Search Results (38)

The Qieliekeqi siderite deposit, located in the Tashkurgan block of western Kunlun, is a carbonate-hosted iron deposit with hydrothermal sedimentary features. This study integrates whole-rock geochemistry, stable isotopes, and zircon U–Pb–Hf data to investigate its metallogenic evolution. Coarse-grained siderite samples, formed in deeper water, exhibit average Al₂O₃/TiO₂ ratios of 29.14, δEu of 2.69, and δCe of 0.83, indicating hydrothermal fluid dominance with limited seawater mixing. Banded samples from shallower settings show an average Al₂O₃/TiO₂ of 17.07, δEu of 3.18, and δCe of 0.94, suggesting stronger seawater interaction under oxidizing conditions. Both types are enriched in Mn, Co, and Ba, with low Ti and Al contents. Stable isotope results (δ¹³C_PDB = −6.0‰ to −4.6‰; δ¹⁸O_SMOW = 16.0‰ to 16.9‰) point to seawater-dominated fluids with minor magmatic and meteoric contributions, formed under open-system conditions at avg. temperatures of 53 to 58 °C. Zircon U–Pb dating yields an age of 211.01 ± 0.82 Ma, with an average εHf(t) of −3.94, indicating derivation from the partially melted ancient crust. These results support a two-stage model involving Late Cambrian hydrothermal sedimentation and Late Triassic magmatic overprinting. Full article

Deep-sea Fe-Mn polymetallic nodules formed nowadays at the deep-sea ocean floor were evaluated as promising critical raw materials (CRMs). Here, we report results of polymetallic nodules from the H22_NE block of the Interoceanmetal (IOM) exploration area in the eastern part of the Clarion–Clipperton Zone (CCZ), NE Pacific Ocean. The polymetallic nodules were studied with X-ray Diffraction, Raman spectroscopy, SEM-EDS, and LA-ICP-MS (bulk nodules and in situ nodule layers). Additionally, we combine geochemical data of polymetallic nodules with the previously reported data of pore waters and sediments from six stations. Our study aims to define the mineral composition and determine the content of CRMs in the polymetallic nodules and to assess the main factors controlling metal deposition and nodule enrichment in some CRMs. Mn content and the Mn/Fe ratio of the nodules classify them mostly as mixed hydrogenetic–diagenetic type. They are also enriched in Ni, Cu, Co, Zn, Mo, W, Li, Tl, and REE. The in situ REE patterns exhibit MREE and HREE enrichment and a variable Ce anomaly that argues for a changing oxic/suboxic environment and periodically changing of diagenetic and hydrogenetic nodule growth. The results of the joint study of the bottom sediments, pore waters, and polymetallic nodules show a complexity of processes that influence the formation of these deposits. The changing oxic and anoxic conditions are well documented in the chemistry of the nodule layers. Probably the most important controlling factors are sedimentation rate, bioturbation, adsorption, desorption, and oxidation. In addition, growth rates, water depth variations, electro-chemical speciation, phosphatization, and the structures of the Fe-Mn adsorbents are also considered. The polymetallic nodule deposits in the IOM contract area are estimated for future mining for Ni, Cu, Co, and Mn resources. They, however, contain additional metals of economic importance, such as REE and other trace elements (referred to as CRMs) that are potential by-products for metal mining. They can significantly increase the economic importance of exploited polymetallic nodules. Full article

The conversion of NO_x and the yield of N₂O during NH₃-SCR-DeNO_x below 473 K over TiO₂-supported transition metal oxide catalysts with equal loading of 20 wt.-% decreases in the following order of the supported oxides: MnO_x > CuO_x > CoO_x > FeO_x > NiO_x > CeO_x. The storage capacity for NH₃, characterized by the acid site density of the catalyst, is not directly correlated with the catalytic activity. Rather, the temperature range for the reduction of the supported transition metal oxides as determined by TPR-H₂ is the main governing factor for high NH₃-SCR-DeNO_x activity, especially in the temperature range below 473 K. At the same time, oxidation temperature range and the density of Lewis acid sites govern the formation of N₂O. The decomposition of NH₄NO₃ as an intermediate in the NH₃-SCR-DeNO_x reaction is determined by the redox property of TMO-based catalysts, which further influences both the windows of the decomposition temperature and the yield of N₂O. The correlation between the redox properties and the activity for NH₃-SCR-DeNO_x was confirmed for a series of MnO_x-CeO_x/TiO₂-SiO₂ mixed transition metal oxide catalysts as a promising combination of the less active and more selective CeO_x with less selective and highly active MnO_x. The linear correlation between reduction temperature range and the NH₃-SCR-DeNO_x activity indicates that the found relation can be transferred to other supported transition metal-containing catalysts for low-temperature NH₃-SCR-DeNO_x. Full article

A series of M(x)CoCeMgAlO mixed oxides with different transition metals (M = Cu, Fe, Mn, and Ni) with an M content x = 3 at. %, and another series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x ranging from 1 to 9 at. % with respect to cations, while keeping constant in both cases 40 at. % Co, 10 at. % Ce and Mg/Al atomic ratio of 3 were prepared via thermal decomposition at 750 °C in air of their corresponding layered double hydroxide (LDH) precursors obtained by coprecipitation. They were tested in a fixed bed reactor for complete methane oxidation with a gas feed of 1 vol.% methane in air to evaluate their catalytic performance. The physico-structural properties of the mixed oxide samples were investigated with several techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental mappings, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction under hydrogen (H₂-TPR) and nitrogen adsorption–desorption at −196 °C. XRD analysis revealed in all the samples the presence of Co₃O₄ crystallites together with periclase-like and CeO₂ phases, with no separate M-based oxide phase. All the cations were distributed homogeneously, as suggested by EDX measurements and elemental mappings of the samples. The metal contents, determined by EDX and ICP-OES, were in accordance with the theoretical values set for the catalysts’ preparation. The redox properties studied by H₂-TPR, along with the surface composition determined by XPS, provided information to elucidate the catalytic combustion properties of the studied mixed oxide materials. The methane combustion tests showed that all the M-promoted CoCeMgAlO mixed oxides were more active than the M-free counterpart, the highest promoting effect being observed for Fe as the doping transition metal. The Fe(x)CoCeMgAlO mixed oxide sample, with x = 3 at. % Fe displayed the highest catalytic activity for methane combustion with a temperature corresponding to 50% methane conversion, T₅₀, of 489 °C, which is ca. 40 °C lower than that of the unpromoted catalyst. This was attributed to its superior redox properties and lowest activation energy among the studied catalysts, likely due to a Fe–Co–Ce synergistic interaction. In addition, long-term tests of Fe(3)CoCeMgAlO mixed oxide were performed, showing good stability over 60 h on-stream. On the other hand, the addition of water vapors in the feed led to textural and structural changes in the Fe(3)CoCeMgAlO system, affecting its catalytic performance in methane complete oxidation. At the same time, the catalyst showed relatively good recovery of its catalytic activity as soon as the water vapors were removed from the feed. Full article

Ba_1−xCe_xMnO₃ (BM-Ce_x) and Ba_1−xLa_xMn_0.7Cu_0.3O₃ (BMC-La_x) perovskite-type mixed oxides were synthesized using the sol–gel method adapted for aqueous media with different values of x (0, 0.1, 0.3, 0.6) to estimate the effect of the degree of the partial substitution of Ba by Ce or La on the structure and properties that are relevant for their use as catalysts for gasoline direct injection (GDI) soot oxidation. The samples were deeply characterized by ICP-OES, XRD, XPS, N₂ adsorption, H₂-TPR, and O₂-TPD, and their potential as catalysts for soot oxidation has been analyzed in various scenarios that replicate the exhaust conditions of a GDI engine. By comparing the catalytic performance for soot oxidation of the two tested series (BM-Cex and BMC-Lax) and in the two conditions used (100% He and 1% O₂ in He), it could be concluded that (i) in the absence of oxygen in the reaction atmosphere (100% He), BMC-La_0.1 is the best catalyst, as copper is also able to catalyze the soot oxidation; and (ii) if oxygen is present in the reaction atmosphere (1% O₂/He), BM-Ce_0.1 is the most-active catalyst as it presents a higher proportion of Mn(IV) than BMC-La_0.1. Thus, it seems that the addition of an amount of Ce or La higher than that corresponding to x = 0.1 in Ba_1−xCe_xMnO₃ and Ba_1−xLa_xCu_0.3Mn_0.7O₃ does not allow us to improve the catalytic performance of BM-Ce_0.1 and BMC-La_0.1 for soot oxidation in the tested conditions. Full article

The sol–gel method, adapted to aqueous media, was used for the synthesis of BaMn_0.7Cu_0.3O₃ (BMC) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A, A = Ce, La or Mg) perovskite-type mixed oxides. These samples were fully characterized by ICP-OES, XRD, XPS, H₂-TPR, BET, and O₂–TPD and, subsequently, they were evaluated as catalysts for CO oxidation under different conditions simulating that found in cars exhaust. The characterization results show that after the partial replacement of Ba by A metal in BMC perovskite: (i) a fraction of the polytype structure was converted to the hexagonal BaMnO₃ perovskite structure, (ii) A metal used as dopant was incorporated into the lattice of the perovskite, (iii) oxygen vacancies existed on the surface of samples, and iv) Mn(IV) and Mn(III) coexisted on the surface and in the bulk, with Mn(IV) being the main oxidation state on the surface. In the three reactant atmospheres used, all samples catalysed the CO to CO₂ oxidation reaction, showing better performances after the addition of A metal and for reactant mixtures with low CO/O₂ ratios. BMC-Ce was the most active catalyst because it combined the highest reducibility and oxygen mobility, the presence of copper and of oxygen vacancies on the surface, the contribution of the Ce(IV)/Ce(III) redox pair, and a high proportion of surface and bulk Mn(IV). At 200 °C and in the 0.1% CO + 10% O₂ reactant gas mixture, the CO conversion using BMC-Ce was very similar to the achieved with a 1% Pt/Al₂O₃ (Pt-Al) reference catalyst. Full article

BaMnO₃ (BM) and Ba_0.9A_0.1MnO₃ (BM-A) (A = Ce, La or Mg) perovskite-type mixed oxides were synthesized by the aqueous sol–gel method; thoroughly characterized by ICP-OES, XRD, H₂-TPR, BET, and O₂-TPD; and tested as catalysts for CO oxidation under simulated automobile exhaust conditions. The characterization results indicate that the main effects of the partial substitution of Ba with A-metal in BM perovskite are the maintenance of the hexagonal structure of the perovskite and the increase in reducibility and oxygen mobility. All samples catalyze the CO to CO₂ oxidation reaction in the different reactant mixtures employed, showing the best performance for the mixture with the lowest CO/O₂ ratio and in the presence of a dopant in the BM perovskite formulation. BM-La is the most active catalyst for improving CO oxidation, as it is the most reducible, and because is able to evolve oxygen at intermediate temperatures. Full article

Ba_0.9A_0.1MnO₃ (BM-A) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) (A = Mg, Ca, Sr, Ce, La) perovskite-type mixed oxides were synthesised, characterised, and used for soot oxidation in simulated Gasoline Direct Injection (GDI) engine exhaust conditions. The samples have been obtained by the sol-gel method in an aqueous medium and deeply characterised. The characterization results indicate that the partial substitution of Ba by A metal in BaMnO₃ (BM) and BaMn_0.7Cu_0.3O₃ (BMC) perovskites: (i) favours the hexagonal structure of perovskite; (ii) improves the reducibility and the oxygen desorption during Temperature-Programmed Desorption (O₂-TPD) tests and, consequently, the oxygen mobility; (iii) mantains the amount of oxygen vacancies and of Mn(IV) and Mn(III) oxidation states, being Mn(IV) the main one; and (iv) for Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) series, copper is partially incorporated into the structure. The soot conversion data reveal that Ba_0.9La_0.1Mn_0.7Cu_0.3O₃ (BMC-La) is the most active catalyst in an inert (100% He) reaction atmosphere, as it presents the highest amount of copper on the surface, and that Ba_0.9Ce_0.1MnO₃ (BM-Ce) is the best one if a low amount of O₂ (1% O₂ in He) is present, as it combines the highest emission of oxygen with the good redox properties of Ce(IV)/Ce(III) and Mn(IV)/Mn(III) pairs. Full article

Dry reforming of methane with ratio CH₄/CO₂ = 1 is studied using supported Ni catalysts on SBA-15 modified by CeMnO_x mixed oxides with different Ce/Mn ratios (0.25, 1 and 9). The obtained samples are characterized by wide-angle XRD, SAXS, N₂ sorption, TPR-H₂, TEM, UV–vis and Raman spectroscopies. The SBA-15 modification with CeMnO_x decreases the sizes of NiO nanoparticles and enhances the NiO–support interaction. When Ce/Mn = 9, the NiO forms small particles on the surface of large CeO₂ particles and/or interacts with CeO₂, forming mixed phases. The best catalytic performance (at 650 °C, CH₄ and CO₂ conversions are 51 and 69%, respectively) is achieved over the Ni/CeMnO_x/SBA-15 (9:1) catalyst. The peculiar CeMnO_x composition (Ce/Mn = 9) also improves the catalyst stability: In a 24 h stability test, the CH₄ conversion decreases by 18 rel.% as compared to a 30 rel.% decrease for unmodified catalyst. The enhanced catalytic stability of Ni/CeMnO_x/SBA-15 (9:1) is attributed to the high concentration of reactive peroxo (O⁻) and superoxo (O₂⁻) species that significantly lower the amount of coke in comparison with Ni-SBA-15 unmodified catalyst (weight loss of 2.7% vs. 42.2%). Ni-SBA-15 modified with equimolar Ce/Mn ratio or Mn excess is less performing. Ni/CeMnO_x/SBA-15 (1:4) with the highest content of manganese shows the minimum conversions of reagents in the entire temperature range (X(CO₂) = 4–36%, X(CH₄) = 8–58%). This finding is possibly attributed to the presence of manganese oxide, which decorates the Ni particles due to its redistribution at the preparation stage. Full article

Novel yttrium-doped CeO₂, MnO_x, and CeMnO_x composites are investigated as catalysts for low-temperature NH₃-SCR. The study involves the preparation of unmodified oxide supports using a citrate method followed by modification with Y (2 wt.%) using two approaches, including the one-pot citrate method and incipient wetness impregnation of undoped oxides. The NH₃-SCR reaction is studied in a fixed-bed quartz reactor to test the ability of the prepared catalysts in NO reduction. The gas reaction mixture consists of 800 ppm NO, 800 ppm NH₃, 10 vol.% O₂, and He as a balance gas at a WHSV of 25,000 mL g⁻¹ h⁻¹. The results indicate that undoped CeMnO_x mixed oxide exhibits significantly higher deNO_x performance compared with undoped and Y-doped MnO_x and CeO₂ catalysts. Indeed, yttrium presence in CeMnO_x promotes the competitive NH₃-SCO reaction, reducing the amount of NH₃ available for NO reduction and lowering the catalyst activity. Furthermore, the physical-chemical properties of the prepared catalysts are studied using nitrogen adsorption/desorption, XRD, Raman spectroscopy, temperature-programmed reduction with hydrogen, and temperature-programmed desorption of ammonia. This study presents a promising approach to enhancing the performance of NH₃-SCR catalysts at low temperatures that can have significant implications for reducing NO emissions. Full article

In the present study CeO₂, MnO₂ and CeMnO_x mixed oxide (with molar ratio Ce/Mn = 1) were prepared by sol-gel method using citric acid as a chelating agent and calcined at 500 °C. The silver catalysts (1 wt.% Ag) over the obtained supports were synthesized by the incipient wetness impregnation method with [Ag(NH₃)₂]NO₃ aqueous solution. The selective catalytic reduction of NO by C₃H₆ was investigated in a fixed-bed quartz reactor using a reaction mixture composed of 1000 ppm NO, 3600 ppm C₃H₆, 10 vol.% O₂, 2.9 vol.% H₂ and He as a balance gas, at WHSV of 25,000 mL g⁻¹ h⁻¹.The physical-chemical properties of the as-prepared catalysts were studied by several characterization techniques, such as X-ray fluorescence analysis, nitrogen adsorption/desorption, X-ray analysis, Raman spectroscopy, transmission electron microscopy with analysis of the surface composition by X-ray energy dispersive spectroscopy and X-ray photo-electron spectroscopy. Silver oxidation state and its distribution on the catalysts surface as well as the support microstructure are the main factors determining the low temperature activity in NO selective catalytic reduction. The most active Ag/CeMnO_x catalyst (NO conversion at 300 °C is 44% and N₂ selectivity is ~90%) is characterized by the presence of the fluorite-type phase with high dispersion and distortion. The characteristic “patchwork” domain microstructure of the mixed oxide along with the presence of dispersed Ag⁺/Ag_n^δ+ species improve the low-temperature catalyst of NO reduction by C₃H₆ performance compared to Ag/CeO₂ and Ag/MnO_x systems. Full article

A 5 wt% BaMnO₃-CeO₂ composite catalyst prepared by the one-pot method exhibits extraordinary catalytic performance for nitrogen monoxide (NO) direct decomposition into N₂ and O₂; however, the reasons for the high activity remain to be explored. Here, the catalyst was prepared by mechanical mixing and then subjected to thermal treatment at different temperatures (600–800 °C) to explore the underlying reasons. The thermal pre-treatment at temperatures higher than 600 °C can improve the catalytic activity of the mechanically mixed samples. The 700 °C-treated 5%BaMnO₃-CeO₂ sample shows the highest activity, with NO conversion to N₂ of 13.4%, 40.6% and 57.1% at 600, 700, and 800 °C, respectively. Comparative activity study with different supports (ZrO₂, TiO₂, SiO₂, Al₂O₃) reveals that CeO₂ is indispensable for the high performance of a BaMnO₃-CeO₂ composite catalyst. The Ce species (mainly Ce³⁺) in CeO₂ components diffuse into the lattice of BaMnO₃, generating oxide ion vacancy in both components as evidenced by X-ray photoelectron spectroscopy and Raman spectra, which accelerates the rate-determining step and thus higher activity. The chemisorption results show that the interaction between BaMnO₃ and CeO₂ leads to higher redox activity and mobility of lattice oxygen. This work demonstrates that engineering the oxide ion vacancy, e.g., by thermal treatment, is an effective strategy to enhance the catalytic activity towards NO direct decomposition, which is expected to be applicable to other heterogeneous catalysts involving oxide ion vacancy. Full article

The MnO_х-ZrO₂-CeO₂ oxide catalysts were synthesized by co-precipitation method with varying (1) Zr/Zr + Ce molar ratio at constant manganese content of 0.3; (2) manganese content at constant Zr/Ce molar ratio of 1; (3) Mn/Mn + Zr molar ratio at constant Ce content of 0.5. Catalysts are characterized by XRD, N₂ adsorption, TPR, and XPS. The catalytic activity of all the series was tested in the CO and propane oxidation reactions. In contrast to the variation of the manganese content, the Zr/Zr + Ce molar ratio does not significantly affect the catalytic properties. The dependence of the catalytic activity in CO oxidation on the manganese content has a «volcano» shape, and the best catalytic performance is exhibited by the catalyst with Mn/(Zr + Ce) = 1. In the case of propane oxidation reaction, there is «sigma» like dependence, activity increases with increase of Mn/(Mn + Zr + Ce) molar ratio up to 0.3, stabilizing with a further increase in the manganese content. XRD and XPS have shown that with an increase of the Mn concentration in the MnO_x-ZrO₂-CeO₂ catalysts, the amount of crystalline manganese oxides such as Mn₂O₃ and Mn₃O₄, as well as the surface concentration of Mn cations, increases. While the content of Mn_xZr_yCe_1-x-yO₂ solid solution decreases, the concentration of manganese cations (x) in volume of Mn_xZr_yCe_1-x-yO₂ mixed oxide grows. The maximum activity in CO oxidation corresponds to the balance between the amount of the solid solution and the concentration of manganese cations in the volume of mixed oxide. The propane oxidation reaction is less sensitive to the state of manganese ion rather than to its amount. In this case, a decrease in the content of the Mn_xZr_yCe_1-x-yO₂ solid solution with increase in manganese amount in catalyst is compensated by an increase in content of crystalline manganese oxides and the surface concentration of manganese. Full article

A set of ceria-manganese mixed metal oxide catalysts with varying Ce:Mn ratios were prepared by coprecipitation using sodium carbonate and were evaluated for the total oxidation of propane and naphthalene. Manganese-rich samples were the most active, with Ce_0.25Mn_0.75O_x having the highest activity. Catalysts were characterised using X-ray diffraction, Brunauer–Emmett–Teller (BET) surface area, Raman spectroscopy, temperature programmed reduction (TPR), electron microscopy, and X-ray photoelectron spectroscopy (XPS), establishing that the high activity of Ce_0.25Mn_0.75O_x was due to the formation of phase-separated Mn-substituted ceria and Mn₂O₃ phases that were not simultaneously present in the other catalysts. The catalyst preparation technique for the most active ratio was investigated using co-precipitation by urea, oxalic acid and citric acid, and mechanochemical grinding. For propane, the mechanochemical and urea catalysts were more active than the carbonate coprecipitated catalyst, due to greater surface area and increased phase separation. This work demonstrates that ceria-manganese mixed metal oxides are more active than the parent oxide, but that preparation technique is also important for controlling activity. Full article

Two Mn_0.15Ce_0.85O_2−δ mixed oxides were synthesized by a co-precipitation method using Mn(NO₃)₂ and KMnO₄ as the manganese precursors, respectively. Structural analyses by X-ray powder diffraction and Raman spectroscopy reveal the formation of MnO_x-CeO₂ solid solutions. The Mn_0.15Ce_0.85O_2−δ catalyst prepared from the high-valent manganese precursor exhibits higher activity for the catalytic oxidation of NO. The advantage of KMnO₄ is related to the improved redox property of the catalyst as supported by H₂ temperature-programmed reduction (TPR) and O₂ temperature-programmed desorption (TPD). The Mn-Ce interactions create more Mn⁴⁺, Ce³⁺ and oxygen vacancies on the KMnO₄-synthesized mixed oxides based on the Raman and X-ray photoelectron spectra (XPS). Full article