A general strategy for the multivariate curve resolution (MCR-ALS) of signals from modulation excitation experiments

MIRTA RAQUEL ALCARAZ; MARÍA JULIA CULZONI; ALEJO AGUIRRE; SEBASTIÁN E. COLLINS; HÉCTOR GOICOECHEA

Estepona

Conferencia; 6th International Conference on Operando Spectroscopy; 2018

Varios organizadores

Rigorous Operando experimentation requires the simultaneous measurement of conversion and selectivity alongside with the spectroscopy data of the catalyst under steady state condition. However, when investigating reaction mechanisms under steady state, it is almost impossible to discriminate between signals from spectators/solvent/supports and those of true reaction intermediates. Thus, transient analytical methods can be exploited to facilitate this task. Particularly, transitory experiments subjected to periodical perturbations around a steady state, e.g. SSITKA and modulation excitation spectroscopy (MES), have been developed and increasingly used. The periodic perturbation (modulation) of an external parameter is employed to stimulate the system around a steady state while time-resolved spectra are collected. MES experiments, in combination with phase sensitive detection (PSD), allow the sensitive and selective detection of reaction intermediates [1]. The quantitative determination of the retardation (phase lag) of each intermediate contains kinetic information. However, frequently, signals after PDS could be convoluted and spectral resolution is needed. Multivariate curve resolution-alternating least squares (MCR-ALS) is a widespread iterative soft-modelling method that allows the bilinear decomposition of a data matrix into two submatrices containing chemically meaningful information for the pure compounds involved in the system [2,3], i.e. their pure spectral profiles and their varying abundance throughout the process. In this work, MCR-ALS is applied to model MES-PSD data. To develop the best strategy, synthetic spectra (including noise) were modelled using an A → B → C system. The implemented strategy consists in conducting successive MCR-ALS analysis to discriminate the individual contributions of each component in terms of both kinetics (concentrations as a function of the phase angles) and characteristic spectral bands. Besides, the selectivity constrain in the spectral mode was implemented to cope with the system ambiguity. In spite of the data complexity, the proposed resolution is simple, fast and does not involve data pre-treatment steps. Finally, this methodology was validated resolving the adsorption-desorption kinetics of oxalic acid on TiO2 monitored by ATR-FTIR spectroscopy.