氧空位修饰的暴露TiO2{001}的Ru/TiO2增强光热协同CO2甲烷化活性和稳定性

利用太阳能在温和条件下实现CO₂还原反应,不仅可以缓解过度消耗化石能源造成的能源危机,还可以改善诸如温室效应和海洋酸化等环境问题.光热协同催化可以有效降低催化反应温度,具有较大的应用前景.本文利用Ru与暴露TiO₂{001}晶面的TiO₂载体产生的金属-载体相互作用,经过高温氢气煅烧后,获得具有丰富表面氧空位的Ru/TiO₂催化剂.活性测试结果表明,具有丰富表面氧空位的Ru/TiO₂表现出优异的CO₂甲烷化活性,反应过程中甲烷的TOF值在300°C时可以达到22 h-1,但该催化剂却表现出较差的稳定性,在反应10小时后,甲烷的TOF值逐渐降低到19 h-1.将紫外光引入到Ru/TiO₂热催化甲烷化体系中,甲烷的TOF值增加到30 h-1,且兼具高稳定性.热催化反应过程中逐渐消失的表面氧空位和部分氧化的Ru是活性降低的主要原因.在光热协同反应中,光生电子的产生稳定了Ru表面的电子密度,同时也再生了催化剂上表面氧空位,这有效地提高了反应的活性和稳定性.程序升温原位红外和X射线光电子能谱实验结果表明,当催化剂表面具有丰富的表面氧空位时,CO₂可以有效地在Ru纳米粒子上解离成CO中间体,随后吸附在Ru上的CO中间体解离成表面碳物种,并加氢产生甲烷.在热催化反应过程中,Ru纳米粒子逐渐被氧化成Ru Ox物种,且表面氧空位被CO中间物种覆盖,降低了催化反应的稳定性.当紫外光引入到上述反应中,催化剂的表面氧空位可有效提高光生载流子的分离能力.TiO₂载体产生的光电子转移至Ru表面,稳定了金属Ru纳米粒子的价态.另外,载体产生的光生空穴加速了H₂质子化,提高了催化剂对氢气的活化迁移能力,促进了CO中间体的加氢甲烷化反应,进而再生表面氧空位.因此在紫外光照下,兼顾提高了热催化CO₂甲烷化的活性和稳定性.值得注意的是,当Ru负载于暴露少量TiO₂{001}晶面的TiO₂载体上时,产生了强金属-载体相互作用并抑制了H₂在催化剂上的吸附活化,不利于产生表面氧空位.因此暴露少量TiO₂{001}晶面的Ru/TiO₂催化剂也不利于光生载流的产生和分离,这导致热催化或光热协同催化反应活性较低.

Photo-enhanced thermal catalytic CO2 methanation activity and stability over oxygen-deficient Ru/TiO2 with exposed TiO2 {001} facets: Adjusting photogenerated electron behaviors by metal-support interactions

In this study, two Ru/TiO2 samples with different TiO2 facets were prepared to investigate their photo-thermal catalytic CO2 + H2 reaction behavior. Without UV irradiation, the Ru/TiO2 with 67％ {001} facet (3RT) displayed improved thermal catalytic activity for CO2 methanation than that of Ru/TiO2 with 30％{001} facet (0RT). After H2 pretreatment, both samples exhibited enhanced thermal catalytic activities, but the H2-treated 3RT (3RT-H) exhibited superior activity to that of the H2-treated 0RT (0RT-H). Under UV irradiation, 3RT-H exhibited apparent photo-promoted thermal catalytic activity and stability, but the enhanced catalytic activity was lower than that of 0RT-H. Based on the characterization results, it is proposed that both the surface oxygen vacancies (Vos) (activating CO2) and the metallic Ru nanoparticles (activating H2) were mainly responsible for CO2 methanation. For 0RT, H2 pretreatment and subsequent UV irradiation did not promote the for-mation of Vos, resulting in low catalytic activity. For 3RT, on the one hand, H2 pretreatment pro-moted the formation of Vos, which were regenerated under UV irradiation; on the other hand, the photogenerated electrons from TiO2 transferred to Ru to maintain the metallic Ru nanoparticles. Both behaviors promoted the activation of CO2 and H2 and enhanced CO2 methanation. Moreover, the photogenerated holes favored the dissociated H at Ru migrating to TiO2, also promoting CO2 methanation. These behaviors occurring on 3RT-H may be attributed to the suitable metal-support interaction between the Ru nanoparticles and TiO2 {001}, resulting in the easy activation of lattice oxygen in TiO2 to Vos. With reference to the analysis of intermediates, a photo-thermal reaction mechanism is proposed for the Ru/TiO2 {001} facet sample.