ATR-FTIR Quantitative Spectrokinetic Analysis at Liquid/Solid Interface

ALEJO AGUIRRE; PABLO A. KLER; CLAUDIO L.A. BERLI; SEBASTIÁN E. COLLINS

Deauville

Congreso; Fifth International Conference on Operando Spectroscopy; 2015

Laboratoire Catalyse et Spectrochimie, ENSICAEN

Attenuated total reflection (ATR) infrared (IR) spectroscopy is a powerful tool to investigate reaction pathways in liquid(reactive)/solid(catalyst) systems, because it provides the detection of adsorbed species on a catalyst under reaction conditions [1]. Catalysts are commonly deposited on an internal reflection elements (IRE) as layers of powders or as films (e.g. metal film), and they are exposed to the liquid phase reactants. In order to study the evolution of adsorbed intermediates, transient techniques are usually applied, such as stop-flow and concentration-modulation excitation spectroscopy (c-MES) [1-2]. However, to obtain quantitative information, that is, determining intrinsic reaction rates, the chemical engineering aspects of an ATR flow-through cell must be developed. Particularly, mass transport in the ATR cell has to be characterized. We present here an optimized flow-through ATR microfluidic cell to assess intrinsic kinetic parameters of reactions under chemical control. This ATR cell possesses no dead-volume and has a uniform fluid velocity profile across the cell. The mass transport dynamic from the bulk solution to the surface of the ATR crystal -where the catalyst is deposited- is described by a convection-diffusion transport model [3]. The mass-transfer limitation in the ATR cell is investigated comparatively using the simplest case of adsorption/desorption in both a non-porous and a porous catalyst layer (Fig. 1). A Pt thin film deposited on a ZnSe IRE by vapor deposition and the layer of Pt(0.38%)/Al2O3 was prepared by evaporation of a dispersion of the catalyst. The operational limit of the cell is characterized by means of a non-dimensional analysis in terms of Péclet (Pe), diffusive Biot (〖Bi〗_D) and Sherwood numbers (〖Sh〗_M) for the porous film case. Suitable criteria to determine kinetic parameters under chemical control were develop considering the geometry and experimental flow rates. Results show that, for the porous film case, the internal mass transport (e.g. the diffusion in the porous layer) could be neglected for low BiD; and the external mass transport (e.g. the transport from the liquid phase to the film) is limiting for BiD/ShM > 0.1 (Fig. 2). This condition allows the determination of elemental kinetic constants under chemical control.