Rare Noble Metals Research Articles

Amorphous Nickel Nanofilms for Efficient Hydrogen Generation from ammonia borane

AbstractClean and sustainable hydrogen production through liquid hydrogen storage material requires highly active and stable earth‐abundant non‐noble metal to replace expensive and rare noble metals. Herein, nickel nanofilms (Ni/NiO‐NFs) were prepared by the ionic liquid/water interface route. The cationic carbon chain length of the ionic liquid affects the phase composition of the nickel nanofilm, and the ionic liquid with [OMIm][PF6] as the anion has good thermal stability during the synthesis process. The efficiency of Ni/NiO‐NFs catalysts was tested by comparative kinetic analysis of the AB hydrolysis for hydrogen production. The as‐prepared Ni/NiO‐NFs catalyst exhibits excellent hydrogen generation performances, including a hydrogen production rate (2917 ml min−1 gNi−1), and a low activation energy (48.1 kJ/mol). The transition of nickel oxide to metallic nickel and the destruction of the catalyst structure is responsible for the decreased durability. This work highlights the significance of amorphous nanofilms catalysts via the ionic interface method on the regulation of activity for AB hydrolysis.

Electrochemical Ultrathin Metal‐Atomic Layer Deposition for Silica Microenvironment‐Assisted Cu‐Based Catalysis

AbstractReplacing commonly used precious and rare noble metals by the abundant copper (Cu)‐based catalysts is highly desired for sustainable fine‐chemical synthesis. However, in the lack of model platforms, complex surface chemistry of randomly nanostructured bulk Cu is notoriously challenging to understand and control. By synthesizing ultrathin 2D‐Cu layer sandwiched inside the bilayer silica template, an unusual but critical cooperative role of Lewis basic amino‐silica microenvironment for [Cu]‐catalyzed selective hydrogenation of unsaturated C─C bonds in diverse alkynes, ene‐ynes, and α,β‐unsaturated (alkene) Michael acceptors is discovered. Newly developed nanospace‐confined electrochemical (eChem) atomic layer deposition (NC‐EAD) technique afforded < 2 nm ultrathin Cu(0)‐layer intimately covered inside silica envelope. This model platform aided the detailed mechanistic study deciphering the unexpected finding – originally non‐reactive Cu‐film, just by a simple silica coating step, turning into an efficient catalyst for scalable fine‐chemical synthesis. The concept of reactive metal surface‐microenvironment manipulation, presents a new paradigm for controlling complex molecular interactions in heterogeneous catalysts.

Effective Activation of Strong C−Cl Bonds for Highly Selective Photosynthesis of Bibenzyl via Homo‐Coupling

AbstractCarbon‐carbon (C−C) coupling of organic halides has been successfully achieved in homogeneous catalysis, while the limitation, e.g., the dependence on rare noble metals, complexity of the metal‐ligand catalylst and the poor catalyst stability and recyclability, needs to be tackled for a green process. The past few years have witnessed heterogeneous photocatalysis as a green and novel method for organic synthesis processes. However, the study on C−C coupling of chloride substrates is rare due to the extremely high bond energy of C−Cl bond (327 kJ mol−1). Here, we report a robust heterogeneous photocatalyst (Cu/ZnO) to drive the homo‐coupling of benzyl chloride with high efficiency, which achieves an unprecedented high selectivity of bibenzyl (93 %) and yield rate of 92 % at room temperature. Moreover, this photocatalytic process has been validated for C−C coupling of 10 benzylic chlorides all with high yields. In addition, the excellent stability has been observed for 8 cycles of reactions. With detailed characterization and DFT calculation, the high selectivity is attributed to the enhanced adsorption of reactants, stabilization of intermediates (benzyl radicals) for the selective coupling by the Cu loading and the moderate oxidation ability of the ZnO support, besides the promoted charge separation and transfer by Cu species.

Effective Activation of Strong C-Cl Bonds for Highly Selective Photosynthesis of Bibenzyl via Homo-Coupling.

Carbon-carbon (C-C) coupling of organic halides has been successfully achieved in homogeneous catalysis, while the limitation, e.g., the dependence on rare noble metals, complexity of the metal-ligand catalylst and the poor catalyst stability and recyclability, needs to be tackled for a green process. The past few years have witnessed heterogeneous photocatalysis as a green and novel method for organic synthesis processes. However, the study on C-C coupling of chloride substrates is rare due to the extremely high bond energy of C-Cl bond (327 kJ mol-1 ). Here, we report a robust heterogeneous photocatalyst (Cu/ZnO) to drive the homo-coupling of benzyl chloride with high efficiency, which achieves an unprecedented high selectivity of bibenzyl (93 %) and yield rate of 92 % at room temperature. Moreover, this photocatalytic process has been validated for C-C coupling of 10 benzylic chlorides all with high yields. In addition, the excellent stability has been observed for 8 cycles of reactions. With detailed characterization and DFT calculation, the high selectivity is attributed to the enhanced adsorption of reactants, stabilization of intermediates (benzyl radicals) for the selective coupling by the Cu loading and the moderate oxidation ability of the ZnO support, besides the promoted charge separation and transfer by Cu species.

Synergistic mechanism and decopperization kinetics for copper anode slime via an integrated ultrasound-sodium persulfate process

An eco-friendly synergistic decopperization method via ultrasound and sodium persulfate was developed from copper anode slime (CAS), a hazardous solid waste. The advanced oxidation process was applied to the field of metal leaching/enrichment in hydrometallurgy. Electron paramagnetic resonance (EPR) spectroscopy was used to trace the free-radical quenching mechanism in the leaching experiment. The influence of different conditions on copper leaching efficiency was studied. A total of 98.11% of copper was removed by the synergistic process from CAS. By comparison, the leaching efficiency of copper by persulfate leaching under the same conditions was only 66.64%. The kinetics model fitting results indicate that ultrasound reduced the threshold of the leaching reaction. The leaching reaction order values of the H2SO4 and Na2S2O8 concentrations are 0.11 and 0.69, respectively, which shows that persulfate plays a major role in leaching efficiency in the decopperization process. Mineralogical analysis demonstrated that the insoluble copper was effectively transformed into copper sulfate via ultrasound and sodium persulfate. In addition, hazardous solid waste was converted to valuable high-grade metallurgical stock through the enrichment of rare noble metals in CAS using a cooperative process. The novel decopperization method via integrated ultrasound/sodium persulfate has wide application prospects in waste management and recycling in a clean and economical hydrometallurgical process.

Implementation of Transition Metal Phosphides as Pt-Free Catalysts for PEM Water Electrolysis

Proton Exchange Membrane (PEM) water electrolysis (WE) produces H2 with a high degree of purity, requiring only water and energy. If the energy is provided from renewable energy sources, it releases “Green H2”, a CO2-free H2. PEMWE uses expensive and rare noble metal catalysts, which hinder their use at a large industrial scale. In this work, the electrocatalytic properties of Transition Metal Phosphides (TMP) catalysts supported on Carbon Black (CB) for Hydrogen Evolution Reaction (HER) were investigated as an alternative to Platinum Group Metals. The physico-chemical properties and catalytic performance of the synthesized catalysts were characterized. In the ex situ experiments, the 25% FeP/CB, 50% FeP/CB and 50% CoP/CB with overpotentials of −156.0, −165.9 and −158.5 mV for a current density of 100 mA cm−2 showed the best catalytic properties, thereby progressing to the PEMWE tests. In those tests, the 50% FeP/CB required an overpotential of 252 mV for a current density of 10 mA cm−2, quite close to the 220 mV of the Pt catalyst. This work provides a proper approach to the synthesis and characterization of TMP supported on carbon materials for the HER, paving the way for further research in order to replace the currently used PGM in PEMWE.

Periodicity in the Electrochemical Dissolution of Transition Metals

In this study, intrinsic metal descriptors based on theoretical considerations are linked to experimentally determined dissolution rates. Clear trends are identified between the dissolution during oxide formation and the metal–metal cohesive energy, as well as between the dissolution during the reduction of an oxide and the metal–oxygen adsorption energy.

Electrocatalytic activity of layered MAX phases for the hydrogen evolution reaction

The hydrogen evolution reaction (HER) is important for the advancement of next-generation electrochemical energy devices. The search for an alternative inexpensive catalyst for energy conversion to replace expensive and rare noble metals is of high priority. There has been a significant push to investigate electrocatalysis of various layered materials for hydrogen evolution. However, the electrocatalytic activity of layered MAX phases remains largely unexplored. Herein, electrocatalytic activity studies of MAX (Ti2AlC, Ta2AlC, Ti2SnC, Ti3SiC2, V2AlC, Mo2TiAlC2, and Cr2AlC) phases are conducted. Material and electrochemical characterization are carried out to understand the morphology and catalytic activity, respectively. From Tafel slope analysis, it was found that proton adsorption is the rate-limiting step for all the MAX phases studied. Double transition-metal MAX carbides (Mo2TiAlC2) showed better catalytic activity for HER than single transition-metal MAX carbides.

Mild olefin formation via bio-inspired vitamin B12 photocatalysis.

Dehydrohalogenation, or elimination of hydrogen-halide equivalents, remains one of the simplest methods for the installation of the biologically-important olefin functionality. However, this transformation often requires harsh, strongly-basic conditions, rare noble metals, or both, limiting its applicability in the synthesis of complex molecules. Nature has pursued a complementary approach in the novel vitamin B12-dependent photoreceptor CarH, where photolysis of a cobalt–carbon bond leads to selective olefin formation under mild, physiologically-relevant conditions. Herein we report a light-driven B12-based catalytic system that leverages this reactivity to convert alkyl electrophiles to olefins under incredibly mild conditions using only earth abundant elements. Further, this process exhibits a high level of regioselectivity, producing terminal olefins in moderate to excellent yield and exceptional selectivity. Finally, we are able to access a hitherto-unknown transformation, remote elimination, using two cobalt catalysts in tandem to produce subterminal olefins with excellent regioselectivity. Together, we show vitamin B12 to be a powerful platform for developing mild olefin-forming reactions.

Manganese catalyzed switchable C-alkylation/alkenylation of fluorenes and indene with alcohols.

The usage of earth-abundant, nontoxic transition metals in place of rare noble metals is a central goal in catalysis. This would be especially interesting when the reactivity and selectivity patterns can be tuned. Herein, we introduced the first Mn-catalyzed selective C-alkylation and olefination of fluorene, and indene with alcohols. Various substrates including benzylic, heteroaromatic, and aliphatic primary and secondary alcohols are employed as alkylating agents. Mechanistic investigations and a kinetic study underpin the involvement of the olefinated intermediate to furnish the alkylated product.

Investigating the Performance of Tantalum Carbide Supported Iridium-Based Catalyst for Polymer Electrolyte Membrane Water Electrolysis

Hydrogen from polymer electrolyte membrane (PEM) water electrolysis has been identified as a key enabler for the transition to low/no-carbon energy from renewable energies. Polymer electrolyte membrane water electrolysis can be used to generate hydrogen from surplus energy generated using renewable energy technologies. Iridium is currently considered a state-of-the-art catalyst for the oxygen evolution reaction. However, it is a rare noble metal and as such a surge in demand would increase the cost of PEM water electrolyser units. One of the main strategies employed is the reduction of the noble metal loading through the use of supports. Tantalum carbide supported iridium based catalysts have been shown to be potential candidates for use as oxygen evolution reaction (OER) catalysts for water electrolysis. In this study, the performance of IrOx/TaC catalyst prepared using a surfactant mediated method is presented. IrOx/TaC catalysts were synthesized and fabricated into membrane electrode assemblies (MEAs). Nafion 115 was used as the membrane for the MEAs. Ir:TaC ratio was varied and its effect on the electrolyser performance was observed. The ionomer loading in the anode was also varied at the different Ir:TaC ratios and the performance observed. The elemental distribution of the IrOx/TaC was determined using scanning transmission electron microscopy (STEM)/energy dispersive spectroscopy (EDS). The conductivities of the catalysts and MEAs were also determined. Scanning electron microscopy (SEM) was used to analyse the morphology and elemental distribution of the electrode surfaces and the cross-section of the MEAs. The electrochemical performance of the MEAs was then tested in a single cell electrolyser equipped with an in-situ reference electrode. Current-voltage (I-V) curves were obtained potentiostatically at 80 °C. It was observed that while the fabricated MEAs had lower loading of iridium in the electrode, their performance was comparable with those found in literature.

Engineered 2D Transition Metal Dichalcogenides—A Vision of Viable Hydrogen Evolution Reaction Catalysis

AbstractThe hydrogen evolution reaction (HER) is an emerging key technology to provide clean, renewable energy. Current state‐of‐the‐art catalysts still rely on expensive and rare noble metals, however, the relatively cheap and abundant transition metal dichalcogenides (TMDs) have emerged as exceptionally promising alternatives. Early studies in developing TMD‐based catalysts laid the groundwork in understanding the fundamental catalytically active sites of different TMD phases, enabling a toolbox of physical, chemical, and electronic engineering strategies to improve the HER catalytic activity of TMDs. This report focuses on recent progress in improving the catalytic properties of TMDs toward highly efficient production of H2. Combining theoretical and experimental considerations, a summary of the progress to date is provided and a pathway forward for viable hydrogen evolution from TMD driven catalysis is concluded.

Adam H. Clark

Adam H. Clark

Exfoliated Mo2C nanosheets hybridized on CdS with fast electron transfer for efficient photocatalytic H2 production under visible light irradiation

Visible-light-driven H2 production by water splitting using photocatalysts has aroused widespread interest. For wide applications, an exploration of low-cost and high-performance cocatalysts to replace expensive and rare noble metals is highly desirable. Herein, highly exfoliated 2D Mo2C nanosheets (Mo2C) were prepared by liquid-phase ultrasonic exfoliation for the first time. As a photocatalyst for H2 production under visible light irradiation, Mo2C was hybridized on CdS nanoparticles (CdS) to be the composite Mo2C/CdS. Mo2C/CdS exhibits a remarkable H2 production rate of 7.7 mmol g−1 h−1 which is 6-, 16-, and 4-folds as high as those for bulk Mo2C/CdS, pure CdS, and Pt/CdS, respectively. The apparent quantum efficiency (AQE) of 3 wt% Mo2C/CdS is 86 % at 460 nm. The enhanced activity is attributed to the rapid transfer of photogenerated electrons from CdS to Mo2C as well as the presence of more active sites of Mo2C than bulk Mo2C.

3d-Metal Catalyzed N- and C-Alkylation Reactions via Borrowing Hydrogen or Hydrogen Autotransfer.

The conservation of our element resources is a fundamental challenge of mankind. The development of alcohol refunctionalization reactions is a possible fossil carbon conservation strategy since alcohols can be obtained from indigestible and abundantly available biomass. The conservation of our rare noble metals, frequently used in key technologies such as catalysis, might be feasible by replacing them with highly abundant metals. The alkylation of amines by alcohols and related C-C coupling reactions are early examples of alcohol refunctionalization reactions. These reactions follow mostly the borrowing hydrogen or hydrogen autotransfer catalysis concept, and many 3d-metal catalysts have been disclosed in recent years. In this review, we summarize the progress made in developing Cu, Ni, Co, Fe, and Mn catalysts for C-N and C-C bond formation reactions with alcohols and amines using the borrowing hydrogen or hydrogen autotransfer concept. We expect that the findings in this field will inspire others to develop new efficient and selective earth-abundant metal catalysts for borrowing hydrogen or hydrogen autotransfer applications or to develop novel alcohol refunctionalization reactions that can be mediated by such metals.

Noble-Metal-Free MoS2 Platelets with Promising Catalytic Performance in Hydrogen Evolution Reaction for the Post-Lithium-Ion Battery

In search of a practical alternative catalyst to costly and rare noble metals for hydrogen evolution reaction (HER), here we propose a lamellar electrode architecture out of engineered 2D MoS2 plat...

A Review of Thiosulfate Leaching of Gold: Focus on Thiosulfate Consumption and Gold Recovery from Pregnant Solution

Thiosulfate leaching is a promising alternative to cyanidation, and the main hindrances for its wide commercial application are the high thiosulfate consumption and the difficult recovery of dissolved gold. In this review, the four solutions to reduce the consumption of thiosulfate, including the control of reaction conditions, the use of additives, the generation of thiosulfate in situ, and the replacement of traditional cupric-ammonia catalysis, are introduced and evaluated after the presentation of background knowledge about thiosulfate consumption. The replacement of cupric-ammonia catalysis with other metals, such as nickel- and cobalt-based catalysts, is proposed. The reason is that it not only reduces thiosulfate consumption observably via decreasing the redox potential of leach solution significantly but also is beneficial to gold recovery mainly owing to eliminating the interference of cuprous thiosulfate [Cu(S2O3)3]5−. Based on the comparative analysis for five common recovery techniques of rare-noble metals from pregnant leach solution, ion-exchange resin adsorption is considered to be the most appropriate to recover aurothiosulfate [Au(S2O3)2]3− because the resin can be employed in the form of resin-in-leach/pulp and, furthermore, is able to be eluted and regenerated simultaneously at ambient temperature. At last, how to reduce the process cost of the resin adsorption technique is discussed. In order to simplify the complex two-stage elution process for loaded resins, the traditional catalysis is suggested to be replaced.

ZnO Photoanode Effect on the Efficiency Performance of Organic Based Dye Sensitized Solar Cell

Dye sensitized solar cell has been emerged as one of the most promising candidates for photovoltaics applications in good quality of their low manufacturing cost and impressive conversion energy. Titanium dioxide (TiO2) which is used as photoanode in the market has the advantage of wide bandgap energy but low in electron mobility (∼10 cm2/(V.s)). Ruthenium in the other hand, as the dye sensitizer is a rare noble metal and harmful to human health. Thus, this article reveals the performance of photo-to-electric conversion efficiency with the usage of Zinc Oxide as photoanode with higher electron mobility (155 cm2/(V.s)) compared to TiO2 utilizing three natural fruit dyes of Prunus domestica, Magnifera indica and Citrus limon. ZnO and TiO2 photoanodes were fabricated using sol gel and dr blade method respectively. The morphology of the photoanodes were characterized using Scanning Electron Microscope and the efficiency of the complete DSSC with all different fruit dyes were characterized using Semiconductor Parametric Analyzer. The different property of electron mobility photoanodes effect in DSSC proved to give better performance with the photoconversion efficiency of 3.082% using ZnO with Prunus domestica dye. This article also reveals that pH indicator does not affect the selection and the performance of DSSC.

Manganese-Catalyzed Sustainable Synthesis of Pyrroles from Alcohols and Amino Alcohols.

The development of reactions that convert alcohols into important chemical compounds saves our fossil carbon resources as alcohols can be obtained from indigestible biomass such as lignocellulose. The conservation of our rare noble metals is of similar importance, and their replacement by abundantly available transition metals, such as Mn, Fe, or Co (base or nonprecious metals), in key technologies such as catalysis is a promising option. Herein, we report on the first base-metal-catalyzed synthesis of pyrroles from alcohols and amino alcohols. The most efficient catalysts are Mn complexes stabilized by PN5 P ligands whereas related Fe and Co complexes are inactive. The reaction proceeds under mild conditions at catalyst loadings as low as 0.5 mol %, and has a broad scope and attractive functional-group tolerance. These findings may inspire others to use Mn catalysts to replace Ir or Ru complexes in challenging dehydrogenation reactions.

Manganese-Catalyzed Multicomponent Synthesis of Pyrimidines from Alcohols and Amidines.

The development of catalytic reactions for synthesizing different compounds from alcohols to save fossil carbon feedstock and reduce CO2 emissions is of high importance. Replacing rare noble metals with abundantly available 3d metals is equally important. We report a manganese-complex-catalyzed multicomponent synthesis of pyrimidines from amidines and up to three alcohols. Our reaction proceeds through condensation and dehydrogenation steps, permitting selective C-C and C-N bond formations. β-Alkylation reactions are used to multiply alkylate secondary alcohols with two different primary alcohols to synthesize fully substituted pyrimidines in a one-pot process. Our PN5 P-Mn-pincer complexes efficiently catalyze this multicomponent process. A comparison of our manganese catalysts with related cobalt catalysts indicates that manganese shows a reactivity similar to that of iridium but not cobalt. This analogy could be used to develop further (de)hydrogenation reactions with manganese complexes.