No abstract

Nitrogen-doped porous carbon nanospheres (PCNs) with a high surface area were prepared by chemical activation of nonporous carbon nanospheres (CNs). CNs were obtained via carbonization of polypyrrole nanospheres (PNs) that were synthesized by ultrasonic polymerization of pyrrole. The catalysts Pt/PCN, Pt/CN, and Pt/PN were prepared by depositing Pt nanoparticles on supports PCNs, CNs, and PNs, respectively, using ethylene glycol chemical reduction. Nitrogen adsorption, X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were employed to characterize samples. It was found that after chemical activation using KOH, PCNs containing N functional groups (mainly N-6 and N-Q) possessed a microporous structure with a high surface area of 1010 m2/g and a particle size of less than 100 nm. The electrochemical properties of samples Pt/PCN, Pt/CN, and Pt/PN, together with commercial catalysts E-TEK (40 wt % Pt loading), were comparatively investigated in methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) for fuel cells. The results showed that the catalytic activity of Pt/PN toward both reactions at room temperature is almost negligible possibly due to the poor conductivity of support PNs proven by impedance spectroscopy, in contrast with some literature reports. Compared to Pt/CN and E-TEK catalyst, Pt/PCN revealed an enhanced mass activity in ORR and MOR because of the high dispersion of small Pt nanoparticles, the presence of nitrogen species, and developed microporous structure of support PCNs.

Supported Pd, Au, and Pd−Au alloy catalysts are characterized with in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (DRIFTS), quantitative powder X-ray diffraction, and X-ray photoelectron spectroscopy. The spectroscopic results presented in the paper demonstrate the existence of electron density transfer between Pd and Au atoms in alloy surfaces. In particular, the modification of the Pd electronic structure by the addition of Au is confirmed probably for the first time by the DRIFT spectra. The relationship between surface composition and catalyst performance in the synthesis of hydrogen peroxide directly from hydrogen and oxygen was established. Preliminary results indicate that the activity and selectivity of Pd−Au alloy catalysts can be significantly enhanced through adjusting the surface structures by changing the Au content in alloys.

In the Heck coupling of bromobenzene and styrene over conventionally prepared Pd particles supported on SiO2 at 135 °C under Ar with Na2CO3 as the base, the catalysis of 2−4 ppm of leached Pd in solution is considerably affected by the supported Pd particles. Although such low concentration leached Pd alone is very active for the Heck reaction, its catalytic activity is suppressed in the presence of the supported Pd particles. Despite this, the variation of TON with leached Pd concentration follows the same trend of a homogeneous Pd catalyst. Homogeneous catalysis in nature is suggested based on the fact that the solid−liquid reaction rate is dependent on the leached Pd concentration rather than the supported Pd loading. The change of homogeneous catalysis relating to the variation of active Pd fraction in the presence and absence of the supported Pd particles is discussed.