Xuezhi Duan

Mechanistic insight into size-dependent activity and durability in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane

We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

Modified carbon nanotubes by KMnO4 supported iron Fischer–Tropsch catalyst for the direct conversion of syngas to lower olefins

Manganese and potassium promoters coated carbon nanotubes (i.e., MnK-CNTs) were synthesized by a redox reaction between CNTs and KMnO4, in which the CNTs act as reducing agent and as substrate for the heterogeneous nucleation of K-doped manganese oxide. The as-synthesized MnK-CNTs were employed to support Fe catalyst (i.e., Fe/MnK-CNTs, the loadings of 7.9 wt% Fe, 15.7 wt% Mn and 1.9 wt% K) for the direct conversion of syngas to lower olefins. It is revealed that Fe/MnK-CNTs catalyst is more active and stable than FeMnK/CNTs catalyst prepared by co-impregnation method using CNTs as a support. Furthermore, under similar CO conversion, the Fe/MnK-CNTs catalyst exhibits higher selectivity of hydrocarbons especially lower olefins. This could be related to the small-sized and uniform nanoparticles, the well-distributed promoters, the weak metal–support interaction and the greater defects on support, which are the consequences of the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere.

Carbon Nanotubes as Support in the Platinum‐Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane

We report remarkable support effects for carbon nanotubes (CNTs) in the Pt/CNT-catalyzed hydrolytic dehydrogenation of ammonia borane. The origin of the support-dependent activity and durability is elucidated by combining the catalytic and durability testing with characterization by a range of spectroscopy and high-angle annular dark-field scanning transmission electron microscopy techniques and ICP analysis. The effects mainly arise from different electronic properties and different abilities for the adsorption of boron-containing species on platinum surfaces and changes in size and shape of the platinum particles during the reaction. Defect-rich CNTs in particular are a promising support material, as it not only enhances the platinum binding energy, leading to the highest hydrogen generation rate, but also inhibits the adsorption of boron-containing species and stabilizes the platinum nanoparticles to resist the agglomeration during the reaction, leading to the highest durability. The insights revealed herein may pave the way for the rational design of highly active and durable metal/carbon catalysts for the hydrolytic dehydrogenation of ammonia borane.

Ir–Re alloy as a highly active catalyst for the hydrogenolysis of glycerol to 1,3-propanediol

In this work, bimetallic Ir–Re catalysts supported on KIT-6 are prepared by tuning the thermal treatment procedures, i.e., conventional calcination and reduction (Ir–Re/KIT-6-CR) and modified direct reduction (Ir–Re/KIT-6-R) after impregnation of two metal precursors. The structure of both catalysts is intensively characterized by H2-TPR, STEM-HAADF-EDX, XPS and CO-DRIFTS. Results indicate that an Ir–Re alloy forms on the KIT-6 support when direct reduction is employed, which exhibits excellent catalytic performance in hydrogenolysis of glycerol. The formation rate of 1,3-propanediol over Ir–Re/KIT-6-R reaches 25.6 mol1,3-PD molIr−1 h−1 at 63% glycerol conversion with the addition of amberlyst-15 under 8 MPa H2, 393 K and 20 wt% glycerol aqueous solution, almost twice that over Ir–Re/KIT-6-CR. It is revealed that Re species without prior calcination treatment could be fully reduced and therefore couple with Ir to form an Ir–Re alloy structure with enhanced resistance against particle aggregation, while the calcination and subsequent reduction leads to the formation of an Ir–ReOx structure since the rhenium oxide species generated during the calcination is difficult to be reduced.

Eco-friendly one-pot synthesis of highly dispersible functionalized graphene nanosheets with free amino groups

An eco-friendly, facile and scalable hydrothermal approach, in which the reduction and functionalization of graphite oxide (GO) are completed in one pot, is proposed for the synthesis of monolayer 3-aminopropyltriethoxysilane (APTES)-functionalized graphenes (A-FGs). Atomic force microscopy, transmission electron microscopy and x-ray diffraction analyses indicate that the as-synthesized A-FGs consist of only one or a few layered graphenes, while x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis reveal that APTES is bonded to graphene by the dehydration reaction between the Si-OH (produced by APTES hydration) and the -OH on the GO surface. As a result, free amino groups are left on the A-FGs. Moreover, A-FGs are highly dispersible in dimethylsulfoxide, APTES and ethylene glycol, and their solubilities are up to 0.89, 4.03 and 0.90 mg ml(-1), respectively.

Discrimination of the mechanism of CH4 formation in Fischer–Tropsch synthesis on Co catalysts: a combined approach of DFT, kinetic isotope effects and kinetic analysis

The mechanism of CH4 formation during Fischer–Tropsch synthesis on cobalt has been studied. DFT, kinetic isotope effect and kinetic analyses are combined to discriminate between the possible reaction routes of CH4 formation on Co catalysts. Nine direct reaction mechanisms were proposed from 21 elementary steps. They were first screened by DFT calculations in which the activation energies as well as the free energy profiles in each direct mechanism were compared, resulting in a reduction to six reaction mechanisms. Additional reduction was based on kinetic analysis where the reaction order was used as a descriptor. Subsequently, the kinetic isotope effect (KIE) values were calculated and compared to our previous SSITKA results. Finally, the dominating reaction route was suggested, which follows the initial elementary steps with H-assisted CO activation to form HCOH via HCO as an intermediate. It then proceeds through HCOH dissociation to CH followed by stepwise hydrogenation to CH4.

Carbon Nanofiber-Supported Ru Catalysts for Hydrogen Evolution by Ammonia Decomposition

Abstract Carbon nanofibers (CNFs) with fish-bone graphene alignment and carbon nanotubes (CNTs) were used to support ruthenium for ammonia decomposition. The Ru nanoparticles on the CNF supports are more active than those on CNT supports. The Ru particle size was adjusted by changing the Ru loading or by introducing oxygen containing groups onto the CNF surface. The site activity increases when the Ru crystal size increases. The oxygen groups on the CNFs have a remarkable effect on ammonia decomposition over the Ru nanoparticles. On identically sized Ru crystals, oxygen on the CNFs clearly enhances ammonia decomposition over the Ru/CNFs.

Understanding Co‐Mo Catalyzed Ammonia Decomposition: Influence of Calcination Atmosphere and Identification of Active Phase

To elucidate the effect of textural and chemical properties of catalysts and to discriminate the active phase are fundamental issues in heterogeneous catalysis, but they are often complicated by the nature of support adding a new dimension. In this work, metal amine metallate (Co(en)3MoO4) precursor was directly treated by calcination and prenitridation to understand these two issues in Co‐Mo catalyzed ammonia decomposition. The calcination atmosphere (i.e., Ar and Air) is found to remarkably affect the textural and chemical properties of catalysts, and air atmosphere is favorable for higher stable reactivity despite incomplete nitridation of the catalyst. Further prenitridation of the sample from the calcination of Co(en)3MoO4 under air gives rise to higher catalytic activity, and Co3Mo3N is suggested as the dominant active phase of Co‐Mo catalysts. The insights revealed here shed new light on the design of Co‐Mo catalysts for ammonia decomposition.

Structural manipulation of the catalysts for ammonia decomposition

Ammonia decomposition is an important reaction in energy and environmental industries. The review is focused on the catalytic decomposition of ammonia as a key step. The performances of the often used catalysts (i.e., Ru, Ni, Fe and bimetallic catalysts) are summarized and the effects of the size and shape of metal nanoparticles, promoters, supports and preparation techniques are reviewed.

The templating effect of an easily available cationic polymer with widely separated charge centers on the synthesis of a hierarchical ZSM-5 zeolite

The epichlorohydrin-N,N-dimethyl-1,3-diaminopropane copolymer (PCA) has been introduced for the first time as a meso-template to successfully synthesize a hierarchical ZSM-5 zeolite (PCA-ZSM-5) with mesopores of 7–50 nm in diameter by using small-sized nanoblocks. However, when its structural analogue epichlorohydrin–dimethylamine polyamine (PCS) is used, the obtained ZSM-5 zeolite (PCS-ZSM-5) has lower mesoporosity than ZSM-5 nanocrystallite aggregates (NCA-ZSM-5) synthesized without the meso-template. The templating effect of the two employed cationic polymers (PCS and PCA) on the synthesis of hierarchical ZSM-5 is valuable and interesting to be evaluated, because they are easily available and have common structural characteristics that their macromolecular structure will be largely endangered by the decomposition of quaternary ammonium groups in the long polymer chain. PCA entrapped in the zeolite partially retains its cationic charges and macromolecular structure under hydrothermal conditions and thus has a significant templating effect on the synthesis of hierarchical ZSM-5, which is benefited from the cationic centers widely separated by more than 3 carbons in PCA. However, PCS decomposes severely into small amine molecules, due to the short separation of cationic centers. Further investigation into the templating effect of PCA shows that the small-sized and negatively charged nanoblocks can easily wrap and assemble with PCA and transform into hierarchical ZSM-5 templated by PCA. However, when using large-sized zeolite seeds to synthesize ZSM-5, PCA shows a negligible templating effect because PCA with limited charge density and low accessibility of cationic charges has insufficient interactions with zeolite seeds. The catalytic activities of PCA-ZSM-5 and NCA-ZSM-5 for acetalization of cyclohexanone with methanol are inferior to that of PCS-ZSM-5 with the highest number of acid sites, but the catalytic activities for aldol condensation of benzaldehyde with n-butyl alcohol follow the order of PCA-ZSM-5 > NCA-ZSM-5 > PCS-ZSM-5, consistent with the order of mesoporosity.