Pt–rGO–TiO2 nanocomposite by UV-photoreduction method as promising electrocatalyst for methanol oxidation

Pt/TiO₂-decorated reduced graphene oxide composite as catalyst for methanol electro-oxidation with three phase junction structure has been synthesized by UV-photoreduction (denoted as p-Pt/rGO@TiO₂). The obtained p-Pt/rGO@TiO₂ has been detailedly characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperometry (CA). XRD and TEM characterizations indicate that photoreduction is favorable to anchoring Pt nanoparticles (NPs) (ca. 2.2 nm) at the interface between TiO₂ and reduced graphene oxide (rGO), and forming the Pt, TiO₂ and rGO three phase junction structure. P-Pt/rGO@TiO₂ exhibits a higher activity for methanol electro-oxidation than m-Pt/rGO and m-Pt/rGO@TiO₂ (prepared by microwave-assisted polyol process). Lifetime tests demonstrate that the electrochemical durability of p-Pt/rGO@TiO₂ is improved by a factor of 2 or more as compared with m-Pt/rGO and m-Pt/rGO@TiO₂. XPS characterizations of p-Pt/rGO@TiO₂ reveal stronger interaction between Pt and support hybrid compared with m-Pt/rGO@TiO₂, which facilitates poisoning species removal and prevents Pt nanoparticles from migrating/agglomerating on or detaching from carbon support. This provides a facile and promising strategy to improve both the activity and durability of electrocatalysts for DMFCs.     

Methanol and ethanol oxidation on carbon supported nanostructured Cu core Pt–Pd shell electrocatalysts synthesized via redox displacement

Six different carbon-supported Cu core Pt–Pd shell (Cu@Pt–Pd) catalysts have been successfully synthesized by the galvanic replacement of Cu atoms by Pt⁴⁺ and Pd²⁺ ions at room temperature and their electrocatalytic activity for methanol and ethanol oxidation have been evaluated in acid media. Cu@Pt–Pd core shell nanoparticles with a narrow size distribution and an average diameter in the range of 3.1–3.5 nm were generated onto the carbon support. The compositional and the structural analysis of the as-prepared materials pointed out that the nanoparticles are formed by a Cu rich core covered by a Pt–Pd rich shell due to the interdiffusion of the metals after the galvanic replacement reaction. The electrocatalytic properties of the Cu@Pt–Pd electrodes in the electro-oxidation of methanol and ethanol was found to be dependent on the electrochemical surface area, lattice strain of the surface, composition and thickness of the Pt–Pd shell surrounding the Cu core. The optimum catalyst composition to obtain the best performance for methanol and ethanol electro-oxidation was determined to be Pt_0.59Pd_0.324Cu_0.167/C (6.2 wt.% Pt, 2.2 wt.% Pd and 0.7 wt.% Cu). This catalyst has a greatly enhanced mass activity, lower onset potential and poisoning rate, and higher turnover number in the MOR and EOR reactions compared to a commercial Pt_0.51Ru_0.49/C (20 wt.% Pt and 10 wt.% Ru). Consequently, this simple preparation method is a viable approach to making a highly active catalyst with low platinum content for application in direct alcohol fuel cells (DAFCs).     

Use of Pd–MnMoO4–graphene hybrids as efficient and CO poisoning tolerant electrocatalysts for methanol oxidation

40 wt%Pd-x wt%MnMoO₄/Graphene (GNS) (0 ≤ x ≤ 20) hybrids have been synthesized for use as efficient and CO poisoning tolerant anode materials in methanol fuel cells. Investigations revealed that the addition of MnMoO₄ increases the electrocatalytic activity of the base electrode (40 wt%Pd/GNS) towards the methanol oxidation reaction (MOR) in 1 M KOH significantly. The catalytic activity of the electrode is found to be the greatest with 8 wt%MnMoO₄. The addition of MnMoO₄ also improves CO poisoning tolerance of the base electrode by 11–73%. The MOR activity and CO poisoning tolerance of the 40 wt%Pd-8 wt%MnMoO₄/GNS hybrid electrode were superior to other electrodes of the investigation.     

3D V–Ni3S2@CoFe-LDH core-shell electrocatalysts for efficient water oxidation

Studying cheap and efficient electrocatalysts is of great significance to promote the sluggish kinetics of oxygen evolution reaction (OER). Here, we adopted a simple two-step method to successfully prepare the 3D V–Ni₃S₂@CoFe-LDH core-shell electrocatalyst. The V–Ni₃S₂@CoFe-LDH/NF shows excellent OER performance with low overpotential (190 mV at 10 mA/cm² and 240 mV at 50 mA/cm²), small Tafel slope (26.8 mV/dec) and good long-term durability. Excitingly, to reach the same current density, V–Ni₃S₂@CoFe-LDH/NF electrode even needs much smaller overpotential than RuO₂. Furthermore, the outstanding OER activity of V–Ni₃S₂@CoFe-LDH/NF is ascribed to the following reasons: (1) V–Ni₃S₂ nanorod cores improve the conductivity and ensure the fast charge transfer; (2) CoFe-LDH nanosheets interconnected with each other provide more exposed active sites; (3) the unique 3D core-shell structures are favorable for electrolyte diffusion and gas releasing. Our work indicates that building 3D core-shell heterostructure will be a useful way to design good electrocatalysts.     

Electrochemical oxidation of alcohols on Pt–TiO2 binary electrodes

In this study Pt–TiO₂ binary electrodes were prepared by means of thermal decomposition of chloride precursors on Ti substrates, characterised by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), electrochemical techniques and CO stripping and used as anodes for alcohol oxidation. The minimization of the Pt loading without electrocatalytic activity losses was also explored. TiO₂ was chosen due to its chemical stability, low cost and excellent properties as substrate for Pt dispersion. It was found that TiO₂ loading up to 50% results in Electrochemically Active Surface (EAS) increase. The EAS of Pt(50%)-TiO₂(50%) was found to be almost one order of magnitude higher than that of pure Pt while the EAS of samples with Pt loading lower than 30% was negligible. The above conclusion has been confirmed both by following the charge of the reduction peak of platinum oxide and by CO stripping experiments. All samples have been evaluated during the electrochemical oxidation of methanol and ethanol. In both cases the Pt(50%)-TiO₂(50%) electrode had better electrocatalytic activity than the pure Pt anode. The observed higher performance of the binary electrodes was mainly attributed to the enhanced Pt dispersion as well as the formation of smaller Pt particles by the addition of TiO₂.     

Heterogeneous Ir3Sn–CeO2/C as alternative Pt-free electrocatalysts for ethanol oxidation in acidic media

For reducing the Pt usage and driving down the cost of fuel cells, it is urgent to develop alternative Pt-free catalysts with high catalytic performance. In this study, an Ir₃Sn–CeO₂/C heterogeneous catalyst is designed as low-price, alternative Pt-free electrocatalyst towards ethanol oxidation reaction (EOR) in acidic conditions. Owing to the strong synergistic effect among Ir, Sn and CeO₂ components, Ir₃Sn–CeO₂/C heterogeneous catalyst exhibits higher catalytic activity and stability for EOR in comparison with commercial Pt/C, as-prepared Ir/C and Ir₃Sn/C. Additionally, kinetics and mechanisms of EOR are also investigated. It proves that ethanol electrooxidation on Ir₃Sn–CeO₂/C catalyst is a diffusion controlled irreversible process. Meanwhile, the H₂SO₄ and ethanol concentrations can affect the EOR activity. All results demonstrate Ir₃Sn–CeO₂/C heterogeneous catalyst is a promising Pt-free choice for EOR.     

CeO2-supported Pt–Cu alloy nanoparticles synthesized by radiolytic process for highly selective CO oxidation

CeO₂-supported Pt–Cu bimetallic catalysts were synthesized by radiolytic process and their PROX activities were evaluated in relation to structural properties of the catalysts. Irradiating the aqueous precursor solution yielded Pt–Cu alloy nanoparticles and amorphous-like CuO on CeO₂ which are thermodynamically stable products formed from reduced Pt and Cu. Addition of Cu to Pt significantly improved CO selectivity in PROX reaction. The Pt–Cu catalysts had wide temperature window for 100% CO conversion in contrast to very narrow window for monometallic Pt and Cu catalysts. Much lower light-off temperature for Pt–Cu catalysts than Cu catalyst revealed that Pt-Cu alloy surface is the active center. Regardless of the amount of CuO phase, the bimetallic catalyst exhibited high catalytic performance, which further revealed that Cu in close contact with Pt is responsible for the improved selectivity. The CuO phase was suggested to promote oxygen supply to CO chemisorbed on Pt–Cu alloy surface.     

Pd decorated Co–Ni nanowires as a highly efficient catalyst for direct ethanol fuel cells

The storage and conversion of energy necessitates the use of appropriate electrochemical systems and chemical reaction catalysts. This work presents newly developed catalysts for electrooxidation of ethanol in an alkaline medium. Nanocatalysts composed of Co–Ni nanowires (Co–Ni NWs) decorated with Pd nanoparticles (Pd NPs) were made at varying metal ratios and their chemical composition and structure was investigated in detail. The synthesis involved a wet chemical reduction assisted by a magnetic field, which led to the generation of NWs, followed by the deposition of spherical Pd NPs on their surface. The best catalytic activity was obtained for the catalyst made of Co₃–Ni₇ decorated with Pd NPs, which exhibited EOR of 8003 mA/mg_Pd for only 0.86 wt% of Pd loading. The results can be explained by the synergistic effect between the morphology of the bimetallic support and the favorable interaction of oxophilic Co, Ni with catalytic Pd.     

Comparison of structure and catalytic performance of Pt–Co and Pt–Cu bimetallic catalysts supported on Al2O3 and CeO2 synthesized by electron beam irradiation method for preferential CO oxidation

In order to investigate the effect of transition metal addition to platinum with different support materials on preferential CO oxidation, structure and chemical properties of supported bimetallic catalysts prepared by electron beam irradiation method were correlated to the catalytic performance. On Al₂O₃, decoration of Pt by small amount of Co (Co/Pt ∼ 0.03) drastically increased CO and O₂ conversions while addition of equimolar Cu to Pt increased them only above 100 °C, where the rate-controlling factor was suggested to change from oxygen transport to CO activation. On CeO₂, either addition of Co or Cu to Pt had minor or negative effect on high O₂ conversion inherent to high oxygen transport at Pt–CeO₂ interface. On Pt–Cu/CeO₂, however, metal-CuO_x interface dominates the reaction characteristics to give improved selectivity, which is suitable for deep CO removal in excess O₂/CO condition. The order of selectivity above 100 °C, Pt–CoO_x > Pt(alloy)–CuO_x > Pt–CeO₂ interfaces, was derived from structural analysis and catalytic tests.     

Novel Ni–ZrO2 catalyst doped with Yb2O3 for ethanol steam reforming

A type of Yb₂O₃ doped Ni–ZrO₂ catalyst for ethanol steam reforming was developed, and displayed excellent catalyzing performance for the selective formation of H₂ and CO₂. Over a Ni_1.25Zr₁Yb_0.8 catalyst, STY(H₂) can maintain stable at the level of 0.396 mol h⁻¹ g⁻¹ (data taken 120 h after the reaction started) under the reaction conditions of 0.5 MPa and 723 K, which was 1.6 times that (0.247 mol h⁻¹ g⁻¹) of the Yb-free counterpart Ni_1.25Zr₁. Characterization of the catalyst revealed that dissolution of an appropriate amount of Yb³⁺ ions in the zirconia host resulted in the formation of the Zr–Yb composite oxide with cubic-ZrO₂ structure, c-(Zr–Yb)O_z, which inhibited effectively the transformation of c-ZrO₂ to thermodynamically more stable m-ZrO₂, thus avoiding sintering of the (Zr–Yb)O_z composite. It was demonstrated that the doping of Yb₂O₃ to Ni–ZrO₂ changed also the valence states or the micro-environments of the Ni-species at the quasi-active surface of the tested catalyst, which was conducive to inhibiting agglomeration of the Ni_x⁰–Niⁿ⁺ species active catalytically, with resulting in maintaining the high metallic nickel dispersion and inhibiting coking. The aforementioned two factors both contributed to improving the activity and operating stability as well as heat-resistant quality of the catalyst.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Pt–Ni bimetallic composite nanocatalysts prepared by using multi-walled carbon nanotubes as reductants for ethanol oxidation reaction

The multi-walled carbon nanotubes (MWNTs) are introduced as reductants to prepare bimetallic PtNi_x composite nanocatalysts via a hydrothermal reaction for the investigation of ethanol oxidation reaction (EOR). The crystal structure and elemental analysis of PtNi_x/MWNTs nanocatalysts are characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), respectively. The morphologies of these nanocatalysts are observed by transmission electron microscopy (TEM) and scanning electron microscope (SEM). The results reveal that the PtNi_x nanocatalysts with a nanoparticle size ranging from 6 to 13 nm are immobilized on the surface of MWNTs. The electrocatalytic activities of the PtNi_x/MWNTs nanocatalysts for EOR in alkaline media are examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). The onset potential of EOR in the nanocatalyst of PtNi₃/MWNTs is negatively shifted for about 190 mV as compared to that in the nanocatalyst of Pt/MWNTs. The current for the forward anodic peak of EOR in the PtNi₃/MWNTs nanocatalyst is about 2.5 times higher than that of pure Pt/MWNTs.     

The graphene-meso-macroporous SiO2 supported Pt–Ni alloy nanocatalyst for preferential oxidation of CO in H2-rich gases

In this work, nanoparticles of Pt–Ni alloy were supported on a new kind of composite which composed of graphene sheets and meso-macroporous SiO₂, and the composite supported Pt–Ni catalyst was applied to the preferential oxidation of CO (CO-PROX) in H₂-rich gases. The bimetallic Pt–Ni alloy catalyst was characterized by using techniques of SEM, TEM, XRD, TPR, CO chemisorptions and XPS. The catalyst showed excellent catalytic performance for CO-PROX with high activity at low temperature, high selectivity and very good stability, which was attributed to the following characters of the catalyst: Pt–Ni nanoparticles were in alloy state and highly dispersed, Pt–Ni nanoparticles were preferentially loaded on the surface of graphene sheets, and the meso-macroporosity of the composite markedly improved the mass transferring ability. This is a case study, and this kind of catalysts can be extended to other gas–solid catalytic reactions.     

Hydrogen production by steam reforming of ethanol over mesoporous Ni–Al2O3–ZrO2 aerogel catalyst

A mesoporous Ni–Al₂O₃–ZrO₂ aerogel (Ni–AZ) catalyst was prepared by a single-step epoxide-driven sol–gel method and a subsequent supercritical CO₂ drying method. For comparison, a mesoporous Al₂O₃–ZrO₂ aerogel (AZ) support was prepared by a single-step epoxide-driven sol–gel method, and subsequently, a mesoporous Ni/Al₂O₃–ZrO₂ aerogel (Ni/AZ) catalyst was prepared by an incipient wetness impregnation method. The effect of preparation method on the physicochemical properties and catalytic activities of Ni–AZ and Ni/AZ catalysts was investigated. Although both catalysts retained a mesoporous structure, Ni/AZ catalyst showed lower surface area than Ni–AZ catalyst. From TPR, XRD, and H₂–TPD results, it was revealed that Ni–AZ catalyst retained higher reducibility and higher nickel dispersion than Ni/AZ catalyst. In the hydrogen production by steam reforming of ethanol, both catalysts showed a stable catalytic performance with complete conversion of ethanol. However, Ni–AZ catalyst showed higher hydrogen yield than Ni/AZ catalyst. Superior textural properties, high reducibility, and high nickel surface area of Ni–AZ catalyst were responsible for its enhanced catalytic performance in the steam reforming of ethanol.     

Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe₂O₃) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm². The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm² at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.     

A highly dispersed Pt/γ-Al2O3 catalyst prepared via deposition–precipitation method for preferential CO oxidation

Highly dispersed Pt/γ-Al₂O₃ catalysts were prepared by deposition–precipitation (DP) method with precursor solutions of various pH. The pH was controlled from 6.5 to 9.5 with 5 wt% NaOH solution. As the pH of precursor solution increases over pH 7.5, the metal dispersion and surface PtO_x species decrease and the Pt particle size increases. PrOx test was carried out with a space velocity of 60,000 mL/h g_cat in temperature ranges from 100 to 200 °C. The [O₂]/[CO] ratio was adjusted between 1 and 2 and the effect of H₂O and CO₂ was examined at [O₂]/[CO] = 2. It is interesting that the CO conversion has good agreement with the Pt metal dispersion. In addition, highly dispersed Pt/γ-Al₂O₃ catalyst prepared by DP with pH 7.5 exhibited good catalytic activity below 150 °C in PrOx due to the improvement of the metal dispersion and reducibility of surface PtO_x species at low temperatures compared with the catalyst prepared by impregnation method.     

A novel catalyst–sorbent system for an efficient H2 production with in-situ CO2 capture

This paper presents an experimental investigation for an improved process of sorption-enhanced steam reforming of methane in an admixture fixed bed reactor. A highly active Rh/Ce_αZr_1−αO₂ catalyst and K₂CO₃-promoted hydrotalcite are utilized as novel catalyst/sorbent materials for an efficient H₂ production with in situ CO₂ capture at low temperature (450–500 °C). The process performance is demonstrated in response to temperature (400–500 °C), pressure (1.5–6.0 bar), and steam/carbon ratio (3–6). Thus, direct production of high H₂ purity and fuel conversion >99% is achieved with low level of carbon oxides impurities (<100 ppm). A maximum enhancement of 162% in CH₄ conversion is obtained at a temperature of 450 °C and a pressure of 6 bar using a steam/carbon molar ratio of 4. The high catalyst activity of Rh yields an enhanced CH₄ conversion using much lower catalyst/sorbent bed composition and much smaller reactor size than Ni-based sorption enhanced processes at low temperature. The cyclic stability of the process is demonstrated over a series of 30 sorption/desorption cycles. The sorbent exhibited a stable performance in terms of the CO₂ working sorption capacity and the corresponding CH₄ conversion obtained in the sorption enhanced process. The process showed a good thermal stability in the temperature range of 400–500 °C. The effects of the sorbent regeneration time and the purge stream humidity on the achieved CH₄ conversion are also studied. Using steam purge is beneficial for high degree of CO₂ recovery from the sorbent.     

Catalytic partial oxidation of CH4–H2 mixtures over Ni foams modified with Rh and Pt

The catalytic partial oxidation (CPO) of methane–hydrogen mixtures in air, intended for the first stage of hybrid radiant catalytic burners, was investigated under self-sustained short contact time conditions on commercial Ni foam catalysts eventually modified with Rh and Pt. The modified catalysts were prepared by a simple novel method based on the spontaneous deposition of noble metals via metal exchange reactions onto those Ni foam substrates. SEM-EDS, electrochemical methods and H₂-TPR analysis were integrated to characterize morphology, surface area of metal deposits and reducibility of foam catalysts before and after exposure to severe conditions in the CPO reactor. In particular Rh forms finely dispersed deposits that retain their high specific surface area at temperatures up ca. 1100 °C. Modification with noble metals enhances stability and reducibility of the Ni foam whereas the overall CPO performance is not significantly improved. Safe operation of the CPO reactor with up to 70% vol. H₂ in the fuel mixture has been achieved by properly increasing the feed equivalence ratio to avoid catalyst overheating, while guaranteeing high methane conversions and a persistent net hydrogen production.     

Three-dimensional reduced graphene oxide–Mn3O4 nanosheet hybrid decorated with palladium nanoparticles for highly efficient hydrogen evolution

A three-dimensional (3D) reduced graphene oxide–Mn₃O₄ nanosheet (Mn₃O₄@rGO) hybrid was achieved by simple electrodeposition technique. Small palladium nanoparticle were homogeneously anchored onto Mn₃O₄@rGO substrate through the reduction of palladium salt. The interpenetrating network architecture of Mn₃O₄@rGO greatly inhibited the aggregation of 2D sheets of Mn₃O₄ and rGO, and the open 3D orientation of the Mn₃O₄@rGO hybrid nanosheets on the electrode facilitated both mass transport and electron transfer as well as maximally exposed active sites. The introduction of Mn₃O₄ enhanced the structural and electrochemical stability of rGO. The as-synthesized Pd/Mn₃O₄@rGO hybrid was employed as an electrocatalyst for electrocatalytic hydrogen evolution reaction (HER). The electrocatalyst showed a low overpotential of 20 mV at 10 mA cm^?2, a small Tafel slope of 48.2 mV dec^?1, and a large exchange current density of 0.59 mA cm^?2. Importantly, the catalyst possessed superior durability with 85.87% of catalytic activity after a long-time test (10 h). This work presents a simple and efficient stratagy to construct high-performance electrocatalysts for energy and environmental applications.