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

CATALYTIC CRACKING OF TOLUENE USING RED MUD: HYDROGEN EVOLUTION AND CARBON FORMATION.

Toluene has been used as a model tar compound in many cracking and reforming studies in the context of biomass gasification and pyrolysis. Low cost alternate catalysts such as biochar, limonite have been studied. Toluene has also been used in chemical vapour deposition of carbon nano tubes. In this study, red mud waste from the aluminium industry without any pre treatment was used as a catalyst for toluene cracking. The hydrogen evolution and nano carbon formation were studied. The reduction behaviour of the catalyst in hydrogen showed conversion of iron hydroxide and oxide to iron above 700 ?C. Hydrogen formation rate was maximum at 800 ?C after around 7 h and subsequently reduced with time. Depending on the carbon content, the post reaction catalyst was comprised of reduced iron oxides (magnetite, wustite) and mixture of iron, iron carbide and graphitic carbon. Carbon content up to 46% was obtained and the deposited carbon was in the form of nano fibers. While red mud has been used in catalytic steam gasification and pyrolysis of biomass, the hydrogen evolution using red mud and toluene has not been reported

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

DFT-D3 study of molecular N2 and H2 activation on Co3Mo3N surfaces

Cobalt molybdenum nitride (Co3Mo3N) is one of the most active catalysts for ammonia synthesis, although the atomistic details of the reaction mechanism are currently unknown. We present a dispersion-corrected (D3) DFT study of the adsorption and activation of molecular nitrogen and hydrogen on Co3Mo3N-(111) surfaces to identify possible activation sites for ammonia synthesis. H2 was found to adsorb both molecularly on the Mo3N framework and dissociatively on Co8 clusters or Mo3 clusters that were exposed due to N-vacancies. We find that there are two possible activation sites for N2 where both N2 and H2 can coadsorb. The first is a Mo3 triangular cluster that resides at 3f nitrogen vacancies, and the second is a surface cavity where N2 is activated by a Co8 cluster, the second being a more efficient activation site. N2 was found to adsorb in three adsorption configurations: side-on, end-on, and an unusual tilt end-on (155°) configuration, and the existence of these three adsorption configurations is explained via MP2 calculations and the sphere-in-contact model

Nitrogen activation in a Mars-van Krevelen mechanism for ammonia synthesis on Co3Mo3N

Co3Mo3N is one of the most active catalysts for ammonia synthesis; however, little is known about the atomistic details of N2 adsorption and activation. Here we examine whether N2 can adsorb and activate at nitrogen surface vacancies. We have identified the most favorable sites for surface nitrogen vacancy formation and have calculated vacancy formation free energies (and concentrations) taking into account vacancy configurational entropy and the entropy of N2 at temperature and pressure conditions relevant to ammonia synthesis (380–550 °C, 100 atm) via a semiempirical approach. We show that 3-fold hollow bound nitrogen-containing (111)-surfaces have surprisingly high concentrations (1.6 × 1016 to 3.7 × 1016 cm–2) of nitrogen vacancies in the temperature range for ammonia synthesis. It is shown that these vacancy sites can adsorb and activate N2 demonstrating the potential of a Mars–van Krevelen type mechanism on Co3Mo3N. The catalytically active surface is one where 3f-hollow-nitrogens are bound to the molybdenum framework with a hexagonal array of embedded Co8 cobalt nanoclusters. We find that the vacancy-formation energy (VFE) combined with the adsorption energy can be used as a descriptor in the screening of materials that activate doubly and triply bonded molecules that are bound end-on at surface vacancies

Modelling the performance of a syngas fuelled engine: effect of excess air and CO2 as combustion diluents

No abstract available

DFT-D3 study of H 2 and N 2 chemisorption over cobalt promoted Ta 3 N 5-(100), (010) and (001) surfaces

The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol−1. H2 was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol−1. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol−1 per H2. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta3N5-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol−1. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta3N5. The studies offer significant new insight with respect to the chemistry of N2 and H2 with tantalum nitride surfaces in the presence of cobalt promoters

Sol–gel preparation of low oxygen content, high surface area silicon nitride and imidonitride materials

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely α-Si3N4 and structural similarities are observed between the amorphous and crystallised materials

A Comparison of the reactivity of the lattice nitrogen in tungsten substituted Co3Mo3N and Ni2Mo3N

The effect of the partial substitution of Mo with W in Co3Mo3N and Ni2Mo3N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co3Mo3N was reduced to Co6Mo6N under a 3 : 1 ratio of H2/Ar. Co3Mo2.6W0.4N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni2Mo3N and Ni2Mo2.8W0.2N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni2Mo3N is more endothermic than Co3Mo3N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N2 adsorption and activation at the vacancy site