Electrocatalytic Glycerol Upgrading into Glyceric Acid on Ni3Sn Intermetallic Compound.

Electrochemical glycerol oxidation features an attractive approach of converting bulk chemicals into high-value products such as glyceric acid. Nonetheless, to date, the major product selectivity has mostly been limited as low-value C1 products such as formate, CO, and CO2, due to the fast cleavage of carbon-carbon (C-C) bonds during electro-oxidation. Herein, the study develops an atomically ordered Ni3Sn intermetallic compound catalyst, in which Sn atoms with low carbon-binding and high oxygen-binding capability allow to tune the adsorption of glycerol oxidation intermediates from multi-valent carbon binding to mono-valent carbon binding, as well as enhance *OH binding and subsequent nucleophilic attack. The Ni3Sn electrocatalyst exhibits one of the highest glycerol-to-glyceric acid performances, including a high glycerol conversion rate (1199 µmol h-1) and glyceric acid selectivity (62 ± 3%), a long electrochemical stability of > 150 h, and the capability of direct conversion of crude glycerol (85% purity) into glyceric acid. The work features the rational design of highly ordered catalytic sites for tailoring intermediate binding and reaction pathways, thereby facilitating the efficient production of high-value chemical products.

Electrocatalytic CO2 Upgrading to Triethanolamine by Bromine-Assisted C2 H4 Oxidation.

The electrocatalytic carbon dioxide (CO2 ) reduction is a promising approach for converting this greenhouse gas into value-added chemicals, while the capability of producing products with longer carbon chains (Cn >3) is limited. Herein, we demonstrate the Br-assisted electrocatalytic oxidation of ethylene (C2 H4 ), a major CO2 electroreduction product, into 2-bromoethanol by electro-generated bromine on metal phthalocyanine catalysts. Due to the preferential formation of Br2 over *O or Cl2 to activate the C=C bond, a high partial current density of producing 2-bromoethanol (46.6 mA⋅cm-2 ) was obtained with 87.2 % Faradaic efficiency. Further coupling with the electrocatalytic nitrite reduction to ammonia at the cathode allowed the production of triethanolamine with six carbon atoms. Moreover, by coupling a CO2 electrolysis cell for in situ C2 H4 generation and a C2 H4 oxidation/nitrite reduction cell, the capability of upgrading of CO2 and nitrite into triethanolamine was demonstrated.

Photocatalytic CO2 conversion: from C1 products to multi-carbon oxygenates.

Photocatalytic CO2 conversion into high-value chemicals has been emerging as an attractive research direction in achieving carbon resource sustainability. The chemical products can be categorized into C1 and multi-carbon (C2+) products. In this review, we describe the recent research progress in photocatalytic CO2 conversion systems from C1 products to multi-carbon oxygenates, and analyze the reasons related to their catalytic mechanisms, as the production of multi-carbon oxygenates is generally more difficult than that of C1 products. Then we discuss several examples in promoting the photoconversion of CO2 to value-added multi-carbon products in the aspects of photocatalyst design, mass transfer control, determination of active sites, and intermediate regulation. Finally, we summarize perspectives on the challenges and propose potential directions in this fast-developing field, such as the prospect of CO2 transformation to long-chain hydrocarbons like salicylic acid or even plastics.

Electrocatalytic methane oxidation to formate on magnesium based metal-organic frameworks.

The electrochemical methane oxidation reaction is a potential approach for upgrading the nature-abundant methane (CH4) into value-added chemicals, while the activity and selectivity have remained substantially low due to the extremely inert chemical property of CH4. Inspired by the methane mono-oxygenase in nature, we demonstrated Mg-substituted metal-organic frameworks (Mg-MOF-74) containing a uniform distribution of Mg-oxo-Mg nodes as efficient catalytic sites. Compared to MgNi-MOF-74 and Mg(OH)2 without the Mg-oxo-Mg nodes, the Mg-MOF-74 presented a much enhanced CH4 electrooxidation performance, with a unique selectivity of producing formate. The maximum Faradaic efficiency of all liquid products reached 10.9% at 1.60 V versus reversible hydrogen electrode (RHE), corresponding to the peak production rate of 126.6 µmol·h-1·g-1.

Electron Localization and Lattice Strain Induced by Surface Lithium Doping Enable Ampere-Level Electrosynthesis of Formate from CO2.

The electrochemical CO2 conversion to formate is a promising approach for reducing CO2 level and obtaining value-added chemicals, but its partial current density is still insufficient to meet the industrial demands. Herein, we developed a surface-lithium-doped tin (s-SnLi) catalyst by controlled electrochemical lithiation. Density functional theory calculations indicated that the Li dopants introduced electron localization and lattice strains on the Sn surface, thus enhancing both activity and selectivity of the CO2 electroreduction to formate. The s-SnLi electrocatalyst exhibited one of the best CO2 -to-formate performances, with a partial current density of -1.0 A cm-2 for producing formate and a corresponding Faradaic efficiency of 92 %. Furthermore, Zn-CO2 batteries equipped with the s-SnLi catalyst displayed one of the highest power densities of 1.24 mW cm-2 and an outstanding stability of >800 cycles. Our work suggests a promising approach to incorporate electron localization and lattice strain for the catalytic sites to achieve efficient CO2 -to-formate electrosynthesis toward potential commercialization.

Electrocatalytic Methane Oxidation Greatly Promoted by Chlorine Intermediates.

Renewable energy-powered methane (CH4 ) conversion at ambient conditions is an attractive but highly challenging field. Due to the highly inert character of CH4 , the selective cleavage of its first C-H bond without over-oxidation is essential for transforming CH4 into value-added products. In this work, we developed an efficient and selective CH4 conversion approach at room temperature using intermediate chlorine species (*Cl), which were electrochemically generated and stabilized on mixed cobalt-nickel spinels with different Co/Ni ratios. The lower overpotentials for *Cl formation enabled an effective activation and conversion of CH4 to CH3 Cl without over-oxidation to CO2 , and Ni3+ at the octahedral sites in the mixed cobalt-nickel spinels allowed to stabilize surface-bound *Cl species. The CoNi2 Ox electrocatalyst exhibited an outstanding yield of CH3 Cl (364 mmol g-1 h-1 ) and a high CH3 Cl/CO2 selectivity of over 400 at room temperature, with demonstrated capability of direct CH4 conversion under seawater working conditions.

Efficient carboxylation of styrene and carbon dioxide by single-atomic copper electrocatalyst.

Electrocarboxylation of olefins with carbon dioxide (CO2) is a potential approach to produce carboxylates as synthetic intermediates of polymer and pharmaceuticals. Nonetheless, due to the intrinsic inertness of CO2 at ambient conditions, the electrocarboxylation efficiency has been quite limited, typically with high applied potentials and low current densities. In this work, we demonstrate that nitrogen-coordinated single-atomic copper sites on carbon framework (Cu/NC) served as an excellent electrocatalyst for electrocarboxylation of styrene with CO2. The Cu/NC catalyst allowed to efficiently activate CO2, followed by nucleophilic attack to carboxylate styrene to produce phenylsuccinic acid, thus leading the reaction toward the CO2 activation pathway. The enhanced CO2 activation capability enabled increased selectivity and activity for electrocarboxylation of styrene. The Faradaic efficiency of electrocarboxylation was 92%, suggesting most of the activated CO2 proceeded to react with styrene rather than direct reduction to CO or CH4. The electrocarboxylation exhibited almost 100% product selectivity toward phenylsuccinic acid, with a high partial current density of 58 mA·cm-2 at -2.2 V (vs. Ag/AgI), corresponding to an outstanding production rate of 216 mg·cm-2·h-1, substantially exceeding previously reported works. Our work suggests an exciting perspective in electrocarboxylation of olefins by rational design of CO2 activation electrocatalysts.

Promoting N2 electroreduction to ammonia by fluorine-terminating Ti3C2Tx MXene.

Two-dimensional MXene-based materials are potential of presenting unique catalytic performances of electrocatalytic reactions. The surface functionalization of MXene-based catalysts is attractive for developing efficient electrocatalysts toward nitrogen reduction reaction. Herein, we reported a Ti3C2Tx MXene with a medium density of surface functionalized fluorine terminal groups, as an excellent N2 reduction reaction electrocatalyst with enhanced adsorption and activation of N2. The Ti3C2Tx MXene catalyst showed a production rate of ammonia as 2.81 × 10-5 µmol·s-1·cm-2, corresponding to a partial current density of 18.3 µA·cm-2 and a Faradic efficiency of 7.4% at - 0.7 V versus reversible hydrogen electrode in aqueous solutions at ambient conditions, substantially exceeding similar Ti3C2Tx MXene catalysts but with higher or lower densities of surface fluorine terminal groups. Our work suggests the capability of developing surface functionalization toolkit for enhancing electrochemical catalytic activities of two-dimensional MXene-based materials.

Electrochemical nitrogen fixation via bimetallic Sn-Ti sites on defective titanium oxide catalysts.

The efficient adsorption and activation of inert N2 molecules on a heterogeneous electrocatalyst surface are critical toward electrochemical N2 fixation. Inspired by the bimetallic sites in nitrogenase, herein, we developed a bi-metallic tin-titanium (Sn-Ti) structure in Sn-doped anatase TiO2 via an oxygen vacancy induced engineering approach. Density functional theory (DFT) calculations indicated that Sn atoms were introduced in the oxygen vacancy sites in anatase TiO2 (101) to form Sn-Ti bonds. These Sn-Ti bonds provided both strong σ-electron accepting and strong π-electron donating capabilities, thus serving as both N2 adsorption and catalytic N2 reduction sites. In 0.1 M KOH aqueous solution, the Sn-TiO2 electrocatalyst achieved a NH3 production rate of 10.5 µgh-1cm-2 and a corresponding Faradaic efficiency (FENH3) of 8.36% at -0.45 V vs. reversible hydrogen electrode (RHE). Our work suggests the potential of atomic-scale designing and constructing bimetallic active sites for efficient electrocatalytic N2 fixation.

Oxygen vacancies enhanced cooperative electrocatalytic reduction of carbon dioxide and nitrite ions to urea.

The electrochemical reduction of carbon dioxide and nitrite ions into value-added chemicals represents one of the most promising approaches to relieve the greenhouse gases, while a critical challenge is to search for a highly effective catalyst with low energy input and high conversion selectivity. In this work, we demonstrated low-valence Cu doped, oxygen vacancy-rich anatase TiO2 (Cu-TiO2) nanotubes as a synergetic catalyst for electrochemical co-reduction of both CO2 and NO2-. The incorporation of Cu dopants in anatase TiO2 facilitated to form abundant oxygen vacancies and bi-Ti3+ defect sites, which allowed for efficient nitrite adsorption and activation. The low-valence Cu dopants also served as effective catalytic centers to reduce CO2 into CO* adsorbate. The close proximity of CO* and NH2* intermediates was beneficial for the subsequent cooperative tandem reaction to form urea via the CN coupling. This oxygen vacancy-rich Cu-TiO2 electrocatalyst enabled excellent urea production rate (20.8 µmol⋅h-1) and corresponding Faradaic efficiency (43.1%) at a low overpotential of -0.4 V versus reversible hydrogen electrode, substantially superior than those of undoped TiO2, thus suggesting an exciting approach for cooperative CO2 and nitrogen fixation.

Fast cooling induced grain-boundary-rich copper oxide for electrocatalytic carbon dioxide reduction to ethanol.

Electrochemical CO2 reduction with rationally designed copper-based electrocatalysts is a promising approach to reduce CO2 emission and produce value-added products. Grain boundaries and micron-strains inside catalysts have been proposed as active catalytic sites, while the controlled formation of these sites has remained highly challenging. In this work, we developed a strategy of creating high-density grain boundaries and micron-strains inside CuO electrocatalysts by fast cooling with liquid nitrogen. Compared to samples with slower cooling rates, the fast cooled CuO showed clear difference in their crystal domain sizes, micro-strain densities, and the chemisorption capacities of CO2 and CO. This micro-strain-rich CuO electrocatalyst exhibited a high total current density over 300 mA·cm-2, and an outstanding Faradaic efficiency for C2 products (with a majority to ethanol) at -1.0 V vs. reversible hydrogen electrode. Our work suggests a facile approach of tuning grain boundaries and micro-strains inside Cu-based electrocatalysts to scale up electrochemical CO2 reduction for high value-added products.

One-dimensional Nanomaterial Electrocatalysts for CO2 Fixation.

Electroreduction of CO2 into valuable molecules or fuels is a sustainable pathway for CO2 reduction as well as energy storage. However, the premature development stage of electrocatalysts with high activity, selectivity, and durability still remains a significant bottleneck that hinders this field. One-dimensional (1D) nanomaterials, including nanorods, nanotubes, nanoribbons, nanowires, and nanofibers, are generally considered as high-activity and stable electromaterials, due to their unique uniform structures, orientated electronic and mass transport, and rigid tolerance to stress variation. During the past several years, 1D nanomaterials and nanostructures have been extensively studied due to their potentials in serving as CO2 electroreduction catalysts. In this minireview, recent studies and advances in 1D nanomaterials for CO2 eletroreduction are summarized, from the viewpoints of both computational and experimental aspects. Based on the composition, the 1D nanomaterials are studied in four categories, including metals, transition-metal oxides/nitrides, transition-metal chalcogenides, and carbon-based materials. Different parameters in tuning 1D materials are also summarized and discussed, such as the crystal facets, grain boundaries, heteroatoms doping, additives and the electrochemical tuning effects. Finally, the challenges and prospects in this direction will be discussed.

Color-tunable emission and energy transfer investigation in Sr3Y(PO4)3:Ce3+,Tb3+ phosphors for white LEDs.

A novel color-tunable phosphor Sr3Y(PO4)3:Ce3+,Tb3+ was synthesized through solid-state reaction method. Several techniques, such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy, were used to investigate the obtained phosphors. Results of luminescence spectra and decay time measurements revealed that an efficient energy transfer occurred from Ce3+ to Tb3+ via a dipole-dipole mechanism, where Ce3+ exhibited a strong excitation band in the near-ultraviolet region. CIE chromaticity coordinates were tuned from deep blue (0.162, 0.090) to green (0.230, 0.411) by adjusting the relative concentrations between Ce3+ and Tb3+ ions. Results revealed that the as-synthesized phosphors had color-tunable characteristics and can be used as promising materials in the field of phosphor-converted white light-emitting diodes.

Dual emission of Ce3+ ,Mn2+ -coactivated Ca3 YNa(PO4 )3 F via energy transfer: a single component white/yellow-emitting phosphor.

A series of Ce3+ ,Mn2+ -coactivated Ca3 YNa(PO4 )3 F phosphors were synthesized via a traditional solid-state reaction under a reductive atmosphere. X-Ray powder diffraction was used to confirm that the crystal structure and diffraction peaks of Ce3+ /Mn2+ -doped samples matched well with the standard data. A spectral overlap between the emission band of Ce3+ and the excitation band of Mn2+ suggested the occurrence of energy transfer from Ce3+ to Mn2+ . With increasing Mn2+ content, the emission intensities and lifetime values of the Ce3+ emission for Ca3 YNa(PO4 )3 F:Ce3+ ,Mn2+ phosphors linearly decrease, whereas the energy transfer efficiencies gradually increase to 89.35%. By adjusting the relative concentrations of Ce3+ and Mn2+ , the emission hues are tuned from blue to white and eventually to yellow. These results suggest that Ca3 YNa(PO4 )3 F:Ce3+ ,Mn2+ phosphors have promising application as white-emitting phosphors for near-ultraviolet light-emitting diodes.

Luminescence and energy transfer of the color-tunable phosphor Li6Gd(BO3)3:Tb³âº/Bi³âº, Eu³âº.

Bi(3+)/Tb(3+), Eu(3+) co-doped Li6Gd(BO3)3 (LGBO) phosphors were synthesized via a traditional solid-state method. Phase purity was investigated using X-ray diffraction, absorption strength of the phosphors were investigated by UV-vis absorption spectra, and the photoluminescence properties of the phosphors were studied systematically. Results showed that the emission intensity of Bi(3+), Eu(3+) co-doped LBOG was 2.76 times higher than that of Eu(3+)-doped LGBO irradiated at 275 nm, thereby implying the possibility of energy transfer from Bi(3+) to Eu(3+). The excitation spectra of Tb(3+), Eu(3+) co-doped LGBO phosphors are broader in comparison with single-doped phosphors and show tunable colors from green to yellow to orange-red when the ratio of Tb(3+) to Eu(3+) is adjusted These results demonstrate that LGBO:Tb(3+), Eu(3+) phosphors have potential use in light-emitting diodes.

Luminescence and energy transfer of color-tunable Li6Gd(BO3)3:Ce(3+), Tb(3+) phosphor.

A series of novel color-tunable phosphors of Ce(3+), Tb(3+)-codoped Li6Gd(BO3)3 was synthesized through a classic solid-state reaction. The color of these phosphors changes from blue to green by adjusting the ratio of Ce(3+) to Tb(3+). The photoluminescence properties of the synthesized phosphors were investigated, and several major emission bands that belong to Ce(3+) and Tb(3+) ions were irradiated with near ultraviolet light. Moreover, the energy transfer mechanism between Ce(3+) and Tb(3+) in Li6Gd(BO3)3 was explored. The photoluminescence decay curves were performed to validate the energy transfer. The analysis demonstrated that the energy transfer from Ce(3+) to Tb(3+) arose from dipole-dipole interaction with a critical distance of approximately 17.6 Å.