•Mesoporous MoO3 is an effective catalyst in the N-ethylcarbazole hydrogenation.•Pd/MoO3 is superior in the catalytic conversion of Octahydro-N-ethylcarbazole.•The superior activity is due to enhanced H spillover and increased H in HxMoO3.Mesoporous MoO3 shows an apparent activity in the catalytic hydrogenation of N-ethylcarbazole (NEC), where a significant amount of tetrahydro-N-ethylcarbazole (4H-NEC) and perhydro-N-ethylcarbazole (PNEC) are detected with the hydrogen uptake of 0.97 wt% after 6 h when the temperature rises to 220 °C. 0.5 wt% Pd/MoO3 catalyst shows a superior catalytic efficiency than the traditional precious metal catalysts 0.5 wt% Ru/Al2O3 and 0.5 wt% Pd/Al2O3, especially in the conversion of Octahydro-N-ethylcarbazole (8H-NEC) to PNEC. The hydrogenation mechanism of MoO3 is completely different from the traditional precious metal catalysts. With the presence of a small amount of Pd, the breaking of HH bond is greatly accelerated, result in the promotion of hydrogen spillover rate and the increase of the concentration of hydrogen molybdenum bronze HxMoO3, which improves the catalytic efficiency of the MoO3 catalyst. Rise the temperature also helps increasing the concentration of H in HxMoO3.Download high-res image (245KB)Download full-size image

Understanding mechanisms of catalyst–substrate interactions is of essential importance for the design and development of novel catalysts with superior performances. In the present density functional theory study, selective hydrogenation of styrene on a polyacetylene (PA)-supported Pd4 catalyst (Pd4/PA) was employed as a model system to address how catalyst–substrate interactions affect the charge state of Pd, which subsequently influences catalytic activity. It was found that the Pd cluster can be anchored strongly on the C═C bond of the polymer substrate through the π–d interaction, which further leads to charge rearrangement on the Pd4 cluster with the top two Pd atoms being more negatively charged. By comparing the calculated minimum energy profiles of styrene hydrogenation on surfaces of both pure Pd4 and Pd4/PA, the mechanism that dictates the catalytic process on Pd4/PA was identified. Charge analysis reveals that the enhanced catalytic activity of Pd4/PA is largely attributed to the negative charges on the two topmost Pd atoms, which facilitates both hydrogenation of styrene and desorption of the product. Nevertheless, PA hydrogenation to produce polyethylene (PE) was also found to be a potentially viable process with a moderate activation barrier of 0.43 eV, which may consequently lead to the formation of a PE-supported Pd4 catalytic system. As a consequence, the absence of π orbitals of the PE substrate may significantly reduce the electronic interaction between Pd4 and PE, which ultimately leads to the catalytic performance similar to the activity on the pure Pd4 cluster.

Utilizing the functional groups of carbon substrates or third-party additives to adhere lithium polysulfides for suppressing their dissolution has been demonstrated to be capable of improving the sulfur cathode stability of lithium–sulfur batteries. In the present first-principles study, we systematically investigated the competitions between polysulfide self-cohesion, solvation, and its anchoring strengths on substrates. The dissolution probability of polysulfides in ether-based electrolytes is evaluated by a defined solvation potential ΔPS-C, which confirms that Li2S8 is the most soluble species; the competition of Li2S8 anchoring strength on different substrates and its solvation energy is described by a stabilizing potential ΔPS-A, which can be used to verify if a certain substrate can effectively stabilize polysulfides in cathodes. Two properties for a feasible substrate, containing affinitive sites with high electron density for anchoring polysulfides and containing sufficient affinitive sites to provide multiple interactions for enhancing the stabilization, are necessarily proposed. Accordingly, phosphorylated chitosan, among several substrates, is predicted to be a promising third-party substrate to preserve polysulfides in cathodes and prevent them from being dissolved. Our computational scheme may provide a reliable procedure for rapidly screening most appropriate candidates for designing the novel architecture of sulfur cathodes.

The development of a heterogeneous catalyst with high catalytic activity and durability for H2O2-mediated oxidation is one of the most important industrial and environmental issues. In this study, a Mn(II)-doped TiO2 heterogeneous catalyst was developed for H2O2-mediated oxidation. The TiO2 substrate-dependent partial-redox behavior of Mn was identified on the basis of our density functional theory simulations. This unique redox cycle was induced by a moderate electron transfer from Ti to Mn, which compensated for the electron loss of Mn and finally resulted in a high-efficiency cycling of Mn between its oxidized and reduced forms. In light of the theoretical results, a Mn(II)-doped TiO2 composite with well-defined morphology and large surface area (153.3 m2 g–1) was elaborately fabricated through incorporating Mn(II) ions into a TiO2 nanoflower, and further tested as the catalyst for oxidative degradation of organic pollutants in the presence of H2O2. Benefiting from the remarkable textural features and excellent Mn cycling property, this composite exhibited superior catalytic performance for organic pollutant degradation. Moreover, it could retain 98.40% of its initial activity even in the fifth cycle. Our study provides an effective strategy for designing heterogeneous catalytic systems for H2O2-mediated oxidations.Keywords: H2O2-mediated oxidation; heterogeneous catalyst; high catalytic activity and durability; Mn(II)-doped TiO2; partial redox;

Glycidyl fatty acid esters (GEs) are by-products of edible oil refinement that have attracted attention globally due to concerns over their possible harmful effects on human health when consumed. It is thus important to improve our understanding of GE formation if we are to suppress GE production during edible oil refinement. In this paper, a first-principles density functional theory study of the formation mechanism of GEs was performed. Triglycerides undergo a self-condensation reaction between two adjacent ester groups to yield GEs and an anhydride as a by-product. This process is energetically unfavorable, having a relatively high activation energy of around 80 kcal/mol, which indicates that GE formation is intrinsically a high-temperature process. Both the thermodynamic and the kinetic energies of the reaction are insensitive to the size of the fatty chain substituents present. If water participates in the self-condensation, the activation barrier is notably decreased by 23.9 kcal/mol, indicating that GE production in the presence of high-temperature water vapor should be more kinetically favorable. Our results suggest that reducing the reaction temperature and avoiding the use of water should suppress GE production during edible oil refinement.

We reported a rhombohedral Na-rich nickel hexacyanoferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na+ ions in the rhombohedral framework in parallel with the low charge density at the Fe2+ ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g–1 at the current density of 80 mA g–1, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral–cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/discharge and the DFT calculations, respectively.Keywords: cathode material; density functional theory study; high discharge voltage; mechanism; Na-ion batteries; rhombohedral nickel hexacyanoferrate;

Nanoparticle catalysts consist of hundreds or thousands of atoms with structures that are essentially unknown. First-principles-based quantum mechanical calculations on this scale of substance would be prohibitively expensive to perform. Consequently, it has become common to use studies of small clusters at the subnano scale to gain insight into the chemical reactivity of nanoparticle catalysts. Recent theoretical and experimental investigations, however, have found that hydrogen reactivity on small Pt clusters is sensitive to the charge states of the clusters. This finding is in contrast to expectations about the reactivity of nanoparticles and casts doubt on whether small clusters can indeed be used to model realistic catalysts. The present case study for Pt clusters provides a systematic analysis of the charge sensitivity of the key catalytic properties for hydrogenation and clarifies the conditions for which a subnanoscale model may be expected to provide meaningful insights into the behavior of nanoparticle catalysts.Keywords: catalysts; cluster charge states; hydrogenation; platinum; subnano clusters

We present a first-principles study using periodic density functional theory on a water gas shift reaction on a Feoct2-tet1-terminated Fe3O4 (111) surface. We show that water can easily undergo dissociative adsorption to form OH and H adatom species on the surface. Three possible reaction mechanisms (i.e., redox mechanism, associative mechanism, and coupling mechanism) were systematically explored based on minimum energy path calculations. It was identified that the redox mechanism is the energetically most favorable pathway for the water gas shift reaction on the Feoct2-tet1-terminated Fe3O4 (111) surface. The COO* desorption was found to be the rate-limiting step with a barrier of 1.04 eV, and the OH dissociation has the second-highest activation barrier (0.81 eV). Our results are consistent with results of kinetic and isotope exchange experiments. Our studies suggest that it is necessary to develop a promoter to reduce the activation barriers of the COO* desorption and OH dissociation steps in order to improve the catalyst performance.

Stabilizing lithium polysulfides in cathodes via interactions between polysulfides and affinitive functional groups could prevent polysulfide dissolution, leading to suppressed “shuttle effect” of lithium/sulfur (Li/S) batteries. Herein, four deoxynucleotides (DNs), including A (adenine-DN), T (thymine-DN), G (guanine-DN), and C (cytosine-DN), which own rich polysulfide affinitive groups, are selected to model the anchoring environments of polysulfides. Using the most soluble Li2S8 as probe, our first-principles simulations suggest that the interactions between polysulfides and substrates are highly correlated to the charges of affinitive sites, H-bonding environments and structural tension. The contributions from each type of interactions are quasi-quantitatively assessed. The electrostatic attractions between Li+ and the strong electron lone-pairs dominate the adsorption energetics, while the H-bonds formed between S82– and substrate give rise to excessive stabilization. In contrast, structural distortion or rearrangement of the substrates is detrimental to the anchoring strengths. The quasi-quantitative resolution on the different interaction modes provides a facile and rational scheme for screening more efficient polysufide affinitive additives to sustain the cathode cyclicity of Li/S batteries.

A detailed reaction mechanism has been proposed for the full ALD cycle of Si3N4 deposition on the β-Si3N4(0001) surface using bis(diethylamino)silane (BDEAS) or bis(tertiarybutylamino)silane (BTBAS) as a Si precursor with NH3 acting as the nitrogen source. Potential energy landscapes were derived for all elementary steps in the proposed reaction network using a periodic slab surface model in the density functional approximation. Although the dissociative reactivity of BTBAS was slightly better than that of BDEAS, the thermal deposition process was still found to be an inherently high temperature process due to the high activation energies during the dissociative chemisorption of both precursors and the surface re-amination steps. These results underline the need to develop new precursors and alternative nitrogen sources when low temperature thermal silicon nitride films are targeted.

The energy landscape of a full atomic layer deposition cycle to grow a layer of SiO2 on the hydroxylated SiO2(001) surface was systematically explored using density functional theory. A monoaminosilane-based compound, di(sec-butylamino)silane (DSBAS), was utilized as the silicon precursor with ozone acting as an oxidizing agent. The ALD cycle includes dissociative chemisorption of DSBAS, oxidation, and condensation for surface regeneration. Our results indicate that the dissociative chemisorption of DSBAS is kinetically facile. Upon oxidation by ozone, the layer grows with a SiO2 crystalline morphology. The entire ALD cycle was found to be thermodynamically and kinetically favorable. This is important for growing dense and conformal SiO2 thin films free of impurities and thus well-suited for low-temperature deposition of SiO2 thin films.

A self-assembling deposition process of SiO2 thin film growth catalyzed by Al with a small silanol precursor was systematically studied using density functional theory. The full catalytic self-assembling deposition (CSD) cycle is divided into two half reactions. In the first half, the trimethylaluminum molecule undergoes a dissociation process on the hydroxylated SiO2(001) surface that results in the anchoring of an −AlCH3 species on the surface and the sequential elimination of two CH4 molecules. Subsequently, in the second half of the reaction, two reaction routes, i.e., the top-down and the bottom-up routes, were examined to address the growth mechanism of the chain extension with bis(methoxyl)-monobutoxylsilanol. Our results suggest that the bottom-up route is energetically preferred with a strong influence by the catalytic effect of the seed layer of the Al species. The sp2 electronic configuration of the Al atom allows its pz orbital to accept electron from the lone pair of the silanol precursor, which facilitates the Al–O formation. The electronic configuration of the Al atom was found to undergo sp2 → sp3 → sp2 evolution cycles along the reaction pathway, each of which produces one layer of a Si–O unit to grow the chain. Our results are consistent with the experimental observations and provide a detailed mechanistic understanding on the CSD processes.

A detailed reaction mechanism has been proposed for the full ALD cycle of Si3N4 deposition on the β-Si3N4(0001) surface using bis(diethylamino)silane (BDEAS) or bis(tertiarybutylamino)silane (BTBAS) as a Si precursor with NH3 acting as the nitrogen source. Potential energy landscapes were derived for all elementary steps in the proposed reaction network using a periodic slab surface model in the density functional approximation. Although the dissociative reactivity of BTBAS was slightly better than that of BDEAS, the thermal deposition process was still found to be an inherently high temperature process due to the high activation energies during the dissociative chemisorption of both precursors and the surface re-amination steps. These results underline the need to develop new precursors and alternative nitrogen sources when low temperature thermal silicon nitride films are targeted.