39 Search Results

In heterogeneous catalysis, unexpected effects from the supposedly inert reactor dilutant are not rare, but related understanding is lacking and inconsistent. Here we report investigations of the impacts of the anatase TiO₂ dilutant on the reverse water-gas shift (rWGS) reaction over Pt/Al₂O₃ catalysts. Combining detailed kinetic data with microscopic and spectroscopic results, we demonstrate that the catalyst-dilutant communication depends on temperature. At high temperature (400 °C), Pt migrate from Al₂O₃ to TiO₂, resulting in higher dispersion and activity, without changing reaction mechanisms. The Pt migration is general to catalysts of different Pt nuclearity and various oxide dilutants. In contrast, atmore » low temperature (250 °C), carboxylic acids (in particular acetic acid) present in the ambient air and adsorbed on TiO₂ are transferred onto Al₂O₃ in close contact, effectively blocking the formate rWGS pathway. As a result, the rWGS can only proceed through the carboxyl pathway, and hence is significantly slower. The acetate transfer affects catalysts of different Pt nuclearity and support, but is unique to anatase TiO₂ dilutant. As the acetate layer is slowly removed under H₂ or the rWGS stream, the activity recovers. This work elucidates the complicated communication between catalysts and dilutants, which has general implications in heterogeneous catalysis, and resolves inconsistency in related reports in the literature. Finally, the impacts that anatase TiO₂ dilution has on the rWGS also unveil mechanistic understanding that further confirms the two co-existing rWGS pathways.« less

CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO₂. Among them, few systems, such as Au/TiO₂, can perform CO oxidation at temperatures as low as -70 °C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambient temperatures representing an essential pathway for life, which enables respiration in the absence of oxygen and leads to carbonate mineral formation. Herein, we report that CO can be oxidized to CO₂ by nitrate at -140 °C within an inorganic, nonmetallic zeolitic system. The transformation ofmore » NO_x and CO species in zeolite as well as the origin of this unique activity is clarified using a joint spectroscopic and computational approach.« less

A hybrid catalytic system comprised of metal oxide and Cu or Fe exchanged zeolite components represents a potential advancement of state-of-the-art for selective catalytic reduction (SCR) of NOx by ammonia, including an elegant solution to the current challenge of improving the low-temperature efficiency of SCR catalysts. The idea is to enable in-situ NO oxidation over metal oxide component and concomitant fast SCR over zeolite component in the so-called “bifunctional mechanism”. This review is presented in the wake of growing interest in this innovative catalytic system. We begin this review by presenting key parameters that contribute to the synergy between metalmore » oxide and zeolite components. Then, we discuss the materials selection for both components and their possible interactions in the system, followed by a summary of recent NH3-SCR investigations over multiple metal oxide–zeolite pairs and additional discussion on pairing techniques potentially explorable in upcoming studies. Finally, we end the review by providing the perspectives on future challenges in the development of this catalytic system.« less

Abstract γ‐alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large‐scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ‐alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid‐state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O) ₅ Al(VI)‐OH sites on the (100) segments of massively restructured (110) facets on typical rhombus‐platelet γ‐alumina as wellmore » as the (100) segments of irrational surfaces (invariably always present in all γ‐alumina samples) responsible for its unique catalytic activity. Such (O) ₅ Al(VI)‐OH sites are also present on the macroscopically defined (100) facets of γ‐alumina with elongated/rod‐like geometry. The mechanism by which these sites lose ‐OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta‐coordinate Al ⁺³ O ₅ sites is clarified. These coordinatively unsaturated penta‐coordinate Al sites produce well‐defined thermally stable Al‐carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ‐alumina and their role as catalytically active sites.« less

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ-alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O)₅Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as the (100)more » segments of irrational surfaces (invariably always present in all γ-alumina samples) responsible for its unique catalytic activity. Such (O)₅Al(VI)-OH sites are also present on the macroscopically defined (100) facets of γ-alumina with elongated/rod-like geometry. The mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al⁺³O₅ sites is clarified. These coordinatively unsaturated penta-coordinate Al sites produce well-defined thermally stable Al-carbonyl complexes. Overall, our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ-alumina and their role as catalytically active sites.« less

The analogy between single-atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. Here we provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria-supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of CeO₂ reducibility. The long-range electrostatic footprints (≈1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in an observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certainmore » Pd density, leading to their permanent removal and a consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single-atom catalysts can be tuned by varying the surface density of single metal atoms.« less

Identifying the role of oxide supports in transition metal catalysis is critical toward our understanding of heterogeneous catalysis. The water-gas shift (WGS) reaction is a prototypical example where oxide support dictates catalytic activity, yet the cause for this remains uncertain. Herein, we show that a single descriptor—the equilibrium constant for hydroxyl formation—relates the WGS turnover frequency across disparate oxide supports. The dissimilar equilibrium constant, or oxophilicity, between early and late transition metals exemplify the utility of metal-support interfacial sites to circumvent adsorption-energy scaling restrictions, thereby providing bifunctional gains for the WGS reaction class. In relation, the equilibrium constant for hydroxylmore » formation is equivalent to the equilibrium constant for the formal heterolytic dissociation of hydrogen, and therefore, reflects the ability of the metal-support interface to participate in hydrogen heterolysis. The ubiquitous coexistence, yet divergent chemical behavior of homo- and heterolytically activated hydrogen renders oxide support identity central toward our understanding of hydrogenation catalysis. This work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences and performed at the Environmental Molecular Sciences Laboratory in (EMSL), which is a DOE Office of Science User Facility located at the Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility. N.N. would like to thank Oliver Y. Gutierrez for critical feedback during the final stage of manuscript preparation.« less

Single-atom catalysts are often reported to have catalytic properties that surpass those of nanoparticles, while a direct comparison of sites common and different for both is lacking. In this work, we show that single atoms of the Pt-group embedded into the surface of Fe₃O₄ have a greatly enhanced interaction strength with CO₂ compared with Fe₃O₄ surface. The strong CO₂ adsorption on single Rh atoms and corresponding low activation energies lead to two-orders-of-magnitude higher conversion rates of CO₂ compared to Rh nanoparticles. This high activity of single atoms stems from the partially oxidic state imposed by their coordination to the support.more » Fe₃O₄-supported Rh nanoparticles follow the behavior of single atoms for CO₂ interaction and reduction, which is attributed to the dominating role of partially oxidic sites at the Fe₃O₄-Rh interface. Thus, we show a likely common catalytic chemistry for two kinds of materials thought to be different, and we show that single atoms of Pt-group metals on Fe₃O₄ are an especially successful material for catalyzed reactions that depend primarily upon sites with the metal-O-Fe environment.« less