Joseph Eng

A VIBRATIONAL STUDY OF ETHANOL ADSORPTION ON SI(100)

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (<1450 cm−1) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C3v axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the obs...

Vibrational study of silicon oxidation : H2O on Si(100)

Abstract The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra ( −1 are acquired using a buried CoSi 2 layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a SiH stretching vibration (2082 cm −1 ), and SiOH vibrations (OH stretch at 3660 cm −1 and the SiOH bend and SiO stretch of the hydroxyl group centered around 820 cm −1 ). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the SiO mode of the surface hydroxyl group, namely, that the SiO stretch of the OH surface species formed upon H 2 O exposure occurs at 825 cm −1 , while the SiO stretch of the OD surface species formed upon D 2 O exposure shifts to 840 cm −1 , contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm −1 , which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H 16 2 O, H 18 2 O and D 2 O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the SiO stretching and SiOH bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.

Controlling surface reactivities of transition metals by carbide formation

Abstract The surface reactivities of transition metals, including the Groups 4–6 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overlayer. More importantly, the reactivities of carbide-modified surfaces frequently demonstrate strong similarities to those of the Pt-group (Pt, Pd, Ir, Rh, Ru and Os) metals. In this paper, we will summarize our recent surface science investigations of the electronic, structural, and catalytic properties of well-characterized model carbide systems. Using several characteristic surface probing reactions, we will provide experimental evidence for the similar reactivities of the carbide-modified surfaces and the Pt-group metals. We will also discuss the underlying structural and electronic properties that are controlling the reactivities of the carbide-modified surfaces, such as the binding sites and chemical nature of the carbon atoms, the ionicity of the metal–carbon bonds, and the band structure of the transition metal carbides. We will also show examples of experimental attempts to correlate findings from the single crystal model carbide systems to amorphous powder carbide catalysts.

NEXAFS determination of electronic and catalytic properties of transition metal carbides and nitrides: From single crystal surfaces to powder catalysts

Abstract In this paper we will provide a brief review of our recent investigations of the physical and chemical properties of transition metal carbides and nitrides using the near-edge X-ray absorption fine structure (NEXAFS) technique. We will first use single crystal model surfaces to demonstrate the capabilities of NEXAFS for determining fundamental electronic and structural properties of transition metal carbides and nitrides, such as the ionicity of the metal–nonmetal bonds, the p-projected density of unoccupied states, and the location of carbon and nitrogen atoms on the surface or interstitial subsurface sites. We will use several examples to correlate these electronic and structural properties to the chemical reactivities of the transition metal carbide and nitride model surfaces. Furthermore, we will compare the general similarities of the NEXAFS investigations between model surfaces and powder catalysts. We will also provide several examples to demonstrate how the fundamental learnings from the model surfaces can be used to understand the catalytic properties of powder materials with complicated catalytic formulations, such as the bimetallic oxycarbide and oxynitride catalysts.

REACTIVITIES OF CARBON- AND NITROGEN-MODIFIED MO(110) : A COMPARISON OF MODIFICATION EFFECTS BY SURFACE AND INTERSTITIAL ADATOMS

The surface properties of carbon‐ and nitrogen‐modified Mo(110) have been investigated using high‐resolution electron energy loss spectroscopy, near‐edge x‐ray absorption fine structure, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and temperature programmed desorption. Exposure of the Mo(110) surface to olefin molecules at 600 K resulted primarily in surface carbon atoms; subsequent annealing to 1200 K produced interstitial/subsurface carbon atoms. Higher concentrations of carbon atoms in the interstitial/subsurface regions could be achieved by repeated dosing/annealing cycles. Exposure to either N2 or NH3 led to the deposition of surface nitrogen atoms. Subsequent annealing of such surfaces resulted in N2 desorption without any detectable production of interstitial/subsurface nitrogen atoms. Reactions with ethylene or cyclopentene as probing molecules demonstrated that the reactivities of carbon‐ and nitrogen‐modified Mo(110) depend strongly on the location of the adatoms. For example,...

A survey of acetylene cyclization on single crystals, supported particles and bimetallic surfaces: new cyclization studies on bimetallic Pd/W(211)

Abstract A review of acetylene cyclotrimerization to form benzene is presented. This structure-sensitive catalytic reaction is of particular interest because C–C bonds can be formed readily on certain surfaces under ultra high vacuum (UHV) conditions. In addition, we present new results for acetylene cyclotrimerization on a bimetallic surface, Pd deposited on W(211). Pd on W is chosen because it is a morphologically unstable system and W(211) facets develop after annealing Pd/W(111) to ≥700 K. Temperature programmed desorption (TPD) data reveal negligible amounts of benzene detected from acetylene adsorption on clean W(211). A monolayer (ML) Pd film on W(211) decreases the high reactivity towards acetylene decomposition and catalyzes the cyclotrimerization reaction; benzene desorbs with TPD peaks at 210 K and ∼470 K. The use of high resolution electron energy loss spectroscopy (HREELS) has shown the desorption of benzene products to be reaction rate-limited on the ∼1.0 ML Pd/W(211) surface.