Jan Paul

A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001) : I. The structural and electronic properties of Cu

We have used high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe to investigate the structural and electronic properties of well annealed copper layers adsorbed on a Ru(001) substrate. Adsorption of CO on annealed Cu–Ru (001) surfaces (θCu<1) at 85 K results in occupation of both Ru and Cu sites which exhibit characteristic C–O stretching frequencies as a function of CO coverage. The latter also indicates formation of Cu islands for 0.25<θCu<1 and chemical modification of the copper film by the underlying Ru, which leads to increased Cu–CO backdonation. From IR line shapes of the C–O stretch from CO‐√3×√3‐R30° on Ru sites, domain sizes are estimated for the bare Ru patches. Weak chemical modification is still observed for a 3 ML thick Cu film, whereas a 8 ML thick film exhibits all properties of bulk Cu(111), i.e., a coverage dependent red shift (2077–2075 cm−1), a (1×1) LEED pattern and characteristic CO‐TDMS features. In contrast to the modification of Cu by Ru, ther...

The interaction of CO and O2 with the (111) surface of Pt3Ti

Abstract The electronic properties of clean and partly oxidized Pt 3 Ti(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 Pt 3 Ti(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 TiO 2 . Flashing with CO shifts the band to 500 cm −1 . Correlated with this shift we observe (i) CO 2 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 TiO 2 ”, segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti 4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti 3+ species. The vibrational shift, 650 to 500 cm −1 , can be understood by the dipole active bands of a triatomic O−Ti 4+ −O vibrator compared to a diatomic Ti 3+ −O vibrator.

Hydrogen adsorption on alkali modified aluminum

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 1:10 4 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.

A FT‐IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001). II. The dispersion of copper

The dispersion of copper adsorbed on a Ru(001) substrate has been investigated by using Fourier transform‐infrared reflection absorption spectroscopy (FT‐IRAS) and carbon monoxide as a molecular probe. Copper films evaporated at 85 K show a drastically different CO adsorption behavior compared to annealed films and exhibit a variety of adsorption sites. Characteristic C–O stretching frequencies allow us to identify small copper clusters of 1–4 atoms (2138–2123 cm−1), two‐dimensional (2120–2110 cm−1) and three‐dimensional (2098 cm−1) copper aggregates. After annealing to 250 K copper films at sub‐ and monolayer coverages form well‐ordered small two‐ and three‐dimensional copper aggregates. Formation of the epitaxial monolayer or islands of copper (2082 cm−1) requires a surprizingly mild annealing temperature of 350 K. Further annealing to 540 K results in increasing domain size of the copper islands or annealing of defect sites of the epitaxial monolayer. Multilayer coverages of copper evaporated at 85 K e...

TDS and EELS observations for CO, O2, and CH3OH bound to Ru(0001)/Cu

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 (O 2 ), methanol (CH 3 OH), and to some extent water (H 2 O) 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 O 2 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/O 2 or O 2 /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 (CH 2 O) relative to CO with increasing copper and methoxy coverages.

Vibrational spectra of CO and CO2 adsorbed on potassium modified Fe(100)

High resolution electron energy loss (EEL) spectra for CO adsorbed on potassium modified Fe(100) are presented. Five CO vibrational bands are clearly distinguishable. Two bands at or above 2000 cm−1 correspond to the α1− and α2−states on the clean surface. Two more bands, residing between 1450 and 1850 cm−1, reveal adsorption at potassium modified sites. The response of these bands to changes in potassium coverage and annealing temperature is highly predictable and similar to the situation on Fe(111) [1]. This suggests a local K: CO coordination. A fifth band is observed around 950 cm−1 and interpreted as CO adsorbed at potassium modified α3−sites, redshifted from 1200 cm−1 at unmofidied sites. The present results are complementary to a previous study of the same adsorption system utilizing photoemission spectroscopy and thermal desorption spectroscopy [2]. Finally, we present novel vibrational and thermal desorption (TDS) data for Fe(100)/K exposed to CO2 and point at similarities between intermediate states following CO and CO2 adsorption on different potassium promoted iron surfaces.

Co2 conversion and oxalate stability on alkali promoted metal surfaces: Sodium modified Al(100)

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.

Alkali promoted CO bond weakening on aluminum: A comparison with transition metal surfaces

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 abil...