the cooperation of Al and Ti species,which further sped up the oxidation reaction rate.?4?At last,we studied the influence of impregnation sequence,methanol content and space velocity on the performance of 0.8%Pd-Pt/CAT catalyst.The sequence that the Pt was loaded after the impregnation of Pd was more appropriate,especially conducive to the activity at low temperatures.Under200?,for conditions of GHSV of 35,000 h^-1,CH₃OH contents ranging from1.0vol.%to 7.0vol.%and 1.0vol.%CH₃OH,GHSV ranging from 35,000 h^-1 to175,000 h^-1,the CH₃OH conversion and CO₂ selectivity for the catalyst kept well for 100%.The results demonstrated that the catalyst possessed excellent removal effects for methanol gas with large content and high velocity.The0.8%Pd-Pt/CAT catalyst was also proved to be active in oxidation reactions of formaldehyde,ethanol and acetaldehyde.