Supported shaped catalysts based on cobalt mixed oxides for N2O decomposition

Disertační práce se zabývá výzkumem katalyzátorů na bázi kobaltových směsných oxidů pro nízkoteplotní rozklad N2O na kyslík a dusíku s potenciální aplikací pro snižování emisí N2O v odpadních plynech z výroby HNO3. V první části práce byl vyhodnocen vliv vnější difúze a axiální disperze na dosažené konverze N2O v laboratorních reaktorech s cílem výběru vhodných podmínek pro testování tvarovaných katalyzátorů, které umožní přímý přenos dat do většího měřítka. V další části práce byly zkoumány kobaltové katalyzátory pro rozklad N2O nanesené na různých komerčních nosičích v peletizované a pěnové formě a hledány souvislosti mezi způsobem přípravy, fyzikálně-chemickými vlastnostmi katalyzátorů s využitím dostupných metod (AAS, BET, XRD, SEM-EDS, FTIR, Ramanová spektroskopie, Rtuťová porozimetrie a H2-TPR) a jejich katalytickou aktivitou. V případě směsného oxidu Co3O4, naneseném na různé druhy tvarovaných komerčních nosičů (TiO2, Al2O3 a směsné oxidy Mg-Al s různým poměrem Mg a Al) bylo zjištěno, že Co3O4 nanesený na nosiči s vysokým obsahem Mg vykazoval nejvyšší katalytickou aktivitu mezi studovanými vzorky, přestože obsahoval hůře redukovatelné složky. Toto lze vysvětlit přítomností snadněji redukovatelných aktivních míst a lepší disperzí aktivní fáze na povrchu nosiče, což přispívá k většímu počtu dostupných aktivních míst. V případě katalyzátorů na bázi směsných oxidů kobaltu, nanesených na keramické pěny, bylo studováno následující: • Vhodná metoda přípravy katalyzátorů: Směsné oxidy Co3O4 a Co4MnAlOx byly naneseny na pěnu SiC suspenzní a impregnační metodou. Použití suspenzní metody vedlo ke vzniku aktivní fáze s větším měrným povrchem a lepší redukovatelností, což vedlo k vyšší konverzi N2O. Nevýhodou vzorků připravených suspenzní metodou byla špatná adheze aktivní vrstvy k nosiči způsobená praskáním vrstvy katalyzátoru po kalcinaci. • Vhodný druh keramického pěnového materiálu: Aktivní fáze (Co3O4-Cs) byla nanesena na různé druhy keramických pěn (Al-Si, Zr-Mg-Al a SiC-Al). Horší redukovatelnost (z důvodu interakcí mezi aktivní fází s nosičem) a horší disperze aktivní fáze na nosiči vedly k nižší katalytické aktivitě aktivní fáze nanesené na SiC-Al a Zr-Mg-Al ve srovnání se vzorkem naneseným na nosič Al-Si. • Optimální počet vrstev aktivní fáze na keramické pěně: Co3O4-Cs a Co4MnAlOx-K byly naneseny na keramickou pěnu Al-Si v různém počtu vrstev pro dosažení většího množství aktivní fáze v katalyzátoru. Bylo zjištěno, že nanesení více vrstev vedlo ke snížení dostupnosti aktivních míst spodních vrstev po nanesení druhé a další vrstvy. Z tohoto důvodu nedošlo v obou případech po nanesení druhé a další vrstvy k očekávanému zvýšení katalytické aktivity. • Vhodnost nanesení různých mezivrstev na keramickou pěnu za účelem zvýšení měrného povrchu katalyzátoru: Co3O4-Cs byl nanesen na keramické pěny Al-Si, které byly již potaženy různými mezivrstvami (MgO, Mn2O3, SiO2 a TiO2). Použití vybraných mezivrstev vedlo k horší disperzi aktivní fáze na nosiči a ke změnám v redukovatelnosti způsobených interakcemi mezi aktivní fází a mezivrstvou. Z tohoto důvodu nebylo pozorováno očekávané zvýšení konverze N2O vlivem vybraných mezivrstev a to i přes větší měrný povrch katalyzátorů s mezivrstvami. Nosičové katalyzátory s nejvyšší katalytickou aktivitou byly pomocí matematického modelování provozního reaktoru pro snížení N2O v odpadním plynu z výrobny HNO3 porovnávány s nenosičovým komerčním katalyzátorem AST-4 se stejným chemickým složením aktivní fáze.This Ph. D. thesis focuses on the research of supported catalysts based on cobalt mixed oxides for low temperature N2O decomposition applicable in the reduction of N2O emissions from HNO3 production plants. The first part of this work focuses on the study of laboratory reactors for shaped catalyst testing. Catalytic test conditions suitable for obtaining kinetic data not affected by macro kinetic elements, which would allow direct data transfer to a larger scale, were determined. In the second part of this thesis, cobalt catalysts deposited on different commercial supports in the pelletized and open-cell foam form were studied for N2O decomposition. The relationship between the method of catalyst preparation, their physico-chemical properties evaluated by available methods (AAS, BET, XRD, SEM, FTIR, Raman, Hg-porosimetry, H2-TPR) and catalytic properties were studied. In the case of Co3O4 deposited on the different kinds of pelletized supports (TiO2, Al2O3, and Mg-Al mixed oxides with various Mg and Al), it was found that Co3O4 deposited on a support with a high content of Mg possessed the highest catalytic activity among the supported samples, in spite of the fact that it contained hardly reducible compounds. This can be explained by the presence of active sites with easier reducibility and better dispersion of active phase on the surface of the support. The part dealing with cobalt based catalyst deposited on the open-cell ceramic foam consists of the following steps: • Determination of the optimal preparation method: Co3O4 and Co4MnAlOx were deposited on the SiC open-cell foams by wet impregnation and suspension methods. Suspension method provides active phase with higher surface areas and sites with better reducibility, both of these factors contribute to higher N2O conversions. However, the layer of active phase prepared by suspension method has worse adhesion to support, due to catalyst layer cracking. • Determination of a suitable kind of ceramic foam material: Co3O4-Cs was deposited on the different kinds of ceramic foams (Al-Si, Zr-Mg-Al and SiC-Al). Samples on the SiC-Al and Zr-Mg-Al supports showed lower catalytic activity in comparison to samples on the Al-Si support, which could be related to worse dispersion of active phase on these supports and interaction of active phase with support material. • Determination of the optimal number of catalyst layers on the ceramic foam supports: Co3O4-Cs and Co4MnAlOx-K were deposited on the ceramic foams with different numbers of deposited catalyst layers. Application of several active layers, in order to increase the amount of active phase in the catalyst, leads to the reduction of active sites' accessibility in catalytic reaction and did not lead to an increase in catalytic activity. • Optimization of catalyst preparation in order to increase catalyst surface area by applying different interlayers: Co3O4-Cs was deposited on the ceramic foams already coated by different interlayers (MgO, Mn2O3, SiO2 and TiO2). The use of chosen interlayers led to worse dispersion of active phase on the support and changes in reducibility. In the last part of the dissertation, supported catalysts with the highest catalytic activity and unsupported commercial pellets AST-4 with the same chemical composition of active phase were compared by mathematical modeling of the full scale reactor for N2O abatement in waste gas from HNO3 production plants to evaluate the catalytic activity in industry conditions.9350 - Institut environmentálních technologiívyhově

Must the best laboratory prepared catalyst also be the best in an operational application?

Three cobalt mixed oxide deN(2)O catalysts, with optimal content of alkali metals (K, Cs), were prepared on a large scale, shaped into tablets, and tested in a pilot plant reactor connected to the bypassed tail gas from the nitric production plant, downstream from the selective catalytic reduction of NOx by ammonia (SCR NOx/NH3) catalyst. High efficiency in N2O removal (N2O conversion of 75-90% at 450 degrees C, VHSV = 11,000 m(3) m(bed)(-3) h(-1)) was achieved. However, a different activity order of the commercially prepared catalyst tablets compared to the laboratory prepared catalyst grains was observed. Catalytic experiments in the kinetic regime using laboratory and commercial prepared catalysts and characterization methods (XRD, TPR-H-2, physisorption, and chemical analysis) were utilized to explain this phenomenon. Experimentally determined internal effectiveness factors and their general dependency on kinetic constants were evaluated to discuss the relationship between the catalyst activity in the kinetic regime and the internal diffusion limitation in catalyst tablets as well as their morphology. The theoretical N2O conversion as a function of the intrinsic kinetic constants and diffusion rate, expressed as effective diffusion coefficients, was evaluated to estimate the final catalyst performance on a large scale and to answer the question of the above article title.Web of Science92art. no. 16

Precipitated K-promoted Co-Mn-Al mixed oxides for direct NO decomposition: Preparation and properties

Direct decomposition of nitric oxide (NO) proceeds over Co-Mn-Al mixed oxides promoted by potassium. In this study, answers to the following questions have been searched: Do the properties of the K-promoted Co-Mn-Al catalysts prepared by different methods differ from each other? The K-precipitated Co-Mn-Al oxide catalysts were prepared by the precipitation of metal nitrates with a solution of K2CO3/KOH, followed by the washing of the precipitate to different degrees of residual K amounts, and by cthe alcination of the precursors at 500 degrees C. The properties of the prepared catalysts were compared with those of the best catalyst prepared by the K-impregnation of a wet cake of Co-Mn-Al oxide precursors. The solids were characterized by chemical analysis, DTG, XRD, N-2 physisorption, FTIR, temperature programmed reduction (H-2-TPR), temperature programmed CO2 desorption (CO2-TPD), X-ray photoelectron spectrometry (XPS), and the species-resolved thermal alkali desorption method (SR-TAD). The washing of the K-precipitated cake resulted in decreasing the K amount in the solid, which affected the basicity, reducibility, and non-linearly catalytic activity in NO decomposition. The highest activity was found at ca 8 wt.% of K, while that of the best K-impregnated wet cake catalyst was at about 2 wt.% of K. The optimization of the cake washing conditions led to a higher catalytic activity.Web of Science97art. no. 59

Co-Mn-Al mixed oxides promoted by K for direct NO decomposition: Effect of preparation parameters

Fundamental research on direct NO decomposition is still needed for the design of a sufficiently active, stable and selective catalyst. Co-based mixed oxides promoted by alkali metals are promising catalysts for direct NO decomposition, but which parameters play the key role in NO decomposition over mixed oxide catalysts? How do applied preparation conditions affect the obtained catalyst's properties? Co4MnAlOx mixed oxides promoted by potassium calcined at various conditions were tested for direct NO decomposition with the aim to determine their activity, stability and selectivity. The catalysts were prepared by co-precipitation of the corresponding nitrates and subsequently promoted by KNO3. The catalysts were characterized by atomic absorption spectrometry (AAS)/inductive coupled plasma (ICP), X-ray photoelectron spectrometry (XPS), XRD, N-2 physisorption, temperature programmed desorption of CO2 (TPD-CO2), temperature programmed reduction by hydrogen (TPR-H-2), species-resolved thermal alkali desorption (SR-TAD), work function measurement and STEM. The preparation procedure affects physico-chemical properties of the catalysts, especially those that are associated with the potassium promoter presence. The addition of K is essential for catalytic activity, as it substantially affects the catalyst reducibility and basicity-key properties of a deNO catalyst. However, SR-TAD revealed that potassium migration, redistribution and volatilization are strongly dependent on the catalyst calcination temperature-higher calcination temperature leads to potassium stabilization. It also caused the formation of new phases and thus affected the main properties-S-BET, crystallinity and residual potassium amount.Web of Science97art. no. 59

How loading of Co3O4-Cs on an open-cell foam influences N2O decomposition

Co3O4 modified with Cs was deposited on an alpha- Al2O3 open-cell foam, characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction by hydrogen (TPR-H2), and scanning electron microscopy-energy-dispersive Xray analysis (SEM-EDAX) and tested for the low-temperature decomposition of N2O. The aim was to study the effect of the amount of active phase on N2O conversion. Three different approaches were used: (i) the application of foam supports with different cell sizes, (ii) influencing catalyst loading using impregnation solutions with different precursor concentrations, and (iii) deposition of the active phase precursor by repeated immersion-calcination cycles. Increasing the geometric surface area of the support, and thus catalyst loading, was successfully done using the support with higher pore densities. A higher loading was also achieved by increasing the nitrate precursor concentration in the impregnation solution. In both cases, the catalyst activity increased with an increase in the amount of the active phase. Compared to that, a repeated impregnation procedure can ensure the deposition of a higher amount of active phase in comparison to that obtained with one-step impregnation, but only the last layer is used in the reaction and the rest of the active phase remains unutilized, which makes this type of preparation unfavorable. The high catalytic activity was preserved at 450 degrees C even in the presence of O2, H2O, and NO.Web of Science6231309130

Washcoated open-cell foam cobalt spinel catalysts for N2O decomposition

The Co3O4 modified with 1 wt.% Cs was deposited on alpha-Al2O3 open-cell foam covered with different washcoats (MgO, MnO2, SiO2, TiO2), investigated by XRD, Raman microspectroscopy, nitrogen physisorption, XPS, SEM and TPR-H2 in order to elucidate interactions between Cs-Co3O4 and the washcoat and their effect on surface area, reducibility, dispersion, and low temperature decomposition of N2O. The samples with SiO2 and TiO2 washcoats had the largest surface areas. Only SiO2 did not interact with the active phase and did not change the reducibility of the catalyst. Although the same Cs concentration was adjusted during preparation of all catalysts, differences in the Cs/Co surface molar ratio were observed due to a different level of Co3O4 particles aggregation and cesium dispersion. Catalytic activity correlated with the surface Cs/Co molar ratio closely interconnected with the surface Co3+/Co2+ molar ratio while there were no direct relationships to the redox properties and surface area. The highest activity was achieved for the MgO washcoat prepared from carbonate with the highest Co3+/Co2+ molar ratio corresponding to the optimal Cs/Co surface molar ratio around 0.1.Web of Science533art. no. 11275

Cobalt oxide catalysts on commercial supports for N2O decomposition

Co3O4 oxide catalysts prepared on different commercial supports, namely, TiO2, Al2O3, and Mg-Al mixed oxides with various Mg and Al concentrations, were characterized by atomic absorption spectrometry, Brunauer-Emmett-Teller method, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and H-2 temperature-programmed reduction, tested for N2O decomposition, and compared with bulk Co3O4. In spite of the fact that Co3O4/70Mg30Al also contained hardly reducible compounds, it possessed the highest catalytic activity, probably due to the presence of active sites with easier reducibility and better dispersion of the active phase on the support contributing to a higher number of active sites. The conversion over Co3O4-supported tablets is comparable with that of the same catalyst bed of bulk Co3O4 tableted catalyst.Web of Science40599098

Oxygen effect in NO direct decomposition over K/Co-Mg-Mn-Al mixed oxide catalyst - Temperature programmed desorption study

Co-Mg-Mn-Al mixed oxide with or without K was prepared and characterized by XPS, TPR H-2, TPD NO, TPD O-2 and TPD NO+O-2. NO decomposition was performed over catalyst activated in different atmosphere (N-2, He, O-2). Presence of oxygen during activation changed the course of surface stabilization but did not affect NO conversion achieved at steady state. O-2 desorbed as a reaction product does not affect the reaction rate for NO inlet con-centration of 400-8000 ppm, but presence of O-2 in inlet gas mixture (0.05 -3 mol.%) has significant inhibiting effect. Transition metals in octahedral coordination represent sites for NO chemisorption, while potassium en -sures NO oxidation to surface nitrites, which are indispensable reactive species. Oxygen desorption was the slowest reaction step, while N-2 desorption was fast. Oxygen inhibition was ascribed to oxidation of NO* species on the octahedral Co3+ and Mn3+ to more stable surface nitrates making NO* species as limiting reactant.Web of Science510art. no. 11169

An investigation on the N2O decomposition activity of Mn x Co1−x Co2O4 nanorods prepared by the thermal decomposition of their oxalate precursors

This investigation aimed to develop a series of new spinel-oxide catalysts with the general formula MnxCo1-xCo2O4 (0.0 <= x <= 1.0) and testing their activity towards N2O direct decomposition. These catalysts have been prepared by the microwave-assisted co-precipitation method and the subsequent calcination at 500 degrees C. Characterization results revealed that the utilized preparation protocol has led to the development of pure spinel phases with nanorods morphology. N2O decomposition experiments indicated that the prepared catalysts were active and the best performance was exhibited by Mn0.75Co0.25Co2O4 catalyst. The activity of the Mn0.75Co0.25Co2O4 was enhanced by doping with K-ions at optimal concentration (n(K)/(n(Co) + n(Mn)) = 0.025). The co-existence of some contaminants (O-2, NO, H2O vapor) in the reactor feed revealed different inhibitory levels on the performance of the K0.025Mn0.75Co0.25Co2O4 catalyst. The inhibitory order was O-2 < H2O < NO < O-2 + H2O < O-2 + H2O + NO.Web of Science9328927

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