Preparation and characterization of Pd doped ceria–ZnO nanocomposite catalyst for methyl tert-butyl ether (MTBE) photodegradation

Highlights

• Novel Pd supported ceria–ZnO photocatalysts were prepared with different amounts of palladium.

• The photocatalytic activity of these catalysts was evaluated for degradation of MTBE in water.

• Near complete removal of MTBE was achieved using 1% Pd doped ceria–ZnO catalyst and UV irradiation.

• Highest rate constant was obtained in case of 1% Pd doped ceria–ZnO catalyst.

• Shape and size of pores are important factors for high photoactivity of catalyst.

Abstract

A series of binary oxide catalysts (ceria–ZnO) were prepared and doped with different amounts of palladium in the range of 0.5%–1.5%. The prepared catalysts were characterized by SEM, TEM, XRD and XPS, as well as by N₂ sorptiometry study. The XPS results confirmed the structure of the Pd CeO_2−x-ZnO. The photocatalytic activity of these catalysts was evaluated for degradation of MTBE in water. These photocatalyst efficiently degrade a 100 ppm aqueous solution of MTBE upon UV irradiation for 5 h in the presence of 100 mg of each of these photocatalysts. The removal of 99.6% of the MTBE was achieved with the ceria–ZnO catalyst doped with 1% Pd. In addition to the Pd loading, the N₂ sorptiometry study introduced other factors that might affect the catalytic efficiency is the catalyst average pore sizes. The photoreaction was determined to be a first order reaction.

Introduction

The steep increase in the concentration of pollutants in the environment has attracted considerable interest from researchers seeking to discover efficient methods for controlling the entry of pollutants and reducing the existing pollutant load in the environment [1], [2], [3], [4], [5]. Several decades ago, fuel oxygenates were introduced to eliminate the use of leaded gasoline, and these molecules helped to improve the octane value of gasoline and provide nearly complete combustion of fuel by supplying extra oxygen during the combustion process [6]. After the passage of the clean air act in 1990, their use increased tremendously [7]. In 1997, after the Kyoto protocol agreement to control the emission of greenhouse gases, fuel oxygenates were in high demand worldwide [8]. There are two types of fuel oxygenates, including aliphatic alcohols and ethers. The blending of alcohols in gasoline required careful handling to avoid water content. However, ether is easy to mix with gasoline without any problems. Therefore, ether based oxygenates, such as ethyl tert-butyl ether and methyl tert-butyl ether, were preferentially blended with gasoline [9], [10]. As the application of MTBE became more common and its consumption increased, MTBE began to appear in certain water sources, which raised concern over human health and its increasing concentration in water bodies [11], [12]. Therefore, MTBE was being considered a potential environmental pollutant because it found its way into the environment via accidents, spills, faulty gas station and leakage from pipelines. The main reason for its accumulation in water bodies was due to its high solubility in water (∼50 mg l⁻¹) [13]. MTBE has very weak partition with the organic fraction in soil, and once released from a source, it has a tendency to spread rapidly in groundwater where its presence in water poses a high risk to human health. MTBE has a high affinity for blood resulting in a tendency to accumulate in the blood stream, which can be detected during breathing. Human exposure to MTBE may result in coughing, dizziness, fever, headaches, muscular aches, vomiting, sleepiness and skin and eye irritation [14], [15]. In the light of available reports, there are no data on human carcinogenicity but the evidences of exposure of MTBE on mice and rats have demonstrated the carcinogenicity and therefore, human carcinogenic nature of MTBE cannot be ruled out. MTBE concentration for carcinogenicity varies depending on subject and condition and it can be broadly grouped as concentration higher than 300 ppm but below this concentration and above the advised concentration by EPA (20–40 ppb) it is toxic to human on long term exposure. Therefore, The United States Environment Protection Agency (USEPA) has also suggested that at higher concentrations, MTBE may be carcinogenic [16], [17]. In addition, MTBE affects the taste and odor of water. Based on taste and odor, the USEPA has issued limits for MTBE in drinking water in the range of 20–40 ppb [18], [19]. The special characteristics of MTBE and its effects on the environment and human health have attracted much interested from scientists seeking to control its entry into the environment and to degrade the MTBE currently in the environment. Previously, different conventional techniques have been used with limited success including activated carbon treatments, aerobic/anaerobic biodegradation, air-stripping to remove MTBE from water [20], [21]. MTBE was determined to be highly resistant to biodegradation due to its ether linkage and tertiary carbon. In addition, MTBE has a high solubility in water, a very low Henry's law constant (5.5 × 10⁻⁴ atm m³ mol⁻¹ at 25 °C), which hinders its partition from the liquid phase to the vapor phase, and a moderate affinity toward carbon, which resulting in the high cost of activated carbon adsorption [22], [23], [24].

Recently, heterogeneous photocatalysis, which is an advanced oxidation process (AOP), has become widely applied to the treatment of toxic and non-biodegradable compounds from the environment. Photocatalysis is a simple and very promising technique for solving various environmental and energy issues. Environmental pollution as well as the problems associated with the presence and ever increasing mass/volume of organic, toxic and nonbiodegradable pollutants provides the impetus for fundamental and applied research to solve these issues [25]. Typically, photocatalysis is initiated by the irradiation of a photocatalyst, which are primarily composed of semiconducting metal oxides, with a light source with sufficient energy to excite an electron from the valence band of the photocatalyst to the conduction band, which creates a hole in the valence band. Therefore, the electron–hole pair is generated due to photoexcitation and reacts with hydroxyl ions/oxygen/water to produce hydroxyl (^•OH) radicals. These hydroxyl (^•OH) radicals react with the organic molecules adsorbed on the photocatalyst and degrade them to CO₂ and H₂O through a series of chemical reactions. Many metal oxides have been reported to be active photocatalysts for the degradation of organic pollutants. However, each of these photocatalysts has its own drawbacks that limit its usage under particular conditions [26], [27]. In addition, a rare earth cerium oxide (CeO₂) has also been studied and applied in heterogeneous catalysis due to its ability to release and absorb oxygen through a fast Ce⁴⁺/Ce³⁺ cycle [28], [29], [30]. Recently, ceria–ZnO composites have been reported to exhibit enhanced photoactivity in the photocatalytic degradation of Rhodamine B by Li et al. [31] compared to their individual components, which might be due to improved separation of the photogenerated electron/hole pairs, larger surface area and enhanced adsorption ability of the surfaces and interfaces in nanosize ceria–ZnO. Noble metal (Pt, Rh, Pd) doping on ceria, which can be used as a support or promoter, is very important due to the unique acid–base and redox properties of ceria that further influences the redox reactions of supported noble metals, the catalytic property of metal crystallites, the thermal resistance of supporting material and dispersion of supported metals [32]. In addition, Pd loading onto ceria has been reported to alter the surface properties of the support material due to the electron-transfers between Ce and Pd [33]. Therefore, encouraged by the properties described above, we synthesized a novel catalyst by combining CeO₂ and ZnO and doped it with different amount of Pd to study the photocatalytic degradation and kinetics of MTBE in the presence of UV radiation.