High-Sensitivity Gas-Phase Raman Spectroscopy for Time-Resolved In Situ Analysis of Isotope Scrambling over Platinum Nanocatalysts

Abstract

In this study, we present a novel approach for time-resolved, in situ analysis of isotope scrambling reactions over platinum nanoparticle catalysts using high-sensitivity gas-phase Raman spectroscopy. A recently developed spectrometer setup enables detection limits in the hundreds of ppm, a dynamic range spanning four orders of magnitude in mole fraction, and a temporal resolution of one second. Experiments were performed by introducing D2 gas to an H2-activated Pt nanoparticle catalyst in a closed sample, resulting in the formation of gaseous HD and H2. The time-resolved gas-phase mole fraction profiles show HD as the dominant product and only minor formation of H2. This observation is consistent with a predominantly associative exchange mechanism, in which D2 reacts directly with surface-bound hydrogen to produce HD. A superimposed exchange involving trace water vapor was also observed, with stepwise conversion of H2O to HDO and D2O via surface-mediated reactions. Mole fractions were quantified using a spectral fitting routine based on simulated Raman spectra derived from literature polarizabilities and energy levels. The reaction quotient of the hydrogen isotopologues converged over time toward literature values of the equilibrium constant, and measurements at defined H2/D2 ratios confirmed relative accuracies better than 2%. This Raman-based quantification method enables simultaneous, in situ detection of all relevant species with high accuracy and is ideally suited for studying transient, catalytic processes.