Dehydrogenation Pathway Research Articles

HCOOH Decomposition on Sub-Nanometer Pd6 Cluster Catalysts: The Effect of Defective Boron Nitride Supports Through First Principles

The catalytic properties of a hexagonal boron nitride- (h-BN) supported Pd6 sub-nanometer cluster in the context of formic acid (HCOOH) decomposition were studied by means of periodic Density Functional Theory (DFT) calculations. The effect of support defectivity – boron (h-BvN) and nitrogen (h-BNv) monovacancies – on the competition between the formate (HCOO)- and carboxyl (COOH)-mediated decomposition pathways was analyzed. Defects are responsible for charge-transfer leading to a positively or negatively charged cluster, and open new reactive channels in which vacancy-mediated dehydrogenation pathways can occur. Pd6 cluster reconstructions, induced by the adsorption of reaction intermediates and by the presence of monovacancies in the support, greatly stabilize the formation of CO from COOH, which could drastically decrease the selectivity towards hydrogen production. A simplified descriptor-based analysis, based on selected thermochemical quantities calculated on charged cluster models, suggests that Pd6 sub-nanometer clusters supported on pristine h-BN and h-BNv can be more selective than Pd6 supported on defective h-BvN towards HCOOH dehydrogenation.

On the Promoting Effects of Te and Nb in the Activity and Selectivity of M1 MoV-Oxides for Ethane Oxidative Dehydrogenation

The pathways of ethane oxidative dehydrogenation and total combustion have been elucidated for M1 phase type Mo–V oxide catalysts with different metal composition. The ethane oxidation mechanism is not affected by the presence of Te or Nb. Conversely, the selectivity is strongly affected by stoichiometry of M1 catalysts. This is attributed to the facile oxidation of ethene to COx upon formation of unselective VOx species in the absence of Te and Nb.

Reactivity of C3Hx Adsorbates in Presence of Co-adsorbed CO and Hydrogen: Testing Fischer–Tropsch Chain Growth Mechanisms

The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongoing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution X-ray photoemission spectroscopy to study the reactivity of C3Hx adsorbates on a Co(0001) single crystal surface in order to explore the stabilities of the different C3Hx surface intermediates and to study elementary reaction steps relevant to chain growth and chain termination. Propene (H3C–CH=CH2) and propyl (H3C–CH2–CH2–) adsorbates react below 200 K already, either by desorption of propene or by dehydrogenation to adsorbed propyne (H3C–C≡CH). Co-adsorbed Had and COad do not affect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of propene desorption. Their high reactivity under simulated FTS conditions disqualifies them as feasible intermediates for FTS, which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C3Hx adsorbate in the absence of COad, is hydrogenated to propylidyne (H3C–CH2–C≡) > 230 K when both COad and Had are present. Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H5C2–C≡C–C2H5) surface intermediate which is hydrogenated to 3-hexene (H5C2–CH=CH–C2H5) above 350 K. These findings are of direct relevance to FTS: they show that the high coverage of COad and Had present during the reaction influence the reactivity of CxHy adsorbates involved in chain growth and ultimately steer product selectivity. The findings provide further experimental support for the previously proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes CxHy growth intermediates in the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH).

Mechanistic insights on aqueous formic acid dehydrogenation over Pd/C catalyst for efficient hydrogen production

Efficient hydrogen production is one of the most important issues in the future energy economy. Dehydrogenation of formic acid has been extensively investigated for safe and easy hydrogen production. Thereby, various catalysts have been developed for aqueous formic acid dehydrogenation (FAD) reaction to efficiently produce hydrogen. However, there is still a lack of understanding of the reaction mechanism. Herein, to fundamentally comprehend the aqueous FAD reaction mechanism, we prepared Pd/C catalyst and performed controlled catalytic tests. The results demonstrated that the aqueous FAD reaction occurs through the formate anion dehydrogenation pathway. DFT calculation explained this by the observation that the charge of formate anion significantly mitigates the activation barrier of H-COO bond cleavage. In addition, computational prediction combined with kinetic isotope effect experiments verified that combinative desorption of hydrogen is the rate-determining step of FAD.

The Potassium-Induced Decomposition Pathway of HCOOH on Rh(111)

Formic acid (FA) can be considered both a CO and a H2 carrier via selective dehydration and dehydrogenation pathways, respectively. The two processes can be influenced by the modification of the active components of the catalysts used. In the present study the adsorption of FA and the decomposition of the formed formate intermediate were investigated on potassium promoted Rh(111) surfaces. The preadsorbed potassium markedly increased the uptake of FA at 300 K, and influenced the decomposition of formate depending on the potassium coverage. The work function (Δϕ) is increased by the adsorption of FA on K/Rh(111) at 300 K suggesting a large negative charge on the chemisorbed molecule, which could be probably due to the enhanced back-donation of electrons from the K-promoted Rh into an empty π orbital of HCOOH. The binding energy of the formate species is therefore increased resulting in a greater concentration of irreversibly adsorbed formate species. Decomposition of the formate species led to the formation of H2, CO2, H2O, and CO, which desorbed at significantly higher temperatures from the K-promoted surface than from the K-free one as it was proven by thermal desorption studies. Transformation of surface formate to carbonate (evidenced by UPS) and its decomposition and desorption is responsible for the high temperature CO and CO2 formation.

Reductive Deamination with Hydrosilanes Catalyzed by B(C6 F5 )3.

The strong boron Lewis acid tris(pentafluorophenyl)borane B(C6F5)3 is known to catalyze the dehydrogenative coupling of certain amines and hydrosilanes at elevated temperatures. At higher temperature, the dehydrogenation pathway competes with cleavage of the C−N bond and defunctionalization is obtained. This can be turned into a useful methodology for the transition‐metal‐free reductive deamination of a broad range of amines as well as heterocumulenes such as an isocyanate and an isothiocyanate.

Metal-organic interface engineering for boosting the electroactivity of Pt nanodendrites for hydrogen production

Abstract Recently, the surface chemical functionalization and morphology control of precious metal nanostructures have been recognized as two efficient strategies for improving their electroactivity and/or selectivity. In this work, 1, 10-phenanthroline monohydrate (PM) functionalized Pt nanodendrites (Pt-NDs) on carbon cloth (CC) (denoted as PM@Pt-NDs/CC) and polyethylenimine (PEI) functionalized Pt-NDs on CC (denoted as PEI@Pt-NDs/CC) are successfully achieved by immersing Pt-NDs/CC into PM and PEI aqueous solutions, respectively. PEI functionalization of Pt-NDs/CC improves its electroactivity for hydrogen evolution reaction (HER) due to local proton enrichment whereas PM functionalization of Pt-NDs/CC improves its electroactivity for formic acid oxidation reaction (FAOR) by facilitating dehydrogenation pathway. With such high activity, a two-electrode electrolyzer is assembled using PM@Pt-NDs/CC as the anodic electrocatalyst and PEI@Pt-NDs/CC as the cathodic electrocatalyst for electrochemical reforming of formic acid, which only requires 0.45 V voltage to achieve the current density of 10 mA cm−1 for high-purity hydrogen production, much lower than conventional water electrolysis (1.59 V). The work presents an example of interfacial engineering enhancing electrocatalytic activity and indicates that electrochemical reforming of formic acid is an energy-saving electrochemical method for high-purity hydrogen production.

Aqueous phase reforming of pilot-scale Fischer-Tropsch water effluent for sustainable hydrogen production

Fischer-Tropsch (FT) synthesis produces an aqueous stream containing light oxygenates as major by-product. The low carbon concentration of the organics makes its thermal recovery unprofitable. Thus, novel processes are needed to utilize this waste carbon content. In this work, the aqueous phase reforming of the wastewater obtained from a 15 kWth Fischer-Tropsch plant was explored as a promising process to produce hydrogen at mild temperatures. The FT product water was firstly characterized and afterward subjected to the reforming at different reaction temperatures and time, using a platinum catalyst supported on activated carbon. It was observed that, besides activity, the selectivity towards hydrogen was favored at higher temperatures; equally, increasing the reaction time allowed to obtain the total conversion of most molecules found in the solution, without decreasing the selectivity and reaching a plateau at 4 hours in the hydrogen productivity. In order to get more insights into the reaction mechanism and product distribution derived from the APR of FT product water, several tests were performed with single compounds, finding characteristic features. The importance of the position of the hydroxyl group in the molecule structure was highlighted, with secondary alcohols more prone to dehydrogenation pathways compared to primary alcohols. Moreover, no interference among the substrates was reported despite the mixture is constituted by several molecules: in fact, the results obtained with the real FT product water were analogous to the linear combination of the single compound tests. Finally, the reuse of the catalyst showed no appreciable deactivation phenomena.

Acceptorless dehydrogenation of amines and alcohols using simple ruthenium chloride

A highly efficient, economic and environmental friendly catalyst system has been developed for the dehydrogenation of alcohols and amines using simple RuCl3·nH2O and N-benzylhexamethylenetetramine. The in situ catalyst system efficiently oxidized the primary and secondary amines and secondary alcohols into nitrile, imine and ketone products, respectively in moderate to excellent yields. The developed catalyst system was also found to be efficient for the dehydrogenation of N-heterocyles. A detailed mechanism study revealed the first example of N-benzylhexamethylenetetramine (HMTA-Bz) being simultaneously acting as base, reducing agent and hydride source to generate the [Ru(II)(H)2] species as the active catalyst. The mechanism studies also revealed both the alcohol and amine oxidation involves dehydrogenative pathway with the evolution of hydrogen as the only by-product. The developed catalyst system also provides possible platform for the release of hydrogen from liquid organic hydrogen carriers (LOHCs).

Effects of Modifying Acidity and Reducibility on the Activity of NaY Zeolite in the Oxidative Dehydrogenation of n-Octane

Non-coking stable alkaline earth metal (M = Mg, Sr, and Ba) modified Ga-NaY catalysts were prepared by ionic-exchange and tested in oxidative dehydrogenation (ODH) of n-octane using air as the source of oxygen. The role of the alkaline earth metals in NaY was to poison the acid sites while enhancing the basic sites responsible for ODH. The exception was the calcium modified NaY, which was more acidic than the parent NaY, coking and unstable under the ODH conditions used in this study. The role of gallium was to enhance the dehydrogenation pathway and improve the stability of NaY. The sequence of increasing selectivity to octenes followed the order: CaGa-NaY < Ga-NaY< MgGa-NaY < SrGa-NaY < BaGa-NaY. The highest octene selectivity obtained was 37% at iso-conversion of 6 ± 1% when BaGa-NaY was used at a temperature of 450 °C. The activity of the catalysts was directly proportional to the reducibility of the catalysts, which is in agreement with expectations.

Effects of van der Waals interactions on the dehydrogenation of n-butane on a Ni(1 1 1) surface

Density functional theory (DFT) including the van der Waals (vdW) correction (DFT-D3) calculations were carried out to study adsorption of n-butane on Ni(1 1 1) and two competitive dehydrogenations of n-butane to form 1-butyne and/or 2-butyne. Inclusion of the vdW correction increases the n-butane adsorption by about 0.8 eV and the adsorption specificity, i.e. adsorption configuration and molecular orientation. Furthermore, the DFT predicted the dehydrogenation pathway to 1-butyne is slightly favoured while DFT-D3 predicted that formations of 1-butyne and 2-butyne are equally possible. Therefore, the results show clearly that vdW correction is necessary to predict reaction pathways with weak adsorptions.

Desulfurization of 2-phenylcyclohexanethiol over transition-metal phosphides

The desulfurization of 2-phenylcyclohexanethiol (2-PCHT) was studied over Ni2P, MoP, and WP under 4.0 MPa H2 or Ar at 240 °C. The hydrodesulfurization of 2-PCHT proceeded through three parallel pathways: β-elimination, hydrogenolysis, and dehydrogenation. Under Ar, the parallel pathways were β-elimination, hydrogenolysis or homolytic C–S bond cleavage, and dehydrogenation. MoP and WP were more active than Ni2P. β-Elimination dominated the hydrodesulfurization of 2-PCHT over Ni2P, while hydrogenolysis was as fast as β-elimination over MoP and WP. Under Ar, β-elimination and dehydrogenation pathways were about equal over Ni2P, whereas β-elimination was the major pathway over MoP and WP. The inhibiting effects of piperidine depended on the reaction and catalyst. Phosphosulfide phases were formed under both H2 and Ar, but the sulfidation behavior of Ni2P was different from that of MoP and WP. Ni2P was more difficult to sulfide than MoP and WP under H2.

Exploration of the Dehydrogenation Pathways of Ammonia Diborane and Diammoniate of Diborane by Molecular Dynamics Simulations Using Reactive Force Fields.

Ammonium aminodiboranate (AADB) and diammoniate of diborane (DADB) are two isomers of ammonia borane (AB), which have been intensively studied for hydrogen storage. Their high hydrogen contents give them the high potential to serve as hydrogen storage materials. To explore their dehydrogenation pathways, molecular dynamics (MD) simulations with a reactive force field (ReaxFF) were applied. Temperature ramping simulations of their thermolysis were carried out. For AADB, at low temperatures, its hydrogen release can be realized mainly via intermolecular dehydrogenations. As the temperature of the simulated system increases, the formations of B-N bonds begin to occur. In the case of DADB, we found that this molecule could release hydrogen at a lower temperature with the cleavage of the B-N bond. The compositional analysis of the simulated systems was also conducted to monitor the potential intermediates along their dehydrogenation pathways. Our current work provides a detailed picture of the initial dehydrogenation steps of AADB and DADB and highlights the difference in their respective dehydrogenation processes.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Alcohols to Carboxylate Salts and Hydrogen

The facile oxidation of alcohols to carboxylate salts and H2 is achieved using a simple and readily accessible cobalt pincer catalyst (NNNHtBuCoBr2). The reaction follows an acceptorless dehydrogenation pathway and displays good functional group tolerance. The amine-amide metal-ligand cooperation in cobalt catalyst is suggested to facilitate this transformation. The mechanistic studies indicate that in-situ-formed aldehydes react with a base through a Cannizzaro-type pathway, resulting in potassium hemiacetolate, which further undergoes catalytic dehydrogenation to provide the carboxylate salts and H2.

Sodium Alanate Dehydrogenation Properties Enhanced by MnTiO3 Nanoparticles

In this study, we investigated the influence of MnTiO3 nanoparticles additive on hydrogen released performance of NaAlH4 for the first time. The MnTiO3 nanoparticles were successfully synthesized using conventional solid-state ceramic route. It was found that the hydrogen released performance of NaAlH4 can be significantly improved by the addition of MnTiO3 nanoparticles. Meantime, the composite of NaAlH4 doped 5 wt% MnTiO3 possessed excellent dehydrogenation properties, the onset dehydrogenation temperature was only 70.6 °C, reduced by about 105 °C in comparison with the pristine NaAlH4, and approximately 5.01 wt% of hydrogen could be released from composite with temperature heated to 220 °C. The isothermal dehydrogenation test results indicated that the amount of hydrogen released by NaAlH4-5 wt% MnTiO3 composite could reach 4.4 wt% under 200 °C within 25 min. According to the analysis of X-ray diffraction, the presence of MnTiO3 nanoparticles did not alter the overall dehydrogenation pathway of NaAlH4, and the Al3 Ti phases formed after dehydrogenation, which enhanced hydrogen desorption performances of NaAlH4 .

DFT Analysis of Ethanol Electro-Oxidation on Fe(110) and Fe3C(110) and its Correlation with the Stress Corrosion Cracking of Carbon Steel

Striking mechanical and morphological similarities of the stress corrosion cracking (SCC) of carbon steel in ethanolic media with those governed by a cleavage-like mechanism in CO-CO2 aqueous solutions, prompted the investigation of the possibility of ethanol electrochemical oxidation into CO on ferrite (Fe) and cementite (Fe3C) surfaces. Density functional theory computations on (110) surfaces reveal that the catalytic activity of Fe and Fe3C through the α dehydrogenation pathway can significantly reduce the energy barrier of electro-oxidation of ethanol and production of CO to 0.575 and 0.480 eV, respectively. These first principle calculations indicate that at the anodic potentials applied during potentiostatic slow strain rate testing, ethanol electrooxidation to CO is thermodynamically viable on carbon steel, giving further credit to the involvement of cleavage type SCC of carbon steel in ethanolic environments.

NHC‐assisted Ni(II)‐catalyzed acceptorless dehydrogenation of amines and secondary alcohols

A novel catalytic system including NiCl2‐NHC ligand precursor has been developed for the preparation of imines from amines and ketones from alcohols. Owing to the acceptorless dehydrogenation pathway for this reaction, the hydrogen gas is liberated as a by‐product. The active catalyst is generated in situ by the reaction of nickel (II) chloride, bis (imidazolium) chlorides and potassium tert‐butoxide. The products were obtained in good to excellent yields and a wide variety of amines and ketones were applied successfully in this protocol.

Improving the electrocatalytic activity for formic acid oxidation of bimetallic Ir–Zn nanoparticles decorated on graphene nanoplatelets

A graphene nanoplatelet (GNP)-supported Ir–Zn catalyst (Ir–Zn/GNP) was fabricated by H2 reduction to discover an alternative for non-platinum and non-palladium catalysts as an anode catalyst in direct formic acid fuel cell (DFAFC). The obtained Ir–Zn/GNP catalyst with ratio of Ir:Zn = 50:50 (Ir50Zn50/GNP) exhibited better electrocatalytic activity than GNP-supported iridium catalyst (Ir/GNP) for formic acid oxidation. Although the oxidation peak current density of Ir50Zn50/GNP was slightly lower than that of Ir/GNP, the oxidation peak potential shifted more negatively (193 mV) than Ir/GNP with higher value of the ratio of forward scan to reverse the scan peak current (If/Ib). The presence of Zn also enhanced the power density and current generation with increased performance stability in a passive DFAFC cell tests. The improvement of the electrochemical performance was ascribed to the ensemble effect where the addition of Zn could modify the Ir atom arrangement, thereby promoting the oxidation through dehydrogenation pathway. However, extremely high Zn content would inhibit oxidation capability because Zn atoms might reduce the Ir catalytic sites. A new alternative for non-Pt and non-Pd anode catalysts for DFAFC applications was successfully achieved.

In Situ Exfoliation and Pt Deposition of Antimonene for Formic Acid Oxidation via a Predominant Dehydrogenation Pathway.

Direct formic acid fuel cell (DFAFC) has been considered as a promising energy conversion device for stationary and mobile applications. Advanced platinum (Pt) electrocatalysts for formic acid oxidation reaction (FAOR) are critical for DFAFC. However, the oxidation of formic acid on Pt catalysts often occurs via a dual pathway mechanism, which hinders the catalytic activity owing to the CO poisoning. Herein, we directly exfoliate bulk antimony to 2D antimonene (Sb) and in situ load Pt nanoparticles onto antimonene sheets with the assistance of ethylenediamine. According to the Bader charge analysis, the charge transfer from antimonene to Pt occurs, confirming the electronic interaction between Pt and Sb. Interestingly, antimonene, as a cocatalyst, alters the oxidation pathway for FAOR over Pt catalyst and makes FAOR follow the more efficient dehydrogenation pathway. The density functional theory (DFT) calculation demonstrates that antimonene can activate Pt to be a lower oxidative state and facilitate the oxidation of HCOOH into CO2 via a direct pathway, resulting in a weakened intermediate binding strength and better CO tolerance for FAOR. The specific activity of FAOR on Pt/Sb is 4.5 times, and the mass activity is 2.6 times higher than the conventional Pt/C.

Anion-cation synergistic metal-free catalytic oxidative homocoupling of benzylamines by triazolium iodide salts.

Triazolium iodide salts are excellent catalysts for the selective oxidative coupling of benzylamines to yield imines. This metal-free reaction proceeds in quantitative spectroscopic yields when run in refluxing 1,2-dichlorobenzene and open to the air. No catalytic activity was observed with related triazolium tetrafluoroborate salts. Variation of catalyst and reaction atmosphere provides mechanistic insights, and revealed dioxygen as the terminal oxidant and the iodine/iodide couple as key redox component in the catalytic dehydrogenation pathway. While molecular iodine is competent as a catalyst in its own right, the triazolium cation triples the reaction rate and reaches turnover frequencies up to 30 h-1, presumably through beneficial interactions of the electron-poor azolium π system and I2, which facilitate the electron transfer from the substrate to iodine and concomitant formation of I-. This acceleration is specific for triazolium cations and represents a hybrid anion/cation catalytic process as a simple and straightforward route towards imine products, with economic advantages over previously reported metal-based catalytic systems.