Surface coupling of methyl radicals for efficient low‐temperature oxidative coupling of methane

Selective coupling of methyl radicals to produce C_(2) species(C2H4 and C2H6)is a key challenge for oxidative coupling of methane(OCM).In traditional OCM reaction systems,homogeneous transformation of methyl radicals in O_(2)‐containing gases are uncontrollable,resulting in limited C_(2) selectivity and yield.Herein,we demonstrate that methyl radicals generated by La_(2)O_(3)at low reaction temperature can selectively couple on the surface of 5 wt%Na2WO4/SiO_(2).The controllable surface coupling against overoxidation barely changes the activity of La_(2)O_(3)but boosts the C_(2)selectivity by three times and achieves a C_(2)yield as high as 10.9%at bed temperature of only 570℃.Structure‐property studies suggest that Na_(2)WO_(4) nanoclusters are the active sites for methyl radical coupling.The strong CH_(3)·affinity of these sites can even endow some methane combustion catalysts with OCM activity.The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst.The bifunctional OCM catalyst system,which composes of a methane activation center and a CH_(3)·coupling center,may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4(~600℃)and is therefore more controllable,safer,and certainly more attractive as an actual process.

Reversible transformation between terrace and step sites of Pt nanoparticles on titanium under CO and O_(2) environments

Understanding the dynamic evolution of active sites of supported metal catalysts during catalysis is fundamentally important for improving its performance,which attracts tremendous research interests in the past decades.There are two main surficial structures for metal catalysts:terrace sites and step sites,which exhibit catalytic activity discrepancy during catalysis.Herein,by using in situ transmission electron microscopy and in situ Fourier transform infrared spectroscopy(FTIR),the transformation between surface terrace and step sites of Pt-TiO_(2) catalysts was studied under CO and O_(2) environments.We found that the{111}step sites tend to form at{111}terrace under O_(2) environment,while these step sites prefer to transform into terrace under CO environment at elevated temperature.Meanwhile,quantitative ratios of terrace/step sites were obtained by in situ FTIR.It was found that this transformation between terrace sites and step sites was reversible during gas treatment cycling of CO and O_(2).The selective adsorption of O_(2) and CO species at different sites,which stabilized the step/terrace sites,was found to serve as the driving force for active sites transition by density functional theory calculations.Inspired by the in situ results,an enhanced catalytic activity of Pt-TiO_(2) catalysts was successfully achieved through tuning surface-active sites by gas treatments.

Hole-Transporting Materials with Rational Combination of Pyridine and Dibenzo[a,c]phenazine as Electron Acceptor for Dopant-Free Perovskite Solar Cells

Perovskite solar cells(PSCs)have been proven to be a promising option for photovoltaic conversion.With the aim to achieve efficient and stable PSCs,it is essential to explore dopant-free hole-transporting materials(HTMs)with high hole mobility.Herein,HTMs bearing electron donor(D)-electron acceptor(A)-electron donor(D)structures have been constructed with strong intramolecular charge transfer(ICT)effect,based on rational combination of dibenzo[a,c]phenazine and pyridine as electronic acceptors and anchoring groups to perovskite layer.Accordingly,high hole mobility(7.31×10^(-5) cm^(2)·V^(-1)·s^(-1))and photoelectric conversion efficiency(20.45%)have been achieved by dopant-free DPyP-based PSC.It afforded an efficient way to design HTMs with high hole mobility by adjustment of molecular configurations and electronic property of conjugated systems.

Recent advances in gas-involved in situ studies via transmission electron microscopy

Gases that are widely used in research and industry have a significant effect on both the configuration of solid materials and the evolution of reactive systems. Traditional studies on gas-solid interactions have mostly been static and post-mortem and unsatisfactory for elucidating the real active states during the reactions. Recent developments of controlled-atmosphere transmission electron microscopy （TEM） have led to impressive progress towards the simulation of real-world reaction environments, allowing the atomic-scale recording of dynamic events. In this review, on the basis of the in situ research of our group, we outline the principles and features of the controlled-atmosphere TEM techniques and summarize the significant recent progress in the research activities on gas-solid interactions, including nanowire growth, catalysis, and metal failure. Additionally, the challenges and opportunities in the real-time observations on such platform are discussed.

Direct observation of Pt nanocrystal coalescence induced by electron-excitation-enhanced van der Waals interactions

Nanocrystal coalescence has attracted paramount attention in nanostructure fabrication in the past decades. Tremendous endeavor and progress have been made in understanding its mechanisms, benefiting from the development of transmission electron microscopy. However, many mechanisms still remain unclear, especially for nanocrystals that lack a permanent dipole moment standing on a solid substrate. Here, we report an in situ coalescence of Pt nanocrystals on an amorphous carbon substrate induced by electron-excitation- enhanced van der Waals interactions studied by transmission electron microscopy and first principles calculations. It is found that the electron-beam-induced excitation can significantly enhance the van der Waals interaction between Pt nanocrystals and reduce the binding energy between Pt nanocrystals and the carbon substrate, both of which promote the coalescence. This work extends our understanding of the nanocrystal coalescence observed in a transmission electron microscope and sheds light on a potential pathway toward practical electron- beam-controlled nanofabrication.

In situ TEM observation of dissolution and regrowth dynamics of MoO2 nanowires under oxygen

Direct observation of the dissolution behavior of nanomaterials could provide fundamental insight to understanding their anisotropic properties and stability. The dissolution mechanism in solution and vacuum has been well documented. However, the gas-involved dissolution and regrowth have seldom been explored and the mechanisms remain elusive. We report herein, an in situ TEM study of the dissolution and regrowth dynamics of MoO2 nanowires under oxygen using environmental transmission electron microscopy （ETEM）. For the first time, oscillatory dissolution on the nanowire tip is revealed, and, intriguingly, simultaneous layer-by-layer regrowth on the sidewall facets is observed, leading to a shorter and wider nanowire. Combined with first-principles calculations, we found that electron beam irradiation caused oxygen loss in the tip facets, which resulted in changing the preferential growth facets and drove the morphology reshaping.

Toward In Situ Atomistic Design of Catalytic Active Sites via Controlled Atmosphere Transmission Electron Microscopy

CONSPECTUS:Heterogeneous catalysts are widely used in a variety of industrial fields,including environmental protection,energy conversion,and chemical production.Their performance during reactions is usually determined by a small fraction of sites at the catalyst surfaces/interfaces,namely,the active sites.Actually,since the concept of the“active sites”was proposed by Hugh Taylor in the 1920s,determining the active site at the atomic level and understanding the molecular processes that happened at the active site have become the top priority in catalysis research.Researchers tried different methods to acquire various information related to the surface/interface active sites,pursuing their rational design at the atomic level.Although great achievements have been made in catalyst surface study,in situ atomistic design of active sites remains challenging,due to the lack of direct information and effective manipulation means concerning the active sites.Specifically,many critical issues regarding the active sites under reaction conditions remain to be solved:(1)precisely identifying the active sites,which is the foundation for understanding the catalytical mechanism and rational design of the catalyst;(2)accurately manipulating the surface/interface active sites at the atomic scale,which is the basis for realizing the desired performance;(3)maintaining these designed active sites operating in a long-term with high efficiency without deactivation,which is extremely important to the practical applications of the catalysts.All these aspects rely on the fundamental understanding of the interactions between different surface/interface configurations of the catalyst and external environments(gas,pressure,temperature,etc.).In this Account,we present the recent progress in our group on the studies of surface/interface active sites via controlled atmosphere transmission electron microscopy(CATEM).We first briefly introduce the advances in CATEM technologies,including the window(closed)approach and aperture(open)approaches.Then,the challenge of identifying active sites is discussed,and our efforts in this target by determining surface atomic structures,tracking the active components,visualizing reacting molecules,and in situ evaluating catalytic performance are demonstrated.The next section focuses on the in situ manipulating active sites by controlling external environmental factors(e.g.,gas,temperature,and pressure),including tailoring the catalyst shape,surface components,and interface with atomic precision.The fourth section discusses the strategies for the long-term stable operation of the active sites based on our in situ studies in understanding the deactivation mechanisms.In the end,we provide our perspectives on the future opportunities and some scientific and technical challenges in this booming area.This Account highlights the in situ atomic level design of the active sites based on the CATEM route,which provides an applicable strategy for deep understanding and the rational design of the catalyst active sites.

Visualizing the toughening origins of gel-grown calcite single-crystal composites

Biogenic single crystals have been widely demonstrated to incorporate macromolecules to achieve extra damage tolerance, spurring investigations on their synthetic analogs with enhanced mechanical properties as well as the enhancement mechanism（s） behind. And the investigations rely on both rational design of the single-crystal composites and, equally importantly, nanoscale and in-situ characterization strategy. Here, composite structures are constructed inside the calcite single-crystal host by incorporating guest materials of agarose fibers, multi-walled carbon nanotubes （MWCNTs）, and graphene oxide （GO）, through crystallization in agarose gel media. Further, transmission electron microscopy-scanning probe microscopy （TEM-SPM） method, coupling compression measurements with nanoscale imaging, shows that the obtained single-crystal composites exhibit improved toughness, compared to the solution-grown pure single crystals. Particularly, the rupture time increases by 1.25 times after the gel-networks and MWCNTs are incorporated. More importantly, the in-situ observation of the crystal deformation suggests that the guest incorporation toughens the single-crystal host by the shielding effect of nanofiber on crack-bridging at nanoscale. As such, this work may have implications for understanding the damage tolerance of biominerals as well as towards the development of new mechanically reinforced single-crystal composite materials.