Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al₂O₃Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium

Abstract

The adsorption of CO on a reduced 1% Ir/Al2O3 catalyst has been studied by using Fourier transform infrared spectroscopy (FTIR) at two constant CO partial pressures P a (200 and 2000 Pa) in the adsorption temperature (denoted by T a) range of 300−740 K. At 300 K, it is observed a strong IR band at 2044 cm-1 ascribed to linear CO species (denoted by L) associated to weak shoulders at ≈2080 and ≈2000 cm-1. The FTIR spectra show how the coverage θL of the L CO species evolves with T a for the two P a values. At T a > 580 K, the curves θL= f(T a) are in very good agreement with an adsorption model based on Temkin's approximations indicating that the heat of adsorption linearly increases with the decrease in θL from E 1 to E 0 at θ L= 1 and 0, respectively. This allows us to evaluate the ratio E 0/(E 0 − E 1) ≈ 2.1 and then E 0 − E 1 ≈ 115 kJ/mol. Moreover, it is shown that, in the T a range of 300−740 K, the curves θL= f(T a) for the two CO partial pressures are in very good agreement with an adsorption model considering the following: (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the increase in θL from E 0= 225 kJ/mol at θL= 0 to E 1= 115 kJ/mol at θL= 1. Finally the two isobars are used to determine the isosteric heats of adsorption at several coverages in the range of 1−0.8 from the Clausius−Clapeyron equation. A good agreement is observed with the values determined using the adsorption model and this validates the various assumptions involved in the model. The heat of adsorption values are compared to the literature data on single crystals and supported Ir catalysts. Moreover, it is shown that the heat of adsorption values allow us to quantify several quantitative data found in the literature on the stability of the L CO species on Ir-containing solids either in a vacuum or in an inert gas.

A Contribution to the Experimental Microkinetic Approach of Gas/Solid Heterogeneous Catalysis: Measurement of the Individual Heats of Adsorption of Coadsorbed Species by Using the AEIR Method

“…The experimental data in Figure 2 are compared to theoretical curves providing the individual heats of adsorption and the mathematical expressions of interest for the adsorption equilibriums of L, B and 3FC CO species. adsorbed species, the validity of Equation 3has been ascertained by different authors [33,35,36,39,50,51] and we have confirmed this point for the L CO species on the reduced metal particles of Cu/Al2O3 [62,63] and Ir/Al2O3 [64]. If the adsorption leads to different adsorbed species such as L, B and 3FC CO species on Pt/Al2O3 (inset Figure 1), the ascertainment of the Beer-Lambert law presents difficulties, due to the fact that at 300 K, volumetric/gravimetric methods provide the total amount of adsorbed species: QT = Σ QX whereas AX depends of a specific X species.…”

Catalytic decomposition of propellant N2O Over Ir/Al2O3 catalyst

“…The main band at 2050 cm −1 is the character of linearly adsorbed CO on metallic Ir sites, Ir 0 (CO) . The doublet peaks at 2075 and 2015 cm −1 are assigned to gem‐carbonyl CO (ad) on Ir I sites with partially positive state, Ir I (CO) 2 . With the adsorption time, these peaks do not shift but the strength of doublet peaks slightly increase.…”

Hydrogenation of α,β-unsaturated carbonyl compounds over Au and Ir supported on SiO2

“…4(a)) showed an asymmetric absorption band with a strong maximum at 2063 cm −1 and a small contribution at 2115 cm −1 . The band around 2115 cm −1 can be related to the formation of CO adsorbed on metallic gold sites (CO Au 0 ) [23]. The low frequency CO band at 2063 cm −1 can be assigned to CO adsorption on negatively charged gold species.…”