Theory of the NO+CO surface reaction model

We derive a pair approximation (PA) for the NO+CO model with instantaneous reactions. For both the triangular and square lattices, the PA, derived here using a simpler approach, yields a phase diagram with an active state for CO-fractions y in the interval y_1 < y < y_2, with a continuous (discontinuous) phase transition to a poisoned state at y_1 (y_2). This is in qualitative agreement with simulation for the triangular lattice, where our theory gives a rather accurate prediction for y_2. To obtain the correct phase diagram for the square lattice, i.e., no active state, we reformulate the PA using sublattices. The (formerly) active regime is then replaced by a poisoned state with broken symmetry (unequal sub- lattice coverages), as observed recently by Kortluke et al. [Chem. Phys. Lett. 275, 85 (1997)]. In contrast with their approach, in which the active state persists, although reduced in extent, we report here the first qualitatively correct theory of the NO+CO model on the square lattice. Surface diffusion of nitrogen can lead to an active state in this case. In one dimension, the PA predicts that diffusion is required for the existence of an active state.Comment: 15 pages, 9 figure

Laboratório caseiro: Um calorímetro alternativo

Fazer experiências que envolvam trocas de calor entre corpos, embora seja algo simples, requer alguns cuidados no que diz respeito ao isolamento térmico do sistema em relação ao meio ambiente. Uma das formas de se obter este isolamento é utilizando um calorímetro. Neste trabalho apresentamos uma forma simples e barata de construção deste equipamento, bem como sugerimos a realização de uma experiência que permite a determinação do calor de fusão do gelo