Study On Preparation Of The Monolithic Catalyst For Catalytic Combustion Of Volatile Organic Compounds By Electroless Plating

Among the technologies developed for the treatment of VOCs, catalytic combustion is considered to be one of the most effective measures to destroy VOCs due to its simple equipment and eliminating VOCs at relatively low temperatures. The catalyst is the key of catalytic combustion. Cordierite honeycomb ceamics was the carrier, precous metal palladium was the active componet, the Pd-based monolithic catalyst was prapared by electroless plating. The catalytic activity behavior and thermal stability of the monolithic catalysts for toluene were investigated. In addition, the Pd-based monolithic catalysts were charactrized by various techniques, and the mechanism of catalytic oxidation was also studied. The main contents and results in the thesis could be summarized as follows:Preparation of the catalysts. Pd/cordierite monolithic catalysts were prepared by electroless plating. The experiment showed that the most optimum conditions of electroless plationg were:concentration of PdCl₂ was form 0.05g/L to 0.5g/L, concentration of NH₃·H₂O (25%) was form 40g/L to 120g/L, concentration of NH4C1 was form 5g/L to 20g/L, concentration of NaH₂PO₂·H₂O was form 1g/L to 10g/L, concentration of HC1 (36-38%) was form 1g/L to 4mL/L, pH was 9 to 10, reaction temperature was form 25℃to 60℃. The results showed that palladium nanoparticles were dispersed symmetrically over cordierite, by using this technology, it had the advantages of simple equipment, easy process of manufacture, high utilization of palladium, short period of preparation, low cost, etc.Characterization and catalytic activity evaluation. The surface morphology, structure and performancethe of the Pd-based monolithic catalysts were charactrized by SEM, XRD, UV, EDX, BET, XPS, H₂-TPR and O₂-TPD techniques. The palladium grains were load evenly on the cordierite ceramic substrate. Toluene and formaldehyde were instead of VOCs in the experiment of catalytic activity evaluation. The experiment showed that the catalysts showed more activity when the loading of active component palladium loading over 0.36wt%. The Pd-based monolithic catalysts were prepared under different calcination temperatures, the one prepared under 500℃showed more activity and the palladium grains size were about 100nm, then it was found the efficiency became lower with the rise of calcination temperature, this result might be attributed to the sintering and decomposition of the PdO particles and decrease in the surface area.It was also studied the influence of space velocity, concentration of VOCs and the temperature. It was found that the efficiency of catalytic combustion for toluene and formaldehyde became lower when space velocity became faster. It had a higher catalytic efficiency of toluene when space velocity under 20000h-1. But space velocity was form under 3000h-1, the monolithic catalyst had a high activity for formaldehyde. It was also found that concentration of toluene had weak influence to the catalytic combustion, the monolithic catalyst had high activity when concentration of toluene within 6.0g/m3. It had high activity when concentration of formaldehyde within 2.0mg/m3, this showed that concentration of formaldehyde had strong influence to catalytic combustion. So elements of VOCs had greater impact to the catalytic combustion, this also showed that the catalyst has selectivity. In the Catalytic combustion process, toluene catalytic efficiency of Pd/cordierite monolithic catalyst was maintained at about 99% within 10h in the endurance test, then catalytic efficiency decreased with the reaction time after 10h, mainly because PdO-Pd redox cycle could not be proceed smoothly, most of PdO was reduced to Pd0 by toluene molecule, led to the amount of PdO were not enough, therefore activity decreased.CeO₂-ZrO₂ mixed oxides were investigated due to their excellent oxygen storage capacity, high thermal stability. CeO₂-ZrO₂ mixed oxides were loaded on the catalyst support, and then active component palladium was plated by electroless plating. Meanwhile the washcoats and their supported-Pd catalysts were characterized by SEM, EDX and UV. In addition, it was studied the influene of palladium loading and calcination temperature. It showed that these catalysts had good catalytic activity and temperature resistance. Relative to the Pd/cordierite monolithic catalyst, compared to the catalyst calcined at 500℃, T99% of the catalyst calcined at 800℃was 255℃, increasing by 25℃. Compared to the monolithic catalyst calcined at 500℃, T99% of the monolithic catalyst calcined at 800℃was 250℃, increasing by 15℃. The main reason was that CeO₂-ZrO₂ mixed oxides could act as oxygen storge resercor giving lattice oxygen to Pd, which could stabilize PdO at high temperatures.