Methane as Alternative in the Selective Reduction of NO over Supported Palladium Catalysts in Lean Conditions: Role of Redox Properties of Support Materials

The reduction of CH₄ by NO has been investigated in the presence of oxygen on palladium supported on alumina, ceria–zirconia mixed oxides and perovskite materials, mainly LaCoO₃. The activation procedure, under oxygen or hydrogen, drastically influences the catalytic performances of both catalysts. The stabilisation of a metallic or oxidic Pd phase leads to poor activity in the conversion of NO in the absence of oxygen. On the other hand, oxygen enhances the activity, particularly on the reduced Pd/LaCoO₃, in the CH₄ + NO reaction. Such results have been explained by different interactions between palladium and the support.     

Pd-doped LaCoO3 regenerative catalyst for automotive emissions control

The effect of partial substitution of Co by Pd in LaCoO₃ perovskite structure (i.e., LaCo_0.95Pd_0.05O₃) and the reductive diffusion of Pd from the bulk of perovskite to its surface, thus forming Pd nanoparticles, on CO and C₃H₈ oxidation present in air (simulated exhaust gas) are reported. X-ray powder diffraction (XRD) analyses confirm the perovskite structure for the catalysts. Scanning electron microscopy (SEM) and BET surface area measurements show that partial substitution of Co by Pd decreases the crystallite size of the perovskite and therefore increases its surface area. H₂-temperature programmed reduction (TPR) experiments reveal that Pd reduces at 135 °C and facilitates the reduction of Co in the perovskite structure. By partial reduction of the Pd containing catalyst at 180 °C for 30 min, the complete oxidation temperatures of CO and C₃H₈ decrease by about 70 and 50 °C, respectively.The reduction duration of the Pd containing catalyst strongly affects the T₅₀ and T₉₀ temperatures (temperatures at which 50 and 90% conversion occurs, respectively) and has an optimum, where it decreases by increasing the reduction temperature of the catalyst.     

Catalytic Post-Treatment of Automotive Exhaust Gas from Natural Gas Combustion Engines: Potential Interest of Perovskite Materials

This study reports an investigation of the surface properties of Pd-modified perovskite catalysts for NO _x removal from Natural Gas engines. H₂ production from reforming and water-gas-shift reactions can be profitably used for the reduction of NO _x . Particular attention has been paid to the nature of interactions between Pd and LaCoO₃ according to in situ reductive thermal treatments. The catalytic properties have been investigated by temperature-programmed NO desorption and temperature-programmed NO/H₂ reaction. Different experiments performed on partially and extensively reduced catalysts lead to changes in surface reactivity. Interestingly, beneficial interactions with a significant selectivity enhancement to the production of nitrogen is observed on pre-reduced Pd/LaCoO₃ under smooth conditions at 250 °C particularly in the presence of oxygen. More extensive reduction at 450 °C leads to the loss of the structural properties of LaCoO₃ accompanied with partial segregation of CoO _x and La₂O₃. Such structural changes lead to a detrimental effect on the catalytic performances.     

Synergistic effect of Pt or Pd and perovskite oxide for water gas shift reaction

The water gas shift (WGS) reaction over Pt and Pd catalysts supported on various perovskite oxides has been investigated at 573 K without catalyst pretreatment. The Pt and Pd catalysts on LaCoO₃ support showed high catalytic activity. Interaction between Pt or Pd and the support is considered to promote the WGS reaction: Pt/LaCoO₃ had high initial activity but deactivated immediately; Pd/LaCoO₃ was less active than Pt/LaCoO₃, but had superior stability. Catalysts were characterized using XRD, STEM, XPS, and H₂-temperature programmed reduction (TPR). Results of this study showed that reduction of the support decreased the CO conversion on Pt/LaCoO₃. On the other hand, Pd/LaCoO₃ showed stable activity for the WGS reaction. Therefore, Pd was added to Pt/LaCoO₃ for stabilizing the catalyst activity, and 0.5 wt.% Pd/1 wt.% Pt/LaCoO₃ catalyst showed higher activity and stability.     

An in situ study of the NO + H2 + O2 reaction on Pd/LaCoO3 based catalysts

In situ X-ray diffraction (XRD) and quasi in situ X-ray photoelectron spectroscopy (XPS) measurements were complementary used to investigate structural and surface modifications of a palladium-supported on LaCoO₃ perovskite catalyst under various controlled atmospheres, particularly during the reduction of NO by hydrogen under lean conditions, in the presence of a large excess of oxygen.

An extensive reduction of the perovskite was evidenced during the pre-activation thermal treatment of the palladium-supported catalyst under hydrogen at 773 K leading to the formation of Pd particles in contact with Co⁰ and La₂O₃. In the presence of an excess of oxygen, the catalyst structure changes during the reaction. The reduced solid is progressively transformed into LaCoO₃ in the range of 873–1173 K. However, such a bulk transformation probably occurs at lower temperatures at the surface of the solid according to XPS analyses. At the same time, the binding energy (BE) level of the Pd 3d_5/2 photopeak increases up to 337.5 eV which reveals the stabilisation of oxidic palladium species in a different chemical environment than that corresponding to PdO. Such changes induced different catalytic properties of the catalyst during the reduction of NO by H₂.     

Zirconia-supported LaCoO3 catalysts for hydrogen production by oxidative reforming of diesel: Optimization of preparation conditions

The influence of calcination temperature and precursor type used in the preparation of ZrO₂-supported LaCoO₃ catalyst on its behaviour for hydrogen production by oxidative reforming of diesel has been analyzed in terms of LaCoO₃ structure. Four samples have been prepared by wetness co-impregnation with cobalt and lanthanum salts and characterized by means of XRD, BET, SEM-EDX, TXRF and XPS. Physicochemical characterization shows a great influence of the nature of precursors and calcination temperature used in the synthesis on the textural, morphological, surface and structural properties of LaCoO₃ deposited over ZrO₂. The use of nitrate precursors and high calcination temperature leads to the formation of LaCoO₃ perovskite structures of high grain and crystallite size on ZrO₂ support. On the contrary, the catalyst prepared from acetate precursors and calcined at low temperature showed perovskite crystallites of lower size. For this sample, the smaller perovskite crystallites on the catalyst at the beginning of the reaction imply higher and more stable hydrogen production for short-term test aging test.     

Vibrationally Excited CO2 Formed in CO + NO Reaction on Pd(110) Surface in High Surface Temperature Range

The infrared chemiluminescence spectra of CO₂ formed during steady-state CO+NO reaction over Pd(110) indicated that the temperature of the bending vibrational mode was much higher than that of the antisymmetric one at higher surface temperatures such as 800–850 K. Especially, in the high temperature range, more vibrationally excited CO₂ was formed from CO+NO reaction than CO+O₂ reaction. On the basis of the result, we propose the model structure of reaction intermediates for CO₂ formation in CO+NO reaction, which is different from that in CO+O₂ reaction.     

Elucidation of CO2 Formation Mechanism in CO + NO Reaction on Pd(111) and Pd(110) Surfaces Using IR Chemiluminescence Method

The infrared (IR) chemiluminescence technique was applied to steady-state CO oxidation by NO on Pd(111) and Pd(110). From a comparison of IR emission spectra of CO₂ between the CO + NO and CO + O₂ reactions, it was found that the vibrational energy states of CO₂ in the CO + NO reaction were similar to those in the CO + O₂ reaction. This indicates that the reaction path of CO₂ formation in CO + NO is the same as that in CO + O₂, although the vibrational states are very dependent on the surface structure.     

Flame-synthesized LaCoO3-supported Pd: 2. Catalytic behavior in the reduction of NO by H2 under lean conditions

A 0.5 wt% Pd/LaCoO₃, prepared by flame-spray pyrolysis (FP), was tested as catalyst for the low-temperature selective reduction of NO by H₂ in the presence of excess O₂. In particular, the effect of the precalcination and prereduction temperature on catalytic activity was compared with that of a similar Pd/LaCoO₃ sample prepared by impregnation with a Pd solution of FP-prepared LaCoO₃. The FP-made catalyst allowed full NO conversion at 150 °C, with 78% selectivity to N₂, thus outperforming the catalytic behavior of the corresponding sample prepared by impregnation. The higher activity of the FP-made catalyst has been attributed to the formation of segregated Co metal particles, not present in the impregnated sample, formed during the precalcination at 800 °C, followed by reduction at 300 °C. Two reaction mechanisms can be deduced from the temperature-programmed experiments. The first of these, occurring at lower temperatures, indicates cooperation between the Pd and Co metal particles, with formation of active nitrates on cobalt, successively reduced by hydrogen spillover from Pd. The second, occurring at higher temperature, allows 50% conversion of NO, with >90% selectivity to N₂, and involves N adatoms formed by dissociative NO adsorption over Pd. Prereduction at 600 °C led to a slight increase in catalytic activity, due to the formation of a PdCo alloy, which is more stable on reoxidization compared with Pd alone. Moreover, the cooperative reaction mechanism seems to be favored by the proximity of Co and Pd in metal particles.     

Understanding the Catalytic Conversion of Automobile Exhaust Emissions Using Model Catalysts: CO + NO Reaction on Pd(111)

The CO + NO reaction is one of the profoundly important reactions that take place on Pd-based industrial three-way catalysts (TWC). In this review, we discuss results from polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and conventional IRAS experiments on CO adsorption, NO adsorption and the CO + NO reaction on a Pd(111) model catalyst surface within a wide range of pressures (10^?6–450 Torr) and temperatures (80–650 K). It will be shown that these studies allow for a detailed understanding of the adsorption behavior of these species as well as the nature of the products that are formed during their reaction under realistic catalytic conditions. CO adsorption experiments on Pd(111) at elevated pressures reveal that CO overlayers exhibit similar adsorption structures as found for ultrahigh vacuum (UHV) conditions. On the other hand, in the case of the CO + NO reaction on Pd(111), the pressure dependent formation of isocyanate containing species' was observed. The importance of this observation and its effects on the improvement of the catalytic NO _x abatement is discussed. The kinetics of the CO + NO reaction on Pd(111) were also investigated and the factors affecting its selectivity are addressed.     

LaCo1−x Pd x O3 Perovskite-Type Oxides: Synthesis, Characterization and Simultaneous Removal of NO x and Diesel Soot

A series of naometric perovskite catalysts LaCo_1?x Pd _x O₃ (x = 0, 0.01, 0.03) were prepared via a solution combustion synthesis route using metal nitrates as oxidizers and urea as fuel. It is essential to add a certain amount of ammonia aqueous solution to Pd²⁺ ions solution in the catalyst preparation process. Homogeneous nanoparticles LaCo_1?x Pd _x O₃ catalysts with the sizes in the range of 68–122 nm were obtained and characterized by using of XRD, BET, H₂-TPR, XPS, SEM and TEM. Pd was successfully introduced into the LaCoO₃ perovskite lattices. Further information was obtained by using XPS upon the LaCo_0.97Pd_0.03O₃ (with NH₄OH) sample after H₂-TPR. The results revealed that surface Pd was reduced to the metallic state at the end of the first step in the H₂-TPR experiment, and some surface Co could be reduced to metallic Co simultaneously. The catalytic properties were investigated for simultaneous NO _x -soot removal reaction. The performance of LaCo_1?x Pd _x O₃ catalysts were greatly improved by the partial substitution of Pd. The maximum NO conversion into N₂ and the ignition temperature of soot are 32.8% and 265 °C, respectively.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Perovskite-Based Lean-Burn NOx Trap Catalysts Without Using Platinum Group Metals: K/LaCoO3/Ce1?xZrxO2

Abstract  

A series of Ce_1−x Zr _x O₂ (x = 0, 0.1, 0.2, 0.3) solid solution supported lean-burn NO _x trap (LNT) catalysts K/LaCoO₃/Ce_1−x Zr _x O₂ were prepared by successive impregnation. After sulfation the supported perovsikte LaCoO₃ was well maintained; reducing treatment partly destroyed the perovsikte, but it can be well recovered by re-oxidation treatment. Based on NO _x storage and sulfur-resisting performance of the catalysts, the optimal atomic ratio of Zr in Ce_1−x Zr _x O₂ support is x = 0.2. The catalyst K/LaCoO₃/Ce_0.8Zr_0.2O₂ exhibits much better NO _x storage capacity than the Pt-based catalyst Pt/K/Ce_0.8Zr_0.2O₂, which is highly related to its stronger capability for NO to NO₂ oxidation. During NO _x storage much larger amounts of nitrate and nitrite species were identified by in situ DRIFTS over perovskite-based catalysts than over Pt-based one. The H₂-TPR results reveal that after deep sulfation little sulfur species were deposited on the catalyst K/LaCoO₃/Ce_1−x Zr _x O₂, showing strong sulfur-resisting ability. As a result, it is thought that the full replacement of Pt by perovskite LaCoO₃ in the corresponding LNT catalysts is feasible.     

Preparation and characterisation of the perovskite oxide superconductor YBa2Cu3O7 − x

A perovskite oxide, YBa₂Cu₃O_{7 ? x} (YBCO), has been prepared by the method of carbonate precipitation from a mixed nitrate solution and oxidised at 950°C in an air atmosphere to obtain a perovskite oxide to be used as a catalyst for H₂/CO reactions. The characterisation of the perovskite oxide was performed by using X-ray diffraction before and after the treatment with different gases, A.C. susceptibility measurements, particle size measurements and temperature programmed reduction (TPR) in which hydrogen was used in order to monitor the changes of the perovskite with temperature in a hydrogen atmosphere. A.C. susceptibility measurements showed that the YBCO perovskite exhibited the typical good superconducting sharp transition temperature at 93 K, indicating that the wet chemical preparation route is suited to the production of this material.     

Mechanism of the ammonia formation from NO-H2. A model study with Pt-Rh alloy single crystal surfaces

The reduction of NO by H₂ was studied over three different Pt-Rh single crystal surfaces, i.e. Pt-Rh(111), (410) and (100). The adsorption and dissociation of NO was studied by HREELS, LEED, XPS, AES and TDS. It was found that the dissociation of NO and its reaction with H₂ is very surface structure sensitive. The selectivity towards nitrogen and the dissociation activity increases in the same order, i.e. Pt-Rh(111) < (100) < 410). Nitrogen atoms were easily hydrogenated at 400 K in hydrogen to NH _x (x = 1 or 2) on the surface. A model is proposed in which the selectivity of the NO-H₂ reaction over Pt-Rh surfaces is determined by the relative amounts of hydrogen, NO and nitrogen adatoms on the surface.     

Designing a structured catalyst for selective reduction of O<Subscript>2</Subscript> by hydrogen in the presence of NO

The influence of the promoter (Pd) modifying additives of oxides of rare-earth (La₂O₃, CeO₂) and transition (NiO, CuO) metal oxides on the catalytic activity of Co₃O₄/cordierite in reactions of O₂ and NO reduction by hydrogen was studied. Introducing Pd and rare-earth metal oxides into the composition of cobalt oxide catalyst results in an increase in its activity in H₂ + 1/2O₂ → H₂O, H₂ + NO → 1/2N₂ + H₂O reactions and an increase in selectivity upon oxygen reduction by hydrogen in the presence of nitric oxide, due possibly to a decrease in the strength of oxygen bounds with the surface and the formation of low-temperature forms of oxygen, which is not typical of unpromoted cobalt oxide catalyst. A structured Pd-Co₃O₄-La₂O₃/cordierite catalyst was developed that surpasses the commercial granulated silver-manganese catalyst used in industry to purify the technological gases used in the production of hydroxylamine sulfate of oxygen impurities with reference to activity and selectivity (in the process of oxygen reduction in the presence of nitric oxide), and to thermal stability.     

Etching Induced Stepped Nanostructure on Pb(Mg(1–x/2)Mn(x/2)W1/2)O3 Ceramics

A new solid solution system, Pb(Mg_(1–x/2)Mn_(x/2)W_1/2)O₃, is formed by replacing Mg²⁺ with Mn²⁺ in the Pb(Mg_1/2W_1/2)O₃ complex perovskite. The solid solution remains in the orthorhombic Pmcn symmetry up to x = 0.1. It is observed that for such a chemical modification, the surface of the grain changes significantly. A stepped surface nanostructure appears in the Mn‐substituted perovskite. Detailed surface morphology of the stepped patterns was revealed by atomic force microscopy (AFM), which determined that the interlayer step height varies from 2.3 to 8.3 nm. Analyse by XRD, SEM, EDX, and AFM suggest that the nature of the stepped surface structure may be the result of etching rather than spiral growth or two‐dimensional nucleation as it was previously believed.     

Surface modification of LiMn2O4 cathode with LaCoO3 by a molten salt method for lithium ion batteries

Spinel LiMn₂O₄ is a promising cathode due to its advantages of low-cost, nontoxicity and thermal stability. However, the dissolution of manganese and the phase transformation induce the rapid capacity fade. Surface coating is an effective method to improve its electrochemical performance. In this work, spinel LiMn₂O₄ modified with perovskite LaCoO₃ was prepared using a novel molten salt method. The resulted samples were characterized by X-ray diffraction (XRD), transmission/scanning electron microscopy (TEM/SEM), Fourier transformation infrared (FT-IR), Raman, and X-ray photoelectronic spectroscopy. The content of Mn³⁺ increased with the LaCoO₃ coating accompanied by the increased concentration of oxygen vacancy. LiMn₂O₄ modified with 2% LaCoO₃ shows a higher capacity and cycling stability than others at 0.2 C, while the cathode with 4% LaCoO₃ shows the best rate performance at a larger current at 2 and 5 C. This enhanced performance can be attributed to improved interfacial conductivity between the cathode and electrolyte and the protective effects of coating.     

Co oxidation on LaCoO3 perovskite

A study of CO oxidation on LaCoO₃ perovskite was performed in an ultrahigh vacuum system by means of adsorption and desorption. All gases were adsorbed at ambient temperature. Two adsorption states (α- and β-) of CO exist. The α-peak at 440 K is attributed to carbonyl species adsorbed on Co³⁺ ions while the β-peak at 663 K likely comes from bidentate carbonate formed by adsorption on lattice oxygens. CO₂ shows a single desorption peak (β-state, 483 K) whose chemical state may be monodentate carbonate. A new CO₂ desorption peak at 590 K can be created by oxidation of CO. O₂ also shows two adsorption states. One desorbs at 600 K, which may reflect adsorption on Co^3- ions. The other apparently incorporates with bulk LaCoO₃ and desorbs above 1000 K. The two adsorption states of CO are oxidized via different mechanisms. The rate determining step in oxidation of a-CO is the surface reaction whereas for that of β-CO, it is desorption of product CO₂.     

Detection of Reduced W n+ Sites on WO3–ZrO2 and Pt/WO3–ZrO2 Catalysts by Infrared Spectroscopy of Adsorbed NO

Adsorption of NO on oxidized WO₃–ZrO₂ catalysts leads to formation of adsorbed N₂O, surface nitrates, NO⁺, and Zr⁴⁺(NO^- ₃)–NO species. When NO is adsorbed on a sample reduced at 523 K, W⁵⁺ –NO (1855 cm^-1) and W⁴⁺(NO)₂ (1785 and 1700 cm^-1) species are formed in low concentration. Reduction at 573 K increases the density of W⁴⁺ sites and this density remains unchanged after reduction up to 673 K. The W⁴⁺(NO)₂ species are stable towards evacuation and in the presence of oxygen; however, they quickly disappear in the simultaneous presence of NO and O₂ as a result of the oxidation of the W⁴⁺ cations to W⁶⁺. The density of the W⁴⁺ sites on a Pt/WO₃–ZrO₂ sample is significant even after reduction at 523 K. This is explained by the promotion effect of platinum on the support reduction. The use of NO as a probe molecule for detection of reduced W ⁿ⁺ sites on tungstated zirconia is discussed.