Methane cracking over cobalt molybdenum carbides

The catalytic behaviour of Co3Mo3C, Co6Mo6C, Co3Mo3N and Co6Mo6N for methane cracking has been studied to determine the relationship between the methane cracking activity and the chemical composition. The characterisation of post-reaction samples showed a complex phase composition with the presence of Co3Mo3C, α-Co and β-Mo2C as catalytic phases and the deposition of different forms of carbon during reaction

Radicals in carbonaceous residue deposited on mordenite from methanol

It is shown that control of the degree of coking can lead to the observation of hyperfine structures in the carbonaceous residues deposited from methanol over mordenite (H-MOR) at temperatures relevant to the conversion of methanol to hydrocarbons. EPR measurements of the catalyst samples at various times on stream have been recorded, with a rich hyperfine splitting pattern observed in the early stages of the reaction. Interpretation of the EPR data with the aid of density functional theoretical calculations has afforded the first definitive assignment of the radical cations formed in high temperature coke. The results detail a shortlist of six species: 2,3/2,6/2,7-dimethylnaphthalenium, 2,3,6-trimethylnaphthalenium, 2,3,6,7-tetramethylnaphthalenium, and anthracenium radical cations whose proton hyperfine splitting profiles match the experimental spectra; 2,3,6,7-tetramethylnaphthalenium showed the best agreement. The observation of these particular isomers of polymethylnaphthalene suggest the formation of more highly branched polyaromatic species is less likely within the confines of the H-MOR 12-membered ring channel. These radicals formed when the catalyst is active may constitute key intermediates in the conversion of methanol to light olefins

Synthesis and methane cracking activity of a silicon nitride supported vanadium nitride nanoparticle composite

The co-ammonolysis of V(NMe2)4 and Si(NHMe)4 with ammonia in THF and in the presence of ammonium triflate ([NH4][CF3SO3]) leads to the formation of monolithic gels. Pyrolysing these gels produces mesoporous composite materials containing nanocrystalline VN and amorphous silicon imidonitride. Elemental mapping indicated a thorough distribution of VN with no evidence of large cluster segregation. Whilst not active for ammonia synthesis, the silicon nitride based materials were found to possess activity for the COx-free production of H2 from methane, which makes them candidates for applications in which the presence of low levels of CO in H2 feedstreams is detrimental

An in situ XAS study of the cobalt rhenium catalyst for ammonia synthesis

A cobalt rhenium catalyst active for ammonia synthesis at 400 °C and ambient pressure was studied using in situ XAS to elucidate the reducibility and local environment of the two metals during reaction conditions. The ammonia reactivity is greatly affected by the gas mixture used in the pre-treatment step. Following H2/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H2/N2 pre-treatment. In situ XAS at the Co K-edge and Re LIII-edge show that cobalt initiates reduction, undergoing reduction between 225 and 300 °C, whereas reduction of rhenium starts at 300 °C. The reduction of rhenium is near complete below 400 °C, as also confirmed by H2-TPR measurements. A synergistic co-metal effect is observed for the cobalt rhenium system, as complete reduction of both cobalt and rhenium independently requires higher temperatures. The phases present in the cobalt rhenium catalyst during ammonia production following both pre-treatments are largely bimetallic Co–Re phases, and also monometallic Co and Re phases. The presence of nitrogen during the reduction step strongly promotes mixing of the two metals, and the bimetallic Co–Re phase is believed to be a pre-requisite for activity

Removal of Arsenic (III) from groundwater applying a reusable Mg-Fe-Cl layered double hydroxide

BACKGROUND: Layered double hydroxide compounds (LDHs) have been applied for the removal of oxyanions including arsenate (As(V)). However, the aim of this present research is to develop a LDH to treat arsenite (As(III)). Both batch and column sorption studies were conducted to assess the effect of LDH dosage, contact time, solution pH and initial As(III) concentrations on the As(III) removal performance. The potential re-use of this sorbent was also investigated.<p></p> RESULTS: For 2 g L−1 of Mg-Fe-Cl LDH, As(III) in test solution can be reduced from 400 µg L−1 to <10 µg L−1 after a contact time of 2 h. High As(III) concentration in Bangladesh groundwater can be reduced to meet the national drinking water standards (<50 µg L−1). The maximum adsorption capacity of As(III) by Mg-Fe-Cl LDH is 14.6 mg g−1-LDH. Further, reusability of this sorbent was at least 20 cycles of regeneration with effective As(III) removal between 93.0 and 98.5%. Moreover, As(III) removal was unaffected by the solution pH but affected by the co-existing competing anions and concentration of As(III). Finally, the main mechanism of As(III) removal by Mg-Fe-Cl LDH was suggested to be chemical sorption together with anion and ligand exchange with interlayer Cl− and OH− ions.<p></p> CONCLUSION: High efficiency of sorption of As(III) by the developed Mg-Fe-Cl LDH was demonstrated in this study which is generally not the case for most other sorbent materials. Pilot-scale trials are needed to explore the suitability of full application of the developed Mg-Fe-Cl LDH for the removal of As(III).<p></p&gt

Iron ochre: a pre-catalyst for the cracking of methane

Background: Iron ochres are gelatinous sludges which can cause problems in terms of water management. In this manuscript, the application of iron ochre obtained from a river has been applied to catalytically crack methane – another potential waste product - into two useful products, hydrogen and a magnetic carbon containing composite.<p></p> Results: The powder XRD pattern of the iron ochre was found to be consistent with the expected 2-line ferrihydrite and EDX analysis showed Fe to be a major component although some Si was present. The sample was observed to contain a fraction with a tubular morphology consistent with the presence of extra-cellular biogenic iron oxide formed by leptothrix. Upon exposure to methane at elevated temperatures, the material was found to transform into an active catalyst for hydrogen production yielding a magnetic carbon containing composite material comprising filamentous carbon and encapsulating graphite.<p></p> Conclusion: The application of two waste products – iron ochre and methane – to generate two useful products – hydrogen and a magnetic carbon containing composite- has been demonstrated. Furthermore, the ochre has been shown to comprise tubular morphology extra-cellular biogenic iron oxide which may be of interest in terms of other applications.<p></p&gt

Nitrides, hydrides and carbides as alternative heterogeneous catalysis for ammonia synthesis: A brief overview

Driven by the desire to develop novel catalyst formulations which are applicable for localised, more sustainable routes, the area of heterogeneously catalysed ammonia synthesis has attracted much attention in the academic literature in recent times. One of the key incentives for this has been the idea that ammonia synthesis for the production of synthetic fertiliser can be achieved on, for example, a farm close to its point of application with the required hydrogen feedstream being derived from sustainable sources such as electrolysis of water accomplished using electricity produced using wind turbines or solar energy sources. Further drivers are the possible application of ammonia as a non-fossil based fuel and also as a means to indirectly store intermittent over-supply of sustainably derived electricity. In the literature, the energy intensive nature of the Haber Bosch Process, frequently quoted to be 1-2% of global energy demand, and its CO2 footprint, stated to comprise 2.5% of fossil fuel based emissions, are statistics that are frequently quoted in justification for the search for new routes to ammonia production [1,2]. However, due recognition has to be given to the highly efficient integration of the Haber Bosch Process as currently operated. In relation to this, large scale synthesis of ammonia is highly optimised and it can be credited with the sustenance of ca 40% of the global population. These considerations, coupled to the recently reported UK CO2 supply chain shortage, related to a reduction in commercial fertiliser production [3], underline the importance of the highly integrated nature of the process

A green approach for the removal of Sr(II) from aqueous media: kinetics, isotherms and thermodynamic studies

Adsorption efficiency of native, NaOH-treated and immobilized peanut husk and sugarcane bagasse for Sr(II) removal was studied in batch mode. In view of promising adsorption efficiency of peanut husk versus sugarcane bagasse, the biosorption behaviour of the Sr(II) ions onto peanut husk (native and modified) was studied as a function of pH, biosorbent dose, contact time, initial metal ion concentration and temperature for the maximum removal of Sr(II) ions. Linear and non-linear pseudo-first and second-order kinetic models were applied and value of R2 and six non-linear regression error functions, namely hybrid fractional error function (HYBRID), Marquardt's percent standard deviation (MPSD), average relative error (ARE), the sum of the errors squared (ERRSQ/SSE), the sum of the absolute errors (EABS) and Chi-square test (χ2) were used to predict the most optimum kinetic model. Sorbent-sorbate reaction nature was estimated by fitting equilibrium data by non-linear and transformed linear forms of the Langmuir, Freundlich and Redlich-Peterson isotherms and most optimum isothermal model was optimized by comparing linear and non-linear R2 value and non-linear regression error functions. HCl proved most successful eluating agents for sorbed Sr(II) ions. Biosorption characteristics and effectiveness of the process was also confirmed by Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX). In view of promising efficiency of peanut husk as an adsorbent, it could possibly be used for the removal of Sr(II) ions from aqueous medium and is also extendable to other radionuclide

The potential of manganese nitride based materials as nitrogen transfer reagents for nitrogen chemical looping

A systematic study was carried out to investigate the potential of manganese nitride related materials for ammonia production. A-Mn-N (A = Fe, Co, K, Li) materials were synthesised by nitriding their oxide counterparts at low temperature using NaNH2 as a source of reactive nitrogen. The reactivity of lattice nitrogen was assessed using ammonia synthesis as a model reaction. In the case of Mn3N2, limited reactivity was observed and only 3.1% of the available lattice nitrogen was found to be reactive towards hydrogen to yield ammonia while most of the lattice nitrogen was lost as N2. However, the presence of a co-metal played a key role in shaping the nitrogen transfer properties of manganese nitride and impacted strongly upon its reactivity. In particular, doping manganese nitride with low levels of lithium resulted in enhanced reactivity at low temperature. In the case of the Li-Mn-N system, the fraction of ammonia formed at 400 °C corresponded to the reaction of 15% of the total available lattice nitrogen towards hydrogen. Li-Mn-N presented high thermochemical stability after reduction with hydrogen which limited the regeneration step using N2 from the gas phase. However, the results presented herein demonstrate the Li-Mn-N system to be worthy of further attention

Low-T mechanisms of ammonia synthesis on Co3Mo3N

Dispersion-corrected periodic DFT calculations have been applied to elucidate the Langmuir–Hinshelwood (dissociative) and an Eley–Rideal/Mars–van Krevelen (associative) mechanism for ammonia synthesis over Co3Mo3N surfaces, in the presence of surface defects. Comparison of the two distinct mechanisms clearly suggests that apart from the conventional dissociative mechanism, there is another mechanism that proceeds via hydrazine and diazane intermediates that are formed by Eley–Rideal type chemistry, where hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions. This result clearly suggests that via surface defects ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis