Catalytic oxidation of saturated hydrocarbons on multicomponent Au/Al2O3 catalysts: Effect of various promoters

The performance of unpromoted and MO_x-(M: alkali (earth), transition metal and cerium) promoted Au/Al₂O₃ catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH₄ (above 400 °C), compared with C₃H₈ (above 250 °C). The addition of various MO_x to Au/Al₂O₃ improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al₂O₃ are FeO_x and MnO_x. For C₃H₈ oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation.     

A spillover approach to oxidation catalysis over copper and manganese mixed oxides

The activity of a hopcalite-type catalyst for H₂ and CO oxidation is compared with that of single-phase oxides CuO, Mn₂O₃ and CuMn₂O₄ (spinel) and a mixture of 1:1 CuO and Mn₂O₃ in order to elucidate the effects that are responsible for the high catalytic activity of the former. The reaction rates over the hopcalite catalyst calcined at 550°C (a mixture of CuO, Mn₂O₃ and CuMn₂O₄) are very close to those of the physical mixture of CuO and Mn₂O₃, being much greater than the rates over the single-phase oxides. CuO, Mn₂O₃ and CuMn₂O₄ show a kinetic compensation behavior both in H₂ and CO oxidation. By taking into account the activation energies and the reducibility measured by TPR it was concluded that the oxidation reactions over CuO follow a redox mechanism using lattice oxygen, while over Mn₂O₃ the mechanism is associative involving adsorbed oxygen species. Based on the TPR and kinetic results, the synergy between copper and manganese oxides in hopcalite and in the CuO–Mn₂O₃ mixture is assigned to a spillover effect.     

The nature of metal oxide on adsorptive and catalytic properties of Pd/MeOx/Al2O3 catalysts

The role of ceria, niobium and molybdenum oxides on the promotion of the NO reduction by CO was studied. A bifunctional mechanism was discussed as a function of both the nature of interaction between metal oxide and palladium and the redox properties of each metal oxide.

The NO dissociation was better on the Pd/MoO₃/Al₂O₃ catalyst than on the Pd/CeO₂/Al₂O₃ and Pd/Nb₂O₅/Al₂O₃ catalysts. The explanation for the very high N₂ production on Pd–Mo catalyst during the TPD analysis may be attributed to the NO+Me^δ+ stoichiometric reaction.

The promoting effect of a reducible oxide for the NO+CO reaction at low temperature can be ascribed mainly to its easiness for a redox interchange and its interaction with the noble metal particles. This would increase the surface redox ability and favor the dynamic equilibrium needed for high N₂ selectivity.     

Physicochemical surface and catalytic properties of CuO–ZnO/Al2O3 system

A series of CuO–ZnO/Al₂O₃ solids were prepared by wet impregnation using Al(OH)₃ solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O₂ at temperatures ranged between 125°C and 200°C.

The results showed that CuO interacts with Al₂O₃ to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl₂O₄ at 600°C while stimulates its production at 800°C. The treatment of CuO/Al₂O₃ solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol⁻¹ in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature.     

铁氧化物在流化床温度和CO气氛下的形态迁移及其对NO催化还原

利用小型固定床实验台实验研究了铁氧化物在典型流化床温度和CO还原性气氛下的形态迁移及其生成物对NO的催化还原作用,采用分级还原结合X射线衍射（XRD）表征分析,确定铁氧化物与CO和NO反应后生成物的价态及各种铁氧化物对NO的还原机制。结果表明,Fe₂O₃在实验条件下可依次被CO还原为Fe₃O₄、FeO和单质铁,反应过程中随着还原度的增加,还原速率逐级下降,从Fe₂O₃还原到Fe₃O₄的速率最高,FeO还原到Fe速率最低,在实验温度范围内,床温升高有利于提高Fe₂O₃到Fe₃O₄的还原速率和还原度。不同形态的铁氧化物对NO的催化还原特性不同,Fe₂O₃及其部分还原后生成的Fe₃O₄都不能直接与NO反应,Fe₂O₃对CO催化还原NO的效果很弱,而Fe₃O₄对CO还原NO的反应却有很强的催化作用,而进一步还原生成FeO与单质铁还可直接与NO反应。     

Influence of metal oxides on the catalytic behavior of Au/Al2O3 for the selective reduction of NOx by hydrocarbons

The reduction of NO by propene in the presence of excess oxygen over mechanical mixtures of Au/Al₂O₃ with a bulk oxide has been investigated. The oxides studied were: Co₃O₄, Mn₂O₃, Cr₂O₃, CuO, Fe₂O₃, NiO, CeO₂, SnO₂, ZnO and V₂O₅. Under lean C₃H₆-SCR conditions, these oxides (with the exception of SnO₂) convert selectively NO to NO₂. When mechanically mixed with Au/Al₂O₃, the Mn₂O₃ and Co₃O₄ oxides and, to a much greater extent, CeO₂ act synergistically with this catalyst greatly enhancing its SCR performance. It was found that their synergistic action is not straightforwardly related to their activity for NO oxidation to NO₂. The exhibited catalytic synergy may be due to the operation of either remote control or a bifunctional mechanism. In the later case, the key intermediate must be a short-lived compound and not the NO₂ molecule in gas-phase.     

A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers

The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO_x/Al₂O₃ and MO_x/TiO₂ supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H₂O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts, it is shown that the presence of MO_x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al₂O₃ or Pt/TiO₂, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO_x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H₂O, 20% H₂ and 6% CO₂, showed that certain composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.     

Formation of niobium double oxides by the glycothermal method

Thermal reaction of niobium alkoxide in organic media at 300°C yielded amorphous niobia, which maintained a surface area above 130 m²/g after calcination at 500°C. Niobium double oxides (LiNbO₃, Zr₆Nb₂O₁₇, CaNb₂O₆, CrNbO₄, FeNbO₄, ZnNb₂O₆ and R₃NbO₇ (R; rare earth)) were directly obtained by the reaction of niobium alkoxide with the corresponding metal alkoxide, acetate or acetylacetonate in 1,4-butanediol at 300°C. In some cases, amorphous product was obtained; however, double oxides (SrNb₂O₆, Co₄Nb₂O₉, etc.) crystallized from the products at low temperatures.     

Comparison of catalytic activity and selectivity of Pd/(OSC + Al2O3) and (Pd + OSC)/Al2O3 catalysts

The effect of the Pd addition method into the fresh Pd/(OSC + Al₂O₃) and (Pd + OSC)/Al₂O₃ catalysts (OSC material = Ce_xZr_1−xO₂ mixed oxides) was investigated in this study. The CO + NO and CO + NO + O₂ model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 °C lower for the fresh Pd/(OSC + Al₂O₃) catalyst than for the fresh (Pd + OSC)/Al₂O₃ catalyst during the CO + NO + O₂ reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than Pd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger Pd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al₂O₃) catalyst was higher than that of the (Pd + OSC)/Al₂O₃ catalyst.     

Influence of the pre-treatment on the structure and reactivity of Ir/γ-Al2O3 catalysts in the selective reduction of nitric oxide by propene

The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al₂O₃ under lean-burn conditions (1000 vpm NO, 2000 vpm C₃H₆, 500 vpm CO, 10 vol.% O₂) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O₂ was found to be necessary to activate Ir/Al₂O₃ while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir⁰ reduced surface sites (νCO at 2060 cm⁻¹) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO₂ particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.     

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La_0.5Sr_xCe_yFeO_z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO₃ and SrFeO_3−x and oxides -Fe₂O₃ and CeO₂ depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH₄+O₂ (“lean-NO_x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La³⁺ cation at A-site with divalent Sr⁺² and tetravalent Ce⁺⁴ cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La_0.5Sr_xCe_yFeO_z materials to reduce NO by CO or by CH₄ under “lean-NO_x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s⁻¹) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al₂O₃ catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La_0.5Sr_xCe_yFeO_z solids and their catalytic activity. O₂ TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O₂ TPD studies was observed for the La_0.5Sr_0.2Ce_0.3FeO_z and La_0.5Sr_0.5FeO_z solids.     

Supported base metal catalysts for the preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX)

Supported base metal catalysts were tested for the preferential oxidation of CO (CO PROX). The catalysts we investigated covered a wide range of transition metals (Co, Cr, Cu, Ni, Zn) supported on oxides with very different acidic, basic and redox properties (MgO, La₂O₃, SiO₂–Al₂O₃, CeO₂, Ce_0.63Zr_0.37O₂). The influence of the metal loading (Cu), the support properties (acidity, basicity, redox, surface area) and the reaction conditions (reaction temperature, feed composition) on the catalyst activity and selectivity was evaluated. The activity of ceria and ceria–zirconia supported copper catalysts was comparable to the performances of noble metal samples classically used for the PROX reaction. In addition, Cu–CeO₂ catalysts showed a practically constant and high selectivity towards CO oxidation in the temperature range of 50–150 °C. Due to the strong synergetic effect between copper and ceria, only a small amount of copper (0.3 wt.%) was necessary to get an active catalyst. The best catalytic performances were obtained for the samples containing 1–3 wt.% copper. The presence of small copper particles in close interaction with the ceria support was shown to be responsible for the enhanced activity. Except for the hydrogen oxidation, no parallel reactions (CO or CO₂ methanation reactions, coking, RWGS) could be detected over these catalysts. Classically, an increase of the oxygen excess led to an increased CO conversion with a simultaneous loss of selectivity towards CO₂. Finally, the presence of CO₂ in the feed negatively affected the catalytic activity. This effect was attributed to the adsorption of CO₂ on the copper sites, probably as CO.     

Regeneration of a vanadium pentoxide supported activated coke catalyst-sorbent used in simultaneous sulfur dioxide and nitric oxide removal from flue gas: Effect of ammonia

A vanadium pentoxide supported activated coke (V₂O₅/AC) catalyst-sorbent has been reported to be very active for simultaneous removal of SO₂ and NO under dry conditions at temperatures of 200 °C and below. Regeneration of the SO₂-captured catalyst-sorbent is a key step in operation of such a process, which influences the catalyst-sorbent's SO₂ and NO removal activities, lifetime, as well as recovery of sulfur. Due to limited information in this regard, this paper studies thermal regeneration of a V₂O₅/AC catalyst-sorbent with emphases on the effect of atmosphere. The optimum regeneration temperature is found to be 380 °C in an Ar stream and 300 °C in a 5% NH₃/Ar stream. Compared to the fresh V₂O₅/AC, the V₂O₅/ACs regenerated in Ar show lower SO₂ adsorption capacities and higher NO removal activities, while the regenerated V₂O₅/ACs by 5% NH₃/Ar show higher and stable SO₂ adsorption capacities and higher NO removal activities. Two types of reactions occur during the regeneration: reduction of the adsorbed sulfur species by carbon to SO₂ and CO₂, and oxidation of carbon by oxygen in the V₂O₅/AC to CO₂. The carbon consumption of the latter is much more than that of the former in an Ar atmosphere, but fully suppressed by the presence of 5% NH₃. Detailed analysis and characterization of the V₂O₅/AC subjected to the regenerations are presented.     

Physico-chemical characteristics and CO oxidation studies over Pd/(Mn2O3 + SnO2) catalyst

The mixed oxide catalyst (Mn₂O₃ + SnO₂) prepared by the coprecipitation method has been impregnated with Pd metal and it's catalytic behaviour for CO oxidation reaction has been investigated. In the coprecipitated material, Mn₂O₃ and SnO₂ were found to crystallise at 875 K and 1175 K, respectively, which are significantly higher than the crystallisation temperatures of individual oxides prepared under similar conditions. Results of catalytic oxidation of CO, carried out using the pulse method for the mixed oxide system and the individual oxides, suggest significant synergistic effects between these two oxides. The impregnation of palladium metal facilitated CO oxidation and the catalyst Pd/(Mn₂O₃ + SnO₂) was found to be quite effective for CO oxidation even at room temperature. Further, the CO disproportionation has been observed on palladium sites in the temperature range 350 to 400 K for the individual oxide systems.     

Low-temperature H2-SCR of NO on a novel Pt/MgO-CeO2 catalyst

The selective catalytic reduction of NO by H₂ under strongly oxidizing conditions (H₂-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La₂O₃, MgO, Y₂O₃, CaO, CeO₂, TiO₂, SiO₂ and MgO-CeO₂). The Pt/MgO and Pt/CeO₂ solids showed the best catalytic behavior with respect to N₂ yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO₂ support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N₂ production (mols N₂/g_cat s). High NO conversions (70–95%) and N₂ selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h⁻¹ with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H₂, 5 vol% O₂ and He as balance gas. Addition of 5 vol% H₂O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO₂ during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO₂ in the feed stream following a given support pretreatment. N₂ selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO₂ catalyst in the 100–200 °C range in the presence of water and SO₂ in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO₂ catalyst towards a new NO_x control technology in the low-temperature range of 100–200 °C using H₂ as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO_x species with H₂ experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO_x intermediate species, NO_x spillover).     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

Interactions NO—CO and O2—NO—CO on CuCo2O4/γ-Al2O3 and on γ-Al2O3- and CuCo2O4/γ-Al2O3-supported Pt, Rh and Pt—Rh catalysts, a transient response study

The interactions NO—CO and O₂—NO—CO have been studied onCuCo₂O₄γ-Al₂O₃ and on γ-Al₂O₃- and CuCo₂O₄γ-Al₂O₃-supported Pt, Rh and Pt—Rh catalysts. The deposition of noble metals (Pt, Rh and Pt—Rh) on CuCo₂O₄γ-Al₂O₃ instead of γ-Al₂O₃ is beneficial in: lowering the temperature at which maximum N₂O is formed and decreasing the maximum N₂O concentration attained; lowering the onset temperature of NO to N₂ reduction, and increasing the N₂ selectivity; preserving the activity towards NO to N₂ reduction on a higher level following the concentration step NO + COO₂+ NO + CO and changing the conditions from stoichiometric to oxidizing (50% excess of oxidants). The reason for this behaviour of the CuCo₂O₄γ-Al₂O₃-based noble metal catalysts is the formation (reversible) of a reduced surface layer on the CuCo₂O₄ supported spinel under the conditions of a stoichiometric NO + CO mixture.     

Structural and chemical promoter effects of alkali (earth) and cerium oxides in CO oxidation on supported gold

Nanostructured Au/Al₂O₃ catalysts prepared by deposition–precipitation with urea were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) and tested for low temperature CO oxidation. The paper is focused on the effect of two different kinds of additives on the structure and reactivity of Au/Al₂O₃ for CO oxidation: (1) alkali (earth) metal oxides and (2) ceria. The structural properties of nanosized Au particles are drastically affected by the presence of additives. The main role of the alkali (earth) metal oxides is to stabilize the small Au particles against sintering, such that when BaO is added to Au/Al₂O₃, full CO conversion is already achieved at room temperature. For this reaction ceria addition does not result in a higher activity. However, when both BaO and CeO_x are added to Au/Al₂O₃, the catalytic performance of the multicomponent catalyst resembles that of Au/BaO/Al₂O₃.     

Direct decomposition of nitric oxide over Ba catalysts supported on CeO2-based mixed oxides

The direct decomposition of nitric oxide (NO) over barium catalysts supported on various metal oxides was examined in the absence and presence of O₂. Among the Ba catalysts supported on single-component metal oxides, Ba/Co₃O₄ and Ba/CeO₂ showed high NO decomposition activities, while Ba/Al₂O₃, Ba/SiO₂, and Ba/TiO₂ exhibited quite low activities. The effect of an addition of second components to Co and Ce oxides was further examined, and it was found that the activities were significantly enhanced using Ce–Mn mixed oxides as support materials. XRD results indicated the formation of CeO₂–MnO_x solid solutions with the cubic fluorite structure. O₂-TPD of the CeO₂–MnO_x solid solutions showed a large desorption peak in a range of relatively low temperature. The BET surface areas of the CeO₂–MnO_x solid solutions were larger than those of pure CeO₂ and Mn₂O₃. These effects caused by the addition of Mn are responsible for the enhanced activities of the Ba catalysts supported on Ce–Mn mixed oxides.