Fast energy delocalization upon vibrational relaxation of a deuterated zeolite surface hydroxyl

Abstract

. (1995). Fast energy delocalization upon vibrational relaxation of a deuterated zeolite surface hydroxyl. Chemical Physics, 102(5), 2181-2185. DOI: 10.1063/1.468740 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. In this time-resolved study of vibrational dynamics of deuterated surface hydroxyls at acid sites in the zeolite Mordenite, we investigate the O-D T 1 vibrational lifetime and transient band shifts. It is shown that after infrared excitation of the stretching mode of a surface hydroxyl, the excess energy is rapidly distributed over delocalized low-energy lattice modes upon de-excitation. This is asserted from the observation that nonexcited hydroxyls are perturbed by the relaxation of their excited counterparts immediately after this relaxation. This observation can be made owing to better resolution in transient transmission spectroscopy obtained by deuteration of the surface hydroxyls. This assignment allows for accurate estimates of lattice temperatures after relaxation of the vibration. Further, from the observation that the vibrational lifetime is dependent on frequency ͑increasing from 25 to 70 ps with increasing frequency͒, it is concluded that the deuterated acidic protons are hydrogen bonded to lattice oxygen atoms in the zeolite.

Direct vibrational energy transfer in zeolites

“…At the O-D wavelength our laser pulses have a much smaller spectral width than at the O-H wavelengths, which results in a better signal-to-noise ratio and an improved frequency resolution 42 ͑see Table I for an overview of the experimental parameters and the results͒. Furthermore, we will show later that for the O-D experiments spectral diffusion is negligible ͑due to lower concentrations of oscillators and smaller transition dipole moments compared to the O-H vibrations͒.…”

“…44 Catalytically active sites in the porous zeolite framework are the hydroxyl groups, which are found near aluminum atoms in the crystalline alumino-silicate structure. The time-resolved infrared saturation experiments provided new information on the different hydroxyl groups and hydrogen bonding to the lattice, 37,38 on the deexcitation mechanism, 42 and on interactions with adsorbates. 43 Besides contributing to the insight in the molecular structure, these studies are the very first step in revealing the reaction dynamics on a molecular scale, since the coordinate of the hydroxyl stretch vibration is parallel with the reaction coordinate for the catalytic proton donation.…”

“…In this context the population dynamics of hydroxyl vibrations in zeolites have been studied in the last few years. [35][36][37][38][39][40][41][42][43] Acid zeolites are industrially important materials because they are widely exploited as catalysts. 44 Catalytically active sites in the porous zeolite framework are the hydroxyl groups, which are found near aluminum atoms in the crystalline alumino-silicate structure.…”

Dynamical studies of zeolitic protons and interactions with adsorbates by picosecond infrared spectroscopy

“…This is demonstrated by the transmission increase after vibrational relaxation, which means that OD... N 2 groups have been converted to bare OD groups with a different frequency. However, experiments with decreasing pump energy revealed that the desorption is indirect, caused by a rapid heating of the zeolite lattice due to the laser pulse [8,9]. Hence, in contrast to other hydrogen-bonded systems, direct dissociation of the hydrogen bond as a one-photon desorption process, does not occur upon relaxation of the vibrationally excited zeolite hydroxyl.…”

“…Knowledge of the interaction between BrOnsted hydroxyls and adsorbed species is therefore essential for a fundamental understanding of zeolite catalysis. Time-resolved infrared spectroscopy has proven to be a powerful tool in the investigation of the vibrational dynamics of the zeolite hydroxyl [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. This paper reports on a time-resolved infrared investigation of the effect of simple adsorbates on the dynamic properties of the catalytic BrOnsted hydroxyl.…”