Kr Tsai

Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a Ni-MgO catalyst

Abstract By using a NiMgO catalyst, carbon nanotubes with small and even diameter could be prepared from catalytic decomposition of CH4 or CO. These carbon nanotubes prepared by this method are more or less twisted, with the outer diameter at 15–20nm, and the tube length up to 10 μm. The results of XRD measurements and pulse reaction testing indicated that the NiO and MgO components in this catalyst precursor formed, due to their highly mutual solubility, a NixMg1 − xO solid solution. The high dispersion of Ni-species in this solid solution and the effect of valence-stabilization by the MgO crystal field would be in favor of inhibiting deep reduction of Ni2+ to Ni0 and aggregation of the Ni0 to form large metal particles at the surface of catalyst, making the carbon nanotubes grown on this catalyst relatively small and even in size of diameter. The experimental results also indicated that, in the growing process of carbon nanotubes, the rate-determining step was dependent upon the conditions of preparation (i.e. feedgas used, reaction temperature, flow-rate of the feedgas, etc.). The growth mechanism of the carbon nanotubes on the NiMgO catalyst is discussed together with the experimental results.

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM-5-based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H2SO4/HZSM−5 catalyst prepared from a H2SO4‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH4)2WO4 showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO6)n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H2‐TPR results showed that the reduction of precursor of the 3% W–H2SO4/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W5+ and W4+, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W5+. The results seem to imply that the observed high DHAM activity on the W–H2SO4/HZSM‐5 catalyst was closely correlated with (WO6)n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H2SO4/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H2SO4/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH4) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used.

THE OXIDATIVE COUPLING OF METHANE AND THE ACTIVATION OF MOLECULAR O2 ON CEO2/BAF2

CeO2/BaF2 was used as the catalyst for the oxidative coupling of methane (OCM). At 800°C and CH4∶O2=2.7∶1,CH4 conversion of 34% with C2 hydrocarbon selectivity of 54.3% was obtained. XRD measurement showed that partial anion (O2−,F−) and/or cation (Ce4+,Ba2+) exchange between CeO2 and BaF2 lattices occurred. ESR study showed that O− species existed on degassed catalyst. XPS study revealed that, when BaF2 was added to CeO2, the binding energy of Be 3d5/2 was 2.2 eV lower than that in CeO2, and the “electron-enriched lattice oxygen” species was detected. XPS, ESR and Raman study showed that, under O2 adsorbing conditions, O22− and O−2 species were detected on CeO2/BaF2.

Study on catalytic synthesis of methanol from syngas via methylformate in heterogeneous ''one-pot'' reactions

Abstract A “one-pot” two-step catalytic synthesis of methanol in single fixed-bed continuous flow reactor from syngas via methylformate (MF) has been studied, and a CuZSM-5- or CuY-supported alkali-metal methoxides liquid/solid phases catalysts has been developed. Under conditions of 373K, 1.1MPa, H 2 /CO/CH 3 0H=8/5.8/1.7 (mol/mol), and GHSV=2250ml(STP)/h-g catal., X CO reached 37.5% and the net yield of CH 3 0H reached 14.3mmol/h-g catal.. The spectroscopic (ESR, XPS-Auger, TPR/TPO, and IR) characterizations of the catalyst systems provided significant experimental evidence for catalytically active species and reaction intermediates existing at the functioning catalysts. The mechanism of the catalysis and the major pathway of the reaction are discussed together with the results of the experimental investigation in the present work.

CHARACTERIZATION OF MOSX-K+/SIO2 CATALYSTS FOR SYNTHESIS OF MIXED ALCOHOLS FROM SYNGAS

Abstract Characteristic studies, by means of XRD, XPS, LRS, IR and TPD methods, of MoS x -K 4 /SiO 2 catalysts for mixed alcohols synthesis provide experimental evidence for formation of Mo-S-K surface phase and presence of disulfide species, (S-S) 2− , at the surface of the catalysts. One of the origins of the promoter action in shifting the selectivity from hydrocarbons to alcohols is likely that addition of the alkali salt led to a decrease in concentration of (S-S) 2− at the surface, which in turn would reduce the generation of the reactive hydrogen.

Carbon nanotubes-supported Rh-phosphine complex catalysts for propene hydroformylation

Two kinds of carbon nanotubes were synthesized from catalytic composition of CH 4 and CO, and employed as a novel carrier for the supported Rh-phosphine complex catalyst. The prepared catalyst displayed excellent performance for propene hydroformylation. The present work also provides an example for the application of the carbon nanotubes as an alternative to middle/arge-pore molecular sieves under certain circumstances.

Studies on methane-oxidative-coupling (MOC) catalysts -II. design, preparation & characterization of rare-earth-based MOC catalysts

Abstract An approach to catalyst design for methane-oxidative-coupling (MOC) reactions is presented. By adjusting and controlling (i) active phases of catalysts, (ii) types of oxygen species on catalyst surfaces, (iii) concentration of surface active oxygen species, and (iv) surface properties towards gas phase oxidation reactions, the consecutive deep oxidation reactions that produced CO x (x = l,2) in the gas phase and on catalyst surfaces can be suppressed, and effective MOC catalysts can be created. Several of effective MOC catalysts containing thoria have been prepared. The Th-La-O x /BaCO 3 catalyst maintained its C 2 yield at -19. 4% together with C2-selectivity at -63. 3% during a 1000 hours operation.

Effects of Formic Acid and CO2 in CO Hydrogenation to Methanol Over Copper-Based Catalysts and Nature of Active Sites

Abstract Effects of small amounts of formic acid and/or CO 2 on methanol yield in CO hydrogenation to methanol over Cu/ZnO, Cu/ZrO 2 , and Cu/MgO catalysts and the in-situ FT-IR spectra of the chemisorbed species have been investigated. Formate adspecies appears to be a chemisorbed intermediate in CO hydrogenation to methanol over the three copper-based catalysts; CO hydrogenation over Cu/MgO is inhibited by even very small proportion of CO 2 ; over Cu/ZnO, the inhibit on by CO 2 becomes apparent at slightly higher CO 2 proportion, while over Cu/ZrO 2 it is unaffected by small amounts of CO 2 . The inhibition is probably due to carbonation of the basic MgO site and the slightly basic ZnO site. The nature of the active site and mechanism of synergistic catalysis in CO hydrogenation to methanol over copper-based catalysts is discussed.

Methane and Light Alkane (C 2 -C 4 ) Conversion Over Metal Fluoride-Metal Oxide Catalyst System in Presence of Oxygen

A novel series of metal fluoride-metal oxide catalysts with good to excellent catalytic performance for oxidative coupling of methane and oxidative dehydrogenation of ethane, propane and iso-butane were developed. XRD analysis suggested that, with the addition of metal fluorides to metal oxides, partial substitution of cations and/or anions happened between fluorides and oxides, leading to the formation of new phases ( e.g. LaOF ) and expansion/contraction of lattices ( e.g. in CeO2-BaF2 ). O2 n- (1 ≤ n ≤ 2) species were detected by the Raman spectroscopy over the catalysts after O2 adsorption. O2- species was observed by in situ FTIR spectroscopies at 650°C over SrF2-La2Q3 catalyst.