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/SiO 2 catalyst) as a function of different experimental parameters such as the duration t a of the adsorption at 300 K, the adsorption temperature T a (300–673 K range) and pressure P a (1–4 kPa range), the reduction temperature T R in hydrogen (423, 523, 673 K) and the presence of H 2 O. 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 2090–2040 cm −1 range with a small contribution of bridged CO species (denoted B CO) with IR bands in the 1880–1830 cm −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 t a at 300 K leads to an increase of the IR band intensity due to an activated reconstruction process. The presence of H 2 O 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 θ L from 90 kJ/mol to 250 kJ/mol at θ L  = 1 and 0 respectively whatever (a) the reduction temperature and (b) the absence or the presence of H 2 O. These values are consistent with previous measurements on others Pt° particles supported on different metal oxides. However the value at θ L  = 0 is higher by about 30 kJ/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.