Dissociative Water Adsorption by Al₃O₄⁺ in the Gas Phase

Abstract

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory (DFT) to study the adsorption of up to four water molecules on AlO. The infrared photodissociation spectra of [AlO(DO)] are measured in the O-D stretching (3000-2000 cm) as well as the fingerprint spectral region (1300-400 cm) and are assigned based on a comparison with simulated harmonic infrared spectra for global minimum-energy structures obtained with DFT. We find that dissociative water adsorption is favored in all cases. The unambiguous assignment of the vibrational spectra of these gas phase model systems allows identifying characteristic spectral regions for O-D and O-H stretching modes of terminal (μ) and bridging (μ) hydroxyl groups in aluminum oxide/water systems, which sheds new light on controversial assignments for solid AlO phases.

Dissociative Water Adsorption on Gas-Phase Titanium Dioxide Cluster Anions Probed with Infrared Photodissociation Spectroscopy

“…Henderson [19], on the other hand, studied the vibrational features of a hydrated TiO 2 surface with electron energy loss spectroscopy and saw only one feature in this region at 3690 cm − 1 . In this work, we report O-D stretches in a fairly narrow 2709-2776 cm − 1 range, which can be scaled by a factor of 1.36 [40,89] to yield comparable O-H stretching frequencies in the 3684-3775 cm − 1 range, in good agreement with the bulk surface assignments. In contrast to the previous study of water adsorption on Al 3 O 4 + [40], we see no major distinction in stretching frequencies between terminal and bridging hydroxyl groups; compare, for instance, the harmonic IR spectra of structures 4-1-a and 4-1-b in Fig.…”

“…[(TiO 2 ) n (D 2 O) m ] − clusters are prepared in a dual gas channel pulsed laser vaporization source [40], based on the fast flow cluster reactor pioneered by Smalley and coworkers [72] O) m ] − mass/charge ratio. Quadrupole mass spectra of (TiO 2 ) n − anions with and without D 2 O vapor added to the clustering channel are shown in Fig.…”

“…Clusters exhibit varied geometries and stoichiometries, with under-coordinated atoms and dangling moieties that mimic reactive point defects on a bulk surface [35,36]. Size-selected clusters can be complexed with a well-defined number of substrate molecules, allowing for the systematic characterization of how cluster size, stoichiometry, and local structure affect reactivity [37][38][39][40]. These systems are also tractable for simulation with high level quantum chemistry calculations, allowing for analysis of geometries and reaction mechanisms with clarity not available in bulk studies [41].…”

“…Vibrational action spectroscopy has proved to be an excellent method to characterize the structure of gas-phase ions [64,65], and has been particularly illuminating for the study of transition metal oxide clusters, which demonstrate complex electronic structure and many low-lying structural isomers [13,66,67]. Most recently, this technique has been adapted to illuminate site-specific water dissociation on Al 3 O 4 + [40]. Here, we present infrared photodissociation (IRPD) spectra of messenger-tagged titanium dioxide-water cluster anions [(TiO 2 ) n (D 2 O) m ] − with n = 2-4 and m = 1-3 in the fingerprint (400-1200 cm − 1 ) and O-D stretching (2600-3000 cm − 1 ) spectral regions.…”

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

“…Different types of model compounds have proven very effective in reducing complexity in order to gain atomistic understanding. These include thin films studied with surface science techniques, gas phase clusters as well as inorganic compounds with well characterized structures . An example for the latter is the molecular compound [Al 3 (μ 2 ‐OH) 3 (THF) 3 (PhSi(OSiPh2O) 3 ) 2 ], 1 , (see Figure ) featuring a cyclic arrangement of three Al IV ‐O(H)‐Al IV units surrounded by a siloxide framework; 1 may serve as a model to investigate the formation of EFA species on zeolite walls during dealumination, a ubiquitous process in zeolite chemistry …”