David S. A. Simakov

Solar thermal catalytic reforming of natural gas: a review on chemistry, catalysis and system design

Solar radiation is an abundant and environmentally benign energy source. However, its capture and effective utilization is one of the most difficult challenges faced by modern science. An effective way to capture solar energy is to convert it to chemical energy using concentrated solar power and thermochemical conversion routes, such as methane reforming. Methane, the main component of natural gas, is poised to become a leading feedstock in the near term, partly due to recent developments in shale gas extraction. Solar-to-chemical energy conversion can be achieved by reforming methane into synthesis gas, a mixture of carbon monoxide and hydrogen, in a single, highly endothermic catalytic process when reacted with steam or carbon dioxide. This review highlights different aspects of solar thermal reforming of methane, including thermodynamics, challenges related to catalyst activity and stability and reactor design. Equilibrium limitations are discussed in detail with respect to solar thermal reforming. Recent developments in methane reforming catalysis are critically reviewed in a broad scope, addressing catalyst deactivation drawbacks and focusing on alternative catalysts. The potential of the low-temperature solar methane steam reforming and the related technological challenges are discussed, including catalyst requirements. Future directions are also outlined.

Noise induced oscillations and coherence resonance in a generic model of the nonisothermal chemical oscillator

Oscillating chemical reactions are common in biological systems and they also occur in artificial non-biological systems. Generally, these reactions are subject to random fluctuations in environmental conditions which translate into fluctuations in the values of physical variables, for example, temperature. We formulate a mathematical model for a nonisothermal minimal chemical oscillator containing a single negative feedback loop and study numerically the effects of stochastic fluctuations in temperature in the absence of any deterministic limit cycle or periodic forcing. We show that noise in temperature can induce sustained limit cycle oscillations with a relatively narrow frequency distribution and some characteristic frequency. These properties differ significantly depending on the noise correlation. Here, we have explored white and colored (correlated) noise. A plot of the characteristic frequency of the noise induced oscillations as a function of the correlation exponent shows a maximum, therefore indicating the existence of autonomous stochastic resonance, i.e. coherence resonance.

EGFR-dependent network interactions that pattern Drosophila eggshell appendages

Similar to other organisms, Drosophila uses its Epidermal Growth Factor Receptor (EGFR) multiple times throughout development. One crucial EGFR-dependent event is patterning of the follicular epithelium during oogenesis. In addition to providing inductive cues necessary for body axes specification, patterning of the follicle cells initiates the formation of two respiratory eggshell appendages. Each appendage is derived from a primordium comprising a patch of cells expressing broad (br) and an adjacent stripe of cells expressing rhomboid (rho). Several mechanisms of eggshell patterning have been proposed in the past, but none of them can explain the highly coordinated expression of br and rho. To address some of the outstanding issues in this system, we synthesized the existing information into a revised mathematical model of follicle cell patterning. Based on the computational model analysis, we propose that dorsal appendage primordia are established by sequential action of feed-forward loops and juxtacrine signals activated by the gradient of EGFR signaling. The model describes pattern formation in a large number of mutants and points to several unanswered questions related to the dynamic interaction of the EGFR and Notch pathways.

Dynamic model for the coordination of two enhancers of broad by EGFR signaling

Significance Temporal regulation of a typical developmental control gene depends on multiple enhancers, which makes it important to understand how enhancers coordinate their activities in time. We describe a mechanism for such coordination in Drosophila oogenesis, where the expression of the transcription factor Broad depends on sequential activities of two enhancers. The early enhancer is essential for activity of the late one. To explain this requirement, we propose a mechanism based on a network with feedforward and feedback loops. This network interprets and modulates the epidermal growth factor receptor (EGFR) signaling gradient that controls both enhancers. Experiments and computational modeling show that understanding Broad expression is impossible without simultaneously considering two enhancers, a dynamic gradient, and a network with extracellular and intracellular components. Although it is widely appreciated that a typical developmental control gene is regulated by multiple enhancers, coordination of enhancer activities remains poorly understood. We propose a mechanism for such coordination in Drosophila oogenesis, when the expression of the transcription factor Broad (BR) evolves from a uniform to a two-domain pattern that prefigures the formation of two respiratory eggshell appendages. This change reflects sequential activities of two enhancers of the br gene, early and late, both of which are controlled by the epidermal growth factor receptor (EGFR) pathway. The late enhancer controls br in the appendage-producing cells, but the function of the early enhancer remained unclear. We found that the early enhancer is essential for the activity of the late enhancer and induction of eggshell appendages. This requirement can be explained by a mechanism whereby the BR protein produced by the early enhancer protects the late enhancer from EGFR-dependent repression. We illustrate this complex mechanism using a computational model that correctly predicts the wild-type dynamics of BR expression and its response to genetic perturbations.

Discrete model of periodic pattern formation through a combined autocrine–juxtacrine cell signaling

We model the formation of periodic patterns of gene expression in epithelial cell sheets driven by autocrine signaling coupled to juxtacrine lateral inhibition. The mathematical model is based on a continuous description of the extracellular matrix and a discrete cell-level description of the layer of cells, coupling the dynamics of diffusible ligands to the threshold-controlled cell-autonomous regulation with randomly fluctuating production rates. The results of numerical simulations indicate that propagating signaling waves emerge in a certain parametric domain, leading to the formation of a variety of either periodic or irregular patterns. For some selections of parameters, a propagating stripe of uniform expression leaves in its wake stationary periodic arrays. Coupling of autocrine and juxtacrine cell communication is essential for the pattern regularity and for the selection of expression patterns. Moreover, weak but non-vanishing noise levels are essential for the formation of regular patterns. Additional autocrine and cell-autonomous regulatory interactions can be introduced to increase the spacing of a periodic pattern.

Effect of noise correlation on noise-induced oscillation frequency in the photosensitive Belousov-Zhabotinsky reaction in a continuous stirred tank reactor.

We report on the experimental study of noise-induced oscillations in the photosensitive Ru(bpy)3(2+)-catalyzed Belousov-Zhabotinsky reaction in a continuous stirred tank reactor (CSTR). In the absence of deterministic oscillations and any external periodic forcing, oscillations appear when the system is perturbed by stochastic fluctuations in light irradiation with sufficiently high amplitude in the vicinity of the bifurcation point. The frequency distribution of the noise-induced oscillations is strongly affected by noise correlation. There is a shift of the noise-induced oscillation frequency toward higher frequencies for an intermediate range of the noise correlation exponent, indicating the occurrence of coherence resonance. Our findings indicate that, in principle, noise correlation can be used to direct chemical reactions toward certain behavior.

Biological Conversion of CO 2

Utilizing CO2, as a concept of converting a greenhouse gas into a value-added feedstock, has gained considerable attention in recent years. In a search for sustainable and economically viable routes, scientists and engineers mainly focused on chemical conversion methods, either thermocatalytic, electrocatalytic, or photocatalytic. However, photosynthetic organisms have been converting CO2 and water into organic matter over more than two billions of years. Over this vast period of time, the sophisticated and, at the same time, robust mechanisms of CO2 fixation into complex biological molecules have evolved. Terrestrial and marine photosynthetic organisms convert more than 100 Gt per year of CO2 into biomass. This huge resource could potentially become a disruptive technology for CO2 conversion into renewable fuels and chemicals.

Electrocatalytic Reduction of CO 2

Renewable sources, such as solar and wind electricity, are attractive alternatives to fossil energy, because they are CO2 neutral and, therefore, do not contribute to the greenhouse effect. However, these types of renewable energy are not available on demand due to their transient character. One option is to store this surplus electricity as a liquid fuel that has high volumetric energy density and can be easily transported, for example methanol. This power-to-fuel chain can be achieved indirectly, using electrolysis and thermocatalytic conversion, or directly by electrochemical reduction.

Thermocatalytic Conversion of CO 2

Current solutions for CO2 emissions reduction mainly rely on capturing CO2 emitted from power stations and its storage. Such processes are based on CO2 separation via either amine scrubbing, pressure swing adsorption, or membrane separation followed by cryogenic liquefaction. These approaches are energy intensive, leading to high capital and operating costs. Conversion of captured CO2 into synthetic fuels and chemicals is an attractive avenue for reduction of CO2 emissions. Thermocatalytic conversion combines the use of high temperatures with a heterogeneous catalyst, providing fast reaction rates and, therefore, allowing for large volume production.

Photocatalytic Reduction of CO2

As compared to other methods for CO2 reduction, the photocatalytic conversion of CO2 has a unique advantage of direct utilization of solar energy. In this sense, this pathway is similar to the biological photosynthesis. However, despites decades of quite intensive research, mechanisms of the catalytic photoreduction of CO2 are still not well-understood, which limits our ability to select novel catalytic systems. The design of an efficient photocatalytic reactor is still challenging. Although the potential of the CO2 photoreduction is large, challenges are still great and this technology is not yet economically viable.