1,007 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
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
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)
Energetics and Vibrational States for Hydrogen on Pt(111)
We present a combination of theoretical calculations and experiments for the
low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111)
surface. The vibrational band states are calculated based on the full
three-dimensional adiabatic potential energy surface obtained from first
principles calculations. For coverages less than three quarters of a monolayer,
the observed experimental high-resolution electron peaks at 31 and 68meV are in
excellent agreement with the theoretical transitions between selected bands.
Our results convincingly demonstrate the need to go beyond the local harmonic
oscillator picture to understand the dynamics of this system.Comment: In press at Phys. Rev. Lett - to appear in April 200
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
The Static and Dynamic Lattice Changes Induced by Hydrogen Adsorption on NiAl(110)
Static and dynamic changes induced by adsorption of atomic hydrogen on the
NiAl(110) lattice at 130 K have been examined as a function of adsorbate
coverage. Adsorbed hydrogen exists in three distinct phases. At low coverages
the hydrogen is itinerant because of quantum tunneling between sites and
exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate
mediated interactions produce an ordered superstructure with c(2x2) symmetry,
and at higher coverages, hydrogen exists as a disordered lattice gas. This
picture of how hydrogen interacts with NiAl(110) is developed from our data and
compared to current theoretical predictions.Comment: 36 pages, including 12 figures, 2 tables and 58 reference
A LEED structural analysis of the Co(100) surface
The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 Ă
, corresponding to a 12.8(Âą0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8(Âą0.2)%
Children of prisoners: exploring the impact of families' reappraisal of the role and status of the imprisoned parent on children's coping strategies
Qualitative data from a larger study on the impact of parental imprisonment in four countries found that children of prisoners face fundamentally similar psychological and social challenges. The ways that children cope, however, are influenced by the interpretative frame adopted by the adults around them, and by how issues of parental imprisonment are talked about in their families. This article argues that families have to reappraise their view of the imprisoned parent and then decide on their policy for how to deal with this publicly. Their approach may be based on openness and honesty or may emphasise privacy and secrecy, or a combination of these. Children are likely to be influenced by their parents'/carers' views, although these may cause conflict for them. Where parents/carers retain a positive view of the imprisoned parent, children are likely to benefit; where parents/carers feel issues of shame and stigma acutely, this is likely to be transmitted to their children. This is important for social workers and practitioners involved in supporting prisoners' families and for parenting programmes
High-efficiency production of the antimicrobial peptide pediocin PA-1 in metabolically engineered Corynebacterium glutamicum using a microaerobic process at acidic pH and elevated levels of bivalent calcium ions
Background Pediocin PA-1 is a bacteriocin of recognized value with applications in food bio-preservation and the
medical sector for the prevention of infection. To date, industrial manufacturing of pediocin PA-1 is limited by high
cost and low-performance. The recent establishment of the biotechnological workhorse Corynebacterium glutamicum as recombinant host for pediocin PA-1 synthesis displays a promising starting point towards more efcient
production.
Results Here, we optimized the fermentative production process. Following successful simplifcation of the production medium, we carefully investigated the impact of dissolved oxygen, pH value, and the presence of bivalent
calcium ions on pediocin production. It turned out that the formation of the peptide was strongly supported by an
acidic pH of 5.7 and microaerobic conditions at a dissolved oxygen level of 2.5%. Furthermore, elevated levels of CaCl2
boosted production. The IPTG-inducible producer C. glutamicum CR099 pXMJ19 Ptac pedACDCg provided 66 mg Lâ1
of
pediocin PA-1 in a two-phase batch process using the optimized set-up. In addition, the novel constitutive strain Ptuf
pedACDCg allowed successful production without the need for IPTG.
Conclusions The achieved pediocin titer surpasses previous eforts in various microbes up to almost seven-fold, providing a valuable step to further explore and develop this important bacteriocin. In addition to its high biosynthetic
performance C. glutamicum proved to be highly robust under the demanding producing conditions, suggesting its
further use as host for bacteriocin production
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