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

Synthèse et caractérisation des oxydes-mixtes nanocristallins de type hexaaluminate : propriétés en mobilité d'oxygène et en catalyse d'oxydation

Depuis les années 1970, les oxydes mixtes de type hexaaluminate suscitent un intérêt certain pour les réactions catalytiques du fait de leur stabilité thermique élevée, leur conférant un fort potentiel pour les réactions se déroulant à haute température. Cependant, la majorité des procédés de synthèse adoptés pour la préparation de ces derniers nécessite au moins une étape de traitement thermique à haute température afin d’achever le processus de cristallisation. Ainsi, les hexaaluminates préparées par voie chimique classique présentent des tailles de particule larges et des aires spécifiques réduites (< 20 m2 g-1). Dans le cadre de ce travail, l’optimisation des paramètres et des conditions de synthèse pour la préparation d’hexaaluminates nanocristallines de grande aires spécifiques a fait l’objet d’une étude détaillée. Le broyage réactif s’est révélé être une méthode de synthèse efficace, et des aires spécifiques très élevées ont été obtenues (> 70 m2 g-1). Il a été observé que la nature et la concentration du métal de transition inséré dans la structure influence fortement les propriétés redox et la mobilité d’oxygène de ces solides. Cependant, pour une même composition chimique, les propriétés redox et la mobilité de l’oxygène sont conditionnées par les propriétés structurales et texturales. L’étude des propriétés catalytiques des hexaaluminates en oxydation de CH4 et de CO a montré que l’activité de ces derniers résulte d’un équilibre complexe entre les propriétés texturales et structurales, l’état de la surface (nature et concentration des sites redox), et les propriétés de réductibilité et de mobilité d’oxygène.Since the beginning of the 1970’s, hexaaluminate mixed oxides were proposed as efficient materials for catalytic reactions at high temperature, e.g. catalytic combustion. Their abilities to maintain phase stability and high surface area are considered as great properties. Unfortunately, most of the reported chemical synthesis methods for hexaaluminate preparation involve at least one calcinations step at high temperature (> 1100°C) to crystallize the desired pure phase, which leads to crystal growth and unavoidable surface reduction. The catalytic performances of hexaaluminate materials depend essentially to the structural and textural properties i.e. surface area, crystal size, and phase purity. Thus, the first part of this study was dedicated to the study of an original synthesis route, the "Activated Reactive Synthesis" process that is evidenced as a promising top down approach to generate nanostructured hexaaluminate with high surface area. Values of surface areas obtained were largely higher (> 77 m2 g-1) to those reported for hexaaluminates prepared by conventional routes (~ 20 m2 g-1). The nature of the transition metal, Mn+, inserted in the hexaaluminate structure, and its concentration, play a key role on the redox behaviours and the oxygen transfer properties. Nevertheless, for a same chemical composition, the redox properties and oxygen mobility were found to be dependent to the structural and textural properties. Activities of hexaaluminate materials in CO and CH4 oxidation reactions are reported to depend on a complex balance between structural and textural properties, surface state, reducibility, and oxygen mobility in the bulk

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

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

DFT-D3 study of H-2 and N-2 chemisorption over cobalt promoted Ta3N5-(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

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co_{3}Mo_{3}N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co₃Mo₃N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases