Spectro-thermal investigation of the decomposition intermediates developed throughout reduction of ammonium paratungstate Nasr E. Fouad * , Ahmed K.H. Nohman, Mohamed I. Zaki 1 Faculty of Science, Chemistry Department, Minia University, El-Minia 61519, Egypt Received 15 June 1999; received in revised form 30 August 1999; accepted 2 September 1999 Abstract The thermal decomposition course of ammonium paratungstate (APT) in hydrogen was studied using thermogravimetric and differential thermal analyses. X-ray diffractometry, infrared spectroscopy and diffuse re¯ectance spectroscopy were used to characterize and identify the intermediate solid products. It was found that ammonium paratungstate decomposes to tungsten metal W 0 through ®ve main steps encompassing different tungsten intermediate compounds. Ammonium tungsten bronze (NH 4 ) 0.33 WO 3 , which precedes the formation of WO 3 , is relatively the most stable intermediate (250±5508C) encountered through reduction of APT to tungsten metal W 0 . The in¯uence of hydrogen spillover on the reduction behavior commences to be effective just after formation of the bronze intermediate. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Ammonium paratungstate; Tungsten bronze; Tungsten oxides; Reduction; TG 1. Introduction As a precursor of tungsten-based catalysts [1], ammonium paratungstate (APT) received a great deal of attention to explore its thermal decomposition course to the onset of WO 3 formation. The later oxide is frequently considered as the active phase in tung- sten-based catalysts [2,3]. However, for preparing supported metal catalysts, supports are directly impregnated with an aqueous solution of APT, fol- lowed by the subsequent calcination and/or reduction. The thermal decomposition course of APT in air and nitrogen, to the onset of formation of WO 3 , was the subject matter of some studies whether to char- acterize the thermal decomposition behavior or to estimate the different kinetic parameters for the decomposition steps [2±4]. In this laboratory, reduc- tion of WO 3 was studied, adopting the isothermal and non-isothermal techniques, to probe the autocatalytic effect induced during the reduction process [5] and to specify the kinetic model of the reaction governing such solid/gas reduction process [6]. However, decom- position of APT in hydrogen was studied non-isother- mally to estimate the different kinetic parameters controlling the decomposition process [4]. The decomposition course and identi®cation of the inter- mediate compounds was beyond the aim of that study. Ammonium tungston bronze has attracted consid- erable attention over the years because of its unique crystalline structure [7]. Alkali and ammonium tung- sten bronzes approach the composition A 0.3 WO 3 which contains two types of structural channels [8]. The ®rst channel is a broad hexagonal one and is Thermochimica Acta 343 (2000) 139±143 * Corresponding author. Fax: 20-86-342601. E-mail address: nfouad@scc-alph1.minia.eun.eg (N.E. Fouad). 1 Present address: Faculty of Science, Chemistry Department, Kuwait University, PO Box 5969, 13060 Safat, Kuwait. 0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(99)00339-1