William S. Epling

The Effects of CO2 and H2O on the NOx Destruction Performance of a Model NOx Storage/Reduction Catalyst

The effects of CO2 and H2O on the NOx storage and reduction characteristics of a Pt/Ba/Al2O3 catalyst were investigated. The presence of CO2 and H2O, individually or together, affect the performance and therefore the chemistry that occurs at the catalyst surface. The effects of CO2 were observed in both the trapping and reduction phases of the experiments, whereas the effect of H2O seems limited to the trapping phase. The data also indicate that multiple types of sorption sites (or mechanisms for sorption) exist on the catalyst. One mechanism is characterized by a rapid and complete uptake of NOx. A second mechanism is characterized by a slower rate of NOx uptake, but this mechanism is active for a longer time period. As the temperature is increased, the effect of H2O decreases compared to that of CO2. At the highest temperatures examined, the elimination of H2O when CO2 is present did not affect the performance.

Selective Catalytic Reduction of NOx with NH3 over a Cu-SSZ-13 Catalyst Prepared by a Solid-State Ion-Exchange Method

A solid‐state ion‐exchange method was developed to synthesize Cu‐SSZ‐13 catalysts with excellent performance in the selective catalytic reduction of NOx with NH3 (NH3‐SCR) and with durable hydrothermal stability. Experiments provide evidence that isolated Cu ions were successfully exchanged into the pores, which are the active centers for the NH3‐SCR reaction.

Effects of CO on Pd/BEA Passive NOx Adsorbers

Cold-start emissions significantly contribute to total vehicular pollutant emission profiles. There has therefore been a significant amount of research focused on catalytic aftertreatment systems that can mitigate CO, hydrocarbon and NOx emissions at low temperatures. Recently, passive NOx adsorbers (PNAs) have been developed, and these can store or trap NOx at low temperature and are designed to release the trapped NOx at higher temperature ranges, where downstream NOx reduction catalysts are active. Pd/BEA is reported to be one such PNA. In this study its NOx storage at low temperatures and release at higher temperatures was characterized using temperature programmed (TP) experiments and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The influence of CO was investigated and was shown to improve the amount stored at low temperature and also induce higher temperature desorption, in a range that is applicable for downstream NOx reduction. Without the reductant, NOx release primarily occurred at temperatures too low to be practically relevant.Graphical Abstract

Reaction Kinetics of C3H6 Oxidation for Various Reaction Pathways Over Diesel Oxidation Catalysts

Reaction kinetics studies of C3H6 oxidation over Pt/Al2O3 and Pt/SiO2 catalysts were characterized using temperature-programmed oxidation with different oxidants: O2, NO2 and surface nitrates. Activation energies and conversion performance were compared in order to determine which hydrocarbon oxidation reaction pathway(s) is relevant in diesel exhaust gas aftertreatment applications. NOx adsorption did not occur on the SiO2 surface so the reaction between C3H6 and NO2 could be isolated, i.e. no nitrate effect would complicate the analysis and their significance could be decoupled. These results were then compared with Pt/Al2O3 where surface nitrates did form upon exposure to NOx. The onset of C3H6 oxidation was observed at a lower temperature with O2 than with NO2, but the activation energy was lower with NO2. This apparent discrepancy is related to the different oxidant concentrations used and the different adsorption pathways. The results indicate that hydrocarbons must be activated first for oxidation to begin, for either NO2 or O2. In analyzing the reaction between C3H6 and nitrates, the reaction did not occur until NOx started to desorb from the catalyst at higher temperatures, i.e. when nitrates became unstable and decomposed, thus providing a readily available oxidant source. However, when O2 was added to the nitrate/C3H6 system, the reaction began at even lower temperature than with just C3H6 and O2. Nitrate consumption was also observed once oxidation began. The presence of the combination of nitrates and O2 resulted in a lower C3H6 oxidation activation energy.

Effect of SO2 on NH3 oxidation over a Cu-SAPO-34 SCR catalyst

The impact of SO2 on NH3 oxidation, an undesired side reaction that can occur during NH3-SCR, was studied using Cu-SAPO-34. Based on kinetic analysis, monomeric Cu++ was active toward NH3 oxidation. In terms of the SO2 effect, significant loss in NH3 oxidation activity was observed at temperatures below 350 °C. However, at higher temperatures, SO2 had no significant effect on NH3 oxidation, which corresponds to the lack of an observed effect on SCR performance. Temperature-programmed-desorption (TPD) data obtained after NH3 and SO2 exposures demonstrate that SO2 inhibited monomeric copper NH3 oxidation activity through formation of ammonium sulfate, whose formation was catalyzed by the Cu. The monomeric copper regained NH3 oxidation activity after ammonium sulfate decomposed.

Hydrocarbon Trapping over Ag-Beta Zeolite for Cold-Start Emission Control

Toluene, hexane, ethane, ethylene and CO adsorption and desorption on/from model Ag-exchanged beta zeolites were evaluated. The presence of water significantly inhibited adsorption of ethylene below 100 °C, whereas hexane and toluene adsorption were not affected. The addition of Ag increased adsorption capacities and desorption temperatures for both ethylene and toluene, while no difference was observed for hexane. Under conditions containing all hydrocarbons, ethylene adsorption was absent, and the toluene adsorption rate decreased. Results with binary mixtures demonstrate the inhibition of ethylene adsorption is due to toluene and the slower rate of toluene adsorption is caused by hexane.Graphical Abstract

Study of NO Formation During NH3 Oxidation Reaction Over a Cu-SAPO-34 SCR catalyst

Ammonia oxidation to N2 over a Cu-SAPO-34 SCR catalyst was studied with the intent of determining if the mechanism is the direct oxidation of NH3 to N2 or proceeds through a two-step pathway where NH3 is oxidized to NO, with the NO then reacting with available NH3 through the SCR reaction to produce N2. NH3 oxidation data show that some NO is indeed formed and the NO selectivity is dependent on the amount of NH3 adsorbed on the surface, and thus on the adsorption time and temperature. This is due to the ratio of NO formed to adsorbed NH3 downstream along the monolith channel. NO selectivity increased as the amount of NH3 adsorbed on the catalyst surface decreased. Overall, the data demonstrate that NH3 to N2 can occur via the NO intermediate pathway, with subsequent reduction by NH3 to N2.Graphical Abstract

Coupled Heterogeneous and Homogeneous Hydrocarbon Oxidation Reactions in Model Diesel Oxidation Catalysts

In testing model diesel oxidation catalysts to treat exhaust from low temperature combustion (LTC) vehicle engines, the simulated exhaust conditions were such that both homogeneous and heterogeneous oxidation of some hydrocarbons took place. When homogeneous oxidation occurred, NO was readily oxidized to NO2 and the larger hydrocarbon species were partially oxidized to aldehyde, alcohol, and alkene intermediates. The homogeneous oxidation potentials of several hydrocarbons were compared in the absence and presence of a model oxidation catalyst. Of the hydrocarbons evaluated, diethyl ether led to the best NO to NO2 oxidation in terms of temperature and extent of conversion, and it did not inhibit the oxidation of other hydrocarbon species. The clear evidence of homogeneous reactions occurring within the monolith catalyst channels, and the formation of associated reaction intermediates, may impact reactions that would normally be considered isolated to the catalyst surface. These findings demonstrate that, in modeling such reactions, the homogeneous reactions need to be considered and may influence future catalyst design, especially considering the importance of NO2 for downstream selective catalytic reduction and particulate filter catalyst systems.

A Summary of Sulfur Deactivation, Desorption, and Regeneration Characteristics of Mono- and Bimetallic Pd-Pt Methane Oxidation Catalysts: Pd:Pt Mole Ratio and Particle Size Dependency

Complete CH4 oxidation (combustion) studies were conducted with fresh (small particle size) and sintered (large particle size) Pd/Pt bimetallic, Al2O3 supported catalysts before and after exposure to SO2. Temperature-programmed oxidation, reduction, and desorption as models for potential catalyst regeneration were evaluated in terms of CH4 oxidation performance recovery. Temperature-programmed desorption studies show that Pd/Pt catalysts with little Pt substitution or small particle sizes tended to form aluminum sulfate species at low temperatures. Aluminum sulfate species thermally decompose at high temperatures, thus requiring high-temperature conditions to recover catalytic activity lost due to sulfate formation. In contrast, Pd/Pt catalysts with higher Pt content or larger particle sizes were less effective at sulfate formation at low temperatures. In this case, low-temperature decomposing sulfur species inhibited the CH4 oxidation reaction over a broader temperature range. For Pd/Pt catalysts with high Pt content and small particle size, the associated sintering effects from the temperature-programmed reduction and desorption methods were more detrimental to catalytic activity than the sulfur exposure used in this study. Sintering bimetallic samples increased the particle size and provided some resistance to further sintering. Sintered, SO2-exposed Pd/Pt catalysts with low Pt content recovered all activity via temperature-programmed desorption regeneration, whereas those with high Pt content catalysts only recovered some activity. Regardless of particle size, the effectiveness of the temperature-programmed desorption regeneration method decreased with increasing Pt content.