1,483 research outputs found
Response of a catalytic reaction to periodic variation of the CO pressure: Increased CO_2 production and dynamic phase transition
We present a kinetic Monte Carlo study of the dynamical response of a
Ziff-Gulari-Barshad model for CO oxidation with CO desorption to periodic
variation of the CO presure. We use a square-wave periodic pressure variation
with parameters that can be tuned to enhance the catalytic activity. We produce
evidence that, below a critical value of the desorption rate, the driven system
undergoes a dynamic phase transition between a CO_2 productive phase and a
nonproductive one at a critical value of the period of the pressure
oscillation. At the dynamic phase transition the period-averged CO_2 production
rate is significantly increased and can be used as a dynamic order parameter.
We perform a finite-size scaling analysis that indicates the existence of
power-law singularities for the order parameter and its fluctuations, yielding
estimated critical exponent ratios and . These exponent ratios, together with theoretical symmetry
arguments and numerical data for the fourth-order cumulant associated with the
transition, give reasonable support for the hypothesis that the observed
nonequilibrium dynamic phase transition is in the same universality class as
the two-dimensional equilibrium Ising model.Comment: 18 pages, 10 figures, accepted in Physical Review
Inflationary and dark energy regimes in 2+1 dimensions
In this work we investigate the behavior of three-dimensional (3D)
cosmological models. The simulation of inflationary and dark-energy-dominated
eras are among the possible results in these 3D formulations; taking as
starting point the results obtained by Cornish and Frankel.
Motivated by those results, we investigate, first, the inflationary case
where we consider a two-constituent cosmological fluid: the scalar field
represents the hypothetical inflaton which is in gravitational interaction with
a matter/radiation contribution. For the description of an old universe, it is
possible to simulate its evolution starting with a matter dominated universe
that faces a decelerated/accelerated transition due to the presence of the
additional constituent (simulated by the scalar field or ruled by an exotic
equation of state) that plays the role of dark energy. We obtain, through
numerical analysis, the evolution in time of the scale factor, the
acceleration, the energy densities, and the hydrostatic pressure of the
constituents. The alternative scalar cosmology proposed by Cornish and Frankel
is also under investigation in this work. In this case an inflationary model
can be constructed when another non-polytropic equation of state (the van der
Waals equation) is used to simulate the behavior of an early 3D universe.Comment: Latex file, plus 9 figures. To appear in General Relativity and
Gravitatio
Transition from accelerated to decelerated regimes in JT and CGHS cosmologies
In this work we discuss the possibility of positive-acceleration regimes, and
their transition to decelerated regimes, in two-dimensional (2D) cosmological
models. We use general relativity and the thermodynamics in a 2D space-time,
where the gas is seen as the sources of the gravitational field. An
early-Universe model is analyzed where the state equation of van der Waals is
used, replacing the usual barotropic equation. We show that this substitution
permits the simulation of a period of inflation, followed by a
negative-acceleration era. The dynamical behavior of the system follows from
the solution of the Jackiw-Teitelboim equations (JT equations) and the
energy-momentum conservation laws. In a second stage we focus the
Callan-Giddings-Harvey-Strominger model (CGHS model); here the transition from
the inflationary period to the decelerated period is also present between the
solutions, although this result depend strongly on the initial conditions used
for the dilaton field. The temporal evolution of the cosmic scale function, its
acceleration, the energy density and the hydrostatic pressure are the physical
quantities obtained in through the analysis.Comment: To appear in Europhysics Letter
Bragg Polaritons: Strong Coupling and Amplification in an Unfolded Microcavity
Periodic incorporation of quantum wells inside a one--dimensional Bragg
structure is shown to enhance coherent coupling of excitons to the
electromagnetic Bloch waves. We demonstrate strong coupling of quantum well
excitons to photonic crystal Bragg modes at the edge of the photonic bandgap,
which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg
polariton branches are in good agreement with the theory and allow
demonstration of Bragg polariton parametric amplification.Comment: 4 pages, 4 figure
A leed analysis of the (2Ă1)H-Ni(110) structure
A monolayer of H atoms adsorbed on Ni(110) below 180 K forms a (2Ă1) structure. The unit cell exhibits a glide symmetry plane and contains two adsorbed atoms. Based on a quantitative comparison between experimental and calculated LEED I/V spectra using standard R-factors the following structure was derived: On the clean Ni(110) surface the separation between the first two atomic layers, d12, is contracted by 8.5%±1.5% with respect to the bulk value; those between the second and third and the third and fourth layer, d23 and d34, are expanded by 3.5%±1.5% and 1%±1.5%, respectivelyâin agreement with recent other results. In the presence of the H adlayer the contraction of d12 is reduced to 4.5%±1.5%, while the expansion of d23 is not affected within the limits of accuracy. The third interlayer spacing d34 returns to its bulk value. The H atoms occupy threefold-coordinated sites formed by two Ni atoms from the first layer and one Ni atom from the second layer which confirms previous more qualitative conclusions based on He diffraction and vibrational spectroscopy. The bond lengths between H and its neighbouring Ni atoms were determined to be equal, namely 1.72±0.1 Ă
Kinetic phase diagram for CO oxidation on Pt(210): Pattern formation in the hysteresis and oscillation regions
The reactive behavior of catalytic CO oxidation on Pt(210) is studied by means of combined reaction rate measurements and photoelectron emission microscopy (PEEM). These methods allow an investigation of the phenomena at macroscopic and mesoscopic level, respectively. The external control parameters (flow rate, CO and oxygen partial pressures, surface temperature and scanning rates of pressure and temperature) are systematically varied to reveal various reactive regions in parameter space. The macroscopic measurements for a given temperature and flow rate (under isothermal conditions) show that lower pressures lead to a pronounced clockwise hysteresis in the production rate of CO2, while increasing pressures cause a systematic narrowing leading to a crossing of the two hysteresis branches into a region of counterclockwise hysteresis. A further pressure increase leads to macroscopic temporal oscillations. Mesoscopic spatiotemporal oscillations appear at the same conditions. The resulting macroscopic isothermal kinetic phase diagram exhibits a cross-shaped characteristic similar to that previously obtained for the Pd(110) surface. The mesoscopic lateral distribution of CO and oxygen adsorbed on the surface is monitored with the photoelectron emission microscope during the reaction at isothermal conditions and different constant oxygen pressures. The observed mesoscopic spatiotemporal patterns, such as islands, waves, target patterns and spirals, are correlated via the external control parameters with different regions in the macroscopic isothermal phase diagram. The results are compared with previous data of CO oxidation on other surfaces, like Pd(110) and Pt(110)
Macroscopic and mesoscopic characterization of a bistable reaction system: CO oxidation on Pt(111) surface
The catalytic oxidation of CO by oxygen on a platinum (111) single-crystal surface in a gas-flow reactor follows the LangmuirâHinshelwood reaction mechanism. It exhibits two macroscopic stable steady states (low reactivity: CO-covered surface; high reactivity: O-covered surface), as determined by mass spectrometry. Unlike other Pt and Pd surface orientations no temporal and spatiotemporal oscillations are formed. Accordingly, CO+O/Pt(111) can be considered as one of the least complicated heterogeneous reaction systems. We measured both the macroscopic and mesoscopic reaction behavior by mass spectrometry and photoelectron emission microscopy (PEEM), respectively, and explored especially the region of the phase transition between low and high reactivity. We followed the rate-dependent width of an observed hysteresis in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust conditions of the external control parameters which totally inhibited the motion of the reaction/diffusion front. By systematic variation of these conditions we could pinpoint a whole region of external control parameters in which the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, our experiments and simulation reveal the existence of an âexperimentalâ bistable region inside the âcomputedâ bistable region of the reactivity diagram (S-shaped curve) leading to a novel dollar ($)-shaped curve
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