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 Ptox 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 Pt 2O2+, balancing negative charges on the zeolite framework. The results of temperature-programmed H2 reduction (H 2-TPR) suggested that Pt2O2+ was reduced to Pt- below 550 K. The Pfn+ cations (n = 1 or 2) became reduced to Pto and Ptox above 550 K. Reduction was introduced by heterolytic H2 dissociation, generating neutral platinum hydride and zeolite Brønsted acid sites, [Pt-nH]° and H -. A similar platinum species, x[Pto-nH], was obtained from homoly tic H2 dissociation on PtOx. When H2 was removed from the system, electrons were transferred from Pto atoms or Ptox nanoparticles to the zeolite protons. When H2 was released, acid sites were annihilated, and the highly dispersed metal again became the zeolite cation Pfn-. 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 H 2-reduced catalyst, was shown to stem from two overlapping component bands. These bands were assigned to CO bound to Pt+ and Pt o. 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 [Pto-H]/ZO-C nH2n+1 (species 1) and [Pto-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 Ptox clusters were shown to catalyze the saturation of the eventually formed alkenes and, thereby, to suppress coke formation and catalyst deactivation.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films