Noble Metal Particles Research Articles

How the Most Neglected Residual Species in MOF-Based Catalysts Involved in Catalytic Reactions to Form Toxic Byproducts.

In recent years, multifarious new materials have been developed for environmental governance. Thereinto, metal organic framework (MOF)-based catalysts have been widely employed for heterogeneous catalysis because of their high porosity to confine noble metal particles faraway from aggregation. However, the potential reactions between residual species from the material synthesis process and target pollutants, which could form highly toxic byproducts, are often neglected. Herein, we took the widely used Zr-MOF, UiO-66, with highly thermal stability supported Pd catalysts as the example to investigate how the residual species in catalysts are involved in aromatic volatile organic compounds (VOCs) degradation reaction. The results showed that residual Cl species originated from the ZrCl4 metal precursor participated in the VOC degradation reaction, leading to the production of various chlorine-containing byproducts, even the hypertoxicity dioxin precursor, dichlorobenzene. Meanwhile, the chlorination mechanism for the formation of chlorine-containing byproducts was revealed by density functional theory calculation. Furthermore, the highly efficient residual Cl removal approaches are proposed. Importantly, the migration and transformation of residual Cl during the degradation of five benzene series VOCs are comprehensively studied and elucidated. We anticipate that these findings will raise alarm about the neglected issue of residual species in MOF-based catalysts for heterogeneous catalysis, especially environmentally friendly catalysis.

The Effect of Loading W&V:TiO2 Nanoparticles with Noble Metals for CH4 Detection

TiO2 nanoparticles (NPs) doped with W (W:TiO2), double-doped with W and V (W&V:TiO2), and loaded with noble metals (W:TiO2 @Pt/Pd/Ag and W&V:TiO2@Pt/Pd/Ag) were synthesized by laser pyrolysis followed by chemical impregnation and reduction. Due to its exceptional properties, TiO2 is considered a key material being used in a wide range of applications. To improve its detection activity, the increase in the specific surface of the material, and the presence of defects in its structure play a decisive role. Doped and double-doped TiO2 nanoparticles with dimensions in the range of 25–30 nm presented a mixture of phases corresponding to titania, with the anatase phase accounting for the majority (95%). By loading these nanoparticles with small particles of noble metals, a significant increase in the specific surface area by three or even five times the original values was achieved. Sensitive thin films for surface acoustic wave (SAW) sensors were made with the NPs, embedded in polyethyleneimine (PEI) polymer and deposited by spin-coating. Each sensor was tested at CH4 concentrations between 0.4 and 2%, at room temperature, and the best results were obtained by the sensor with NPs doped with V and decorated with Pd, with a limit of detection (LOD) of 17 ppm, due to the strong catalytic effect of Pd.

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano‐Islands

AbstractCeO2‐supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2‐Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano-Islands.

CeO2-supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2-Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Dielectric Engineering of Perovskite BaMnO3 for the Rapid Heterogeneous Nucleation of Pt Nanoparticles for Catalytic Applications

AbstractMicrowave heating provides a rapid method for the heterogeneous nucleation of noble metal particles on perovskite support materials for electrocatalytic purposes. To succeed, dielectric tuning of perovskite materials becomes fundamental. Herein, the dielectric engineering of the BaMnO3 perovskite system is carried out through the use of B‐site doping to give BaTi0.5Mn0.5O3. Using a combination of atomic‐scale imaging and electron energy loss spectroscopy (EELS), the preferential filling of the M1 and M3 B‐sites with Mn and Ti ions in the 12R‐rhombohedral perovskite structure is established. While the addition of Ti in the BaMnO3 system has no detrimental effects on the presence of the oxygen reduction reaction (ORR) active Mn3+ states at the surface, it does alter the dielectric constant and loss tangent, thus facilitating the heterogeneous nucleation of Pt nanoparticles on BaTi0.5Mn0.5O3 via rapid microwave heating. Higher Pt loading regimes are found to increase the size and aggregation of the nucleated particles, thus reducing their ORR activity. Therefore, lower Pt loading not only reduces costs but improves overall activity. This work represents future possibilities for the dielectric engineering of perovskite and similar support materials to aid in the quick and easy formation of stable noble metal‐support catalytic systems.

Gold particles modified 3D printed carbon black nanonetwork electrode for improving the detection sensitivity of dopamine

Using 3-dimensional (3D) printing to create a fast and accurate electrochemical sensor has attracted the attention of analytical scientists. This study utilized a carbon black/PLA composite filament as the raw material for the construction of 3D electrodes (3DEs). Au particles were modified onto the activated surface of the carbon black nanonetwork 3DE. The physical and chemical properties of the 3DEs were evaluated through scanning electron microscopy (SEM) and X-ray spectroscopy (XPS). Additionally, a range of electrochemical techniques were employed to investigate the electrochemical performance of the 3DEs. The sensor is capable of detecting DA in the concentration range of 0.01–20 μM with the limit of detection (LOD) of 0.084 μM. The approach is also capable of being applied to the actual sample detection of DA with excellent selectivity. Hence, the method of combining noble metal particles and carbon black nanonetwork for constructing 3DE presents novel prospects towards the creation of electrochemical sensors with exceptional sensitivity.

Thiol-functionalized mesoporous silica-embedded AuNPs with highly sensitive substrates for surface-enhanced Raman scattering detection

Surface-enhanced Raman spectroscopy (SERS) provides a fast, simple, and label-free approach that can directly detect various analytes attached to SERS substrates and provide qualitative and quantitative information based on the SERS spectra of the analyte. The signal enhancement provided by noble metal particles allows us to achieve high sensitivity and good stability even at low concentrations in water quality testing. Over the past decade, SERS has become increasingly popular in the detection field, highlighting its significant application potential. However, the signal enhancement of single AuNPs is inferior to that of AgNPs, but we can achieve an overall enhancement by uniformly arranging multiple AuNPs. The hot-spot effect, which is crucial for Raman signal enhancement, occurs only when NMNPs are within a specific distance. However, when noble metal particles come into contact with each other, the enhancement effect of the hot spot disappears. Therefore, in this study, AuNPs@MPS-SH substrates with surface-enhanced Raman scattering (SERS) activity were successfully synthesized by immobilizing AuNPs on thiol-functionalized mesoporous silica (MPS-SH). MPS nanospheres with tailored pore size and surface-bound -SH groups were synthesized for the controlled embedding of Au nanoparticles (AuNPs). By adjusting the ratio of reactants, uniform AuNP dispersion and strong MPS-AuNP interactions were achieved. This optimized structure creates localized surface plasmon resonance (LSPR) hotspots, significantly enhancing the surface-enhanced Raman scattering (SERS) effect for ultrasensitive analyte detection. Transmission electron microscopy (TEM) revealed that the average diameter of MPS nanoparticles is 260 nm, with a pore size of 8 nm. Optimal Raman enhancement was achieved at an AuNPs concentration of 2.6 × 1018 particles/mL. The lowest detection limit concentration for Rhodamine 6G was below 10−6 M, while for uremic toxin (urea) it was below 10−3 M. The ability to directly detect low concentrations by mixing with the analyte demonstrates the ultra-high sensitivity and label-free detection potential of this study in the fields of water quality and disease detection.

Substrate types and applications of MXene for surface-enhanced Raman spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) has been widely used in the analysis of analytes because of its unique fingerprint characteristics, high sensitivity, and fast detection response. MXene is widely used in SERS studies among the various substrates due to its ultra-high chemical stability, excellent conductivity, hydrophilicity, and low fabrication cost. This mini-review summarizes MXene's research in the SERS field from two aspects. We reviewed MXene materials used as SERS substrates alone and combined with noble metal particles primarily. Subsequently, we outlined representative applications of MXene-based SERS in biomedicine, food safety, and environmental monitoring. Moreover, we discussed the technical bottleneck and the prospect of future development in this field.

A universal strategy for green and surfactant-free synthesis of noble metal nanoparticles.

We propose a universal, green, and surfactant-free strategy to synthesize noble metal particles with high monodispersity using gaseous H2 as a reducing agent in a solution at 60 °C. The prepared Pt nanoparticles have a 24 mV more positive half-wave potential than the commercially available Pt/C in the oxygen reduction reaction, while showing high durability with negligible half-wave potential decay after 10 000 cycles of testing.

Amorphous Pt-decorated α-Fe2O3 sensor with superior triethylamine sensing performance prepared by a one-step impregnation method

Noble metals are widely used additives to improve the sensing performance of conductometric metal oxide gas sensors. Although amorphous noble metal particles are expected to further improve the gas sensor response, common techniques often result in crystalline nanoparticles. Herein, an amorphous Pt nanoparticle-decorated α-Fe2O3 sensor was prepared via a temperature-controlled facile one-step impregnation method without the use of a reductant, surfactant, or subsequent thermal reduction. The amorphous Pt-functionalized α-Fe2O3 exhibited superior sensing performance to triethylamine, achieving 100 °C and 140 °C decreases in the optimal working temperature, and had better selectivity, with 22-fold and 14-fold improvements in gas response when compared to those of crystallized Pt-functionalized α-Fe2O3 and pristine α-Fe2O3, respectively. The as-prepared amorphous Pt-decorated α-Fe2O3 sensor showed great promise as a promising triethylamine sensor, and the newly developed facile impregnation method was shown to be an effective strategy to synthesize amorphous noble metal-decorated metal oxide gas sensors.

Construction of g- C3N4/Au/MgAl2O4 photocatalysts with different coupling methods to improve the photodegradation behavior and performance prediction

A novel g-C3N4/Au/MgAl2O4 (CN-Au-MAO) system have been developed to improve the photocatalytic activity of CN and MAO for the simulated sunlight -driven chlortetracycline hydrochloride (CTC-HCl) removal. The Au particles act as a surface plasmon resonance center to separate and accumulate electrons from the conduction band of CN and MAO, promoting the transfer of photo-generated carriers, thereby maintaining high oxidative capacities of MAO. The prepared photocatalysts revealed that the plasma resonance wavelength and photocatalytic efficiency of CN -Au -MAO photocatalysts strongly depend on the coupling order of CN, Au, and MAO. The effects of mass percentages of Au, CN and Au-CN, catalyst content, drug concentration and pH value on the degradation of CTC-HCl were explored in depth. The CN-Au-MAO photocatalysts exhibited a high photocatalytic efficiency of 92.56% toward the photodegradation of CTC-HCl with the optimum catalyst content of 1 g/L, drug concentration of 200 mg/L and pH= 11. The genetic algorithm was used to predict the photocatalytic activity of CN-Au-MAO photocatalyst, and the minimum data set that can effectively predict its photocatalytic activity was explored. The active species detection experiments, degradation pathways, and toxicity estimates were also performed. A reasonable mechanism of the CN-Au-MAO system was proposed. This work effectively solves the problem of coupling MAO with the noble metal particles, which makes MAO easy to hydrolyze, and the problem of predicting the effective minimum data set required for the photocatalytic activity through neural network algorithms.

Simulation of noble metal particle growth and removal in the molten salt fast reactor

In the Molten Salt Fast Reactor (MSFR), fuel is dissolved in a liquid salt, causing the formation and decay of fission products, such as noble metals, to be inside the salt. Noble metals have very low solubility and do not dissolve in the salt. They are known to agglomorate into particles which, in turn, diffuse, drift or sediment to interfaces such as walls, free surfaces and bubbles, where they deposit. Leveraging this in the MSFR, online or offline helium bubbling is foreseen to act as a means to extract noble metal particles from the salt. This can potentially limit strong deposition of particles onto walls (i.e., plating), preventing dangerous decay heat hotspots in the heat exchangers or other primary circuit components. In this paper, we investigate the efficiency of particle removal by helium bubbles in the MSFR using Computational Fluid Dynamics (CFD). Theoretical estimates of equilibrium particle size distributions are used as initial condition, based on previous work. CFD calculations are performed in a simplified MSFR geometry. Bubble expansion, turbulent bubble coalescence and turbulent break-up are accounted for using the poly-disperse Log-normal Method of Moments (LogMoM) model, embedded in the two-fluid framework in OpenFOAM. The CFD analyses presented in this paper, in conjunction with the developed theory of noble metal particle formation, growth and capture, will help in the understanding and optimization of the fuel cycle in the MSFR or any other molten salt reactor design. The precise knowledge of how noble metal particle populations in the salt can be controlled, and how plating of noble metal particles onto primary circuit reactor components can be reduced, contributes to safe molten salt reactor operation.

Quantifying Nanoparticle Layer Topography: Theoretical Modeling and Atomic Force Microscopy Investigations.

A comprehensive method consisting of theoretical modeling and experimental atomic force microscopy (AFM) measurements was developed for the quantitative analysis of nanoparticle layer topography. Analytical results were derived for particles of various shapes such as cylinders (rods), disks, ellipsoids, hemispheres (caps), etc. It was shown that for all particles, their root-mean-square (rms) parameter exhibited amaximum at the coverage about 0.5, whereas the skewness was a monotonically decreasing function of the coverage. This enabled a facile determination of the particle coverage in the layer, even if the shape and size were not known. The validity of the analytical results was confirmed by computer modeling and experimental data acquired by AFM measurements for polymer nanoparticle deposition on mica and silica. The topographical analysis developed in this work can be exploited for a quantitative characterization of self-assembled layers of nano- and bioparticles, e.g., carbon nanotubes, silica and noble metal particles, DNA fragments, proteins, vesicles, viruses, and bacteria at solid surfaces. The acquired results also enabled a proper calibration, in particular the determination of the measurement precision, of various electron and scanning probe microscopies, such as AFM.

Combined use of in situ and operando-FTIR, TPR and FESEM techniques to investigate the surface species along the simultaneous abatement of N2O and NO on Pt,Pd,Rh/TiO2-ZrO2 and Pt,Pd,Rh/TiO2-ZrO2-CeO2 catalysts

Pt,Pd,Rh supported on TiO2-ZrO2 and TiO2-ZrO2-CeO2 show a promising catalytic behavior for the simultaneous selective catalytic reduction of NO and N2O with CH4 in the presence of O2 (SCRsim). The nature of the catalytic species and the formation of intermediate species on the catalyst surface were investigated by combining several techniques. The co-existence of fully and partly reduced noble metals are strictly dependent on the O2/CH4 ratio in the feed which is crucial for a good catalytic activity. When the O2/CH4 ratio is less than 1, NO and N2O were efficiently and simultaneously reduced with negligible formation of by-products. As the O2/CH4 ratio was higher than 1, CH4 combustion prevailed. The CeO2 made the catalyst slightly more efficient, likely improving the solid redox properties at the interface between noble metal particles and gaseous reactants. The operando FT-IR analysis identified some surface intermediate species involved in the reaction pathways. In the presence of NO, surface nitrite/nitrate species that are strongly adsorbed on the catalysts surface competed with the N2O adsorption sites. An assay of the SCRsim catalytic behaviour in the presence of water revealed that the catalyst with ceria has a good catalytic activity and stability along tests simulating nearly real conditions.

Surface graft for effective PdO nanoparticles anchoring on Beta zeolite with outstanding lean methane combustion performance

A post-synthetic modification strategy for anchoring PdO nanoparticles on the Beta zeolite surface and obtaining stable and highly dispersed PdO sites with abundant active Pdn+ (0 < n < 2) species was reported. The modified Beta supported Pd catalyst with a 0.94 wt% Pd loading amount shows (>) 99% methane conversion at 375 °C with a space velocity of 30,000 mL h−1 g−1 and a satisfied long-term (>30 h) CH4 oxidation stability at 340 °C. Such a post-synthetic strategy provides a new way to disperse and stabilize noble metal particles on zeolite support for harsh catalytic process.

Catalytic oxidation degradation of volatile organic compounds (VOCs) – a review

Abstract Volatile organic compounds (VOCs) are considered one of the significant contributors to air pollution because they are toxic, difficult to remove, come from a wide range of sources, and can easily cause damage to the environment and human health. There is an urgent need for effective means to reduce their emissions. The current treatment technologies for VOCs include catalytic oxidation, adsorption, condensation, and recovery. Catalytic oxidation technology stands out among the others thanks to its high catalytic efficiency, low energy requirement, and lack of secondary pollution. The difficulty of this technology lies in the development of efficient catalysts. The research on loaded noble metal catalysts and non-noble metal oxide catalysts in this area over the past few years is briefly described in this work. Firstly, the catalytic destruction mechanism of organic volatile compounds is introduced. Secondly, the effects of structural modulation during catalytic oxidation, such as the adjustment of noble metal particle size and morphology, metal doping, and defect engineering, on the conformational relationships are discussed. Finally, the challenges faced by thermal catalytic oxidation for the degradation of VOCs are discussed, and the prospects for its development are presented.

NIR enhancement induced by refractive index differences between noble metal particles and nanocrystals in glass-ceramics

The use of optically active ions doped into photonic media containing noble metals is an intriguing area of photoluminescence (PL) research. In this paper, we present a new approach using nickel ions doped into photonic glass ceramics (GCs) containing Au metal NCs. We exploit the unique PL properties of Ni2+ embedded in ZnAl2O4 nanocrystals (NCs). Under the same conditions, the Au-ZAO duplex samples exhibit 2–3 times higher near-infrared (NIR) emission intensity and higher absorption intensity compared to the samples without Au metal. We provide direct evidence of energy harvesting around NCs through numerical simulations. The significant NIR PL properties of Ni2+ in Au-ZAO GCs can be explained by the significant dielectric coefficient difference between the Au metal, the surrounding medium, and the ZAO NCs. Furthermore, the intense and broad emission spectra of our samples cover the entire second biological NIR region, which makes them particularly useful for biomedical applications.

Formation of nanograined Ag-Co3O4 core@shell structure to achieve enhanced xylene sensing characteristics

Xylene (C8H10) is a volatile organic compound as well as a pollutant which needs to be detected for various demands and applications. Herein, we report a unique core@shell (C-S) structure with core noble metal particles to incorporate catalytic elemnts. A nanograined Ag-Co3O4 C-S structure was prepared using a chemical synthesis method. Systematic xylene sensing studies were undertaken to prove the excellence of nanograined C-S structure. Various characterization techniques were used to confirm the synthesis of desired products with the expected features. The pristine sensor offered the highest performance to xylene at 350 °C, whereas nanograined Ag-Co3O4 C-S structure gas sensor worked at a lower temperature namely 250 °C. In addition, the maximum response of the pristine sensor to 50 ppm-xylene was about 1.26, whereas that of the nanograined Ag-Co3O4 C-S structure gas sensor was 2.47. Moreover, the latter sensor showed higher selectivity to xylene than the former sensor. The improved xylene sensing characteristics of the nanograined Ag-Co3O4 C-S structure gas sensor were caused by its high surface area, formation of Ag-Co3O4 heterojunctions, and catalytic effects of Ag nanoparticles. This study demonstrates the potential of this new system for sensing of xylene.

Pt Nanoparticles Supported on Ultrathin Ni(OH)2 Nanosheets for Highly Efficient Reduction of 4-Nitrophenol

The synthesis of highly efficient heterogeneous catalysts with uniformly dispersed noble metal particles and a suitable size is crucial for various industrial applications. However, the high cost and rarity of noble metals limit their economic efficiency, making it essential to improve the catalytic performance with lower noble metal loading. Herein, a two-step method was developed for the synthesis of uniformly dispersed ~3 nm Pt nanoparticles (NPs), strongly anchored on Ni(OH)2 nanosheets (NSs), which was proven by adequate structural characterizations. XPS analysis demonstrated that Ni(OH)2 NSs with abundant oxygen vacancies provided sufficient anchor sites for Pt NPs and prevented their agglomeration. The catalytic performance of Ptn/Ni(OH)2 (n (represents the addition amount of Pt precursors during the synthesis, μmol) = 5, 10, 15, and 20) NSs with controllable Pt loading were evaluated via the reduction of 4-nitrophenol to 4-aminophenol as a model reaction. The Pt10/Ni(OH)2 NSs exhibited the best activity and stability, with a reaction rate constant of 0.02358 s−1 and negligible deterioration in ten reaction cycles. This novel synthetic method shows potentials for the synthesis of highly efficient noble-metal-supported catalysts for heterogeneous catalysis.

Convenient hydrothermal treatment combining with “Ship in Bottle” to construct Yolk-Shell N-Carbon@Ag-void@mSiO2 for high effective Nano-Catalysts

More active sites will be exposed when noble metal particles are loaded on the core of yolk-shell nanospheres (NM-YSNS) to give higher metal utilization rate, catalytic activity, and good recycle stability. Herein, we adopt an efficient strategy to access the high-performance NM-YSNS via “ship in bottle” mindset on loading Ag nanoparticle on the core surface of core–shell structure accurately and the core shrinkage on forming yolk-shell structure. First, we systematically investigated the mechanism of SiO2 coating 3-aminophenol-co-formaldehyde nanoparticles to generate 3-aminophenol-co-formaldehyde@mesoporous SiO2 core–shell nanospheres (3-APF@mSiO2 CSNS). Interestingly, the core surface of 3-APF@mSiO2 CSNS became uneven after hydrothermal process, which provided positions for Ag, then additional Ag+ could reach the 3-APF surface through the mSiO2 shell and be in situ reduced into Ag particles by –NH- and –NH2 there. Similarly, the 3-APF@Au@mSiO2 CSNS were prepared successfully. After carbonizing 3-APF@Ag@mSiO2 CSNS, the N-Carbon@Ag-void@mSiO2 YSNS was obtained. The NM-YSNS catalyst had a high specific surface area of 258.8 m2g−1, mSiO2 shell of 3.8 nm mesoporous size, and core loaded with Ag particles firmly, showing a high turnover frequency of 137.08 h−1 and good recycle stability in catalytic reduction of 4-nitrophenol for preparing 4-aminophenol.