Particle Size Effect Of Pd/ZnO Catalyst In The Hydrogenation Of CO₂ To Methanol

CO₂ hydrogenation is an important way to effectively reduce the concentration of CO₂ in the atmosphere and obtain methanol,a chemical raw material with high added value.In this reaction,the Pd/ZnO catalyst has received a lot of attention due to its excellent catalytic performance and stability.In this research paper,by taking the advantages of atomic layer deposition（ALD）technology in fine-tuning the metal particle size,the size of Pd particles was precisely controlled,and the size-dependent strong metal-support interactions（SMSI）was studied in depth,as well as the influence of SMSI effect on the activity and selectivity for hydrogenation of CO₂ to methanol.The results obtained are as follows:We found that in the Pd/ZnO catalyst system,SMSI was size-dependent,which had an important impact on the activity and selectivity for hydrogenation of CO₂ to methanol.Firstly,we used ALD method to deposit a series of Pd particles with various sizes（1.6 nm to 7.9 nm）on regular ZnO nanodisk supports.By means of CO chemisorption and TEM characterization,we found the Pd particles with diameter over CO₂.5 nm were more prone to be encapsulated by ZnO than 1.6 nm Pd particles after reduction in hydrogen at CO₂50℃.At the same time,XPS results showed that the electronic structure of 1.6-nm Pd/ZnO catalyst was significantly different from other Pd/ZnO catalysts with larger-size Pd particles.In the reaction of CO₂ hydrogenation to methanol,Pd/ZnO catalysts with Pd particles larger than CO₂.5 nm exhibited higher intrinsic activity（TOF）and methanol selectivity than 1.6-nm Pd/ZnO catalyst.These differences were attributed to the different extents of formation of Pd-ZnO interfaces induced by the size-dependent SMSI as well as the varied CO-binding strength by the changed electronic structures of Pd nanoparticles on particle size.In addition,to artificially increase the Pd-ZnO interface without changing the particle size,ZnO ALD was performed on the 1.6-nm Pd/ZnO catalyst for eight cycles to further reinforce the encapsulation of the small Pd nanoparticles（1.6-nm Pd/ZnO@8cZnO）.Comparative experiment results showed that the activity of 1.6-nm Pd/ZnO@8cZnO catalyst was much higher than that of 1.6-nm Pd/ZnO catalyst,but methanol selectivity of 1.6-nm Pd/ZnO@8cZnO catalyst was still far lower than other Pd/ZnO catalysts with larger-size Pd particles.This result again verified the understanding of the particle size effect of Pd/ZnO catalysts mentioned above.