Experimental and numerical investigation of fractal-tree-like heat exchanger manufactured by 3D printing

© 2018 Elsevier Ltd The manufacturing difficulties of complex fractal-tree-like heat exchangers have limited their industrial applications, although many evidences have shown that they have significant advantages in heat transfer. Nevertheless, the emerging 3D printing technology has brought great opportunity for the development of complex structured device. In the present study, three-dimensional (3D) fractal-tree-like heat exchangers were designed and manufactured using 3D printing technology. Their performance was evaluated from both thermal and hydrodynamic perspectives, the flow characteristics were investigated in detail. The results show that a fractal-tree-like heat exchanger can improve hydrodynamic performance, reduce pressure drops and has great heat transfer ability. In general, the fractal-tree-like heat exchanger has a comprehensive advantage over the traditional spiral-tube exchangers as it has a higher value of coefficient of performance (COP). Furthermore, the 3D printing provides a visual, efficient, and precise approach in the present research

Ni-Doping Effects on Carbon Diffusion and Oxidation over Mo₂C Surfaces

Spin-polarized periodic density functional theory calculations have been performed to study the adsorption, diffusion, and oxidation of carbon on the Mo-terminated β-Mo₂C­(001) surface as well as on a model Ni-doped β-Mo₂C­(001) surface with a surface Ni:Mo ratio of 1:2. The most stable adsorption sites for O and CO were found to be similar on the two surfaces, whereas those for C are different in that C prefers to adsorb at the step interface on the model Ni-doped surface. The adsorption energies for all three species were found to be less negative on the Ni-doped surface. The energy barriers and reaction energies for the diffusion and oxidation of carbon on the above β-Mo₂C­(001) surfaces were calculated. On the pure β-Mo₂C­(001) surface, C diffusion from its most stable adsorption site has a much smaller energy barrier of ∼1.0 eV than C oxidation of ∼2.6 eV, with both processes being quite endothermic. Upon Ni doping, the lowest energy barrier for C diffusion from its most stable adsorption site remains ∼1.0 eV, whereas the lowest energy barrier for C oxidation is ∼1.6 eV, much lower than that of ∼2.6 eV on the pure β-Mo₂C­(001) surface. The energy barrier difference between C diffusion and oxidation of ∼0.6 eV on the Ni-doped surface is much smaller than that of ∼1.6 eV on the pure β-Mo₂C­(001) surface, and this can be beneficial for preventing carbon deposition and increasing CO selectivity

CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces

Periodic density functional theory calculations were carried out to investigate CO dissociation pathways on the Fe(100) surfaces covered with up to one monolayer of Cu atoms, which serve as the simple models for the Cu/Fe catalysts for higher alcohol synthesis (HAS) from syngas. For all the model catalyst surfaces, H-assisted CO dissociation was predicted to have lower energy barriers than direct CO dissociation. The difference in the energy barriers between the two dissociation pathways increases as Cu surface coverage increases, suggesting reduced contribution of direct CO dissociation on Cu-rich surfaces. A further thermodynamic analysis also reaches the same conclusion. Several reaction properties for CO dissociation, including CO physisorption and chemisorption energies, and energy barriers for direct and H-assisted CO dissociations, were found to scale linearly with Cu surface coverage, and these reaction properties were predicted to depend largely on the structure of the surface layer, which can be expected to also apply to other metal alloy catalysts. Cu doping was found to reduce the activity of the Fe­(100) surface in catalyzing direct and H-assisted CO dissociations, so CO dissociations should occur primarily on Fe-rich surfaces, leading to CH_<i>x</i> formation, whereas Cu-rich surfaces are potential sources for physisorbed CO molecules. This is also expected to apply to other Cu/M catalysts and is consistent with the dual site mechanism previously proposed for these bimetallic catalysts. A synergy between these two types of active sites is beneficial for the formation of higher alcohols, which may be the reason for the superior performance of the Cu/Fe catalysts for the HAS reaction

Methane Activations by Lanthanum Oxide Clusters

Density functional theory and coupled cluster theory were employed to study the activations of CH₄ by neutral lanthanum oxide clusters (LaO­(OH), La₂O₃, La₃O₄(OH), La₄O₆, La₆O₉) as models for the La₂O₃ catalysts for the oxidative coupling of methane (OCM) reaction. The physisorption energies (Δ<i>H</i>_298 K) of CH₄ on the lanthanum oxide clusters were predicted to be −4 to −3 kcal/mol at the CCSD­(T) level. CH₄ is activated by hydrogen transfer to one of the O sites on the lanthanum oxide clusters, and the energy barriers (Δ<i>E</i>_0 K) from the physisorption structures were calculated to be modest at ∼20 kcal/mol for La₂O₃ and ∼25 kcal/mol for the other clusters. This is accompanied by the formation of a La–CH₃ bond, whose bond dissociation energy (Δ<i>E</i>_0 K) was calculated to be 53 to 60 kcal/mol. CH₄ chemisorption is slightly exothermic on LaO­(OH) and La₂O₃, whereas it becomes increasingly endothermic for the larger lanthanum oxide clusters. The formation of the CH₃ radical was predicted to be substantially endothermic, by ∼50 kcal/mol for LaO­(OH) and La₂O₃ and 64 to 76 kcal/mol for La₃O₄(OH) and La₄O₆ (Δ<i>H</i>_298 K). Calculations on the activation of CH₄ by La₆O₉ with a higher coordination number for both the La and O sites than La₄O₆ yield an energy barrier slightly higher by <1 kcal/mol, suggesting that the effects of the coordination numbers on the reaction energetics are rather small. The energy barrier for hydrogen abstraction does not correlate well with the negative charge on the O site, and a linear relation between the energy barrier and the chemisorption energy was not found for all the lanthanum oxide clusters, which is attributed to the strong dependency of their correlation on the specific chemical environment of the reactive site. Cluster reaction energies, physisorption and chemisorption energies, energy barriers, and La–CH₃ bond energies calculated at the DFT level with the B3LYP and PBE functionals were compared with those calculated at the CCSD­(T) level showing that the B3LYP functional yields better cluster reaction energies, chemisorption energies, and energy barriers. Although the PBE functional yields better physisorption energies, the DFT results can deviate substantially from the CCSD­(T) values. Although the O^2– sites in these cluster models were predicted to be less reactive toward CH₄ than the O^– sites modeled by the nonstoichiometric La₂O_3.33(001) surface (Palmer, M. S. et al. <i>J. Am. Chem. Soc.</i> <b>2002</b>, <i>124</i>, 8452), they are more reactive than the O₂^2– site modeled on the stoichiometric La₂O₃(001) surface, which suggests the relevance of the lattice oxygen sites on the La₂O₃ catalyst surfaces in the OCM reaction

Investigations of the Low-Frequency Spectral Density of Cytochrome c upon Equilibrium Unfolding

The equilibrium unfolding process of ferric horse heart cytochrome c (cyt c), induced by guanidinium hydrochloride (GdHCl), was studied using UV–vis absorption spectroscopy, resonance Raman spectroscopy, and vibrational coherence spectroscopy (VCS). The unfolding process was successfully fit using a three-state model which included the fully folded (N) and unfolded (U) states, along with an intermediate (I) assigned to a Lys bound heme. The VCS spectra revealed for the first time several low-frequency heme modes that are sensitive to cyt c unfolding: γ_a (∼50 cm^–1), γ_b (∼80 cm^–1), γ_c (∼100 cm^–1), and ν_s(His-Fe-His) at 205 cm^–1. These out-of-plane modes have potential functional relevance and are activated by protein-induced heme distortions. The free energies for the N–I and the I–U transitions at pH 7.0 and 20 °C were found to be 4.6 kcal/M and 11.6 kcal/M, respectively. Imidazole was also introduced to replace the methionine ligand so the unfolding can be modeled as a two-state system. The intensity of the mode γ_b∼80 cm^–1 remains nearly constant during the unfolding process, while the amplitudes of the other low frequency modes track with spectral changes observed at higher frequency. This confirms that the heme deformation changes are coupled to the protein tertiary structural changes that take place upon unfolding. These studies also reveal that damping of the coherent oscillations depends sensitively on the coupling between heme and the surrounding water solvent

Role of Peroxides on La₂O₃ Catalysts in Oxidative Coupling of Methane

Density functional theory and coupled cluster theory [CCSD­(T)] calculations reveal an important pathway for the one-step CH₃OH formation upon CH₄ activation at the peroxide (O₂^2–) site of La₂O₃-based catalysts for the oxidative coupling of methane (OCM) reaction. Using modest-sized La₄O₇ and La₆O₁₀ clusters as catalyst models, two types of structures for the O₂^2– site were predicted, with the less stable structure (type <b>II</b>) more reactive with CH₄ than the more stable structure (type <b>I</b>). CH₄ activation at the O₂^2– site can always occur via the above pathway, and for the smaller La₂O₄ cluster and the type <b>I</b> structure of La₄O₇, an alternative pathway leading to La–CH₃ bond formation was also predicted, similar to that at the oxide (O^2–) site from our previous study. For the type <b>I</b> structure of La₄O₇, the energy barrier for La–CH₃ bond formation is lower than that for CH₃OH formation, but both are higher than that for CH₃OH formation for the type <b>II</b> structure of La₄O₇. The O₂^2– site was predicted to be much less reactive with CH₄ than the oxide (O^2–) site, and can lead to CH₃OH formation, which is considered as a side reaction. Thus, our calculations do not appear to support the central role previously proposed for the O₂^2– site for La₂O₃-based catalysts for the OCM reaction. However, considering the catalytic and redox nature of this reaction, both the O^2– and O₂^2– sites may still play important roles in the whole catalytic cycle

Investigations of the Low Frequency Modes of Ferric Cytochrome <i>c</i> Using Vibrational Coherence Spectroscopy

Femtosecond vibrational coherence spectroscopy is used to investigate the low frequency vibrational dynamics of the electron transfer heme protein, cytochrome <i>c</i> (cyt <i>c</i>). The vibrational coherence spectra of ferric cyt <i>c</i> have been measured as a function of excitation wavelength within the Soret band. Vibrational coherence spectra obtained with excitation between 412 and 421 nm display a strong mode at ∼44 cm^–1 that has been assigned to have a significant contribution from heme ruffling motion in the electronic ground state. This assignment is based partially on the presence of a large heme ruffling distortion in the normal coordinate structural decomposition (NSD) analysis of the X-ray crystal structures. When the excitation wavelength is moved into the ∼421–435 nm region, the transient absorption increases along with the relative intensity of two modes near ∼55 and 30 cm^–1. The intensity of the mode near 44 cm^–1 appears to minimize in this region and then recover (but with an opposite phase compared to the blue excitation) when the laser is tuned to 443 nm. These observations are consistent with the superposition of both ground and excited state coherence in the 421–435 nm region due to the excitation of a weak porphyrin-to-iron charge transfer (CT) state, which has a lifetime long enough to observe vibrational coherence. The mode near 55 cm^–1 is suggested to arise from ruffling in a transient CT state that has a less ruffled heme due to its iron d⁶ configuration

CO₂ Chemisorption and Its Effect on Methane Activation in La₂O₃‑Catalyzed Oxidative Coupling of Methane

Density functional theory and coupled cluster theory calculations were carried out to study the formation of the carbonate species on La₂O₃ catalyst using the cluster model and its effect on subsequent CH₄ activation. Physisorption and chemisorption energies as well as energy barriers for the reaction of CO₂ and La₂O₃ clusters, and the reaction of CH₄ with the CO₃^2– site on the resulting clusters, were predicted. Our calculations show that CO₂ chemisorption at the La³⁺–O^2– pair sites is thermodynamically and kinetically very favorable due to the strong basicity of the O^2– site on La₂O₃, which leads to the formation of the La³⁺–CO₃^2– pair sites. In addition, CH₄ activation at the La³⁺–CO₃^2– pair sites is similar to that at the La³⁺–O^2– pair sites, which results in the formation of the bicarbonate species and the La–CH₃ bond, although the La³⁺–CO₃^2– pair sites are much less reactive with CH₄ in terms of both thermodynamics and kinetics. Further thermodynamical calculations show that the CO₃^2– species in these clusters dissociate between 500 to 1250 K, with half of them completely dissociated at 873 K, consistent with the experimental observation. Our studies suggest that the CO₃^2– site is unlikely to be the active site in La₂O₃-catalyzed oxidative coupling of methane, and CO₂ as a major byproduct is likely to act as a poison to the La₂O₃-based catalysts especially at modest reaction temperature

Investigations of Ferric Heme Cyanide Photodissociation in Myoglobin and Horseradish Peroxidase

The photodissociation of cyanide from ferric myoglobin (MbCN) and horseradish peroxidase (HRPCN) has definitively been observed. This has implications for the interpretation of ultrafast IR (Helbing et al. <i>Biophys. J</i>. <b>2004</b>, <i>87</i>, 1881–1891) and optical (Gruia et al. <i>Biophys. J</i>. <b>2008</b>, <i>94</i>, 2252–2268) studies that had previously suggested the Fe–CN bond was photostable in MbCN. The photolysis of ferric MbCN takes place with a quantum yield of ∼75%, and the resonance Raman spectrum of the photoproduct observed in steady-state experiments as a function of laser power and sample spinning rate is identical to that of ferric Mb (metMb). The data are quantitatively analyzed using a simple model where cyanide is photodissociated and, although geminate rebinding with a rate of <i>k</i>_BA ≈ (3.6 ps)⁻¹ is the dominant process, some CN^– exits from the distal heme pocket and is replaced by water. Using independently determined values for the CN^– association rate, we find that the CN^– escape rate from the ferric myoglobin pocket to the solution at 293 K is <i>k</i>_out ≈ (1–2) × 10⁷ s^–1. This value is very similar to, but slightly larger than, the histidine gated escape rate of CO from Mb (1.1 × 10⁷ s^–1) under the same conditions. The analysis leads to an escape probability <i>k</i>_out/(<i>k</i>_out + <i>k</i>_BA) ∼ 10^–4, which is unobservable in most time domain kinetic measurements. However, the photolysis is surprisingly easy to detect in Mb using cw resonance Raman measurements. This is due to the anomalously slow CN^– bimolecular association rate (170 M^–1 s^–1), which arises from the need for water to exchange at the ferric heme binding site of Mb. In contrast, ferric HRP does not have a heme bound water molecule and its CN^– bimolecular association rate is larger by ∼10³, making the CN^– photolysis more difficult to observe

Bimodal Mesoporous Carbon-Coated MgO Nanoparticles for CO₂ Capture at Moderate Temperature Conditions

Bimodal mesoporous carbon supported MgO (MgOMC) materials were prepared to provide a candidate for trapping CO₂ in flue gas at high temperature by an impregnation method. Textural properties of MgOMC were characterized by X-ray diffraction, N₂ adsorption/desorption isotherm, scanning electron microscopy, transmission electron microscopy and an element mapping method, and their CO₂ adsorption behaviors in both pure CO₂ and 10 vol % CO₂ simulated flue gas were evaluated by a thermal gravimetric method at ambient temperature. Effects of MgO content in composite and calcination temperature on CO₂ adsorption capacity were studied. Results showed that MgOMC samples with high content of basic MgO sites and well-developed porous structure exhibit high CO₂ uptakes at 75 °C, rapid adsorption kinetics, excellent CO₂ selectivity in flue gas and good regenerability under mild temperature below 300 °C, indicating their potential application in the CO₂ capture from flue gas