Behaviour of Platinum-Tin during CO2-assisted propane dehydrogenation: Insights from quick X-ray absorption spectroscopy

Abstract CO2-assisted propane dehydrogenation has been studied on Pt-Sn/MgAl2O4 catalysts with support surface area of ~ 127 m2/g or ~ 5 m2/g, 3 wt% Pt and a Pt/Sn molar ratio of 3/1. In situ XAS was employed to track the dynamic changes occurring to the catalyst in presence of a reductive (H2) or oxidative (CO2) atmosphere. Reduction leads to the formation of a Pt-Sn alloy, the active compound for propane dehydrogenation. Oxidation by CO2 led to the loss of the Pt-Sn alloy due to firstly oxidation of Sn to SnO and subsequent oxidation of SnO to SnO2. The electronic and structural properties of the catalyst were determined by modelling of the EXAFS data. The Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method was used in conjunction with the XAS data to determine the amount of Sn present in the Pt-Sn alloy phase and the phase of the alloy itself: after a single step reduction 42% of all Sn goes into Pt3Sn alloy, participating in the reaction, with the remainder being SnOx. The percentage of Sn going into the Pt3Sn alloy increased after 10 H2/O2 redox cycles to 72%. A combination of in-situ XAS with CO2-PDH activity data covering CO2:C3H8 ratios from 0.25:1 to 1:1 allowed to show that CO2 helps to improve conversion of propane by means of the reverse water gas shift reaction, wherein the product H2 generated from PDH reacts with feed CO2 to shift the equilibrium towards products. The reaction performed better at lower ratios of CO2:C3H8. Increasing ratios of CO2:C3H8 induced faster deactivation of the catalyst by the oxidation of Sn to SnOx, leading to loss of Pt-Sn alloy. Suppression of carbon accumulation occurred by means of the reverse Boudouard reaction with the carbon formed during PDH. As possible reaction network for the entire CO2-PDH reaction, a combination of Langmuir-Hinshelwood (L-H) and the Mars-van Krevelen mechanism (MvK) was proposed. The MvK steps were the oxidation of Sn to SnO by CO2, which would then subsequently react with the H2 and C generated from PDH that takes place on Pt sites by the L-H mechanism, to go back to the Pt3Sn alloy.