Application of potassium ion deposition in determining the impact of support reducibility on catalytic activity of au/ceria-zirconia catalysts in CO oxidation, NO oxidation, and C3H8 combustion

The purpose of the study was to show how a controlled, subtle change of the reducibility of the support by deposition of potassium ions impacts the activity of gold catalysts. Since the activity of supported gold catalysts in carbon monoxide oxidation is known to strongly depend on the reducibility of the support, this reaction was chosen as the model reaction. The results of tests conducted in a simple system in which the only reagents were CO and O2 showed good agreement with the CO activity trend in tests performed in a complex stream of reagents, which also contained CH4, C2H6, C3H8, NO, and water vapor. The results of the X-ray Diffraction (XRD) studies revealed that the support has the composition Ce0.85Zr0.15O2, that its lattice constant is the same for all samples, and that gold is mostly present in the metallic phase. The reducibility of the systems was established based on Temperature Programmed Reduction (TPR) and in situ XRD measurements in H2 atmosphere. The results show that the low temperature reduction peak, which is due to the presence of gold, is shifted to a higher value by the presence of 0.3 at% potassium ions on the surface. Moreover, the increase of the potassium loading leads to a more pronounced shift. The T50 of CO oxidation in the simple model stream was found to exhibit an excellent linear correlation with the maximum temperature of the low temperature reduction peak of Au catalysts. This means that stabilizing oxygen with a known amount of potassium ions can be numerically used to estimate the T50 in CO oxidation. The results in the complex stream also showed a similar dependence of CO conversion on reducibility, though there was no substantial difference in the activity of the catalysts in other reactions regardless of the potassium loading. These studies have shown that the influence of potassium varies depending on the reaction, which highlights differences in the impact of reducibility and importance of other factors in these reactions

Surface Intermediates, Mechanism, and Reactivity of Soot Oxidation

Factors that may govern diesel particulate matter (DPM) oxidation at low temperatures (~200°C) were studied using reactivity and TP-ToFSIMS analysis. Best-case scenarios that give maximum gasification rates were determined for DPM impregnated with KOH and non-catalyzed DPM using temperature programmed oxidation and isothermal experiments. Conditions of intimate catalyst-carbon contact (K/C molar ratio=1/50) and high NO2 concentrations (1%) to improve the reactivity of the carbon reactive sites were unable to meet the steady state gasification rate needed for particulate filter regeneration for a modern diesel engine at 200°C. Oxygen-free thermal annealing (>500°C) caused reactivity losses of a maximum of 40% that correspond to changes to surface morphology and/or concentration of oxygen-containing functional groups. TP-ToFSIMS identified surface functional group changes with temperature on non-dosed and NOX pre-dosed (1.5%NO, 1%NO2, 4.5%O2, balance helium) diesel soot and sucrose char. Detailed analysis of the NOX dosed sucrose char spectra using both inspection and principal component analysis techniques revealed that the 1200 ion fragments created could be reduced to five sets of ions that are chemically and kinetically distinct. These sets presumably represent surface functional groups on the carbon. For example, Set IV may represent carboxylic acid, lactone, or carboxylic anhydride functional groups. Based on these results a mechanism for the surface reaction of NO2 with carbon under vacuum conditions was postulated. At temperatures less than 200°C the ion fragments contain primarily carbon-NO2 type ions. As temperature increases between 200 and 400°C the ion fragments are primarily carbon-NO and carbon-N type fragments. At higher temperatures (>500°C) the surface is enriched with nitrogen containing functional groups. A surface reaction mechanism is proposed where NO2 is bonded to an armchair site and with increasing temperatures and molecular rearrangements the N is incorporated into the carbon ring. The initial surface composition of NOx containing functional groups changes within the area of relevance of low temperature soot regeneration (i.e. between 25° and 200°C). Further studies are needed to understand the effect of N-incorporation on carbon reactivity. No rate processes either in reactor studies or based on surface functional groups met the rate criteria for low temperature DPM oxidation.Ph

ICEF2006-1538 FLOW-THROUGH FILTER TECHNOLOGY FOR HEAVY DUTY DIESEL ENGINES

ABSTRACT Reducing particulate emissions from diesel engines has become a major challenge for regions of Europe, Japan and the United States. Many mobile applications have been successfully addressed with passively regenerating wall flow filters. However stationary engines, locomotives and other large constant speed engines often require a different approach to particulate filtration. Flow-through filter technologies have merit for these applications due to their low maintenance requirements, tolerance to misfueling and suitability for engines with high specific PM emissions. When considering the application of a particulate filter to any diesel engine the means of regeneration, or combustion of the accumulated soot, is of critical importance. In the case of filters which are regenerated through the use of a catalytic coating the duty cycle of the engine, and characteristics of the exhaust gas itself dictate the potential success or failure of the system. In many cases interruption of operation, whether due to insufficient regeneration rates, or for scheduled service to remove accumulated ash, is relatively more difficult to accept for locomotive and non-mobile engine operations. Locomotives, power generators and the like often accumulate large number of service hours between scheduled maintenance events and perform tasks where interruption of service can have costly consequences. Details of an investigation into the suitability of a flow-through filter for heavy-duty constant speed engines are presented. Aspects of the design, including materials selection, catalyst coating and performance under various conditions are discussed. Results from CFD and micro-dilution tunnel particulate sampling of full-scale devices support the progressive refinement of the design

CFD Modelling of 3-Way Catalytic Converters with Detailed Catalytic Surface Reaction Mechanism

Copyright © 2004 SAE International This paper presents a 3-D CFD modelling of flow and heterogeneous reactions in catalytic converters. The pressure and velocity fields in the catalytic converters are calculated by the state of the art modelling technique for the flow resistance of catalyst substrate. A surface reaction model is applied to predict the performance of a three-way Pt/Rh catalyst. A reaction mechanism with detailed catalytic surface reactions for the 3-way catalyst is applied. The novelty of this approach is the use of a surface chemistry solver coupled with a 3-D CFD code in the entire computational domain of the catalyst substrate that allows flow distribution for complex configurations to be accounted for. The concentrations of the gas species and the site species are obtained. A comparison between the simulation results and the experimental data of a three-way catalyst was made

Activity of Ag/CeZrO2, Ag+K/CeZrO2, and Ag-Au+K/CeZrO2 Systems for Lean Burn Exhaust Clean-Up

Herein, the activity of Ag and bimetallic Au-Ag catalysts, supported over Ce0.85Zr0.15O2 (CZ), was investigated in a complex stream, whose components included CO, C3H8, NO, O2, and, optionally, an injection of water vapor. In such a stream, three of the possible reactions that can occur are CO oxidation, propane combustion, and NO oxidation. The aim of these studies was to explore whether silver, due to its strong affinity to oxygen, will counteract the stabilization of oxygen by potassium. The effect of the presence of potassium ions on the activity of the monometallic silver catalysts is beneficial in the complex stream without water vapor in all three studied reactions, although it is negligible in the model CO stream. It has been shown that water vapor strongly suppresses the activity of the Ag+K/CZ catalyst, much more so than that of the Ag/CZ catalyst. The second purpose of the work was to determine whether the negative effect of potassium ions on the activity of nanogold catalyst can be countered by the addition of silver. Studies in a model stream for CO oxidation have shown that, for a catalyst preloaded with gold, the effect of potassium is nulled by silver, and the activity of AuAg + 0.15 at%K/CZ and AuAg + 0.30 at%K/CZ is the same as that of the monometallic Au catalyst. Conversely, when the reaction is carried out in a complex stream, containing CO, C3H8, NO, O2, and water vapor, the presence of water vapor leads to higher CO conversion as well as increased NO2 formation and slightly suppresses the C3H8 combustion

Reducibility Studies of Ceria, Ce_0.85Zr_0.15O₂ (CZ) and Au/CZ Catalysts after Alkali Ion Doping: Impact on Activity in Oxidation of NO and CO

The aim of these studies was to perform thorough research on the influence of alkali metal ions (Li, Na, K and Cs) on the properties of nanogold catalysts supported on ceria–zirconia. The addition of alkali metal ions onto CeO2 further affected the reducibility, which was not noted for the Zr-doped support (Ce0.85Zr0.15O2). Despite the substantial impact of alkali metal ions on the reducibility of ceria, the activity in CO oxidation did not change much. In contrast, they do not have a large effect on the reducibility of Au/CZ but suppressed the activity of this system in CO oxidation. The results show that for CO oxidation, the negative effect of potassium ions is greater than that of sodium, which corresponds to the shift in the Tmax of the reduction peak towards higher temperatures. The negative effect of Li+ and Cs+ spans 50% CO conversion. The negative effect was visible for CO oxidation in both the model stream and the complex stream, which also contained hydrocarbons and NO. In the case of NO oxidation to NO2, two temperature regimes were observed for Au + 0.3 at% K/CZ, namely in the temperature range below 350 °C; the effect of potassium ions was beneficial for NO oxidation, whereas at higher temperatures, the undoped gold catalyst produced more NO2