Insights into the Ligand Shell, Coordination Mode, and Reactivity of Carboxylic Acid Capped Metal Oxide Nanocrystals

Abstract

A detailed knowledge of surface chemistry is necessary to bridge the gap between nanocrystal synthesis and applications. Although it has been proposed that carboxylic acids bind to metal oxides in a dissociative NC(X)2 binding motif, this surface chemistry was inferred from indirect evidence on HfO2 nanocrystals (NCs). Here, a more detailed picture of the coordination mode of carboxylate ligands on HfO2 and ZrO2 NC surfaces is shown by direct observation through solid‐state NMR techniques. Surface‐adsorbed protons are clearly distinguished and two coordination modes of the carboxylic acid are noted: chelating and bridging. It is also found that secondary ligands penetrate the ligand shell and have the same orientation with respect to the surface as the primary ligands, indicating that the ionic or hydrogen‐bonding interactions with the surface are more important than the van der Waals interactions with neighboring ligands. During ligand exchange with amines, the chelating carboxylate is removed preferentially. Finally, it is shown that the HfO2 and ZrO2 NCs catalyze imine formation from acetone and oleylamine. Together with the previously reported catalytic activity of HfO2, these results put colloidal metal oxide nanocrystals squarely in the focus of catalysis research.

Kinetics of Valeric Acid Ketonization and Ketenization in Catalytic Pyrolysis on Nanosized SiO2, γ‐Al2O3, CeO2/SiO2, Al2O3/SiO2 and TiO2/SiO2

“…The presence of these bands in the spectra of acetic acid on the surface of CeO 2 and Al 2 O 3 is confirmed by the work of other authors . A recent report has also confirmed the existence of the weakly bound acids on the surface of zirconium and hafnium oxides by using NMR techniques …”

Stabilization of Colloidal Ti, Zr, and Hf Oxide Nanocrystals by Protonated Tri-n-octylphosphine Oxide (TOPO) and Its Decomposition Products

“…A resonance at a similar chemical shift was previously assigned to a hydroxyl group on the surface of ZrO2. 60,62 In contrast, the pyrophosphonate signal has little cross correlation with the 1 H NMR spectrum.…”

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“…For a more exhaustive overview of the chemical principles behind surface ligation and ligand exchange reactions as well as for a detailed discussion of the different binding motifs, the reader is referred to more dedicated review articles …”