No abstract

The application of small and weakly interacting probe molecules for the characterization of acidic and basic properties by FTIR spectroscopy is exempliÐed by using H-and alkali cation-exchanged zeolites as typical solid Bro nsted and Lewis acids and Lewis bases. Criteria for the selection of probe molecules are given. Bro nsted acidity can be characterized by the H-bonding method when CO and are used as molecular probes. Quantum chemical calculations are shown to provide important additional N 2 information on the electronic nature of the adsorption interaction and the vibrational behaviour of the probe molecule. Lewis acidity dominates in cation-exchanged zeolites for small cations (Li`, Na`) whereas basic properties develop with increasing cation radius. CO, and interact with cation centers, the interaction energy decreasing with increasing cation radius. CO 2 , N 2 CH 4 CO at very low equilibrium pressures permits a siting of Na`, and the Al distribution in six-rings can be probed. (S II -sites) CH 4 interacts with cations in the conÐguration having symmetry. CH-acids such as acetylene and M`É É ÉH 3 CH C 3v Cl 3 CH(D), methylacetylene, are shown to be potentially suitable probe molecules for basic properties using the H-bonding method. All three molecules undergo H-bonding and the induced red-shift of the CwH stretching frequency permits a ranking of the O z 2~É É ÉHwC base strength of a given series of materials.

Zirconia-supported tungsten oxide catalysts were prepared by suspending hydrous zirconium oxide in aqueous solutions of ammonium metatungstate at their natural pH. The suspensions were refluxed at 383 K followed by evaporation of the water, drying and calcination. The WO3 loadings were chosen between 3.6 and 32 wt % and the calcination temperatures were in the range 773 e T e 1273 K. The resulting materials were structurally characterized by X-ray diffraction, differential thermal analysis, and surface area measurements, by laser Raman and Fourier transform infrared (FTIR) spectroscopy, and by UV-vis diffuse reflectance spectroscopy. Their acidic properties were tested by FTIR spectroscopy using carbon monoxide as a probe molecule. The experimental results indicate that the presence of WOx retards the crystallization of the zirconia material and stabilizes the tetragonal ZrO2 phase and the intrinsic BET surface area. W-O-W linkages are detected even at the lowest WO3 loadings indicating that oligomeric WOx clusters are initially anchored to the ZrO2 surface which grow in extension with increasing loading. Near the theoretical monolayer coverage a relatively dense overlayer has formed that eventually forms a three-dimensional network of interconnected WO6 octahedra. WdO groups are present and presumably connected with peripheral WO6 octahedra. This structural model is supported by the number of nearest W neighbors, which varies between two and five depending on the WO3 loading as determined from the edge position in UV-Vis diffuse reflectance spectra. It is inferred that the WO6 network may be structurally described as a pseudo-heteropolytungstate that might have some Zr 4+ polyhedra incorporated. FTIR spectroscopy showed an extremely broad quasi-continuous absorption in the O-H stretching region, which is suggestive of the presence in the overlayer network of delocalized protons. These are considered to be responsible for the detected protonic acidity of the materials.

Adsorption of CO on Ni-ZSM-5 reveals the existence of two kinds of Ni 2+ ions, with the respective stretching frequencies being at 2220 and 2212 cm -1 . The carbonyl complexes are resistant to evacuation at room temperature, which is explained by the high electrophilicity of cations in a ZSM matrix. At 85 K and in the presence of CO, part of the Ni 2+ -CO species are converted into Ni 2+ (CO) 2 dicarbonyls (2204 cm -1 ) which are characterized by a very low stability and easily lose one of their CO ligands. Interaction of Ni-ZSM-5 with CO at 673-773 K results in a preferential reduction of the Ni 2+ ions which form the carbonyls characterized by the band at 2220 cm -1 . CO adsorption at room temperature on the reduced sample leads to the formation of Ni + (CO) 2 gem dicarbonyls (ν s at 2136 cm -1 and ν as at 2092 cm -1 ) which are converted, during evacuation, to linear Ni + -CO species (2109 cm -1 ). The dicarbonyl structure is proved by 12 CO-13 CO coadsorption, and the mixed Ni + ( 12 CO)( 13 CO) species are characterized by ν( 12 CO) at 2123 cm -1 and ν( 13 CO) at 2058 cm -1 . At low temperature, a third CO molecule is coordinated to the Ni + cations, thus producing tricarbonyls (2156, 2124, and 2109 cm -1 ). The latter species lose one of their CO ligands during evacuation at 85 K and are converted back into dicarbonyls. The Ni + sites are easily oxidized in the presence of O 2 . However, the Ni 2+ ions produced differ in properties from the initially deposited cations and are easily reduced to Ni + even at ambient temperature.