Dehydrogenation Pathway Research Articles

Theoretical study of the structure and dehydrogenation mechanism of sodium hydrazinidoborane

Alkali-metal hydrazinidoboranes have been recently investigated as a new stable high-capacity material for hydrogen storage, necessitating an exploration of the dehydrogenation mechanism for further developments in this field. Herein, we present a first systematic study of the structure and dehydrogenation mechanism of sodium hydrazinidoborane (NaHB) with three possible pathways considered: pathway A, corresponding to unimolecular dehydrogenation; pathway B, featuring dehydrogenation of the (NaHB)2 dimer via two different sub-pathways, and pathway C, corresponding to direct dehydrogenation (as compared to B). The calculated rate of the most probable dehydrogenation pathway (B, 3.28[Formula: see text]min[Formula: see text] is similar to that obtained experimentally (12.26[Formula: see text]min[Formula: see text], supporting the validity of our findings.

Conducting polymers inducing catalysis: Enhanced formic acid electro-oxidation at a Pt/polyaniline nanocatalyst

Research is moving rapidly to sustain convenient energy resources fulfilling the global climate legislations. Herein, a novel catalyst of platinum nanoparticles (PtNPs) dispersed onto polyaniline (PANi) is recommended for formic acid electro-oxidation (FAO); the fundamental anodic reaction in direct formic acid fuel cells (DFAFCs). The catalyst's preparation scheme allows a sequential electrodeposition of fibril PANi and spherical PtNPs (ca. 65 nm in size) on a glassy carbon (GC) substrate and permits a precise control over the deposition sequence and loading. Interestingly, incorporation of PANi into the catalyst's ingredients can significantly (ca. 16 times) improve the catalytic activity of the catalyst towards FAO by shifting the mechanism towards the desirable dehydrogenation pathway and mitigating the undesirable poisoning dehydration pathway. The catalytic efficiency is tuned by manipulating the deposition order and loading of different catalyst's ingredients. Several techniques are employed to confirm the successful deposition of the catalyst and to evaluate its morphology, composition and crystal structure. While PtNPs are essential for FA adsorption, PANi improves the dispersion of PtNPs and mediates FAO to facilitate the charge transfer and mitigate CO poisoning. A promising catalytic stability is achieved in a long continuous (150 CVs) electrolysis experiment.

Probing the effect of the Pt-Ni-Pt(111) bimetallic surface electronic structures on the ammonia decomposition reaction.

We report a detailed investigation of elementary catalytic decomposition of ammonia on the Pt-Ni-Pt(111) bimetallic surface using in situ near ambient pressure X-ray photoelectron spectroscopy. Under the near ambient pressure (0.6 mbar) reaction conditions, a different dehydrogenation pathway with a reduced activation energy barrier for recombinative nitrogen desorption on the Pt-Ni-Pt(111) bimetallic surface is observed. The unique surface catalytic activity is correlated with the downward shift of the Pt 5d band states induced by the Ni subsurface atoms via charge redistribution of the topmost Pt layer. Our results provide a practical understanding of the unique chemistry of bimetallic catalysts for facile ammonia decomposition under realistic reaction conditions.

Palladium-catalyzed decarboxylative, decarbonylative and dehydrogenative C(sp2)-H acylation at room temperature.

Over the past few decades, an impressive array of C-H activation methodology has been developed for organic synthesis. However, due to the inherent inertness of the C-H bonds (e.g. ∼110 kcal mol-1 for the cleavage of C(aryl)-H bonds) harsh reaction conditions have been realized to overcome high energetic transition states resulting in a limited substrate scope and functional group tolerance. Therefore, the development of mild C-H functionalization protocols is in high demand to exploit the full potential of the C-H activation strategy in the synthesis of a complex molecular framework. Although, electron-rich substrates undergo electrophilic metalation under relatively mild conditions, electron-deficient substrates proceed through a rate-limiting C-H insertion under forcing conditions at high temperature. In addition, a stoichiometric amount of toxic silver salt is frequently used in palladium catalysis to facilitate the C-H activation process which is not acceptable from the environmental and industrial standpoint. We report herein, a Pd(ii)-catalyzed decarboxylative C-H acylation of 2-arylpyridines with α-ketocarboxylic acids under mild conditions. The present protocol does not require stoichiometric silver(i) salts as additives and proceeds smoothly at ambient temperature. A novel decarbonylative C-H acylation reaction has also been accomplished using aryl glyoxals as acyl surrogates. Finally, a practical C-H acylation via a dehydrogenative pathway has been demonstrated using commercially available benzaldehydes and aqueous hydroperoxides. We also disclose that acetonitrile solvent is optimal for the acylation reaction at room temperature and has a prominent role in the reaction outcome. Control experiments suggest that the acylation reaction via decarboxylative, decarbonylative and dehydrogenative proceeds through a radical pathway. Thus we disclose a practical protocol for the sp2 C-H acylation reaction.

Elucidating the mechanism of MgB2 initial hydrogenation via a combined experimental-theoretical study.

Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed.

Changing the dehydrogenation pathway of LiBH4–MgH2via nanosized lithiated TiO2

Nanosized lithiated titanium oxide (LixTiO2) noticeably improves the kinetic behaviour of 2LiBH4 + MgH2. The presence of LixTiO2 reduces the time required for the first dehydrogenation by suppressing the intermediate reaction to Li2B12H12, leading to direct MgB2 formation.

Clarifying the dehydrogenation pathway of catalysed Li4(NH2)3BH4-LiH composites.

The effect of different metal oxides (Co3O4 and NiO) on the dehydrogenation reaction pathways of the Li4(NH2)3BH4-LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B-H bonds in the Li-B-N-H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH2 and Li3BN2 formation as the end product with a strong reduction of NH3 emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li4(NH2)3BH4-LiH-NiO and Li4(NH2)3BH4-NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li3BN2H2 at the early stages of dehydrogenation and the end products LiNH2 and monoclinic Li3BN2. We provided evidence for the interaction between NiO and LiNH2 during heating and proposed that the presence of Li facilitates a NHx-rich environment and the Ni catalyst mediates the electron transfer to promote NHx coupling.

Pt-Au Film Catalyst for Formic Acid Oxidation

Direct formic acid fuel cells (DFAFCs) have shown promise as efficient and renewable power sources for portable electronics. However, pure Pt catalysts exhibit poor activity for the formic acid oxidation reaction (FAOR) due to the formation of poisoning species, which slow down the rate of oxidation. Here, we report on the novel synthesis of a bimetallic, Pt-Au film electrocatalyst for the oxidation of formic acid (HCOOH). A bilayer stack composed of individual Pt and Au metal layers was deposited using magnetron sputtering, resulting in a smooth surface consisting of tightly packed nanocrystallites. At room temperature, the bilayer stack shows the electrochemical characteristics of a pure Pt surface. A post-synthesis heat treatment was carried out to form a bimetallic film consisting of the two metals. As a result, the heat-treated film showed electrochemical characteristic of both metals, indicating a mixed metal surface. The Pt-Au catalyst surface exhibited greatly enhanced activity for formic acid oxidation compared to a commercial bulk Pt catalyst. The presence of Au on the catalyst surface promotes the oxidation of formic acid through the direct dehydrogenation pathway, via the ensemble effect. Therefore, the heat treatment of a sputtered bilayer stack provides a simple method for forming stable bimetallic Pt-Au catalysts with high activity for the oxidation of formic acid.

A Facile and Efficient Method to Fabricate Highly Selective Nanocarbon Catalysts for Oxidative Dehydrogenation.

Carbon nanotubes (CNTs) were used in oxidative dehydrogenation (ODH) reactions. Quinone groups on the CNT surface were identified as active sites for the dehydrogenation pathway. Liquid-phase oxidation with HNO3 is one way to generate various oxygen functionalities on the CNT surface but it produces a large amount of acid waste, limiting its industrial application. Here, a facile and efficient oxidative method to prepare highly selective CNT catalysts for ODH of n-butane is reported. Magnesium nitrate salts as precursors were used to produce defect-rich CNTs through solid-phase oxidation. Skeleton defects induced on the CNT surface resulted in the selective formation of quinone groups active for the selective dehydrogenation. The as-prepared catalyst exhibited a considerable selectivity (58 %) to C4 olefins, which is superior to that of CNTs oxidized with liquid HNO3 . Through the introduction of MgO nanoparticles on the CNT surface, the desorption of alkenes can be accelerated dramatically, thus enhancing the selectivity. This study provides an attractive way to develop new nanocarbon catalysts.

Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward Formic Acid Oxidation with Predominant Dehydrogenation Pathway.

For direct formic acid fuel cells (DFAFCs), the dehydrogenation pathway is a desired reaction pathway, to boost the overall cell efficiency. Elaborate composition tuning and nanostructure engineering provide two promising strategies to design efficient electrocatalysts for DFAFCs. Herein, we present a facile synthesis of porous AgPt bimetallic nanooctahedra with enriched Pt surface (denoted as AgPt@Pt nanooctahedra) by a selective etching strategy. The smart integration of geometric and electronic effect confers a substantial enhancement of desired dehydrogenation pathway as well as electro-oxidation activity for the formic acid oxidation reaction (FAOR). We anticipate that the obtained nanocatalyst may hold great promises in fuel cell devices, and furthermore, the facile synthetic strategy demonstrated here can be extendable for the fabrication of other multicomponent nanoalloys with desirable morphologies and enhanced electrocatalytic performances.

Mechanisms of Metal-Free Aerobic Oxidation To Prepare Benzoxazole Catalyzed by Cyanide: A Direct Cyclization or Stepwise Oxidative Dehydrogenation and Cyclization?

The detailed mechanism of the cyanide-catalyzed synthesis of benzoxazole from 2-aminophenol and benzaldehyde was investigated in depth using density functional theory (DFT). The metal-free aerobic oxidative process as well as the dehydrogenation detail were examined and described. Cyanide anion was proved to assist the oxygen atoms to undergo an aerobic dehydrogenation rather than direct cyclization which was predicted according to Baldwin's 5-exotet rule. The dehydrogenation pathway was thermodynamically favored over cyclization with an activation energy difference of 24.2 kcal/mol. Another point noteworthy was the observation that a trace amount of water could reduce the activation energy effectively during the condensation step (ΔΔG⧧ = 22.3 kcal/mol).

DFT Study on the Synergistic Effect of Pd–Cu Bimetal on the Adsorption and Dissociation of H2O

Aiming at exploring the effect of surface composition on the catalytic activity by density functional theory, the adsorption and complete dehydrogenation mechanisms of H2O on five Pd–Cu (100) surfaces have been investigated. With regard to adsorption, it is found that the adsorption OH, O, and H on Pd–Cu0–4, Pd–Cu1–3, Pd–Cu2–2, Pd–Cu3–1, and Pd–Cu4–0 surfaces are chemisorption and that of H2O may be considered as physisorption. Interestingly, it is revealed that the difference of Pd–Cu doping ratios has no effect on the adsorption configurations of all intermediates; nevertheless, it has an evident influence on the interaction between adsorbates and substrates. In addition, the energy pathways for the complete dehydrogenation of H2O on five Pd–Cu (100) surfaces are analyzed to explore the dissociation mechanisms. It is found that the doped Pd atoms on the first layer of Cu (100) surface are beneficial for the scission of H–O bond of H2O except for that on the Pd–Cu4–0 and Pd–Cu3–1 surfaces and can promote...

New dehydrogenation pathway of LiBH4 + MgH2 mixtures enabled by nanoscale LiBH4

This study reports a new dehydrogenation pathway for the LiBH4 + MgH2 system. We find that in the presence of nanoscale LiBH4, the first dehydriding reaction is the decomposition of LiBH4 and the product from the LiBH4 decomposition in turn causes the hydrogen release from MgH2. The new reaction pathway has resulted in a large amount of hydrogen release from MgH2 below 150 °C. The discovery made in this study confirms the ab initio density functional theory calculations reported in the literature and a previous study showing that the new reaction pathway observed in this study can be achieved during high-energy ball milling of MgH2 at ambient temperature in conjunction with aerosol spraying of LiBH4 solution. The establishment of this new reaction pathway opens the door to make LiBH4 + MgH2 as a viable system for reversible hydrogen storage applications near 150 °C in the future.

A pharmacokinetic evaluation and metabolite identification of the GHB receptor antagonist NCS-382 in mouse informs novel therapeutic strategies for the treatment of GHB intoxication.

Gamma‐aminobutyric acid (GABA) is an endogenous inhibitory neurotransmitter and precursor of gamma‐hydroxybutyric acid (GHB). NCS‐382 (6,7,8,9‐tetrahydro‐5‐hydroxy‐5H‐benzo‐cyclohept‐6‐ylideneacetic acid), a known GHB receptor antagonist, has shown significant efficacy in a murine model of succinic semialdehyde dehydrogenase deficiency (SSADHD), a heritable neurological disorder featuring chronic elevation of GHB that blocks the final step of GABA degradation. NCS‐382 exposures and elimination pathways remain unknown; therefore, the goal of the present work was to obtain in vivo pharmacokinetic data in a murine model and to identify the NCS‐382 metabolites formed by mouse and human. NCS‐382 single‐dose mouse pharmacokinetics were established following an intraperitoneal injection (100, 300, and 500 mg/kg body weight) and metabolite identification was conducted using HPLC‐MS/MS. Kinetic enzyme assays employed mouse and human liver microsomes. Upon gaining an understanding of the NCS‐382 clearance mechanisms, a chemical inhibitor was used to increase NCS‐382 brain exposure in a pharmacokinetic/pharmacodynamic study. Two major metabolic pathways of NCS‐382 were identified as dehydrogenation and glucuronidation. The K m for the dehydrogenation pathway was determined in mouse (K m = 29.5 ± 10.0 μmol/L) and human (K m = 12.7 ± 4.8 μmol/L) liver microsomes. Comparable parameters for glucuronidation were >100 μmol/L in both species. Inhibition of NCS‐382 glucuronidation, in vivo, by diclofenac resulted in increased NCS‐382 brain concentrations and protective effects in gamma‐butyrolactone‐treated mice. These initial evaluations of NCS‐382 pharmacokinetics and metabolism inform the development of NCS‐382 as a potential therapy for conditions of GHB elevation (including acute intoxication & SSADHD).

Carbon nanotubes supported platinum–gold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction

Improving the electrocatalytic activity, durability, and utilization of anode eletrocatalysts are crucial for accelerating commercialization of direct formic acid fuel cells. In this work, the multiwall carbon nanotubes (MWCNTs) supported PtAu alloy nanocrystals (MWCNTs/PtAu) composites are synthesized by a one-pot wet-chemical method using polyethyleneimine as the complexant and surfactant. The physicochemical properties of the as-prepared MWCNTs/PtAu composites are characterized detailedly by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, etc. These structural investigations display the PtAu alloy nanocrystals are highly dispersed on the surface of the MWCNTs. Cyclic voltammetry and chronoamperometry measurements show the as-prepared MWCNTs/PtAu composites significantly enhance the direct dehydrogenation pathway of the formic acid oxidation reaction, resulting in the improved electocatalytic activity and durability for the formic acid electrooxidation.

Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols.

Design of an efficient new catalyst that can mimic the enzymatic pathway for catalytic dehydrogenation of liquid fuels like alcohols is described in this report. The catalyst is a nickel(II) complex of 2,6-bis(phenylazo)pyridine ligand (L), which possesses the above requisite with excellent catalytic efficiencies for controlled dehydrogenation of alcohols using ligand-based redox couple. Mechanistic studies supported by density functional theory calculations revealed that the catalytic cycle involves hydrogen atom transfer via quantum mechanical tunneling with significant kH/kD isotope effect of 12.2 ± 0.1 at 300 K. A hydrogenated intermediate compound, [NiIICl2(H2L)], is isolated and characterized. The results are promising in the context of design of cheap and efficient earth-abundant metal catalyst for alcohol oxidation and hydrogen storage.

Dehydrogenation of Ammonia Borane by Metal Nanoparticle Catalysts

Ammonia borane (AB), having a high hydrogen density of 19.6 wt %, has attracted much attention as a promising chemical hydrogen storage material. In the past few years, a number of highly active metal nanoparticle (NP) catalysts, which are easy to handle and separate, have been developed for AB dehydrogenation. In this Perspective, we summarize new progress in the development of metal NP catalysts, which are categorized into monometallic and heterometallic catalysts, with excellent activity and high recyclability for different AB dehydrogenation pathways, including solvolysis (hydrolysis and methanolysis) in protic solvents and dehydrocoupling in nonprotic solvents, and we survey the corresponding methods for the regeneration of AB. Moreover, the merits and drawbacks of solvolysis and dehydrocoupling are discussed.

Discovering Pd-B/C As a Potential Bifunctional Catalyst in the Formic Acid ↔ Carbon Dioxide Conversion Cycle

It is well known that facile interconversion of HCOOH and CO2 is very important in hydrogen production, direct formic acid fuel cell, as well as neutral emission of CO2. In the past few years, we have investigated chemical and electrochemical dehydrogenation of formic acid over Pd-B/C and Pd/C, revealing that significantly enhanced dehydrogenation of HCOOH can be achieved on the home-synthesized Pd-B/C[1-3]. B-doping in the Pd lattice can be attained during the reduction of a Pd(II) precusor from an aqueous solution using dimethylamine borane as the reducing agent and boron source[1]. The XRD result suggested the expansion of Pd lattice and the XPS result indicated the partial electron transfer from B to Pd [3], lowering down the d-band center of Pd, favoring the dehydrogenation pathway, as supported by latest DFT calculation by Studt and Norskov[4]. In situ ATR-IR spectroscopy on Pd-B/C and Pd/C was comparatively conducted in conditions mimicking hydrogen production or direct formic acid fuel cell, it is revealed that desired dehyrogenation pathway forming CO2 is enhanced accompanied with inhibited accumulation of CO poissoning specie on Pd-B/C [2-3]. Since both forward and backward conversions may involve the weakly adsorbed formate as an active intermediate, it is thus interesting to explore the use of Pd-B/C as the catalyst for electroreducing CO2 to form HCOOH. Indeed, our recent preliminary analysis on the products of CO2 electroreduction over Pd-B/C and Pd/C catalysts in a KHCO3 electrolyte indicates that the formation of HCOOH on Pd-B/C is greatly enhanced as compared to that on Pd/C. Acknowledgements ：Financial supports from NSFC (grant No. 21273046 and 21473039）and the 973 Program (No. 2015CB932303) of MOST are highly appreciated References ： [1] J.Y. Wang, Y.Y. Kang, H. Yang, and W.B. Cai, J. Phys. Chem. C , 113, 8366 (2009). [2] K. Jiang, K. Xu, S.Z. Zou, and W.B. Cai, J. Am. Chem. Soc., 136, 4861 (2014). [3] K. Jiang, J.F. Chang, H. Wang, S. Brimaud, W. Xing, R. J. Behm, and W.B. Cai , ACS Appl. Mater. Interfaces, 8, 7133(2016) [4] J. S.Yoo, Z.-J. Zhao, J. K. Nørskov, and F. Studt, ACS Catal., 5, 6579 (2015).

The interaction characteristics controlling dispersion mode-catalytic functionality relationship of silica–modified montmorillonite-anchored Ni nanoparticles in petrochemical processes

This study has dealt with different interaction events during the synthesis of silica-modified montmorillonite (SMM) nanocomposites. TEOS precursor was intercalated into the clay (M) gallery (in M/TEOS mass ratios = 1:2 and 1:6), assisted by dodecylamine (DDA) or CTAB driving agents (at constant surfactant/M ratio = 1.8). Ni NPs (of 2, 4, 6 wt%) were immobilized into the gallery of H-SMM sample (M/TEOS = 1:6). Various characteristics were studied via XRD, EDX, FTIR, N2 physisorption, H2-pulse titration, DSC-TGA, TEM and DLS techniques. The formed Silica NPs were attached on platelet surface through the interaction with edge aluminol sites in octahedral sheet. Considerable delamination of clay platelets was observed by increasing the M/TEOS ratio and in presence of CTAB, exhibiting higher surface area and pore parameters. Ni NPs were anchored onto the bonded silica at clay layer edges through interactions involving DDA, interlayer sites and silica NPs. Ni nanocatalysts were tested in ethyl benzene (EB) conversion, considering possible reaction pathways according to various interaction events affecting the catalytic behavior. TOF of the reaction to selectively produce styrene remained constant with Ni loadings, suggesting that dehydrogenation pathway is structure insensitive. The constancy of TOF to produce optimized quantities of benzene favored the involvement of edge interlayer acid sites in cracking pathway, independent of Ni NPs content. TOF to selectively produce ethane in cracking pathway increased with Ni loading, where enlarged surface Ni NPs (reaching to 202 nm) seemed to control this structure sensitive pathway.

Picomole‐Scale Real‐Time Photoreaction Screening: Discovery of the Visible‐Light‐Promoted Dehydrogenation of Tetrahydroquinolines under Ambient Conditions

AbstractThe identification of new photocatalytic pathways expands our knowledge of chemical reactivity and enables new environmentally friendly synthetic applications. However, the development of miniaturized screening procedures/platforms to expedite the discovery of photochemical reactions remains challenging. Herein, we describe a picomole‐scale, real‐time photoreaction screening platform in which a handheld laser source is coupled with nano‐electrospray ionization mass spectrometry. By using this method, we discovered an accelerated dehydrogenation pathway for the conversion of tetrahydroquinolines into the corresponding quinolines. This transformation is readily promoted by an off‐the‐shelf [Ru(bpy)3]Cl2⋅6 H2O complex in air at ambient temperature in direct sunlight, or with the aid of an energy‐saving lamp. Moreover, radical cations and trans‐dihydride intermediates captured by the screening platform provided direct evidence for the mechanism of the photoredox reaction.