Impact of the Reconstruction of Gold Particles on the Heats of Adsorption of Linear CO Species Adsorbed on the Au Sites of a 1% Au/Al2O3 Catalyst

The adsorption of CO at 300 K on a reduced (H-2, 713 K) 1% Au/Al2O3 solid leads to the formation of a linear CO species adsorbed on the Au particles (denoted L1) characterized by an IR band at 2098 cm(-1) that slightly shifts to higher wavenumbers with the decrease in the coverage. For adsorption pressures P-a > 5 kPa, it is shown that there is a progressive reconstruction of the Au surface with the increase in the duration of the adsorption t(a) leading to the disappearance of the Au sites associated with the L1 CO species and to the formation of new Au sites forming a linear CO species (denoted L2) characterized by an IR band at 2070 cm(-1). The heats of adsorption of the two adsorbed species are determined as the function of their respective coverages by using the adsorption equilibrium infrared spectroscopy procedure previously developed. It is shown that they linearly decrease with the increase in their coverages from 62 to 43 kJ/mol for the L1 and from 100 to 50 kJ/mol for the L2 CO species at coverages 0 and 1, respectively. The values for the L1 CO species are consistent with a previous study on Au/TiO2 and with literature data on model Au particles. The higher heat of adsorption of the L2 CO species as compared to that of L1 can be a driving force for the reconstruction of the Au particles. Moreover, it is shown that the L2 CO species presents a property similar to that of adsorbed CO species on reduced supported metal particles: the position of its IR band shifts to lower wavenurnbers with the decrease in its coverage (the singleton is situated at 2025 cm(-1)). The sintering of the gold particles as a function of the number of pretreatment/reconstruction cycles leads to the detection of a third linear CO species characterized by an IR band at 2048 cm(-1) at 300 K with heats of adsorption of 80 and 60 kJ/mol at low and high coverages, respectively.