Role of Carbon Surface Chemistry in Catalysis

The catalytic role of carbon surfaces has been discussed in excellent reviews by Leon y Leon and Radovic [53] and by Radovic and Rodriguez-Reinoso [43] and is discussed extensively throughout this book. In this section, examples of the specific catalytic effect of carbon surface functionality are introduced only briefly. 

Besides providing high-energy adsorption sites for physical or specific adsorption, carbon, which consists of both small pores and functional groups, is able to catalyze surface reactions. A simple example is oxidation of sulfur dioxide where it was found that basic functional groups present on the surface of carbons 

A well-known example of a complex catalytic reaction that takes place on the surface of carbon is the oxidation of hydrogen sulfide [329,330], When water is present on the carbon surface and the surface has the basic pH required for dissociation of H2S, oxidation of the HS ions by active oxygen occurs either to elemental sulfur or sulfuric acid. The latter is formed when the reaction takes place in very small pores, where only sulfur radicals very susceptible for further oxidation to SO3 are formed. Catalytic oxidation also occurs in the case of methyl mercaptan adsorption [331], where on basic carbon, thiolate ions formed as a result of dissociation are further oxidized to dimethyldisulfide strongly adsorbed in the pore system. In the case of desulfurization, inorganic constituents of carbon such as iron and calcium also play a crucial role. Those elements, present even in small amounts, contribute significantly to the oxidation reactions as catalysts [332,333], 

Catalytic oxidation of hydrogen sulfide or methyl mercaptan is also enhanced in the presence of nitrogen-containing functionalities [57], Besides providing the basic pH needed for effective dissociation of HS , they were proposed to activate oxygen via formation of superoxide ion, which participates in the oxidation of thiolate ions to sulfur and sulfuric acid [95], 

Another important catalytic reaction crucial for environmental remediation is reduction of NO , When activated carbons are used as removal media, the elimination process includes adsorption combined with either oxidation or reduction, with carbon acting as the reducing agent and perhaps even as a catalyst [332,333], Oxidation usually leads to the formation of nitric acid, whereas N2 is the product of NO c reduction. It was found that surface chemistry affects NO removal performance, and an optimal amount of oxygen-functional groups on the surface of char is needed [334-337], 

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