Light-off Temperature Research Articles

Dynamic Catalyst Conversion Measurement Using One Single Sensor Device

IntroductionIn-situ monitoring of catalytic conversion with only one single (gas) sensor component represents a novel approach in gas sensor technology. The background of this idea is a sensor device that "compares" two gas atmospheres before (upstream) and after (downstream) of a catalyst. The sensor element itself separates two gas compartments from each other, but connects two (identical) electrodes in each of these gas compartments by means of an ion bridge. According to the mixed- potential principle, a half-cell potential is generated at each individual electrode. The potential difference between the two electrodes is evaluated as a sensor signal and, according to the model of mixed-potential formation, should depend directly on the ratio of the concentrations of a specific gas species between the two gas spaces, i.e., in this case directly on the conversion of the gas over the catalyst [1].The sensor device itself is the basis for reliable and reproducible measurements in that field. It is made from oxygen ion-conducting yttria-stabilized zirconia (YSZ) and designed as a full ceramic disc (figure 1a). Furthermore, it comprises an internal heating structure. So, the sensor can be brought to an appropriate measuring temperature (up to 650 °C) locally in the middle of the disc – exactly in that region where the planar electrodes are integrated on both sides and where the ionic conductivity is necessary. In the outer region, the sensor disc remains cold enough to use simple sealing materials (e.g. Viton). More details on the sensor setup can be found in [2]. A special housing (made from PEEK-cells) acts as sensor chamber with gas feed lines connected to the up- and downstream atmospheres of the catalyst (details can be found in [3]).In the present contribution, dynamic tests concerning the catalyst temperature and the inlet concentration are discussed. In these investigations, it should be verified that the sensor signal is independent from the inlet concentration.Experimental SetupMeasurements were conducted in the lab. The inlet gas was mixed with mass-flow-controllers (MFCs) from propene in lean (e.g. oxygen containing) gas atmosphere (N2 balance). The gas flows over an oxidation catalyst (Pt as precious metal component on cordierite honeycomb substrate, diameter 1”, located in a quartz tube) inside a horizontal furnace (Carbolite). The gas / catalyst temperature was controlled by a thermocouple (upstream) inside the reactor. During the experiment, the furnace was heated up and – by that – conversion over the catalyst is increased, as long as the catalyst light-off temperature is reached (exothermic conversion is visible by a small temperature increase, measured with a downstream thermocouple inside the reactor, figure 2 first graph). The sensor device (disc inside the housing) was connected to the gas atmospheres up- and downstream the catalyst – each purging one compartment of the sensor housing with contact to each electrode. Simultaneously to the catalyst heating, the inlet (upstream) concentration was changed (see figure 2, second graph), which might be typical for real applications in the automotive field. The downstream HC-concentration was measured by FID. Out of this information, the conversion was calculated and plotted as (1 – conv.) in figure 2 (third graph). Figure 2 (last graph) shows the sensor raw signal that is a voltage signal as a difference of both half-cell potentials at the up- and downstream electrodes (in this case made from zinc chromite) measured at a sensor temperature of 500 °C. Results and Conclusions If one has a look on the raw data first (figure 2), the following results can be found. Regarding the up- and downstream temperature one assumes a light-off of the catalyst at about 150 °C (t = 15000 s). Starting from here, the downstream temperature increases more than the upstream temperature. The gas data indicate a small conversion even before (lower downstream concentration @ t > 7500 s). Although the calculated conversion is still “0”, (i.e. (1 - conv.) = 1), the sensor signal shows a slight increase in that area. In the medium temperature range (15000 – 30000 s, 150 °C < T < 175 °C) the conversion depends on the inlet concentration. Independently from that fact, the sensor signal follows the calculated conversion factor. In the higher temperature range (t > 30000 s, T > 175 °C), conversion is near 100 % (i.e. (1 - conv.) approaches “0”); the sensor shows its maximum signal.Now, in a second data evaluation, the sensor signal is plotted against the calculated conversion data in a half-logarithmic representation (figure 3, thick line including all measured data points in comparison to the thin line that was obtained in stationary experiments). We found an impressing correlation between both data sets, although the experimental sequence changed dynamically two parameters (temperature and inlet concentration) simultaneously and in a wide range. Therefore, the presented novel sensor device seems to be an appropriate candidate for catalyst conversion control.

Development of catalytic ceramic filter candles for tar conversion

A catalyst with a high activity in tar conversion, impregnated on the ceramic hot gas filter material was developed. The aim of the experiments was to estimate the light-off temperature of the catalytic filter elements for naphthalene (tar model compounds) conversion and the long-term catalytic stability at a temperature of 700 °C. Configuration of the catalyst was optimized through improvements in coking resistance and long-term stability. The composition and morphology parameters of the filter material were considered. Both the impregnation methods and the composition of the impregnation solution were investigated and validated. The catalyst composed of Ni, Fe, Cr oxides, promoted with Pt (AlSi-Cat43-Pt), and impregnated on the ceramic-fiber filter composed of Al2O3(44%)/SiO2(56%) was found to be the most active catalyst. The designated catalyst was catalytically active at temperatures of about 700 °C, with a naphthalene conversion of around 93% over 95 h without catalyst deactivation. We found that the steam and gas compositions had an influence on the catalytic activity of the filter elements. The same catalytic filter was catalytically active for 115 h at a low concentration of H2O (10 vol%) and H2 (3 vol%) with a naphthalene conversion of 98% at 790 °C without significant deactivation.

Sensitivity of pollutants abatement in oxidation catalysts to the use of alternative fuels

The aim to reduce well to wheel CO2 emissions incentives the utilisation of alternative fuels (low to zero carbon content and/or low well to tank CO2 emissions) as well as the enhancement of engine efficiency. In parallel, the reduction of engine tailpipe CO2 emissions brings new challenges such as the decrease of the exhaust gas temperature. This trend penalises the ability of the exhaust aftertreatment system to eliminate pollutant emissions. In addition, the combustion of alternative fuels and new combustion modes induce changes in the nature and concentration of the exhaust species, which is known to affect the pollutants abatement mechanisms. This investigation provides new understanding on the sensitivity of pollutants abatement in oxidation catalysts to the use of alternative fuels. The studied fuels are conventional diesel, alternative fuels (rapeseed methyl ester and gas to liquid) as well as propane using a dual-fuel combustion strategy. The research combines experimental conversion efficiency from genuine exhaust gases with modelling work useful to explain the reasons for the change in light-off temperature as a function of the fuel. In addition to the CO and NO impact, HC surrogates are proposed distinguishing species of different reactivity for each fuel based on the experimental HC speciation. The results highlight the role of the engine-out emissions on the pollutants conversion efficiency. Their fashion with different fuels contributes to evidence the interest for low engine-out emissions along with low light alkanes content in total HC, as promoted by alternative fuels, to reduce the oxidation light-off temperature.

Characterization of Pt-based oxidation catalyst – Deactivated simultaneously by sulfur and phosphorus

Simultaneous poisoning of sulfur + phosphorus on a platinum-based diesel oxidation catalyst was studied to gain a deeper understanding on the catalyst deactivation. Compared to a single poisoning (sulfur or phosphorus), the simultaneous poisoning had a severe effect on the catalyst activation: light-off temperature for 90% conversion of propene was not reached, this of carbon monoxide was much higher than expected, and the maximum conversion of nitrogen monoxide collapsed. With very comprehensive structural characterization by various methods (e.g. STEM-EDS, XPS, DRIFTS) used, we achieved to conclude an explanation for this. In the case of the S + P-poisoning of the catalyst, formed aluminum phosphate was found to block adsorption sites for sulfur species on alumina and sulfur adsorbs mainly on cerium oxides. In addition, sulfur species remain with and in the vicinity of the platinum particles blocking active sites.

Effects of CO and H2O Co-Feed on the Adsorption and Oxidation Properties of a Pd/BEA Hydrocarbon Trap

Hydrocarbon traps for exhaust emissions control adsorb hydrocarbons in low temperature exhaust and release them as the exhaust warms up. In this work, a Pd/BEA hydrocarbon trap was tested under lean exhaust conditions using ethylene and dodecane as model hydrocarbons. Ethylene uptake was partially inhibited by CO and H2O when fed separately. When both were added, the loss in ethylene uptake was 90% relative to the condition with no H2O or CO. Dodecane uptake was unchanged under all conditions tested. During a temperature ramp, ethylene desorbed and was combusted to CO2 and H2O over active Pd centers. Further, oxidation light-off of dodecane generated an exotherm which caused rapid desorption of the remaining hydrocarbon species from the zeolite. For both hydrocarbons, CO co-feed led to a decreased oxidation light-off temperature, and therefore lower desorption temperature. By pretreating the catalyst in CO and H2O at 80 °C, and even after removing CO from the feed, the enhanced oxidation light-off behavior was observed. DRIFTS characterization shows that some form of oxidized Pd was reducible to Pd0 by CO at 80 °C only in the presence of H2O. Further, this reduction appears reversible by high temperature oxygen treatment. We speculate that this reduced Pd phase serves as the active site for low temperature hydrocarbon oxidation.

How CO2 poisons La2O3 in an OCM catalytic reaction: A study by in situ XRD-MS and DFT

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly into more valuable C2 products (ethane and ethylene). The mechanism related to selectivity and conversion is still not clear. Previous results suggested that the bulk structure change of the La2O3 catalyst is related to OCM activation performance. This work is focused on investigation of carbonates and the reaction products simultaneous formation over La2O3 catalyst utilizing coupled in situ XRD-MS. Semi quantitative analysis clearly reveals a positive dependence between CO2 and C2 products light-off temperature and the initial bulk La2O2CO3 concentration, indicating a strong correlation between carbonates and La2O3 catalyst poisoning. The activity of the La based catalyst is associated to its oxide form rather than to its oxycarbonate form. Based on operando results a two-step bulk La2O2CO3 decomposition model resulting in surface carbonates presenting on the catalyst surface between 500 °C and 800 °C is proposed. DFT calculations further confirm the proposed model and indicate surface carbonates do inhibit catalytic methane activation. It is concluded that CO32− species located in surface region of the catalyst operate as a direct La2O3 catalyst poison in the highly motivating OCM reaction.

Analysis of the Driving Altitude and Ambient Temperature Impact on the Conversion Efficiency of Oxidation Catalysts

Worldwide emission standards are extending their requirements to cover engine operation under extreme ambient conditions and fill the gap between the type-approval and real driving conditions. The new ambient boundaries affect the engine performance and raw emissions as well as the efficiency of the exhaust aftertreatment systems. This study evaluates the impact of high altitude and low ambient temperature on the light-off temperature and conversion efficiency of an oxidation catalyst. The results are compared in a common range of exhaust mass flow and temperature with the baseline sea-level operation at 20 °C. A reduction of CO and HC conversion efficiencies was found at 2500 m and −7 °C, with a relevant increase of the light-off temperature for both of the pollutants. The analysis of the experimental data was complemented with the use of a catalyst model to identify the causes leading to the deterioration of the CO and HC light-off. The use of the model allowed for identifying, for the same exhaust mass flow and temperature, the contributions to the variation of conversion efficiency caused by the change in engine-out emissions and tailpipe pressure, which are, in turn, manifested in the variation of the reactants partial pressure and dwell time as governing parameters.

Active oxygen center in oxidative coupling of methane on La2O3 catalyst

La2O3 catalyzed oxidative coupling of methane (OCM) is a promising process that converts methane directly to valuable C2 (ethylene and ethane) products. Our online MS transient study results indicate that pristine surface without carbonate species demonstrates a higher selectivity to C2 products, and a lower light-off temperature as well. Further study is focused on carbonate-free La2O3 catalyst surface for identification of active oxygen species associated with such products behavior. XPS reveals unique oxygen species with O 1 s binding energy of 531.5 eV correlated with OCM catalytic activity and carbonates removal. However, indicated thermal stability of this species is much higher than the surface peroxide or superoxide structures proposed by earlier computation models. Motivated by experimental results, DFT calculations reveal a new more stable peroxide structure, formed at the subsurface hexa-coordinate lattice oxygen sites, with energy 2.18 eV lower than the previous models. The new model of subsurface peroxide provides a perspective for understanding of methyl radicals formation and C2 products selectivity in OCM over La2O3 catalyst.

Ternary multifunctional catalysts of polymeric carbon nitride coupled with Pt-embedded transition metal oxide to enhance light-driven photothermal catalytic degradation of VOCs

Light driven photothermal catalysis has been carried out by converting the light energy into heat to reach the light-off temperature of the reaction. Herein we have synthesized the ternary multifunctional catalysts of polymeric carbon nitride coupled with Pt-embedded transition metal oxide (Pt-Cox/CN), for the catalytic degradation of toluene. Under the condition of space velocity of 30,000 mL/(gh) and concentration of 210 ppm, toluene conversion and CO2 mineralization can reach 90% and 83% over Pt-Co20/CN, respectively. The introduction of an appropriate proportion of CoO enhances the light absorption of nanocomposites and improves the adsorption for toluene. Meanwhile, CoO promotes the proportion and mobility of adsorbed oxygen on the surface, which are conducive to the catalytic oxidation reaction according to the Mars-van Krevelen mechanism. The results also suggest that light irradiation serves as a source of heat to initiate photo-induced chemical reactions and promote photothermal catalytic oxidation by promoting the activation of lattice oxygen.

Essential role of B metal species in perovskite type catalyst structure and activity on toluene oxidation

The perovskite type materials with transition metals are getting more attention especially as catalysts in total oxidation reaction. This work explores the B metal effect on the catalytic activity of LaBO3 structured perovskites in total oxidation of toluene. The perovskite type oxides were obtained by Pechini method and characterized by X-ray diffraction, nitrogen adsorption/desorption isotherms, thermogravimetric analysis and differential scanning calorimetry, temperature-programmed reduction (H2-TPR), Raman spectroscopy, Fourier transform infrared spectroscopy and particle size analysis. The results showed that LaFeO3 catalyst contained a single orthorhombic LaFeO3 phase, while LaMnO3 contained LaMn2O5 species besides cubic LaMnO3 phase. Both catalysts show very narrow distributions and average values of 55.59 μm and 51.43 μm for LaMnO3 and LaFeO3, respectively. With regard to the H2-TPR profile for the LaMnO3, Mn4+ to Mn3+ reduction and Mn3+ to Mn2+ reduction. Consequently, the redox performance of ABO3 perovskites was found as mainly driven by the B-site transition-metal element character. According to the catalytic tests, the LaMnO3 catalyst was more active for toluene oxidation than LaFeO3 and achieved the lowest light-off temperatures. An excellent agreement between the experimental data and the proposed one-dimensional pseudo-homogeneous model was achieved and corresponding kinetic parameters (estimated rate constants, k, activation energies, EA, and frequency factors, Ar) were estimated. Lower activation energy was estimated for LaMnO3 catalyst (84 kJ mol−1 vs. 99 kJ mol−1 for LaFeO3) confirming that LaMnO3 catalyst was more active for toluene oxidation under reaction conditions presented in this paper.

CO oxidation activity of Pt/CeO2 catalysts below 0 °C: platinum loading effects

Reducing the operating temperature of oxidation catalysts is important for designing energy efficient processes, extending catalyst lifetime, and abating pollutants, especially in cold climates. Herein, high CO oxidation activity at sub-ambient temperatures is reported for Pt/CeO2 catalysts with high content of Pt in the form of dispersed Pt2+ and Pt4+ centers. Whereas the reference 1 wt%Pt catalyst was active for CO oxidation only above 100ᵒC, the 8 and 20 wt%Pt catalysts converted 60 and 90 % of CO, respectively, below 0ᵒC. Ionic platinum was shown to facilitate oxygen release from ceria and lower the light-off temperature of the reaction occurring through the Mars-van-Krevelen mechanism. However, the remarkable activity observed at sub-ambient temperatures for the ≥8 wt%Pt catalysts is proposed to involve O2 and CO reactants weakly adsorbed on PtOx clusters. The synergies between ionic platinum and nanostructured ceria reported in this work advance the knowledge-driven design of catalysts for low-temperature oxidation reactions.

Forced Unsteady State Operation of a Catalytic Converter during Cold Start-up for Oxidizing CO Over Pt/γ-Al2O3Catalyst

CO oxidation in the catalytic converter hasn’t showed best performance particularly during cold start-up, since the catalyst is not active during this period. The purpose of this experiment was to develop the forced unsteady state operation procedure of CO oxidation using 0.05%-w Pt/γ-Al2O3and space velocity of 0.406 mmol/s/gram. The catalytic converter was gradually ramped-up, while introducing the feed gas containing CO in the air. The feed gas was modulated following a square wave model with switching time variation at 3, 6, 15, and 30 s and various operation modes. To gain the intrinsic reaction rate, the external mass transfer criterion was determined. Ramping-up the temperature from 50 until 150°C increased the CO conversion with different profiles between steady state and dynamic flow rate. The dynamic system with modulated CO feed flow gave lower light-off temperature and higher average CO conversion than the steady state system which gave light off temperature 115°C and average CO conversion of 48.86%. The switching time of 3 s gave highest average CO conversion during ramping-up, which was 79.35%. Meanwhile the dynamic operation system with modulated feed flow gave higher lightoff temperature and lower average CO conversion than steady state system.

Yttrium stabilization and Pt addition to Pd/ZrO2 catalyst for the oxidation of methane in the presence of ethylene and water.

Catalytic oxidation is the most efficient method of minimizing the emissions of harmful pollutants and greenhouse gases. In this study, ZrO2-supported Pd catalysts are investigated for the catalytic oxidation of methane and ethylene. Pd/Y2O3-stabilized ZrO2 (Pd/YSZ) catalysts show attractive catalytic activity for methane and ethylene oxidation. The ZrO2 support containing up to 8 mol% Y2O3 improves the water resistance and hydrothermal stability of the catalyst. All catalysts are characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), O2-temperature-programmed desorption (O2-TPD), and CO-chemisorption techniques. It shows that high Pd dispersion and Pd–PdO reciprocation on the Pd/YSZ catalyst results in relatively high stability. In situ diffuse reflectance infrared Fourier-transform (DRIFT) experiments are performed to study the reaction over the surface of the catalyst. Compared with bimetallic catalysts (Pd : Pt), the same amounts of Pd and Pt supported on ZrO2 and Y2O3-stabilized ZrO2 catalysts show enhanced activity for methane and ethylene oxidation, respectively. A mixed hydrocarbon feed, containing methane and ethylene, lowers the CH4 light-off temperature by approximately 80 °C. This shows that ethylene addition has a promotional effect on the light-off temperature of methane.

Kinetics of NH3 oxidation on Pt/Al2O3: Rate enhancement and NH3 inhibition

We describe a combined experimental kinetics and modeling study for NH3 oxidation on Pt/Al2O3 and development of a predictive microkinetic model valid over a range of conditions. The NH3 oxidation rate (TOF) dependence is reported for NH3 concentrations between 10 and 25,000 ppm and temperatures below 250°C for Pt/Al2O3 powder and washcoated monoliths. The data reveal a shift from positive- to negative-order rate dependence on NH3 with the rate maximum dependent on the processing of Pt/Al2O3; unmilled Pt/Al2O3 powder exhibits a rate maximum at 500 ppm NH3, while ball-milled Pt/Al2O3 has a maximum at ~10,000 ppm. This unexpected enhancement of Pt/Al2O3 activity from milling results in a lowering of the light-off temperature (T50) by up to 100°C. Further examination rules out the extent of pore diffusion limitations as the root cause, but rather an increase in the fraction of stepped crystalline planes and destabilization of Pt oxide shown from XRD and H2-TPR characterization. The dependence of a shift in the rate maximum to higher NH3 concentrations with milling extent is shown to require single- and dual-site reaction pathways. The kinetic scheme also captures a subtle shift in the apparent NH3 reaction order for both unmilled and milled Pt/Al2O3. The microkinetic scheme is incorporated into fixed-bed and monolith reactor models which show excellent agreement with the NH3 conversion and product distribution data.

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO Oxidation on Pt/Ceria.

Pt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The state of Pt has often been proposed to differ depending on experimental conditions, for example, metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast to all previous reports, we show herein that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of the metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and platinum's evolving nature under reaction conditions.

Comparison of real driving emissions from Euro VI buses with diesel and compressed natural gas fuels

Public transport bus service is extended in big cities and cleaner bus fleets are encouraged to reduce greenhouse and air pollutant emissions in high populated areas. In this work, real driving emissions produced by two Euro VI-C buses, one fueled by diesel fuel and one fueled by compressed natural gas, were measured. Both vehicles with different engine technologies but with the same pollutant emission certification, were tested in Madrid (Spain), in order to compare their emissions under the same route. The buses were instrumented with a modular on-board Portable Emissions Measurement System, the OBS-ONE by Horiba. Regulated gaseous emissions and particle number were registered together with different vehicle parameters: speed, exhaust mass flow rate, air and fuel consumption, environmental conditions and global position coordinates during the route. THC emissions from CNG bus doubled those obtained with Diesel bus. Slightly higher NOx emissions from the Diesel bus, with selective catalytic reduction, were obtained during those parts of the circuits (mainly urban) where both the exhaust and the operation temperature of the after-treatment device were below the light-off temperature. However, the CNG bus emitted a greater number of particles, mainly on the rural sector of the circuit. Finally, NOx and particle number concentrations along the route showed that traffic is a determining factor in creating areas with high concentrations of pollutants.

Size effect in two-dimensional oxide-on-metal catalysts of CO oxidation and its connection to oxygen bonding: An experimental and theoretical approach

Transition metal oxide layers on metal surfaces often show superior catalytic performance as compared to conventional, i.e., metal-on-oxide, systems. In this work, we studied the CO oxidation reaction over FeO supported by Pt(111) and observed strong size effects on the reactivity. The monolayer islands below 5 nm in size showed a light-off temperature for CO2 production 200 K lower than for the 30 nm islands. In an attempt to rationalize the size effect, we have performed oxygen desorption experiments and have combined them with an extended DFT analysis to provide insight into the bonding of various oxygen species in such systems. The theoretical results on isolated FeO2 islands indicate a substantially lower stability of the boundary oxygen atoms as compared with the same islands embedded into an FeO layer in qualitative agreement with experiment showing that the smaller the island, the lower the desorption temperature. To the best of our knowledge, the results demonstrate the first example of a size effect in oxidation catalysis on two-dimensional systems.

Barium promoted manganese oxide catalysts in low-temperature methane catalytic combustion

In this study, a series of BaO-MnOx mixed oxide catalysts were synthesized by the mechanochemical method and employed in lean methane catalytic combustion (MCC) at low temperatures. The synthesized catalysts were characterized by XRD, BET, TGA, FT-IR, H2-TPR, O2-TPD, and FESEM analyses. The results indicated that the 10 wt% BaO-MnOx catalyst with a BET surface area of 25 m2 g−1 possessed the best catalytic performance. The higher activity of the 10 wt% BaO-MnOx catalyst was due to the higher ability to supply oxygen through the components during the MCC process. The light-off temperature corresponding to 50% of the methane conversion was about 330 °C, which was about 50 °C lower than the pure MnOx. Moreover, for the BaO(10)-MnOx catalyst, the 10 and 90% of methane conversion temperatures were about 305 and 427 °C, respectively. Also, the 10 wt% BaO-MnOx catalyst exhibited high catalytic stability under dry feed condition at 450 °C for 50 h. Furthermore, the influence of various parameters such as calcination temperature, feed ratio, GHSV, pretreatment condition, and presence of water vapor in the feedstock was studied on the catalytic performance.

Preparation of Pt/Al2O3 and PtPd/Al2O3 catalysts by supercritical deposition and their performance for oxidation of nitric oxide and propene

Pt/Al2O3 and bimetallic PtPd/Al2O3 catalysts were prepared via supercritical deposition method using supercritical carbon dioxide. The effects of Pt loading of Pt/Al2O3 and Pd addition to Pt/Al2O3 on particle size, particle size distribution (PSD) and activity for NO and C3H6 oxidation and C3H6-selective catalytic reduction (C3H6-SCR) were investigated. Pt/Al2O3 catalysts were prepared with Pt loadings of 0.6, 1.2 and 2.1 wt% and a bimetallic PtPd/Al2O3 catalyst was prepared with total metal loading of 1.4 wt% and Pt:Pd molar ratio of 1.3:1. A small fraction of the particles agglomerated after calcination at 550 °C. Around 98 % of the particles had an average particle size of ∼1 nm. The rest of the particles were larger and average size of these larger particles was ∼10 nm for monometallic catalysts and ∼6.5 nm for PtPd/Al2O3. All catalysts were found to be active for NO and C3H6 oxidation and C3H6-SCR reactions. NO oxidation performance of 1.2 wt% Pt/Al2O3 catalyst was the highest. C3H6 oxidation activity increased with increasing metal content. Light-off temperature for C3H6 oxidation shifted to higher temperature in the presence of NO, suggesting competitive oxidation of C3H6 and NO. Concentration profiles indicated that C3H6-SCR started when C3H6 conversion by oxidation reached 50 %; C3H6 was consumed both by oxidation and C3H6-SCR at higher conversions.

Oxidation of Dichloromethane over Au, Pt, and Pt-Au Containing Catalysts Supported on γ-Al2O3 and CeO2-Al2O3.

Au, Pt, and Pt-Au catalysts supported on Al2O3 and CeO2-Al2O3 were studied in the oxidation of dichloromethane (DCM, CH2Cl2). High DCM oxidation activities and HCl selectivities were seen with all the catalysts. With the addition of Au, remarkably lower light-off temperatures were observed as they were reduced by 70 and 85 degrees with the Al2O3-supported and by 35 and 40 degrees with the CeO2-Al2O3-supported catalysts. Excellent HCl selectivities close to 100% were achieved with the Au/Al2O3 and Pt-Au/Al2O3 catalysts. The addition of ceria on alumina decreased the total acidity of these catalysts, resulting in lower performance. The 100-h stability test showed that the Pt-Au/Al2O3 catalyst was active and durable, but the selectivity towards the total oxidation products needs improvement. The results suggest that, with the Au-containing Al2O3-supported catalysts, DCM decomposition mainly occurs via direct DCM hydrolysis into formaldehyde and HCl followed by the oxidation of formaldehyde into CO and CO2.