Catalysts Prepared by Microemulsion

1 Introduction. - During the past few years, a number of papers dealing with the preparation of metallic particles from microemulsions have appeared. For example Pd, Ni, Pt, Ag and bimetallic Au/Pd were successfully prepared by this method . For this reason, it is of interest to report the research work done on this subject and its input on catalysis research. 

In this section we will focus on the description of several aspects relevant to the preparation of both catalytically active metal particles and metal-supported catalysts via the microemulsion technique. Regarding the metal supported catalysts, in some cases both the metallic particles and the support were synthesized by microemulsions. However, in general metallic particles prepared from microemulsions were deposited on commercial supports. The catalytic behaviour of these microemulsion-derived materials will be commented and, when possible, compared to catalysts obtained from traditional techniques under similar reaction conditions. Selected results concerning the study of the strong metal-support interaction effect (SMSI) obtained with catalysts prepared by microemulsion will be detailed . Several papers dealing with the preparation of immobilized metal particles on supports have been described although the catalytic behaviour of the solids was not studied. However, their potential catalytic ability led us to include those papers within this chapter. 

the preparation and catalytic activity of some non-supported metal catalysts prepared from microemulsions will be described. 

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Martinez-Arias, A., Fernandez-Garcia, M., Iglesias-Juez, A. et al. (2001) New Pd/CexZrj x02/Al203 three-way catalysts prepared by microemulsion Part 2. In situ analysis of CO oxidation and NO reduction under stoichiometric CO + NO + 02, Appl. Catal. B Environ., 31, 51. 

Agrell, J., Germani, G., Jaras, S.G., and Boutonnet, M. Production of hydrogen by partial oxidation of methanol over ZnO-supported palladium catalysts prepared by microemulsion technique. Applied Catalysis. A, General, 2003, 242 (2), 233. 

A study of the metal-support interaction effect has been carried out for a Pt-Ti02 catalyst prepared by microemulsion. Isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane were chosen as model reactions for studying the influence of the catalyst nature on the strong metal-support interaction (SMSI) effect. A comparison between the cata-lyticbehaviour of similar catalysts prepared by microemulsion and incipient wetness method,was reported. The catalysts were reduced at 200 and 390 °C since it is well known that the reduction temperature plays a predominant role in the SMSI effect l The behaviour of thePt-TiO microemulsion-prepared catalyst was found to be similar to that of a Pt/AbOs catalyst. Also, it was found that the microemulsion-prepared catalysts require a higher temperature of reduction to induce metal-support interactions. 

Obviously, it is necessary to remove the carbon residues from the catalysts prepared by microemulsion. This is usually achieved by submitting the catalysts to a controlled calcination step. Since this is mostly done under air, a subsequent reduction step is needed to obtain the reduced metal. Hayashi et al. studied the amount of carbon left on Fe/Si02 catalysts, prepared by microemulsion, after submitting the samples to calcination in air during 5 hours at different temperatures. At temperatures above 500 °C the amount of carbon in such catalysts was similar to the amount of carbon detected in a Fe/SiOa catalyst prepared following a conventional impregnation procedure after calcination under the same conditions. 

Microemulsion vs Traditional Techniques. - The performance of catalysts prepared by microemulsion has been studied in heterogeneous catalytic processes. Here we will describe some examples in which catalysts prepared by microemulsion display a performance different from that of catalysts prepared by traditional synthesis routes. 

Figure 5 TEM photographs of Rh-Si02 catalysts prepared by microemulsion and impregnation methods, adapted from reference 96...

Table 8 Metal-based catalysts prepared by microemulsion...

Tables 8 and 9 summarize the most relevant results obtained with catalysts prepared by microemulsion as found in the literature and patents, respectively.

Ritzkopf I, et al. Decreased CO production in methanol steam reforming over Cu/Zr02 catalysts prepared by the microemulsion technique. Appl Catal A Gen. 2006 302(2) 215-23. 

Figure 2.7. TEM images of (a) Ni/Si02 catalyst prepared by impregnation and (b) Si02-coated Ni catalyst prepared by water-in-oil microemulsion methods. Adapted from Takenaka et al.112...

Self-assembly of molecules and nanoparticles to build well-defined structures, constitutes another approach to make model catalysts [33,34]. Here, nano-structured surfaces are made from nanoscale building blocks that are synthesized from atoms and molecules by chemical means. There has been a tremendous development in this field during the past decade, which includes a number of different strategies, including microemulsions [33], (micellar) block copolymers [35,36], and template CVD growth [37]. Relatively little work has, however, so far been directed toward heterogeneous catalysis in the sense described in this chapter, i.e., to make supported catalysts [38]. There are many reports on preparations but relatively much fewer on evaluations of catal3dic activity, trends, or reactivity versus particle size, etc. A main issue for model catalysts prepared by self-assembly is whether they maintain the well-defined character after, e.g., template removal and calcinations and other pretreatment steps, before they can be used as model catalysts. 

Platinum catalysts were prepared by impregnation of formed supports with reverse microemulsions (water-in-oil) or water containing chloroplatinic acid. Microemulsion catalysts were more active in combustion of toluene in toluene-air mixtures than those prepared classically from water solutions. The size of platinum in classically impregnated catalysts was three times higher than that of the catalysts prepared from microemulsions. In the case of microemulsion preparation method, platinum is located near the pellet surface or its position in the pellet can be optimised. The viscosity of the microemulsion affects the concentration profile of platinum in the catalysts. 

This contribution describes some physical properties of reverse w/o microemulsions, physical-chemical properties of platinum catalysts prepared by classical impregnation from water solutions of HaPtCle and from reverse w/o microemulsions comprising chloroplatinic acid and their activity in combustion of volatile organic compounds (VOC). In our study we concentrated on TweenSO (polyoxlethylen(20)sorbitan monoleate), as the surfactant does not comprise any potentially harmful ions (like sodium in AOT or bromide in CTAB) that could affect the activity of platinum in catalytic combustion. 

Fig. 4 Histograms of platinum particles determined by TEM in the catalysts prepared from microemulsions (0.1 % Pt)...

We will describe the current state-of-the-art of the microemulsion method for the preparation of metal-based catalysts. First, some general considerations concerning the nature of a microemulsion and its relation to the preparation of particles will be given. Then, both the preparation of solid oxides and metal-supported catalysts by microemulsion will be detailed. When possible, the properties of the solids prepared by microemulsion will be compared with those of their counterparts prepared by traditional techniques. Particular attention will be paid to the description of the catalytic properties of these solids. There is a large body of work in the field of organic synthesis, and enzyme catalysis in which microemulsion techniques play an important role. However this topic is not included in this paper, for that purpose several reviews are available, see for example those by Holmberg and Lawrence Rees... 

The preparation of sulphated zirconia designed for catalyst supports was studied by Boutonnet et al. . Zirconia prepared in microemulsion showed a pure tetragonal structure compared with zirconia prepared by an impregnation -precipitation procedure which also contained monoclinic phase. Platinum-promoted sulphated zirconia catalysts were prepared both in anionic and non-ionic microemulsions. Furthermore, the catalytic activity and selectivity for the isomerization of hexanes were tested. The catalysts produced by the microemulsion method showed a higher selectivity towards isomers but a lower activity when compared to catalysts prepared by impregnation technique. More recently, a study of zirconia synthesis from micro and macroemulsion systems has been conducted . Spherical ZrOa particles ranging from tens of nanometers to a few micrometers were produced. 

Another example dealing with the use of a microemulsion as the reaction medium can be found in a patent issued by Tinucci Platone . Although in this patent the catalyst was not prepared by microemulsion, the reaction takes place in a microemulsion system. The catalyst, a water-soluble rhodium complex, was formed in situ. The reactants, i.e. alkenes, were also the constituents of the oil phase of the microemulsion. Conversions higher than 70 % were obtained while the structure of the microemulsion was still maintained. Beyond that conversion level, the microemulsion was destroyed, probably because the oil phase was modified during the catalytic reaction. 

The methodology was first reported by Boutonnet et al.. They obtained a supported metal catalyst by depositing ultra-fine particles synthesized in a w/o microemulsion on pumice. The activity of this catalyst was tested in the but-1-ene hydrogenation reaction and compared with the activity of similar catalysts prepared by the traditional impregnation method both from aqueous and alcoholic solutions. The particle size was found to be in the 20-30 A range for the particles prepared from microemulsions and above 200 A for the classical impregnation counterparts. The behaviour of the Pt-based catalyst was found to depend on the preparation method. However, such a correlation between the activity and the preparation method could not be established in the case of Rh and Pd-based catalysts. 

During the study of the effect of the metallic content of Rh/Si02 catalysts, prepared by w/o microemulsion, in the CO hydrogenation reaction, Tago et al. found that the selectivity towards C2+ oxygenated products increased with the rhodium content The selectivity was above 40 % for a catalyst with a Rh content of about 3.5 wt %. From their results, it was established that the product selectivity and the turn-over frequencies (TOF) were dependent on the rhodium content of the catalyst. This effect was neither ascribed to the particle size nor to diffusion phenomena. The catalysts displayed similar particle sizes, irrespective of the Rh content, and it was evident from the Thiele modulus that the reactions were carried out under reaction rate-controlled conditions. 

The catalytic activity of the microemulsion catalyst was lower than the one obtained with the catalysts prepared by impregnation and poorly dispersed. 

As stated in this paper as well as is shown in Figure 5, when metal-supported catalysts are prepared by microemulsion technique, the metal particle size is smaller and the particle size distribution is sharper than that observed for analogous catalysts prepared by conventional methods such as impregnation or incipient wetness . 

Recently, the microemulsion technique has been explored as an alternative route to prepare Cu/ZnO catalysts. The Cu/ZnO catalyst was prepared in a microemulsion system using the oxalate route. In spite of the low surface are and poor Cu dispersion, the catalysts obtained by this procedure, when compared to catalysts prepared by a classical co-impregnation method, were more active in H2 production through partial oxidation of methanol. The authors propose this enhancement in the activity to be due either to the presence of some specific... 

In this paper, a detailed description of the state-of-the-art of catalyst preparation by the microemulsion technique has been given. It has been shown that one of the major advantages of the microemulsion technique is that it allows the preparation of metal-based catalysts displaying a narrow particle size distribution in the nanosize range. 

Godoi DRM, Perez J, Mercedes Villullas H (2007) Influence of particle size on the properties of Pt-RuC catalysts prepared by a microemulsion method. J Electrochem Soc 154 B474-B479... 

Xiong, L. Manthiiam, A. Nanostructured Pt-M/C (M=Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel-cells. Electrochim. Acta 50 (2005), pp. 2323-2329. 

T. Herranz, S. Rojas, F. J. Perez-Alonso, M. Ojeda, P. Terreros, and J. L. G. Fierro, Hydrogenation of carbon oxides over promoted Fe-Mn catalysts prepared by the microemulsion methodology, Applied Catalysis A, vol. 311, no. 

Figure 1. TEM images of Pd/Ti02 catalysts prepared by the microemulsion method under three different synthesis conditions.

The XRD and TEM showed that the bimetallic nanoparticles with Ag-core/Rh-shell structure spontaneously form by the physical mixture of Ag and Rh nanoparticles. Luo et al. [168] carried out structure characterization of carbon-supported Au/Pt catalysts with different bimetallic compositions by XRD and direct current plasma-atomic emission spectroscopy. The bimetallic nanoparticles were alloy. Au-core/Pd-shell structure of bimetallic nanoparticles, prepared by co-reduction of Au(III) and Pd(II) precursors in toluene, were well supported by XRD data [119]. Pt/Cu bimetallic nanoparticles can be prepared by the co-reduction of H2PtClg and CuCl2 with hydrazine in w/o microemulsions of water/CTAB/ isooctane/n-butanol [112]. XRD results showed that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (111) plane of the PtCu3 bulk alloy. 

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