NH3 plasma synthesis of N-doped activated carbon supported Pd catalysts with high catalytic activity and stability for HCOOH dehydrogenation

Plasma is a simple and effective method to prepare N-doped carbon materials and supported metal catalysts. In this work, Pd/C–C(NH₃) and Pd/C–P(NH₃) catalysts are prepared by heat treatment and cold plasma methods using Ar and NH₃ as the working gas. The activity and stability of obtained catalysts are tested by formic acid dehydrogenation reaction. The results show that TOF_initial of Pd/C–P(NH₃) is 527.1 h⁻¹ at 50 °C, and the HCOOH decomposition rate is about 89.2% at 4 h. The hydrogen production of Pd/C–P(NH₃) when used in first and third cycle are 1.14 and 1.14 times than that of Pd/C–C(NH₃), and 1.24 and 13.24 times than that of commercial Pd/C. Various characterization techniques are used to characterize the structure of the prepared Pd/C catalysts. The results indicate that NH₃ plasma is milder than NH₃ thermal treatment. The high activity and stability of Pd/C–P(NH₃) are mainly due to the NH₃ cold plasma effectively achieving N-doping of the carbon support, and Pd nanoparticles with a small size and high dispersion. Atmospheric pressure NH₃ cold plasma provides an effective method to prepare high-performance Pd/C catalysts for HCOOH dehydrogenation and plays a guiding role in the preparation of high-performance carbon-supported noble metal catalysts.     

Synthesis of highly active and stable Pd/C catalysts toward HCOOH dehydrogenation by a simple NH3 adsorption method

Pd catalysts supported on activated carbon (Pd/C–NH₃) toward HCOOH dehydrogenation were prepared by a simple adsorption method using ammonia (NH₃) and Ar as the working gas. The results show that the TOF_initial of Pd/C–NH₃ was 459.8 h⁻¹ at 50 °C. When the reaction was carried out for 4 h, the HCOOH dehydrogenation ratio over Pd/C–NH₃ was about 81.2%, which was 1.15 and 1.13 times, respectively, as that of the as-prepared Pd/C catalyst without any treatment (Pd/C–As) and the Pd/C catalyst purchased from Sigma-Aldrich (Pd/C-CM). The total amount of H₂ and CO₂ produced by using Pd/C–NH₃ to decompose HCOOH in the third cycle was 99.4% of the gas produced by the first reaction cycle, and 1.80 and 12.60 times, respectively, as that of Pd/C–As and Pd/C-CM. The characterization results indicated that the Pd active species in Pd/C–NH₃ migrated to the outer surface of the carbon support during the reaction, and the pore volume of the carbon support became larger, which were beneficial to the reaction. These factors made Pd/C–NH₃ exhibit excellent HCOOH dehydrogenation activity and stability. NH₃ adsorption is a simple and effective method for preparing high-performance Pd/C HCOOH dehydrogenation catalysts, and has important guiding significance for the preparation of other carbon supported noble metal catalysts.     

Cu-assisted induced atomic-level bivalent Fe confined on N-doped carbon concave dodecahedrons for acid oxygen reduction electrocatalysis

Atomically dispersed transition metals anchored on N-doped carbon have been successfully developed as promising electrocatalysts for acidic oxygen reduction reaction (ORR). Nonetheless, how to introduce and construct single-atomic active sites is still a big challenge. Herein, a novel concave dodecahedron catalyst of N-doped carbon (FeCuNC) with well confined atomically dispersed bivalent Fe sites was facilely developed via a Cu-assisted induced strategy. The obtained catalyst delivered outstanding ORR performance in 0.5 M H₂SO₄ media with a half-wave potential (E_1/2) of 0.82 V (vs reversible hydrogen electrode, RHE), stemming from the highly active bivalent Fe-N_x sites with sufficient exposure and accessibility guaranteed by the high specific surface area and curved surface. This work provides a simple but efficient metal-assisted induced strategy to tune the configurations of atomically dispersed active sites as well as microscopy structures of carbon matrix to develop promising PGM-free catalysts for proton exchange membrane fuel cell (PEMFC) applications.     

Metal organic framework derived nitrogen-doped carbon anchored palladium nanoparticles for ambient temperature formic acid decomposition

Well-dispersed palladium nanoparticles (NPs) anchored on a porous N-doped carbon is prepared by wet chemical method, using metal organic frameworks (ZIF-8) as a precursor to derive the porous N-doped carbon support. Benefitting from the N-doping and the porous structure of the carbon materials, the final Pd NPs are in high dispersion and exhibit reduced particle sizes, with electronic structure and chemical status tuned to favor the formic acid decomposition (FAD). The prepared Pd/C_ZIF-8-950 catalysts show enhanced catalytic performance and selectivity for FAD, the turnover of frequency (TOF) and the mass activity up to 1166 h⁻¹ and 11.01 mol H₂ g⁻¹ _pd h⁻¹ were obtained at 30 °C. This work provides an effective and easy way for synthesis the Pd-based catalyst, which has enormous application prospects for the next generation hydrogen energy preparation and storage.     

Pd nanoparticles assembled on Ni- and N-doped carbon nanotubes towards superior electrochemical activity

Metal-based catalysts within single-atom to 1–2 nm size range are attracting considerable attention recent years. Carbon-based materials with their excellent electro- and photo-chemical properties are ideal candidates as supporting substrate for constructing of metal catalyst. Here we report a palladium (less than 5 nm in average diameter) deposited Ni carbon nanotubes (CNTs) with Ni metal nanoparticles (NPs) to be around single atom to 1–2 nm on average. Mono-dispersed Pd NPs are homogeneously immobilized on both synthesized Ni- and N-doped CNTs and N-doped commercial made CNTs using poly(diallyldimethyl ammonium) chloride (PDDA) as the key bonding components. Enhanced electrocatalytic activity is observed in measurements including hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and methanol oxidation reaction (MOR), with some of the samples having higher HER (under acidic condition) and OER (under basic condition) activity comparing with the commercial Pd/C (40 wt%) sample. The result provides a forward-looking strategy for fabricating efficient and low-cost catalysts.     

Synergistic catalysis of Pd–Ni(OH)2 hybrid anchored on porous carbon for hydrogen evolution from the dehydrogenation of formic acid

The development of a facile yet efficient strategy to boost the catalytic performance of supported Pd nanoparticles (NPs) toward the dehydrogenation of formic acid (FA) is essential but remains challenging. Here, a novel hybrid nanocatalyst comprising Pd and Ni(OH)₂ supported on porous carbon (PC) is developed. The obtained Pd–Ni(OH)₂/PC nanocatalyst exhibits an excellent catalytic performance for FA dehydrogenation to produce hydrogen. The introduction of Ni(OH)₂ in PC support can significantly promote the catalytic activity of Pd NPs toward FA dehydrogenation. Additionally, the catalytic property of Pd–Ni(OH)₂/PC is correlated with the Pd/Ni ratio. The ₂Pd–₁Ni(OH)₂/PC with the optimum Pd/Ni ratio of 2/1 exhibits the maximum turnover frequency (TOF) of 3409 h⁻¹ at 60 °C for FA dehydrogenation. The highly dispersed ultrafine Pd–Ni(OH)₂ hybrid NPs with numerous accessible active sites and Ni(OH)₂−induced positive synergetic effects with Pd NPs considerably boost the catalytic performance for FA dehydrogenation.     

Catalytic performance of dioxide reforming of methane over Co/AC-N catalysts: Effect of nitrogen doping content and calcination temperature

The nitrogen doped activated carbon (AC-N) has been successfully prepared with commercial activated carbon as carbon material followed by a simple N-doping method using melamine as nitrogen sources. Using AC-N as the supports, cobalt supported on N-doped activated carbon (Co/AC-N) were developed and used as catalyst for dry reforming reaction (DRM). It was discovered that the Co/AC-N catalysts revealed much higher catalytic performance for DRM reaction in comparison to activated carbon supported cobalt catalyst (Co/AC). Moreover, the catalytic activity was influenced by preparation conditions of AC-N such as calcination temperature and the doping amount of nitrogen. The catalysts were characterized by BET, XRD, XPS, H₂-TPR, Raman spectroscopy and TEM. It was found that catalytic activities of the catalysts with different calcination temperature and nitrogen doping were influenced by catalyst surface defects and disorders, Co²⁺/Co³⁺ molar ratio, the content of nitrogen function groups (graphitic N, pyrrolic-N and pyridinic-N) and interaction between active metal and support. The Raman spectroscopy illustrated that the N-doped catalyst surface defects and disorders increased, which improved the performances of the Redox catalysts. The XPS valence band also revealed that higher Co²⁺/Co³⁺ molar ratio and nitrogen function groups was achieved by decreasing calcination temperature and increasing nitrogen doping. In short, the doping of nitrogen increased the structural defects and the interaction between active metals and supports, modified the surface electronic structure, which were facilitated the oxidation and reduction of methane and carbon dioxide.     

Enhancement of oxygen reduction activity with addition of carbon support for non-precious metal nitrogen doped carbon catalyst

An improved synthesis scheme of non-precious metal N-doped carbon catalysts for oxygen reduction reaction is reported. The non-precious metal N-doped carbon catalysts were prepared by pyrolysis of the mixture (phenol resin, Ketjen black carbon support and cobalt phenanthroline complex). The drastic improvement of distribution state of Ketjen black supported non-precious metal N-doped carbon catalysts was observed by means of transmission electron microscopy (TEM). In addition, the non-precious metal N-doped carbon catalyst synthesized with Ketjen black carbon support showed much higher oxygen reduction reaction (ORR) activity relative to unsupported non-precious metal N-doped carbon catalyst in O₂-saturated 0.5 mol l⁻¹ H₂SO₄ at 35 °C. Moreover, the highest ORR activity was obtained with addition of optimum amount of Ketjen black carbon support was thirtyfold compared to unsupported non-precious metal N-doped carbon catalyst at 0.7 V. Similarly, the performance of a polymer electrolyte fuel cell (PEFC) using the non-precious metal N-doped carbon catalyst as the cathode electrode catalyst was obviously better than that of the non-precious metal N-doped carbon catalyst before optimization. Microstructure, specific surface area and surface composition of the non-precious metal N-doped carbon catalysts were analyzed by XRD, XPS and BET measurement with nitrogen physisorption, respectively.     

A facile synthesis of Pd/C cathode electrocatalyst for proton exchange membrane fuel cells

A carbon supported palladium (Pd/C-NaBH₄-NH₃) catalyst was synthesized via modified sodium borohydride reduction method using ammonia as the complexing reagent. The Pd/C catalysts were characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Rotating disk electrode (RDE), cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and single cell measurements were employed to evaluate the activities of the catalysts. The as-prepared catalysts with face-centered cubic (fcc) structure are uniformly dispersed on the carbon supports. Twinned and polycrystalline structures are observed in the HRTEM image of Pd/C-NaBH₄-NH₃. The results indicate that the Pd/C-NaBH₄-NH₃ catalyst shows high activity for the oxygen reduction reaction. Single cell with Pd/C-NaBH₄-NH₃ as the cathode displays a maximum power density of 508 mW cm⁻². The favorable performance of the Pd/C-NaBH₄-NH₃ catalyst may be attributed to the uniformly dispersed nanoparticles and more crystalline lattice defects.     

Atomically dispersed palladium supported on nitrogen-doped mesoporous carbon for drastic electrocatalytic hydrogen evolution

The electrocatalysis of water to hydrogen is expected to play an essential and significant role in the development of future electrochemical energy conversion and storage technologies, together with the exploration of green energy. However, the high cost of noble metal catalysts remains a key challenge and it still requires further investigations to fabricate high mass activity and stable electrocatalysts. Herein, we report a facile and economical approach to achieve atomically dispersed palladium on the nitrogen-doped mesoporous carbon matrix (Pd₁/NMC) as the electrocatalyst for hydrogen evolution, which exhibits an overpotential of 37 and 118 mV at the current density of 10 and 100 mA cm^?2, respectively, superior to the commercial platinum/carbon (Pt/C) and palladium/carbon (Pd/C) catalysts. Moreover, the mass activity of the Pd₁/NMC catalyst surpasses that of Pt/C and Pd/C at 100 mV versus RHE in HER. Systematic characterizations demonstrate that the Pd atoms are atomically dispersed on the surface of NMC and stabilized by active nitrogen sites, inducing the isolated Pd atoms to form a favorable bivalent oxidation state. This method provides an atomic-level insights into preparing superior single-atom catalysts for energy-related applications and devices.     

Highly dispersed Pd-MnOx nanoparticles supported on graphitic carbon nitride for hydrogen generation from formic acid-formate mixtures

Highly dispersed Pd and MnO_x nanoparticles supported on the graphitic carbon nitride with different composition have been prepared by a simple liquid deposition-reduction method and used as an efficient FA decomposition catalyst for hydrogen generation. The catalytic activity depends on the Pd-MnO_x composition of catalyst, the FA/SF ratio and the reaction temperature. The Pd-MnO_x/CN-1 catalyst exhibited excellent catalytic activity for hydrogen generation with the initial TOF of 465 h^?1 from formic acid-sodium formate mixture aqueous solution with a FA/SF ratio of 1:8 at 348 K. The synergetic effect between Pd nanoparticles and the carbon nitride support, the Mn/Pd ratios, the good dispersion of nanoparticles and the nature of the carbon nitride support were suggested to be responsible for the efficient catalytic performance of the Pd-MnO_x/CN nanocatalysts.     

Carbon nanotube-copper oxide-supported palladium anode catalysts for electrocatalytic enhancement in formic acid oxidation

Pd/xCuO–10CNT (x = 1, 2, 3, 4) catalysts were synthesized using an improved polyol method. Uniformly prepared catalyst structures and chemical compositions of the catalysts delivered a high oxidation performance. The prepared catalysts were characterized via transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The formation of homogenous active Pd metal and CuO nanoparticle-modified CNT surfaces was found. Meanwhile, the electrocatalytic activity and the long-term stability performance of the prepared catalysts toward formic acid oxidation reaction (FAOR) were also employed via cyclic voltammogram (CV) and chronoamperometry (CA), respectively. Prominently, the prepared Pd/xCuO–CNT nanocomposite catalyst presented an outstanding electrocatalytic performance with a higher maximum forward peak current density (26.9 mA cm^?2) than those of catalysts Pd/CNT (3.4 mA cm^?2) and Pd/C (2.3 mA cm^?2) toward FAOR in the H₂SO₄ electrolyte, representing high conductivity CNT, and dispersed Pd nanoparticles with a large active surface area, on the CuO-CNT support. Additionally, the prepared catalysts also had outstanding stability and an excellent CO poisoning tolerance through the modified Pd structures on CuO-supported CNT. The insertion of CuO onto the CNT surface before Pd loading provided additional electrochemical active sites due to the enhanced geometric and bifunctional system. CuO supports the adsorption of oxygen-containing species (OH_ads) on the catalyst surface, and the electron effect among Pd and Cu metals is beneficial for charge transfer.     

Improved hydrogen storage performance of defected carbon nanotubes with Pd spillover catalysts dispersed using supercritical CO2 fluid

Hydrogen storage properties of carbon nanotubes (CNTs) modified by oxidative etching and decoration of Pd spillover catalysts are investigated. A mixed H₂SO₄/H₂O₂ solution containing ferrous ions (Fe²⁺) is useful to open the caps, to shorten the length, and to generate defects on CNTs. The Pd catalysts are deposited on the CNTs with the aid of supercritical carbon dioxide (scCO₂); as a result, a highly dispersed Pd nanoparticles and an intimate connection between Pd and carbon surface can be obtained. Combination of the two approaches can optimize a hydrogen spillover reaction on CNTs, resulting in a superior hydrogen storage capacity of 1.54 wt% (at 25 °C and 6.89 MPa), which corresponds to an enhancement factor of ∼4.5 as compared to that of pristine CNTs.     

2-Dimensional metal-organic framework derived N-doped carbon immobilized Pd nanoparticles for hydrogen release from formic acid

Hydrogen release from formic acid is a significant energy supply route. However, the current catalysts suffer from low catalytic efficiency and stability. Herein, a porous N-doped carbon material with high N content (16.87%) and a large surface area (1544 m²·g^?1) were designed using a 2-dimensional metal-organic framework and etching agent potassium chloride. Due to its high N content and large surface area, ultrafine Pd particles are uniformly distributed on the porous N-doped carbon support, which effectively enhances excellent reactivity and enables a TOF value of 2365 h^?1 under additive-free conditions. The research also revealed that the strong interaction between Pd particles and pyridinic-N species can significantly slow metal agglomeration. Hence, the Pd/NC_ZIF-L (KCl) displayed good activity even after 10 cycling experiments, and it is a particularly competitive catalyst for hydrogen release from formic acid.     

Noble-metal-free Co@Co2P/N-doped carbon nanotube polyhedron as an efficient catalyst for hydrogen generation from ammonia borane

Developing an efficient catalyst for hydrogen (H₂) generation from hydrolysis of ammonia borane (AB) to significantly improve the activity for the hydrogen generation from AB is important for its practical application. Herein, we report a novel hybrid nanostructure composed of uniformly dispersed Co@Co₂P core-shell nanoparticles (NPs) embedded in N-doped carbon nanotube polyhedron (Co@Co₂P/N–CNP) through a carbonization-phosphidation strategy derived from ZIF-67. Benefiting from the electronic effect of P doping, high dispersibility and strong interfacial interaction between Co@Co₂P and N-CNTs, the Co@Co₂P/N–CNP catalyst exhibits excellent catalytic performance towards the hydrolysis of AB for hydrogen generation, affording a high TOF value of 18.4 mol H₂ mol metal^?1 min^?1 at the first cycle. This work provides a promising lead for the design of efficient heterogeneous catalysts towards convenient H₂ generation from hydrogen-rich substrates in the close future.     

Decomposition and coupling of methane over Pd–Au/Al2O3 catalysts to form COx-free hydrogen and C2 hydrocarbons

Catalytic decomposition of methane (CDM; CH₄ → C + 2H₂) is expected to be used for clean hydrogen production because CDM does not emit carbon dioxide. Recently, it was reported that Pd–based catalysts promotes CDM, simultaneously facilitating coupling of CH₄ to form C₂ hydrocarbons. In this study, varieties of supported Pd–M alloy catalysts (M = Fe, Co, Ni, Cu, Zn, Ga, In, Sn, Au, Pb, and Bi) were synthesized and their activities for the CDM and CH₄ coupling were examined. The catalytic activity for CH₄ strongly depended on the types of Pd–M. Pd–M/Al₂O₃ (M = Ni, Fe, Co, Au) showed high activity for CDM. In addition to the production of hydrogen by the CDM, Pd–Au/Al₂O₃ formed C₂ hydrocarbons such as ethane and ethylene via the coupling of CH₄. Effects of Pd/Au ratio and reaction temperatures were examined and the role of Au for the CH₄ conversion reaction was discussed.     

Effect of Pt–Pd/C coupled catalyst loading and polybenzimidazole ionomer binder on oxygen reduction reaction in high-temperature PEMFC

Polybenzimidazole (PBI) was studied as an ionomer binder at varying ratios (1–7) in a 20–40 wt% Pt–Pd/C cathode-coupled catalyst layer for the oxygen reduction reaction (ORR) in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). Catalytic activity was examined by CV and LSV, while the properties of the catalysts were characterized by FESEM-EDX, N₂ adsorption–desorption, XRD and FTIR. The results showed that the distribution of metals on the carbon surface, carbon wall thickness and the interaction between ionomer and coupled catalysts affected the ORR performance. The fabricated membrane electrode assembly with 5:95 PBI: 30 wt% Pt–Pd/C catalyst ratio exhibited the best performance and highest durability for HT-PEMFC at 170 °C, yielding a power density of 1.30 Wcm⁻² with 0.02 mg_Pt/cm Pt loading. This performance of ultra-low metal loading of coupled Pt–Pd/C electrocatalyst with PBI binder was comparable to those reported by other studies, highlighting a promising catalyst for fuel cell application.     

Improving the activity and CO tolerance by the nitrides-modified Pd/C electrocatalysts for methanol and formic acid oxidation reactions

Direct methanol and formic acid fuel cells attracted extensive attention with high specific energy and low operating temperature. The efficient electrocatalyst was one of the important factors to limit their commercial applications. Here, the nitrides-modified Pd/C catalysts (Pd–NbN/C, Pd–Mo₂N/C, Pd-VN/C and Pd-BN/C) were prepared using sodium citrate as a reducing agent in ethylene glycol solution. The mass activities followed the trend of Pd–NbN/C > Pd–Mo₂N/C > Pd-VN/C > Pd-BN/C > Pd/C, where Pd–NbN/C exhibited the highest mass activity (4052.19 A g_Pd^?1) for MOR, 19.2 times that of the commercial Pd/C (210.53 A g_Pd^?1). Electrochemical measurements (LSV, Tafel, EIS, CA) indicated that the addition of nitrides increased the charge-transfer kinetics, reaction rate and CO tolerance of catalysts for MOR in alkaline media. The rate-determining step was CO_ads oxidation process. Combined the structural analysis (HRTEM, XRD, XPS, ICP) with electrochemical results, the enhanced catalytic performance was ascribed to that higher ECSAs and the charge transfer between Pd and nitrides, leading to the negative shift of the d-band center towards the Fermi level. The linear correlations were found for the d-band center VS the mass activities and the onset potential of CO oxidation VS the d-band center, indicating that the activity and anti-CO poisoning ability could be enhanced by controlling the d-band structure of catalysts for MOR. Furthermore, the higher mass activity for FAOR in acidic media suggested that nitrides-modified Pd/C may be the promising bi-functional materials.     

Methanol-reduced Pd nanoparticles anchored on B,N-CDs@CNT hybrid for oxygen reduction reaction

The development of highly active oxygen reduction reaction (ORR) catalysts with low-loading of precious metals is imperative but remains a great challenge. Herein, Pd/B,N-CDs@CNT composite catalysts was designed with Pd nanoparticles immobilized on hybrid support composed of B,N-doped carbon dots (B,N-CDs) and the multi-wall carbon nanotubes (CNT) using a simple methanol reduction method. The deposited Pd nanoparticles exhibit clean surface, low crystallinity and surface distortion. The Pd/B,N-CDs@CNT catalyst shows significantly enhanced ORR activity, with the mass activity about 5–6 times higher than that of commercial Pt/C. Thus prominent ORR performance is mainly attributed to the unique microstructure of Pd nanoparticles. Moreover, the composited B,N-CDs and the doped B, N heteroatoms further improve the ORR performance, in which B,N-CDs supply more absorption sites, the formation of N–Pd facilitates the electron transfer, B doping can promote the adsorption of oxygenated species and weaken the strong interaction between Pd and C. Furthermore, the B, N co-doping plays a synergistic effect. This strategy provides a simple and mild method for designing highly efficient electrocatalysts with ultra-low precious metals in alkaline electrolytes.     

Tuning morphology and structure of Fe–N–C catalyst for ultra-high oxygen reduction reaction activity

Exploring efficient and durable non-precious metal catalysts for oxygen reduction reaction (ORR) has long been pursued in the field of metal-air batteries, fuel cells, and solar cells. Rational design and controllable synthesis of desirable catalysts are still a great challenge. In this work, a novel approach is developed to tune the morphologies and structures of Fe–N–C catalysts in combination with the dual nitrogen-containing carbon precursors and the gas-foaming agent. The tailored Fe–N₁/N₂–C-A catalyst presents gauze-like porous nanosheets with uniformly dispersed tiny nanoparticles. Such architectures exhibit abundant Fe-N_x active sites and high-exposure surfaces. The Fe–N₁/N₂–C-A catalyst shows extremely high half-wave potential (E_1/2, 0.916 V vs. RHE) and large limiting current density (6.3 mA cm⁻²), far beyond 20 wt% Pt/C catalyst for ORR. This work provides a facile morphological and structural regulation to increase the number and exposure of Fe-N_x active sites.