TPR and XRD studies of yttria-doped ceria/γ-alumina-supported copper oxide catalyst

Yttria-doped ceria (YDC) and pure ceria (CeO₂), respectively, were deposited on γ-alumina (γ-Al₂O₃) using the impregnation method; then, copper oxide was also supported on them by employing the impregnation method. For comparison, CuO/γ-Al₂O₃ catalysts were prepared in this work. The catalysts were characterized by temperature-programmed reduction (TPR) and X-ray diffraction (XRD). For CuO/γ-Al₂O₃ catalysts, two TPR peaks, namely β and γ, were observed. These have been attributed to the reduction of highly dispersed copper oxide species and bulk-like copper oxide, respectively. For CuO/CeO₂/γ-Al₂O₃ and CuO/YDC/γ-Al₂O₃ catalysts, four TPR peaks, namely ₁, ₂, β′ and γ′, could be observed. The peaks with lower peak temperatures as compared to those of β′ and γ′ peaks have been attributed to the reduction of interface-boundary copper oxide species that contact closely and interact strongly with the supported ceria or YDC. Crystal sizes calculated from XRD measurements confirmed that yttria (Y₂O₃) addition could lead to crystal growth of ceria and correspondingly enhance the dispersion of the supported copper oxide due to the partition of YDC crystallite. Hence, this work shows that supported YDC and ceria can act bi-functionally as a textural promoter as well as a structural promoter.     

Study of Ag/La0.6Sr0.4MnO3 catalysts for complete oxidation of methanol and ethanol at low concentrations

Ag-modified La_0.6Sr_0.4MnO₃-based catalysts with the perovskite-type structure were prepared by using a citric acid sol–gel method, and their catalytic performance for complete oxidation of methanol and ethanol was evaluated and compared with that of the γ-Al₂O₃-supported catalysts, Ag/γ-Al₂O₃, Pt/γ-Al₂O₃, and Pd/γ-Al₂O₃. The results showed that the Ag-modified La_0.6Sr_0.4MnO₃-based catalysts with the perovskite-type structure displayed the activity significantly higher than that of the supported precious metal catalysts, 0.1%Pd/γ-Al₂O₃ and 0.1%Pt/γ-Al₂O₃ in the temperature range of 370–573 K. Over a 6%Ag/20%La_0.6Sr_0.4MnO₃/γ-Al₂O₃ catalyst, the T₉₅ temperature for methanol oxidation can be as low as 413 K. Even at such low reaction temperature, there were little HCHO and CO detected in the reaction exit-gas. However, for the 0.1%Pd/γ-Al₂O₃ and 0.1%Pt/γ-Al₂O₃ catalysts, the HCHO content in the reaction exit-gas reached 200 and 630 ppm at their T₉₅ temperatures. Over a 6%Ag/La_0.6Sr_0.4MnO₃ catalyst, the T₉₅ temperature for ethanol oxidation can be as low as 453 K, with a corresponding content of CH₃CHO in the exit-gas at 782 ppm; when ethanol oxidation is performed at 493 K, the content of acetaldehyde in the exit-gas can be below 1 ppm. Characterization of the catalysts by X-ray diffraction (XRD), TEM, XPS, laser Raman spectra (LRS), hydrogen temperature-programmed reduction (H₂-TPR) and oxygen temperature-programmed desorption (O₂-TPD) methods revealed that both the surface and the bulk phase of the perovskite La_0.6Sr_0.4MnO₃ played important roles in the catalytic oxidation of the alcohols, and that γ-Al₂O₃ as the bottom carrier could be beneficial in creating a large surface area of catalyst. Moreover, a small amount of Ag⁺ doped onto the surface of La_0.6Sr_0.4MnO₃ was able to partially occupy the positions of La³⁺ and Sr²⁺ due to their similar ionic radii, and thus, became stabilized by the perovskite lattice, which would be in favor of preventing the aggregation of the Ag species on the surface and enhancing the stability of the catalyst. On the other hand, modification of the Ag⁺ to the surface of La_0.6Sr_0.4MnO₃ resulted in an increase in relative content of the surface O₂²⁻/O⁻ species highly reactive toward the alcohols and aldehydes as well as CO. Besides, solution of low-valence metal oxides SrO and Ag₂O with proper amounts in the lattice of the trivalent metal perovskite-type oxide LaMnO₃ would also lead to an increase in the content of the reducible Mnⁿ⁺ and the formation of anionic vacancies, which would be favorable for the adsorption-activation of oxygen on the functioning catalyst and the transport of the lattice and surface oxygen species. All these factors would contribute to the pronounced improvement of the catalyst performance.     

A new insight in the unusual adsorption properties of Cu cations in Cu-ZSM-5 zeolite

ZSM-5 zeolites modified with Cu⁺ ions were prepared either by the high-temperature chemical reaction of hydrogen form with CuCl vapour or by the wet ion exchange with subsequent reduction of the modified samples in CO at 873 K. Adsorption of H₂, N₂ or C₂H₆ by Cu⁺ ions was studied by DRIFTS and by volumetric technique. The conclusions about the structure of adsorption complexes were supported by the DFT cluster quantum chemical calculations. The obtained results indicated that in addition to the previously reported strong adsorption of nitrogen, the univalent copper also unusually strongly adsorbs molecular hydrogen and ethane. Adsorption of hydrogen is the most amazing since the observed low-frequency shifts of the HH stretching vibrations were as high as about 1000 cm⁻¹. This is quite different from much weaker H₂ perturbation by Cu²⁺ cations. Adsorption of ethane by Cu⁺ ions also resulted in the low-frequency shifts of some of CH IR stretching bands up to 400 cm⁻¹. The DFT cluster modelling indicated that both adsorption of hydrogen and ethane could be explained by interaction with the isolated Cu⁺ ions localized at the sites of the ZSM-5 framework. Quantum chemical calculations indicated the important role in the bonding of adsorbed hydrogen and ethane of electron back donation from d_π-orbitals of Cu⁺ ions to the σ^*HH or CH orbitals. The overall yield of Cu⁺ sites of the strong H₂ or N₂ adsorption is about twice lower than the total copper content.     

Characterization of Pd-based FCC CO/NOx control additives by in situ FTIR and extended X-ray absorption fine structure spectroscopies

Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃-type materials used as FCC additives for CO/NO_x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H₂ at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO_x species on the catalyst surface. However, the Na⁺ species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm⁻¹. Finally, hydroxyls from the support are also actively participating in the formation of HNO_x type compounds with characteristic stretching vibrations in the 3500–3572 cm⁻¹ region.     

Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM5

Copper ion-exchanged zeolites ZSM5 with SiO₂:Al₂O₃ molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h⁻¹). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO₂ formation in the gas phase.

A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO₂:Al₂O₃ ratio, enhances the conversion of NO. A vibrational band at 1631 cm⁻¹ is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu²⁺–O–Cu²⁺ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N₂O₃, also binds in a bridge configuration to the same Cu²⁺–O–Cu²⁺ dimers.

A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm⁻¹ probes isolated copper ions.

A third infrared band at 2130 cm⁻¹ confirms previous observations of -ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for adsorption are blocked by Cu²⁺ ion-exchange. The 2130 cm⁻¹ species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice.

Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers.

A high SiO₂:Al₂O₃ ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.     

Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N₂, increased as Cu loading increased. N₂ selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H₂ (H₂-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu²⁺ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H₂-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu⁺ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO₂ suggested that CuO could form N₂, but, independent of the CuO dispersion, nitrogen oxides (NO_x) were formed above 350 °C. Cu⁺ and the square-pyramidal and distorted square-pyramidal forms of Cu²⁺ showed low activity for AN decomposition. Temperature-programmed desorption of NH₃ suggested that N₂ formation from NH₃ proceeded on Cu²⁺, resulting in the formation of Cu⁺. The Cu⁺ ions were oxidized to Cu²⁺ at around 300 °C. Thus, high N₂ selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu²⁺, which can be reversibly reduced and oxidized.     

Surface properties and reactivity of Cu/γ-Al2O3 catalysts for NO reduction by C3H6: Influences of calcination temperatures and additives

The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al₂O₃ catalysts on the surface properties and reactivity for NO reduction by C₃H₆ in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al₂O₃ catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al₂O₃ and 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalysts calcined at 1073 K possess a CuAl₂O₄ phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C₃H₆ shows that the CuAl₂O₄ is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K, although these catalysts possess the same CuAl₂O₄ phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K is attributed to the formation of less active CuAl₂O₄ phase with high aggregation and preferential promotion of C₃H₆ combustion to CO_x by MnO₂. The engine dynamometer test for NO reduction shows that the C₃H₆ is a more effective reducing agent for NO reduction than the C₂H₅OH. The maximum reactivity for NO reduction by C₃H₆ is reached when the NO/C₃H₆ ratio is one.     

Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2H4 as a reducing agent

This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO₂ with C₂H₄ as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C₂H₄/SO₂, and pretreatment are all important factors affecting catalyst activity. Fe₂O₃/γ-Al₂O₃ was found to be the most active catalyst among six γ-Al₂O₃-supported metal oxide catalysts tested. With Fe₂O₃ as the active species, of the supports tested, CeO₂ is the most suitable one. Using this Fe₂O₃/CeO₂ catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C₂H₄/SO₂ is 1:1, and the catalyst presulfidized by H₂+H₂S exhibits a higher performance than those pretreated with H₂ or He. Although the feed concentrations of C₂H₄:SO₂ being 3000:3000 ppm provide a higher conversion of SO₂, the sulfur yield decreases drastically at temperatures above 300 °C. With higher feed concentrations, maximum yield appears at higher temperatures. The C₂H₄ temperature-programmed desorption (C₂H₄-TPD) and SO₂-TPD desorption patterns illustrate that Fe₂O₃/CeO₂ can adsorb and desorb C₂H₄ and SO₂ more easily than can Fe₂O₃/γ-Al₂O₃. Moreover, the SO₂-TPD patterns further show that Fe₂O₃/γ-Al₂O₃ is more seriously inhibited by SO₂. These findings may properly explain why Fe₂O₃/CeO₂ has a higher activity for the reduction of SO₂.     

The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3

The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C₃H₆ over CuO/γ-Al₂O₃ has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu⁰/Cu⁺ to Cu²⁺ by NO and O₂, (ii) the co-adsorption of NO/NO₂/O₂ to produce Cu²⁺(NO₃⁻)₂, and (iii) the reaction of Cu²⁺(NO₃⁻)₂ with C₃H₆ to produce N₂, CO₂, and H₂O. Increasing the O₂/NO ratio from 25.0 to 83.4 promotes the formation of NO₂ from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO₂/O₂. This reactant mixture allows the formation of Cu²⁺(NO₃⁻)₂ and its reaction with the C₃H₆ to occur at a higher rate with a higher selectivity toward N₂ than the low O₂/NO flow. Both the high and low O₂/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C₃H₇---NO₂, C₃H₇---ONO, CH₃COO⁻, Cu⁰---CN, and Cu⁺---NCO intermediates toward N₂, CO₂, and H₂O products. High O₂ concentration in the high O₂/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O₂/NO SCR at 523–698 K. High O₂ concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O₂ concentration at temperatures above 723 K.     

Surface and bulk properties of Cu–ZSM-5 and Cu/Al2O3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al₂O₃, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H₂, O₂) and on their DeNO_x activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu²⁺ and Cu⁰, while Cu⁺ is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation.

The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu⁺ ions; the isolated Cu²⁺ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N₂ are characterized by the presence of isolated Cuⁿ⁺ ions beside the additional influence of the zeolitic framework; after the formation of Cu⁺ ions the redox cycles are reversible but, after the formation of Cu⁰, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions.     

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO₂-promoted γ-Al₂O₃ with various CeO₂ (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO₂ structure and dispersion on the γ-Al₂O₃ support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO₂ structure. Pd is present on the oxidized CeO₂-promoted catalysts as Pd⁰, Pd⁺ and Pd²⁺, at ratios strongly dependent on CeO₂ structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO₂ and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO₂. FTIR spectra of adsorbed CO revealed that after pre-treatment under H₂ or in the presence of amorphous CeO₂, partial encapsulation of Pd particles occurs. CeO₂ structure influences the CH₄ steam reforming reaction rates. Crystalline CeO₂ and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO₂ as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al₂O₃ or on amorphous (reduced) CeO₂/Al₂O₃ catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO₂/γ-Al₂O₃ than on Pd/γ-Al₂O₃, which is attributed to a catalytic synergism between Pd and CeO₂. The low rates on the reduced Pd/CeO₂/Al₂O₃ catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.     

In situ FTIR characterization of the adsorption of CO and its reaction with NO on Pd-based FCC low NOx combustion promoters

The adsorption of CO and its reaction with NO in the 400–600 °C temperature range on Ceⁿ⁺/Na⁺/γ-Al₂O₃ and Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃ type materials used commercially as FCC additives were monitored by FTIR spectroscopy. Exposure of both types of samples to CO leads to the formation of carboxylates and carbonates. The concentration of these species was higher in samples containing Pd, indicating that palladium catalyzes their formation. The Pdⁿ⁺ cations initially present in these samples undergo partial reduction to form metallic Pd in the presence of CO even at room temperature. More complete reduction of Pd, along with some aggregation, was observed after exposure to CO at elevated temperatures. Exposure of both types of samples to NO/CO mixtures in the 400–600 °C temperature range leads to the formation of surface isocyanate species. Both Na⁺ and Ceⁿ⁺ promote the formation of such NCO species. However, surface isocyanate species were formed with substantially higher rates in the presence of palladium. The formation of the isocyanate species strongly correlates with changes observed in the ν_OH region, indicating that hydroxyls actively participate in the surface chemistry involved and are capable of protonating the NCO species. The isocyanates are also reactive towards O₂ and NO yielding CO₂ and N₂. These results suggest that isocyanates are possibly involved as intermediates in the CO–NO reaction over the materials examined.     

An investigation of NO/CO reaction over perovskite-type oxide La0.8Ce0.2B0.4Mn0.6O3 (B = Cu or Ag) catalysts synthesized by reverse microemulsion

The perovskite-type oxides La_0.8Ce_0.2Cu_0.4Mn_0.6O₃ and La_0.8Ce_0.2Ag_0.4Mn_0.6O₃ prepared by reverse microemulsion and sol–gel methods (denoted as R and S, respectively), have been investigated on their catalytic performance for the (NO + CO) reaction, and characterized by means of temperature-programmed desorption (TPD), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). XRD measurements proved the presence of the perovskite phase with a considerable amount of CeO₂ phase and the formation of CeO₂ phase was restrained with the reverse microemulsion method. TEM investigations revealed that the La_0.8Ce_0.2Cu_0.4Mn_0.6O₃-R nanoparticles were uniform spheres in shape with diameters ranging from 40 to 50 nm, whereas an aggregation of particles was found for the La_0.8Ce_0.2Cu_0.4Mn_0.6O₃-S catalyst. The activity of NO reduction with CO decreased in the order of La_0.8Ce_0.2Cu_0.4Mn_0.6O₃-R > La_0.8Ce_0.2Cu_0.4Mn_0.6O₃-S > La_0.8Ce_0.2Ag_0.4Mn_0.6O₃-R > La_0.8Ce_0.2Ag_0.4Mn_0.6O₃-S. In NO-TPD experiments, the principal desorbed species detected in the effluent was NO with a trace amount of O₂ and N₂O, suggesting that the non-dissociated adsorption of NO on the surface of the perovskite-type oxides was dominant. The XPS results revealed that Ce⁴⁺ and Cu⁺ was the predominant oxidation state for Ce and Cu components in La_0.8Ce_0.2Cu_0.4Mn_0.6O₃ and La_0.8Ce_0.2Ag_0.4Mn_0.6O₃ catalysts. The existence of Cu⁺ ions and its redox reaction (Cu⁺ ↔ Cu²⁺) would benefit the NO adsorption and reduction by CO.     

Interaction between Pt and MoO3 dispersed over alumina

Pt–xMo/γ-Al₂O₃ catalysts of different molybdenum loading (2–20 wt.%) and with 1 wt.% of platinum were prepared by successive wet impregnation after intermediate calcination. The structure, morphology and surface were characterized by various methods. The DRS results indicate the presence of octahedral Mo⁶⁺ and tetrahedral Mo⁶⁺ phases. It also evidences the presence of polymeric MoO_x species, responsible for the formation of a well dispersed surface sublayer and bulk MoO₃ crystalline phase. XPS results after reduction and passivation of the 1Pt and 1Pt2Mo revealed the presence of residual chlorine, in the form of surface species such as [Pt(OH)_xCl_y]_s and [PtO_xCl_y]_sfavoring the formation of well dispersed platinum particles. The TPD and FTIR results are consistent with the existence of new active sites of Pt in the presence of molybdenum loading. For low Mo content there is a H₂ spillover effect. These results confirm the decoration model of Pt encapsulation by partially reduced Mo species as well as H₂ storage and backspillover due to the generation of a bronze compound.     

Performance and characterization of Ru/Al2O3 and Ru/SiO2 catalysts modified with Mn for Fischer–Tropsch synthesis

Mn effect and characterization on γ-Al₂O₃-, -Al₂O₃- and SiO₂-supported Ru catalysts were investigated for Fischer–Tropsch synthesis under pressurized conditions. In the slurry phase Fischer–Tropsch reaction, γ-Al₂O₃ catalysts showed higher performance on CO conversion and C₅⁺ selectivity than -Al₂O₃ and SiO₂ catalysts. Moreover, Ru/Mn/γ-Al₂O₃ exhibited high resistance to catalyst deactivation and other catalysts were deactivated during the reaction. From characterization results on XRD, TPR, TEM, XPS and pore distribution, Ru particles were clearly observed over the catalysts, and γ-Al₂O₃ catalysts showed a moderate pore and particle size such as 8 nm, where -Al₂O₃ and SiO₂ showed highly dispersed ruthenium particles. The addition of Mn to γ-Al₂O₃ enhanced the removal of chloride from RuCl₃, which can lead to the formation of metallic Ru with moderate particle size, which would be an active site for Fischer–Tropsch reaction. Concomitantly, manganese chloride is formed. These schemes can be assigned to the stable nature of Ru/Mn/γ-Al₂O₃ catalyst.     

Effect of phosphorus addition on the hydrotreating activity of NiMo/Al2O3 carbide catalyst

A series of phosphorus promoted γ-Al₂O₃ supported NiMo carbide catalysts with 0–4.5 wt.% P, 13 wt.% Mo and 2.5 wt.% Ni were synthesized and characterized by elemental analysis, pulsed CO chemisorption, BET surface area measurement, X-ray diffraction, near-edge X-ray absorption fine structure, DRIFT spectroscopy of CO adsorption and H₂ temperature programmed reduction. X-ray diffraction patterns and CO uptake showed the P addition to NiMo/γ-Al₂O₃ carbide, increased the dispersion of β-Mo₂C particles. DRIFT spectra of adsorbed CO revealed that P addition to NiMo/γ-Al₂O₃ carbide catalyst not only increases the dispersion of Ni-Mo carbide phase, but also changes the nature of surface active sites. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activities of these P promoted NiMo/γ-Al₂O₃ carbide catalysts were performed in trickle bed reactor using light gas oil (LGO) derived from Athabasca bitumen and model feed containing quinoline and dibenzothiophene at industrial conditions. The P added NiMo/γ-Al₂O₃ carbide catalysts showed enhanced HDN activity compared to the NiMo/γ-Al₂O₃ catalysts with both the feed stocks. The P had almost no influence on the HDS activity of NiMo/γ-Al₂O₃ carbide with LGO and dibenzothiophene. P addition to NiMo/γ-Al₂O₃ carbide accelerated CN bond breaking and thus increased the HDN activity.     

Application of Ce0.33Zr0.63Pr0.04O2-supported noble metal catalysts in the catalytic wet air oxidation of 2-chlorophenol: Influence of the reaction conditions

In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto θ-Al₂O₃. These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1 wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al₂O₃ less than 0.6 (xLa_0.2Sr_0.3Ba_0.5MnAl₁₁O₁₉ + (1−x)·Al₂O₃, with x = 0.25, 0.60), samples with high specific surface area and θ-Al₂O₃ structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al₂O₃ catalyst. The highest activity was obtained for the samples presenting a θ-Al₂O₃ structure (with x = 0.60) and a high surface area. Impregnation of 1 wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700 °C) were obtained on the stabilized θ-Al₂O₃ solids (x = 0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700 °C), due to a progressive reduction of the PdO_x active phase into the less active Pd° phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported θ-Al₂O₃ samples. This clearly showed the beneficial effect of the support for the stabilization of the PdO_x active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn³⁺/Mn²⁺ redox properties (in the case of the hexaaluminate and stabilized θ-Al₂O₃ samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface.     

Characterization and activity of novel copper-containing catalysts for selective catalytic reduction of NO with NH3

Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH₃, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu²⁺ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu²⁺ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V₂O₅–WO₃TiO₂ catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH₃ combustion on the free Mg/Al mixed oxide surface.     

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

A series of palladium-substituted La₂CuO₄, corresponding to the formula La₂Cu_{1 −x}Pd_xO₄ (x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m²/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C₃H₆ decreased markedly when increasing the palladium content, the activity of catalysts La₂Cu_0.9Pd_0.1O₄ and La₂Cu_0.8Pd_0.2O₄ being comparable to that of a Pt-Rh/CeO₂–Al₂O₃ catalyst for NO reduction, and higher for CO and C₃H₆ oxidation.

All the La₂Cu_{1 − x} Pd_xO₄ catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd⁰ ions atomically dispersed, surrounded by Cu⁺ and Cu²⁺ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La₂Cu_{1 −x}Pd_xO₄ structure.     

NO reduction by CH4 in the presence of O2 over La2O3 supported on Al2O3

Dispersing La₂O₃ on δ- or γ-Al₂O₃ significantly enhances the rate of NO reduction by CH₄ in 1% O₂, compared to unsupported La₂O₃. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La₂O₃ precursor used, the pretreatment, and the La₂O₃ loading. The most active family of catalysts consisted of La₂O₃ on γ-Al₂O₃ prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m²) occurred between the addition of one and two theoretical monolayers of La₂O₃ on the γ-Al₂O₃ surface. The best catalyst, 40% La₂O₃/γ-Al₂O₃, had a turnover frequency at 700°C of 0.05 s⁻¹, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La₂O₃/Al₂O₃ catalysts exhibited stable activity under high conversion conditions as well as high CH₄ selectivity (CH₄ + NO vs. CH₄ + O₂). The addition of Sr to a 20% La₂O₃/γ-Al₂O₃ sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO⁼₄ to the latter Sr-promoted La₂O₃/Al₂O₃ catalyst decreased activity although sulfate increased the activity of Sr-promoted La₂O₃. Dispersing La₂O₃ on SiO₂ produced catalysts with extremely low specific activities, and rates were even lower than with pure La₂O₃. This is presumably due to water sensitivity and silicate formation. The La₂O₃/Al₂O₃ catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications.