Nano-structured PdxPt1−x/Ti anodes prepared by electrodeposition for alcohol electrooxidation

Nano-structured Pd_xPt_1−x (x = 0-1) composite catalysts supported on Ti substrate are successfully prepared by electrodeposition method, and the morphology and phase of the catalysts are analyzed by field emission scanning electron microscope (FE-SEM) and X-ray energy dispersion spectroscopy (EDS). The activity and stability of the Pd_xPt_1−x/Ti composite catalysts are assessed for the electrooxidation of alcohols (methanol, ethanol and 2-propanol) in alkaline medium using cyclic voltammetry and chronoamperometry techniques. The results show that the Pd and Pt form Pd_xPt_1−x nano-structured composite catalysts, uniformly distributed on the Ti substrate. The electrocatalytic activity and stability of the Pd_xPt_1−x nanocatalysts depend strongly on the atomic ratios of Pd and Pt. Among the synthesized catalysts, the Pd_0.8Pt_0.2/Ti displays the best catalytic activity and stability for the electrooxidation reaction of alcohols investigated in alkaline medium under conditions in this study, and shows the potential as electrocatalysts for direct alcohol fuel cells.     

Electrospray Ionization–Mass Spectrometry in the Enantioselective Hydrogenation of Ethyl Pyruvate Catalyzed by Dihydrocinchonidine Modified Pt/Al2O3 in Acetic Acid

The enantioselective hydrogenation of ethyl pyruvate (EtPy) on Pt–alumina (E 4759), Pt black, and Pt black+alumina (mixture) catalysts modified by dihydrocinchonidine (DHCD) in acetic acid was studied by electrospray ionization–mass spectrometry (ESI–MS). Application of the ESI–MS technique led to the recognition of a novel-type compound, O⁺[Al(OAc)₂]₃ (oxonium ions). The effect of the DHCD concentration, temperature, and oxonium cations on the reaction rate and the enantioselectivity has been studied. Using the Engelhard 4759 catalyst in acetic acid under mild experimental conditions (room temperature, hydrogen pressure 1 bar, DHCD concentration 0.01 mM/L) an optical yield of 92% can be achieved. The high enantioselectivity is accompanied by the following turnovers: EtPy/DHCD>43,000, EtPy/Pt_surface>1000, TOF=1–2 s⁻¹, and DHCD/Pt_surface ratio=0.0072. The enantioselectivity reducing factor is identified by ESI–MS as the gradual hydrogenation of the quinoline skeleton of DHCD that becomes more pronounced with increasing temperature and hydrogenation time. The discovery of oxonium cations, the extremely low DHCD/Pt_surface ratio, and the new data obtained by the Pt black + alumina mixture made possible an interpretation of the mechanism of the heterogeneous enantioselective hydrogenation of α-ketoesters.     

On the composition and structure of thin layers of titanium oxide on platinum surfaces

This work reviews the available data about the structure and composition of thin layers of titanium oxide on the surface of platinum. These systems can be prepared by vapor deposition and subsequent oxidation of Ti on a massive Pt substrate or by oxidation at low pressure of the Pt₃Ti alloy. For both types of substrate, the XPS data show that two distinct Ti states in the oxide layer can be detected. The structure and the composition of the oxide layer varies depending on the substrate composition and on the conditions of preparation. On pure Pt, the preferential formation of a defective TiO_2–x oxide was observed. On Pt₃Ti, the oxide film a layer was formed of TiO of thickness of the order of a single atomic layer and of crystallites of nearly stoichiometry TiO₂.     

Characterization of Pt/SiO2 Model Catalysts at UHV and Near Atmospheric Pressures

Pt/SiO₂ model catalyst samples, prepared under UHV conditions in a contiguous high pressure reactor cell surface analysis chamber, have been characterized via CO oxidation reaction kinetics under elevated pressure conditions (approaching 1 atm). Reaction kinetics are studied as a function of Pt coverage (θ _Pt = 1–10 mL), along with measurements on a Pt(110) single crystal for direct comparison. CO desorption measurements and STM measurements on Pt/SiO₂ films at T = 300 K have been obtained at various θ _Pt. Kinetic results show agreement between observations on single crystal and catalyst samples, and general agreement and correlation is obtained for site calculations across the various methods. Results demonstrate the utility of well characterized model catalyst samples in obtaining qualitative and quantitative reactivity data at elevated pressures.     

Phase and Texture Evolution in Chemically Derived PZT Thin Films on Pt Substrates

The crystallization of lead zirconate titanate (PZT) thin films was evaluated on two different platinum‐coated Si substrates. One substrate consisted of a Pt coating on a Ti adhesion layer, whereas the other consisted of a Pt coating on a TiO₂ adhesion layer. The Pt deposited on TiO₂ exhibited a higher degree of preferred orientation than the Pt deposited on Ti (as measured by the Full Width at Half Maximum of the 111 peak about the sample normal). PZT thin films with a nominal Zr/Ti ratio of 52/48 were deposited on the substrates using the inverted mixing order (IMO) route. Phase and texture evolution of the thin films were monitored during crystallization using in situ X‐ray diffraction at a synchrotron source. The intensity of the Pt₃Pb phase indicated that deposition on a highly oriented Pt/TiO₂ substrate resulted in less diffusion of Pb into the substrate relative to films deposited on Pt/Ti. There was also no evidence of the pyrochlore phase influencing texture evolution. The results suggest that PZT nucleates directly on Pt, which explains the observation of a more highly oriented 111 texture of PZT on the Pt/TiO₂ substrate than on the Pt/Ti substrate.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

Guaiacol HDO on La-modified Pt/Al2O3: Influence of rare-earth loading

The stability of the catalyst used in hydrodeoxygenation (HDO) of biomass-derived oils needs improvement. La has been applied in delaying Al₂O₃ phase-change under reaction conditions. Lanthanum (0.5–8 wt.%)-γ-alumina was studied as Pt (1 wt.%) carrier aimed at guaiacol (GUA) HDO. Materials characterization included N₂ physisorption, X-ray diffraction (XRD), thermal analysis, FTIR, UV–vis, and TPR. Solids pore size (~8–10 nm) was suitable for GUA (kinetic diameter~0.668 nm) hydrotreating. Mixed carriers were amorphous (XRD), suggesting well-dispersed La domains; meanwhile, carbonates/bicarbonates were formed (from CO₂) due to the basic surface properties of modified supports (FTIR). That could impart catalyst stability by inhibiting coking through the passivation of Lewis acidity on Al₂O₃. Pt reducibility increased with La loading in various formulations. However, that was not reflected in enhanced GUA HDO (T = 488 K and P = 3.2 MPa, batch reactor), presumably due to the strong metal–support interaction (SMSI), where LaO_x covered the metallic Pt particle surface. GUA HDO on various catalysts was approximated by pseudo-first-order kinetics (integral regime, k), where deviations were observed as La loading increased, presumably by an SMSI state that could affect the rate-determining step of the reaction mechanism. Basic sites provided by rare-earth could contribute to altering HDO reaction pathways as well. At 1 wt.% rare-earth, GUA HDO was maximized (k~25% higher than that on Pt/Al₂O₃), with that material also exhibiting similar deoxygenation (85%–90% at total GUA conversion) to the latter Pt over pristine alumina. Conversely, both parameters significantly diminished over the catalyst of the highest La content. Materials at low rare-earth concentrations deserve further studies focused on catalyst stability under HDO conditions.     

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 and electrocatalytic oxygen reduction activity of graphene-supported Pt3Co and Pt3Cr alloy nanoparticles

Graphene-supported Pt and Pt₃M (M = Co and Cr) alloy nanoparticles are prepared by ethylene glycol reduction method and characterized with X-ray diffraction and transmission electron microscopy. X-ray diffraction depicted the face-centered cubic structure of Pt in the prepared materials. Electron microscopic images show the high dispersion of metallic nanoparticles on graphene sheets. Electrocatalytic activity and stability of the materials is investigated by rotating-disk electrode voltammetry. Oxygen reduction activity of the Pt₃M/graphene is found to be 3–4 times higher than that of Pt/graphene. In addition, Pt₃M/graphene electrodes exhibited overpotential 45–70 mV lower than that of Pt/graphene. The high catalytic performance of Pt₃M alloys is ascribed to the inhibition of formation of (hydr) oxy species on Pt surface by the alloying elements. The fuel cell performance of the catalysts is tested at 353 K and 1 atm. Maximum power densities of 790, 875, and 985 mW/cm² are observed with graphene-supported Pt, Pt₃Co, and Pt₃Cr cathodes, respectively. The enhanced electrocatalytic performance of the Pt₃M/graphene (M = Co and Cr) compared to that of Pt/graphene makes them a viable alternative to the extant cathodes for energy conversion device applications.     

Preferential CO Oxidation on Bimetallic Pt<Subscript>0.5</Subscript>M<Subscript>0.5</Subscript> Catalysts (M = Fe,Co, Ni) Prepared from Double Complex Salts

Properties of Pt_0.5M_0.5 nanopowders (M = Fe, Co, Ni) of alloys obtained via the decomposition of double complex salts [Pt(NH₃)₅Cl][Fe(C₂O₄)₃] ? 4H₂O, [Pt(NH₃)₄][Co(C₂O₄)₂(H₂O)₂] ? 2H₂O, and [Pt(NH₃)₄][Ni(C₂O₄)₂(H₂O)₂] ? 2H₂O, respectively, are studied in the reaction of preferential CO oxidation. It is shown that bimetallic Pt_0.5M_0.5 catalysts (M = Fe, Co, Ni) are much more active in the low temperature range than Pt nanopowder. The activity of the catalysts decreases in the order Pt_0.5M_0.5 ≥ Pt_0.5M_0.5 > Pt_0.5M_0.5 @ Pt. The higher activity of bimetallic Pt0.5M0.5 catalysts in the reaction of preferential CO oxidation in the low-temperature range under conditions of dense Pt surface coverage by adsorbed CO molecules is most likely caused by the activation of CO on Pt atoms, the activation of O₂ on atoms of the second metal (Fe, Co, Ni), and the reaction that occurs at the sites of contact between the atoms of platinum and the atoms of the second metal on the surfaces of the alloy’s nanoparticles. The bimetallic systems investigated in this work can be used to improve catalysts of practically important preferential CO oxidation reaction. These systems have considerable potential in the afterburning reactions of CO and hydrocarbons; hydrogenation reactions; electrochemical reactions; and many others. The means used in the preparation of bimetallic nanopowders based on the decomposition of double complex salts is simple, does not require the use of expensive or complex reagents, and can be easily adapted to produce supported catalysts containing Pt_0.5M_0.5 metal alloys (M = Fe, Co, Ni).     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

The effectiveness of platinum/carbon electrocatalysts: Dependence on catalyst layer thickness and Pt alloy catalytic effects

In our previous work, we have proposed a new method to estimate the effective Pt utilization or “effectiveness” (Ef_Pt) using the ratio of the mass activity (MA) for the oxygen reduction reaction in the membrane-electrode assembly (MEA) in the polymer electrolyte fuel cell to that in the channel flow double electrode measurement, MA_max, under similar conditions. In the present research, applying this method, we have focused on elucidating the effect of the thickness of the catalyst layer (CL), the effect of Pt-based alloy catalysts, and effect of the state of dispersion of the Pt/C catalysts in the CL in measurements carried out at 80 °C and various relative humidities (RH), in either O₂ or air. The effect of a thin CL (0.04 mg cm⁻², Pt/C) has improved Ef_Pt by a factor of four, going from 3% to 12%, and the integrated effect of a thin CL and alloying (0.05 mg cm⁻² Pt₃Co) has improved Ef_Pt by a factor of six, going from 3% to 17% for air at 0.85 V, T_cell = 80 °C, and 30% RH. Furthermore, we found that the Ef_Pt values were dependent upon the state of Pt dispersion in the CL. The highest Ef_Pt value obtained thus far for air at 0.85 V, T_cell = 80 °C, and 100% RH was ca. 22%, shown by a low Pt loading CL diluted with added uncatalyzed carbon black (0.04 mg cm⁻², overall average 30 wt%-Pt).     

Pt and Sn Doped Sputtered CeO2 Electrodes for Fuel Cell Applications

The interaction of Pt with CeO₂ layers was investigated by using photoelectron spectroscopy. Thirty‐nanometre‐thick Pt and Sn doped CeO₂ layers were deposited simultaneously by rf‐magnetron sputtering on a Si(001) substrate and a carbon diffusion layer of a polymer membrane fuel cell by using a composite CeO₂–Pt–Sn target. The laboratory XPS and synchrotron radiation soft X‐ray and hard X‐ray photoemission spectra showed the formation of cerium oxide with completely ionised Pt^2+,4+ species, and with Pt⁴⁺ embedded in the film bulk. Hydrogen/air fuel cell activity measurements normalised to the amount of Pt used revealed high specific power of up to 5.4 × 10⁴ mW mg^–1 (Pt). The activity of these materials is explained by the strong activity of embedded Ptⁿ⁺ cations.     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

Substrate–support interactions in metal-catalyzed carbon nanofiber growth

Catalyst–support interactions are critical in CVD processes for nanotube synthesis. In this article, the relative contributions of the catalyst electronic structure and support chemical composition are evaluated with Cu, Fe and Ni as catalysts and Al₂O₃, CaO, SiO₂ and TiO₂ as support media. The impact of the interaction is judged qualitatively based on nanotube growth and structure. Results are interpreted in terms of electron charge donation to the metal nanoparticle enabled by either strong-metal support interaction (SMSI) or by interaction of the catalyst nanoparticle with exposed Lewis base sites on the support material. The role of the physical structure of the support medium is explored by comparison of nanotubes grown upon powdered and fumed phases of the support oxides. Carbon nanotubes catalyzed by metal nanoparticles generated in-situ or preformed illustrate the advantage of presynthesized particles for size uniformity with attendant greatly lessened dependence upon catalyst preparation conditions. Catalyst retention and dispersion under rapid heating conditions is evaluated for the same support-catalyst systems listed above as a preliminary test for flame synthesis. Results show that SMSI interaction is critical to using the supported catalyst method in a flame.     

Activity,selectivity, and methanol tolerance of novel carbon-supported Pt and Pt<Subscript>3</Subscript>Me (Me = Ni,Co) cathode catalysts

The activity, selectivity, and methanol tolerance of novel, carbon supported high-metal loading (40 wt.%) Pt/C and Pt₃Me/C (Me = Ni, Co) catalysts for the O₂ reduction reaction (ORR) were evaluated in model studies under defined mass transport and diffusion conditions, by rotating (ring) disk and by differential electrochemical mass spectrometry. The catalysts were synthesized by the organometallic route, via deposition of pre-formed Pt and Pt₃Me pre-cursors followed by their decomposition into metal nanoparticles. Characteristic properties such as particle sizes, particle composition and phase formation, and active surface area, were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For comparison, commercial Pt/C catalysts (20 and 40 wt.%, E-Tek, Somerset, NJ, USA) were investigated as well, allowing to evaluate Pt loading effects and, by comparison with the pre-cursor-based catalyst with their much smaller particle sizes (1.7 nm diameter), also particle size effects. Kinetic parameters for the ORR were evaluated; the ORR activities of the bimetallic catalysts and of the synthesized Pt/C catalyst were comparable and similar to that of the high-loading commercial Pt/C catalyst; at typical cathode operation potentials H₂O₂ formation is negligible for the synthesized catalysts. Due to their lower methanol oxidation activity the bimetallic catalysts show an improved methanol tolerance compared to the commercial Pt/C catalysts. The results indicate that the use of very small particle sizes is a possible way to achieve reasonably good ORR activities at an improved methanol tolerance at DMFC cathode relevant conditions.     

Study of electrochemical bleaching of <Emphasis Type="Italic">p</Emphasis>-nitrosodimethylaniline and its role as hydroxyl radical probe compound

In the present paper, research on the electrochemical bleaching of p-nitrosodimethylaniline (RNO) in different electrolyte systems is presented with special attention to the role of RNO as a selective hydroxyl radical probe compound. At a Ti/Pt₉₀–Ir₁₀ anode, RNO was found to be bleached in 0.050 M sodium sulphate electrolyte due to lattice active oxygen without hydroxyl radicals being intermediately present. In 0.050 M sodium chloride, the bleaching rate was greatly enhanced due to indirect bulk oxidation by active chlorine species, again without the presence of hydroxyl radicals in the oxidation mechanisms. Under galvanostatic electrolysis, a linear relationship was found between the concentration of added chloride to a supporting sodium sulphate electrolyte and the first order rate constant of the bleaching reaction, showing the importance of the indirect bulk chlorine bleaching in chloride electrolyte systems. In this fashion both the chemically bonded active oxygen and the chemical bulk oxidation by active chlorine species proved to be valid bleaching pathways of RNO that according to these findings cannot be regarded as a fully selective hydroxyl radical probe compound. In addition, the difference in the mechanisms of chloride electrolysis at Ti/Pt₉₀–Ir₁₀ and Si–BDD anodes was clearly demonstrated using t-BuOH as hydroxyl radical scavenger.     

Activity-stability relationships of ordered and disordered alloy phases of Pt3Co electrocatalysts for the oxygen reduction reaction (ORR)

We report on synthesis-structure-activity-stability relationships of Pt₃Co nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). We have synthesized Pt₃Co alloy electrocatalysts using liquid impregnation techniques followed by reductive annealing at high and low temperatures. We have performed detailed structural X-ray diffraction (XRD)-based structural characterization (symmetry, lattice parameters and composition) of individual Pt-Co alloy phases before and, importantly, after electrochemical rotating disk electrode (RDE) measurements. This enables us to directly evaluate the corrosion stability of various Pt-Co alloy phases under typical fuel cell cathode conditions.Pt₃Co prepared at low annealing temperatures (600 °C) resulted in multiple phases including (i) a disordered face-centered cubic (fcc) Pt₉₅Co₅ phase and (ii) an ordered face-centered tetragonal (L1₀) Pt₅₀Co₅₀ phase; high temperature annealing (950 C) resulted in a single ordered primitive cubic (L1₂) Pt₃Co phase. The ordered alloy phases in both catalysts were not stable under electrochemical treatment: The ordered face-centered tetragonal (fct) phase showed corrosion and dissolution, while the ordered primitive cubic (L1₂) Pt₃Co phase transformed into a disordered structure. The ordered primitive cubic structure exhibited higher resistance to sintering.Low annealing temperatures resulted in higher Pt surface-area specific activities for ORR. Kinetic Tafel analysis confirmed a general shift in the formation potential of oxygenated surface species, such as Pt-OH, for both alloy catalysts. Reduced OH coverage alone proved insufficient to account for the observed activity trends of the two alloy catalysts.     

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.     

Further insights into the durability of Pt3Co/C electrocatalysts: Formation of “hollow” Pt nanoparticles induced by the Kirkendall effect

This paper provides further insights into the degradation mechanisms of nanometer-sized Pt₃Co/C particles under various proton-exchange membrane fuel cell (PEMFC) operating conditions. We confirm that Co atoms are continuously depleted from the mother Pt₃Co/C electrocatalyst because they can diffuse from the bulk to the surface of the material. The structure of the Pt–Co/C nanoparticles in the long-term is determined by a balance between Co surface segregation and formation of oxygenated species from water splitting. When the PEMFC is operated at high current density (low cathode potential, below the onset of surface oxide formation from water), a steady-state is reached between the rate of Co dissolution at the surface and Co surface segregation. Consequently, Co and Pt atoms remain homogeneously distributed within the Pt–Co/C particles and the thickness of the Pt-shell is maintained to a small value not detectable by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. When the PEMFC is operated at low current density (high cathode potential), the formation of surface oxides from water and the resulting “place-exchange” mechanism enhance the rate of diffusion of Co atoms to the surface. Consequently, the fresh Pt₃Co/C particles form core/shell particles with thick Pt-shells and Co content < 5 at% and, ultimately, “hollow” Pt nanoparticles (Kirkendall effect). To the best of our knowledge, this is the first report on the formation of “hollow” Pt particles in a PEMFC.