ATR-FTIR Quantitative Spectrokinetic Analysis at Liquid/Solid Interface

ALEJO AGUIRRE; CLAUDIO, BERLI; SEBASTIAN COLLINS

Deuville

Congreso; 5th International Congress on Operando Spectroscopy; 2015

Attenuated total reflection (ATR) infrared (IR) spectroscopy is apowerful tool to investigate reaction pathways inliquid(reactive)/solid(catalyst) systems, because it provides the detection of adsorbedspecies on a catalyst under reaction conditions [1]. Catalysts are commonlydeposited on an internal reflection elements (IRE) as layers of powders or asfilms (e.g. metal film), and they are exposed to the liquid phase reactants. Inorder to study the evolution of adsorbed intermediates, transient techniquesare usually applied, such as stop-flow and concentration-modulation excitationspectroscopy (c-MES) [1-2]. However, to obtain quantitative information, thatis, determining intrinsic reaction rates, the chemical engineering aspects ofan ATR flow-through cell must be developed. Particularly, mass transport in theATR cell has to be characterized. We present here an optimized flow-through ATRmicrofluidic cell to assess intrinsic kinetic parameters of reactions underchemical control. This ATR cell possesses no dead-volume and has a uniformfluid velocity profile across the cell. The mass transport dynamic from thebulk solution to the surface of the ATR crystal -where the catalyst isdeposited- is described by a convection-diffusion transport model [3]. Themass-transfer limitation in the ATR cell is investigated comparatively usingthe simplest case of adsorption/desorption in both a non-porous and a porouscatalyst layer (Fig. 1). A Pt thin film deposited on a ZnSe IRE by vapordeposition and the layer of Pt(0.38%)/Al2O3 was preparedby evaporation of a dispersion of the catalyst. The operational limit of thecell is characterized by means of a non-dimensional analysis in terms of Péclet (Pe),diffusive Biot (Bi) and Sherwood numbers (Sh) for the porous film case. Suitable criteria todetermine kinetic parameters under chemical control were develop considering thegeometry and experimental flow rates. Results show that, for the porous filmcase, 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 transportfrom the liquid phase to the film) is limiting for BiD/ShM > 0.1 (Fig. 2). This conditionallows the determination of elemental kinetic constants under chemical control.