ArticleThe effects of TiO2 crystal-plane-dependent Ir-TiOx interactions on the selective hydrogenation of crotonaldehyde over Ir/TiO2 catalysts
Graphical Abstract
Ir-TiOx interactions related to the crystal plane of the TiO2 support could influence the crucial properties of the catalysts, which consequently alters their reaction behavior during crotonaldehyde hydrogenation on the Ir-TiOx interface.
Introduction
The selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is an important process in the fine chemicals, pharmaceutical, and fragrance industries [1, 2]. Crotonaldehyde (CRAL) is a typical α,β-unsaturated aldehyde; its hydrogenation involves several reaction pathways which result in different products (Scheme 1). Achieving high selectivity to the desired unsaturated alcohol (crotyl alcohol, CROL) on commonly employed noble metal catalysts is rather difficult, because the hydrogenation of the C=C bond to form butanal (BUAL) is both thermodynamically and kinetically favored [3, 4, 5]. Over the past few decades, great efforts have been made to develop efficient catalysts for the selective hydrogenation of CRAL. Generally, noble metals supported on reducible oxides (i.e., TiO2 [6, 7, 8], CeO2 [9, 10, 11], ZrO2 [12], Co3O4 [13], and ZnO [14]) are more active and selective for the reaction than those supported on non-reducible oxides (i.e., SiO2). Although Pt- [8, 10, 11, 12, 13, 14, 15] and Au-based catalysts have attracted the most attention [9, 16], Ir-based catalysts are more selective for the hydrogenation of CRAL [7, 17, 18, 19, 20] and other α,β-unsaturated aldehydes [21], while supported Ir/TiO2 catalysts are appealing in the hydrogenation of a variety of carbonyl compounds [7, 22, 23, 24, 25]. It has also been established that the metal-support interfacial sites act as the active structures during the hydrogenation of CRAL [6, 7], which could influence the catalyst structure and catalytic performance [26, 27].
As the catalysts used in the selective hydrogenation of CRAL are usually subjected to a pre-reduction procedure, the reduction temperature could have a significant effect on the resulting catalytic performance because the metal-support interactions could be altered upon the pre-treatment [27]. A higher reduction temperature (i.e., 500 °C) is more beneficial to the catalytic performance of Ir catalysts in CRAL hydrogenation compared to a lower reduction temperature (i.e., 200 °C), which can be attributed to surface decoration of the metal component on the reducible oxide supports via strong metal-support interactions (SMSI) [6, 7]. However, the complexity of the support oxide surface structure (such as multiple exposed crystal planes) makes the investigation of such SMSI extremely challenging. Since the crystal planes of the support have different morphological and electronic structures, it is meaningful to study the crystal-plane effect of the support on the metal-support interactions. Although there are some reports regarding the TiO2 crystal-plane-dependent metal-support interactions [28, 29], the TiO2 crystal-plane-dependent Ir-TiOx interactions and their effect on the hydrogenation of CRAL has not yet been reported; this could be owing to the difficulty associated with synthesizing TiO2 nanocrystals with specifically exposed crystal planes in the past.
In contrast, it has been reported that the surface Lewis acid sites are key factors in the CRAL hydrogenation reaction as the surface Lewis acid sites could enhance the adsorption strength of the C=O bond, thus resulting in promotion of the selectivity to crotyl alcohol. These Lewis acid sites could be generated by the combination of a reducible oxide support, such as TiO2, and a chlorine-containing metal precursor, such as H2IrCl6 [30, 31]. Recently, we have reported that the oxygen vacancies in TiO2 serve as surface acid sites, which provides sites for the adsorption of C=O groups, thus is responsible for the observed activity and selectivity of the catalyst [32]. However, the effect of crystal-plane-dependent surface acidity as a function of the pre-treatment on the crotonaldehyde hydrogenation is still not clear.
In the current study, TiO2 nanocrystals, with different exposed crystal planes ({101}, {100}, and {001}), were prepared as supports for Ir nanoparticles (NPs) and the subsequent catalysts subjected to various pre-treatments (reduction in a H2 atmosphere at different temperatures and O2 re-oxidation) to tune the metal-support interactions and investigate in detail their effects on the adsorption and selective hydrogenation of CRAL. The results suggest that the chemical properties of the catalysts can be remarkably tuned by the pre-treatment, which consequently changes the catalytic behavior.
Section snippets
Synthesis of anatase TiO2-{101}, TiO2-{100}, and TiO2-{001} nanocrystals
Anatase TiO2-{101} and TiO2-{100} nanocrystals (NCs) were synthesized following the procedures reported by Chen et al. [33] and Liu et al. [28, 34]. Anatase TiO2-{001} was synthesized following a hydrothermal procedure reported by Han et al. [35]. To prepare the TiO2-{101} and TiO2-{100} nanocrystals (NCs), a TiCl4 (6.6 mL) solution was added dropwise to a 0.43 mol l-1 HCl aqueous solution (20 mL) at 0 °C. After stirring for an additional 30 min, the solution was added dropwise to a 5.5 wt% NH3
Catalyst characterization
Fig. S1 shows the TEM and HRTEM images of the as-synthesized TiO2-{101}, TiO2-{100}, and TiO2-{001} NCs. The morphologies of these nanocrystals are quite uniform, but their crystallite sizes differ significantly. Measurements of the lattice fringes further confirm the presence of anatase TiO2. According to a previously proposed procedure [33, 34] and based on the TEM and HRTEM images, the dominant crystal facets of the crystals are identified as the {100}, {101,} and {001} facets, and the
Conclusions
In this study, we investigated the effects of the Ir-TiO2 interactions on the crotonaldehyde hydrogenation over Ir/TiO2 catalysts. These interactions could be adjusted by thermal treatments (i.e., reduction at different temperatures and subsequent re-oxidation), which alter the nature of the Ir species (morphologies and electronic properties) as well as the surface acidity. The interactions are also closely related to the crystal plane of TiO2, which further explains the observed reaction
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Boosting chiral carboxylic acid hydrogenation by tuning metal-MO<inf>x</inf>-support interaction in Pt-ReO<inf>x</inf>/TiO<inf>2</inf> catalysts
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