温室效应引起的全球变暖对人类生存环境造成了严重威胁。CH4-CO2重整反应(DRM)以两大温室气体为原料，在消耗利用CH4和CO2的同时，产生H2/CO物质的量比接近1的合成气，是费托合成生产液体燃料或高值化学品的理想原料。镍基催化剂是DRM反应应用最广泛的催化剂，但在高温反应条件下易因烧结和积碳而失活，阻碍了其工业应用。针对镍基催化剂因积碳而失活的不足，综述了DRM反应热力学和反应机理，对催化剂积碳失活机理进行分析，论述了催化剂活性组分、载体、助剂以及制备方法等。CH4-CO2重整热力学分析表明高温低压有利于平衡向生成合成气方向移动。逆水煤气变换反应会消耗原料中CO2产生CO，因此一般情况下CO2转化率高于CH4转化率，H2/CO物质的量比小于1。反应温度557～700 ℃时，积碳主要来源于CO歧化反应和CH4裂解反应。反应温度超过700 ℃，不利于CO歧化反应的发生，积碳主要来源于CH4裂解反应。积碳依据活性不同可分为无定形碳和石墨碳，无定形碳在低于573 K即可被H2或含氧物种消除。而石墨碳需要在较高温度才能被气化消除，是造成催化剂失活的主要原因。增加原料气中CO2/CH4物质的量比、在原料气中添加水蒸气或氧气可以在一定程度上减少积碳，但解决积碳问题的核心是催化剂的研究。双金属镍基催化剂在一定比例下形成合金，在活化CH4及消碳过程起协同作用。载体介孔结构的限域效应使镍颗粒尽可能存在于催化剂孔道中，有利于减小金属镍颗粒尺寸且在一定程度上增强金属-载体相互作用力，从而提高催化剂对DRM反应催化活性和抗积碳能力。具有特殊氧化还原性质和超常储氧能力(OSC)的载体可利用氧空位来促进CO2活化和解离，通过将表面碳氧化为CO来减少由于碳沉积而导致的催化剂失活。使用碱性载体或助剂可适当增加催化剂碱性，使CH4裂解反应的碳沉积速率与碳消除反应速率相当，从而减少积碳。通过研发新的催化剂制备方法，实现反应过程强化和反应过程耦合，均可有效降低和消除反应积碳。说明适当调变催化剂组成和结构，可以明显提高催化剂在DRM反应中的抗积碳能力。

Global warming caused by the greenhouse effect poses a serious threat to the human living environment. Dry reforming of methane (DRM) reaction uses two major greenhouse gases as feedstock to produce syngas with H2/CO molar ratio close to 1 while consuming methane and carbon dioxide,making it an ideal feedstock for Fischer-Tropsch synthesis to produce liquid fuels or high value chemicals. Nickel-based catalysts are the most widely used catalysts for DRM reaction,but they are prone to deactivation due to sintering and carbon accumulation under high temperature,hindering their industrial application. In view of the inactivation of nickel-based catalysts due to carbon deposition,thermodynamics and reaction mechanisms of DRM reactions were reviewed,carbon deposition inactivation mechanisms was analysed,and catalyst active components,supports,auxiliaries and preparation methods were discussed. Thermodynamic analysis of DRM reaction shows that high temperature and low pressure favor a shift in equilibrium towards syngas production. Reverse water-gas shift reaction consumes CO2 from the feedstock to produce CO,so in general the CO2 conversion rate is higher than the CH4 conversion rate,and the H2/CO molar ratio is less than 1. At a reaction temperature of 557-700 ℃,the carbon deposition mainly comes from the carbon monoxide disproportionation reaction and the methane cracking reaction. Reaction temperature above 700 ℃ are not conducive to carbon monoxide disproportionation reactions and carbon deposition will mainly originate from methane cracking reaction. Amorphous carbon can be eliminated by hydrogen or oxygenated species at temperatures below 573 K. On the other hand,graphite carbon requires gasification at higher temperature to be eliminated and it is the main cause of catalyst deactivation. Increasing the CO2/CH4 molar ratio in the feed gas or adding water vapour or oxygen to the feed gas can reduce carbon deposition to some extent,but to solve the problem of carbon accumulation is the study of catalysts. Bimetallic nickel-based catalysts form alloys at certain ratios,which act synergistically in the activation of CH4 and the decarbonisation process. The limiting effect of the mesopore structure in support allows the nickel particles to be present in the catalyst pores as much as possible,which helps to reduce the size of nickel metal particles and to enhance the metal-support interaction to a certain extent,thus improving the catalytic activity and resistance to carbon depositionn in the DRM reaction. Supports with special redox properties and extraordinary oxygen storage capacity (OSC) can use oxygen vacancies to facilitate the activation and dissociation of CO2,reducing catalyst deactivation due to carbon deposition by oxidation of surface carbon to CO. The use of alkaline supports or auxiliaries can increase the catalyst alkalinity appropriately so that the carbon deposition rate of the methane cracking reaction is comparable to the carbon elimination reaction rate,thereby reducing carbon accumulation. New catalyst preparation methods,reaction process enhancement and reaction process coupling can all be effective in reducing and eliminating reaction carbon accumulation. Studies show that appropriate modifications to the composition and structure of catalyst can significantly improve the resistance of the catalyst to carbon accumulation in the DRM reaction.

0 引言

1 CH4-CO2重整热力学

2 CH4-CO2重整反应机理

3 催化剂积碳失活机理分析

4 CH4-CO2重整催化剂

   4.1 活性组分

   4.2 载体

   4.3 助剂

   4.4 制备方法

5 总结及展望