Hydrogenolysis of alkanes has been widely reported as structure sensitive reaction on transition metal heterogeneous catalysts with metal particle sizes ranging between 1 and 20 nm. In this work, a series of Ir/MgAl2O4 and Ir/SiO2 catalysts with different Ir particle sizes ranging from subnanometer clusters (<1 nm) to nanoparticles (1-3 nm) were prepared and tested for n-butane hydrogenolysis. Our results show that the activity towards n-butane hydrogenolysis increases as Ir particle size increases in the lower particle size range, goes through a maximum at similar to 1.4-1.6 nm and then drops with a further increase in particle size. The product distribution at low temperature (170-190 degrees C) is dominated by central and terminal C-C bond cleavage of n-butane, and less by two C-C bond cleavage or further hydrogenolysis of the propane and ethane products. 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 similar to 1.4 nm but remains constant with further decrease in size. The results show that an Ir size of similar to 1.4 nm is optimum for n-butane hydrogenolysis activity and selectivity towards ethane. (C) 2020 Elsevier Inc. All rights reserved.