Influence of the preparation method on the morphological and composition properties of Pd–Au/ZrO2 catalysts and their effect on the direct synthesis of hydrogen peroxide from hydrogen and oxygen
Graphical abstract
The preparation method of Pd–Au/zirconia catalysts strongly influences H2O2 direct synthesis. Au changes the chemical composition of the metallic particles, their morphology, and charge of the exposed Pd sites.
Introduction
The direct reaction of H2 + O2 → H2O2 is clearly the most atom-efficient method to form hydrogen peroxide, but none of the presently available processes has solved the productivity vs. safety dilemma. In fact, the major problem of the direct route to hydrogen peroxide is the poor selectivity to H2O2 vs. H2O that can be achieved with known catalysts. As shown in Fig. 1, the process also involves the thermodynamically highly favoured but undesirable parallel and consecutive water-forming reactions. Another serious problem that has limited the implementation for this process is the significant risk of handling the explosive hydrogen/oxygen gas mixture over an active catalyst. Therefore, despite several patents [1], [2], [3], [4], [5], [6], [7] and recent literature [8], [9], [10], [11], [12], the direct synthesis of hydrogen peroxide has not yet found the way to commercialization.
For a long time it has been known that alloying or combination of two metals can lead to materials with special chemical properties due to an interplay of “ensemble” and “electronic” effects [13]. In particular for the direct synthesis H2O2, from its elements, for which it is generally agreed that palladium is the most effective metal [8], [9], [14], [15], [16], it has been reported that the combination of Pd with Au [17], [18], [19], Ir [20], Ag [21], and Pt [17], [21], [22], [23] improves the catalytic performance with respect to monometallic palladium samples. As far as gold is concerned, since the pioneering work of Haruta et al. [24], gold nanoparticles supported on metal oxides are known to be active in some important industrial reactions [25], [26], [27], [28], the present reaction being not included. The reasons for the activity of small gold particles are still a matter of debate. It has been shown that the catalytic activity of gold critically depends on the preparation method, on the support type, and on the pre-treatment procedure. The most widely accepted explanation for the variability of gold catalytic properties focuses on the size of gold particles and on the amount of low coordination sites of gold. In addition, other factors have been considered in the literature: metal-support interface, and charge transfer from the support or metal cationic sites [29]. Several methods have been tested for making suitably small gold particles: the deposition–precipitation (DP) method has been qualified as the best so far. In this method [30] the pH of a solution of HAuCl4 is raised by the addition of a base to the point where the adsorption of the species in solution can react with or be deposited on the support.
We have already shown [31] that zirconia is a good support for Pd-based catalysts for the direct synthesis of hydrogen peroxide and that it is possible to prepare highly dispersed gold on zirconia by DP [32]. We have also recently demonstrated [33], [34] for both plain and sulfated zirconia and ceria supports that while the monometallic gold catalysts are inactive under mild experimental conditions, the addition of a 1:1 amount of gold to a monometallic Pd sample improves the productivity and especially the selectivity of the process. In these samples gold must be in close contact with Pd, as its presence profoundly changes both Pd dispersion and its morphology and charge. However, no evidence for small Au particles was found, probably because of the preparation conditions. In the present work, we wish to report the preparation by different methods of a series of Pd–Au samples supported on zirconia in order to fully understand the nature of palladium–gold interactions. These catalysts were also tested for the direct synthesis of hydrogen peroxide both under very mild conditions (1 bar and 20 °C and outside the explosion range) and at higher pressure (10 bar) using solvents expanded with CO2 in order to increase productivity further.
Section snippets
Materials
ZrOCl2 (Fluka) was used as received for sample synthesis. All kinetic tests were performed in anhydrous methanol (SeccoSolv, Merck, [H2O] < 0.005%). Commercial standard solutions of Na2S2O3 (Fixanal [0.01], Hydranal-solvent E, and Hydranal-titrant 2E, all from Riedel-de Haen) were used for iodometric and Karl–Fischer titrations.
Catalyst preparation
Zirconia was prepared by precipitation from ZrOCl2 at pH 10, aged under reflux conditions for 20 h [35], [36], washed free from chloride (AgNO3 test), and dried at 383 K
Catalyst morphology and analytical properties
N2 physisorption analysis has been carried out in order to determine the surface area and pore size distribution of the zirconia prepared as a support. In this investigation, the choice of a mesoporous material as a support is very important, since the presence of micropores could cause mass transfer problems, while a low surface area would not allow a good dispersion of the Pd and Au active phases. The N2 physisorption isotherm for the calcined zirconia sample is shown in Fig. 2, with its BJH
Conclusions
In this work the influence of the preparation method of bimetallic Pd–Au on zirconia catalysts for the direct synthesis of hydrogen peroxide has been evidenced. It is quite clear from the reactivity data that the order and the method by which the two metals are deposited on the support are critical for obtaining a high activity and a high selectivity in H2O2 formation. It has been proved that depositing firstly Au by DP and subsequently Pd by IW is a method that allows to obtain catalysts with
Acknowledgment
We thank MIUR (Rome) for financial support.
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