Structure Sensitivity of Alkanes Hydrogenolysis and Alkynes Hydrogenation on Supported Ir Catalysts

In many catalytic systems, the activity and selectivity of supported metal catalysts or extended metal surface catalysts would be affected by the metal surface structure, and this phenomenon is called structure sensitivity. Generally, structure sensitivity is led by the change of geometric and electronic properties of the metal on the surface. The variation of metal nuclearity and metal-support interactions are effective ways to change the geometric and electronic properties of the supported metal catalyst, leading to different types of the active sites exposing on the support that would take effect on catalyzing the reaction. In this work, a series of supported Ir catalysts (on MgAl2O4 and SiO2) with different particle sizes less than 3 nm were utilized for hydrogenolysis of n-butane and ethane to study the structure sensitivity as well as the potential reaction pathways. The results indicate that the activity of n-butane hydrogenolysis increases as Ir particle size increases in the small particle size range (0.7–1.4 nm) and then drops when the Ir particle size further increases and the Ir single atoms might be inactive for hydrogenolysis after the post-reaction analysis. The selectivity of n-butane hydrogenolysis is dominated by central and one terminal C–C bond cleavage on the n-butane molecules at low temperature range. The selectivity to central C–C bond cleavage is highly dependent on the size of Ir and increases with a decrease in particle size down to ~1.4 nm but remains constant with further decrease in size. The hydrogenolysis of ethane shows a similar trend in the small size range but the activity is much lower than n-butane, which supports the low level of series reaction pathway in the case of n-butane hydrogenolysis. In addition to Ir nuclearity, the effect of electronic properties was also studied on another series of Ir catalysts supported on ZnAl2O4, in which zinc replace the magnesium within the same spinel structure. The characterization results including HAADF-STEM and volumetric CO chemisorption show the difference of Ir nuclearity in the subnanometer regime and nanoparticles (~1.4 nm), while XPS and DRIFTS indicate the difference of electronic properties from metal-support interaction on the two Ir catalysts with the same nuclearity but reduced at different temperatures. Acetylene hydrogenation is structure sensitive on Ir/ZnAl2O4 catalysts and the activity and selectivity are mainly determined by Ir nuclearity instead of the difference in electronic properties. The Ir single atoms and subnanometer clusters are more selective to the target product of C2H4 but less active than large Ir nanoparticles as there might be more π-bonded adsorption than di-σ bonded adsorption for C2H2 on the Ir single atoms and subnanometer clusters.