Tungsten carbide was synthesized by calcination of carbon cryogel with
embedded tungsten in a form of metatungstate. This material was used as a
support for Pt nanoparticles. XRD pattern of W-C support indicates the
presence of WC, W2C, and unreacted W, as well as graphitized carbon.
According to the previous TEM analysis of W-C support, it contains particles
with core-shell structure, where W particle was covered with the shell of a
mixture of WC and W2C. The average Pt grain size calculated from XRD pattern
was about 6 nm. Cyclic voltammogram of W-C support was recorded within
potential range relevant for its application as a catalyst support in fuel
cells. Pair of anodic/cathodic peaks close to the negative potential limit
could be ascribed to the intercalation of hydrogen within hydrous tungsten
oxide, which is always present on the surface of WC in aqueous solutions.
Cyclic voltammogram of Pt/W-C indicated that tungsten oxide species are
present on tungsten carbide shell as well as on the surface of Pt
nanoparticles. Pt surface is only partially covered by hydrous tungsten
oxide. Hydrogen intercalation in hydrous tungsten oxide is enhanced in the
presence of Pt nanoparticles. Also, the presence of hydrated tungsten oxide
leads to the decrease of OH chemisorbed on Pt surface. Stripping of
underpotentially deposited copper was used for the assessment of Pt surface
area and the specific surface area of Pt was estimated to 41 m2 g-1.
Electrochemical oxygen reduction reaction was examined on the synthesized
Pt/W-C catalyst and compared with the results on the commercial Pt/C
catalyst. It was found that the current densities at Pt/W-C are almost double
as those on Pt/C. The Tafel plots for both catalysts are characterized with
two Tafel slopes: -0.060 V dec-1 at low current densities, and -0.120 V dec-1
at high current densities. From the rotational dependence of the reaction
rate, it was found that oxygen reduction on both Pt/W-C and Pt/C follows the
first order kinetics with respect to O2 and that four electrons are
transferred per O2 molecule. The results show that the presence of tungsten
carbide in support material i.e. hydrous tungsten oxide on Pt surface, leads
to promotion of oxygen reduction on the Pt/W-C catalyst. It was assumed that
oxophilic hydrated tungsten oxide hinders OH adsorption on Pt surface, thus
increasing Pt surface area available for O2 adsorption and its
electrochemical reduction.
Structural Effects of Tungsten Oxide Electro-Catalyst Towards High Stable CO Tolerance Mechanism
