Kinetics of reduction of a RuO2 film on Ru by atomic hydrogen. Microelectronic Engineering , , Scientific Reports , 2 1 DOI: Hess, P. Krause, S. Rohrlack, J. Farkas, H. One-dimensional confinement in heterogeneous catalysis: Trapped oxygen on RuO2 model catalysts. Surface Science , , LL Hangyao Wang, William F.
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Comparative chemistries of CO and NO oxidation over RuO 2 : insights from first-principles thermodynamics and kinetics. Molecular Simulation , 38 , Beatriz Roldan Cuenya. Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects. Thin Solid Films , 12 , Rules for selectivity in oxidation processes on RuO2 Physical Chemistry Chemical Physics , 12 38 , Nature and role of surface carbonates and bicarbonates in CO oxidation over RuO2.
Physical Chemistry Chemical Physics , 12 24 , Molecular origins of surface poisoning during CO oxidation over RuO2 Surface Science , 16 , LL Ari P. Seitsonen, Herbert Over. Intimate interplay of theory and experiments in model catalysis. Jorge J. European Journal of Inorganic Chemistry , 5 , Heterogeneous oxidation catalysis on ruthenium: bridging the pressure and materials gaps and beyond.
Journal of Physics: Condensed Matter , 20 18 , Over, Y. He, A. Farkas, G. Mellau, C. Korte, M. Knapp, M. Chandhok, M. Long-term stability of Ru-based protection layers in extreme ultraviolet lithography: A surface science approach.
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Theodore E. Madey, Nadir S. Faradzhev, Boris V. Yakshinskiy, N. Surface phenomena related to mirror degradation in extreme ultraviolet EUV lithography. Applied Surface Science , 4 , Ping Liu, James T. Muckerman, Radoslav R. Adsorption of platinum on the stoichiometric RuO2 surface. The Journal of Chemical Physics , 14 , Adsorption of platinum on the stoichiometric RuO[sub 2] surface.
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Or, is it hydrophobic? We show that it is not at all hydrophobic, but largely catalytic. Inside the interior environment of C 70 the H 2 O becomes significantly more basic compared to its isolated counterpart. We examined the role played by the cage in facilitating this process and also address the question of whether the resulting species formed between H 2 O and HX is unaffected by or innocent of the cage environment.
If the guest s is are not innocent, does the intermolecular interaction of the guest with the cage interior modify their fundamental geometric, electronic and vibration properties? What are the vibrational bands that should be taken into account for rationalizing whether the guest molecule is inert or electroactive inside the cage?
Are these vibrational bands affected by the dipolar screening effect of the cage, thereby preventing their experimental observation? Is the proton transfer feature associated with the dimers in C 70 comparable to what might be inferred from the geometries of the same isolated dimers in the first excited state, or in their anionic ground states? We answer these questions below. The smaller red spheres at the center of five-membered rings of C 70 represent the centroids. There are significant differences between the intermolecular geometries of these two systems.
Proton-Transfer Reactions Involve Electron-Pair Transfer
This is in agreement with experiment Fig. Their encapsulation inside the C 70 cage, however, caused the bonds in HCl and HBr to stretch sufficiently such that the resulting geometries Fig. The relative change in the free energies associated with the geometric reorganization of these three dimers is 2. The stretching of HCl and HBr bonds in C 70 is presumably driven by dissociative electron density transfer from H to the Cl and Br atoms, respectively. This is the likely scenario inside the cage as the HX bond breaking in the presence of the H 2 O molecule is due to the encapsulation efficiency of the C 70 cage.
The proton transfer from the acids HX was made possible because the cage substantially minimized the potential barrier between HX and H 2 O Fig.
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The proton transfer is essentially a one-dimensional process in these systems. Ma et al. This parameter is positive when the H atom is bound to the halogen atom, and negative when the H atom is detached and moves away For instance, the HCl and HBr bond distances in the monomers are 1. Its encapsulation inside the C 70 markedly increases these component energies by 1. Such large values are expected given that complex formation is predominantly electrostatically driven, with a significant transfer of charge between the interacting partners.
Binding energies of ion-pairs of similar magnitudes have been reported in many recent studies 34 , 35 , 36 , 37 , 38 , 39 , 40 , The charge transfer for these dimers is 0. Depending on the charge polarity of a particular atom in the guest, a specific portion of the cage is polarized with an opposite sign so as to facilitate a Coulombic attraction between them. Because of this, the charge-transfer increases significantly with cage residence, suggesting a further transfer of charge between the monomers inside the host.
This result suggests that all the three individual dimers in C 70 are Mulliken inner type complexes 34 , However, this is not so when these dimers are enclosed inside C Previous studies have shown that anionic C 60 -mediated reduction reactions involving H 2 O are very promising for proton transfer processes 46 ; similar results have been reported for non-fullerene based compounds as well 47 , It was therefore expected that the polarity of the ion-pair would increase significantly. The anomaly could thus be attributed to the effect of the cage interior that effectively screens the dipolar electric field of an entrapped species irrespective of whether this is an ion-pair, a simple hydrogen bonded dimer, or simply an H 2 O molecule.
These are significantly smaller A fundamental question in endohedral systems is whether the guest species is innocent inside the cage interior, or whether it experiences significant intermolecular interactions We examined the bond critical point bcp and bond path topologies of potential bonding interactions Fig. Each hydrogen atom in H 2 O is involved in a single hydrogen bonding interaction with carbon, whereas each halogen atom is involved in a cone of interaction with the neighboring 5-membered aromatic ring of the C 70 cage interior; these are revealed by the bond path and bond critical point topologies of the charge density.
Bond paths are shown in dotted and solid lines in atom color, and bond critical points as tiny spheres in green. The intermolecular interaction between the host and the guest for each system is elucidated by the bond path topologies shown as solid and dotted lines in atom color. In each of these systems, the halogen is involved in a five-center, six bond topology, forming a cone of intermolecular interactions reminiscent of that found for cyclopentadienyl and substituted dienyl bonding to titanium Such a topology is of significance in the understanding of non-classical metal-to-saturated-carbon atom interactions.
This is true not only for the isolated guest molecules viz. There are several spikes in each of these plots. The first represents attraction between the host and guest species and the second corresponds to repulsion in the RDG picture. RDG 2D plots 0. Two distinct features are reminiscence of these plots. RDG plots 0. This innovative approach automates the identification of intermolecular interactions using actual and pro-molecular electron densities.
As can be seen from a -c and d -f , the isosurfaces blue, green and mixed color are centered around the bond critical point regions in the 3D maps of all the isolated and endohedral systems, signifying stabilizing interactions between interacting basins. However, the isosurface plots suggest that attraction between the guest and the fullerene C 70 cage is significantly dispersed, and is in excellent agreement with the QTAIM description of bond path topologies that suggest many-fold interactions between the host and the guest in each of the endohedral systems.
However, our geometry optimization of the endohedral system with the dimer along the minor axis forced the dimer to rotate back to a configuration illustrated in Fig. This also explains why the positional exchange of the H 2 O and HF was not detected experimentally at room temperature From the Tauc plot, the optical energy gap of the C 70 thin film was reported to be 1. This change is predominantly due to the energy of the LUMO level that becomes more negative This geometric modification increases the overlap between molecular states and reduces the gap between the frontier orbitals.
However, this does not lead to a change in the nature of both these frontier orbitals, as evidenced in the density of states DOS spectra Fig. In other words, our analysis of the atom-projected DOS spectra suggests that the encapsulated species does not contribute to the frontier orbitals see Fig. That the guest species does not contribute to the frontier orbitals is analogous with the organic-inorganic methylammonium lead triiodide MAPbI 3 semiconductor.
In this, the organic cation MA, entrapped inside the inorganic cage, does not contribute either to the conduction band minimum CBM or to the valence band maximum VBM that are built from significant contributions from s and p orbitals of the Pb and I atoms. As is seen in the simulated IR spectra shown in Fig. Important vibrational modes are marked by arrows.
The H 2 O species in these three endohedral systems offer very interesting IR characteristics. According to the literature 65 , the intensities of the corresponding vibrational modes should increase. However, we found them to decrease significantly. Because of this apparent anomaly in the intensity profile, analytical second derivative calculations were performed on the endohedral geometries of all the three guest dimers by removing the C 70 cage. Positive frequencies were found in all cases, indicative of these being in meta-stable states. The simulated IR spectra are shown in Fig.
Although the frequency centers in the simulated spectra were somewhat shifted because of the removal of the cage, the intensities of most of the vibrational bands were found to be markedly larger than those shown in Fig. This provides unequivocal proof that the reduction in the band intensity is a consequence of the profound electrostatic shielding effect of the C 70 cage. It explains why in many previous experimental studies the stretching and bending vibrational bands of HF and H 2 O, as well as those associated with the intermolecular interactions of the encaged species 6 , were not detected.
Our calculations suggest that there are three such reasonably strong vibrations in isolated C In the simulated complex geometries Fig. The centroid-to-centroid distance between the two five-membered rings of the C 70 cage placed at the opposite extremes of the major axis Fig. This shows that there is a marginal elongation of the major axis of the cage due to the enclosed dimers. This is accompanied by a contraction along the minor axis so that overall the volume of the cage decreases by The 0.
When the 0. Our calculations show that the reduction in the 0. It is calculated to be This suggests a decrease in polarizability i.
It is a result of the prevailing pressure of carbon cage that compresses the guest, resulting in a decrease in the total polarizability This suggests a marginal decrease in the dielectric constant of the encapsulated species. Previous studies demonstrate that provided the dipole cannot reorient fast enough in composites, this causes the dielectric constant to decrease Others propose that the low dielectric constant of organic photovoltaics assists the exciton to present at larger distances At the same time, we have demonstrated using geometrical, electron density and vibrational characteristics that the C 70 cage interior serves as a super-catalyst for HCl and HBr bond dissociation, enhancing the acidity of these acids by bond cleavage and assisting in complete proton transfers to H 2 O.
The C 70 cage interior is not hydrophobic and the guest species are not inert, as often contended 6 , 7 , 11 , This observation is in line with others in which the effective pair interaction is not hydrophobic, yet the solvation properties are; hence fullerenes serve as an example in which hydrophobic interaction cannot be deducted from hydrophobic solvation Because the C 70 cage interior is polarizable, its effect plays a vital role that is largely responsible for the development of many anomalous features that were undetectable experimentally in previous studies e.
Because of its nature as stated in 2 and 3, C 70 prefers to serve as a cationic host upon encapsulation of two guest molecules. It has the potential to provide a terrace to the guest species for facilitating efficient proton transfer reactions between them. The C 70 cage interior has the ability to screen the electrostatic dipolar field of the guest species, hence limiting the observability of many vibrational bands that are IR active. This explains why experiments often fail to show the IR spectra of the entrapped species inside the host. Charge rearrangement, bond polarization and ion-pair formation are likely consequences of an accommodation of a host species, especially for dimer molecules.
The C 70 cage interior provides an elegant and innovative terrace to electrogenerate reactive species between H 2 O and HX. The mechanistic details involved in this and other similar systems will surely uncover the novel physical chemistry and catalytic detail of these materials. The Cartesian coordinates of these systems were used for energy-minimizations using the Gaussian 09 26 package. For reasons discussed in the Results and Discussion section, the PBE functional implanted in Gaussian 09, together with the G d,p basis set, was used.
S1 and the Results and Discussion section. The same method was used for calculations to obtain the eigenvalues of the Hessian second derivative matrix; in all cases, positive eigenvalues were obtained. Several factors influence the process and the most important are: residence time, temperature and the type of pyrolysis agent. When the residence time and temperature increase, the composition of the obtained product shifts to more thermodynamically stable compounds [ 2 , 8 , 20 , 32 ].
The pyrolysis products can be used as an alternative fuel or as a source of chemicals [ 30 ].
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The composition of the product also depends on the presence of catalysts including concentrations and types. Higher temperatures decrease the yield of hydrogen, methane, acetylene and aromatic compounds, whereas lower temperatures favor the generation of gas products [ 32 ].
Previous experiments to evaluate the polymer degradation process are important because they provide information on the feasibility of recycling these polymers raw materials and even fuels. However, most studies are focused on pyrolysis of pure polymers and unmixed [ 7 ]. During the pyrolysis, the polymer materials are heated to high temperatures and thus, their macromolecules are broken into smaller molecules, resulting in the formation of a wide range hydrocarbons.
The products obtained from the pyrolysis can be divided into non-condensable gas fraction, liquid fraction consisting of paraffins, olefins, naphthenes and aromatics and solid waste. From the liquid fraction can be recovered hydrocarbons in the gasoline range C4-C12 , diesel CC23 , kerosene CC18 and motor oil CC40 [ 1 , 3 , 18 , 20 , 33 - 35 ]. The thermal cracking usually produces a mixture of low value hydrocarbons having a wide variety of products, including hydrogen to coke. In general, when the pyrolysis temperature is high, there is increased production of non-condensable gaseous fraction and a lower liquid fuels such as diesel.
The yield and composition of the products obtained are not controlled only by the temperature but also the duration of the reaction [ 33 ]. The thermal pyrolysis proceeds according to the radical chain reactions with hydrogen transfer steps and the gradual breakdown of the main chain. This mechanism provides many oligomers by hydrogen transfer from the tertiary carbon atom along the polymer chain to the radical site [ 18 ]. This is due to high content of tertiary carbons of PP. The initiation step comprises homolytic breaking of carbon-carbon bond, either by random chain scission as by cleavage at the end of the chain, resulting in two radicals [ 36 , 37 ].
For PP and PE the chain scission occurs at random [ 37 ]. The termination reactions can occur, for example, by disproportionation, which can produce different olefins and alkanes or a combination of radicals can lead to the same products. Branched products can be formed from the interaction between two secondary radicals or between a secondary radical with a primary [ 36 , 37 ]. As a consequence of these mechanisms, the thermal pyrolysis leads to a wide distribution of hydrocarbon, a C5-C80 range, each fraction being mainly composed of diene, 1-olefin and n-paraffin.
At high temperatures hydrogen is formed in significant amounts. Products obtained by thermal cracking are of limited commercial value, especially being applied as fuel. For heavy oils, it has been proposed its use as a wax [ 36 ]. These factors severely limit its applicability and increase the cost of recycling raw material of plastic waste [ 23 ].
The thermal pyrolysis requires high temperatures due to the low thermal conductivity of polymers [ 20 ] , which is not very selective and a possible solution to reduce these reaction conditions is the use of catalyzed pyrolysis. Catalytic pyrolysis is an alternative to the recycling of pure or mixed plastics waste [ 30 ]. The catalyst can promote:. Homogeneous and heterogeneous catalyst systems have been employed in the cracking polymers.
In general, heterogeneous catalysts have been more used due to the ease of their separation and recovery of the reaction [ 36 , 39 ]. The homogeneous catalysts especially used are Lewis acids, as AlCl 3 , fused metal tetracloroaluminatos M AlCl 4 n , where the metal may be lithium, sodium, potassium, magnesium, calcium or barium and n can be 1 or 2 [ 36 ]. A wide variety of heterogeneous catalysts has been used and among them are: conventional solid acids such as zeolites, silica-alumina, alumina and FCC catalysts Fluid Catalytic Cracking , mesostructured catalysts such as MCM etc.
Many studies have been carried out describing the cracking of pure polyolefins over various solid acids such as zeolites, clays, among others. The use of zeolites has been shown to be effective in improving the quality of products obtained in the pyrolysis of polyethylene and other addition polymers. That is, these features allow milder operating conditions lower temperatures and reaction times than a thermal pyrolysis [ 4 , 25 , 30 , 45 ]. Differences in the catalytic activity of these solids are related to their acidic properties, especially the strength and number of acidic sites.
The properties of these solid structures, as the specific area, particle size and pore size distribution, also have a crucial role in their performance, they control accessibility of voluminous molecules of the polyolefin internal catalytically active sites. While most work on catalytic cracking of polymers has been performed with pure polymers, it is accepted that the decomposition process can be affected by the presence of contaminants as well as chemical changes that occur in the polymer structure during use [ 11 , 20 , 21 , 34 , 42 , 46 ]. As mentioned, the catalyst pore size and acidity are important factors in the catalytic cracking of polymers [ 40 , 43 , 47 ].
Generally, the level of catalytic activity in the polyolefin pyrolysis increases with increasing the number of acidic sites. Thus, it is known that zeolite catalysts achieve higher conversions acids non-zeolitic catalysts [ 42 ]. This determines the products obtained in these reactions. Solid acid catalysts such as zeolites, favor hydrogen transfer reactions due to the presence of many acid sites [ 11 , 18 , 22 , 36 , 42 , 44 ]. In the case of crystalline solid acids, it is believed that most of the acid sites are located inside the pores of the material, as in the case of zeolites [ 11 , 42 ].
Cracking is processed either by random chain scission medium or weak acidity , for scission at the end of the chain strong acidity to give waxes and distillates gasoil, gasoline or light hydrocarbons C3-C5 olefins , respectively. These primary cracking products may be removed from the reaction medium or subjected to secondary reactions such as oligomerization, cyclization and aromatization.
The relative extent of these reactions is connected to the acidity and properties of catalyst, but also to experimental variables employed such as reactor type, temperature, residence time, etc. Catalysts having acidic sites on the surface and with the possibility of donating hydrogen ion increase rate of the isomerization products and increase the yield of hydrocarbon isomers and the quality of the fuel formed.
Catalysts containing strong acid sites, higher density, are more effective in cracking polyolefins. However, the strong acidity and high pore size cause rapid deactivation of the catalyst. Thus, according to literature, it is preferable to carry out the pyrolysis of polyolefins in the presence of a catalyst with light acidity and long life [ 33 ]. Other types of catalysts which may be used in the pyrolysis process are catalysts with Lewis acid sites which are electron pair acceptors. These catalysts may be dissolved in molten polymer, which substantially increases the cracking efficiency while reducing its consumption.
These types of catalysts have acidic sites on their surfaces that change the charge distribution in the carbon chain, making them capable of abstracting hydride ions of hydrocarbons to produce carbonium ions. This increases the catalytic effect, enabling a reduction in pyrolysis temperature and promoting the generation of ions for olefinic and aromatic compounds [ 32 ]. However, the cost of the catalyst can greatly affect the economy of the process, even if it shows a good performance. To reduce this cost and make it even more attractive process, you can reuse the catalyst or use it in smaller quantities [ 23 , 42 , 48 ].
The biggest problem in the use of catalysts in the pyrolysis of plastics is that coke formation deactivates the catalyst over time, thereby decreasing its life cycle [ 33 ]. Seo et al. As shown in Table 1 , the pyrolysis performed with the zeolite ZSM-5 had higher yield of the gaseous fraction and smaller liquid fraction when compared with thermal cracking.
Hydrogen Transfer Reactions - Reductions and Beyond by Guillena and Ramon
According to the authors, this is explained by the properties of the catalyst. Most zeolites, including ZSM-5, showed excellent catalytic efficiency in cracking, isomerization and aromatization due to its strong acidic property and its microporous crystalline structure. The ZSM-5 zeolite has a three-dimensional pore channel structure with pore size of 5.
Thus, initially degraded material on the external surface of the catalyst can be dispersed in the smaller internal cavities of the catalyst thus decomposed gaseous hydrocarbons molecules with smaller sizes. Marcilla et al. As can be seen, the condensable products were the major fraction for the thermal process and no solid fraction coke was detected. For the catalytic process an increase of the gas fraction, and this is due to the HZSM-5 catalyst present, which has strong and weak acid sites and an average pore size small. As mentioned above, this facilitates cracking leading to compounds with small sizes gas fraction.
The results for the batch reactor are similar. However, there are studies where the values for each product obtained are different. This is because in this type of reactor the heat transfer is not as favored and, consequently, other factors such as the size and quantity of the sample or the carrier gas flow can determine the type of product formed.
Moreover, in such reactors the extent of secondary reactions is smaller than the fluidized bed reactor.
Using fixed beds where polymer and catalyst are contacted directly leads to problems of blockage and difficulty in obtaining intimate contact over the whole reactor. Without effective contact the formation of large amounts of residue are likely, and scale-up to industrial scale is not feasible [ 15 ]. The low thermal conductivity and high viscosity of the plastic may lead to a difficulty in mass transfer and heat. These factors influence the distribution of products, in conjuction with the operating conditions [ 50 ]. Currently it is known the existence of minerals which have all essential requirements to be classified as zeolites, however, instead of aluminum Al and silicon Si occupying the tetrahedral positions are present elements such as phosphorus P , beryllium Be , among others [ 51 , 52 ].
They are composed of tetrahedra of SiO 4 , AlO 4 and PO 4 as primary structural units, which are linked through oxygen atoms. Each oxygen atom is shared by two silicon or aluminum atoms, thus giving rise to a three-dimensional microporous structure [ 46 , 53 ]. The combination of these two primary structures is found in the common zeolites, developing cavities of various shapes and sizes which are interconnected [ 42 , 51 , 53 , 54 ].
When these cations are exchanged for protons, zeolite acid sites are formed. This exchange allows modification of the original properties of zeolites. These channels and cavities are occupied by ions, water molecules or other adsorbates which, due to high mobility, allow the ion exchange [ 51 , 53 ]. The pore size corresponding to two-dimensional opening zeolite is determined by the number of tetrahedral atoms connected in sequence. The three-dimensional interactions lead to the most different geometries, forming from large internal cavities to a series of channels crossing the whole zeolite [ 55 ].
The pores of zeolites function as molecular sieves, blocking the free diffusion of large, bulky molecules inside the internal surface of the catalyst [ 41 , 54 ]. These molecular sieves combine high acidity with selectivity form. The reactivity and the selectivity of zeolites as catalysts are determined by its high number of active sites, which are caused by an imbalance of charge between the silicon and aluminum atoms in the crystal, making the zeolite of the structural unit has a charge balance total least one [ 42 , 51 ].
However, the process of rupture of the polymer molecules starts on the external surface of the zeolites, since the polymer chains must be broken before penetrating the internal pores of the zeolites, due to its small pore size. The zeolites have a specific pore size and the access of polymer molecules to internal reactive sites of the catalyst, as well as the final products within the pores are limited by their size.
Generally, the level of catalytic activity in the pyrolysis of polyolefins increases with increasing the number of acidic sites. Thus, it is known that zeolite catalysts achieve higher conversions than non-zeolitic catalysts acids [ 42 ]. In addition, branching of the polymer or end chain of polyethylene can penetrate the pores of the zeolites, reacting the acid sites located there and so increasing the activity [ 34 ].
During the catalyzed pyrolysis, the polymer melts and is dispersed around the catalyst. The molten polymer is drawn into the spaces between the particles and therefore the active sites on the external surface of the catalyst. Reactions at the surface produce a low molecular weight materials, which are sufficiently volatile at the temperature of the reaction can diffuse through the polymer film as a product or may react even more in the pores.
These reactions proceed via carbocation as transition state. The reaction rate is governed both by the nature of the carbocation formed as the nature and strength of the acid sites involved in catalysis. Regardless of how the carbocation is formed, it may be subjected to any of the following methods: load isomerization, the isomerization chain, hydride transfer, transfer of alkyl groups and formation and breaking of carbon-carbon bonds.
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