Sintering of chlorinated Pt/gamma-Al2O3 catalysts: An in situ study by X-ray absorption spectroscopy

In situ EXAFS spectroscopy was used for identifying the surface species inv olved in the sintering of alumina-supported Pt catalysts under dried oxidiz ing atmospheres. A Pt/gamma-Al2O3 catalyst (0.62 wt% Pt, 0.88 wt% Cl) was h eated in a 2% O-2/N-2 gaseous mixture from 300 to 525 degrees C for about 1 20 min and then kept at this temperature for up to 720 min. The main observ ation, which is in good agreement with ex situ TPR experiments, was that ch lorine is always present in the surrounding of platinum during the oxidizin g treatment, The metal sintering process involved three successive steps du ring which the chlorine and oxygen coordinations passed through a maximum, whereas platinum coordination exhibited a minimum. Formation of Pt(OH)(4)Cl -2 species was detected at the end of the first step, i.e., when the temper ature reached 500 degrees C. After about 4 h of treatment, we deduced that platinum species are made up of a metal platinum core surrounded by a doubl e coating of oxychlorinated species. More precisely, EXAFS experiments sugg ested that surface platinum oxide is made of rigid PtO6 octahedra, but thei r assembly led to a largely disordered structure. The absence of a long-ran ge order allows the location of residual chlorine species either between th e PtO6 octahedra or at the Pt-oxidized surface shell. (C) 1999 Academic Pre ss.