Probing active species for CO hydrogenation over ZnCr_(2)O_(4) catalysts  

ZnCr_(2)O_(4)表面CO加氢反应活性位点的探测

作　　者：Yunjian Ling Yihua Ran Weipeng Shao Na Li Feng Jiao Xiulian Pan Qiang Fu Zhi Liu Fan Yang Xinhe Bao 凌云健;冉义华;邵伟鹏;李娜;焦峰;潘秀莲;傅强;刘志;杨帆;包信和(中国科学院大连化学物理研究所催化基础国家重点实验室,辽宁大连11602;中国科学院大学,北京100039;上海科技大学物质科学与技术学院,上海201210;上海科技大学上海市高分辨电子显微学重点实验室,上海201210)

机构地区：[1]State Key Laboratory of Catalysis,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian 116023,Liaoning,China [2]University of Chinese Academy of Sciences,Beijing 100039,China [3]School of Physical Science and Technology,ShanghaiTech University,Shanghai 201210,China [4]Shanghai Key Laboratory of High-resolution Electron Microscopy,ShanghaiTech University,Shanghai 201210,China

出　　处：《Chinese Journal of Catalysis》2022年第8期2017-2025,共9页催化学报（英文）

基　　金：国家自然科学基金(21972144,92045303,91945302,21991152);上海市科委(20JC1416700,21DZ2260400).

摘　　要：Oxide catalysts are increasingly employed for hydrogenation reactions,among which ZnCrOx is a major catalyst for the oxide-zeolite(OXZEO)process and for the hydrogenation of C1 molecules in general.Owing to the complex nature of ternary oxides,the surface and catalytic properties of ZnCr_(2)O_(4) spinel have remained controversial for CO hydrogenation.Combining in-situ Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy,we examined the adsorption and reaction of CO/H_(2) on the ZnCr_(2)O_(4) catalysts,which were pre-treated under oxidative or reductive conditions.The reduced ZnCr_(2)O_(4) catalyst was found to expose more surface sites for CO adsorption/reaction than the oxidized ZnCr_(2)O_(4) catalyst.Exposing the reduced ZnCr_(2)O_(4) to H_(2) at room temperature led to the formation of surface hydride species,which would transform into hydroxyl species at elevated temperatures.The reduced ZnCr_(2)O_(4) surface exhibited much stronger interaction with CO and H_(2) than ZnO and Cr_(2)O_(3).Exposing the reduced ZnCr_(2)O_(4) to the CO and H_(2)(1:1)mixture gas led to the hydrogenation of CO.However,CO was oxidized by the hydroxyl species via the water-gas-shift reaction,whereas the hydrogenation of CO could only be achieved by surface hydride species on the reduced ZnCr_(2)O_(4) to formyl or formate species at 373-473 K.Our study has thus shed light on the active species that control elementary reaction process of CO hydrogenation on complex oxide surfaces.CO加氢转化为高附加值化学品是煤、天然气和生物质等清洁利用的核心过程之一.近年来,氧化物催化剂越来越多地被用于加氢反应,其中ZnCrO_(x)催化剂广泛应用于Cl分子催化加氢反应,并且是氧化物分子筛(OXZEO)合成气转化催化剂的主要活性组分之一.由于复合氧化物表面结构的复杂性,在CO加氢反应中,ZnCrO_(x)上的活性位点和活化过程尚存在争议.本文采用原位傅里叶变换红外光谱(FT-IR)和X射线光电子能谱(XPS)研究了不同条件预处理的ZnCr_(2)O_(4)尖晶石催化剂在CO/H_(2)中的原位吸附和反应过程.XPS结果表明,相较氧化的ZnCr_(2)O_(4),使用H_(2)还原的ZnCr_(2)O_(4)中Cr^(6+)的含量会下降,同时带来更多的表面氧空位或羟基物种.Cr^(6+)的还原也能由FT-IR谱中位于1013 cm^(-1)铬酸盐的Cr=O振动峰的消失来证明.利用CO作为探针分子,FT-IR结果表明还原的ZnCr_(2)O_(4)比氧化的ZnCr_(2)O_(4)具有更多的CO吸附位点.通过对比ZnO和Cr_(2)O_(3)上的CO吸附波数发现,这些CO吸附位点并非还原ZnCr_(2)O_(4)相分离产生的ZnO和Cr_(2)O_(3),而是通过直接还原表面Cr位点,在其周围产生氧空位而产生的新位点.在室温下,ZnCr_(2)O_(4)与CO能反应形成碳酸盐,还原的ZnCr_(2)O_(4)产生碳酸盐的表面吸附量以及结合碳酸盐的稳定性要明显高于氧化的ZnCr_(2)O_(4).真空室温下,H_(2)能在还原的ZnCr_(2)O_(4)上异裂解离形成氢化物,并可在373 K稳定存在;而ZnO表面H_(2)异裂产生的氢化物在低于室温即已脱附.还原的ZnCr_(2)O_(4)表面形成的氢化物在423 K以上会有部分转化为羟基,这些羟基具有较高的热稳定性,在673 K以上才开始脱附.相比ZnO或Cr_(2)O_(3),还原的ZnCr_(2)O_(4)表面与CO和H_(2)的作用都要更强.在CO/H_(2)(1:1)混合气氛下,还原的ZnCr_(2)O_(4)表面能发生CO加氢反应,在373 K发现CO加氢产物甲酸盐形成.然而,在只有羟基存在的还原的ZnCr_(2)O_(4)表面上,羟基只能�

关 键 词：ZnCr_(2)O_(4) Fourier-transformed infrared spectroscopy CO adsorption HYDRIDE HYDROXYL 

分 类 号：TQ426[化学工程]