Catalytic Effect of Potassium Compounds in Soot Oxidation

The Effect of Potassium Inclusion in a Silver Catalyst for N2O-Mediated Oxidation of Soot in Oxidising Exhaust Gases

It has previously been shown that an Ag/CZA catalyst can simultaneously remove NOx and soot from an oxygen-rich exhaust gas at low temperatures, by utilising the N2O generated preferentially during incomplete NOx reduction. Here, we examine the effect of reformulating the catalyst to include potassium, which is a known promoter of soot combustion. On including 2 wt% K, NOx-reduction occurs both in the absence and presence of soot, but the N2O formed does not play a part in the oxidation of soot. At higher K loadings (5, 10 and 15 wt%), NOx reduction is almost completely disabled, and only contributes to the activity of the catalyst containing 5 wt% K when tested in the presence of soot. At a loading of 20 wt% K, the potassium phase segregates, leaving NO and NH3 adsorption sites exposed. In the absence of soot, this catalyst can remove NOx by reduction on the Ag/CZA component and through nitration of the potassium phase. Although the presence of potassium lowers the onset temperature for soot oxidation to within the range of NOx reduction over Ag/CZA, the mobile K species prevents the desirable C+N2O reaction.

Application Prospect of K Used for Catalytic Removal of NOx, COx, and VOCs from Industrial Flue Gas: A Review

NOx, COx, and volatile organic compounds (VOCs) widely exist in motor vehicle exhaust, coke oven flue gas, sintering flue gas, and pelletizing flue gas. Potassium species have an excellent promotion effect on various catalytic reactions for the treatment of these pollutants. This work reviews the promotion effects of potassium species on the reaction processes, including adsorption, desorption, the pathway and selectivity of reaction, recovery of active center, and effects on the properties of catalysts, including basicity, electron donor characteristics, redox property, active center, stability, and strong metal-to support interaction. The suggestions about how to improve the promotion effects of potassium species in various catalytic reactions are put forward, which involve controlling carriers, content, preparation methods and reaction conditions. The promotion effects of different alkali metals are also compared. The article number about commonly used active metals and promotion ways are also analyzed by bibliometric on NOx, COx, and VOCs. The promotion mechanism of potassium species on various reactions is similar; therefore, the application prospect of potassium species for the coupling control of multi-pollutants in industrial flue gas at low-temperature is described.

Influence of the Preparation Method of Ag-K/CeO2-ZrO2-Al2O3 Catalysts on Their Structure and Activity for the Simultaneous Removal of Soot and NOx

Ag/CeO2-ZrO2-Al2O3, a known catalyst for the simultaneous removal of NOx and soot, was modified by the addition of K, and was prepared using various techniques: wet impregnation, incipient wetness, and chemical vapor impregnation at different temperatures. The effect of the preparation method on catalyst activity was studied. It was found that catalysts prepared via wet impregnation, incipient wetness, and chemical vapor impregnation at 80 °C were able to utilize in situ formed N2O at low temperatures, to simultaneously remove NOx and soot. The difference in preparation method affected the catalyst’s ability to produce and use N2O as an oxidant for soot. The temperature at which chemical vapor impregnation was performed greatly influenced the catalyst’s ability to oxidize soot. The introduction of K to the Ag/CeO2-ZrO2-Al2O3 vastly improved the soot oxidation activity, particularly for the catalyst prepared via wet impregnation. However, the incorporation of K had an adverse effect on the reduction of NOx.

Improved Hydrothermal Stability in Glass Diesel Soot Oxidation Catalysts

The hydrothermal stability of K-Ca-Si-O glass soot oxidation catalysts has been improved by substitution of Ce and Zr for Ca. This work demonstrates that glasses can be tailored to withstand the challenging diesel exhaust hydrothermal environment by considering the field strengths and partial molar free energies of the hydration reactions (ΔGi) of the cation species in the glass. The result is a glass that shows less formation of precipitates after 2 h hydrothermal exposure in air with 7% H2O at temperatures ranging from 300–700 °C. A K-Ca-Si-O glass with a soot T50 (the temperature when 50% of the soot is oxidized) of 394 °C was found to degrade to 468 °C after a 2 h, 700 °C hydrothermal exposure, whereas the improved K-Ce-Zr-Si-O glass only changed from 407 °C to 427 °C after the same treatment.

Promotion of graphitic carbon oxidation via stimulating CO2 desorption by calcium carbonate

Carbon oxidation has two stages, the first is the formation of surface oxides and the second is the gasification of the surface oxides to CO₂. Calcium carbonate (CaCO₃) was used to catalyze the gasification of the surface oxides. The catalytic effect of on graphite oxidation and its catalytic mechanism were studied by using thermogravimetric technique and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was found that characteristic temperature (T₅₀) of graphite oxidation with CaCO₃ was 946 K, 113 K lower than that of graphite only. DRIFTS analysis results show that surface oxides (adsorbed CO₂ and carbonate CO₃^2-) were formed on the graphite surface at a temperature above 473 K, carbonate products on graphite surface disappeared when CaCO₃ was present; formation of CO₃^2- on CaCO₃ surface was confirmed, this CO₃^2- may be more easily gasified into gaseous CO₂. The kinetic analysis results showed that CaCO₃ promoted graphite oxidation has an activation energy of 74.3 kJ mol^-1, far lower than that of graphite (148 kJ mol^-1).

Dipole-moment-driven diesel soot oxidation in the presence of alkali metal chlorides

The dipole moments of alkali metal chlorides drive the oxidation of soot by promoting electron transfer, justifying their excellent activities despite their poor redox abilities.