Promotion in ammonia synthesis: A pressure dependent phenomenon

The origin of the pressure dependence of potassium promotion of iron catalysts has been investigated by measuring the turnover numbers for ammonia synthesis on polycrystalline iron, potassium-doped polycrystalline iron and two triply promoted (K₂O, Al₂O₃, C_aO) industrial ammonia synthesis catalysts at 1, 2 and 31 bar in the temperature range 550–800 K. At 1 and 2 bar the turnover numbers of polycrystalline iron and potassium doped polycrystalline iron are greater than those of the industrial catalysts, suggesting that the promoters are merely decorating the surface of the iron, blocking off part of it for reaction. It is only at 31 bar and at temperatures above 670 K, that the promoting effect of the additions to the industrial catalyst becomes apparent, the turnover numbers of these materials being roughly 20 times greater than that of polycrystalline iron. Potassium doped polycrystalline iron, however, has the same turnover number as those of the industrial catalysts. It is concluded that at temperatures above 670 K in a hydrogen/ nitrogen (3 : 1) mixture and at 31 bar the morphology of the surface of the industrial catalysts is changed with the iron, probably as iron nitride, coating the potassium aluminate, the identity of the turnover numbers of the catalysts and of potassium doped polycrystalline iron showing that potassium plays a key role in this promotion. The surface restructuring of the industrial catalysts does not produce dominantly the (111) surface, the observed turnover numbers being 10³ times lower than that of the (111) surface. The activation energy for ammonia synthesis in the temperature range 670–770 K at 31 bar on these materials is much higher (32–37 kcal mol^–1) than that of iron (111) (19.2 kcal mol^–1) since it incorporates the activation energy for surface reconstruction of iron. The molecular nitrogen desorption spectra from the industrial catalysts exhibit three states: (i) N₂, bonded end on, (ii) N₂, -bonded and (iii) -bonded N₂, vicinal to some form of potassium.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

Iron catalyst supported on carbon nanotubes for Fischer-Tropsch synthesis: Effects of Mo promotion

Our studies on the application of carbon nanotubes (CNTs) as support have shown that iron catalysts supported on CNTs are active and selective catalysts for Fischer-Tropsch synthesis (FTS). However, these catalysts experienced deactivation as a result of active site agglomerations. In order to control the agglomeration of active site, which is an important step in developing a novel catalyst supported on carbon-based supports, the effects of Mo promotion on deactivation behavior of iron catalysts supported on CNTs were studied. In this work the properties and catalytic performance of unpromoted iron catalysts were compared with a promoted catalyst with different Mo contents (0.5, 1, 5, and 12 wt%). Based on TEM and XRD analyses, promotion of the catalysts with Mo resulted in production of smaller metal particles compared to the unpromoted iron catalyst. According to XRD analysis, Mo species were deposited in their amorphous structure. TPR analyses showed that addition of Mo increased reduction temperature significantly. Based on TEM and XRD analyses, the particle size of the iron oxides in the unpromoted catalyst increased from 16 to 25 nm under FT operating conditions, while the particle size of the iron oxide in the Mo promoted catalysts (∼12-14 nm) did not change noticeably under the same operating conditions. Activity, selectivity and stability of the unpromoted and Mo promoted catalysts showed that addition of 0.5-1 wt% Mo resulted in a more stable catalyst. Higher contents of Mo (5 and 12 wt%) decreased the activity of the catalysts due to catalytic site coverage and lower extent of reduction. Mo promotion (0.5-12 wt%) increased the selectivity of the catalysts toward lighter hydrocarbons. The promotion of the iron catalyst with 0.5 wt% of Mo stabilized the activity of the catalyst with minimal increase (2%) in methane selectivity.     

Neopentane cracking catalyzed by iron- and manganese-promoted sulfated zirconia

Cracking of neopentane was catalyzed by a sulfated oxide of zirconium promoted with iron and manganese. Reaction at 300–450°C, atmospheric pressure, and neopentane partial pressures of 0.00025–0.005 bar gave methane as the principal product, along with C₂ and C₃ hydrocarbons, butenes, and coke. The order of reaction in neopentane was determined to be 1, consistent with a monomolecular reaction mechanism and with the formation of methane andt-butyl cations; the latter was presumably converted into several products, including only little isobutylene. At 450°C and a neopentane partial pressure of 0.005 bar, the rate of cracking at 5 min onstream was 5×10^–8 mol/(g of catalyst s). Under the same conditions, the rates observed for unpromoted sulfated zirconia and USY zeolite were 3×10^–8 and 6×10^–9 mol/ (g of catalyst s), respectively. The observation that the promoted sulfated zirconia is not much more active than the other catalysts is contrasted to published results showing that the former catalyst is more than two orders of magnitude more active than the others forn-butane isomerization at temperatures <100°C. The results raise a question about whether the superacidity attributed to sulfated zirconia as a low-temperature butane isomerization catalyst pertains at the high temperatures of cracking.     

High Pressure Desorption of K+ from Iron Ammonia Catalyst – Migration of the Promoter Towards Fe Active Planes

The thermal desorption of potassium ions from industrial iron catalysts was studied in situ in the wide pressure range of 10^?8–10 bar of Ar, N₂ and synthesis gas mixture of N₂:3H₂. While high activation energy of 284 ± 1 kJ/mol, for K⁺ was determined for the catalyst precursor, in the reaction conditions it drops down to 231 ± 5 kJ/mol, corresponding well to that found for iron single crystals in UHV studies. The results are rationalized in terms of potassium migration from oxide storage phases towards the iron facets developed during the catalyst activation.     

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10^–3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10^–2 molecules/sites, under the same reaction conditions. The initial C₂ formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.     

Distribution of alkaline promoters within the structure of iron oxide catalyst for dehydrogenation

Along with potassium, cesium is an efficient promoter of catalytic activity of iron oxide catalysts for dehydrogenation of olefins and alkylaromatic hydrocarbons. In the reaction medium, a catalyst is a ferrite system consisting of potassium β″-polyferrite, potassium and cesium monoferrites, and magnetite. The character of the distribution of alkaline promoters within the catalyst structure is studied to provide the theoretically substantiated calculation of the optimal composition of this type of catalysts. The preferred location of cesium ions is shown to be the structure of β″-polyferrite of K_{2 ? z} Cs _z Fe²⁺Fe ₁₀ ³⁺ O₁₇ composition. The catalytic activity of this system with different contents of cesium in the dehydrogenation of ethylbenzene to styrene (flow-type reactor; 0.1 MPa; 600°C; hourly space velocity of ethylbenzene, 1 h^?1; ethylbenzene : steam weight ratio, 1 : 3) was tested. The maximum specific rate of styrene formation is attained at a Cs : Fe ratio in the interval 0.023–0.027, corresponding to the coefficient z = 0.26–0.30. It is impractical to introduce more cesium. Theoretical propositions for targeted transporting of a promoting agent to a given phase of a catalytically active ferrite system are developed. The content of expensive cesium compounds in iron oxide catalyst is optimized.     

Kinetics and catalysis of the reaction of coal char and steam

The kinetics and catalysis of the reaction between steam and two different coal chars were investigated in a small fixed-bed reactor. Atmospheric pressure and temperatures between 600 and 850 °C were used. Various possible catalysts were tested, and the effect of catalyst concentration was examined. Of the catalysts used, potassium carbonate was the most effective. It was followed in order of decreasing effectiveness by sodium carbonate, lithium carbonate, potassium chloride, sodium chloride, and copper (II) oxide. Calcium oxide and iron (III) oxide were totally ineffective. The catalysts were more effective at higher concentration. Also, addition of 10% potassium carbonate to the coal char lowered the apparent activation energy of the reaction from 254.2 to 144.5 kJ/mol and the frequency factor from 2.41 × 10⁸ to 1.32 × 10⁴ s^?1. The water-gas shift reaction reached equilibrium during gasification in the presence of alkali-metal carbonates and the product gas was primarily carbon dioxide and hydrogen. Without catalyst present, the water-gas shift reaction was far from equilibrium.     

Ion cyclotron resonance study of CO oxidation in the gas phase in the presence of rhenium cations with carbonyl and oxygen ligands. Comparison with heterogeneous catalysis

Gas-phase oxidation of CO in the presence of rhenium cations with carbonyl and oxygen ligands has been studied by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Rhenium cations have been generated by the electron impact of Re₂(CO)₁₀ vapour. Contrary to the unreactive rhenium ions, rhenium monocarbonyi ions have been found to react with O₂ molecules yielding rhenium monoxide ions and CO₂ molecules. ReO⁺ ions are subsequently oxidized with O₂ to di- and trioxide ions. The bond energies in rhenium oxide ions were estimated as D°(Re⁺–O)=104±14, D°(ReO⁺–O)<118, D°ReO ₂ ⁺ –O)=122±4 kcal/mol. Simultaneous addition of CO and O₂ molecules to the reaction volume leads to the gas-phase catalytic oxidation of CO with pairs of rhenium oxide ions ReO ₃ ⁺ /ReO ₂ ⁺ serving as the oxidized and reduced forms of the catalyst. The mechanisms of the above reactions are discussed in connection with that for oxidation of CO over solid oxide catalysts.     

The oxidative dehydrogenation of ethane over molybdenum-vanadium-niobium oxide catalysts: the role of catalyst composition

The oxidative dehydrogenation of ethane has been studied at atmospheric pressure using molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350–450 °C. The presence of all three oxides together is necessary in order to have active and selective catalysts. The best results have been obtained using a mixture having a Mo V Nb ratio of 19 5 1. Our studies of the variation of oxide composition suggest that the active phase is based on molybdenum and vanadium. Niobium enhances the intrinsic activity of the molybdenum-vanadium combination and improves the selectivity by inhibiting the total oxidation of ethane to carbon dioxide. The apparent activation energies for the conversion of ethane to ethylene, carbon monoxide and carbon dioxide were 18, 27 and 17 kcal/mol, respectively. The addition of water vapor to the gas stream does not affect the product distribution on this catalyst.     

Selective oxidation of propane on MgO/γ‐Al2O3‐supported molybdenum catalyst: influence of promoters

The influence of promoters, potassium and samarium, on molybdenum supported over MgO–γ‐Al₂O₃ catalyst has been investigated in the oxidative dehydrogenation of propane. The acidities of catalysts were determined by temperature‐programmed desorption of NH₃ and by decomposition of 2‐propanol. The K‐promoted catalyst showed the lower acidity followed by the Sm, whereas the unpromoted sample showed the highest acidity. The higher the acid character of the catalyst, the lower the selectivity to propene. Redox properties determined from EPR spectra change with the addition of the promoter. A parallelism between Mo⁶⁺ reducibility and catalytic activity was found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Iron catalyst for ammonia synthesis doped with lithium oxide

Iron catalysts doped with Al₂O₃, CaO were obtained by melting iron oxide with 2.2 wt.% of Al₂O₃, 2.1 wt.% of CaO. The reduced catalyst was impregnated with lithium hydroxide water solution. Activity measurements were carried out in the laboratory installation in the temperature range 623–773 K under the pressure of 10 MPa. The activity of catalyst containing 0.79 wt.% of Li₂O and reduced at 773 K was similar to the activity of an industrial iron catalyst doped with potassium oxide. After reduction at 923 K the catalyst containing 0.48 wt.% of Li₂O was about 15% more active than the industrial catalyst. Increasing Li₂O concentration results in the decrease of the surface area of a catalyst reduced at 923 K. The most active catalysts doped with lithium oxide were more active than the industrial catalyst when their activity was calculated and scaled down to surface area units.     

Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst

Rubidium promoted iron Fischer–Tropsch synthesis (FTS) catalysts were prepared with two Rb/Fe atomic ratios (1.44/100 and 5/100) using rubidium nitrate and rubidium carbonate as rubidium precursors. Results of catalytic activity and deactivation studies in a CSTR revealed that rubidium promoted catalysts result in a steady conversion with a lower deactivation rate than that of the corresponding unpromoted catalyst although the initial activity of the promoted catalyst was almost half that of the unpromoted catalyst. Rubidium promotion results in lower methane production, and higher CO₂, alkene and 1-alkene fraction in FTS products. Mössbauer spectroscopic measurements of CO activated and working catalyst samples indicated that the composition of the iron carbide phase formed after carbidization was χ-Fe₅ C₂ for both promoted and unpromoted catalysts. However, in the case of the rubidium promoted catalyst, ?′-Fe_2.2C became the predominant carbidic phase as FTS continued and the overall catalyst composition remained carbidic in nature. In contrast, the carbide content of the unpromoted catalyst was found to decline very quickly as a function of synthesis time. Results of XANES and EXAFS measurements suggested that rubidium was present in the oxidized state and that the compound most prevalent in the active catalyst samples closely resembled that of rubidium carbonate.     

Furfural hydrogenation over carbon‐supported copper

Furfural hydrogenation over copper dispersed on three forms of carbon – activated carbon, diamond and graphitized fibers – were studied. Only hydrogenation of the C=O bond to form either furfuryl alcohol or 2‐methyl furan occurred at temperatures from 473 to 573 K. Reduction at 573 K gave the most active catalysts, all three catalysts had activation energies of 16 kcal/mol, and turnover frequencies were 0.018–0.032 s^-1 based on the number of Cu⁰ + Cu⁺ sites, which were counted by N₂O adsorption at 363 K and CO adsorption at 300 K, respectively. The Cu/activated carbon catalyst showed no deactivation during 10 h on stream, in contrast to the other two catalysts. A simple Langmuir–Hinshelwood model invoking two types of sites was able to fit all kinetic data quite satisfactorily, thus it was consistent with the presence of both Cu⁰ and Cu⁺ sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of the support and of the preparation method for copper-based catalysts in the 2-propanol decomposition

Pure copper oxide and mixed CuO/ZnO catalysts with different Cu:Zn atomic ratios were tested for the 2-propanol decomposition in order to investigate the nature of the active site and the role of the ZnO support. Fresh catalysts as well as catalysts oxidized in pure oxygen did not exhibit any catalytic activity below 373 K. When reduced either in pure hydrogen or in reaction mixture (helium plus alcohol) both copper oxide and mixed two-phase catalysts showed a dehydrogenating activity in the temperature range 323–423 K. The apparent activation energy for both reduced CuO and reduced CuO/ZnO catalysts was 60 ± 8 kJ mol^–1. The first order rate constants were found to be a linear function of the exposed zero-valent copper area. The comparison of Cu(0) turnover frequency in unsupported Cu(0) and in Cu(0)/ZnO samples did not show any synergic effect of the support. The role of the preparation method on the Cu(0) dispersion is also discussed.     

The oxidative dehydrodimerization of propylene over BiSn oxide catalysts: III. Reduction and temperature-programmed reoxidation of the catalyst

A bismuth-tin oxide catalyst, which is active for the oxidative dehydrodimerization of propylene, was studied with the objective of characterizing its redox properties. The redox properties were investigated by kinetic methods and temperature-programmed reoxidation. The initial rate of reduction of the catalyst exhibited a first-order dependence on propylene partial pressure and an activation energy of 22 kcal/mol. The results of the temperature-programmed reoxidation investigation suggested a low-temperature reoxidation region and a high-temperature reoxidation region. The activation energy for the low-temperature reoxidation was 23 kcal/mol; the activation energy for the high-temperature reoxidation was 45 kcal/mol. Additional information regarding the physicochemical changes which occur in the catalyst in the two temperature regions was obtained from an examination of the catalyst by Auger and ESCA. The results obtained from these investigations suggested that the tin cations are more resistent to reduction than the bismuth cations. In addition, the low-temperature reoxidation appears to be associated with the transformation of Sn⁰ to Sn⁴⁺ and Bi⁰ to possibly an intermediate oxidation state. The high-temperature reoxidation appears to be associated with the full reoxidation of Bi to Bi³⁺. The redox properties and the physicochemical changes also correlated with the mechanism suggested for the oxidative dehydrodimerization of propylene in the earlier studies.     

Reprint of: Oxidative dehydrogenation of cyclohexane and cyclohexene over supported gold, –palladium catalysts

Supported gold, palladium and gold–palladium catalysts have been used to oxidatively dehydrogenate cyclohexane and cyclohexenes to their aromatic counterpart. The supported metal nanoparticles decreased the activation temperature of the dehydrogenation reaction. We found that the order of reactivity was Pd ≥ Au–Pd > Au supported on TiO₂. Attempts were made to lower the reaction temperature whilst retaining high selectivity. The space-time yield of benzene from cyclohexane at 473 K was determined to be 53.7 mol/kg_cat/h rising to 87.3 mol/kg_cat/h at 673 K for the Pd catalyst. Increasing the temperature in this case improved conversion at a detriment to the benzene selectivity. Oxidative dehydrogenation of cyclohexene over AuPd/TiO₂ or Pd/TiO₂ catalysts was found to be very effective (conversion >99% at 423 K). These results indicate that the first step in the reaction sequence of cyclohexane to cyclohexene is the slowest step. These initial results suggest that in a fixed-bed reactor the oxidative dehydrogenation in the presence of oxygen, palladium and gold–palladium catalysts are readily able to surpass current literature examples and with further modification should yield even higher performance.     

Comparative energy barriers for hydrogen activation by homogeneous and heterogeneous metal oxide catalysts

Triethylene glycol solutions of alkali and alkaline earth metal hydroxide complexes are well-defined soluble oxide water-gas shift catalysts which equilibrate the reaction of carbon monoxide and water to yield hydrogen and carbon dioxide at temperatures ranging from 150 ° to 250 °C and carbon monoxide pressures of 1 to 300 atm. Significantly, catalysis proceeds cleanly, even in the complete absence of a metal center in the soluble oxide system. Thus, the rate of hydroxide ion catalyzed hydrogen evolution is highest in the presence of a noncoordinating organic cation: Bu_ΔN⁺>Cs⁺>Na⁺>H⁺>Ca⁺². Furthermore, the activation energy for the homogeneous sodium hydroxide catalyst in triethylene glycol solution, 26±1 kcal, is comparable to that exhibited by a commercially used heterogeneous iron oxide catalyst, 27±0.2 kcal. The alkali metal hydroxide system may be modified for metal cocatalysis. Thus, lead (II) oxide dissolves in the triethylene glycol solutions to yield a new species which exhibits a²⁰⁷Pb NMR resonance shifted 3350 ppm downfield from lead perchlorate. The activity of this lead modified system is improved by three orders of magnitude. Yet, the activation energy is unchanged, 26±1 kcal, suggesting that entropic factors may be important in these homogeneous metal oxide hydrogen evolution/activation systems.     

Deactivated iron catalyst in the fischer-tropsch synthesis

The selectivity for higher hydrocarbons (C₁₁–C₁₇) has been studied in the Fischer-Tropsch synthesis using fresh and used fused iron catalysts under different reaction conditions. On increasing the temperature higher hydrocarbon products were formed in the C₁₁–C₁₇ range. The deactivated fused iron catalyst is less active but selective to heavier hydrocarbon chain molecules. The product distribution is shifted towards heavier hydrocarbons due to the effects of the pore volume, presence of potassium and site densities at the surface.     

Wet air oxidation of p-coumaric acid over promoted ceria catalysts

The catalytic wet air oxidation (WAO) of p-coumaric acid (PCA) has been investigated over Fe- and Zn-promoted ceria catalysts. The catalysts have been prepared by the coprecipitation method and have been characterized by X-ray diffraction (XRD), BET surface area, SEM–EDX and temperature programmed reduction (TPR). The oxidation reaction was carried out in a batch reactor under an air pressure of 2 MPa and in the temperature range 353–403 K. Fe-CeO₂ catalysts, with 20–50 mol% of iron, were found more effective than the unpromoted and Zn-promoted ceria catalysts. On the basis of characterization data, it has been suggested that the higher activity of the Fe-promoted catalysts is related to the modification of the structural and redox properties of the ceria oxide catalyst on addition of iron.