K-modified Co-Mn-Al mixed oxide-effect of calcination temperature on N2O conversion in the presence of H2O and NOx

The effect of calcination temperature (500-700 degrees C) on physico-chemical properties and catalytic activity of 2 wt. % K/Co-Mn-Al mixed oxide for N2O decomposition was investigated. Catalysts were characterized by inductively coupled plasma spectroscopy (ICP), X-ray powder diffraction (XRD), temperature-programmed reduction by hydrogen (TPR-H-2), temperature-programmed desorption of CO2 (TPD-CO2), temperature-programmed desorption of NO (TPD-NO), X-ray photoelectron spectrometry (XPS) and N-2 physisorption. It was found that the increase in calcination temperature caused gradual crystallization of Co-Mn-Al mixed oxide, which manifested itself in the decrease in Co2+/Co3+ and Mn3+/Mn4+ surface molar ratio, the increase in mean crystallite size leading to lowering of specific surface area and poorer reducibility. Higher surface K content normalized per unit surface led to the increase in surface basicity and adsorbed NO per unit surface. The effect of calcination temperature on catalytic activity was significant mainly in the presence of NOx, as the optimal calcination temperature of 500 degrees C is necessary to ensure sufficient low surface basicity, leading to the highest catalytic activity. Observed NO inhibition was caused by the formation of surface mononitrosyl species bonded to tetrahedral metal sites or nitrite species, which are stable at reaction temperatures up to 450 degrees C and block active sites for N2O decomposition.Web of Science1010art. no. 113

Application of calcined layered double hydroxides as catalysts for abatement of N2O emissions

The results of catalytic decomposition of N2O over mixed oxide catalysts obtained by calcination of layered double hydroxides (LDHs) are summarized. Mixed oxides were prepared by thermal treatment (500 °C) of coprecipitated LDH precursors with general chemical composition of MII1-xMIIIx(OH)2(CO3)x/2·yH2O, where MII was Ni, Co, Cu and/or Mg, MIII was Mn, Fe and/or Al, and the MII/MIII molar ratio was adjusted to 2. The influence of chemical composition of the MII-MIII mixed oxide catalysts on their activity and stability in N2O decomposition was examined. The highest N2O conversion was reached over Ni-Al (4:2) and Co-Mn-Al (4:1:1) catalysts. Their suitability for practical application was proved in simulated process stream in the presence of O2, NO, NO2 and H2O. It was found that N2O conversion decreased with increasing amount of oxygen in the feed. The presence of NO in the feed caused a slight decrease in N2O conversion. A strong decrease in the reaction rate was observed over the Ni-Al catalyst in the presence of NO2 while no N2O conversion decrease was observed over the Co-Mn-Al catalyst. Water vapor inhibited the N2O decomposition over all tested catalysts. The obtained kinetic data for N2O decomposition in a simulated process stream over the Co-Mn-Al catalyst were used for a preliminary reactor design. The packed bed volume necessary for N2O emission abatement in a HNO3 production plant was calculated as 35 m3 for waste gas flow rate of 30 000 m3 h-1

Modification of Co-Mn-Al mixed oxide with promoters and their effect on properties and activity in VOC total oxidation

The activity and selectivity of the Co-Mn-Al mixed oxide catalyst modified with promoters (Pt, Pd, K and La) in total oxidation of volatile organic compounds (toluene and ethanol) were studied. The promoters were introduced at the stage of coprecipitation of a layered double hydroxide (LDH) precursor or impregnation of the mixed oxide obtained by LDH precursor calcination. In total oxidation of toluene, the most active Co-Mn-Al catalysts were those containing low amounts of potassium regardless of the mode of modification, while in total oxidation of ethanol the catalyst impregnated with a higher potassium concentration (3 wt.%) was the most active. Introduction of Pt and Pd in an amount of 0.5 or 0.1 wt.% into the Co-Mn-Al mixed oxide did not improve its catalytic activity. The impregnation method appears to be a more effective mode for preparation of active catalysts than the method using an addition of promoters at the stage of coprecipitation of the LDH precursor. Undesirable reaction intermediate (benzene) was formed when toluene oxidation was carried out over lanthanum- or palladium-containing catalysts. In total oxidation of ethanol, a number of reaction intermediates were produced acetaldehyde being the main one. The catalysts modified at the stage of LDH precursor coprecipitation exhibited a better selectivity (i.e., a lower acetaldehyde formation) than those modified by impregnation. The best results were obtained with the Co-Mn-Al mixed oxide catalyst modified with potassium

N2O catalytic decomposition — effect of pelleting pressure on activity of Co-Mn-Al mixed oxide catalysts

The effect of pelleting pressure (0–10 MPa) during the preparation of Co-Mn-Al mixed oxide catalyst on its texture and activity for N2O catalytic decomposition was examined for small grain sizes used in laboratory experiments, and for model industry catalyst particles. Adsorption/desorption measurements of nitrogen, mercury porosimetry and helium pycnometry were used for detail characterization of porous structure. A volume of micropores of about 20 mm3 g−1 was evaluated using modified BET equation. This value did practically not change with the increasing pelletization pressure except that of the sample formed at the pressure of 10 MPa. Although an increase of pelleting pressure caused an increase in bulk density and a decrease in pore size and pore volume of the prepared catalyst (resulting in lower values of N2O effective diffusion coefficient), no direct correlation between pelleting pressure used and catalyst activity has been found. In contrary, estimation of the internal diffusion limitation according to the Weisz-Prater criterion indicated that even laboratory experimental data obtained for catalyst grains with particle size lower than 0.315 mm pelletized at higher pressures could be influenced by internal diffusion. Estimation of the internal mass transfer limitation in industrial catalyst particles described by the effectiveness factor showed that effectiveness factor of about 0.07 and 0.2 can be obtained for spheres with the radius of 1.5 mm and 0.5 mm, respectively, if pelleting pressure of about 6 MPa was used for the catalyst preparation

Cobalt oxide catalysts supported on CeO2-TiO2 for ethanol oxidation and N2O decomposition

Cobalt oxide catalysts deposited on titania-ceria supports were examined in deep ethanol oxidation and N2O decomposition. Supports with various molar ratio of CeO2/TiO2 were prepared by the sol-gel method and cobalt components were introduced by impregnation and subsequent calcination. The supports and catalysts were examined by chemical analysis, X-ray diffraction, nitrogen physisorption, H-2-TPR, and NH3-TPD. It was found out that the ethanol conversion at 200 degrees C is proportional to the CeO2/(CeO2 + TiO2) molar ratio in the supports, and temperature T-50 of ethanol oxidation is proportional to the amount of components reducible in the temperature range of 20-500 degrees C. A comparison of specific catalytic activities in both ethanol oxidation and N2O decomposition proved a lower rate of N2O decomposition than that of oxidation of ethanol (approximately 25 times). The findings confirmed the great importance of the supports surface areas on specific activity of cobalt catalysts in both reactions. The obtained results showed that ceria is the best support of cobalt oxides for both deep ethanol oxidation and N2O decomposition when reaction rates are related to unit amount of active component in the catalysts.Web of Science121113912

Cobalt oxides supported over ceria-zirconia coated cordierite monoliths as catalysts for deep oxidation of ethanol and N2O decomposition

Cordierite monoliths coated with ceria-zirconia supporting cobalt oxide were prepared, examined in the deep oxidation of ethanol and N2O decomposition, and compared with pelletized commercial cobalt oxide catalyst. Interaction of Co3O4 with ceria-zirconia washcoat led to formation of Co3O4 particles with slightly worse structure ordering resulting in better reducibility than that observed for the commercial Co3O4 catalyst. In oxidation of ethanol, activity of the Co3O4-containing monoliths was comparable with that of pelletized cobalt oxide catalyst with nearly seven times higher content of active components. However, conversions of N2O over the monolith catalysts were lower. Nevertheless, incorporation of Co3O4 onto ZrO2-CeO2 washcoat increased rate of both catalytic reactions, i.e., N2O decomposition and deep ethanol oxidation.Web of Science14761391137

Optimization of Cs content in Co-Mn-Al mixed oxide as catalyst for N2ON_2O decomposition

A series of Co–Mn–Al mixed oxide catalysts with different Cs contents (0.5–4.6 wt%) was prepared by calcination of Co–Mn–Al hydrotalcite (Co:Mn:Al = 4:1:1), followed by impregnation by cesium salt (CsNO3, Cs2CO3) using the pore filling method. Chemical analysis, N2 sorption, temperature programmed reduction (TPR)-H2, temperature programmed desorption (TPD)-CO2 and TPD-NH3 and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalysts. All prepared catalysts were tested for N2O catalytic decomposition in inert gas and in the presence of oxygen, water vapor and nitric oxide. The influence of Cs salts used for catalyst preparation and cesium content on catalyst activity were studied. A significant increase in catalytic activity with increasing amount of cesium promoter was observed without respect to the Cs precursor. The strong promotional effect of cesium is electronic in nature and is discussed in term of changes in surface composition and catalyst reducibility.Web of Science41129332931

Co–Mn–Al mixed oxides as catalysts for ammonia oxidation to N2ON_2O

Mixed oxide catalysts containing Co, Mn, and Al in a molar ratio of 4:1:1 were prepared by heating precursors obtained by a mechanochemical method (when appropriate nitrates were milled with ammonium hydrogen carbonate) or by coprecipitation. The precursors and related mixed oxides obtained at 500 °C were characterized by XRD, TG/DTA, UV–Vis, IR spectroscopy, TPR and NH3-TPD, and were tested for activity and selectivity in ammonia oxidation to N2O. The precursors prepared by mechanochemical reaction showed very similar phase composition to that of the coprecipitated product. All examined mixed oxide catalysts were active in the low temperature range (100 % conversion of NH3 was achieved at 250 °C), but the selectivity was sensitive to catalyst composition and the method used for preparation of the precursors. Non-modified Co–Mn–Al mixed oxide catalyst obtained from the coprecipitated precursor exhibited higher selectivity towards N2O formation than the calcined product prepared by the mechanochemical method. Modification of the latter mixed oxide catalysts with cesium promoter (1 wt%) significantly increased their selectivity to nitrous oxide. The yield of N2O at 250 °C was close to 100 %.Web of Science4232690266

Catalytic activity of cobalt grafted on ordered mesoporous silica materials in N2O decomposition and CO oxidation

Three different ordered mesoporous silica materials, such as MCM-41, Al incorporated into the silica framework MCM-41 (mass ratio Si/Al = 9) and SBA-15, were prepared. Furthermore, aluminum was grafted on the surface of MCM-41 (mass ratio Si/AI =16) and SBA-15 (mass ratio Si/Al= 23) by the molecular designed dispersion (MDD) method. In a next step, cobalt (Co =10-12 wt%), as an active metal for redox reactions, was introduced by the MDD technique. The prepared catalysts were characterized by atomic absorption spectroscopy, microwave plasma-atomic emission spectroscopy, N-2 physisorption, thermogravimetric analysis, X-ray diffraction, diffuse reflectance UV-vis spectroscopy, infrared and Raman spectroscopies, transmission electron microscopy, temperature programmed reduction by hydrogen and their catalytic properties were evaluated for two model reactions: N2O decomposition and CO oxidation. All prepared catalysts showed poor activity in the reaction of N2O decomposition and the use of reducing agent (carbon monoxide) improved it only little. Contrary to this, all catalysts were significantly more active for CO oxidation by oxygen. The highest activity was reached over the SBA + Co catalyst which is proposed to be due to the smallest size of tetrahedral Co(II) species on SBA-15 silica support.Web of Science437725

Effect of preparation method on catalytic properties of Co-Mn-Al mixed oxides for N2O decomposition

Co-Mn-Al mixed oxides (Co:Mn:Al molar ratio of 4:1:1) were prepared by three different methods (i) calcination of hydrotalcite-like precursors (Co-Mn-Al-HT-ex), (ii) calcination of corresponding nitrates (Co-Mn-Al-nitr) and (iii) calcination of the product of mechanochemical reaction of Co, Mn, Al nitrates with NH4HCO3 (Co-Mn-Al-carb). The catalysts were characterized by AAS, XRD, SEM, Raman spectroscopy, FTIR, TPR-H2, TPD-N2O, step response experiments and tested for N2O decomposition in inert gas and simulated waste gas from HNO3 production. Different conditions of synthesis led to the formation of spinel-like phase with different structural properties leading to different catalytic activity. N2O conversions decreased with decreasing specific surface area in order Co-Mn-Al-carb > Co-Mn-Al-HT-ex > Co-Mn-Al-nitr. The synthesis from carbonate precursor led to the less ordered structure manifested itself in smaller crystallite size causing (i) higher surface area (ii) better reducibility and (iii) increase of mean (Co + Mn) valence. However, its highest catalyst activity was determined by the lowest bond strength of active site − oxygen while specific surface area and amount of active sites per unit surface were not decisive parameters for activities order.Web of Science42524723