02 Mar 2021-Catalysis Science & Technology (The Royal Society of Chemistry)-Vol. 11, Iss: 4, pp 1395-1406

Abstract: The CO2 reforming of methane (dry reforming) is a challenging reaction because it requires high temperature and is always accompanied by various side reactions, such as steam reforming, the reverse water gas shift reaction, and coke formation. In this study, density functional theory was used to study dry reforming and its related processes on the Ru(0001) surface. The results showed that CO2 dissociation occurred mainly by direct dissociation and O* produced by CO2 dissociation was the main oxidant of intermediates on the surface of Ru(0001). The activation energy of CO2 was lower than that of CH4, and CH–O oxidation and C–O oxidation were likely. The free energy spans of dry reforming, steam reforming, the reverse water gas shift, CH4 cracking, and the Boudouard reaction were 158, 155, 123, 176, and 245 kJ mol−1, respectively. We compared the same key process on the surfaces of Ni(111) and Ru(0001), and the results showed that Ru was more resistant to coke than Ni due to its strong “oxophilic” nature, which resulted in a facile activation of CO2. Interestingly, it was found that the binding energy difference between and O* increased following the order Ru > Co > Ni > Pd > Pt, and the same trend for the carbon deposition resistance could be expected. The results of the microkinetic simulation under catalytic reaction conditions (973.15 K, 1 bar) and low-pressure conditions (973.15 K, 160 Pa) showed that the ratio of the generation rate of H2 and CO was slightly less than 1 : 1 and a small amount of H2O was generated, which indicates that the reverse water gas shift reaction and steam reforming had little effect on the dry reforming. The activation of CH4 was the rate-limiting step in dry reforming, which was confirmed in the sensitivity analysis under catalytic reaction conditions. The dry reforming rate positively correlated with the CH4 pressure and was almost independent of the CO2 pressure under low-pressure conditions. In addition, the effect of carbon deposition on the catalyst was very small throughout the simulation process. Our present results may provide a theoretical guide for the experiment and design of a highly efficient dry reforming catalyst by alloying a typical DRM catalyst, such as Ni, with smaller noble Ru to promote its carbon deposition resistance.

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Topics: Carbon dioxide reforming (70%), Steam reforming (64%), Boudouard reaction (58%) ... show more

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10 results found

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Md. Imteyaz Alam^{1}, Raffaele Cheula^{1}, Gianluca Moroni^{1}, Luca Nardi^{1} +1 more•Institutions (1)

Abstract: The increasing environmental concerns due to anthropogenic CO2 emissions have called for an alternate sustainable source to fulfill rising chemical and energy demands and reduce environmental problems. The thermo-catalytic activation and conversion of abundantly available CO2, a thermodynamically stable and kinetically inert molecule, can significantly pave the way to sustainably produce chemicals and fuels and mitigate the additional CO2 load. This can be done through comprehensive knowledge and understanding of catalyst behavior, reaction kinetics, and reactor design. This review aims to catalog and summarize the advances in the experimental and theoretical approaches for CO2 activation and conversion to C1 products via heterogeneous catalytic routes. To this aim, we analyze the current literature works describing experimental analyses (e.g., catalyst characterization and kinetics measurement) as well as computational studies (e.g., microkinetic modeling and first-principles calculations). The catalytic reactions of CO2 activation and conversion reviewed in detail are: (i) reverse water-gas shift (RWGS), (ii) CO2 methanation, (iii) CO2 hydrogenation to methanol, and (iv) dry reforming of methane (DRM). This review is divided into six sections. The first section provides an overview of the energy and environmental problems of our society, in which promising strategies and possible pathways to utilize anthropogenic CO2 are highlighted. In the second section, the discussion follows with the description of materials and mechanisms of the available thermo-catalytic processes for CO2 utilization. In the third section, the process of catalyst deactivation by coking is presented, and possible solutions to the problem are recommended based on experimental and theoretical literature works. In the fourth section, kinetic models are reviewed. In the fifth section, reaction technologies associated with the conversion of CO2 are described, and, finally, in the sixth section, concluding remarks and future directions are provided.

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2 Citations

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Utsab Guharoy^{1}, Utsab Guharoy^{2}, Utsab Guharoy^{3}, Tomas Ramirez Reina^{2} +5 more•Institutions (4)

Abstract: It is worthwhile to invest in the development of CO2 reforming of methane, as it presents a promising alternative for transforming two global warming gases into a very versatile product such as syngas. A syngas rich feed gas presents extensive prospects for existing downstream industrial processes for producing valuable fuels and chemicals. The commercialization of the DRM process greatly depends upon the development of low cost, non-precious transition metal-based catalysts, to provide a desirable balance between catalytic activity and stability. In this review, the progress in the advancements of non-precious catalytic materials have been discussed from a theoretical point of view. A theoretical perspective gives an opportunity to gain fundamental information at the atomic level, such as the interaction of reaction intermediates with particular crystal facets (typically active sites in the reaction), combined with electronic structure insights, directly influencing the kinetic behaviour of the catalyst system. Theoretical insights into the DRM reaction mechanisms on non-precious Ni-based bimetallic and transition metal phosphide catalysts are extensively discussed, together with the mitigation mechanisms to avoid carbon deposition and catalyst deactivation under DRM reaction conditions. Prospects of future development of DRM are also provided, highlighting the importance of computational chemistry studies in the development of the next-generation advanced DRM catalysts.

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Topics: Carbon dioxide reforming (51%), Syngas (50%)

2 Citations

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Abstract: Density functional theory (DFT) calculation and microkinetic modeling were performed to study the methanol synthesis from CO2 hydrogenation on indium-terminated In2O3(100), defective In2O3(110) and In2O3(111) surfaces. It is found that these surfaces can adsorb and activate CO2 and drive dissociative H2 adsorption. As for adsorbed CO2, it may be transformed into HCOO, COOH and CO. Based on the calculation results, adsorbed CO2 prefers to be hydrogenated to HCOO on these surfaces compared to decomposition of CO2 and CO2 hydrogenation to COOH. According to transient state calculation, indium-terminated In2O3(100) surface displays low catalytic activity due to the high energy barrier. On the defective In2O3(110) and In2O3(111) surfaces, CO2 hydrogenation to methanol will undergo HCOO, H2CO and H3CO intermediates because this pathway has the lowest energy barriers. Microkinetic analysis reveals defective In2O3(110) is the optimal surface for methanol synthesis from CO2 hydrogenation and high ratio of H2/CO2 favors CH3OH formation. Download : Download high-res image (152KB) Download : Download full-size image Density functional theory calculation and microkinetic modeling were performed to study the methanol synthesis from CO2 hydrogenation on In2O3 surfaces.

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Topics: Catalysis (52%)

1 Citations

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Abstract: Periodic density functional theory (RPBE) computation has been used to assess dissociative adsorption of H2O and CO2 on the clean and O-pre-covered Ru(11−21) and Ru(20−21) surfaces. It is found that surface H2O prefers desorption on the clean Ru(11−21) and Ru(20−21) surfaces, while dissociative adsorption on O-pre-covered Ru(11−21) and Ru(20−21) surfaces, in contrary to that on O-pre-covered Ru(0001) surface. The H2O dissociation capability on these clean and O pre-covered Ru surfaces follows a sequence of Ru(11−21) > (20−21) > (0001). In addition, CO2 desorption energy is comparable with the barrier of CO2 dissociation on Ru(11−21) and Ru(20−21) (0.81 vs. 0.77 eV, 0.14 vs. 0.20 eV, respectively), indicating that CO2 would not desorb in quantity before it occurs dissociation, which is contrary to the case on Ru(0001) (-0.32 vs. 0.23 eV). For surface O removed via H2O or CO2 formation, H2O formation has lower effective barrier than CO2 formation on these three Ru surfaces, in line with the experiments, and the effective barrier of H2O and CO2 formation follows the increasing order of Ru(11−21)

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Abstract: Hydrogenation of CO2 to methanol, ethanol and other oxygenates is an emerging attractive process in C1 chemistry but remains a great challenge not least because of the intrinsic inertness of CO2, difficulty in C–C bond coupling, and complexity in product distribution. Identifying the dominant reaction mechanism is therefore urgent but still lacking under real reaction conditions. In this work, by combining density functional theory calculations with microkinetic modeling, we predicted the activity plots of C1 & C2 oxygenates as a function of temperature and pressure on both stepped Pd(211) and flat Pd(111) surfaces according to the reaction network consisting of ∼150 elementary steps. We found that Pd(211) is more active than Pd(111), and the incremental effect is more remarkable for the production of C2 oxygenates. An optimal reaction temperature of around 500 K is theoretically rationalized. COOH is the key intermediate in CO2 activation, and the CO insertion with CHx highly contributes to the C–C bond coupling. Formation of ethanol is directly competitive with that of methane. The activity dependences on reaction conditions are different between CO2 and CO hydrogenation. The formation energy scaling relations of intermediates and transition states between the different Pd surfaces were established, providing a simplified strategy to estimate transition state energies on other surfaces for microkinetic simulation. All these constitute the key foundation for the rational design of metal catalysts and optimization of reaction conditions for CO2 hydrogenation to C1 & C2 oxygenates.

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Topics: Transition state (54%)

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61 results found

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Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif

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Topics: Local-density approximation (60%), Orbital-free density functional theory (57%), Density functional theory (56%) ... show more

117,932 Citations

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Abstract: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.

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Topics: Projector augmented wave method (66%), Pseudopotential (58%)

48,474 Citations

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Abstract: The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Bl\"ochl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules $({\mathrm{H}}_{2}{,\mathrm{}\mathrm{H}}_{2}{\mathrm{O},\mathrm{}\mathrm{Li}}_{2}{,\mathrm{}\mathrm{N}}_{2}{,\mathrm{}\mathrm{F}}_{2}{,\mathrm{}\mathrm{BF}}_{3}{,\mathrm{}\mathrm{SiF}}_{4})$ and several bulk systems (diamond, Si, V, Li, Ca, ${\mathrm{CaF}}_{2},$ Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.

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Topics: Cauchy stress tensor (53%)

46,297 Citations

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Abstract: A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.

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Topics: Brillouin zone (62%)

42,677 Citations

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Abstract: We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order Natoms2 scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special ‘preconditioning’ optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.

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Topics: Projector augmented wave method (55%), Conjugate gradient method (55%), Iterative method (54%) ... show more

40,008 Citations