Yanxin Chen

Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al2O3 catalyst

Abstract The mechanism of catalytic partial oxidation of methane to syngas (POM) over a Ni/α-A12O3 catalyst was studied by using a pulse reactor and temperature-programmed surface reaction (TPSR) techniques. Over a reduced nickel catalyst (Ni0/A12O3), methane activation follows the dissociation mechanism; while on oxidic nickel catalysts (NiO/Al2O3), methane is first oxidized to carbon dioxide and water, and simultaneously, NiO is reduced to Ni0. CH4 dissociation occurs over Ni0 active sites, generating hydrogen and surface C species. A transient process was observed during the CH4/O2 reaction. The nickel valence was transformed from NiO to Ni0 at a certain critical temperature and simultaneously, the reaction was transformed rapidly from the complete oxidation of methane to the partial oxidation of methane. It has been found that the POM reaction takes place over a thin layer of the catalyst bed. This reaction zone is nearly isothermal, over which almost 100% of oxygen and more than 90% of methane are converted. The temperature drop in the downstream of the catalyst bed does not imply that the steam or carbon dioxide reforming reaction occurs in the lower part of the bed. Ni0 species constitute the active sites for the partial oxidation of methane to syngas. Both methane and oxygen are activated on Ni0 sites, generating surface Ni⋯C and Niδ+⋯Oδ− species. These two kinds of intermediates have been proposed to account for the mechanism of methane partial oxidation. The Niδ+⋯Oδ− species over Ni0 catalyst surface is considered to be a kind of weakly bounded, mobile oxygen species. The reaction between Niδ+⋯Oδ− and Ni⋯C intermediates generate the primary product of CO. However, the presence of NiO over the catalyst surface significantly reduces the CO selectivity. Thus, the NiO species are not possible to be the intermediate for the POM reaction. The mechanism of partial oxidation of methane should follow the direct oxidation route.

Effect of La2O3 added to NiO/Al2O3 catalyst on partial oxidation of methane to syngas

Abstract Partial oxidation of methane to syngas has been studied over NiO/Al 2 O 3 catalyst promoted with La 2 O 3 . Addition of La 2 O 3 can lower the ignition temperature of the reaction, which plays an important role in the initial reaction process. Furthermore, it is indicated by TPR and XRD measurements that addition of La 2 O 3 undermines the interaction between NiO and Al 2 O 3 to form new species of LaNiO 3 after calcination at 800°C in air. 2%La 2 O 3 added to NiO/Al 2 O 3 not only is the optimal content for catalytic activity, but also efficiently inhibits the carbon deposition on the surface of catalyst in combination with CaO.

Light alkenes preparation by the gas phase oxidative cracking or catalytic oxidative cracking of high hydrocarbons

The preparation of light alkenes by the gas phase oxidative cracking (GOC) or catalytic oxidative cracking (COC) of model high hydrocarbons (hexane, cyclohexane, isooctane and decane in the GOC process and hexane in the COC process) was investigated in this paper. The selection for the feed in the GOC process was flexible. Excellent conversion of hydrocarbons (over 85%) and high yield of light alkenes (about 50%) were obtained in the GOC of various hydrocarbons including cyclohexane at 750°C. In the GOC process, the utilization ratio of the carbon resources was high; CO dominated the produced COX (the selectivity to CO2 was always below 1%); and the total selectivity to light alkenes and CO was near or over 70%. In the COC of hexane over three typical catalysts (HZSM-5, 10% La2O3/HZSM-5 and 0.25% Li/MgO), the selectivity to COX was hard to decrease and the conversion of hexane and yield of light alkenes could not compete with those in the GOC process. With the addition of H2 in the feed, the selectivity to COX was reduced below 5% over 0.1% Pt/HZSM-5 or 0.1% Pt/MgAl2O4 catalyst. The latter catalyst was superior to the former catalyst due to its perfect performance at high temperature, and with the latter, excellent selectivity to light alkenes (70%) and the conversion of hexane (92%) were achieved at 850°C (a yield of light alkenes of 64%, correspondingly).

Production of light alkenes with low CO2 emission from gas phase oxidative cracking (GOC) of hexane

Compared with non-oxidative pyrolysis of hexane, the rupture of C-C bonds of hexane in the GOC process becomes rather easy due to the change of thermodynamics and kinetics of the reaction with introduction of oxygen. CO predominates in the COx produced, and the ratio CO/CO2 can be as high as 16. The GOC process could be operated in an autothermic way, which would minimize energy consumption of the whole process and greatly decrease the CO2 emission. GOC seems to be an environmentally benign and promising alternative route for light alkenes production using heavy feedstock.

Reducibility of Mo species in Pt/MoO3 and PtMo/Al2O3 catalysts

Recucibility of Mo species in Pt/MoO3 and PtMo/Al2O3 has been investigated by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (H2-TPD) and temperature programmed electronic conductivity (TPEC) techniques. In Pt/MoO3 at H2 atmosphere, it was found by TPEC and TPR that, a slight amount of Pt could activate the transfer of the species and H atoms between H2 and MoO3, and thus accelerate the reduction of MoO3. In PtMo/Al2O3, TPR and H2-TPD revealed that the reduction of surface Mo species could also be facilitated by Pt. Two kinds of hydrogen molybdenum species were proposed on the surface of the catalyst after prereduction.

Correlations between p-Type Semiconductivity and C2 Selectivity for Oxidative Coupling of Methane (OCM) Over Acceptor Doped SrTiO3

Abstract In this work some acceptor doped SrTiO3 were prepared and tested by OCM reaction, XRD, TPD-MS, electrical conductivity and XPS. C2 selectivity of the catalyst is improved effectively and the p-type semiconductivity is enhanced upon doping. Correlation between them is discussed. The best catalyst prepared is a SrTi0.9Li0.1O3-δ sample possessing C2 selectivity 57.3% and C2 yield 16.9%.