The graphene-NO reaction: initial insights from computational quantum chemistry

OYARZÚN, A.; GARCÍA, X.; RADOVIC, L.:
Carbon 2013 World Conference on Carbon, Rio de Janeiro, Brazil, July 2013.

Introduction

The carbon-NO reaction has been of increasing interest because of its relevance to both problems (e.g., soot interactions with NOx) and solutions (e.g., activated carbon sorbent injection) in air quality management. As society, especially in rapidly developing nations, struggles to control both acid rain and smog , the use of carbon as a reducing agent (e.g., 2C+2NO→C + CO₂ + N₂) or, even better, as a catalyst (2C + 2NO→O₂ N₂) offers much promise. The apparently simple stoichiometry, like that of the C-O₂ reaction, can be misleading, however, and, much more so than the C-O₂ reaction, its practical utilization has been hampered by a lack of understanding of key details of the reaction mechanism.

Experimentally, the C-O₂ and the C-NO reactions pose similar challenges: there is more than one path to the formation of CO and/or CO₂, and this is very much dependent on the reaction conditions, to the extent that different mechanisms may apply in different temperature ranges. Use of the Langmuir-Hinshelwood formalism is common for gas-carbon reactions, but it does require the determination of several kinetic parameters.

Theoretical approaches are increasingly reliable and useful, particularly those rooted in quantum mechanics. Here we adopt such methodology and focus on the adsorption step of the reaction sequence. Our working assumption is that, once the carbon-oxygen surface complexes are formed, the similarities between the C-NO and C-O₂ reactions are likely to increase (e.g., 2C(O)↔ C + C₂2), and the extensive knowledge from the latter can be used to elucidate the surface rearrangement and desorption steps in the former. The aim of this ongoing study is to address the key specific questions whose discussion is available, but whose answers do not seem to be firmly established, in the literature: Is NO adsorption dissociative? Which configuration, O-down or N- down, leads more readily to the reaction products? Is single- site or dual-site adsorption favored? Is the most abundant adsorbate the NO monomer or the dimer? Which (re)active sites are responsible for N₂, CO and/or CO₂ formation, and are they different from those involved in the other oxygen- transfer reactions?