Effect of Ag⁺ Cations on Nonoxidative Activation of Methane to C₂-Hydrocarbons

Abstract

Nonoxidative activation of methane was carried out over Ag/ZSM-5 catalysts, prepared by the wetness impregnation method, and the catalysts were characterized by various techniques. Theoretical calculation was further employed to clarify the reaction mechanism of methane coupling over the catalysts. Ag + ions were found to be the main Ag species at low loading (1-5 wt %) whereas higher loading (10 wt %) resulted in metal clusters. Combined with experimental evidence and computer modeling, it is concluded that isolated Ag + ions play a crucial role in the catalytic coupling of methane under nonoxidative conditions.

On the possibility of AgZSM-5 zeolite being a partial oxidation catalyst for methane

“…The silver ions are the active centers in the catalyst for the heterolytic dissociation of the CH bond in CH 4 . A similar mechanism was also reported by another research group in recent years [14]. In accordance with the mechanisms proposed by the two groups, we propose that CH 4 is activated on the Ag + ions exchanged in the ZSM-5 zeolite to form H − and CH + 3 ions.…”

“…As the results show, the Ag + exchanged on Brønsted acid sites may play an important role in the reaction. This result corresponds with the results of activation observed by Baba and Sawada, indicating the same mechanism at work, i.e., Ag-CH 3 and Ag-H formation on Ag + exchanged in the ZSM-5 sample [12][13][14]. One of the reasons for the lower efficiency observed in the Ag/SiO 2 ·Al 2 O 3 sample may be the difference in the acid strength of the Ag(H)ZSM-5 sample, as is apparent from the observed positions of IR bands for CO molecules adsorbed onto the respective samples [7].…”

“…Several characteristic bands are observed; the bands in the range between 40,000 and 52,000 cm −1 were assigned to the transition due to Ag + ions, and the band in the range 25,000-40,000 cm −1 are ascribable to silver clusters with different sizes and oxidation states. A main band around 30,000-32,000 cm −1 is assignable to the species composed of Ag δ+ n clusters and the band at around 20,000 cm −1 is the species of Ag metal particles [14,15,20,21]. These data clearly indicate that the Ag + , Ag δ+ n , and Ag clusters coexist in the presence of a mixture of CH 4 and O 2 ; the catalytic reaction will be a continuous process in the present condition that causes the partial oxidation reaction of CH 4 .…”

Surface properties and catalytic performance of Pt–Ag supported on silica – The effect of preparation methods

“…290 nm is recorded for both above mentioned bimetallic materials. It can correspond to different cationic species made with both, platinum and silver [46][47][48][49][50].…”

Silver catalysts supported over activated carbons for the selective oxidation of CO in excess hydrogen: effects of different treatments on the supports

“…It is well known that the properties of the supports have great influence on the catalytic performance. When loading active components onto/into different supports, the charges and sizes of active components [1,2], the particle shapes, the crystallographic structures [3] as well as the specific active sites at the metal-support boundary [4] can be controlled. A large volume of works have already been done on elucidating the effect of supports on the structure of metal catalysts and, subsequently, their catalytic performances [5][6][7].…”