Electrocatalytic ammonia synthesis: Role of cathode materials and reactor configuration

Ammonia was synthesized by reducing / over several bimetallic surfaces, supported on H⁺ conducting Nafion-117 polymer. When Pd was used alone as a cathode, the cent percent product was ammonia but the process was kinetically unfavorable (2.3 × 10⁻³ min⁻¹) during nitrite reduction. Under the same experimental conditions, Pt–Pd, Ni–Pd, Ag–Pd and Cu–Pd cathodes were mainly selective to (69–82%) but nitrite reduction activity was still low (8.1 × 10⁻³ min⁻¹). The Rh–Pd was only one cathode, which simultaneously enhanced nitrate (21.1 × 10⁻³ min⁻¹) and nitrite (25.6 × 10⁻³ min⁻¹) reduction yielding ca. 75% of . The cyclic voltammetric investigations as well as catalytic hydrogenation experiments indicated that the surface to substrate electron transfer reaction was the main reason of ammonia formation. It was noticed that multi functional roles of the Nafion membrane enhanced the ammonia synthesis performance of the electrolyte reactor.     

Growth of carbon nanotubes in six orthogonal directions on spherical alumina microparticles

Multi-walled carbon nanotubes (MWCNTs) of uniform diameter grow in six orthogonal directions on micrometer spherical alumina particles by chemical vapor deposition. A mixture of xylene and acetylene is used as the carbon source for the growth of CNTs. Detailed investigation shows that the MWCNT arrangement is greatly dependent on the geometry, size and crystallography of the alumina microparticles. Similar CNT growth is found on the pristine and high-temperature (1350 °C) treated particles under the experimental conditions used. Characterization of the treated particles reveals that the growth of CNTs occurs at the crystal step sites, which also exist on the surface of pristine microparticles. The CNT organization varies with the orientation of the crystal steps. The specific surface structures potentially determine the growth of CNTs in six orthogonal directions. The directions of the CNT bundles are normal to {0 0 0 2} or planes, according to selected area diffraction analysis by transmission electron microscopy.     

Universal relation between the boundary temperatures of a basic cycle of sorption heat machines

For analyzing a basic cycle of an adsorption heat machine (AHM) an empiric rule was suggested, which manifests that adsorption isosters and equilibrium line for the pure sorbate intersect at T approaching infinity. This rule prompts how to plot the cycle, gives a link between the boundary temperatures of the cycle and allows estimation of a minimal temperature of an external heat source that is necessary to drive the cycle. In this paper the validity of the estimation was justified for working pairs which are most commonly used for adsorption units: water–silica gel, water–zeolite 13X, water–zeolite 4A, water–selective water sorbents (SWSs), CO₂–carbon, methanol–carbons (AC-35, TA90), methanol–hydrophobic zeolite CBV 901 Y and ammonia–carbon PX31. Four main working pairs for absorption heat machines—ammonia–water, water–LiBr, methanol–LiBr and R22–isobutylacetate—are also analyzed. This allowed the formulation of requirements to an optimal adsorbent to be used in a single-effect non-regenerative cycle of an AHM. The accuracy of the estimation was examined for each pair. Moreover, it was shown that Trouton's rule is always valid if sorption equilibrium obeys the Polanyi potential theory, i.e., the equilibrium sorption is a unique function of the sorption potential ΔF=-RTln(P/P₀). For chemical reactions between various salts and sorbates this rule is violated because of a large difference between the standard changes of the entropy and enthalpy in the course of reaction and evaporation. In this case can be calculated from the Clausius–Clapeyron and Vant-Hoff equations.     

Selective mesoporous adsorbents for and Cu separation

NH₂–MCM-41 and COONa–MCM-41 were prepared by grafting aminopropyls and propionates on the pore wall of the mesoporous silica MCM-41 for the separation of and Cu²⁺. Single and binary component adsorptions show that has a 100% selectivity for adsorption at low pH (<3.5 i.e., point of zero charge, p.z.c.), while COO⁻Na⁺–MCM-41 adsorbs only Cu²⁺ over the entire pH range of the study (i.e., pH 1.5–5.5). Adsorption uptakes were rapid on both adsorbents reaching equilibrium in less than 3 h and in most cases less than 10 min. The adsorption capacity of for is high, approaching the theoretical values of two aminopropyls for one , the Cu²⁺ adsorption on COO⁻Na⁺–MCM-41 was less efficient requiring five propionates to adsorb one Cu²⁺. High purity hydrogen dichromate and copper chloride solutions were respectively recovered by a simple acid wash and the regenerated adsorbents were reused without a loss of performance.     

Influence of carbon dioxide partial pressure and fluidization velocity on activated carbons prepared from scrap car tyre in a fluidized bed

The effect of carbon dioxide partial pressure and fluidization velocity on activated carbons produced by carbon dioxide activation of scrap car tyre rubber in a fluidized bed has been studied. The method consisted of carbonization at under nitrogen followed by activation at . Three types of activated carbons were produced using activated gas concentrations of 20, 60 and 100% carbon dioxide by volume, the rest nitrogen, at a constant fluidization velocity (0.0393 m/s) to investigate the influence of carbon dioxide partial pressure. Within the experimental setup and activation time of 4 h, it was observed that BET surface area and total pore volume increased with carbon dioxide partial pressure reaching and , respectively, for 100% activation with carbon dioxide. Three other types of activated carbons were produced using 100% carbon dioxide at two (0.0393 m/s), three (0.0589 m/s) and four (0.0786 m/s) times the minimum fluidization velocity (U_mf). The BET surface area and total pore volume were observed to increase with fluidization velocity (which can be viewed as an indicator of the intensity of mixing in the bed), reaching and , respectively, at four times the minimum fluidization velocity.     

Hierarchical structures in agar hydrogels

Small angle neutron scattering experiments were performed on agar solutions and gels to explore their differential microscopic structures. In solution state, the wave vector, q, dependence of static structure factor, I(q), could be described by . Statistical analysis gave: R_g = 18 nm and α = 0.85 ± 0.07 indicating the existence of rod-like rigid structures of length, R_g ≈ 63 nm. In gels, which had discernible Gaussian, power-law and Kratky–Porod regimes in the low, intermediate and high-q regions. Regression analysis yielded a characteristic length, Ξ = 3.3 − 4 nm for gels with agar concentration, c = 0.1 − 0.3% (w/v). The exponent β = 1.2 ± 0.2 and the cross-sectional radius of cylindrical fibres, R_c = 1.5 ± 0.3 nm remained invariant of agar concentration. This assigned a value 5 nm to the persistence length of the fibres in the solution phase that reduced to 3 nm in the gel phase indicating differential hydration of the fibres.     

Synthesis of helical carbon nanotubes, worm-like carbon nanotubes and nanocoils at 450 °C and their magnetic properties

High purity (99.21 wt.%) helical carbon nanotubes (HCNTs) were synthesized in large quantity over Fe nanoparticles (fabricated using a coprecipitation/hydrogen reduction method) by acetylene decomposition at 450 °C. Field-emission and transmission electron microscope images reveal that the selectivity to HCNTs (with two or three coiled nanotubes connected to a catalyst nanoparticle) is up to ca. 93%. The yield of HCNTs (as defined by the equation: ) is ca. 7474% in a run of 6 h, higher than any of those reported in the literature. If hydrogen was introduced during acetylene decomposition for ca. 30 min, the HCNTs mainly consisted of two coiled tubes connected to a catalyst nanoparticle, and carbon nanocoils (CNCs) of different structures were generated. If hydrogen was present throughout acetylene decomposition, worm-like carbon nanotubes (CNTs) as well as CNCs were produced in large quantities. Because the HCNTs and worm-like CNTs are attached to Fe nanoparticles, the nanomaterials are high in magnetization.     

Flame nanotube synthesis in moderate electric fields: From alignment and growth rate effects to structural variations and branching phenomena

The flame synthesis of carbon nanotubes (CNTs) coupled with application of moderate electric fields is studied experimentally as a means to control CNTs growth rates and morphology. The nanotubes are grown on a conductive metal-based catalytic probe positioned at the fuel side of the opposed flow oxy-flame. The probe was connected to an external voltage source to generate radial electric fields on its surface. At low applied voltages (from 0.3 to 2 V), the effect of the electric field on alignment and growth rate enhancement revealed the generation of vertically aligned carbon nanotube (VACNT) arrays with uniform distribution of CNT diameters. Further increases of the applied voltage resulted in structural modifications of the generated nanotubes. In particular, helically coiled CNTs were observed at applied voltages of 3 V. At higher voltages the arrays contained multi-walled CNTs with fascinating modified morphologies such as Y, T, and multi-junction patterns. Analysis of the samples generated at applied voltage of 5 V showed the presence of particle sprouting and early CNT junctions in the form of small bumps extruding from the outer surface of the CNTs. Analysis of material samples synthesized at 12 and 25 V showed the presence of fully branched CNT structures.     

Oriented growth of single-walled carbon nanotubes on a MgO(0 0 1) surface

Oriented growth of carbon nanotubes lying on a single-crystal MgO(0 0 1) surface is reported. The nanotubes were grown at 900 °C by chemical vapour deposition using a mixture of carbon monoxide and hydrogen, with Co catalyst nanoparticles formed by self-assembling on MgO under ultra high vacuum conditions. Field emission scanning electron microscopy measurements reveal that the nanotubes grow parallel to the MgO(0 0 1) plane and preferentially along the two perpendicular directions, [1 1 0] and . The interactions between oxygen atoms of the MgO substrate and carbon nanotubes would drive the directional growth. The Raman spectra support that most nanotubes with a diameter ranging from 1.5 to 4 nm, observed by transmission electron microscopy and atomic force microscopy, are single-walled. The observed directional growth requires a very good surface quality of the MgO substrate. Interestingly, rough areas damaged by water vapour prevent the formation of nanoparticles necessary to the nucleation of nanotubes.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media: Part II. Carbon-supported nanoparticles

We studied the borohydride oxidation reaction (BOR) by voltammetry in 0.1 M NaOH/10⁻³ M BH₄⁻ on carbon-supported Pt, Ag and alloyed PtAg nanoparticles (here-after denoted as Pt/C, Ag/C and Pt–Ag/C). In order to compare the different electrocatalysts, we measured the BOR kinetic parameters and the number of electrons exchanged per BH₄⁻ anion (faradaic efficiency). The BOR kinetics is much faster for Pt/C than for Ag/C (i_Pt=0.15, i_Ag=3.1×10⁻⁴ A cm⁻² at E=−0.65 V vs. NHE at 25 °C), but both materials present similar Tafel slope values. The n value involved in the BOR depends on the thickness of the active layer of electrocatalysts. For a “thick layer” (approximately 3 m), n is nearly 8 on Pt/C and 4 on Ag/C, whereas n decreases for thinner Pt/C active layers (n2 for thickness <1 m). These results are in favour of the sequential BH₄⁻ hydrolysis (yielding H₂) followed by hydrogen oxidation reaction (HOR), or direct sequential BOR on Pt/C, whereas Ag/C promotes direct but incomplete BOR (Ag has no activity regarding hydrogen evolution reaction, HER). The n value close to 8 for the thick Pt/C layer displays the sufficient residence time of the molecules formed (H₂ by heterogeneous hydrolysis or BOR intermediates) within the active layer, which favours the complete HOR and/or BOR. Two PtAg/C nanoparticles alloys have been tested (noted APVES-4C and APVES-E1). They show different behavior; the borohydride oxidation reaction kinetics is faster on APVES-E1 than on APVES-4C (b=0.15, and b=0.31 V dec⁻¹,  A cm⁻², respectively, at 25 °C), but the n values are higher on APVES-4C than APVES-E1 (nearly 8 vs. 3, respectively, at 25 °C). These discrepancies probably originate from the heterogeneity of such bimetallic materials, as observed from physicochemical characterizations.     

A mesoporous silica functionalized by a covalently bound naphthalene-based receptor for selective optical detection of fluoride ion in water

The naphthalene urea fluorescent receptor 1 immobilized mesoporous silica (FMS) and the fluorescent receptor 1 immobilized silica particles (FSP) were prepared via sol–gel reaction. The sensing ability of FMS and FSP was studied by addition of the anions F^–, Cl^–, Br^–, I^– and to water suspensions of the assayed solid. Of all the anions tested, addition of fluoride ion to a suspension of FMS resulted in the largest decrease in the fluorescence intensity of naphthalene of FMS. Thus, fluoride ions bind with four urea N–H protons of receptor 1 onto FMS. On the other hand, addition of Cl^–, I^–, Br^– and did not reduce the fluorescence of FMS. In case of FSP, the sensitivity for fluoride ion was 8 times lower than that of FMS due to immobilization of smaller amounts of receptor 1 in silica particles. The result implies that mesoporous silica with its large surface area is useful as a supporting material.     

Os(dppe)(dppe monoxide)(CO)Cl2 as an active intermediate in the synthesis of strongly luminescent divalent osmium complexes

Reaction of K₂OsCl₆ with 1,2-bis(diphenylphosphino)ethane (dppe) in 1:2 ratio afforded the osmium(II) Os(dppe)(dppeO)(CO)Cl₂ complex (where dppeO = 1,2-bis(diphenylphosphino)ethane monoxide) with the molecular structure confirmed by X-ray investigations. The Os(dppe)(dppeO)(CO)Cl₂ intermediate can be simply converted to species in reaction with N ∩ N chelating diimines like 1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline or 3,4,7,8-tetramethyl-1,10-phenanthroline. The obtained mixed complexes feature strong (with _em up to 0.57) long lived (with τ_em up to 25.2 μs) MLCT emission.     

The role of interfacial interactions in the dynamic mechanical response of functionalized SWNT-PS nanocomposites

The dynamic mechanical properties of nanocomposites of functionalized single-walled carbon nanotubes dispersed in polystyrene are reported as a function of temperature. For nanocomposites containing less than 0.1 wt% nanotubes, the enhancement of the magnitude of the complex modulus , where E′ and E″ are the elastic and loss moduli, respectively, exceeded the predictions of the Halpin-Tsai model (this model is often used to predict the properties of macroscopic composites). In this concentration range, however, the loss tangent, tan δ = E″/E′, of the nanocomposite remained comparable to that of pure PS, i.e., comparable elasticity. At larger concentrations the mechanical response became more elastic than PS, but the magnitude of the complex modulus fell appreciably below predictions based on the Halpin-Tsai model. An alternate hypothesis that relies on the role of attractive interactions between the nanotubes and polystyrene chains, and consistent with previous Raman scattering and glass transition temperature data, is proposed to describe the observed dynamic response.     

Anion exchanged polymerized ionic liquids: High free volume single ion conductors

In this study, we investigate the isolated effect of anion type on the chemical, thermal, and conductive properties of imidazolium-based polymerized ionic liquids (PILs). PILs with various anions at constant average chain length were prepared by ion exchange with a water-soluble PIL precursor, (poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (poly(MEBIm-Br)). NMR, IR, and elemental analysis confirm that anion exchange of ploy(MEBIm-Br) with bis(trifluoromethanesulfonyl) imide (TFSI), tetrafluoroborate (BF₄), trifluoromethanesulfonate (Tf), and hexafluorophosphate (PF₆) in water resulted in nearly fully exchanged PILs. As a function of anion type, the glass transition temperature plays a dominant role, but not the sole role in determining ion conductivity. Other factors affecting ionic conductivity include the size and symmetry of the anion and dissociation energy of the ion pair. Both the Vogel-Fulcher-Tammann (VFT) and Williams-Landel-Ferry (WLF) equations were employed to investigate the temperature dependent ionic conductivities. The (9.03) and (168 K) values obtained from the WLF regression of these PILs greatly deviate from the classical WLF values originally obtained from the mechanical relaxation of uncharged polymers ( = 17.44,  = 51.6 K) and the WLF values obtained from the conductive properties of other polymer electrolytes. This suggests that the fractional free volume (f (T_g) = B/(2.303)) and Vogel temperature (T₀ = T_g − ) are strong functions of ion concentration, where high free volume allows for ion mobility at temperatures farther below the glass transition temperature of the polymer.     

Preferential oxidation of CO in excess hydrogen over a nanostructured CuO–CeO2 catalyst with high surface areas

CuO–CeO₂ is prepared by coprecipitation and ethanol washing and characterized using BET, HR-TEM, XRD and TPR techniques. The results show that CuO–CeO₂ is nanosized (r_TEM = 6.5 nm) and possesses high surface area (S_BET = 138 m² g⁻¹). Furthermore, some lattice defects in the surface of CuO–CeO₂ are found, which are beneficial to enhance catalytic performance of CuO–CeO₂ in preferential oxidation of CO in excess hydrogen (PROX). Consequently, the nanostructured CuO–CeO₂ exhibits perfect catalytic performance in PROX. Namely, CO content can be lowered to less than 100 ppm at 150 °C with 100% selectivity of O₂ in the presence of 8% CO₂ and 20% H₂O at .     

Laser desorption-ionization time of flight mass spectrometry of various carbon materials

The possibilities of commercially available MALDI-TOF mass spectrometric instrumentation equipped with 337 nm nitrogen laser in carbon materials analysis have been examined. Laser desorption-ionization of synthetic diamond, graphite, glassy carbon, carbon nano-tubes, and diamond-like thin layer results in the formation of two positively charged ( and odd-numbered 44-346) and one negatively charged carbon cluster set (). Fullerene C₆₀ was analyzed for comparison. From the clusters mass spectra, several similarities, concerning “magic number” ions with high signal intensities, can be observed. It is concluded that differing carbon materials are subjected by UV-laser pulses to similar physico-chemical changes. Heavy carbon clusters of low stability and characteristic loss of neutral C₃ particle in case of the smaller clusters during post-source decay (PSD) implies that the species observed do not possess stable fullerene structure, which might have been expected for C_n species with n ? 20.     

Anomalous IR optical properties of aggregates of Pd nanoparticles induced through electrochemical cyclic voltammetry

IR optical properties of Pd nanoparticles with different size and aggregation state were studied in the current paper. The dispersed Pd nanoparticles () stabilized with poly(N-vinylpyrrolidone) (PVP) were synthesized by the seeding growth method, in which the seeds were formed step by step through reducing H₂PdCl₄ with ethanol. The dispersed Pd nanoparticles of much large size () were grown from the by keeping the colloid of undisturbed for 150 days at room temperature around 20 °C. The aggregates of () were prepared through an agglomeration process induced during a potential cyclic scanning between −0.25 V and 1.25 V for 20 min at a scan rate of 50 mV s⁻¹. Scanning electron microscope (SEM) patterns confirmed such aggregation of . Fourier transform infrared (FTIR) spectroscopy together with CO adsorption as probe reaction was employed in studies of IR optical properties of the prepared Pd nanoparticles. The results demonstrated that CO adsorbed on films substrated on CaF₂ IR window or glassy carbon (GC) electrode yielded two strong IR absorption bands around 1970 cm⁻¹ and 1910 cm⁻¹, which were assigned to IR absorption of CO bonded on asymmetric and symmetric bridge sites, respectively. Similar IR bands were observed in spectra of CO adsorbed on films, except the IR bands were much weak, whereas CO adsorbed on film produced an IR absorption band near 1906 cm⁻¹, and an anomalous IR absorption band whose direction has been completely inverted around 1956 cm⁻¹. The direction inversion of the IR band of CO bonded to asymmetric bridge sites on was ascribed to the interaction between Pd nanoparticles inside the aggregates. Based on FTIR spectroscopic and cyclic voltammetric results, the aggregation mechanism of Pd nanoparticles from to has been suggested that the agglomeration of Pd nanoparticles was driven by the alteration of electric field across electrode-electrolyte interface, when the PVP stabilizer was stripped via oxidation during cyclic voltammetry.     

Electrochemical promotion of CO oxidation on Pt/YSZ: The effect of catalyst potential on the induction of highly active stationary and oscillatory states

We have investigated the kinetics, rate oscillations and electrochemical promotion of CO oxidation on Pt deposited on YSZ using a standard oxygen reference electrode at temperatures 300–400 °C. We have found that electropromotion is small (ρ < 3) when the catalyst potential U_WR, is below 0.4 V and very pronounced (ρ  9, Λ  1500) when U_WR exceeds 0.4 V. This sharp transition in the electropromotion behavior is accompanied by an abrupt change in reaction kinetics and in catalyst potential. For fixed temperature this transition, which leads to a highly active electropromoted state, occurs at specific ratio and catalyst potential. It is shown via comparison with independent catalyst potential–catalyst work function measurements that the transition corresponds to the onset of extensive O²⁻ spillover from YSZ onto the catalyst surface, and concomitant establishment of an effective double layer at the catalyst–gas interface, which is the cause of the highly active electropromoted state.     

Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.