Heat of adsorption of CO on EUROPT-1 using the AEIR method: Effect of analysis parameters, water and sample mode

Abstract

Fourier transform infrared spectroscopy (FTIR) in transmission and in diffuse reflectance (DRIFT) modes is used to characterize the adsorbed species formed during the adsorption of CO on EUROPT-1 (a 6.3% Pt/SiO2 catalyst) as a function of different experimental parameters such as the duration ta of the adsorption at 300K, the adsorption temperature Ta (300–673K range) and pressure Pa (1–4kPa range), the reduction temperature TR in hydrogen (423, 523, 673K) and the presence of H2O. Whatever the experimental conditions the adsorption of CO on EUROPT-1 is dominated by linear CO species (denoted L CO) characterized by IR bands in the 2090–2040cm−1 range with a small contribution of bridged CO species (denoted B CO) with IR bands in the 1880–1830cm−1 range. It is shown that the intensity, shape and positions of the IR bands of the L CO are strongly dependent on the experimental conditions. Increasing the duration of adsorption ta at 300K leads to an increase of the IR band intensity due to an activated reconstruction process. The presence of H2O leads to a shift of the L CO species IR band towards lower wavenumbers. Transmission and DRIFT modes provide similar qualitative data. However, the measurement of the heats of adsorption of the L CO species through the adsorption equilibrium infrared spectroscopy (AEIR) method, which is based on the quantification of the IR bands of adsorbed species, reveals the limitations of the DRIFT technique at high temperatures. Using transmission mode, it is shown that the heats of adsorption of the L CO species linearly vary which its coverage θL from 90kJ/mol to 250kJ/mol at θL=1 and 0 respectively whatever (a) the reduction temperature and (b) the absence or the presence of H2O. These values are consistent with previous measurements on others Pt° particles supported on different metal oxides. However the value at θL=0 is higher by about 30kJ/mol, possibly due to the high Pt dispersion (D≈0.9) of the reduced-reconstructed EUROPT-1. This study emphasizes that the use of CO to characterize supported Pt° particles is not straightforward considering the different experimental parameters that may affect the Pt structure and IR band features of the adsorbed species.