Heats of adsorption of linear NO species on a Pt/Al2O3 catalyst using in situ infrared spectroscopy under adsorption equilibrium

The adsorption of 1% NO/He on a reduced 2.9% Pt/Al2O3 catalyst is studied at several adsorption temperatures T-a in the range 300-680 K using in situ FTIR spectroscopy. At 300 K several adsorbed NO species are formed on the Pt particles and on the support leading to numerous IR bands in the range 1850-1200 cm(-1). After a short duration of the adsorption, two main IR bands are detected at 1764 and 1710 cm(-1) assigned to two linear NO species (denoted by L) on defect and terrace sites of the Pt particles, respectively. The IR band at 1710 cm(-1) is not modified with the duration of the adsorption at 300 K while that at 1764 cm-1 progressively decreases. Several IR bands are detected in the range 1650-1200 cm(-1) mainly due to the formation of nitrite and nitrate-adsorbed species on the support. The increase in T-a from 300 to 540 K leads to a strong decrease in the IR band at 1710 cm(-1) while that at 1764 cm(-1) increases and shifts to 1778 cm(-1). For higher adsorption temperatures (T-a < 680 K), the IR band at 1778 cm(-1) progressively decreases with the increase in T-a. Cooling of the catalyst in 1% NO/He from 640 K leads to an increase in the IR band at 1778 cm(-1) and for the same value of T-a, the IR band intensities are similar during the heating and cooling stages. Similar to the analytical method previously developed for the determination of the heats of adsorption of linear and bridged CO species on several supported metal catalysts, the change in the intensity of the IR band of the L species adsorbed on defect sites with T. permits determination of the evolution of the coverage theta (L) of this adsorbed species with T. at the constant NO pressure P-a = 10(3) Pa. It is shown that the curve theta (L) = f(T) is in agreement with an adsorption model which considers: (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the increase in theta (L) in the range 0-1. This allows us to determine that the heat of adsorption of the L species on defect sites linearly varies with its coverage from E-0 = 135 kJ/mol at theta (L) = 0 to E-1 = 105 kJ/mol at theta (L) = 1. These values are in reasonable agreement with the literature data on Pt single crystals and polycrystalline Pt solids (foil and filament). It seems that it is the first time that the heats of adsorption of adsorbed NO species on supported Pt catalysts are determined probably because the present procedure is affected neither by the adsorption of other NO species nor by the NO dissociation; both processes create difficulties when using other classical analytical methods (temperature-programmed desorption and calorimetry). (C) 2001 Academic Press.