Carbon-supported cobalt catalyst for ammonia synthesis: Effect of preparation procedure

Supported cobalt catalysts were synthesised, characterised (by H₂ TPD, XRD, TEM), and tested in ammonia synthesis at 9.0 MPa (400–470 °C; H₂:N₂ = 3:1). Partly graphitised carbon of high surface area (840 m²/g), cobalt nitrate, and barium nitrate were used as a support, a precursor of the active phase, and a promoter precursor, respectively. Both cobalt dispersion in the Ba-doped catalyst and, to a greater extent, catalytic properties of the promoted Co surfaces (TOF) proved to be dependent on the unpromoted material pretreatment (reduction in H₂, subsequent calcination in air). The kinetic studies of NH₃ synthesis have shown explicitly that Ba-promoted cobalt on carbon is very active and less inhibited by the ammonia product than the commercial magnetite-based material.     

Effect of K, Cs and Ba on the kinetics of NH3 synthesis over carbon-based ruthenium catalysts

The kinetics of NH₃ synthesis over carbon-based ruthenium catalysts promoted with barium or alkali was studied. Both the ammonia partial pressure dependencies of the reaction rates (T = 400°C, p = 63 bar, H₂ : N₂ = 3 : 1) and the pressure variations of the activity (T = 370°C, p= 4–63 bar, H₂ : NN₂ = 3 : 1) were found to be different for Ba and for the alkali (K, Cs). Ba–Ru/C proved to be more sensitive to the NH₃ content and to the total pressure. The rate of synthesis over the alkali-promoted catalysts is, in turn, much stronger influenced by the ruthenium dispersion. TOFs of NH₃ synthesis for the promoted samples at 370°C and 4 bar (Ba 0.085 1/s, Cs 0.05 1/s, K 0.035 1/s) are significantly higher than that for the Ru(0001) basal plane (0.0085 1/s results from the literature data at 370°C, 2 bar). The most active Ru/C samples (Ba or Cs) exceed significantly the fused iron catalyst, especially at high conversions.     

Characterization and reactivity of aerogel sulfated zirconia-ceria catalyst for n-hexane isomerization

Sulfated zirconia doped with cerium has been prepared by sol gel process using the supercritical evacuation way of the solvent. The physicochemical and catalytic properties of aerogel solids AZC were compared as a function of both the Ce/Zr molar ratio and the calcination temperature. The samples were characterized by N₂ adsorption desorption, XRD, FTIR, RAMAN, XPS spectroscopy and TPD of NH₃. The catalyst prepared with molar ratio Ce/Zr = 0.4 exhibits the high activity and selectivity toward isomerization product in the n-hexane isomerization reaction. This result is probably due to its important total acidity.     

Selective epoxidation of styrene with air catalyzed by CoOx and CoOx/SiO2 without any reductant

Cobalt oxide (CoOx) prepared by a direct calcination of cobalt nitrate was considerably active for the epoxidation of styrene with air in DMF under mild conditions. A substrate conversion of 75.8 mol% with an epoxide selectivity of 82.1% was achieved at 353 K over 10 mg of cobalt oxide catalyst. Once CoOx was loaded on the support SiO₂ through a simple procedure consisting of wet impregnation, drying and calcination, the as-prepared catalyst presented higher catalytic activity and epoxide selectivity than cobalt oxide itself. Over the optimized catalyst CoOx/SiO₂ (1.0 wt% Co), 85.7 mol% of styrene was effectively converted at 363 K within 4 h, with a high epoxide selectivity up to 86.0%. The results showed that many factors influenced the performance of the catalyst, such as the Co loading, the support, the temperature and the atmosphere, etc. The leaching of cobalt from the catalyst CoOx/SiO₂ was negligible, indicating the applicability of the catalyst CoOx/SiO₂ as a true heterogeneous catalyst. The control test and UV–vis spectra revealed a synergic interaction among solvent, oxygen and substrate over CoOx/SiO₂.     

Carbon-supported cobalt–iron catalysts for ammonia synthesis

The cobalt, iron and Co–Fe catalysts deposited on carbon were prepared, characterised (XRD, H₂ TPD) and studied in ammonia synthesis at 90 bar (H₂:N₂ = 3:1). Partly graphitised carbon material obtained via high temperature treatment (1900 °C) of commercial activated carbon was used as a support for the active metals (10 wt.%) and barium or potassium were used as promoters. XRD studies of unpromoted materials have shown that cobalt (5–20% in Co + Fe) dissolves in the iron phase (alloy formation); the average sizes of crystallites (20–30 nm) are roughly independent of the metal kind (Co, Fe, Co–Fe). The effect of Ba and that of K on the catalyst performance proved to be strongly dependent on the choice of an active phase (Co or Fe or Co–Fe). In the case of Co/C, the promotional effect of barium was extremely large. Furthermore, the Ba–Co/C system was found to be less inhibited by the ammonia product than Ba–Fe/C. At low temperature (400 °C) and at high conversion (8% NH₃ in the gas), the surface-based reaction rate (TOF) for Ba–Co/C is about six times higher than that for Ba–Fe/C.     

Effect of the dimensions of carbon nanotube channels on copper–cobalt–cerium catalysts for higher alcohols synthesis

A series of carbon nanotube (CNT)-supported copper–cobalt–cerium catalysts were prepared and investigated for higher alcohols synthesis. The superior selectivity for the formation of ethanol and C_2 + alcohols achieved using the CuCoCe/CNT(8) catalyst was 39.0% and 67.9%, respectively. The diameters of CNTs considerably influence the distribution of metal particles and the electronic interaction between the tube surface and the active species. The electronic effect between the encapsulated Co species and the inner surface is greatly improved in the narrowest CNT channel, which is expected to facilitate the reduction of cobaltous oxide and promote the alcohols yield remarkably (291.9 mg/g_cath).     

Synthesis of Highly Active Nano-Sized Pt/CeO<Subscript>2</Subscript> Catalyst via a Cerium Hydroxy Carbonate Precursor for Water Gas Shift Reaction

Abstract  

A novel precipitation/digestion route has been developed to synthesize crystalline cerium hydroxy carbonate (CHC: Ce(OH)CO₃) by using an equimolar quantity of cerium nitrate (Ce(NO₃)₃·6H₂O) and mixed precipitants (KOH + K₂CO₃) at room temperature. Nano-sized CeO₂ supports could be prepared by the pre-calcination of CHC at 400 °C for 4 h. A highly active water gas shift (WGS) catalyst, 1 wt.% Pt/CeO₂ catalyst showed almost equilibrium CO conversion with 100% CO₂ selectivity at 320 °C even at the gas hourly space velocity (GHSV) of 45,625 h⁻¹.     

Ammonia decomposition on tungsten carbide

Decomposition of NH₃ is an important reaction in the cleaning of syngas obtained from the gasification of biomass as well as for the production of hydrogen for fuel cells from easily condensed NH₃. To the best of our knowledge, this paper reports for the first time a detailed study of NH₃ decomposition on tungsten carbide (WC). Results for a commercially available Fe ammonia synthesis catalyst (Amomax-10) are also reported for comparison.The WC catalyst was characterized by BET, XRD, SEM, EDX and temperature programmed reaction (TPRx). The catalytic behavior of WC strongly depended on pretreatment conditions. The highest activity was obtained with WC samples pretreated in an 80/20 mixture of H₂–CO. Complete decomposition of NH₃ was observed at 550 °C for 4000 ppm of NH₃ at a space velocity of 1,884,000 h⁻¹. At lower temperatures, the activity of the WC catalyst reached steady-state after an induction period that decreased in time with increasing temperature. Reconstruction of the surface during pretreatment and during decomposition of NH₃ is suggested to be responsible for the behavior of the catalyst observed during TPRx and time-on-stream (TOS) isothermal reaction. The commercial Fe NH₃ synthesis catalyst, although active for NH₃ decomposition, showed rapid partial deactivation following an induction period with a steady-state conversion of only 35% at 650 °C and the space velocity used. Thus, WC appears to be an excellent catalyst for use in ammonia decomposition.     

Properties of zeolite Y in various forms and utilization as catalysts or supports for cerium oxide in ethanol oxidation

Zeolite Y in sodium form (NaY) was synthesized using silica source from rice husk, transformed to ammonium form (NH₄Y), and calcined to convert to proton form (HY). The direct conversion of NH₄Y to HY resulted in loss of the zeolite crystallinity and lower surface area. Thus, the NH₄Y was further used in the preparation of cerium (Ce) catalysts. The NH₄Y was also treated with a basic solution in an attempt to generate mesopores but only site defects were likely formed. The supported Ce catalysts with good Ce dispersion were prepared by wetness impregnation of Ce precursor solution on NaY, NH₄Y, and base-treated NH₄Y. Upon calcination, the generated catalysts were notated as Ce/NaY, Ce/HY, and Ce/YB. In catalytic testing on ethanol oxidation at varying temperature in a continuous laboratory-scale fixed-bed reactor, all the zeolites gave low ethanol conversion at 100–300 °C. The catalytic activity significantly improved with the presence of Ce. The Ce/YB showed the higher ethanol conversion and CO₂ yield than Ce/NaY and Ce/HY probably because of the presence of more local site defects on the zeolite.     

助剂对活性炭负载钴催化剂氨合成活性的影响

采用四槽高压连续流动反应器研究了添加助剂Ba、K和Sm对活性炭负载钴催化剂氨合成活性的影响,结果发现,添加助剂Ba、K和Sm可以提高催化剂的氨合成活性,其中,Ba的促进效果最好,Ba与Co物质的量比为0.3时,催化活性最高。在Ba-Co/AC催化剂中,助剂Sm的加入降低了催化剂的氨合成活性,而少量K助剂(K与Co物质的量比为0.25～0.5)可以提高其催化性能,在10 MPa、10 000 h^-1和450 ℃条件下,双助剂催化剂的氨合成活性可达120 mmol·(g·h）^-1,进一步增加K的量,其氨合成活性下降。     

The effect of cobalt on the activation of fused iron catalyst for ammonia synthesis

BET, scanning electron microscopy, X-ray diffraction, Mössbauer spectroscopy, X-ray fluorescence spectroscopy and McBain thermobalance were used to investigate the effect of cobalt on the reduction behavior and activity of used iron catalyst for ammonia synthesis. Activity tests were carried out under 10 MPa in the 350–450°C temperature rang. Studies were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approx. 5.5% wt Co. Cobalt changed the reduction behavior of the catalyst. The rate of the surface change during reduction was higher for the case of the ‘cobalt catalyst’; however the rate of mass change was higher for a typical iron catalyst. The process of reduction was probably followed by the formation of an Fe₃Co compound and by the surface faceting, with the exposure of an Fe(111) plane.     

Low-Temperature SCR of NO with NH3 over USY-Supported Manganese Oxide-Based Catalysts

A series of catalysts of manganese oxide, manganese–cerium and iron–manganese oxide supported on USY (ultra-stable Y zeolite) were studied for the low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. It was found that MnO_x/USY have high activity and high selectivity to N₂ in the temperature range 80-180 °C. The addition of iron and cerium oxide increased NO conversion significantly although the single-component Fe/USY and Ce/USY catalysts had low activities. Among the catalysts studied in this work, the 14% Ce-6% Mn/USY showed the highest activity. The results showed that this catalyst yielded nearly 100% NO conversion at 180 °C at a space velocity of 30 000 cm³ g^-1 h^-1. The only product is N₂ (with no N₂O) below 150 °C. The effects of the concentration of oxygen, NO and NH₃ were studied and the steady-state kinetics were also investigated. The reaction order is 1 with respect to NO and zero with respect to NH₃ on the 14% Ce-6% Mn/USY catalyst at 150 °C.     

Catalytic properties of Rh, Ni, Pd and Ce supported on Al-pillared montmorillonites in dry reforming of methane

A natural Maghnia clay was pillared by Al₁₃ and impregnated by 3–10 wt.% Me (Me = Rh, Ni, Pd, Ce) to be used as catalysts in the reforming of methane with carbon dioxide to synthesis gas. The structural and textural properties of materials calcined at 450 °C were determined by several techniques (XRD, FT-IR, ²⁷Al magic angle spinning (MAS) NMR, X-ray photoelectron spectroscopy (XPS), BET, thermogravimetric analysis (TGA)–DSC, H₂-temperature programmed reduction (TPR) and NH₃-TPR). Although impurities are present in the Al-pillared layered clay (PILC) support, most properties are close to those of pure Al-pillared Na-montmorillonite. Impregnation and calcination leads to the plugging of most micropores by clusters or microparticles of oxides. The NMR resonances of Al_VI and Al_IV specie are not modified after impregnation, and Al_VI/Al_IV ratio only varies on loading when compared to Al-PILC. Catalytic experiments show that the most active catalyst is 3% Rh/Al-PILC on which 88 mol.% of methane is converted at 650 °C with a minimum amount of carbon deposit. The conversions decrease along the 3% Rh ≈ 10% Ni > 3% Pd > 3% Ni > 3% Ce series. The H₂/CO ratio amounts to 1.1 with Rh and to 0.85 with Pd which are metallic at the temperature of reaction, but it has a lower value with Ni and Ce due to the RWGS reaction known to proceed in the presence of oxides.     

The Kinetics of Ammonia Synthesis over Ruthenium-Based Catalysts: The Role of Barium and Cesium

The effect of barium and cesium on the kinetic behavior of Ru/MgO in ammonia synthesis was studied. The activity measurements were performed in a differential reactor at 400°C under a pressure of 6.3 MPa and supplemented with chemisorption measurements and temperature-programmed surface reaction experiments (TPSR). The latter were performed by titrating preadsorbed atomic nitrogen (N_ads) with hydrogen. Both promoted systems proved to be much more active in NH₃ synthesis than the unpromoted one: Ba-Ru/MgO>Cs-Ru/MgO»Ru/MgO. The kinetic behavior of Ba-Ru/MgO was found to be different from that of Cs-Ru/MgO which was much less sensitive to changes in the ammonia content (x_NH3). The dependence of the turnover frequencies (TOF) on x_NH3 for Ba-Ru/MgO and Cs-Ru/MgO was found to be analogous to that for Ba-Ru/C and Cs-Ru/C, respectively. For Ba-Ru/MgO and Ba-Ru/C the differences in TOF did not exceed 10 to 30% over the range of x_NH3 studied. It is therefore suggested that cesium acts as an electronic promoter, whereas barium plays the role of a structural promoter that controls the concentration of active sites which are most likely B₅-type sites, the effect of the support being negligible. The same onset temperature of ammonia formation during the TPSR experiment observed for Ru/MgO and Ba-Ru/MgO supports this hypothesis. Furthermore, a strong increase in activity was observed for Ru/MgO and Ba-Ru/MgO when heating in synthesis gas up to 520°C. This can be attributed to the sintering of very small Ru particles and to the removal of water traces from both the MgO and the Ba + O adlayer. In contrast, the activity of Cs-Ru/MgO decreased significantly after heating at 520°C. Thus, due to the very high activity, very high thermal stability, and absence of methanation problems, barium-promoted ruthenium catalysts supported on magnesia are considered excellent ammonia synthesis catalysts.     

Characterization of Silica-Supported Cobalt Catalysts Prepared by Decomposition of Nitrates Using Dielectric-Barrier Discharge Plasma

Abstract  

Low temperature decomposition of precursors usually leads to higher cobalt dispersion. In this study, we present a method to decompose cobalt precursors by using dielectric-barrier discharge (DBD) plasma without requiring a thermal calcination process. Cobalt (Co) catalysts prepared by DBD plasma were characterized by a range of techniques. The results indicate that the DBD decomposition method can not only reduce the decomposition time but also achieve an increased Co dispersion, small Co₃O₄ cluster size and uniform distribution compared to traditional calcination method. It was observed that the DBD-treated catalysts performed well in Fischer–Tropsch synthesis and were favorable for heavy hydrocarbon formation.     

Alcohol Synthesis via Fischer–Tropsch Synthesis over Activated Carbon Supported Alkaline Earth Modified Cobalt Catalyst

Although numerous efforts have been made in direct syngas conversion to higher alcohols via Fischer–Tropsch synthesis, the higher alcohols distribution remains a challenge. Here, we introduce alkaline earth metal oxide as promoter into activated carbon supported cobalt catalyst to tune distribution of higher alcohols. With the addition of Mg, the distribution of C_2-5 alcohols increase from 41.2 to 75.8% accompanying with distribution of C_6-18 alcohols decrease from 52.8 to 14.0%. Ba-promoted Co based catalyst (CoBa/AC) presents similar alcohols distribution to un-promoted catalyst, while the alcohol selectivity over CoBa/AC is higher than Co/AC. For promoted catalysts, the distribution of C_6-18 alcohols increased in the order of Mg?<?Ca?<?Sr?<?Ba. The characterization results exhibit that the promoter addition facilitates the cobalt carbide formation, which leads to enhancement of selectivity to higher alcohols. The available active cobalt sites of promoted Co based catalysts increase in the same above order of Mg?<?Ca?<?Sr?<?Ba.

Graphic Abstract

     

NOx storage/reduction catalysts based in cobalt/copper hydrotalcites

The use of materials based on hydrotalcites as NO_x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H₂O and SO₂. The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H₂O and SO₂, than a Pt–Ba/Al₂O₃ reference catalyst.     

Synthesis, characterization and performance of CuO/La2O3 composite catalyst for ammonia catalytic oxidation

This work considers the oxidation of ammonia (NH₃) by selective catalytic oxidation (SCO) over a CuO/La₂O₃ composite catalyst at temperatures between 150 and 400 °C. A CuO/La₂O₃ composite catalyst was prepared by co-precipitation of copper nitrate and lanthanum nitrate at various molar concentrations. This study also considers how the concentration of influent NH₃ (C₀ = 1000 ppm), the space velocity (GHSV = 92,000 l/h), the relative humidity (RH = 12%) and the concentration of oxygen (O₂ = 4%) affect the operational stability and the capacity for removing NH₃. The catalysts that were characterized using FTIR, XRD, UV-Vis, BET and PSA, have shown that the catalytic behavior is related to the copper (II) oxide, while lanthanum (III) oxide may serve only to provide active sites for the reaction during a catalyzed oxidation run. The experimental results show that the extent of conversion of ammonia by SCO in the presence of the CuO/La₂O₃ composite catalyst was a function of the molar ratio. The ammonia was removed by oxidation in the absence of CuO/La₂O₃ composite catalyst, and around 93.0% NH₃ reduction was achieved during catalytic oxidation over the CuO/La₂O₃ (8:2, molar/molar) catalyst at 400 °C with an oxygen content of 4.0%. Moreover, the effect of the reaction temperature on the removal of NH₃ in the gaseous phase was also monitored at a gas hourly space velocity of under 92,000 h^− 1.     

Synthesis of hydrocarbons from carbon monoxide and hydrogen in the presence of cobalt catalysts promoted with cerium oxide

The effect of the promotion of Co/SiO₂ catalysts with cerium oxide on their physicochemical characteristics and activity in the synthesis of hydrocarbons from CO and H₂ was studied. According to X-ray diffraction data, the average size of Co₃O₄ crystallites weakly depends on the introduced amount of the promoter. The results of temperature-programmed reduction and oxygen titration showed that the degree of cobalt reduction in the hydrogen-activated catalysts increased with the concentration of the CeO₂ promoter. The activity of the promoted samples increased proportionally to the content of cerium, and selectivity for the target higher hydrocarbons reached a maximum in the Co–20Ce/SiO₂ sample.     

Comparative Study on Catalytic Performances for Low-temperature CO Oxidation of Cu–Ce–O and Cu–Co–Ce–O Catalysts

Two series of Cu–Ce–O and Cu–Co–Ce–O catalysts were prepared by co-precipitation method. The prepared catalysts were characterized by XRD, IR, TPR, XPS, BET and ICP-AES. The catalytic activities of the catalysts for low-temperature CO oxidation were evaluated through a microreactor-GC system. TPR results indicate that the addition of cobalt to the Cu–Ce–O can increase the dispersion of copper oxide, and the interaction between cobalt and copper can enhance the reducibility of each other. XPS analysis show that Ce⁴⁺, Cu²⁺, along with Co₃O₄, are present on the surface of Cu_0.4Co_0.6Ce₄ catalyst. The Co/Cu atomic ratio and the calcination temperature have significant effect on the activities of the catalysts. Compared with Cu₁Ce₄ catalyst, the Cu_0.4Co_0.6Ce₄ catalyst has better activity and thermal stability.