35 Search Results

Chloroaluminate ionic liquid catalyzes the disproportionation of alkanes, a reaction readily initiated by carbenium-ion precursors such as tert-butyl chloride, resulting in equimolar amounts of isobutane and methylpentanes. The carbenium ion-AlCl4- ion-pairs stabilized by the ionic liquid are the key intermediates in two distinctive kinetic phases, i.e., a transi-ent phase (0-5 minutes) and a steady-state phase (after 5 minutes). The transient phase constitutes the majority of iso-pentane conversion and is governed by the initial carbenium ion concentration. In the steady-state phase, disproportiona-tion occurs at a considerably lower rate, affected by the carbenium ion concentration, the concentration of the ionic liq-uid, andmore » the reaction temperature. The formation of olefins observed in the 1H NMR spectra of the reacting substrates, along with the DFT calculations, suggests that dehydrochlorination of active carbenium ion-pairs reduces their concentra-tion, decreasing, in turn, the reaction rate. Kinetic modeling indicates that the transient phase is significantly controlled by the hydride transfer (kHT) and the dehydrochlorination rate constants (kDC), while the steady-state phase is additional-ly influenced by the hydrochlorination rate constant (k–DC). The overall activation energy of the reaction at the steady state, expressed as Ea,steady-state = Ea,HT – Ea,DC + Ea,–DC, was 54 kJ/mol. The reaction mechanism and the kinetics highlight the potential of Lewis acid-catalyzed conversions of hydrocarbons under remarkably mild conditions.« less

Polyolefins and their chlorinated derivatives such as polyvinyl chloride (PVC) are among the most prevalent plastics in global production and waste streams. Traditional waste-to-energy methods such as incineration and pyrolysis, as well as most chemical upcycling methods for PVC utilization, require thorough, high-temperature dechlorination to prevent the release of toxic chlorinated compounds. Here, we present here a strategy for upgrading discarded PVC into chlorine-free fuel range hydrocarbons and hydrogen chloride in a single-stage process catalyzed by chloroaluminate ionic liquids. This approach offsets endothermic dechlorination and carbon-carbon bond cleavage with exothermic alkylation and hydrogen transfer by isobutane or isopentane in amore » low-temperature tandem process. The light isoalkanes are available from refinery processes and partly from recycling of the product stream. This process is suitable for handling real-world mixed and contaminated PVC and polyolefin waste streams.« less

The shape of supported metal particles regulates their catalytic reactivity and is determined by the degree of wetting between the metal particle and the support surface. Flattened particles that wet support surfaces were reported in various catalytic systems, particularly in the subnanometer size regime. Such consequential metal–support wetting phenomena are poorly understood, and methods to study them on powder catalysts under realistic conditions are lacking. Here, we investigate the size-dependent wetting behaviors of Ru particles on two reducible-oxide supports, anatase TiO₂ (TiO₂-A) and CeO₂, under reducing catalytic conditions. X-ray absorption spectroscopy (XAS), low-energy ion scattering (LEIS), and density functional theorymore » (DFT) are combined to determine the shape of Ru particles. Ru particles remain three-dimensional without wetting the TiO₂-A support within the coverage range studied (0.06–0.98 Ru nm^–2). In contrast, at low coverages (<0.25 Ru nm^–2), Ru wets the CeO₂ support to form flat, disordered structures. The higher wettability of CeO₂ than TiO₂-A is attributed to oxygen vacancies in the near-surface region. The shape difference between small Ru particles or clusters on the two supports leads to drastically contrasting catalytic reactivities in polyolefin hydrogenolysis, despite similar diameters. As a result, this work highlights the implications of metal–support wetting, or cluster shape, on catalytic behaviors of small metal clusters, while establishing the foundation for future systematic studies of such a phenomenon in realistic systems, by delivering a multitechnique methodology and revealing governing fundamental principles.« less

Ru/C-catalyzed polyethylene (PE) and hydrocarbon hydrogenolysis under quasi-supercritical fluid of isopentane was kinetically and mechanistically investigated. PE hydrogenolysis with C–C and C–H cleavage showed zeroth order, suggesting strong adsorption of hydrocarbons. PE yielded broad product distribution of heavy (C_21–40) and diesel-range (C_11–20) hydrocarbons in the primary step of hydrogenolysis due to stochastic C–C cleavage over Ru surface. Catalytic hydrogenolysis of n-hexadecane, squalane, and light hydrocarbons such as n-pentane, iso-pentane, and n-hexane further described C–C cleavage reactivity between primary and secondary carbons, i.e., ¹C–²C and ²C–²C, which has an order of magnitude higher hydrogenolysis rate than that involving a tertiary carbon.more » The PE saturated Ru surface and lower C–C cleavage reactivity of tertiary carbon in iso-pentane, therefore, imited sovlent conversion during hydrogenolysis, whereas leading to selective PE conversion. Using hexadecane, we observed comparable hydrogenolysis rates between H₂ and D₂ (k_H/k_D ~ 1), indicating the kinetically relevant step of C–C cleavage with facilitating C–H cleavage and rehydrogenation. However, the normal kinetic isotope effect between hexadecane and deuterated hexadecane (k_C16H34/k_C16D34 ~ 5) revealed that the dehydrogenation, i.e., C–H cleavage, can be kinetically involved in the hydrogenolysis kinetic. By considering the 8-fold lower H-D exchange rate with deuterated hexadecane compared to n-hexadecane, the lower rate for hydrogenolysis and H-D exchange with deuterated hexadecane can be attributed to the C–D bond dissociation energy being 3 kJ/mol higher than that of the C–H bond. Increasing H₂ pressure favors internal C–C bond cleavage over terminal one. This minimizes the formation of lower hydrocarbons, particularly methane. However, the increase in H₂ pressure increases the coverage of adsorbed hydrogen on the Ru particles due to competitive adsorption of H₂ and polyethylene, which, in turn, reduces the polyethylene conversion rates.« less

NO_x abatement from H₂ internal combustion engines (H₂-ICEs) is challenging due to high H₂O content and unburned H₂ in the exhaust. This study examines Cu-SSZ-13 SCR catalysts, focusing on the effects of high H₂O and H₂ levels on its activity and stability. High H₂O content typical of H₂-ICE exhaust hinders low-temperature SCR activity by impeding Cu migration and oxidation half cycle efficacy. H₂ slip decreases high-temperature SCR activity by reducing active Cu sites to the inactive CuI state. Combined, high H₂O and H₂ slip reduce SCR performance across all temperatures, making it less effective than in diesel applications. Additionally, agingmore » under high H₂O and H₂ contents induce a severe deterioration of Cu-SSZ-13 via CuO_x formation and dealumination, further degrading catalyst performance. This suggests Cu-SSZ-13 may not be suitable for H₂-ICE aftertreatment, especially given the ongoing development of H₂-ICE itself. Parallel efforts in H₂-ICE and catalyst development are essential to accelerate H₂-ICE deployment.« less

Synthesis of robust and hydrothermally stable PGM/ceria materials for NO, CO, and hydrocarbon abatement remains a formidable challenge, as ceria and PGMs are known to sinter severely >800 °C under hydrothermal conditions, leading to irreversible activity loss. In this work, we tackle this challenge by synthesizing well-defined catalysts with atomically dispersed rhodium supported on ceria with varying abundance of (100), (101), and (111) facets. Evaluation of these catalysts for NO reduction by CO as well as CO and propylene oxidation under model and industrially relevant conditions reveals pronounced reactivity and stability differences. Different modes of interaction of Rh ions withmore » the ceria facets and their facile reducibility were shown to be the crucial parameters controlling reactivity, resulting in pronounced activity and stability variations. Facet-dependent poisoning of surfaces by nitrites was identified as the main reason for deactivation of the catalysts at low temperature, which is mitigated for (111) ceria facets. (111)-enriched ceria nanoparticles survive very harsh hydrothermal aging at 950 °C by maintaining and preserving (111) facets, unlike other ceria nanoparticles which sinter into poorly defined shapes. Thus, putting atomically dispersed PGM sites on (111) ceria facets lead to the catalytic material with the highest activity and stability for all studied reactions, providing the pathway to catalysts that can endure extremely harsh hydrothermal aging conditions.« less

The complicated dynamics of active sites on single-atom catalysts under reducing conditions limits their applications in hydrogenation reactions and mechanistic understanding. Herein, we report that on Rh₁/TiO₂, *H spillover during room-temperature ethylene hydrogenation hydroxylates and reduces TiO₂, enhancing the intrinsic activity of Rh₁ by 9-fold. Spectroscopic and kinetic evidence suggests that the spillover of *H is suppressed by their facile reaction with C₂H₄, most of the spilled *H are nonreactive spectators, and >99% turnovers occur on a small subset (<20%) of exposed “active Rh₁”. Steady-state kinetics indicates competitive adsorption between H and C₂H₄, H₂ dissociation is the rate-determining step, andmore » the apparent activation barrier (E_a,app) of the reaction is ~48 kJ/mol. The evolution of Rh₁ under H₂ was further tracked by spectroscopic and microscopic techniques at elevated temperatures. At 200 °C, more Rh₁ are exposed, but these Rh₁ are at least 5-fold less active than that of the “active Rh₁”. At 300 °C, Rh clusters derived from Rh₁ become the main active sites, shifting E_a,app to 62 kJ/mol, characteristic of Rh nanoparticles. At ≥400 °C, larger and more active Rh particles in the strong metal–support interaction state are created. In conclusion, this work revealed the unexpected regulation effects of *H spillover on M₁ active sites under ambient conditions, differentiated the minority active M₁ sites, and demonstrated how the stability of M₁ under reducing atmospheres affects hydrogenation catalysis.« less

Ultra small Pd₄ clusters form in the micropores of FER zeolite during low temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100°C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations we provide the molecular level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures which effectively blocks O₂ activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in themore » presence of CO, which significantly increases their affinity for binding O₂. In conclusion, our study shows a pathway for achieving low temperature CO oxidation activity on the basis of well-defined Pd/zeolite system.« less

Research interest in single-atom catalysts (SACs) has been continuously rising. However, the lack of understanding of the dynamic behaviors of SACs during applications hinder catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in-situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. Themore » activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ~1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at high temperature, also re-disperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ~5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways co-exist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar with the ones formed under H₂. Finally, this work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pre-treatments, and how their activity is modulated by these behaviors. These insights on SAC dynamics and structure-function relationship are valuable to mechanistic understanding and catalyst design.« less

Atom trapping leads to catalysts with atomically dispersed Ru₁O₅ sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru₁/CeO₂ is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru₁/CeO₂ shows very high NO_x storage properties due to formation of stable Ru–NO complexes as well as amore » high spill-over rate of NO_x onto CeO₂. Only ~0.05 wt % of Ru is required for excellent NO_x storage. Ru₁O₅ sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO₂ nanoparticles. Here we clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru₁/CeO₂ for NO reduction by CO at low temperatures: only 0.1–0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru₁/CeO₂ and its propensity to form oxygen vacancies/Ce⁺³ sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.« less