Electrocatalytic Oxidation Reaction Research Articles

Electrocatalytic valorization into H2 and hydrocarbons of an aqueous stream derived from hydrothermal liquefaction

Electrocatalytic oxidation is an attractive process for valorizing the organic compounds and removing the nitrogen in aqueous waste streams at ambient conditions. We evaluated the electrocatalytic oxidation reaction as a function of applied potential over Pt electrodes of an aqueous stream generated via hydrothermal liquefaction. We quantified the conversion of particular organic compounds (e.g., carboxylic acids, alpha hydroxyacids, alcohols, ketones, and amides) and the removal of carbon, nitrogen, sulfur, and chemical oxygen demand. Organic nitrogen and sulfur were oxidized to nitrates and sulfates. The main reaction products from the electrocatalytic oxidation at the anode were short chain volatile hydrocarbons (i.e., olefins, and paraffins) and CO2, while H2 was generated at the cathode. Unidentified compounds were converted to short chain carboxylic acids, alcohol and ketones, while ammonia was oxidized into N2. Studies with model compounds showed that amides and alpha hydroxyacids yielded carboxylic acids that convert further via (non-)Kolbe chemistry. Simultaneous denitrification, valorization of organic compounds, and H2 generation from the aqueous stream can potentially simplify the unit operations currently used in a hydrothermal process. The cost of the electricity required to drive the electrocatalytic operation can be partially mitigated by selling the excess H2 produced.

Prussian blue analogue-derived porous bimetallic oxides Fe3O4–NiO/NF as urea oxidation electrocatalysis

Electrocatalytic urea oxidation reaction (UOR), as an important half-reaction of the direct urea fuel cell, has an important application in the fields of electricity generation, hydrogen production from urea-containing wastewater and urea wastewater treatment. However, the intrinsically sluggish reaction rate of UOR restircts its application. It is urgent to prepare advanced electrocatalysts with high catalytic activity and durability. Herein, we report a bimetallic oxide catalyst Fe3O4–NiO/NF with porous nanostructure derived from Prussian blue analogue. The prepared catalyst has a high BET surface area (79 m2 g−1) and mesoporous structure. Fe3O4–NiO/NF has remarkable UOR activity compared with NiFe(CN)6/NF, the required potential for Fe3O4–NiO/NF decreased from 1.5 to 1.44 V to reach current density of 10 mA cm−2, and the Tafel slope decreased from 38.7 to 32.5 mV dec−1. The advanced porous nanostructure and abundant redox couple of Ni3+/Ni2+ and Fe3+/Fe2+ make Fe3O4–NiO/NF to be a promising UOR catalyst. This study provides a universal optimization strategy of structure and composition for designing advanced UOR electrocatalysts.

Evaluation of electrochemical properties for water splitting by NiO nano-cubes synthesized using Olea ferruginea Royle

Low-cost and efficient electro-catalysts can play a key role in electro-catalytic water oxidation reaction. Hereinto, highly stable semiconducting nickel oxide (NiO) nanoparticles were synthesized by a facile, biomimetic and cost effective one pot synthesis route. Nickel (II) acetate tetrahydrate was used as a precursor and medicinal plant Olea ferruginea Royle as a reducing agent. The biosynthesized NiO NPs were scrutinized by UV–Vis absorption spectroscopy UV Vis.), Fourier transform infrared spectroscopy (FTIR), gas chromatography mass spectroscopy (GC–MS), X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average particle size of the NiO NPs was 13 ± 1 nm which was calculated by Scherrer equation, with uniform cubic shape. NiO supported on Ni-foam was investigated as an electro-catalyst for water splitting application. The results revealed an over potential of 0.41 V and Tafel slope of 97 mVdec−1 at 10 mA/cm2 which is comparable to benchmark catalysts. These results suggests that the organic framework derived NiO could be a competitor to the chemically synthesized electrode material that comprises of costly reagents and require advanced synthesis approaches. Thus, in present study we believe that bio-assisted NiO is an efficient electrode material for water splitting studies and could be a sustainable step towards the replacement of noble metal-based electro-catalysts for energy conversion.

Crystal phase-controlled growth of PtCu and PtCo alloys on 4H Au nanoribbons for electrocatalytic ethanol oxidation reaction

Crystal phase can greatly affect the physicochemical properties and applications of nanomaterials. However, it still remains a great challenge to synthesize nanostructures with the same composition and morphology but different phases in order to explore the phase-dependent properties and applications. Herein, we report the crystal phase-controlled synthesis of PtCu alloy shells on 4H Au nanoribbons (NRBs), referred to as 4H-Au NRBs, to form the 4H-Au@PtCu core-shell NRBs. By tuning the thickness of PtCu, 4H-PtCu and face-centered cubic (fcc) phase PtCu (fcc-PtCu) alloy shells are successfully grown on the 4H-Au NRB cores. This thickness-dependent phase-controlled growth strategy can also be used to grow PtCo alloys with 4H or fcc phase on 4H-Au NRBs. Significantly, when used as electrocatalysts for the ethanol oxidation reaction (EOR) in alkaline media, the 4H-Au@4H-PtCu NRBs show much better EOR performance than the 4H-Au@fcc-PtCu NRBs, and both of them possess superior performance compared to the commercial Pt black. Our study provides a strategy on phase-controlled synthesis of nanomaterials used for crystal phase-dependent applications.

Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru-Doped TiO2 /RuO2 Electrocatalysts.

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru-doped TiO2 /RuO2 (abbreviated as Ru/TiO2 ) as a proof-of-concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ruδ + can function as the main active center for the NOR process. Remarkably doping Ru into the TiO2 lattice can induce an upshift of the d-band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N2 to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO2 on the TiO2 surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO2 in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h-1 gcat -1 (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO2 catalyst (with the hybrid composition of Rux Tiy O2 and extra RuO2 by 2.79 wt% Ru addition amount) in 0.1 m Na2 SO4 electrolyte.

Electrocatalytic activity enhancement of Au NPs-TiO2 electrode via a facile redistribution process towards the non-enzymatic glucose sensors

The global market of glucose sensors has greatly developed in recent decades due to the increasing demands in environmental monitoring, medical community, and food industries. Composite Au nanoparticles (NPs)-TiO2/Polyaniline (PANI) electrodes are prepared via a facile, time-saving, and of low waste solution emission processes in this study for practicing the non-enzymatic glucose sensor applications. A redistribution phenomenon of the TiO2 nanoparticles on the electrode is found in this study by simply immersing and drying the composite electrodes. The redistribution effect on the active sites is enhanced by using TiO2 beyond its saturated concentration and refreshing the accessible surface area of the electroactive sites via immersing-and-drying processes. By controlling the TiO2 concentration on the electrode surface, the usage of the noble metals is reduced to 50 %, while the activity to electrocatalytic glucose oxidation reaction is preserved. The cost for the electrode synthesis for glucose sensor is greatly reduced by replacing the Au NPs with TiO2. In addition, fundamental electrochemical investigations were also conducted in this work. The sensitivity of the optimized Au NPs-TiO2/PANI composite electrode was 313.6 μA mM−1 cm−2, the limit of detection was 0.15 μM, and the long-term stability was up to 70 days.

Crystalline Cobalt/Amorphous LaCoOx Hybrid Nanoparticles Embedded in Porous Nitrogen-Doped Carbon as Efficient Electrocatalysts for Hydrazine-Assisted Hydrogen Production

Hydrazine electro-oxidation has received substantial attention owing to its high energy density, low onset potential and wide applications in hydrazine-assisted hydrogen production and direct hydrazine fuel cells (DHFC). In this work, crystalline cobalt/amorphous LaCoOx hybrid nanoparticles embedded in porous nitrogen-doped carbon (N-C) were fabricated via pyrolytic decomposition of the dual-metal lanthanum-incorporated zeolitic-imidazolate framework (La/ZIF-67), which exhibit high activity and stability towards electrocatalytic hydrazine oxidation reaction (HzOR). The hybrid nanoparticles based on metallic cobalt and amorphous LaCoOx could provide abundant active sites for HzOR catalysis, while the highly conductive and porous N-C could act as both robust skeleton for anchoring the active hybrid nanoparticles and facile charge transport pathway for HzOR process, thereby resulting in enhanced HzOR activity. With the synergistic merits of enriched active sites, large surface area, enhanced charge transfer ability and intimate catalyst anchoring, promoted HzOR performance with high activity and stability was achieved for the optimized catalyst, which shows ultralow onset potential of -0.17 V vs. RHE, high HzOR current density of 69.2 mA cm-2 at 0.3 V vs. RHE and superior stability for 20 h continuous catalysis, making the catalyst a promising electrode material for hydrazine-assisted hydrogen production.

Green bio-synthesis of Ni/montmorillonite nanocomposite using extract of Allium jesdianum as the nano-catalyst for electrocatalytic oxidation of methanol

In this work, synthesis of Ni nanoparticles was carried out successfully by water extract of Allium jesdianum as a biochemical reducing agent in the presence of montmorillonite clay (MMT) as a natural solid support for the first time. Then the electrochemical activity of the synthesized nanocomposite was investigated in methanol electrocatalytic oxidation. MMT with high cation exchange capacity and nano layer structure was exposed to ion exchange conditions in nickel solution. Then Ni 2+ ion exchanged form was used in this process as a source of ions and also capping agent. Water extract of Allium jesdianum used as a reducing agent due to abundant availability of phenolic and flavonoid contents. The synthesized Ni/MMT nanocomposite was characterized using UV–Vis spectroscopy (UV–Vis), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission electron microscopy (TEM) and Energy-dispersive X-ray spectroscopy (EDX). The surface of prepared modified electrode has been characterized using SEM to evaluate the morphology, showing uniform dispersion of Ni nanoparticles with mean diameter of 12 to 20 nm. The modified carbon paste electrode was then used in methanol electrocatalytic oxidation reaction. Methanol oxidation on the proposed modified electrode surface occurs at 0.6 V and 0.3 V in alkaline and acidic medium respectively. Also, the results showed the better performance of modified electrode toward methanol electrocatalytic oxidation in comparison with carbon paste electrode that is modified by ion exchanged MMT. Charge transfer coefficients and apparent charge transfer rate constant for the modified electrode in the absence of methanol in alkaline medium were respectively found as: α a = 0.53, α c = 0.37 and k s = 1.6 × 10 −1 s −1 . Also, the average value of catalytic rate constant for the electrocatalytic oxidation of methanol by the prepared nano-catalyst was estimated to be about 0.9 L·mol -1 ·s -1 by chronoamperometry technique. The prepared electrode was also effective for electrocatalytic oxidation of ethanol and formaldehyde in alkaline medium. Ni/MMT nanocomposite which was synthesized by green method has the suitable electrocatalytic activity to methanol electrocatalytic oxidation. • Ni/MMT nanocomposite was synthesized by green method using Allium jesdianum water extract. • Ni/MMT nanocomposite was characterized by UV–Vis, XRD, SEM, TEM and EDX techniques. • The particles size of Ni/MMT was in the range of 12–20 nm. • Ni/MMT nanocomposite has electrochemical activity for oxidation of methanol in alkaline and acidic medium.

Enhanced catalytic performance and changed reaction chemistry for electrochemical glycerol oxidation by atomic-layer-deposited Pt-nanoparticle catalysts

Atomically controlled thin TiO2 layers were coated onto a carbon-supported Pt nanoparticle (Pt/C) surface using an atomic layer deposition (ALD) technique. Then, the resulting TiO2 coated Pt/C (ALD(TiO2)-Pt/C) was heat-treated at 150 °C under N2 atmosphere. The heat-treated ALD(TiO2)-Pt/C (ALD(TiO2)-Pt/C(150 H T)) catalyst demonstrated improved electrocatalytic glycerol oxidation reaction (GOR) activity with a turnover frequency (TOF) 3-times higher than for uncoated Pt/C catalyst. Interestingly, the ALD(TiO2)-Pt/C(150 H T) catalyst maintained its catalytic GOR performance after a cyclic voltammetry (CV) stability test of 1200 cycles, preventing Pt sintering due to strong Pt-TiO2 interaction. The reaction chemistry for GOR on the ALD(TiO2)-Pt/C(150 H T) catalyst also changed due to the newly created Pt-TiO2 interfaces, producing more oxidized chemicals such as glyceric acid and glycolic acid. The enhanced GOR performance is related to the formation of new Pt-O-Ti bonds and to change in the physicochemical properties of the Pt by interaction between the Pt nanoparticles and ALD-coated TiO2.

Pt decorated 2D/3D heterostructure of Bi2WO6 nanosheet/Cu2S snowflake for improving electrocatalytic methanol oxidation with visible-light assistance

Photo-assisted electrocatalysis based on noble metal/semiconductor electrode has attracted great interest recently. In this paper, a new two dimensional 2D/3D heterostructure of Bi2WO6 nanosheet/Cu2S snowflake was facile synthesized firstly and then employed as a support for the depositing of Pt nanoparticles. The as-synthesized Pt-Bi2WO6/Cu2S composite was further used as working electrode in the application of electrocatalytic methanol oxidation reaction (MOR). The electrocatalytic performances were evaluated by cyclic voltammetry, CO stripping voltammetry, chronoamperometry, chronopotentiometry, photocurrent response, electrochemical impedance spectra and photo&electrocatalytic organic pollutants degradation. It is noted that the introduction of Cu2S improved electrocatalytic activity and stability of Pt-Bi2WO6 for MOR under visible light illumination. The introduced 3D snowflake like Cu2S not only provides more activate sites for depositing Pt nanoparticles and trapping the reactivated molecules, but also acts as a hole transport channel, which can effectively separate the photogenerated electron-hole pairs excited by Bi2WO6, thereby improving the photo-assisted electrocatalytic performance. The present results provide a desirable noble metal electrocatalyst support by using 2D/3D heterojunction in the application of traditional electrocatalytic reactions.

Noble-metal-free cobalt hydroxide nanosheets for efficient electrocatalytic oxidation

Cobalt hydroxide has been emerging as a promising catalyst for the electrocatalytic oxidation reactions, including the oxygen evolution reaction (OER) and glucose oxidation reaction (GOR). Herein, we prepared cobalt hydroxide nanoparticles (CoHP) and cobalt hydroxide nanosheets (CoHS) on nickel foam. In the electrocatalytic OER, CoHS shows an overpotential of 306 mV at a current density of 10 mA·cm−2. This is enhanced as compared with that of CoHP (367 mV at 10 mA·cm−2). In addition, CoHS also exhibits an improved performance in the electrocatalytic GOR. The improved electrocatalytic performance of CoHS could be due to the higher ability of the two-dimensional nanosheets on CoHS in electron transfer. These results are useful for fabricating efficient catalysts for electrocatalytic oxidation reactions.

Pt3Ag alloy wavy nanowires as highly effective electrocatalysts for ethanol oxidation reaction

Direct ethanol fuel cell (DEFC) has received tremendous research interests because of the more convenient storage and transportation of ethanol vs. compressed hydrogen. However, the electrocatalytic ethanol oxidation reaction typically requires precious metal catalysts and is plagued with relatively high over potential and low mass activity. Here we report the synthesis of Pt3Ag alloy wavy nanowires via a particle attachment mechanism in a facile solvothermal process. Transmission microscopy studies and elemental analyses show highly wavy nanowire structures with an average diameter of 4.6 ± 1.0 nm and uniform Pt3Ag alloy formation. Electrocatalytic studies demonstrate that the resulting alloy nanowires can function as highly effective electrocatalysts for ethanol oxidation reactions (EOR) with ultrahigh specific activity of 28.0 mA/cm2 and mass activity of 6.1 A/mg, far exceeding that of the commercial Pt/carbon samples (1.10 A/mg). The improved electrocatalytic activity may be partly attributed to partial electron transfer from Ag to Pt in the Pt3Ag alloy, which weakens CO binding and the CO poisoning effect. The one-dimensional nanowire morphology also contributes to favorable charge transport properties that are critical for extracting charge from catalytic active sites to external circuits. The chronoamperometry studies demonstrate considerably improved stability for long term operation compared with the commercial Pt/C samples, making the Pt3Ag wavy nanowires an attractive electrocatalyst for EOR.

An advanced, efficient and highly durable of reduced graphene oxide/platinum nanoparticles nanocomposite electrocatalyst fabricated via one-step method of the hydrothermal-assisted formic acid process for the electrocatalytic oxidation reaction of methanol

The high-quality of reduced graphene oxide (rGO) supported platinum nanoparticles (PtNPs) was synthesized by a simple, efficient, rapid, clean, surfactant-free, and single-step of hydrothermal-assisted formic acid process. The as-synthesized rGO/PtNPs catalyst was extensively characterized in which demonstrated that the PtNPs have successfully anchored on the surface of rGO with the small average particle size of 4 nm. The evaluation of electrocatalytic activity and durability performance of the as-synthesized rGO/PtNPs nanocomposite catalyst towards methanol oxidation reaction (MOR) as well as the determination of electrochemical surface area (ECSA) were carried out by cyclic voltammogram and chronoamperometry. Besides, the as-prepared rGO/PtNPs nanocomposite catalyst has further proved the remarkably higher electrocatalytic property which exhibited superior maximum forward peak current density (64.04 mA/cm2) toward MOR in acidic media compared with Vulcan XC72/PtNPs (47.54 mA/cm2) and rGO/PdNPs (6.21 mA/cm2) catalysts owing to homogenous distribution and synergic effects of PtNPs with rGO as well as improved electron transfer by rGO. Moreover, the rGO/PtNPs nanocomposite catalyst still achieve the high current density even after 2900 s of continuous catalyst at 0.6 V. This study provides new insights into the production of superior electrocatalytic activity and durability of anode catalyst through a facile, low cost and clean synthesis approach for the enhancement of direct methanol fuel cell performance.

Surface elemental distribution effect of Pt-Pb hexagonal nanoplates for electrocatalytic methanol oxidation reaction

Bimetallic Pt-based catalysts have been extensively investigated to enhance the performance of direct methanol fuel cells (DMFCs) because CO, a by-product, reduces the activity of the pure Pt catalysts. Herein, we synthesized Pt-Pb hexagonal nanoplates as a model catalyst for the methanol oxidation reaction (MOR) and further controlled the Pt and Pb distributions on the surface of the nanoplates through acetic acid (HAc) treatment. As a result, we obtained Pt-Pb nanoplates and HAc-treated Pt-Pb nanoplates with homogeneous and heterogeneous distributions of the Pt-Pb alloy surfaces, respectively. We showed that the MOR activity and stability of the Pt-Pb nanoplates improved compared to those of the HAc-treated Pt-Pb nanoplates, mainly due to the enhanced CO tolerance and the modified electronic structure of Pt under the influence of the oxophilic Pb.

Atomic-layer-deposited SnO2 on Pt/C prevents sintering of Pt nanoparticles and affects the reaction chemistry for the electrocatalytic glycerol oxidation reaction

Here we demonstrate that atomic-layer-deposited SnO2 on Pt/C enhances the catalytic stability and changes product selectivity for the electrocatalytic glycerol oxidation reaction.

Surface reconstruction of AgPd nanoalloy particles during the electrocatalytic formate oxidation reaction.

Formate is a kind of carbon-neutral fuel that can be synthesized by electrochemical conversion of CO2, however, the generated aqueous formate electrolyte is still short of potential application. Here, formate solution is proposed to be utilized as anode fuels of direct formate fuel cells through the formate oxidation reaction (FOR), and graphene supported AgPd nanoalloys (AgPd/rGO) are prepared to catalyze the FOR. Specifically, the mass activity of the as-prepared Ag49Pd51/rGO catalyst is 4.21 A mg-1Pd and the retention activity of Ag49Pd51/rGO is 49.1% of initial activity after successive 500 cycles, which is 2.48 and 3.03 times higher than that of unsupported Ag51Pd49 nanoalloys. When increasing the positive scan limit from 0.0 to 0.8 V, the mass activity of the Ag49Pd51/rGO catalyst increases from 2.32 to 6.03 A mg-1Pd and Pd surface coverage increases from 51.87% to 62.42%, indicating the occurrence of surface reconstruction where Pd atoms migrate to the surface of AgPd nanoalloys, and less intensive reconstruction is observed in unsupported Ag51Pd49 nanoalloys, whose mass activity increases from 1.35 to 2.49 A mg-1Pd. The driving force and kinetic path are calculated for the surface reconstruction induced by the adsorption of H, O and C atoms, in the case of C atoms on graphene, the segregation energy of surface Pd atoms in the AgPd nanoalloy is -1.16 eV, and the activation energy for the migration of subsurface Pd atoms to the surface is 0.54 eV, which are lower than the segregation (0.03 eV) and activation (2.06 eV) energy on a clean alloy surface.

Engineering the morphology of palladium nanostructures to tune their electrocatalytic activity in formic acid oxidation reactions.

Pd nanomaterials can be cheaper alternative catalysts for the electrocatalytic formic acid oxidation reaction (FAOR) in fuel cells. The size and shape of the nanoparticles and crystal engineering can play a crucial role in enhancing the catalytic activities of Pd nanostructures. A systematic study on the effect of varying the morphology of Pd nanostructures on their catalytic activities for FAOR is reported here. Palladium nanoparticles (Pd0D), nanowires (Pd1D) and nanosheets (Pd2D) could be synthesized by using swollen liquid crystals as ‘soft’ templates. Swollen liquid crystals are lyotropic liquid crystals that are formed from a quaternary mixture of a surfactant, cosurfactant, brine and Pd salt dissolved in oil. Pd1D nanostructures exhibited 2.7 and 19 fold higher current density than Pd0D and Pd2D nanostructures in the FAOR. The Pd1D nanostructure possess higher electrochemically active surface area (ECSA), better catalytic activity, stability, and lower impedance to charge transfer when compared to the Pd0D and Pd2D nanostructures. The presence of relatively higher amounts of crystal defects and enriched (100) crystal facets in the Pd1D nanostructure were found to be the reasons for their enhanced catalytic activities.

Unexpectedly high stability and surface reconstruction of PdAuAg nanoparticles for formate oxidation electrocatalysis

High-performance Pd-based nanocatalysts for alkaline methanol and formate fuel cells have stimulated widespread attention. Hence, a series of ternary Pd-Au-Ag nanoalloys have been synthesized on carbon nanotubes, which demonstrate promising activity and unexpectedly high stability for the formate oxidation reaction (FOR) in alkaline medium. The ternary Pd3Au3Ag1 nanoalloy catalyst showed an initial mass activity of 4.51 A mgPd-1 and a retained mass activity of 1.32 A mgPd-1 after chronoamperometric measurement for 3600 s, which are superior to the best values for all FOR catalysts reported so far. The Pd3Au3Ag1 catalyst also showed a good specific activity of 4.32 mA cm-2 for the methanol oxidation reaction. Furthermore, surface reconstruction of the Pd3Au3Ag1 nanoalloy was observed during FOR, where the activity of Pd3Au3Ag1 catalysts increased up to 33% and the cycling durability retained 55% after cyclic voltammetry with the upper potential of 1.7 V. The FOR enhancement is attributed to the formation of mixed oxidation-state Ag sites and the increase in the Pd surface coverage, and provides a new prospect for the design of ternary nanoalloy electrocatalysts for various fuel oxidation reactions.

Multinary PtPdNiP truncated octahedral mesoporous nanocages for enhanced methanol oxidation electrocatalysis

Multinary PtPdNiP TOMNs have been synthesized for the electrocatalytic methanol oxidation reaction with a superior electrocatalytic performance.

Pore structure dependent activity and durability of mesoporous rhodium nanoparticles towards the methanol oxidation reaction.

This work reports a significant effect of porous structures on the electrocatalytic methanol oxidation performances of Rh-based catalysts. It is demonstrated that a three-dimensional net-like mesoporous structure with ultrathin subunits is beneficial to exposing more active sites and boosting electron transfer inside particles, thus presenting the highest activity.