Synthetic Exhaust Gas Research Articles

Effects of Excess Cobalt Oxide Nanocrystallites on LaCoO3 Catalyst on Lowering the Light off Temperature of CO and Hydrocarbons Oxidation

Catalysts with the formula of LaCo(1+x)O(3+δ), where 0 ≤ x ≤ 1, were studied for oxidation of CO and C2H6 in a synthetic exhaust gas, comprising 6.0 % CO and 0.2 % C2H6 in Ar. Ethane was selected as a model for hydrocarbons in the exhaust gas. The performance of catalysts is correlated to their properties, particularly their redox in oxidizing and reducing atmospheres. XRD patterns show perovskite structure for all catalysts. The Co3O4 crystallite size was calculated using the Scherrer’s equation. TPR results show the reduction of Co3O4 and cobalt oxide in perovskite structure in the range of 330 - 475 °C and 550 - 650 °C, respectively. Disappearance of the cobalt oxide structure in XRD patterns of LaCo(1≤x≤1.3)O(3+δ) catalysts are attributed to small size of the cobalt oxide crystallites. Redox properties of catalysts were also studied by electrical conductivity measurements. Similar Arrhenius-type electrical conductivity behaviors of catalysts with that of cobalt oxide indicates that the cobalt component is essential for charge carrier mobility in LaCo(1+x)O(3+δ) catalysts. Catalyst with 0.3 mole excess cobalt, i.e. LaCo1.3O(3+δ), which shows the lowest activation energy of electrical conductivity (Ec) and the lowest ratio of conductivities in reducing to oxidizing atmospheres, has the lowest light off temperatures for oxidation of both CO and ethane. High ability of catalysts in gas phase-lattice oxygen transfer is evidenced by the fast reduction and oxidation behaviors of catalysts in CO and air atmosphere, respectively.

Auto-cyclic reactor: Design and evaluation for the removal of unburned methane from emissions of natural gas engines

A non-adiabatic fixed bed auto-cyclic reactor (ACR) consisting of two counter-current concentric compartments was designed and built for removing low concentrations of methane from exhaust gases from natural gas engines. The length was based on simulations by a simple heterogeneous one dimensional model using literature parameters and kinetic data, while the diameter was selected to assure a linear fluid velocity between 0.5 and 2 m/s. Its innovative design consists of a judicious combination of 14 longitudinal fins welded to the outlet part of inner reactor compartment to maximize the heat transfer to the inlet section and highly active pellet type catalyst filling the space between fins to lower the ignition temperature. The experimental ACR pilot unit was loaded by a combination of highly active laboratory prepared catalysts: palladium/alumina pellets and palladium/alumina coated cordierite monoliths. The efficiency of methane removal from air and from synthetic exhaust gas containing 7 vol% CO 2 and 14 vol% H 2O was evaluated under a wide range of operating conditions: temperature from 290 to 500 °C, methane concentration between 500 and 3800 ppm. The reactor performance was monitored in terms of axial temperature profiles and methane conversion both in transient and steady state conditions. Reproducible performance of the ACR was observed even after 1200 h of cumulative operation and complete methane removal was obtained at relatively low temperatures. To simulate the obtained experimental data, a heterogeneous one-dimensional model was developed to suit the final reactor configuration using actual laboratory determined kinetic data. The model described adequately the experimental temperature profiles and methane conversion when heat transfer between the reactor compartments and heat loss were taken into account.

NOx reduction performance of lean NOx catalyst and lean NOx adsorber using DME as reducing agent

Commercially available lean NOx catalyst and lean NOx adsorber samples were tested in a synthetic exhaust gas bench with DME as reducing agent. The LNA catalyst gave a very high NOx conversion (> 80 %) over a wide temperature range (275–450°C), and with low concentration of by-products such as NH3, N2O and methanol. The Cu-zeolite based LNC catalyst, however, showed practically no NOx conversion.

Reduction of NO in the exhaust gas by reaction with N radicals

N atoms can serve as a reducing agent for NO in the exhaust gases from fossil fuel power plants. In order to investigate the reduction of NO, synthetic exhaust gas was plasma treated in a dielectric barrier discharge (DBD) (direct treatment) as well as treated by adding nitrogen atoms generated in a pure nitrogen DBD (remote treatment). A DBD with a coaxial electrode geometry was used. Oxygen could be added to the synthetic exhaust. A complete NO reduction was achieved in dry, oxygen free exhaust gas with direct treatment, but oxygen and humidity in the exhaust gas promotes formation of N x O y and reduces the efficiency of NO reduction. A 90% NO reduction was achieved with remote treatment. Remote treatment is insensitive to oxygen in the exhaust, but requires large amounts of nitrogen and energy. Fourier transform infrared spectroscopy and ultraviolet absorption spectroscopy were employed in order to detect NO, NO 2, and N 2O in the treated exhaust gas stream.

Nitride precipitation during high temperature corrosion of ductile cast irons in synthetic exhaust gases

Internal nitrides form in two ductile cast irons (SiMo and Ni-Resist) intended for exhaust systems in vehicles. Samples oxidised at 650–1050 °C for 50 h in modified synthetic exhaust gases were analysed by using AES and FEG-SEM. No nitrides formed in absence of NO x . In dry petrol gas coarse nitrides (<20 μm) precipitated heterogeneously deep (600 μm at 950 °C) into SiMo. It is argued that the accommodation of volume change, preferential diffusion paths and increased N solubility as Si was depleted contribute to a self-accelerating process. The Si depletion around the coarse nitrides lowered the microhardness and the corrosion resistance of the alloy. In diesel and in normal petrol gases μ-sized MgSiN 2 form in SiMo in cell boundaries where Mg segregates. This also occurs in Ni-Resist in both dry and normal petrol whereas no nitrides were observed in Ni-Resist exposed to diesel gases.

NOx remediation in oxygen-rich exhaust gas using atmospheric pressure non-thermal plasma generated by a pulsed nanosecond dielectric barrier discharge

It is clearly seen that the application of non-thermal plasmas (NTP) to remove NOx from gas mixture containing a large amount of oxygen (O2) is dominated by NO to NO2 oxidation. Experiments have been conducted using a NTP generated by a nanosecond pulsed dielectric barrier discharge in synthetic exhaust gas, prepared from N2, O2, NO, H2O, and C3H6, over a large range of gas temperature (20-300\\r{}C). Results show that the NOx removal rate significantly increased with increasing specific energy deposition. For example, at a temperature of 100\\r{}C and an energy deposition of 27 J l-1, 92% of the NO molecules have been removed. The W values for NO is dramatically reduced to values scaling from ≈15 eV at 27 J l-1 down to ≈4 eV at 7 J l-1. NOx removal efficiency around 43% was obtained at a temperature of 260\\r{}C and a space velocity of 60 000 h-1 for a specific input energy of 27 J l-1. W values for NOx were less than ≈30 eV. Such treatments in exhaust gas with and without the presence of water vapour induced reactions leading to the production of a large variety of by-products such as acetaldehyde, propylene oxide, formic acid, methyl nitrate, and nitromethane.

Pulsed microwave discharge at atmospheric pressure for NOx decomposition

A 3.0 GHz pulsed microwave source operated at atmospheric pressure with apulse power of 1.4 MW, a maximum repetition rate of 40 Hz, and a pulse lengthof 3.5 µs is experimentally studied with respect to the ability toremove NOx from synthetic exhaust gases. Experiments in gas mixturescontaining N2/O2/NO with typically 500 ppm NO are carried out. Thedischarge is embedded in a high-Q microwave resonator, which provides areliable plasma ignition. Vortex flow is applied to the exhaust gas toimprove gas treatment. Concentration measurements by Fourier transform infraredspectroscopy confirm an NOx reduction of more than 90% in the case of N2/NO mixtures. The admixture of oxygen lowers the reductive potential ofthe reactor, but NOx reduction can still be observed up to 9% O2 concentration. Coherent anti-Stokes Raman scattering technique isapplied to measure the vibrational and rotational temperature of N2.Gas temperatures of about 400 K are found, whilst the vibrationaltemperature is 3000-3500 K in pure N2. The vibrational temperaturedrops to 1500 K when O2 and/or NO are present. The randomlydistributed relative frequency of occurrence of selected breakdownfield intensities is measuredby a calibrated, short linear-antenna. The breakdown field strengthin pure N2amounts to 2.2×106 V m-1, a value that is reproduciblewithin 2%. In the case of O2 and/or NO admixture, the frequencydistribution of the breakdown field strength scatters more and extends overa range from 3 to 8×106 V m-1.

NOx Storage Reduction over PtBa/γ-Al2O3 Catalyst

A transient study on the NOx storage-reduction properties of a PtBaO/γ-Al2O3 catalyst is performed by using a synthetic exhaust gas containing oxygen and nitrogen oxides (storage phase) and a reducing gas containing hydrogen (reduction phase). The influence of water and carbon dioxide is also investigated. It is found that (i) NOx is stored in the form of nitrites and nitrates; (ii) during storage nitrites are oxidized to nitrates, and nitrates are most abundant when the storage process is completed; (iii) 0.3–3% CO2 has a marked inhibiting effect on the storage of NOx, particularly at low temperature, whereas 1% H2O has a promoting effect at low temperature and an inhibiting effect at high temperature. In the presence of 0.3–3% CO2+1% H2O the process is inhibited at any temperature; (iv) the storage of NOx occurs preferentially in the order at BaO, Ba(OH)2, and BaCO3. The abundance of the different Ba sites at the catalyst surface depends on the composition of the exhaust gas and of the reducing gas; (v) considerable amounts of NOx are stored up to catalyst saturation and up to the NOx breakthrough in He +3% O2 atmosphere that correspond to 24% Ba and 13–15% Ba to the best, respectively; (vi) in the presence of 0.3–3% CO2 and 1% H2O in the exhaust these quantities diminish by 20–40% for NOx stored up to catalyst saturation and by 50% for NOx stored up to the NOx breakthrough; (vii) the reduction of the stored NOx is fast and is limited by the concentration of the reducing agent at any temperature in He +2000 ppm H2; (viii) the reduction of the stored NOx is very selective to N2 (95–100%); (ix) the reduction is slower in the presence of 0.3–3% CO2 and 0.3–3% CO2 +1% H2O; (x) once all the stored reactive NOx groups have been reduced, in the presence of 0.3–3% CO2 and at sufficiently high temperature (T≥300°C) CO is formed through the reverse WGS reaction. This reaction, however, is of lesser importance when water is present in the exhaust due to thermodynamic constraints. The complete set of reactions involved in the storage-reduction cycle is identified and used to account quantitatively for the bulk of experimental data and to provide a comprehensive chemistry of the process.

An approach to characterization and after-treatment of particulate emissions from gasoline engines

The growing importance of understanding and controlling particulate emissions from gasoline engines is outlined, and an experimental simulation approach is described which has the potential for exploring particle deposition/capture and oxidation phenomena under well-controlled conditions, thus avoiding some of the uncertainties involved in interpreting engine experiments. Artificially generated sub-100 nm carbon particles are introduced into a synthetic exhaust gas stream, simulating engine-out soot emissions. Initial experiments in insulated pipe flows have shown very little particle deposition or change in mean size for residence times up to 1–2 s at uniform temperatures up to 400°C and hydrocarbon concentrations up to 4000 ppm C1′ in line with the results of supporting analytical and computational studies. Further numerical predictions suggested that it might be possible to capture some 20–30 per cent of the particles by turbulent diffusion alone at higher gas flowrates more representative of those in engine exhaust pipes. The effect of two different catalysts and an uncoated monolith on particle size and concentration has been investigated at space velocities of 50000 and 100000 h−1, with an inlet gas temperature of 400°C. Particle penetration through the catalysts increased with particle size but in all cases exceeded 70 per cent, with no evidence of significant changes in particle mean size. In the next phase of experimental and modelling work, the contribution of thermophoresis to particle deposition in uninsulated pipes will be examined.

Macro-kinetic investigation on phenol uptake from air by biofiltration: Influence of superficial gas flow rate and inlet pollutant concentration

The macro-kinetic behavior of phenol removal from a synthetic exhaust gas was investigated theoretically as well as experimentally by means of two identical continuously operating laboratory-scale biological filter bed columns. A mixture of peat and glass beads was used as filter material. After sterilization it was inoculated with a pure strain of Pseudomonas putida, as employed in previous experimental studies. To determine the influence of the superficial gas flow rate on biofilter performance and to evaluate the phenol concentration profiles along the column, two series of continuous tests were carried out varying either the inlet phenol concentration, up to 1650 mg . m(-3), or the superficial gas flow rate, from 30 to 460 m(3) . m(-2) . h(-1). The elimination capacity of the biofilter is proved by a maximum volumetric phenol removal rate of 0.73 kg . m(-3) . h(-1). The experimental results are consistent with a biofilm model incorporating first-order substrate elimination kinetics. The model may be considered a useful tool in scaling-up a biofiltration system. Furthermore, the deodorization capacity of the biofilter was investigated, at inlet phenol concentrations up to 280 mg . m(-3) and superficial gas flow rates ranging from 30 to 92 m(3) . m(-2) . h(-1). The deodorization of the gas was achieved at a maximum inlet phenol concentration of about 255 mg . m(-3), operating at a superficial gas flow rate of 30 m(3) . m(-2) . h(-1). (c) 1996 John Wiley & Sons, Inc.

Catalytic NO x reduction in lean burn exhaust over Pt/silica catalysts with controlled Pt particle size

Platinum-on-silica catalysts with 1.5 wt.% platinum and average metal particle diameters of <3, 3.5, 8.7 and 22.5 nm prepared by sol–gel processing of platinum organo(alkoxy)silane complexes were evaluated as lean DeNO x catalysts. The catalytic experiments were run with a synthetic exhaust gas containing 500 ppm NO, 175 ppm propane, 175 ppm propene, 350 ppm CO, 6% O 2, 10% CO 2 and 12% water, with a volumetric space velocity of 60 000 h −1 and at temperatures in the range 150–550°C. Catalysts with platinum particle sizes of 3.5 nm and larger showed only little platinum metal sintering during catalytic testing, whereas significant sintering was observed in the catalyst with the smallest platinum particle size of <3 nm. The lightoff temperature of NO x conversion, propene and propane conversion, and NO into NO 2 oxidation increases with increasing Pt particle size. The maximum NO x conversion reached at the optimum reaction temperature is 20–23% and does not depend on the Pt particle size. These catalysts convert NO x selectively into N 2O. The absence of N 2 formation can be explained by a lack of sufficiently strong Brönsted acid sites on the silica support.

Reduction of NO x in diesel exhaust with methanol over alumina catalyst

NO x reduction in diesel exhaust using selective catalytic reduction by methanol over alumina catalyst was investigated. Although a slight decrease in catalytic activity took place at the initial stage of the catalyst evaluation, more than 40% NO x conversion was obtained at 400°C and a space velocity of 10 000 h −1 by using a ball type alumina catalyst and methanol ( CH 3OH/NO x mole ratio = 2) as a reducing agent. The experimental results using synthetic exhaust gases containing SO 2 indicated that the initial catalytic deactivation is ascribed to the formation of Al 2(SO 4) 3-like species on alumina surface through reaction with SO 2. However, the formation stopped at a certain point in time. The stable activity following the initial deactivation did not depend on SO 2 concentration. A durability test of a honeycomb type alumina was also conducted, and more than 70% of NO x conversion maintained for more than 4000 h at a CH 3OH/NO x mole ratio of 6. In this case a gradual decrease in the activity of alumina occurred due to catalyst sintering.

Lambda detection with thin-film metal oxides using synthetic exhaust gas mixtures

New exhaust gas sensors based on the gas dependence of the electric conductivity of metal oxide thin films have been developed to realize a cylinder-selective lambda measurement in internal combustion engines. To understand these results, the lambda characteristics of the sensors were measured in synthetic gas mixtures with concentrations similar to the exhaust of an internal combustion engine. The typical lambda characteristics of the sensors show a strong change in resistance near the stoichiometric point, and a slight dependence on the lambda value in the rich (lambda <1) and lean (lambda > 1) regions. The lambda dependence is similar to that of the computed oxygen partial pressure of the used gas mixtures in equilibrium. This allows us to assume that on the sensor surface the thermodynamic equilibrium is established between the gases and the metal oxide determined by the concentration of oxygen vacancies. From a comparison of the computed with the measured lambda characteristic of the sensor materials, it can be assumed that at temperatures of about 1000 °C the resistance of thin-film metal oxides is determined by the concentration of oxygen vacancies as a function of the oxygen partial pressure. A dispersion of catalytically active platinum can enhance the reaction rate to establish the equilibrium even at temperatures lower than 1000 °C.

Lambda measurement with Ga 2O 3

Sensors based on high-temperature-stable semiconducting gallium oxide thin films may be used to monitor the composition of ot exhaust gases from internal combustion engines or furnace installations. The electrical d.c. conductance of these low-cost devices, operable in the temperature range 700–1000 °C, represents the sensor signal. Investigations have been performed in the laboratory, using a special gas-mixing system that supplies a mixture of the main components of real exhaust gas (N 2, O 2, CH 4, CO, NO, water vapour) to produce a synthetic exhaust gas with very precisely defined composition. Additional investigations have been performed in real exhaust gas on an engine bench test-bed. In the temperature range 1000–900 °C the gallium oxide sensors respond to the oxygen partial pressure of the mixture's thermodynamic equilibirum. With knowledge of the fuel composition (carbon-hydrogen ratio), this yields a simple λ measurement for λ=1.2–0.85 with a resistance jump of about three decades at the stoichiometric point. The main mechanism in this temperature regime is a setting of the bulk defect equilibirum. At lower temperatures, the influence of surface-located mechanisms becomes more important: between 800 and 700 °C the jump at the stoichiometric points becomes smaller and broader, but there is still a monotonic λ dependency. At 600 °C or below, the monotonic λ dependency is lost.

Adherence behavior of oxide grown in air and synthetic exhaust gas on Fe-Cr-Al alloys containing strong sulfide-forming elements: Ca, Mg, Y, Ce, La, Ti, and Zr

This work evaluated the adherence of oxide grown in air and synthetic exhaust gas on Fe-20Cr-5Al alloys containing strong sulfide-forming elements: Ca, Mg, Y, Ce, La, Ti, and Zr. Results support the theory that reactive elements provide good oxide adherence on alumina-forming materials primarily by tying up sulfur as stable Sulfides; however, other influences on adherence were found. Highly volatile elements, such as Ca and Mg, lose their sulfur-controlling ability by diffusing out of the matrix and into the growing oxide scale. Zirconium results in the growth of an extensive network of oxide pegs into the substrate which improves adherence. Titanium segregates to the alumina scale and acts as a sink for S in the matrix. In synthetic exhaust gas (N2+CO2+H2O), local oxide spalling was observed and was shown to be caused by H2O in the atmosphere. The added benefits of Ti and Zr, i.e., forming oxide pegs and sinks for S, improve adherence in this environment.

Laboratory investigations into a new method of reducing nitrogen oxide in the presence of free oxygen

This paper reports on investigations into a new method of reducing nitrogen oxide in the exhaust gases of engines which are operated under lean air/fuel conditions, as are, for instance, hydrogen-operated engines. The proposed method is called the “ARD-technique”, which means Adsorption, Reduction or Reaction and Desorption. As compared with today's conventional catalytic processes, application of the “ARD-technique” means a completely new procedure in exhaust gas treatment. We present the results of laboratory research work using the “ARD-technique” in combination with commercially available adsorbers, oxygen-containing synthetic exhaust gases and gaseous hydrogen as the reduction medium. Depending on the relevant experimental parameters such as the temperature, the flow conditions and gas compositions, the measurements yield values between 50% and more than 90% for the obtainable NO x conversions.

Physicochemical properties of Ce-containing three-way catalysts and the effect of Ce on catalyst activity

The activity of Pt,Rh,Ce/γ-Al 2O 3 and Pt,Rh/CeO 2 catalysts has been studied in a full complement synthetic exhaust gas mixture consisting of H 2, CO, C 3H 6, C 3H 8, NO, O 2, N 2, CO 2, H 2O, and SO 2. Direct interaction between Pt and CeO 2 was shown to lead to large improvements in catalyst performance after activation of the catalyst in the synthetic exhaust gas. Catalyst activation was shown to be due to reduction of the noble metals and surface Ce and this state of the catalyst was found to be particularly effective for CO oxidation. The key component responsible for activation was shown to be H 2; even at the low H 2 level present in the exhaust gas (≤.26 vol%), reduction is shown to be very easy. The degree of Pt/Ce interaction, and thus activity, after catalyst activation could be controlled by varying the CeO 2 crystallite size. Decreasing the CeO 2 crystallite size led to greater Pt/Ce interaction as shown by TPR and STEM analysis and resulted in greater activity for both fresh and laboratory aged catalysts. Direct Pt/Ce interaction also led to a synergistic reduction of Pt and surface Ce which we found to qualitatively correlate with catalyst performance after activation.

Oxidation behavior of Fe-20Cr-5Al rare earth alloys in air and synthetic exhaust gas

Fe-20Cr-5Al alloy foils are used in automotive catalytic converters. This work examines oxidation behavior of four production-processed alloy foils in both air and synthetic exhaust gas environments. Oxidation tests were performed between 750° C and 1150° C for times to 96 hrs. Weight gain results in both atmospheres were similar, an indication that the same mechanism controls oxidation in both environments. At high temperatures (>-950° C) both atmospheres produce weight gains consistent with α-alumina growth. Activation energies of 323 kJ/gmole and 271 kJ/gmole were calculated for oxidation in air and synthetic exhaust gas, respectively. At lower temperatures (<-850° C), accelerated weight gains can occur from growth of transition alumina. Despite similar weight gain results, the two atmospheres produce different oxide morphologies: at 950° C and above, air produces a rounded, porous oxide while synthetic exhaust produces a more compact, angular oxide. Unexpectedly, oxide spalling occurred on foils oxidized in synthetic exhaust at 1050° C and above.