Search

A novel sonochemical approach to synthesize platinum (Pt) nanoparticles has been developed to produce smaller, and more monodisperse particles than traditional chemical reduction methods such as the sodium borohydride (NaBH4) reduction method. It was also found that careful experimental manipulation of the ultrasonic conditions such as the ultrasonic frequency (f), the acoustic power (Pacoustic) and the solution volume (V) may be employed to tailor the sonochemical radical yield to the same value for two different ultrasonic systems (20 kHz provided by an ultrasonic probe or sonifier, and 408 kHz by an ultrasonic plate transducer) leading to a similar reduction rate, and as a consequence, to Pt-nanoparticles with similar sizes. The reduction of Pt(IV) was found to be a first order reaction with a rate constant (k) of 0.062 min−1 for both ultrasonic systems, and the particle sizes from TEM measurements were found to be (2.2 ± 0.5) nm and (2.3 ± 0.4) nm for the 20 kHz and the 408 kHz samples, respectively. In comparison, the chemical reduction method resulted in particle sizes of (3.0 ± 0.5) nm. It was also found that the sonochemical synthesis at 408 kHz was more efficient than at 20 kHz with an initial sonochemical efficiency of (0.20 ± 0.04) μmol kJ−1 at 408 kHz and (0.060 ± 0.004) μmol kJ−1 at 20 kHz. The sonochemical synthesis at 408 kHz also yielded nanoparticles of higher purity. Interestingly, direct sonication into the reaction vessel using the 20 kHz sonication probe produced impurities of Ti, V and Al in the solution, indicating that direct sonication at lower frequencies led to (i) probe material (made of Ti alloy – Ti-6Al-4V) erosion induced by cavitation and (ii) a continuous supply of micro meter sized impurities. It was also observed that the introduction of impurities increased the reduction rate constant of Pt(IV) through heterogeneous nucleation (k = 0.12 min−1) as was demonstrated for the 408 kHz system under controlled addition of Ti/Al/V seed microparticles. Direct sonication using a sonifier should therefore be avoided when high purity catalytic materials are required.

Iridium oxide powders with a surface area of more than 1 m2 g-1 (4 m2 g-2 from the H-UPD charge) and iridium-oxide crystallites less than 10 nm across were synthesized by heat treating gels formed from citric acid, ethylene glycol and dihydrogen hexachloroiridate(iv) in air. The characteristics of the resulting material was found to be strongly dependent on the heat-treatment step in the synthesis. A single heat-treatment of the gel resulted in a material with a substantial fraction of elemental iridium metal, i.e. iridium in oxidation state zero (Ir0). Post-synthesis modification of the powder by potential cycling resulted in oxidation peaks consistent with the conversion of the metal phase to iridium oxide. Linear combination of the near-edge part of the X-ray absorption data (X-ray absorption near-edge spectroscopy, XANES) collected in situ during potential cycling and an analysis of the extended X-ray fine-structure (EXAFS) part of the spectrum showed that the overall metal fraction was not significantly affected by the cycling. The oxidation of the metal phase is therefore limited to a thin layer of oxide at the metal surface, and a significant part of the iridium is left inactive. A modification of the heat treatment procedure of the sample resulted in iridium oxide containing only insignificant amounts of elemental iridium metal.

Iridium and ruthenium oxide are active electrocatalysts for oxygen evolution. The relation between preparation method, structure, and behavior of mixed oxides of iridium and ruthenium are of interest in order to obtain active and stable catalysts. In this work the structure of mixed Ru–Ir oxides synthesized by the polymeric precursor method, which involves the formation of a gel containing the metal precursors and subsequent heat-treatment in air, was studied for the IrxRu1−xO2 system. An in-depth analysis of X-ray diffraction (XRD) and X-ray absorption (XAS) data, including EXAFS and linear combination of XANES, shows that the polymeric precursor synthesis method is capable of providing an intimate mixing of Ir and Ru in the catalyst. In addition to the oxide phase, metal phases, i.e. with Ru or Ir or both in oxidation state zero (Ir(fcc) and Ru(hcp)), were also found in the product materials. Facing complex structures such as some of those synthesized here, we have shown that a representation of shells with more than one atom type are efficiently represented using mixed sites, i.e. including scattering contributions from several elements in a site corresponding to a partial occupancy of the site by these elements, this method forming a very efficient basis for analyzing EXAFS data.