Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts
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This paper describes the investigations of the catalytic activity in soot oxidation over well-defined iron oxide based materials. The nanostructuration of iron oxide by potassium into tunnelled (KFeO2) and layered (K2Fe22O34) ferrites and the surface promotion with CeO2 results in the marked increase in the catalytic activity (decrease of the ignition temperature down to 210 °C and T 10 % to 310 °C). The measurements of the catalysts work function showed that both nanostructuration and surface promotion with ceria of the best KFeO2 phase led to increase of the electron availability (decrease of the work function). Strong correlation of the catalytic activity in soot combustion of the Ce–K–Fe–O systems with the work function value was revealed for the first time in the model studies, and can be used as a guideline for optimisation of the real catalytic filters.
KeywordsIron oxide Potassium Cerium oxide Soot Diesel Oxidation Promotion Ferrite Work function
Soot particles emitted from diesel engines, due to their submicron size and specific chemical composition, are among the most hazardous urban air pollutants . Native soot self-ignites at temperatures higher than 550 °C, but it could be burned in catalytic filters already at the temperature of the exhaust gases (200–400 °C), provided an efficient catalyst is applied. The desired catalyst should not only assure complete combustion at relatively low temperatures, but also be cheap and stable at broad range of operating conditions. Facile washcoating and the use of ecologically benign raw materials are another practical requirements. In addition to catalyst developments, several approaches have been adopted in the field of soot filtration  and filter regeneration , as they may affect the desired morphology of the catalyst.
A large number of catalytic formulations, including single and mixed transition metal oxides, perovskite- and spinel-type oxides, and low melting point compounds, have been tested for the catalytic combustion of diesel soot [4, 5, 6, 7]. Despite of the progress made, the reaction mechanism for this process has been scarcely addressed and therefore a lot of work is still needed in order to understand and advance a rational catalyst design. The investigated catalytic systems operate satisfactory only above 500 °C, but upon impregnation with alkali, among which potassium [5, 8] is the most extensively investigated, their activity can be substantially enhanced . Its positive role on soot combustion has been discussed in terms of: (i) an electron donor effect, increasing the oxygen reactivity; (ii) autogenic formation of low melting point compounds, or eutectics with other components of the catalyst, wetting the soot surface to increase the contact with the catalyst, (iii) the formation of a superficial carbonate intermediate and (iv) nanostructurization of the parent oxide. The latter case is may be illustrated by the transformation of Fe2O3 into tunnelled and layered ferrites of KFeO2, and K2Fe22O34 structures. Such materials exhibiting low work function and semiconductivity are promising generic materials for model redox catalytic studies since electronic properties can be easily tuned by Fermi level adjustment .
On the other hand CeO2 alone or in combination with other metal oxides can be also highly active in oxidation of soot and it is suggested that the mechanism involved in such a case is based of the redox properties of ceria related to its ability to deliver the structural oxygen for the reaction .
In the present work we described the nanostructuration of iron oxide into layered and tunneled K–Fe–O catalyst. The catalytic of the resultant ferrites can be further enhanced by doping with ceria nanoparticles. Both effects were discussed in terms of electronic factor-structure–activity relationship.
2 Experimental Part
Several iron containing oxides were used in the study: α-Fe2O3, K2Fe22O34 as well as promoted and unpromoted KFeO2. α-Fe2O3 was obtained from Merck and used without further processing. The samples of K2Fe22O34 ferrite were prepared by solid-state reaction of a stoichiometric mixture of K2CO3 with α-Fe2O3 at 1200 °C as described previously . The KFeO2 sample was synthesized from K2CO3 with α-Fe2O3 according to the procedure described in . In brief, the mixtures of finely ground powders were placed in a porcelain crucible and heated with the rate of 5 °C/min in static air up to the final temperatures of 800 °C for KFeO2 and 1200 °C for K2Fe22O34. For 0.5–4 wt% Ce-doped KFeO2 samples, the appropriate fraction of α-Fe2O3 powder was replaced by CeO2. Four CeO2 types of precursors were used for KFeO2 promotion: micrometric commercial cerium(IV) oxide (Merck), nanometric CeO2 obtained by sol–gel method, solid ammonium cerium(IV) nitrate (Merck) forming an oxide upon calcination and ammonium cerium(IV) nitrate water solution with subsequent impregnation of KFeO2, drying in 100 °C for 1 h and calcination in 400 °C for 4 h. All such obtained samples were mixed with carbon black, Printex 80 (Degussa AG) with catalyst to soot ratios: 1/1, 4/1, 8/1. Both “loose” than a “tight” conditions  were tested in preliminary experiments, but since the results were better reproducible in the latter conditions the “tight contact” was applied in the comparative tests. Catalytic activity of the samples was determined by temperature-programmed oxidation (TPO) tests, carried out with the catalyst/soot mixtures as described above. The amount of sample loaded into the reactor varied from 50 to 100 mg. The soot combustion experiments were carried using a heating rate of 10 °C/min, in the flow of 5 % O2 in He. The gas phase composition (O2, CO, CO2, H2O) was monitored by the QMS spectrometer (SRS RGA200). The ignition temperatures were determined with the use of the Arrhenius-like plot (lnX vs. 1/T), which gives rise to a sharp turning point defining the start of the oxidation process in a non-arbitrary way in contrast to more subjective yet commonly use method of T 5 % or T 10 %.
The samples were characterized by X-ray diffraction (XRD) (X’pert Pro Philips) for phase composition, SEM (Philips XL 30 ESEM) for grain size morphology, N2-BET (Quantasorb Jr) and XPS (Prevac/VG SCIENTA R3000) for surface area and composition, and Kelvin Probe (McAllister KP6500) for evaluation of electronic properties. The contact potential difference (V CPD) measurements were carried out by the dynamic condenser method of Kelvin. To standardize the catalyst surface the measurements were carried out in vacuum of 10−7 mbar after annealing the sample to 400 °C. The work function values were obtained from a simple relation e⋅V CPD = Φ reference – Φ sample, using a standard stainless steel plate as a reference electrode (d = 3 mm, Φ reference = 4.1 eV).
3 Results and Discussion
The introduction of ceria (at the level of 2 %) did not lead to any appreciable changes in the diffraction pattern. The SEM images of the bare ferrites reveal different morphologies of the KFeO2 and K2Fe22O34 grains (Fig. 1). Whereas for the KFeO2 crystallites of 3–5 μm are interconnected forming coarse blunt aggregates of about 10 μm, in the case of K2Fe22O34 the hexagonal well-shaped crystallites of 0.5–2 μm in diameter and thickness of ~0.2 μm were observed, and resembled that of a single K+-β-ferrite with stoichiometric composition . The EDX analysis showed that the distributions of K and Fe are strongly correlated as expected for the ferrite phases, whereas ceria segregates at the surface in the form of spherical smaller nanocrystals with the size in the range of 10–500 nm, depending on the applied preparation method. The XPS surface composition of the investigated samples indicated that within the surface layer only the constituent elements were found with the diagnostic binding energies for KFeO2 (BEFe2p3/2 = 709.7–710.0 eV, BEK2p3/2 = 292.2–292.4 eV, BEO1s = 530.4–530.8 eV) and K2Fe22O34 (BEFe2p3/2 = 710.2–710.4 eV, BEK2p3/2 = 294.4–294.5 eV, BEO1s 529.4–529.5 eV). Small concentration of CeO2 combined with the overlapping Fe LMM Auger signal made it impossible to determine the Ce 3d binding energies.
It is worth mentioning that for the whole series of the investigated samples a strong correlation of the catalytic activity gauged by both the ignition (T ig) and 10 % conversion (T 10 %) temperatures of soot combustion with the work function value exists (Fig. 3a). The observed correlations reveal that the electronic effect (availability of electrons for oxygen activation) governs the soot combustion process. It is valid with the same spectacular trend either in the case of nanostructuration of iron oxide by potassium (gray points) and for subsequent surface modification of the best performing KFeO2 ferrite by 2 % of CeO2 (green points). Such level of doping was found to be optimal, as it can be inferred from the Fig. 3b. The characteristic Topping-like behavior confirms that the observed changes of T ig can actually account for by the associated work function changes. As a result, those findings show that the unique, concise parameter—the catalyst work function—plays the key role in the soot combustion process. The observations of a prime importance of catalyst work function modification for the activity was also observed for other redox reactions [15, 16].
As mentioned, the electronic promotion effect on soot combustion can be explained in terms of enhanced energetic accessibility of the electrons for activation of the dioxygen molecule by the catalyst. Indeed, the facilitation of the electron transfer from the catalyst surface to O2 molecules gives rise to the formation of strongly reactive surface oxygen species, such as O x − radicals (O− and O2 −), which are known as efficient soot oxidation agents taken into account in kinetic modeling of soot oxidation . It is well known that the work function controls the nature of surface oxygen speciation. Two main scenarios related to dioxygen chemisorption include singly ionized molecular (O2 −) and atomic (O−) species as revealed elsewhere . Nonetheless, at this stage the established quantitative work function—combustion temperature relationship can be used as a guidelining principle for the optimization of the catalyst for soot combustion, and searching for new promising washcoats of DPF filters.
Strong correlation of the catalytic activity of the Ce–K–Fe–O systems (ignition and 10 % conversion temperatures of soot combustion) with the catalyst work function value was revealed for the first time. The nanostructuration of iron oxide by potassium into tunnelled (KFeO2) and layered (K2Fe22O34) ferrites, together with the surface promotion by ceria can be interpreted in the unique term of the electronic promotion, gauged by the work function lowering. It is proposed that the enhanced electron availability is beneficial for the generation of the surface reactive oxygen species, initializing the soot combustion process. The established correlation was used as a guideline for systematic optimization of the KFeO2 catalysts by ceria resulting in soot ignition temperature of 210 °C.
This study was sponsored by National Science Centre Polish Grant number 2011/01/B/ST4/00574.
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