A sonochemical process was developed to treat carbon nanotubes in nitric and sulfuric acids to create surface functional groups for metal nanoparticle deposition. Carbon nanotubes treated in the sonochemical process are shown to lead to the deposition of uniformly dispersed high loading Pt nanoparticles, which have not been achieved with carbon nanotubes treated in reflux processes. Pt nanoparticles of a size less than 5 nm and loading up to 30 wt % with little aggregation were synthesized on the sonochemically treated carbon nanotubes. Cyclic voltammetry measurements in 1.0 M H2SO4 showed that the Pt nanoparticles on carbon nanotubes are more than 100% active in the electrochemical adsorption and desorption of hydrogen than the Pt nanoparticles supported on carbon black. This enhancement of electrochemical activity is attributed to the unique structures of carbon nanotubes and the interactions between the Pt nanoparticles and the carbon nanotube support. The ability to synthesize high loading Pt on carbon nanotubes using the sonochemical technique makes it possible to prepare high loading catalysts for the cathode of polymer electrolyte membrane (PEM) fuel cells.

Functionalization of carbon nanotubes (CNTs) is important for enhancing deposition of metal nanoparticles in the fabrication of supported catalysts. A facile approach for oxidizing CNTs is presented using a sonochemical method to promote the density of surface functional groups. This was successfully employed in a previous study [J. Phys. Chem. B 2004, 108, 19255] to prepare highly dispersed, high-loading Pt nanoparticles on CNTs as fuel cell catalysts. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, cyclic voltammetry, and settling speeds were used to characterize the degree of surface functionalization and coverage. The sonochemical method effectively functionalized the CNTs. A mixture of -C-O-/-C=O and -COO- was observed along with evidence for weakly bound CO at longer treatment times. The integrated XPS C 1s core level peak area ratios of the oxidized-to-graphitic C oxidation states, as well as the atom % oxygen from the O 1s level, showed an increase in peak intensity (attributed to -CO(x)()) with increased sonication times from 1 to 8 h; the increase in C surface oxidation correlated well with the measured atom %. Most of the CNT surface oxidation occurred between 1 and 2 h. The sonochemically treated CNTs were also studied by cyclic voltammetry and settling experiments, and the results were consistent with the XPS observations.

To create new catalyst materials for fuel cell applications, multiwalled carbon nanotubes (CNTs) were functionalized with sCdO, sCsOsCs, sCOOs, and sCsOH groups using a sonochemical treatment method under acidic aqueous solution (HNO 3 /H 2 SO 4 ) conditions to make them amenable to deposition of highly dispersed, ∼4 nm diameter Pt nanoparticles. The Pt-CNT interface was probed with X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS), and Raman and attenuated total reflection infrared (ATR-IR) spectroscopies to elucidate the nature of the Pt cluster-CNT surface binding. The degree of disorder of the sp 3 -hybridized C from the CNTs, as measured by the Raman D-to-G integrated peak area ratios, increased with the degree of surface oxidation of the CNTs. EXAFS of the Pt L III edge showed Pt coordination with oxygen (in the form of PtO x ) at the outermost perimeter of the Pt clusters while the majority of the bulk, as shown by the XPS Pt 4f core level, was in the metallic form. Infrared measurements showed that the carbonyl CdO stretching at 1700 cm -1 red shifted to ∼1550 cm -1 following Pt cluster deposition. In addition, changes in the CsO structural features at ∼1030 and 1150 cm -1 were observed, indicative of Pt cluster binding with the ionic form of carboxylate, COO(Pt), or ester-like, C(dO)CO(Pt), O atoms.

Bimetallic Pt-Ru alloy catalysts have been demonstrated to be more active than pure Pt catalysts in the electrooxidation of methanol. We report here a study on Pt-Ru nanoparticle catalysts supported on sonochemically functionalized carbon nanotubes. The catalysts were prepared by directly reducing the corresponding salts, K 2 PtCl 4 and K 2 RuCl 5 , in an ethylene glycol aqueous solution containing dispersed carbon nanotubes. Three catalysts of different Pt to Ru atomic ratios, namely, Pt 53 Ru 47 , Pt 69 Ru 31 , and Pt 77 Ru 23 , were prepared for investigation of the compositional effects. It was shown that highly dispersed bimetallic Pt-Ru alloy nanoparticles with no agglomeration can be synthesized on the carbon nanotubes with average particle sizes of less than 3.0 nm in diameter. The Pt-Ru nanoparticles are uniform and cover only the outside of the carbon nanotubes. It was found that the polyol process produced alloy compositions that are not consistent with the metal ratios in the precursors. It was also found that the lattice spacings of these catalysts are different due to the different compositions of the catalysts. Cyclic voltammetry showed that the catalysts were electrocatalytically active in the electrooxidation of methanol. Among the three catalysts, the Pt 53 Ru 47 catalyst produced the best performance. This catalyst was found to be the most stable, while the other two deactivated faster in the oxidation of methanol. All three Pt-Ru catalysts have higher electrocatalytic activities than a commercial catalyst of Pt 50 Ru 50 supported on carbon black. However, the Pt 69 Ru 31 and Pt 77 Ru 23 catalysts showed poorer stability that can be justified by the bifunctional mechanism of bimetallic Pt-Ru alloys.

New Pt monolayer electrocatalysts were prepared using galvanic displacement of a copper monolayer deposited at underpotentials on a Pd core. By performing underpotential deposition twice, two monolayers were deposited, forming a core-shell structure with double shells. The double shells consist of an outermost shell of Pt monolayer and a sublayer shell of Pd-Au alloy. It was found that by adjusting the compositions of the alloy sublayer, it is possible to mediate the oxygen reduction reaction (ORR) activity of the Pt catalysts. An alloy with 10% (atomic) Au was found to be the most active among the catalysts tested. Furthermore, the catalysts showed good cycling stability that may be due to stabilizing effect of Au. Since different alloys can be used as the sublayer for mediation, this work may open up various opportunities to tailor electrocatalysts for best ORR activity.