Recent Submissions

  • A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion.

    Bailleul, Simon; Yarulina, Irina; Hoffman, Alexander E J; Dokania, Abhay; Abou-Hamad, Edy; Chowdhury, Abhishek Dutta; Pieters, Giovanni; Hajek, Julianna; De Wispelaere, Kristof; Waroquier, Michel; Gascon, Jorge; Van Speybroeck, Veronique (Journal of the American Chemical Society, American Chemical Society (ACS), 2019-08-30) [Article]
    A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(μ-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.
  • Tetracrystalline Tetrablock Quarterpolymers: Four Different Crystallites under the Same Roof.

    Hadjichristidis, Nikolaos; Ladelta, Viko; Zapsas, Georgios; Abou-Hamad, Edy; Gnanou, Yves (Angewandte Chemie (International ed. in English), Wiley, 2019-08-27) [Article]
    Multicrystalline block copolymers having three or more crystalline segments are essential materials for the advancement of physics in the field of crystallinity. Due to the challenging synthesis of multicrystalline polymers only a limited number of tricrystalline terpolymers are reported until now. We report, for the first time, the synthesis of poly(ethylene)-b-poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(L-Lactide) (PE-b-PEO-b-PCL-b-PLLA) tetracrystalline tetrablock quarterpolymer, by combining polyhomologation, ring-opening polymerization, and an organic/metal “catalyst switch” strategy.1H NMR spectroscopy and gel-permeation chromatography confirm the formation of the tetrablock quarterpolymer, while differential scanning calorimetry, X-ray diffraction, and wide-line separation solid-state NMR spectroscopy reveal the existence of four different crystalline domains.
  • Performance and Stability Improvement of Layered NCM Lithium-Ion Batteries at High Voltage by a Microporous Al2O3 Sol–Gel Coating

    Wu, Yingqiang; Li, Mengliu; Wahyudi, Wandi; Sheng, Guan; Miao, Xiaohe; Anthopoulos, Thomas D.; Huang, Kuo-Wei; Li, Yangxing; Lai, Zhiping (ACS Omega, American Chemical Society (ACS), 2019-08-19) [Article]
    A simple and low-cost polymer-aided sol–gel method was developed to prepare γ-Al2O3 protective layers for LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials. The selected polyvinyl alcohol polymer additive not only facilitates the formation of a uniform and thin γ-Al2O3 layer on the irregular and rough cathode particle surface to protect it from corrosion but also serves as a pore-forming agent to generate micropores in the film after sintering to allow fast transport of lithium ions, which guaranteed the excellent and stable battery performance at high working voltage. Detailed studies in the full battery mode showed that the leached corrosion species from the cathode had a more profound harmful effect to the graphite anode, which seemed to be the dominating factor that caused the battery performance decay.
  • Heterostructured MXene and g-C3N4 for high-rate lithium intercalation

    Zhu, Yun Pei; Lei, Yongjiu; Ming, Fangwang; Abou-Hamad, Edy; Emwas, Abdul-Hamid M.; Hedhili, Mohamed N.; Alshareef, Husam N. (Nano Energy, Elsevier Ltd, 2019-08-16) [Article]
    A critical limitation to conventional electrochemical double-layer capacitors is their low energy densities. This has triggered significant interest in developing new pseudocapacitive materials, which utilize faradaic mechanisms to increase their energy densities. In this work, graphitic carbon nitride (g-C3N4) and Ti3C2Tx MXene are hybridized to form a unique two-dimensional (2D) heterostructure, which delivers remarkable pseudocapacitive characteristics and robust stability towards lithium storage. Interestingly, the improved kinetics is reflected by insignificant influence of (dis)charge rates on the pseudocapacitance even when testing at a 120C rate, and small peak potential offsets at high scan rates, revealing that there are no significant diffusion limitations in the heterostructure. This unexpected fast kinetics is related to the intrinsic chemical and electronic coupling effects between g-C3N4 and MXene, which can synergistically improve both electron transfer and lithium diffusion kinetics compared to MXene itself.
  • Mechanistic study of hydroamination of alkyne through tantalum-based silica-supported surface species

    Aljuhani, Maha A.; Zhang, Ziyun; Barman, Samir; El Eter, Mohamad; Falivene, Laura; Ould-Chikh, Samy; Guan, Erjia; Abou-Hamad, Edy; Emwas, Abdul-Hamid M.; Pelletier, Jeremie; Gates, Bruce C.; Cavallo, Luigi; Basset, Jean-Marie (ACS Catalysis, American Chemical Society (ACS), 2019-08-06) [Article]
    Selective hydroamination of terminal alkynes with primary aryl amines is catalyzed by an unprecedented well-defined silica-supported tantalum complex [(≡Si-O-)Ta(η1σ-NEtMe)2(=NtBu)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization tech-niques including as in situ infrared, in situ elemental microanal-ysis, 1H and 13C solid-state NMR (including double and triple quanta sequence), and X-ray absorption spectroscopies. These were complemented by the state-of-the-art DNP-SENS 15N characterization. Several catalytic intermediates have been isolated in particular the 4-membered metallacycle ring inter-mediate resulting from the anti Markovnikov addition of the alkyne to the surface tantalum imido. The mechanism proposed was based on the isolation of all intermediates. A DFT calcula-tion has confirmed all the elementary steps and intermediates that were fully characterized.
  • Quantum-Dot-Derived Catalysts for CO2 Reduction Reaction

    Liu, Min; Liu, Mengxia; Wang, Xiaoming; Kozlov, Sergey; Cao, Zhen; De Luna, Phil; Li, Hongmei; Qiu, Xiaoqing; Liu, Kang; Hu, Junhua; Jia, Chuankun; Wang, Peng; Zhou, Huimin; He, Jun; Zhong, Miao; Lan, Xinzheng; Zhou, Yansong; Wang, Zhiqiang; Li, Jun; Seifitokaldani, Ali; Dinh, Cao Thang; Liang, Hongyan; Zou, Chengqin; Zhang, Daliang; Yang, Yang; Chan, Ting Shan; Han, Yu; Cavallo, Luigi; Sham, Tsun Kong; Hwang, Bing Joe; Sargent, Edward H. (Joule, Elsevier BV, 2019-07-17) [Article]
    Defect sites are often proposed as key active sites in the design of catalysts. A promising strategy for improving activity is to achieve a high density of homogeneously dispersed atomic defects; however, this is seldom accomplished in metals. We hypothesize that vacancy-rich catalysts could be obtained through the synthesis of quantum dots (QDs) and their electrochemical reduction during the CO2 reduction reaction (CO2RR). Here, we report that QD-derived catalysts (QDDCs) with up to 20 vol % vacancies achieve record current densities of 16, 19, and 25 mAcm−2 with high faradic efficiencies in the electrosynthesis of formate, carbon monoxide, and ethylene at low potentials of –0.2, –0.3, and –0.9 V versus reversible hydrogen electrode (RHE), respectively. The materials are stable after 80 hr of CO2RR. These CO2RR performances in aqueous solution surpass those of previously reported catalysts by 2×. Together, X-ray absorption spectroscopy and computational studies reveal that the vacancies produce a local atomic and electronic structure that enhances CO2RR.
  • Turning a Methanation Co Catalyst into an In–Co Methanol Producer

    Bavykina, Anastasiya; Yarulina, Irina; Al Abdulghani, Abdullah; Gevers, Lieven; Hedhili, Mohamed N.; Miao, Xiaohe; Galilea, Adrian; Pustovarenko, Alexey; Dikhtiarenko, Alla; Cadiau, Amandine; Aguilar-Tapia, Antonio; Hazemann, Jean-Louis; Kozlov, Sergey; Oud-Chikh, Samy; Cavallo, Luigi; Gascon, Jorge (ACS Catalysis, American Chemical Society (ACS), 2019-07-05) [Article]
    The direct hydrogenation of CO2 to methanol using hydrogen is regarded as a potential technology to reduce greenhouse gas emissions and the dependence on fossil fuels. For this technology to become feasible, highly selective and productive catalysts that can operate under a wide range of reaction conditions near thermodynamic conversion are required. Here we combine a CO-producing In oxide catalyst with a methane-producing Co catalyst to obtain an In/Co catalyst for CO2 reduction to methanol. Density functional (DFT) simulations demonstrate that the charge transfer between the Co support and the In oxide film leads to enrichment of the surface of indium oxide with O vacancies, which serve as active sites for selective conversion of CO2 to methanol. Moreover, our simulations suggest that CO2 reduction on Co-supported In2O3–x films will preferentially yield methanol, rather than CO and methane. As a result, the prepared In@Co catalysts produce methanol from CO2 with high selectivity (>80%) and productivity (0.86 gCH3OH gcatalyst–1 h–1) at conversion levels close to thermodynamic equilibrium, even at temperatures as high as 300 °C and at moderate pressures (50 bar).
  • Negative differential resistance and magnetotransport in Fe3O4/SiO2/Si heterostructures

    Liu, Xiang; Mi, Wenbo; Zhang, Qiang; Zhang, Xixiang (Applied Physics Letters, AIP Publishing, 2019-06-17) [Article]
    The electronic transport and magnetotransport properties of Fe3O4/SiO2/Si heterostructures were investigated with a current source. Negative differential resistance is observed in Fe3O4/SiO2/p-Si heterostructures. The measurement circuit with four electrodes that I+ (I−) and V+ (V−) came into contact with the Fe3O4 (Si) layer introduces an in-plane transport into the heterostructures. By decreasing the temperature, the in-plane conductive channel switches from Fe3O4 to p-Si. However, the in-plane current is still carried by Fe3O4 in Fe3O4/SiO2/n-Si heterostructures. The formation of an accumulation layer in p-Si facilitates conductive channel switching (CCS), while the depletion layer in n-Si hampers the CCS. At 150 K, a magnetic-field-independent magnetoresistance (MR) in Fe3O4/SiO2/p-Si heterostructures manifests the conductive channel in the space charge region of p-Si. A positive MR generated from the increased electronic scattering in a trapezoidal space charge region reshaped by the magnetic field has been detected.
  • Bimetallic Pt-Sn nanocluster from the hydrogenolysis of a well-defined surface compound consisting of [([tbnd]AlO–)Pt(COD)Me] and [([tbnd]AlO–)SnPh3] fragments for propane dehydrogenation

    Xu, Zhikang; Xu, Rui; Yue, Yuanyuan; Yuan, Pei; Bao, Xiaojun; Abou-Hamad, Edy; Basset, Jean-Marie; Zhu, Haibo (Journal of Catalysis, Academic Press, 2019-06-01) [Article]
    The bimetallic Pt-Sn catalysts are known to be the most efficient ones for propane dehydrogenation reaction; however, the synthesis of bimetallic Pt-Sn nanocluster (below 1 nm) with an alloy structure still remains a formidable challenge. An approach for the controllable synthesis of Pt-Sn nanocluster based on Surface Organometallic Chemistry on oxide concept is developed here, which involves two basic steps: the sequential grafting of Pt(COD)Me2 and HSnPh3 at the surface of dehydroxylated θ-Al2O leading to the formation of well-defined bimetallic surface compound consisting of [([tbnd]AlO–)Pt(COD)Me] and [([tbnd]AlO–)SnPh3] fragments, and the hydrogenolysis of this surface compound under a mild condition giving rise to the ultra-small bimetallic Pt-Sn cluster around 0.75 nm. The achieved Pt-Sn/θ-Al2O3 catalysts exhibit high activity, selectivity and stability in propane dehydrogenation to propylene.
  • Effect of Zeolite Topology and Reactor Configuration on the Direct Conversion of CO2 to Light Olefins and Aromatics

    Ramirez, Adrian; Dutta Chowdhury, Abhishek; Dokania, Abhay; Cnudde, Pieter; Caglayan, Mustafa; Yarulina, Irina; Abou-Hamad, Edy; Gevers, Lieven; Ould-Chikh, Samy; De Wispelaere, Kristof; Van Speybroeck, Veronique; Gascon, Jorge (ACS Catalysis, American Chemical Society (ACS), 2019-05-29) [Article]
    The direct transformation of CO2 into high-value-added hydrocarbons (i.e., olefins and aromatics) has the potential to make a decisive impact in our society. However, despite the efforts of the scientific community, no direct synthetic route exists today to synthesize olefins and aromatics from CO2 with high productivities and low undesired CO selectivity. Herein, we report the combination of a series of catalysts comprising potassium superoxide doped iron oxide and a highly acidic zeolite (ZSM-5 and MOR) that directly convert CO2 to either light olefins (in MOR) or aromatics (in ZSM-5) with high space–time yields (STYC2-C4= = 11.4 mmol·g–1·h–1; STYAROM = 9.2 mmol·g–1·h–1) at CO selectivities as low as 12.8% and a CO2 conversion of 49.8% (reaction conditions: T = 375 °C, P = 30 bar, H2/CO2 = 3, and 5000 mL·g–1·h–1). Comprehensive solid-state nuclear magnetic resonance characterization of the zeolite component reveals that the key for the low CO selectivity is the formation of surface formate species on the zeolite framework. The remarkable difference in selectivity between the two zeolites is further rationalized by first-principles simulations, which show a difference in reactivity for crucial carbenium ion intermediates in MOR and ZSM-5.
  • Tandem conversion of CO2 to valuable hydrocarbons in highly concentrated potassium iron catalysts

    Ramirez, Adrian; Ould-Chikh, Samy; Gevers, Lieven; Dutta Chowdhury, Abhishek; Abou-Hamad, Edy; Aguilar, Antonio; Hazemann, Jean-Louis; Wehbe, Nimer; Al Abdulghani, Abdullah; Kozlov, Sergey; Cavallo, Luigi; Gascon, Jorge (ChemCatChem, Wiley, 2019-04-29) [Article]
    The alarming atmospheric concentration and continuous emissions of carbon dioxide (CO2) require immediate action. As a result of advances in CO2 capture and sequestration technologies (generally involving point sources such as energy generation plants), large amounts of pure CO2 will soon be available. In addition to geological storage and other applications of the captured CO2, the development of technologies able to convert this carbon feedstock into commodity chemicals may pave the way towards a more sustainable economy. Here, we present a novel multifunctional catalyst consisting of Fe2O3 encapsulated in K2CO3 that can transform CO2 into olefins via a tandem mechanism. In contrast to traditional systems in Fischer-Tropsch reactions, we demonstrate that when dealing with CO2 conversion (in contrast to CO), very high K loadings are key to activate CO2 via the well-known ‘potassium carbonate mechanism’. The proposed catalytic process is demonstrated to be as productive as existing commercial processes based on synthesis gas while relying on economically and environmentally advantageous CO2 feedstock.
  • A strategy to convert propane to aromatics (BTX) using TiNp4 grafted at the periphery of ZSM-5 by surface organometallic chemistry

    Almaksoud, Walid; Gevers, Lieven; Vittenet, Jullian; Ould-Chikh, Samy; Telalovic, Selvedin; Bhatte, Kushal Deepak; Abou-Hamad, Edy; Anjum, Dalaver H.; Hedhili, Mohamed N.; Vishwanath, Vinu; Alhazmi, Abdulrahman; Almusaiteer, Khaled; Basset, Jean-Marie (Dalton Transactions, Royal Society of Chemistry (RSC), 2019-04-24) [Article]
    The direct conversion of propane into aromatics (BTX) using modified ZSM-5 was achieved with a strategy of
  • Aqueous Zinc-Ion Storage in MoS2 by Tuning the Intercalation Energy

    Liang, Hanfeng; Cao, Zhen; Ming, Fangwang; Zhang, Wenli; Anjum, Dalaver H.; Cui, Yi; Cavallo, Luigi; Alshareef, Husam N. (Nano Letters, American Chemical Society (ACS), 2019-04-15) [Article]
    Aqueous Zn-ion batteries present low-cost, safe, and high-energy battery technology but suffer from the lack of suitable cathode materials because of the sluggish intercalation kinetics associated with the large size of hydrated zinc ions. Herein we report an effective and general strategy to transform inactive intercalation hosts into efficient Zn2+ storage materials through intercalation energy tuning. Using MoS2 as a model system, we show both experimentally and theoretically that even hosts with an originally poor Zn2+ diffusivity can allow fast Zn2+ diffusion. Through simple interlayer spacing and hydrophilicity engineering that can be experimentally achieved by oxygen incorporation, the Zn2+ diffusivity is boosted by 3 orders of magnitude, effectively enabling the otherwise barely active MoS2 to achieve a high capacity of 232 mAh g–1, which is 10 times that of its pristine form. The strategy developed in our work can be generally applied for enhancing the ion storage capacity of metal chalcogenides and other layered materials, making them promising cathodes for challenging multivalent ion batteries.
  • Nanomechanical DNA resonators for sensing and structural analysis of DNA-ligand complexes

    Stassi, Stefano; Marini, Monica; Allione, Marco; Lopatin, Sergei; Marson, Domenico; Laurini, Erik; Pricl, Sabrina; Pirri, Candido Fabrizio; Ricciardi, Carlo; Di Fabrizio, Enzo M. (Nature Communications, Springer Nature, 2019-04-12) [Article]
    The effect of direct or indirect binding of intercalant molecules on DNA structure is of fundamental importance in understanding the biological functioning of DNA. Here we report on self-suspended DNA nanobundles as ultrasensitive nanomechanical resonators for structural studies of DNA-ligand complexes. Such vibrating nanostructures represent the smallest mechanical resonator entirely composed of DNA. A correlative analysis between the mechanical and structural properties is exploited to study the intrinsic changes of double strand DNA, when interacting with different intercalant molecules (YOYO-1 and GelRed) and a chemotherapeutic drug (Cisplatin), at different concentrations. Possible implications of our findings are related to the study of interaction mechanism of a wide category of molecules with DNA, and to further applications in medicine, such as optimal titration of chemotherapeutic drugs and environmental studies for the detection of heavy metals in human serum.
  • Light-Induced Self-Assembly of Cubic CsPbBr3 Perovskite Nanocrystals into Nanowires

    Liu, Jiakai; Song, Kepeng; Shin, Yongwoo; Liu, Xin; Chen, Jie; Yao, Kexin; Pan, Jun; Yang, Chen; Yin,Jun; Xu, Liang-Jin; Yang, Haoze; El-Zohry, Ahmed; Xin, Bin; Mitra, Somak; Hedhili, Mohamed N.; Roqan, Iman S.; Mohammed, Omar F.; Han, Yu; Bakr, Osman (Chemistry of Materials, American Chemical Society (ACS), 2019-04-10) [Article]
    The rapid development of halide perovskite synthesis offers the opportunity to fabricate high-quality perovskite nanocrystals (NCs), whose structural uniformity can lead to assembled supra-structures with improved device performance and novel collective properties. Light is known to significantly affect the structure and properties of halide perovskites and plays a crucial role in the growth and assembly of their crystals. Nevertheless, the light-induced growth mechanisms of perovskite NCs are not yet clearly understood. In this work, we performed a systematic study of the visible-light-induced template-free synthesis of CsPbBr3 nanowires (NWs) generated through self-assembly of cubic (in phase and close to cubic morphology) NCs. Using atomic-resolution electron microscopy, we visualized the cubic-to-orthorhombic phase transition in NCs and the interface between coalesced NCs. Remarkably, the images of the interface revealed the coexistence of CsBr and PbBr2 surface terminations in halide perovskites. Our results shed light on the mechanism underlying the observed anisotropic assembly of halide perovskites and elucidate the vital role of light illumination during this process. More importantly, as an elegant and promising green-chemistry approach, light-induced self-assembly represents a rational method for designing perovskites.
  • A 0D Lead-Free Hybrid Crystal with Ultralow Thermal Conductivity

    Haque, Mohammed; Gandi, Appala; Mohanraman, Rajeshkumar; Weng, Yakui; Davaasuren, Bambar; Emwas, Abdul-Hamid M.; Combe, Craig; Baran, Derya; Rothenberger, Alexander; Schwingenschlögl, Udo; Alshareef, Husam N.; Dong, Shuai; Wu, Tom (Advanced Functional Materials, Wiley, 2019-02-07) [Article]
    Organic–inorganic hybrid materials are of significant interest owing to their diverse applications ranging from photovoltaics and electronics to catalysis. Control over the organic and inorganic components offers flexibility through tuning their chemical and physical properties. Herein, it is reported that a new organic–inorganic hybrid, [Mn(C2H6OS)6]I4, with linear tetraiodide anions exhibit an ultralow thermal conductivity of 0.15 ± 0.01 W m−1 K−1 at room temperature, which is among the lowest values reported for organic–inorganic hybrid materials. Interestingly, the hybrid compound has a unique 0D structure, which extends into 3D supramolecular frameworks through nonclassical hydrogen bonding. Phonon band structure calculations reveal that low group velocities and localization of vibrational energy underlie the observed ultralow thermal conductivity, which could serve as a general principle to design novel thermal management materials.
  • Direct Growth of Single Crystalline GaN Nanowires on Indium Tin Oxide-Coated Silica

    Prabaswara, Aditya; Min, Jung-Wook; Subedi, Ram; Tangi, Malleswararao; Holguin Lerma, Jorge Alberto; Zhao, Chao; Priante, Davide; Ng, Tien Khee; Ooi, Boon S. (Nanoscale Research Letters, Springer Nature, 2019-02-05) [Article]
    In this work, we demonstrated the direct growth of GaN nanowires on indium tin oxide (ITO)-coated fused silica substrate. The nanowires were grown catalyst-free using plasma-assisted molecular beam epitaxy (PA-MBE). The effect of growth condition on the morphology and quality of the nanowires is systematically investigated. Structural characterization indicates that the nanowires grow in the (0001) direction directly on top of the ITO layer perpendicular to the substrate plane. Optical characterization of the nanowires shows that yellow luminescence is absent from the nanowire's photoluminescence response, attributed to the low number of defects. Conductive atomic force microscopy (C-AFM) measurement on n-doped GaN nanowires shows good conductivity for individual nanowires, which confirms the potential of using this platform for novel device applications. By using a relatively low-temperature growth process, we were able to successfully grow high-quality single-crystal GaN material without the degradation of the underlying ITO layer.
  • Quantifying the impact of dispersion, acidity and porosity of Mo/HZSM-5 on the performance in methane dehydroaromatization

    Vollmer, Ina; Mondal, Amantrita; Yarulina, Irina; Abou-Hamad, Edy; Kapteijn, Freek; Gascon, Jorge (Applied Catalysis A: General, Elsevier BV, 2019-01-29) [Article]
    The catalytic performance of the bifunctional catalyst Mo/HZSM-5 for methane dehydroaromatization (MDA) depends on the Mo dispersion and on zeolite acidity. Here we separately quantify the effect of dispersion and the effect of acidity on aromatic yields and coke selectivity. Also, the effect of porosity on the same is quantitatively assessed. For that, a suite of 17 samples with varying Mo dispersion were synthesized by means of several methods, including chemical vapor deposition with MoCl5, MoO2Cl2 and Mo(CO)6 as precursors and the conventional methods, incipient wetness impregnation and solid ion exchange. These catalysts were characterized with pyridine IR-spectroscopy, XPS, UV–vis spectroscopy, N2 adsorption, XRD, TGA and 27Al MAS NMR. The combined results yielded a measure of how much Mo is anchored to the zeolite as well-defined cationic species and how much is present as bigger clusters on the outer surface of the zeolite. Through relating these characterization results to the catalytic behavior of the catalysts, it was found that the maximum instantaneous benzene and naphthalene yields as well as the integral selectivities during methane dehydroaromatization linearly increase with the amount of Mo present as mono- or dimeric species. At the same time, the selectivity to coke increases with the amount of Mo present as bigger clusters or nanoparticles on the outer surface of the zeolite. The number of Mo cationic sites is the most important factor determining the activity of Mo/HZSM-5 for low loadings of Mo. But at higher loadings, the high rate of aromatics formation requires an easily accessible pore structure as well.
  • A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation

    Vollmer, Ina; Kosinov, Nikolay; Szécsényi, Ágnes; Li, Guanna; Yarulina, Irina; Abou-Hamad, Edy; Gurinov, Andrei; Ould-Chikh, Samy; Aguilar-Tapia, Antonio; Hazemann, Jean-Louis; Pidko, Evgeny; Hensen, Emiel; Kapteijn, Freek; Gascon, Jorge (Journal of Catalysis, Elsevier BV, 2019-01-22) [Article]
    The active sites on the methane dehydroaromatization (MDA) catalyst Mo/HZSM-5 are very hard to characterize, because they are present in various geometries and sizes and only form under reaction conditions with methane at 700 °C. To address these issues an experimental strategy is presented that enables distinguishing different active sites for MDA present on Mo/HZSM-5 and helps determining the Mo charge, nuclearity and chemical composition. This approach combines a CO pretreatment to separate the active Mo site formation from coke formation, quasi-in situ spectroscopic observations using DNP, 13C NMR, CO IR and theory. This allows the discrimination between three different types of active sites. Distinct spectroscopic features were observed corresponding to two types of mono- or dimeric Mo (oxy-)carbide sites as well as a third site assigned to Mo2C nanoparticles on the outer surface of the zeolite. Their formal Mo oxidation state was found to be between 4+ and 6+. Dynamic nuclear polarization (DNP) measurements of samples carburized in CO as well as in CH4 confirm the assignment and also show that accumulated aromatic carbon covers the bigger Mo nanoparticles on the outer surface of the zeolite, causing deactivation. It was previously observed that after an initial period where no desired products are formed yet, benzene starts slowly forming until reaching its maximum productivity. Direct observation of the active site with 13C NMR confirmed that Mo-sites do not transform further once benzene starts forming, meaning that they are fully activated during the period where no desired products are observed yet. Therefore the slow increase of the benzene formation rate cannot be attributed to a further transformation of Mo sites.
  • Twofold Porosity and Surface Functionalization Effect on Pt-Porous GaN for High-Performance H2-Gas Sensors at Room Temperature

    Shafa, Muhammad; Priante, Davide; Elafandy, Rami T.; Hedhili, Mohamed N.; Mahmoud, Saleh T.; Ng, Tien Khee; Ooi, Boon S.; Najar, Adel (ACS Omega, American Chemical Society (ACS), 2019-01-18) [Article]
    The achievement of H2 detection, up to 25 ppm, at room temperature using sulfur-treated, platinum (Pt)-decorated porous GaN is reported in this study. This achievement is attributed to the large lateral pore size, Pt catalyst, and surface treatment using organic sulfide. The performance of H2-gas sensors is studied as a function of the operating temperature by providing an adsorption activation energy of 22 meV at 30 ppm H2, confirming the higher sensitivity of the sulfide-treated Pt-porous GaN sensor. Furthermore, the sensing response of the sulfide-treated Pt-porous GaN gas sensor increases with the increase in porosity (surface-to-volume ratio) and pore radii. Using the Knudsen diffusion-surface reaction equation, the H2 gas concentration profile is simulated and fitted within the porous GaN layer, revealing that H2 diffusion is limited by small pore radii because of its low diffusion rate. The simulated gas sensor responses to H2 versus the pore diameter show the same trend as observed for the experimental data. The sulfide-treated Pt-porous GaN sensor achieves ultrasensitive H2 detection at room temperature for 125 nm pore radii.

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