Individual Heat of Adsorption of Adsorbed CO Species on Palladium and Pd-Sn Nanoparticles Supported on Al2O3 by Using Temperature-Programmed Adsorption Equilibrium Methods

摘要

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x = 2 or 5 wt %) in the 300-713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as "B"), which are the dominant species on Pd-0 particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as "L"). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3, in parallel to the formation of two strongly adsorbed B CO species, two linear L1(pd0) and L2(pd0) CO species are formed, which exhibit different heats of adsorption. For the dominant L1(pd0) CO species, the heat of adsorption decreases linearly, from 92 kJ/mol to 54 kJ/mol, as its coverage increases at coverage 0 and 1, respectively, while that of the L2(pd0) species is >165 kJ/mot at coverage 1. On the two Pd-Sn containing particles, two linear CO species are formed denoted L1(2pd-xSn) and L2(2Pd-xSn) with x = 2 or 5. The L1(2pd-xSn) species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (which is slightly dependent on x) linearly varies with its coverage: similar to 90 kJ/mol and similar to 50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1(pd0) indicates that the electronic effect of tin is very modest, compared to its geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover, (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd3Sn and Pd2Sn are present and (b) the impact of tin on the H-2 chemisorption on Pd-0 sites are presented.