30 Figure 1.321H NMR spectrum of precursors (neutral azide, positively charged alkyne and negatively charged alkyne) before reaction (in DMSO-d6). 33 Figure 1.371H NMR spectrum of precursors (positively charged azide, negatively charged azide and neutral alkyne) before reaction (in D2O).
Background
Catalysis and enzymes
The active site of the enzyme is the site of catalysis, and this is very specific to its substrates. As a result, the substrate easily forms its transition state and the reaction proceeds.3 Other molecules with different functional groups or shapes may not fit well and it is difficult to bind to the active site.
Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC)
Introduction
On the downside, many of these systems achieve only moderate selectivities (especially with respect to equivalent functionalities), are tailored to very specific substrates (Figure 1.6b), and often require elaborate synthesis. Even for gates based on weaker van der Waals interactions, selectivities are still notable, 6−7 times; in this case, hydrophobic entry units preferentially accept more hydrophobic substrates (Figure 1.11g−i).
Results and discussion
- Synthesis of Au NPs catalyst
- Selectivity for negatively- vs positively- charged substrates
- Experiments with “competing” substrates
- Site-selective, on-nanoparticle catalysis
- Theoretical model
- Different effects influencing gating and selectivity
In this model, we consider a reaction + → , where a charged dialkyne can have two orientations in relation to the charged, catalytic surface of the nanoparticles (in Figure 1.9a,c, respectively noted and ; M stands for the azide) . On the other hand, NPs electric surface potential ϕ(r) increases with particle size, which in turn increases the magnitude of the electric potential energy difference (ϕ(rA) − ϕ(rB)) between the two orientations of a substrate and therefore improves the selectivity at ∼ exp- (−q(ϕ(rA) − ϕ(rB))/kT); this effect is actually seen in the distribution of products shown in Figure 1.12c.
Conclusion
Although we expected that this smaller difference might result in decreased selectivity between 4A and 4B , we were surprised to find that the main effect was a dramatically increased formation of the double product 4C (in a ratio of ∼5:2 relative to the amount of reacted products; see the orange bars in Figure 1.12d). This effect is clearly due to NP since, in the CuSO4/NaAs system, single-reaction products 4A and 4B are dominant.
Experimental section
General methods
Even in the NP system, the product ratio of 5:2 is already established in about 1 h, which suggests that the reactions on both alkynes take place in rapid succession. These results can be rationalized by the MD simulations shown in Fig. 1.12e,f, namely the substrate is oriented such that the charged NMe3+ group on the phenyl ring faces away from the NP, while the two alkynes are arranged in a pincer shape, thus that they can enter the monolayer of particles and react at the catalytic sites in it.
Nanoparticle synthesis and representative TEM images
Gold nanoparticles were imaged with high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F) at an accelerating voltage of 200 kV.
Nanoparticle functionalization, purification, control experiments, and reaction setup
In the first experiment, only the methanol/water mixture (8 mL) was added to a vial containing the same amount of solid CuI, and this mixture was subjected to the same centrifugation procedure (5 times). Reaction setup: First, methanol – important during purification of NPs – was removed from the water/methanol NP suspension (1:1 v/v, 8 mL in total).
Individual and competing reactions of triazoles
Synthesis of “0+” & “0−” triazoles via click reaction in competitive environment (neutral azide, positively charged alkyne and negatively charged alkyne). Synthesis of "+0" and "−0" triazoles via the click reaction in the competitive environment (positively charged azide, negatively charged azide and neutral alkyne).
Synthesis of dialkyne substrates
The mixture was then poured into a separatory funnel containing water and extracted with diethyl ether (2 x 10 mL). This mixture was stirred overnight at room temperature and then acidified to pH 4-5 by HClaq (2 M). After stirring at rt for 24 h, diethyl ether (10 mL) was added and the mixture was kept in a refrigerator for several hours.
The flask was fitted with a reflux condenser covered with a rubber septum, an inert Ar atmosphere was placed, and the Ar balloon was placed on top of the reflux condenser. After completion of the reaction (monitored by TLC, hexane:AcOEt = 4:1), the reaction mixture was cooled to room temperature and the solids were filtered and rinsed with AcOEt. Aqueous, 10% LiCl solution (50 mL) was added to the filtrate and, after separation, the organic phase was dried over anhydrous MgSO4, filtered and the solvents removed in vacuo.
Water (20 mL) was added to the mixture and the aqueous phase was extracted with Et 2 O (3 x 20 mL), dried over anhydrous MgSO 4 , filtered and the solvents removed in vacuo. After completion of the reaction (monitored by TLC, hexane:AcOEt = 7:2) AcOEt (50 mL) was added and organic phases were washed in a separatory funnel with 10% aqueous solution of LiCl (2 × 30 mL), dried over anhydrous MgSO 4 , filtered and the solvents were removed in vacuo.
Site-selective synthesis of triazoles from dialkynes
Dialkyne 1 was reacted with excess azide in the presence of CuSO4/NaAs to produce di-substituted product 1C. Dialkyne 2 was reacted with excess azide in the presence of CuSO4/NaAs to produce the di-substituted product 2C. Dialkyne 3 was reacted with excess azide in the presence of CuSO4/NaAs to produce the di-substituted product 3C.
Dialkyne 4 was reacted with excess azide in the presence of CuSO4/NaAs to give the di-substituted product 4C.
Nanostructured rhenium–carbon composites as hydrogen-evolving catalysts effective
Background
- Electrocatalysis
- Hydrogen Evolution Reaction (HER) and metals
Electrocatalysts participate in electrochemical reactions and are specific forms of catalysts that function at the surface of the electrode.2 They can be heterogeneous or homogeneous. Second, it is important to improve the intrinsic activity of the active site.1 Because there is a physical limit to the amount of materials that can be loaded onto an electrode, it is important to increase the efficiency of the electrochemical reaction by increasing the intrinsic improve activity. The hydrogen evolution reaction (HER, 2H++ 2e−→ H2) – the main process of water splitting (cathodic reaction) – is a multi-step electrochemical reaction that takes place on the electrode surface and generates hydrogen gas.7 The electrocatalytic evolution of hydrogen gas is an alternative method for hydrogen production for internal combustion engines and fuel cells.8 The HER is a simple example of a two-electron transfer chemical reaction involving one Had intermediate.
The generally accepted reaction mechanisms of the Volmer, Heyrovsky and Tafel steps are different in acidic and alkaline solutions. Here “M” represents the surface of the metal electrode and “Had” represents a hydrogen atom adsorbed on the active site of the electrode. As described in reaction (1) to (6), both adsorption and desorption processes of the hydrogen atom on an electrode surface are crucial steps for the HER.
Importantly, the bond must also be weak enough to allow for easy bond breaking to effectively release H2 hydrogen gas. This volcano plot shows the dependence of the HER exchange current on metals and highlights the role of the M-Had bond strength.
Introduction
Results and discussion
- Synthesis of Re-based nanocomposites and characterization
- Electrocatalytic activity toward Hydrogen Evolution Reaction (HER)
- Durability test
As illustrated in Figure 2.4a-c, the first step produced ∼200–500 nm berry-shaped clusters made of ∼5 nm nanoparticles. In the acidic electrolyte, ReO2−NPC/a-C exhibits moderate HER activity with an initial overpotential of η = 218 mV and overpotential at 10 mA cm−2η10= 243 mV vs. the reversible hydrogen electrode (RHE) (green line in Figure 2.6a) . In contrast to these oxide-based materials, metallic Re-NPC/a-C under acidic conditions show much lower values of η = 102.7 mV and η10= 133 mV (purple line in Figure 2.6a), proving their increased activity in HER.
The SEM images in Figure 2.15 show that in the materials thus made, the Re nanoclusters are entangled in a web of MWNTs. The Re−NPC/a-C catalyst shows an Rct value of 19.8 Ω in acid solution (Figure 2.6g) and exhibits a much lower value of Rct. In acidic media, the catalytic currents per ESCA for Re-NPC/a-C are much larger than those for the bulk Re powder (Figure 2.17a).
The enhanced catalytic activity for Re−NPC/a-C is also verified by a lower Tafel slope showing faster kinetics (Figure 2.17d). Enhanced intrinsic catalytic activity is also observed in basic and neutral media (Figure 2.17b,c,e,f), although it is not as pronounced as in the acidic media.
Conclusion
In addition, we confirmed the presence of the generated hydrogen by gas chromatography and quantified the amounts of H2 gas evolved as well as the Faradaic efficiency (Figure 2.23).
Experimental section
- Materials
- Characterization
- Electrochemical measurements
- Electrochemically active surface area (ECSA)
- Supplementary figures
The morphology and structure of the product samples were imaged using scanning electron microscopy (SEM, Hitachi SU-8220) and transmission electron microscopy (TEM, JEOL JEM-2100). Oxidation states in the NPCs were examined by X-ray photoelectron spectroscopy (XPS; ESCALAB 250XI, Thermo Fisher Scientific), and the deconvolution analysis with an associated database was performed using the XPSPEAK program and Thermo Scientific Avantage software. BET surface areas of ReO2−NPC/a-C and Re−NPC/a-C were measured using a volumetric adsorption system (ASAP2420, Micromeritics Instruments) with N2 adsorptive at 77 K.
Then, 5 μL of ink was applied to the GC electrode (diameter 3 mm, geometric area 0.0707 cm2) and air dried at room temperature. In the case of the nanocomposite mixture of Re−NPC/a-C and MWNT (Re−NPC/a-C/MWNT), 4 mg of Re−NPC/a-C and MWNT at a ratio of 5:1 w/w was mixed and sonicated together in the same ink solution (1 ml 3:1 v/v water/ethanol with 40 μl Nafion solution). Before electrochemical measurements, all electrolytes were bubbled with N2 gas (99.999%) for 30 min, and then cyclic voltammetry (CV) in the range of 0.1 to −1.0 V (vs RHE) was performed for electrochemical cleaning (20 repetitions, scan rate of 100 mV s−1).
Electrochemical impedance spectroscopy (EIS) analyzes were performed at an overpotential of 173 mV (vs. RHE) from 100 kHz to 0.1 Hz in the same setup. According to the equation of ic= vCdl, 82−85 the difference in linear slopes is equal to twice Cdl, and thus it is divided by 2 to obtain the average value of Cdl from the cathodic and anodic charging currents.
Regulated synthesis of Mo sheets and their derivative MoX sheets (X: P, S or C) as efficient electrocatalysts for hydrogen evolution reactions. Rhenium Electrodeposition Process on p-Si (100) and Electrochemical Behavior of the Hydrogen Evolution Reaction on p-Si/Re/0.1 M H2SO4Interface. Correlate the hydrogen evolution reaction activity in alkaline electrolytes with the hydrogen bonding energy on monometallic surfaces.
New insights into the hydrogen evolution reaction under buffered near-neutral pH conditions: Enthalpy and entropy of activation. An electrochemical study of the hydrogen evolution reaction at YNi2Ge2 and LaNi2Ge2 electrodes in alkaline solution. Cactus-like hollow Cu2SRu nanosheets as excellent and robust electrocatalysts for the alkaline hydrogen evolution reaction.
Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction. Design of an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends.
