Journal of Catalysis 246 (2007) 66–73 www.elsevier.com/locate/jcat In situ ATR-IR study of CO adsorption and oxidation over Pt/Al 2 O 3 in gas and aqueous phase: Promotion effects by water and pH Sune D. Ebbesen, Barbara L. Mojet, Leon Lefferts ∗ Catalytic Processes and Materials, Faculty of Science and Technology, Institute of Mechanics Processes and Control Twente (IMPACT), University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands Received 9 June 2006; revised 25 September 2006; accepted 18 November 2006 Available online 22 December 2006 Abstract The adsorption and oxidation of carbon monoxide over a Pt/Al 2 O 3 catalyst layer deposited on a ZnSe internal reflection element was in- vestigated both in gas phase and water using attenuated total reflection infrared spectroscopy. A preparation method is described that results in a strongly attached layer that is stable for many days in a water flow. Both adsorption and oxidation of CO are largely affected by the presence of liquid water. It influences the metal particle potential as well as the CO molecule directly, which is reflected in large red shifts (45 cm −1 ) and a fourfold higher intensity when the experiments are carried out in water. Furthermore, the rate of CO oxidation changes significantly when carried out in water compared with gas phase. Finally, with increasing pH, CO stretching frequencies shift to lower wavenumbers, accompanied by a large increase in CO oxidation rate.  2006 Elsevier Inc. All rights reserved. Keywords: ATR-IR spectroscopy; CO oxidation; Carbon monoxide; Adsorption; Pt/Al 2 O 3 ; Liquid phase; Water; pH effect 1. Introduction The chemical industry is continuously searching for more cost- and performance-efficient processes, and thus the trend is toward using water as a solvent. The selection of water as a solvent offers many benefits, including low cost, environmen- tal friendliness, availability, and safety. The chemical industry is nevertheless a major contributor to environmental pollution, largely due to the use of hazardous solvents. Of the top 10 chemicals released or disposed of by the chemical industry in the mid-1990s, five were solvents: namely, methanol, toluene, xylene, methyl ethyl ketone, and methylene chloride [1]. For this reason, also catalytic wastewater and groundwater treatments are receiving increasing attention. Water purifica- tion in general includes especially oxidation processes and to a lesser extent, hydrogenation processes as well [2]. As an ex- ample, over several catalysts, wet-air oxidation can be applied for removal of organic compounds from water [2]. Denitrifica- * Corresponding author. Fax: +31 53 489 4683. E-mail addresses: s.d.ebbesen@utwente.nl (S.D. Ebbesen), b.l.mojet@utwente.nl (B.L. Mojet), l.lefferts@utwente.nl (L. Lefferts). tion (nitrite and nitrate removal) is one of the most investigated hydrogenation reactions in groundwater treatment. The denitri- fication is catalyzed by supported bimetallic noble metals [3]. Technology in this area is developing, and at the current stage the formation of ammonia as a side product is a problem be- cause it is undesirable in drinking water [4]. The selectivity to nitrogen is known to depend on, among other things, the type of reducing agent or pH. So far, detailed mechanistic studies are lacking, because it is difficult to study heterogeneous catalysts in situ if the reaction is carried out in water. In gas phase, vibrational spectroscopy is a versatile tool for studying adsorption and reaction on cat- alytic surfaces. However, in liquid-phase reactions, application of normal transmission infrared spectroscopy is not suitable un- less the path length of the light is very short (i.e., in the order of a few microns), because liquids (particularly water) normally are strong absorbers of infrared radiation. Attenuated total re- flection infrared spectroscopy (ATR-IR), however, is ideally suited for studying molecular vibrations at the solid–liquid in- terface, because the evanescent wave is restricted to the region near the interface, thereby minimizing the contribution from the liquid [5]. As a result, the correct design of the ATR cell allows 0021-9517/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2006.11.019