No abstract

Characterization of CO species adsorbed on well-degassed surfaces of polycrystalline MgO at low CO pressures (< 1300 Pa) and at and below room temperature was studied by using temperature-programmed desorption (TPD) and infrared (IR) spectroscopies. Mutual transformation between adsorbed species stable at room temperature, and their reactivities with 0 2 were also investigated. Adsorbed species are classified into five groups, KO, K1 (and K1' ), Kz, K3, and &. Among them species KO and K1 (and K1' ) are stable only below room temperature while species KZ to & are stable above this temperature. Species KO is a linear-type CO monomer with the C atom bonded to surface Mg2+ and has five subspecies depending on its circumstance, while all the other species K1 to & are formed on surface 02-to which the C atom is linked. A chained-type CO monomer, K1, is reversibly transformed at ca. 230 K in the presence of gaseous CO into a linear-type CO monomer, K1'. Species KZ is a chained-type tetramer of CO with a carbonyl CO bond and resonanced bonds. There are two subspecies, K~A and Km, which are formed by the addition of one CO molecule to corresponding trimer species, K~A and K~x , respectively, in the presence of gaseous CO. K~A and Kzx are reversibly transformed into K~A (at 400 K) and K~x (in 300-400 K), respectively, by the elimination of the CO added.There are two more subspecies, K~B and K~c , in species K3, but they have no corresponding KZ subspecies, and hence no transformations occur. All the K3 species seem to be a linear-type trimer with a ketenic group, >C=O=O, and are desorbed in 520-580 K. Species &, though detailed information is not obtained, consists of three subspecies &A, &B, and &c, but no transformations between species K3 and & are observed.

The C–H bond of methane is easily activated on the MgO surface even below room temperature. Active sites consist of intrinsic ion pairs, O3C2−–Mg3C2+ and O4C2+, of MgO in low coordination states on which a methane molecule adsorbs in heterolytically dissociated form.

Partial oxidation of CH, by 0, on MgO pretreated at 673 and 1123 K has been investigated near room temperature by temperature-programmed desorption (TPD), infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy. The UV-irradiation effect on this reaction was also studied. In the dark, this oxidation reaction proceeded only on MgO pretreated at 1123 K and is initiated by the heterolytic dissociation of CH, into CH, and Hf on a low-coordinated surface ion pair, Mg:;-Of,.CH; thus formed reacts with 0, and then an oxide ion 0'to give OCH,. Upon subsequent heating in vacuo, part of the OCH; decomposes into CO and H, , and the rest is further oxidized into COZ-. Under UV-irradiation, the oxidation reaction proceeded on MgO irrespective of its pretreatment temperature. This reaction seems to be initiated by an active oxygen species, 0 -, which can be formed on MgO in the presence of 0, under UV-irradiation. The initial event in 0-formation is photon absorption at OHand MgfS-0;; sites on MgO pretreated at 673 and 1123 K, respectively. Such a site reacts with 0, to produce 0 -. The 0 -so formed can abstract an H atom from CH, to give a CH, radical. This CH, radical reacts with a surface oxide ion 0'-to produce OCH,, which is mainly further oxidized into HCO; by adsorbed oxygen species during UV-irradiation. A part of the HCO; decomposes to give CO during subsequent heating in vacuo, and the remainder is further oxidized into COS-.Partial oxidation of CH, by 0, on metal oxides has been investigated by many researchers.' Ito has shown that Lidoped MgO has high activity for this reaction., However, all the catalysts that have high activity for this reaction require high reaction temperatures, usually above 900 K. High temperatures are considered necessary to produce active sites, such as 0 -, which can abstract H from CH,3 or to activate CH, molecules in the vibrational mode.4 If this reaction can proceed at lower temperatures, especially near room temperature, we can save energy and suppress complete oxidation, i.e., the production of CO, and H,O.Garrone et aL5 reported that MgO could not adsorb CH, in dissociated form. However, we have already shown that MgO which had previously been outgassed at high temperatures (above 973 K) can dissociate CH, into CH; and Hf at room temperature.6 This fact indicates that MgO outgassed above 973 K can activate CH, at room temperature without excess energy. In fact, we have already reported that previously adsorbed CH;, which had been formed by heterolytically dissociative adsorption of CH, on low-coordinated surface ions, was easily oxidized by subsequent oxygen admission even at 223 K.7 Therefore, we may expect to obtain valuable partially oxidized products in greater amounts if the reaction is carried out using a mixture of CH, and 0, over MgO.Recent reports from our group have indicated that UVirradiation of the MgO surface in the presence of a gas gives pronounced effects on the adsorptive nature : oxygen species such as 0 -, 0, and O,, which can react with hydrocarbons...

Partial oxidation by 0, of methane, which had previously been adsorbed at room temperature on MgO pretreated at 1123 K in uucuo, has been examined near room temperature by t.p.d., i.r. and e.s.r. spectroscopies and compared with a methanol adsorption system. The oxidation reaction of methane preadsorbed either in the dark or under u.v.-irradiation proceeds according to the same reaction mechanism, where low-coordination surface ions of MgO play an important role. Methane is adsorbed in a heterolytically dissociated form (CH;+H+), and then oxidized to a methoxide species, OCH;(Z), on the admission of 0, at room temperature. On heating under evacuation, OCHi(2) is either decomposed into H, and CO in the temperature range 495-535 K or further oxidized by coexisting 0; into HCO; below 473 K. The HCO; formed gives either CO or bidentate COiat cu. 600 K. Another more stable methoxide species, OCH;(l), is also sometimes formed either directly on room-temperature oxidation or through a transformation of the less stable species, OCH;(2), on heating at ca. 500 K. Part of the OCHi(1) present then decomposes into H, and CO at 760 K and the rest changes into bidentate CO:-. The COi-species thus formed ultimately are desorbed as CO, above 600-700 K.