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Register a new userEffect of CO desorption and coadsorption with O on the phase diagram of a Ziff-Gulari-Barshad model for the catalytic oxidation of CO
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We study the effect of coadsorption of CO and O on a Ziff-Gulari-Barshad (ZGB) model with CO desorption (ZGB-d) for the reaction CO + O --> CO_2 on a catalytic surface. Coadsorption of CO on a surface site already occupied by an O is introduced by an Eley-Rideal-type mechanism that occurs with probability p, 0 <= p <= 1. We find that, besides the well known effect of eliminating the second-order phase transition between the reactive state and an O-poisoned state, the coadsorption step has a strong effect on the transition between the reactive state and the CO-poisoned state. The coexistence curve between these two states terminates at a critical value k_c of the desorption rate k which now depends on p. Our Monte Carlo simulations and finite-size scale analysis indicate that k_c decreases with increasing values of p. For p=1, there appears to be a sharp phase transition between the two states only for k at(or near) zero.
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We present results of kinetic Monte Carlo simulations of a modified Ziff-Gulari-Barshad model for the reaction CO+O --> CO_2 on a catalytic surface. Our model includes impurities in the gas phase, CO desorption, and a modification known to eliminate the unphysical O poisoned phase. The impurities can adsorb and desorb on the surface, but otherwise remain inert. In a previous work that did not include CO desorption [G. M. Buendia and P. A. Rikvold, Phys. Rev. E, 85 031143 (2012)], we found that the impurities have very distinctive effects on the phase diagram and greatly diminish the reactivity of the system. If the impurities do not desorb, once the system reaches a stationary state, the CO_2 production disappears. When the impurities are allowed to desorb, there are regions where the CO_2 reaction window reappears, although greatly reduced. Following experimental evidence that indicates that temperature effects are crucial in many catalytic processes, here we further analyze these effects by including a CO desorption rate. We find that the CO desorption has the effect to smooth the transition between the reactive and the CO rich phase, and most importantly it can counteract the negative effects of the presence of impurities by widening the reactive window such that now the system remains catalytically active in the whole range of CO pressures.
We present a study of the dynamical behavior of a Ziff-Gulari-Barshad model with CO desorption and lateral diffusion. Depending on the values of the desorption and diffusion parameters, the system presents a discontinuous phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve between these phases. Inclusion of the diffusion term produces a significant increase in the CO_2 production rate. We further applied a square-wave periodic pressure variation of the partial CO pressure with parameters that can be tuned to modify the catalytic activity. Contrary to the diffusion-free case, this driven system does not present a further enhancement of the catalytic activity, beyond the increase induced by the diffusion under constant CO pressure.
We present a computational study of the dynamic behavior of a Ziff-Gulari-Barshad model of CO oxidation with CO desorption on a catalytic surface. Our results provide further evidence that below a critical desorption rate the model exhibits a non-equilibrium, first-order phase transition between low and high CO coverage phases. Our kinetic Monte Carlo simulations indicate that the transition process between these phases follows a decay mechanism very similar to the one described by the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by nucleation and growth. We measure the lifetimes of the metastable phases on each side of the transition line and find that they are strongly dependent on the direction of the transformation, i.e., from low to high coverage or vice versa. Inspired by this asymmetry, we introduce a square-wave periodic external forcing, whose two parameters can be tuned to enhance the catalytic activity. At CO desorption rates below the critical value, we find that this far-from-equilibrium system undergoes a dynamic phase transition between a CO_2 productive phase and a nonproductive one. In the space of the parameters of the periodic external forcing, this nonequilibrium phase transition defines a line of critical points. The maximum enhancement rate for the CO_2 production rate occurs near this critical line.
We study by kinetic Monte Carlo simulations the catalytic oxidation of carbon monoxide on a surface in the presence of contaminants in the gas phase. The process is simulated by a Ziff-Gulari-Barshad (ZGB) model that has been modified to include the effect of the contaminants and to eliminate the unphysical oxygen-poisoned phase. The impurities can adsorb and desorb on the surface, but otherwise remain inert. We find that, if the impurities can not desorb, no matter how small their proportion in the gas mixture, the first order transition and the reactive window that characterize the ZGB model disappear. The coverages become continuous, and once the surface has reached a steady state there is no production of CO$_2$. This is quite different from the behavior of a system in which the surface presents a fixed percentage of impurities. When the contaminants are allowed to desorb, the reactive window appears again, and disappears at a value that depends on the proportion of contaminants in the gas and on their desorption rate.
The phase diagram of water harbours many mysteries: some of the phase boundaries are fuzzy, and the set of known stable phases may not be complete. Starting from liquid water and a comprehensive set of 50 ice structures, we compute the phase diagram at three hybrid density-functional-theory levels of approximation, accounting for thermal and nuclear fluctuations as well as proton disorder. Such calculations are only made tractable because we combine machine-learning methods and advanced free-energy techniques. The computed phase diagram is in qualitative agreement with experiment, particularly at pressures $lesssim$8000 bar, and the discrepancy in chemical potential is comparable with the subtle uncertainties introduced by proton disorder and the spread between the three hybrid functionals. None of the hypothetical ice phases considered is thermodynamically stable in our calculations, suggesting the completeness of the experimental water phase diagram in the region considered. Our work demonstrates the feasibility of predicting the phase diagram of a polymorphic system from first principles and provides a thermodynamic way of testing the limits of quantum-mechanical calculations.
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