Redispersion of Sintered Pt/Al 2 O 3 Naphtha Reforming Catalysts: An in Situ Study Monitored by X-ray Absorption Spectroscopy Franc ¸ ois Le Normand* LERCSI, UniVersite ´ Louis Pasteur, Institut Le Bel, URA 423 du CNRS, 4 rue Blaise Pascal, F 67070 Strasbourg Ce ´ dex, France Armando Borgna, Teresita F. Garetto, and Carlos R. Apesteguia INCAPE, Facultad de Ingenierı ´a Quı ´mica (UNL), CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina Bernard Moraweck Institut de Recherches sur la Catalyse, UPR 5401 du CNRS, 2 aVenue Albert Einstein, F 69626 Villeurbanne Ce ´ dex, France ReceiVed: January 5, 1996; In Final Form: March 5, 1996 X Redispersion of sintered Pt (0.6-1.0%)/Al 2 O 3 catalysts at 773 K under HCl/H 2 O/O 2 /N 2 atmospheres was studied by hydrogen chemisorption, temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and in situ X-ray absorption spectroscopy (EXAFS) experiments. TPR and H 2 chemisorption results showed that the concentration of hydroxychlorided Pt species on the alumina carrier and, as a consequence, the metallic dispersion constantly augmented during the 8 h redispersion treatment. In situ EXAFS experiments indicated that Pt(OH) 4 Cl 2 2- species, which are formed by the attack of gaseous/ surface chlorided species to partially oxidized metallic Pt particles, are responsible for Pt redispersion. EXAFS studies also suggested that once the Pt-Cl/Pt-O coordination number ratio was stable, the Pt atoms were present on the alumina carrier either in the form of small rafts containing Cl and O atoms in a slighly distorted octahedral environment or in relatively large metallic particles covered by Pt(OH) 4 Cl 2 2- species. This suggestion was supported by the bimodal distribution of metal platinum particles observed by TEM when the redispersed samples where examined after its reduction with H 2 . Introduction Bifunctional Pt/Al 2 O 3 -Cl naphtha reforming catalysts are deactivated by side reactions such as coking, sintering, and poisoning in the course of industrial operation. 1 The in situ regeneration of the catalyst involves two main steps: (i) burning of the coke in O 2 /N 2 mixtures of low oxygen content and at temperatures lower than 773 K; because the reaction is highly exothermic and forms water, this step causes an additional sintering of the platinum particles; 2,3 the growth of the platinum crystallites may be partially prevented by introducing chlorided compounds to the oxidizing mixture; 4 (ii) rejuvenation of the catalyst by redispersing the metallic fraction and restoring the chlorine content on the alumina carrier using hydroxychlori- nating atmospheres at 673-773 K. 4,5 In both steps, the chemical nature of the surface intermediate species plays a key role in the sintering/redispersion process of the metallic fraction. 5 Particularly, several papers employing TPR, UV-vis, XRD, and TEM techniques have reported that the formation of surface oxychlorided species of platinum is a requisite for redispersing it. 5-7 However, knowledge on the exact chemical nature of these species and on its formation mechanism, particularly under industrial conditions, is still lacking. Extended X-ray absorption fine structure (EXAFS) spectroscopy seems to be a preferential technique for obtaining insight on this topic, since the local environment around platinum may be continuously monitored in situ under operative conditions approaching the real ones. 8-10 In a previous work, 9 we have used the EXAFS technique to investigate the chemical nature of the species involved during the sintering of unchlorided Pt/Al 2 O 3 catalysts. In this paper, we employed the same technique for in situ investigating the redispersion kinetics of sintered Pt/Al 2 O 3 catalysts under gaseous HCl/H 2 O/O 2 /N 2 mixtures. The aim was twofold: (i) to establish the chemical nature of the surface species responsible for redispersion, and (ii) to study the effect the redispersing atmosphere composition has on the redispersion kinetics. Experimental Section Samples Preparation. Two alumina-supported platinum catalysts containing 0.95% Pt (sample S 1 ) and 0.62% Pt (sample S 2 ) were prepared as previously described. 11 A high-purity γ-Al 2 O 3 powder (175 m 2 /g; Cyanamid Ketjen CK-300) was impregnated with an aqueous solution of H 2 PtCl 6 and HCl. After impregnation the samples were dried 12 h at 393 K and then calcined with flowing air at 773 K for 4 h. Sample S 1 was sintered in a O 2 (2%)/N 2 atmosphere at 923 K for 8 h, whereas sample S 2 was treated in the same gaseous mixture at 883 K for 12 h. It was checked that the sintering treatments did not modify the platinum content of the samples. The main characteristics of sintered samples S 1 and S 2 are given in Table 1. Redispersion Procedures. In all the cases, redispersion treatments were carried out in a flow system at 773 K and 1 atm (1 atm ) 101.325 kPa) using a gaseous mixture of HCl/ * Address correspondence to this author. Present address: IPCMS-GSI, UMR 1046 du CNRS, Ba ˆtiment 69, 23, rue du Loess, F 67037 Strasbourg Cedex, France. X Abstract published in AdVance ACS Abstracts, May 1, 1996. 9068 J. Phys. Chem. 1996, 100, 9068-9076 S0022-3654(96)00080-9 CCC: $12.00 © 1996 American Chemical Society