Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequent...

Single-atom catalysts: a new frontier in heterogeneous catalysis.

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Atomically dispersed noble metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low (usually below 0.5%) to avoid the formation of metal nanoparticles through sintering. We report a photochemical strategy to fabricate a stable atomically dispersed palladium-titanium oxide catalyst (Pd1/TiO2) on ethylene glycolate (EG)-stabilized ultrathin TiO2 nanosheets containing Pd up to 1.5%. The Pd1/TiO2 catalyst exhibited high catalytic activity in hydrogenation of C=C bonds, exceeding that of surface Pd atoms on commercial Pd catalysts by a factor of 9. No decay in the activity was observed for 20 cycles. More important, the Pd1/TiO2-EG system could activate H2 in a heterolytic pathway, leading to a catalytic enhancement in hydrogenation of aldehydes by a factor of more than 55.

/pdf/photochemical-route-for-synthesizing-atomically-dispersed-1yjhc201ew.pdf

Photochemical route for synthesizing atomically dispersed palladium catalysts

Facile dissociation of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resolution scanning tunneling microscopy, we show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissociation at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.

http://science.sciencemag.org/content/sci/335/6073/1209.full.pdf

Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations

The recent explosive growth in research on catalysis by supported single metal atoms proves the scientific interest in this new frontier of heterogeneous catalysis. A supported single-atom catalyst (SAC) contains only isolated individual atoms dispersed on, and/or coordinated with, the surface atoms of an appropriate support. SACs not only maximize the atom efficiency of expensive metals but also provide an alternative strategy to tune the activity and selectivity of a catalytic reaction. When single metal atoms are strongly anchored onto high-surface-area supports, SACs offer a great potential to significantly transform the field of heterogeneous catalysis, which has been critical to enabling many important technologies. In this Perspective, I discuss the most recent advances in preparing, characterizing, and catalytically testing SACs with a focus on correlating the structural perspective of the anchored single metal atoms to the observed catalytic performances. The grand challenge to successfully devel...

Catalysis by Supported Single Metal Atoms

We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) used for producing H 2 . The alkali ion–associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-O x (OH) y species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.

http://science.sciencemag.org/content/sci/329/5999/1633.full.pdf

Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions.

We demonstrate ways to prepare active and stable Pt–CeO
 x
 catalysts for the water-gas shift reaction (WGSR). Various synthesis protocols are shown to work; the best being the coprecipitation/gelation method, which suppresses the crystal growth of ceria during calcination by the incorporation of some platinum in the bulk oxide. Metallic platinum nanoparticles are not necessary for an active WGSR catalyst. Reaction light-off occurs at ∼120 °C, where Pt2+ species bound to ceria are still present. No activation period, and no hysteresis phenomena were found. During reaction at reducing conditions, some Pt is reduced, but it is reoxidizable. The stability of these low-content (<2 at.%) Pt/CeO
 x
 catalysts is high even in realistic reformate gas streams. To avoid cerium(III) hydroxycarbonate formation at room temperature-shutdown with water condensation, a small amount (∼1%) of gaseous oxygen is added to the reaction gas mixture. Cyclic shutdown/startup operation is thus possible without catalyst degradation.

/pdf/the-importance-of-strongly-bound-pt-ceo-x-species-for-the-2g8zkp0om7.pdf

The Importance of Strongly Bound Pt–CeO x Species for the Water-gas Shift Reaction: Catalyst Activity and Stability Evaluation

Encapsulated platinum in the form of core–shell Pt@SiO 2 prepared by reverse microemulsion, can be rendered catalytically active for the low-temperature water-gas shift (WGS) reaction by the addition of alkali ions during the synthesis steps or by impregnation of the dried (uncalcined) core–shell materials. We report that positively charged platinum species, embedded throughout the silica shell and stabilized by the alkali ions are the active sites for the WGS reaction. Water dissociation and hydroxyl regeneration take place on the Na-promoted Pt–O x sites. It is shown by cyclic CO-TPR experiments with intermittent ambient rehydration that the activity remains constant, and the onset temperature of the reaction is the same, ∼150 °C, with cycling. The apparent activation energies for the WGS reaction in realistic fuel gas mixtures over the various encapsulated Pt-based catalysts evaluated here and those on other supports are all in the same range, 70 ± 5 kJ/mol. Hence any difference in the reaction rates between the open- and encapsulated-Pt catalysts is attributed to their different number of active sites accessible to the reactants. The Pt-Na@SiO 2 catalyst structures show remarkable stability with time-on-stream at 350 °C.

Silica-encapsulated platinum catalysts for the low-temperature water-gas shift reaction

A dramatic enhancement of the catalytic activity of Pt-Al2O3 and Pt-SiO2 for the low-temperature (about 100 °C) water-gas shift reaction occurs by addition of Na+ or K+ ions.

Danny Pierre

Papers

Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions.

The Importance of Strongly Bound Pt–CeO x Species for the Water-gas Shift Reaction: Catalyst Activity and Stability Evaluation

Silica-encapsulated platinum catalysts for the low-temperature water-gas shift reaction

Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions.