Nanosized Ag/α-MnO₂ catalysts highly active for the low-temperature oxidation of carbon monoxide and benzene

Abstract

Nanosized α-MnO₂-supported silver catalysts (xAg/nano-MnO₂, x = 0–10.0 wt%) were prepared by the incipient wetness impregnation method and characterized by means of numerous analytical techniques. Catalytic activities of the materials were evaluated for the oxidation of CO and benzene. It is shown that the loading of silver on nano-MnO₂ could significantly modify the catalytic activities and the catalytic performance of xAg/nano-MnO₂ strongly depended upon the Ag loading, among which 5Ag/nano-MnO₂ performed the best for the addressed reactions. The excellent performance of 5Ag/nano-MnO₂ was associated with the highly dispersed Ag, good low-temperature reducibility, and synergism at the interface of Ag and MnO₂ nanodomains.

Introduction

Catalytic oxidation of carbon monoxide is of considerable interest due to its relevance in many industrial applications, such as gas purification in CO₂ lasers, CO sensors, air-purification devices for respiratory protection, and pollution control devices for reducing industrial and environmental emissions [1]. Most of volatile organic compounds (VOCs) emitted from transportation and industrial activities are harmful to the atmospheric environment and human health [2], [3]. Therefore, it is of significance to eliminate the VOCs [4]. The conventional incineration of organic pollutants usually requires high temperatures (>1000 °C), resulting in a rise in energy consumption. Catalytic oxidation of VOCs, however, has been generally accepted to be one of the most effective pathways for the destruction of VOCs at lower temperatures (<500 °C). Such a catalytic strategy possesses advantages of high feasibility, low operation cost, and high destruction efficiency [5]. The catalyst employed is a key issue determining the effectiveness of catalytic oxidation technology.

In the past years, a number of materials, including supported noble metals (Pt, Pd, Rh, and Au) [6], [7], [8] and base transition-metal (Cr, Co, Cu, Ni, and Mn) oxides [6], [9], [10], have been used as catalyst for the combustion of VOCs. The former are highly active at lower temperatures, but their applications are limited due to the high cost and involved problems related to sintering, volatility, and susceptibly poisoning tendency [6], [11], [12]. Hence, there is an urgent need to develop cheaper and active catalysts that exhibit high thermal stability and great resistance to poisoning.

Bulk and/or supported manganese oxides have long been used as an active catalyst for the oxidation of carbon monoxide, methane, and hydrocarbons [13]. Since a single-component catalyst is usually hard to rival a precious metal catalyst, attempts for enhancing catalytic activity have been made by combining two or more transition-metal elements. For example, Mn–Fe composite oxides showed a higher activity than the zeolite-supported Pt material in catalyzing the oxidation of oxygen-containing organic compounds [14]. Loading a small amount of gold nanoparticles on manganese oxide (i.e., Au/MnO_x) could give rise to a significant enhancement in catalytic activity for the oxidation of CO [15] and alcohol [16].

Although silver is not a good oxidation catalyst in most cases, it can be utilized as an active component of catalysts for some deep or partial oxidation. It has been demonstrated that supported silver catalysts showed good catalytic performance for the selective oxidation of ethylene to ethylene oxide [17], the oxidation of acetone and pyridine [18], and the oxidation of VOCs [19]. To the best of our knowledge, no reports on the use of nanosized MnO₂-supported Ag as catalyst for benzene combustion have been seen in the literature. Recently, we have prepared numerous MnO₂-supported transition-metal catalysts and found these materials performed well in the combustion of some typical VOCs. Herein, we report the preparation, characterization, and catalytic properties of nanosized Ag/MnO₂ with various Ag loadings for the complete oxidation of carbon monoxide and benzene.