Influence of Potassium and NO Addition on Catalytic Activity in Soot Combustion and Surface Properties of Iron and Manganese Spinels

Abstract

Two series of (0-4 wt%) potassium doped oxide catalysts based on iron and manganese spinel were prepared. The synthesized materials were characterized in terms of their structure (XRD, Raman spectroscopy) and surface electronic properties (work function measurements). The catalytic activity towards soot combustion was determined by temperature programmed oxidation of a physical mixture of soot and catalyst in tight contact in gas oxygen mixtures with and without NO addition. For iron spinel based materials, where potassium is localized at the surface, the catalytic activity correlates with the work function lowering upon K doping, while for manganese spinel based materials, where potassium is incorporated into the bulk (formation of KMn 4 O 8 or KMn 8 O 16 ), the correlation was not found. The presence of NO in the gas mixture leads to a systematic decrease of soot ignition temperature for all samples.

Strong Enhancement of deSoot Activity of Transition Metal Oxides by Alkali Doping: Additive Effects of Potassium and Nitric Oxide

“…It was found that NO has a positive effect on catalyst activity in soot combustion as it forms NO 2 , which is a stronger oxidant for carbon particles than the oxygen [22]. A systematic shift of the temperature window of soot combustion on potassium-promoted spinels in the presence of NO was also reported [23]. It was found that potassium-containing catalysts can be used for the simultaneous removal of NO x and soot [24][25][26], so similar catalytic systems were also investigated in bench-scale experiments [26][27][28].…”

“…The surface doped catalysts were prepared by the wet impregnation of 1 g of the iron (Aldrich) and cobalt spinels (Fluka) with a potassium carbonate solution, to achieve the optimal alkali loading corresponding to the theoretical 0.5 monolayer, as described in [23]. The impregnated spinel was dried at 100°C for 1 h and subsequently calcined at 400°C for 4 h. These samples were further designated as K/Fe 3 O 4 and K/Co 3 O 4 , respectively.…”

How to Efficiently Promote Transition Metal Oxides by Alkali Towards Catalytic Soot Oxidation

“…Surface promotion: the surface doped catalysts were prepared by wet impregnation of 1 g of the investigated spinels (Mn 3 O 4 , Fe 3 O 4 -Aldrich, Co 3 O 4 -Fluka) with a potassium carbonate (POCh) to achieve optimal alkali loading corresponding to theoretical 0.5 monolayer, as described in [15]. Impregnated spinels were dried at 100°C for 1 h, and subsequently calcined at 400°C for 4 h. The temperature was optimal for potassium cations to disperse equally on the surface, and low enough not to change the structure or surface area of investigated spinels.…”

“…Some simple transition metals or oxides also exhibit good catalytic activity and stability [12]. The most promising materials in terms of activity, price and environmental friendliness are based on perovskites [13,14] and spinels [15,16]. The research presented in this paper is focused on the catalytic activity of manganese, iron and cobalt spinels in soot combustion as they are commonly known as the catalysts for many oxidation reactions, for example methane [17], carbon oxide [18] or aromatic compounds [19].…”

“…Surface modifications are believed to have a number of effects. Firstly, they result in the modification of the electrodonor properties of the surface [15], which may lead to the formation of active forms of oxygen by means of the electron transfer. Secondly, the formation of low melting point compounds or superficial carbonate intermediates may improve soot-catalyst contact [9].…”