Precisely Applying TiO₂ Overcoat on Supported Au Catalysts Using Atomic Layer Deposition for Understanding the Reaction Mechanism and Improved Activity in CO Oxidation

For TiO₂ supported Au catalysts, the Au particle size and the interfacial perimeter sites between Au particles and the TiO₂ support both play important roles in CO oxidation reaction. However, changing the Au particle size inevitably accompanied by the change of the perimeter length makes it extremely difficult to identify their individual roles. Here we reported a new strategy to isolate them by applying TiO₂ overcoat to Au/Al₂O₃ and Au/SiO₂ catalysts using atomic layer deposition (ALD) where the new Au–TiO₂ interfacial length was precisely tuned to different degrees while preserving the particle size. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption all confirmed that the TiO₂ overcoat preferentially decorates the low-coordinated sites of Au nanoparticles and generates Au–TiO₂ interfaces. In CO oxidation, we demonstrated a remarkable improvement of the catalytic activities of Au/Al₂O₃ and Au/SiO₂ catalysts by the ALD TiO₂ overcoat. More interestingly, the activity as a function of TiO₂ ALD cycles obviously showed a volcano-like behavior, providing direct evidence that the catalytic activities of TiO₂ overcoated Au catalysts strongly correlate with the total length of perimeter sites. Finally, our work suggests that this strategy might be a new method for atomic level understanding the reaction mechanism and high performance catalyst design

Precisely Controlled Porous Alumina Overcoating on Pd Catalyst by Atomic Layer Deposition: Enhanced Selectivity and Durability in Hydrogenation of 1,3-Butadiene

Metal catalyst in selective hydrogenation reactions often suffers from low selectivity and especially poor durability due to heavy coke formation. Here we report that precisely controlled porous alumina overcoating on a Pd catalyst using atomic layer deposition (ALD) not only remarkably enhances the selectivity to butenes, especially to 1-butene, but also achieves the best ever durability against deactivation in selective hydrogenation of 1,3-butadiene in the absence (or presence) of propene. Therein no visible activity declines or selectivity changes were observed during a total 124 h of reaction time on stream

Catalysts Transform While Molecules React: An Atomic-Scale View

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO₂(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the VO vanadyl group. Catalytic tests on the powder-supported VO_<i>x</i> revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts

First-Principles Predictions and <i>in Situ</i> Experimental Validation of Alumina Atomic Layer Deposition on Metal Surfaces

The atomic layer deposition (ALD) of metal oxides on metal surfaces is of great importance in applications such as microelectronics, corrosion resistance, and catalysis. In this work, Al₂O₃ ALD using trimethylaluminum (TMA) and water was investigated on Pd, Pt, Ir, and Cu surfaces by combining <i>in situ</i> quartz crystal microbalance (QCM), quadrupole mass spectroscopy (QMS), and scanning tunneling microscopy (STM) measurements with density functional theory (DFT) calculations. These studies revealed that TMA undergoes dissociative chemisorption to form monomethyl aluminum (AlCH₃*, the asterisk designates a surface species) on both Pd and Pt, which transform into Al­(OH)₃* during the subsequent water exposure. Furthermore, the AlCH₃* can further dissociate into Al* and CH₃* on stepped Pt(211). Additional DFT calculations predicted that Al₂O₃ ALD should proceed on Ir following a similar mechanism but not on Cu due to the endothermicity for TMA dissociation. These predictions were confirmed by <i>in situ</i> QCM, QMS, and STM measurements. Our combined theoretical and experimental study also found that the preferential decoration of low-coordination metal sites, especially after high temperature treatment, correlates with the differences in free energy between Al₂O₃ ALD on the (111) and stepped (211) surfaces. These insights into Al₂O₃ growth on metal surfaces can guide the future design of advanced metal/metal oxide catalysts with greater durability by protecting the metal against sintering and dissolution and enhanced selectivity by blocking low-coordination metal sites while leaving (111) facets available for catalysis

Singlet Oxygen-Engaged Selective Photo-Oxidation over Pt Nanocrystals/Porphyrinic MOF: The Roles of Photothermal Effect and Pt Electronic State

The selectivity control toward aldehyde in the aromatic alcohol oxidation remains a grand challenge using molecular oxygen under mild conditions. In this work, we designed and synthesized Pt/PCN-224­(M) composites by integration of Pt nanocrystals and porphyrinic metal–organic frameworks (MOFs), PCN-224­(M). The composites exhibit excellent catalytic performance in the photo-oxidation of aromatic alcohols by 1 atm O₂ at ambient temperature, based on a synergetic photothermal effect and singlet oxygen production. Additionally, in opposition to the function of the Schottky junction, injection of hot electrons from plasmonic Pt into PCN-224­(M) would lower the electron density of the Pt surface, which thus is tailorable for the optimized catalytic performance via the competition between the Schottky junction and the plasmonic effect by altering the light intensity. To the best of our knowledge, this is not only an unprecedented report on singlet oxygen-engaged selective oxidation of aromatic alcohols to aldehydes but also the first report on photothermal effect of MOFs

Activating Edge Sites on Pd Catalysts for Selective Hydrogenation of Acetylene via Selective Ga₂O₃ Decoration

Pd catalysts are industrially used in the selective hydrogenation of acetylene to ethylene. Terrace Pd atoms of the closely packed {111} facets on supported Pd particles are generally considered to be the catalytically active sites. We herein report that deposition of an appropriate amount of Ga₂O₃ adlayers on Pd particles supported on alumina by the atomic layer deposition (ALD) technique substantially enhanced the catalytic activity, selectivity, and stability in the selective hydrogenation of acetylene to ethylene. Structural characterization results demonstrate that Ga₂O₃ is preferentially deposited at the edges and open facets of Pd particles with the ALD technique. This transforms the poisoning edge sites of the {111} facets into the catalytically active terrace-like sites, leading to an increase in the number of active sites and subsequently the enhancement of the catalytic activity; this also suppresses the formation of poisoning carbonaceous deposits on the open facets and blocks the migration of carbonaceous deposits from the open facets to the neighboring active {111} facets, leading to a significant improvement in catalytic stability. These results demonstrate a concept of selective oxide decoration to comprehensively improve the performance of supported metal catalysts and provide a practical strategy

Water-Mediated Mars–Van Krevelen Mechanism for CO Oxidation on Ceria-Supported Single-Atom Pt₁ Catalyst

In water-promoted CO oxidation, water was thought not to directly participate in CO₂ production. Here we report that via a water-mediated Mars–van Krevelen (MvK) mechanism, water can directly contribute to about 50% of CO₂ production on a single-atom Pt₁/CeO₂ catalyst. The origin is the facile reaction of CO with the hydroxyl from dissociated water to yield the carboxyl intermediate, which dehydrogenates subsequently with the help of a lattice hydroxyl to generate CO₂ and water. The water-mediated MvK type reaction found here provides new insights in the promotion role of water in heterogeneous catalysis

Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions: Combination of Host–Guest Cooperation and Bimetallic Synergy in Catalysis

Metal nanoparticles (NPs) stabilized by metal–organic frameworks (MOFs) are very promising for catalysis, while reports on their cooperative catalysis for a cascade reaction have been very rare. In this work, Pd NPs incorporated into a MOF, MIL-101, have jointly completed a tandem reaction on the basis of MOF Lewis acidity and Pd NPs. Subsequently, ultrafine PdAg alloy NPs (∼1.5 nm) have been encapsulated into MIL-101. The obtained multifunctional PdAg@MIL-101 exhibits good catalytic activity and selectivity in cascade reactions under mild conditions, on the basis of the combination of host–guest cooperation and bimetallic synergy, where MIL-101 affords Lewis acidity and Pd offers hydrogenation activity while Ag greatly improves selectivity to the target product. As far as we know, this is the first work on bimetallic NP@MOFs as multifunctional catalysts with multiple active sites (MOF acidity and bimetallic species) that exert respective functions and cooperatively catalyze a one-pot cascade reaction

Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions: Combination of Host–Guest Cooperation and Bimetallic Synergy in Catalysis

Metal nanoparticles (NPs) stabilized by metal–organic frameworks (MOFs) are very promising for catalysis, while reports on their cooperative catalysis for a cascade reaction have been very rare. In this work, Pd NPs incorporated into a MOF, MIL-101, have jointly completed a tandem reaction on the basis of MOF Lewis acidity and Pd NPs. Subsequently, ultrafine PdAg alloy NPs (∼1.5 nm) have been encapsulated into MIL-101. The obtained multifunctional PdAg@MIL-101 exhibits good catalytic activity and selectivity in cascade reactions under mild conditions, on the basis of the combination of host–guest cooperation and bimetallic synergy, where MIL-101 affords Lewis acidity and Pd offers hydrogenation activity while Ag greatly improves selectivity to the target product. As far as we know, this is the first work on bimetallic NP@MOFs as multifunctional catalysts with multiple active sites (MOF acidity and bimetallic species) that exert respective functions and cooperatively catalyze a one-pot cascade reaction

Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions: Combination of Host–Guest Cooperation and Bimetallic Synergy in Catalysis

Metal nanoparticles (NPs) stabilized by metal–organic frameworks (MOFs) are very promising for catalysis, while reports on their cooperative catalysis for a cascade reaction have been very rare. In this work, Pd NPs incorporated into a MOF, MIL-101, have jointly completed a tandem reaction on the basis of MOF Lewis acidity and Pd NPs. Subsequently, ultrafine PdAg alloy NPs (∼1.5 nm) have been encapsulated into MIL-101. The obtained multifunctional PdAg@MIL-101 exhibits good catalytic activity and selectivity in cascade reactions under mild conditions, on the basis of the combination of host–guest cooperation and bimetallic synergy, where MIL-101 affords Lewis acidity and Pd offers hydrogenation activity while Ag greatly improves selectivity to the target product. As far as we know, this is the first work on bimetallic NP@MOFs as multifunctional catalysts with multiple active sites (MOF acidity and bimetallic species) that exert respective functions and cooperatively catalyze a one-pot cascade reaction