28 Search Results

Anchoring divalent metal ions in the same zeolite framework with similar Si/Al ratio selectively as zeolite-bound M⁺² or [M⁺²-OH]⁺¹ cationic species enables critical comparison of the species’ intrinsic reactivity for industrially and fundamentally relevant reactions. H-BEA zeolites with similar Si/Al ratios but differing framework Al siting were used to anchored multiple divalent metal cations (Ni, Pd, Pt, Cr, Cu) in the zeolite micropores. State-of-the-art infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) measurements, including two-dimensional pulsed HYSCORE EPR, extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations together provide unambiguous evidence for the selective formation of divalent metalmore » cations as M⁺²/2Al species (for H-BEA prepared in the conventional hydroxide media), and [M⁺²OH]⁺¹/1Al species for H-BEA prepared in HF. Solid-state proton-decoupled triple-quantum magic-angle spinning (3Q MAS) NMR measurements confirmed contrasting Al distributions in the two H-BEA zeolites, which led to a contrasting divalent cation speciation. The reactivities of the two cationic species were explored for catalytic and adsorptive applications in both organometallic homogeneous and heterogeneous catalysis. This work demonstrates their divergent reactivity in ethylene dimerization, ethylene oxidation (Wacker process), selective catalytic reduction (SCR) of NO, NO adsorption, and methane oxidation. Both M⁺²/2Al and [M⁺²OH]⁺¹/1Al cations are both active for ethylene dimerization, but [M⁺²OH]⁺¹/1Al species show higher reaction rates for each Pd, Ni, Pt. [M⁺²OH]⁺¹/1Al is active for acetaldehyde formation in Wacker ethylene oxidation. A new active site for ethylene oligomerization is proposed that possesses a terminal OH group (Cr-OH) in Phillips catalysts evident by a nearly inactive isolated Cr⁺²/2Al species that contrast an active Cr─OH motif.« less

Understanding the quantitative interactions among zeolite pore walls, Bro̷nsted acid sites, and molecules with both polar and nonpolar regions is essential for scoping out the potential of zeolites as sorbents and catalysts. Purely siliceous zeolites (MFI and Beta in the present study) are hydrophobic, whereas those containing aluminum are considered hydrophilic, preferentially adsorbing organic molecules even in aqueous environments. To characterize these interactions, we use primary alcohols of increasing molecular weight, quantifying their specific interactions in the confined pore space of the alkyl (CH_x) and OH groups. Three types of interactions were identified: (i) alkyl CH_x groups interacting with themore » zeolite pore walls (approximately 10 kJ mol⁻¹ per carbon), (ii) alcohol OH groups interacting with the pore walls (30−35 kJ mol⁻¹), and (iii) alcohol OH groups interacting with Bro̷nsted acid sites (37 kJ mol⁻¹). All three interactions were well mirrored by computational simulations. The contribution of the alkyl CH_x groups was inferred from the incremental increase in sorption enthalpy with increasing molecular weight; the interaction strength of the OH groups was determined by extrapolating the global adsorption enthalpy of the alcohols to a hypothetical OH group without an alkyl group. This value was identical to the adsorption enthalpy of water. The experiments demonstrated that only water has an adsorption enthalpy on zeolite pore walls lower than its condensation enthalpy (30−35 kJ mol⁻¹ vs 45 kJ mol⁻¹), limiting the concentration of water that can be adsorbed.« less

Here, in this study, we investigated the effect of the support (rutile vs anatase titania) and of the Pt size of Pt nanoparticles and supports (rutile vs anatase) in the CO oxidation reaction. The steady-state specific activity of Pt/Rutile gradually increases with Pt dispersion, while that of Pt/Anatase is invariant with respect to Pt dispersion and is lower than the activity of Pt/Rutile. Our kinetic studies demonstrated that the reaction order for CO based on steady state is positive for sub-nm Pt clusters stabilized on rutile titania and it gradually changes to zero for large Pt clusters (>1 nm), asmore » known in agreement with other studies for for supported Pt catalysts. In contrast, Pt/Anatase catalysts always show zero order dependence irrespective of Pt the size of Pt nanoparticles. Scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and CO chemisorption results confirm that the notable sintering of sub-nm Pt clusters supported on anatase occurs with time-on-stream, which we correlate with might be related with the change of the CO reaction order from positive (order initially) tobut zero order for larger particles after sintered states. On the contrary, Pt/Rutile does not show serious sintering during the reaction, and it shows positive reaction order even during the steady state. Thus Pt/Rutile shows positive order at steady state and higher specific activity. Our study reveals the origin of profound catalytic differences between Pt nanoparticles supported on anatase and rutile titania and highlights better stability and activity of Pt/Rutile catalysts. than Pt/Anatase due to its better stability.« 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

Abstract Low‐loaded platinum‐group single‐atom catalysts on CeO ₂ (M ₁ /CeO ₂ ) were synthesized via high‐temperature atom trapping (AT) and tested for the NO+CO reaction under dry and wet conditions. The activity of these catalysts for NO+CO reaction follows the order Rh>Pd≈Ru>Pt>Ir. For Rh, Ru, and Pd single‐atom catalysts, the N ₂ O byproduct is formed but not clearly observed in Ir and Pt cases, which may result from the higher reaction temperature (>200 °C) required for Pt and Ir catalysts. The presence of water can promote the activity of these M ₁ /CeO ₂ catalysts for the NO+CO reaction.more » Under wet conditions, significant NH ₃ formation occurred during the reaction, which is due to the co‐existence of water‐gas‐shift reaction on these catalysts. Compared with Pt, Pd and Ir, the Rh and Ru single‐atom catalysts show higher selectivity to NH ₃ species, resulting from the hydride species on the surface. Among all tested catalysts, Ru ₁ /CeO ₂ shows the highest production of ammonia and highest CO conversion due to excellent water‐gas‐shift activity, whereas Pd ₁ /CeO ₂ shows lowest ammonia production. Rh ₁ /CeO ₂ shows the best low temperature NO reduction activity among all tested catalysts.« less

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis gel during the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH₄-form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of an increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO₄– motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements and show their utility for NOx pollutantmore » adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and, thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with an increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH)₂ does not lose its PNA capacity after extremely harsh aging at 850 and 900 °C (10 h in 10% H₂O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 °C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 °C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications.« 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

Nitrous oxide N₂O reduction is a big challenge due to high global warming potential of N₂O. (~300 times higher compared with CO₂). The best known catalysts, such as Rh/ceria, require relatively high temperatures for N₂O decomposition. Herein, we report that Ru/ceria catalysts with low Ru loading of ~0.25 wt% efficiently catalyze low temperature N₂O reduction by CO starting at 100 °C (full N₂O conversion below 200 °C) under industrially relevant flow rates and gas concentrations. Further, this remarkable performance stems from maintaining isolated Ru cations even on reduced ceria surface and, simultaneously, the propensity of Ru to affect ceria surfacemore » to form labile surface oxygen thereby creating large number of oxygen vacancies (Ce⁺³ cations) in the presence of CO. In contrast, for Rh/CeO₂ catalysts with equivalent metal loading, the activity is much lower because atomically dispersed Rh sinters into metallic clusters at the onset reaction temperature (~200 °C): these clusters are much less effective than isolated single Ru ions, with lower Ce⁺³ concentration maintained on reduced Rh/CeO₂ catalyst. Our study highlights the benefits of gaining molecular-level insight into the dynamic nature of catalytically active sites under reaction conditions for preparing catalysts containing low loading of precious metals with unsurpassed low temperature activity.« less

In this work, we utilized an ultramicroporous metal–organic framework (MOF) named [Ni₃(pzdc)₂(ade)₂(H₂O)₄]·2.18H₂O (where H₃pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H₂) adsorption. Upon activation, [Ni₃(pzdc)₂(ade)₂] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer–Emmett–Teller (BET) surface area of 160 m²/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H₂ uptake saturation at 2.11 mmol/g atmore » ~0.15 bar. The affinity of this MOF for H₂ was determined to be 9.7 ± 1.0 kJ/mol. In situ H₂ loading experiments supported by molecular simulations confirmed that H₂ does not bind to the open Ni(II) sites of [Ni₃(pzdc)₂(ade)₂], and the high affinity of the MOF for H₂ is attributed to the interplay of pore size, shape, and functionality.« less