Pd nanoparticles supported on carbon-modified rutile TiO2 as a highly efficient catalyst for formic acid electrooxidation

La2O3 Promoted Pd/rGO Electro-catalysts for Formic Acid Oxidation

High activity, a low rate of CO poisoning, and long-term stability of Pd electro-catalysts are necessary for practical use as an anode material in direct formic acid fuel cells. Achieving a high degree of Pd nanoparticle dispersion on a carbon support, without agglomeration, while maintaining a facile electron transfer through the catalyst surface are two challenging tasks to be overcome in fulfilling this aim. Herein, we report the effect of addition of La/La-oxides on the efficiency of Pd nanoparticles supported on reduced graphene oxide (rGO) for formic acid electro-oxidation reaction. A series of electro-catalysts with different Pd-La molar ratios were successfully synthesized and characterized using a range of techniques including PXRD, XPS, TEM, FTIR, and Raman spectroscopy and then tested as anode materials for direct formic acid fuel cells. We explore that the lanthanum species (La/La-oxide) significantly promote the activity and stability of Pd catalyst toward electrocatalytic oxidation of formic acid. The metallic ratio is found to be critical, and the activity order of various catalysts is observed as follows; Pd₃₀La₇₀/rGO > Pd₈₀La₂₀/rGO > Pd₇₀La₃₀ rGO. The obtained mass specific activity for Pd₃₀La₇₀/rGO (986.42 A/g) is 2.18 times higher than that for Pd/rGO (451 A/g) and 16 times higher than that for Pd/C (61.5 A/g) at given onset peak potentials. The high activity and stability of the electro-catalysts are attributed to the uniform dispersion of Pd nanoparticles over the rGO support, as evidenced from TEM images. It is believed that the role of La species in promoting the catalyst activity is to disperse the catalyst particles during synthesis and to facilitate the electron transfer via providing a suitable pathway during electrochemical testing.

Enhancement of Oxidation of Formic Acid in Acid Medium on Zirconia-Supported Phosphotungstate-Decorated Noble Metal (Pd, Pt) Nanoparticles

Palladium and platinum nanoparticles have been modified at sub-monolayer level with Keggin-type phosphododecatungstate (by spontaneous adsorption of H3PW12O40) and considered as catalysts for the electrooxidation of formic acid in sulfuric acid electrolyte. The presence of phosphotungstate adsorbates has been confirmed using Fourier transform infrared spectroscopy (by reflectance). The enhancement effect (described in terms of the oxidation current increases) is even more pronounced when the catalytic centres have been supported on nanostructured ZrO2. In the case of Pd catalysts, a large population of hydroxyl groups and a high mobility of interfacial protons (on zirconia) seem to favour the direct oxidation path to CO2, whereas in the case of Pt-based systems, the enhancement effect is related to the oxidative removal of otherwise passivating CO adsorbates (indirect oxidation path). The role of polytungstate adsorbates on Pd or Pt nanoparticles relates to their ability to interact competitively with the CO adsorbates at noble metal catalytic sites (‘third-body’ effect).

Graphene-based transition metal oxide nanocomposites for the oxygen reduction reaction

The development of low cost, durable and efficient nanocatalysts to substitute expensive and rare noble metals (e.g. Pt, Au and Pd) in overcoming the sluggish kinetic process of the oxygen reduction reaction (ORR) is essential to satisfy the demand for sustainable energy conversion and storage in the future. Graphene based transition metal oxide nanocomposites have extensively been proven to be a type of promising highly efficient and economic nanocatalyst for optimizing the ORR to solve the world-wide energy crisis. Synthesized nanocomposites exhibit synergetic advantages and avoid the respective disadvantages. In this feature article, we concentrate on the recent leading works of different categories of introduced transition metal oxides on graphene: from the commonly-used classes (FeOx, MnOx, and CoOx) to some rare and heat-studied issues (TiOx, NiCoOx and Co-MnOx). Moreover, the morphologies of the supported oxides on graphene with various dimensional nanostructures, such as one dimensional nanocrystals, two dimensional nanosheets/nanoplates and some special multidimensional frameworks are further reviewed. The strategies used to synthesize and characterize these well-designed nanocomposites and their superior properties for the ORR compared to the traditional catalysts are carefully summarized. This work aims to highlight the meaning of the multiphase establishment of graphene-based transition metal oxide nanocomposites and its structural-dependent ORR performance and mechanisms.