The effect of heteroatoms in carbonaceous surfaces: computational analysis of H chemisorption on to a PANH and Si-doped PAH

2019 ◽  
Vol 490 (1) ◽  
pp. 172-180 ◽  
Author(s):  
César Barrales-Martínez ◽  
Soledad Gutiérrez-Oliva
Keyword(s):  
Activation Energy ◽  
Computational Analysis ◽  
Rate Coefficient ◽  
Internal Position ◽  
Lower Activation ◽  
Polycyclic Aromatic ◽  
External Site ◽  
Si Doped ◽  
Photodissociation Regions ◽  
H2 Formation

ABSTRACT In this work, we studied the effect of a heteroatom (nitrogen and silicon) inside the main skeleton of the carbonaceous surface in the H chemisorption reaction. The process taking place on to an N-doped polycyclic aromatic hydrocarbon (PAH), known as PANHs, shows differences in the energetic parameters only when the process is carried out on to the N atom. When N is located in an external site of the surface, the process is barrierless, whereas if N is in an internal position of the surface the activation energy drastically increases. The aromaticity of these N-doped systems does not change much concerning pristine coronene. In a Si-doped PAHs, the chemisorption on to the Si atom takes place in the absence of activation energy, regardless the position of Si on the surface. Moreover, the adsorption on to their neighbour carbon atoms is carried out with lower activation energies than those found in the reaction on to pristine PAH, indicating that the presence of silicon atoms in the surface favours H chemisorption. This might be due to a loss of aromaticity on the surface. In both cases, the reactions become significantly more exoenergetic. Finally, the presence of heteroatoms favours kinetically the reaction, where the rate coefficient of H2 formation process, calculated considering all of the sites of every PAH studied in this work, reaches a close value to the reported for diffuse interstellar medium and photodissociation regions ($R_{_{\mathrm{ H}\mathrm{ }_2}} = 1 \times 10^{-17}$ cm3 s−1 at 40 K).

scholarly journals Development of a PtSn bimetallic catalyst for direct fuel cells using bio-butanol fuel

2015 ◽  
Vol 51 (69) ◽  
pp. 13412-13415 ◽  
Author(s):  
V. K. Puthiyapura ◽  
D. J. L. Brett ◽  
A. E. Russell ◽  
W. F. Lin ◽  
C. Hardacre
Keyword(s):  
Activation Energy ◽  
Fuel Cells ◽  
Bimetallic Catalyst ◽  
Lower Activation ◽  
Lower Activation Energy

PtSn showed a higher activity and lower activation energy towards butanol electrooxidation compared to pure Pt.

scholarly journals Photoinduced polycyclic aromatic hydrocarbon dehydrogenation The competition between H- and H2-loss

2018 ◽  
Vol 616 ◽  
pp. A166 ◽  
Author(s):  
P. Castellanos ◽  
A. Candian ◽  
J. Zhen ◽  
H. Linnartz ◽  
A. G. G. M. Tielens
Keyword(s):  
Density Functional ◽  
Carbon Budget ◽  
Multidisciplinary Approach ◽  
Fragmentation Channel ◽  
Functional Theory ◽  
Medium Carbon ◽  
Polycyclic Aromatic ◽  
Atom Loss ◽  
Photodissociation Regions ◽  
Sequential Fragmentation

Polycyclic aromatic hydrocarbons (PAHs) constitute a major component of the interstellar medium carbon budget, locking up to 10–20% of the elemental carbon. Sequential fragmentation induced by energetic photons leads to the formation of new species, including fullerenes. However, the exact chemical routes involved in this process remain largely unexplored. In this work, we focus on the first photofragmentation steps, which involve the dehydrogenation of these molecules. For this, we consider a multidisciplinary approach, taking into account the results from experiments, density functional theory (DFT) calculations, and modeling using dedicated Monte-Carlo simulations. By considering the simplest isomerization pathways — i.e., hydrogen roaming along the edges of the molecule — we are able to characterize the most likely photodissociation pathways for the molecules studied here. These comprise nine PAHs with clearly different structural properties. The formation of aliphatic-like side groups is found to be critical in the first fragmentation step and, furthermore, sets the balance of the competition between H- and H2-loss. We show that the presence of trio hydrogens, especially in combination with bay regions in small PAHs plays an important part in the experimentally established variations in the odd-to-even H-atom loss ratios. In addition, we find that, as PAH size increases, H2 formation becomes dominant, and sequential hydrogen loss only plays a marginal role. We also find disagreements between experiments and calculations for large, solo containing PAHs, which need to be accounted for. In order to match theoretical and experimental results, we have modified the energy barriers and restricted the H-hopping to tertiary atoms. The formation of H2 in large PAHs upon irradiation appears to be the dominant fragmentation channel, suggesting an efficient formation path for molecular hydrogen in photodissociation regions (PDRs).

The diffusion of Fe2+ ions in spinels with relevance to the process of maghemitization

1980 ◽  
Vol 43 (331) ◽  
pp. 889-899 ◽  
Author(s):  
W. Freer ◽  
R. O'Reilly
Keyword(s):  
Activation Energy ◽  
Spinel Structure ◽  
Single Phase ◽  
Sea Water ◽  
Magnetic Minerals ◽  
Magnetic Studies ◽  
Water Leaching ◽  
Lower Activation ◽  
Diffusion Activation ◽  
Lower Activation Energy

SummaryThe maghemitization process, by which magnetic minerals with spinel structure become progressively oxidized but remain single phase spinels, seems to be an important feature of submarine weathering. Whether the process takes place by the minerals acquiring oxygen from the sea-water or by the sea-water leaching out iron, the controlling process is the diffusion of Fe2+ in the spinel structure. Magnetic studies have suggested that during maghemitization the availability for oxidation of Fe2+ in the tetrahedral (A) sites of the spinel structure is much less than that in octahedral (B) sites. In this study the Fe2+-containing spinels FeAl2O4, FeCr2O4, FeGa2O4, and Fe2GeO4, in which Fe2+ is predominantly in either A or B sites were prepared, and the diffusion of Fe2+ was studied by (1) interdiffusion experiments with the Mg2+ counterparts and (2) oxidation experiments in air. Fe2GeO4 (Fe2+ in B sites) was found to be associated with a higher interdiffusion coefficient and lower activation energy than FeAl2O4 (75% Fe2+ in A sites). Oxidation/diffusion activation energies of 0.27 and 0.71 eV were assigned to Fe2+ in B and A sites respectively. The experiments thus provide support for the maghemitization model in which Fe2+ in B sites is preferentially oxidized.

Persistent photoconductivity in AlGaN films Grown by mocvd

1998 ◽  
Vol 512 ◽  
Author(s):  
A. Y. Polyakov ◽  
N. B. Smirnov ◽  
A. V. Govorkov ◽  
J. M. Redwing
Keyword(s):  
Activation Energy ◽  
Light Intensity ◽  
High Resistivity ◽  
Persistent Photoconductivity ◽  
Quasi Fermi Level ◽  
Decay Times ◽  
Long Time ◽  
Light Excitation ◽  
Si Doped ◽  
Kinetics Of

ABSTRACTPhotocurrent transients due to illumination by above-bandgap and subbandgap light were studied for Si doped and undoped films of AlGaN grown by MOCVD on sapphire and having compositions ranging from 0% to 60% of Al. It is shown that in Si doped layers the decay of photoconductivity takes extremely long time (hundreds and thousands seconds, depending on temperature, composition and illumination conditions). Both the kinetics of rise and fall of photoconductivity are best described by stretched exponents. The characteristic decay times are virtually temperature independent for temperatures below 270–290K and have activation energy of 0.14–0.26 eV (depending on composition) for higher temperatures. The decay times become longer with decreased light intensity and increase when above-bandgap light excitation is replaced by subbandgap light excitation (the photocurrent values from which the decay starts being equivalent). The results cannot be quantitatively explained by the effects of changing of the quasi-Fermi level position well known for DX-centers in AlGaAs. No persistent photoconductivity could be observed in high resistivity undoped AlGaN films with 5%, 15% and 25% of Al.

scholarly journals Electrical Properties of Cubic InN And GaN Epitaxial Layers as a Function of Temperature

2000 ◽  
Vol 5 (S1) ◽  
pp. 216-222
Author(s):  
J.R.L. Fernandez ◽  
V.A. Chitta ◽  
E. Abramof ◽  
A. Ferreira da Silva ◽  
J.R. Leite ◽  
...  
Keyword(s):  
Activation Energy ◽  
Electrical Properties ◽  
Carrier Concentration ◽  
Doping Level ◽  
Type Conductivity ◽  
Si Doping ◽  
Donor And Acceptor ◽  
Cubic Gan ◽  
Si Doped ◽  
P Type

Carrier concentration and mobility were measured for intrinsic cubic InN and GaN, and for Si-doped cubic GaN as a function of temperature. Metallic n-type conductivity was found for the InN, while background p-type conductivity was observed for the intrinsic GaN layer. Doping the cubic GaN with Si two regimes were observed. For low Si-doping concentrations, the samples remain p-type. Increasing the Si-doping level, the background acceptors are compensated and the samples became highly degenerated n-type. From the carrier concentration dependence on temperature, the activation energy of the donor and acceptor levels was determined. Attempts were made to determine the scattering mechanisms responsible for the behavior of the mobility as a function of temperature.

scholarly journals The release of iron from horse spleen ferritin by reduced flavins

1974 ◽  
Vol 143 (2) ◽  
pp. 311-315 ◽  
Author(s):  
Somjai Sirivech ◽  
Earl Frieden ◽  
Shigemasa Osaki
Keyword(s):  
Activation Energy ◽  
Specific Binding ◽  
Entropy Change ◽  
Degree Of Polymerization ◽  
Total Iron ◽  
Fast Reaction ◽  
Lower Activation ◽  
Ferritin Iron ◽  
Horse Spleen Ferritin ◽  
Protein Portion

Ferritin-Fe(III) was rapidly and quantitatively reduced and liberated as Fe(II) by FMNH2, FADH2 and reduced riboflavin. Dithionite also released Fe(II) from ferritin but at less than 1% of the rate with FMNH2. Cysteine, glutathione and ascorbate gave a similar slower rate and yielded less than 20% of the total iron from ferritin within a few hours. The reduction of ferritin-Fe(III) by the three riboflavin compounds gave complex second-order kinetics with overlapping fast and slow reactions. The fast reaction appeared to be non-specific and may be due to a reduction of Fe(III) of a lower degree of polymerization, equilibrated with ferritin iron. The amount of this Fe3+ ion initially reduced was small, less than 0.3% of the total iron. Addition of FMN to the ferritin–dithionite system enhanced the reduction; this is due to the reduction of FMN by dithionite to form FMNH2 which then reduces ferritin-Fe(III). A comparison of the thermodynamic parameters of FMNH2–ferritin and dithionite–ferritin complex formation showed that FMNH2 required a lower activation energy and a negative entropy change, whereas dithionite required 50% more activation energy and showed a positive entropy change in ferritin reduction. The effectiveness of FMNH2 in ferritin–Fe(III) reduction may be due to a specific binding of the riboflavin moiety to the protein portion of the ferritin molecule.

The Entry of Free Radicals Into Polystyrene Latex Particles

1988 ◽  
Vol 41 (12) ◽  
pp. 1799 ◽  
Author(s):  
ME Adams ◽  
M Trau ◽  
RG Gilbert ◽  
DH Napper ◽  
DF Sangster
Keyword(s):  
Activation Energy ◽  
Free Radicals ◽  
Free Radical ◽  
Rate Coefficient ◽  
Sodium Dodecyl ◽  
Polystyrene Latex ◽  
Entry Rate ◽  
Latex Particles ◽  
Polystyrene Latex Particles ◽  
Emulsion Polymerizations

Mechanistic understanding of the processes governing the kinetics of emulsion polymerization has both scientific and technical interest. One component of this process that is poorly understood at present is that of free radical entry into latex particles. Measurements were made of the entry rate coefficient as a function of temperature for free radicals entering polystyrene latex particles in seeded emulsion polymerizations initiated by γ-rays. The activation energy for entry was found to be less than 24�3 kJ mol-1, consistent with entry being controlled by a physical (e.g., diffusional ) rather than a chemical process. Measurement of the entry rate coefficient as a function of the γ-ray dose rate suggested that the factors that determine the entry rate when the primary free radicals are uncharged are similar to those that determine the entry rate for charged free radicals derived from chemical initiation by peroxydisulfate. This result was consistent with measurements of the entry rate coefficient of charged free radicals derived from peroxydisulfate; these data were found to be virtually independent of both the extent of the latex surface coverage by the anionic surfactant sodium dodecyl sulfate and the ionic strength of the continuous phase. The data refute several proposals given in the literature for the rate-determining step for entry, being inconsistent with control by (1) collision of free radicals with the latex particles, (2) surfactant desorption , and (3) an electrostatic barrier arising from the colloidal nature of the entering free radical. The origin of the activation energy for entry remains obscure.

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