Change In Reaction Pathway Research Articles

Soft and dispersed interface-rich aqueous systems that promote and guide chemical reactions

Although aqueous solutions are considered to be sustainable, environmentally friendly reaction media, their use is often limited by poor reactant solubility. This limitation can be overcome by converting aqueous solutions into soft, dispersed interface-rich systems such as polyelectrolyte solutions, micellar solutions, oil-in-water microemulsions or vesicle dispersions. All consist of homogeneously distributed dynamic structures that, in a fashion reminiscent of enzymes, provide local environments that are different from the bulk solution. The presence of soft, dispersed interface-rich structures leads to not only selective reaction accelerations but also changes in reaction pathways, whereby chemical reactions are guided towards desired products. Once again, the analogy to enzyme-catalysed transformations is enticing. In this Review, we illustrate the general concepts applied in such systems and illustrate them with selected examples, ranging from enzyme mimics, the preparation of conductive polymers and transition-metal-catalysed organic syntheses on the industrial scale to the chemistry of prebiotic systems. Aqueous media containing homogeneously distributed soft dynamic structures can promote a wide range of synthetic and degradative chemical reactions. This promotion is illustrated by selected examples from academia and industry, as well as from the field of prebiotic chemistry.

Shape selectivity vapor-phase conversion of lignin-derived 4-ethylphenol to phenol and ethylene over acidic aluminosilicates: Impact of acid properties and pore constraint

Selective dealkylation of alkylphenols, the opposite reaction of the more commonly studied phenol alkylation, may represent an important reaction in the production of base chemicals like phenol and olefins from fossilized and raw lignocellulosic matter. This study reports the first thermodynamics and kinetics studies of the vapor-phase conversion of ethylphenol (EP) over acidic γ-Al2O3, amorphous (ASA) and crystalline aluminosilicates (like ferrierite, ZSM-22, ZSM-5, beta, and USY) in the absence of hydrogen and noble metals, as a way to produce phenol and ethylene. The reaction was studied deliberately in presence of steam to get stable time on stream catalysis. The thermodynamic analysis shows an endothermal EP conversion to phenol and ethylene, favoured at high reaction temperature, while isomerisation, disproportionation and transalkylation are thermodynamically preferred at low temperature. The kinetic study examines the role of the catalytically active sites; it reveals the importance of site constraint on the activity, selectivity and stability, and shows the complex temperature dependency of the dealkylation. Both Brønsted and Lewis acid sites are active, but multifactor dependency (such as acid strength and site accessibility) complicates the establishment of simple quantitative relationships with the acid type. EP does not enter the micropores of ferrierite and ZSM-22, as suggested by adsorption experiments. Kinetics without significant diffusion limitations were obtained with ZSM-5, beta and USY. Thus, in absence of intracrystalline diffusion limitation (as verified by calculations using reported effective diffusivities, and substantiated by a comparably high apparent activation energy for all zeolites), the increased reaction turnover rate with increasing pore size from medium to large pore zeolites is largely explained by a change in reaction pathway (from monolecular to bimolecular) to convert EP to phenol and ethylene. A pathway proceeding through fast thermodynamically favourable bimolecular reactions occurs in the spatially non-constrained pores and crystal surface, whereas monomolecular reactions take place in the micropores of ZSM-5. Despite the lower rate, the selectivity over ZSM-5 strongly benefits from active site confinement, being responsible to achieve quantitative formation of phenol and ethylene from ethylphenol. The excellent performance of ZSM-5 thus accords with its shape selective property that avoids undesired side reactions such as the sterically demanding bimolecular reactions like disproportionation, transalkylation and C-C cracking, and severe cokes formation.

Effect of carbon nanotubes on the microstructural evolution and hydrogen storage properties of Mg(BH4)2

As an important member of complex hydrides, Mg(BH4)2 has a high gravimetric capacity (14.9 wt%). In this study, the Mg(BH4)2 was synthesized by the ion exchange method. Afterwards, the Mg(BH4)2 composites with different amounts (2, 5, 10, 25 and 50 wt%) of carbon nanotubes (CNTs) were prepared by mechanical milling. Effects of the CNTs on the microstructural evolution and hydrogen storage properties were investigated. The onset desorption temperature of the Mg(BH4)2-5 wt% CNTs is decreased to lower temperature around 120 °C from 275 °C. The desorption plateau of Mg(BH4)2-5 wt% CNTs is the highest, which ascribes that CNTs causes the reaction pathway change. Additionally, the Mg(BH4)2-5 wt% CNTs shows the best dehydriding kinetics properties, and can desorb 6.04 wt% hydrogen within 2000 s at 300 °C. The dehydrogenation activation energy of the Mg(BH4)2-5 wt% CNTs is decreased to 130.2 kJ/mol H2, which is much lower than that of pure Mg(BH4)2.

Production of hydrogen-rich syngas using Zr modified Ca-Co bifunctional catalyst-sorbent in chemical looping steam methane reforming

High purity H2 could be produced through modified chemical looping steam methane reforming (CL-SMR) process using calcium asan essential component in the structure of bifunctional catalyst-sorbents. Herein, we present an effective approach to the production of high purity hydrogen in chemical looping reforming of methane via modification of a bifunctional Ca-Co-Zr catalyst-sorbent. The synthesis of the samples including pure Ca, Ca-Co and zirconium modified Ca-Co with different molar ratios was performed using co-precipitation method. The influence of reaction variables such as steam to carbon molar ratio (S/C=1–5), reaction temperature (500–750°C) and lifetime of the samples are investigated on methane conversion and hydrogen yield. Moreover, Ca/Co mass ratio (0.11–9) is optimized and subsequently, different amounts of zirconium promoter (mass ratio=2.25–18) are added to improve the bifunctional catalyst-sorbent structure. The characterization of samples was performed using XRD, FESEM, BET and EDX techniques. It is found that the addition of zirconium to the sample could improve the textural features and stability of bifunctional catalyst-sorbent. The results reveal that Ca-Co-Zr (via mass ratios of CaCo=9 and CaZr=4.5) exhibits the highest catalytic activity among all tested samples and shows 98.3% methane conversion and 84.5% hydrogen yield at 700°C. The cyclic life time results at 700°C indicated that the Ca-Co-Zr (9,4.5) bifunctional catalyst-sorbent remained stable up to 16 cycles, while non-promoted samples showed fast deactivation after about 10 redox cycles. The coke formation is inhibited, while the reaction pathway changes to a combined reaction. In summary, the obtained results show the suitable efficiency of the synthesized catalyst in the modified CL-SMR process.

Tunneling Effect That Changes the Reaction Pathway from Epoxidation to Hydroxylation in the Oxidation of Cyclohexene by a Compound I Model of Cytochrome P450.

The rate constants of the C═C epoxidation and the C-H hydroxylation (i.e., allylic C-H bond activation) in the oxidation of cyclohexene by a high-valent iron(IV)-oxo porphyrin π-cation radical complex, [(TMP•+)FeIV(O)(Cl)] (1, TMP = meso-tetramesitylporphyrin dianion), were determined at various temperatures by analyzing the overall rate constants and the products obtained in the cyclohexene oxidation by 1, leading us to conclude that reaction pathway changes from the C═C epoxidation to C-H hydroxylation by decreasing reaction temperature. When cyclohexene was replaced by deuterated cyclohexene (cyclohexene-d10), the epoxidation pathway dominated irrespective of the reaction temperature. The temperature dependence of the rate constant of the C-H hydroxylation pathway in the reactions of cyclohexene and cyclohexene-d10 by 1 suggests that there is a significant tunneling effect on the hydrogen atom abstraction of allylic C-H bonds of cyclohexene by 1, leading us to propose that the tunneling effect is a determining factor for the switchover of the reaction pathway from the C═C epoxidation pathway to the C-H hydroxylation pathway by decreasing reaction temperature. By performing density functional theory (DFT) calculations, the reaction energy barriers of the C═C epoxidation and C-H bond activation reactions by 1 were found to be similar, supporting the notion that small environmental changes, such as the reaction temperature, can flip the preference for one reaction to another.

4+2] or [4+1] Annulation: Changing the Reaction Pathway of a Rhodium‐Catalyzed Process by Tuning the Cp Ligand

AbstractA change in reaction pathway was achieved for the first time by tuning the cyclopentadienyl (Cp) ligand used for the rhodium‐catalyzed cyclization of benzamides with conjugated enynones. Depending on the Cp ligand, the reaction pathway switched between [4+2] and [4+1] annulation. Electronic effects turned out to be crucial for the product distribution. The dichotomy was attributed to the alteration of the Lewis acidity of the resultant Cp‐bound rhodium species.

4+2] or [4+1] Annulation: Changing the Reaction Pathway of a Rhodium-Catalyzed Process by Tuning the Cp Ligand.

A change in reaction pathway was achieved for the first time by tuning the cyclopentadienyl (Cp) ligand used for the rhodium-catalyzed cyclization of benzamides with conjugated enynones. Depending on the Cp ligand, the reaction pathway switched between [4+2] and [4+1] annulation. Electronic effects turned out to be crucial for the product distribution. The dichotomy was attributed to the alteration of the Lewis acidity of the resultant Cp-bound rhodium species.

Pt Supported on Titanium Nitride/Graphene Nanocomposite As a Beneficial Catalyst for Oxygen Reduction Reaction in Acidic and Alkaline Solutions

In this study, titanium nitride/graphene (TiN/G) and supported 2.5 wt.% Pt on TiN/G nanocomposites are synthesized and tested as electrocatalysts for oxygen reduction reaction (ORR). The kinetics of ORR on the synthesized catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH solutions using electrochemical methods, including the cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating disk electrode (RDE). Moreover, ORR activity of smooth Pt electrode is compared with the synthesized samples. It is found that the ORR activity of TiN/G and Pt/TiN/G are a strong function of pH of the solution. Fig. 1 shows the cathodic linear sweep voltammograms for different electrodes with catalyst loading of 0.46 mg cm-2 in air saturated in 0.5 M H2SO4 and 0.1 NaOH electrolytes. For the acidic solution (Fig. 1(a)), the onset potential on Pt/TiN/G for ORR, comparing with smooth Pt, shifts to the positive potentials for about 180 mV and the limiting current enhances up to 20%. Moreover, the number of transferred electrons calculated as 3.94 with 4% H2O2 production and ORR current experiences 15% of decay after 1200 cycles. Meanwhile, in alkaline media (Fig. 1(b)), the onset potential shifts to more negative potentials for about 150 mV compare with Pt electrode and ORR proceeds via transferring of 3.65 electrons with 17% H2O2 production. Consequently, the kinetics of ORR on 2.5 wt.% Pt/TiN/G are much higher in acid than in alkaline solution. The pH effect is associated with the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. Unique Tafel slope of 120 mV dec-1 characterized in acidic solution (Fig. 2(a)), shows that the active surface of the Pt/TiN/G is not blocked by OH species and indicates that the rate determining step is the exchange of the first electron. However, the adsorption of OH species in alkaline solution and their coverage variation with potential leads to lower slope value and transition from the low slope (Temkin adsorption condition) to the high Tafel slope (Langmuir adsorption condition) (Fig. 2(b)). The change of the Tafel slopes with potential in alkaline media implies the change in reaction pathway from acidic to alkaline solutions. CV results reveal that in acidic solution the potential of PtOH formation and Pt oxidation shifts to higher overpotentials compared with alkaline media because of the presence of HSO4ˉ/SO42ˉ anions in double layer instead of adsorption of OHˉ anions on the electrode surface. The absence of OH on the surface facilitates the interaction of O2 with Pt/TiN/G catalyst and increases the ORR activity. As a result of the change in oxygen adsorption conditions for TiN/G that arise from a change in the electronic structure of Pt/TiN/G catalyst connected to the electronic interactions between Pt and TiN/G as the support material, TiN/G shows a reverse trend. The presence of OHˉ anions in alkaline media helps the ORR proceeds via a two stages pathway with 17% H2P2 production and the onset potential shifts to negative potentials for about 60 mV compare with Pt electrode. However, the alterations in surface protonation conditions and oxidation of TiN/G in acidic media leads to reduction of ORR activity in such a way that the produced H2O2increases up to 52% and the onset potential shifts to negative potentials for about 230 mV toward Pt electrode and the electrode stability decreases up to 51% after 1200 cycles. The high ORR activity and stability of the TiN/G in the alkaline electrolyte are mainly due to (i) N-doped graphene structure that combines through a hybrid state with TiN nanoparticles, (ii) unstoichiometric nitrogen and oxygen doped into TiN lattice, and (iii) high electrochemical surface area. By modifying catalyst surface with Pt nanoparticles, the ORR activity remarkably promotes in both acidic and alkaline solutions as a result of modification of electronic structure and increasing electrochemical surface area. Finally, the Pt/TiN/G catalyst shows a superior performance in acidic media than does in alkaline media because of changing the chemical in the surface at different pH values. Figure captions: Fig. 1. Voltammograms for ORR of GC electrode coated with TiN/G and 2.5 wt.% Pt/TiN/G in (a) 0.5 M H2SO4and (b) 0.1 M NaOH. The vertical axis shows difference in current between air and Ar atmospheres. The sweep rate was 5 mV/s (cathodic scan) and RDE at 1600 rpm. Fig. 2. Tafel plots for the O2 reduction on different electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M NaOH constructed from the data in Fig. 1. Figure 1

Improved ethanol steam reforming on Rh/Al2O3 catalysts doped with CeO2 or/and La2O3: Influence in reaction pathways including coke formation

In this paper, Rh catalysts supported over bare γ-Al2O3 and γ-Al2O3 modified with different promoters (15wt%La2O3 and/or 2.5wt%CeO2) were reported. The catalysts were prepared by successive impregnation: first the rare earth oxide promoter and then Rh. Hydrogen production by ethanol steam reforming (ESR) over these catalysts was investigated. Among the catalysts, Rh/La2O3–CeO2–γ-Al2O3 catalyst shows the highest hydrogen yield as well as the best stability during the reaction. Reaction pathways of the ESR over these Rh catalysts are investigated from the analysis of the adsorbed species and gas phase products formed during temperature programmed desorption of ethanol and ethanol steam reforming reaction. The differences in the reaction pathways and products distribution are as a result of the modification of the stability of some reaction intermediates caused by the presence of a small quantity of CeO2 and La2O3 on the support surfaces: in effect, acetaldehyde was formed as an intermediate over the modified-γ-Al2O3 supported catalysts while in the case of the bare Al2O3 support the main route of the steam reforming reaction is the direct ethanol decomposition. Analysis of the used catalysts by XPS, TGA and RAMAN showed that changes in reaction pathways associated to the presence of promoters in the Al2O3 support also modifies the extension and type of carbon deposited on catalyst surfaces.

Kinetics Enhancement, Reaction Pathway Change, and Mechanism Clarification in LiBH4 with Ti-Catalyzed Nanocrystalline MgH2 Composite

A composite of 2LiBH4 + nano-MgH2* (Ti-catalyzed) shows significantly enhanced desorption kinetics compared to a conventional mixture of 2LiBH4 + MgH2. The desorption mechanism was studied in the temperature range between 304 and 383 °C and under different pressure conditions. Desorption temperatures are 50–70 °C lower compared to conventional 2LiBH4 + MgH2 mixtures. During the hydrogen release from a mixture of 2LiBH4 + nano-MgH2* at a hydrogen back-pressure of 0.4 MPa, MgB2 is formed and three different plateaus of equilibrium were detected. The reaction pathway is changed at 360 °C for the 2LiBH4 + MgH2 system when the nano-MgH2* is used.

Methanol–Alkene Reactions in Zeotype Acid Catalysts: Insights from a Descriptor-Based Approach and Microkinetic Modeling

We recently proposed the ammonia heat of adsorption as a reactivity descriptor in solid acid catalysis, using it to predict the activity of zeotype catalysts in the propene–methanol reaction (J. Phys. Chem. Lett. 2014, 5, 1516–1521). Here we extend the approach to a series of alkene reactants, establishing transition state energy scaling relations for ethene and butenes. Using these relations in connection with microkinetic modeling, we predict a change in reaction pathway as a function of acid-site reactivity and alkene size. The results illustrate the potential of the descriptor-based approach to model acid-catalyzed reactions and efficiently screen for improved solid acid catalysts.

Computation and Experiment Reveal That the Ring-Rearrangement Metathesis of Himbert Cycloadducts Can Be Subject to Kinetic or Thermodynamic Control

Unusual observations in the ring-rearrangement metathesis of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provides a reasonable explanation for the previously irreconcilable results.

The effect of rhodium nanoislands on the electrocatalytic activity of gold for oxygen reduction in perchloric acid solution

The effect of spontaneously deposited rhodium nanoislands on the electrocatalytic activity of polycrystalline gold, Au(poly), for oxygen reduction reaction (ORR) was studied using rotating disc electrode technique in 0.1M HClO4 solution. Surface topography atomic force microscopy (AFM) images of obtained Rh/Au(poly) surfaces revealed that both the size and coverage of rhodium nanoislands increase with the deposition time, for a given depositing conditions. The exact values of rhodium coverage ranging from 50% to the full coverage, and the qualitative changes in the surface chemical composition with respect to bare gold were estimated from phase AFM images. Deposited Rh was also identified by the changes in cyclic voltammetry (CV) profiles from which potential limits for ORR were determined. Obtained Rh/Au(poly) surfaces have shown a significant shift of the initial potential for ORR to the more positive potentials with respect to pure gold already for 50% Rh coverage, indicating that a pronounced electronic effect is responsible for the lowering of the activation energy for oxygen adsorption. The initial potential increases only slightly with the further increase of Rh coverage. Reaction pathway changes from 2e− reduction at lower overpotentials to a partial 4e− reduction at higher overpotentials. The later is attributed to the increased activity of obtained Rh/Au(poly) surfaces towards hydrogen peroxide reduction (HPRR), which appear as an intermediate during ORR. The fraction of 4e− reduction slightly increases with the increase of Rh coverage.

An Ionic Liquid Dependent Mechanism for Base Catalyzed β-Elimination Reactions from QM/MM Simulations

Ionic liquids have been proposed to induce a mechanistic change in the reaction pathway for the fundamentally important base-induced β-elimination class compared to conventional solvents. The role of the reaction medium in the elimination of 1,1,1-tribromo-2,2-bis(3,4-dimethoxyphenyl)ethane via two bases, piperidine and pyrrolidine, has been computationally investigated using methanol and the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate [BMIM][BF(4)] and [BMIM][PF(6)], respectively. QM/MM Monte Carlo simulations utilizing free-energy perturbation theory found the ionic liquids did produce a reaction pathway change from an E1cB-like mechanism in methanol to a pure E2 route that is consistent with experimental observations. The origin of the ionic liquid effect has been found as: (1) a combination of favorable electrostatic interactions, for example, bromine-imidazolium ion, and (2) π-π interactions that enhance the coplanarity between aromatic rings maximizing the electronic effects exerted on the reaction route. Solute-solvent interaction energies have been analyzed and show that liquid clathrate solvation of the transition state is primarily responsible for the observed mechanistic changes. This work provides the first theoretical evidence of an ionic liquid dependent mechanism and elucidates the interplay between sterics and electrostatics crucial to understanding the effect of these unique solvents upon chemical reactions.

A laboratory pyrolysis study to investigate the effect of water pressure on hydrocarbon generation and maturation of coals in geological basins

This study investigates the effect of water pressure on hydrocarbon generation and maturation of coals. Using a 25ml Hastalloy pressure vessel, two high-volatile coals (Longannet, UK 0.75% Ro, and perhydrous Svalbard (Spitsbergen), Norway 0.68% Ro) were pyrolysed under non-hydrous, hydrous at 175bar pressure, and high water pressure hydrous (500bar and 900bar) conditions at 350°C for 24h. The bitumen yield obtained during pyrolysis, together with the Rock–Eval S2, hydrogen index (HI) and vitrinite reflectance (VR) results from the pyrolysed coal residues indicated that water under relatively low pressure (175bar) hydrous conditions promoted hydrocarbon generation and coal maturation in relation to non-hydrous conditions, consistent with previous work. However, under high water pressure (500 and 900bar) conditions, a combination of the hydrocarbon gas (C1–C4) and bitumen yields, Rock–Eval S2, HI, VR and solid state 13C NMR results demonstrated that the changes in reaction pathways occurring with increasing pressure resulted in both hydrocarbon generation and maturation being retarded. The observed effect of pressure implies that for Type III source rocks, hydrocarbon generation will be retarded in high pressure geological basins, with gas yields being proportionally reduced more than bitumen yields. Source rock maturation (or coalification) is also retarded, with the decreases in vitrinite reflectance and carbon aromaticity being relatively small but significant in terms of explaining retardation in geological basins.

Effect of C3H6 on selective catalytic reduction of NOx by NH3 over a Cu/zeolite catalyst: A mechanistic study

The effects of C3H6 on key SCR reactions over a model Cu/beta zeolite catalyst were characterized using step-response method reactor testing. Under standard SCR conditions, C3H6 clearly inhibited the reduction reaction at 200°C and above. The inhibition was not caused by competitive adsorption between C3H6 and NH3, but by surface intermediate species formed during C3H6 oxidation, including acrolein-like and coke species as indicated by in situ DRIFTS. Similar to the standard SCR reaction, C3H6 also had a negative effect on the fast SCR reaction. Spaci-FTIR (spatially resolved capillary-inlet Fourier transform infrared spectroscopy) results indicated that NO2 was quickly reduced to NO by C3H6, leading to the occurrence of some standard SCR instead of purely fast SCR. However, C3H6 had a positive effect on NO2 SCR. The reduction of NO2 to NO by C3H6 resulted in the occurrence of the fast SCR reaction combined with NO2 SCR instead of pure NO2 SCR. The reaction pathway change also decreased N2O formation significantly.

Organophosphazenes. 26. Factors Controlling the Pathways Observed in the Reactions of Ethynyl Lithium Reagents with Hexafluorocyclotriphosphazene

In contrast to previously studied reactions of ethynyl lithium reagents, the reactions of propynyl and hexynyl lithium with N(3)P(3)F(6) lead to predominantly nongeminal isomers. A modest cis stereo selectivity was observed. The sequential addition of a lithio acetylene reagent which follows a predominately geminal pathway (lithiophenylacetylene) and an aryl lithium reagent (p-propenylphenyl lithium) which follows a predominantly nongeminal pathway were examined. The relative order of addition of the two reagents was interchanged, resulting in a change of reaction pathway demonstrating ring substituent control of the regio and stereo chemical pathways. Hydrogenation of the ethynyl unit in the phosphazene derivatives provides a facile pathway to the difficult to prepare alkylphosphazenes. The chemical shift of the organosubstituted phosphorus center undergoes a remarkably large change (46-47 ppm) on going from the ethynyl to the alkyl derivatives, which reduces the complex (31)P and (19)F NMR spectra of the ethynyl derivatives to easily interpretable first-order spectra, thus allowing for structure assignment. The (13)C NMR data shows that nongeminal regio selectivity increases with the amount of s character on the ethynyl carbon atom attached to the phosphorus center. These observations allow for an understanding of the factors controlling regio and stereo chemical control in the reactions of carbanionic nucleophiles with N(3)P(3)F(6).

Change in reaction pathway of nickel(II) complex induced by magnetic fields

Three identical hydrothermal self-assembly reactions in a mixture of nickel nitrate hexahydrate, nicotinic acid, sodium azide were carried out with the applied magnetic fields of 0, 0.15, 0.3T, respectively. It was found that [Ni1.5(N3)(nic)2(Hnic)]n can be obtained at 160°C for 48h without an applied magnetic field, while the final product totally changed into [Ni2(nic)4(H2O)] as a 0.3T weak magnetic field was employed in the reaction. Both of the two products show three-dimensional frameworks, however, they display two different coordination geometries with different ligands. We consider the mechanism base on changes of magnetic susceptibilities and structures of [Ni2(nic)4(H2O)], and propose a view that the magnetic field can reduce the activation energy of reaction pathways according to transition state theory. The possible reason of the decrease of activation energy is that a magnetic field can mediate coupling among the Ni2+ ions during the formation of [Ni2(nic)4(H2O)], which is indicated by the slight changes of microstructure of crystal, such as bond angle and length.

Catalysis of oxygen reduction on Au modified by Pd nanoislands in perchloric acid solution

Oxygen reduction reaction (ORR) was studied on polycrystalline gold, Au(poly), modified by nanosized palladium islands in 0.1M HClO4 solution using rotating disc electrode technique. Paladium was spontaneously deposited from 1mM PdSO4·2H2O+0.5M H2SO4 solution for various deposition times at a submonolayer coverage. Topography of obtained Pd/Au(poly) surfaces was observed by tapping mode atomic force microscopy (AFM), while chemical surface composition changes were detected by simultaneously performed phase AFM. Deposited Pd islands were nonuniform in size and randomly distributed over the gold substrate. Size and number of Pd islands increase with the increase of the deposition time, and consequently surface coverage increases too. Deposited Pd was also identified by the changes in cyclic voltammetry (CV) profiles from which an active surface area was estimated. Obtained Pd/Au(poly) surfaces have shown a significant catalytic activity towards oxygen reduction reaction which increases with the increase of Pd islands coverage. The initial potential of ORR was shifted positively, while reaction pathway changes from 2e-reduction on pure gold to 4e-reduction on Pd modified gold. Additionally, obtained Pd/Au(poly) surfaces have shown a significant activity towards hydrogen peroxide reduction (HPRR), which appear as an intermediate during ORR. This supports the assumption that ORR on Pd/Au(poly) occurs partly through 4e-series reduction pathway, the fraction of which increases with the increase of Pd coverage.

Alkaline hydrothermal conversion of cellulose to bio-oil: Influence of alkalinity on reaction pathway change

The effects of alkalinity on alkaline hydrothermal conversion (alkaline-HTC) of cellulose to bio-oil were investigated in this study. The results showed that the initial alkalinity greatly influenced the reaction pathways. Under initial strong alkaline conditions with final pH greater than 7, alkaline-HTC only followed the alkaline pathway. However, under initial weak alkaline conditions with final pH of less than 7, acidic as well as alkaline pathways were involved. The main mechanism behind this change of reaction pathways under weak alkaline conditions was that carboxylic acids were first formed from cellulose via the alkaline pathway and then neutralized/acidified the alkaline solutions. Once the pH of the alkaline solutions decreased to less than 7, the acidic instead of the alkaline reaction pathway occurred. This change of the reaction pathways with initial alkalinity partly explained the inconsistent results in the literature of alkaline-HTC bio-oil compositions and yields.