Purification Of HCN Gases By Catalytic Hydrolysis Over Cu-Al Mixed Oxides Derived From Hydrotalcite-like Compounds Under Lower Temperature

Hydrogen cyanide (HCN) is a kind of typical poisonous and harmful gaseous pollutants, having high toxicity and strong corrosiveness, so purification for HCN can prevent the air pollution, avoid the human health harm, and realize resource utilization of varieties of industrial gases and waste gases. Catalytic hydrolysis method is that in appropriate catalyst HCN and water reacts to ammonia and carbon monoxide which have lower toxicity and are more liable to subsequent processing and utilization. It can deep purify HCN, having no effect for resource utilization of CO and so on. However, so far it’s still lack of specifical HCN hydrolysis catalyst at home and abroad, there existing the problem of higher catalytic temperature of 400-500℃ and lower HCN conversion of less than 80% for the catalysts which have been reported. It’s the key to promote the catalytic hydrolysis application of research and development of catalysts having lower catalytic temperature and higher hydrolysis activity for HCN.The catalytic activity of the hydrotalcite-like compounds (HTLCs) and derived composite oxides for hydrolysis of HCN was studied in this paper. In the thesis, with NaOH and Na2CO3 as a precipitant, Mg-Al, Zn-Al, Ni-Al, Cu-Al, Co-Al, Cu-Co-Al hydrotalcite-like compounds were synthesized by co-precipitation method and composite oxides were derived by calcining in a certain temperature. The effects of metal species, Cu/Al molar ratio, pH value, synthesis method, calcination temperature on the catalytic activity were investigated. In this study, catalysts calcined at different temperature were characterized by XRD, TG-DTA, FT-IR, N2-BET, SEM and TEM techniques to analyze the effect of calcination temperature on the structure and properties of catalysts. The effect of reaction temperature on the conversion and hydrolysis efficiency of HCN were also studied. The effects of Co element to Cu-Al hydrotalcite-derived oxide catalyst were explored by comparison of Cu-Al, Co-Al, Cu-Co-Al hydrotalcite-derived oxide catalysts, the relationship between structure and properties of the catalysts and the catalyst activity was explored by XRD, XPS and H2-TPR techniques.The main results and conclusions of the study are as follows:(1) Mg-Al, Zn-Al, Ni-Al, Cu-Al and Co-Al hydrotalcite-derived oxides have high HCN catalytic activity by hydrolysis, and Cu-Al hydrotalcite-derived oxide has higher activity than other catalysts. After react in 240 min at 200℃, the conversion of HCN is over 95% for Cu-Al hydrotalcite-derived oxide catalyst.(2) Cu-Al catalyst having better hydrotalcite-like structure was synthetized by single titration method. Cu-Al hydrotalcite-derived oxides shows the best catalyst activity at lower temperature when Cu/Al molar ratio is about 2, synthetic pH value is 8.0, calcination temperature is 400℃. Under optimum conditions, after 240 min at a reaction temperature of 200℃ HCN conversion for Cu-Al hydrotalcite-derived oxide catalyst was more than 95%.(3) The calcination temperature has a major impact on the structure and performance of the catalyst. As the increasing calcination temperature, the hydrotalcite-like compounds gradually loses interlayer water molecules, interlayer carbonate ions, laminates hydroxy, and ultimately the formation of mixed metal oxide catalyst is formed. Catalysts calcined at 400℃ has a higher specific surface area, relatively uniform pore structure, and therefore it has a higher catalytic activity.(4) With the increasing reaction temperature, HCN conversion and hydrolysis efficiency of HCN increases. At 100℃, HCN has a lower conversion and hydrolysis efficiency, there existing adsorption reaction in the system in spite of hydrolysis reaction; At 200℃ HCN hydrolysis reaction mainly occurs in the system when HCN conversion and hydrolysis are all over 95%; Above 300℃, HCN conversion and hydrolysis efficiency are all close to 100%, exclusively hydrolysis reaction occurs in the system.(5) Cu-Co-Al mixed oxide derived from hydrotalcite-like compounds has the highest catalytic activity, compared to Cu-Al and Co-Al hydrotalcite-derived oxide. In terms of catalyst activity, the performance of the three catalysts follows the order: Cu-Co-Al > Cu-Al> Co-Al. After react in 240 min at 200℃, the conversion of HCN is over 97.5% for Cu-Co-Al,95.9% for Cu-Al and 11% for Co-Al; NH3 generation is 116.2mg/m3 for Cu-Co-Al,113.4mg/m3 for Cu-Al and 11.4mg/m3 for Co-Al.(6) By comparing of the relationship between structure and properties of catalysts and catalytic activity for Cu-Co-Al, Cu-Al and Co-Al hydrotalcite-derived oxides, it is found that it’s beneficial of lower crystallinity, higher dispersion catalyst to the hydrolysis of HCN at lower temperature. Cu-Co-Al hydrotalcite-derived oxide catalyst shows the best catalyst activity at lower temperature due to its appropriate surface adsorbed molecular oxygen and lattice oxygen content, higher Cu2+ and Co3+content and lower reduction temperature.(7) In the reaction of HCN hydrolysis to NH3 and CO, Gibbs free energy AG is less than 0 in the experimental temperature range, indicating that the reaction can take place on the thermodynamics. And the Gibbs free energy AG of the reaction decreases as the reaction temperature increases, indicating the reaction occurs more easily with increasing reaction temperature. As the main active component of the catalyst CuO and CU2O reacts with HCN to CuCN, but CuCN itself is instability and immediately converted to the active CuO. In the case of the presence of trace amounts of O2 or CO, HCN preferentially reacts with them to a small amount of H2O, promoting HCN hydrolysis and improving purification efficiency of HCN.