W. K. Kuhn

CO adsorption on clean and C-, O- and H-covered Mo(110) surfaces: an IRAS study

The adsorption of CO on clean and O-, C- and H-precovered Mo(110) surfaces has been studied using infrared reflection absorption spectroscopy (IRAS). For a low CO exposure (⩽ 2 L) at a sample temperature of 90 K, CO on a Mo(110) surface shows no IR features in the 1800–2200 cm−1 frequency region, and is thus likely bound to the substrate with the C-O axis tilted with respect to the surface normal. By an exposure of ≈4 L, CO features at 2004 and 1888 cm−1 are evident, and interpreted to correspond to linear and bridging CO, respectively. Saturation CO on Mo(110) at 90 K shows a single peak at 2033 cm−1 which splits into two peaks at 2031 and 2014 cm−1 upon heating to 160 K due to the formation of ordered CO islands. Adsorption of O2; induces two CO IR peaks at 2062 and 1983 cm−1 which are believed to arise from CO molecules that have an indirect (via the Mo substrate) and a direct electronic interaction with O, respectively. CO on a C-covered Mo(110) surface displays an IR peak at 2085 cm−1. The invariant frequency of this peak at various C coverages indicates that the C forms two-dimensional islands which grow via a mechanism of expansion at the island edges. Coadsorption of H with CO on a Mod 10) surface results in a splitting of the linear CO band. The peak induced at 2008 cm−1 is attributed to CO adsorbed in the vicinity of H.

Infrared vibrational studies of CO adsorption on ultrathin Cu films on a Rh(100) surface

Adsorption of CO on ultrathin Cu films supported on a Rh(100) surface has been studied using infrared reflection absorption spectroscopy (IRAS). At low Cu coverage (<0.3 ML), CO on Cu shows a higher vibrational frequency than the gas phase CO stretching frequency (2143 cm−1). This blue shift is interpreted as arising from CO adsorption on Cu atoms and/or two‐dimensional atom clusters with a small positive charge, i.e., Cuδ+n, formed via charge transfer at the Cu–Rh(100) interface. As the Cu coverage increases, the CO vibrational frequency for Cu–CO red shifts. On an annealed Cu overlayer (≳0.3 ML), the IR spectra show a sharp peak at 2095 cm−1 which is interpreted to correspond to CO adsorbed on well‐ordered pseudomorphic Cu domains. CO adsorption on Rh(100) was found to be blocked by 0.6 ML Cu. For a high coverage of Cu (∼7 ML) annealed to 850 K, a peak at 2076 cm−1 is observed and identified as corresponding to CO adsorption on three‐dimensional Cu clusters.

An IR study of CO surface diffusion on submonolayer metal overlayers

Abstract The adsorption and surface diffusion of CO on Pd/W (110) and Cu/Rh (100) surfaces have been studied using infrared reflection—absorption spectroscopy (IRAS). For CO adsorbed on 0.14 MP Pd on W (110), the CO is found to diffuse from Pd to W at approximately 200 K. Likewise, CO adsorbed on ≈ 0.1 ML Cu on Rh (100) will diffuse from the Cu to the Rh at approximately 250 K. In addition, the CO adsorbed on the copper is blue-shifted relative to the gas phase CO stretching frequency of 2143 cm−1.

Cu titration of tilted CO on a Mo(110) surface

Abstract Upon CO exposure to a Mo(110) surface at 95 K, the initial ≈0.3 monolayer (ML) of CO does not exhibit a vibrational frequency in the 1800–2200 cm− region, implying that the CO axis is tilted with respect to the surface normal. Upon subsequent deposition of ≈0.9 ML Cu at 95 K followed by heating, infrared-reflection-adsorption-spectroscopy (IRAS) shows that a portion of the tilted CO molecules diffuses onto the Cu overlayer and assumes an upright bonding geometry, with the remainder dissociating on the Mo(110) surface into atomic C and O.

Interaction of CO with Cu/Rh(100), Cu overlayer structures and CO surface diffusion

The interaction of CO with ultrathin Cu films on a Rh(100) surface has been studied using infrared reflection–absorption spectroscopy (IRAS). The CO vibrational frequency on Cu/Rh(100) at low Cu coverage [∼0.1 monolayer (ML)] shows a blue shift relative to its gas phase stretching frequency (2143 cm−1 ). This blue shift likely arises from CO adsorbed onto Cu adatoms which are either slightly charged due to charge transfer from Cu to Rh or self‐polarized because of a strong Cu–Rh interaction. It is further shown that disordered Cu islands, well‐ordered Cu pseudomorphic islands, and Cu three‐dimensional clusters are distinctively characterized by CO peaks at 2122, 2093, and 2076 cm−1, respectively. In addition, it is found that CO adsorbed onto ∼0.1 ML Cu/Rh(100) diffuses from Cu to Rh at a sample temperature (Ts) of 150–270 K, and that Cu deposition onto CO‐covered Rh(100) at Ts=90 K causes CO spill over from Rh to Cu, thus forming a CO/Cu/Rh(100) structure.