Effect of the addition of Au in zirconia and ceria supported Pd catalysts for the direct synthesis of hydrogen peroxide
Introduction
Hydrogen peroxide has always been considered a very interesting and environmentally friendly oxidant with applications confined mainly in unselective sectors such as the paper and textile industry and the treatment of waste waters. More recent applications in the chemical industry are related to the discovery of TS-1 molecular sieve and its ability to promote large scale selective oxidation processes such as the epoxidation of olefins, the hydroxylation of aromatics, and the synthesis of cyclohexanone oxime, a key intermediate in the production of nylon-6 [1]. These and other possible smaller scale applications of hydrogen peroxide as oxidant would greatly benefit from on-site, moderate scale production facilities that would avoid transport costs.
The increasing demand for hydrogen peroxide [2], [3] and the general need for greener oxidants is revamping the interest for a possible alternative to the auto oxidation of alkyl anthraquinone route, based on the direct combination of H2 and O2 [4]. The anthraquinone process, although used on a multi-million tonne scale annually, suffers from several limitations: (i) it produces a significant amount of organic waste due to the over-reduction of anthraquinone, (ii) it needs several costly separation and concentration steps, and (iii) it is economically feasible only on large scale plants. The synthesis of hydrogen peroxide by direct reaction of hydrogen and oxygen has long been an attractive alternative but has never found industrial application because of two major drawbacks. First, H2/O2 gas mixtures are explosive over a wide range of concentrations (4–94% H2 in O2), causing serious safety problems. Second, good yields and selectivities to hydrogen peroxide rather than water are very difficult to obtain, since the same catalysts used to produce H2O2 are also active for its decomposition and its hydrogenation to water as well as for water direct synthesis.
Several patents concerning the direct synthesis of H2O2 [5], [6], [7], [8], [9], [10], and a number of scientific works have been published in the open literature over the last decades [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Most patents and papers deal with explosive hydrogen/oxygen mixtures and high pressures, needed to improve H2O2 yield. However, operating commercially in the explosion region is extremely dangerous, thus more recent studies have concentrated on carrying out the reaction with diluted H2/O2 mixtures well away from the explosion regime [26], [27]. Until very recently, the catalysts used in these investigations were based predominantly on palladium. In the past few years, Hutchings and co-workers have reported that catalysts based on Au–Pd alloys supported on alumina [14], iron oxide [17], titanium oxide [18], or carbon [28] can significantly improve the rate of hydrogen peroxide formation when compared with the Pd only catalyst. However all these investigations were performed using moderately high pressures (37 bar).
The present paper reports the preparation and use of mono- and bimetallic palladium–gold catalysts for the direct synthesis of hydrogen peroxide under very mild conditions (20 °C and 1 bar) and outside the explosion range. Zirconia and ceria, both plain and sulfated, were chosen as supports. Since H2O2 is more stable under acidic conditions, a strongly acidic support can prove useful, in principle, for the direct synthesis of H2O2. It is known [29] that the surface acidity/basicity of zirconia can be controlled by addition of different dopants, SO2−4 being the most investigated one. On the contrary there are almost no studies regarding sulfated ceria [30]. The latter support is characterized by a high oxygen storage capacity (OSC) and reducibility [31] and it is chosen in this work because it has been previously shown [24], [25], [26] that partially oxidized Pd seems to play an important role in the reaction.
Section snippets
Materials
ZrOCl2 (Fluka), (NH4)2Ce(NO3)6 (Sigma–Aldrich), (NH4)2SO4 (Merck), were used for sample synthesis as received. All kinetic tests were performed in anhydrous methanol (SeccoSolv, Merck, ). 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 support was prepared by precipitation from ZrOCl2 at constant pH (pH 10), aged under reflux conditions [32],
N2 physisorption analyses
The use of mesoporous materials is very important in this investigation, since the presence of micropores could bear mass transfer problems, while a low surface area would not allow a good dispersion of the active phase. So N2 physisorption analyses were carried out in order to determine surface areas and pore size distributions of the supports.
The N2 physisorption isotherms for the zirconia calcined samples are shown in Fig. 1a and data are reported in Table 1. As can be seen, whether plain or
Conclusions
In this work mono- and bimetallic palladium–gold catalysts supported on zirconia and ceria both sulfated and non-sulfated were successfully tested for the direct synthesis of hydrogen peroxide under very mild conditions (1 bar and 20 °C) and outside the explosion range. Support sulfation was found to be beneficial for productivity especially in bimetallic samples. Although gold by itself is not active in catalysis and gold small particles are not evidenced by HRTEM it must be in close contact
Acknowledgments
We thank MIUR (Rome) for financial support through PRIN 2006 program.
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