FeO x supported single‐atom Pd bifunctional catalyst for water gas shift reaction

A Multitechnique Study of C2H4 Adsorption on Fe3O4(001)

The adsorption/desorption of ethene (C2H4), also commonly known as ethylene, on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature-programmed desorption (TPD), scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT)-based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C2H4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C2H4 binds atop surface Fe sites up to a coverage of 2 molecules per (√2 × √2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C2H4 adsorption is found to be close to 4 molecules per (√2 × √2)R45° unit cell.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Noble-metal based single-atom catalysts for the water-gas shift reaction

Single-atom catalysts (SACs) have attracted great attention in heterogeneous catalysis. In this Feature Article, we summarize the recent advances of typical Au and Pt-group-metal (PGM) based SACs and their applications in the water-gas shift (WGS) reaction in the past two decades. First, oxide and carbide supported single atoms are categorized. Then, the active sites in the WGS reaction are identified and discussed, with SACs as the positive state or metallic state. After that, the reaction mechanisms of the WGS are presented, which are classified into two categories of redox mechanism and associative mechanism. Finally, the challenges and opportunities in this emerging field for the collection of hydrogen are proposed on the basis of current developments. It is believed that more and more exciting findings based on SACs are forthcoming.

α-MoC Supported Noble Metal Catalysts for Water-Gas Shift Reaction: Single-Atom Promoter or Single-Atom Player

In this work, we study the water-gas shift (WGS) reaction catalyzed by α-MoC(100) supported typical platinum group metal (PGM) single atoms (Rh₁, Pd₁, and Pt₁) and Au₁ via density functional theory calculations. The adsorption energies of key reaction intermediates and the kinetic barriers of the proposed rate-determining step in the WGS were systematically investigated. It is found that Rh₁, Pd₁, and Pt₁ can serve as single-atom promoters (SAPs) to improve the WGS performance of surface Mo atoms on α-MoC(100). The enhanced activity originates from the fact that SAP modifies the electronic structure of Mo active sites. Comparatively, the Au₁ species not only acts as an SAP but also directly participates in the catalysis as a single-atom player. The additional experiments with single-atom catalyst performance and kinetic studies confirm the theoretical calculation conclusions. This study can provide a basis to further develop efficient WGS catalysts by tuning the activity of the substrate with intercalation of SAPs.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

Surface Density Dependent Catalytic Activity of Single Palladium Atoms Supported on Ceria*

The analogy between single-atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. We provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria-supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of CeO₂ reducibility. The long-range electrostatic footprints (≈1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in an observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certain Pd density, leading to their permanent removal and a consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single-atom catalysts can be tuned by varying the surface density of single metal atoms.

DFT calculations for single-atom confinement effects of noble metals on monolayer g-C3N4 for photocatalytic applications

Graphitic carbon nitride, as a very promising two-dimensional structure host for single atom catalysts (SACs), has been studied extensively due to its significant confinement effects of single atoms for photocatalytic applications. In this work, a systematic investigation of g-C3N4 confining noble metal single atoms (NM1@g-C3N4) will be performed by using DFT calculations. The geometric structure calculations indicate that the most favorable anchored sites for the NM1 is located in the six-fold cavity, and the deformed wrinkle space of g-C3N4 helps the NM1 to be stabilized in the six-fold cavity. The electronic structure calculations show that the conduction band of NM1@g-C3N4 moved down and crossed through the Fermi level, resulting in narrowing the band gap of the NM1@g-C3N4. Moreover, the confined NM1 provide a new channel of charge transport between adjacent heptazine units, resulting in a longer lifetime of photo-generated carriers except Ru, Rh, Os and Ir atoms. Furthermore, the d-band centres of NM1 in NM1@g-C3N4 show that Rh1@, Pd1@, Ir1@ and Pt1@g-C3N4 SACs may have better photocatalytic performance than other NM1@g-C3N4 SACs. Finally, Pt1@g-C3N4 SACs are considered to have higher photocatalytic activity than other NM1@g-C3N4 SACs. These results demonstrate that the confinement effects of noble metals on monolayer g-C3N4 not only makes the single atom more stable to be anchored on g-C3N4, but also enhances the photocatalytic activity of the system through the synergistic effect between the confined NM1 and the monolayer g-C3N4. These detailed research may provide theoretical support for engineers to prepare photocatalysts with higher activity.

Mo single atoms in the Cu(111) surface as improved catalytic active centers for deoxygenation reactions

The surface Mo-doped Cu(111) catalyst feature improved performance towards deoxygenation reactions, acting as a single-atom alloy capable of breaking Brønsted–Evans–Polanyi relations for carbonyl bond scissions.

A synergetic effect between a single Cu site and S vacancy on an MoS2 basal plane for methanol synthesis from syngas

Compared to MoS₂(001), the synergetic effect between the single Cu site and S vacancy on Cu/MoS₂(001) destabilizes O, which not only increases the CO hydrogenation rate by 5 orders of magnitude, but leads to the selectivity switch from CH₄ to CH₃OH.

A perspective on oxide-supported single-atom catalysts

Single-atom catalysts (SACs) can not only maximize the metal atom utilization efficiency, but also show drastically improved catalytic performance for various important catalytic processes. Insights into the working principles of SACs provide rational guidance to design and prepare advanced catalysts. Many factors have been claimed to affect the performance of SACs, which makes it very challenging to clarify the correlation between the catalytic performance and physicochemical characteristics of SACs. Oxide-supported SACs are one of the most extensively explored systems. In this minireview, some latest developments on the determining factors of the stability, activity and selectivity of SACs on oxide supports are overviewed. Discussed also are the reaction mechanisms for different systems and methods that are employed to correlate the properties with the catalyst structures at the atomic level. In particular, a recently proposed surface free energy approach is introduced to fabricate well-defined modelled SACs that may help address some key issues in the development of SACs in the future.

Catalytic Semi-Water-Gas Shift Reaction: A Simple Green Path to Formic Acid Fuel

Formic acid (FA) is a promising CO and hydrogen energy carrier, and currently its generation is mainly centered on the hydrogenation of CO₂ . However, it can also be obtained by the hydrothermal conversion of CO with H₂ O at very high pressures (>100 bar) and temperatures (>200 °C), which requires days to complete. Herein, it is demonstrated that by using a nano-Ru/Fe alloy embedded in an ionic liquid (IL)-hybrid silica in the presence of the appropriate IL in water, CO can be catalytically converted into free FA (0.73 m) under very mild reactions conditions (10 bar at 80 °C) with a turnover number of up to 1269. The catalyst was prepared by simple reduction/decomposition of Ru and Fe complexes in the IL, and it was then embedded into an IL-hybrid silica {1-n-butyl-3-(3-trimethoxysilylpropyl)-imidazolium cations associated with hydrophilic (acetate, SILP-OAc) and hydrophobic [bis((trifluoromethyl)sulfonyl)amide, SILP-NTf₂ ] anions}. The location of the alloy nanoparticles on the support is strongly related to the nature of the anion, that is, in the case of hydrophilic SILP-OAc, RuFe nanoparticles are more exposed to the support surface than in the case of the hydrophobic SILP-NTf₂ , as determined by Rutherford backscattering spectrometry. This catalytic membrane in the presence of H₂ O/CO and an appropriate IL, namely, 1,2-dimethyl-3-n-butylimidazolium 2-methyl imidazolate (BMMIm⋅MeIm), is stable and recyclable for at least five runs, yielding a total of 4.34 m of free FA.

On the Controlled Loading of Single Platinum Atoms as a Co-Catalyst on TiO2 Anatase for Optimized Photocatalytic H2 Generation

Single-atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic-scale defect engineering approach to form and control traps for platinum SA sites as co-catalyst for photocatalytic H₂ generation is described. Thin sputtered TiO₂ layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO₂ layers are reduced in Ar/H₂ under different conditions (leading to different but defined Ti³⁺ -O_v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF-STEM results show that only on the thin-film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA-Pt decoration can enhance the normalized photocatalytic activity of a TiO₂ sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO₂ surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO₂ . Importantly, the density of the relevant surface exposed defect centers-thus the density of Pt-SA sites, which play the key role in photocatalytic activity-can be precisely optimized.

Sub-nanocatalysis for Efficient Aqueous Nitrate Reduction: Effect of Strong Metal-Support Interaction

Magnetic ferroferric oxide-supported bimetallic Pd-In cluster sub-nanoparticles were used for the first time for the catalytic reduction of nitrates. Due to the unique properties of the FeO_x support, the PdIn active centers could be highly dispersed in both nano- and sub-nanoscales. A variety of characterizations and the charge density difference model confirm that a strong metal-support interaction exists between the active sites and the support. The PdIn nanoparticles on FeO_x show high selectivity toward nitrogen and excellent cyclic activity due to ferromagnetism, which broaden its prospect in practical water treatment. Moreover, the active centers in the sub-nanoscale are used in the nitrate reduction process for the first time and they show a distinct higher activity in denitration. The rate constant for nitrate conversion on PdIn sub-nanoparticles is larger than that for its nanometer counterpart based on the Langmuir-Hinshelwood model. High turnover frequency value and ammonia selectivity are achieved for the small-sized sub-nanocatalyst. The FeO_x-supported PdIn nanoparticles and sub-nanoparticles have two application areas in water purification and ammonia recovery, respectively. Density functional theory calculations on the adsorption energies of elemental reactions are also performed, which shed some light on the catalysis mechanism and catalyst design.

Solar-Driven Water-Gas Shift Reaction over CuOx /Al2 O3 with 1.1 % of Light-to-Energy Storage

Hydrogen production from coal gasification provides a cleaning approach to convert coal resource into chemical energy, but the key procedures of coal gasification and thermal catalytic water-gas shift (WGS) reaction in this energy technology still suffer from high energy cost. We herein propose adopting a solar-driven WGS process instead of traditional thermal catalysis, with the aim of greatly decreasing the energy consumption. Under light irradiation, the CuO_x /Al₂ O₃ delivers excellent catalytic activity (122 μmol g_cat ^-1  s^-1 of H₂ evolution and >95 % of CO conversion) which is even more efficient than noble-metal-based catalysts (Au/Al₂ O₃ and Pt/Al₂ O₃ ). Importantly, this solar-driven WGS process costs no electric/thermal power but attains 1.1 % of light-to-energy storage. The attractive performance of the solar-driven WGS reaction over CuO_x /Al₂ O₃ can be attributed to the combined photothermocatalysis and photocatalysis.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

Experimental Study on Catalytic Combustion of Methane in a Microcombustor with Metal Foam Monolithic Catalyst

Utilizing catalysts in microcombustors is probably an excellent practical solution to stabilize fuel combustion because of the relatively fast reaction speed. In the present work, the monolithic catalyst Pd/A2O3/Fe-Ni with metal foam as matrix was used inside a 5 mm in diameter microcombustor. Then the effects of inlet velocity and equivalent ratio on catalytic combustion characteristics of methane were studied experimentally. The results showed that the methane and air mixture with the stoichiometric ratio Φ = 1.0 could be ignited at v = 0.2–0.6 m/s. The velocity of premixed mixture had a great influence on the catalytic combustion of methane. The larger the inlet velocity, the higher the temperature and the brighter the flame were. The experiment results also showed that the equivalence ratio had a large essential impact on the catalytic combustion, especially for the lean mixture of methane and air. It seemed the addition of the porous matrix with catalysts could significantly extend the limits of stable combustion. In the detection of exhaust gas, CO selectivity increased and CO2 selectivity decreased with the equivalence ratio. When Φ was between 0.94 and 1.0 m/s, a little amount of hydrogen was produced due to the lack of oxygen. The measured conversion of methane to CO and CO2 was very high, usually greater than 99%, which indicated the excellent performance of the catalyst.

Cu/CeO2 Catalyst for Water-Gas Shift Reaction: Effect of CeO2 Pretreatment

CuO/CeO₂ is a kind of promising catalysts for the water-gas shift (WGS) reaction. Efforts were put in to improve its performance through modification of CeO₂ support. In this study, portions of CeO₂ prepared by a co-precipitation method were separately annealed at 300 °C in air, under vacuum and with H₂ , and were used as supports for the fabrication of CuO/CeO₂ catalysts. The physicochemical properties of the catalysts were characterized by X-ray diffraction, N₂ -physisorption, inductively coupled plasma, Raman spectroscopy, CO₂ temperature-programmed desorption, and H₂ temperature-programmed reduction techniques. The relation between catalytic performances and physicochemical properties of the CuO/CeO₂ catalysts were discussed. Among the three catalysts, the one with CuO supported on H₂ -reduced CeO₂ shows the highest catalytic activity, mainly due to strong CuO-CeO₂ synergetic interaction and high concentration of Frenkel-type oxygen vacancies. The superior catalytic activities can also be attributed to the Cu⁰ crystals of small size and the oxygen vacancies in non-stoichiometric CeO_2-x .