Elsevier

Journal of Catalysis

Volume 275, Issue 1, 30 September 2010, Pages 108-116
Journal of Catalysis

Au on MgAl2O4 spinels: The effect of support surface properties in glycerol oxidation

https://doi.org/10.1016/j.jcat.2010.07.022Get rights and content

Abstract

Here, we investigated the properties of Au nanoparticles prepared via three different techniques and supported on three different MgAl2O4 spinels. After careful characterization of bare and gold-loaded supports (XPS, BET, XRD, STEM) and catalytic test for the selective oxidation of glycerol, we concluded that the surface composition and area of the spinel play an important role in determining the selectivity of the catalyst as well as gold particle size. When supported on surface characterized by a similar Al/Mg ratio, gold clusters selectivity is not mediated by particle dimension. For example, large gold particles on MgAl2O4, which typically produce high selectivity to glycerate when supported on aluminum-rich surfaces instead, enhance the C–C bond cleavage reaction. Accordingly, the selectivity of similarly sized AuNPs on MgAl2O4 spinels with the same surface Al/Mg ratio is similar but we demonstrate that the activity depends on gold surface exposure (at.% Au by XPS) and on support surface area.

Graphical abstract

The catalytic activity of AuNPs on spinels is dependent on Au size and accessibility by the reactant, whereas selectivity is ruled out by the Mg/Al ratio at the surface.

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Introduction

Gold nanoparticles (AuNPs) have attracted attention since ancient times for their beautiful color, but recently there has been increasing interest in gold NPs for different applications, in particular as catalysts [1], [2], [3]. Indeed, supported or non-supported AuNPs are catalytically active in many reactions, including CO oxidation, water–gas shift reaction, propylene epoxidation and acetylene hydrochlorination [4], [5], [6]. Arguably, one of the potential applications of AuNPs is the catalytic oxidation of alcohols to carbonyl compounds or carboxylic acids, which are valuable organic synthesis precursors [7], [8], [9], [10], [11]. While colloidal or colloid-like systems have shown catalytic activity for the liquid-phase oxidation of alcohols [12], [13], [14], [15], supported AuNPs are far more investigated for alcohol oxidations as they are easier to handle and recover. Glycerol is an intriguing starting reagent since it is a major co-product of biodiesel production. Its conversion into valuable products is of upmost importance for biomass valorization and it can be considered a polyfunctional material, which is very useful as a feedstock for fine chemical synthesis. Glycerol transformations have been well studied, with the oxidative transformation one of the most investigated [16], [17], [18]. Catalysts normally suffer from deactivation mainly ascribed to the irreversible adsorption of reaction products that, especially in the case of glycerol, have strong chelating properties (Scheme 1) due to the co-presence of OH and COOH functional groups. The selective oxidation of glycerol is normally carried out in aqueous solution under mild conditions (50–60 °C, 3–10 bar O2) and in the presence of a base that prolongs the catalyst life [7], [9], [11], [19], [20], [21], [22], [23], [24], [25], [26]. Recently, gold-catalyzed selective glycerol oxidation has been performed with H2O2 as the oxidant, resulting in a different set of products’ distributions (glycolic acid as main product) [27] (Scheme 1).

A suitable support material is crucial for catalytic activity as well as for catalyst stability. The brutal conditions present in liquid-phase reactions lead to more stringent requirements for the choice of support material and catalyst chemistry to maintain activity and selectivity. While one of the main roles of the support is to avoid coalescence and agglomeration of the AuNPs by reducing their mobility, metal-support interaction can also play an important role in the reaction mechanism. Nanoparticles supported on reducible oxides such as TiO2 and CeO2, where the transition metal ion exists in two different oxidation states and vacancy chemistry plays a role in catalytic mechanisms, are classic examples of support-induced properties of the catalyst [2], [10].

The MgAl2O4 spinel is a widely used refractory material due to its high melting temperature (2135 °C) and thermal stability [28]. These properties combined with its hydrothermal stability make it a promising catalyst support for applications such as environmental catalysis and fine chemical production [29]. For catalytic purposes, high surface areas are often desirable. Fortunately over the last few years, spinels have been successfully prepared by different methods, allowing one to obtain materials with tunable surface areas and chemical compositions. Au/MgAl2O4 appeared promising in gas-phase CO to CO2 oxidation and in aqueous-phase ethanol oxidation, providing a simple and green route to acetic acid or ethyl acetate [30], [31], [32]. Due to the properties of MgAl2O4 described above, the application of MgAl2O4 as a catalyst support has been explored.

Here, we investigated the structure and properties of Au nanoparticles, prepared via three different techniques, and supported on MgAl2O4 spinels that, due to their different properties, could be used to tune the evolution of the glycerol selective oxidation in terms of both activity and selectivity.

Section snippets

Materials

Gold of 99.99% purity in sponge from Fluka was used as a gold source for the preparation of HAuCl4 for gold sols. Tetrakis(hydroxymethyl)phosphonium chloride (THPC, 80% solution) from Aldrich was used. NaOH and urea (purity >99%) were from Fluka. Gaseous oxygen from SIAD was 99.99% pure. Glycerol (86–88% solution) from Aldrich was used. Samples of potential reaction products from the oxidation of glycerol were obtained from Fluka and used as standard reference samples for product analysis.

Support materials

MgAl2O

Support characterization

XRD spectra of the samples are reported in Fig. 1. All the three supports exhibited a spinel-type MgAl2O4 crystalline phase, though crystallite sizes clearly differ as indicated by the width of the peaks. The commercial material has extra diffraction peaks from a secondary oxide phase(s) and is not phase pure. Using the Scherrer formula, crystal size at 2θ = 37° was estimated to be ≫30 nm for the commercial support, 3.5 nm for the spinel obtained by coprecipitation, and 18.5 nm for the material

Conclusions

Gold nanoparticles supported on three spinel-type MgAl2O4 supports prepared using different routes have been explored in the selective liquid-phase oxidation of glycerol. The increasing of Al/Mg surface ratio by XPS after wet gold deposition revealed that gold selectively deposited onto the more basic Mg sites. On the contrary, the Al/Mg ratio was almost the same as before gold deposition in magnetron sputtered gold catalysts showing that in these cases no preferential adsorption of the gold to

Acknowledgments

Authors gratefully acknowledge Fondazione Cariplo for financial support. Microscopy studies at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences were sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. This research (GMV, KvB) was sponsored by the Materials Sciences and Engineering Division, US Department of Energy under contract with UT-Battelle, LLC.

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    Previously at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States.

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