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A novel approach to optimize the cold start performance of Pd- and Pt-based three-way catalysts in stoichiometric natural gas engine tailpipes is presented, utilizing temperature-dependent frequency control of lambda switches. In comparison to Pt/Al 2 O 3 , Pd/Al 2 O 3 exhibits a lower optimal frequency at the same inlet temperature, which can be attributed to the differing noble metal-specific adsorption–desorption energies of CO and O, which can inhibit the catalytic activity by surface poisoning, lower exothermicity evolving due to its lower catalytic activity and its higher oxygen storage capacity. Catalyst operation at a fixed frequency during light-off resulted in improved pollutant conversion only within a specific temperature range and relevant NH 3 slip was observed at elevated temperatures compared to conventional stoichiometric steady-state operation. In contrast, controlling the frequency as a function of the catalyst inlet temperature enabled a substantial decrease of the light-off temperature of CH 4 and NO x and an increase in N 2 selectivity. [Display omitted] • T 50 of CH 4 and NO x conversion can be decreased by 60 K and 335 K on Pd/Al 2 O 3. • T 50 of CH 4 and NO x conversion can be decreased by 120 K and 210 K on Pt/Al 2 O 3. • Optimal frequency depends on different adsorption–desorption energies. • Optimal frequency depends on the applied gas mixtures. [ABSTRACT FROM AUTHOR]

• Ultrastable Pt/CeO 2 -Al 2 O 3 catalysts were designed using a Sabatier principle. • Anchor CeO 2 on penta-site rich γ-Al 2 O 3 nanosheets can mitigate CeO 2 sintering. • Optimal CeO 2 coverage on γ-Al 2 O 3 nanosheets can stop Pt sintering. • Pt clusters on CeO 2 "nano-islands" are the most active species for three-way catalyst. • Pt/60wt.%CeO 2 -Al 2 O 3 performs comparable with the state-of-the-art Rh catalysts. Designing Rh-free, especially Pt-only, three-way catalysts with improved low-temperature activity/stability is highly desirable. Herein, we demonstrate that ultrastable Pt/CeO 2 -Al 2 O 3 nanosheet catalysts can be obtained based on a Sabatier principle of metal-support interaction. Tuning the surface coverage of penta-site rich γ-Al 2 O 3 nanosheets (AlNS, weak adhesion to Pt) by CeO 2 "nano-islands" (strong adhesion to Pt) can lead to the synthesis of Pt/60 wt.%CeO 2 -AlNS that have lower light-off temperatures for CO, hydrocarbons, and NO compared to conventional Pt/CeO 2 and Pt/Al 2 O 3 catalysts by 100–200 °C and a similar performance with the state-of-the-art Rh-based catalyst. Incorporating CeO 2 on the surface of AlNS can retard the sintering of CeO 2 during harsh redox hydrothermal aging, associate with the strong interaction between CeO 2 "nano-island" and penta-site rich γ-Al 2 O 3 nanosheets. Moreover, the improved activity/stability of Pt/CeO 2 -AlNS catalysts can be attributed to tuning of the Pt detachment and migration from and back to the CeO 2 "nano-islands", respectively, that keeps Pt as nanoclusters on CeO 2 , the most active species for three-way catalyst applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Intra-catalyst species measurement was performed on NG-TWC under λ-modulation. • Impact of λ-modulation parameters on reaction pathways was investigated. • Lean-Rich phases cycle catalyst oxidation state and prevailing reaction pathways. • Lean-phase induced oxidized catalyst environment favors total oxidation reactions. • Gradient of catalyst oxidation state exits along the catalyst flow-axis. The performance of a three-way catalyst (TWC) in natural gas-powered vehicles is enhanced by periodic changes in air-to-fuel ratio (λ-modulation). The reaction networks and sequences inside the catalyst that facilitate such enhanced performance have not been extensively investigated. This work applied intra-catalyst measurements (SpaciMS) to analyze the transient spatiotemporal gas concentrations inside a Pd-based TWC to establish relationships between CH 4 , NO x , CO and H 2 conversion pathways. Steam reforming and partial oxidation were revealed to be the main CH 4 conversion routes. The cyclic rich-lean conditions combined with the oxygen storage capacity (OSC) of the TWC generate reduced and oxidized zones that are constantly moving within the catalyst, changing the dominant chemical reactions occurring on the surface. In the reduced zones, OSC is depleted while CH 4 is converted through steam reforming and NO x is converted through reactions with H 2 , CO, and other surface-bound reducing fragments formed by CH 4 conversion. In the oxidized zones, OSC is replenished, CH 4 is converted by partial oxidation, and H 2 , CO, and NH 3 are oxidized. The length of lean-rich phases impacts the catalyst performance significantly; too short or too long of a rich or lean phase can lower the overall conversion of reactive species. An inhibition of CH 4 conversion was observed during the rich phase possibly due to CO-poisoning of active sites. The intra-catalyst measurements revealed that the catalyst consists of three distinct reaction zones and their lengths vary with modulation conditions. Various modulation frequencies, amplitudes, λ-centers, and temperatures were investigated which allowed an understanding of how these parameters affect the reaction zones and catalyst utilization. Understandings from this work can enable adaptive λ-control strategies to optimize the overall TWC performance over a range of vehicle operating conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxygen isotope labeling and reaction quenching technique is applied to model TWC. • Oxygen storage process in model TWC is visualized with 100-nm-scale resolution. • Presence of Pd enhances surface 18O concentration and diffusion depth into CZ bulk. • Favored oxygen vacancy formation on CZ near the Pd cluster is supported by DFT. • Oxygen release/storage simulation provides reasonable local kinetic parameters. Elucidation of oxygen dynamics and transport in oxygen storage component of three-way catalyst (TWC) is a crucial issue for clarifying the reaction mechanism and precise kinetic modeling. Herein, we report a direct visualization of the oxygen storage process in a Pd/CeO 2 -ZrO 2 (CZ) via a combined method of oxygen isotope labeling and reaction quenching. 18O 2 is stored in a model TWC comprising a dense CZ substrate with Pd at 600 °C, the catalyst is quenched, and the 18O distribution in the surface and bulk is visualized by secondary-ion mass spectrometry with a 100-nm-scale spatial resolution. The results show that the presence of Pd enhances both the surface 18O concentration and the 18O diffusion depth into the CZ bulk. 18O concentration on the Pd surface is higher near the Pd/CZ interface, indicating that oxygen adsorption near the Pd/CZ interface is preferred to that close to the Pd center. The promoting effect of Pd on oxygen vacancy formation in CZ is supported by density functional theory calculations. In addition, the comparison between the 18O distribution and oxygen release/storage simulation based on diffusion equation provides the local oxygen release/storage rates. These are of the same order of magnitude and qualitatively consistent with the trend of conventional oxygen storage capacity measurements. The visualization technique reported here is a promising tool for directly investigating the oxygen dynamics and transport at the metal/oxygen storage material system from both qualitative and quantitative aspects. [ABSTRACT FROM AUTHOR]

[Display omitted] • O 2 -dithering improves CH 4 and NO conversion over Pd/Al 2 O 3. • Effect of O 2 pulses on the catalytic activity can be predicted reliably. • The prediction relies on the O 2 concentration present in the reactive feed. • The O 2 pulses are beneficial to the activity when CH 4 steam reforming is promoted. Under periodic lean/rich operation generated by repeated O 2 pulses (O 2 -dithering), the abatement of CH 4 and NO is enhanced compared to static operation. However, it is not well understood how the dithering parameters (pulse amplitude and frequency) affect CH 4 conversion, making it difficult to anticipate the impact of such pulses on the catalytic performance. Here, we show that the effect of O 2 -dithering on the activity of Pd/Al 2 O 3 can be reliably assessed as a function of dithering parameters, temperature, and exhaust gas composition. Operando X-ray absorption spectroscopy was used to support the conclusions based on considerations of the Pd oxidation state. Our work relies on explicit calculations of the O 2 concentration present in the reactive feed, which is based on the amount of O 2 pulsed to the system and the pollutants' conversion. Since NO contributes to the amount of oxygen available during reaction, its concentration and conversion need to be correctly evaluated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Deactivation mechanisms revealed for catalytic natural gas combustion over real-world TWCs. • Thermal aging induces redistribution of Pd species on Pd-based TWCs. • Sulfur exposure blocks active sites but also creates some new sites on Pd-based TWCs. Thermal aging and sulfur poisoning are major problems influencing the efficiency and lifetime of natural gas three-way catalysts (TWCs) during real-world operation. In the present study, thermal aging and sulfur-induced deactivations on CH 4 combustion were investigated over commercial monolithic honeycombs, i.e., Pd/Ce x Zr 1-x O 2 (CZ)-based TWCs that are thermally aged in the bench (HTA), followed by being operated behind the stoichiometric natural gas engine in high sulfur fuel (HTA + S). Both HTA and HTA + S samples show degraded performance in CH 4 combustion relative to the fresh one, which are caused by the reconstruction of Pd species induced by thermal aging and the decreased reducibility of the support. The 18O 2 labelling experiments show that in all cases CH 4 combustion proceeds via a MvK mechanism in which lattice O (O L) plays a key role. The sulfation aging (HTA + S sample) promotes the activation of CH 4 and O 2 by forming Pdδ+-(SO 4 2-)δ- and Ce3+-V O couples at the interface as compared to HTA sample. However, such a promotion effect could be compromised by the blocking effect of sulfate on the exchange of O L from the bulk to the surface and across the surface. The fundamental understanding of thermal and S deactivation mechanisms from this study could help to predict a real-world lifetime use curve of natural gas TWCs. [ABSTRACT FROM AUTHOR]

The overall reaction kinetics for a modern double-layered Pd/Rh bimetallic TWC has been developed by combining the activity functions derived separately for the Pd-based ( a Pd ) and Rh-based ( a Rh ) TWCs with their respective reference reaction kinetics. A 3D activity function for the commercial Rh-based TWC ( a Rh ) was developed on the basis of the metal surface area (MSA) of Rh, and subsequently used in combination with the detailed reaction kinetics of the Rh reference catalyst in predicting the catalytic performance variation with respect to the Rh loading from 2.5 to 15 g/ft 3 and the catalyst mileage from 4 k to 100 k miles obtained from the laboratory aging program. Finally, the catalytic performance of a modern double-layered Pd80/Rh10 TWC monolith reactor was predicted with a reasonable accuracy over a wide range of the catalyst mileage from 4 k to 100 k by using the overall reaction kinetics for Rh developed in this work along with that for Pd developed previously. The excellent capability of the overall reaction kinetic model in predicting the catalytic performance of the Pd/Rh bimetallic TWC is attributable to the successful combination of the accurate initial activity profiles of Pd and Rh at 4 k miles, the accurate characterization of the deactivation kinetics of Pd and Rh, the detailed surface reaction kinetics employed, and the spatial separation of Pd and Rh particles in a double-layered catalyst configuration. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reaction kinetics of Pd/Rh TWC was developed by combining kinetics of each catalyst. [•] Combined kinetics well predicted TWC performances, independent of washcoat location. [•] This may be understood by the thin catalyst layers less than 50μm. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of carbon monoxide and propylene oxidation as well as nitrous oxide reduction on a Pd–Rh catalyst were determined with synthetic gas mixtures between 500 and 700K. The overall reaction rate expression was constructed from elementary reaction steps and the kinetic parameters were determined from the experiments. A transient mathematical model has been developed to simulate automobile exhaust gas conversion based on the kinetics model equations. A comparison of simulated results with engine experiments proved to be successful. Results indicate that the reaction mechanisms proposed in this study are capable to describe the behavior of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary kinetic steps. [Copyright &y& Elsevier]

Abstract: This study concentrates on the external and internal mass transfer with multiple reactions in the catalytic layer of a three-way catalyst (TWC). A single channel model accounting for the species diffusion inside the washcoat using the effectiveness factor was developed. Validation and calibration of the model was achieved by comparing predictions against experimental data obtained previously by the same authors. The model was then applied to study the importance of both turbulent monolith structures and controlled washcoat structures on TWC conversions. The numerical results show that: (i) increasing the transport coefficients using turbulent monolith structures can produce either positive or negative effects on the TWC conversions; (ii) overall, the net effect of increasing the transport coefficients on the TWC conversions is positive; (iii) at high inlet gas temperatures and high space velocities the turbulent monolith structures present important improvements in the TWC conversions; (iv) the TWC conversions can be significantly improved enhancing the transport properties of the porous washcoat structure; (v) enhancements in the transport properties of the washcoat structure have deeper impacts on the TWC conversions than improvements in the monolith channel structure. [Copyright &y& Elsevier]

• Methane conversion pathway is analyzed in engine exhaust and model gas reactor. • CH 4 conversion efficiency is more than fivefold higher in transient than in steady. • In steady states, direct oxidation is the only CH4 conversion pathway. • Transient high conversion is due to activation of steam reforming. • Methane steam reforming diminishes with time due to surface carbonaceous species. Methane abatement pathways in Pd/Rh three-way catalysts have been investigated in three scales ranging from a vehicle application size catalyst, a model gas reactor and the catalyst in powder form. A special test rig was designed for the investigation of vehicle size catalysts, allowing sampling along the catalyst at discrete spatial locations, which are subject to different feed compositions. Dependent on the history of chemical environment of the catalyst, significant differences in methane conversion rate at identical feed have been identified. At steady state methane conversion rate was low and the reaction pathway was identified as limited to only direct oxidation by oxygen. Following a rich-to-lean transition, the catalyst exhibited more than 8 times higher methane conversion rates compared to steady state. The high methane conversion rates have been identified and attributed to the activation of methane steam reforming (SR) related to transient reduction of ceria. Methane SR efficiency decreased with time and the conversion rate finally converged to steady state levels. The findings were validated using a model gas reactor enabling analysis under well-defined feed compositions. The deactivation of SR was further analyzed with infrared spectroscopy (DRIFTS). Evidences from DRIFTS measurements showed that the deactivation was linked to the formation of carbonaceous species on the catalyst surface, most likely carbonates. The coherent results from engine exhaust analysis, model gas reactor and DRIFTS study give important insights in the activation and deactivation of methane reaction pathways. The results of this study suggest that catalyst formulation and operation strategies of methane conversion should focus on the stimulation of SR and the maintenance of catalyst activity towards SR through targeted periodic lean/rich transitions. [ABSTRACT FROM AUTHOR]