Electronegative Cations Research Articles

Softeners doped with asphaltene dispersants acting in synergy with resins: Molecular mechanisms underlying asphalt rejuvenation

In confronting the global climate change, reusing waste asphalt materials stands out as a key strategy for the transportation sector to preserve natural resources and reduce the carbon footprint. Recycling additives or rejuvenators are usually required in this practice, especially for boosting the recycling rate. Given the imperative need for a general compositional design principle of rejuvenators, we previously proposed an innovative formulation concept by combining softeners with asphaltene dispersants, and reported that the use of dispersants largely reduced the total quantity of rejuvenators needed and thus would minimize the environmental impacts. The present study is aimed for probing the molecular mechanisms underlying this observation, with additional interest focused on the synergistic effect of the dispersants with resins and also on the impacts due to their self-association. Selection of representative materials consisted of three types of dispersants including an ionic liquid, a phenolic amphiphile, and an ester plasticizer, in combination with an aromatic oil as the softener. Long-time molecular dynamics (MD) simulation was employed to inspect the aggregation of asphaltenes. Quantum chemical (QC) computation was performed to provide insights into the source of different dispersing behaviors. The results indicated that the three dispersants all acted in concert with resins in dispersing asphaltenes. The dispersion characteristics of asphaltenes could be viewed as a net result of the two competing mechanisms of the dispersing capability and self-association of the dispersant used. The ionic liquid exhibited much higher propensity to self-association at increasing concentrations than the phenolic dispersant, and yet the optimum performance was achieved for both when they were added at low dosages. The ester plasticizer showed minimal self-association and yielded improved dispersion with increasing concentrations. In all cases, the dispersing functionality mainly originated from the presence or formation of significant electron-withdrawing sites, collectively due to the highly electronegative cation and anion, hydrogen bonding, and polar groups. Comparison of the dispersing power was also conducted from different perspectives. The findings are expected to provide molecular insights for formulating versatile and more environmentally benign asphalt rejuvenators.

Optimizing the Oxygen‐Catalytic Performance of Zn–Mn–Co Spinel by Regulating the Bond Competition at Octahedral Sites

AbstractBy using the more electro‐negative Mn3+ ion to partially replace Co3+ at the octahedral site of spinel ZnCo2O4, i.e., forming ternary Zn–Mn–Co spinel oxide, the electrocatalytic oxygen reduction/evolution activity is found to be significantly increased. Considering the physical characterization and theoretical calculations, it demonstrated that the bond competition played a key role in regulating the cobalt valence state and the electrocatalytic activity. The partial replacement of octahedral‐site‐occupied Co3+ by Mn3+ can effectively modulate the adjacent Co–O bond and induce the Jahn–Teller effect, thus changing the originally stable crystal structure and optimizing the binding strength between the active center and reaction intermediates. Certainly, the Mn‐substituted ZnMn1.4Co0.6O4/NCNTs exhibit higher electrocatalytic oxygen reduction reaction (ORR) activity than that of ZnCo2O4/NCNTs and ZnMn2O4/NCNTs, supporting that the Co–O bond covalency determines the ORR activity of spinel ZnCo2O4. This study offers the competition between adjacent Co–O and Mn–O bonds via the BOh–O–BOh edge‐sharing geometry. The ion substitution at octahedral sites by less electronegative cations can be a new and effective way to improve the electrocatalytic performance of cobalt‐based spinel oxides.

Improved Regioselective Mononitration of Naphthalene over Modified BEA Zeolite Catalysts

HBEA zeolite modified with highly electronegative cations is a highly efficient and reusable catalyst for the nitration of naphthalene with nitric acid, which has been successfully prepared in this work. Catalytic selective mononitration of naphthalene was investigated. The ratio of 1-nitronaphthalene isomer to 2-nitronaphthalene could reach 19.2, with a moderate yield of 68.2%, when the reaction was carried out in 1,2-dichloroethane, with 1.0 mmol naphthalene, 0.22 mL nitric acid (95%), and 0.10 g HBEA-25 at −15 °C. The effects of reaction temperature and the quantity of zeolites on 1-nitronaphthalene were also studied. The catalyst is readily recyclable, and we believe this to be a major step forward in the area of clean technology for aromatic nitration.

Research progress of two-dimensional covalent bond substructure Zintl phase thermoelectric materials

Thermoelectric materials can realize the direct conversion between thermal energy and electrical energy, and thus having important applications in semiconductor refrigeration and heat recovery. Zintl phase is composed of highly electronegative cations and anions, which accords with the concept of “phonon glass, electron crystal” (PGEC). Thermoelectric properties of Zintl phase have attracted extensive interest, among which the two-dimensional (2D) covalent bond structure featured Zintl phases have received more attention for their outstanding electrical properties. In this review, Zintl phase materials with two-dimensional covalent bond substructures are reviewed, including 1-2-2-type, 9–4+<i>x</i>–9-type, 2-1-2-type and 1-1-1-type Zintl phase. The 1-2-2-type Zintl phase is currently the most widely studied and best-performing Zintl material. It is worth mentioning that the maximum <i>ZT</i> value for the Mg<sub>3</sub>Sb<sub>2</sub>-based n-type Zintl material with the CaAl<sub>2</sub>Si<sub>2</sub> structure has been reported to reach 1.85, and the average <i>ZT</i> value near room temperature area also reaches 1.4. The 9–4+<i>x</i>–9-type Zintl material with a mass of atoms in unit cell contributes to lower thermal conductivity thus relatively high <i>ZT</i> value. The 2-1-2-type Zintl material has extremely low thermal conductivity due to the intrinsic vacancies, which has been developing in recent years. The 1-1-1-type Zintl material with the same ZrBeSi structure as the 2-1-2-type Zintl material, shows better electrical transport performance. In sum, this review summarizes the recent progress and optimization methods of those typical Zintl phases above. Meanwhile, the future optimization and development of Zintl phase with two-dimensional covalent bond substructures are also prospected.

An Effective Way to Improve Bifunctional Electrocatalyst Activity of Manganese Oxide via Control of Bond Competition

A critical role of bond competition in tailoring Mn valence state and bifunctional electrocatalyst activity of manganese oxide is evidenced by the remarkable improvement of the electrocatalyst activity of α-MnO2 upon the partial substitution of electronegative Ru4+ ion. The replacement of Mn4+ ion with more electronegative Ru4+ one is quite effective in weakening adjacent (Mn−O) bonds in terms of bond competition, leading to the stabilization of Jahn-Teller active Mn3+ species, as well as in providing electrocatalytically active Ru sites. The resulting Ru-substituted α-Mn1−xRuxO2 nanowires show much higher electrocatalyst activities for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) than does the physical mixture of α-MnO2 and RuO2, indicating the main role of (Mn−O) bond covalency in the optimization of the bifunctional electrocatalyst activity of manganese oxide. The present study underscores that, like the previous strategy of structural disorder enhancement, the substitution of highly electronegative cation can provide a novel efficient way of improving the electrocatalyst performance of manganese oxide via the bond competition between adjacent (Ru−O) and (Mn−O) bonds.

Metal salts with highly electronegative cations as efficient catalysts for the liquid-phase nitration of benzene by NO2 to nitrobenzene

Metal salts with highly electronegative cations have been used to effectively catalyze the liquid-phase nitration of benzene by NO2 to nitrobenzene under solvent-free conditions. Several salts including FeCl3, ZrCl4, AlCl3, CuCl2, NiCl2, ZnCl2, MnCl2, Fe(NO3)3∙9H2O, Bi (NO3)3∙5H2O, Zr(NO3)4∙5H2O, Cu(NO3)2∙6H2O, Ni (NO3)2∙6H2O, Zn(NO3)2∙6H2O, Fe2(SO4)3, and CuSO4 were examined and anhydrous FeCl3 exhibited the best catalytic performance under the optimal reaction conditions. The benzene conversion and selectivity to nitrobenzene were both over 99%. In addition, it was determined that the metal counterion and the presence of water hydrates in the salt affects the catalytic activity. This method is simple and efficient and may have potential industrial application prospects.

Dielectric abnormality and ferroelectric asymmetry in W/Cr co-doped Bi4Ti3O12 ceramics based on the effect of defect dipoles

In this paper, a sort of W/Cr co-doped Bi4Ti3O12 ceramics with different additional amount of Cr2O3: Bi4Ti2.95W0.05O12.05 + y wt. % Cr2O3 (BITW-yCr, y = 0–0.4) were prepared by a conventional ceramic process. XRD analysis for BITW-yCr ceramics firstly reveal that the formation of orthorhombic Bi4Ti3O12 was accompanied by the output of less cubic Bi2Ti2O7 as the second phase. On the dielectric response curves of samples, there are two abnormal variations that a dielectric “hunch” with the relaxation characteristic occurs around 500 °C at low frequencies, and the dielectric peak at Curie temperature presents an obvious dispersity as its intensity is significantly decreasing with the increase of frequency. Here, a physical mode for describing the formation process and acting mechanism of defect dipoles was established. In the pseudo-perovskite layer of BITW-yCr ceramics, the electronegative cation defects tend to associate with the electropositive oxygen vacancies, forming the electric defect dipoles. The orientated polarization of defect dipoles driving by the external electric filed contributed to the dielectric relaxation process based on the activated motion of oxygen vacancy at high temperatures, besides the inability of ionic polarization, the interference effect of defect dipoles on the short-range interaction of intrinsic dipoles may be another reason for the dielectric dispersion. On the other hand, the internal bias field built by the reorientated defect dipoles is considered to cause the asymmetry of hysteresis loops for ferroelectrics, and the addition of Cr2O3 (y) is found to increase the internal bias field of BITW-yCr ceramics. Additionally, it is also observed to decrease the temperature coefficient of dielectric constant of samples. Finally, profited from a lower coercive field and a larger spontaneous polarization, the resulting large remanent polarization (Pr = 17 μC/cm2) contributes to a high piezoelectricity (d33 = 28 pC/N) for the ample at y = 0.2.

A route to ultrathin quantum gases at polar perovskite heterointerfaces

AbstractOxide interfaces are attracting interest in recent years due to special functionalities of two‐dimensional quantum gases. However, with typical thicknesses of at least 10–12 Å the gases still extend considerably in the third dimension, which compromises the size of quantum effects. To overcome this limitation, we propose incorporation of highly electronegative cations, such as Ag. By ab initio calculations, we demonstrate the formation of a mobile two‐dimensional hole gas in AgNbO3/SrTiO3 that is confined to an ultrathin slab of only 5.6 Å thickness. Electronegative cations therefore are a promising way to enhance the quantum nature of hole gases. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Doped-vanadium oxides as sensing materials for high temperature operative selective ammonia gas sensors

Abstract Resistive electrochemical sensors based on vanadium oxides equipped with a pair of interdigital Au electrodes can detect NH3 gas selectively at high temperature (500 °C). NH3 addition in a base gas increased the relative conductance (σ/σ0). Addition of less electronegative cation (Ce, Zr, Mg) to V2O5 increased the response and recovery rates, while electronegative cation (Al, Fe, Ni) increased sensor response magnitude. Among the samples tested, Al and Ce co-doped sample (VAlCe) was the most suitable sensor. The VAlCe sensor responded rapidly and linearly to change in concentration of NH3 in the oxygen rich gas mixture and showed high selectivity in the presence of coexisting gases (NO, CO, H2). The presence of water vapor did not markedly decrease the response magnitude but increased the response rate; the 90% response and 50% recovery times were less than 15 s. Based on the in situ UV–vis results, a possible sensing mechanism is proposed; adsorbed NH3 causes reduction of V5+ to V4+, which results in the conductivity increase. Role of surface acidity on the selective detection of NH3 as a basic molecule is also discussed.

Influences of A- and B-site cations on the physicochemical properties of perovskite-structured A(In 1/3Nb 1/3B 1/3)O 3 (A = Sr, Ba; B = Sn, Pb) photocatalysts

We have investigated the effects of A- and B-site cation substitution on the physicochemical properties of perovskite-structured A(In 1/3Nb 1/3B 1/3)O 3 (A = Sr, Ba; B = Sn, Pb) photocatalysts. X-ray diffraction, X-ray absorption and diffuse UV–vis spectroscopic analyses reveal that tetravalent Pb IV or Sn IV ions can be successfully incorporated into the octahedral B-site of the perovskite lattice, leading to a narrowing of bandgap energy ( E g). The substitution of such electronegative cations gives rise to the enhancement of photocatalytic activity to effectively decompose organic molecules. Interestingly, alkaline earth metal ions in the dodecahedral A-site with ionic bonding environment also affect significantly the band structure and photoefficiency of the perovskite compounds; a larger cation is beneficial for creating visible light driven photocatalytic activity through a decrease of E g. This observation could be understood in terms of the weakening of transition metal–oxygen bond upon the expansion of unit cell. The present relationship between the chemical bonding nature of substituent cation and the band structure provides an efficient tool for designing and developing new efficient visible light active photocatalysts.

A study of the bonded interactions in nitride molecules in terms of bond critical point properties and relative electronegativities

Bond critical point properties calculated for the MN bonds in a number of geometry optimized nitride molecules containing first- and second-row M cations are compared with those calculated for a number of oxide molecules. As reported for the oxides, the value of the electron density, ρ(r c ), at the bond critical points, r c , increases with decreasing bond length while for the more electronegative cations, the local energy density, H(r c ) decreases nonlinearly in value as the relative electronegativities of the M-cations, χ M , tend to increase. In the majority of cases, χM, |λ1|/λ3 and ∇2ρ(r c ) increase with decreasing minimum energy bond lengths. The bond lengths adopted by the molecules are indicated to be an important determinant of the critical point properties of the electron density distributions. The relative electronegativities derived from the electron density distributions of the nitrides agree with those derived for the oxides and Pauling’s electronegativities to within ∼5%, on average.

Atomic and ionic radii: a comparison with radii derived from electron density distributions

The bonded radii of anions obtained in topological analyses of theoretical and experimental electron density distributions differ from atomic, ionic and crystal radii in that oxide-, fluoride-, nitride- and sulfide-anion radii are not constant for a given coordination number. They vary in a regular way with bond length and the electronegativity of the cation to which they are bonded, exhibiting radii close to atomic radii when bonded to a highly electronegative cation and radii close to ionic radii when bonded to a highly electropositive cation. The electron density distributions show that anions are not spherical but exhibit several different radii in different bonded directions. The bonded radii of cations correlate with ionic and atomic radii. But unlike ionic radii, the bonded radius of a cation shows a relatively small increase in value with an increase in coordination number. In contrast to atomic and ionic radii, the bonded radius of an ion in a crystal or molecule can be used as a reliable and well-defined estimate of its radius in the direction of its bonds.

Promotion effects of various oxides on oxidation of benzene over vanadium pentoxide catalysts

The effect of promoters on the activity and selectivity of the active sites was investigated in the selective oxidation of benzene over vanadium pentoxide catalysts. The activity per surface area is increased by addition of MoO 3, WO 3, P 2O 5 and SnO 2, but the activity of each redox sites, i.e., turnover frequency (TF), was not. This indicates that the promoters enhanced the activity by increasing the redox sites, such as the surface V O species. The selectivity to maleic anhydride (MA) was improved by the addition of MoO 3, WO 3 and P 2O 5, while it was lowered by SnO 2. The effect on the selectivity was discussed in the light of the electronegativity of promoter cations. The additions of MoO 3, WO 3 and P 2O 5, of which the cations are more electronegative than V 5+, decreased TF of the production of carbon oxides more seriously than that of MA. On the other hand, the addition of less electronegative cation, i.e., Sn 4+, increased TF of the production of carbon oxides at the sacrifice of MA.

Nature of the Effect of Counter Cations on the Redox Property of 12-Molybdophosphates

Abstract The role of counter cations of 12-molybdophosphates in determining the redox property of the catalysts has been studied systematically. It is suggested that the electronegative cations act as large electron reservoirs to facilitate electron transfer in the redox process, as supported by the XPS results.