Introducing trace Co to molybdenum carbide catalyst for efficient ammonia synthesis

Development of the cost-effective catalysts with excellent catalytic performance is highly demanded for ammonia synthesis, and the strong adsorption of ammonia greatly hinders the design of cost-effective and high-performance ammonia synthesis catalysts. Herein, we report that the addition of a small amount of Co species (0.1 wt%) into Mo₂C catalyst, which can provide electrons to Mo₂C, not only leads to improvement of the adsorption and migration of hydrogen, but also facilitates the adsorption and desorption of ammonia. Consequently, Mo₂C catalyst with 0.1 wt%Co offers a 40% higher ammonia synthesis activity and a lower negative reaction order with respect to NH₃ in comparison to Mo₂C. This work stresses the importance of the minor components in improving the ammonia synthesis activity by accelerating the migration of reactants over the catalyst surface and the escape of products from the catalyst.     

Enhanced CeO2-supported Pt catalyst for water-gas shift reaction

Sodium-promoted, CeO₂-supported, Pt catalyst were synthesized and investigated for water-gas shift (WGS) reaction. The ceria supports were synthesized by conventional precipitation method. The effect of the basic aqueous solutions used in the preparation procedure on the nature of Pt/CeO₂ catalyst was examined by physical and chemical characterization analyses. The catalytic activity for the WGS reaction was observed. In the Pt/CeO₂ catalyst prepared by NaOH, the presence of sodium (1.64 wt.%) on the support modified the base and electronic properties and consequently enhanced the catalytic activity of Pt/CeO₂. Based on temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared spectroscopy (DRIFTS) analyses, we concluded that sodium introduced through the ceria preparation procedure played an important role in the WGS reaction as a promoter to induce the fast build-up of surface intermediates of the WGS reaction and also as an electron donor to modify the adsorption strength between CO and Pt.     

Electronic Metal–Support Interactions and the Production of Hydrogen Through the Water-Gas Shift Reaction and Ethanol Steam Reforming: Fundamental Studies with Well-Defined Model Catalysts

The electronic properties of Ni and Pt nanoparticles deposited on CeO₂(111) have been examined using core and valence photoemission. The results of valence photoemission point to a new type of metal–support interaction which produces large electronic perturbations for small Ni and Pt particles in contact with ceria. The Ni/CeO₂(111) and Pt/CeO₂(111) systems exhibited a density of metal d states near the Fermi level that was much smaller than that expected for bulk metallic Ni or Pt. The electronic perturbations induced by ceria on Ni made this metal a very poor catalyst for CO methanation, but transformed Ni into an excellent catalyst for the production of hydrogen through the water-gas shift and the steam reforming of ethanol. Furthermore, the large electronic perturbations seen for small Pt particles in contact with ceria significantly enhanced the ability of the admetal to adsorb and dissociate water made it a highly active catalyst for the water-gas shift. The behaviour seen for the Ni/CeO₂(111) and Pt/CeO₂(111) systems illustrates the positive effects derived from electronic metal–support interactions and points to a promising approach for improving or optimizing the performance of metal/oxide catalysts.     

Characterization of Cu/CeO2/γ-Al2O3 Thin Film Catalysts by Thermal Desorption Spectroscopy

Thermal desorption spectroscopy (TDS) under ultra high vacuum (UHV) condition has been used to investigate the desorption characteristics of Cu/CeO₂/γ-Al₂O₃ thin film catalysts coated onto the microchannel of a microreactor. TDS results demonstrate that surface desorption profiles and chemical properties (acid–base and redox properties) are remarkably influenced by the catalyst composition, i.e. the loading of copper and ceria. The enhanced basicity with the increase of ceria loading and the decrease of copper loading is evident from the shifted desorption maximum of CO₂ in TDS spectra. Three oxygen species, ranging from weakly bound oxygen desorbed at low temperature to the strongly held lattice oxygen desorbed at high temperature, are easily discernible and clearly identified by O₂ TDS spectra, depending on the catalyst compositions. The concomitant thermal desorption of O₂, CO₂, and H₂O at low temperature indicates the unique chemical properties of copper/ceria catalyst with appropriate copper and ceria contents. The observed low-temperature feature is ascribed to the role of porthole of copper/ceria interfacial area for several desorbed species. The weakly bound oxygen species is attributed to the enhanced abundance of copper/ceria interfacial anionic vacancies created by the intimate contact between copper and ceria entities and its impact on steam reforming of methanol (SRM) reaction is tentatively discussed in terms of reverse oxygen spillover.     

Highly efficient Ru/Sm2O3-CeO2 catalyst for ammonia synthesis

A series of Ru/Sm₂O₃–CeO₂ catalysts were prepared by using a co-precipitation (CP) method and characterized by XRD, BET, SEM, H₂-TPD-MS, H₂-TPR and CO chemisorption. The activity test shows that ammonia concentration of the catalyst with 7% Sm is 13.4% at 10 MPa, 10,000 h^− 1, 425 °C, which is 21% higher than that of Ru/CeO₂. Such high catalytic activity was due to three effects: the morphology changes of catalyst, electrodonating property of partially reduced CeO_2 − x to Ru metal and the property of easily hydrogen desorption derived from the presence of Sm³⁺ in ceria.     

Effect of chlorine on Ir/CeO2 catalyst behavior for preferential CO oxidation

Effect of chlorine on Ir/CeO₂ catalyst behavior for preferential CO oxidation is investigated by high-resolution transmission electron microscopy, X-ray photoemission spectroscopy, and diffuse reflectance infrared spectroscopy. The presence of chlorine favors the dispersion of Ir particles. On ceria support, the replacement of the lattice oxygen by chloride ions would produce CeOCl species, which could hinder the formation of hydroxyl groups and carbonate and/or carboxylate species on the ceria surface. These features could explain the decreased activity of the Cl-containing Ir/CeO₂ sample.     

In-situ XPS Study on the Reducibility of Pd-Promoted Cu/CeO2 Catalysts for the Oxygen-assisted Water-gas-shift Reaction

Cu/CeO₂, Pd/CeO₂, and CuPd/CeO₂ catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO₂ catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO₂ components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO₂ greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce³⁺ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO₂ catalysts, possibly by helping to maintain Cu in a reduced state during reaction.     

Synergistic Effect of MgO and CeO2 as a Support for Ruthenium Catalysts in Ammonia Synthesis

Nanocomposite materials in MgO–CeO₂ (Ce=0–100 mol%) system were prepared as the support materials for Ru-based ammonia synthesis catalysts, so that the resultant catalytic activity was enhanced by mixing them together compared with that of the MgO or CeO₂ component was solely used as a support. Such high catalytic activity was due to two effects: electrodonating property of partially reduced CeO_2-δ to Ru metal and strong metal-support interaction derived from the unique catalyst morphology in this study.     

Hydrogen production from ethanol steam reforming over Rh/CeO2 catalyst

Hydrogen production from ethanol steam reforming over an Rh/CeO₂ catalyst was investigated with a stoichiometric feed composition. Ethanol was entirely converted to hydrogen and C₁ products (CO, CO₂, CH₄) at 400 °C due to the remarkable C–C bond cleavage capacity of Rh species. The Rh/CeO₂ catalyst exhibited stable activity and selectivity without the obvious deactivation during 70 h on stream test. Structural analysis of the aged catalysts indicated that the strong interaction between Rh and ceria support efficiently inhibited Rh particles sintering (stable at around 2 nm) and coke formation to guarantee catalyst stability.     

SOx storage and release kinetics for ceria-supported platinum

The SO_x storage and release kinetics on CeO₂ have been studied by lean SO_x adsorption and temperature programmed desorption for different pairwise configurations of individual monolith samples, i.e., Pt/CeO₂ + SiO₂, Pt/SiO₂ + CeO₂, CeO₂ + Pt/SiO₂ and CeO₂ + SiO₂. In the case of sole ceria, SO_x adsorption proceeds both via SO₂ and SO₃ adsorption although the latter channel is kinetically favored. Hence, the rate of SO₂ oxidation is crucial for the overall SO_x storage kinetics. It is also found that physical contact between Pt and ceria is important for the storage process. This is attributed to efficient transport routes for SO_x (surface diffusion and spill-over processes) and/or specific adsorption sites at the platinum–ceria interface. The main route for SO_x release is found to be thermal decomposition where the effect of platinum is minor, although an indirect effect cannot be ruled out. Different mechanistic scenarios for SO_x adsorption are discussed, which may serve as a guide for future experiments.     

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (MNi, Co, and Fe), as well as Mo-free catalysts, M/MgO (MNi, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH₄ over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni₃C, Co₂C, and Fe₃C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC_1−x into Mo₂C. The significant effect of the addition of Mo is concerned with the formation of Mo₂C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo₂C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH₄.     

Effects of potassium on the chemisorption of CO2 and CO on the Mo2C/Mo(100) surface

The interaction of CO₂ with K-promoted Mo₂C/Mo(100) has been studied with high-resolution electron energy loss spectroscopy, work function measurements and temperature-programmed desorption. Pre-adsorbed potassium dramatically affects the adsorption behavior of CO₂ on the Mo₂C/Mo(100) surface. It increases the rate of adsorption, the binding energy of CO₂ and it induces the dissociation of CO₂ through the formation of negatively charged CO₂. Potassium adatoms also promote the dissociation of adsorbed CO over Mo₂C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and characterization of massive molybdenum carbides and supported carbide-containing catalysts Mo2C/C prepared through mechanical activation

Procedures for the synthesis of massive molybdenum carbide by the mechanical activation of a mixture of MoO₃, commercial carbon, and Zn in air and the synthesis of the supported carbide-containing catalyst Mo₂C/C by the mechanical activation of commercial carbon impregnated with a 16% aqueous solution of ammonium paramolybdate in an inert atmosphere were developed for the first time. With the use of a set of physicochemical methods, the metal contents, particle sizes, specific surface areas, and phase compositions of the mechanically activated composites were determined. The structure of the carbide-containing supported catalyst was studied by electron microscopy, and its acidic properties were studied by the temperature-programmed desorption of ammonia; catalytic tests in the model reactions of dibenzothiophene (DBT) and alkane aromatization were performed. It was found that the Mo₂C/C catalyst exhibited high activity in these reactions: the conversion of DBT at a contact time of 3–6 h was 80–85%. The conversion of n-heptane at a contact time of 2 h was 31.2%, and 100% toluene was the reaction product. An increase in the contact time to 6 h led to a decrease in the conversion of n-heptane to 1.3%, and to 47% C₆-C₇ cycloalkanes were present in the reaction products. The results of this work are indicative of the high catalytic activity of the Mo₂C/C catalyst obtained by mechanical activation.     

Nano-crystalline Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons

Nano-crystalline cerium oxide catalysts have been prepared by precipitation and evaluated for the total catalytic oxidation of naphthalene, which is a polycyclic aromatic hydrocarbon (PAH). Ceria synthesised by precipitation with urea was the most active catalyst for oxidation of naphthalene to carbon dioxide. The urea precipitated CeO₂ demonstrated over 90% naphthalene conversion to carbon dioxide at 175°C (100 ppm naphthalene, GHSV=25,000 h⁻¹), whilst ceria precipitated via a carbonate only gave 90% conversion at 275°C. Comparison with known high activity total oxidation catalysts, Mn₂O₃ and 0.5% Pt/γ-Al₂O₃, showed that the urea precipitated CeO₂ was a more effective catalyst for naphthalene total oxidation. At temperatures below those required to achieve catalytic activity the adsorption capacity of urea precipitated ceria for naphthalene was considerably greater than any of the other catalysts examined. The high adsorption capacity of the material provides the advantage that it can be used as a combined catalyst and adsorbent to remove PAHs from waste streams.     

Catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst

The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst was studied. Adsorption and temperature programmed desorption of NO/O₂ on Au/CeO₂ reveal that the catalyst adsorbs and desorbs NO over a large range of temperature. A maximum of 26% conversion of NO _x was obtained around 210 °C, with a selectivity of 50% to N₂.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria–zirconia catalysts

Heats of adsorption were measured calorimetrically for O₂ adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria–zirconia catalysts, all with 1 wt% Pd. Significantly more O₂ adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria–zirconia were also 200 kJ/mol, much lower than the heat of reaction for Ce₂O₃ oxidation to CeO₂, but in reasonable agreement with estimates from O₂ desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.     

The supported CeO2/Co3O4–MnO2/CeO2 catalyst on activated carbon prepared by a successive-loading approach with superior catalytic activity and selectivity for CO preferential oxidation in H2-rich stream

The supported CeO₂/Co₃O₄–MnO₂/CeO₂ catalyst on activated carbon (AC) prepared by a successive loading approach to support ceria, cobalt-manganese oxide and ceria on activated carbon exhibits superior catalytic activity and selectivity to Co₃O₄–MnO₂–CeO₂/AC prepared by a one-step loading for CO preferential oxidation in H₂-rich stream, although the same loading of Co, Mn and Ce was used, which illustrates that the addition of ceria doesn't always enhance catalytic performance in CO PROX reaction, and appreciate supporting method is essential. The superior catalytic activity and selectivity of developed catalyst can be ascribed to high reducibility, well dispersion, unique porous structure, and strong interaction between Co₃O₄–MnO₂ and CeO₂.     

The effect of ceria content on the properties of Pd/CeO2/Al2O3 catalysts for steam reforming of methane

The effect of CeO₂ loading on the surface properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pd catalysts was studied in the process of steam reforming of methane. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), UV–vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The XRD measurements indicated that palladium particles on the surface of fresh and reduced catalysts are well dispersed. TPR experiments revealed a heterogeneous distribution of PdO species over CeO₂–Al₂O₃ supports; one fraction of large particles, reducible at room temperature, another fraction interacting with CeO₂ and Al₂O₃, reducible at higher temperatures of 347 and 423 K, respectively. The PdO species reducible at room temperature showed lower CO adsorption relative to the PdO species reducible at high temperature. In contrast to Pd/Al₂O₃, the FTIR results revealed that CeO₂-containing catalyst with CeO₂ loading ≥12 wt.% show lower ratio (LF/HF) between the intensity of the CO bands in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). This ratio was constant with increasing the temperature of reduction. The FTIR spectra and the measurement of Pd dispersion suggested that Pd surface becomes partially covered with ceria at all temperature of reduction and with increasing ceria loading in Pd/CeO₂–Al₂O₃ catalysts. Although the PdO/Al₂O₃ showed higher Pd dispersion compared to that of CeO₂-containing catalysts, the addition of ceria resulted in an increase of the turnover rate and specific rate to steam reforming of methane. The CH₄ turnover rate of Pd/CeO₂–Al₂O₃ catalysts with ceria loading ≥12 wt.% was around four orders of magnitude higher compared to that of Pd/Al₂O₃ catalyst. The increase of the activity of the catalysts was attributed to various effects of CeO₂ such as: (i) change of superficial Pd structure with blocking of Pd sites; (ii) the jumping of oxygen (O^*) from ceria to Pd surface, which can decrease the carbon formation on Pd surface. Considering that these effects of CeO₂ are opposite to changes of the reaction rate, the increase of specific reaction rate with enhancing the ceria loading suggests that net effect results in the increase of the accessibility of CH₄ to metal active sites.     

Hydrothermal Biotemplated Synthesis of Biomorphic Porous CeO2 and Their Catalytic Performance

Biomorphic porous CeO₂ powder was synthesized by the hydrothermal method using stems of clover as biotemplates. Thermogravimetric and differential thermal analysis, X-ray diffraction, N₂ adsorption–desorption, field emission scanning electron microscopy and Fourier transfer infrared spectroscopy were applied to characterize the samples. The oxygen storage/release capacity (OSC) and catalytic oxidation performance of the biomorphic porous CeO₂ for acid magenta were also investigated. Results show that the as-synthesized CeO₂ powders exhibit a cubic phase and have porous structures with pore size ranging from several to dozens of micrometers. The results of N₂ adsorption–desorption measurement suggest that the biomorphic CeO₂ contains a large number of mesopores on the surface of CeO₂ framework; and, the pore diameter is from 15–35 nm. The OSC value of the biomorphic porous CeO₂ is 174.6 μmol O₂/g ceria, which is two-times higher than powdered CeO₂. After catalytic oxidation for 300-min by the biomorphic porous CeO₂, the decolorizing rate of acid magenta is close to 95 %, which is higher than for powdered CeO₂ (64 %).