14462
J. Phys. Chem. 1996, 100, 14462-14467
Thermal Conversion of Methoxy Species on Dimethyl Ether Adsorbed CeO2 Michikazu Hara, Miyuki Kawamura, Junko N. Kondo, Kazunari Domen, and Ken-ichi Maruya* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama 226, Japan ReceiVed: March 25, 1996; In Final Form: June 6, 1996X
The behavior of methoxy species on dimethyl ether (DME) adsorbed CeO2 was investigated using temperatureprogrammed desorption (TPD) and infrared (IR) spectroscopy. TPD experiments showed that the thermal desorption peaks at 470-500 and 570 K are all DME from DME/CeO2 adsorbed at 300 K. These DME desorptions are attributed to the recombination of the surface methoxy species on CeO2. IR spectra of DMEadsorbed CeO2 showed two bands at 1158 and 1070 cm-1 at 300 K. These bands were assigned to the C-O stretching (ν(CO)) bands of surface methoxy species bound to one Ce ion (terminal-OCH3) and two Ce ions (bridged-OCH3), respectively. These bands decreased with increasing the temperature of CeO2. The increase of the temperature to 490 K led to the appearance of a new band at 1025 cm-1 which was identified as ν(CO) of µ-OCH3 bound to more than three Ce ions. The TPD and IR results suggest that only the µ-OCH3 is decomposed into carbonate via a formate intermediate. The reactions of surface methoxy species on CeO2 are discussed.
1. Introduction
2. Experimental Section
The development of a viable catalytic CO-H2 reaction on metal oxides has been anticipated since the oil crisis due to the demand of the products involving branched hydrocarbons. In the reaction, surface methoxy species are regarded as key intermediates leading to products such as hydrocarbons and oxygenates. This has stimulated the investigation of surface methoxy species, and valuable information has been gradually accumulated.1-7 However, the relationship of surface methoxy species to products higher than C2 has not yet been clearly understood because of the complex reactions of the methoxy species. We have been investigating the catalytic reactions of COH2 over ZrO2 and CeO2, which selectively produce isobutene.7-10 It was suggested that surface methoxy species are possible intermediates and the products depend not only on the catalyst but also on the reaction temperature. The CO-H2 reaction over ZrO2 selectively produces methanol and isobutene at 523 and 623 K, respectively.8 The reaction over CeO2 at 523 K produces oxygenates such as methanol, 2-methylpropanol, and diisopropyl ketone, in addition to small amount of hydrocarbons, whereas only hydrocarbons are produced at temperatures higher than 623 K.8 This complex behavior on CeO2 may reflect that surface methoxy species give rise to at least: two reactions one produces hydrocarbons and the other produces oxyhydrocarbons. To overcome this complex reaction, it would be necessary to simplify the system at first only to the reaction of the surface methoxy species. In this study, the surface methoxy species on CeO2 formed by DME adsorption were investigated using temperatureprogrammed desorption (TPD) and infrared (IR) spectroscopy. Methanol has been generally used to prepare surface methoxy species on the surfaces of metal oxides. However, hydroxyl groups formed simultaneously with the methoxy species by the dissociative adsorption of methanol would bring about the complexity of the reaction of the methoxy species. Therefore, DME was used as a source of the methoxy species to simplify the reaction.
CeO2 catalyst with a specific surface area of 29 m2 g-1 was used for TPD and IR experiments. The preparation of the CeO2 catalyst is described elsewhere.8-10 Thermal desorption spectra (TDS) were measured using a conventional flow system with N2 carrier gas (60 cm3 min-1). A quartz reactor with CeO2 catalyst (2.0 g) was allowed to be evacuated (ca. 1 Pa) apart from the flow system. The CeO2 in the reactor was heated at 873 K for 30 min before DME adsorption. DME added to N2 carrier through a mass flow valve was adsorbed onto CeO2 at 300 K. The flow rate of DME was 8.9 µmol min-1, and the amount of DME was controlled by the exposure time. TDS were obtained by heating DME-adsorbed CeO2 to 873 K at a rate of 20 K min-1 while detecting the desorbed species using a thermal conductivity detector (TCD). Desorbed species were identified by an on-line gas chromatograph equipped with TCD, flame ionization detector, and two kinds of columns (P1 and 5A molecular sieve). For IR experiments, CeO2 (150 mg) was pressed into a selfsupporting disk with a diameter of 20 mm. The CeO2 disk was set in an IR cell with NaCl windows connected to a glass closed gas circulation system.6,7 Carbon contamination on the surface was removed by exposing the disk to oxygen (4 × 104 Pa) for 1 h at 873 K followed by evacuation under vacuum (
