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We report remarkable support effects for carbon nanotubes (CNTs) in the Pt/CNT-catalyzed hydrolytic dehydrogenation of ammonia borane. The origin of the support-dependent activity and durability is elucidated by combining the catalytic and durability testing with characterization by a range of spectroscopy and high-angle annular dark-field scanning transmission electron microscopy techniques and ICP analysis. The effects mainly arise from different electronic properties and different abilities for the adsorption of boron-containing species on platinum surfaces and changes in size and shape of the platinum particles during the reaction. Defect-rich CNTs in particular are a promising support material, as it not only enhances the platinum binding energy, leading to the highest hydrogen generation rate, but also inhibits the adsorption of boron-containing species and stabilizes the platinum nanoparticles to resist the agglomeration during the reaction, leading to the highest durability. The insights revealed herein may pave the way for the rational design of highly active and durable metal/carbon catalysts for the hydrolytic dehydrogenation of ammonia borane., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Incineration fly ash could be contaminated with NH3 that was slipped from the ammonia-based selective non-catalytic reduction(SNCR) process and from evaporation of municipal solid wastes' leachate involved in the wastes. This research was conducted to investigate the impacts of ammonia on leaching of dissolved organic carbon (DOC) and metals from incineration fly ash in the pH range of 3.66-12.44 with an active ammonia spike. A geochemical modeling software Visual MINTEQ was adopted to calculate the chemical speciation of metals under the leaching conditions to reveal the mechanism behind the impacts. It was proved that at pH > 9, the leaching of DOC increased significantly in the presence of high concentrations of ammonia (> or = 1 357 mg x L(-1)), but there was little effect when the ammonia level in eluates was not higher than 537 mg x L(-1). At pH < 6, metals in fly ash were released mainly in the form of free metal ions and chloride complexes, which were little influenced by ammonia; while at pH 8-12, higher concentrations of ammonia (> or = 3 253 mg x L(-1)) mobilized Cd, Cu, Ni and Zn significantly due to the formation of soluble metal-ammonia complexes, and the leaching rates reached their peaks at pH around 9; however, ammonia had little impacts on Al and Pb leaching within this pH range. At pH > 12, for Cd, Cu, Ni and Zn, their leaching species were predominantly in the form of hydroxide complexes. Under the ammonia concentration of 3253 mg x L(-1), the Visual MINTEQ modeling results were compared with the experimental data, and it was proved that the leaching of Al, Pb and Zn was mainly controlled by precipitation/dissolution modeling, while Cd, Cu and Ni were controlled by precipitation/dissolution and surface complexation/precipitation processes; Visual MINTEQ modeling could well describe the leaching behaviors of Al, Cu, Pb and Zn from incineration fly ash.

To elucidate the effect of textural and chemical properties of catalysts and to discriminate the active phase are fundamental issues in heterogeneous catalysis, but they are often complicated by the nature of support adding a new dimension. In this work, metal amine metallate (Co(en)3MoO4) precursor was directly treated by calcination and prenitridation to understand these two issues in Co-Mo catalyzed ammonia decomposition. The calcination atmosphere (i.e., Ar and Air) is found to remarkably affect the textural and chemical properties of catalysts, and air atmosphere is favorable for higher stable reactivity despite incomplete nitridation of the catalyst. Further prenitridation of the sample from the calcination of Co(en)3MoO4 under air gives rise to higher catalytic activity, and Co3Mo3N is suggested as the dominant active phase of Co-Mo catalysts. The insights revealed here shed new light on the design of Co-Mo catalysts for ammonia decomposition. [ABSTRACT FROM AUTHOR]

Catalytic hydrolysis of ammonia borane has tremendous potential as an energy-efficient approach to supply hydrogen for energy vehicles and portable electronic devices. Herein, DFT calculation is first performed on electronic properties of Ni 2 P and Ni 5 P 4 nanocatalysts. It is found that more electrons are transferred from Ni to P for Ni 5 P 4 , indicating that Ni 5 P 4 may show superior performance based on the electron effect. Therefore, Ni 2 P and Ni 5 P 4 with high purity are synthesized by the phase-controlled thermal decomposition approach. Gratifyingly, the Ni 5 P 4 catalyst exhibits the as-expected better catalytic activity than that of Ni 2 P catalyst. It also shows low activation energy and good stability. Furthermore, the structures and morphologies of both catalysts are characterized by multi-techniques such as XRD, HRTEM and XPS. The better performance could be ascribed to the higher positive charge of Ni together with the stronger ensemble effect of P. The insights sheds new light on the design of efficient Ni P catalysts for hydrogen generation. [ABSTRACT FROM AUTHOR]

Ni nanoparticles at the tips of carbon nanofibers (Ni-CNFs) were synthesized by catalytic chemical vapor deposition (CCVD) method using CH 4 as the carbon source, and then employed as catalysts for the generation of H 2 from ammonia decomposition. The morphology of Ni catalysts is highly sensitive to the ratio of CH 4 to H 2 . Especially for the CH 4 /H 2 ratio of 4, the as-obtained Ni-CNFs catalyst shows higher H 2 formation rate, which could be due to more accessible facets to the reactants and unique shape effect. Meanwhile, this catalyst also shows good thermal stability, possibly owing to the highly dispersed and spatially isolated Ni nanoparticles by CNFs. Moreover, effect of the surface carbon coverage on the orientation of Ni crystal facets and the matching degree between graphene sheet and Ni crystal facet were investigated by DFT calculations. Finally, a possible formation mechanism of shaped Ni catalysts was discussed by combining experimental and theoretical results. [ABSTRACT FROM AUTHOR]

Density functional theory calculationshave been performed to investigatethe recombinative desorption of N on terraced Fe(100) and Fe(110)as well as stepped Fe(111) and Fe(211) surfaces. The results showedthat the stepped surfaces, especially the Fe(111) surface, are moreactive than the terraced surfaces, which could be related to the presenceof so-called C7sites for the stepped surfaces. Carbonatoms were found to stabilize the thermodynamically unstable steppedsurfaces to be the energetically favored facets and thus facilitatethe preferential exposure. As these carbon atoms incorporated intothe stepped surfaces in the form of surface or subsurface carbon,the activation energy for the recombinative desorption of N was lowered,which was mainly ascribed to the decrease in the d band center ofsurface Fe atoms. In addition, a relation between the electronic andstructural properties of Fe catalysts in the absence or presence ofcarbon and ammonia decomposition activity was also correlated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fe-CNFs/CMFs catalysts were prepared by using CCVD method. [•] Fe particles located on the tops of carbon nanofibers. [•] Fe crystal phases were changed from Fe3C to α-Fe(C) when from 0.20 to 0.33atm. [•] Fe3C and α-Fe(C) crystal phases were easily nitrided into Fe2N. [•] Structured polyhedral Fe2N catalyst was more active for ammonia decomposition. [ABSTRACT FROM AUTHOR]

Abstract: Ni catalysts are widely used to catalyze ammonia decomposition. In this study, first-principles calculations are performed to investigate the preferred adsorption sites and the adsorption energies of NHx (x=0–3) and H, and the transition states, activation energies and rate constants of NH3 stepwise dehydrogenation and associative desorption of N on the stepped Ni(211) and the close-packed Ni(111) surfaces. The results show that the barrier of associative desorption of N, which is higher than those of NH3 stepwise dehydrogenation, on Ni(211) are 1.10eV higher than that on Ni(111), indicating that the associative desorption of N on Ni(211) is difficult to take place. Hence the strongly adsorbed N atoms would block some of the step sites of Ni(211). The N-blocking Ni(211) surface is constructed, and then NH3 decomposition on 2N–Ni(211) is investigated. The results indicate that Ni catalysts with too many or too few step sites, which can be associated with Ni particle sizes, show low ammonia decomposition activities. Therefore, different sized Ni-based catalysts are employed to catalyze ammonia decomposition, and TOFH2 as a function of Ni particle size is correlated. Finally, the structure sensitivity of ammonia decomposition over Ni catalysts is rationally interpreted. [Copyright &y& Elsevier]

Ni-carbon nanofibers (Ni-CNFs) catalysts were synthesizedin situby the decomposition of carbon-containing gases (i.e., CH4, CO, and C2H4) over Ni/Al2O3catalyst and directly used to catalyze ammonia decomposition.The results showed that Ni nanoparticles were found to locate at thetips of CNFs when using CH4and CO as carbon sources, whilethey located at the roots of CNFs when using C2H4as a carbon source. For ammonia decomposition, Ni catalysts at thetips of CNFs showed higher activity, which could be due to the moreaccessible surfaces to the reactants. Interestingly, the Ni catalystat the tips of CNFs with CH4as a carbon source exhibitedhigher activity than that with CO as a carbon source, even thoughthe former catalyst had a larger average particle size. The possiblemechanism was given by combining characterization results with ourprevious simulation results. Finally, when using CH4asa carbon source, the effect of the Ni-CNFs catalysts with differentgrowth times on the activity was further studied. [ABSTRACT FROM AUTHOR]

Abstract: MCM-41 supported Co, Mo and Collic catalysts with different Co/Mo atomic ratios were prepared by incipient wetness impregnation, and the channels of MCM-41 acted as nano-reactor. The catalysts without pre-nitridation treatments were directly used to catalyze ammonia decomposition. The characterization of the catalysts was carried out by N2 physisorption, H2-TPR, Raman, XRD, TEM and XPS. The results show that the catalysts have the similar size distribution of metallic particles. Due to the synergistic effect between Co and Mo, MCM-41 supported Collic catalysts show higher ammonia decomposition activity compared to Co or Mo alone. The Collic catalyst with the Co/Mo atomic ratio of about 7/3 exhibits the highest NH3 conversion. Moreover, the Co7Mo3/MCM-41 catalyst shows good thermal stability. [Copyright &y& Elsevier]

Abstract: Ammonia decomposition on transition metal surfaces is of great importance in energy and environmental industries. With the aim to obtain a systematic knowledge about the reaction mechanism of NH3 decomposition on the close-packed surfaces of the 3d-late transition metals, we perform first-principles calculations to determine the preferred adsorption sites and the adsorption energies of NH x (x =0–3) and H, and identify the transition states of the NH3 stepwise dehydrogenation reactions and the N recombination reactions on the close-packed Fe(110), Co(111) and Ni(111) surfaces. The results show that the calculated adsorption energies and activated energies mainly depend on the d-band center of metal surfaces. Barrier decomposition analysis reveals that the interaction among binding species in transition states will increase the energy barrier while the bonding to the surface of the species in the initial states and the transition states will decrease the energy barrier. Moreover, a linear relationship, known as the Brϕnsted–Evans–Polanyi relation, also exists between the activation energy of the N recombination reaction and the adsorption energy of N on the close-packed surfaces of the 3d-late transition metals. [Copyright &y& Elsevier]

Abstract: First-principles calculations based on density functional theory (DFT) have been performed to study the adsorption and decomposition of NH3 on Ni(110). The adsorption sites, the adsorption energies, the transition states and the activation energies of the stepwise dehydrogenation of NH3 and the associative desorption of N are determined, and the zero point energy correction is included, which makes it possible to compute the rate constants of the elementary steps in NH3 decomposition. Combined DFT calculations and kinetic analysis at 350K indicate that the associative desorption of N has a reaction rate lower than NHx dehydrogenation and is therefore the rate determining step. The distinctly different rate constants over Ni(110), Ni(111) and Ni(211) imply that ammonia decomposition over Ni-based catalyst is a structure-sensitive reaction. [Copyright &y& Elsevier]

Abstract: Fe nanoparticles on the top of carbon nanofibers (CNFs) were synthesized by catalytic CVD on a purpose as catalysts for ammonia decomposition. The size and shape of Fe particles on the top of CNFs depended on the Fe particle reconstruction and CNF morphology. On a mica support Fe catalyst, platelet structure of CNFs was obtained with small, uniform, and unwrapped Fe particles on the top. HRTEM illustrated that the Fe particle surface is covered by a few graphene layers, which redissolves into the Fe particle and does not prevent the access of NH3 to active surface sites at reaction conditions. The catalyst is highly active and stable because the Fe particles are highly dispersed and physically isolated by CNFs, and the surfaces are largely exposed to the reactants. [ABSTRACT FROM AUTHOR]