Elisabet Ahlberg

A novel strategy to direct the oxygen reduction reaction to preferentially produce H(2)O(2) is formulated and evaluated. The approach combines the inertness of Au nanoparticles toward oxidation, with the improved O(2) sticking probability of isolated transition metal "guest" atoms embedded in the Au "host". DFT modeling was employed to screen for the best alloy candidates. Modeling indicates that isolated alloying atoms of Pd, Pt, or Rh placed within the Au surface should enhance the H(2)O(2) production relative to pure Au. Consequently, Au(1-x)Pd(x) nanoalloys with variable Pd content supported on Vulcan XC-72 were prepared to investigate the predicted selectivity toward H(2)O(2) production for Au alloyed with Pd. It is demonstrated that increasing the Pd concentration to 8% leads to an increase of the electrocatalytic H(2)O(2) production selectivity up to nearly 95%, when the nanoparticles are placed in an environment compatible with that of a proton exchange membrane. Further increase of Pd content leads to a drop in H(2)O(2) selectivity, to below 10% for x = 0.5. It is proposed that the enhancement in H(2)O(2) selectivity is caused by the presence of individual surface Pd atoms surrounded by gold, whereas surface ensembles of contiguous Pd atoms support H(2)O formation. The results are discussed in the context of exergonic electrocatalytic H(2)O(2) synthesis in Polymer Electrolyte Fuel Cells for the simultaneous cogeneration of chemicals and electricity, the latter a credit to production costs.

Experimental interest in the possible curvature dependence of particle charging in electrolyte solutions is subjected to theoretical analysis. The corrected Debye−Hückel theory of surface complexation (CDH-SC) and Monte Carlo (MC) simulation are applied to investigate the dependence of surface charging of metal oxide nanoparticles on their size. Surface charge density versus pH curves for spherical metal oxide nanoparticles in the size range of 1−100 nm are calculated at various concentrations of a background electrolyte. The surface charge density of a nanoparticle is found to be highly size-dependent. As the particle diameter drops to below 10 nm there is considerable increase in the surface charge density as compared with the limiting values seen for particles larger than 20 nm. This increase in the surface charge density is due to the enhanced screening efficiency of the electrolyte solution around small nanoparticles, which is most prominent for particles of diameters less than 5 nm. For example, the surface charge densities calculated for 2 nm particles at 0.1 M concentration are very close to the values obtained for 100 nm particles at 1 M concentration. These predictions of the dependence of surface charge density on particle size by the CDH-SC theory are in very good agreement with the corresponding results obtained by the MC simulations. A shift in the pH value of the point of zero charge toward higher pH values is also seen with a decreasing particle size.

Cr 2 O 3 nanoparticles, widely used in the industry, can be obtained by calcination of the nanoparticles synthesized via the hydrothermal method. The chemical nature and the morphology of as‐prepared and calcined nanoparticles are investigated by scanning electron microscopy, X‐ray diffraction and Raman spectroscopy. Our results indicate that the as‐prepared nanoparticles mainly consist of amorphous and hydrated Cr(OH) 3 , with only minor amounts of Cr 2 O 3 . By contrast, and as already known before, calcined nanoparticles consist of Cr 2 O 3 . We also demonstrate the effect of inappropriately chosen experimental conditions, because the use of laser intensities above 0.7 mW during the Raman experiments causes a local heating and thus induces the transformation of Cr(OH) 3 into Cr 2 O 3 . The correlation between the laser power and a local heating is further corroborated by thermogravimetric analyses, which show that upon increased temperature, Cr(OH) 3 first dehydrates and then partially condensates to the intermediate CrO(OH) form, to finally attain the crystalline form of Cr 2 O 3 at about 409 °C. Copyright © 2017 John Wiley & Sons, Ltd.