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Abstract:
A Pt black sample reduced from H2PtCl6 and presintered at 473 K was characterized by X-ray photoelectron spectroscopy and UV photoelectron spectroscopy (UPS) in three stages of its life: before catalytic runs, after a known length of use at 600 K with n-hexane model compound and after “regenerating” it with H2 and O2 in the ultrahigh vacuum apparatus. Spectra measured at 300 and 600 K, respectively, reveallikely three-dimensionalcarbonaceous deposits, mostly graphite and also CxHy polymer. More polymer and less graphite was present at 600 K. Oxygenated species including “PtOads”, OH, and H2O were adsorbed likely on clean Pt islands. Less water and more “PtOads” were observed at 600 K. The presence of surface “PtOads” was confirmed by UPS immediately after regeneration. They transform slowly into chemisorbed OH and/or H2O when Pt is kept at 600 K for several hours. A part of these latter entities may be clustered around ions of potassium impurity. n-Hexane reacted almost entirely to give methane in the first moments of its contact with this catalyst. This may be due to the hydrogen excess on the regenerated Pt surface. Nondegradative reactions prevailed in steady state. Hydrogen pressure dependence of turnover frequencies and selectivities in the steady state is reported. Marked selectivity difference over “fresh” Pt and “used” Pt after several regenerations is reported. Aromatization became a primary reaction after several runs only. Although clean Pt patches are responsible for all reactions, they may be influenced by the presence of K+ and by the differences of the chemical state of oxygen on Pt or around K+ ions. Benzene selectivity was higher in the presence of K+. One of the effects of chemisorbed O can be the shielding of the electric charge of K+ promoting aromatization. This shielding was largely removed when oxygen transforms to surface OH/H2O; in this state aromatization selectivity increased again.
