Catalytic Wet Oxidation of Ammonia Solution with Platinum- Palladium-rhodium Composite Oxide Catalyst

This study adopted high concentration aqueous solutions of ammonia for use in catalytic liquid-phase oxidation in a trickle-bed reactor with platinum-group metals (platinum-palladiumrhodium), prepared by the co-precipitation of H2PtCl6, Pd(NO3)3 and Rh(NO3)3. The experimental results revealed that a minimal amount of ammonia was removed from the solution by wet oxidation in the absence of any catalyst, while around 99% of the ammonia was removed by wet oxidation over the platinum-palladium-rhodium composite oxide catalyst at 230 °C with an oxygen partial pressure of 2.0 MPa. A synergistic effect is evident in the platinum-palladium-rhodium composite oxide structure, which has the greatest capacity to reduce ammonia. The particles were characterized by UV-Vis and particle size analyzer. The effluent streams were at a liquid hourly space velocity of under 9 h in the wet catalytic processes, and a reaction pathway was linked the oxidizing ammonia to nitric oxide, nitrogen and water. The solution contained by-products, including nitrate and nitrite. Nitrite selectivity was minimized and ammonia removal maximized when the feed ammonia solution had pH about 8.2. *Corresponding author Email: [email protected] INTRODUCTION Ammonia is utilized extensively and in large quantities for various purposes. Since ammonia is a useful chemical in the manufacture of ammonium nitrate, ammonia, urea, ammonium phosphate, petroleum refineries and coke, it is commonly present in industrial wastewaters. Additionally, wastewaters that contain ammonia are commonly either toxic or have concentrations or temperatures that prevent direct biological treatment. Aqueous solutions primarily contain two forms of ammonia, unionized ammonia and ammonium ion; both of which are in equilibrium. The removal of ammonia from industrial effluent represents an important and dynamic field of research. Conventional biological, physical and chemical treatment processes, including biological nitrification, activated carbon fiber adsorption, ozonation and ion exchange processing, achieve only phase transformations and may yield contaminated sludge and/or adsorbent, both of which require further disposal. Therefore, the removal of ammonia from air and waste streams is important. Several nitrogen-containing compounds are present in the environment, and affect the acidification and eutrophication of ecosystems. The nitrogen cycle characterizes the motion and transformation of these nitrogen compounds through the biosphere. The key concern associated with the use of ammoniacontaminated water with excessive nitrate concentrations involves its effect on human health, especially that of infants [1]. Wet oxidation (WO) technology was originally designed to oxidize organic substances, yielding intermediate products with small molecular weights at temperatures between 125 and 350 °C and pressures of between 0.5 and 20.0 MPa in the aqueous phase. However, ammonia is usually an end product of wet oxidation and is difficult to oxidize. In their review of the WO process, Mishra et al. [2] and Bhargava et al. [3] also offered suggestions for future work. However, catalytic WO (CWO) is known to increase the range of uses of WO technology if appropriate catalysts are used; such catalysts have the potential to promote oxidation with shorter reaction times and under milder operating conditions. The selective catalytic oxidation 86 J. Environ. Eng. Manage., 18(2), 85-91 (2008) of ammonia-containing water to yield molecular nitrogen and water can be adopted to eliminate ammonia pollution [4-7]. Various catalysts have also been used for ammonia oxidation in the liquid phase. For instance, Levec and Pintar [8] showed that the WO of aqueous solutions of organics from wastewaters at low temperatures and pressures was easier in the presence of heterogeneous catalysts than in a non-catalytic WO processes. Also, Imamura et al. [9], who designed various heterogeneous catalysts of WO, found that the Mn/Ce (1:1, molar/molar) composite catalyst was more active than a Co/Bi catalyst for the WO of ammonia. These catalysts were active in reactions at temperatures of over 260 °C with 4.0 MPa. Imamura [10] published a review paper that summarized recent works on CWO and noncatalytic WO. Platinum-based group metal additives as threeway catalysts are the most active components in hydrocarbon oxidation and are also active in all other reactions [11,12]. However, little research has been performed on the use of platinum-palladium-rhodium (Pt-Pd-Rh) based alumina-supported catalyst to elucidate the reactive characteristics of these active metals in CWO. The activity of the Pt-Pd-Rh composite oxide catalyst in the oxidation of ammonia solutions, given various values of relevant parameters, and its effect on the removal of ammonia in CWO, were studied. MATERIALS AND METHODS 1. Materials and Chemicals The Pt-Pd-Rh composite oxide catalysts that were used in this study were prepared by the coprecipitation, which involved aqueous H2PtCl6, Pd(NO3)3 and Rh(NO3)3 (all grade, Merck, Darmstadt, Germany). A Pt-Pd-Rh catalyst was washcoated on gamma-alumina substrate with a high surface area with platinum, palladium and rhodium in weight ratios of 4:3:1, maintaining the ratio of catalytic active metals at a constant 6.4%. The catalysts were then calcined at 500 °C in an air stream for 4 h. The resulting powder was formed into tablets using acetic acid as a binder. 2. Experimental Methods All feed solutions were made using Millipore (Bedford, MA) water (18 MΩ cm), and the pH value of the ammonia aqueous solution was adjusted to 12.0 using 1 M sodium hydroxide. The CWO was conducted in a continuous trickle-bed reactor (TBR) that was constructed from 45 cm of stainless steel tubing (SS-316), 25 mm I.D., which was resistant to high pressures and temperatures of up to 10 MPa and 350 °C, respectively, and to corrosion at pHs of between 5 and 12.5. The main operating parameters were a liquid feed rate given by a liquid hourly space velocity (LHSV) of 1.5 to 9 h, reaction temperatures of 150, 200 and 230 °C, and ammonia concentrations in the inlet between 400 and 1000 mg L. The partial pressure of oxygen in the reactor was maintained at a constant 2.0 MPa, and the pH of the buffer ammonia solution in the inlet was maintained at 12.0. Before each experiment, the reactor was heated for around 2 h to a steady reaction temperature. Each temperature was maintained for 60 min to allow the system to enter a steady state. When the desired reaction temperature was reached, the desired feed flow rate was initiated. After a constant reaction temperature had been reached, the reaction pressure was adjusted to the desired value using a backpressure regulator. This change took about 15 min. Additionally, the sample cooling system started at the same time as the heating system of the reactor. Reducing the temperature of the recirculation took about 15 min. The cooled mixed stream was directed to the sampling vessel and separated immediately. Therefore, the liquid and gas samples could be collected. The reproducibility of experiements was acceptable, since differences between ammonia conversions from duplicate experiences were found always to be less than 5%.