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Transition metal phosphides (TMP) have gained great interest as catalysts for hydrogenation reactions due to their physical and chemical properties. Among these reactions, the hydrogenation of nitroarenes to the corresponding anilines is of great industrial importance, as these compounds are precursors of high value-added chemicals. In this work, we present the microwave-assisted synthesis of cobalt phosphide nanoparticles supported on carbon, and its application in the hydrogenation of 1-chloro-4-nitrobenzene. The catalysts were characterised by transmission electron microscopy (TEM), temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and N2 adsorption at −196 °C. It could be observed that the catalytic activity depends on the reduction temperature, since it determines the crystalline phase of the phosphide. Thus, we found that the most active phase for the hydrogenation of 1-chloro-4-nitrobenzene corresponds to a phosphorus-rich cobalt phosphide obtained at low reduction temperatures.

Efficient catalysts for the oxygen evolution reaction (OER) are essential for advancing renewable energy technologies. This study focuses on the synthesis and characterization of CoxPy, CoO, CoP, NiCoP, and NiCo/C nanoparticles as catalysts for the oxygen evolution reaction (OER). The CoxPy nanoparticles exhibited a sea-urchin-like morphology, providing a high surface area for electrocatalytic activity. Other catalysts exhibited distinct morphologies, including spherical particles and porous structures. Characterization techniques, such as transmission electron microscopy and X-ray diffraction, confirmed the morphology and composition of the synthesized nanoparticles. The electrocatalytic performance of the catalysts was evaluated through polarization curve measurements, and a microkinetic steady-state model based on the Electrochemical Oxide Path was developed to understand the OER mechanism. The impact of adventitious Fe on catalyst activity was also investigated, revealing the high sensitivity of CoxPy to Fe. The findings suggest the dominance of the Electrochemical Oxide Path in the OER mechanism for all catalysts, with NiCoP showing the lowest onset potential.

Transition metal phosphides have been recognized as efficient co-catalysts to boost the activity of semiconductor photocatalysts. However, a rigorous and quantitative understanding is still to be developed about how transition metal phosphides influence photoexcited electron dynamics. Here, we present a nanosecond time-resolved transient absorption spectroscopy (TAS) study of the photoexcited electron dynamics in carbon nitrides (g-C3N4) before and after Co and/or P modifications. Our spectroscopic study showed that Co or P lowered the initial electron density, whereas they promoted the photoexcited electron relaxation of g-C3N4, with their half-life times (t50%) of 2.5 and 1.8 ns, respectively. The formation of a CoP co-catalyst compound promoted the electron relaxation (t50%=2.8 ns) without significantly lowering the charge separation efficiency. Density functional theory (DFT) calculations were undertaken to explore the underlying fundamental reasons and they further predicted that CoP, compared to Co or P modification, better facilitates photoexcited electron transfer from g-C3N4 to reactants.

Heteroatom doping is considered an efficient strategy when tuning the electronic and structural modulation of catalysts to achieve improved performance towards renewable energy applications. Herein, we synthesized a series of carbon-based hierarchical nanostructures through the controlled pyrolysis of Co-MOF (metal organic framework) precursors followed by in situ phosphidation. Two kinds of catalysts were prepared: metal nanoparticles embedded in carbon nanotubes, and metal nanoparticles dispersed on the carbon surface. The results proved that the metal nanoparticles embedded in carbon nanotubes exhibit enhanced ORR electrocatalytic performance, owed to the enriched catalytic sites and the mass transfer facilitating channels provided by the hierarchical porous structure of the carbon nanotubes. Furthermore, the phosphidation of the metal nanoparticles embedded in carbon nanotubes (P-Co-CNTs) increases the surface area and porosity, resulting in faster electron transfer, greater conductivity, and lower charge transfer resistance towards ORR pathways. The P-Co-CNT catalyst shows a half-wave potential of 0.887 V, a Tafel slope of 67 mV dec−1, and robust stability, which are comparatively better than the precious metal catalyst (Pt/C). Conclusively, this study delivers a novel path for designing multiple crystal phases with improved catalytic performance for energy devices. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Catalysts based on earth-abundant non-noble metals are interesting candidates for alkaline water electrolysis in sustainable hydrogen economies. However, such catalysts often suffer from high overpotential and sluggish kinetics in both the hydrogen and oxygen evolution reactions (HER and OER). In this study, a hybrid catalyst made of iron-doped cobalt phosphide (Fe-CoP) nanowire arrays and Ni(OH)2 nanosheets is introduced that displays strong electronic interactions at the interface, which significantly improves the interfacial reactivity of reactants and/or intermediates with the hybrid catalyst surface. The combined experimental and theoretical study further confirms that the hybrid catalyst promotes the sluggish rate-limiting steps in both the HER and OER. Full water electrolysis is thus enabled to take place at such a low cell voltage as 1.52 V to reach the current density of 10 mA cm−2 along with superior durability and high conversion efficiency.

Molecular hydrogen (H2) is a clean and sustainable energy carrier that has the potential to meet the ever-growing global energy demands at no environmental cost. Water splitting is one of the most promising ways to produce hydrogen but it is a thermodynamically uphill reaction and thus requires external energies such as electricity to initiate the reaction. Efficient water splitting requires highly active, earth-abundant and robust catalysts to promote the two half reactions, namely the hydrogen evolution and oxygen evolution reactions (HER and OER). Cobalt phosphides have been shown to be active toward HER, especially in acidic medium, and OER in alkaline medium. However, they show much inferior HER activity in alkaline medium due to the extra water dissociation step. Thus it is still a challenge to improve the catalytic activity of cobalt based phosphides for both HER and OER in alkaline solution. My thesis is focused on the nanohybridization of metal with phosphide for enhancing water splitting activity in alkaline medium and the mechanism of their performance. This thesis contains 5 chapters. Chapter 1 mainly introduces electorcatalytic water splitting, the development of cobalt based phosphides as electrocatalysts, nanohybridization and the objectives of my thesis. Chapter 2 introduces the techniques used in my experiments. Major findings of my study are presented and discussed in chapter 3 and 4. And the conclusions and outlooks are shown in chapter 5. In chapter 3, to increase the electrocatalytic HER activity of cobalt phosphide in alkaline solution and figure out the mechanism, I synthesized 3D porous Co-P/Co nanohybrids nanosheets array grown on carbon cloth. This catalyst showed efficient HER performance in alkaline medium. It showed a low overpotential (128 mV at 10 mA cm-2), low Tafel slope (58 mV dec-1), and prominent electrochemical stability. Moreover, it exhibited superior activity and stability than Co-P/CC. Most im

Molecular hydrogen (H2) is a clean and sustainable energy carrier that has the potential to meet the ever-growing global energy demands at no environmental cost. Water splitting is one of the most promising ways to produce hydrogen but it is a thermodynamically uphill reaction and thus requires external energies such as electricity to initiate the reaction. Efficient water splitting requires highly active, earth-abundant and robust catalysts to promote the two half reactions, namely the hydrogen evolution and oxygen evolution reactions (HER and OER). Cobalt phosphides have been shown to be active toward HER, especially in acidic medium, and OER in alkaline medium. However, they show much inferior HER activity in alkaline medium due to the extra water dissociation step. Thus it is still a challenge to improve the catalytic activity of cobalt based phosphides for both HER and OER in alkaline solution. My thesis is focused on the nanohybridization of metal with phosphide for enhancing water splitting activity in alkaline medium and the mechanism of their performance. This thesis contains 5 chapters. Chapter 1 mainly introduces electorcatalytic water splitting, the development of cobalt based phosphides as electrocatalysts, nanohybridization and the objectives of my thesis. Chapter 2 introduces the techniques used in my experiments. Major findings of my study are presented and discussed in chapter 3 and 4. And the conclusions and outlooks are shown in chapter 5. In chapter 3, to increase the electrocatalytic HER activity of cobalt phosphide in alkaline solution and figure out the mechanism, I synthesized 3D porous Co-P/Co nanohybrids nanosheets array grown on carbon cloth. This catalyst showed efficient HER performance in alkaline medium. It showed a low overpotential (128 mV at 10 mA cm-2), low Tafel slope (58 mV dec-1), and prominent electrochemical stability. Moreover, it exhibited superior activity and stability than Co-P/CC. Most im

Cobalt phosphides are an emerging earth-abundant alternative to platinum-group-metal-based electrocatalysts for the hydrogen evolution reaction (HER). Yet, their stability is inferior to platinum and compromises the large-scale applicability of CoPx in water electrolyzers. In the present study, we employed flat, thin CoPx electrodes prepared through the thermal phosphidation (PH3) of Co3O4 films made by plasma-enhanced atomic layer deposition to evaluate their stability in acidic water electrolysis by using a multi-technique approach. The films were found to be composed of two phases: CoP in the bulk and a P-rich surface CoPx (P/Co>1). Their performance was evaluated in the HER and the exchange current density was determined to be j0=−8.9 ⋅ 10−5 A/cm2. The apparent activation energy of HER on CoPx (Ea=81±15 kJ/mol) was determined for the first time. Dissolution of the material in 0.5 M H2SO4 was observed, regardless of the constantly applied cathodic potential, pointing towards a chemical instead of an electrochemical origin of the observed cathodic instability. The current density and HER faradaic efficiency (FE) were found to be stable during chronoamperometric treatment, as the chemical composition of the HER-active phase remained unchanged. On the contrary, a dynamic potential change performed in a repeated way facilitated dissolution of the film, yielding its complete degradation within 5 h. There, the FE was also found to be changing. An oxidative route of CoPx dissolution has also been proposed.

Molecular hydrogen (H2) is a clean and sustainable energy carrier that has the potential to meet the ever-growing global energy demands at no environmental cost. Water splitting is one of the most promising ways to produce hydrogen but it is a thermodynamically uphill reaction and thus requires external energies such as electricity to initiate the reaction. Efficient water splitting requires highly active, earth-abundant and robust catalysts to promote the two half reactions, namely the hydrogen evolution and oxygen evolution reactions (HER and OER). Cobalt phosphides have been shown to be active toward HER, especially in acidic medium, and OER in alkaline medium. However, they show much inferior HER activity in alkaline medium due to the extra water dissociation step. Thus it is still a challenge to improve the catalytic activity of cobalt based phosphides for both HER and OER in alkaline solution. My thesis is focused on the nanohybridization of metal with phosphide for enhancing water splitting activity in alkaline medium and the mechanism of their performance. This thesis contains 5 chapters. Chapter 1 mainly introduces electorcatalytic water splitting, the development of cobalt based phosphides as electrocatalysts, nanohybridization and the objectives of my thesis. Chapter 2 introduces the techniques used in my experiments. Major findings of my study are presented and discussed in chapter 3 and 4. And the conclusions and outlooks are shown in chapter 5. In chapter 3, to increase the electrocatalytic HER activity of cobalt phosphide in alkaline solution and figure out the mechanism, I synthesized 3D porous Co-P/Co nanohybrids nanosheets array grown on carbon cloth. This catalyst showed efficient HER performance in alkaline medium. It showed a low overpotential (128 mV at 10 mA cm-2), low Tafel slope (58 mV dec-1), and prominent electrochemical stability. Moreover, it exhibited superior activity and stability than Co-P/CC. Most im

Cobalt phosphides are an emerging earth-abundant alternative to platinum-group-metal-based electrocatalysts for the hydrogen evolution reaction (HER). Yet, their stability is inferior to platinum and compromises the large-scale applicability of CoPx in water electrolyzers. In the present study, we employed flat, thin CoPx electrodes prepared through the thermal phosphidation (PH3) of Co3O4 films made by plasma-enhanced atomic layer deposition to evaluate their stability in acidic water electrolysis by using a multi-technique approach. The films were found to be composed of two phases: CoP in the bulk and a P-rich surface CoPx (P/Co>1). Their performance was evaluated in the HER and the exchange current density was determined to be j0=−8.9 ⋅ 10−5 A/cm2. The apparent activation energy of HER on CoPx (Ea=81±15 kJ/mol) was determined for the first time. Dissolution of the material in 0.5 M H2SO4 was observed, regardless of the constantly applied cathodic potential, pointing towards a chemical instead of an electrochemical origin of the observed cathodic instability. The current density and HER faradaic efficiency (FE) were found to be stable during chronoamperometric treatment, as the chemical composition of the HER-active phase remained unchanged. On the contrary, a dynamic potential change performed in a repeated way facilitated dissolution of the film, yielding its complete degradation within 5 h. There, the FE was also found to be changing. An oxidative route of CoPx dissolution has also been proposed.

In order to elucidate the dependence of the magnetic ground state on the Ca content (x) in Sr1-xCaxCo2P2 (0 <= x <= 1, ThCr2Si2-type structure), we have performed muon spin rotation and relaxation (mu+SR) experiments on Sr1-xCaxCo2P2 powder samples mainly in a zero applied field. The end member compound, SrCo2P2, is found to be paramagnetic down to 19 mK. As x increases, such a paramagnetic ground state is observed down to 1.8 K until x = 0.45. Then, as x increases further, a short-range antiferromagnetic (AF) ordered phase appears at low temperatures for 0.48 <= x <= 0.75, and finally, a long-range AF ordered phase is stabilized for x > 0.75. The internal magnetic field of the other end member compound, CaCo2P2, is well consistent with that of the A-type AF order state, which was proposed from neutron scattering experiments. The phase diagram determined with mu+SR is different from that proposed by macroscopic measurements. For an isostructural compound, LaCo2P2, static magnetic order is found to be formed below similar to 130 K., QC 20161005