Atomically dispersed Au-(OH)x species bound on titania catalyze the low-temperature water-gas shift reaction.
01 Mar 2013-Journal of the American Chemical Society (J Am Chem Soc)-Vol. 135, Iss: 10, pp 3768-3771
TL;DR: A new method for stabilizing appreciable loadings of isolated gold atoms on titania is reported and these catalyze the low-temperature water-gas shift reaction and may catalyze a number of other reactions that require oxidized active metal sites.
Abstract: We report a new method for stabilizing appreciable loadings (~1 wt %) of isolated gold atoms on titania and show that these catalyze the low-temperature water-gas shift reaction. The method combines a typical gold deposition/precipitation method with UV irradiation of the titania support suspended in ethanol. Dissociation of H2O on the thus-created Au-O-TiO(x) sites is facile. At higher gold loadings, nanoparticles are formed, but they were shown to add no further activity to the atomically bound gold on titania. Removal of this "excess" gold by sodium cyanide leaching leaves the activity intact and the atomically dispersed gold still bound on titania. The new materials may catalyze a number of other reactions that require oxidized active metal sites.
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TL;DR: Recent advances in preparation, characterization, and catalytic performance of SACs are highlighted, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene, offering the potential for applications in a variety of industrial chemical reactions.
Abstract: 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...
3,051 citations
01 Jun 2018
TL;DR: A review of single-atom catalysts can be found in this paper, where the authors discuss the utility of SACs in a broad scope of industrially important reactions and highlight the advantages these catalysts have over those presently used.
Abstract: Single-atom catalysis has arguably become the most active new frontier in heterogeneous catalysis. Aided by recent advances in practical synthetic methodologies, characterization techniques and computational modelling, we now have a large number of single-atom catalysts (SACs) that exhibit distinctive performances for a wide variety of chemical reactions. This Perspective summarizes recent experimental and computational efforts aimed at understanding the bonding in SACs and how this relates to catalytic performance. The examples described here illustrate the utility of SACs in a broad scope of industrially important reactions and highlight the advantages these catalysts have over those presently used. SACs have well-defined active centres, such that unique opportunities exist for the rational design of new catalysts with high activities, selectivities and stabilities. Indeed, given a certain practical application, we can often design a suitable SAC; thus, the field has developed very rapidly and afforded promising catalyst leads. Moreover, the control we have over certain SAC structures paves the way for designing base metal catalysts with the activities of noble metal catalysts. It appears that we are entering a new era of heterogeneous catalysis in which we have control over well-dispersed single-atom active sites whose properties we can readily tune. Single-atom catalysts are heterogeneous materials featuring active metals sites atomically dispersed on a surface. This Review describes methods by which we prepare and characterize these materials, as well as how we can tune their catalytic performance in a variety of important reactions.
2,306 citations
TL;DR: A photochemical strategy to fabricate a stable atomically dispersed palladium–titanium oxide catalyst (Pd1/TiO2) on ethylene glycolate–stabilized ultrathin TiO2 nanosheets containing Pd up to 1.5%.
Abstract: 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.
1,339 citations
01 Jan 2018
TL;DR: In this paper, a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging, was reported.
Abstract: Single-atom catalysts (SACs) have recently attracted broad research interest as they combine the merits of both homogeneous and heterogeneous catalysts. Rational design and synthesis of SACs are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging. The unambiguous structure determination allows density functional theoretical prediction of MN4C4 moieties as efficient oxygen evolution catalysts with activities following the trend Ni > Co > Fe, which is confirmed by electrochemical measurements. Determination of atomistic structure and its correlation with catalytic properties represents a critical step towards the rational design and synthesis of precious or nonprecious SACs with exceptional atom utilization efficiency and catalytic activities.
1,305 citations
TL;DR: An electrocatalyst for hydrogen generation based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene is reported, which is robust and highly active in aqueous media with very low overpotentials.
Abstract: Reduction of water to hydrogen through electrocatalysis holds great promise for clean energy, but its large-scale application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. Here we report an electrocatalyst for hydrogen generation based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene. This catalyst is robust and highly active in aqueous media with very low overpotentials (30 mV). A variety of analytical techniques and electrochemical measurements suggest that the catalytically active sites are associated with the metal centres coordinated to nitrogen. This unusual atomic constitution of supported metals is suggestive of a new approach to preparing extremely efficient single-atom catalysts.
1,262 citations
References
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TL;DR: Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.
Abstract: Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H-2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.
4,446 citations
TL;DR: It is reported here that for the class of nanostructured gold– or platinum–cerium oxide catalysts, which are active for the water-gas shift reaction, metal nanoparticles do not participate in the reaction.
Abstract: Traditional analysis of reactions catalyzed by supported metals involves the structure of the metallic particles. However, we report here that for the class of nanostructured gold- or platinum-cerium oxide catalysts, which are active for the water-gas shift reaction, metal nanoparticles do not participate in the reaction. Nonmetallic gold or platinum species strongly associated with surface cerium-oxygen groups are responsible for the activity.
2,616 citations
TL;DR: In this article, the steady-state, water-gas-shift kinetics were measured on model, ceria-supported, Pd, Pt, and Rh catalysts and compared to rates obtained on alumina-supported catalysts.
Abstract: Steady-state, water-gas-shift kinetics were measured on model, ceria-supported, Pd, Pt, and Rh catalysts and compared to rates obtained on alumina-supported catalysts. When ceria was calcined at low temperatures prior to addition of the precious metal, the specific rates were found to be identical for each of the metals, with an activation energy of 11 ± 1 kcal/mol and reaction orders of zero and one for CO and H 2 O respectively. For comparison, specific rates on Rh/alumina were at least two orders of magnitude lower. However, ceria structure strongly affected the results. When ceria was calcined to high temperatures to increase crystallite size, prior to the addition of Pd, specific rates were a factor of 50 lower at 515 K and the activation energy was found to be much higher, 21 ± 1 kcal/mol. By comparison with results from an earlier study of CO oxidation [17], we propose that water-gas shift on ceria-supported metals occurs primarily through a bifunctional mechanism in which CO adsorbed on the precious metal is oxidized by ceria, which in turn is oxidized by water. Deactivation of the catalyst following growth in the ceria crystallite size is due to the decreased reducibility of large ceria crystallites. The implications of these results for automotive, emission-control catalysts is discussed.
807 citations
TL;DR: Activity correlations with the shape (rod, cube, polyhedron) and crystal plane of nanoscale ceria as a support for gold catalysts for the water–gas shift (WGS) reaction are presented.
Abstract: The water–gas shift (WGS) reaction (CO+H2OQCO2+H2) plays an important role in fuel processing for polymer electrolyte membrane (PEM) fuel-cell applications. The hydrogen in the reformate gas is upgraded by removal of the carbon monoxide, which is a strong poison of the anode catalysts in current PEM fuel cells. Active shift catalysts that are also stable under the operating conditions of practical fuel-cell systems are under intense study, and nanostructured Au-CeO2, first reported by Fu et al. as a promising lowtemperature shift catalyst, holds a prominent position. This catalyst exploits the strong interaction of ceria with finely dispersed and stabilized gold atoms and clusters on the surface of ceria. Gold nanoparticles and clusters that interact strongly with oxide supports were first described by Haruta et al. to be extremely active CO oxidation catalysts. Numerous studies since then have reaffirmed the activity of well-dispersed gold for CO oxidation and many other reactions. While a full mechanism of this catalytic process still needs to be established, even for the simplest of these reactions (CO oxidation), a careful investigation of the reported strong metal–support interaction through structural studies may provide further mechanistic insights as well as rationalize the design of practical catalysts. For the WGS reaction on Au-CeO2, the importance of nanoscale ceria as a support that stabilizes active gold species has been demonstrated recently. Hydrolysis methods for the synthesis of ceria nanocrystals have proven to be powerful for controlling particle size and crystal shape. For example, Yan et al. have obtained single-crystalline CeO2 nanopolyhedra ({111} and {100}), nanorods ({110} and {100}), and nanocubes ({100}) by hydrolysis of cerium(III) salts, combined with a hydrothermal treatment, and have further found that oxygen storage takes place both at the surface and in the bulk for nanorods and nanocubes but is restricted to the surface for nanopolyhedra, just like its bulk ceria counterpart. Trovarelli et al. have studied the rearrangement of CeO2 crystallites under airaging and the exposure of more reactive {100} surfaces for CO oxidation. Very little is known for Au-CeO2 composite polycrystalline nanomaterials with respect to the shape/crystal plane effect of CeO2 on the gold species< activity/stabilization as highly active catalysts for the WGS reaction. Herein, we present activity correlations with the shape (rod, cube, polyhedron) and crystal plane of nanoscale ceria as a support for gold catalysts for this reaction. The interaction between deposited gold and different crystal orientations is investigated at ambient pressure andmonitored by several analytical techniques, including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction by hydrogen (H2-TPR). Figure 1 depicts our two-step preparation process, which includes hydrothermal synthesis of ceria nanorods, nano-
742 citations
TL;DR: Alkali ions added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2.
Abstract: 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.
603 citations