Plasmachemical preparation of lead-tungstate bronze

Brožek, Vlastimil

Tungsten bronzes are materials based on tungsten oxide that exhibit different colours, excellent chemical resistivity, metallic appearance and good conductivity. The doped tungsten bronzes may by used as anodic catalyst in fuel cells. The tungsten bronzes are prepared by reduction of tungstates. The reduction process is known for very long time, but the correlation between their stoichiometry, conductivity and colour are not yet completely understood. The published results about colour of products are very often misleading. In this paper, we study chemical reactions between lead-tungstate and hydrogen plasma. The reduction was performed in an inductively coupled r.f. discharge. The r.f. generator operated at the frequency of 27 MHz with the power output of 1 kW. The PbWO4 was heated by the discharge to the temperature of 900°C. The resulting samples were analysed by SEM, XRD and three point L*a*b* method. The XRD spectra of our plasmachemical product show diffraction patterns of metal tungsten and lead-tungstate bronze with a cubic phase similar to Pb0,261WO3 (P4/mbm). The diffraction peaks of original lead-tungstate bronze are higher for the sample reduced only 60 min than for 120 min. To obtain data for process kinetics observation, further experiments were performed with ground lead-tungstate powder at two size fractions. Analytical balance, after determining the W and Pb content, showed the bronze composition to be most likely close to Pb 1-X = 0,225.

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