新型V2O5/TiO2-石墨烯复合催化剂的制备及其低温脱硝机理研究

Nitrogen oxides emitted from the burning of fossil fuels and mobile and stationary sources is one of the main atmospheric pollutants that leads to a lot of environmental problems. Selective catalytic reduction (SCR) of NOx by NH3 in the presence of oxygen is one of the most effective methods to decrease the NOx levels in gaseous emissions. A well-known commercialized catalyst for this process is V2O5/TiO2. However, there are still some inevitable problems with this catalyst system including the toxicity of vanadium species, the narrow temperature window of 300-400 °C, and this go against the denitration engineering reform. So it is very necessary to develop a low-temperature SCR process and catalysts. Graphene (GE) as a new carbon nanomaterial has many exceptional properties, such as high electron mobility, and large theoretical specific surface area. The high-efficiency process of the gain and loss of electrons has resulted in the improvement of the redox performance and the catalytic reduction ability of the catalyst. Therefore, in this work, GE will be introduced into V2O5/TiO2 catalyst to improve the loading of the active component (V2O5), increase the adsorption of NH3 and the reaction ratio, while the low temperature catalytic activity will be enhanced. Meanwhile, in order to achieve and enhance the ability to improve the SCR performance at low temperature, thermal catalysis and photo catalysis of the composites catalysts will be combined. And by these, this work will put forward the mechanisms and provides a scientific theory basis for the development of our country’s gas denitration technologies.

燃煤造成大量的NOx排放，是造成大气污染的主要来源之一，给人类生存环境带来巨大危害。选择性催化还原技术作为比较成熟且高效的烟气脱硝技术已得到广泛的应用，但商用的钒钛系催化剂的活化温度较高且温度窗口较窄，不利于现有脱硝工程的改造。因而亟待研究和开发低温抗中毒能力强的催化剂。石墨烯作为一种新型碳材料具有比表面积大、电导率高等特点，有利于提高催化剂的氧化还原性及催化还原能力。因此，本项目将石墨烯与钒钛催化剂进行复合，以提高活性物质的分散性增强气体吸附，加快反应速率，达到降低催化剂活性温度的目的。同时将催化剂的热催化与光催化相结合，实现催化剂的低温高效脱硝，并探明机理，为我国烟气脱硝的发展提供科学理论依据。

传统的SCR催化剂（V2O5-WO3/TiO2）在我国推广应用面临的一大挑战在于其对锅炉负荷的适应性，开发具有宽温度窗口的中低温SCR催化剂势在必行。石墨烯作为碳材料家族的新成员，具有更大的比表面积、强的电子迁移能力、优异的热稳定性好以及良好的机械性等特点，适合作为催化剂载体。还可能与活性组分发生协同作用，促进催化反应的进行。.本项目以溶胶凝胶法将改进的Hummers法制备的氧化石墨烯与TiO2复合，制备TiO2-石墨烯复合载体，通过超声浸渍法负载VOx活性组分，制备V2O5/TiO2-石墨烯系列复合催化剂。分别考察了石墨烯含量、VOx活性组分负载量对V2O5/TiO2-石墨烯催化剂的结构、表面物化性质、活性的影响。结果表明，催化剂中VOx和TiO2以纳米颗粒形式均匀分散在石墨烯表面，且VOx以多种氧化物共同存在。当石墨烯含量为2 wt.%，VOx负载量为10 wt.%的催化剂中V4+和表面吸附氧比例增加，低温区酸性位点数量增多，且出现了更多的中强酸位点，氧化还原能力增强，使其获得了最佳的低温NH3-SCR活性，拓宽了活性温窗。在此基础上考察H2O和SO2对其低温SCR活性的影响。研究结果表明，V2O5/TiO2-石墨烯复合催化剂的抗中毒能力显著优于V2O5/TiO2催化剂，主要原因是石墨烯中丰富的OH基团的疏水性和预硫化作用抑制了H2O和SO2在催化剂表面的吸附。此外，由于V2O5、TiO2和石墨烯三者之间的强相互作用，使得催化剂光催化氧化NOX的能力得到明显改善。.为了进一步探讨V2O5/TiO2-石墨烯复合催化剂的低温脱硝机理，通过原位漫反射傅里叶变换红外光谱技术研究了NH3和NOX等反应气体在催化剂表面的吸脱附行为、各种反应气体之间的暂态反应和稳态反应，分析催化剂表面SCR反应路径，揭示石墨烯在NH3-SCR反应中的促进机制。结果表明，NH3分别以NH4+和分子配位态吸附在Bronsted酸位和Lewis酸位上，由于催化剂表面丰富的活性氧物种，NOx在催化剂表面主要形成NO2，可以促进快速SCR反应的发生，从而提升催化活性。因此，石墨烯复合催化剂比V2O5/TiO2催化剂拥有更好的低温SCR催化活性。这些研究可为新型低温SCR催化剂的开发和应用提供新的契机。