Carolyn Parks Hubbard

Raman spectroscopy of palladium oxide on γ-alumina applicable to automotive catalysts: Nondestructive, quantitative analysis; oxidation kinetics; fluorescence quenching

Abstract Raman scattering is used to analyze a series of model catalysts consisting of γ-alumina powder with up to 20 wt.-% Pd, and two monolithic catalyst samples with palladium concentrations typically employed in automotive emission control. By using the 514.5 nm Ar laser line to take advantage of a resonant Raman effect, the 651-cm −1 Raman line in PdO can be detected in the oxidized catalysts with palladium loadings as small as 0.2 wt.-%. A strong fluorescence signal is observed in reduced pure γ-Al 2 O 3 . This signal is quenched by the addition of palladium, admixed either from aqueous solution or from Pd black, provided the palladium was allowed to interact with the γ-Al 2 O 3 during at least one oxidation treatment at 600°C, followed by reduction in hydrogen at 300°C. Raman spectroscopy is found to be applicable for a quantitative, nondestructive analysis of PdO on γ-alumina, including automotive formulations, within certain limitations. At low concentrations, some palladium may interact with the support and thus may not contribute to the signal associated with crystalline PdO. The analysis is also suited for studying sintering and reduction/oxidation kinetics in situ.

Stabilization of the ethane oxidation catalytic activity of Cu-ZSM-5

Abstract The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Treatment by steam at 630°C for 17 h or high-temperature dry calcination at 850°C induce an irreversible change in coordination for practically all square-planar Cu2+ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu2+ ions remain accessible to gas-phase molecules, but the catalytic reactivity of these altered copper sites decreases drastically. A stabilizing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part of the copper ions (20–30%) preserves the parent square-planar Cu2+ state even after calcination at 850°C for 0.5 h. The effect of ca. 1% La or Ce is much less pronounced. The catalytic activity in the complete oxidation of ethane correlates well with the number of square-planar cupric cations retained by the samples after different treatments. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500–650°C.

In situ ESR of CrH-ZSM-5 up to 500°C in flowing gas containing O2, H2O, CCl4, NO, NO2, and C3H6

A flow cell was used for the in situ ESR monitoring of the state and reactivity of chromium ions in Cr-ZSM-5. Calcination of Cr(NO3)3/NH4-ZSM-5 in air at 500°C is accompanied by migration of chromium ions inside the zeolitic channels and stabilization ofisolated Cr5+ cations near lattice A13+ ions. Calcination of Cr-ZSM-5 at 750°C leads to a gradual disappearance of the isolated Cr5+ cations and formation of α-Cr2O3 microcrystals. All the Cr5+ cations are accessible to gas-phase molecules: O2 strongly broadens the dipole-dipole signal; H2O sorption increases the local crystal field symmetry; admission of CCl4 results in a small change of the Cr5+ local coordination; strongly stabilized complexes on Cr-ZSM-5 are observed upon sorption of either NO or NO2. The sorption of C2H6 on Cr-ZSM-5 at 20°C is accompanied by a gradual reduction of the Cr5+ sites. At 500°C in [C3H6 + O2 + He] flow, even at a large excess of oxidant, the reduction of a noticeable part of Cr5+ ions takes place. At 400°C, in the same gas mixtures, a deeper reduction of Cr5+ occurs. Closer to stoichiometric conditions, in a [C3H6 + NO + He] flow with 120% excess of oxidant the Cr5+ is completely reduced at 500°C. The oxidation of propene is accompanied by coke deposition on the surface of the catalyst. The implications of the results are discussed.

Modification and stabilization of Cu-ZSM-5 by introduction of a second cation

Publisher Summary Cu-ZSM-5 has been extensively investigated in the low-temperature decomposition of NO to dements, in the selective catalytic reduction (SCR) of NOX in excess oxygen by organic compounds, and in the total oxidation of alkanes. In the past few years, Cu-ZSM-5 has been examined by a multitude of physical methods but the characterization often yields contradictory results about the changes in the coordinative and valence states of the copper after treatment in different gas mixtures. Hence, the nature of the active sites in Cu-ZSM-5 catalysts has not been completely elucidated. Little is also known about the mechanism of Cu-ZSM-5 deactivation and the stabilizing effect of promoters. This chapter summarizes the results of investigation of H-ZSM-5 containing copper and one other cation (Ba, Ga, La, Ce, Co, Nb). The results in the chapter provide evidence of a noticeable influence of different additives on treatment-induced bonding change between the isolated Cu2+ cations and tile framework oxygens of H-ZSM-5 linked, in turn, to the Al3+ ions.

Effects of MMT® Fuel Additive on Emission System Components: Comparison of Clear- and MMT®-fueled Escort Vehicles from the Alliance Study

Emission studies were carried out on clear-fueled and MMT-fueled 100,000-mile Escort vehicles from the Alliance study [SAE 2002-01-2894]. Alliance testing had revealed substantially higher emissions from the MMTfueled vehicle, and the present study involved swapping the engine cylinder heads, spark plugs, oxygen sensors, and catalysts between the two vehicles to identify the specific components responsible for the emissions increase. Within 90% confidence limits, all of the emissions differences between the MMTand Clearvehicles could be accounted for by the selected components. NMHC emission increases were primarily attributed to the effects of the MMT cylinder head and spark plugs on both engine-out and tailpipe emissions. CO emission increases were largely traced to the MMT cylinder head and its effect on tailpipe emissions. NOx emission increases were linked to the MMT catalyst. In addition to heavy deposits on the valves and spark plugs from the MMT vehicle, substantial deposits were also observed on the catalyst, with approximately 20% of the channels totally blocked.

The Effect of Different Ageing Conditions on Spatial Variations in Emissions Across the Radius of a Close-coupled After-treatment System

Using local emissions measurements immediately downstream of a close-coupled catalyst, spatial variations in emissions have been analysed for close-coupled catalysts with different ageing histories. Comparison of the radial emissions profiles between a uniformly-aged (oven-aged) catalyst and two vehicle-aged parts suggests that the vehicle-aged parts have substantial variations in catalyst damage across the radius of the catalyst. The radial variations in damage were confirmed by bench reactor and post-mortem studies. The radial catalyst damage profiles inferred from engine-based evaluations of vehicle aged catalysts show broad correlation with high flow areas identified by CFD predictions and high temperature regions as measured during engine tests. Assessment of the results obtained from CFD analyses, engine-based evaluations, bench reactor evaluations and post-mortem suggests that flow non-uniformity creates high thermal gradients, leading to localised damage of the brick, thus degrading overall catalyst performance.