Atomic-Scale Understanding of Catalyst Activation: Carboxylic Acid Solutions, but Not the Acid Itself, Increase the Reactivity of Anatase (001) Faceted Nanocatalysts

Abstract

Our ability to predict nanocatalyst reactivity has been hindered by our lack of atomic-scale understanding of nanocatalyst surface structure. Do nanocatalyst surfaces adopt a bulk-terminated structure or do they reconstruct to minimize their free energy, thereby lowering their reactivity as often observed in vacuum? Similarly, do nanocatalysts processed at high temperatures maintain their low reactivity, reconstructed surfaces when used at low temperatures? Using a new technique for the preparation of anatase nanocatalysts suitable for atomic-scale imaging and surface spectroscopy, we show that solution-prepared anatase is terminated by a monolayer of fluorine, which acts as an atomic-scale oleophobic coating, preventing the accumulation of adventitious carbon. We further show that the most common TiO2 functionalization chemistry, a carboxylic acid solution, causes the spontaneous reorganization of a reconstructed anatase nanocatalyst, leading to a five-fold increase in reactive sites. This reorganization is not observed when carboxylic acids are deposited from the gas phase, suggesting that model experiments in vacuum environments can lead to a nonequilibrium, kinetically trapped state that may not be catalytically relevant. Aqueous carboxylic acid solutions produce densely packed carboxylate monolayers with richer adsorption geometries than previously predicted. Ab initio simulations show that although the carboxylate termination is somewhat less effective at removing surface stress than the reconstruction, it is more effective in lowering the surface energy. This observation suggests that bulk-terminated metal-oxide nanocrystals may be common in reactive environments, even if high temperatures are used to process the nanocatalyst or if the reactant is later rinsed off. As such, the assumption of a bulk-terminated surface may be a reasonable starting point for “materials-by-design” approaches to computationally engineered nanocatalysts.

Shape-controlling effects of hydrohalic and carboxylic acids in TiO2 nanoparticle synthesis

“…III D, Table II). However, the adsorption structure at half coverage agrees well with the experimental and theoretical results from DeBenedetti et al 33…”

“…The difference of the findings may be due to the different functionals and computational settings used. On the other hand, in accordance with the results presented here, a combined experimental and theoretical study found that the reconstruction is not the thermodynamic ground state in benzoic acid and HF aqueous solutions and only formed at temperatures above 550 ○ C. 33 Furthermore, the aforementioned substitution of surface hydroxyl groups by halide ions also releases the tensile stress on the unreconstructed (001) surface. Since this eliminates the driving force for the reconstruction, the substitution process will also lift the reconstruction.…”

“…For the smaller acetic acid, this effect is less pronounced, leading to a stronger adsorption than for benzoic acid in this case. It has to be noted that for the (001) surface, full coverage with benzoic acid is not observed experimentally, 33 which can be attributed to the low adsorption energy and the corresponding higher surface energy (see Sec. III D, Table II).…”

Facet Dependent Catalytic Activities of Anatase TiO$_2$ for CO$_2$ Adsorption and Conversion

“…Furthermore, due to higher surface energy of (001) facet, it undergoes 1×4 reconstruction at ultrahigh vacuum and higher temperature [39][40][41][42][43] . The reactivities of the reconstructed (001)-(1×4) surface needs to be understood further for its practical implications [44][45][46][47][48] .…”

2D Surface Phase Diagram of a Multicomponent Perovskite Oxide: La$_{0.8}$Sr$_{0.2}$MnO$_3$(110)

“…The surface structure of metal oxides dictates their surface properties through the coordination, arrangement, and electronic structure of the topmost atoms [13][14][15][16][17][18][19][20]. Hence, it is important to map out the parameter space of the equilibrium surface phases of these materials, including their evolution with µ O .…”