Oxidation and Corrosion of Platinum–Nickel and Platinum–Cobalt Nanoparticles in an Aqueous Acidic Medium

We report on the synthesis, characterization, and corrosion behavior of randomly oriented platinum–nickel and platinum–cobalt nanoparticles (PtNi-NPs, PtCo-NPs). Unsupported and carbon-supported nanocatalysts were synthesized using the “water-in-oil” microemulsion method. X-ray diffraction (XRD) was used to examine their average crystallite size, which was 2.3 nm in both cases. The shape, size, and size distribution of the nanoparticles were evaluated using transmission electron microscopy (TEM); they were determined to be spherelike with an average size of 3.3 and 3.2 nm for PtNi-NPs and PtCo-NPs, respectively, and a narrow size distribution. Comparison of the XRD and TEM data indicated that the nanocatalysts were polycrystalline in nature. Thermogravimetric analysis (TGA) measurements were carried out to evaluate the metal loadings of the carbon-supported nanocatalysts, which were 38.1 wt % for PtNi-NPs and 40.8 wt % for PtCo-NPs. Static Density Functional Theory (DFT) calculations were performed to analyze the structures and energetics of the PtNi and PtCo systems; it was found that the presence of Ni and Co introduced ca. 8% of volume reduction, as compared to the volume of pure, bulk Pt. Cyclic voltammetry (CV) measurements at potential scan rates of 5.0 and 50.0 mV s^–1 in 0.50 M aqueous H₂SO₄ indicated that the specific surface areas (A_s) of the PtNi-NPs and PtCo-NPs were 74.5 m² g_Pt^–1 and 33.1 m² g_Pt^–1, respectively. In situ confocal Raman spectroscopy was successfully used to monitor the formation and reduction of surface oxide layers in the submonolayer and monolayer ranges. Corrosion of the nanocatalysts was studied using anodic and cathodic potentiodynamic polarization (PDP) measurements at a very low potential scan rate of s = 0.10 mV s^–1 in 0.50 M aqueous H₂SO₄ saturated with different gases (N₂(g), O₂(g), or H₂(g)). The nature of the dissolved gas had a profound impact on the corrosion characteristics of the nanoparticles. The nanocatalysts were stable in the electrolyte saturated with H₂(g) and underwent slight corrosion in the electrolyte saturated with N₂(g) and significant corrosion in the electrolyte saturated with O₂(g). The carbon support was also observed to undergo corrosion and porosity changes. The degradation of the nanocatalysts was more pronounced in the case of the anodic PDP measurements than the cathodic ones. Cyclic voltammetry was employed to analyze the loss of A_s of the nanocatalysts as a result of PDP measurements, and the results were found to corroborate the corrosion data. Evolution of the value of A_s of the nanocatalysts in 0.50 M aqueous H₂SO₄ outgassed using N₂(g) was examined by recording 500 CV transients in the 0.05 V ≤ E ≤ 1.45 V range at s = 50.0 mV s^–1. It was found that in both cases this treatment resulted in a 50% reduction in A_s.