| 1341. |
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| 1342. |
- Paul, Jan, et al.
(författare)
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Alkali hydride formation on aluminum
- 1988
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Ingår i: Catalysis: theory to practice. - : Chemical Inst. of Canada. ; , s. 1890-1897
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Konferensbidrag (refereegranskat)
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| 1343. |
- Paul, Jan, et al.
(författare)
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Alkali promoted CO bond weakening on aluminum : a comparison with transition metal surfaces
- 1987
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 86:9, s. 5188-5195
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Tidskriftsartikel (refereegranskat)abstract
- Data on the adsorption and decomposition of carbon monoxide on alkali promoted Al(100) are presented. CO dissociates on the potassium or sodium promoted surface and aluminum oxide and aluminum carbide form after annealing to 700 K. At intermediate temperatures EELS show alkali-CO complexes with vibrational frequencies ranging from 1060 to 2060 cm-1. A band at 1750 cm-1 was assigned to CO molecules coordinated to bulk potassium. CO vibrational spectra as well as work function measurements reveal an altered alkali dispersion as a function of preannealing temperature. Comparisons are made between the surfaces of aluminum and transition metals with respect to (i) alkali adsorption, (ii) hybridization between metal d states and CO π orbitals, (iii) the magnitude of unscreened (long-range) perturbations, and finally (iv) the energetics of carbide and oxide formation. Potassium but not sodium atoms bind strongly to aluminum carbide (Td>700 K). We suggest that potassium is rare among alkali metals not in its ability to promote CO dissociation but in preventing a downshift of the C 2pz orbital and thus carbide to graphite transformation.
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| 1344. |
- Paul, Jan, et al.
(författare)
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Carbon dioxide activation and alkali compound formation. I : Vibrational characterization of oxalate intermediates
- 2003
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 531:3, s. 244-264
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Tidskriftsartikel (refereegranskat)abstract
- The activation of CO2 in thin potassium layers adsorbed on Cu(1 1 1) has been studied with time-evolved Fourier transform-infrared reflection absorption spectroscopy. The growth of thin layers by reactive evaporation of potassium in a CO2 atmosphere permits control of the K:CO2 stoichiometry, which strongly affects the selectivity in the formation of intermediates and the decomposition pathways of the layer. Layers grown in a CO2 rich atmosphere exhibit the preferential growth of stoichiometric potassium oxalate K2C2O4 (D2h). The molecular identity of oxalate with D2h symmetry is confirmed by vibrational spectra utilizing isotopic substitution methods (13CO2 and C18O2) and by the use of isotopic mixtures of CO2/C18O2 and CO2/13CO2. A comparison of the isotope data with theoretical calculations gives an estimated OCO bond angle in oxalate of 132°. Far-IR spectra obtained with synchrotron radiation indicate the equivalent interaction of all oxygen atoms with the potassium. A comparison of the vibrational data with theoretical ab initio calculations confirms the structural model of an oxalate species that is bulk coordinated with no strong directional bonding and all oxygen atoms equally interacting with potassium. At medium and low CO2:K ratios, very complex vibrational spectra are observed, indicating the formation of an oxalate surface species with C2v symmetry in addition to D2h- oxalate, CO2- and CO22- species.
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| 1345. |
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| 1346. |
- Paul, Jan, et al.
(författare)
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Co2 conversion and oxalate stability on alkali promoted metal surfaces : sodium modified Al(100)
- 1988
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Ingår i: Catalysis Letters. - 1011-372X .- 1572-879X. ; 1:12, s. 445-455
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Tidskriftsartikel (refereegranskat)abstract
- CO2 conversion on alkali promoted metals in aprotic systems has been followed with surface sensitive spectroscopies. New results on sodium modified aluminum(100) are presented and compared with previous studies on magnesium [1], aluminum [2], and bulk alkali metals [3]. Electron energy loss spectra reveal two different states of CO2 adsorption at 100 K and monolayer sodium coverage. Vibrational bands at 650 cm-1 and 2325 cm-1 correspond to weakly bound molecular CO2 and a multitude of bands between 2300 cm-1 and 460 cm-1 to oxalate ions with low, possibly unidentate, coordination. Gentle annealing increases the coordination as apparent by vibrational shifts. This corresponds to oxalate to carbonate conversion, a process which is completed around room temperature. CO desorption was detected at 285 K and Auger measurements reveal a 1:3 C/O stoichiometry after high temperature annealing. We observe no release of CO2 above 110 K but an additional weak state of CO desorption around 470 K. High temperature annealing causes decomposition of all intermediates and leaves the aluminum surface covered with an irreducible carbide and oxide overlayer. We suggest that CO2 reduction and dimerization to C2O4 -2 is a common path to yield carbon deposition on all alkali promoted surfaces in hydrogen deficient systems. In contrast, oxalate decomposition is related to the specific chemistry of each substrate.
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| 1347. |
- Paul, Jan, et al.
(författare)
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Decomposition of water on clean and oxidized aluminum(100)
- 1986
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 90:21, s. 5321-5324
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents electron energy loss spectra (EELS) and thermal desorption (TDS) traces following the adsorption and subsequent annealing of water on A1(100), with and without the presence of a surface oxide. A water molecule will in both cases either (1) reversibly bind at a hydrogen-bonded site or (2) decompose, preferentially to a surface-bound hydroxyl species. The H20 dissociation occurs via H2 evolution only on the bare surface and also via hydrogen abstraction in the presence of a surface oxide. Neither dihydrogen nor water desorption are observed as the hydroxyl species diminish during annealing. Instead, the aluminum hydroxide transforms into aluminum oxide and "trapped" hydrogen atoms, depicted as an Al/A10, hydride.
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| 1348. |
- Paul, Jan, et al.
(författare)
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Hydrogen adsorption on alkali modified aluminum
- 1988
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 194:3, s. 419-437
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Tidskriftsartikel (refereegranskat)abstract
- Coadsorption experiments of alkali metals and hydrogen on Al(100) are reported. The probability for H-H (D-D) dissociation and subsequent atomic adsorption is less than 1:104 at all alkali coverages. As a consequence, each overlayer had to be prepared by exposing the sodium or potassium modified surface to a beam of hydrogen (deuterium) atoms. We observed an attractive alkali-hydrogen interaction at all coverages. This interaction is best described as the formation of an alkali hydride in the presence of excess alkali atoms. The formation of the hydride shifts the recombination and desorption temperature of hydrogen adatoms on Al(100) from around 350 to around 500 K. While no isotope effect was detected on the clean aluminum surface (diffusion limited recombination), a significant 10-15 K shift was observed for both alkali hydrides (bond breaking). Furthermore, both alkali hydrides observed intense vibrational losses, thus revealing the ionic character of the metal-hydrogen bond. Electron energy loss spectra of an annealed monolayer of sodium hydride (deuteride) showed metal-hydrogen stretching bands at 1850 (1350) cm-1 and 1715 (1250) cm-1, a deformation band at 800 (600) cm-1, and a metal-metal band at 200 (200) cm-1. Corresponding peak positions for potassium hydride (deutride) were 1650 (1200) cm-1, 1500 (1100) cm-1, and 775 (585) cm-1. The metal-metal vibration could not be separated from the elastic peak because of the larger mass of potassium compared to sodium.
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| 1349. |
- Paul, Jan, et al.
(författare)
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TDS and EELS observations for CO, O2, and CH3OH bound to Ru(0001)/Cu
- 1986
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 172:1, s. 151-173
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of monolayers of copper atoms adsorbed onto a Ru(0001) single crystal surface have been studied with thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS) utilizing carbon monoxide (CO), dioxygen (O2), methanol (CH3OH), and to some extent water (H2O) as chemical probes. Whereas a three-monolayer-thick film exhibits most properties of a Cu(111) crystal distinct deviations are found at lower Cu coverages. TDS as well as EELS show a weakened Ru---CO bond and a strengthened Cu---CO bond as a result of metal-metal interaction. The stronger Cu---CO bond is accompanied by a higher probability for O2 dissociation. The mobilities of copper and oxygen atoms are such that annealing to 650 K produces an overlayer structure which is independent of adsorption sequence: Cu/O2 or O2/Cu, but where Ru---O as well as Cu---O vibrations can be identified. Methanol adsorbs reversibly on a monolayer of copper atoms. Metal bound methoxy species are formed in the presence of oxygen atoms. The decomposition paths of such methoxy intermediates alter towards more formaldehyde (CH2O) relative to CO with increasing copper and methoxy coverages.
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| 1350. |
- Paul, Jan, et al.
(författare)
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The interaction of CO and O2 with the (111) surface of Pt3Ti
- 1986
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 177:1, s. 121-138
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of clean and partly oxidized Pt3Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt3Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) "reconstruction" of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm-1 which coincides with infrared absorption spectra of TiO2. Flashing with CO shifts the band to 500 cm-1. Correlated with this shift we observe (i) CO2 desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates "islands of TiO2", segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti3+ species. The vibrational shift, 650 to 500 cm-1, can be understood by the dipole active bands of a triatomic O-Ti4+ -O vibrator compared to a diatomic Ti3+-O vibrator.
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