Study Of COS Hydrolysis Over Mixed Oxides Derived From Hydrotalcite-like Compounds

Carbonyl sulfide (COS), widely exists in natural gas, water gas and industrial tail gas, is normally regarded as a significant poison for industrial catalysis. Trace amount of COS can result in the deactivation of catalysts and lead to corrosion of reaction equipments. Furthermore, not only does COS provides economic problems, but also affects the environment. It has been proven to be a major source of acid rain when oxidized to sulfur oxide and to promote photochemical reactions. Hydrolysis is recognized as the most promising process due to higher removal efficiency among the methods for COS removal.The catalytic activity of the hydrotalcite-like compounds (HTLCs) derived composite oxides for hydrolysis of COS was studied in this paper. In the thesis, Zn/Al, Ni/Al and Zn/Ni/Al HTLCs were prepared by co-precipitation method, from the corresponding nitrate solutions and the mixed precipitants (NaOH and Na2CO3). The effects of the divalent metal ratio, n(M2+)/n(M3+), pH value and calcination temperature were studied. The products were characterized using XRD, BET, FTIR, SEM and CO2-TPD. The effects of the reaction temperature, the gas hourly space velocity (GHSV), relative humidity, the inlet concentration of COS and the content of O2were also studied. Causes of deactivation of the catalyst were analyzed systematically. Hydrolysis kinetics and the reaction mechanism of COS hydrolysis was also studied in this paper.The research results showed that the Zn/Al HTLCs derived oxides has great catalytic activity in the COS hydrolysis at low temperature. The optimum preparation conditions of the catalyst is Zn/Al mole ratio of0.5-1, pH of9.5-10and calcined at400℃. The reaction temperature is beneficial to improve the activity of catalyst. The amount of water is conducive to catalyze the hydrolysis of COS reaction, however, excess water will hinder the COS hydrolysis. Oxygen will lead to catalyst poisoning due to the deterioration of the oxidation of H2S. The NI/Al HTLCs derived oxides has great catalytic activity in the COS hydrolysis and the optimum preparation conditions of the catalyst is Ni/Al mole ratio of2, pH of10and calcined at300℃. Zn/Ni/Al HTLCs derived catalyst were perpared on the basis of previous studies, the optimum preparation conditions of the catalyst is Zn/Ni mole ratio of0.2-0.3, M2+/M3+mole ratio of3-4, pH of10and calcined at350℃for3h. Addition of Ce enhanced the catalytic activities significantly, but excessive Ce-doping had a negative effect on COS hydrolysis. When the n(Al):n(Ce) was50, the catalysts modified by rare earth gave higher activity.The catalyst activity decreased gradually as the reaction progresses. The XRD results showed that the hydrotalcite-like structure does not restore on the deactivated Catalyst. Pore structure and surface area measurement showed that the main cause of catalyst deactivation was the micropore decreases. The reason for this phenomenon may be due to the stoppage of pore structure by elemental sulfur and metal sulfate. After the regeneration process of immersing Na2CO3and NaOH solution and calcined at350℃, the activity could be restored effectively.The reaction kinetics of catalytic hydrolysis of COS over mixed oxides derived from Zn/Ni/Al HTLCs as follows:The desk study of reaction mechanism of COS hydrolysis showed that there were two steps on the surface of catalyst in the hydrolysis process:hydrolysis of COS process and the oxidation of H2S. The rate of COS hydrolysis was the control step of COS removal. The COS was continuously hydrolysised to H2S on the basic position with the reaction and H2S was oxidized to elemental sulfur and sulfate on the oxidation position.