High performance of Ir-promoted Ni/TiO2 catalyst toward the selective hydrogenation of cinnamaldehyde

Ir-CoO Active Centers Supported on Porous Al2 O3 Nanosheets as Efficient and Durable Photo-Thermal Catalysts for CO2 Conversion

Photo-thermal catalytic CO₂ hydrogenation is currently extensively studied as one of the most promising approaches for the conversion of CO₂ into value-added chemicals under mild conditions; however, achieving desirable conversion efficiency and target product selectivity remains challenging. Herein, the fabrication of Ir-CoO/Al₂ O₃ catalysts derived from Ir/CoAl LDH composites is reported for photo-thermal CO₂ methanation, which consist of Ir-CoO ensembles as active centers that are evenly anchored on amorphous Al₂ O₃ nanosheets. A CH₄ production rate of 128.9 mmol g_cat⁻ ¹ h⁻¹  is achieved at 250 °C under ambient pressure and visible light irradiation, outperforming most reported metal-based catalysts. Mechanism studies based on density functional theory (DFT) calculations and numerical simulations reveal that the CoO nanoparticles function as photocatalysts to donate electrons for Ir nanoparticles and meanwhile act as "nanoheaters" to effectively elevate the local temperature around Ir active sites, thus promoting the adsorption, activation, and conversion of reactant molecules. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) demonstrates that illumination also efficiently boosts the conversion of formate intermediates. The mechanism of dual functions of photothermal semiconductors as photocatalysts for electron donation and as nano-heaters for local temperature enhancement provides new insight in the exploration for efficient photo-thermal catalysts.

General Strategy toward Hydrophilic Single Atom Catalysts for Efficient Selective Hydrogenation

Well dispersible and stable single atom catalysts (SACs) with hydrophilic features are highly desirable for selective hydrogenation reactions in hydrophilic solvents towards important chemicals and pharmaceutical intermediates. A general strategy is reported for the fabrication of hydrophilic SACs by cation-exchange approach. The cation-exchange between metal ions (M = Ni, Fe, Co, Cu) and Na⁺ ions introduced in the skeleton of metal oxide (TiO₂ or ZrO₂ ) nanoshells plays the key role in forming M₁ /TiO₂ and M₁ /ZrO₂ SACs, which efficiently prevents the aggregation of the exchanged metal ions. The as-obtained SACs are highly dispersible and stable in hydrophilic solvents including alcohol and water, which greatly facilitates the catalysis reaction in alcohol. The Ni₁ /TiO₂ SACs have been successfully utilized as catalysts for the selective C=C hydrogenation of cinnamaldehyde to produce phenylpropanal with 98% conversion, over 90% selectivity, good recyclability, and a turnover frequency (TOF) of 102 h^-1 , overwhelming most reported catalysts including noble metal catalysts.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

γ-Valerolactone Production from Levulinic Acid Hydrogenation Using Ni Supported Nanoparticles: Influence of Tungsten Loading and pH of Synthesis

γ-Valerolactone (GVL) has been considered an alternative as biofuel in the production of carbon-based chemicals; however, the use of noble metals and corrosive solvents has been a problem. In this work, Ni supported nanocatalysts were prepared to produce γ-Valerolactone from levulinic acid using methanol as solvent at a temperature of 170 °C utilizing 4 MPa of H₂. Supports were modified at pH 3 using acetic acid (CH₃COOH) and pH 9 using ammonium hydroxide (NH₄OH) with different tungsten (W) loadings (1%, 3%, and 5%) by the Sol-gel method. Ni was deposited by the suspension impregnation method. The catalysts were characterized by various techniques including XRD, N₂ physisorption, UV-Vis, SEM, TEM, XPS, H₂-TPR, and Pyridine FTIR. Based on the study of acidity and activity relation, Ni dispersion due to the Lewis acid sites contributed by W at pH 9, producing nanoparticles smaller than 10 nm of Ni, and could be responsible for the high esterification activity of levulinic acid (LA) to Methyl levulinate being more selective to catalytic hydrogenation. Products and by-products were analyzed by ¹H NMR. Optimum catalytic activity was obtained with 5% W at pH 9, with 80% yield after 24 h of reaction. The higher catalytic activity was attributed to the particle size and the amount of Lewis acid sites generated by modifying the pH of synthesis and the amount of W in the support due to the spillover effect.

Tuning CO2 hydrogenation selectivity on Ni/TiO2 catalysts via sulfur addition

Although sulfur has long been identified as a poison for Ni catalysts in CO-methanation, its association with Ni on a reducible support allows the selective formation of CO in CO2 hydrogenation.

H-spilled storage to maximize the catalytic performances of Pd-based bimetals@Ti3C2Tx MXene in selective semihydrogenations

Hydrogen spillover is an important theme for hydrogen storage and H-involving catalytic reactions. This work shows that catalytic reactivity and selectivity can be revealed by differentiating energetic characteristics of the...

LED-driven controlled deposition of Ni onto TiO2 for visible-light expanded conversion of carbon dioxide into C1-C2 alkanes

Photocatalytic gas-phase hydrogenation of CO₂ into alkanes was achieved over TiO₂-supported Ni nanoparticles under LED irradiation at 365 nm, 460 nm and white light. The photocatalysts were prepared using photo-assisted deposition of Ni salts under LED irradiation at 365 nm onto TiO₂ P25 nanoparticles in methanol as a hole scavenger. This procedure yielded 2 nm Ni particles decorating the surface of TiO₂ with a nickel mass content of about 2%. Before the photocatalytic runs, Ni/TiO₂ was submitted to thermal reduction at 400 °C in a 10% H₂ atmosphere which induced O-defective TiO_2-x substrates. The formation of oxygen vacancies, Ti³⁺ centers and metallic Ni sites upon photocatalytic CO₂ hydrogenation was confirmed by operando EPR analysis. In situ XPS under reaction conditions suggested a strong metal-support interaction and the co-existence of zero and divalent Ni states. These photoactive species enhanced the photo-assisted reduction of CO₂ below 300 °C to yield CO, CH₄ and C₂H₆ as final products.

Nickle nanocrystals decorated on graphitic nanotubes with broad channels for fire hazard reduction of epoxy resin

Carbon nanotubes (CNTs) are a sort of carbon-based nanofillers blended into polymer nanocomposites to improve both of the flame retardancy and mechanical properties. However, the CNTs tend to entangle into bundles and the channels are too narrow to allow the entry of polymer chains, harmful to the dispersion and interaction within the polymer matrix. Therefore, by means of a facile pyrolysis method, boron and nitrogen co-doped larger-diameter graphitic nanotubes with decorated nickel nanocrystals (Ni/GNTs) were developed as flame retardant for epoxy resin (EP). The nanotubes are short but with large specific surface area. Compared to the commercial CNTs, the epoxy chain could infiltrate into the channels of Ni/GNTs which was approved by different techniques. The unique nanostructure endowed the product with strong interaction with the polymer matrix. The fire behaviors were examined by cone calorimeter tests, and the results showed that with the addition of 2 wt% Ni/GNTs, the peak of heat release rate and the total smoke production values of the nanocomposites were reduced by 43.5 % and 22.8 % compared with those of pure epoxy, respectively. Meanwhile, the flexural and tensile properties of EP/Ni/GNTs were also enhanced.

Identification of electron-rich mononuclear Ni atoms on TiO2-A distinguished from Ni particles on TiO2-R in guaiacol hydrodeoxygenation pathways

Electron-rich mononuclear Ni atoms located at the oxygen vacancies on TiO₂-A are the active sites for selective hydrodeoxygenation of guaiacol to phenolics, while the reduced Ni particles on TiO₂-R catalyze hydrogenative aromatic ring saturation.

Reaction Analyses Based on Quaternary Metal/Metal Oxide Catalyst Testing in Micro-Structured Reactors Using Combinatorial High-Throughput Methods for Power-to-Gas Applications

To optimize the Sabatier process, quaternary supported catalyst materials are synthesized and tested. The syntheses are performed by the industrially established, reproducible and automated methods of impregnation and sol–gel synthesis. The screening of the 588 quaternary catalysts is carried out in a specially designed 10-fold parallel gas flow micro-structured reactor as wall catalysts in sequential operation mode at a temperature of T = 573 K and a pressure of p = 15 bar. For the description of the activity, the reaction parameters CO2 conversion, CH4 yield, and CH4/CO2 selectivity are used. These are determined by analyses of the gas phase composition using µGC-FID. The catalysts with the highest activities are validated in a micro-structured reactor with similar characteristics as the screening reactor in the temperature range between T = 573–673 K and a pressure of p = 15 bar. Characterization by powder X-ray diffraction, Raman spectra, and scanning transmission electron microscopic images data on the phase and element distribution after calcination and reduction was conducted.

Pt-Ni Nanoalloys for H2 Generation from Hydrous Hydrazine

Hydrous hydrazine (N2H4∙H2O) is a candidate for a hydrogen carrier for storage and transportation due to low material cost, high hydrogen content of 8.0%, and liquid stability at room temperature. Pt and Pt nanoalloy catalysts have been welcomed by researchers for the dehydrogenation of hydrous hydrazine recently. Therefore, in this review, we give a summary of Pt nanoalloy catalysts for the dehydrogenation of hydrous hydrazine and briefly introduce the decomposition mechanism of hydrous hydrazine to prove the design principle of the catalyst. The chemical characteristics of hydrous hydrazine and the mechanism of dehydrogenation reaction are briefly introduced. The catalytic activity of hydrous hydrazine on different supports and the factors affecting the selectivity of hydrogen catalyzed by Ni-Pt are analyzed. It is expected to provide a new way for the development of high-activity catalysts for the dehydrogenation of hydrous hydrazine to produce hydrogen.

Pd Nanoparticles Supported on Triangle-Shaped La2 O2 CO3 Nanosheets: A New Highly Efficient and Durable Catalyst for Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde

A catalyst in which Pd nanoparticles are supported on triangle-shaped La₂ O₂ CO₃ nanosheets exposing predominantly the (001) planes (Pd/La₂ O₂ CO₃ -TNS; where TNS denotes triangular nanosheets) was prepared by a facile solvothermal method. The Pd/La₂ O₂ CO₃ -TNS catalysts exhibited excellent catalytic activity and recycling stability for hydrogenation of cinnamaldehyde to hydrocinnamaldehyde with turnover frequency of up to 41 238 h^-1 . This enhanced activity of Pd/La₂ O₂ CO₃ -TNS results from strong metal-support interactions. Structure analysis and characterization demonstrated that surface-oxygen-enriched La₂ O₂ CO₃ -TNS supports exposing (001) planes are beneficial to charge transfer between the Pd nanoparticles and triangle-shaped La₂ O₂ CO₃ nanosheets and increase the electron density of Pd. Moreover, the modulated electronic states of the Pd/La₂ O₂ CO₃ -TNS catalysts can enhance the adsorption and activation of hydrogen to enhance the hydrogenation activity.

High performance of Mo-promoted Ir/SiO2 catalysts combined with HZSM-5 toward the conversion of cellulose to C5/C6 alkanes

In this study, the Mo-promoted Ir/SiO₂ (Ir-MoO_x/SiO₂) catalysts combined with the zeolite HZSM-5 were used for the direct conversion of microcrystalline cellulose (MCC) to liquid fuel (C₅/C₆ alkanes) in n-dodecane/H₂O system. A synergistic effect was formed between the partially reduced MoO_x species and the Ir particles, which effectively promoted the catalytic activity of Ir/SiO₂ catalyst. When the Mo/Ir molar ratio was 0.5, a high yield of C₅/C₆ alkanes (91.7%) was achieved at 210 ℃ for 12 h. In addition, the main component of C₅/C₆ alkanes was n-hexane, which was proven to be obtained by the hydrogenolysis of the key intermediate, sorbitol, formed from the hydrolysis and hydrogenation of MCC.

Formation Combined with Intercalation of Ni and Its Alloy Nanoparticles within Mesoporous Silica for Robust Catalytic Reactions

Intercalation of silica-supported nickel nanoparticles within mesoporous silica has been achieved through chemical reduction of nickel silicate with mesoporous silica ( mSiO₂) coated on inner and outer surfaces. Formation of nickel nanoparticles was controlled at nickel silicate-silica interface and was well-confined by mSiO₂ coating. Doping of other transition metals has been accomplished at the stage of nickel silicate formation, because of similarity in critical stability constants of respective metal salts. Doped nickel silicates were able to produce nickel-based bimetallic and trimetallic alloy nanoparticles within the final dual-shell configuration. This type of catalyst has been tested for both liquid- and gas-phase reactions, all showing good activity and selectivity. Ni nanoparticles could serve as the active catalyst or activity enhancer to other alloyed metals for different reactions. Especially for selective hydrogenation of trans-cinnamaldehyde, 100% selectivity toward hydrocinnamaldehyde at full conversion has been achieved without using noble metals. Spent catalysts in all cases showed no changes in terms of morphology and crystal structure, indicating this type of catalyst was robust under such reaction conditions, including gas-solid reaction systems.

Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Ni Supported on Zirconium Phosphate Catalysts

Crystal α-zirconium phosphate (α-ZrP) was prepared by a hydrothermal method and exfoliated into a layered structure by n-hexylamine (C₆H₁₃NH₂). Ni-based catalyst (Ni/ZrP) was promoted by loading nickel on the layered α-ZrP via ion exchange. The catalyst was performed to catalyze hydrodeoxygenation of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF), and a 68.1% yield of DMF and 100% conversion of HMF were achieved at 240 °C, 5 MPa H₂, and 20 h. The DMF yield can still retain 52.8% after five cycles. The characteristics of the catalyst were investigated via N₂ adsorption-desorption, X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, pyridine-adsorbed Fourier transform infrared (FTIR) spectra, FTIR spectra, inductively coupled plasma mass spectrometry, and thermogravimetric analysis-mass spectrometry, as well as Raman spectroscopy. A pathway from HMF to DMF was found with MF as the intermediate product, and DMF production was preferable via the -CH₂OH group hydrogenolysis of HMF over Lewis acidic sites of Ni/ZrP, which is caused by the zirconium vacant orbits.

Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst

The surface of Pd particles is modified by pyridine and this promotes the hydrogenation of CO in CAL over Pd/C.