Pt(Pd)负载催化剂上NOx的H2-SCR研究

The selective catalytic reduction of NO by H2 (H2-SCR), being known as a new method for eliminating nitric oxide in excess O2, has the advantage of working at low temperature (80-180 oC). Hence, it is expected to be a promising method for application, so as to resolve the problems of NH3-SCR, such as deactivation of catalyst resulted from ammonium sulfate deposition, NH3-slip and high running cost. Furthermore, the investigation of H2-SCR may have a special meaning for our country. The dust being contained in the exhaust of thermoelectric power plant must be removed before the abatement of nitric oxide. The dust treatment in electrostatic solid precipitator unit makes the exhaust temperature largely decreased and so that the NH3-SCR can not worked. On the other hand, for H2-SCR at present state, a serious shortcoming of it is that it produces large portion (20-50%) of N2O, the more harmful contaminant than NO and NO2. To efficiently modify the catalyst components for increasing the selectivity of NO being reduced to N2 in H2-SCR, so as to put the method to industrial application, the routes of N2 and N2O formation over catalysts and the factors influencing the their formation must be deeply understand. This subject investigates the active structure of Pt (Pd) supported catalysts for NO reduction to N2, the routes of N2 and N2O formation on the active sites, and the mechanism of the doping materials promoting NO reduction to N2 in the catalysts, and by which, give a reference for the investigation of industrial H2-SCR catalyst.

H2选择催化还原消除富氧中氮氧化物(H2-SCR)作为一种新的脱硝方法，具有在低温(80-180℃)条件下高选择性地还原消除富氧中NO的优点，有望取代目前应用的NH3-SCR，从而解决NH3-SCR法存在的硫酸铵沉积使催化剂失活、氨泄露造成二次污染以及运行成本过高等问题。因除尘处理使尾气大幅度降温导致NH3-SCR法难以应用于热电厂尾气脱硝，因而H2-SCR法对于我国热电厂尾气脱硝更具有特殊的研究意义。面向应用，H2-SCR法目前的主要难题是有较多污染物N2O(20-50%)生成。要提高NO生成N2的选择性，其研究关键在于认知NO还原成N2和N2O的途径，从而有效调变催化剂活性组分的电性因素，抑制N2O的生成。本研究课题，旨在探讨载Pt(Pd)催化剂的H2-SCR反应活性结构形态、N2和N2O的形成途径、以及催化助剂促进 NO还原生成N2的机制，为高选择性应用催化剂研究给出理论依据。

H2选择催化还原消除富氧中氮氧化物(H2-SCR)具有在低温条件下消除NO的优点。面向应用，其主要难题是N2生成选择性较差。因而开发H2-SCR技术的关键在于认知N2O的生成机制。本课题主要研究了Pt(Pd)基催化剂的活性结构、N2O的形成途径、以及有利于目的反应的催化助剂。研究发现，金属态的Pt(Pd)为催化活性位。当Pt(Pd)担载量 低于0.5 wt.%时，Pt(Pd)的化学形态强烈地受载体表面酸碱性的影响。酸性载体有利于Pt(Pd)以金属态的形式存在，因而相应催化剂具有较高的低温活性。H2-SCR反应同时发生在Pt表面和Pt晶粒/载体交界处。在Pt表面上，Had还原NOx吸附物种生成N2或N2O强烈受Pt表面Had和吸附NOx比值的影响。该比值越小，NOx越易还原为N2O。在Pt晶粒/载体交界处，溢流Had还原NOx吸附物种主要生成N2O。在该界面处还原的NOx吸附物种越多，N2总选择性就越低。因此，适度限制活性氢从Pt表面溢流向载体的助剂，以及对氮氧化物吸附能力适度的载体都是提高催化剂选择性的关键要素。研究发现，H2-SCR并不经过NO的氧化步骤，这与HC-SCR及NH3-SCR截然不同。W改性Pt/HZSM-5显著地提高了催化剂的H2-SCR活性和选择性。其机制为，W主要存在Pt粒子的周围而有利于其保持金属态、并减少Pt晶粒/载体界面处氮氧化物的吸附、以及抑制氢溢流。在110 oC、反应气组成为910 ppm NO + 90 ppm NO2 + 5000 ppm H2 + 10% O2 + N2、空速为36000 h-1条件下，0.1wt.%Pt-1wt.%W/HZSM-5催化剂给出了NOx转化率可高达91.1%，N2选择性为74.2%的稳定催化结果。