Deconvolution and Curve Fitting of IR Spectra for CO Adsorbed on Pt/K-LTL: Potassium Promoter Effect and Adsorption Site Distribution

Abstract

Diffuse reflectance infrared (DRIFT) spectra have been obtained for carbon monoxide adsorbed with and without H2O on Pt/K-LTL zeolite. The complex spectra of CO linearly adsorbed on highly dispersed Pt have been analyzed by the combination of Fourier self-deconvolution and curve fitting. The linear-CO spectrum is composed of two broad multiplets: a high-frequency band (HFB) at 2100−1980 cm-1 and a low-frequency band (LFB) at 1990−1860 cm-1. The relative intensities of the HFB and LFB depend on the amount of coadsorbed water in the zeolite pores. The LFB is associated with an ion−dipole interaction between the oxygen atoms of CO species with potassium cations from the zeolite support. The HFB is associated with linear-CO unperturbed by the alkali promoter effect. It is proposed that water preferentially adsorbs on the K+ ions, shielding the ion−dipole interaction. The individual components of the complex HFB and LFB are proposed to be due to CO molecules adsorbed on platinum atoms with different coordination numbers. The average coordination number of the Pt atoms in K-LTL, determined in the applied deconvolution and curve-fitting program, corresponds well with that obtained by EXAFS, indicating a metal particle size distribution with particles containing between 1 and 13 platinum atoms. By applying the newly developed method, several literature IR CO spectra of Pt/K-LTL with different relative intensities in the HFB region have been analyzed successfully by adjusting the particle size distribution only. In view of the new results showing the prominent effects of particle size distribution and potassium promotion on the IR spectra of adsorbed CO, previously reported conclusions concerning metal−support interactions in PT/K-LTL should be reconsidered when based on infrared spectroscopic data.