Preparation Of Palladium Supported On Molecular Sieve And Its Application In Efficiently Catalytic Combustion Of VOCs

As the precursors of fine particles and photochemical smog,volatile organic compounds（VOCs）are regarded as important substances in the atmosphere.In addition,industrial emissions are one of the main sources of VOCs.Therefore,it is particularly important to control industrial emissions of VOCs.Supported Pd catalyst is extensively applied in VOCs catalytic combustion due to its superior catalytic activity and high stability.The current application of Pd-based catalysts has the following problems:Firstly,the cost of the precious metal Pd is high and its supply is insufficient;Secondly,the active specie Pd has a high loading amount and large size resulting in that most of the Pd atoms are not involved in the reaction process for most catalytic reactions,which seriously affects the utilization rate of Pd and reduces the catalytic performance of the catalyst.The investigation of the reaction pathway and reaction mechanism for the catalytic combustion of VOCs can provide theoretical guidance for the design and preparation of high-efficiency catalysts.Furthermore,cyclohexane is widely used as the raw materials in chemical production processes such as rubber and coatings and is harmful to human health.Thus,the preparation of high-efficiency and low-cost Pd nanoparticles supported mesoporous molecular sieve catalysts for the catalytic oxidation of cyclohexane and the investigation of its reaction mechanism are of great significance.The main contents of this paper as follows:（1）Pd was supported on SBA-15（Pd SCA/SBA-15）and Ti-doped SBA-15 molecular sieve（Pd SAC/Ti-SBA-15）through the weak reduction method,and Pd NPs/Ti-SBA-15 was obtained by changing the reduction time to control the size of the Pd nanoparticles.The prepared catalysts were applied to the catalytic combustion of cyclohexane in order to investigate their catalytic performances.The results showed that Pd SAC/Ti-SBA-15（0.04wt%）exhibited excellent catalytic activity(T₉₀=270 ^oC)in constrast to Pd NPs/Ti-SBA-15with high Pd loading（0.5 wt%）(T₉₀=370 ^oC)and Pd SAC/SBA-15 without Ti substation(T₉₀=500 ^oC).The experimental results and theoretical calculation demonstrated that the excellent catalytic activity of Pd SAC/Ti-SBA-15 may be attributed to the larger number of acidic sites derived from Ti substitution which promoted the atomically dispersed Pd sites on the Ti-doped-SBA-15 and achieved the maximum utilization of Pd,thus improving the catalytic oxidation ability and stability of Pd SAC/Ti-SBA-15 for cyclohexane.（2）The noble metal Pd was supported on Ti-doped SBA-15 molecular sieve through the weak reduction method to obtain the Pd catalyst with low loading and high dispersion of Pd.The catalytic combustion of cyclohexane,styrene and n-hexane was carried out to investigate the catalytic performance of the catalyst for different types of VOCs.The results showed that Pd SAC/Ti-SBA-15 could achieve the complete conversion of styrene,cyclohexane and n-hexane at 260 ^oC,320 ^oC and 360 ^oC,respectively.Besides,the stability test results showed that Pd SAC/Ti-SBA-15 can maintain the conversion rate of styrene,cyclohexane and n-hexane up to 95%within 10 h.The superior catalytic activity of styrene was mainly ascribed to the Lewis acidity which promoted the adsorption and activation of styrene.In addition,PTR-TOF-MS was used to detect the gas phase intermediates produced during the catalytic oxidation process of styrene,cyclohexane and n-hexane with the aim to deduce their possible catalytic oxidation pathways.The catalytic oxidation of styrene mainly started with the catalytic oxidation of styryl groups,and the oxidation of cyclohexane was the oxidative dehydrogenation of cyclohexane,while the catalytic oxidation of n-hexane was mainly initiated by its catalytic cracking.Finally,the catalytic oxidation of styrene on Pd SAC/Ti-SBA-15 follows the L-H mechanism,while the catalytic combustion of cyclohexane and n-hexane follows the E-R model.（3）Na BH₄ was used as the reducing agent to load Pd Ag with a certain total loading but different mass ratios of Pd and Ag on Ti doped SBA-15 molecular sieve,and the same method was used to prepare Pd/Ti-SBA-15 and Ag/Ti-SBA-15 as the comparison.The catalytic performances of the catalysts were investigated through the catalytic combustion of cyclohexane,and the reaction mechanism of Ag doping to promote the catalytic combustion of cyclohexane was discussed.The results showed that Pd Ag（13:1）/Ti-SBA-15 with the Pd and Ag mass ratio of 13:1,possessed the best catalytic combustion activity of cyclohexane and was superior to that of Pd/Ti-SBA-15 and Ag/Ti-SBA-15.Besides,Pd Ag（13:1）/Ti-SBA-15maintained 100%cyclohexane mineralization for 72 h at 270 ^oC.The excellent catalytic performance of Pd Ag（13:1）/Ti-SBA-15 was ascribed to the synergistic effect between Pd and Ag and its superior adsorption of cyclohexane.Besides,the addition of Ag promoted the formation of benzene at low temperatures and its degradation at high temperatures.