Microstructure characterization and propane oxidation over supported Ru nanoparticles synthesized by the microwave-polyol method
Graphical abstract
Research highlights
▶ The supported Ru nanoparticles are very active and stable for propane oxidation. ▶ The 1.6 nm Ru nanoparticles exhibit higher activity than 6 nm nanoparticles. ▶ In oxygen atmosphere Ru nanoparticles possesses good stability up to 250 °C. ▶ The metallic and oxide Ru species plays important role for propane oxidation.
Introduction
In recent years, there has been increasing concern for the environment due to the emission of different pollutants to the atmosphere. The emission of volatile organic compounds (VOCs) has received particular attention as they have been established with the increase in photochemical smog, depletion of stratospheric ozone and the production of ground-level ozone [1]. Catalytic oxidation has been identified as one of the most efficient way to destroy VOCs at low concentrations. Numerous studies have shown that short-chain hydrocarbons, including propane are amongst the most difficult to destroy [2], [3]. Currently, the most active catalysts for total oxidation of propane are those based on platinum [3], [4], [5], [6], [7] and palladium [8], [9]. The higher volatility of the other noble metals makes them less attractive because of the high reaction temperatures reached during combustion [10]. Usually, γ-Al2O3 is commonly used as a support, in spite of many studies have demonstrated that the catalytic activities of alumina-supported catalysts are slightly lower than those of catalysts supported on CeO2, TiO2 or ZrO2 [11]. For the metal oxides, which are also frequently tested in oxidation of propane, cobalt oxide has been shown to be the most active [12], [13]. However, the search of a new catalytic materials that can allow efficient oxidation of propane at lower temperatures still remains a major challenge.
Catalysts based on ruthenium are of great importance for processes such as ammonia synthesis/decomposition [14], [15], CO and NO oxidation [16], [17], N2O decomposition [18] and other industrially important reactions [19], [20], [21], [22]. Also, Ru catalysts are highly active for wet oxidation of organic compounds containing oxygen [23], [24], [25]. However, the application of the Ru-based catalysts in the alkanes/alkenes combustion is very scarce [26], [27]. Recently, oxidation of carbon black and VOCs such as propene and toluene over Ru/CeO2 catalysts has been investigated by Aouad et al. [28]. Mitsui et al. [29] studied the combustion of ethyl acetate, acetaldehyde, and toluene over ruthenium supported on CeO2, ZrO2, SnO2 and Al2O3. Very recently, Kamiuchi et al. [30] reported that activity of Ru/SnO2 catalysts in combustion of ethyl acetate was strongly influenced by the structural changes of active sites such as sintering and redispersion. The combustion of ethyl acetate has been also investigated over various Me/CeO2 catalysts, and it was found that catalytic activity of ruthenium was higher then platinum, palladium and rhodium [31].
Recently, we have studied the low temperature oxidation of butane [32] and propane [33] over Ru/γ-Al2O3 catalysts prepared by the incipient wetness impregnation method using RuCl3 as a metal precursor. It was found that in the presence of large amount of chlorine on the catalyst surface, activities of the catalysts towards the C3–C4 alkanes oxidation were greatly suppressed. Also, the pre-treatment in oxygen at 250 °C and especially at 600 °C induce significant loss activity of the Ru/γ-Al2O3 catalysts in both reactions. The activity loss was attributed to the formation of crystalline RuO2 oxide and to some sintering of the active phase [32], [33].
It is well known that conventional methods of the catalyst preparation generally yield in a wide distribution of metal particle sizes and provided lack of uniform particle composition and morphology. Hence, in this paper we have extended previous studies by performing the oxidation of propane over the Ru catalyst which is based on alumina supported colloidal Ru nanoparticles prepared using a microwave-polyol method. The obtained Ru nanocatalyst, although prepared also from RuCl3, was free of chlorine contamination. Moreover, the structural changes of the catalyst prepared from metal colloid, as opposite to conventional preparation methods, are more easy to investigate not only due to narrow distribution of metal particle size but also due to homogeneous distribution of the metal nanoparticles on the support. The synthesis of the γ-alumina-supported Ru catalysts from the Ru colloid and its catalytic activity have been reported only in a few papers [34], [35], [36], [37], [38]. However, to the best of our knowledge, results presented in this study are the first data available on the activity of the Ru nanoparticles in the oxidation of propane. Also, interaction of the oxygen with the supported Ru nanoparticles over the wide temperature range was investigated. Such studies are very important because metal nanoparticles with size on the order of 1–3 nm are difficult to handle or recycle and may coagulate. Different chemical and physical methods were applied to characterize the Ru nanoparticles and to investigate morphology changes created upon heat treatment in hydrogen and oxygen atmosphere.
Section snippets
Synthesis of the supported ruthenium nanoparticles
High purity γ-alumina (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 calcination at 550 °C, was used as support material. The support was prepared in another laboratory by hydrolysis of aluminum isopropoxide. The microwave-polyol synthesis of ruthenium nanoparticles consisted of the reduction of RuCl3 (Fluka) with ethylene glycol (EG) in the presence of γ-alumina using a special autoclave with a microwave heating (MW) (MW Reactor ERTEC, Poland)
Structure characterization of ruthenium nanoparticles supported on γ-Al2O3
The as prepared colloidal catalyst contained 4.9 ± 0.1 wt.% Ru and this value was close to the expected metal loading (5 wt.%). Chemical and X-ray photoelectron spectroscopy (XPS) analyses showed that the as prepared catalyst was free of chlorine contamination. Therefore, chlorine ions could not affect catalytic properties of ruthenium nanoparticles. Moreover, only trace of sodium ions and C-containing species were detected at the surface of this catalyst.
The morphological characteristics of the
Conclusions
In this study, we prepared ruthenium nanoparticles supported on γ-alumina by a microwave-polyol method and investigated their microstructure/morphology evolution after heat treatment at hydrogen and oxygen atmosphere. Catalytic performances for propane total oxidation of colloidal Ru/Al2O3 catalysts were also studied. It was found that the propane oxidation turnover rate on as prepared system with mean Ru particle size of 1.6 nm and narrow size distribution was about two times higher than the
Acknowledgments
The authors thank very much Mrs. L. Krajczyk for TEM investigations and A. Cielecka for chemisorption measurements. This study was financially supported by the Polish State Committee for Scientific Research which founded the project N N209 335537 (2009–2011).
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