Microstructure characterization and propane oxidation over supported Ru nanoparticles synthesized by the microwave-polyol method

Abstract

Ruthenium nanoparticles deposited on γ-Al₂O₃ were prepared in one step by a microwave-polyol method and tested in the complete oxidation of propane. The oxidation reaction was carried out under oxygen rich-conditions over the as prepared colloidal 4.9 wt.% Ru/γ-Al₂O₃ catalyst and heated in H₂ at 500 °C for 15 h. The as prepared catalyst contained Ru nanoparticles with mean size of 1.6 nm and narrow size distribution, while hydrogen treated metal particles with mean size of 6 nm. Before examining catalytic properties, the Ru nanoparticles were subjected to heat treatment in oxygen atmosphere to study their microstructure evolution. HRTEM, SAED, XRD, BET, XPS, as well as hydrogen chemisorption and O₂ uptake techniques were applied to characterize the supported Ru nanoparticles. It was established that catalyst with the 1.6 nm Ru nanoparticles exhibited slightly higher specific activity than the catalyst with the 6 nm Ru nanoparticles. The superior catalytic performance of the Ru nanoparticles could be correlated with a high metallic dispersion and low particle sizes. It was evidenced that the most active sites in the propane oxidation reaction, consist small Ru_xO_y clusters without well-defined stoichiometry. Such surface species were formed at 100–200 °C, and as a result the as prepared Ru/γ-Al₂O₃ catalyst reached 100% propane conversion below 200 °C. Moreover, the Ru nanoparticles under oxidative atmosphere up to 250 °C, both in oxygen and in reaction of propane oxidation, possesses good stability and the ruthenium phase was not agglomerated. In consequence, recycling of the supported Ru nanoparticles results only in an insignificant loss of the catalytic activity. The very good catalytic performances of supported Ru nanoparticles prepared under microwave-polyol conditions, preserved after consecutive runs, make them promising for practical application in the purification of environmental pollutions.

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.