Guo-Dong Lin

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.

Raman spectra of MWCNTs and MWCNT-based H2-adsorbing system

Abstract Raman spectra were taken with an UV–vis Raman system on two types of multiwalled carbon nanotubes (MWCNTs, CH4-based) and MWCNTs (CO-based), grown catalytically with CH4 and CO as the carbon sources, respectively, and their H2-adsorbing systems. The strongest peak at 1580 cm−1 and the second-strongest band at 1416 cm−1 could be attributed to the fundamental frequency modes E2g and D (induced by defects) analogous to those of graphite, and the bands at 2832, 2996, 3160 cm−1 and the region 4410–4422 cm−1 may be due to their second- and third-order combination frequencies: 2D, D+E2g, 2E2g, and 2D+E2g, respectively. By a comparative study with the different laser excitation lines (325 nm vs. 514 nm), it is found that the position of the D-band depends strongly on the laser excitation frequency. The Raman-spectroscopic investigation of the H2–MWCNTs adsorption systems showed that adsorption of H2 on the MWCNTs could occur in associative and dissociative forms. The observed Raman peaks at 2856, 2967 and 3950 cm−1 may be assigned to symmetric C–H stretch of surface C H2, asymmetric C–H stretch of surface C H3, and H–H stretch of molecularly adsorbed H2(a), respectively. It was also shown that the concentration of the observed hydrogen-containing ad-species on the MWCNTs (CH4-based) was higher than those on the MWCNTs (CO-based), and that doping of a proper amount of K+ in the MWCNTs enhanced somewhat H2 adsorption. Adsorption of trace amount of moisture on the purified MWCNTs was below the detectable limit, but pronounced on the K+-doped MWCNTs. On the H2/K+–MWCNTs (CH4-based) adsorption system, the observed strong Raman bands in the region 3370–3470 cm−1 may be ascribed to O–H stretching of surface hydroxyls generated from adsorption of the trace amount of moisture.

Highly Active CNT-Promoted Cu–ZnO–Al2O3 Catalyst for Methanol Synthesis from H2/CO/CO2

With types of in-house-synthesized multi-walled carbon nanotubes (CNTs) and the nitrates of the corresponding metallic components, highly active CNT-promoted Cu–ZnO–Al2O3 catalysts, symbolized as CuiZnjAlk-x%CNTs, were prepared by the co-precipitation method. Their catalytic performance for methanol synthesis from H2/CO/CO2 was studied and compared with the corresponding CNT-free co-precipitated catalyst, CuiZnjAlk. It was shown experimentally that appropriate incorporation of a minor amount of the CNTs into the CuiZnjAlk could significantly increase the catalyst activity for methanol synthesis. Under the reaction conditions of 493 K, 5.0 MPa, H2/CO/CO2/N2 = 62/30/5/3 (v/v), GHSV = 8000 h-1, the observed CO conversion and methanol formation rate over a co-precipitated catalyst of Cu6Zn3Al1-12.5%CNTs reached 36.8% and 0.291 μmol CH3OH s-1 (m2-surf. Cu)-1, which was about 44 and 25% higher than those (25.5% and 0.233 μmol CH3OH s-1 (m2-surf. Cu)-1) over the corresponding CNT-free co-precipitated catalyst, Cu6Zn3Al1. Addition of a minor amount (10–15 wt%) of the CNTs to the Cu6Zn3Al1 catalyst was found to considerably increase specific surface area, especially Cu surface area of the catalyst. H2-TPD measurements revealed that the CNTs and the pre-reduced CNT-promoted catalyst systems could reversibly adsorb and store a considerably greater amount of hydrogen under atmospheric pressure at temperatures ranging from room temperature to ∼573 K. This unique feature would be beneficial for generating microenvironments with higher stationary-state concentration of active hydrogen adspecies on the surface of the functioning catalyst, especially at the interphasial active sites since the highly conductive CNTs might promote hydrogen spillover from the Cu sites to the Cu/Zn interphasial active sites, and thus be favorable for increasing the rate of the CO hydrogenation reactions. Alternatively, the operation temperature for methanol synthesis over the CNT-promoted catalysts can be 15–20 degrees lower than that over the corresponding CNT-free contrast system. This would contribute considerably to an increase in equilibrium CO conversion and CH3OH yield. The results of the present work indicated that the CNTs could serve as an excellent promoter.

Development of coking-resistant Ni-based catalyst for partial oxidation and CO2-reforming of methane to syngas

Abstract Addition of small amount of trivalent-metal oxides, Cr 2 O 3 and La 2 O 3 , to a Ni Mg O (Ni/Mg=1/1, mol/mol) catalyst for partial oxidation of methane (POM) and CO 2 -reforming of methane (MCR) reactions has been found to improve the performance of the catalyst for coking-resistance. The POM operation at 1053 K for 50 h, or the MCR operation at 1100 K for 6 h, did not leave any detectable amount of carbon deposit on the surface of the catalyst. Studies of XRD, XPS, and H 2 -TPR spectroscopies showed that the doping of small amounts of Cr 3+ and La 3+ to the Ni Mg O system led to the formation of a host-dopant-type Ni Mg Cr La O solid solution, with a considerable number of Schottky defects in the form of cationic vacancies. An increase in the degree of disorder in the solid solution due to Cr 2 O 3 and La 2 O 3 dissolved in Ni x Mg 1− x O lattice would be expected to enhance the mobility of the lattice oxygen anions. This would be in favor of speeding up the reaction between the carbon-containing species and reactive oxygen species via migration of the lattice O 2− so as to inhibit the deposition of carbon on the surface of the catalyst. On the other hand, part of the Schottky defects in the form of cationic vacancies may diffuse to the surface, where Ni + -species can be well accommodated and stabilized, thus, forming a rich-in-Ni (with mixed valence states) surface layer. As a result, the proportion of the reducible Ni-species was pronouncedly increased, but the temperature for their reduction was considerably raised, so that the surface Ni-species were maintained with higher possibility in positive valence states under POM and MCR reaction conditions. This would, to some extent, lead to the reduction of the rate of deep dehydrogenation of methane to carbon, therefore tending to reduce, if not avoid, coking caused by an excess of carbon on the surface.

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. II. Action of Promoters Zn and Li

By correlating the results of the NH3-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydro-aromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg2+, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn2+ or Li+ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg2+ and Zn2+, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn2+ or Li+ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W6+ species and an increase in quantity of the reducible W6+ species at the reaction temperature for DHAM, as has been evidenced by the results of a H2-TPR study on the reducibility of the Zn2+ (or La3+, Li+, Mn2+)-promoted W/HZSM-5 system. The above two roles that Zn2+ and Li+ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.

Carbon nanotube-promoted Co–Cu catalyst for highly efficient synthesis of higher alcohols from syngas

The development of a type of carbon-nanotube-promoted Co-Cu catalyst, which displays excellent performance for highly effective and selective formation of the C(2-4)-oxygenates, especially BuOH and DME, from syngas, is reported.

Novel MWCNT-Support for Co-Mo Sulfide Catalyst in HDS of Thiophene and HDN of Pyrrole

Abstract With home-made multi-walled carbon nanotubes (MWCNTs, simplified as CNTs in later text) as support, CNT-supported Co-Mo-S catalysts, denoted as x% (mass percentage) MoiCoj/CNTs, were prepared. Their catalytic performance for thiophene hydrodesulfurization (HDS) and pyrrole hydrodenitrification (HDN) reactions was studied, and compared with the reference system supported by AC. Over the 7.24%Mo3Co1/CNTs catalyst at reaction condition of 1.5 MPa, 613 K, C4H4S/H2=3.7/96.3 (molar ratio) and GHSV≈8000 mlSTp/(g-cat.h), the specific HDS activity of thiophene reached 3.29 mmolc4H4s/(s-molMo), which was 1.32 times as high as that (2.49 mmolc4H4s/(s-molMo)) of the AC-based counterpart, and was 2.47 times as high as that (1.33 mmolc4H4s/(s-molMo)) of the catalysts supported by AC with the respective optimal Mo3Co1-loading amount, 16.90%Mo3Co1/AC. Analogous reaction-chemical behaviours were also observed in the case of pyrrole HDN. It was experimentally found that using the CNTs in place of AC as support of the catalyst caused little change in the apparent activation energy for the thiophene HDS or pyrrole HDN reaction, but led to a significant increase in the concentration of catalytically active Mo-species (Mo4+) at the surface of the functioning catalyst. On the other hand, H2-TPD measurements revealed that the CNT-supported catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures ranging from room temperature to about 673 K. This unique feature would help to generate microenvironments with higher stationary-state concentration of active hydrogen-adspecies at the surface of the functioning catalyst. Both factors mentioned above were favorable to increasing the rate of thiophene HDS and pyrrole HDN reactions.

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. I. Performance of Catalysts

With incorporation of Zn (or Mn, La, Zr ) into the W/HZSM-5 catalyst, highly active and heat-resisting W/HZSM-5-based catalysts were developed and studied. Under reaction conditions of 0.1 MPa, 1073 K, GHSV of feed-gas CH4+10% Ar at 960 h−1, the conversion of methane reached 18–23% in the first 2 h of reaction, and the corresponding selectivity to benzene, naphthalene, ethylene and coke was 56–48, ∼18, ∼5 and ∼22%, respectively. Addition of a small amount of CO2 (≤2%) to the feed-gas was found to significantly enhance the conversion of methane and the selectivity of benzene, and to improve the performance of coke-resistance of the W/HZSM-5-based catalysts. Heavy deposition of carbon on the surface of the functioning catalyst was the main reason leading to deactivation of the catalyst. Reoxidation by air may regenerate the deactivated catalyst effectively. In comparison with the Mo/HZSM-5 catalyst, the promoted W/HZSM-5-based catalyst can operate under reaction temperature of 1073 K, and gain a methane conversion approximately two times as high as that of the Mo/HZSM-5 catalyst operating at 973 K. It can also operate at 973 K and have about the same methane conversion as that of the Mo/HZSM-5 catalyst at the same reaction temperature. Its main advantage is its heat-resistant performance; the high reaction temperature did not lead to loss of W component by sublimation.

Pt catalyst supported on multiwalled carbon nanotubes for hydrogenation-dearomatization of toluene

Abstract Using homemade multiwalled carbon nanotubes (CNTs) as the support, the Pt/CNTs catalyst was prepared by an incipient wetness method. Performance of the catalyst for hydrogenation-dearomatization (HDA) of toluene was evaluated and compared with the reference catalysts supported on γ-Al 2 O 3 and activated carbon (AC). Over the 1.0%Pt/CNTs catalyst under the reaction conditions of 0.4 MPa, 373 K, PhCH 3 /H 2 = 6/94 (mol/mol), and GHSV = 120 L/(h·g), the observed conversion of toluene HDA reached 100%, and the corresponding specific reaction rate was 0.0523 mmol/(s·m 2 ). This value was 1.17 and 1.18 times that of the 1.4%Pt/γ-Al 2 O 3 and 2.4%Pt/AC catalysts with the respective optimal Pt loading, respectively. It was experimentally found that using CNTs in place of γ-Al 2 O 3 or AC as the support of the catalyst did not cause a significant change in the apparent activation energy for the toluene HDA reaction but led to a slight increase in concentration of catalytically active Pt species (Pt 0 ) at the surface of the functioning catalyst. In addition, the Pt/CNTs catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures from room temperature to 573 K. This unique feature would help to generate a microenvironment with higher stationary state concentration of active hydrogen adspecies at the surface of the functioning catalyst. These effects favored the toluene HDA reaction.

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.