Partial oxidation of methane with oxygen over magnesium oxide at low temperatures

Abstract

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...

Structural and optical properties of 6CaO·6SrO·7Al2O3 thin films derived by sol–gel dip coating process

“…The absorption peaks in the 550-850 cm − 1 region are attributed to the C12A7 characteristic absorption corresponding to the Al-O stretching and bending modes in AlO 4 tetrahedral[26,27]. There is a distinguishable and repeatable peak near 3540 cm − 1 ; assigned to the stretching vibration of O-H on the films surface[28][29][30].…”

Hydrogen Production by Steam Reforming of Ethanol on Potassium-Doped 12CaO · 7Al2O3 Catalyst

“…Particularly, negligible carbon deposition was formed even for S/C ratio equal to the stoichiometric one, which would be more appropriate to the hydrogen production from ethanol steam reforming. It was noticed that the active Oappeared both in the bulk and on the surface of C12A7-O -or K-doped samples [18][19][20] and have a high reactivity, particularly in strong oxidation for hydrocarbons [27][28][29][36][37][38]. The active species of O -may be involved in the steam reforming of ethanol, because the intermediate of OH -has been observed by FT-IR.…”

“…Because the 3592 cm -1 band is close to the m(O-H -) [28,49,50], it was assigned to the stretching vibration of O-H -on the sample surface. We have confirmed that the O -anion radicals existed on the surfaces of C12A7-O -and K-doped C12A7-O - [18,19], and it is well known that active O -anion have strong oxidation power, particularly in the oxidation of hydrocarbons [27][28][29][36][37][38]. Thus it may indicate that the observed OH -species on Databases.…”

“…The microporous crystal 12CaO Á 7Al 2 O 3 (C12A7) has a cubic structure with a lattice constant of 1.1989 nm, which is characterized by a positive charged lattice framework [Ca 24 Al 28 O 64 ] 4+ including 12 subnanometer sized cages [13][14][15][16][17]. The two remaining O 2-ions are clathrated in the cages.…”

“…We also observed that O 2-was able to substituted by other mono-charge anion X -(X -= OH -, Hetc.) to form the derivatives [Ca 24 Al 28 O 64 ] 4+ Á 4(X -) (C12A7-X -) [21][22][23]. It is well known that active O -anion is a key intermediate in the anion chemistry [24][25][26][27][28][29][30], particularly in low-temperature oxidation of hydrocarbons [27][28][29].…”