Study of the reduction behavior of W/TiO2 catalysts by XPS using curve fitting, deconvolution and factor analysis

X‐ray photoelectron spectroscopy (XPS, ESCA) was used to determine the tungsten oxidation states in a 5 wt.% WO3/TiO2 catalyst reduced in hydrogen in the temperature range 400–700°C. Because, in the region of interest, W 4f overlaps with the Ti 3p energy level, a multiple analysis procedure including deconvolution, non‐linear leastsquares curve fitting and factor analysis was used to analyze the data. Deconvolution was better able to resolve the peaks in the W 4f region and thus elucidate the positions of the tungsten components for each reduction temperature. It was shown that there is a maximum of three W 4f doublets. The W 4f7/2 peaks of these components were located at 35.5, 33.3 and 31.1 eV and they were assigned to W+6, W+4 and Wo, respectively. Factor analysis verified the presence of three spectral tungsten components. X‐ray diffraction verified the presence of Wo on reduction at 675°C. Non‐linear least‐squares curve fitting, using the positions from deconvolution, was able to determine the distribution of tungsten oxidation states as a function of reduction temperature: W+6 steadily decreased with increasing reduction temperature, W+4 initially appears at 408°C and goes through a maximum around 580°C; Wo was first detected following reduction at 550°C and was the only species present for reduction temperatures higher than 665°C. From the distribution, it is proposed that, unlike the molybdenum systems, tungsten undergoes a more simplified reduction process in which only one intermediate oxidation state (presumably W+4) is detected.