Influence of the composition of perovskites based on SrMnO3 on their catalytic properties in the oxidative coupling of methane

Abstract

The oxidative coupling of methane (OCM) in a periodic regime over the SrMnO 3 and its derivatives has been investigated. It has been established that partial replacement of the strontium ion by alkali metal ions leads to an increase in both catalytic activity and in selectivity with respect to higher hydrocarbons. Comparison of the results obtained in the present work with those obtained earlier for catalysts based on SrCoO 3 led to the conclusion that the Co-containing catalysts were preferable to the Mn-containing catalysts for carrying out oxidative coupling of methane in a periodic regime.Key words: oxidative coupling of methane, higher hydrocarbons, perovskite catalysts, periodic regime.Perovskites of the general formula ABO 3 , where A is a rare earth or alkaline earth metal and B is a transition metal (most frequently manganese or cobalt), are widely used as catalysts in oxidation reactions in which some perovskite catalysts couple high activity with stability when working under drastic conditions [1][2][3][4][5][6][7]. Factors associated with the high activity of the perovskites may be unusual oxidation states of the transition metals, a considerable amount of non-stoichiometric oxygen, and defects in the lattices of the catalysts [8]. Replacement of the ions A in the perovskites by ions of lower valency increases the concentration of oxygen vacancies in the lattice, as a result of which diffusion of oxygen ions into the catalysts may be facilitated, and an increase in catalytic activity of the perovskites is achieved [9]. This method of improving the catalytic properties of perovskites was used successfully in our previous work [10] where it was shown that replacement of strontium ions by alkali metal ions in the perovskite SrCoO 3-x led to a considerable increase in the yield of C 2+ hydrocarbons in the oxidative coupling of methane (OCM).To test the generality of this method of modification of perovskite catalysts with alkali metals and to study the influence of the nature of the transition metal in position B of the perovskite structure on the properties of the catalyst, we have carried out an analogous study of perovskites based on SrMnO 3 in the present work. EXPERIMENTALFor the synthesis of perovskites based on SrMnO 3 we used aqueous solutions of Sr(NO 3 ) 2 (analytical grade), Mn(NO 3 ) 2 ·6(H 2 O) (analytical grade), NaHCO 3 (titration grade), K 2 C 2 O 4 (titration grade), ammonium citrate (analytical 32 0040-5760/05/4101-0032

Amino Acid-Aided Synthesis of a Hexagonal SrMnO3 Nanoperovskite Catalyst for Aerobic Oxidation

“…It should be noted that these specific surface areas for SMO-AA and SMO-MA are much larger than those (2–25 m 2 g –1 ) for SMOs synthesized by conventional methods such as the PC, acetate, sol–gel combustion, cellulose templating, and citrate methods (Table S1). 4,7 In addition, synthesis using the AA and MA precursors yield pure SMO at lower calcination temperatures (by 100–400 K), possibly because the amorphous precursors with relatively low carbon contents are readily decomposed. The pH of the starting solution for the AA precursor (pH 4.23) was higher than those for the HA and MA precursors (pHs 1.64 and 3.70, respectively), which indicates that the use of metal acetates and AA containing NH 2 amino groups significantly accelerates the ligand exchange reaction without pH adjustment.…”

Synthesis of cobalt loaded double perovskite Sr 2 TiFeO 6 ‐δ ( STF ) as a stable catalyst for enhanced hydrogen production via methane decomposition

“…57 The Co 3 O 4 peak was associated with the cobalt oxide impregnated onto the STF backbone having a crystallite size of 0.7 nm, while SrCoO 3-δ was identified as an unwanted product formed as a result of increased Co impregnation concentration. 58,59 Moreover, unit cell length of STF, 2.5% Co/STF, 5% Co/STF, 7.5% Co/STF, and 10% Co/STF were calculated and were found to be 3.89 Å. This further confirms that, after occupying the deficient B-sites, Co was loaded onto the STF backbone and not doped inside the STF lattice and even the formation of SrCoO 3-δ did not deteriorate the STF lattice.…”

“…Furthermore, the decrease in hydrogen selectivity and yield can also be attributed to the formation of SrCoO 3-δ as it tends to catalyze methane to form higher hydrocarbons by oxidative coupling of methane. 58,59 The catalytic stability test was carried out for 5% Co/STF where the catalytic activity was tested continuously for 46.5 hours, as shown in Figure 11. As can be seen from the graph, the CH 4 conversion increases steadily for the first 10 hours and starts to stabilize.…”

Divide to Conquer: On the Use of Distributed Feed Strategies in Oxidative Coupling of Methane

“…Most stoichiometric perovskites were mainly tested in powder form in packed bed reactors. Table S3 adopted from refs − shows a summary of the reported results. However, nonstoichiometric perovskites are of greater interest in the context of this article, as they are more suitable materials for MIEC membranes for oxygen permeation.…”