(679a) Structure Sensitivity of n-CnH2n+2 hydrogenolysis on Supported Ir Catalysts

Hydrogenolysis of alkanes has been widely reported as structure sensitive reaction on transition metal heterogeneous catalysts. For example, for n-butane hydrogenolysis, the selectivity to ethane drastically increases with decreasing Ir particle size in the range of 1 – 10 nm. ¹ However, the particle size effect in the subnanometer regime has not been investigated. In this work, a series of Ir/MgAl₂O₄ and Ir/SiO₂ catalysts with different particle sizes ranging from single atoms, subnanometer clusters to nanoparticles (<10nm) were prepared and tested for n-C_nH_2n+2 hydrogenolysis (n=2 and 4) to investigate the structure sensitivity. The catalysts were characterized by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The kinetic measurement results show the activity for n-butane and ethane hydrogenolysis increases with increase in Ir particle size then decreases above ~2 nm. Additionally, the results show that n-butane hydrogenolysis selectivity to ethane is insensitive to sizes below ~2nm and proceeds primarily by cleavage of the central C-C bond. On the other hand, the selectivity decreases with increasing particle size (~2-10nm). Lastly, the effect of temperature and conversion on selectivity on the different size Ir catalysts will be presented and discussed with respect to the reaction pathways (terminal vs. central C-C cleavage).

1. Engstrom, J.R., D.W. Goodman, and W.H. Weinberg, Hydrogenolysis of n-butane over the (111) and (110)-(1. times. 2) surfaces of iridium: a direct correlation between catalytic selectivity and surface structure. Journal of the American Chemical Society, 1986. 108(15): p. 4653-4655.