The Construction And Process Characteristic Study On New Catalyst Systems Of Heteropoly Compounds With Adsorption-Decomposition Function For NO_x Conversion

With the development of modern society, many proceses, such as steel making, electric power generation, the operation of car engines and incineration of municipal refuse are emitting more and more gases, such as SO₂, NO_x, CO, et al, which are harmful to ecological environment and human health. In recent years, both academia and industry all over the world have paid deep concern on the problem of air pollution. The removal of nitrogen oxides (NO_x) is one of the hottest points in the area of enrionmental pollution treatment because NO_x are very harmful and hard to be removed.Direct catalytic decomposition of NO_x has been recognized as one of the most attractive methods for eliminating pollution derived from NO_x, since it does not require addition of any reducing agent and the only products are N₂ and O₂. This method, simple and cheap, would avoid a secondary pollution. Heteropoly acids (HPAs) or heteropoly compounds (HPCs), as a new type of catalyst for NO_x direct decomposition, have attracted the attention of worldwide researchers during the past years. HPCs are environment-friendly caltalysts and have been applied successfully in organic synthesis, but there has been no report on the industrial application of the NO_x catalytic decomposition by HPCs, to date.Seventeen kinds of solid-state heteropoly acids or their salts and eight kinds of supported heteropoly acids (mostly HPW with eight kinds of supports) with different loadings were prepared and evaluated in this study. All the catalyst systems were evaluated using a fixed-bed catalytic reactor. The adsorption efficiencies of H₃PW₁₂O₄₀ (HPW), H₄SiW₁₂O₄₀ and (NH₄)₃PMo₁₂O₄₀ can reach upto 66.3%, 62.9% and 69.5%, respectively. HPW/SnO₂, HPW/ZSM-5, HPW/USY and HPW/SiO₂ were selected as efficient catalyst systems, with NO_x adsorption efficiencies being 77.3%, 70.5%, 65.7 and 62.3%. respectively, corresponding to the HPW loadings of 50%, 50%, 16.7% and 28.6% These four catalysis systems are characterized by IR, XRD, TEM and BET. Taking HPW/SnO₂ (50% loading) as an example, the process characteristics were carried out. Factors such as gas humidity, oxygen content, adsorption temperature, space velocity and initial NO_x concentration, were studied. NO_x saturated adsorption curve and accumulated quantity curve for HPW and HPW/SnO₂ (50% loading) were gained. The saturated amounts of NO_x adsorbed were equal to 50.5 mg NO₂/g-HPW for H₃PW₁₂O₄₀ and 85.4 mg NO₂ /g-HPW for HPW/SnO₂ (50% loading). It was showed that there was an obvious synergetic effect between HPW and supports. Based on IR study and literatures, the NO_x adsorption mechanism involving the combination of NO_x with bulk proton to form (NOH)⁺ which can be incorporated into the secondary structure of HPCs was discussed. It is concluded that the process of NO_x adsorption on HPCs is the bulk diffusion of NO_x into HPCs by "pseudoliquid phase" effect and the interaction between NO_x and HPCs depends on the Br(?)nsted acidic strength of the solid-state HPCs. With the aid of GC-MS analysis, the process effectiveness for NO_x conversion to N₂ was confirmed by the temperature programmed desorption experiments for HPW and HPW/SnO₂ (50% loading).