Highly Selective Photocatalytic Synthesis of Acetic Acid at 0-25oC.

Acetic acid (AA), a vital compound in chemical production and materials manufacturing, is conventionally synthesized by starting with coal or methane through multiple steps including high-temperature transformations. Here we present a new synthesis of AA from ethane through photocatalytic selective oxidation of ethane by H2O2 at 0-25°C. The catalyst designed for this process comprises g-C3N4 with anchored Pd1 single-atom sites. In-situ studies and computational simulation suggest the immobilized Pd1 atom becomes positively charged under photocatalytic condition. Under photoirradiation, the holes on the Pd1 single-atom of OH-Pd1Å/g-C3N4 serves as a catalytic site for activating a C-H instead of C-C of C2H6 with a low activation barrier of 0.14 eV, through a concerted mechanism. Remarkably, the selectivity for synthesizing AA reaches 98.7%, achieved under atmospheric pressure of ethane at 0°C. By integrating photocatalysis with thermal catalysis, we introduce a highly selective, environmentally friendly, energy-efficient synthetic route for AA, starting from ethane, presenting a promising alternative for AA synthesis. This integration of photocatalysis in low-temperature oxidation demonstrates a new route of selective oxidation of light alkanes.

Atomic-Scale Mechanism of Platinum Catalyst Restructuring under a Pressure of Reactant Gas.

Heterogeneous catalysis is key for chemical transformations. Understanding how catalysts' active sites dynamically evolve at the atomic scale under reaction conditions is a prerequisite for accurately determining catalytic mechanisms and predictably developing catalysts. We combine in situ time-dependent scanning tunneling microscopy observations and machine-learning-accelerated first-principles atomistic simulations to uncover the mechanism of restructuring of Pt catalysts under a pressure of carbon monoxide (CO). We show that a high CO coverage at a Pt step edge triggers the formation of atomic protrusions of low-coordination Pt atoms, which then detach from the step edge to create sub-nano-islands on the terraces, where under-coordinated sites are stabilized by the CO adsorbates. The fast and accurate machine-learning potential is key to enabling the exploration of tens of thousands of configurations for the CO-covered restructuring catalyst. These studies open an avenue to achieve an atomic-scale understanding of the structural dynamics of more complex metal nanoparticle catalysts under reaction conditions.

Integrated in situ spectroscopic studies on syngas production from partial oxidation of methane catalyzed by atomically dispersed rhodium cations on ceria.

Catalytic reforming of methane to produce syngas is an important strategy for producing value-added chemicals. The conventional reforming catalyst relies on supported nickel nanoparticles. In this work, we investigated singly dispersed Rh cations anchored on a CeO2 catalyst (Rh1/CeO2) for high activity and selectivity towards the production of syngas via partial oxidation of methane (POM) in the temperature range of 600-700 °C. The yields of H2 and CO at 700 °C are 83% and 91%, respectively. The anchored Rh1 atoms on CeO2 of Rh1/CeO2 are in the cationic state, and on an average each Rh1 atom coordinates with 4-5 surface lattice oxygen atoms of CeO2. Compared to inert CeO2 for POM, via the incorporation of single-atom sites, Rh1 modifies the electronic state of oxygen atoms proximal to the Rh1 atoms and thus triggers the catalytic activity of CeO2. The high activity of single-atom catalyst Rh1/CeO2 suggests that the incorporation of single atoms of transition metals to the surface of a reducible oxide can modulate the electronic state of proximal anions of the oxide support toward forming an electronic state favorable for the selective formation of ideal products.

C-N Coupling through Hydroaminoalkylation on a Single-Atom Rh Heterogeneous Catalyst.

C-N coupling is significant for the synthesis of fine chemicals toward various applications. Hydroaminoalkylation of olefins is a tandem reaction of C-N coupling involving first the formation of an aldehyde through hydroformylation of an olefin and then the production of amine through reductive amination of the aldehyde. Here we report a stable, supported catalyst of singly dispersed Rh1 atoms anchored on TiO2 (P25) nanoparticles designated as Rh1 /P25. Its high activity for C-N coupling was demonstrated by six hydroaminoalkylations of olefins and amines with selectivity of higher than 90% for producing tertiary amines. The singly dispersed Rh1 O4 on P25 exhibit activity and selectivity for hydroaminoalkylation comparable or even higher than some reported molecular catalysts. In contrast to molecular catalysts, the Rh-based single-atom Rh heterogeneous catalysis (Rh1 /P25) can be readily separated from reactants and products, reused for multiple runs of hydroaminoalkylation, and recycled with a low cost.

Single-Atom High-Temperature Catalysis on a Rh1O5 Cluster for Production of Syngas from Methane.

Single-atom catalysts are a relatively new type of catalyst active for numerous reactions but mainly for chemical transformations performed at low or intermediate temperatures. Here we report that singly dispersed Rh1O5 clusters on TiO2 can catalyze the partial oxidation of methane (POM) at high temperatures with a selectivity of 97% for producing syngas (CO + H2) and high activity with a long catalytic durability at 650 °C. The long durability results from the substitution of a Ti atom of the TiO2 surface lattice by Rh1, which forms a singly dispersed Rh1 atom coordinating with five oxygen atoms (Rh1O5) and an undercoordinated environment but with nearly saturated bonding with oxygen atoms. Computational studies show the back-donation of electrons from the dz2 orbital of the singly dispersed Rh1 atom to the unoccupied orbital of adsorbed CHn (n > 1) results in the charge depletion of the Rh1 atom and a strong binding of CHn to Rh1. This strong binding decreases the barrier for activating C-H, thus leading to high activity of Rh1/TiO2. A cationic Rh1 single atom anchored on TiO2 exhibits a weak binding to atomic carbon, in contrast to the strong binding of the metallic Rh surface to atomic carbon. The weak binding of atomic carbon to Rh1 atoms and the spatial isolation of Rh1 on TiO2 prevent atomic carbon from coupling on Rh1/TiO2 to form carbon layers, making Rh1/TiO2 resistant to carbon deposition than supported metal catalysts for POM. The highly active, selective, and durable high-temperature single-atom catalysis performed at 650 °C demonstrates an avenue of application of single-atom catalysis to chemical transformations at high temperatures.

Understanding Catalyst Surfaces during Catalysis through Near Ambient Pressure X-ray Photoelectron Spectroscopy.

Heterogeneous catalysis occurs on the surface of a catalyst particle in a gas or liquid environment of reactants. The surface of the catalyst particle acts as an active chemical agent directly participating in a chemical reaction performed at a solid-gas or solid-liquid interface. Thus, authentic surface chemistry and the structure of a catalyst particle during catalysis are key descriptors for understanding catalytic performance of this catalyst. However, identification of the authentic surface of a catalyst particle during catalysis is not a simple task. We are far from knowing the fact. Photoelectron spectroscopy is one of the main techniques for characterizing surface of a catalyst since it's a surface sensitive technique. When used to track the surface of a catalyst particle at relatively high temperature in gas phase in the torr pressure range, it is called near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) or AP-XPS for simplicity. In the last several years, AP-XPS has been used to observe surface chemistry of catalysts of single crystals and nanoparticles of metal, metal oxide, and carbide. In this review, instrumentation of the near ambient pressure X-ray photoelectron spectrometers and observation of catalyst surfaces in gases phase under reaction conditions and during catalysis with AP-XPS are discussed with the following objectives: (1) to present how the surface of a catalyst particle can be characterized in gas phase, (2) to interpret how surface chemistries observed during catalysis are correlated with measured catalytic performances, (3) to demonstrate how the uncovered correlations between surface structures and catalytic performances help to understand catalytic mechanisms at a molecular level, and (4) to discuss challenges and prospects of using AP-XPS to explore the authentic surface of a catalyst under a condition near to an industrial catalytic condition. This review focuses on the application of AP-XPS to studies of catalysis and how the insights gained from AP-XPS studies can be used to achieve fundamental understanding of the catalytic mechanism at a molecular level.

Synergy of Single-Atom Ni1 and Ru1 Sites on CeO2 for Dry Reforming of CH4.

Heterogeneous catalysis performs on specific sites of a catalyst surface even if specific sites of many catalysts during catalysis could not be identified readily. Design of a catalyst by managing catalytic sites on an atomic scale is significant for tuning catalytic performance and offering high activity and selectivity at a relatively low temperature. Here, we report a synergy effect of two sets of single-atom sites (Ni1 and Ru1) anchored on the surface of a CeO2 nanorod, Ce0.95Ni0.025Ru0.025O2. The surface of this catalyst, Ce0.95Ni0.025Ru0.025O2, consists of two sets of single-atom sites which are highly active for reforming CH4 using CO2 with a turnover rate of producing 73.6 H2 molecules on each site per second at 560 °C. Selectivity for producing H2 at this temperature is 98.5%. The single-atom sites Ni1 and Ru1 anchored on the CeO2 surface of Ce0.95Ni0.025Ru0.025O2 remain singly dispersed and in a cationic state during catalysis up to 600 °C. The two sets of single-atom sites play a synergistic role, evidenced by lower apparent activation barrier and higher turnover rate for production of H2 and CO on Ce0.95Ni0.025Ru0.025O2 in contrast to Ce0.95Ni0.05O2 with only Ni1 single-atom sites and Ce0.95Ru0.05O2 with only Ru1 single-atom sites. Computational studies suggest a molecular mechanism for the observed synergy effects, which originate at (1) the different roles of Ni1 and Ru1 sites in terms of activations of CH4 to form CO on a Ni1 site and dissociation of CO2 to CO on a Ru1 site, respectively and (2) the sequential role in terms of first forming H atoms through activation of CH4 on a Ni1 site and then coupling of H atoms to form H2 on a Ru1 site. These synergistic effects of the two sets of single-atom sites on the same surface demonstrated a new method for designing a catalyst with high activity and selectivity at a relatively low temperature.

Methane Chemisorption on Oxide-Supported Pt Single Atom.

Methane chemisorption has been recently demonstrated on the rutile IrO2 (110) surface. However, it remains unclear how the general requirements are for methane chemisorption or complexation with a single atom on an oxide surface. By exploring methane adsorption on Pt1 substitutionally doped on many rutile-type oxides using hybrid density functional theory, we show that the occupancy of the Pt dz2 orbital is the key to methane chemisorption. Pt single atom on the semiconducting or wide-gap oxides such as TiO2 and GeO2 strongly chemisorbs methane, because the empty Pt dz2 orbital is located in the gap and can effectively accept σ-electron donation from the methane C-H bond. In contrast, Pt single atom on metallic oxides such as IrO2 and RuO2 does not chemisorb methane, because the Pt dz2 orbital strongly mixes with the support-oxide electronic states and become more occupied, losing its ability to chemisorb methane. This study sheds further light on the impact of the interaction between a Pt single atom and the oxide support on methane adsorption.

C-C Coupling on Single-Atom-Based Heterogeneous Catalyst.

Compared to homogeneous catalysis, heterogeneous catalysis allows for ready separation of products from the catalyst and thus reuse of the catalyst. C-C coupling is typically performed on a molecular catalyst which is mixed with reactants in liquid phase during catalysis. This homogeneous mixing at a molecular level in the same phase makes separation of the molecular catalyst extremely challenging and costly. Here we demonstrated that a TiO2-based nanoparticle catalyst anchoring singly dispersed Pd atoms (Pd1/TiO2) is selective and highly active for more than 10 Sonogashira C-C coupling reactions (R≡CH + R'X → R≡R'; X = Br, I; R' = aryl or vinyl). The coupling between iodobenzene and phenylacetylene on Pd1/TiO2 exhibits a turnover rate of 51.0 diphenylacetylene molecules per anchored Pd atom per minute at 60 °C, with a low apparent activation barrier of 28.9 kJ/mol and no cost of catalyst separation. DFT calculations suggest that the single Pd atom bonded to surface lattice oxygen atoms of TiO2 acts as a site to dissociatively chemisorb iodobenzene to generate an intermediate phenyl, which then couples with phenylacetylenyl bound to a surface oxygen atom. This coupling of phenyl adsorbed on Pd1 and phenylacetylenyl bound to Oad of TiO2 forms the product molecule, diphenylacetylene.

X-ray Photoelectron Spectroscopy Studies of Nanoparticles Dispersed in Static Liquid.

For nanoparticles active for chemical and energy transformations in static liquid environment, chemistries of surface or near-surface regions of these catalyst nanoparticles in liquid are crucial for fundamentally understanding their catalytic performances at a molecular level. Compared to catalysis at a solid-gas interface, there is very limited information on the surface of these catalyst nanoparticles under a working condition or during catalysis in liquid. Photoelectron spectroscopy is a surface-sensitive technique; however, it is challenging to study the surfaces of catalyst nanoparticles dispersed in static liquid because of the short inelastic mean free path of photoelectrons traveling in liquid. Here, we report a method for tracking the surface of nanoparticles dispersed in static liquid by employing graphene layers as an electron-transparent membrane to separate the static liquid containing a solvent, catalyst nanoparticles, and reactants from the high-vacuum environment of photoelectron spectrometers. The surfaces of Ag nanoparticles dispersed in static liquid sealed in such a graphene membrane liquid cell were successfully characterized using a photoelectron spectrometer equipped with a high vacuum energy analyzer. With this method, the surface of catalyst nanoparticles dispersed in liquid during catalysis at a relatively high temperature up to 150 °C can be tracked with photoelectron spectroscopy.

Atomic-Scale Observations of Catalyst Structures under Reaction Conditions and during Catalysis.

Heterogeneous catalysis is a chemical process performed at a solid-gas or solid-liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.

Interactions of gaseous molecules with X-ray photons and photoelectrons in AP-XPS study of solid surface in gas phase.

Studies of the surface of a catalyst in the gas phase via photoelectron spectroscopy is an important approach to establish a correlation between the surface of a catalyst under reaction conditions or during catalysis and its corresponding catalytic performance. Unlike the well understood interactions between photoelectrons and the atomic layers of a surface in ultrahigh vacuum (UHV) and the well-developed method of quantitative analysis of a solid surface in UHV, a fundamental understanding of the interactions between X-ray photons and gaseous molecules and between photoelectrons and molecules of the gas phase in ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is lacking. Through well designed experiments, here the impact of the interactions between photoelectrons and gaseous molecules and interactions between X-ray photons and gaseous molecules on the intensity of the collected photoelectrons have been explored. How the changes in photoelectron intensity resulting from these interactions influence measurement of the authentic atomic ratio of element M to A of a solid surface has been discussed herein, and methods to correct the measured nominal atomic ratio of two elements of a solid surface upon travelling through a gas phase to its authentic atomic ratio have been developed.

Low-temperature activation of methane on doped single atoms: descriptor and prediction.

Catalytic transformation of methane under mild conditions remains a grand challenge. Fundamental understanding of C-H activation of methane is crucial for designing a catalyst for the utilization of methane at low temperature. Recent experiments show that strong methane chemisorption on oxides of precious metals leads to facile C-H activation. However, only a very few such oxides are capable (for example, IrO2 and PdO). Here we show for the first time that strong methane chemisorption and facile C-H activation can be accomplished by single transition-metal atoms on TiO2, some of which are even better than IrO2. Using methane adsorption energy as a descriptor, we screened over 30 transition-metal single atoms doped on TiO2 for the chemisorption of methane by replacing a surface Ti atom with a single atom of another transition metal. It is found that the adsorption energies of methane on a single atom of Pd, Rh, Os, Ir, and Pt doped on rutile TiO2(110) are greater than or similar to those on rutile IrO2(110), a benchmark for the chemisorption of methane on transition oxides. Electronic structure analysis uncovered orbital overlap and mixing between methane and the single atom, as well as significant localization of the charge between the molecule and the surface, demonstrating chemical bonding of CH4 to doped single atoms. Facile C-H dissociation has been found on the single-atom sites with the transition state energies lower than desorption energies. Our computational studies predict that Pd, Rh, Os, Ir, and Pt single atoms on rutile TiO2(110) can activate C-H of methane at a temperature lower than 25 °C.

Transition of surface phase of cobalt oxide during CO oxidation.

In situ/operando studies of a heterogeneous catalyst are particularly valuable for achieving a fundamental understanding of catalytic mechanisms at a molecular level by establishing a correlation between the observed catalytic performance and the corresponding surface chemistry during catalysis. Herein, CO oxidation on cobalt oxides was studied via ambient pressure X-ray photoelectron spectroscopy (AP-XPS). During CO oxidation on CoO in the temperature range of 140-180 °C, the active surface phase of CoO progressively transforms to Co3O4. Kinetic studies of CO oxidation on the surface phase CoO at 80-120 °C and on the formed Co3O4 at 160-220 °C show that CoO and Co3O4 exhibit different activation barriers: 49.3 kJ mol-1 for CoO and 36.9 kJ mol-1 for Co3O4. This study demonstrates the transition of the active surface phase of a transition metal oxide-based catalyst under catalytic conditions with no change in the bulk phase of the catalyst.

Operando chemistry of catalyst surfaces during catalysis.

Chemistry of a catalyst surface during catalysis is crucial for a fundamental understanding of mechanism of a catalytic reaction performed on the catalyst in the gas or liquid phase. Due to the pressure- or molecular density-dependent entropy contribution of gas or liquid phase of the reactants and the potential formation of a catalyst surface during catalysis different from that observed in an ex situ condition, the characterization of the surface of a catalyst under reaction conditions and during catalysis can be significant and even necessary for understanding the catalytic mechanism at a molecular level. Electron-based analytical techniques are challenging for studying catalyst nanoparticles in the gas or liquid phase although they are necessary techniques to employ. Instrumentation and further development of these electron-based techniques have now made in situ/operando studies of catalysts possible. New insights into the chemistry and structure of catalyst nanoparticles have been uncovered over the last decades. Herein, the origin of the differences between ex situ and in situ/operando studies of catalysts, and the technical challenges faced as well as the corresponding instrumentation and innovations utilized for characterizing catalysts under reaction conditions and during catalysis, are discussed. The restructuring of catalyst surfaces driven by the pressure of reactant(s) around a catalyst, restructuring in reactant(s) driven by reaction temperature and restructuring during catalysis are also reviewed herein. The remaining challenges and possible solutions are briefly discussed.

Achieving Atomic Dispersion of Highly Loaded Transition Metals in Small-Pore Zeolite SSZ-13: High-Capacity and High-Efficiency Low-Temperature CO and Passive NOx Adsorbers.

The majority of harmful atmospheric CO and NOx emissions are from vehicle exhausts. Although there has been success addressing NOx emissions at temperatures above 250 °C with selective catalytic reduction technology, emissions during vehicle cold start (when the temperature is below 150 °C), are a major challenge. Herein, we show we can completely eliminate both CO and NOx emissions simultaneously under realistic exhaust flow, using a highly loaded (2 wt %) atomically dispersed palladium in the extra-framework positions of the small-pore chabazite material as a CO and passive NOx adsorber. Until now, atomically dispersed highly loaded (>0.3 wt %) transition-metal/SSZ-13 materials have not been known. We devised a general, simple, and scalable route to prepare such materials for PtII and PdII . Through spectroscopy and materials testing we show that both CO and NOx can be simultaneously completely abated with 100 % efficiency by the formation of mixed carbonyl-nitrosyl palladium complex in chabazite micropore.

Size- and shape-dependent catalytic performances of oxidation and reduction reactions on nanocatalysts.

Heterogeneous catalysis is one of the most important chemical processes of various industries performed on catalyst nanoparticles with different sizes or/and shapes. In the past two decades, the catalytic performances of different catalytic reactions on nanoparticles of metals and oxides with well controlled sizes or shapes have been extensively studied thanks to the spectacular advances in syntheses of nanomaterials of metals and oxides. This review discussed the size and shape effects of catalyst particles on catalytic activity and selectivity of reactions performed at solid-gas or solid-liquid interfaces with a purpose of establishing correlations of size- and shape-dependent chemical and structural factors of surface of a catalyst with the corresponding catalytic performances toward understanding of catalysis at a molecular level.

Formation of Second-Generation Nanoclusters on Metal Nanoparticles Driven by Reactant Gases.

Heterogeneous catalysis occurs at the interface between a solid catalyst and the reactants. The structure of metal catalyst nanoparticles at the metal-gas interface is a key factor that determines catalytic selectivity and activity. Here we report that second-generation nanoclusters are formed on the initial catalyst nanoparticles as a result of interaction with the reactant molecules when the nanoparticles are in a gas phase at Torr pressure or higher. The formation of the second-generation nanoclusters is manifested by a decrease of the average coordination number of the metal atoms and a shift of their core level energies in the presence of gases. The formation of second-generation nanoclusters increases the number of undercoordinated sites, which are the most active for catalysis in many cases.

Integrated In†Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation.

Sodium-based catalysts (such as Na2 WO4 ) were proposed to selectively catalyze OH radical formation from H2 O and O2 at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na2 WO4 , which can form OH radicals, using in situ techniques including X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). As a result, Na2 O2 species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas-phase hydrocarbon reactions, including the selective transformation of methane to ethane.

Excavation of Precious-Metal-Based Alloy Nanoparticles for Efficient Catalysis.

Methods have recently been developed for the synthesis of excavated alloy nanoparticles. However, various challenges still need to be overcome for a broad range of excavated nanoparticles with different sizes, surface structures, compositions, and constituent elements to be available for chemical and energy transformations through thermal catalysis and electrocatalysis.