Carbon nanotubes-promoted Co–B catalysts for rapid hydrogen generation via NaBH4 hydrolysis

In this work, multiwalled carbon nanotubes (MWCNTs) promoted Co–B catalysts for NaBH₄ hydrolysis have been designed and synthesized. The structural features of as-prepared catalysts have been investigated and discussed as a function of MWCNTs contents by X-ray diffraction, X-ray photoelectron spectra, N₂ adsorption/desorption isotherms, scanning electron microscope. The results show that the catalysts still maintain an amorphous structure with the addition of carbon nanotubes promoter. However, the appropriate amount of MWCNTs promoter in Co–B catalysts leads to large specific surface area, fine dispersion of active components, increased active sites and high electron density at active sites. Moreover, hydrogen spillover on the catalyst is promoted, which contributes to regeneration of active sites and accelerating catalytic cycle. Among all the experimental samples, it is found that the Co–B catalyst promoted by 10 wt% carbon nanotubes exhibits optimal catalytic activity with remarkably high hydrogen generation rate of 12.00 L min⁻¹·g_catalyst⁻¹ and relatively good stability.     

Pd nanoparticles supported on functionalized multi-walled carbon nanotubes (MWCNTs) and electrooxidation for formic acid

To improve the utilization and activity of anodic catalysts for formic acid electrooxidation, palladium (Pd) particles were loaded on the MWCNTs, which were functionalized in a mixture of 96% sulfuric acid and 4-aminobenzenesulfonic acid, using sodium nitrite to produce intermediate diazonium salts from substituted anilines. The composition, particle size, and crystallinity of the Pd/f-MWCNTs catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) measurements. The electrocatalytic properties of the Pd/f-MWCNTs catalysts for formic acid oxidation were investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in 0.5 mol L⁻¹ H₂SO₄ solution. The results demonstrated that the catalytic activity was greatly enhanced due to the improved water-solubility and dispersion of the f-MWCNTs, which were facile to make the small particle size (3.8 nm) and uniform dispersion of Pd particles loading on the surface of the MWCNTs. In addition, the functionalized MWCNTs with benzenesulfonic group can provide benzenesulfonic anions in aqueous solution, which may combine with hydrogen cation and then promote the oxidation of formic acid reactive intermediates. So the Pd/f-MWCNTs composites showed excellent electrocatalytic activity for formic acid oxidation.     

Development of Pd and Pd-Co catalysts supported on multi-walled carbon nanotubes for formic acid oxidation

Pd-Co and Pd catalysts were prepared by the impregnation synthesis method at low temperature on multi-walled carbon nanotubes (MWCNTs). The nanotubes were synthesized by spray pyrolysis technique. Both catalysts were obtained with high homogeneous distribution and particle size around 4 nm. The morphology, composition and electrocatalytic properties were investigated by transmission electron microscopy, scanning electron microscopy-energy dispersive X-ray analysis, X-ray diffraction and electrochemical measurements, respectively. The electrocatalytic activity of Pd and PdCo/MWCNTs catalysts was investigated in terms of formic acid electrooxidation at low concentration in H₂SO₄ aqueous solution. The results obtained from voltamperometric studies showed that the current density achieved with the PdCo/MWCNTs catalyst is 3 times higher than that reached with the Pd/MWCNTs catalyst. The onset potential for formic acid electrooxidation on PdCo/MWCNTs electrocatalyst showed a negative shift ca. 50 mV compared with Pd/MWCNTs.     

Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites

Hydrogen storage capacity of a pristine multi-walled carbon nanotubes is increased 10-fold at 298 K and an equilibrium hydrogen pressure of ～23 atm, upon addition of a hydrogen spillover catalyst cobalt- and copper oxide, from 0.09 to 0.9 wt.%. In situ reduction method is utilized to synthesize Co-oxide/MWCNTs and Cu-oxide/MWCNTs composite. Blocking of channels and pores of MWCNTs by oxide nanoparticles during preparation method is responsible for low BET specific surface area of composites compared to pristine sample. X-ray diffraction, scanning, and transmission electron microscopy demonstrates nanostructural characterization of MWCNTs and composites. Thermogravimetric analysis of two oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage of both composites is attributed to the spillover mechanism due to decoration of Co and Cu-oxide nanoparticles on the outer surface of MWCNTs.     

Enhanced electrochemical hydrogen storage by catalytic Fe-doped multi-walled carbon nanotubes synthesized by thermal chemical vapor deposition

Hydrogen storage capacities of raw, oxidized, purified and Fe-doped multi-walled carbon nanotubes (MWCNTs) were studied by electrochemical method. Based on transmission electron microscopy and Raman spectroscopic data, thermal oxidation removed defective graphite shells at the outer walls of MWCNTs. The analysis results indicated that the acid treatment dissolved most of the catalysts and opened some tips of the MWCNTs. Thermal gravimetric analysis and differential scanning calorimetry results illustrated that by oxidation and purification of MWCNTs, the weight loss peak shifts toward a higher temperature. N₂ adsorption isotherms of the purified and oxidized MWCNTs showed an increase in N₂ adsorption below P/P_o = 0.05, suggesting that microporous structures exist in the purified and oxidized MWCNTs. The electrochemical results revealed that the Fe-doped MWCNTs produced the highest hydrogen storage capacities compared to the other samples in various sweep rates. According to electrochemical analyses, the peak currents of hydrogen adsorption/desorption increased by increasing the catalyst's active surface.     

Hydrogen co-reduction synthesis of PdPtNi alloy nanoparticles on carbon nanotubes as enhanced catalyst for formic acid electrooxidation

A novel electrocatalyst of PdPtNi ternary alloy nanoparticles supported on multi-walled carbon nanotubes (MWCNTs) for formic acid oxidation (FAO) reaction is synthesized by a simple hydrogen co-reduction process. The as-synthesized catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). It is found that highly dispersed PdPtNi alloy nanoparticles of ca. 2.56 nm are homogeneously deposited on the MWCNT surface, and the alloying with Pt and Ni alters the electronic structure of Pd atoms with the downshift of Pd d-band center. Studies of cyclic voltammetry and chronoamperometry indicate that the electrocatalytic activity and durability of the PdPtNi/MWCNT for FAO are significantly enhanced as compared with the PdPt/MWCNT and commercial Pd/C catalysts. This study implies that the prepared PdPtNi/MWCNT composite is a promising anode electrocatalyst of direct formic acid fuel cells.     

Carbon-catalyzed oxygen-mediated dehydrogenation of formaldehyde in alkaline solution for efficient hydrogen production

We report an efficient process to dehydrogenate formaldehyde in alkaline solution, catalyzed by carbon nanotubes (CNTs) via a unique reaction mechanism involving molecular O₂. The superior catalytic performance of carbon nanotubes (CNTs) compared to the other carbon-based catalysts is attributed to their sp²-carbon-rich surface, hydrophilicity and abundant surface defects, which are the most plausible active sites. Peroxide species originating from the activation of adsorbed molecular oxygen on the CNTs is found to be a key to C–H activation, leading to efficient hydrogen production. The cost-effective carbon-based dehydrogenation catalysts offer new opportunities to the development of novel liquid organic hydrogen carrier technologies.     

ZrO2 anchored core-shell Pt–Co alloy particles through direct pyrolysis of mixed Pt–Co–Zr salts for improving activity and durability in proton exchange membrane fuel cells

Highly active and durable Pt-based catalysts for oxygen reduction reaction (ORR) are very important and necessary for the proton exchange membrane fuel cells (PEMFCs). In this paper, we report the preparation and performance study of ORR catalysts composed of core-shell Pt–Co alloy nanoparticles (NPs) on multi-walled carbon nanotubes (MWCNTs) anchored with ZrO₂ NPs (denoted as Pt–Co–ZrO₂/MWCNTs). Thanks to the unique three-phase structure, the mass activity of Pt–Co–ZrO₂/MWCNTs for ORR at 0.9 V versus reversible hydrogen electrode (RHE) is1577 mA mg_Pt^?1, which is ～6.6-fold higher than that of the commercial Pt/C (238 mA mg_Pt^?1). After 50,000 cycles for durability test, the mass activity of Pt–Co–ZrO₂/MWCNTs for ORR remained 88% of its initial value. In stark contrast, that of Pt/C kept only about 56.3% of its initial value. More importantly, its catalytic performance was fully observed/verified in a H₂-air PEMFC single cell test. When the Pt loading of Pt–Co–ZrO₂/MWCNTs loaded cathode was one fourth of that with commercial Pt/C as the cathode catalyst, comparable cell performance was achieved. More impressively, the MEA with Pt–Co–ZrO₂/MWCNTs underwent only 24.5% degradation in maximum power density after 30,000 accelerated durability tests (ADTs). However, the MEA with Pt/C after 30,000 ADTs exhibited 39.6% performance loss in maximum power density. The enhanced mass activity and catalytic durability of Pt–Co–ZrO₂/MWCNTs could be attributed to the core-shell Pt–Co alloy NPs with Pt-rich surface and the interface effect between Pt–Co alloy NPs and oxygen vacancy-rich ZrO₂ NPs. In addition, this research also provided a solution to the durability issue of cathodes without sacrificing ORR mass activity, which would promote practical application of PEMFCs.     

Structure and catalytic properties of Ni/MWCNTs and Ni/AC catalysts for hydrogen production via ammonia decomposition

The structure and catalytic properties of nickel catalysts supported on multi-wall carbon nanotubes (MWCNTs) and on three different types of activated carbon (AC) were studied. The surface areas of AC carriers were defining the size of supported nickel particles. Large surface area of AC led to small Ni nanoparticles and high Ni dispersion. Turnover frequency (TOF_NH3) of ammonia decomposition decreased with decreasing of Ni particle size. The highest degree of ammonia conversion was observed on Ni/AC prepared by using of AC support with largest surface area. The catalytic activity of Ni/MWCNTs was much higher than catalytic activity of the studied Ni/AC catalysts. The synergic nickel-support interaction and special electronic conductivity properties of MWCNTs were responsible for high catalytic activity of Ni/MWCNTs catalyst.     

Photocatalytic hydrogen generation over alkaline-earth titanates in the presence of electron donors

The efficiency of alkaline-earth titanate-based compounds (Ca, Sr, Ba) for catalysts in photocatalytic hydrogen generation has been investigated. In this report, we have shown that the addition of organic donors (such as formic acid, acetic acid, methanol, 2-propanol and formaldehyde) enhanced the efficiency of the studied process. The systematic study has shown that the most efficient organic donor in regards to its hydrogen generation efficiency is formic acid. Of the catalysts explored, the highest photocatalytic activity was shown by SrTiO₃:TiO₂. Additionally, the effects of photocatalyst quantity and formic acid concentration on hydrogen evolution have been investigated.     

Ultrathin MoSSe alloy nanosheets anchored on carbon nanotubes as advanced catalysts for hydrogen evolution

The activity of transition metal dichalcogenides (TMD) toward hydrogen evolution reaction (HER) derives from the active sites at the edges, but the basal surface still remain catalytic insert. Herein, ultrathin MoSSe alloy nanosheets array on multiwalled carbon nanotubes (MWCNTs) to form a core shell structure via a simple solvothermal process. These three-dimensional (3D) MoSSe hybrids show a high activity in hydrogen evolution reaction (HER) with a small Tafel slope of 38 mV dec⁻¹ and a low overpotential of 102 mV at 10 mA cm⁻². In addition, their HER activity remains remarkably stable without significant decay after 100 h polarization. Such superior catalytic HER activity springs from the 3D hierarchical heterostructure, which is abundant of catalytic edge sites, and the alloy effect between S and Se, which will create huge defects and strain to form vacancy sites on the basal plane. This strategy may open a new avenue toward the development of nonprecious high-performance HER catalysts.     

Ultrasonically treated multi-walled carbon nanotubes (MWCNTs) as PtRu catalyst supports for methanol electrooxidation

In the quest of fabricating supported catalysts, experimental results of transmission electron microscopy, Raman and infrared spectroscopy indicate that ultrasonic treatment effectively functionalizes multi-walled carbon nanotubes (MWCNTs), endowing them with groups that can act as nucleation sites which can favor well-dispersed depositions of PtRu clusters on their surface. Ultrasonic treatment seems to be superior than functionalization via regular refluxing. This is confirmed by the determination of the electrochemistry active surface area (ECA) and the CO-tolerance performance of the PtRu catalysts, measured by adsorbed CO-stripping voltammetry in 0.5 M sulfuric acid solution, and the real surface area of the PtRu catalysts, evaluated by Brunauer–Emmett–Teller (BET) measurements. Finally, the effectiveness for methanol oxidation is assessed by cyclic voltammetry (CV) in a sulfuric acid and methanol electrolyte.     

Synthesis of well-dispersed Pt/carbon nanotubes catalyst using dimethylformamide as a cross-link

In synthesizing carbon nanotubes supported catalysts, a significant challenge is how to deposit metal nanoparticles uniformly on the surface of carbon nanotubes due to the inherent inertness of carbon nanotube walls. This study reports a facile procedure using N,N-dimethylformamide as a dispersant, ligand and reductant, with which Pt nanoparticles can be deposited uniformly on pristine carbon nanotubes. X-ray photoelectron spectroscopy measurements reveal that metallic Pt nanoparticles are successfully prepared with this method. Transmission electron microscopy and X-ray diffraction analyses confirm the formation of face-centered cubic crystal Pt particles with a size that ranges from 2.0 to 4.0 nm. The support-dependent catalytic properties of the prepared Pt catalyst are characterized by cyclic voltammetric studies of the formic acid electro-oxidation reaction. The results show that a pristine carbon nanotubes supported Pt catalyst has a higher catalytic activity than both carbon powder and modified carbon nanotubes supported catalysts.     

Porous carbon supported Pd as catalysts for boosting formic acid dehydrogenation

The design of efficient catalysts is the essential to realize formic acid (FA) as a hydrogen carrier. However, it remains a challenging task. Herein, the porous carbon was prepared using ZnCo-containing zeolitic imidazole frameworks (ZIFs) as a precursor, which supported Pd as an effective catalyst for FA dehydrogenation. Porous carbon containing Co and N was synthesized by one-step method, and the Co and N promoted the activity of Pd by modifying its electron state. The catalytic performance was further improved by doping Zn into the predesigned bimetallic ZnCo-ZIFs. The addition of Zn increased the dispersion of PdCo nanoparticles, N content and specific surface area of the catalysts. When Zn/Co molar ratio was 2, the prepared catalyst (Pd/Co@CN-2) with an average diameter of PdCo about 2.6 nm exhibited the best catalytic activity, showing an initial turnover frequency value (TOF) as high as 2302 h⁻¹ even at 30 °C.     

Highly dispersed Pd nanoparticles supported on 1,10-phenanthroline-functionalized multi-walled carbon nanotubes for electrooxidation of formic acid

Functionalization step is generally prerequisite to immobilize metal nanoparticles on multi-walled carbon nanotubes (MWCNTs) for production of a high efficient electrocatalyst. We herein report a novel method to functionalize MWCNTs with 1,10-phenanthroline (phen-MWCNTs) as a catalyst support for Pd nanoparticles. Raman spectroscopic analysis results reveal that this phen functionalization method can preserve the integrity and electronic structure of MWCNTs and provide the highly effective functional groups on the surface for Pd nanoparticles. According to the transmission electron microscopy (TEM) measurements, the as-prepared Pd nanop articles are evenly deposited on the surface of the phen-MWCNTs without obvious agglomeration, and the average particle size of the Pd nanoparticles is 2.3 nm. Electrochemical measurements demonstrate that the as-prepared Pd/phen-MWCNTs catalyst has a better electrocatalytic activity and stability for the oxidation of formic acid than Pd catalyst on acid-treated MWCNTs. It is concluded that the as-prepared Pd/phen-MWCNTs would be a potential candidate as an anode electrocatalyst in direct formic acid fuel cell (DFAFC).     

Hydrogen uptake of manganese oxide-multiwalled carbon nanotube composites

Hydrogen uptake of pristine multi-walled carbon nanotubes is increased more than three-fold at 298 K and hydrogen pressure of 4.0 MPa, upon addition of hydrogen spillover catalyst manganese oxide, from 0.26 to 0.94 wt%. Simple and convenient in situ reduction method is used to prepare Mn-oxide/MWCNTs composite. XRD, FESEM, and TEM demonstrates nanostructural characterization of pristine MWCNTs and composite. TGA analysis of Mn-oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage capacity of composite is attributed to spillover mechanism owing to decoration of Mn-oxide nanoparticles on outer surface of MWCNTs. Hydrogen uptake follows monotonous dependence on hydrogen pressure. Oxide-MWCNTs composite not only shows high hydrogen storage capacity as compared to pristine, but also exhibit significant cyclic stability upon successive adsorption–desorption cycles.     

Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells

A novel Pd electrocatalyst is developed by self-assembly of Pd nanopartilces on phosphomolybdic acid (HPMo)-poly(diallyldimethylammonium chloride) (PDDA)-functionalized multiwalled carbon nanotubes supports (Pd/HPMo-PDDA-MWCNTs). The as-synthesized Pd/HPMo-PDDA-MWCNTs were characterized by TEM, EDS mapping, Raman spectra, X-ray photoelectron spectroscopy, electrochmeical CO stripping and cyclic voltammetry techniques. Pd nnaoparticles deposited on HPMo-PDDA-MWCNTs are in the range of 3.1 nm with uniform distributon. Pd/HPMo-PDDA-MWCNT catalysts have lower overpotential for CO_ad oxidation manifested as lower peak and onset potentials as compared to acid-treated MWCNTs supported Pd (Pd/AO-MWCNTs) and carbon supported Pd catalysts (Pd/C). Pd/HPMo-PDDA-MWCNTs catalysts also exhibit a much higher electrocatalytic activity and stability for formic acid oxidation reaction as compared to that on Pd/AO-MWCNTs and Pd/C. The high electrocatalytic activities of Pd/HPMo-PDDA-MWCNTs catalysts are most likely related to highly dispersed and fine Pd nanoparticles as well as synergistic effects between Pd and HPMo immobilized on PDDA-functionalized MWCNTs.     

Pd nanoparticles supported on copper phthalocyanine functionalized carbon nanotubes for enhanced formic acid electrooxidation

The hydrothermal synthesis of a novel Pd electrocatalyst using copper phthalocyanine-3,4′,4″,4′″-tetrasulfonic acid tetrasodium salt (TSCuPc) functionalized multi-walled carbon nanotubes (MWCNTs) composite as catalyst support for Pd nanoparticles is reported. The prepared nanocomposites were characterized by UV–vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical tests. It is found that Pd nanoparticles are uniformly deposited on the surface of TSCuPc-MWCNTs, and their dispersion and electrochemical active surface area (ECSA) are significantly improved. Studies of cyclic voltammetry and chronoamperometry demonstrate that the Pd/TSCuPc-MWCNTs exhibits much higher electrocatalytic activity and stability than the Pd/AO-MWCNTs catalyst for formic acid oxidation. This study implies that the as-prepared Pd/TSCuPc-MWCNTs will be a promising candidate as an anode electrocatalyst in direct formic acid fuel cell (DFAFC).     

Graphene modified Co–B catalysts for rapid hydrogen production from NaBH4 hydrolysis

Graphene oxide (GO) modified Co–B catalysts for NaBH₄ hydrolysis have been synthesized by the chemical reduction in this work. The structural features and catalytic performance of as-prepared samples have been investigated and discussed as a function of amounts of GO. According to structure characterization, the catalysts still retain the amorphous structure of Co–B alloy with the addition of GO, while GO exists as reduced GO (r-GO). The textural analysis and morphology observation indicate that the appropriate amount of GO in Co–B catalyst results in the obvious increase of specific surface area and uniform clustered morphology, which contributes to improve active surface area for catalytic reactions. The results of surface species characterization show that the electron density at active Co sites increases due to an electron transfer from B to Co facilitated by r-GO. It has been found that 50 mg GO modified Co–B catalyst exhibits especially high activity with a hydrogen generation rate of 14.34 L min⁻¹·g_catalyst⁻¹ and much lower activation energy of 26.2 kJ mol⁻¹ for hydrolysis reaction of NaBH₄. Meanwhile, the reusability evaluations show that the catalyst preserves high stability which can still maintain 81.5% of its initial activity after 5 catalytic cycles.     

Ameliorated microstructure and hydrogen absorption/desorption properties of novel Mg–Ni–La alloy doped with MWCNTs and Co nanoparticles

Based on the positive influence of carbon materials and transition metals, a new type of Mg-based composites with particle size of ～800 nm has been designed by doping hydrogenated Mg–Ni–La alloy with multi-walled carbon nanotubes (MWCNTs) and/or Co nanoparticles. The microstructures, temperature related hydrogen absorption/desorption kinetics and dehydrogenation mechanisms are investigated in detail. The results demonstrate that MWCNTs and Co dispersedly distribute on the surface of Mg–Ni–La particles after high-energy ball milling due to powders’ repeated cold welding and tearing. The experimental samples exhibit improved hydrogen storage behaviors and the addition of MWCNTs and Co can further accelerate the de-/hydriding kinetics. For instance, the Mg–Ni–La–Co sample can absorb 3.63 wt% H₂ within 40 min at 343 K. Dehydrogenation analyses demonstrate that the positive effect of MWCNTs is more obvious than that of Co nanoparticles for the experimental samples. The addition of MWCNTs and Co leads to the average dehydrogenation activation energy of experimental samples decreasing to 82.1 and 84.5 kJ mol^?1, respectively, indicating a significant decrease of dehydrogenation energy barriers. In addition, analyses of dehydrogenation mechanisms indicate that the rate-limiting steps vary with the addition of MWCTNs and Co nanoparticles.