On-surface synthesis has been one of the hottest research fields in surface science in the last decade, owing to its great potential for bottom-up synthesis of functional molecules and covalent ...nanomaterials. Compared to classical in-solution chemistry, all of the on-surface reactions are done without solvent, thus very minimal byproducts and no limitation of solubility are involved. However, because of its typically required ultra-high vacuum conditions, where only limited catalysts can be used, a key challenge for on-surface synthesis is the precise control of the reaction pathway. Countless efforts have been made for controllable synthesis of target chemical structures on surfaces by distinct strategies. These strategies can be summarized under following aspects: 1) rational choice of surfaces; 2) template effects based on two-dimensional (2D) environments; 3) on-surface thermodynamic and kinetic controls; 4) the participation of chemisorbed nonmetal adatoms on surfaces. This report reviews the recent progress toward the control of on-surface synthesis and raises a series of questions at the end, which deserve further explorations in the future.
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Abstract
The lack of highly efficient, inexpensive catalysts severely hinders large-scale application of electrochemical hydrogen evolution reaction (HER) for producing hydrogen. MoS
2
as a low-cost ...candidate suffers from low catalytic performance. Herein, taking advantage of its tri-layer structure, we report a MoS
2
nanofoam catalyst co-confining selenium in surface and cobalt in inner layer, exhibiting an ultra-high large-current-density HER activity surpassing all previously reported heteroatom-doped MoS
2
. At a large current density of 1000 mA cm
−2
, a much lower overpotential of 382 mV than that of 671 mV over commercial Pt/C catalyst is achieved and stably maintained for 360 hours without decay. First-principles calculations demonstrate that inner layer-confined cobalt atoms stimulate neighbouring sulfur atoms while surface-confined selenium atoms stabilize the structure, which cooperatively enable the massive generation of both in-plane and edge active sites with optimized hydrogen adsorption activity. This strategy provides a viable route for developing MoS
2
-based catalysts for industrial HER applications.
Abstract
The photofixation and utilization of CO
2
via single-electron mechanism is considered to be a clean and green way to produce high-value-added commodity chemicals with long carbon chains. ...However, this topic has not been fully explored for the highly negative reduction potential in the formation of reactive carbonate radical. Herein, by taking Bi
2
O
3
nanosheets as a model system, we illustrate that oxygen vacancies confined in atomic layers can lower the adsorption energy of CO
2
on the reactive sites, and thus activate CO
2
by single-electron transfer in mild conditions. As demonstrated, Bi
2
O
3
nanosheets with rich oxygen vacancies show enhanced generation of •CO
2
–
species during the reaction process and achieve a high conversion yield of dimethyl carbonate (DMC) with nearly 100% selectivity in the presence of methanol. This study establishes a practical way for the photofixation of CO
2
to long-chain chemicals via defect engineering.
Molybdenum disulfide is naturally inert for alkaline hydrogen evolution catalysis, due to its unfavorable water adsorption and dissociation feature originated from the unsuitable orbital orientation. ...Herein, we successfully endow molybdenum disulfide with exceptional alkaline hydrogen evolution capability by carbon-induced orbital modulation. The prepared carbon doped molybdenum disulfide displays an unprecedented overpotential of 45 mV at 10 mA cm
, which is substantially lower than 228 mV of the molybdenum disulfide and also represents the best alkaline hydrogen evolution catalytic activity among the ever-reported molybdenum disulfide catalysts. Fine structural analysis indicates the electronic and coordination structures of molybdenum disulfide have been significantly changed with carbon incorporation. Moreover, theoretical calculation further reveals carbon doping could create empty 2p orbitals perpendicular to the basal plane, enabling energetically favorable water adsorption and dissociation. The concept of orbital modulation could offer a unique approach for the rational design of hydrogen evolution catalysts and beyond.
Carbon-based nanostructures have attracted tremendous interest because of their versatile and tunable properties, which depend on the bonding type of the constituting carbon atoms. Graphene, as the ...most prominent representative of the π-conjugated carbon-based materials, consists entirely of sp2-hybridized carbon atoms and exhibits a zero band gap. Recently, countless efforts were made to open and tune the band gap of graphene for its applications in semiconductor devices. One promising method is periodic perforation, resulting in a graphene nanomesh (GNM), which opens the band gap while maintaining the exceptional transport properties. However, the typically employed lithographic approach for graphene perforation is difficult to control at the atomic level. The complementary bottom-up method using surface-assisted carbon–carbon (C–C) covalent coupling between organic molecules has opened up new possibilities for atomically precise fabrication of conjugated nanostructures like GNM and graphene nanoribbons (GNR), although with limited maturity. A general drawback of the bottom-up approach is that the desired structure usually does not represent the global thermodynamic minimum. It is therefore impossible to improve the long-range order by postannealing, because once the C–C bond formation becomes reversible, graphene as the thermodynamically most stable structure will be formed. This means that only carefully chosen precursors and reaction conditions can lead to the desired (non-graphene) material. One of the most popular and frequently used organic reactions for on-surface C–C coupling is the Ullmann reaction of aromatic halides. While experimentally simple to perform, the irreversibility of the C–C bond formation makes it a challenge to obtain long-range ordered nanostructures. With no postreaction structural improvement possible, the assembly process must be optimized to result in defect-free nanostructures during the initial reaction, requiring complete reaction of the precursors in the right positions. Incomplete connections typically result when mobile precursor monomers are blocked from reaching unsaturated reaction sites of the preformed nanostructures. For example, monomers may not be able to reach a randomly formed internal cavity of a two-dimensional (2D) nanostructure island due to steric hindrance in 2D confinement, leaving reaction sites in the internal cavity unsaturated. Wrong connections between precursor monomers, here defined as intermolecular C–C bonds forcing the monomer into a nonideal position within the structure, are usually irreversible and can induce further structural defects. The relative conformational flexibility of the monomer backbones permits connections between deformed monomers when they encounter strong steric hindrance. This, however, usually leads to heterogeneous structural motifs in the formed nanostructures. This Account reviews some of the latest developments regarding on-surface C–C coupling strategies toward the synthesis of carbon-based nanostructures by addressing the above-mentioned issues. The strategies include Ullmann coupling and other, “cleaner” alternative C–C coupling reactions like Glaser coupling, cyclo-dehydrogenation, and dehydrogenative coupling. The choice of substrate materials and precursor designs is crucial for optimizing substrate reactivity and precursor diffusion rates, and to reduce events of wrong linkage. Hierarchical polymerization is employed to steer the coupling route, which effectively improves the completeness of the reaction. Effects of byproducts on nanostructure formation is comprehended with both experimental and theoretical studies.
Visible‐light‐driven conversion of CO2 into chemical fuels is an intriguing approach to address the energy and environmental challenges. In principle, light harvesting and catalytic reactions can be ...both optimized by combining the merits of homogeneous and heterogeneous photocatalysts; however, the efficiency of charge transfer between light absorbers and catalytic sites is often too low to limit the overall photocatalytic performance. In this communication, it is reported that the single‐atom Co sites coordinated on the partially oxidized graphene nanosheets can serve as a highly active and durable heterogeneous catalyst for CO2 conversion, wherein the graphene bridges homogeneous light absorbers with single‐atom catalytic sites for the efficient transfer of photoexcited electrons. As a result, the turnover number for CO production reaches a high value of 678 with an unprecedented turnover frequency of 3.77 min−1, superior to those obtained with the state‐of‐the‐art heterogeneous photocatalysts. This work provides fresh insights into the design of catalytic sites toward photocatalytic CO2 conversion from the angle of single‐atom catalysis and highlights the role of charge kinetics in bridging the gap between heterogeneous and homogeneous photocatalysts.
Single‐atom Co sites coordinated on partially oxidized graphene nanosheets can serve as a highly active and durable heterogeneous catalyst for CO2 conversion, wherein the graphene bridges homogeneous light absorbers with single‐atom catalytic sites for the efficient transfer of photoexcited electrons. This design enables a turnover frequency of 3.77 min−1, superior to those obtained with conventional heterogeneous photocatalysts.
Abstract
As diversified reaction paths exist over practical catalysts towards CO
2
hydrogenation, it is highly desiderated to precisely control the reaction path for developing efficient catalysts. ...Herein, we report that the ensemble of Pt single atoms coordinated with oxygen atoms in MIL-101 (Pt
1
@MIL) induces distinct reaction path to improve selective hydrogenation of CO
2
into methanol. Pt
1
@MIL achieves the turnover frequency number of 117 h
−1
in DMF under 32 bar at 150 °C, which is 5.6 times that of Pt
n
@MIL. Moreover, the selectivity for methanol is 90.3% over Pt
1
@MIL, much higher than that (13.3%) over Pt
n
@MIL with CO as the major product. According to mechanistic studies, CO
2
is hydrogenated into HCOO* as the intermediate for Pt
1
@MIL, whereas COOH* serves as the intermediate for Pt
n
@MIL. The unique reaction path over Pt
1
@MIL not only lowers the activation energy for the enhanced catalytic activity, but also contributes to the high selectivity for methanol.
Engineering electronic properties by elemental doping is a direct strategy to design efficient catalysts towards CO2 electroreduction. Atomically thin SnS2 nanosheets were modified by Ni doping for ...efficient electroreduction of CO2. The introduction of Ni into SnS2 nanosheets significantly enhanced the current density and Faradaic efficiency for carbonaceous product relative to pristine SnS2 nanosheets. When the Ni content was 5 atm %, the Ni‐doped SnS2 nanosheets achieved a remarkable Faradaic efficiency of 93 % for carbonaceous product with a current density of 19.6 mA cm−2 at −0.9 V vs. RHE. A mechanistic study revealed that the Ni doping gave rise to a defect level and lowered the work function of SnS2 nanosheets, resulting in the promoted CO2 activation and thus improved performance in CO2 electroreduction.
Nickel in thin tin: Atomically thin SnS2 nanosheets were modified by Ni doping for efficient electroreduction of CO2. Introduction of Ni into SnS2 nanosheets enhanced current density and Faradaic efficiency for carbonaceous product relative to pristine SnS2 nanosheets. When the Ni content was 5 at. %, Faradaic efficiency was 93 % with a current density of 19.6 mA cm−2 at −0.9 V vs. RHE.
Quantum dots (QDs), a class of promising candidates for harvesting visible light, generally exhibit low activity and selectivity towards photocatalytic CO2 reduction. Functionalizing QDs with metal ...complexes (or metal cations through ligands) is a widely used strategy for improving their catalytic activity; however, the resulting systems still suffer from low selectivity and stability in CO2 reduction. Herein, we report that doping CdS QDs with transition‐metal sites can overcome these limitations and provide a system that enables highly selective photocatalytic reactions of CO2 with H2O (100 % selectivity to CO and CH4), with excellent durability over 60 h. Doping Ni sites into the CdS lattice leads to effective trapping of photoexcited electrons at surface catalytic sites and substantial suppression of H2 evolution. The method reported here can be extended to various transition‐metal sites, and offers new opportunities for exploring QD‐based earth‐abundant photocatalysts.
Quantum dots (QDs), a class of promising nanoparticles for visible‐light harvesting, commonly possess low activity and selectivity towards photocatalytic CO2 reduction. Doping CdS QDs with transition‐metal cations, which can trap photoexcited electrons and suppress H2 evolution, provides an approach to visible‐light‐driven highly selective CO2 reduction with excellent durability.
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