The size of the particles gives rise to novel physical properties including modified diffusional behaviour and zeta potential, quantum photo and electronic effects, and quantum tunnelling that can be exploited in catalysed processes.
Examples using palladium and rhodium nanocatalysts were given, including a chiral diphosphate-modified rhodium 0 nanoparticle catalyst for use in styrene hydroformylation 6. In the last decade they have contributed to the industrial application of three catalyst systems.
It has seldom been possible to achieve this, as cyanide sources are usually highly toxic, with the associated negative impact on the environment and implications for health and safety. Cyanides are also efficient catalyst poisons. Professor Beller described the advantages of K 4 [Fe CN 6 ], a complex characterised by low toxicity and low manufacturing cost: properties that make it a convenient choice for palladium- and copper-catalysed processes even on an industrial scale.
Paul Murray AstraZeneca, UK described the use of principal component analysis PCA and experimental design to optimise palladium-catalysed coupling reactions, including a redox neutral coupling reaction, a Buchwald-Hartwig reaction and a Suzuki reaction. The optimisation of a catalysed process is complicated by the number of variables at play including, for the type of reactions being studied, the choice of catalyst precursor, phosphine complex, amines, solvent, and other reaction conditions. The descriptors are used in a PCA that is ultimately a variable reduction procedure.
By reducing the redundancy in the data set, PCA can guide the chemist to the most efficient way to explore the chemical space.
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Michael Krische University of Texas at Austin, USA focused his lecture on the formation of C—C bonds via hydrogenation and transfer hydrogenation, stressing the fact that these transformations provide more atom-economic processes than the classic C—C bond formation reactions such as aldol condensation.
Importantly, primary alcohols can be used in place of aldehydes in these condensations, by exploiting the fact that the iridium catalyst is able to convert them to the aldehyde in situ This is an advantage considering that aldehydes are usually susceptible to oxidation and polymerisation.
Cole-Hamilton's group has developed a bidentate phosphine ligand bis- di- tert -butylphosphinomethyl benzene, 3 15 which can be used with [Pd 2 dba 3 ] in palladium-catalysed carbonylations. Simon Beaumont University of Cambridge, UK reported on the heterogeneously catalysed asymmetric hydrogenation of a carbon—carbon double bond, Figure 1 The chiral induction was obtained by linking chiral pyrrolidine-based sulfide ligands to the palladium nanoparticles.
All these factors contribute in different ways to the enhancement of energy efficiency of the plasma process as well as the catalyst stability, due to a synergy that occurs between the catalyst and the plasma [ 58 ]. This novel technique combines the advantages of high product selectivity from thermal catalysis and the fast startup from plasma technique.
Plasma catalysis has been widely investigated for many applications. Figure 1 briefly summarizes the main application areas of plasma catalysis. In the domain of energy applications, the use of plasma catalysis for dry reforming, CO 2 reduction, hydrogen production, methanation and ammonia NH 3 synthesis has been intensively studied. In this chapter, however, we focus only on their application for CO 2 conversion into value-added chemicals and fuels.
As mentioned in Introduction, nonthermal plasma shows a great potential for an efficient CO 2 utilization. Different routes for CO 2 conversion have been investigated using plasma-catalytic process.
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Table 2 summarizes some of the main reactions usually considered in plasma chemistry for CO 2 reduction using different pathways such as dry reforming of methane, hydrogenation of CO 2. Significant attention has been given to plasma-catalytic dry reforming of methane DRM using supported Ni catalysts. However, most of these studies focused primarily on identifying plasma-catalytic chemical reactions to maximize process performance. Optical emission spectroscopy and plasma chemical kinetic modeling should be used to achieve a better understanding on the formation of a wide range of reactive species in this plasma-catalytic reforming process.
As explained by Fridman [ 3 ], cumulative vibrational excitations of the CO 2 molecule can result in a highly energy-efficient stepwise dissociation. Thus, CO 2 splitting using nonthermal plasmas has been considered as another promising pathway to produce synthetic fuels via CO, as an intermediate product. As well-accepted in the literature, dissociation of a CO 2 molecule in plasma is represented by the following global reaction [ 3 ]:.
The main pathways for decomposition of CO 2 molecule include the electron impact dissociation:. In addition to this, the vibrationally excited CO 2 molecules may also undergo decomposition via the collisions with atomic O:. Traditionally, to characterize the process efficiency, two main parameters reflecting the conversion efficiency and energy efficiency are used.
Here the specific energy input SEI per molecule is given by the ratio of the discharge power P to the gas flow rate F through the discharge volume. When catalysts are combined with plasmas, they can be classified into three systems, i. These three configurations are illustrated in Figure 2. In all cases, the plasma can be used to supply energy for catalyst activation, and it can also provide the reactive gas species needed for reactions on the catalyst surface.
The single-stage type is constructed by coating catalyst on the surface of electrode s or packing catalyst within the plasma zone, which is also called in-plasma catalysis IPC. The catalysts could completely or just partially overlap with the plasma zone. In this manner, the plasma and catalysis could directly interact with each other.
This single-stage system is also easy to combine with the UV irradiation, which is known as plasma photo catalysis, as shown in Figure 2. For the two-stage type, the catalyst is placed after the plasma discharge region; it is also called post-plasma catalysis PPC. The plasma provides chemically reactive species for catalysis or pre-converts reactants into the easier-to-convert products to accelerate the catalysis.
In the nonthermal plasma catalysis system, the long-lived reactive species produced by plasma, e. The multistage plasma catalysis system is a promising option for the industrial use in the future. Different functions of the catalysts can be combined to achieve certain expected reaction in the multistage system. Schematic diagram of different plasma-catalyst configurations according to the catalyst bed position and number. In the context of plasma catalysis, the synergy is referring to a surplus effect of combining the plasma with catalyst, namely, when the resulting effect has a higher impact than the sum of their individual impacts.
In several studies, the combination of plasma and catalysts has been found to have synergistic effects [ 34 , 35 , 61 , 62 ]. A highly important synergistic effect of plasma catalysis is promotion of catalyst activity at reduced temperatures, and hence, a significant reduction in the energy cost for activating the catalyst [ 34 ]. Typical synergistic effect factors of 1.
The effect was observed on the conversions of CH 4 and CO 2 , where the result for the plasma-catalytic reaction was greater than the sum of the catalyst-only or plasma-only results. The selectivity towards H 2 and CO production was also enhanced by the use of plasma catalysis. In general, the enhanced performance of plasma catalysis can in part be attributed to vibrational excitation of CO 2 in the plasma, which enables easier dissociation at low temperature on the catalyst surface.
The plasma electrons in turn affect the catalyst properties chemical composition or catalytic structure. Synergistic effects in the plasma and catalyst are illustrated in Figure 3. Plasma can alter the physicochemical characteristics of catalyst via several routes, which are induced mainly by energetic electron generation.
In the meantime, a catalyst can induce electric field concentration due to its pore structure and dielectric properties. Hence, both electric field distribution and catalyst characteristics are modified to have better DRM performance. Nonthermal plasma technology provides an attractive alternative to the other classical technologies for converting inert carbon emissions. Different types of plasmas have already been used for CO 2 reduction, including dielectric barrier discharges DBDs , glow discharges, radio frequency RF discharges, microwave MW discharges and gliding arc plasma GAP and corona discharges [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 ].
In this section, the most widely used discharges for CO 2 conversion are presented. DBDs have been known for more than a century. The DBD is the most widely used discharge type for CO 2 conversion among the variety of other plasma sources because it is easy to handle with relatively cheap equipment and it operates at atmospheric pressure [ 34 ]. Even though the conversion efficiencies obtained in DBDs are generally quite low [ 1 , 4 , 5 , 8 , 9 , 12 , 13 ], the possibility to work at atmospheric pressure under non-equilibrium conditions is still a very strong advantage of these discharges.
Combined with plasma catalysis, these discharges should also improve the selective production of the targeted compounds. An atmospheric pressure GAP discharge can be formed between two flat knife-shaped electrodes with a gas flowing between them. The gliding arc plasma can be operated in the thermal and nonthermal regime depending on the applied power and flow rate. Furthermore, the arc can be operated in the transition regime, which is an evolving arc starting in the thermal regime going to the nonthermal regime.
This transition regime makes the discharge energy efficient for gas treatment. The high level of efficiency can be attributed to non-equilibrium vibrational excitation of CO 2 and a high-temperature gradient between the gliding arc and the surrounding gas that results in fast quenching. Plasmas generated by the injection of microwave power, i. MW discharges are commonly generated using frequencies of 2.
They can be operated over a wide pressure range from few mTorr to the atmospheric pressure. The properties of the MW discharges operating at atmospheric pressure are close to those of thermal plasma. However, the MW discharges are far from thermodynamic equilibrium at low pressure. The performance of a microwave discharge in terms of efficiency of CO 2 dissociation process depends heavily on the plasma parameters such as power and operating pressure.
The ability to create a strong non-equilibrium environment in microwave discharges possesses highly vibrational states of CO 2 molecules, which are energy-efficient for CO 2 decomposition [ 3 ]. In general, the high efficiency of microwave plasmas is attained due to the high absorption of the applied power by electrons as well as relatively high excitation of the CO 2 asymmetric mode [ 24 ], which plays a key role for CO 2 decomposition [ 22 ]. This plot is calculated based on the cross sections of the corresponding electron impact reactions [ 1 , 37 , 66 ].
In microwave plasma, the reduced electric field is typically around 50 Td, which is most appropriate for the vibrational excitation of CO 2. Moreover, the purple curve in Figure 4 has its particular importance as it represents the first vibrational level of the asymmetric vibrational mode of CO 2 , which represents the most important channel for the dissociation [ 66 ].
The energy efficiency for the dissociation of CO 2 is quite limited in a DBD plasma [ 3 , 4 , 11 , 12 , 13 ]. As a result of previous studies on CO 2 decomposition in plasma, it was concluded that higher pressures and lower values of reduced electric field make the vibrational excitation mechanism more favorable than the electronic excitation mechanism, explaining the higher energy efficiency of these types of discharges e.
In this chapter we have summarized the results from the recent publications on plasma set-ups with and without combining a catalyst for CO 2 conversion in Table 2 and discussed the current research status on this topic. The best combination of conversion Their simulation results suggest that the increased CO 2 conversion is caused by the presence of strong electric fields and thus high electron energies at the contact points, which thereby lowers the breakdown voltage.