Formation of the V Center on H-Type Zeolites with Thermal Treatment

Abstract

The effect of thermal treatment on H-MCM-22 and H-ZSM-5 zeolites was investigated using the electron spin resonance technique. A six-line signal (denoted as A, g ) 2.048, A ) 22.15 G) was detected on H-MCM-22 after He purging at high temperatures, whose intensities increased with the treating temperature. The same signal was also found on H-ZSM-5 zeolites with different crystal sizes. The paramagnetic center was identified as a V center, namely, a hole of an electron trapped on an oxygen atom bonding to a nearby aluminum atom. These signals appeared only on a dealuminated sample or a sample concomitantly with dealumination. The formation of the hole might involve an electron transferring from the lattice oxygen to a nonframework aluminum species, and the hyperfine splitting is caused by the interaction between the electron hole locating on the p orbit of oxygen and the framework aluminum bonding with the oxygen. The signal disappeared after the sample was exposed to air or oxygen at room temperature. However, the process was reversible. A new set of signals (denoted as B, g1 ) 2.008, g2 ) 2.003, g3 ) 1.9985) was observed after oxygen adsorption on the H-MCM-22 pretreated with He at 973 K or He purging at 973 K on the H-MCM-22 pretreated with oxygen at 813 K, which was attributed to the O -species.

Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

“…2B) that silver particles do not sinter after H 2 pretreatment. It has been known that the thermal stability of the silver particles is significantly lower under oxidizing conditions, and heating at higher temperatures under oxidizing conditions will induce the migration of the silver particles [27,28]. A high catalytic activity has also been found on a physical mixture of Ag with silica powder after oxygen activation at higher temperatures [26].…”

“…The composition of the effluent gas was monitored with an on-line GC-14B gas chromatograph equipped with a Molsieve 5 Å column and a Porapak Q column (T = 30 • C, helium as the carrier gas at 30 ml/min). The conversions of CO, oxygen, and the selectivity toward CO oxidation were calculated by a previously described method [25,26]. The CO conversion was calculated from the change in CO concentration, and selectivity toward CO oxidation was calculated from the oxygen mass balance.…”

“…It is found that both the ratio I (2170 cm −1 )/I (2187 cm −1 ) and the overall peak intensity increase with the temperature of oxygen pretreatment. It has been shown that larger silver particles tend to form on the silver catalyst with higher loading [25]. TEM observations were made on these catalysts with different silver loadings to obtain the distributions of particle size (Fig.…”

Restructuring and Redispersion of Silver on SiO2 under Oxidizing/Reducing Atmospheres and Its Activity toward CO Oxidation

“…The conversions of CO and O 2 , as well as the selectivity, were calculated by a method similar to that described previously. 9 Figure 2 (square symbols). The concentration of CO in the feed was 1%, and the O 2 concentration in pure He was 0.5%.…”

“…We have shown that the silver catalysts have a relatively high activity for CO selective oxidation at low temperatures. [8][9][10][11] It was found that the activity of the Ag catalyst varies greatly with the pretreatment process. A silver catalyst deactivated remarkably after pretreatment in H 2 at high temperatures, whereas treatment in oxygen at high temperatures could reactivate the catalyst.…”