Catalytic properties of trivalent rare-earth oxides with intrinsic surface oxygen vacancy

Oxygen vacancy (Ov) is an anionic defect widely existed in metal oxide lattice, as exemplified by CeO2, TiO2, and ZnO. As Ov can modify the band structure of solid, it improves the physicochemical properties such as the semiconducting performance and catalytic behaviours. We report here a new type of Ov as an intrinsic part of a perfect crystalline surface. Such non-defect Ov stems from the irregular hexagonal sawtooth-shaped structure in the (111) plane of trivalent rare earth oxides (RE2O3). The materials with such intrinsic Ov structure exhibit excellent performance in ammonia decomposition reaction with surface Ru active sites. Extremely high H2 formation rate has been achieved at ~1 wt% of Ru loading over Sm2O3, Y2O3 and Gd2O3 surface, which is 1.5–20 times higher than reported values in the literature. The discovery of intrinsic Ov suggests great potentials of applying RE oxides in heterogeneous catalysis and surface chemistry

Cascade NH3 Oxidation and N2O Decomposition via Bifunctional Co and Cu Catalysts

The selective catalytic oxidation of NH3 (NH3-SCO) to N2 is an important reaction for the treatment of diesel engine exhaust. Co3O4 has the highest activity among non-noble metals but suffers from N2O release. Such N2O emissions have recently been regulated due to having a 300× higher greenhouse gas effect than CO2. Here, we design CuO-supported Co3O4 as a cascade catalyst for the selective oxidation of NH3 to N2. The NH3-SCO reaction on CuO-Co3O4 follows a de-N2O pathway. Co3O4 activates gaseous oxygen to form N2O. The high redox property of the CuO-Co3O4 interface promotes the breaking of the N-O bond in N2O to form N2. The addition of CuO-Co3O4 to the Pt-Al2O3 catalyst reduces the full NH3 conversion temperature by 50 K and improves the N2 selectivity by 20%. These findings provide a promising strategy for reducing N2O emissions and will contribute to the rational design and development of non-noble metal catalysts

Subsurface Single-atom Catalyst Enabled by Mechanochemical Synthesis for Oxidation Chemistry

Single-atom catalysts have garnered significant attention due to their exceptional atom utilization and unique properties. However, the practical application of these catalysts is often impeded by challenges such as sintering-induced instability and poisoning of isolated atoms due to strong gas adsorption. In this study, we employed the mechanochemical method to insert single Cu atoms into the subsurface of Fe2O3 support. By manipulating the location of single atoms at the surface or subsurface, catalysts with distinct adsorption properties and reaction mechanisms can be achieved. It was observed that the subsurface Cu single atoms in Fe2O3 remained isolated under both oxidation and reduction environments, whereas surface Cu single atoms on Fe2O3 experienced sintering under reduction conditions. The unique properties of these subsurface single-atom catalysts call for innovations and new understandings in catalyst design

Rational Design and Improvement of Transition Metal Catalysts for Selective Ammonia Oxidation

Various nitrogen-containing pollutants (NH3 and NOX) have raised concerns about public health and environmental protection, which has led to increasingly strict emission standards. As one of the most promising methods for removing ammonia, the selective catalytic oxidation of NH3 (NH3-SCO) to nitrogen has received increasing attention and will play a crucial role in the upcoming EU7 standard. However, achieving high activity and nitrogen selectivity simultaneously remains a challenge. Noble metals generally exhibit high activity, while excessive oxidation leads to low nitrogen selectivity above 300 °C in NH3-SCO. In comparison, transition metal catalysts are favourable for the formation of N2 but suffer from low activity. To replace expensive noble metals, a series of binary transition metal oxide catalytic systems were designed, and a wide range of characterization techniques was applied to study the structures and performances of the catalysts. Increasing the oxidation rates is an effective method for enhancing the NH3-SCO activity of transition metals, which can be achieved by activating the surface lattice oxygen of the catalysts. Cu-CeO2 single-atom catalysts (SACs) with different shapes are prepared and tested for the NH3-SCO. The interaction between Cu and CeO2 is crucial to regulating the interface structure and the content of oxygen vacancies. Among them, Cu-CeO2 SACs prepared by the one-step flame spray pyrolysis (FSP) method exhibited a high extent of ceria reduction at low temperatures and a drastically improved catalytic activity in the N-H bond dissociation. Altogether, the obtained results demonstrate that FSP is an appealing strategy for the synthesis of highly active SACs. Reducing the emission of NOX at high temperatures is an effective method for transition metals with high activities to achieve high N2 productivity. N2O is a major by-product of active Co-based catalysts in NH3-SCO. However, there remains an insufficient understanding of the fundamental methods for achieving satisfying low-temperature activity with less N2O emission. CuO-Co3O4 catalysts were designed to catalyze a cascade reaction that first forms N2O at an unprecedented rate from NH3 and then decomposes N2O back to N2, achieving high NH3-SCO performance. The ability to remove N2O at high temperatures can be further exploited to reduce N2O emissions from noble metal catalysts. Synthesizing innovative catalysts is essential for enhancing low-temperature catalytic activity and N2 selectivity. The mechanochemical method was employed to insert single Cu atoms into the subsurface of Fe2O3 support. The subsurface single-atom catalyst design strategy opens promising perspectives to initiate the lattice distortion and facilitate the activation of the inactive lattice oxygen. Controlling single atoms on the surface and subsurface resulted in very different adsorption properties of catalysts. The subsurface Cu single atoms in Fe2O3 remained isolated under either oxidative or reductive environments, while surface Cu single atoms on Fe2O3 sintered under reduction. The unique properties of these subsurface single-atom catalysts call for innovations and understandings in catalyst design

Research progress in stabilization of interface and bulk structure of lithium metal anodes

With the rapid development of information technology，electrification and new energy technologies， portable electric devices，electric vehicles and energy storage facilities require rechargeable batteries with higher energy density.However， the energy density of widely used lithium-ion batteries is approaching the limit， which cannot meet the above demands. Therefore it is urgent to explore new electrochemical systems with higher energy density. Lithium metal anode is a promising candidate for achieving next-generation high-energy-density batteries due to its ultrahigh theoretical capacity （3860 mAh·g-1） and most negative electrochemical potential （-3.04 V vs SHE）. However， during the few decades， the practical application of lithium metal batteries has been hindered by short lifetime and safety issues. In this paper， the history and development of lithium metal batteries were introduced， and the current issues and corresponding mechanisms were analyzed， such as high reactivity， lithium dendrites， dead lithium and volume expansion. Some strategies to deal with the above problems in terms of interface and bulk structure design， including the protection layers formed ex situ/in situ， lithium-based alloys and 3D composite lithium metal anodes，were proposed. Finally，the future developments of practical lithium metal anodes based on constraints for actual batteries， crosstalk of electrodes and failure mechanisms of large-capacity batteries were discussed

Polyphenylene as an Active Support for Ru-Catalyzed Hydrogenolysis of 5-Hydroxymethylfurfural

Selective transformation of biomass feedstocks to platform molecules is a key pursuit for sustainable chemical production. Compared to petrochemical processes, biomass transformation requires the defunctionalization of highly polar molecules at relatively low temperatures. As a result, catalysts based on functional organic polymers may play a prominent role. Targeting the hydrogenolysis of the platform chemical 5-hydroxymethylfurfural (5-HMF), here, we design a polyphenylene (PPhen) framework with purely sp2-hybridized carbons that can isolate 5-HMF via π−π stacking, preventing hemiacetal and humin formation. With good swellability, the PPhen framework here has successfully supported and dispersed seven types of metal particles via a newly developed swelling-impregnation method, including Ru, Pt, Au, Fe, Co, Ni, and Cu. Ru/PPhen is studied for 5-HMF hydrogenolysis, achieving a 92% yield of 2,5-dimethylfuran (DMF) under mild conditions, outperforming the state-of-the-art catalysts reported in the literature. In addition, PPhen helps perform a solventless reaction, achieving direct 5-HMF to DMF conversion in the absence of any liquid solvent or reagent. This approach in designing support−reactant/solvent/metal interactions will play an important role in surface catalysi

Cu speciation in CHA framework and its impact in selective catalytic reduction of NO

The selective reduction of NO with NH3 (SCR) is an important reaction for emission control. The mobility of a metal cation in a confined space is a classic problem in transport phenomena. When the metal cation is an active centre of a chemical reaction, its mobility will be decisive to its catalytic performance, such as the Cu in chabazite (CHA) zeolite catalyst for selective catalytic reduction of NO (SCR) in diesel exhaust. Here we identify electron paramagnetic resonance (EPR) as an effective tool to quantify Cu2+ at different locations of CHA, and study the mobility of hydrated Cu2+. The Cu2+ transfers from one adjacent Al3+ site (1AlCu) to two adjacent Al3+ site (2AlCu) during dehydration. In the hydrothermal aging process, the transition is recorded from CuOx clusters to isolated Cu2+ sites and from Cu at 8-O member ring to 6-O member ring. The result indicates that the Cu – framework (fw) O interaction is thermodynamic drive for the migration of Cu species. For SCR, this means that the reduced Cu-Ofw interaction will lead to mobile Cu species in the catalytic circle

Oxygen vacancy formation as the rate-determining step in the Mars-van Krevelen mechanism

The Mars-van Krevelen mechanism (MvK) is a widely recognized model for describing the role of lattice oxygen in catalysis. Following the MvK mechanism, the formation and conversion of surface oxygen vacancy (VO) are considered as the key steps. CeO2-ZrO2 (CZ) mixed oxides are the typical catalyst support in MvK mechanism. They have the unique property of hosting remarkable amount of VO without significant change in lattice structure, offering O storage and release capability that maintains the required concentration of active O on the catalytic surface. In this regard, the rate of VO formation and conversion directly affect their catalytic performance. In this work, we obtained the VO formation and conversion kinetics by measuring the rate of the Ce4+ reduction and oxidation via operando energy dispersive Extended X-ray Absorption Fine Structure (EDE). The main conclusions are: 1) VO formation is 10 times faster than VO conversion; 2) VO formation rates are comparable with the CO oxidation rates, thereby serving as the rate-determining step in CO oxidation; 3) Pd and Cu serve as catalysts for VO formation by significantly improving its rate by 50 times at 250 C by weakening the metal-O bonding strength, whereas the activation energy have been reduced to 58.4 kJ/mol and 36.5 kJ/mol, respectively. Our method in measuring and analysing partial reaction rates within a turnover is therefore important for all chemical reactions

The decisive role of CuI-framework O binding in oxidation half cycle of selective catalytic reduction

Cu-exchanged zeolite is an efficient catalyst to remove harmful nitrogen oxides from diesel exhaust gas through the selective catalytic reduction (SCR) reaction. The SCR performance is structure dependent, in which a Cu with one adjacent framework Al (1AlCu) has lower activation energy in oxidative half-cycle than Cu with two adjacent framework Al (2AlCu). Using a combination of operando X-ray absorption spectroscopy, valence to core - X-ray emission spectroscopy and density functional theory calculations, here we showed that 1AlCu proceeds with nitrate mechanism, in which side-on coordination of O2 at a CuI(NH3)xOfw (fw = framework) is the rate-limiting step in the oxidation half-cycle. As a result, the CuI(NH3)xOfw at 1AlCu can easily yield a transient CuIINOx intermediate upon breaking of Cu-Ofw after interaction with NO. In the meantime, 2AlCu has high barriers for Cu-Ofw bond breaking and proceeds with dimer mechanism. Our results show the coexisting of both dimer and nitrate mechanism, in particular at high Cu loadings, in which controlling the strength of the Cu-Ofw coordination is key for the O-O split in the nitrate pathway

Designing Reactive Bridging O^2– at the Atomic Cu–O–Fe Site for Selective NH₃ Oxidation

Surface oxidation chemistry involves the formation and breaking of metal–oxygen (M–O) bonds. Ideally, the M–O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O2– species within the atomic Cu–O–Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O2– has a lower antibonding orbital energy and thus weaker Cu–O/Fe–O strength. In selective NH3 oxidation, the weak Cu–O/Fe–O bond enables fast Cu redox for NH3 conversion and direct NO adsorption via Cu–O–NO to promote N–N coupling toward N2. As a result, 99% N2 selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O2– in catalysis