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These aspects are discussed in the following sections. Electric current is the main output from renewable technologies such as solar, wind, or hydroelectric energy. In addition, it is possible to harvest thermal energy directly from solar or geothermal energy, or to indirectly produce it from renewable electricity.
Another option is to look for other energy forms, such MWs, plasmas, or light, which open up new opportunities for efficient and flexible transformations. These methods of unconventional activation of chemical reactions have attracted recent attention owing to four main advantages: 32 selectivity, bypass of reactions bottlenecks, new products or properties added value , and improved process conditions. Electromagnetic energy deployment to a catalyst is a radically new approach to activate catalytic reactions.
In the following sections, a brief description of these three unconventional energy forms is given. It is intended to give a broad view of the potential and results of these strategies for CO 2 conversion. MW heating takes place because of dipole rotation and ionic polarization during the interaction of the electromagnetic field and the matter exposed. The generation of MWs occurs in magnetrons. Once the electromagnetic field is generated, it is deployed by means of a MW applicator.
Examples of MW activation of catalytic reactions for CO 2 conversion are not abundant, but there are some interesting examples. Owing to the localized thermal effects of MWs and the low energetic level of CO 2 , endothermic reactions were investigated in particular. Similarly, during homogeneous reactions, an absorbing liquid phase is used to transform and transmit the MW energy. Part of the potential of MWs is related to the possibility of selective energy delivery directly into the catalytic structures.
E-Poster Payments will not be refunded. Deactivation and regeneration of catalysts Catalyst deactivation, the loss of catalytic activity and selectivity over time, is one of the major problems concerning industrial application of catalyst in pyrolysis process [ 25 , 53 ]. Chandra, M. If you decide to participate, a new browser tab will open so you can complete the survey after you have completed your visit to this website. The results of rice husk pyrolysis with ZnO studied by Zhou et al.
This mere effect will increase reaction yields by diminishing the conduction—convection losses present in a conventional heating system. Therefore, more initiatives are expected with reactions that use this technology to transform CO 2. Given the high conversion rates possible, it would be interesting to research the behavior with real flue gases or a representative mixture of industrially relevant cases. Plasma is called the fourth state of matter, after solid, liquid, and gas.
Some examples of natural plasmas are the sun, other stars, the aurora borealis, and lightning. Plasma can, however, also be created artificially. These plasmas are fully ionized and resemble the conditions of the sun.
In particular, nonthermal and warm plasmas are of interest for CO 2 conversion. They are also called gas discharge plasma and they are typically partially ionized, consisting of a large number of molecules, but also ions and electrons, as well as radicals and excited species. All of these species can interact with each other, making this type of plasma a highly reactive chemical cocktail with high potential for affordable processes, certainly given its low temperature and cheap materials.
Simply speaking, such a gas discharge plasma is created by applying electric power to a gas, causing breakdown of the gas into the formation of electrons and ions. The electrons are easily heated by the applied electric power because of their small mass, whereas the gas itself typically remains near room temperature. The energetic electrons will then collide with the gas molecules, causing excitation, ionization, and dissociation collisions, thereby creating excited species, ions, and radicals. The latter species can easily react with each other, forming new molecules.
Thus, due to the electron activation of the gas molecules, thermodynamically unfavorable reactions, such as CO 2 splitting and the DRM, can proceed under mild reaction conditions i. Thus, plasma technology can contribute to the solution for the current imbalance between the supply and demand of energy, and for the integration of intermittent RE into the existing electricity grid, by using excess RE for the conversion of CO 2 into new fuels.
Solar light is an immense and distributed energetic resource. It is the main energy input for living organisms and captured through photosynthesis. As a result of this complex set of processes, the overall reaction is the conversion of carbon dioxide and water into oxygen and carbohydrates.
Artificial photosynthesis technologies mimic these natural photosynthetic CO 2 conversion systems. The conversion of light into chemical energy for such fuels can be accomplished by using catalytic strategies that involve light capture, concentration, and catalytic reaction.
More efficient light use for reactions involve the capture of the visible spectrum. Usually, this is achieved by means of organic or inorganic sensitization strategies. In particular, localized catalytic activation has enabled more efficient light energy use.
Furthermore, recent developments of artificial photosynthesis involve the incorporation of other species, such as nitrogen to the reaction pathway, with the objective of producing derived substances, such as nitrogen compounds. The transformation of CO 2 into fuels and chemical intermediates represents a breakthrough to reduce carbon emissions. Both changes in the morphology of the solids and localized heating have been claimed as explanations for the increase in either selectivity or reaction rate when using MW energy in catalytic reactions.
Detailed studies have identified two phenomena that give a rational explanation for these observations: selective heating and local hot spots. Briefly, when two or more phases are involved, one material will absorb MWs more efficiently selectively than the others.
This is the case for processes involving liquid—liquid, gas—solid, and solid—liquid systems. This selective heating in solid catalysts leads to the formation of hot spots in the catalyst bed and catalyst superheating, which altogether result in higher reaction rates as a consequence of local temperature increase. Left: Preferential absorption of MWs in graphite covering a much colder pellet reproduced from Ref. Reprinted with permission from Ref.
Copyright American Chemical Society. Another interesting finding is that the difference in temperature between the nanoparticles and fluid is not that big, but the superposition of the heating of a group of nanoparticles makes the temperature ramp up after a given characteristic time [Eq. This theoretical finding is in agreement with experimental evidence, for which small differences were found.
Another experimental evidence of the effect of localized heating came from studies to understand carbon formation on catalysts. This principle has been applied for the local control of nanoscale reactions. Localized catalyst heating unit. Reproduced with permission from Ref. Copyright , Elsevier. Indeed, because plasma is such a reactive environment, a variety of different compounds chemicals, fuels can be formed, but without selectivity.
As a matter of fact, these phenomena can increase the effectiveness of a reaction, but they can also modify the catalyst. Generation of gas discharges between catalyst pellets. The interactions of plasmas and catalysts take place in both ways. Electrons and active species can undergo both elastic and inelastic interactions with a catalyst. Inelastic interactions require energy transfer from the particles to the surface and are therefore of interest for chemical reactions. In this range, the inelastic collisions of electrons with matter can activate secondary electron emissions and phonons.
When an electron impacts on a material, resulting in an inelastic collision, the main result can be heating through phonon activation i. An interesting effect of electron interactions with semiconductors might be the generation of an electron—hole pair.
This could be an interesting opportunity to bridge disciplines through a deeper understanding of catalytic processes in plasmas by the study of photocatalytic processes. Usually, photocatalytic systems based on a simple combination of a semiconductor and a cocatalyst can only operate over narrow light wavelength ranges.
For example, it is known that TiO 2 is active in the UV range, but shows very limited activity when irradiated with light in the visible wavelength range. Control of the particle size of the semiconductor provides the first mechanism for adjusting the wavelength of light absorption. Despite these efforts, in general, inorganic systems are limited with respect to their ability to absorb light over a wide wavelength range, and particularly in the visible range. An alternative to overcome this limitation is to employ organic light sensitizers, which are usually inspired by natural pigments that perform this function in green plants.
The stabilization of charge separation induced by light is a key aspect that determines the photocatalytic activity of the material. An important method to stabilize charge separation is to provide a nanostructure to the catalytic materials with appropriate and controlled sizes and shapes suitable for stabilization of charge separation. This catalytic material was able to induce water splitting under ambient conditions when exposed to natural sunlight.
Titania nanotubes have also been used as supports for nanostructured photocatalysts, 96 with enhanced properties when doped with elements such as nitrogen, 97 metal cocatalysts, 98 or porphyrin light sensitizers. Careful control of catalyst particle size can also enhance the catalytic activity of the material through a plasmon resonance effect. Increased energy absorption at narrow wavelength ranges is achieved with a proper particle size and shape of the catalyst nanoparticles; this depends on the surrounding media.
Ethanol steam reforming over gold nanoparticles as a result of the incidence of light. Left: Scheme of the vapor formed by the localized heating of the nanoparticles on the microfluidic channel. Right: Image of bubble formation when a laser is focused in the microchannel. The first concept to keep in mind when designing reactors for CO 2 transformation with MWs is that MW heating is not ruled by the transport of thermal energy, but by heat generation. Therefore, reactions taking place owing to MW heating cannot be studied in the same way as that of conventional reactions.
Usually, chemical reactions are driven by heat transfer through a solid wall or by the convection of a fluid. Under MWs, heating occurs first inside the sample and then it is transmitted to the rest of the system. Influence of mixing and location on temperature measurements. The graph shows how the temperatures at different locations are similar when intense agitation is present.
One relevant aspect is the kind of thermometer used in the experiments. Metallic probes cannot be used due to their interaction with the MWs. Commercial apparatus usually installs IR detectors for external measurement of temperature, but, as already revealed, this is not advisable.
Several reactor configurations have been proposed to study chemical reactions under MWs. This allowed close control of the reaction evolution and, at the same time, diminished the effects of transport phenomena. More recently, traveling electromagnetic waves have been proposed to overcome the interference phenomena and nonuniformities of resonant fields to allow precise control and optimization of the MW field. To realize the latter, the plasma should be combined with a catalyst, 42 , 55 , 81 as mentioned above, because the plasma itself is too reactive an environment, and thus, produces a wealth of reactive species, which easily recombine to form new molecules, without any selectivity.
Plasma catalysis is mostly performed in a DBD reactor. In plasma catalytic reactors, the most common way to introduce the catalyst is by means of a packed bed, but other catalyst structures include coatings, powder, and monoliths. The integration of plasmas and catalysts allows an increase in the selectivity of the chemical reactions, while keeping the operating conditions at relatively low temperatures; this permits an increase in efficiency and diminishes the erosion and deactivation of both catalyst and electrodes.
Nevertheless, it can also result in a decay of the catalytic action due to erosion and transformation of the active sites. Thus, through the study of the dynamic evolution of the active species, it could be possible to obtain the residence times necessary for them to reach the solid.
The increase of greenhouse gases in the atmosphere and the decrease of the available amount of fossil fuels necessitate finding new alternative and sustainable energy sources in the near future. This book summarizes the role and the possibilities of catalysis in the production of. Catalysis for Alternative Energy Generation. Chapter · March with 1, Reads. DOI: /_7. Cite this publication.