Oxidative coupling of methane: Influence of the phase composition of silica-based catalysts

Characterization of Mn–Na2WO4/SiO2 and Mn–Na2WO4/MgO catalysts for the oxidative coupling of methane

“…The increase in surface concentration of W, Na, and Mn after reaction is in agreement with previously published data [14]. However, other authors have reported very little variation between fresh and spent catalysts [7,9], and some authors report a decrease mainly in W concentration at the surface of the catalysts [23,25]. The lack of trend in surface composition before and after reaction is consistent with the Mn-Na 2 WO 4 /n-SiO 2 catalyst being highly active over rather large concentration ranges of Mn, Na and W [7].…”

“…The lower BE side of the peak has a pronounced shoulder (at approximately 530.2 eV), which arises due to the MnO x and WO y oxygen species, and potentially also from non-bridging oxygens in Na 2 O-SiO 2 compounds [31]. This shoulder is not present in the XPS spectrum obtained from MnO x /n-SiO 2 catalyst (Figure 3b) [13,14], but in all reported XPS data from SiO 2 -(and MgO-) supported catalysts the Na:W surface concentration ratio is significantly higher than the stoichiometric 2:1 of Na 2 WO 4 [7,9,13,14,[23][24][25]. This indicates that the surface is enriched in sodium compared to the bulk concentration of the Mn-Na 2 WO 4 /n-SiO 2 catalyst.…”

“…While some studies indicate that manganese is not necessary for an active catalyst [6], other studies reveal that there is a correlation between the activity plus selectivity and the surface concentration of manganese [7]. While most studies include XRD data from these types of catalysts, fewer investigations have used XPS to characterize the Mn-Na 2 WO 4 /SiO 2 catalysts [5,7,9,[13][14][23][24][25]. These references contain important XPS data, but present only tabulated data, such as tables with binding energies or surface compositions, and/or only selected XPS peaks.…”

Partial oxidation of methane to produce syngas over a neodymium–calcium cobaltate-based catalyst

“…11b may be due to pyrolytic carbon. These agglomerated particles form a porous, easily penetrated layer on the coarse grains of the metal/oxide composites that could not prevent access of CH 4 and O 2 to the surface of the catalyst, which should not significantly affect the catalytic properties of the composite.…”

“…The conversion processes that convert natural gas to hydrogen, synthesis gas, or ethylene have attracted significant attention in recent years [1][2][3][4][5]. The production of hydrogen and synthesis gas has been widely studied due to their potential application as strategic sources of clean energy [6][7][8].…”