Study of the cap structure of (3,3), (4,4) and (5,5)-SWCNTs: application of the sphere-in-contact model

We have applied the sphere-in-contact model supported by hybrid Density Functional Theory (DFT) calculations to elucidate the cap geometry of the sub-nanometer in dimension (3,3), (4,4) and (5,5) single-wall carbon-nanotubes (SWCNTs). Our approach predicts certain cap-geometries that do not comprise of the commonly known for their stability combination of pentagonal and hexagonal carbon rings but also tetragonal, trigonal and all-pentagonal structures. Based on hybrid-DFT calcula-tions carbon atoms in these new cap geometries have similar stability to carbon found in other fullerene-like capped zig-zag and arm-chair nanotubes (i.e., (5,5), (6,6), (9,0) and (10,0)) that are known to be stable and synthetically accessible. We find that the cap structure of the (3,3)-CNTs is a pointy carbon geometry comprised of six pentagonal rings with a single carbon atom at the tip apex. In this tip geome-try the carbon atom at the tip apex does not have the usual sp2 or sp3 geometry but an unusual trigonal pyramidal configuration. DFT calculations of the molecular orbitals and density-of-states of the tip show that this tip structure apart from being stable can be used in scanning probe microscopies such as STM for very high resolution imaging

The integration of experiment and computational modelling in heterogeneously catalysed ammonia synthesis over metal nitrides

In this perspective we present recent experimental and computational progress in catalytic ammonia synthesis research on metal nitrides involving a combined approach. On this basis, it suggested that the consideration of nitrogen vacancies in the synthesis of ammonia can offer new low energy pathways that were previously unknown. We have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies on their surfaces. These vacancies adsorb dinitrogen much more strongly than the defect-free surfaces and can efficiently activate the strong N–N triple bond. These fundamental studies suggest that heterogeneously catalysed ammonia synthesis over metal nitrides is strongly affected by bulk and surface defects and that further progress in the discovery of low temperature catalysts relies on more careful consideration of nitrogen vacancies. The potential occurrence of an associative pathway in the case of the Co3Mo3N catalytic system provides a possible link with enzymatic catalysis, which will be of importance in the design of heterogeneous catalytic systems operational under process conditions of reduced severity which are necessary for the development of localised facilities for the production of more sustainable “green” ammonia

An experimental and computational IR and hybrid DFT-D3 study of the conformations of l-lactic and acrylic acid: new insight into the dehydration mechanism of lactic acid to acrylic acid

We have studied using hybrid Density Functional Theory (DFT) with an aug-cc-pVTZ basis set and D3 dispersion corrections the intra-molecular hydrogen bond of L-lactic acid and L-lactic-acid analogs with the hydroxyl group on the alpha carbon atom substituted by α-XH (where X = S, Se, Te) as well as the conformations of acrylic acid. The results show that there are three types of intramolecular hydrogen bonds that can form only when α-OH is present, whereas other less electronegative functional groups such as –SH, –SeH and –TeH do not exhibit the formation of an intramolecular H-bond. We show that the intra-molecular H-bond formed between the alpha-OH hydrogen and the COOH carbonyl oxygen would enhance the rate of nucleophilic substitution of alpha-OH at the K+ sites in the previously suggested dehydration mechanism of L-lactic to acrylic acids. We find that a temperature range between 190 and 210 °C would be optimum to maximise the rate of nucleophilic substitution of the alpha-OH group at the potassium sites during the dehydration mechanism of L-lactic acid to acrylic acid. Additionally, our hybrid-DFT simulation of the infrared spectrum of the various conformers shows that the lowest energy conformer can be identified by a single vibrational band at 3734 cm−1 whereas for the other conformers, this vibrational band is split with Δν that ranges between 6 cm−1 and 176 cm−1. We also find that the various conformers of acrylic acid can be identified by a double peak for the C[double bond, length as m-dash]O and O–H vibrations, which have Δν′ and Δν′′ values of 24 and 42 cm−1, respectively. This computational study is useful for spectroscopic experimental efforts that try to identify the various conformers of L-lactic acid and acrylic acid and to gain mechanistic insight into the dehydration mechanism over K substituted NaY zeolites

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

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

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

On the possibility of an Eley-Rideal mechanism for ammonia synthesis on Mn6N5+x (x=1)-(111) surfaces

Recently we reported an Eley–Rideal/Mars–van Krevelen mechanism for ammonia synthesis on cobalt molybdenum nitride (Co3Mo3N). In this mechanism hydrogenation of activated dinitrogen occurs directly from the gas phase in a low barrier step forming a hydrazinylidene intermediate [double bond, length as m-dash]NNH2. In this paper we study whether such a mechanism of ammonia synthesis could occur on the (111) surface of another metal nitride, Mn6N5+x (x = 1), as this would explain the low-T ammonia synthesis activity of Co3Mo3N. We find that although N2 adsorbs more strongly than H2 on the (111) surface, having also examined the (110) and the (100) surface, N2 is not significantly activated when adsorbed in an end-on configuration. The hydrogenation reactions via an Eley–Rideal mechanism are all high barrier processes (>182 kJ mol−1) and therefore an Eley–Rideal mechanism for ammonia synthesis is predicted to not occur on this material unless there are high temperatures. Our study indicates that the fact that an Eley–Rideal/Mars–van Krevelen mechanism occurs on Co3Mo3N is a result of the stronger activation of dinitrogen at nitrogen vacancies when dinitrogen is adsorbed in an end-on configuration

Mechanisms of ammonia and hydrazine synthesis on η-Mn3N2-(100) surfaces

Understanding the mechanism of catalytic reactions is crucial for the future development of catalysts. In this computational study, dispersion-corrected Density Functional Theory (DFT) theory was used to calculate the various mechanistic pathways for ammonia and hydrazine synthesis on η-Mn3N2-(100) surfaces. A simple Lewis structure representation algorithm was used in order to locate various possible NxHy intermediates. Hydrogenation of dinitrogen results in significant activation of the inert triple bond and these intermediates have a significant role in the ammonia and hydrazine synthesis reaction on manganese nitrides via a Langmuir-Hinschelwood mechanism. It is anticipated that these findings are significant in developing new catalysts for hydrazine synthesis using η-Mn3N2 (100) catalysts

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