Modification of the preparation procedure for increasing the hydrodesulfurisation activity of the CoMo/γ-alumina catalysts

In this work we have examined whether the re-impregnation of CoMo/γ-alumina catalysts or the replacement of the conventional non-dry impregnation step by “equilibrium deposition filtration” (EDF) may be used for improving their surface characteristics and thus their catalytic activity.

Two samples were prepared. In the first sample (EDF) the molybdenum species were mounted by “equilibrium deposition filtration” whereas in the second sample these species were mounted by non-dry impregnation (NDI). In both cases the Co was deposited on the calcined Mo/γ-Al₂O₃ precursor solid by simple dry impregnation. An aliquot of each sample was impregnated again with an amount of pure water equal to its pore volume and then it underwent drying and calcination.

The catalysts prepared were characterized using N₂ adsorption measurements (BET), UV–vis diffuse reflectance spectroscopy (DRS), laser Raman spectroscopy (LRS) and NO chemisorption. The hydrodesulfurization (HDS) activities over the catalysts studied were determined using a continuous-flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure.

It was confirmed that the replacement of the conventional impregnation by equilibrium deposition filtration results to catalysts with relatively high active surface and high portion of the well-dispersed octahedral cobalt and thus, to catalysts with 30% higher HDS activity. The re-impregnation resulted to partial dissolution and re-dispersion of the Mo and Co supported oxidic phases. Concerning the NDI catalyst re-impregnation resulted to an increase of the active surface and of the portion of the well-dispersed octahedral cobalt and thus to 25% higher catalytic activity. The opposite effects were observed for the EDF catalyst which exhibited almost 7% lower activity after re-impregnation.     

Supported MoO3 catalysts: preparation by the new “slurry impregnation” method and activity in hydrodesulphurization

A new preparation of supported MoO₃ is described. Slurry MoO₃/water is used instead of the solution (NH₄)₆Mo₇O₂₄. Preparation and HDS activity are illustrated for MoO₃ supported over Al₂O₃, active carbon and ZrO₂. Another application of the new principle is the preparation of high surface area MoO₃/MgO by the reaction of MgO with slurry (NH₄)₆Mo₇O₂₄/methanol. Texture of MgO that is deteriorated in aqueous solution of (NH₄)₆Mo₇O₂₄ is stable in that slurry. “Slurry impregnation” is a special case of equilibrium adsorption impregnation. It is simple and it provides monolayer dispersion of molybdena.     

A comparative study of supported MoO3 catalysts prepared by the new “slurry” impregnation method and by the conventional method: their activity in transesterification of dimethyl oxalate and phenol

A new preparation method for supported MoO₃ catalyst, slurry impregnation, has been described and compared with the conventional impregnation method. Slurry MoO₃/water is used instead of the solution ammonium heptamolybdate, AHM [(NH₄)₆Mo₇O₂₄]. The MoO₃/γ-alumina, MoO₃/active carbon, and MoO₃/silica catalysts with different Mo loadings were prepared by slurry and by conventional method. The low solubility of MoO₃ was sufficient to transport molybdenum species from solid MoO₃ to the adsorbed phase. The equilibrium was achieved after several hours at 95 °C based on the loading amount of molybdenum. Only the process of drying was needed; calcination was not necessary and was left out. This is an important advantage for active carbon support because oxidative degradation of active carbon impregnated by molybdena starts at a relatively low temperature of about 250 °C during calcination on air. The activity was tested in the transesterification of dimethyl oxalate (DMO) and phenol at 180 °C. The dependences of catalytic activity on Mo loadings for the slurry prepared catalysts were similar to the dependences for the samples prepared by the conventional impregnation method with AHM. The activities of the slurry impregnation MoO₃/γ-Al₂O₃ catalysts were almost the same as those of catalysts prepared conventionally. Although the performances of slurry impregnation MoO₃/SiO₂ catalysts for transesterification of DMO were slightly better than those of the corresponding catalysts prepared by conventional impregnation, no waste solution and no calcining nitrogenous gases were produced. Therefore, we conclude that the new slurry impregnation method for preparation of supported molybdenum catalysts is an environmentally friendly process and a simple, clean alternative to the conventional preparation using solutions of (NH₄)₆Mo₇O₂₄. The present work will lead to a remarkable improvement in the catalyst preparation for the transesterification reaction.     

A study on the preparation of supported metal oxide catalysts using JRC-reference catalysts. I. Preparation of a molybdena–alumina catalyst. Part 4. Preparation parameters and impact index

The effects of the volume and pH of the impregnation solution and of the calcination conditions were examined on the physicochemical and catalytic properties of a 13 wt% MoO₃/Al₂O₃ extrudate catalyst. The Al₂O₃ support and drying procedures (static conditions without flowing air) were fixed in the preparations. In the present series of catalysts, the amount of crystalline MoO₃ was marginally small. It was found that the dispersion of Mo oxide species increased as the volume of the impregnation solution increased, gradually approaching a maximum value. The increase in pH (2–8) of the impregnation solution was found to reduce the dispersion of Mo oxide species. The Mo dispersion increased slightly for the impregnation catalysts as the calcination temperature increased (673–873 K), whereas it decreased for the equilibrium adsorption catalysts. The effects of the calcination atmosphere (with or without flowing air, or with flowing humid air) were very small on the dispersion of Mo oxide species under the present preparation conditions. On the other hand, the methanol oxidation activity of MoO₃/Al₂O₃ was sensitive to the preparation parameters examined here. It was demonstrated by means of EPMA and XPS that a considerable migration of Mo took place during the calcination.

In the present study on the preparation of a 13 wt% MoO₃/Al₂O₃ catalyst, an impact index is proposed to measure the magnitude of the effects of the respective parameter(s) on the physicochemical and catalytic properties. With the Mo dispersion, the effects of the preparation parameter decreased in the order, surface area of the support >> drying process > volume of the impregnation solution > pH, calcination temperature and atmosphere. The size of the impact index for the dispersion of Mo sulfide species is 70–75% of that for the Mo oxide species. The HDS activity of the catalyst was less affected by the preparation parameters than the Mo sulfide dispersion. The preparation parameters affected the segregation of Mo on the outer surface of extrudates in a decreasing order: drying process > volume of the impregnation solution > pH, calcination conditions. It was found that the oxidation of methanol was affected most intensely by the drying procedures. The volume of the impregnation solution, calcination conditions and pH of the impregnation solution also strongly affected the oxidation activity. The impact index suggests that the sensitivity to the preparation variables of the physicochemical and catalytic properties of MoO₃/Al₂O₃ decreases in the order, methanol oxidation activity > surface Mo segregation > Mo oxide dispersion > Mo sulfide dispersion > HDS activity.     

Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction

The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, and Pd) for the WGS reaction is investigated with respect to the physichochemical properties of the metallic phase and the support. It has been found that, for all metal-support combinations investigated, Pt is much more active than Pd, while Rh and Ru exhibit intermediate activity. The turnover frequency (TOF) of CO conversion does not depend on metal loading, dispersion or crystallite size, but depends strongly on the nature of the metal oxide carrier. In particular, catalytic activity of Pt and Ru catalysts, is 1-2 orders of magnitude higher when supported on “reducible” (TiO₂, CeO₂, La₂O₃, and YSZ) rather than on “irreducible” (Al₂O₃, MgO, and SiO₂) metal oxides. In contrast to what has been found in our previous study over Pt/TiO₂ catalysts, catalytic activity of dispersed Pt does not depend on the structural and morphological characteristics of CeO₂, such as specific surface area or primary crystallite size.     

Selective reduction of nitric oxide by methanol and dimethyl ether over promoted alumina catalysts in excess oxygen

Selective catalytic reduction (SCR) activity for NO conversion to N₂ over γ-alumina, vanadia/alumina and molybdena/alumina catalysts has been investigated with methanol (MeOH) and dimethyl ether (DME) as reductants under lean conditions. Molybdena/alumina catalysts showed high efficiency for NO reduction with either reductant, especially at low temperature, which may involve surface formyl produced by oxidative dehydrogenation. Sulphated γ-alumina remains active for NO reduction with MeOH, while sulphated 5 wt.% MoO₃/Al₂O₃ remains active with both MeOH and DME over a broad temperature range.     

Methanol oxidation on LaCo mixed oxide supported onto MCM-41 molecular sieve

Lanthanum cobaltate LaCoO_x supported onto MCM-41 mesoporous molecular sieve was prepared by in-situ oxidative decomposition of mixed LaCo citrate complexes inside the mesopores of this support. The prepared materials were characterized by EDX, EPR and UV-Vis DRS techniques as well as by N₂-BET measurements. The nanosized LaCoO_x particles within the mesopores of MCM-41 matrix contains cobalt atoms in lower than Co(III) average oxidation state. Also, the supported cobaltate does not form short-range order species of LaCoO₃ but presents as the highly disordered, oxygen deficient Co oxide nanophase. Catalytic activity in MeOH oxidation was tested using both conventional fixed-bed flow system and “operando” mode of DRIFT technique. The very high activity of MCM-41-supported cobaltate is not only due to highly dispersed LaCoO₃ phase but also because of rather low oxidation state of Co.     

A comparative study of the partial oxidation of methane to formaldehyde on bulk and silica supported MoO3 and V2O5 catalysts

The mechanism of the partial oxidation of methane to formaldehyde with O₂ has been investigated on bulk and differently loaded silica supported (4–7 wt%) MoO₃ and (5–50 wt%) V₂O₅ catalysts at 600–650°C in a pulse reactor connected to a quadrupole mass spectrometer. The reaction rate and product distribution in the presence and in the absence of gas-phase O₂ have been evaluated. On bare SiO₂, low and medium loaded silica supported MoO₃ and V₂O₅ catalysts the reaction proceeds via a concerted mechanism involving the activation of gas-phase oxygen on the reduced sites of the catalyst surface as proved by the direct correlation between catalytic activity and density of reduced sites evaluated in steady-state conditions, while on highly loaded catalysts as well as on bulk MoO₃ and V₂O₅ the reaction rate drops dramatically and the reaction pathway via redox mechanism becomes predominant. The results indicate that the surface mechanism is essentially more effective than the redox mechanism enabling also a higher selectivity to HCHO.     

Carbon dioxide reforming of methane over Ni/Al2O3 treated with glow discharge plasma

Ni/Al₂O₃ catalyst was first treated by argon glow discharge plasma followed by calcination in air. The catalyst prepared this way exhibits an improved low-temperature activity for carbon dioxide reforming of methane, compared to the catalyst prepared without plasma treatment. The catalyst characterization using XRD, chemisorption and TEM analyses show that the plasma treatment followed by calcination thermally induces a generation of specific nickel species on the support. This kind of “plasma” metal species is highly dispersed on the support and can remain stable during reforming reactions. The average size of the “plasma” metal particles is ca. 5 nm. The plasma treatment can also enhance the anti-carbon deposition performance of the catalyst. The formation of carbon species that is responsible for catalyst deactivation can be inhibited. The catalyst stability is therefore improved.     

Measurement of the Surface Free Energy of Amorphous Cellulose by Alkane Adsorption: A Critical Evaluation of Inverse Gas Chromatography (IGC)

Surface energies of amorphous cellulose “beads” were measured by IGC at different temperatures (50 to 100°C) using n-alkane probes (pentane to undecane). The equation of Schultz and Lavielle was applied which relates the specific retention volume of the gas probe to the dispersive component of the surface energy of the solid and liquid, γ^d_s and γ^d_l, respectively, and a parameter (“a”) which represents the surface area of the gas probe in contact with the solids. At 50°C, γ^d_s was determined to be 71.5 mJ/m², and its temperature dependence was 0.36 mJ m^-2 K^-1. Compared with measurements obtained by contact angle, IGC results were found to yield higher values, and especially a higher temperature dependence, d(γ^d_s)/dT. Various potential explanations for these elevated values were examined. The surface energy, as determined by the Schultz and Lavielle equation, was found to depend mostly on the parameter “a”. Two experimental conditions are known to affect the values of “a”: the solid surface and the temperature. While the surface effect of the parameter “a” was ignored in this study, the dependence of the surface energy upon temperature and probe phase was demonstrated to be significant. Several optional treatments of the parameter “a” were modeled. It was observed that both experimental imprecision, but mostly the fundamental difference between the liquid-solid vs the gas-solid system (and the associated theoretical weakness of the model used), could explain the differences between γ^d_s and d(γ^d_s)/dT measured by contact angle and IGC. It was concluded that the exaggerated temperature dependence of the IGC results is a consequence of limitations inherent in the definition of parameter “a”.     

SINTERING KINETICS AND TRANSPORT PROPERTY EVOLUTION OF LARGE MULTI-PARTICLE AGGREGATES

Ultrafine (“nano”-) particles produced from highly supersaturated vapors or liquids are usually aggregated, often containing thousands of small 'primary' particles bound together in tenuous structures characterized by mass fractal dimensions less than 3. Such aggregates have large initial surface area but are metastable with respect to more compact configurations. Available restructuring mechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (“primary”) particles. We here exploit the notion that, provided an aggregate is sufficiently large, it can be treated like a spatially non-uniform porous medium, undergoing finite-rate surface energy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after a D_ƒ ≌s const stage of sintering [associated with a corresponding increase in mean apparent primary particle ('grain') size], we use an extension of the sintering rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977), treating the material of the restructuring aggregate to be a Newtonian viscous fluid. We predict and report here the time-dependent increase in fractal dimension, D_ƒ, and associated decreases in: aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimension-less time [real time in multiples of the characteristic sintering time, μ (R₁)_t=0/σ cr, where u is the material's viscosity (R_l)_t=0 is the effective initial grain radius and a the material surface tension]. In these units, we find that the total required coalescence time does not increase with N as sensitively as N^1/3 an important observation for processes involving very large aggregates. With validation and the indicated extensions, our pseudo-continuum methods are efficient enough to be used for estimating the morphological- and transport property-evolution of entire populations of restructuring aggregates, perhaps characterized by some non-separable probability density function pdf(N, D_ƒ,R₁,) locally, in non-isothermal combustion-synthesis reactors.     

Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts

The influence of the ZrO₂ support modification by Y₂O₃ and the presence of ethylenediamine (“en”) during the preparation of Co/ZrO₂ were studied and compared with a reference catalyst conventionally prepared by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. The effect of the en/Co molar ratio (x = 1–3) was studied. Activation of cobalt species was followed by differential thermal and thermogravimetric analyses (DTA/TG) analyses and by specific surface area measurements which evidence the complete cobalt precursor decomposition at 450 °C, whatever the support composition and the en/Co molar ratio. The addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase in the catalytic activity of the activated solids for the toluene deep oxidation as compared to the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion (X-ray diffraction (XRD)) and also in terms of Co-support interaction (H₂-temperature-programmed reduction (TPR)). The generated cobalt species were reducible at much lower temperatures and were more active in the toluene total oxidation. Finally, an efficient catalyst was produced combining the modifications of the support by yttrium oxide and of the precursor (use of ethylenediamine).     

Support effects on structure and activity of molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of MoO₃ catalysts supported on ZrO₂, Al₂O₃, TiO₂ and SiO₂ with Mo surface densities, n_s, in the range of 0.5–18.5 Mo/nm² were studied for the oxidative dehydrogenation (ODH) of ethane by in situ Raman spectroscopy and catalytic activity measurements at temperatures of 400–540 °C. The molecular structure of the dispersed surface species evolves from isolated monomolybdates (MoO₄ and MoO₅, depending on the support) at low loadings to associated MoO_x units in polymolybdate chains at high loadings and ultimately to bulk crystalline phases for loadings exceeding the monolayer coverage of the supports used. The nature of the oxide support material and of the Mo–O–support bond has a significant influence on the catalytic behaviour of the molybdena catalysts with monolayer coverage. The dependence of reactivity on the support follows the order ZrO₂ > Al₂O₃ > TiO₂ > SiO₂. The oxygen site involved in the anchoring Mo–O–support is of relevance for the catalytic activity.     

Mechanistic evidence of the partial oxidation of methane to formaldehdye over silica based oxide catalysts by temperature programmed reaction studies

The catalytic behaviour of SiO₂ supported MoO₂ and V₂O₅ catalysts in the partial oxidation of methane to formaldehyde with O₂ (MPO) in the range 400–800°C has been investigated by temperature programmed reaction (TPR) tests. Both the sequence of the onset temperature of product formation and the product distribution patterns signal that MPO on silica based oxide catalysts occurs mainly via a consecutive reaction path: CH₄ → HCHO → CO → CO₂. At T >/ 700°C a parallel surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C₂ products both on SiO₂ and MoO₃/SiO₂ catalysts. The redox properties of MoO₃/SiO₂ and V₂O₅SiO₂ catalysts have been systematically evaluated by H₂ and CH₄ temperature programmed reduction (H₂-TPR, CH₄-TPR) measurements. H₂-TPR results do not account for the reactivity scale of oxide catalysts in the MPO. CH₄-TPR measurements indicate that the enhancement in the specific activity of the silica is controlled by the capability of MoO₃ and V₂O₅ promoters in providing ‘active’ lattice oxygen species.     

The catalytic transformation of chlorofluorocarbons in hydrogen on metal-based catalysts supported on inorganic fluorides

The use of metal halides as carriers for supported metal catalysts allows to obtain stable and selective materials for the hydrodechlorination of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) to hydrofluorocarbons (HFCs). The most used catalyst is Pd/AlF₃, but unexpected selectivities have been obtained with Pd on “ZrF₄” oxyfluoride materials or KMgF₃ perovskite-like structure. From a survey of the kinetics, mechanism and surface complexes occurring in the transformation of CFC on Pd, explanations of the beneficial use of metal fluorides as carriers are provided. It is proposed that the good hydrodechlorination selectivity observed on Pd/fluoride comes from an electronic modification of Pd by substoichiometric fluoride species, e.g. AlF_x (x<3), in decoration onto the metal particles. The electron withdrawing effect of these species decreases the disponibility of Pd d electrons and favors the desorption of pallado-fluorocarbenes, e.g. =CF₂, CF₃–CF=, etc., to yield HFC compounds. It is also demonstrated that the dilution of the Pd surface by the decorating AlF_x species decreases the probability of occurrence of surface complexes exchanging multiple bonds with Pd, e.g. CF₃–C=, and leading to deeply hydrogenated compounds. Several methods, alloying, coprecipitation, etc. allow to prepare Pd/fluoride with enhanced interaction between Pd and these substoichiometric species.     

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH₃ in the presence of O₂ has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T_R<513 K, 2503 (ppm) < 4000, 12 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O₂, and essentially 0 with respect to NH₃. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH₃ was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu^I is oxidized by O₂ to “Cu^II-oxo”, (ii) “Cu^II-oxo” reacts with NO to yield “Cu^II-N_xO_y”, and (iii) finally “Cu^II-N_xO_y” is reduced by NH₃ to give N₂, H₂O, and the regeneration of Cu^I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu^I or “Cu^II-oxo” species constitute the rate-determining active center.     

Catalytic oxidation of hydrogen sulfide by dioxygen on CoN4 type catalyst

A new catalyst based on the formation of carbon supported “CoN₄” structures by heat treatment of Co(CH₃COO)₂ and imidazole impregnated carbon black is introduced. The performance of the new catalyst is compared to the performance of other catalysts including activated carbon powder, granular activated carbon, and pyrolysed cobalt(II) mesotetra-4-methoxyphenylporphyrin. The optimized form of the new catalyst outperforms all these catalysts. The underlying concept is borrowed from fuel cell technology, which uses electrocatalysts that activate triplet dioxygen and facilitate its reduction. The same concept is used to reduce oxygen by hydrogen sulfide. In electrochemistry oxygen reduction is carried out by heterogeneous electron transfer, whereas in dehydrosulfurization the reduced sulfur moieties serve as the electron donors. We postulate that the reason for the improved performance of the new catalyst compared to pyrolysed carbon supported cobalt porphyrin, which is also a Me–N₄ type catalyst, stems from the ability to load a larger amount of cobalt chelates rather than from a change of oxidation mechanism.     

Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane

Aerosol particles of amorphous hydrogenated silicon resulting from thermal decomposition of silane were investigated by hydrogen evolution, IR-, EPR-, NMR spectroscopy, and transmission electron microscopy.

The experimental data show that aerosol particles contain to a various extent {SiH₂}_n polymer structures and two types of monohydride groups SiH- “clustered” and “dilute” monohydride groups. The hydrogen atoms of the “clustered” monohydride groups are located close to each other. The “clustered” monohydride groups are inaccessible to the ambient because they are embedded in the amorphous network. The “dilute” monohydride groups are relatively isolated from each other. The majority of “dilute” monohydride groups are open to the ambient. They are located on the surface of preferentially interconnected microchannels and microvoids.

Interaction between the “dilute” SiH groups and atmospheric oxygen results in formation of OSiH groups in which hydrogen and oxygen are bonded to a common silicon atom. Evidently, the interaction occurs throw the oxygen reaction with weak bonds associated with “dilute” monohydride groups. There is no interaction between oxygen and both “clustered” SiH groups and {SiH₂}_n chain because the former are inaccessible to atmospheric oxygen and the latter has presumably no weak bonds in the chains.     

Optimization of the pretreatment procedure in the design of cobalt silica supported Fischer–Tropsch catalysts

The effect of cobalt precursor, catalyst pretreatment and promotion with ruthenium and rhenium on the formation of cobalt metal nanoparticles and catalytic performance of supported Fischer–Tropsch (FT) catalysts was studied using a combination of techniques (DSC–TGA, UV–vis spectroscopy, XPS, XRD, EXAFS–XANES, in situ magnetization measurements, propene chemisorption and catalytic measurements). The cobalt promoted and unpromoted catalysts were prepared by aqueous co-impregnation using cobalt nitrate or acetate, ruthenium nitrosyl nitrate or perrhenic acid. In both unpromoted and Ru and Re-promoted cobalt catalysts after impregnation and drying, cobalt is present mainly in octahedrally coordinated complexes. The repartition of cobalt species between Co₃O₄ and cobalt silicate depends essentially on the exothermicity of precursor decomposition. Cobalt nitrate precursor, with an endothermic decomposition, favors Co₃O₄ crystallites. Lower temperature of cobalt nitrate decomposition and catalyst calcination generally leads to higher dispersion of supported cobalt oxide. Cobalt acetate precursor, with an exothermic decomposition, favors cobalt silicate. By optimizing the conditions of cobalt acetate decomposition, the fraction of cobalt silicate can be decreased favoring a more reducible Co₃O₄ phase. For the catalysts prepared from cobalt nitrate, promotion with ruthenium increases the cobalt dispersion, while maintaining high reducibility. For the catalyst prepared via low temperature decomposition of cobalt acetate, addition of ruthenium increases the fraction of Co₃O₄ crystalline phase and decreases the concentration of barely reducible cobalt silicate. The Fischer–Tropsch reaction rates over unpromoted and promoted cobalt catalysts were found to be primarily a function of the number of cobalt metal sites, which are generated by the reduction of Co₃O₄ crystallites.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.