High NOx Storage Research Articles

BaTi0.8Cu0.2O3 Catalysts for NO Oxidation and NOx Storage: Effect of Synthesis Method

The effect of the synthesis method (sol–gel or hydrothermal) in the physicochemical properties of a BaTi0.8Cu0.2O3 catalyst with perovskite structure was analyzed and the catalytic performance for the NO–NO2 oxidation and for the NOx storage was tested. The characterization of catalysts by BET, ICP-OES, XRD, XPS and H2-TPR reveals that only the sol–gel process allows the incorporation of copper into the perovskite structure, and that, by using the hydrothermal method, highly dispersed CuO is deposited on the surface of the BaTiO3. The NOx storage cyclic (NSC) experiments, carried out using both catalysts, indicate that sol–gel catalyst shows a high NOx storage capacity at 400 °C whilst the hydrothermal catalyst exhibits a high NO to NO2 oxidation activity. For this reason, a double bed reactor configuration was tested for NSC experiments using the hydrothermal catalyst as the oxidation catalyst and the sol–gel catalyst as the NOx trap.

Tailoring the properties of BaTi0.8Cu0.2O3 catalyst selecting the synthesis method

The effect of the synthesis method (hydrothermal and sol-gel) on the properties of BaTi0.8Cu0.2O3 perovskites as catalysts for NOx and soot removal has been analyzed. X-ray powder diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), ICP-AES, N2 adsorption at −196°C, Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM) and temperature programmed reduction with hydrogen (H2-TPR) have been used for catalysts characterization. To test their catalytic activity, NOx storage and soot combustion temperature programmed reaction tests have been carried out.The results allow to conclude that the synthesis method determines the position of copper on the perovskite structure and, therefore, the catalytic applications. When the hydrothermal method is used the copper is highly dispersed on the perovskite surface, obtaining a catalyst with a high activity for the NO to NO2 oxidation reaction, which can be used as oxidation catalyst for soot removal. Nevertheless, using the sol-gel method, copper is incorporated into the perovskite structure and, consequently, the catalyst presents a high NOx storage capacity

Non-thermal plasma-assisted NOx storage and reduction over cobalt-containing LNT catalysts

Based on their high NO oxidation capacity (NOC) and NOx storage capacity (NSC), cobalt-containing LNT catalysts of the type Co/Ba/Al and Pd/Co/Ba/Al were tested under NOx storage-reduction (NSR) conditions, as well as in an H2-plasma assisted NSR process. For comparison purposes, a traditional Pt/Ba/Al catalyst was tested under the same conditions. The Co/Ba/Al catalyst exhibited similar catalytic properties to those of the Pt/Ba/Al catalyst when an H2-plasma was employed in the rich phase to assist reduction of the stored NOx. However, the NOx removal efficiency was greatly depressed by the presence of H2O and CO2 in the feed. Notably, the addition of Pd to the Co/Ba/Al sample greatly lessened the inhibiting effects of H2O and CO2 on plasma-assisted cycle-averaged NOx conversion, this being due to the excellent NOx storage performance of the Pd/Co/Ba/Al sample even in the presence of H2O and CO2. The results of the present study show that by combining the high NOx storage capacity of Pd/Co/Ba/Al in the lean phase with non-thermal plasma-assisted activation of the reductant in the rich phase, high NOx conversion can be obtained over a broad temperature window of 150–350°C.

Pt-free, non-thermal plasma-assisted NOx storage and reduction over M/Ba/Al2O3 (M = Mn, Fe, Co, Ni, Cu) catalysts

Based on their high NOx storage capacities (NSCs), a series of M/Ba/Al (M=Mn, Fe, Co, Ni, Cu) catalysts were evaluated in NOx storage-reduction catalysis. Although the M/Ba/Al catalysts exhibited high NSC values, they exhibited much lower activity during lean-rich cycling compared with a traditional Pt/Ba/Al catalyst, indicating that regeneration of stored NOx is the rate limiting step for the M/Ba/Al samples. By employing an H2-plasma in the rich phase to assist reduction of the stored NOx, the NOx conversion was greatly improved, especially at low temperature.

TiO2–Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement

TiO2–Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2–Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05–3.10eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.

Effects of Ti-doping on the NOx storage and the sulfur resistance of the BaFe1 − xTixO3 − y perovskite-type catalysts for lean-burn exhausts

Doping with Ti in the BaFeO3−x perovskite resulted in the significantly improved NOx adsorbability and the sulfur resistance, as compared with the BaFeO3−x. The Ti loading and residual carbonate could enhance the NOx storage capacity of these catalysts. However, during the sulfation, the sulfates were readily formed over the undissolved titanium oxides and the carbonates on perovskite. The complete Ti-doping in perovskite would reduce the amount of the residual carbonate, as well as the undissolved titanium, consequently leading to the relatively smaller NOx storage capacity (NSC) but the further improved sulfur resistance. Among these Ti incorporated perovskites, the BaFe0.8Ti0.2O3−y perovskite prepared by the impregnation method had the most complete Ti-doping degree and the highest sulfur resistance: only 5.1% of the NSC dropped (from 1000 to 949μmolg−1) after the sulfation pretreatment at 400°C. It is a possible NOx trapping material with the advantages of the high NOx storage capacity and sulfur tolerance for lean-burn exhausts.

Non-thermal plasma-assisted NOx storage and reduction on a LaMn0.9Fe0.1O3 perovskite catalyst

Based on its high NOx storage capacity (NSC), a study of the properties of LaMn0.9Fe0.1O3 in NOx storage-reduction catalysis was undertaken. The microstructure of the LaMn0.9Fe0.1O3 catalyst was characterized by XRD, XPS and H2-TPR techniques and compared with that of LaMnO3. The role of Fe doping in LaMnO3 on the perovskite structure and on NOx storage capacity was clarified. Though the LaMn0.9Fe0.1O3 perovskite has high NSC values even at relatively low temperature (<300°C), it shows much lower activity during lean-rich cycling compared with a traditional Pt/BaO/Al2O3 catalyst, indicating that regeneration of stored NOx is the rate limiting step for the perovskite catalyst. By employing an H2-plasma in the rich phase to assist reduction of the stored NOx, the NOx conversion is greatly improved, especially at low temperature. The results of the present study show that by combining the high NOx storage capacity of the perovskite in the lean phase with non-thermal plasma-assisted activation of the reductant in the rich phase, high NOx conversion can be obtained at low temperature.

High-Temperature NO x Storage and Sulfur-Resistance of the Lithium-Based Lean-Burn NO x Trap Catalyst Pt/Li/TiO2–Al2O3

A series of lithium-based lean-burn NO x trap catalysts Pt/Li/TiO2–Al2O3 were prepared by sequential impregnation. The doping of TiO2 into the support Al2O3 significantly enhances the sulfur-resistance performance of the catalyst Pt/Li/Al2O3. On TiO2–Al2O3 mixed oxides, the Pt and lithium species are more highly dispersed, giving higher NO x storage capacity, as compared with those on Al2O3 and TiO2. In situ DRIFTS reveals that the NO x storage on Pt/Li/TiO2–Al2O3 mainly proceeds on –OLi sites forming bidentate nitrate species at 500 °C. The catalyst Pt/Li/TiO2–Al2O3 with 40% TiO2 in the support is the most promising one applicable to the lean-burn NO x abatement. The sulfated NSR catalyst Pt/Li/TA(40) is easier to reduce than the sulfated Pt/Ba/TA(40), showing better regeneration ability.

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

Three MnOx–SnO2 catalysts were successfully prepared by hetero-redox, homo-redox and common hydrothermal methods. NO adsorption and desorption were performed to investigate the NOx storage capacity at 100 °C. All the samples showed good performances on NOx 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 NOx storage capacity.

Novel Ru−Mg−Al−O Catalyst Derived from Hydrotalcite-like Compound for NO Storage/Decomposition/Reduction

Ru-Mg-Al hydrotalcite-like anionic clay (Mg/Al/Ru) 90: 29: 1) was successfully prepared with a constant-pH coprecipitation method. Calcination of a hydrotalcite-like precursor at 600 degrees C in the air gave rise to the well-mixed oxide Ru-Mg-Al-O that possesses a good dispersion of Ru species. Ru-Mg-Al-O catalyst after suitable pretreatment exhibits quite high NOx storage capability in the temperature range of 250-400 degrees C, and the highest NOx storage capability of about 220 mu mol g(-1) is obtained at 350 degrees C with flowing 790 ppm NO and 8% O-2 in N-2 stream. Meanwhile, the decomposition of 25-60% NO to N-2 as well as N2O is clearly observed on the catalyst at 300-400 degrees C. In situ diffuse reflectance Fourier transform ( DRIFT) spectra indicate that NOx is adsorbed and stored on a catalyst mainly in the form of various coordinated nitrites/ nitrates. On the basis of the NOx adsorption-desorption profiles as well as the in situ DRIFTS spectra, we have proposed a schematic outline for NOx storage and NO decomposition. Finally, the reduction of stored NOx species on the catalyst by H-2 was carried out at 350 degrees C, indicating that all adsorbed NOx species can be readily reduced by hydrogen.

Rare earths based oxides as alternative materials to Ba in NOx-trap catalysts

The reduction of NOx from Diesel Engines is a major issue of automotive catalysis. In this paper, we investigate basic rare earths oxides (such as La2O3, Nd2O3 or Pr6O11) used as NOx-trap sites in replacement of Ba. These species are intimately mixed with (CeZr) solid solutions and the thus obtained mixed oxides are considered as Pt carrier. Investigations on powder model Pt-(CeZr)/RE2O3 (or Pr6O11) catalysts show promising results. These model catalysts present high NOx storage capacity, especially at low temperature, and improved resistance to sintering at 700 °C under lean hydrothermal conditions. Moreover, they show total desulfation at 600 °C i.e. 150 °C lower than current Pt-(BaO/Al2O3) technologies and are thus considered as more durable technologies.

Role of cobalt on γ-Al2O3 based NO x storage catalyst

Co/Pt/Ba/γ-Al2O3, Co/Ba/γ-Al2O3, Pt/Ba/γ-Al2O3, Co/Pt/γ-Al2O3, Ba/γ-Al2O3, Pt/γ-Al2O3, and Co/γ-Al2O3 type catalysts were prepared by a conventional impregnation method, and their NO x storage capacities were evaluated by colorimetric assay. Co-containing catalysts had a higher NO x storage capacity than that of Co-free counterparts. The role of each component, especially Co, for the catalysts prepared was investigated by using in-situ FTIR. The high NO x storage for Co-containing catalysts including Co/Ba/γ-Al2O3 and Co/Pt/Ba/γ-Al2O3 is mainly due to the formation of Co3O4 on the catalyst surface identified by XAFS.

Comparison Between a Pt–Rh/Ba/Al2O3and a Newly Formulated NOX-Trap Catalysts Under Alternate Lean–Rich Flows

In situ FT-IR spectroscopy coupled with mass spectrometry have been used to study the mechanism of nitrates formation and reduction over a common Pt–Rh/Ba/Al2O3 NO x storage catalyst, compared with a different alumina-based compound. The experimental device used consists of a transmission reactor cell (having a very small dead volume) dedicated to the evolution of surface species, and of a mass spectrometer combined with a FT-IR micro-cell for gas analysis, allowing time resolved analysis in stationary and transient conditions. At the first time the nitration properties of the catalysts under a lean flow have been studied in the appropriate temperature window (473–673 K). The dynamics of nitrates formation has been pointed out, as well as the different coordination sites on the compounds surface. Then the catalysts have been alternatively exposed to rich and lean flows very close to the real exhaust composition. This has allowed the identification of reduction pathway, active sites, intermediate species and by-products for NO X -trap reaction. In particular, we have differentiated the role of the support and of the noble metal in the mechanism, as well as of isocyanate adspecies and ammonia among the detected species. The very high NO X storage properties and the selectivity (near 100%) in nitrogen of the newly designed catalyst have been pointed out.