Spectroscopy under the Surface. In-Situ ATR-IR Studies of Heterogeneous Catalysis in Water.

The chemical industry is always searching for more cost- and performance-efficient processes and the trend is therefore to use water as a solvent. The selection of water as a solvent offers many benefits; beside low cost, the advantages are the fact that it is environmentally benign, its availability and safety. The chemical industry is a major contributor to environmental pollution, largely due to the use of hazardous solvents. These solvents often end up in wastewater and can be removed by for example wetair oxidation over several catalysts. At the same time, nitrite and nitrate levels in groundwater have increased over the last few decades. Denitrification (nitrite and nitrate removal) is therefore one of the most investigated hydrogenation reactions in ground-water treatment. So far, detailed mechanistic studies of reactions carried out in water are lacking because it is difficult to study heterogeneous catalysis in-situ when the reaction is carried out in water. Vibrational spectroscopy is a versatile tool to study adsorption and reaction on catalytic surfaces in gas phase. Because liquids and water in particular, are strong absorbers of infrared radiation, normal transmission infrared spectroscopy is not applicable unless the pathlength of the light is very short, i.e. in the order of a few microns. Attenuated Total Reflection Infrared Spectroscopy (ATRIR), however, is ideally suited for studying molecular vibrations at the solid-liquid interface since the evanescent wave is restricted to the region near the interface, thereby minimising the contribution from the liquid. The work described in this thesis demonstrates the application of ATR-IR infrared spectroscopy to study adsorption and catalytic reactions on metal surfaces during heterogeneous catalytic reactions in aqueous phase. By coating an Internal Reflection Element (IRE) with a stable, thin (in the order of a few microns) catalyst layer, adsorption and reaction at the solid-aqueous interface was studied successfully. Properties of the technique and the influence of water on the supported metal catalysts were demonstrated using two reactions (i) CO oxidation and (ii) nitrite hydrogenation. CO is a widely applied molecule to characterise supported noble metal catalysts and CO oxidation is a very simple reaction. Therefore CO adsorption and oxidation was used to demonstrate the technique and to give insight in the behaviour of supported catalysts in water. Denitrification, nitrite hydrogenation, is becoming more important due to the high nitrite and nitrate levels found in ground-water in Europe and the increasingly strict regulations for drinking water quality. The hydrogenation can be performed over single noble metal catalysts such as platinum and palladium catalysts. The heterogeneous hydrogenation of nitrite was investigated by ATR-IR, in order to resolve the hydrogenation mechanism.