Quantitative spectrokinetic analysis of gas/solid reactions using a micro-reactor with simultaneous detection by DRIFT and mass spectrometry

ALEJO AGUIRRE; SEBASTIÁN E. COLLINS

Estepona

Conferencia; 6th International Conference on Operando Spectroscopy; 2018

Varios organizadores

Understanding of reaction mechanisms and structure-activity relationships under working ?Operando- conditions is of central importance to perform a rational design of new and optimized catalytic systems. Diffuse reflectance infrared spectroscopy -DRIFT- is a powerful technique to monitor the reaction intermediates at the gas(reactant)/solid(catalyst) interfase. However, the most popular design of DRIFT cells features a (heated) cup where the catalyst powder is placed and the gas feed is, in the best situation, forced to flow though the catalyst bed. This design has some problems that can hamper the correct correlation between intermediates (IR signals) and gas phase concentration (e.g. CG or MS signals): i) temperature gradient in the catalyst bed, ii) IR "sensing" at the top/beginning of the catalyst bed, iii) possibility of by-pass of the gases, iv) large difference of temperature between the catalytic bed and the ?cup?, (v) large volume to exchange in transient studies. In this work, the design, modelling and use of a DRIFT cell that features a plug-flow micro-reactor, coupled with mass spectrometry to perform Operando and transient studies in a quantitative manner is described. A scheme of the cell/microreactor is shown in Fig. 1. The fluid dynamic and reaction limits of the cell were theoretically and experimentally investigated. It is demonstrated that this cell resembles a plug-flow reactor, without temperature gradients inside the catalytic bed. An analysis of the operational limits by calculating the mass and energy transfer coefficients inter- and intraparticle is presented. This cell was experimentally validated to obtain kinetic parameters of reactions under true chemical control conditions during the oxidation of CO on a Au/CeO2 catalyst. TOF and apparent activation energy were identical to the measured using a conventional reactor. Transient and excitation modulation spectroscopy (MES) experiments were also carried out. The simultaneous detection of gas phase concentrations by MS and the intensity of the IR signals allowed the quantification of surface species (Fig. 2): i) active in the reaction Au0-CO (2110 cm-1), Au+-CO (2125 cm-1), ii) carbonate adsorbed on the ceria support (1700-1200 cm-1). Moreover, the amount of adsorbed oxygen could also be quantified. Kinetic constants of CO adsorption and oxidation were measured.