Highlights of the major progress in single-atom catalysis in 2015 and 2016

The idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in2011when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2.The last six years have witnessed tremendous progress in the field of single‐atom catalysis.Here we introduce the major achievements on this topic in2015and2016.Some particular aspects of single‐atom catalysis are discussed in depth,including new approaches in single‐atom catalyst(SAC)synthesis,stable gold SACs for various reactions,the high selectivity of Pt and Pd SACs in hydrogenation,and the superior performance of non‐noble metal SACs in electrochemistry.These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.

Electronic interaction between single Pt atom and vacancies on boron nitride nanosheets and its influence on the catalytic performance in the direct dehydrogenation of propane

The electronic metal-support interaction(EMSI)is one of most intriguing phenomena in heterogeneous catalysis.In this work,this subtle effect is clearly demonstrated by density functional theory(DFT)calculations of single Pt atom supported on vacancies in a boron nitride nanosheet.Moreover,the relation between the EMSI and the performance of Pt in propane direct dehydrogenation(PDH)is investigated in detail.The charge state and partial density of states of single Pt atom show distinct features at different anchoring positions,such as boron and nitrogen vacancies(Bvac and Nvac,respectively).Single Pt atom become positively and negatively charged on Bvac and Nvac,respectively.Therefore,the electronic structure of Pt can be adjusted by rational deposition on the support.Moreover,Pt atoms in different charge states have been shown to have different catalytic abilities in PDH.The DFT calculations reveal that Pt atoms on Bvac(Pt-Bvac)have much higher reactivity towards reactant/product adsorption and C–H bond activation than Pt supported on Nvac(Pt-Nvac),with larger adsorption energy and lower barrier along the reaction pathway.However,the high reactivity of Pt-Bvac also hinders propene desorption,which could lead to unwanted deep dehydrogenation.Therefore,the results obtained herein suggest that a balanced reactivity for C–H activation in propane and propene desorption is required to achieve optimum yields.Based on this descriptor,a single Pt atom on a nitrogen vacancy is considered an effective catalyst for PDH.Furthermore,the deep dehydrogenation of the formed propene is significantly suppressed,owing to the large barrier on Pt-Nvac.The current work demonstrates that the catalytic properties of supported single Pt atoms can be tuned by rationally depositing them on a boron nitride nanosheet and highlights the great potential of single-atom catalysis in the PDH reaction.

Orbital symmetry matching:Achieving superior nitrogen reduction reaction over single-atom catalysts anchored on Mxene substrates

The nitrogen reduction reaction(NRR)under ambient conditions is still challenging due to the inertness of N2.Herein,we report a series of superior NRR catalysts identified by examining Ti2NO2 MXenes embedded with 28 different single-atom catalysts using first-principles calculations.The stability of this system was first verified using formation energies,and it is discovered that N2 can be effectively adsorbed due to the synergistic effect between single atom catalysis and the Ti atoms.Examination of the electronic structure demonstrated that this design satisfies orbital symmetry matching where“acceptor-donor”interaction scenario can be realized.A new“enzymatic-distal”reaction mechanism that is a mixture of the enzymatic and distal pathways was also discovered.Among all of the candidates,Ni anchored on MXene system achieves an onset potential as low as–0.13 V,which to the best of our knowledge is the lowest onset potential value reported to date.This work elucidates the significance of orbital symmetry matching and provides theoretical guidance for future studies.

Single Pt Atoms Supported on Oxidized Graphene as a Promising Catalyst for Hydrolysis of Ammonia Borane

Based on density functional theory calculations,the full hydrolysis of per NH3BH3 molecule to produce three hydrogen molecules on single Pt atoms supported on oxidized graphene(Pt1/Gr-O)is investigated.It is suggested that the first hydrogen molecule is produced by the combination of two hydrogen atoms from two successive B-H bonds breaking.Then one H2O molecule attacks the left*BHNH3 group(*represents adsorbed state)to form*BH(H2O)NH3 and the elongated O-H bond is easily broken to produce*BH(OH)NH3.The second H2O molecule attacks*BH(OH)NH3 to form*BH(OH)(H2O)NH3 and the breaking of O-H bond pointing to the plane of Pt1/Gr-O results in the desorption of BH(OH)2NH3.The second hydrogen molecule is produced from two hydrogen atoms coming from two H2O molecules and Pt1/Gr-O is recovered after the releasing of hydrogen molecule.The third hydrogen molecule is generated by the further hydrolysis of BH(OH)2NH3 in water solution.The rate-limiting step of the whole process is the combination of one H2O molecule and*BHNH3 with an energy barrier of 16.1 kcal/mol.Thus,Pt1/Gr-O is suggested to be a promising catalyst for hydrolysis of NH3BH3 at room temperature.

Interaction of CO and O2 with Supported Pt Single-Atoms on TiO2(110)

In view of the high activity of Pt single atoms in the low-temperature oxidation of CO,we investigate the adsorption behavior of Pt single atoms on reduced rutile TiO2(110)surface and their interaction with CO and O2 molecules using scanning tunneling microscopy and density function theory calculations.Pt single atoms were prepared on the TiO2(110)surface at 80 K,showing their preferred adsorption sites at the oxygen vacancies.We characterized the adsorption configurations of CO and O2 molecules separately to the TiO2-supported Pt single atom samples at 80 K.It is found that the Pt single atoms tend to capture one CO to form Pt-CO complexes,with the CO molecule bonding to the fivefold coordinated Ti(Ti5 c)atom at the next nearest neighbor site.After annealing the sample from 80 K to 100 K,CO molecules may diffuse,forming another type of complexes,Pt-(CO)2.For O2 adsorption,each Pt single atom may also capture one O2 molecule,forming Pt-O2 complexes with O2 molecule bonding to either the nearest or the next nearest neighboring Ti5c sites.Our study provides the single-molecule-level knowledge of the interaction of CO and O2 with Pt single atoms,which represent the important initial states of the reaction between CO and O2.

Unveiling structural evolution of Fe single atom catalyst in nitrate reduction for enhanced electrocatalytic ammonia synthesis

Atomic transition metal–nitrogen–carbon electrocatalysts exhibit outstanding activity in various electrocatalytic reactions.The challenge lies in predicting the structure of the active center,which may undergo changes under applied potential and interact with reactants or intermediates.Advanced characterization techniques,particularly in-situ X-ray absorption spectroscopy(XAS),provide crucial insights into the structural evolution of the metal active center during the reaction.In this study,nitrate reduction to ammonia(NO_(3)RR)was selected as a model reaction,and we introduced in-situ XAS to reveal the structural evolution during the catalytic process.A novel single atom catalyst of iron loaded on three-dimensional nitrogen–carbon nanonetwork(designated as Fe SAC/NC)was successfully synthesized.We unraveled the structural transformations occurring as pyrrole-N_(4)-Fe transitions to pyrrole-N_(3)-Fe throughout the NO_(3)RR process.Notably,the Fe SAC/NC catalyst exhibited excellent catalytic activity,achieving a Faradaic efficiency of 98.2% and an ammonia generation rate of 22,515μg·h^(−1)·mgcat−1 at−0.8 V versus reversible hydrogen electrode.Theoretical calculations combined with in-situ spectroscopic characterization showed that pyrrole-N_(3)-Fe reduced the energy barrier from *NO to*NHO and improved the selectivity of ammonia.This provides a robust reference for the design of efficient nitrate-to-ammonia synthesis catalysts.

Heterogeneous catalysis under flow for the 21st century fine chemical industry

Due to metal leaching and poor catalyst stability, the chemical industry's fine chemical and pharmaceutical sectors have been historically reluctant to use supported transition metal catalysts to manufacture fine chemicals and active pharmaceutical ingredients. With the advent of new generation supported metal catalysts and flow chemistry, we argue in this study, this situation is poised to quickly change. Alongside heterogenized metal nanoparticles, both single-site molecular and single-atom catalyst will become ubiquitous. This study offers a critical outlook taking into account both technical and economic aspects.

Enhancing sintering resistance of atomically dispersed catalysts in reducing environments with organic monolayers

Atomically dispersed precious metal catalysts maximize atom efficiency and exhibit unique reactivity.However,they are susceptible to sintering.Catalytic reactions occurring in reducing environments tend to result in atomically dispersed metals sintering at lower temperatures than in oxidative or inert atmospheres due to the formation of mobile metal-H or metal-CO complexes.Here,we develop a new approach to mitigate sintering of oxide supported atomically dispersed metals in a reducing atmosphere using organophosphonate self-assembled monolayers(SAMs).We demonstrate this for the case of atomically dispersed Rh on Al_(2)O_(3) and TiO_(2) using a combination of CO probe molecule FTIR,temperature programmed desorption,and alkene hydrogenation rate measurements.Evidence suggests that SAM functionalization of the oxide provides physical diffusion barriers for the metal and weakens the interactions between the reducing gas and metal,thereby discouraging the adsorbate-promoted diffusion of metal atoms on oxide supports.Our results show that support functionalization by organic species can provide improved resistance to sintering of atomically dispersed metals with maintained catalytic reactivity.

Enhanced nitrogen electroreduction performance by the reorganization of local coordination environment of supported single atom on N(O)-dual-doped graphene

Developing stable and efficient catalysts for the electroreduction of nitrogen remains a huge challenge and single atom catalysts(SACs)are expected to achieve relatively high ammonia selectivity at low applied potential.Based on density functional theory calculations,the potential application of 27 single transition metal(TM=Sc-Zn,Y-Ag,Hf-Au)atoms supported by N(O)-dualdoped graphene(TM-O_(2)N_(2)/G)for the electroreduction of nitrogen is intensively investigated.At low nitrogen coverage,W(Mo,Nb,Ta)-O_(2)N_(2)/G are predicted to yield low ammonia selectivity(<13%)at limiting-potential of-0.58,-0.53,-0.56,and-0.76 V starting from adsorbed nitrogen with side-on mode,respectively.With the increasing N_(2)coverage,the TM-O_(2)N_(2)/G is reconstructed as TM-(N_(2))2N_(2)/graphene.The electroreduction of nitrogen proceeds from end-on adsorbed nitrogen molecule with high ammonia selectivity,and the limiting-potentials are theoretically predicted as-0.20,-0.40,-0.29,and-0.21 V on W(Mo,Nb,Ta)-(N_(2))2N_(2)/G,respectively.It is suggested that utilizing the reorganization of local coordination environments of SACs by high coverage of reactant molecules under reaction condition can not only enhance the activity at lower limiting-potential but also improve the ammonia selectivity.

Synthesis of cobalt single atom catalyst by a solid-state transformation strategy for direct C-C cross-coupling of primary and secondary alcohols

Atomic engineering of single atom catalysts(SACs)with high-density available active sites and optimized electronic properties can substantially boost catalytic efficacy.Herein,we report a solid-state transformation strategy to access Co SACs by introducing Co species from commercial CO_(2)O3 powders into nitrogen-doped carbon support.The catalyst exhibited excellent catalytic activity,with a turnover frequency(TOF)of 2,307 h^(-1)and yield of 95%,in the direct C-C cross-coupling of benzyl alcohol and 1-phenylethanol(1 atm O2@80℃)to yield chaicone.Density functional theory(DFT)calculations demonstrate the coordination environment and electronic metal-support interaction impact the catalytic pathway.In particular,a wide substrate scope and a broad functional-group tolerance of this SAC were validated,and the employment of this strategy for large-scale synthesis was also shown to be feasible.This work might shed light on the facile and scalable synthesis of highly active,selective,and stable SACs for heterogeneous catalysis.

Defect engineering for high-selection-performance of NO reduction to NH3 over CeO_(2)(111)surface:A DFT study

To reduce the greenhouse effect caused by the surgery of nitrogen-oxides concentration in the atmosphere and develop a future energy carrier of renewables,it is very critical to develop more efficient,controllable,and highly sensitive catalytic materials.In our work,we proposed that nitric oxide(NO),as a supplement to N_(2) for the synthesis of ammonia,which is equipped with a lower barrier.And the study highlighted the potential of CeO_(2)(111)nanosheets with La doping and oxygen vacancy(OV)as a high-performance,controllable material for NO capture at the site of Vo site,and separation the process of hydrogenation.We also reported that the E_(ads) of-1.12 eV with horizontal adsorption and the Bader charge of N increasing of 0.53|e|and O increasing of 0.17|e|at the most active site of reduction-OV predicted.It is worth noting thatΔG of NORR(NO reduction reaction)shows good performance(thermodynamically spontaneous reaction)to synthesize ammonia and water at room temperature in the theoretical calculation.

Kinetically rate-determining step modulation by metal-support interactions for CO oxidation on Pt/CeO_(2)

Rational design and performance promotion are eternal topics and ultimate goals in catalyst preparation.In contrast,trial–and–error is still the common method people take.Therefore,it is important to develop methods to intrinsically enhance the performance of catalysts.The most effective solutions are the one from a kinetic perspective based on clear knowledge of the reaction mechanism.This paper describes rate-determining step cognition and modulation to promote CO oxidation on highly dispersed Pt on CeO_(2).The different degrees of metal–support interactions due to variation of hydroxyl density of support could alter the structure of active species and the ability of oxygen activation apparently,further shift the rate-determining step from oxygen activation to oxygen reverse spillover kinetically.The transformation of rate-determining step could enhance the intrinsic activity significantly,and decrease the T_(50) approximately 140℃.The findings of this research exemplify the universal and effective method of performance elevation by rate-determining step modulation,which is promising for application in different systems.

Highly Efficient Oxygen Reduction Reaction Fe-N-C Cathode in Long-durable Direct Glycol Fuel Cells

The oxygen reduction reaction in direct glycol fuel cells heavily relies on noble metal-based electrocatalysts.In this work,novel Pt group metal-free catalysts based on porous Fe-N-C materials are successfully synthesized as catalysts with high activity and durability for the cathode oxygen reduction reaction(ORR).Through the encapsulation of NH_(4)SCN salt,the surface elements and pore structure of the catalyst are effectively changed,and the active sites of Fe effectively are increased.The half-wave potential of the best Fe-N-C catalyst was-0.02 V vs.Hg/HgO in an alkaline environment.The porous Fe-N-C catalyst possesses a large specific surface area(1158 m^(2)/g)and shows good activity and tolerance to glycol.The direct glycol fuel cell with the Fe-N-C cathode achieved a maximum power density of 62.2 mW/cm^(2) with 4 mol/L KOH and 4 mol/L glycol solution at 25°C and maintained discharge for more than 250 h at a 50 A/cm^(2) current density.