Characterisation of Pt,Sn/Mg(Al)O catalysts for light alkane dehydrogenation by FT-IR spectroscopy and catalytic measurements

The Pt-function in Pt,Sn/Mg(Al)O and in Pt/Mg(Al)O catalysts has been studied by a combination of catalytic testing for ethane dehydrogenation at 450-650ºC under a C2H6 : H2 : CO2 : N2 : Ar = 10 : 1.6 : 6.8 : 5.9 : 75.7 flow, and Fourier Transform Infrared Spectroscopy (FT-IR), using CO as probe molecule. The acid-base properties of the support were also investigated by FT-IR, using CH3CN as probe molecule. CO adsorption experiments revealed the presence of Pt terraces, as well as Pt sites with low coordination number (steps, edges, corners or defects) on Pt/Mg(Al)O. The same experiments on Pt,Sn/Mg(Al)O revealed that Sn covers the steps, corners, edges and defects of the Pt particles, thus developing simultaneously a geometric and a chemical effect on the surface properties of the exposed Pt atoms. Accordingly, the ethane dehydrogenation reaction proceeds with a lower activation energy over Pt,Sn/Mg(Al)O compared to Pt/Mg(Al)O. Further, Sn addition leads to more selective and more stable ethane dehydrogenation catalysts. The higher dehydrogenation selectivity of Pt,Sn catalysts was correlated to the masking of low-coordinated Pt sites. The Pt/Mg(Al)O and Pt,Sn/Mg(Al)O catalysts were subjected to an activation procedure consisting of several test-regeneration cycles. Good correlation was found between the number of accessible Pt sites and the catalytic activity after each cycle.