The use of hydrogen chemisorption for the determination of Ru dispersion in Ru/γ-alumina catalysts
Graphical abstract
Ru nanoparticles on γ-Al2O3 were prepared by reduction of RuCl3 in ethylene glycol with using microwave irradiation. Chemisorptive properties of this system were compared with those of the Ru/γ-Al2O3 catalyst prepared by impregnation. Superior chemisorptive properties of the former system are assigned to the higher dispersion of the smaller ruthenium particles and weaker interaction of Ru nanoparticles with the support.
Introduction
There is a growing interest in understanding the adsorptive and catalytic properties of ruthenium catalysts due to their very high activity for ammonia synthesis [1], [2]. Supported Ru catalysts can also be used in other industrial processes, such as the F–T synthesis of paraffins, methanation of CO or in the partial hydrogenation of benzene to cycloheksene [3], [4]. The conventional ruthenium-based catalysts are usually prepared by impregnation of the supports with an aqueous solution of RuCl3·3H2O [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], or other precursors, if Cl− retention in the catalyst is to be avoided [2], [4], [12], [13], [14], [15], [16], [17]. The impregnation method is simple but has severe drawbacks, most important being not uniform distribution of metal particles in size and shape [18]. For Ru/Al2O3 catalysts prepared by impregnation metal dispersion is generally low, and additionally, if chlorine precursor is used, contamination of the surface of metal with chlorine occurs [5], [6], [9], [11]. Another source of metal contamination in such system may be alumina support dissolved in highly acidic solution used for impregnation [19].
An alternative method for obtaining supported catalysts with well-defined metal particles is that using metal colloids. Polyol reduction method has been widely applied as an effective way to synthesize the highly dispersed metal colloids [20], but only few studies were devoted to ruthenium [20], [21], [22], [23], [24]. There are also only few studies concerning the deposition of ruthenium nanoparticles on the supports [21], [25], [26], [27]. Miyazaki et al. [21], [25] prepared Ru/Al2O3 catalysts by deposition colloidal particles obtained by reduction of RuCl3 in ethylene glycol. It was found that almost all of the ruthenium ions could be reduced to metallic state by the polyol agent, thus, the chlorine ions remained in solution could be easily removed.
Dispersion of ruthenium in traditionally prepared supported catalysts has been studied repeatedly during the past 30 years. However, there is no universally accepted method to measure the exposed ruthenium surface area, but selective gas chemisorption (H2, O2 and CO) is mostly used for this purpose. H2 chemisorption has been extensively applied [1], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], because the chemisorption stoichiometry H/Rus = 1/1 is well established [28], [29], [30]. In the case of oxygen adsorption the surface stoichiometry is more complex and O/Rus values of 1/2 [31], 1/1 [32], [33], [34] or 2/1 [8], [29], [30], [35] were proposed. Moreover, it was shown in few papers that the stoichiometry for O2 adsorption varies with Ru particle size [33], [36]. The formation of oxygen subsurface and a progress of the chemisorption to a bulk oxidation can lead to overestimation, especially in the case of small particles [33], [36]. CO chemisorption has also been frequently used to determine the dispersion of supported ruthenium catalyst [19], [28], [32], [37], however, its application has been questioned because of its tendency to form multiple adsorption bonds, the number of which depends on surface morphology [33]. Moreover, there is a suggestion that CO oxidizes Ru0, i.e. corrosive adsorption of CO takes place [38]. It was reported also that the adsorption of CO on the supported ruthenium catalyst causes disruption of some Ru–Ru bonds in the metal clusters increasing the dispersion of ruthenium [38], [39].
In contrast to the selective chemisorption of O2 and CO, it was reported that the stoichiometry of H2 chemisorption on Ru is not particle size dependent [30], provided equilibrium is established and only the quantity of irreversibly chemisorbed gas is considered. However, there are still problems and inconsistencies concerning the conditions for saturation of a hydrogen monolayer corresponding to H/Rus = 1/1. According to many authors the time needed to reach the equilibration is 1–2 h, though a period of 4–24 h was required at lower pressures and at room temperature [5], [6], [28], [30]. More recent studies have shown that the chemisorption equilibrium can be reached much quicker, in just 10 or even 5 min at temperature of 60–90 or 100 °C [11], [13], [16]. Several effects complicating the utilization of H2 chemisorption to determine Ru dispersion, such as: hydrogen spillover from Ru to the support at elevated temperature [5], [13], formation of subsurface hydrogen species [40], and presence of a “hydrogen fog” over the ruthenium particles [12] have been also reported.
In this work a new method of preparation of the colloidal Ru/γ-Al2O3 catalyst, based on a solvothermal process with using microwave irradiation, has been applied. Thanks to the microwave irradiation the heating time was much shorter than in traditional polyol methods, such as an oil bath used by Miyazaki et al. [21], [25] and thus smaller, more uniform Ru particles could be obtained. The catalyst, in the “as prepared state”, as well as after thermal heating in hydrogen, was characterized by various physical methods including BET, XRD, XPS and TEM. Such well-characterized, model catalyst was used as an object of thorough studies of the H2 chemisorption. The main goal was re-evaluation of existing data and determination optimal procedures for using H2 chemisorption as reliable tool for proper measurement the fraction of surface Ru atoms in highly dispersed catalysts. In this study, static volumetric H2 chemisorption method was applied. In order to evaluate the effect of the preparation method and thus possible contaminations on H2 chemisorption a conventional catalyst with similar loading of the metal, prepared by incipient wetness impregnation with RuCl3 solution was also studied.
Section snippets
Experimental
RuCl3·3H2O (Koch & Ligth) and γ-Al2O3 (K, Fe, Mg and Si < 10−3%) with BET surface area of 245.9 m2/g and pore volume of 0.21 cm3/g, after previous calcination in air at 550 °C for 20 h, were used for catalysts preparation. The support was prepared in another laboratory by hydrolysis of aluminum isopropoxide and Cl− ions were not detected by XPS and chemical analysis.
Two preparation methods of the catalysts were applied. The first – the polyol reduction method – consisted of the reduction of RuCl3
Overall characteristics of the catalysts
Table 1 summarizes the physicochemical and textural properties of the catalysts. The Ru content of 5.1 and 4.6 wt.% for colloidal and impregnated Ru/γ-Al2O3 catalyst, respectively, did not change upon heat treatment procedures. Since the catalysts were prepared from RuCl3·3H2O precursor we first determined an overall and surface content of residual chlorine ions. Fig. 1 shows X-ray photoelectron spectra (Cl 2p line) of the as-prepared colloidal Ru/Al2O3 catalyst, the impregnated catalyst reduced
Conclusions
Good correspondence between H2 chemisorption and XRD and TEM data, obtained in this work for the model, colloidal system, indicates that traditional volumetric H2 chemisorption at room temperature or at 100 °C, can be used to measure accurately the dispersion of Ru in Ru/γ-Al2O3 catalysts. The model, highly dispersed Ru catalyst, prepared from RuCl3 precursor by the microwave-polyol method, is free of chlorine contamination and thus the interfering effect of chlorine on the gas chemisorption
Acknowledgements
This work was financially supported by Polish Committee for Scientific Research (Grant No. 1 T09B 084 30). The authors thanks A. Cielecka for chemisorption measurements.
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