Study Of Mechanism For Catalytic Soot Combustion On Alkali Metals Promoted Oxides

Diesel vehicles are receiving a growing share of light-duty vehicle market due to their highefficiency and low operating costs, but one of the emissions of their pollutants—particulatematerials （PM） have caused severe environmental problems and threatened our livingconditions. In the latest days, continuous somg spread all over the most provinces of China andthe major pollution is PM with the diameter smaller than2.5μm （PM2.5）, which is generallyoriginated from diesel vehicles. Demands of modern technologies for the control of PM arevery urgent. Catalytic combustion is one of the most efficient motheds to minimize sootdischarge, and the most important parameter is catalysts. Nowadays alkali metals have beenstudied widely due to their low price, large amount and outstanding activities, however, themechanism and essential catalytic performance of alkali metals are still in dispute.In this work, alkali metals such as K, Na and Cs are loaded on five different oxides （Al₂O₃,MgO, CeO₂, ZrO₂and TiO₂） by impregnation method using the corresponding carbonates asprecursors. BET surfaces and pore sizes of alkali metal-supported catalysts decreasedobviously comparing with the oxides themselves. This is mainly because that the supported K,Na and Cs species blocked some pores and caused dense surface coverage. Further studiessuggested that the combination between alkali metals and oxides depends on the Sandersonelectronegativity of oxides. The larger of the Sanderson electronegativity, the stronger of theiracidity are, and the easier they combine with alkali metals. So we can conclude from the resultsof XRD that after Na, K and Cs doping on TiO₂, new phases such as Na2Ti6O13, K2Ti6O13andCs2Ti6O13were formed. The orresponding meta-aluminate also appeared after doping on Al₂O₃with the less acidity. As on MgO, CeO₂and ZrO₂, there were only weak interaction betweenalkali metals and oxides.Catalysts were characterized by X-ray absorption fine structure （XAFS）, X-rayphotoelectron spectroscopy analysis （XPS）, Temperature Programmed Desorption with CO₂（CO₂-TPD）, in-situ FTIR of CO₂-adsorption and so on. Infrared transmission was alsoemployed in order to study structures of catalysts. Results showed that three different types ofK-related species formed on the catalysts:①K anchored on the support;②the quasi free KO_xspecies③K₂CO₃formed due to the adsorption of CO₂on KO_x. Similar conclusion can be made on Na and Cs-supported catalysts according to the results of X-ray powder diffraction（XRD）, XAFS and CO₂-TPD.Comparing with the oxides themselves, the presence of alkali metals significantlyimproved the activity of soot combustion, and results of O₂-TPO were in accordance with TOFon K and Cs supported catalysts, with the order of Zr>Ce>Mg>Al>Ti. Further studies onK-supported catalysts showed that both catalysts can exhibit the same catalytic activities whenK exists in the form of KO_xor K₂CO₃.The mechanism for soot combustion with O₂on different alkali metal supported catalystswas proved using in-situ IR. The reaction intermediate of soot combustion named ketene groupwith different peak position was detected on K and Cs supported catalysts: Ketene groupinfluenced by K arose at2162cm^-1, while Cs at2148cm^-1. No peak was observed onNa-catalysts in similar place. The differences can be explained by Sanderson electronegativityof alkali metals. Elements with larger electronegativity tend to attract electrons of C-O bond inthe C=C=O group much easier, leading to the blueshift in wavenumber of IR. Keteneinfluenced by Na was too active to be detected because Na exhibited the largestelectronegativity. Thus we can conclude that the oxygen spillover mechanism is universallyapplicable to different alkali metal supported oxides in the existence of free alkali metals.