Stepwise Dehydrogenation Research Articles

Model catalytic studies on the thermal dehydrogenation of the benzaldehyde/cyclohexylmethanol LOHC system on Pt(111).

We investigated the dehydrogenation reaction and the thermal robustness of the liquid organic hydrogen carrier (LOHC) couple benzaldehyde/cyclohexylmethanol on a Pt(111) model catalyst in situ in synchrotron radiation photoelectron spectroscopy- and complementary temperature-programmed desorption experiments. The system stores hydrogen in a cyclohexyl group and a primary alcohol functionality and achieves an attractive hydrogen storage capacity of 7.0 mass%. We observed a stepwise dehydrogenation mechanism, characterized by a low temperature dehydrogenation of the alcohol group at 235 K. However, stability limitations challenge the system's applicability as reversible hydrogen storage solution, as the resultant aldehyde was found to decompose during the dehydrogenation of its cyclohexyl group (between 250 and 350 K). A comparison of cyclohexylmethanol with the structurally related secondary alcohol (1-cyclo-hexylethanol; 6.3 mass% hydrogen) revealed a parallel stepwise dehydrogenation pattern for both compounds, but a technically relevant superior thermal robustness of the latter. This demonstrates the influence of the alcohol group's substitution degree on the dehydrogenation characteristics of alcohol-functionalized LOHC compounds. Density functional theory calculations are in good agreement with the experimentally observed stability trend.

On-Surface Stepwise Double Dehydrogenation for the Formation of a para-Quinodimethane-Containing Undecacene Isomer.

The on-surface synthesis of an isomer of undecacene, bearing two four-membered rings and two para-quinodimethane moieties, starting from a tetramethyl-substituted diepoxy precursor, is presented. The transformation implies a thermal double deoxygenation followed by a stepwise double dehydrogenation reaction on the Au(111) surface, locally induced by inelastic tunneling electrons. This results in the transformation of para-dimethylbenzene moieties into non-aromatic para-quinodimethanes. The structures and electronic properties of the intermediate and final products are investigated at the single molecule level with high spatial resolution, using both scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy. The experimental results are supported by density functional theory calculations.

Plasma-catalytic dry reforming of CH4: Effects of plasma-generated species on the surface chemistry

By means of steady-state experiments and a global model, we studied the effects of plasma-generated reactive species on the surface chemistry and coking in plasma-catalytic CH4/CO2 reforming at reduced pressure (8–40 kPa). We used a hybrid ZDPlasKin-CHEMKIN model to predict the species densities over time. The detailed plasma-catalytic mechanism consists of the plasma discharge scheme, a gas-phase chemistry set and a surface mechanism. Our experimental results show that the coupling of Ni/SiO2 catalyst with plasma is more effective in CH4/CO2 activation and conversion than unpacked DBD plasma, with syngas being the main products. The highest total conversion of 16 % was achieved at 8000 V and 473 K, with corresponding CO and H2 yields of 15 % and 12 %, respectively. The reactants conversion and product selectivity are well captured by the kinetic model. Our simulation results suggest that vibrational species and radicals can accelerate the dissociative adsorption and Eley-Rideal (E-R) reactions. Path flux analysis shows that E-R reactions dominate the surface reaction pathways, which differs from thermal catalysis, indicating that the coupling of non-equilibrium plasma and catalysis can effectively shift the formation and consumption pathways of important adsorbates. For instance, our model suggests that HCOO(s) is primarily generated through the E-R reaction CO2(v) + H(s) → HCOO(s), while the hydrogenation reaction HCOO(s) + H → HCOOH(s) is the main source of HCOOH(s). Carbon deposition on the catalyst surface is primarily formed through the stepwise dehydrogenation of CH4, while the E-R reactions enhanced by plasma-generated H and O atoms dominate the consumption of carbon deposition. This work provides new insights into the effects of reactive species on the surface chemistry in plasma-catalytic CH4/CO2 reforming.

Design of TiO2-based bi-functional catalyst and the catalytic mechanism of enhanced hydrazine borane hydrolysis for hydrogen production

In this study, a scheme of photocatalytic coupled metal catalytic synergistic enhancement of hydrazine borane hydrolysis is proposed, the catalytic reaction mechanism for hydrogen production from hydrazine borane hydrolysis is revealed. In addition, the effects of solvent property, external electric field conditions on the stability of N2H4BH3 molecule are also investigated systematacially. The results show that the band structure of pure TiO2 can be modulated by heterogeneous element N, C, Ni, Ru, the modification effect of co-doping with heterogeneous alloy elements (eg. NiPt@N–TiO2) is best in all doping schemes. The hydrolysis reaction of -BH3 of N2H4BH3 catalyzed by Pt metal is a stepwise dehydrogenation reaction, and the energy barrier of the reaction rate-determining of the B–N bond breakage is 50.49 kcal/mol. There are two possible cleavage reaction pathways for N2H4, which are hydrogen-producing pathway and ammonia-producing pathway, the energy barrier for the reaction rate-determining step is 67.06 kcal/mol and 82.51 kcal/mol, respectively, the hydrogen production reaction takes precedence over the ammonia production reaction. OH− solution is best among the neutral aqueous solution, rich H+, NH4+ and NaOH solution in B–H bond activation of N2H4BH3 molecule, the external electric field can promote the activation of B–H bond in hydrazine borane hydrolysis system, which is positive factor in improving the performance of hydrazine borane hydrolysis.

Mechanistic investigation of ammonia adsorption and reforming over Ni, Ru loaded catalysts toward green H2 production of marine engines: DFT calculation and experimental evaluation

Highly active catalysts suitable for ammonia reforming reactions have been comprehensively investigated by density functional theory (DFT) calculations and experimental tests. Non-precious metal Ni, precious metal Ru and Ru-Ni composite metal are selected as catalytic activity surfaces, and a series of three-dimensional metal-packed catalyst models are constructed parallelly. Then, to elucidate the rate-determining step of reforming reaction, in-depth analysis of microscopic reaction mechanisms for amino groups (including H and NHx, x=0–3) adsorption, dissociation, recombination and desorption are carried out by simulation parameters and experimental data. The results indicate that NH3 exhibits highly stable adsorption configurations (Bridge site) on both Ni, Ru-based catalyst surfaces, with the adsorption position having a relatively minor effect on adsorption energy. It is worth noting that Ni-based catalyst exhibits better adsorption behaviors towards NH3 compared to Ru. Whereas, during stepwise dehydrogenation of NH3 and subsequent N2 and H2 recombination and desorption processes, the reaction energy barriers confirm experimental observations of higher activity displayed by Ru-based catalyst in low-temperature range of 400–500 °C. Interestingly, N2 synthesis behavior of innovative Ru-Ni bimetallic tandem catalyst surface possesses the lowest reaction energy barrier than other models, which is attributed to the adsorption capability of Ni layers and the catalytic activity of Ru atoms. The experimental results also demonstrate that under typical engine exhaust conditions, the H2 production efficiency of 1 %Ru-3 %Ni/Al2O3 surpasses that of 1 %Ru/Al2O3 by 6.2 %, showcasing exceptional stability.

Unveiling proton-responsive sites and reaction mechanisms in formic acid dehydrogenation catalyzed by Cp*Ir(III)-Pyridylpyrrole complexes: A DFT study

Transition metal catalysts based on proton-responsive ligands are crucial in the realm of chemical hydrogen storage. Nevertheless, the existence of multiple proton-responsive active sites on the ligand adds complexity to the mechanism, limiting our understanding of mechanistic preferences. In this study, density functional theory calculations are used to clarify the mechanistic preferences of iridium catalysts with multiple proton-responsive sites on the ligand for formic acid dehydrogenation. The calculated results show that, with the assistance of the α carbon atom (C2) of the pyrrole ligand site, the catalyst undergoes through an inner-sphere stepwise dehydrogenation mechanism. Moreover, the proton shuttle mechanism efficiently reduces the activation energy needed for H2 production, and solvent-assisted dehydrogenation shows lower energy barriers than substrate-assisted dehydrogenation. The reasons for mechanistic selectivity are further elucidated through Fukui function analysis, molecular plane parameter analysis, and the distortion-interaction model. These findings are anticipated to establish a theoretical foundation for the future design of high-performance catalysts for formic acid dehydrogenation.

High performance catalyst for metal catalysis coupled with photocatalysis and its catalytic ammonia borane methanolysis for hydrogen production

In the study, the influence and mechanisms of metal and non-metal dopants on the photochemical properties of the semiconductor catalyst g-C3N4 are investigated. The results reveal that doping Ni, Ru, and B on g-C3N4 results in significant alterations to the energy band structure, the band gap value decreases to 0.78 eV (Ru@g-C3N4), 0.69 eV (Ni@g-C3N4), 0.5 eV (RuB@B-g-C3N4), and 0.1 eV (NiB@B-g-C3N4) from the original value of 1.139 eV. The doping of metal and non-metal atoms can significantly enhance the light absorption capability and photothermal conversion efficiency of B-g-C3N4. The mechanism of ammonia borane (AB) dehydrogenation follows the stepwise dehydrogenation principle. AB molecules adsorb onto the active metal centers, and with the assistance of the catalytic sites, CH3OH molecules activate and attack the B–N bond in AB molecules, leading to the initial breakage of the B–N bond, the interaction between O atoms in CH3OH and N atoms in AB continues, forming NH3BH2(OCH3). Interactions between H(B) and H(O) result in the production of H2, the energy barrier for this reaction is 34.14 kcal/mol. Compared to dark conditions, RuB@B-g-C3N4 exhibits higher catalytic activity under light irradiation, the reaction time from 9′20″ reduce to 8′. This shows that there is a synergistic effect between photocatalytic AB methanolysis and conventional metal catalyzed AB methanolysis in the RuB@B-g-C3N4 catalytic system. The study provides the novel way for the development of dual-function catalysts combining metal catalysis and photocatalysis through Ru and B modification of g-C3N4.

Positive Electronic Nickel Active Site Enhances N─H/C─H Bonds Breaking for Electrooxidation of Amines to Nitriles Coupling with Hydrogen Production

AbstractElectrocatalytic conversion of amines to nitriles can be used as biomass upgrading to assist hydrogen production in water electrolysis. However, achieving efficient N─H/C─H bond breaking in amines oxidation remains a challenge. Herein, the nitrogen‐doped carbon supported NiO nanoparticles with positive electronic nickel active site are simply synthesized to achieve ∽99% selectivity and complete conversion in electrooxidizing amines to nitriles, and the onset potential of the whole reaction is as low as 1.32 V vs. RHE. Detailed in situ characterizations and theoretical calculations prove that the carbon substrate enhances the interaction between the N 2p orbital of amines and the Ni 3d orbital, which favors the construction of an indirect oxidation intermediate state R‐CH2NH2···NiO(OH)abs and promotes the subsequent stepwise dehydrogenation to nitriles. This whole reaction produces 16.4 times more hydrogen than pure water splitting, facilitating the development of the chemicals industry and energy utilization.

Defect-Confinement Strategy for Constructing CuO Clusters on Carbon Nanotubes for Catalytic Oxidation of AsH3 at Room Temperature.

The efficient removal of the highly toxic arsine gas (AsH3) from industrial tail gases under mild conditions remains a formidable challenge. In this study, we utilized the confinement effect of defective carbon nanotubes to fabricate a CuO cluster catalyst (CuO/ACNT), which exhibited a capacity much higher than that of CuO supported on pristine multiwalled carbon nanotubes (MWCNT) (CuO/PCNT) for catalytically oxidizing AsH3 under ambient conditions. The experimental and theoretical results show that nitric acid steam treatment could induce MWCNT surface structural defects, which facilitated more stable anchoring of CuO and then improved the oxygen activation ability, therefore leading to excellent catalytic performance. Density functional theory (DFT) calculations revealed that the catalytic oxidation of AsH3 proceeded through stepwise dehydrogenation and subsequent recombination with oxygen to form As2O3 as the final product.

Neutral Silylyne Complex of Molybdenum: Synthesis, Properties, and Access to Silaiminoacyl Complexes via [2+3] Cycloaddition.

A neutral silylyne complex of molybdenum was synthesized by the stepwise dehydrogenation method and its properties were compared with those of the tungsten analog. The complex takes a dimeric form as crystals but afford a monomer-dimer equilibrium in solution. The replacement of the central metal from W to Mo led to a monomer dominant (~98 %) solution at room temperature. The monomer-dimer dynamics was investigated based on thermodynamic parameters. The molybdenum silylyne complex underwent [2+2] cycloaddition with alkynes much faster than the tungsten analog. The reactions with organic azides led to the formation of the first example of silaiminoacyl complexes through [2+3] cycloaddition. The structures and bonding aspects of the products were clarified by multiple measurements and DFT calculations.

Theoretical insight into the acceptorless dehydrogenation catalyzed by the transition metal bifunctional catalysts: Mechanism and metal effect

In this paper, the density functional theory study was performed to investigate the reaction mechanism of acceptorless dehydrogenation catalyzed by bifunctional catalysts. The calculation results indicate that the catalyst bearing the Co(III) center adopts the inner-sphere stepwise dehydrogenation mechanism. The H2 formation was found to be the rate-determining step, and the proton-shuttle type transition state effectively reduces the activation energy barrier. Furthermore, the catalytic activities of the noble metals (Rh and Ir) were investigated and compared. The results suggest that the noble metals prefer the outer-sphere bifunctional transfer mechanism for the dehydrogenation reaction. Frontier molecular orbital analysis shows that the HOMO-LUMO gap of the active species increases along with the periodic number, which leads to a weakening of the coordination strength of the substrate to the metal center, resulting in the different mechanistic selectivity toward dehydrogenation reaction. In addition, based on the distortion-interaction analysis, the different activation energy barrier in the H2 formation step is illuminated. The reduced ionic radii of the Co result in lower deformation energies of the catalyst and substrate structures in the H2 formation transition state, thus lowering the activation energy barrier. Our results reveal the advantages of base metal in dehydrogenation reactions, which is expected to provide theoretical guidance for future catalyst design.

Liquid Organic Hydrogen Carriers: Model Catalytic Studies on the Thermal Dehydrogenation of 1-Cyclohexylethanol on Pt(111)

The molecule pair of 1-cyclohexylethanol and acetophenone represents an interesting system for a chemical hydrogen storage cycle as it combines two different classes of so-called liquid organic hydrogen carriers (LOHCs), namely hydrogenated, formerly aromatic cyclic hydrocarbons and alcohols, i.e., hydrogenated carbonyls. In particular, the latter have recently attracted much attention due to their favorable dehydrogenation temperatures and the possibility to convert them directly into electricity in specially designed direct-LOHC fuel cells. Herein, we investigate the temperature-triggered dehydrogenation reaction of 1-cyclohexylethanol to acetophenone on a Pt(111) model catalyst using synchrotron-based temperature-programed X-ray photoelectron spectroscopy. To obtain a complete picture of the reaction mechanism, we consider not only the individual surface reactions of 1-cyclohexylethanol and acetophenone but also those of two potential dehydrogenation intermediates, namely, the partially dehydrogenated 1-cyclohexylethanone and 1-phenylethanol. We find a stepwise dehydrogenation of 1-cyclohexylethanol: the first step around ∼210 K at the alcohol moiety yields the ketone 1-cyclohexylethanone. The second step above ∼260 K at the cyclohexyl group is accompanied by the loss of an H-atom at the molecule’s methyl group and leads to the formation of an acetophenone-like phenyl–C(O)–CH2 species at ∼340 K. The same species are also identified in the surface reactions of acetophenone, 1-phenylethanol, and 1-cyclohexylethanone in this temperature range. Overall, the system shows good thermal robustness: a complete dehydrogenation of the hydrogen-rich carrier occurs without damage to the carbon framework.

Isopropylcyclohexane pyrolysis at high pressure and temperature: Part 1- theoretical study

The thermal decomposition of isopropylcyclohexane (IPCH) has been investigated at the CBS-QB3 and CASSCF/MRCI levels of theory. The pyrolysis of IPCH follows a radical chain mechanism, which mainly includes the C–C bond scission, H-atom abstraction, dissociation of alkene and alkyl-cyclohexene, decomposition, and isomerization of alkyl-cyclohexane radicals, decomposition and isomerization of alkenyl radicals, and stepwise dehydrogenation of cyclic intermediates. The rate constants for all elementary reactions have been evaluated with conventional transition state theory (TST/VTST) in the temperature range of 800–2000 K. The energy barriers and rate constants have been compared with previous theoretical reports of other cyclohexanes (like cyclohexane, methylcyclohexane, and ethylcyclohexane) to see the effect of isopropyl group in the cyclohexane dissociation. The final products of IPCH thermal decomposition are methane (CH4), ethylene(C2H4), propylene (C3H6), 1,3-butadiene (1,3-C4H6), and 1,3-pentadiene (1,3-C5H8), benzene, cyclopentadiene etc. The main goal of this work has been to come up with a detailed description of the IPCH thermal decomposition using high-level quantum chemical methods and provide reliable kinetic information to help in understanding the results from shock tube investigations discussed in part 2 of this work.

Dispersed surface Ru ensembles on MgO(111) for catalytic ammonia decomposition

Ammonia is regarded as an energy vector for hydrogen storage, transport and utilization, which links to usage of renewable energies. However, efficient catalysts for ammonia decomposition and their underlying mechanism yet remain obscure. Here we report that atomically-dispersed Ru atoms on MgO support on its polar (111) facets {denoted as MgO(111)} show the highest rate of ammonia decomposition, as far as we are aware, than all catalysts reported in literature due to the strong metal-support interaction and efficient surface coupling reaction. We have carefully investigated the loading effect of Ru from atomic form to cluster/nanoparticle on MgO(111). Progressive increase of surface Ru concentration, correlated with increase in specific activity per metal site, clearly indicates synergistic metal sites in close proximity, akin to those bimetallic N2 complexes in solution are required for the stepwise dehydrogenation of ammonia to N2/H2, as also supported by DFT modelling. Whereas, beyond surface doping, the specific activity drops substantially upon the formation of Ru cluster/nanoparticle, which challenges the classical view of allegorically higher activity of coordinated Ru atoms in cluster form (B5 sites) than isolated sites.

Nickel single atoms/cerium oxide hybrid for hydrogen production via solar-heating catalytic dehydrogenation of methyl Cyclohexane

Methylcyclohexane (MCH)-toluene-hydrogen cycle can realize the recycling of matter and energy, owning a wide foreground in industrial application. The MCH dehydrogenation process is highly endothermic, and a heating temperature is needed to stimulate this reaction. The combination of thermal catalytic reactor and solar-to-heat system can realize solar-heating catalytic MCH dehydrogenation reaction. Whereas, such a highly desirable advanced system has not yet been reported. In this study, a hybrid of single-atom nickel anchored on cerium oxide nanosheets (Ni SA/CeO2 NS) has been constructed successfully. The hybrid exhibits remarkable thermal catalytic activity in MCH dehydrogenation, owning an H2 generation rate of 2756 mmol g−1 h−1 at 400 °C. The stepwise MCH dehydrogenation mechanism on the hybrid is investigated via density functional theory calculations. Moreover, we have developed a novel type of solar-to-heat device based on Ti2O3 thin film/Cu. Based on this device and the Ni SA/CeO2 NS hybrid, a solar-heating catalysis mode has been developed, with the H2 generation rates of 404 and 2604 mmol g−1 h−1 from MCH dehydrogenation under 1 and 2 kW m−2, respectively. This strategy provides the possibility to realize scalable MCH dehydrogenation under solar irradiation without secondary energy input.

Exploring the pyrolysis chemistry of 1,3,5-trimethylcyclohexane with insight into fuel isomeric and multiple substitution effects

To reveal insights into the combustion mechanism of multiple alkyl substituent cycloparaffins, this work reports an experimental and modeling study of 1,3,5-trimethylcyclohexane (T135MCH) pyrolysis in an extended flow reactor at low and atmospheric pressures. More than 30 species were detected and quantified employing synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry, and a detailed kinetic model developed based on reaction classes and update kinetic data was validated against the measured species profiles with a reasonable agreement. The reaction flux analyses were performed to reveal the key pathways of the fuel decomposition, intermediates production and aromatics formation. For the primary decomposition, the branching ratios of reaction types show strong dependence on changes of pressures and temperatures, including unimolecular methyl elimination, unimolecular ring-opening isomerization and H-abstraction. Besides the direct dissociation channels, major intermediate hydrocarbons are formed via stepwise dehydrogenation, recombination with ĊH3 radical or “formally direct” chemically activated reactions triggered by Ḣ atom addition. Monocyclic aromatic hydrocarbons such as benzene and toluene can be produced by traditional H-abstraction/β-C-H scission sequence, cyclopentadiene-related pathways, or recombination mechanism from small linear products. The formations of indene and naphthalene are controlled by C5+C5 and C5+C4 mechanism respectively. The comparison work of species profiles combined with theoretical calculations of bond dissociation enthalpies (BDEs) was performed to reveal the multiple CH3-group substituent and isomeric effects of methylcyclohexane (MCH), 1,2,4-trimethylcyclohexane (T124MCH) and T135MCH on pyrolysis activity and ethylene/benzene formation. Besides the increased reaction active sites, the added CH3-group and ortho-substitution can both weaken the strength of CC and CH bonds, leading to the promoting decomposition activity. The different formation tendencies of products are caused by different BDEs, length of carbon skeleton, as well as complex fuel-specific pathways.

Real-Space Identification of Non-Noble Single Atomic Catalytic Sites within Metal-Coordinated Supramolecular Networks.

With regard to the development of single atom catalysts (SACs), non-noble metal-organic layers combine a large functional variability with cost efficiency. Here, we characterize reacted layers of melamine and melem molecules on a Cu(111) surface by noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and ab initio simulations. Upon deposition on the substrate and subsequent heat treatments in ultrahigh vacuum (UHV), these precursors undergo a stepwise dehydrogenation. After full dehydrogenation of the amino groups, the molecular units lie flat and are strongly chemisorbed on the copper substrate. We observe a particularly extreme interaction of the dehydrogenated nitrogen atoms with single copper atoms located at intermolecular sites. In agreement with the nc-AFM measurements performed with an O-terminated copper tip on these triazine- and heptazine-based copper nitride structures, our ab initio simulations confirm a pronounced interaction of oxygen species at these N-Cu-N sites. To investigate the related functional properties of our samples regarding the oxygen reduction reaction (ORR), we developed an electrochemical setup for cyclic voltammetry experiments performed at ambient pressure within a drop of electrolyte in a controlled O2 or N2 environment. Both copper nitride structures show a robust activity in irreversibly catalyzing the reduction of oxygen. The activity is assigned to the intermolecular N-Cu-N sites of the triazine- and heptazine-based copper nitrides or corresponding oxygenated versions (N-CuO-N, N-CuO2-N). By combining nc-AFM characterization on the atomic scale with a direct electrochemical proof of performance, our work provides fundamental insights about active sites in a technologically highly relevant reaction.

Urea-modified Cu-based materials: Highly efficient and support-free adsorbents for removal of H2S in an anaerobic and dry environment

In this work, we report a novel urea-modified copper-based material (XUrea@Cu) obtained by directly calcinating a mixture of urea and copper nitrate. This material enables the efficient purification of H2S under dry and anaerobic conditions. The performance test results showed that the H2S capacity of the optimal adsorbent (2Urea@Cu) could reach 364.2 mg(H2S)∙gsorbent-1, which is much superior to those of other adsorbents in the literature. Further study demonstrates that the ultra-desulfurization performance of 2Urea@Cu is mainly attributed to the extensive alkaline site, the high concentrations of active oxygen, and the abundance of oxygen vacancy; which are quite effective for capturing H2S (acidic and strong reducibility). Interestingly, the deactivation adsorbent (De-2Urea@Cu) is mainly composed of high-purity CuS, which means that the Urea@Cu adsorbent can simultaneously achieve efficient purification of H2S and recovery of sulfur resources since CuS is a valuable product. In addition, in-situ FT-IR and theoretical calculation results indicate that H2S molecules dissociate on the CuO(111) surface by stepwise dehydrogenation, while S combines with adjacent Cu to form CuS. The consumption of active components (CuO) and the accumulation of reaction products (CuS) are the primary reasons for the deactivation of adsorbents. Considering the simple preparation process, low cost, and excellent H2S adsorption activity, these urea-modified copper-based materials are very promising H2S adsorbents.

Theoretical insight into the reaction mechanism of ammonia dehydrogenation on iron-based clusters

Ammonia (NH3) molecule decomposition is a promising approach for H2 production with COx-free. Density functional theory (DFT) calculations are employed to investigate the NH3 molecule stepwise dehydrogenation on Fe6M (M = Fe, Mo, Ni, and Co) clusters. The stability of these clusters has been investigated, and the result shows that the stability of Fe-based clusters is enhanced with doping of other metals. The catalytic activity of metal clusters for ammonia decomposition is systematically investigated by analyzing the adsorption property and reaction barriers. NH3 prefers to bind to the atop metal atom through the N end, which is agreement with the electrostatic potential analysis. Moreover, the adsorption energies of NH3 are related to the d-band center of Fe-based clusters. The reaction energy barriers show that the last dehydrogenation step (NH* + * ⇌ N* + H*) is the rate-determining step for most investigated clusters. The overall dehydrogenation process is exothermic reaction, and by comparing the rate-determining step, the Fe6Mo(l) and the Fe6Ni(l) clusters can efficiently promote the NH3 dissociation reaction.

Effect of molecular structure on pyrolysis coking performances of hydrocarbons at different temperatures

Methylcyclohexane (MCH) and n-heptane (nC7) were used to perform experimental studies in high-temperature oxidation pretreated STS304 reactor (ϕ2 × 0.5 × 1000 mm) at 873–1073 K, 1.0 MPa, and 1.0 mL/min feed rate. The heat sinks of the two fuels were calculated based on the conversion and product distribution. MCH was found to have difficulty in achieving high heat sink at low and high temperatures. The formation pathways of ethylene and benzene were then compared. Results showed their main precursors were similar, but the contribution proportion of each path was different affected by initial cracking reactions of MCH and nC7. The coking rate of MCH was lower than that of nC7 at low temperatures, while the trend was the opposite at high temperatures. The variation in C/H ratio of coke for MCH and nC7 with temperature was the same, whereas the C/H ratio of the former was significantly lower. The unique unimolecular demethylation and intramolecular stepwise dehydrogenation reactions of MCH caused its coking rate to be higher than that of nC7. An experimental method was proposed to determine the coking active site density based on composition and structure of coke. Finally, a coking mechanism based on the growth of spherical coke particles was proposed.