Site energy distribution of copper catalytic surfaces from volumetric data collected at various temperatures

Abstract

Two catalysts prepared on a titanium silicate (ETS-10) matrix by loading 6 and 11 wt.% of copper were studied by employing nitrogen monoxide as adsorbate to probe the surface properties of copper centres. Volumetric isotherms of NO adsorption were collected at different temperatures (19–70°C). A thermodynamic model was applied to the isotherms in order to describe the behaviour of the NO–Cu system in terms of surface energy distribution of Cu sites. The higher loading copper sample had more energetic interaction with NO ($\text{−171<}\text{Δ}{}_{\mathrm{a}}\text{H}\text{(}\text{kJ}\text{/}\text{mol}\text{)}\text{<−44}$), with higher heterogeneity, than the less loaded one ($\text{−111<}\text{Δ}{}_{\mathrm{a}}\text{H}\text{(}\text{kJ}\text{/}\text{mol}\text{)}\text{<−41}$). The properties of the Cu sites were related with the activity measured in the SCR of NO with ethylene.

Introduction

The catalytic properties of heterogeneous solid catalysts are governed by their surface characteristics. Gas-phase adsorption methods using suitable probes, chosen on the basis of their chemical and electronic characteristics, appear to be the most promising in order to study the thermodynamic properties of the surface sites.

The collection of experimental adsorption isotherms and/or curves of differential heats of adsorption can reveal the more or less marked degree of heterogeneity of the surface sites. However, the temperature at which the adsorption is performed can deeply influence the results obtained. Higher temperatures depress the adsorption on the more energetic sites, so that definite types of surface sites can only be observed in a certain temperature interval. In order to overcome this problem, the authors recently have proposed a model [1], [2], [3] based on a fitting procedure applied to the volumetric and/or calorimetric adsorption data collected at different temperatures. The model allows the determination of the thermodynamic parameters of adsorption (temperature of half-coverage at unit pressure, T_1/2⁰, equilibrium constant of adsorption, and entropy of adsorption) for each type of site. The model interprets the adsorption on the heterogeneous surface as a summation of single isotherms, each one relevant to a definite type of site. The dependence of the adsorption constant for each type of site is obtained on the basis of the van’t Hoff equation. The determination of the pre-exponential factor in the van’t Hoff equation was obtained through the new parameter T_1/2,i⁰ which represents the temperature of half-coverage at unit pressure for a given type of site. The model allows the site distribution to be obtained in terms of number of sites versus enthalpy of adsorption, which reflects the heterogeneous character of the surface of the solid with respect to the adsorption of a chosen adsorbate.

In this study, the adsorption of NO on copper catalysts was taken into account. NO has already been used by the authors as probe molecule to study the surface copper centres of zeolitic (Cu/ZSM-5) and oxidic (Cu/SiO₂–Al₂O₃) catalysts [4]. NO is a strong Lewis base, able to stabilise the formation of both mono- and dinitrosyls of Cu⁺ and Cu²⁺ cations [5]. In recent years, large and medium pore zeolite structures and other molecular sieves exchanged with Cu and various cations have been studied for reducing NO with hydrocarbons (SCR process) [6]. The synthesised microcrystalline ETS-10 material (titanium silicate) [7] was used as support matrix for the preparation of copper based catalysts, in comparison with the Cu–ZSM-5 system [4]. A very large number of papers on the characterisation of the species formed by interaction of reactant molecules (i.e. NO, O₂, CO) with the surface centres appeared in the literature [8], [9], [10], [11], [12], [13], [14], most of them employing infrared spectroscopy. However, only few papers are devoted to the study of the energy of surface centres and their distribution on the surface [4], [15].

The adsorption of NO on two copper titanium–silicate catalysts was studied with the aim of determining the energy distribution of Cu surface centres. Correlations between both the nature and the energy of the Cu surface species and the catalyst activity in NO reduction with C₂H₄ in excess oxygen are proposed.