Influence of Potassium and NO Addition on Catalytic Activity in Soot Combustion and Surface Properties of Iron and Manganese Spinels
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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 KMn4O8 or KMn8O16), 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.
KeywordsSoot combustion Iron Manganese Spinel Potassium
Soot is a product of the incomplete combustion of hydrocarbons, coal or other such materials, comprised of various arrangements of carbon molecules. The composition of soot depends highly on the conditions and substance being burned. Typical size of soot particles/aggregates between 50 and 500 nm, is large enough to be harmful for human health. Soot has also been proven to have carcinogenic and mutagenic properties . Jacobson , from Stanford University, claims in “Journal of Geophysical Research” that soot pollution is one of the main negative influences causing climate change (global warming). Soot being emitted by factories can stay aloft in the air for as long as a dozen weeks, creating an aerosol which absorbs solar radiation, which can have a direct effect on the climate. According to Jacobson’s calculations, the lowering of soot emissions into the atmosphere would cause the lowering of yearly temperatures by 1 °C in 15 years. Soot, which has settled on the surface of ice, lowers the ability of the ice to reflect solar light—part of the solar radiation is absorbed by the soot and converted into thermal energy. An atmosphere content of a few parts per billion is enough for the settling soot to change the snows reflecting ability by 1 %. This phenomenon has negative consequences for the climate, since such interference into the global energy balance could result in a rise in temperature.
To prevent human illness and climate change soot must be removed or its emissions stopped. Several ways have been developed for this purpose. Reactors or engines may be designed to prevent the formation of soot, yet this method has technological hindrances. On the other hand soot may be removed by use of filters. A disadvantage of this method is that the filters must be replaced and the soot is not permanently eliminated. A third method is to use a catalyst which would lower the temperature of ignition of soot, triggering its combustion in the reactor .
Existing catalysts are either ineffective or based on expensive noble metals. One of such catalysts is based on platinum applied to a cerium–zirconium support. The catalyst is based on technology which places a catalytically active material on a diesel particulate filter (DPF) . Promising substitutes are catalysts based on transition metals, which are relatively cheap and harmless to the environment. Oxides containing transition metals such as perovskite-type  and spinel-type  are being investigated intensively. Moreover, it is widely accepted that the catalytic activity of the material in the soot combustion process can be enhanced by alkali doping [6, 7]. Nitrogen oxides are present in diesel exhaust fumes and some papers report that NO in particular has a positive effect on catalyst activity in soot combustion, as it forms NO2, which is a stronger oxidant for carbon particles than oxygen . The aim of this study is to investigate how alkali doping influences the catalytic activity of iron and manganese oxides with a spinel structure in soot combustion. Moreover, the effect of NO addition will also be addressed.
2.1 Catalyst Preparation
The catalysts were prepared by weighing 1 g of commercial spinel (Mn3O4 and Fe3O4, Aldrich) and adding an appropriate amount of potassium carbonate solution to achieve the desired alkali loading (0–4 wt%). Afterwards, the catalysts were dried for 1 h at 100 °C and subsequently calcined at 400 °C for 4 h.
2.2 Physicochemical Characterization
The Raman spectra were recorded at room temperature in ambient conditions using a Renishaw InVia spectrometer equipped with a Leica DMLM confocal microscope and a CCD detector, with an excitation wavelength of 785 nm. The laser power at the sample position was 1.5 or 3 mW (0.5 or 1 % of total power) with a magnification of 20×. The Raman scattered light was collected in the spectral range of 150–850 cm−1. At least 6 scans, 10 s each, were accumulated to ensure a sufficient signal to noise ratio. XRD patterns were recorded by a Philips X’Pert APD powder diffractometer with Cu Kα radiation at 10 mA and 10 kV, 2θ step scans of 0.02° and a counting time of 1 s per step. Electron donor properties (work function) of the samples were determined by Kelvin Probe (McAllister, KP6500). For each measurement 0.1 g of sample was pressed under 8 MPa for 1 min to form a pellet. The measurement was performed in a vacuum (10−6 mbar) at 150 °C after heating the sample to 400 °C to standardize the catalyst surface. A stainless steel electrode (d = 3 mm, Φ = 4.1 eV) was used as a reference.
2.3 Soot Combustion Tests
The activity of the catalysts in soot combustion was measured by means of temperature programmed reaction (TPR). The quartz fixed-bed reactor was heated (10 °C/min) from RT to 900 °C and two gas mixtures were used during the tests: 5 % O2 in He, as well as 3.3 % O2, 0.3 % NO in He, both at a 60 ml/min flow. The samples for the soot combustion tests were prepared by weighing 0.05 g of soot (Degussa–Printex80) and the catalyst in an 1:8 ratio and then grinding them in a mortar for 10 min following the tight contact method. The changes in the gas composition upon the soot oxidation reaction were monitored by a quadrupole mass spectrometer (SRS RGA200, lines for m/z = 46 (NO2), 44 (CO2), 32 (O2), 30 (NO), 28 (CO), 18 (H2O)).
3 Results and Discussion
The minor effect of addition of NO for the catalysts with high potassium loading is related to the formation of surface potassium carbonate. Indeed, we found, that if pure K2CO3 is used as a catalyst for soot oxidation the addition of NO does not induce any changes in its reactivity. This is in contrast to the redox and noble metal based catalysts where the NO addition usually enhances the catalytic activity .
Two series of (0–4 wt%) potassium promoted iron and manganese spinel based catalysts were investigated in model soot oxidation processes. The temperatures of soot ignition for K-promoted iron spinels were found to be in the range of 320–360 °C, while for K-promoted manganese spinels, in the range of 250–300 °C. We found that presence of NO in the gas feed leads to a decrease of the soot ignition temperature independently of the base metal oxide or potassium loading, suggesting that NO acts as better oxidizing agent than O2. The lowest determined ignition temperature with the presence of NO was found to be ca. 290 °C for K–Fe3O4 and ca. 240 °C for K–Mn3O4. In the case of iron spinel based materials the changes of the observed work function upon K addition correlate with soot oxidation reactivity, while for manganese spinel based materials such correlation is not observed. Whereas, for iron oxide catalyst the addition of potassium leads to the surface promotional effect, for manganese oxides potassium invokes more severe structural changes (formation of KMn4O8 and KMn4O16).
The project was financed by the Polish National Science Center awarded by decision number DEC-2011/01/B/ST4/00574
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