Catalytic CO2 hydrogenation to methanol is a promising route to mitigate the negative effects of anthropogenic CO2.To develop an efficient Pd/ZnO catalyst,increasing the contact between Pd and ZnO is of the utmost importance,because "naked" Pd favors CO production via the reverse water-gas shift path.Here,we have utilized a ZnO@ZIF-8 core-shell structure to synthesize Pd/ZnO catalysts via Pd immobilization and calcination.The merit of this method is that the porous outer layer can offer abundant "guest rooms" for Pd,ensuring intimate contact between Pd and the postgenerated ZnO.The synthesized Pd/ZnO catalysts (PZZ8-T,T denotes the temperature of calcination in degree Celsius) is compared with a ZnO nanorod-immobilized Pd catalyst (PZ).When the catalytic reaction was performed at lower reaction temperatures (250,270,and 290 ℃),the highest methanol space time yield (STY) and highest STY per Pd achieved by PZ at 290 ℃ were 0.465 g gcat-1 h-1 and 13.0 g gPd-1 h-1,respectively.However,all the PZZ8-T catalysts exhibited methanol selectivity values greater than 67.0％ at 290 ℃,in sharp contrast to a methanol selectivity value of 32.8％ for PZ at the same temperature.Thus,we performed additional investigations of the PZZ8-T catalysts at 310 and 360 ℃,which are unusually high temperatures for CO2 hydrogenation to methanol because the required endothermic reaction is expected to be severely inhibited at such high temperatures.Interestingly,the PZZ8-T catalysts were observed to achieve a methanol selectivity value of approximately 60％ at 310 ℃,and PZZ8-400 was observed to maintain a methanol selectivity value of 51.9％ even at a temperature of 360 ℃.Thus,PZZ8-400 attains the highest methanol STY of 0.571 g gcat-1 h-1 at 310 ℃.For a better understanding of the structure-performance relationship,we characterized the catalysts using different techniques,focusing especially on the surface properties.X-ray photoelectron spectroscopy (XPS) results indicated a linear relationship between the methanol selectivity and the surface PdZn:Pd ratio,proving that the surface PdZn phase is the active site for CO2 hydrogenation to methanol.Furthermore,analysis of the XPS O 1s spectrum together with the electronic paramagnetic resonance results revealed that both,the oxygen vacancy as well as the ZnO polar surface,played important roles in CO2 activation.Chemisorption techniques provided further quantitative and qualitative information regarding the Pd-ZnO interface that is closely related to the CO2 conversion rate.We believe that our results can provide insight into the catalytic reaction of CO2 hydrogenation from the perspective of surface science.In addition,this work is an illustrative example of the use of novel chemical structures in the fabrication of superior catalysts using a traditional formula.%近年来由于环境问题CO2加氢制甲醇催化反应重新回归为研究热点.对于Pd/ZnO催化剂,研究表明PdZn合金相是制甲醇反应的活性中心,而单独Pd利于CO生成.为了实现Pd和ZnO的充分接触,本工作以一种ZnO@ZIF-8核壳型结构为载体负载Pd纳米颗粒后经由高温煅烧制得PZZ8-T催化剂(T为不同煅烧温度),同时制备了ZnO纳米棒负载Pd的PZ催化剂作为对比.在随后的CO2加氢反应中,相比于PZ,PZZ8-T展现出极高的甲醇选择性.之后我们通过一系列表征探究了催化剂的构效关系,发现催化剂的甲醇选择性与表面Pd的化学态有关,更多的Pd以PdZn合金的形式存在将会带来更高的甲醇选择性.XPS O 1s谱图和EPR分析表明CO2的活化与催化剂表面的氧空穴和ZnO极性面含量直接相关.而化学吸附手段进一步对Pd-ZnO界面进行了表征,揭示了其与CO2转化速率的关联.本工作的意义在于,一是展现了利用新材料制备更优的传统催化剂的方法,二是通过表面分析手段加深了对催化剂构效关系的理解.
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