Химия и химические технологии. 8. Кинетика и катализ

Kalinichenko O.O., PhD Misnyankin D.O., Dr. Sci. Snizhko L.O.

Ukrainian state university of chemical engineering, Ukraine

Catalytic burning of diesel soot

The removal of soot solid particles consisted of (% mas): C (59-84), H (1-4), O (10-25), N (1-3), S (1-4) and emitted from diesel engines into the atmosphere is an important environmental problem. Special filters used nowadays for reducing of soot burn-out temperature as a rule have the surface catalytic layers. However, the high cost of such systems that partly made from noble metals, hinders the distribution of such filters. In this regard, the development of more accessible catalysts for diesel soot burn-out acquires exceptional importance.

In many catalytic reactions occurring on solid catalysts, the irregular pore system of the catalyst may negatively influence the selectivity of a desired intermediate product by slowing down the diffusion of the product molecules in the tortuous pores, thus allowing their further conversion to undesired by-products. The working conditions of catalysts to neutralize the extreme vehicle emissions are non-stationary due to fluctuations of the input parameters of the gas flow, in particular due to exhaust gas temperature fluctuations that vary from 120-360⁰C in cold start and idling up to 50-700⁰C in steady-state normal regime. At the same time the catalyst must be capable of withstanding heat up to 900-110⁰C without loss of activity.

Aluminium oxide is often used as a carrier in catalytic systems thanks to the large surface area and relatively low cost. The structure of the porous anodic alumina films is characterised as a close-packed array of hexagonal cells, each of them containing a central cylindrical pore directed towards the bottom of the oxide film. In order to bring catalytically active species on the alumina carrier, the anodised metals must be immersed in solutions containing the desired metal salts or complexes.

In this work, catalytic layers were obtained on aluminium (Сu 3.8-4.9, Mg 1.2-1.8, Mn 0.3-0.9, Fe-0.5, Si-0.3, Zn 0.1, Ti, titanium - 0.1, Al - balance) and titanium (N₂- 0.1, C - 0.015, H₂ - 0.015, Fe - 0.5, O₂ - 0.5, Ti - balance) by anodic spark deposition. The solution of sodium silicate (2.5 % vol.) was used as electrolyte. The time of electrolysis was 10 min, galvanostatic current density - 400 A/m², half-periodical positive voltage - 550-600 V. In this technique, the oxide formation takes place by applying an intense electric discharge on the metal surface acting as anode; the surface metal melts and at the same time is oxidised, forming characteristic textures consisting of regular pores. By the appropriate selection of the electrolyte, active phase precursor, stabiliser and working conditions (potential, current density, frequency of pulses, time) oxide layers of different thickness (5–50 m) and pore density (1–3×10⁴ pores/cm²) containing the active component were obtained. The carriers used for anodisation were aluminium and titanium. Metallic substrates like wires, foils or foils and other more complicated specimens of aluminium and titanium with a geometric surface of about 3 cm² were connected to the anode. Obtained oxide layer was soaked by the 2 mol/l solutions of Cu(NO₃)₂⋅3H₂O, Ni(NO₃)₂⋅6H₂O, Mn(NO₃)₂⋅6H₂O, Co(NO₃)₂⋅6H₂O. After every treatment, samples were dried at 130⁰C for 5 min and heated at 550°С for 3 hours. Samples with soaked oxide layers and samples with clean oxide films (for comparison), were covered with soot in burner flame combustion of diesel fuel (GOST 305-82) for 5 min each. After infliction, soaked oxide layers together with soot were removed from the substrates and analysed together with samples of clean soot (Fig. 1).

Fig.1. Soot deposition on metal plates with catalytic layers

Catalytic combustion of soot was studied in a dynamic and isothermal regimes using derivatograph "Paulic-Paulic-Erdey Q-1000" with a heating rate 5°C per minute. Ignition temperature of soot with a precision of ± 30°C were estimated from differential thermal analysis (DTA) curves at the maximum burning rate achieved. Sweep interval of temperatures was 20-500°C. The temperature of the beginning of soot burning was determined by thermogravimetric (TG) curves, the maximal temperature of the process determined by the extremum of the differential thermogravimetric (FGD) curves, with an accuracy of ± 2°C.

Treatment of the thermogravimetric curves was done through every 5 degrees. The conversion degree of reaction were determined as a ratio of the mass loss of samples (due to oxidation of soot) to mass content of soot deposited on the sample. Reaction rate was calculated by the equation

,

were α is conversion degree of the soot burning, K is constant of reaction rate, К₀ is coefficient, E_a is activation energy, T is temperature, t is time of burning, f(α) is function which depends on the reaction mechanism.

For typical topochemical reactions that can be described by the processes of diffusion, embryos formation, moving of the phase borders and grain growth, the f(α) can be represented by the equation

were m, n, p are numerical constants, correspond to the specific reaction mechanism.

As a result of processing the experimental data, the following values were found: for activation energy Е_А = 69±5 kJ/mol, for lnK₀=3,4±0,5. Non-catalytic burning of diesel soot occurs at 612⁰C, but the soot deposited on Al₂O₃ oxide burns between 500 and 640⁰C. So pure aluminium oxide does not show visible catalytic activity. When soot burns on the clean cuprum oxide CuO, the burning temperature is 416⁰C. The cuprum oxide deposited on Al₂O₃ decreases the temperature of soot burning on 60-70⁰C.