CO Adsorption Energy Research Articles

Catalytic activity of 1D chains of gold oxide on a stepped gold surface from density functional theory.

The rich surface chemistry of gold at the nanoscale has made it an important catalyst for low-temperature applications. Recent studies point to the possible role of self-organized structures formed by chemisorbed O atoms on the surface of gold catalysts for their catalytic activity and/or deactivation. In this study, we investigate the reactivity of a double O chain running along a step on a Au(221) surface with oxygen vacancies as a prototypical model of a 1D surface gold oxide. We compare CO and O2 adsorption on such a chain with the oxygen-free Au(221) surface model. A systematic study of the reactivity of the double chain with O vacancies was done with respect to the regular Au(221) surface using CO as a probe. The CO oxidation was investigated assuming dissociative and associative mechanisms. Remarkably, O2 adsorbs stronger on the double oxygen vacancy than on the regular Au(221) surface, and its dissociation barrier reduces significantly from 1.84 eV to 0.87 eV, whereas the CO adsorption energy is similar on these surfaces. Calculations suggest that CO oxidation should occur more efficiently on the double O vacancy than on the regular Au(221) surface due to stronger adsorption of O2 and a low activation barrier for O2 + CO surface reaction.

DFT calculations for adsorption of H2S and other natural gas compounds on (Fe, Co, Ni, Cu and Zn)–Y zeolite clusters

The acid compounds removal from natural gas by adsorption on zeolites is an environmentally friendly alternative technology for its purification and commercialization. However, despite numerous theoretical studies determining the adsorption energies for gases on zeolites are found, for important natural gas compounds adsorption on Y zeolite they are poorly reported, especially with respect to long range van der Waals interactions, the description of their nature and related thermodynamic properties. Thus, this work aims to pre-select transition metal-exchanged Y zeolite for selective removal of H2S and other acid compounds from natural gas streams by DFT calculations for H2S, COS, CO2 and CH4 adsorption on Fe–Y, Co–Y, Ni–Y, Cu–Y and Zn–Y zeolite clusters. Adsorption energies, thermodynamic properties, chemical hardness, electronegativity, Natural Bond Orbital (NBO) and Energy Decomposition Analysis (EDA) were determined using B3LYP hybrid functional and Grimme's D3 empirical dispersion correction. Comparison between H2S and other compounds adsorption energies indicates that Ni–Y zeolite has potential for selective removal of H2S, despite Co–Y zeolite presents stronger adsorption energy for all compounds. Moreover, the NBO and EDA analyzes reveal that there is a transfer of d electrons from H2S molecule to unoccupied orbitals of the cation. Charge transfer and polarization phenomena contribute mainly to the adsorption energy of H2S, COS and CO2 in transition metal-exchanged Y zeolite clusters, showing the presence of surface chemisorption.

From fundamental to CO2 and COCl2 gas sensing properties of pristine and defective Si2BN monolayers.

In this work, the capability of Si2BN monolayers (Si2BN-MLs) to sense CO2 and COCl2 molecules was investigated by analyzing the structural, electronic, mechanical and gas sensing properties of defect-free and defective Si2BN-ML structures. Electronic property analysis revealed that the Si2BN-ML retains its metallicity in the presence of vacancy defects. The computed vacancy formation energies of Si, B and N monovacancies are 3.25 eV, 2.27 eV and 2.55 eV, respectively, which indicate that the B monovacancy is thermodynamically more feasible. Besides, both pristine and defective Si2BN-ML structures show good mechanical stability. To validate the gas sensing properties, the adsorption energy and charge transfer were analysed, showing that both pristine and defective Si2BN-ML structures receive considerable charges from the CO2 and COCl2 molecules via a stable physisorption process. The work function analysis revealed that a minute increase <0.10 eV is responsible for the enhanced selectivity and sensitivity of Si2BN-ML structures in detecting CO2 and COCl2 molecules. The low adsorption energies of both CO2 and COCl2 gas molecules during the interaction with Si2BN-ML structures signify the possibility of a large number of adsorption-desorption cycles with an ultra-low recovery time, 0.174 ns for CO2 and 0.016 ns for COCl2, suitable for efficient gas sensing applications.

CO2 adsorption enhancement over Al/C-doped h-BN: A DFT study

Reducing energy barriers of CO2 being chemisorbed on hexagonal boron nitride (h-BN) is a kernel step to efficiently and massively capture CO2. In this study, aluminum/carbon (Al/C) atoms are used as dopants to alter surface potential fields of h-BN, which aims at lowering energy barriers of adsorption processes. Through theoretical calculations, direct-adsorption structures/properties of CO2, joint-adsorption structures/properties of CO2/H2O, transition state (TS) energy barriers, effects of temperatures on adsorption energies/TS energy barriers and changes of reaction rate constants over different adsorbents are investigated in detail in order to reveal how doping of Al/C atoms promotes CO2 adsorption strength over doped h-BN. According to DFT calculation results, the average adsorption energy of CO2 being directly adsorbed on Al/C-doped h-BN arrives at −59.43 kJ/mol, which is about 5 times as big as that over pure h-BN. As to the average adsorption energy of CO2/H2O and relevant TS energy barrier, they are modified to −118.89 kJ/mol and 40.23 kJ/mol over Al/C-doped h-BN in contrast with −33.91 kJ/mol and 1695.11 kJ/mol over pure h-BN, respectively. What is more, based on thermodynamic analyses and reaction dynamics, the average desorption temperatures of CO2(/H2O) are promoted over doped h-BN and the temperature power exponent is negatively correlated with the activation energy in the Arrhenius equation form. The complete understanding of this study would supply crucial information for applying Al/C-doped h-BN to effectively capturing CO2 in real industries.

Can Charge-Modulated Metal-Organic Frameworks Achieve High-Performance CO2 Capture and Separation over H2 , N2 , and CH4 ?

CO2 capture and separation by using charge-modulated adsorbent materials is a promising strategy to reduce CO2 emissions. Herein, three TM-HAB (TM=Co, Ni, and Cu; HAB=hexa-aminobenzene) metal-organic frameworks (MOFs) were evaluated as charge-modulated CO2 capture and separation materials by using density functional theory and grand canonical Monte Carlo simulations. The results showed that each TM-HAB presented a high electrical conductivity and structural stability when injecting charges. The CO2 adsorption energy increased from 0.211 to 2.091 eV on Co-HAB, 0.262 to 2.119 eV on Ni-HAB, and 0.904 to 2.803 eV on Cu-HAB, respectively, with the increase in charge state from 0.0 to 3.0 e- . Co-HAB and Ni-HAB were better charge-modulated CO2 capture materials with less structure deformation based on energy decomposition analyses. The kinetic process demonstrated that considerably low energy consumptions of 0.911 and 1.589 GJ ton-1 CO2 were observed for a complete adsorption-desorption cycle on Co-HAB and Ni-HAB. All charged MOFs, especially Co-HAB and Ni-HAB, exhibited higher CO2 adsorption energies and adsorption capacities than those of H2 , N2 , and CH4 , thereby exhibiting high CO2 selectivities. Interaction analysis confirmed that the injecting charges had a more pronounced enhancement in the coulombic interactions between CO2 and MOFs. The results of this work highlight Co-HAB and Ni-HAB as promising charge-modulated CO2 capture and separation materials with controllable CO2 capture, high selectivity, and low energy consumption.

Boosting CO desorption on dual active site electrocatalysts for CO2 reduction to produce tunable syngas

The electrochemical conversion of CO 2 to generate syngas (H 2 and CO) is regarded as a promising alternative technique to facilitate CO 2 reduction in the ambient atmosphere. However, it is still a great challenge to acquire high catalytic activity with an adjustable H 2 /CO ratio over a wide range. Here, we construct an electrocatalyst with Fe-containing dual active sites on N-doped porous carbon (Fe/FeN 4 C) to promote a CO 2 reduction reaction for tunable syngas production. The Fe/FeN 4 C catalysts have a positive onset potential (−0.18 V RHE ), approximately 100% of the sum of the Faradaic efficiency (FE) of CO and H 2 , a high total current density (>39.33 mA cm −2 ), and a wide H 2 /CO ratio (1.09∼7.08). Density functional-theory calculations suggest that the Fe single atoms dispersed into the N-doped carbon structure, along with the incorporation of Fe nanoparticles, may decrease the adsorption energy of ∗CO, thus synergistically enhancing the catalytic activity. Fe/FeN 4 C serves as a dual active site catalyst for CO 2 RR to syngas The Fe/FeN 4 C catalyst displays superior CO 2 RR performance toward syngas DFT study confirms the decrease in ∗CO adsorption energy benefits syngas production There is interest in developing active catalysts for the electrochemical conversion of carbon dioxide to syngas with a tunable H 2 /CO ratio. Here, Hua et al. report Fe-containing, N-doped, porous carbon to promote CO 2 reduction for tunable syngas production.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Sc-functionalized porphyrin-like porous fullerene for CO2 storage and separation: A first-principles evaluation

In recent years, there has been a lot of interest in capturing and storing carbon dioxide (CO2) on porous materials as an efficient method for decreasing the adverse effects of this greenhouse gas on the environment and climate change. The current work introduces a Sc-decorated porphyrin-like porous fullerene (Sc6@C24N24) as an efficient material for CO2 capture, storage, and separation using density functional theory calculations. While CO2 is physisorbed over pristine C24N24, the addition of Sc atoms on the N4 sites of C24N24 greatly enhances CO2 adsorption energy. Each Sc atom in Sc6@C24N24 may adsorb up to three CO2 molecules, resulting in a gravimetric density of 48%. Moreover, temperature may be used to modulate CO2 adsorption/desorption over the substrate. The Sc-decorated C24N24 fullerene exhibits a lower affinity for adsorbing N2, CH4, and H2 molecules than CO2. As a consequence, this material might be considered for purifying CO2 molecules from CO2/N2, CO2/CH4, and CO2/H2 mixtures. This study also sheds light on the nature of the Sc-CO2 interaction as well as the underlying mechanism of selective CO2 adsorption on Sc decorated C24N24.

Effect of Mn-doped CaO on NO reduction by CO in carbonation stage of calcium looping: A density functional theory study

Calcium looping uses CaO carbonation/calcination cycles to capture CO2, which is a promising carbon capture technology for coal-fired power plants. However, after CaO absorbs CO2 to generate CaCO3, the catalytic performance of CaO for NOx reduction by CO decreases drastically. In this work, Mn-doped CaO was used to improve NO reduction by CO in carbonation of calcium looping and the mechanism of Mn-doped CaO on NO reduction by CO was determined by the density functional theory. The experimental results show that Mn doping not only improves the CO2 capture capacity of CaO, but also increases NO reduction by CO. Density functional theory calculation results show that the Mn atom provides new adsorption sites for CO and NO on CaO surface, respectively. After addition of Mn, the values of co-adsorption energy for CO and NO on CaO surface increases by 1.5 and 6.9 times in the carbonation stage, respectively. In addition, the total energy barrier for the reaction of NO reduction by CO under the catalysis of CaO in carbonation stage decreases from 11.08 to 8.66 eV due to the addition of Mn. The negative effect of the carbonation of CaO on NO removal by CO is greatly mitigated by Mn. Therefore, NO removal by CO in the presence of Mn-doped CaO is significantly improved in the carbonation stage of the calcium looping.

Single Ni atom doped WS2 monolayer as sensing substrate for dissolved gases in transformer oil: A first-principles study

Insulation defects and equipment failure often exist in power transformer, and insulating oil in transformer will experience decomposition with a series reaction. By detecting the dissolved gases in insulating oil, the operation status of transformer can be evaluated. In this study, the adsorption and gas sensing of several kinds of typical dissolved gas molecule (H2, C2H2, and CO) on pristine and single Ni atom doped WS2 monolayer (Ni-WS2) was theoretically investigated based on first-principles density functional theory. To discuss the adsorption behavior, the structures, adsorption energies, and electron transfers were calculated and compared. As to the electronic properties and chemical interactions between gas molecule and Ni-WS2, the band structures, density of states (DOS), and work functions were studied. The results show that, after the doping of Ni, the adsorption energies of H2, C2H2 and CO on WS2 monolayer increase dramatically, from −0.06 eV to −0.52 eV, −0.23 eV to −1.44 eV, and −0.15 eV to −1.68 eV, respectively. On Ni-WS2, H2 and C2H2 lose electrons while CO shows little electron transfer. Only the adsorption of C2H2 obviously reduces the band gap of Ni-WS2. For all three kinds of gas adsorption, hybridization of states between Ni 3d and atomic orbitals in molecule can be observed in DOS, indicating considerable chemical interactions. In addition, the adsorption of CO increases the work function of Ni-WS2 from 5.56 eV to 5.80 eV. Based on the change of different physical and chemical properties, the Ni-WS2 as sensing material in different gas sensors was evaluated and the desorption time was compared in order to shed light on the experimental realization to detect dissolved gas molecule in insulating oil with high sensitivity and selectivity.

Balancing the CO2 adsorption properties and the regeneration energy consumption via the functional molecular engineering hierarchical pore-interface structure

How to combine the green fabrication process of advanced materials with the engineering of microscope surface functionalization, simultaneously constructing the relationship between the micro-structure and macroscopic energy consumption from the molecular-level, is the significant issue for revealing the process energy consumption towards ultra-low emission flue gas CO2 storage in science-engineer field. Herein, following the research-line of ‘molecular interface structure → CO2 adsorption properties → regeneration energy consumption’, we facilely synthesized an amine-grafted hierarchical porous material (1,2-epoxyhexane (EH) modified Polyethyleneimine (PEI) grafted into glucose-based carbon sphere (GCS)), which presents excellent performance on CO2 adsorption and lower regeneration energy consumption. As for (EH modified PEI)-grafted glucose-based carbon sphere (EH@PEI-GCS), the strategy of increasing the effective amines ratio and reducing the pore blockage is helpful to construct a molecular-modified porous 3D-interface achieving a CO2 adsorption capacity of 5.34 mmol/g and a CO2/N2 (the ratio of Volume: 15%:85%) separation factor of 100.5 at 313 K and 1 bar. Impressively, compared with the different modified molecule configurations, the EH@PEI-GCS can dramatically improve the water resistance, thermal/cycle stability and lower the regeneration energy consumption, simultaneously without remarkable adsorption capacity losing. Comprehensively, it is critical to declare the relationship between the molecular-modified pore surface structure and regeneration energy consumption, and even to bridge the Micro-hierarchical materials science with Macro-energy engineer field.

Diffusion Barriers for Carbon Monoxide on the Cu(001) Surface Using Many-Body Perturbation Theory and Various Density Functionals.

First-principles calculations play a key role in understanding the interactions of molecules with transition-metal surfaces and the energy profiles for catalytic reactions. However, many of the commonly used density functionals are not able to correctly predict the surface energy as well as the adsorption site preference for a key molecule such as CO, and it is not clear to what extent this shortcoming influences the prediction of reaction or diffusion pathways. Here, we report calculations of carbon monoxide diffusion on the Cu(001) surface along the [100] and [110] pathways, as well as the surface energy of Cu(001), and CO-adsorption energy and compare the performance of the Perdew-Burke-Ernzerhof (PBE), PBE + D2, PBE + D3, RPBE, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), HSE06 density functionals, and the random phase approximation (RPA), a post-Hartree-Fock method based on many-body perturbation theory. We critically evaluate the performance of these methods and find that RPA appears to be the only method giving correct site preference, overall barrier, adsorption enthalpy, and surface energy. For all of the other methods, at least one of these properties is not correctly captured. These results imply that many density functional theory (DFT)-based methods lead to qualitative and quantitative errors in describing CO interaction with transition-metal surfaces, which significantly impacts the description of diffusion pathways. It is well conceivable that similar effects exist when surface reactions of CO-related species are considered. We expect that the methodology presented here will be used to get more detailed insights into reaction pathways for CO conversion on transition-metal surfaces in general and Cu in particular, which will allow us to better understand the catalytic and electrocatalytic reactions involving CO-related species.

Computational Screening and Experimental Synthesis of Doped TiO2 for Propane Dehydrogenation

Low-cost, environmentally friendly, and highly efficient catalysts are urgently needed for the direct propane dehydrogenation (PDH) to propylene. Studies have shown that defective TiO2 obtained by in situ reduction shows excellent catalytic PDH performance. Doping metals in the lattice of TiO2 is a plausible method for tuning the vacancy content and electronic property. In this paper, doping with fifth period transition metals was screened using a computational method, followed by experimental synthesis to further improve the PDH activity of TiO2-based catalysts. The TiO2 (101) surface with doping metals was first studied to determine the positive effect on the vacancy formation. Then, the two disputed reaction paths, that is, the concerted and stepwise path, were compared extensively on all of the doping models. Step one is found to be more preferable for all. The microkinetic modeling was employed to obtain the turnover frequency (TOF), based on which the linear relationship is found for TOF and binding energy of dissociated H2 or co-adsorption energy of 2-propyl&H. The abovementioned screening process shows Ru-doped TiO2, which shows the highest PDH activity and is justified by experimental results.

Adsorption of CO2 on Terrace, Step, and Defect Sites on Pt Surfaces: A Combined TPD, XPS, and DFT Study

The interactions of CO2 with terrace, step, and defect or kink sites on Pt surfaces were investigated using temperature-programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations. Desorption peaks of CO2 on Pt(997) were observed at ∼79, 88–89, ∼92 , and ∼103 K and were respectively assigned to desorption of CO2 from multilayer CO2 (amorphous CO2), CO2 from terrace, CO2 from step, and CO2 from defect sites. The defect sites, step sites, and terrace sites were saturated in that order before multilayer adsorption occurred. The adsorption energies of CO2 on the terrace, step, and defect sites were estimated to be around −0.23, −0.28, and −0.34 eV, respectively. The experimentally measured adsorption energies of CO2 on Pt were successfully reproduced using the optB86b-vdW, rev-vdW-DF2, and PBE-D2 functionals, and the actual adsorption energies were found to be between those calculated with rev-vdW-DF2 and optB86b-vdW. Additionally, it was found that CO2 adsorption is energetically more stable at higher CO2 coverage than at lower coverage because of CO2–CO2 lateral attractive interactions.

Density functional theory studies on ortho-position adsorption of SO3 at step sites of a CaO surface with SO2 and CO2

With the application of high-sulfur coal and oxy-fuel combustion in power plants, the formation of SO3 in flue gas has strongly enhanced. Hence, the removal of SO3 has received growing attention. Within this context, SO3 sorption using calcium-based sorbents is an interesting approach. In this study, we analyzed the steps on a CaO surface and examined their influence on the adsorption of SO3 molecules using density function theory calculations. The results indicate that steps with O and Ca atoms at the edges are the most stable terminations and also the best terminations for SO3 adsorption. SO3 molecules bind to OCaO sites by SSO3 atoms and weak bonds between Ca and OSO3 are formed. At edge sites, the adsorption energy of SO3, SO2 and CO2 shows a remarkable increase compared to that on a flat surface, whereas it decreases at the top site nearby the edge. The adsorption energy of SO3 is also affected by the OCaO that interacts with SSO3 and the Ca near the OCaO. Interestingly, the SO3 adsorption energy can increase if an SO3, SO2 or CO2 molecule previously adsorbed at a neighboring site. SO3 adsorption is more stable than SO2 and CO2 absorption on step surface and has an advantage in competitive adsorption with SO2 or CO2. The calculation results thus indicate that the formation of a step surface could be an effective method to enhance SO3 sorption on calcium-based sorbents.

Enhanced CO2 Adsorption Capability for Highly Selective Electroreduction of CO2 to Formate

Electrochemical reduction of CO2 (CO2RR) has been regarded as a promising approach to reduce CO2 emission and produce value-added fuels simultaneously. However, the low reaction kinetics, high overpotential for the target products, and the competitive hydrogen evolution reaction significantly limit the performance for CO2RR. Thus, it is desirable to design highly active and selective electrocatalysts to advance the electrocatalytic performance. Herein, we prepared bismuth subcarbonate nanoflowers (BOC NFs) as a highly active and selective electrocatalyst for CO2RR to formate formation. The increased formate selectivity of over 90 % is achieved in a wide potential range with good stability and high energy efficiency. The in situ DRIFTS measurement reveals the strong CO2 adsorption capability of BOC NFs that ensures the good accessibility of CO2 during CO2RR. Also, the abundant active sites and fast charge transfer ability of BOC NFs jointly contribute to the improved catalytic performance to formate formation. The density functional theory calculation further confirms the increased adsorption energy of CO2 and intermediate that enhances the performance for CO2RR to formate on BOC NFs. Overall, this electrocatalyst holds a promising future as a candidate for highly selective and effective CO2RR to formate.

Supported Metal Oxide Catalysts for CO2 Fischer–Tropsch Conversion to Liquid Fuels─A Review

The global increase of greenhouse gas carbon dioxide (CO2) affects the balance of the atmosphere by contributing vastly to global warming. CO2 hydrogenation is an attractive way to use CO2 for value-added product generation. Direct CO2 conversion to synthetic fuels or gasoline fuel, although challenging, is deemed effective in reducing CO2. The adsorption energy of CO2 on the catalyst surface is the limiting factor; hence, the metal–support interaction is established as key in formulating a catalyst with superior performance. Typically iron oxide catalysts were commonly used as a Fischer–Tropsch catalyst, although the CO2 conversion is relatively low. This review aims to highlight the importance of choosing the most suitable support with desired acidity and reduction capability for adequate interaction with catalyst active sites to enhance the activity and selectivity of the existing metal oxide catalyst. Support incorporation methods on the surface metal–support interaction are also scrutinized and accompanied by the reaction parameters for higher conversion, and reactor setup parameters that may have influenced the selectivity are analyzed.

Two-Dimensional Graphdiyne-Confined Platinum Catalyst for Hydrogen Evolution and Oxygen Reduction Reactions.

Pt-based materials are the state-of-the-art catalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR); however, there is still much room to improve the catalytic activity and enhance the stability of Pt-based catalysts. In this work, two-dimensional (2D) graphdiyne (GDY) with uniform distributed pores was applied to cover the Pt surface for catalyzing HER and ORR through density functional theory (DFT) calculations. The 2D confinement induced by GDY was found to improve the catalytic performance of the Pt catalyst from three aspects: (1) the 2D covering layer increases the stability of the Pt catalyst through forming the heterogeneous interface of GDY/Pt(111); (2) GDY/Pt(111) shows better catalytic activities of HER and ORR, with the smaller average overpotential values of 0.26 and 0.51 V, respectively, compared with those (0.29 V for HER, 0.62 V for ORR) on the Pt catalyst; (3) the confinement effect of GDY weakens the adsorption energy of CO to -1.81 eV (average value) from -2.14 eV on Pt(111), inhibiting CO poisoning. This work sheds new light on 2D confinement effects for HER and ORR, which opens up a new strategy for improving the catalytic performance of Pt-based catalysts.

The sorption and separation mechanism of CO2 in covalent organic frameworks with different aligned structures: DFT and molecular dynamic simulations

Insight into the sorption and separation mechanism of CO2 in covalent organic frameworks (COFs) is vital for efficient CO2 capture and separation. It is noted that COFs with amine functional groups facilitates CO2sorption compared with the other COFs. Firstly, COFs with hydrazone-based functional groups as an example is used to analyze the interactions of CO2 with hydrazone-based COFs (N-COFs) and the effect of stacking pattern (AA and AB staking) on CO2 sorption and separation property in different mixed gases by DFT and molecular dynamic simulation. Secondly, the CO2 separation effects of N-COFs with different layer spacing and pore size were compared. The results showed that the stronger interaction of CO2 with N-COFs result in higher sorption capacity of CO2 compared with that of the other gas, which facilitate the storage and separation of CO2. Moreover, the stacking pattern of N-COFs result in the different migration path ofgases, and the CO2 separation effect of N-COFs are improved by adjusting its layer spacing and pore size. In conclusion, the separation of N-COFs for CO2 is affected by the combined action of CO2 adsorption energy, the species of the mixed gas and the size sieving effect.

DFT and kinetic evidences of the preferential CO oxidation pattern of manganese dioxide catalysts in hydrogen stream (PROX)

The oxidation functionality of Mn(IV) sites has been assessed by density functional theory (DFT) analysis of adsorption and activation energies of CO, H2 and O2 on a model Mn4O8 cluster. DFT calculations indicate that Mn(IV) atoms prompt an easy CO conversion to CO2 via a reaction path involving both catalyst and gas-phase oxygen species, while much greater energy barriers hinder H2 oxidation. Accordingly, a MnCeOx catalyst (Mnat/Ceat, 5) with large exposure of Mn(IV) sites shows a remarkable CO oxidation performance at T ≥ 293 K and no H2 oxidation activity below 393 K. Empiric kinetics disclose that the catalyst-oxygen abstraction step determines both CO and H2 oxidation rate, although different activation energies favor the preferential oxidation (PROX) pattern of the studied catalyst (353–423 K). Conversion-selectivity of 100%, high stability during 72 h reaction time and moderate inhibiting effects of water and CO2 feeding reveal the potential of MnO2 materials as efficient, low-cost and robust PROX catalysts.