Application of the Temkin Model to the Adsorption of CO on Gold

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A treatment of the Temkin model was developed for describing the adsorption of CO on gold. In this presentation, it is made clear that the Temkin thermodynamic model is an extension of the Langmuir model that incorporates a linear variation of the adsorption enthalpy. It is also stressed that the Temkin model can be interpreted in terms of two distinct physical situations: the first assumes equivalent binding sites and an adsorption enthalpy that varies with coverage due adsorbate interactions, while the second assumes a uniform distribution of heterogeneous binding sites and an adsorption enthalpy that varies due to the heterogeneity of sites. In addition, a midrange approximation, where the surface is roughly half covered with adsorbates, is commonly employed. While each of these situations is similar, they are not equivalent and yield slightly different analytical expressions that describe the adsorption coverage as a function of pressure and temperature. Fitting data with the different analytical expressions therefore produces slightly different thermodynamic values for the adsorption enthalpy and entropy. These different cases are explicitly defined and developed. For the adsorption of CO on gold, the adsorbate interaction case is shown to be most consistent with the current body of experimental and theoretical evidence. The various analytical expressions are used to apply the Temkin model to data for CO adsorption on 1% Au/TiO2 real-world catalysts (from the World Gold Council) studied under catalytically relevant isothermal (T = 275–325 K) and isobaric (PCO = 0–10 Torr) experimental conditions. Infrared transmission spectroscopy was the analytical technique used for quantitatively measuring the adsorption coverage. The coverage as a function of pressure (θ,P) and as a function of temperature (θ,T) was fit with the Temkin adsorption models. The models yield the thermodynamic adsorption enthalpy at zero and full coverage as well as the adsorption entropy. The values of these thermodynamic parameters differ slightly depending upon the particular Temkin situation considered (cf. adsorbate interaction, heterogeneous surface, and midrange approximation cases). While all three Temkin cases produced excellent fits to both the isothermal and isobaric data sets, the adsorbate interaction case is most consistent with experimental and theoretical evidence describing the adsorption of CO on gold. The average enthalpy values from fitting the isothermal and isobaric data sets using the adsorbate interaction model are −ΔH0 = 59.2 kJ/mol and −ΔH1 = 54.6 kJ/mol for zero and full coverage, respectively. The adsorption entropy, −ΔS = 142 J/(K mol), was determined by fitting the data sets from multiple isothermal experiments with the adsorbate interaction case. These thermodynamic adsorption values are in excellent agreement with previously reported values. The validity of the Temkin adsorbate interaction model was further supported by fitting isothermal and isobaric data for the adsorption of CO on a well-defined gold surface, as reported previously by Gottfried et al. This new treatment of the Temkin adsorption model is theoretically and experimentally straightforward and applicable to both isothermal and isobaric data sets. It provides meaningful thermodynamic values of adsorption enthalpy and entropy, which can be used to characterize and explain differences between various catalysts. The model should also be applicable to the adsorption of other small molecules on metal surfaces and particles that show coverage-dependent adsorption properties.

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Journal of Physical Chemistry C

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