Highly Active La1−x K x CoO3 Perovskite-type Complex Oxide Catalysts for the Simultaneous Removal of Diesel Soot and Nitrogen Oxides Under Loose Contact Conditions

The nanometric La_1?x K _x CoO₃ (x = 0–0.30) perovskite-type oxides were prepared by a citric acid-ligated method. The catalysts were characterized by means of XRD, IR, BET, XPS and SEM. The catalytic activity for the simultaneous removal of soot and nitrogen oxides was evaluated by a technique of the temperature-programmed oxidation reaction. In the LaCoO₃ catalyst, the partial substitution of La³⁺ at A-site by alkali metal K⁺ enhanced the catalytic activity for the oxidation of soot particle and reduction of NO _x . The La_0.70K_0.30CoO₃ oxides are good candidate catalysts for the simultaneous removal of soot particle and NO _x . The combustion temperatures for soot particles over the La_0.70K_0.30CoO₃ catalyst are in the range from 289 to 461 °C, the selectivity of CO₂ is 98.4% and the conversion of NO to N₂ is 34.6% under loose contact conditions. The possible reasons that can lead to the activity enhancement for the K-substitution samples compared to the unsubstituted sample (LaCoO₃) were given. The particle size has a large effect on its catalytic performance for the simultaneous removal of diesel soot and nitrogen oxides.     

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.     

Structural regeneration of LaCoO3 perovskite-based catalysts during the NO + H2 + O2 reactions

In situ X-ray diffraction (XRD) analysis was used to investigate structural evolutions of LaCoO₃ catalysts and then further modified by palladium (Pd) addition, under various controlled atmospheres, particularly during the reduction of NO by hydrogen in lean conditions. Complementary, XPS measurements provided information about changes in the chemical environments of Pd, Co and nitrogen during sequential temperature-programmed reactions. A preactivation thermal treatment under hydrogen led to the destruction of the perovskite structure while in the course of the NO + H₂ + O₂ reactions, the regeneration of the perovskite structure evidenced by XRD at 873 K started at lower temperature (573 K) at the surface. Palladium has been incorporated in order to evidence its effective role in the surface modifications of LaCoO₃ and its consequence on the catalytic activity.     

Diesel Soot Catalyzes the Selective Catalytic Reduction of NOx with NH3

The catalytic activity of soot samples for the selective catalytic reduction (SCR) of NO_x with NH₃ was investigated in dependence of the NO₂, NO and NH₃ concentration in the temperature range between 200 and 350 °C. The highest NO_x reduction of up to 25 % was measured in the presence of both NO₂ and NO at a GHSV of 35,000 h^?1. Decreasing space velocities resulted in an increase of the SCR activity. In the absence of NO₂, NO_x reduction was not observed. Carbon oxidation and SCR reaction occurred in parallel due to the presence of NO₂ and O₂, but hardly influenced each other, which suggested that in the NO_x reduction on soot most probably physisorbed species were involved. The observed stoichiometries indicated the action of the fast SCR reaction in the presence of NO and the NO ₂ SCR reaction in the absence of NO, while the observed gas phase and surface species pointed at reaction steps similar to those on classical SCR catalysts.     

The Influence of O2, Hydrocarbons, CO, H2, NO x , SO2, and Water Vapor Molecules on Soot Combustion over LaCoO3 Perovskite

The effect of various gases (O₂, hydrocarbons, CO, H₂, NO _x , SO₂, and H₂O vapor) presenting in the diesel exhaust on soot combustion using LaCoO₃ as a catalytic material was investigated in this paper. A significant promotion of the combustion rate was found following a trend of 10% H₂O addition > 3,000 ppm NO _x  > 1% H₂ or 3,000 ppm C₃H₆ addition, while the improvement in soot oxidation due to the introduction of 3,000 ppm CO or 3,000 ppm CH₄ into the reactant gas is relatively less. The wet pretreatment of LaCoO₃ with 10% steam before soot oxidation hardly affects the combustion behavior. Interestingly, 10% water vapor in the reaction feed produced a significant promoting effect on combustion. In contrast, 30 ppm SO₂ treating led to an obvious deactivation likely owing to the coverage of active sites by sulfate compounds.     

Simultaneous catalytic removal of NOx and soot particulate over Co–Al mixed oxide catalysts derived from hydrotalcites

Co–Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NO_x and diesel soot particulates by the temperature-programmed reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500 °C and 800 °C, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800 °C were higher than that at 500 °C. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.     

Hydrothermally Stable WO3/ZrO2–Ce0.6Zr0.4O2 Catalyst for the Selective Catalytic Reduction of NO with NH3

A new catalyst WO₃/ZrO₂–Ce_0.6Zr_0.4O₂ (15 wt % WO₃/ZrO₂:Ce_0.6Zr_0.4O₂ = 50:50) has been developed for the selective catalytic reduction of NO with NH₃. The redox component Ce_0.6Zr_0.4O₂ was dispersed on the surface of acidic WO₃/ZrO₂ by the solution combustion method showing the best NO _x reduction efficiency among the catalysts prepared by various modes of mixing of the components. The catalyst has been characterized by XRD, Raman spectroscopy and NH₃-TPD. A NO _x reduction efficiency of more than 90 % was obtained between 300 and 500 °C at α = NH_3,in/NO _x,in = 1. The catalyst showed stable NO _x reduction efficiency after hydrothermal ageing at 700 °C. Sulfur poisoning promoted the NO _x reduction efficiency at high temperatures at the expense of a reduced activity at lower temperatures, but the catalyst could be fully regenerated by heating in O₂ at 650 °C.     

Novel Y2O3 Doped MnO x Binary Metal Oxides for NO x Storage at Low Temperature in Lean Burn Condition

MnO _x -Y₂O₃ binary metal oxide catalysts are synthesized by a constant-pH co-precipitation method, their ability of NO _x storage capacity and absorbing process were investigated. The pure MnO _x and Y₂O₃ calcined at 500 °C for 4 h in static air are both of body-centre structure, while the binary metal oxides containing Mn and Y are mainly of amorphous phase. The adulteration of Y₂O₃ can remarkably improve the specific surface areas of the catalysts, which probably result of the enhancement on NO storage capacity and catalytic oxidation ability of NO at 100 °C. The XPS results indicate that both Mn and Y have 3+ chemical states in the binary oxides. FT-IR spectra could be beneficial to explain the NO storage process on the binary metal oxide: NO can be adsorbed on the MnO _x and Y₂O₃ sites as nitrates and nitrites, respectively, and then the nitrites on Y₂O₃ site are shifted to Mn₂O₃ site and then is oxidized to nitrates. As a result, the NO storage capacity is enhanced due to the adulteration of Y₂O₃, finally the NO _x are adsorbed on the Mn₂O₃ site as nitrate species.     

NH3 and urea in the selective catalytic reduction of NOx over oxide-supported copper catalysts

The temperature-programmed activity of a series of oxide-supported (TiO₂, Al₂O₃ and SiO₂) Cu catalysts formed from two different Cu precursors (Cu(NO₃)₂ and CuSO₄) for the selective catalytic reduction of NO_x using solutions of urea as a reductant have been determined. These activities are compared to those found using NH₃ as a reducing agent over the same catalysts in the presence of H₂O and it is found that catalysts that are active for the selective reduction of NO_x with NH₃ are inactive for its reduction using solutions of urea. Poisoning of the surface by H₂O_ads is not responsible for all of this decrease in activity and it is postulated that the urea is not hydrolysing to form NH₃ over the catalysts but rather is oxidising to form N₂ or forming passivated layers of polymeric melamine complexes on the surface. The catalysts were characterised by temperature-programmed reduction while temperature-programmed desorption and oxidation of NH₃ and temperature programmed decomposition of urea are used to characterise the interaction of both reductants with the various catalysts.     

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.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

Two Reaction Pathways in the Selective Catalytic Reduction of NO x by C6H14 Over Ag–Al2O3 with H2 Co-Feeding

The NO _x reduction with n-C₆H₁₄ was studied over a 3% Ag/Al₂O₃ catalyst in the presence of hydrogen. The catalyst performance was evaluated by varying the H₂ concentration from 0 to 1600 ppm and by comparing the results with blank runs in which an empty reactor with no catalyst was used. Two distinct reaction pathways were revealed: one at low-temperature (T_react < 370 °C) and another one at high-temperature (T_react > 370–390 °C). Co-feeding of H₂ promotes the reaction within the 150–360 °C interval. The high-temperature pathway (T_react > 370–390 °C) seems to be almost independent of hydrogen co-feeding. The homogeneous gas-phase NO _x oxidation initiated by NO presumably plays an important role in this high-temperature pathway.     

NO–H2–CO–O2 Reactions Over Pt Catalysts Supported on Ln-incorporated FSM-16 (Ln = La, Ce and Pr)

Catalytic NO–H₂–CO–O₂ reaction was studied over Pt-supported Ln-incorporated FSM-16 (Ln = La, Ce and Pr). Pr-FSM-16 exhibited the highest activity for NO _x reduction at ≤200 °C. Pr has an effect of increasing the basicity to promote the oxidative adsorption of NO, which is a key for efficient de-NO _x .     

Lean NOx Reduction with H2 and CO in Dual-Layer LNT–SCR Monolithic Catalysts: Impact of Ceria Loading

A series of monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of lean NO_x trap (LNT) catalyst were synthesized for lean reduction of NO_x (NO&NO₂) with H₂ and CO. The LNT catalyst exhibited a rather low NO_x conversion below 250 °C due to CO inhibition. The top SCR layer comprising Cu/ZSM5 significantly increased the NO_x conversion at low temperature by its reaction with NH₃ formed during the regeneration phase. The addition of CeO₂ to the LNT layer promoted the water gas shift reaction (CO + H₂O ? H₂ + CO₂). The WGS reaction mitigated the CO inhibition and the generated H₂ enhanced the low-temperature catalyst regeneration. The ceria addition decreased the performance at high temperatures due to increased oxidation of NH₃. The ceria loading was optimized by applying a non-uniform axial profile. A dual-layer catalyst with an increasing ceria loading axial profile improved the performance over a wide (low and high) temperature range.     

Removal NO x from Lean Exhaust Gas by Absorption on Oxi-Anionic Materials

NO _x sorption/desorption capacities of Oxi-Anionic materials were measured under representative exhaust gas mixture conditions at 300 °C, with a NO _x trap containing CaO and CuO _x impregnated over a commercial γ-Al₂O₃. These systems absorb (or adsorb) NO _x in a gas stream containing both CO₂ at 300 °C. In the case of a thermal desorption, the trapped NO _x should desorb at a temperature below 550 °C.     

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NO _x with DME occurs with high selectivity to N₂ over Al₂O₃ between 300 °C and 550 °C with a maximum of 47% at 380 °C, and with lower selectivity over Ag/Al₂O₃ between 250 °C and 400 °C due to the catalysts’ sensitivity to gas phase radical reactions and activity for NO _x reduction with methanol.     

NO x Storage at Low Temperature over MnO x –SnO2 Binary Metal Oxide Prepared Through Different Hydrothermal Process

Three MnO _x –SnO₂ catalysts were successfully prepared by hetero-redox, homo-redox and common hydrothermal methods. NO adsorption and desorption were performed to investigate the NO _x storage capacity at 100 °C. All the samples showed good performances on NO _x storage, especially the sample prepared by homo-redox hydrothermal method. The XRD, BET, TPR, XPS measurements were used to characterize the structure of the catalysts. The results revealed that the oxidation state of Mn and the defect oxygen species could be responsible for the high NO _x storage capacity.     

H2-Induced NO x Adsorption/Desorption over Ag/Al2O3: Transient Experiments and TPD Study

NO adsorption/desorption over 1 wt% Ag/Al₂O₃ was studied by a combination of isothermal transient adsorption/desorption and NO _x temperature-programmed desorption (NO _x -TPD) methods. NO _x -TPD profiles obtained for Ag/Al₂O₃ were identified by comparison with decomposition profiles of “model” AgNO₃/Al₂O₃ and Al(NO₃)₃/Al₂O₃ prepared by impregnation of Al₂O₃ with individual AgNO₃ and Al(NO₃)₃ compounds. The data obtained indicate that H₂-induced NO adsorption leads to the formation of surface Ag and Al-nitrates. Their accumulation on the catalyst surface is accompanied by an intensive NO₂ evolution, which proceeds primarily via reaction of surface nitrates with NO. Thus, NO₂ formation appears to result from an intrinsic stage of the H₂-induced NO _x adsorption process, rather than from the direct oxidation of NO by gaseous oxygen catalyzed by Ag.     

The Effect of Copper Substitution on La2Ni1−x Cu x O4 Catalysts Activity for Simultaneous Removal of NOx and Diesel Soot

A series of the La₂Ni_1?x Cu _x O₄ (0 ≤ x ≤ 1.0) perovskite-like complex oxide catalysts, prepared and characterized by XRD, H₂-TPR, O₂-TPD and XPS, and catalytic activity tests, proved to be effective in the simultaneous removal of NOx and soot. The results indicated that the catalysts show the single orthorhombic K₂NiF₄-like phase. The doping of Cu led to the increase of orthorhombic distortion and the decrease of capability to accommodate non-stoichiometric oxygen, as well as the increase of the reducibility of lattice oxygen, resulting in improving catalytic activities. The La₂Ni_0.4Cu_0.6O₄ catalyst showed the highest activity. The maximum conversion of nitrogen oxide to molecular nitrogen was 15.6% and the ignition temperature decreased from 440 to 246 °C, as compared with the uncatalyzed soot combustion reaction.     

Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NO x Storage–Reduction Performance

The effect of Pt location in Pt/Ba/CeO₂ catalysts for NO _x storage–reduction (NSR) was analyzed. The Pt location on BaCO₃ or CeO₂ support was controlled by changing the angle (φ) between the two flame sprays producing these two components. As-prepared flame-made catalysts contain PtO _x which must be reduced during the fuel rich phase to become active for NO _x storage and reduction of NO _x . For Pt on BaCO₃ this process was significantly faster than for Pt on CeO₂. The increased reduction ability of Pt on Ba is reflected in the light off temperatures: for Pt on CeO₂ temperatures around 330 °C were needed to combust 20% of C₃H₆ in air while for Pt on BaCO₃ only 250 °C were required for the same conversion. The ability to control the location of Pt or other noble metals is, therefore, essential to optimize the catalysts for a given Pt/Ba/CeO₂ weight ratio. The best performance was observed when most of the Pt constituent was located near Ba-containing sites.