Erich Knözinger

Hydroxyl groups as IR active surface probes on MgO crystallites

Highly dispersed MgO was produced under diverse non-equilibrium conditions, among them those of chemical gasphase deposition. The resulting powders were then degassed in HV at temperatures up to 800°C. These procedures permit the production of MgO samples which strongly differ in both specific surface area and surface topography. Correspondingly an enormous variety of surface sites is made available. Their distribution was studied by FT-IR spectroscopy using OH groups as surface probes. A unique correlation could be established between the characteristic absorption pattern in the OH stretching region on one hand and the experimental conditions applied in the course of the production and of the pretreatment procedure on the other. This required an assignment of the IR spectra in terms of coordination and hydrogen bonding of surface OH groups on MgO. The strategy of assignment involved adsorption experiments with HX (X = OH, H, SH, NH2), thermal desorption studies, the evaluation of both the isotope effect ∼ν(OH)/∼ν(OD) and the first overtone 2∼v(oh) as well as ab initio calculations related to hydrogenated and hydrated MgO clusters.

Wavelength selective excitation of surface oxygen anions on highly dispersed MgO

Monochromatic UV light in the spectral interval between 4.0 and 5.5 eV is used in order to selectively excite 3- and 4-coordinated oxygen anion sites on the surface of MgO nanoparticles exposed to O2 gas. As a result, two different paramagnetic O− surface species and also ozonide anions O3− are observed by electron paramagnetic resonance (EPR) spectroscopy. The relative abundance of each of the O− species exhibits a specific dependence on the energy of the exciting photons. EPR data together with the results of theoretical modeling suggest that both O− species are located at 3-coordinated sites having different local environments. At sufficiently high O2 pressures molecular oxygen does not only act as an electron trap, favoring the O− formation, but it also contributes to UV induced O3− formation with a maximum efficiency at 4.2 eV.

Far infrared spectra of strongly polar molecules in solid solution. I. Acetonitrile

Strongly polar molecules such as acetone, nitromethane, and acetonitrile imbedded in an inert gas matrix give rise to characteristic absorption patterns below 150 cm−1. The bands observed in the case of acetonitrile are due to an activated phonon band, to two vibrations of a monomer molecule within the polarized cage of inert gas atoms, and to two intermolecular vibrations of an antiparallel dimer.

Chemical vapour deposition — a new approach to reactive surface defects of uniform geometry on high surface area magnesium oxide

Abstract Chemical vapour deposition (CVD) is particularly well suited for the preparation of high surface area magnesium oxide which exhibits a considerably reduced surface heterogeneity. This is shown in the present study for three types of surface defects: low coordinated anions and cations as well as anion vacancies. For each of them, only two to three different species with discrete geometries are observed. The characterization of these defects is based on specific EPR active molecular surface probes, namely, electron deficient oxygen anions O − , superoxide anions O 2 − and surface colour centres F S + , respectively. Some of the O 2 − and F S + centres exhibit a dipolar magnetic interaction with the proton of a closely spaced OH group. The electronic interaction is reflected by a change of the intramolecular force field of the respective OH group. Consequently, there is an IR spectroscopic access to the characterization of the defects in question. On the other hand, the surface O − species are absolutely insensitive to the hydroxylation state of the MgO surface.

Structure, morphology and surface properties of nanostructured ZrO2 particles

Nanocrystalline zirconium dioxide particles have been produced by means of thermal decomposition of a liquid metal organic precursor in a flow reactor system. X-Ray diffraction patterns show that the powder consists of a mixture of the monoclinic and tetragonal phase and that the phase fractions depend on the experimental conditions during the preparation as well as on the subsequent thermal treatment of the sample. The mean particle diameter of the synthesized powders is ca. 40-50 A as estimated from the line width of Bragg reflections and from transmission electron micrographs. The crystallites are non-porous and exhibit a rough surface incorporating a high concentration of low-coordinated surface sites. In agreement with the small particle size, the powder has a high specific surface area (>200 m 2 g –1 ), which surmounts that of commercially available materials by a factor >3. The structure and morphology of the particles is essentially preserved during extensive thermal treatment up to 500°C. Hydroxyl groups as IR active surface probes clearly indicate that the phase fractions on the surface and in the bulk strongly differ from each other. Low-coordinated OH groups on monoclinic surface domains give rise to H/D exchange reactions with D 2 already at room temperature. They are evidenced and monitored with time by FTIR spectroscopy.

Site selective hydroxylation of the MgO surface

On the surface of MgO nanoparticles 1- and multiple (n = 3 and 4) coordinated hydroxyl groups are generated site-selectively and studied by transmission infrared spectroscopy. Their formation is based either on the stepwise dehydration of previously hydrated samples or on simple surface reactions involving molecular hydrogen on the one hand, and molecular oxygen, nitrous oxide or properly selected UV quanta on the other as reactants. 4-Coordinated OH groups may interact (a) with adjacent surface adsorbants such as hydrides (Mg2+H−) or superoxide anions (Mg2+⋯O−2) and (b) with surface defects like oxygen anions on steps (O2−) or paramagnetic surface colour centers. In each case the frequency of the respective IR-active OH stretching vibration is indicative of a specific local surface environment to which the OH probe is exposed. On the other hand, only perfectly free and isolated 3-coordinated OH groups have so far been observed. For sterical reasons they are exempt from any interaction with adjacent surface groups or surface defects.

UV induced local heating effects in TiO2 nanocrystals

When isolated TiO(2) nanocrystals are subjected to UV light at 77 K and pressures below 10(-6) mbar, trapping of photogenerated hole centers occurs on the surface of the nanocrystals and can be tracked by time-resolved electron paramagnetic resonance spectroscopy. Irrespective of the selected UV irradiance used, the maximum concentration of trapped charges was found to be constant for a given number of nanocrystals ( approximately 10(15)) and corresponds to one electron-hole pair per particle. On a time scale of seconds to minutes the dynamics for the trapping process depend on the number of photons with supra band gap energy. A local temperature rise of the TiO(2) nanocrystals was observed for irradiances above 1.55 mW cm(-2) (10(15) photons cm(-2) s(-1)). This is attributed to enhanced nonradiative recombination of photogenerated charge carriers via heat production and points to a substantial contribution of thermal chemistry in photocatalytic reaction cycles.

Disordered phases in vapor deposited rare gases

Neutron and x‐ray diffraction from vapor deposited krypton is reported. Strong diffuse intensity between the positions of the (111) and (200) reflections and between the (311) and (222) reflections of the fcc lattice is observed and ascribed to long range perturbations of the lattice.

Far infrared spectroscopic studies of acetonitrile and other polar molecules in liquid phase

Abstract A composite absorption band with a maximum below 100 cm −1 is observed for polar non-hydrogen bonding molecules in the pure liquid and in binary mixtures with CCl 4 . In the case of acetonitrile it consists of three contributions. A band at 90 cm −1 has to be attributed to orientational motions of molecules within certain clusters prevailing at high concentrations. On dilution with CCl 4 , the clusters are preserved even at concentrations as low as 20 mole %. Below 20 mole % the model of clusters has successively to be replaced by the model of an equilibrium between · antiparallel dimer molecules absorbing at 75 cm −1 owing to intermolecular vibrations and · monomer molecules which absorb at 65 cm −1 owing to librations about temporary equilibrium positions within the cage of solvent molecules (Poley absorption).

Chemisorption of H2 and H2—O2 on polymorphic zirconia

Powders of polymorphic zirconia contain crystallites incorporating monoclinic and tetragonal domains. Thermal activation in high vacuum results in a significant amount of highly reactive radical surface centres (O–, Zr3+) mainly located in tetragonal surface sites. Their exposure to molecular H2 leads uniquely to homolytic dissociation processes clearly evidenced by IR spectroscopy. These processes give rise to an irreversible formation of multiple-coordinated surface OH groups on tetragonal surface sites (3668 cm–1) and to a reversible formation of simple (ZrH; 1565 cm–1) and bridged hydrides (Zr2H; 1360 cm–1). Simple hydrides are very reactive towards any kind of oxygen species (O2, O). Their reaction with physisorbed O2 leads to irreversible formation of multiple-coordinated OH groups and O2.– radical anions [ν(O—O).–; 1118 cm–1] evidenced by 18O2 isotopic experiments. The appearance of these OH groups necessarily implies surface mobility of intermediately existing H atoms on both monoclinic and tetragonal surface domains.