Bao Xinhe

The oxidation of methanol on electrolytic silver catalyst

The oxidation of methanol on the surface of electrolytic silver catalyst has been investigated with Temperature-Programmed Reaction Spectroscopy (TPRS). It has been found that on a clean surface of electrolytic silver no adsorption of CH3OH or H2 occurred except the adsorption of small amounts of H2O or CO2, but the ability of silver to chemisorb the gases mentioned above was greatly enhanced by oxygen preadsorbed on the surface. In the reaction of methanol with preadsorbed oxygen-18, three distinct groups of product were observed at 400, 500 ± 20, and 620 K, respectively. At room temperature, the selective oxidation of methanol led to adsorbed surface species CH3O(a). During desorption, CH3O(a) decomposed and HCHO, CO18O, and H218O were evolved simultaneously at 400 K as the first group of products. At high oxygen coverage, surface oxygen interacted with the carbon atom of CH3O(a) to form another type of stable surface intermediate H3CO18O(a), which decomposed with a pseudo-second-order kinetics at 600 K, yielding only HCHO and H218O with a stoichiometric ratio of 2:1. The evolution of the second group of products CH3OH, CO18O, and H218O at 500 K was rate-limited by desorption of the products.

TDS and XPS studies of the adsorption of O2 on electrolytic silver

Abstract Thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption of oxygen on electrolytic silver. In the pressure range of 1−10 −8 Torr, several kinds of oxygen species have been observed on the surface: (A) Adsorbed molecular oxygen ( E d = 101 kJ/mol, v = 4 × 10 11 s −1 ), (B) surface bound atomic oxygen ( E d = 134 kJ/mol, v = 4.7 × 10 13 s −1 ) and (C) sub-surface atomic oxygen. Results of XPS show that the binding energies of 1s orbitals of these species of oxygen on silver are 528.3, 529.7, 520.2 and 532.2 eV. Two bands centered at 2.8–3.0 and 9.0 eV below the Fermi level appear on the difference spectrum of the valence band which again indicates that both atomic and molecular oxygen exist on the surface of silver.

Hreels of methanol oxidation on polycrystalline silver: Spectroscopic evidence for adsorbed intermediates in the conversion of methoxide to formaldehyde and water

The adsorption and the decomposition of methanol on an oxygen precovered polycrystalline silver surface were characterized by high resolution electron energy loss spectroscopy. A stable methoxy species and subsequently two new intermediates related to the decomposition of methoxy have been isolated and identified by temperature profiled ELS.

Adsorption of oxygen on electrolytic silver by UPS and work function measurement

The adsorption of oxygen on an electrolytic silver surface has been investigated by ultra-violet photoelectron spectroscopy (UPS), and by the measurements of work function change and kinetics. UPS difference spectra show three positive peaks at about 3.0, 5.3 and 9.0 eV below the Fermi level, which are due to the adsorbate resonances of atomic and molecular oxygen on the silver surface. The peak at 5.3 eV indicates that the orbitals derived from the πg∗ orbital of molecular oxygen are electronically filled in the adsorbed dioxygen complex and the peak at 9.0 eV corresponds to electron feedback from the πu orbital of adsorbed dioxygen species to valence orbitals of silver. The bonding configuration is also supported by the changes of work function. Owing to the transfer of electrons from metal to adsorbate, this adsorption of the atomic oxygen increases the work function (Δφsat = +0.5 eV), and O− (ads) species are formed; with the saturated surface the constant dipole moment per adsorbed oxygen is 0.24 Debye. The kinetic results show the presence of a maximum (Smax = 9.3 × 10−3) in the sticking coefficient curve at room temperature. The relation, S = k exp(550/T), has been obtained by measurement of the kinetics, and the results are discussed in terms of precursor adsorption models.

Enhanced Ammonia Synthesis Activity of Ru Supported on Nitrogen-Doped Carbon Nanotubes

Nitrogen-doped carbon nanotubes (NCNTs) were synthesized via chemical vapor deposition using acetonitrile as the carbon and nitrogen sources. Transmission electron microscopic characterization showed clearly tubular morphology of NCNTs with inner diameter of 10-15 nm and wall thickness of 10-20 nm. X-ray photoelectron spectroscopy (XPS) indicated that nitrogen species were incorporated into CNTs and the predominating type of nitrogen species were pyridinic and quaternary nitrogen. XPS, X-ray diffraction, and Raman spectroscopy showed that with increasing temperature the total amount of nitrogen species decreased but the nanotubes were better graphitized. Ru supported on NCNTs exhibited an obviously enhanced activity in ammonia synthesis as compared with the CNT-supported Ru catalyst. In particular, in the temperature range of 550 similar to 750 degrees C, the NCNTs obtained at 650 degrees C after deposition of Ru yielded a better activity, which was most likely attributed to the promoting effect of nitrogen doping and the graphitization of nanotubes.

The poisoning action of iron on electrolytic silver catalysts used for partial oxidation of methanol

Abstract Poisoning of silver catalysts used for partial oxidation of methanol has been investigated by combined surface techniques including XPS, SIMS, SEM, TDS, TPRS, work function measurement, and microreactor studies. Results show that the oxidation path is governed mainly by surface iron loading. Once Fe 2 O 3 is present at the catalyst surface, the catalytic performance degrades rapidly. The surface characteristics of iron depend strongly on the iron loading on the surface.