Mechanistic investigation on Hg^0 capture over MnO_x adsorbents: effects of the synthesis methods

Abstract

目的 针对当前单质汞高效脱除难题, 本文旨在探讨不同制备方法对锰氧化物吸附剂单质汞捕集性能的影响, 并通过对吸附剂物理化学性质的研究, 揭示锰氧化物吸附剂的构效关系及脱汞机理。 创新点 1. 揭示锰氧化物吸附剂脱汞性能与物化性质之间的构效关系; 2. 阐明锰氧化物吸附剂表面脱汞机理, 并揭示其中的活性位点。 方法 1. 采用常规的沉淀法、水热法及溶胶凝胶法合成锰氧化物吸附剂; 2. 采用微型固定床反应器研究系列锰吸附剂的脱汞性能; 3. 采用物化表征研究系列锰氧化物汞吸附剂的物化特性, 揭示其中的构效关系; 4. 结合动力学计算, 揭示锰氧化物汞吸附剂表面脱汞机理, 并阐明影响吸附剂脱汞性能的关键因素。 结论 1. 与采用沉淀法及水热法制备的汞吸附剂相比, 采用溶胶凝胶法制备的汞吸附剂在150~250 °C的温度区间内具有更为优异的脱汞性能; 2. 采用溶胶凝胶法制备的锰氧化物吸附剂具有丰富的表面酸性位点, 可促进单质汞的吸附, 并为接下来的单质汞氧化步骤提供充足的反应物, 这也是该吸附剂具有优异脱汞性能的主要原因; 3. 脱汞过程中, 初始吸附阶段是主要的速控阶段, 而Mn4+及化学吸附氧为该反应中的主要活性位点。 This study demonstrated the impacts of the synthesis methods on the textural structures, chemical properties, and Hg^0 capture capability of the MnO_ x system. Compared with the samples synthesized using the precipitation (PR) and hydrothermal (HT) methods, the adsorbent prepared via the sol-gel (SG) technique gave the best performance. At 150 °C, ca. 90% Hg^0 removal efficiency was reached after 7.5 h for MnO_ x prepared by the SG method, ca. 40% higher than that of the other two methods. The specific surface area of the adsorbent synthesized via the SG technique (23 m^2/g) was almost double that of the adsorbent prepared by the HT method (12 m^2/g) and three times that of the one prepared by the PR method (7 m^2/g). The presence of plentiful acid sites from the SG method facilitated the physisorption of Hg^0, making more Hg^0 available to be oxidized to HgO by the redox sites and thus giving the adsorbent prepared by the SG method the highest Hg^0 removal efficiency. The strong oxidative ability accelerated the oxidation of the physically adsorbed Hg^0 to HgO, which explained the higher Hg^0 removal efficiency of the sample prepared using the HT method than that of the one synthesized by the PR technique. During the whole Hg^0 removal cycles, chemisorption dominated, with the initial adsorption stage and the external mass-transfer process playing important roles.