O. Seiferth

Infrared spectroscopic investigation of CO adsorbed on Pd aggregates deposited on an alumina model support

Abstract We show that CO adsorption on Pd aggregates of varying size and order gives rise to several absorption bands in the range of CO stretching frequencies which we assign to different absorption sites. At low temperature (90 K) and saturation coverage we find the population of terminal as well as bridging sites. The CO molecules are preferentially terminally bound, but these exhibit the lower binding energies in agreement with earlier TDS studies. The CO-molecules on two-fold bridging sites are more tightly bound and the relative intensity of the corresponding absorption band increases with increasing size and order of the Pd aggregates. The observed bands may be assigned according to IR results on Pd(111) single crystals which is the orientation of the aggregate surfaces observed for the present deposits. The bands previously assigned to Pd(100), which gain intensity for the well-ordered aggregates with preferentially (111) oriented surfaces, we reassign to CO molecules bound to edges of the (111) facets.

IR investigations of CO2 adsorption on chromia surfaces: Cr2O3 (0001)/Cr(110) versus polycrystalline α-Cr2O3

Abstract The adsorption of carbon dioxide has been studied on a single-crystalline Cr 2 O 3 (0001) film as well as on samples of polycrystalline α -Cr 2 O 3 . The Cr 2 O 3 (0001) film has been grown on the (110) surface of a chromium single crystal. Upon CO 2 dosage, two chemisorbed and two weakly bound adsorption states are identified. We associate the strongly bound carbon dioxide with carboxylates, bent CO δ− 2 species adsorbed on top of the chromium ions of the ‘polar’ (0001) surface. An assignment to surface carbonate formed upon adsorption on surface oxygen ions is not compatible with the vibrational data. The IR spectra of CO 2 chemisorbed on polycrystalline α-Cr 2 O 3 differ substantially from the IRAS spectra of the CO 2 /Cr 2 O 3 (0001)/Cr(110) system. As the faces of the microcrystals show mainly non-(0001) termination, we consider different modes of coordination of the CO 2 chemisorbates. In particular, bidentate carbonate species are discussed.

Adsorption on a polar oxide surface: O2, C2H4 and Na on Cr2O3(0001)/Cr(110)

A polar Cr2O3(0001) surface is prepared as an epitaxial film on a Cr(110) substrate. The film is thick enough to represent the bulk surface. Applying a variety of surface sensitive techniques [thermal desorption spectroscopy (TDS), reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (PES)] we have studied adsorption of molecular oxygen, ethene and sodium.

Ultraviolet-laser induced desorption of NO from the Cr2O3(0001) surface: Involvement of a precursor state?

NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra o...

Molecules on oxide surfaces

Abstract Metal oxides may be prepared as thin (5–50 A) films on top of metallic substrates. By such means oxide substrates with properties identical to bulk oxides may be formed which can be studied via electron spectroscopies without being hindered by charging, as well as cooling problems. We report here on results on NiO and on Cr 2 O 3 surfaces. We discuss some structural aspects of oxide surfaces such as surface reconstruction of polar rock salt-type surfaces, and structural phase transitions on corundum type structures. The nature of the phase transition will be discussed with respect to the magnetic properties of the oxide. Furthermore we report on the interaction of those surfaces with molecules from the gas phase. In particular we study the interaction with small molecules such as CO, NO, O 2 , CO 2 , H 2 O and C 2 H 4 . We observe via various surface sensitive techniques such as thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), angle resolved photoemission (ARUPS), electron energy loss spectroscopy (HREELS), infrared-reflection-absorption-spectroscopy (IRAS), and near-edge-X-ray-absorption-fine-structure spectroscopy (NEXAFS), associative as well as dissociative adsorption and in the case of ethylene also polymerization reactions. Via isotopic labelling techniques combined with IRAS we study in detail the interaction of oxygen with the oxide surfaces, a process of general interest in connection with oxidation reactions.

Electron, photon and thermally induced chemistry in alkali-NO coadsorbates on oxide surfaces

The effect of alkali metals on the thermal, photon and electron induced chemistry of nitric oxide at metal oxide surfaces is investigated, using XPS, TPD and IRAS. Alkali nitrosyl salts are observed on both NiO(1 1 1) and Cr2O3(0 0 0 1) surfaces, with evidence in the latter case for the presence of a hyponitrite species. Nitrite species are only formed via photon or electron induced reactions.

IR spectroscopy of a Pd-carbonyl surface compound

Pd is deposited onto a model alumina surface in the presence of a CO atmosphere of 3 × 10 -6 mbar at liquid nitrogen temperatures. According to the IR spectra the palladium carbonyl compound formed contains weakly bound terminal and more strongly bound bridging CO molecules, the spectra being similar to the IR spectra of colloidal metal particles in solution and the IR spectra of CO adsorbed on small deposited Pd particles. In line with thermal desorption studies of the stability of the compound, the IR spectra reveal the transition from the compound to a situation where CO is adsorbed on larger compact metal aggregates upon heating to room temperature.

CO2 adsorption on Na precovered Cr2O3(0001)

CO2 adsorption and reaction on Cr2O3(0 0 0 1) is considerably modified by the presence of Na on the surface. Depending on Na coverage, different modes of interaction and reaction have been observed. At low coverage Na adsorbs by transferring an electron to the Cr2O3(0 0 0 1) surface. In this case, the formation of bent CO � (carboxylate) can be observed similar to what was observed for the clean Cr2O3(0 0 0 1) surface [see Surf. Sci. 421 (1999) 176]. Distinct differences with respect to physisorbed CO2 are identified. CO2 desorbs as an intact molecule from the Na-covered surface at temperatures higher than that found for the clean surface. NaCO2 salts form and also Na2CO3 can be observed. Carbonate forms via disproportionation of two CO2 molecules into carbonate and CO with the latter being released into the gas phase. The intermediate formation of an oxalate species, its geometry on the surface, and CO � � CO2 solvation are discussed. 2002 Elsevier Science B.V. All rights reserved.

Electronic Excitations at Oxide Surfaces

The present paper summarizes the possibilities to use electron energy loss spectroscopy (EELS) to investigate the electronic properties of systems with highly localized electronic states, such as oxide surfaces. Surface excitations may be clearly distinguished from bulk excitations in EELS, and eventually a ligand field spectroscopy may be developed on this basis. This ligand field spectroscopy is used to study surface phase transitions at oxide surfaces. Adsorbates influence the surface properties of oxides dramatically. Examples are discussed.