Urea catalytic oxidation for energy and environmental applications.

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Local reaction environment in electrocatalysis.

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Ethylene Electrooxidation to 2-Chloroethanol in Acidic Seawater with Natural Chloride Participation.

Ethylene oxidation to oxygenates via electrocatalysis is practically promising because of less energy input and CO2 output compared with traditional thermal catalysis. However, current ethylene electrooxidation reaction (EOR) is limited to alkaline and neutral electrolytes to produce acetaldehyde and ethylene glycol, significantly limiting cell energy efficiency. Here, we report for the first time an EOR to 2-chloroethanol product in a strongly acidic environment with natural seawater as an electrolyte. We demonstrate a 2-chloroethanol Faradaic efficiency (FE) of ∼70% with a low electrical energy consumption of ∼1.52 × 10-3 kWh g-1 over a commercial Pd catalyst. We establish a mechanism to evidence that 2-chloroethanol is produced at low potentials via direct interaction of adsorbed chloride anions (*Cl) with ethylene reactant because of the high coverage of *Cl during reaction. Importantly, this differs from the accepted multiple step mechanism of subsequent chlorine oxidation and ethylene chlorination reactions at high potentials. With highly active Cl- participation, the production rate for 2-chloroethanol in acidic seawater is a high 26.3 g m-2 h-1 at 1.6 V operation. Significantly, we show that this is 223 times greater than that for ethylene glycol generation in acidic freshwater. We demonstrate chloride-participated EOR in a proton exchange membrane electrolyzer that exhibits a 68% FE for 2-chloroethanol at 2.2 V operation in acidic seawater. This new understanding can be used for designing selective anode oxidation reactions in seawater under mild conditions.

Mild Methane Electrochemical Oxidation Boosted via Plasma Pre-Activation.

Obtaining partial methane oxidation reaction (MOR) with various oxygenates via a mild electrochemical method is practically difficult because of activation of stable C─H bond and consequent reaction pathway regulation. Here, a real-time tandem MOR with cascaded plasma and electrocatalysis to activate and convert the methane (CH4 ) synergistically is reported for the first time. Boosted CH4 conversion is demonstrated toward value-added products including, alcohols, carboxylates, and ketone via use of commercial Pd-based electrocatalysts. Compared with hash industrial processes, a mild condition, that is, anode potential < 1.0 V versus RHE (reversible hydrogen electrode) is used that mitigates overoxidation of oxygenates and obviates competing reaction(s). One evidence that Pd(II) sites and surface adsorbed hydroxyls are important in facilitating activated-CH4 species conversion, and establish a reaction mechanism for conversion(s) that involves coupling reactions between adsorbed hydroxyls, carbon monoxide and C1 /C2 alkyls. One conclude that pre-activation is important in boosting electrochemical partial MOR under mild conditions and will be of benefit in the development of sustainable CH4 conversion technology.

Organic pH Buffer for Dendrite-Free and Shuttle-Free Zn-I2 Batteries.

Aqueous Zn-Iodine (I2 ) batteries are attractive for large-scale energy storage. However, drawbacks include, Zn dendrites, hydrogen evolution reaction (HER), corrosion and, cathode "shuttle" of polyiodines. Here we report a class of N-containing heterocyclic compounds as organic pH buffers to obviate these. We evidence that addition of pyridine /imidazole regulates electrolyte pH, and inhibits HER and anode corrosion. In addition, pyridine and imidazole preferentially absorb on Zn metal, regulating non-dendritic Zn plating /stripping, and achieving a high Coulombic efficiency of 99.6 % and long-term cycling stability of 3200 h at 2 mA cm-2 , 2 mAh cm-2 . It is also confirmed that pyridine inhibits polyiodines shuttling and boosts conversion kinetics for I- /I2 . As a result, the Zn-I2 full battery exhibits long cycle stability of >25 000 cycles and high specific capacity of 105.5 mAh g-1 at 10 A g-1 . We conclude organic pH buffer engineering is practical for dendrite-free and shuttle-free Zn-I2 batteries.

Electrocatalytic CO2-to-C2+ with Ampere-Level Current on Heteroatom-Engineered Copper via Tuning *CO Intermediate Coverage.

An ampere-level current density of CO2 electrolysis is critical to realize the industrial production of multicarbon (C2+) fuels. However, under such a large current density, the poor CO intermediate (*CO) coverage on the catalyst surface induces the competitive hydrogen evolution reaction, which hinders CO2 reduction reaction (CO2RR). Herein, we report reliable ampere-level CO2-to-C2+ electrolysis by heteroatom engineering on Cu catalysts. The Cu-based compounds with heteroatom (N, P, S, O) are electrochemically reduced to heteroatom-derived Cu with significant structural reconstruction under CO2RR conditions. It is found that N-engineered Cu (N-Cu) catalyst exhibits the best CO2-to-C2+ productivity with a remarkable Faradaic efficiency of 73.7% under -1100 mA cm-2 and an energy efficiency of 37.2% under -900 mA cm-2. Particularly, it achieves a C2+ partial current density of -909 mA cm-2 at -1.15 V versus reversible hydrogen electrode, which outperforms most reported Cu-based catalysts. In situ spectroscopy indicates that heteroatom engineering adjusts *CO adsorption on Cu surface and alters the local H proton consumption in solution. Density functional theory studies confirm that the high adsorption strength of *CO on N-Cu results from the depressed HER and promoted *CO adsorption on both bridge and atop sites of Cu, which greatly reduces the energy barrier for C-C coupling.

Metallic nanostructures with low dimensionality for electrochemical water splitting.

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure-property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.

A Top-Down Strategy to Realize Surface Reconstruction of Small-Sized Platinum-Based Nanoparticles for Selective Hydrogenation.

Over the past decades, despite the substantial efforts that have been devoted to the modifications of Pt nanoparticles (NPs) to tailor their selectivities for hydrogenation reactions, there are still a lack of facile strategies for precisely regulation of the surface properties of NPs, especially for those with small sizes. In this work, we propose a top-down thermal annealing strategy for tuning the surface properties of Pt-based NPs (≈4 nm) without the occurrence of aggregation. Compared to conventional bottom-up methods, the present top-down strategy can precisely regulate the surface compositions of Pt-Cd NPs and other ternary Pt-Cd-M NPs (M=Fe, Ni, Co, Mn, and Sn). The optimized Pt-Cd NPs exhibit excellent selectivity toward phenylacetylene and 4-nitrostyrene hydrogenations with a styrene selectivity and 4-aminophenyl styrene selectivity of 95.2 % and 94.5 %, respectively. This work provides a general strategy for the surface reconstructions of Pt-based NPs, and promotes fundamental research on catalyst design for heterogeneous catalysis.

Low Dimensional Platinum-Based Bimetallic Nanostructures for Advanced Catalysis.

The development of renewable energy storage and conversion has been greatly promoted by the achievements in platinum (Pt)-based catalysts, which possess remarkable catalytic performance. However, the high cost and limited resources of Pt have hindered the practical applications and thus stimulated extensive efforts to achieve maximized catalytic performance with minimized Pt content. Low dimensional Pt-based bimetallic nanomaterials (such as nanoplates and nanowires) hold enormous potential to realize this target owing to their special atomic arrangement and electronic structures. Recent achievements reveal that strain engineering (e.g., the compressive or tensile strain existing on the Pt skin), surface engineering (e.g., high-index facets, Pt-rich surface, and highly open structures), and interface engineering (e.g., composition-segregated nanostructures) for such nanomaterials can readily lead to electronic modification, more active sites, and strong synergistic effect, thus opening up new avenues toward greatly enhanced catalytic performance. In this Account, we focus on recent advances in low dimensional Pt-based bimetallic nanomaterials as promising catalysts with high activity, long-term stability, and enhanced selectivity for both electrocatalysis and heterogeneous reactions. We begin by illustrating the important role of several strategies on optimizing the catalytic performance: (1) regulated electronic structure by strain effect, (2) increased active sites by surface modification, and (3) the optimized synergistic effect by interfacial engineering. First of all, a difference in atomic bonding strength can result in compressive or tensile force, leading to downshift or upshift of the d-band center. Such effects can be significantly amplified in low-dimensionally confined nanostructures, producing optimized bonding strength for improved catalysis. Furthermore, a high density of high-index facets and a Pt-rich surface in shape-controlled nanostructures based on surface engineering provide further enhancement due to the increased Pt atom utilization and optimal adsorption energy. Finally, interfacial engineering of low dimensional Pt-based bimetallic nanomaterials with high composition-segregation can facilitate the catalytic process due to a strong synergetic effect, which effectively tunes the electronic structure, modifies the coordination environment, and prevents catalysts from serious aggregation. The rational design of low dimensional Pt-based bimetallic nanomaterials with superior catalytic properties based on strain, surface, and interface engineering could help realize enhanced catalysis, gain deep understanding of the structure-performance relationship, and expand access to Pt-based materials for general communities of materials science, chemical engineering, and catalysis in renewable energy research fields.

Crystal-Phase-Engineered PdCu Electrocatalyst for Enhanced Ammonia Synthesis.

Crystal phase engineering is a powerful strategy for regulating the performance of electrocatalysts towards many electrocatalytic reactions, while its impact on the nitrogen electroreduction has been largely unexplored. Herein, we demonstrate that structurally ordered body-centered cubic (BCC) PdCu nanoparticles can be adopted as active, selective, and stable electrocatalysts for ammonia synthesis. Specifically, the BCC PdCu exhibits excellent activity with a high NH3 yield of 35.7 µg h-1 mg-1 cat , Faradaic efficiency of 11.5 %, and high selectivity (no N2 H4 is detected) at -0.1 V versus reversible hydrogen electrode, outperforming its counterpart, face-centered cubic (FCC) PdCu, and most reported nitrogen reduction reaction (NRR) electrocatalysts. It also exhibits durable stability for consecutive electrolysis for five cycles. Density functional theory calculation reveals that strong orbital interactions between Pd and neighboring Cu sites in BCC PdCu obtained by structure engineering induces an evident correlation effect for boosting up the Pd 4d electronic activities for efficient NRR catalysis. Our findings open up a new avenue for designing active and stable electrocatalysts towards NRR.

Phase Modulating of Cu-Ni Nanowires Enables Active and Stable Electrocatalysts for the Methanol Oxidation Reaction.

The design and development of non-noble metal alternatives with superior performance and promising long-term stability that is comparable or even better than those of noble-metal-based catalysts is a significant challenge. Here, we report the thermal-induced phase engineering of non-noble-metal-based nanowires with superior electrochemical activity and stability for the methanol oxidation reaction (MOR) under alkaline conditions. The optimized Cu-Ni nanowires deliver an unprecedented mass activity of 425 mA mg-1 , which is 4.3 times higher than that of the untreated one. Detailed catalytic investigations show that the enhanced performance is due to the large active area, the increased number of active sites (NiOOH), and fast methanol electrooxidation kinetics. In addition, the generated hollow feature in the nanowires provides a unique void space to release the volume expansion, where the activity can be maintained for 5 h without a distinct activity decay. The present work emphasizes the importance of precisely phase modulating of nanomaterials for the design of non-noble metal electrocatalysts towards the MOR, which opens up a new pathway for the design of cost-effective electrocatalysts with promising activity and long-term stability.

Large-Scale, Bottom-Up Synthesis of Binary Metal-Organic Framework Nanosheets for Efficient Water Oxidation.

Ultrathin metal-organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top-down, low-yield exfoliation methods. Herein, Ni-M-MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large-scale, bottom-up solvothermal method. The solvent mixture of N,N-dimethylacetamide and water plays key role in controlling the formation of these two-dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni-Fe-MOF NSs deliver a current density of 10 mA cm-2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec-1 , and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni-Fe-MOF NSs.

Porous Pt-Ni Nanowires within In Situ Generated Metal-Organic Frameworks for Highly Chemoselective Cinnamaldehyde Hydrogenation.

Although chemoselective hydrogenation of unsaturated aldehydes is the major route to highly valuable industrially demanded unsaturated alcohols, it is still challenging, as the production of saturated aldehydes is more favorable over unsaturated alcohols from the view of thermodynamics. By combining the structural features of porous nanowires (NWs) and metal-organic frameworks (MOFs), a unique class of porous Pt-Ni NWs in situ encapsuled by MOFs (Pt-Ni NWs@Ni/Fex-MOFs) is designed to enhance the unsaturated alcohols selectivity in the cinnamaldehyde (CAL) hydrogenation. A detailed catalytic study shows that the porous Pt-Ni NWs@Ni/Fex -MOFs exhibit volcano-type activity and selectivity in CAL hydrogenation as a function of Fe content. The optimized porous PtNi2.20 NWs@Ni/Fe4 -MOF is highly active and selective with 99.5% CAL conversion and 83.3% cinnamyl alcohol selectivity due to the confinement effect, appropriate thickness of MOF and its optimized electronic structure, and excellent durability with negligible activity and selectivity loss after five runs.

Highly Active and Selective Hydrogenation of CO2 to Ethanol by Ordered Pd-Cu Nanoparticles.

Carbon dioxide (CO2) hydrogenation to ethanol (C2H5OH) is considered a promising way for CO2 conversion and utilization, whereas desirable conversion efficiency remains a challenge. Herein, highly active, selective and stable CO2 hydrogenation to C2H5OH was enabled by highly ordered Pd-Cu nanoparticles (NPs). By tuning the composition of the Pd-Cu NPs and catalyst supports, the efficiency of CO2 hydrogenation to C2H5OH was well optimized with Pd2Cu NPs/P25 exhibiting high selectivity to C2H5OH of up to 92.0% and the highest turnover frequency of 359.0 h-1. Diffuse reflectance infrared Fourier transform spectroscopy results revealed the high C2H5OH production and selectivity of Pd2Cu NPs/P25 can be ascribed to boosting *CO (adsorption CO) hydrogenation to *HCO, the rate-determining step for the CO2 hydrogenation to C2H5OH.

Trimetallic Oxyhydroxide Coralloids for Efficient Oxygen Evolution Electrocatalysis.

Trimetallic oxyhydroxides are one of the most effective materials for oxygen evolution reaction (OER) catalysis, a key process for water splitting. Herein, we describe a facile wet-chemical method to directly grow a series of coralloid trimetallic oxyhydroxides on arbitrary substrates such as nickel foam (NF) and carbon nanotubes (CNTs). The amount of iron in these oxyhydroxide sponges on NF and CNTs was precisely controlled, revealing that the electrocatalytic activity of the WCoFe trimetallic oxyhydroxides depends on the Fe amount in a volcano-like fashion. The optimized W0.5 Co0.4 Fe0.1 /NF catalyst exhibited an overpotential of only 310 mV to deliver a large current density of 100 mA cm-2 and a very low Tafel slope of 32 mV dec-1 . It also showed superior stability with negligible activity decay after use in the OER for 21 days (>500 h). X-ray photoelectron spectroscopy revealed that the addition of Fe leads to an on average lower Co oxidation state, which contributes to the enhanced OER performance.

Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution.

The design of high-performance electrocatalysts for the alkaline hydrogen evolution reaction (HER) is highly desirable for the development of alkaline water electrolysis. Phase- and interface-engineered platinum-nickel nanowires (Pt-Ni NWs) are highly efficient electrocatalysts for alkaline HER. The phase and interface engineering is achieved by simply annealing the pristine Pt-Ni NWs under a controlled atmosphere. Impressively, the newly generated nanomaterials exhibit superior activity for the alkaline HER, outperforming the pristine Pt-Ni NWs and commercial Pt/C, and also represent the best alkaline HER catalysts to date. The enhanced HER activities are attributed to the superior phase and interface structures in the engineered Pt-Ni NWs.

Ordered PdCu-Based Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts.

The development of superior non-platinum electrocatalysts for enhancing the electrocatalytic activity and stability for the oxygen-reduction reaction (ORR) and liquid fuel oxidation reaction is very important for the commercialization of fuel cells, but still a great challenge. Herein, we demonstrate a new colloidal chemistry technique for making structurally ordered PdCu-based nanoparticles (NPs) with composition control from PdCu to PdCuNi and PtCuCo. Under the dual tuning on the composition and intermetallic phase, the ordered PdCuCo NPs exhibit better activity and much enhanced stability for ORR and ethanol-oxidation reaction (EOR) than those of disordered PdCuM NPs, the commercial Pt/C and Pd/C catalysts. The density functional theory (DFT) calculations reveal that the improved ORR activity on the PdCuM NPs stems from the catalytically active hollow sites arising from the ligand effect and the compressive strain on the Pd surface owing to the smaller atomic size of Cu, Co, and Ni.

Recent Advances in Electrocatalytic Hydrogenation Reactions on Copper-Based Catalysts.

Hydrogenation reactions play a critical role in the synthesis of value-added products within the chemical industry. Electrocatalytic hydrogenation (ECH) using water as the hydrogen source has emerged as an alternative to conventional thermocatalytic processes for sustainable and decentralized chemical synthesis under mild conditions. Among the various ECH catalysts, copper-based (Cu-based) nanomaterials are promising candidates due to their earth-abundance, unique electronic structure, versatility, and high activity/selectivity. Herein, recent advances in the application of Cu-based catalysts in ECH reactions for the upgrading of valuable chemicals are systematically analyzed. The unique properties of Cu-based catalysts in ECH are initially introduced, followed by design strategies to enhance their activity and selectivity. Then, typical ECH reactions on Cu-based catalysts are presented in detail, including carbon dioxide reduction for multicarbon generation, alkyne-to-alkene conversion, selective aldehyde conversion, ammonia production from nitrogen-containing substances, and amine production from organic nitrogen compounds. In these catalysts, the role of catalyst composition and nanostructures toward different products is focused. The co-hydrogenation of two substrates (e.g., CO2 and NOx n, SO3 2-, etc.) via C─N, C─S, and C─C cross-coupling reactions are also highlighted. Finally, the critical issues and future perspectives of Cu-catalyzed ECH are proposed to accelerate the rational development of next-generation catalysts.

Atomic-Level Engineered Cobalt Catalysts for Fenton-Like Reactions: Synergy of Single Atom Metal Sites and Nonmetal-Bonded Functionalities.

Single atom catalysts (SACs) are atomic-level-engineered materials with high intrinsic activity. Catalytic centers of SACs are typically the transition metal (TM)-nonmetal coordination sites, while the functions of coexisting non-TM-bonded functionalities are usually overlooked in catalysis. Herein, the scalable preparation of carbon-supported cobalt-anchored SACs (CoCN) with controlled Co─N sites and free functional N species is reported. The role of metal- and nonmetal-bonded functionalities in the SACs for peroxymonosulfate (PMS)-driven Fenton-like reactions is first systematically studied, revealing their contribution to performance improvement and pathway steering. Experiments and computations demonstrate that the Co─N3C coordination plays a vital role in the formation of a surface-confined PMS* complex to trigger the electron transfer pathway and promote kinetics because of the optimized electronic state of Co centers, while the nonmetal-coordinated graphitic N sites act as preferable pollutant adsorption sites and additional PMS activation sites to accelerate electron transfer. Synergistically, CoCN exhibits ultrahigh activity in PMS activation for p-hydroxybenzoic acid oxidation, achieving complete degradation within 10 min with an ultrahigh turnover frequency of 0.38 min-1, surpassing most reported materials. These findings offer new insights into the versatile functions of N species in SACs and inspire rational design of high-performance catalysts in complicated heterogeneous systems.