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- W2018493600 abstract "Various Pt species were shown to be present in Pt/H-mordenite catalyst preparations containing 1.5 and 2.6 wt % Pt. Air calcination of the catalyst precursor generated large Pt0x metal particles (average size of >10 nm) by autoreduction, whereas a fraction of the platinum was converted to cations and oxycations, such as Pt2+ and Pt2O2+, balancing negative charges on the zeolite framework. The results of temperature-programmed H2 reduction (H2—TPR) suggested that Pt2O2+ was reduced to Pt+ below 550 K. The Ptn+ cations (n = 1 or 2) became reduced to Pt0 and Pt0x above 550 K. Reduction was introduced by heterolytic H2 dissociation, generating neutral platinum hydride and zeolite Brønsted acid sites, [Pt—nH]0 and H+. A similar platinum species, x[Pt0—nH], was obtained from homolytic H2 dissociation on Pt0x. When H2 was removed from the system, electrons were transferred from Pt0 atoms or Pt0x nanoparticles to the zeolite protons. When H2 was released, acid sites were annihilated, and the highly dispersed metal again became the zeolite cation Ptn+. The oxidation state and the chemical environment of the platinum were characterized by the vibrational spectra of chemisorbed CO. The spectral feature in the 2090−2100 cm−1 range, present in the spectrum of each H2-reduced catalyst, was shown to stem from two overlapping component bands. These bands were assigned to CO bound to Pt+ and Pt0. The results confirm that the active surface intermediates of alkane hydroisomerization are platinum hydride/carbenium ion and platinum hydride/zeolite proton pair sites, such as [Pt0—H]/ZO−CnH2n+1+ (species 1) and [Pt0—H]/ZO−H+ (species 2) sites, in dynamic equilibrium with gas-phase alkane and H2. Hydrogen promotes release of the alkane from species 1 by generating species 2 (hydride transfer). If the rate of isomer formation is governed by the transformation rate of the carbenium ion, this suggested mechanism corresponds to kinetics that is first-order in hexane and negative-order in hydrogen. The large Pt0x clusters were shown to catalyze the saturation of the eventually formed alkenes and, thereby, to suppress coke formation and catalyst deactivation." @default.
- W2018493600 created "2016-06-24" @default.
- W2018493600 creator A5021850856 @default.
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- W2018493600 date "2009-05-19" @default.
- W2018493600 modified "2023-10-16" @default.
- W2018493600 title "Activation of Hydrogen and Hexane over Pt,H-Mordenite Hydroisomerization Catalysts" @default.
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- W2018493600 doi "https://doi.org/10.1021/jp810716f" @default.
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