The development of indigo reduction methods and pre‐reduced indigo products

The continuing importance of blue denim maintains indigo as an important vat dye industrially. In this review, we examine the various methods that have been used in the past and are currently used to reduce and dissolve indigo for dyeing. We discuss recent insights into the bacterial fermentation technology, the advantages and disadvantages of the direct chemical methods that have predominated for the last century and potentially cleaner technologies of catalytic hydrogenation and electrochemistry, which are becoming increasingly important. With considerations of environmental impact high on the dyeing industry’s agenda, we also discuss the developments that have led to the production of pre‐reduced indigo.     

Direct electrochemical reduction of indigo

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fulfilled. The application of direct electrochemical reduction of indigo as a novel route has been investigated by spectrophotometric and voltammetric experiments in laboratory cells. Experiments yield information about the reaction mechanism and the kinetics, and they show the possibility of this new route for production of water-soluble indigo, which offers tremendous environmental benefits.     

Electrochemical decolourisation of dispersed indigo on boron-doped diamond anodes

In denim production the decolourisation of intensively coloured, indigo-particulate containing waste water is a factor of major environmental concern. Successful anodic decolourisation of solutions containing 0.29 mM dispersed indigo and 0.070 M Na₂SO₄ could be achieved on boron-doped diamond electrodes. Current densities were varied from 0.36 to 80 mA cm^− 2. Relative current efficiency decreases with increased current density from 43% to less than 1% at 80 mA cm^− 2. At higher concentration of dispersed indigo of 25.1 mM, increased relative current efficiency is observed.Diffusion limited current density for decolourisation of 0.292 M indigo dispersion can be calculated with 0.166 ± 0.007 mA cm^− 2. At the experimental conditions studied, peroxodisulfate, formed as by product of the electrolysis, was found to be ineffective for dyestuff destruction.Experiments in presence of small amounts of chloride proved, that the observed decolourisation of the dispersed indigo cannot be attributed to hypochlorite, formed by anodic oxidation of chloride. While anodic decolourisation of a soluble reactive dye proceeds rapidly, commercial vat dyes resist to oxidative anodic attack at the conditions chosen.Bleach experiments of dyed fabric failed, which supports the model of indigo oxidation in the diffusion layer of the anode.     

Voltammetry and spectroelectrochemistry of solid indigo dispersed in carbon paste

Cyclic voltammetry, square wave voltammetry and double potential step UV-vis spectroelectrochemistry were used to study the redox processes of indigo microparticles dispersed in a solid carbon paste electrode. The spectroelectrochemical measurements were performed in a home-made long-path-length thin-layer electrolysis cell. Both the indigo/leucoindigo and indigo/dehydroindigo redox couples underwent reversible 2e⁻/2H⁺ surface-confined reactions. Alkaline electrolytes showed more negative effect on the reduction of indigo to leucoindigo than on its oxidation to dehydroindigo. A new species (probably indolone) was monitored in the re-oxidation of leucoindigo, while isatin was found in the oxidation of indigo at enough positive potential. More detailed electrochemical mechanisms were proposed for the two redox systems, respectively. The present work shows that the microparticle-dispersed carbon paste is an attractive electrode material not only for solid state voltammetry but also for stripping spectroelectrochemistry.     

Holographic polymer dispersed liquid crystal reflection gratings formed by visible light initiated thiol-ene photopolymerization

Visible laser light of wavelengths 442, 488 and 532 nm was used to write holographic reflection gratings with notches into the infra-red spectral region in polymer dispersed liquid crystals (H-PDLC). The gratings were formed by the spatially periodic phase separation of nematic liquid crystal during thiol-ene photopolymerization. An organic titanocene based complex, Rhodamine 6G (Rh6G) and Pyrromethene 597 (Pym) dyes in combination with benzoyl peroxide, were used as initiators. Using visible laser light and a single prism, electrically switchable reflection gratings with notches in the blue, green, yellow, red, and near infra-red (NIR) spectral regions were fabricated using angle tuning. The photophysics of the initiator dyes was studied by absorption and fluorescence spectroscopy. Morphology studies by TEM showed a periodic distribution of discrete nematic droplets of nearly spherical shape. The development of visible photoinitiator systems broadens the range of fabrication wavelengths of dynamic reflection filters using inexpensive visible lasers.     

Electrochemical reduction of indigo in fixed and fluidized beds of graphite granules

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled and lead to problematic waste products. The electrochemical reduction of indigo on a fixed bed cathode consisting of graphite granules has been investigated by spectrophotometric experiments in laboratory cells. Experiments yield information about the kinetics and show the possibility of this process for production of water soluble leuco indigo, which offers environmental benefits. The influence of noble metals deposited on the granules and of different pretreatment methods of the graphite is demonstrated. In addition, the immobilization of quinoid molecules on the graphite surface has been investigated.     

Evolution of oxygen reduction current and biofilm on stainless steels cathodically polarised in natural aerated seawater

The aim of a series of works recently performed at ISMAR was to provide new useful information for a better understanding of the mechanisms by which bacteria settlement causes corrosion on Stainless Steels (SS) and similar active-passive alloys exposed to seawater. In this work, the evolutions of cathodic current, bacteria population, and electronic structure of the passive layer were investigated on SS samples polarised at fixed potentials during their exposure to natural seawater. It was found that, during the first phase of biofilm growth, cathodic current increase is proportional to the number of settled bacteria at each fixed potential. However, the proportionality factor between settled bacteria and cathodic current depends on imposed potential. In particular, the proportionality factor strongly decreases when the potential is increased above a critical value close to −150 mV Ag/AgCl. This effect seems to be correlated with the electronic structure of the passive layer. Indeed, the outer part of the passive layer on tested SS was found to behave like a conductor at potentials more active than −150 mV Ag/AgCl, and like an n-type semiconductor at more noble potentials.     

Direct electrochemical reduction of indigo: process optimization and scale-up in a flow cell

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fulfilled. The industrial feasibility of the direct electrochemical reduction of indigo as a novel method has been determined and a preliminary optimization of electrolytic conditions was performed using a laboratory-scale flow-cell system. The role of current density, pH, temperature and the rate of mass transport are discussed. The influence of particle size reduction by the application of ultrasound is critically considered.     

The nature of the passive film on cathodically modified stainless steels

This paper discusses the effect of additions of platinum group metals (PGMs) on the composition of the passive surface layer formed on stainless steels in reducing acid media. It has been shown that there is an accumulation of these noble metals on the surface of the alloy during the period of active dissolution. The morphology and factors influencing this accumulation of PGMs on the surface are discussed. It was also found that there are differences between passive films formed spontaneously and those formed under potentiostatic control. An interesting fact emerging from the investigations is that different PGMs form different types of oxides on the passivated surfaces. Areas for possible future research are mentioned.     

The synthesis of ionic-liquid-functionalized multiwalled carbon nanotubes decorated with highly dispersed Au nanoparticles and their use in oxygen reduction by electrocatalysis

Multiwalled carbon nanotube (MWCNT)/ionic liquid/gold nanoparticle hybrid materials have been prepared by a chemical route that involves functionalization of MWCNT with amine-terminated ionic liquids followed by deposition of Au. Transmission electron microscopy revealed well-distributed Au with a narrow size distribution centered around 3.3 nm. The identity of the hybrid material was confirmed through Raman and X-ray photoelectron spectroscopy. It showed good electrocatalytic behavior toward oxygen reduction, relative to glassy carbon electrode. The results indicate that modification of MWCNT with ionic liquids and Au could play an important role in increasing the electrocatalytic activity of MWCNT.     

An electron microscopic study on the turbostratic carbon formed in phenolic resin carbon by catalytic action of finely dispersed nickel

Very fine particles of certain substances are known to promote the catalytic graphitization of carbonaceous material, resulting in a unique carbon whose X-ray parameters, lying in the range of turbostratic carbon, remain almost unchanged even after more severe heat-treatments. To elucidate the actual structure of this type of carbon, its formation mechanism, and the reason for its high thermal-stability, a high resolution electron microscopic investigation was carried out on carbons derived from phenolic resin lightly and heavily doped with an organo-nickel compound. The electron microscopic texture of this carbon was found to consist of entangled lattice fringes of a stack number comparable with its X-ray L_c-value (110 Å). This texture was analogous to that of some types of the hard carbon heated to very high temperatures, but the former was less sinuous in its lattice fringe and more open in its structure. The formation mechanism appeared to differ from that accepted for conventional nickel-catalyzed graphitization, and to be particularly related with nickel particles in a certain size range. The evidence obtained so far was not enough to suggest a new mechanism, but only to illustrate part of some of the possible, hitherto little known mechanisms which might proceed along with it. Thermal stability of the carbon was understood to be due to its structure being similar to the hard carbon, but the ultimate reason why this type of structure was thermally stable could be made clear only to a limited extent.     

Electrochemically instantaneous reduction of conducting polyaniline-coated latex particles dispersed in acidic solution

A cathodic voltammetric wave was observed in an aqueous suspension of mono-dispersed, spherical polyaniline-coated polystyrene particles, whereas no anodic wave was detected. This irreversibility was common to particles with eight different diameters ranging from 0.2 to 7.5 μm. Such irreversibility cannot be found at polyaniline-coated electrodes, and thus is a property of the dispersion of polyaniline latex. The reduction current was controlled by diffusion of dispersed particles. The reduction, being the conversion from the electrical conducting state to the resistive one, should begin at a point of contact between the conducting particle and the electrode in order to be propagated to the whole particle rapidly. In contrast, the oxidation proceeds slowly with the propagation of conducting zone, during which Brownian motion lets the particle detach from the electrode. The number of loaded aniline units per particle, determined by weight analysis, ranged from 6 × 10⁶ (Ø 0.2 μm) to 3 × 10¹¹ (Ø 7.5 μm) and was proportional to 2.9 powers of the particle diameter. The diffusion-controlled current of the cathodic wave was proportional to 2.4 powers of the diameter. The difference in these powers, 0.5, agreed with a theoretical estimation of the diffusion-controlled current, the diffusion coefficient for which was given by the Stokes-Einstein equation.     

Highly dispersed carbon-supported Pd nanoparticles catalyst synthesized by novel precipitation–reduction method for formic acid electrooxidation

The highly dispersed and ultrafine carbon-supported Pd nanoparticles (Pd/C) catalyst is synthesized by using an improved precipitation–reduction method, which involves in Pd^II → PdO·H₂O → Pd⁰ reaction path. In the method, palladium oxide hydrate (PdO·H₂O) nanoparticles (NPs) with high dispersion is obtained easily by adjusting solution pH in the presence of 1,4-butylenediphosphonic acid (H₂O₃P-(CH₂)₄-PO₃H₂, BDPA). After NaBH₄ reduction, the resulting Pd/C catalyst possesses high dispersion and small particle size. As a result, the electrochemical measurements indicate that the resulting Pd/C catalyst exhibits significantly high electrochemical active surface area and high electrocatalytic performance for formic acid electrooxidation compared with that prepared by general NaBH₄ reduction method.     

高分散性纳米级Fe0/BAC-cH2-t催化剂催化还原NO

以椰壳活性炭为载体,采用氢气还原法制备出高分散性纳米级零价铁催化剂,采用固定床反应器研究了其对NO催化还原能力。采用XRD、TEM、SEM、XPS等分析手段对催化剂的微纳结构进行表征,考察了催化剂制备过程中H2浓度及煅烧还原温度对催化剂分散性、催化还原NO性能影响,催化剂的再生以及CO对催化剂还原NO的影响,并对CO还原NO反应机理进行推测。结果表明,催化剂活性随着H2浓度的增加逐渐增强,随着煅烧还原温度的升高先升高后降低。当H2浓度为100%时,在700℃煅烧温度下制备出的催化剂,Fe0粒径达到9 nm且均匀分散在椰壳活性炭中。Fe0/BAC-100H2-700催化剂在325℃时,NO转化率可以达到100%,表现出了良好的NO脱除效果。还原NO过程中,Fe0逐渐被氧化成Fe3O4导致催化剂最终失活,失活后的催化剂经再生处理后可恢复活性。NO还原反应过程中CO的加入可以还原Fe3O4再次生成Fe0,提供活性位点,有效的延长催化剂的寿命,减缓催化剂失活的速率。     

Enhanced oxygen reduction performance of Pt catalysts by nano-loops formed on the surface of carbon nanofiber support

We demonstrate a modification of the exposed graphite edges on herringbone carbon nanofiber (CNFs) into graphitic loops with nanosized curvature through thermal treatment. It was found that Pt catalysts supported on the surface modified CNFs showed much enhanced performance on oxygen reduction reaction. Such enhancement was possibly originated from an enhanced surface diffusion of oxygen on the modified CNFs.     

Effect of zirconia phase on the reduction behaviour of highly dispersed zirconia-supported copper oxide

Reduction processes of copper oxide supported both on tetragonal and on monoclinic zirconia were investigated by means of hydrogen temperature-programmed reduction (H₂-TPR). Results show that there exists apparent difference between the reduction behaviour of highly dispersed CuO supported on these two-phase zirconia; while two reduction peaks appear in the H₂-TPR profile of monoclinic zirconia-supported monolayer CuO, only one reduction peak at low temperature is developed for tetragonal zirconia-supported counterpart.     

Surface characterization by X-ray photoelectron spectroscopy and cyclic voltammetry of products formed during the potentiostatic reduction of chalcopyrite

Surface characterization of the transient products that precede chalcocite formation during chalcopyrite reduction was carried out. The experimental strategy employed in the present work consisted of the application of different potential pulses (fixed energetic conditions) on the surface of chalcopyrite electrodes in 1.7 M H₂SO₄. The chemical products formed at different potential pulses were characterized by cyclic voltammetry (CV) and XPS. Each electrogenerated species presented a specific voltammetric behavior and an XPS spectrum, in which the values of principal photoelectronic peak bond energies for Cu 2p_3/2, Fe 2p_3/2 and S 2p_3/2 and the atomic concentrations were considered. Several potential intervals could be identified: in 0.115 ≥ E_cat ≥ −0.085 V vs. SHE, an intermediate copper sulfide is formed whose composition is between those of chalcopyrite and bornite, such as talnakhite. The reduction of this product occurs slowly, giving bornite at potentials less than −0.085 V. In the applied potential region −0.085 ≥ E_cat > −0.185 V, the bornite gradually decomposes causing the incomplete conversion to chalcocite. In the potential interval −0.185 > E_cat ≥ −0.285 V, energetic conditions are large enough to allow the immediate decomposition of bornite, forming chalcocite in a more quantitative manner.     

The reduction of perchlorate by hydrogenation catalysts

Perchlorate competes with thyroid uptake of iodide, an essential nutrient for the production of thyroid hormones. Despite the extensive attempts to reduce perchlorate in aqueous solution, the process is slow and requires high temperatures even in the presence of catalysts. Therefore, perchlorate reduction under hydrogen atmosphere was employed. Monometallic and bimetallic Pt based catalysts (e.g., Pt/C, Ni-Pt/C, Co-Pt/C, and W-Pt/C) supported on activated carbon were prepared by successive incipient wet impregnation method and used for gas-phase reduction of perchlorate pre-adsorbed onto the activated carbon. The catalysts were characterized for hydrogen chemisorption and XPS. Hydrogen uptake was in the order: Co-Pt/C > W-Pt/C ≈ Pt/C > Ni-Pt/C. The least H₂ uptake by Ni-Pt/C was likely due to lower dispersion on activated carbon surface with 11.5% vs. 36% for Pt/C. There was a good correlation between hydrogen uptake by impregnated activated carbon (IAC) and perchlorate reduction. The Co-Pt/C exhibited the highest hydrogen uptake and the best perchlorate reduction while Ni-Pt/C was the least reducing system. When Co-Pt/C was used almost 90% of perchlorate was reduced at 25 °C and initial surface concentration of perchlorate of 11.47 mg g⁻¹ in 24 h. The reaction rate increased 10-folds when the reaction temperature was raised to 75 °C. In 24 h reaction time, increase of temperature from 25 to 75 °C resulted in additional 10% (Co-Pt/C) and 30% (Ni-Pt/C) increase in perchlorate reduction for Co-Pt/AC and Ni-Pt/AC, respectively, which brought the reduction efficiency close to 100%. The only reaction product that evolved was Cl⁻, indicating that the cleavage of the first oxygen atom of perchlorate was the rate-limiting step. The lowest activation energy for the reduction of perchlorate was 39.5 kJ mol⁻¹ for Co-Pt/C. Results also showed that the activation of gaseous hydrogen molecules on metal catalysts was the major reducing step, although deposited metals also participated in the perchlorate reduction directly. Results of XPS analysis revealed that during adsorption/reduction some portion of the second metal in the bimetallic catalysts was lost due to dissolution while Pt was very stable.