富氢气中 CO 选择性甲烷化 Ni/TiO2 催化剂的制备与性能研究

质子交换膜燃料电池（Proton Exchange Membrane Fuel Cell, PEMFC）是一种在交通、分布式发电等领域具备广阔应用前景的清洁发电技术。然而，由于H₂是体积能量密度最小且易燃易爆的气体分子，使用高纯氢气作为氢源时对储存、运输有着严格的安全要求和高昂的成本，这限制了PEMFC的广泛应用。尽管采用甲醇在线制氢技术能够解决氢气的储运问题，但制得的富氢气还需要经过纯化处理，方能避免杂质CO对PEMFC的铂电极造成不可逆的损害。CO选择性甲烷化（Selective Methanation of CO, CO-SMET）有着工艺简单高效、能耗低、反应产物对PEMFC无害等优点，为满足富氢气在线纯化处理供给燃料电池的需求，其关键在于研制出具有高活性、高选择性和高稳定性的催化剂。本文探索出一种可以低温活化Ni/TiO₂催化剂的制备策略，通过浸渍法合成了一种高效的Ni/TiO₂催化剂，其可以将富氢气中1%CO降低至10 ppm。采用XRD、TEM、XPS、H₂-TPR、EPR和Raman等表征技术研究了活性金属颗粒尺寸和载体形貌对催化性能的影响，揭示了其电子-金属-载体相互作用（Electronic Metal-support Interactions, EMSIs）调节CO-SMET催化性能的机制。

通过控制焙烧温度制备出不同Ni晶粒尺寸的Ni/TiO₂催化剂，研究了EMSIs的粒径效应，即Ni晶粒尺寸越大，TiO_2−x对Ni的包覆作用和电子迁移越强，导致活性位点减少和Ni化学态降低，进而影响活性和选择性。为了更深入探究TiO₂形貌对CO-SMET反应性能的影响，本文基于形貌工程分别合成了{101}、{100}和{001}三种不同暴露晶面的TiO₂载体，研究了EMSIs的形貌依赖效应。Ni/TiO₂-{101}和Ni/TiO₂-{100}不仅结构稳定，能够提供更多的氧空位作为重要的活性位点；而Ni/TiO₂-{001}的晶面结构不稳定且对高温敏感，同时过强的金属-载体相互作用促进了TiO_2−x对Ni的包覆，进而降低活性。不仅揭示了Ni/TiO₂催化剂在CO-SMET体系中的活性结构，还阐明了Ni/TiO₂的EMSIs影响CO-SMET催化活性和选择性的机理，为Ni/TiO₂催化剂在CO-SMET的工业应用提供了制备策略和理论依据。

Proton exchange membrane fuel cell (PEMFC) is a prospective clean power technology in transportation and distributed generation system. However, H₂ is the gas molecule with the lowest volumetric energy density, yet it is also flammable and explosive. Therefore, the storage and transportation of pure hydrogen bring about the strict safety requirements and expensive cost, which limits the widespread applications of PEMFC. This problem could be solved by the in-situ hydrogen generation technology of methanol, while the H₂-rich gas needs to be purified to avoid irreversible damage to the Pt electrodes of PEMFC from the by-product carbon monoxide.

Selective methanation of CO (CO-SMET) provides many advantages, such as easy-to-use procedure, low energy consumption, and harmless reaction products to PEMFC, which could meet the requirement of H₂-rich gas online purification and supplement to PEMFC. The key is the development and preparation of effective catalysts with high activity, selectivity and stability. In this work, the effective Ni/TiO₂ catalysts were prepared via the impregnation method and a novel strategy for preparation was explored to activate the Ni/TiO₂ catalysts at low reduction temperature, which can reduce CO concentration from 1% to 10 ppm in the reaction of CO-SMET. The structure-activity relationship of metal’s particle size and supports’ morphology on CO-SMET catalytic reaction were characterized by XRD, TEM, XPS, H₂-TPR, EPR and Raman techniques, and the mechanism was found that electronic metal-support interactions (EMSIs) have a crucial influence in tuning CO-SMET catalytic performance.

Ni/TiO₂ catalysts with different Ni particle size have been synthesized by controlling at different calcination temperatures, and the metal size-dependent EMSIs was investigated on Ni/TiO₂. The enhancement of TiO_2−x layer physically covering the Ni particles and the electron transfer from support to active metal, which results in the decrease of active sites and Ni chemical state along with the Ni particle size increasing, then it influences activity and selectivity of catalysts. In order to further explore the effect of TiO₂ morphology on the performance of CO-SMET reaction, an advanced investigation to morphology of support was carried out. Respectively, three kinds of TiO₂ nanocrystals with mainly {101}, {100} and {001} facets exposed were synthesized by morphology engineering as the supports of Ni catalysts, and the morphology-dependent effect of EMSIs were investigated. Both of Ni/TiO₂-{101} and Ni/TiO₂-{100} are stable and provide more oxygen vacancies as important active sites. However, the labile structure of Ni/TiO₂-{001} make it vulnerable to high temperature, and the activity decreases due to the excessively strong metal-support interactions and TiO_2−x covering the active sites of Ni. This study has revealed there are the active structures of Ni/TiO₂ catalyst on CO-SMET. and clarified the EMSIs mechanism of Ni/TiO₂ catalysts’ activity and selectivity. It provides the preparation strategy and theoretical basis for the industrial application of Ni/TiO₂ catalyst in CO-SMET.

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