No abstract

Single-walled nanotubes (SWNTs) produced by plasma laser vaporization (PLV) and containing oxidized surface functional groups have been studied for the first time with NEXAFS. Comparisons are made to SWNTs made by catalytic synthesis over Fe particles in high-pressure CO, called HiPco material. The results indicate that the acid purification and cutting of single-walled nanotubes with either HNO3/H2SO4 or H2O2/H2SO4 mixtures produces the oxidized groups (O/C = 5.5-6.7%), which exhibit both pi*(CO) and sigma*(CO) C K-edge NEXAFS resonances. This indicates that both carbonyl (C=O) and ether C-O-C functionalities are present. Upon heating in a vacuum to 500-600 K, the pi*(CO) resonances are observed to decrease in intensity; on heating to 1073 K, the sigma*(CO) resonances disappear as the C-O-C functional groups are decomposed. Raman spectral measurements indicate that the basic tubular structure of the SWNTs is not perturbed by heating to 1073 K, based on the invariance of the ring breathing modes upon heating. The NEXAFS studies agree well with infrared studies which show that carboxylic acid groups are thermally destroyed first, followed by the more difficult destruction of ether and quinone groups. Single-walled nanotubes produced by the HiPco process, and not treated with oxidizing acids, exhibit an O/C ratio of 1.9% and do not exhibit either pi*(CO) or sigma*(CO) resonances at the detection limit of NEXAFS. It is shown that heating (to 1073 K) of the PLV-SWNTs containing the functional groups produces C K-edge NEXAFS spectra very similar to those seen for the HiPco material. The NEXAFS spectra are calibrated against spectra measured for a number of fused-ring aromatic hydrocarbon molecules containing various types of oxidized functional groups present on the oxidized SWNTs.

The decomposition of water and CO over clean and carbide-modified W(111) is studied by using temperatureprogrammed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and auger electron spectroscopy (AES). On both clean and modified W(111) surfaces, the activity toward the decomposition of water is found to be significantly higher than Pt group metals. For the CO experiments, both molecular and dissociative adsorption are observed on W(111) and C/W(111). Approximately 52% and 10% of the adsorbed CO dissociates to produce atomic oxygen and carbon on W(111) and C/W(111), respectively. In contrast, CO molecules undergo reversible desorption on oxygen-modified C/W(111) at temperatures as low as 242 K. Finally, coadsorption experiments of water and CO on C/W(111) show that the presence of surface hydroxyls hinders the adsorption of CO, and that only trace amount of gas-phase CO 2 is detected.

The stability of the Pt-3d-Pt(111) (3d = Ti, V, Cr, Mn, Fe, Co, or Ni) bimetallic surface structures in the presence of adsorbed oxygen has been investigated by means of density functional theory (DFT). The dissociative binding energies of oxygen on Pt-3d-Pt(111) (i.e., subsurface 3d monolayer) and 3d-Pt-Pt(111) (i.e., surface 3d monolayer) were calculated. All of the Pt-3d-Pt(111) surfaces were found to have weaker oxygen binding energies than pure Pt(111) whereas all of the 3d-Pt-Pt(111) surfaces were found to have stronger oxygen binding energies than pure Pt(111). The total heat of reaction was calculated for the segregation for 3d metal atoms from Pt-3d-Pt(111) to 3d-Pt-Pt(111) when exposed to a half monolayer of oxygen. All of the Pt-3d-Pt(111) subsurface structures were predicted to be thermodynamically unstable with adsorbed oxygen. In addition, the segregation of subsurface Ni and Co to the surfaces of Pt-Ni-Pt(111) and Pt-Co-Pt(111) was investigated experimentally using Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). AES and HREELS confirmed the trend predicted by DFT modeling and showed that both the Pt-Ni-Pt(111) and Pt-Co-Pt(111) surface structures were unstable in the presence of adsorbed oxygen. The activation barrier of the segregation of surbsurface Ni and Co atoms was determined to be 15 +/- 2 and 7 +/- 1 kcal/mol, respectively. These results are further discussed for their implication in the design and selection of cathode bimetallic electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrode membrane (PEM) fuel cells.

We have utilized the dehydrogenation and hydrogenation of cyclohexene as probe reactions to compare the chemical reactivity of Ni overlayers that are grown epitaxially on a Pt(111) surface. The reaction pathways of cyclohexene were investigated using temperature-programmed desorption, high-resolution electron energy loss (HREELS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our results provide conclusive spectroscopic evidence that the adsorption and subsequent reactions of cyclohexene are unique on the monolayer Ni surface as compared to those on the clean Pt(111) surface or the thick Ni(111) film. HREELS and NEXAFS studies show that cyclohexene is weakly pi-bonded on monolayer Ni/Pt(111) but di-sigma-bonded to Pt(111) and Ni(111). In addition, a new hydrogenation pathway is detected on the monolayer Ni surface at temperatures as low as 245 K. By exposing the monolayer Ni/Pt(111) surface to D2 prior to the adsorption of cyclohexene, the total yield of the normal and deuterated cyclohexanes increases by approximately 5-fold. Furthermore, the reaction pathway for the complete decomposition of cyclohexene to atomic carbon and hydrogen, which has a selectivity of 69% on the thick Ni(111) film, is nearly negligible (<2%) on the monolayer Ni surface. Overall, the unique chemistry of the monolayer Ni/Pt(111) surface can be explained by the weaker interaction between adsorbates and the monolayer Ni film. These results also point out the possibility of manipulating the chemical properties of metals by controlling the overlayer thickness.

For Abstract see ChemInform Abstract in Full Text.

The decomposition of methanol over clean and carbide-modified W( 111) is studied by using temperatureprogrammed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES). The chemistry of methanol on unmodified W( 111) is predominantly complete decomposition to produce atomic carbon and H 2 , with slightly less than 15% of the adsorbed methanol dissociating to form CO, CH 4 , and H 2 . Once the W(111) surface is carbide-modified, however, the most dominant reaction pathway is still the complete decomposition of CH 3 OH at ∼55%, but with significantly more CO and CH 4 desorbing as gas-phase products. If the carbide surface is further modified with oxygen, the activity toward the production of CO is further enhanced and becomes the dominant pathway, while the yield of gas-phase CH 4 is slighted reduced compared to the unmodified C/W(111) surface. These results will be compared to the activity of Pt group metal surfaces to explore the potential application of using tungsten carbides as an alternative to Pt group metal electrodes in fuel cells.

The reactions of methanol, water, and carbon monoxide over clean and carbide-modified W(110) are studied by using temperature-programmed desorption, high-resolution electron energy loss spectroscopy, and Auger electron spectroscopy . The product selectivity of methanol on unmodified W(110) is 67.5% toward complete decomposition, 8.5% toward CO, and 24% toward CH 4 . After the W(110) surface is modified by carbon, the complete decomposition pathway decreases to 58%, with the remaining methanol dissociating to produce approximately equal amounts of CO and CH 4 . On W(110), the number of H 2 O molecules undergoing dissociation is determined to be 0.320 water molecules per W atom. Upon carbon modification, the activity of water decreases by half to 0.153 molecules per W atom. The study of CO on W(110) shows three reaction pathways: decomposition to surface C and O, formation of gas-phase CO 2 , and molecular desorption at 284 and 335 K. On the C/W(110) surface, only 7% of the adsorbed CO decomposes to produce surface C and O; additionally, no CO 2 desorption is detected. The preadsorption of water onto C/W(110) does not appear to affect the amount of CO adsorption, but does lead to CO desorbing at the lower temperature of 271 K. These results are compared to our previous studies on W(111) and C/W(111) to determine the effect of substrate structure on the reaction pathways of methanol, water, and CO.