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Platinum Group Metal-based Sustainable Catalysts for Emission Control & Efficient CH4 Conversion:
08:00am - 11:25am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
Division/Committee: [ENVR] Division of Environmental Chemistry

Platinum group metals (PGMs) are at the core of most catalytic processes ranging from the synthesis of fine chemicals to large scale production of commodity substrates and products. The transportation sector is the major consumer of PGMs. As trends move toward electrifying transportation, changes are expected in PGM synthesis, use and recycling/sustainability. This trend and resulting marketing factors drive the need for exploring sustainable approaches to using these metals efficiently, including PGM dilution and substitution during synthesis. Not only synthetic aspects of such materials need to be understood but also sustainable chemistries such as efficient conversion/activation of CH4 in addition to existing emission control challenges associated with – and N–oxide abatement must be increasingly addressed, especially as methane is a rising concern. This symposium invites contributions on any of these topics from researchers across academia, national laboratories, and industrial sectors for facilitating a well-rounded discussion of the field. Work can be both experimental and theoretical in nature.

Thursday
Introductory Remarks
08:00am - 08:05am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid

Thursday
3739869 - Effect of Pd speciation on CH4 oxidation and passive NOx adsorption/desorption performance over zeolite-based catalysts
08:05am - 08:50am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
Pd-based zeolites are versatile catalysts in automotive exhaust aftertreatment as passive nitrogen oxide (NOx) adsorbers (PNAs) and methane (CH4) oxidation catalysts (MOCs). The versatility is derived from the Pd speciation facilitated by the zeolite framework in the forms of ion-exchanged Pd and PdO nanoparticles. Ion-exchanged Pd species are identified to be the adsorption sites for PNAs, whereas PdO nanoparticles are the active sites for CH4 oxidation. Maintaining the desired Pd species is essential for ensuring stable long-term performance in both PNAs and MOCs; however, Pd speciation is dynamic under long term exposure to ambient conditions. Herein, 0.5 wt.% Pd/zeolite catalysts were synthesized as Pd(0.5)/BEA (Si/Al = 12.5), Pd(0.5)/ZSM-5 (15), and Pd(0.5)/SSZ-13 (12.5) and aged under ambient conditions. Evaluation of the PNA and CH4 oxidation performance of the fresh and aged catalysts was used to probe ion-exchanged Pd and PdO nanoparticles, respectively. The NOx adsorption capacity of fresh Pd(0.5)/BEA (NOx/Pd molar ratio = 0.78) decreased after aging (NOx/Pd = 0.25), while the CH4 conversion at 600 oC improved from 10 to 60%. The probe reactions indicate a decrease in ion-exchanged Pd and an increase in PdO nanoparticles of the fresh and aged Pd(0.5)/BEA, respectively. Pd speciation was also monitored using H2-TPR for ion-exchanged Pd and O2-TPD for PdO. CO-diffuse reflectance FT-IR spectroscopy (DRIFTS) of the fresh and aged Pd(0.5)/BEA catalyst (Fig. 1) showed a decrease in ion-exchanged Pd at 2134 cm-1 and an increase in pore confined Pd ions at 2213 and 2192 cm-1. The presence of the pore confined Pd ions implies solvation by H2O of the ion-exchanged Pd and detachment from the zeolite framework to become mobile. The mobility of Pd as promoted by H2O is a possible mechanism for the increased formation of PdO nanoparticles suggesting that ambient moisture plays a role in Pd speciation.
<b>Figure 1.</b> CO-DRIFTS of fresh and aged Pd(0.5)/BEA (Si/Al = 12.5) catalyst.

Figure 1. CO-DRIFTS of fresh and aged Pd(0.5)/BEA (Si/Al = 12.5) catalyst.


Thursday
3754099 - Kinetic and Thermodynamic Factors Influencing Palladium and Platinum Nanoparticle Redispersion into Mononuclear Cations in Zeolite Supports
08:50am - 09:15am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
While there is a lot of interest in Platinum group metal single atom catalysts and subnanometric clusters for CH4 oxidation, Wacker oxidation, and passive NOx adsorption (PNA), the dynamic transformation between metal particles and isolated atoms is not well understood. Under reducing conditions, Pd cations exchanged in zeolites sinter to particles, causing the loss of active sites for Wacker oxidation and PNA, but regeneration is feasible via redispersion of Pd agglomerates by NO or hydrothermal treatment. Redispersion of Pt particles in oxidizing conditions causes loss of catalytic activity for CH4 oxidation, while reduction treatments facilitate agglomeration. An in-depth understanding of the structural interconversion between cations and particles is thus essential for assessing stability and regeneration of these materials. Here, we use experiments and computational modeling to explore the thermodynamic and kinetic factors influencing the redispersion of Pd and Pt particles in zeolites. Using density functional theory calculations and thermodynamic models, we analyzed the effect of particle size and reaction environment (gas pressures, temperature) on the extent of ion-exchange. In dry air, it is thermodynamically feasible to completely redisperse Pd particles of all sizes. However, experiments showed partial redispersion of larger particles, suggesting kinetic limitations. Increased H2O pressures led to a shift in the thermodynamic equilibrium, favoring PdO formation from cations at T< 800K, evincing its deteriorating effect for PNA. Results from wet air treatments of synthesized Pd particles showed reduced cation exchange as well; also, we noted that samples with exchanged cations showed a propensity for PdO agglomeration. Redispersion rate experiments revealed particle-size dependence, with small particles redispersing faster. Kinetic Monte Carlo (kMC) simulations for Pd redispersion via support-mediated Ostwald ripening and subsequent atom trapping at Brønsted acid sites, accurately captured the observed size-dependent kinetics. Our kMC results indicated faster redispersion of monodispersed particles than those with log-normal distribution where the long tail of larger particles disintegrated the slowest. Analogous theoretical study of Pt/zeolites revealed similar thermodynamic results, with our kinetic model for gas-phase Ostwald ripening and atom trapping being qualitatively consistent with previously reported experimental studies.
Thursday
3742277 - Effect of water and CO on the NOx operating cycle in Pd/CHA passive NOx adsorbers
09:15am - 09:40am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
H2O and CO affect the adsorption/desorption temperature and storage capacity of Pd/CHA passive NOx adsorbers. The presence of various Pd sites (Pd2+, Pd1+, [PdOH]+, and [Pd-O-Pd]2+) makes it challenging to decipher the mechanisms of the adsorption-desorption cycle. Density functional theory (DFT) simulations are performed to understand the impact of H2O and CO on NO interaction with various Pd sites, and explain the mechanisms of NO adsorption, oxidation, and desorption in their presence. Calculations show that while H2O has a negligible effect on the NO adsorption energetics for various Pd sites (Pd2+, Pd1+, and [Pd-O-Pd]2+), it significantly weakens the NO binding on [PdOH]+. However, the presence of CO strengthens NO binding on [PdOH]+ with pre-adsorbed H2O. The presence of H2O facilitates the reduction of [PdOH]+ and [Pd-O-Pd]2+ by NO but negatively impacts the desorption of NO2 (formed by NO oxidation) from the Pd1+ sites (Edes = 198 kJ/mol in the presence of H2O as compared to 123 kJ/mol in its absence). In addition, CO is shown to be a better reductant as compared to NO when both CO and NO are co-adsorbed on [Pd-O-Pd]2+, thus resulting in the formation of Pd1+. The Pd1+ sites can reoxidize to PdII species under an oxidizing atmosphere. However, the presence of H2O favours NO oxidation over the reoxidation of Pd1+. The high NO2 desorption energy along with inhibition of Pd1+ reoxidation in the presence of H2O can explain the experimentally-observed increase in the NOx desorption temperature in the presence of H2O.
Thursday
Intermission
09:40am - 09:50am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid

Thursday
3743092 - Novel zeolite-based Cu-compositions for synergistic oxidation and hydrocarbon trapping behaviors
09:50am - 10:15am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
Ever-increasing and dynamic prices of platinum group metals (PGMs) are major driving force toward exploring alternatives for catalytic processes including emission control reactions. While catalytic versatility of PGMs is unapparelled, their price volatility poses serious challenges both from sustainability and economic considerations. Deactivation issues upon prolonged usage is yet another bottleneck specifically so under both industrial and real-world reaction conditions. One of the approaches for simultaneously addressing such issues is via PGM-dilution. Two or more metals can be strategically selected yielding hybrid compositions which may exhibit catalytic behaviors superior to the monometallic counterparts as well as better long-term durability.
Scope of this study is to explore applicability of Cu-based bi- and tri-metallic compositions for oxidation of CO and hydrocarbons (HCs), and for attaining desired trapping behaviors for NO and HCs under real-world conditions. The specific goal is to attain ≥ 90% conversion of criterial pollutant gases by 150 °C under the simultaneous presence of oxides of nitrogen and carbon as well as hydrocarbons, and water. Firstly, we studied the oxidation performance of Pt50Cu50/SSZ-13 catalysts and observed promising oxidation behavior for both CO and HCs with T90 being ~180 °C in both the cases. This strategy of ours was derived from our ongoing work using PdCu/SSZ-13 passive NOx adsorbers which exhibit remarkably higher, ~2x higher activity than pure Pd/SSZ-13. Pt50Cu50/SSZ-13 catalyst was however unstable upon hydrothermal aging at 800 °C for 25 h leading in almost quantitative activity decline. Based on our previous studies where we had observed beta-zeolites being hydrothermally more robust than chabazite analogues, evaluation of Pt50Cu50/BEA was carried out next. Improved catalytic behaviors were observed in this case however, >50% activity deterioration was again observed upon hydrothermal aging at 800 °C for 25 h. To solve such hydrothermal aging-induced activation decline, trimetallic PdPtCu/BEA combinations were evaluated next. Results obtained from this novel trimetallic composition will be presented along with a comparative analysis of activity/trapping behavior changes occurring upon hydrothermal aging of our Cu-based traps and catalytic materials.

Thursday
3755079 - Modification of three-way catalyst (TWC) formulation for gasoline vehicles with the goal of improving its thermal stability
10:15am - 10:40am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
Three-way catalysts (TWCs) deactivation due to exposure to severe operating conditions has been an important challenge in automotive catalysis. Thus, development of TWCs with enhanced thermal stability is of great interest. A few studies have focused on the effect of promoters on the TWC activity and thermal stability at the temperature of 1050°C. In most of the previous studies, simplified catalyst formulations and operating conditions have been employed, thus their results may not be realistic for commercial applications. In addition, the effect of promoters on the textural properties and stability of ceria-zirconia-based TWCs are not well understood. This work aims to investigate the effects of rare earth metals, as promoters, on the properties of ceria-zirconia-based supports under practical operating conditions of TWCs. A set of commercial support formulations, including ceria, zirconia, alumina, La, Y and Nd, were selected based on their specific surface area before and after aging. The Euro V aging protocol using 10% H2O in air at 1050°C for 8 h was employed for aging. The selected supports were impregnated with Pd (1 wt%), Pt (2 wt%), and Rh (0.2 wt%). The properties and performance of the fresh and aged samples were investigated using XRD, H2-TPR, BET, FESEM, and reactor tests. For the alumina-containing samples, a direct relationship was observed between the hydrogen consumption and the catalyst activity toward conversion of CO, hydrocarbon (HC) and NO. Table 1 shows the temperatures corresponding to 50% conversion (T50) of CO, HC and NO for the fresh and aged samples. Among the aged samples, Ce-Zr-Y catalyst represented the highest activity due to redispersion of active sites after aging. Among the fresh samples, the Ce-Zr-La-Y-Nd catalyst had the best light-off performance. However, its activity significantly decreased after aging due to strong metal-support interactions. Our results can facilitate development of TWC formulations with enhanced hydrothermal stability.
Table 1- T50 of CO, HC and NO for the fresh and aged samples

Table 1- T50 of CO, HC and NO for the fresh and aged samples


Thursday
3758110 - Reactivity of low-life cycle carbon fuels on commercial emissions control catalysts
10:40am - 11:05am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid
Utilization of low-life cycle carbon fuels (LLCFs) in on-road vehicles as well as hard-to-electrify rail, marine and off-road engines will play a critical role in decarbonization of the transportation sector. For commercialization, LLCFs running on engines must meet the stringent emissions regulations. Prior work has shown that LLCFs can introduce emissions control challenges and opportunities. The reactivity of a fuel on a catalyst can be sensitive to its chemical structure and to the exhaust composition (stoichiometric vs. lean).
The current effort involves measuring the impact of these LLCFs on a commercial three-way catalyst and diesel oxidation catalyst reactivity under engine-exhaust relevant conditions. Prior to performance evaluation, the catalyst has been hydrothermally aged as per industry guidelines. A synthetic exhaust flow-reactor has been used to measure the light-off and light-down temperatures of a wide range of fuels as per protocol set by industry. The influence of functional groups of the fuels such as alkanes, alkenes, alcohols, ketones, esters, and non-oxygenated aromatic hydrocarbons, on the catalyst light- off/light-down temperature has been investigated. The impact of these LLCFs on other regulated pollutants such as CO and NOx has also been studied.

Thursday
Panel Discussion
11:05am - 11:25am USA / Canada - Central - August 25, 2022 | Location: Great Lakes C (Marriott Marquis Chicago)
Division: [ENVR] Division of Environmental Chemistry
Session Type: Oral - Hybrid