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Nonoxidative conversion of ethane to ethylene and/or BTX (benzene, toluene , and xylene) suffers rapid deactivation due to coke deposition. We report here the effects of phosphorus modification on the stability and activity of Mo/ZSM5 for nonoxidative conversion of ethane. The results show that the ethylene and BTX yield and stability are significantly enhanced upon modification with 2.5 wt.% P. NH 3 TPD, pyridine FTIR, 1H MAS NMR, 27Al MAS NMR, 31P MAS NMR, 129 Xe NMR, XPS, UV–visible diffuse reflectance spectra (UV–vis DRS), and nitrogen physisorption were carried out to understand the effects of P on the structure of Mo/ZSM5 and its correlation with catalytic performance. The presence of P reduces the acid strength and density, changes the channel system of ZSM5 by forming thermally stable SAPO-like interfaces with the framework Al, and improves the dispersion of molybdenum . Rapid deactivation still occurs on Mo/ZSM5 with 1 wt.% P due to the existence of denser silanol groups, more isolated Mo species, and reduced aperture size with little change in effective micropore volume. A higher P loading (2.5 wt.%) leads to less dense silanol groups and less reduced but stable molybdenum species, and simultaneously reduces channel diameter and internal volume. Consequently, the ethylene selectivity is enhanced and the formation of coke precursors is restricted, resulting in improved stability.

(La,Sr)(Cr,Mn)O3 perovskite (LSCM) is one of the most promising cathode materials for solid oxide electrolysis cell (SOEC) at high temperature, but suffers from poor electrocatalytic activity towards CO2 reduction. Here we report that a modified LSCM based composite cathode fabricated via infiltrating Ce0.9Mn0.1O2−δ (CMO) nanoparticles onto (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3−δ-Ce0.8Gd0.2O1.9 (LSCM-GDC) composite materials, shows greatly improved electrocatalytic activity and stability towards CO2 reduction compared with the unmodified LSCM cathode. Physicochemical characterizations and electrochemical impedance spectroscopy analysis of electrocatalytic CO2 reduction in SOEC show that CO2 adsorption and the following carbonate intermediate dissociation processes on the CMO nanoparticles-modified LSCM-GDC cathode are significantly improved, which is attributed to the increased active three phase boundaries and surface oxygen vacancies by the infiltration of CMO nanoparticles on the LSCM-GDC cathode.

High field quantitative 27Al single pulse (SP) MAS NMR combined with temperature programmed desorption (TPD) of ethanol is used to study the surface of γ-Al2O3 during phase transformation processes induced by calcination in the temperature range of 500–1300 °C. Following ethanol adsorption, ethylene is generated during TPD with a desorption temperature above 200 °C. The amount of ethylene decreases monotonically with increasing calcination temperature prior to TPD. Significantly, 27Al SP MAS NMR reveals that the amount of penta-coordinated Al3+ ions also decreases with increasing calcination temperature. A quantitative (within experimental error) correlation between the amount of penta-coordinated Al3+ ions and the amount of strongly adsorbed ethanol molecules (i.e., the ones that convert to ethylene during TPD) is obtained. These results provide good evidence for a proposal that the penta-coordinated aluminum sites are the catalytic active sites on alumina surfaces during ethanol dehydration reaction across the entire course of γ-to-α Al2O3 phase transformations.

Nitrogen-doped graphene (N-graphene) was reported to exhibit a good activity experimentally as an electrocatalyst of oxygen reduction reaction (ORR) on the cathode of fuel cells under the condition of electropotential of ∼0.04 V (vs. NHE) and pH of 14. This material is promising to replace or partially replace the conventionally used Pt. In order to understand the experimental results, ORR catalyzed by N-graphene is studied using density functional theory (DFT) calculations under experimental conditions taking the solvent, surface adsorbates, and coverages into consideration. Two mechanisms, i.e., dissociative and associative mechanisms, over different N-doping configurations are investigated. The results show that N-graphene surface is covered by O with 1/6 monolayer, which is used for reactions in this work. The transition state of each elementary step was identified using four different approaches, which give rise to a similar chemistry. A full energy profile including all the reaction barriers shows that the associative mechanism is more energetically favored than the dissociative one and the removal of O species from the surface is the rate-determining step.

The layered silicate RUB-39 can be transformed by topotactic condensation into RUB-41 (RRO), a zeolite with 8- and 10- ring pores. If the layered RUB-39 is first silylated with dichlorodimethylsilane (DCDMS) or hexamethyldisiloxane (HMDS), an interlayer expanded structure is created after calcination. The DCDMS expanded material contains 10- and 12-ring pores instead of 8- and 10-ring pores. Detailed physicochemical characterization showed that the Al content is not significantly changed during the expansion. In the hydroconversion of decane, the expanded materials have a significantly increased activity, as demonstrated by the lower temperatures at which isomerization and cracking occur. Detailed comparison of the product selectivities obtained with RUB-41 or with its expanded analogs shows that the void structure of the expanded materials is significantly less constrained, as reflected in the distribution of methylnonane isomers, of the ethyloctane vs. methylnonane isomers, and in the ratio of monobranched vs. dibranched isomers.

Aluminum was incorporated into the layered silicate RUB-39, which is transformed by calcination into RUB-41. This new zeolite with RRO topology contains 8- and 10-ring pores, and the acid sites in the aluminated material catalyze the synthesis of methylamines, in particular mono- and dimethylamine, by amination of methanol. Owing to the shape-selective catalytic properties of (H)Al-RUB-41, low selectivity to the thermodynamically favored trimethylamine product is obtained in comparison with results on RUB-39 or non-shape-selective materials. Both activity and selectivity are highest for RUB-41 catalysts with a high Si to Al ratio. Silylation reduces the number of unselective sites and results in a further suppression of trimethylamine formation. The introduction of acidity in the intact RUB-41 structure is supported by Al-MAS NMR and NH3-TPD data. Additional characterization by XRD and SEM is provided.

In-situ 13 C MAS NMR techniques were used to investigate the effects of different solvent systems on the formation of phenylacetaldehyde (PADH) over TS-1 zeolite. Protic solvents such as water and methanol provide acidic centers during the reaction. These acidic species catalyzed the transformation of the intermediates to PADH. Whereas in the presence of aprotic solvents such as acetone, the precursor of PADH remains stable on the framework of TS-1 and accordingly there is no PADH formation.

Complete reduction of Mo species interacting strongly with a HZSM-5 zeolite support in methane is difficult. and the reduction product, MoCxOy, is a key active species in methane dehydroaromatization (MDA) under nonoxidative conditions. Pretreatment in H-2 at 623 K could lead to topotactic transformation of the Mo species from hexagonally close packing (hcp) to face-centered cubic (fee) structures; the fee MoCxOy species are more active and more stable than the hcp MoCxOy in MDA. After initial H-2 pretreatment at 623 K, followed by a CH4-treatment, the Mo species on a MoO3/HZSM-5 catalyst prepared by mechanical mixing was reduced and carburized to form fee alpha-MoC1-x species on the extra Surface of the zeolite. Methane molecules activated by the alpha-MoC1-x were converted mainly into carbon deposits instead of benzene. Both the most active Mo species and the neighboring Bronsted acid sites are essential in obtaining a perfect catalyst in the reaction of MDA. (c) 2006 Elsevier Inc. All rights reserved.

We report an interesting finding that the catalytic performance of supported Ag/SiO2 catalysts toward selective catalytic oxidation of CO in hydrogen at low temperatures can be greatly enhanced by pretreatment of the SiO2 support before catalyst preparation. Calcination of SiO2 at appropriate temperatures preferentially removes the H-bonded SiOH, which results in the highly dispersive Ag/SiO2 catalyst and thus improves the catalytic performance. (c) 2005 Elsevier Inc. All rights reserved.

Silver is well known to show peculiar catalytic activities in several oxidation reactions. in the present paper, we investigate the catalytic activity of silver catalysts toward co-gelective oxidation in h-2. xrd, tem, tpd, and in situ ftir techniques were used to characterize the catalysts. the pretreatment of the catalysts was found to have great influence on their performance. the pretreatment in 02 improves the activity of the silver catalyst, whereas he pretreatment at 700 degreesc or direct hydrogen pretreatment shows an inverse effect. silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures (> 500 degreesc), and there is solid evidence for the formation of subsurface oxygen species. the existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for co oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. it is observed, for the first time, that adsorbed co on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., rt), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with co. this finding provides a new reaction pathway for co oxidation on silver catalyst. (c) 2004 published by elsevier inc.

By using the solid-state mas nmr technique, the hydrothermal stabilities (under 100% steam at 1073 k) of hzsm-5 zeolites modified by lanthanum and phosphorus have been studied. they are excellent zeolite catalysts for residual oil selective catalytic cracking (rscc) processes. it was indicated that the introduction of phosphorus to the zeolite via impregnation with orthophosphoric acid led to dealumination as well as formation of different al species, which were well distinguished by al-27 3q mas nmr. meanwhile, the hydrothermal stabilities of the zeolites (p/hzsm-5, la-p/hzsm-5) were enhanced even after the samples were treated under severe conditions for a prolonged time. it was found that the si-o-al bonds were broken under hydrothermal conditions, while at the same time the phosphorous compounds would occupy the silicon sites to form (sio)(x)al(op)(4 - x) species. with increasing time, more silicon sites around the tetrahedral coordinated al in the lattice can be replaced till the aluminum is completely expelled from the framework. the existence of lanthanum can partially restrict the breaking of the si-o-al bonds and the replacement of the silicon sites by phosphorus, thus preventing dealumination under hydrothermal conditions. this was also proved by p-31 mas nmr spectra. (c) 2004 elsevier inc. all rights reserved.

In this paper, the oxidation of trimethylphosphine as an in situ probe reaction was performed to monitor the oxidation ability of different Ti species in TS-1 zeolite by P-31 MAS NMR spectra, which found for the first time direct evidence about the oxidation center of TS-1 zeolite. (C) 2003 Elsevier Inc. All rights reserved.

Direct conversion of methane into hydrogen and valuable chemicals under nonoxidative conditions is a process severely limited thermodynamically. However, the movement from the present era of fossil fuels into the coming hydrogen energy age makes it an interesting and important approach compared with the direct conversion of methane under the aid of oxidants. This paper gives a brief overview of the direct conversion of CH 4 under nonoxidative conditions. At the same time, our understanding of methane dehydroaromatization over Mo/HZSM-5 catalysts for the simultaneous formation of hydrogen and light aromatics is discussed in general, while the bifunctionality of Mo/HZSM-5 catalysts and the role of carbonaceous deposits formed during the reaction are reviewed in more detail. A perspective of the topic from both academic points of view and potential industrial applications is also presented.

NaCl-modified VCexCu1-x mixed oxide catalysts were found to be effective for propylene epoxidation using molecular oxygen as the oxidant. Hydrogen addition in the feedstock can significantly enhance the PO selectivity and the stability of the catalyst. XPS results also show that Cu0 may be the active phase for epoxidation.

TPO (temperature-programmed oxidation), TPSR (temperature-programmed surface reaction), and TPH (temperature-programmed hydrogenation) techniques are used to investigate carbonaceous deposition on Mo/HMCM-22 catalysts for methane aromatization. Different carbonaceous species and their amounts are distinguished. There are at least three kinds of cokes: carbidic carbon in molybdenum carbide, molybdenum-associated coke, and aromatic-type cokes on acid sites. It is suggested that the aromatic-type cokes on Bronsted acid sites are responsible for catalyst deactivation. Catalytic activity can be restored after temperature-programmed hydrogenation.

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts were investigated by the TPR (temperature programmed reaction) method, and the active sites were studied by CO-TPD, TPSR (temperature programmed surface reaction of preadsorbed CO by H2) and XPS techniques. The TPR results indicated that ethanol and acetaldehyde might be formed through different intermediates, whereas ethanol and methanol might result from the same intermediate. Results of CO-TPD, TPSR, and XPS showed that on the Rh-based catalyst, the structure of the active sites for the formation of C2-oxygenates is (Rh0xRh+y)–O–Mn+ (M=Mn or Zr, x⪢y, 2≤n≤4). The tilt-adsorbed CO species is the main precursor for CO dissociation and the precursor for the formation of ethanol and methanol. Most of the linear and geminal adsorbed CO species desorbed below 500 K. Based on the suggested model of the active sites, detailed mechanisms for the formation of acetaldehyde and ethanol are proposed. Ethanol is formed by direct hydrogenation of the tilt-adsorbed CO molecules, followed by CH2 insertion into the surface CH2–O species and the succeeding hydrogenation step. Acetaldehyde is formed through CO insertion into the surface CH3–Rh species followed by hydrogenation, and the role of the promoters was to stabilize the intermediate of the surface acetyl species.

The behavior of different species during the temperature-programmed surface reaction (TPSR) of methane over various catalysts is traced by an online mass spectrometer. It is demonstrated that the transformation of MoO3 to molybdenum carbide hinders the activation of methane as well as the succeeding aromatization in the TPSR. If this transformation process is done before the reaction, the temperature needed for methane activation and benzene formation will be greatly lowered (760 and 847 K, respectively). On the basis of comparison of the catalytic behavior of molybdenum supported on different zeolites, it is suggested that the initial activation of methane is the rate-determining step of this reaction. For the cobalt catalysts supported on HMCM-22 or Mo catalysts supported on TiO2, no benzene formation could be observed during the TPSR. However, the prohibition of benzene formation is different in nature over these two catalysts: the former lacks the special properties exhibited by molybdenum carbide, which can continuously activate methane even when multiple layers of carbonaceous species are formed on its surface, while the latter cannot accomplish the aromatization reaction since there are no Bronsted acid sites to which the activated intermediates can migrate, although the activation of methane can be achieved on it. Only for the catalysts that possess both of these properties, together with the special channel structure of zeolite, can efficient methane aromatization be accomplished.

EPR characterization of the Mo species on/in the HZSM-5 zeolite is described. Four different EPR signals, denoted as signals A, B, D, and E, respectively, have been recorded on the Mo/HZSM-5 sample during its stepwise reduction by methane. Different Mo species were identified on the basis of their reducibility and the nature of the signal concerned. There are two kinds of Mo species on the Mo/HZSM-5 sample, which are located at different positions on/in the HZSM-5 zeolite. The first kind of the Mo species is polynuclear and located on the external surface. They are either in the octahedral-coordinated MoO3 crystallite form (denoted as MoO3oct) or in the MoOx form with asquare-pyramidal coordination (denoted as MoOxsqu). The second kind of the Mo species is associated with the Al atom in the lattice channels of the zeolite. The Mo species associated with Al are mononuclear species, which migrate or diffuse into the channels of the HZSM-5 during calcination. The EPR signals of migrating Mo ion (signals D and E) have hyperfine structures which are caused by the interaction between the Mo species and the lattice Al atom. And the corresponding Mo species are located at two different positions close to the Bronsted Al atoms: Al(I)…MoOx and Al(II)…MoOx. The order of reducibility of these four kinds of species is as follows: MoO3oct≈MoOxsqu>Al(I)…MoOx>Al(II)…MoOx. It was found that the O2− species might exist on the surface of the Mo/HZSM-5 catalyst during the induction period of the reaction. The variation of the different Mo species in a 6%Mo/HZSM-5 sample during the reaction course is illustrated, and the relationship between the catalytic performance and the intensity of the ESR spectra is discussed. It is proposed that Mo2C is located at the external surface, while partial reduced Mo species associated with the Al atom is inside the channel during the reaction. Both of these Mo species play a key role in methane dehydro-aromatization.

The interaction between Mo species and a conventionally microsized and particularly nanosized HZSM-5 support was studied by high-resolution multinuclear solid-state NMR techniques. As proved by 27Al and 29Si MAS as well as CP/MAS NMR investigations, this interaction was so strong that the framework aluminum of both microsized and nanosized HZSM-5 zeolites could be extracted. With increasing Mo loading, more nonframework aluminum, resonanced at ca. 30 ppm, appeared in the 27Al MAS NMR spectrum of the Mo-loaded nanosized HZSM-5 catalyst. Meanwhile, this strong interaction led to the formation of more new Al2(MoO4)3 crystallines on the nanosized HZSM-5 support than on the microsized HZSM-5 support. The appearance of Al2(MoO4)3 crystallines resulted in fewer active catalysts for the methane dehyro-aromatization. The results of 1H MAS NMR using perfluorotributyl amine as a probe molecule demonstrated that Mo species preferentially reacted with the silanols and nonframework AlOH on the external surface of microsized and nanosized HZSM-5 zeolites. In addition, impregnated Mo species remained predominantly on the external surface of the nanosized HZSM-5 zeolite, although there was a possibility that they might migrate into the lattice channels of the microsized HZSM-5 zeolite. The migration of some Mo species into the zeolite channels might be beneficial for the conversion of methane to aromatics in the absence of oxygen.