Selective catalytic hydrogenation has wide applications in both petrochemical and fine chemical industries, however, it remains challenging when two or multiple functional groups coexist in the substrate. To tackle this challenge, the "active site isolation" strategy has been proved effective, and various approaches to the site isolation have been developed. In this review, we have summarized these approaches, including adsorption/grafting of N/S-containing organic molecules on the metal surface, partial covering of active metal surface by metal oxides either via doping or through strong metal-support interaction, confinement of active metal nanoparticles in micro- or mesopores of the supports, formation of bimetallic alloys or intermetallics or core@shell structures with a relatively inert metal (IB and IIB) or nonmetal element (B, C, S, etc.), and construction of single-atom catalysts on reducible oxides or inert metals. Both advantages and disadvantages of each approach toward the site isolation have been discussed for three types of chemoselective hydrogenation reactions, including alkynes/dienes to monoenes, α,β-unsaturated aldehydes/ketones to the unsaturated alcohols, and substituted nitroarenes to the corresponding anilines. The key factors affecting the catalytic activity/selectivity, in particular, the geometric and electronic structure of the active sites, are discussed with the aim to extract fundamental principles for the development of efficient and selective catalysts in hydrogenation as well as other transformations.

Selective Hydrogenation over Supported Metal Catalysts: From Nanoparticles to Single Atoms

Single-atom alloys (SAAs) play an increasingly significant role in the field of single-site catalysis and are typically composed of catalytically active elements atomically dispersed in more inert and catalytically selective host metals SAAs have been shown to catalyze a range of industrially important reactions in electro-, photo-, and thermal catalysis studies Due to the unique geometry of SAAs, the location of the transition state and the binding site of reaction intermediates are often decoupled, which can enable both facile dissociation of reactants and weak binding of intermediates, two key factors for efficient and selective catalysis Often, this results in deviations from transition metal scaling relationships that limit conventional catalysts SAAs also offer reduced susceptibility to CO poisoning, cost savings from reduced precious metal usage, opportunities for bifunctional mechanisms via spillover, and higher resistance to deactivation by coking that plagues many industrial catalysts In this review, we begin by introducing SAAs and describe how model systems and nanoparticle catalysts can be prepared and characterized We then review all available SAA literature on a per reaction basis before concluding with a description of the general properties of this new class of heterogeneous catalysts and presenting opportunities for future research and development

Single-Atom Alloy Catalysis

Metal atoms dispersed on the oxide supports constitute a large category of single-atom catalysts. In this review, oxide supported single-atom catalysts are discussed about their synthetic procedures, characterizations, and reaction mechanism in thermocatalysis, such as water-gas shift reaction, selective oxidation/hydrogenation, and coupling reactions. Some typical oxide materials, including ferric oxide, cerium oxide, titanium dioxide, aluminum oxide, and so on, are intentionally mentioned for the unique roles as supports in anchoring metal atoms and taking part in the catalytic reactions. The interactions between metal atoms and oxide supports are summarized to give a picture on how to stabilize the atomic metal centers, and rationally tune the geometric structures and electronic states of single atoms. Furthermore, several directions in fabricating single-atom catalysts with improved performance are proposed on the basis of state-of-the-art understanding in metal-oxide interactions.

Single-Atom Catalysts Based on the Metal-Oxide Interaction.

Carbon-based materials are widely employed as metal-free catalysts or supports in catalysis, energy, and ecological applications because of their interesting properties. Generally, their high surfa...

Carbon-Based Single-Atom Catalysts for Advanced Applications

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance.

Pd–Ag alloy catalysts with very dilute amounts of Pd were synthesized. EXAFS results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H2 dissociation. In addition, for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein molecule for facile hydrogenation of the aldehyde functionality.

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Selective hydrogenation of acrolein on supported silver catalysts: A kinetics study of particle size effects

PdZn catalysts have been proposed as an alternative to Cu low temperature water gas shift (WGS) catalysts due to their similar reactivity but higher thermal stability. Unfortunately, Pd based alloys are likely to be considerably more expensive than Cu catalysts. Therefore, we explore NiZn as a potentially cheaper alternative to PdZn. Both PdZn(111) and NiZn(111) have similar potential energy surfaces for WGS as previous work on Cu(111) suggesting that they may be effective WGS catalysts. However, unlike PdZn (and Cu), the primary mechanism for WGS shifts from the carboxyl mechanism to a redox mechanism over NiZn(111) (although they are similar in magnitude).

A Comparative Density Functional Theory Study of Water Gas Shift Over PdZn(111) and NiZn(111)

We report a transformative, all inorganic synthesis method of preparing supported bimetallic Pd3Ag alloy nanoparticles. The method involves breaking down bulk Pd3Ag alloy into the nanoparticles in liquid lithium, converting metallic Li to LiOH, and transferring Pd3Ag nanoparticles/LiOH mixture onto non-water-soluble supports, followed by leaching off the LiOH with water under ambient conditions. The size of the resulting Pd3Ag nanoparticles was found narrowly distributed around 2.3 nm characterized by transmission electron microscope (TEM). In addition, studies by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy showed that the resulting Pd3Ag nanoparticles inherited similar atomic ratio and alloy structure as the starting material. The synthesized Pd3Ag nanoparticles exhibited excellent catalytic activity toward hydrogenation of acrolein to propanal.

Direct Synthesis of Bimetallic Pd3Ag Nanoalloys from Bulk Pd3Ag Alloy

This thesis aims to investigate multiple critical properties of catalysts which can affect the activity and selectivity greatly in heterogeneous catalysis. One reaction of interest was chosen as a model reaction for this study: the selective hydrogenation of α,β-unsaturated aldehyde to α,β-unsaturated alcohol (using acrolein as a model reactant). 
For the selective hydrogenation of acrolein to allyl alcohol, it is a particularly difficult reaction to obtain selectivity towards C=O bond hydrogenation as the hydrogenation of C=C bond is easier than that of C=O bond thermodynamically. The lack of substituents at C=C bond makes it especially vulnerable to hydrogenation. Thus kinetic control in this system is required. Previous research has made great progress in understanding some of the factors (choice of metal, process conditions) which may improve selectivity. However, no catalyst exists with both high activity and selectivity. Therefore we studied this reaction systematically from four aspects: 1) particle size effects; 2) support effects; 3) alloy effects (utilizing single atom alloy to improve the performance of catalysts); and 4) employing single site heterogeneous catalysts. We found that both selectivity and activity of the catalyst increased as the particle size increased for Ag/SiO2 catalysts. It is speculated that the adsorption mode of the acrolein molecule is critical to high selectivity and that the extended terraces of the larger particles allows for favorable adsorption configurations. Although this trend in selectivity was found to extend to other supports (Al2O3, TiO2), the activity of Ag/Al2O3 dropped with increasing particle size. This could be related to activation (and subsequent spillover) of H2 on the support. Further demonstrating the importance of H2 activation in this reaction, the activity of dilute alloys was assessed. In this case, very low loadings of a d-band transition metal (eg. Pd) was impregnated into the Ag nanoparticle. Even at loadings of 0.01wt%, the presence of Pd had a profound effect on the catalyst performance, greatly increasing activity. Finally, we have evaluated a novel Zn2+ single site catalyst for acrolein hydrogenation which must activate hydrogen through a completely different mechanism. Suggestions for further experiments will be discussed.

Haojuan Wei

Papers

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Selective hydrogenation of acrolein on supported silver catalysts: A kinetics study of particle size effects

A Comparative Density Functional Theory Study of Water Gas Shift Over PdZn(111) and NiZn(111)

Direct Synthesis of Bimetallic Pd3Ag Nanoalloys from Bulk Pd3Ag Alloy

Selective Hydrogenation of Acrolein over Supported Silver and Silver Alloy Catalysts