Structure and chemical composition of supported Pt-Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnO_x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO_x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO_x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO₂ nanoparticles, characterized with the nominal 0.264 nm spacing of SnO₂ (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO_x catalyst. PtSnO_x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO_x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.     

Single-step polyol synthesis of alloy Pt<Subscript>7</Subscript>Sn<Subscript>3</Subscript> versus bi-phase Pt/SnO<Subscript>x</Subscript> nano-catalysts of controlled size for ethanol electro-oxidation

Disordered alloy and bi-phase PtSn nanoparticles of nominal Pt:Sn ratio of 70:30 atomic % with controlled size and narrow size distribution were synthesized using a single-step polyol method. By adjusting the solution pH it was possible to obtain Pt₇Sn₃ nanoparticles of various sizes from 2.8 to 6.5 nm. We found that the presence of NaOH in the synthesis solution not only influenced the nanoparticle size, but as it was revealed by XRD, it apparently also dictated the degree of Pt and Sn alloying. Three catalysts prepared at lower NaOH concentrations (C_NaOH < 0.15 M) showed disordered alloy structure of the nominal composition, while the other three catalysts synthesized at higher NaOH concentrations (C_NaOH > 0.15 M) consisted of bi-phase nanoparticles comprising a crystalline phase close to that of pure Pt together with an amorphous Sn phase. These observations are plausibly due to the phase separation and formation of monometallic Pt and amorphous SnO_x phases. A proposed reaction mechanism of Pt₇Sn₃ nanoparticle formation is presented to explain these observations along with the catalytic activities measured for the six synthesized carbon-supported Pt₇Sn₃ catalysts. The highest catalytic activity towards ethanol electro-oxidation was found for the carbon-supported bi-phase catalyst that formed the largest Pt (6.5 nm) nanoparticles and SnO_x phase. The second best catalyst was a disordered alloy Pt₇Sn₃ catalyst with the second largest nanoparticle size (5 nm), while catalysts of smaller size (4.5–4.6 nm) but different structure (disordered alloy vs. bi-phase) showed similar catalytic performance inferior to that of the 5 nm disordered alloy Pt₇Sn₃ catalyst. This work demonstrated the importance of producing bi-metallic PtSn catalysts with large Pt surfaces in order to efficiently electro-oxidize ethanol.     

Preparation of SnO2-CNTs supported Pt catalysts and their electrocatalytic properties for ethanol oxidation

SnO₂-carbon nanotubes (CNTs) composites were prepared by sol-gel method, and characterized by scanning electron microscopy and X-ray diffraction. Due to high stability in diluted acidic solution, SnO₂-CNTs composites were selected as the catalyst support and second catalyst for ethanol electrooxidation. The electrocatalytic properties of the SnO₂-CNTs supported platinum (Pt) catalyst (Pt/SnO₂-CNTs) for ethanol oxidation have been investigated by typical electrochemical methods. Under the same mass loading of Pt, the Pt/SnO₂-CNTs catalyst shows higher electrocatalytic activity and better long-term cycle stability than Pt/SnO₂ catalyst. Additionally, the effect of the mass ratio of CNTs to SnO₂ on the electrocatalytic activity of the electrode for ethanol oxidation was investigated, and the optimum mass ratio of CNTs to SnO₂ in the Pt/SnO₂-CNTs catalyst is 1/6.3.     

Electrochemical oxidation of ammonia on carbon-supported bi-metallic PtM (M = Ir, Pd, SnOx) nanoparticles

Ammonia electro-oxidation was studied in alkaline solution on carbon-supported Pt and bimetallic Pt_yM_1−y (M = Pd, Ir, SnO_x and y = 70, 50 at.%) nanoparticles. Catalysts were synthesized using the modified polyol method and deposited on carbon, resulting in 20 wt.% of metal loading. Particle size, structure and surface composition of the particles were investigated using TEM, XRD and XPS. Mean size of PtM bi-metallic nanoparticles varied between 2.0 and 4.7 nm, depending on the second metal (M). XRD revealed the structure of all bi-metallic particles to be face-centered cubic and confirmed alloy formation for Pt_yPd_1−y (y = 70, 50 at.%) and Pt₇Ir₃nanoparticles, as well as partial alloying between Pt and SnO_x. Electrochemical behaviour of ammonia on Pt and PtM nanoparticles is comparable to that expected for bulk Pt and PtM alloys. Addition of Pd to Pt at the nanoscale decreased the onset potential of ammonia oxidation if compared to pure platinum nanoparticles; however stability of the catalyst was poor. For Pt₇(SnO_x)₃, current densities were similar to Pt, whereas catalyst stability against deactivation was improved. It is found that carbon supported Pt₇Ir₃ nanoparticles combine good catalytic activity with enhanced stability for ammonia electro-oxidation. Electronic effect generated between two metals in the bimetallic nanoparticles might be responsible for increase in the catalytic activity of Pd- and Ir-containing catalysts, causing weakening of the adsorption strength of poisonous N_ads intermediate.     

Synthesis of well-dispersed PtRuSnOx by ultrasonic-assisted chemical reduction and its property for methanol electrooxidation

PtRuSnO_x supported on multi-wall carbon nanotubes (MWCNTs) was prepared by ultrasonic-assisted chemical reduction method. The as-prepared catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that Pt exists as the face-centered cubic structure, Ru is alloyed with platinum, while non-noble metal oxide SnO_x exists as an amorphous state. From TEM observation, PtRuSnO_x is well dispersed on the surface of MWCNTs with the particle size of several nanometers. The electrochemical properties of the as-prepared catalyst for methanol electrooxidation were studied by cyclic voltammetry (CV) and chronoamperometry (CA). The onset potential of methanol oxidation on PtRuSnO_x and PtRu catalysts is much more negative than that on Pt catalyst, shifting negatively by about 0.20 V, while the peak current density of methanol oxidation on PtRuSnO_x is higher than that on PtRu. Electrochemical impedance spectroscopy (EIS) studies also show that the reaction kinetics of methanol oxidation is improved with the presence of SnO_x. The addition of non-noble metal oxide SnO_x to PtRu promotes the catalytic activity for methanol electrooxidation and the possible reaction mechanism is proposed.     

Polyelectrolyte‐aided synthesis of gold and platinum nanoparticles: Implications in electrocatalysis and sensing

An electrochemical method for depositing redispersible, lower size gold nanoparticles from a novel polyelectrolyte‐gold complex is described. The size of gold nanoparticles is in the range 6.2–15.4 nm. The gold nanoparticles, first deposited on platinum surface are transferable into water. They can also be directly in situ‐electrodeposited on to materials like carbon, carbon nanotubes or conducting polymers for an end use as electro catalysts. The composites Au‐MWCNT, Pt‐MWCNT, Au‐Carbon, and Pt‐Carbon are synthesized and tested for their electrocatalytic activity. The composites exhibit good catalytic activity in sensing dopamine or electrooxidation of methanol. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Influence of the Temperature for the Ethanol Oxidation Reaction (EOR) on Pt/C,Pt‐Rh/C and Pt‐Rh‐SnO2/C

The electrocatalytic activity of Pt/C, Pt‐Rh/C and Pt‐Rh‐SnO₂/C electrocatalysts toward the ethanol oxidation reaction (EOR) was investigated in a three‐electrode assembly at 25 °C, 40 °C and 70 °C in acidic medium. The 10 wt.% electrocatalysts were prepared with a modified polyol method and physically characterized by both X‐Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The CO‐stripping study revealed that CO_ad oxidation initiates at lower potential on Pt‐Rh/C and Pt‐Rh‐SnO₂/C than on Pt/C and shifts negatively when the temperature increases. The positive effect of the temperature is maintained for the EOR: the three electrocatalysts exhibit a higher activity and a negative shift of the EOR wave at higher temperature. Pt‐Rh‐SnO₂/C demonstrates the lowest EOR onset potential of the three electrocatalysts. The steady‐state Tafel slopes and apparent activation energies E_a were determined in 0.5 M H₂SO₄ + 0.1 M EtOH between E = 0.4 and 0.7 V vs. RHE in the temperature range 25–70 °C. The results show rather comparable rate determining steps (rds) for the ethanol electrooxidation on Pt/C and Pt‐Rh/C in the ranges of potential and temperature studied. The EOR on Pt‐Rh‐SnO₂/C seems less influenced by the potential than on Pt/C and Pt‐Rh/C electrocatalysts, but is more temperature sensitive.     

Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt:Sn ratio

Novel carbon supported Pt/SnO_x/C catalysts with Pt:Sn atomic ratios of 5:5, 6:4, 7:3 and 8:2 were prepared by a modified polyol method and characterized with respect to their structural properties (X-ray diffraction (XRD) and transmission electron microscopy (TEM)), chemical composition (XPS), their electrochemical properties (base voltammetry, CO_ad stripping) and their electrocatalytic activity and selectivity for ethanol oxidation (ethanol oxidation reaction (EOR)). The data show that the Pt/SnO_x/C catalysts are composed of Pt and tin oxide nanoparticles with an average Pt particle diameter of about 2 nm. The steady-state activity of the Pt/SnO_x/C catalysts towards the EOR decreases with tin content at room temperature, but increases at 80 °C. On all Pt/SnO_x/C catalysts, acetic acid and acetaldehyde represent dominant products, CO₂ formation contributes 1-3% for both potentiostatic and potentiodynamic reaction conditions. With increasing potential, the acetaldehyde yield decreases and the acetic acid yield increases. The apparent activation energies of the EOR increase with tin content (19-29 kJ mol⁻¹), but are lower than on Pt/C (32 kJ mol⁻¹). The somewhat better performance of the Pt/SnO_x/C catalysts compared to alloyed PtSn_x/C catalysts is attributed to the presence of both sufficiently large Pt ensembles for ethanol dehydrogenation and C-C bond splitting and of tin oxide for OH generation. Fuel cell measurements performed for comparison largely confirm the results obtained in model studies.     

Controlled Synthesis of Pt3Sn/C Electrocatalysts with Exclusive Sn–Pt Interaction Designed for Use in Direct Methanol Fuel Cells

Alloy-type Sn–Pt/C electrocatalysts with Pt/Sn = 1.8–3.0 ratios and exclusive Sn–Pt interaction have been prepared by means of controlled surface reactions (CSRs). As demonstrated by XRD, the incorporation of Sn onto Pt/C was achieved satisfactorily yielding a near-stoichiometric fcc Pt₃Sn alloy phase along with a certain amount of the Pt_(1 ? x)Sn_x solid solution. The content and dispersion of the fcc Pt₃Sn phase within the electrocatalysts can be controlled by tuning the reaction conditions of CSRs. No evidence of the presence of SnO₂ phases in the Sn-modified Pt/C samples was found by means of the XRD and EDS analysis. According to in situ XPS studies, the pre-treatment in hydrogen at 350 °C resulted in complete reduction of tin to Sn⁰. These results demonstrate that the method of CSRs is a powerful tool to create of Pt–Sn bimetallic nanoparticles exclusively, without tin introduction onto the carbon support. The performance of the intermetallic SnPt/C catalysts in the CO and methanol electrooxidation reactions depends on the actual composition of the exposed surface and the size of bimetallic particles. In the consecutive tin introduction the decrease of the amount of SnEt₄ precursor added per period, accompanied with an increase of the number of anchoring periods, resulted in an increase of the activity in both electrooxidation reactions as a consequence of an optimal balance of Pt/C ratio, the content of fcc Pt₃Sn phase and metal particle size. It was demonstrated that the increasing tin content above a certain (optimal) amount gives rise to a negative effect on the catalyst performance in the CO and methanol electrooxidation.     

Morphology-dependent activity of Pt nanocatalysts for ethanol oxidation in acidic media: Nanowires versus nanoparticles

The morphology of nanostructured Pt catalysts is known to affect significantly the kinetics of various reactions. Herein, we report on a pronounced morphology effect in the electrooxidation of ethanol and carbon monoxide (CO) on Pt nanowires and nanoparticles in an acidic solution. The high resolution transmission electron microscopy analysis showed the inherent morphology difference between these two nanostructured catalysts. Voltammetric and chronoamperometric studies of the ethanol electrooxidation revealed that these nanowires had a higher catalytic activity by a factor of two relative to these nanoparticles. The rate for CO monolayer oxidation exhibits similar morphology-dependent behavior with a markedly enhanced rate on the Pt nanowires. In situ infrared reflection–absorption spectroscopy measurements revealed a different trend for chemisorbed CO formation and CO₂-to-acetic acid reaction product ratios on these two nanostructures. The morphology-induced change in catalytic activity and selectivity in ethanol electrocatalysis is discussed in detail.     

Improving the Stability and Ethanol Electro‐Oxidation Activity of Pt Catalysts by Selectively Anchoring Pt Particles on Carbon‐Nanotubes‐Supported‐SnO2

To improve the stability and activity of Pt catalysts for ethanol electro‐oxidation, Pt nanoparticles were selectively deposited on carbon‐nanotubes (CNTs)‐supported‐SnO₂ to prepare Pt/SnO₂/CNTs and Pt/CNTs was prepared by impregnation method for reference study. X‐ray diffraction (XRD) was used to confirm the crystalline structures of Pt/SnO₂/CNTs and Pt/CNTs. The stabilities of Pt/SnO₂/CNTs and Pt/CNTs were compared by analyzing the Pt size increase amplitude using transmission electron microscopy (TEM) images recorded before and after cyclic voltammetry (CV) sweeping. The results showed that the Pt size increase amplitude is evidently smaller for Pt/SnO₂/CNTs, indicating the higher stability of Pt/SnO₂/CNTs. Although both catalysts exhibit degradation of electrochemical active surface area (EAS) after CV sweeping, the EAS degradation for the former is lower, further confirming the higher stability of Pt/SnO₂/CNTs. CV and potentiostatic current–time curves were recorded for ethanol electro‐oxidation on both catalysts before and after CV sweeping and the results showed that the mass specific activity of Pt/CNTs increases more than that of Pt/SnO₂/CNTs, indicating that Pt/CNTs experiences more severe evolution and is less stable. The calculated area specific activity of Pt/SnO₂/CNTs is larger than that of Pt/CNTs, indicating SnO₂ can co‐catalyze Pt due to plenty of interfaces between SnO₂ and Pt.     

The effect of Sn content on the electrocatalytic properties of Pt–Sn nanoparticles dispersed on graphene nanosheets for the methanol oxidation reaction

Direct methanol fuel cell (DMFC) electrode catalysts with improved electrochemical properties have been prepared by dispersing platinum–tin (Pt–Sn) nanoparticles onto graphene sheets. During the deposition, a majority of the oxygenated functional groups on the graphene oxide nanosheets were removed, resulting in the formation of graphene. Microstructural characterization shows that metallic Pt, Pt–Sn alloy and tin dioxide (SnO₂) nanoparticles were distributed on the graphene sheets, representing different lattice planes during the synthetic process. In terms of the electrocatalytic properties, graphene-supported Pt–Sn and graphene-supported Pt catalysts exhibited much higher current densities compared with that of commercial carbon black-supported Pt catalysts. Graphene-supported Pt–Sn increased the electrocatalytic activity, which is strongly influenced by the addition of Sn in its alloyed and oxidized forms, boosting the reaction more readily because of the lower oxidation potential.     

The effect of plasma pre-treatment of carbon used as a Pt catalyst support for methanol electrooxidation

Vulcan XC-72 carbon black for use as a catalyst support was treated in three different plasma atmospheres, H₂, Ar and O₂. The results showed that the microstructure and surface functional groups were significantly changed after plasma treatment. Pt/C catalysts were prepared by chemical reduction of H₂PtCl₆ with HCHO and those with untreated and plasma treated carbon black supports were characterized and tested for methanol electrooxidation. TEM showed that the platinum nanoparticles on H₂ and Ar plasma treated carbon were uniform and well distributed. Those on untreated carbon were uniform in most regions but coalesced in others. On O₂ plasma treated carbon agglomeration of the platinum nanoparticles was significant. XRD showed that the catalysts were composed of face-centered cubic Pt nanoparticles and XPS showed that they were metallic with no oxides present. Cyclic voltammetry and chronoamperometry were used to study methanol electrooxidation on the Pt/C catalysts in a solution of 0.5 M H₂SO₄ + 0.5 M CH₃OH, and showed that the catalytic activity those using H₂ and Ar plasma treated carbon was higher than for the untreated one. Catalysts supported by O₂ plasma treated carbon showed no catalytic activity. The treatment atmosphere of carbon therefore had a large effect on the catalyst performance, with the H₂ plasma being the best.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Ethanol oxidation on the ternary Pt-Rh-SnO2/C electrocatalysts with varied Pt:Rh:Sn ratios

Ternary Pt-Rh-SnO₂/C electrocatalysts with the atomic ratio Pt:Rh:Sn = 3:1:x, where x varies from 2 to 6, were synthesized using the modified polyol method followed by thermal treatment. Several techniques used to characterize these electrocatalysts showed they were composed of homogeneous PtRh alloy and SnO₂, having all three constituents coexisting in single nanoparticles with the average particle size around 1.4 nm and a narrow size distribution. While all the electrocatalysts investigated exhibited high catalytic activity for ethanol oxidation, the most active one had the composition with the Pt:Rh:Sn = 3:1:4 atomic ratio. These ternary-electrocatalysts effectively split the C-C bond in ethanol at room temperature in acidic solutions, which is verified using the in situ IRRAS technique.     

The Nanocrystalline SnO2–TiO2 System—Part I: Structural Features

The phases present and their crystal structure and microstructure in the nanocrystalline SnO₂–TiO₂ system were studied in the compositional range Sn_1?xTi_xO₂ (0.0 ≤ x ≤ 0.9). There is an apparent increase in the solubility limits in the solid solution compared to bulk crystalline SnO₂–TiO₂. No two phase region was observed with increasing TiO₂ content. Electron energy loss spectroscopy, infrared spectroscopy (FTIR), and X‐ray diffraction (XRD) of the nanopowders showed that the apparent increase in solubility is related to the systematic Ti⁴⁺ segregation on the particle surface (surface excess) at the SnO₂‐rich side, avoiding the nucleation of a second phase even at high Ti⁴⁺ contents. Is this finding in accord with Raman spectra, which suggest localized Ti‐rich sites in the absence of a second crystalline phase. Ti⁴⁺ surface excess is also lead to a modification of the surface hydroxyls and a decrease in the crystallite size of the nanoparticles (with a concomitant increase in surface area), with expected implications to catalytic and sensorial properties of these nanoparticles.     

Platinum–tin alloy electrodes for direct methanol fuel cells

Pt electrodes, modified by partial electrodeposited tin, were used as anodes for the catalytic electrooxidation of methanol in acid medium. Sn was electrodeposited galvanostatically and potentiostatically. Cyclic voltammetry was used to study the methanol electrooxidation. Pt modified with Sn proved superior to pure platinum as shown from the methanol peak current densities. Sn also improved the performance regarding the stability of the anode over repeated cycles. It was found that electrodeposited Sn facilitates the oxidation of intermediate poison products through a mixed homogeneous–heterogeneous catalytic mechanism.     

A General Protocol for the Synthesis of Pt–Sn/C Catalysts for the Ethanol Electrooxidation Reaction

A general protocol for the synthesis of Pt–Sn/C catalysts for ethanol electrooxidation by the polyol method is developed after a systematic variation of the preparation variables. This protocol enables the complete transfer of all catalytic elements in the preparation solution to the catalyst support; thereby providing a convenient means of catalyst composition control. Water is a necessary co‐solvent for ethylene glycol in the polyol synthesis of Pt–Sn/C catalysts. The best preparation medium for controlling the particle size to small sizes is 0.1 M NaOH solution in a mixture of equal volumes of water and ethylene glycol. With this medium composition Pt–Sn/C catalysts with the optimized target Pt:Sn atomic ratio of 3:1 could be expeditiously prepared for ethanol electrooxidation.     

TiO2@carbon core-shell nanostructure supports for platinum and their use for methanol electrooxidation

Nanostructures consisting of TiO₂ particles as a core and carbon as a shell (TiO₂@C) were prepared by heat treatment of TiO₂ nanoparticles at 700 or 900 °C in a methane atmosphere. X-ray diffraction and transmission electron microscopy showed that a carbon shell layer was formed whose thickness increased with increasing reaction temperature. These structures were used as supports for platinum nanoparticles and the hybrid particles exhibit improved catalytic activity and stability toward methanol electrooxidation compared to Pt on a carbon black (Vulcan XC-72R). It is likely that enhanced catalytic properties of the Pt on TiO₂@C could be due to the stability of the core-shell support in comparison with carbon black support.     

Development of composite anode electrocatalyst for direct methanol fuel cells

Different effects of support hydrophilicity and metal-oxide on the performance of Pt-based catalysts were investigated with the aim of improving the mass activities toward methanol electrooxidation. Both potentiodynamic and potentiostatic measurements revealed that improved surface hydrophilicity of multi-wall carbon nanotubes (MWCNTs) could promote the dispersion of Pt nanoparticles and, consequently, promote the Pt utilization and reduce the polarization in methanol electrooxidation. In addition, WO₃ was shown to play a supportive role in enhancing catalytic activity. The interaction between Pt and WO₃ was examined by CO-stripping and CO oxidation transient experiments. The results suggested that the activity and the kinetics of monolayer CO_ads electrooxidation of Pt nanoparticles are enhanced by the adjacent WO₃ via a bifunctional mechanism, which accounts for improved activity in methanol electrooxidation.