In situ XPS investigations of Cu1−xNixZnAl-mixed metal oxide catalysts used in the oxidative steam reforming of bio-ethanol

A series of CuNiZnAl-multicomponent mixed metal oxide catalysts with various Cu/Ni ratios were prepared by the thermal decomposition of Cu_1−xNi_xZnAl-hydrotalcite-like precursors and tested for oxidative steam reforming of bio-ethanol. Dehydrogenation of EtOH to CH₃CHO is favored by Cu-rich catalyst. Introduction of Ni leads to CC bond rupture and producing CO, CO₂ and CH₄. H₂ yield (selectivity) varied between 2.6–3.0 mol/mol of ethanol converted (50–55%) for all catalysts at 300 °C. The above catalysts were subjected to in situ XPS studies to understand the nature of active species involved in the catalytic reaction. Core level and valence band XPS as well as Auger electron spectroscopy revealed the existence of Cu²⁺, Ni²⁺ and Zn²⁺ ions on calcined materials. Upon in situ reduction at reactions temperatures, the Cu²⁺ was fully reduced to Cu⁰, while Ni²⁺ and Zn²⁺ were partially reduced to Ni⁰ and Zn⁰, respectively. On reduction, the nature of ZnO on Cu-rich catalyst changes from crystalline to amorphous, relatively inert and highly stabilized electronically. Relative concentration of the Ni⁰ and Zn⁰ increases upon reduction with decreasing Cu-content. Valence band results demonstrated that the overlap between 3d bands of Cu and Ni was marginal on calcined materials, and no overlap due to metallic clusters formation after reduction. Nonetheless, the density of states at Fermi level increases dramatically for Ni-rich catalysts and likely this influences the product selectivity.     

Electrical resistivity of transition metal ion doped Mullite

The electrical resistivity of pure mullite (3Al₂O₃.2SiO₂) varies from 10¹³ ohm-cm at room temperature (r.t.) to 10⁴ ohm-cm at 1400 °C. It was observed that by doping mullite with the 3d-type transition metal ions, e.g. Mn, Fe, Cr and Ti, the resistivity of mullite could be reduced to 10¹¹ ohm-cm, i.e. 1/100 that at r.t. and 1/5 that at 1400 °C. The resistivity of doped and undoped mullite decreased by 6–5 orders at about 500–600 °C but 4–3 orders between this temperature and 1400 °C. The 3d orbital electrons, the oxidation states and the concentration of the transition metal ions as well as the sites of mullite lattice occupied by the ions were found responsible for lowering of resistivity of mullite. Evidence of the presence of Mn²⁺, Mn³⁺, Fe³⁺, Cr³⁺ and Ti⁴⁺ ions in mullite had been obtained which entered the octahedral site. The Ti⁴⁺ ion which substituted Al³⁺ ion in the octahedral site of mullite structure appeared to be the most efficient one to reduce the resistivity. This has been confirmed by the results of activation energy of resistivity/band gap energy, Eg which was the lowest for mullite doped with 1·0 wt% Ti⁴⁺ ion. At 1·0 wt% concentration level, these ions lowered the resistivity of mullite to minimum.     

Physicochemical surface and catalytic properties of CuO–ZnO/Al2O3 system

A series of CuO–ZnO/Al₂O₃ solids were prepared by wet impregnation using Al(OH)₃ solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O₂ at temperatures ranged between 125°C and 200°C.

The results showed that CuO interacts with Al₂O₃ to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl₂O₄ at 600°C while stimulates its production at 800°C. The treatment of CuO/Al₂O₃ solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol⁻¹ in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature.     

Extensive intrazeolitic hydrolysis of Zn(II): partial structures of partially and fully hydrated Zn(II)-exchanged zeolite X

Complete Zn²⁺ exchange of two single crystals of zeolite X (Na₉₂Si₁₀₀Al₉₂O₃₈₄) was attempted at 80°C from aqueous Zn(NO₃)₂ (pH=5.5 at 23°C). The structures of crystal 1 (partially dehydrated by evacuation at 23°C and 10⁻³ Torr for two days) and crystal 2 (fully hydrated) were determined by X-ray diffraction techniques in the cubic space group Fd at 23°C (a_o=24.750(5) and 24.872(6) Å, respectively). They were refined using all intensities to the final error indices R₁=0.126 and 0.116 based on the 428 and 348 reflections, respectively, for which F_o>4σ(F_o). Each crystal has about 54 Zn²⁺ ions per unit cell, indicating the uptake of eight excess Zn(OH)₂ molecules. In both crystals, further extensive hydrolysis of Zn²⁺ is seen. Many non-framework oxygens were not found. In crystal 1, 34 Zn²⁺ ions per unit cell occupy conventional cationic sites: 10 are at site I^′, 12 at site II, and 12 at site III^′. Three Zn²⁺ ions each coordinate to a framework oxygen at a non-conventional site in the supercages. Three Zn²⁺ ions at the centers of sodalite cavities each coordinate tetrahedrally to four non-framework oxygens to give (likely) Zn(OH)₂(H₂O)₂ which hydrogen bonds multiply to the zeolite framework. At three supercage positions, about 14 Zn²⁺ ions that do not coordinate to the zeolite framework are found. Per unit cell, 37 H₃O⁺ ions are found: 20 at site I^′ and 17 at site II. It is presumed, considering the number of H₃O⁺ ions, that the latter 14 Zn²⁺ ions are hydrolyzed Zn²⁺ ions, likely hydrated Zn(OH)₂ molecules, some likely bridging. In crystal 2, 33 Zn²⁺ ions per unit cell are found at conventional cationic sites: two at site I, 14 at two different sites I^′, seven at site II, and 10 at site III^′. As in crystal 1, three Zn²⁺ ions each coordinate to a framework oxygen at a non-conventional site in the supercage. At three supercage positions, about 18 Zn²⁺ ions that do not coordinate to the zeolite framework are found. Per unit cell, 40 H₃O⁺ ions are found: 18 at site II^′ and 22 at site II. Only about 16 non-framework oxygens were found per unit cell: eight water molecules in the supercages and, in the sodalite cages, eight hydroxide ions which participate in the formation of two nearly cubic Zn₄(OH)₄⁴⁺ clusters.     

Recovery of fluoride values from spent pot-lining: Precipitation of an aluminium hydroxyfluoride hydrate product

Treatment of spent pot-lining (SPL) from aluminium smelting cells by a two-stage leaching scheme comprising a water wash and an Al³⁺ leach and fluoride recovery as an aluminium hydroxyfluoride product has been studied for extraction of fluoride and then recovery as smelter grade AlF₃. The NaF content of a −1.18 mm size fraction was removed by the water wash, while the more refractory Na₃AlF₆ and CaF₂ were removed by treatment with 0.34 M Al³⁺ solution at 25 °C for 24 h, which yielded an overall fluoride extraction of 76–86 mol%. Mathematical modelling using experimental stability constant data was carried out to predict the effect of combining solutions and identify ways to manipulate the solution equilibria to maximise fluoride precipitation yields. The predictions were then tested experimentally. In the 4.5–5.5 pH range, selective precipitation of fluoride as an aluminium hydroxyfluoride hydrate product was achieved by neutralisation of the combined solutions with addition of 2 M NaOH solution. Higher pH values lead to the co-precipitation of hydrolysed sodium fluoroaluminates. Characterisation of precipitates using X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, thermogravimetric analysis, differential thermal analysis, aluminium and fluoride determination have pointed out both the AlF₂(OH)·1.4H₂O stoichiometry and possible thermal decomposition pathways to yield a dehydrated aluminium hydroxyfluoride product, AlF₂(OH), that could be used for smelter grade AlF₃ production. The kinetics of hydrolysis are such that nucleation dominates while particle growth is restricted. Techniques to allow slow hydrolysis are necessary to form smelter grade AlF₃.     

Cation distribution between cristobalite, tridymite, and coexisting glass phase in used silica bricks

The chemical composition of cristobalite, tridymite, glass, and accessory phases of different zones of used silica bricks taken from the roof of a glass tank was studied with a high resolution microprobe. Tridymite and cristobalite contain as impurities TiO₂ (≤ 0.36 wt%), Al₂O₃ (≤ 0.37 wt%), and Na₂O (≤ 0.27 wt%). Main constituents of the glass phase coexisting with crystalline silica are: SiO₂ (74 to 60 wt%), TiO₂ (0.4 to 9 wt%), Al₂O₃ (1 to 5 wt%), Fe₂O₃ (0.3 to 3 wt%), CaO (5 to 20 wt%), and Na₂O (8 to 17 wt%). Temperature curves within the bricks during operation of the glass tank have been estimated using direct temperature measurement at the hot front of the bricks, and the transition temperatures of cristobalite to tridymite ( 1450°C), and of - to β-wollastonite ( 1200°C). Microchemical data and supposed temperatures were correlated with the Nernst distribution law. The applicability of the Nernst law shows that local equilibrium conditions were reached during the use of the bricks; they have been preserved during cooling of bricks. The results of the Nernst law cation distribution imply that structural saturation with Al₂O₃, TiO₂, and Na₂O was not reached in the investigated composition range. Al³⁺ is believed to substitute Si⁴⁺ at tetrahedral lattice sites. Al³⁺ substitution is favoured with decreasing temperature in relation to the Al₂O₃ content in the glass phase. Al³⁺ → Si⁴⁺ substitution produces charge deficiency, which is compensated by interstitial entry of Na⁺ into structural channels and voids of tridymite and cristobalite. Ti⁴⁺ incorporation into the cristobalite and tridymite structures is favoured at higher temperatures with respect to the TiO₂ content of the glass phase. The close reciprocal dependence between Al³⁺ and Ti⁴⁺ in silica may indicate that Ti⁴⁺ is tetrahedrally incorporated as well.     

Characterization of Al-doped spinel LiMn2O4 thin film cathode electrodes prepared by Liquid Source Misted Chemical Deposition (LSMCD) technique

A novel process called Liquid Source Misted Chemical Deposition (LSMCD) was used to synthesize Al-doped LiMn₂O₄ cathode films for Lithium microbatteries. The cathode films were characterized by XRD, SEM, cyclic volatmmetry, and charge/discharge test. LiMn_1.8Al_0.2O₄ film crystallized at 800 °C in rapid thermal annealing (RTA) for 5 min under oxygen atmosphere exhibited more improved electrochemical rechargeability than spinel LiMn₂O₄ film because the substitution of Al³⁺ for Mn³⁺ increased Mn---O bonding strength in the spinel framework and suppressed the two-phase behavior of the unsubstituted spinel during the intercalation/deintercalation that is the origin of the failure mechanism in the 4 V region. As a result, LiMn_1.8Al_0.2O₄ film showed an initial discharge capacity of 52 μAh/cm² μm and no capacity fade over 100 cycles.     

Studies of the activation process over Pd perovskite-type oxides used for catalytic oxidation of toluene

Calcined and reduced catalysts Pd/LaBO₃ (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T₅₀ values (temperature at which 50% of toluene is converted): Pd/LaFeO₃ > Pd/LaMnO_3+δ > Pd/LaCoO₃ > Pd/LaNiO₃. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H₂-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO₃ (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 °C for Pd/LaFeO₃ and either 200 or 250 °C for LaCoO₃. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 °C for 2 h (2 L/h, 1 °C/min). As LaFeO₃ was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B³⁺ (B = Mn) or B²⁺ even to B⁰ (B = Co, Ni). In the reactive stream (0.1% toluene in air), Pd⁰ reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd²⁺/Pd⁴⁺ and Pd⁰/Pd²⁺/Pd⁴⁺ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co³⁺ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO₃ for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity.     

Effective Au-Au-Clx/Fe(OH)y catalysts containing Cl for selective CO oxidations at lower temperatures

Supported Au catalysts Au-Au⁺-Cl_x/Fe(OH)_y (x < 4, y ≤ 3) and Au-Cl_x/Fe₂O₃ prepared with co-precipitation without any washing to remove Cl⁻ and without calcining or calcined at 400 °C were studied. It was found that the presence of Cl⁻ had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au⁺-Cl_x/Fe(OH)_y was further calcined at elevated temperatures, such as 400 °C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au⁺, while after calcined at 400 °C it reduced to Au⁰ completely. It also showed that catalysts precipitated at 70 °C could form more Au⁺ species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 °C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO₃)₃ and HAuCl₄ hydrolyzed partially to form Fe(OH)₃ and [AuCl_x(OH)_4−x]⁻ (x = 1–3), respectively, at 70 °C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O₂ were more easily adsorbed on Au⁺ and Au⁰, but was very difficultly absorbed on Au⁻. It also indicated that when Cl⁻ was adsorbed on Au⁰, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl⁻ was adsorbed on Au⁺ there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity.     

Gold supported on ceria and ceria–alumina promoted by molybdena for complete benzene oxidation

New gold–molybdena catalysts supported on ceria and ceria–alumina in reaction of complete benzene oxidation were studied. The catalysts were characterized by means of XRD, TPR, XPS and Raman spectroscopy. High and stable catalytic activity was established in the temperature region 200–240 °C. The presence of gold causes a modification in ceria structure leading to an increase of Ce³⁺ and oxygen vacancies formation. The loading of Al³⁺ increases additionally the oxygen vacancies, while a tendency of decrease of Ce³⁺ amount was observed. The presence of alumina results also in a larger share of active oxygen species proved by analysis of O 1s XPS spectra. The differences in the activities within the starting temperature range (150–180 °C) and in the region of 100% conversion (200–240 °C) could be explained by supposing that in the LT region the electron transfer between nanosized gold and ceria particles via oxygen vacancies has a crucial role. In the HT region the oxygen mobility, provoked by the defective structure of ceria due to the presence of Al³⁺, becomes of prevailing importance. It was also concluded that alumina prevents the gold and ceria agglomeration, which is the main factor to avoid deactivation under extreme reaction conditions.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Absence of E. coli regrowth after Fe and TiO2 solar photoassisted disinfection of water in CPC solar photoreactor

Field disinfection of water in a large solar compound parabolic collector (CPC) photoreactor (35–70 l) was conducted at 35 °C by different photocatalytic processes: sunlight/TiO₂, sunlight/TiO₂/Fe³⁺, sunlight/Fe³⁺/H₂O₂ and compared to the control experiment of direct sunlight alone. Experiments were carried out using a CPC and natural water spiked with E. coli K 12. Under these conditions, total disinfection by bare sunlight irradiation was not reached after 5 h of treatment; and bacterial recovery was observed during the subsequent 24 h in the dark.

The addition of TiO₂, TiO₂/Fe³⁺ or Fe³⁺/H₂O₂ to the water accelerates the bactericidal action of sunlight, leading to total disinfection by solar-photocatalysis. No bacterial regrowth was observed during 24 h after stopping sunlight exposure. For some samples, the decrease of bacteria continues in the dark. A “residual disinfection effect” was observed for these samples before reaching the total inactivation. The effective disinfection time (EDT₂₄), defined as the treatment time required to prevent any bacterial regrowth during the subsequent 24 h in the dark, after stopping the phototreatment, was reached in the presence but not in the absence of different photocatalytic systems. EDT₂₄ was 2 h 30 min, 2 h and 1 h 30 min for sunlight/TiO₂, sunlight/TiO₂/Fe³⁺ and sunlight/Fe³⁺/H₂O₂ systems, respectively. The post irradiation events observed when the phototreated water is poured into an optimal growth medium are also discussed.     

Low-temperature prepared highly effective ferric hydroxide supported gold catalysts for carbon monoxide selective oxidation in the presence of hydrogen

Ferric hydroxide supported Au catalysts prepared with co-precipitation method at room temperature without any heat treatment hereafter exhibited high catalytic activity and selectivity for CO oxidation in air and CO selective oxidation in the presence of H₂. With calcination temperature rising, both activity and selectivity decreased. X-ray Photoelectron Spectra (XPS) indicated that Au existed as Au⁰ and Au⁺ in the catalyst without heat treatment and even after being calcined at 200 °C, while after being calcined at 400 °C, Au existed as Au⁰ completely. X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopic (HRTEM) investigations indicated that both the supports and Au species were highly dispersed as nano or sub-nano particles even after being calcined at 200 °C, but after being calcined at 400 °C the supports transformed to crystal Fe₂O₃ with typical diameter of 30 nm and Au species aggregated to nano-particles with typical diameter of 2–4 nm. HRTEM investigations also suggested that the supports calcined at 200 °C were composed of amorphous ferric hydroxide and crystal ferric oxide. Results of computer simulation (CS) showed that O₂ was adsorbed on Au crystal cell and then were activated, which should be the key factor for the subsequent reaction. It also suggested that O₂ species were more easily adsorbed on Au⁺ than on Au⁰, indicating that higher positive charge of the Au species possessed the higher activity for CO oxidation.     

Influence of isovalent and aliovalent substitutions at Ti site on bismuth sodium titanate-based compositions on piezoelectric properties

The effect of isovalent and aliovalent substitutions in Bi_0.5Na_0.485La_0.005TiO₃ (BNLT) compounds were studied within the additive ranges of 0–2.5 at%. The Zr⁴⁺, Nb⁵⁺ and Fe³⁺ ions were selected as the substituents. The modified BNLT compounds were prepared by conventionally mixed-oxide method. The calcination and sintering were performed at the temperatures of 750–850 °C and 1050–1150 °C, respectively. An increase in the substituents contents affected the physical and piezoelectric properties. The BNLT compositions with the addition of 1 at% Zr⁴⁺, Nb⁵⁺ and Fe³⁺ ions exhibited high relative permittivities (_r) at 730, 735 and 660, respectively. The modified-BNLT with an addition of 1.0 at% Fe provided a piezoelectric coefficient (d₃₃) of 155 pC/N, Curie temperature (T_c) of 320 °C and electromechanical coupling factors in planar (k_p) and thickness (k_t) modes of 15 and 45%, respectively.     

Evaluation of the microporosity of pillared [Fe(CN)6]–MgAl–LDHs

Layered double hydroxides (LDHs) containing Mg²⁺ and Al³⁺ in the basic layers and NO⁻₃ as an interlayer anion were synthesized by the method of coprecipitation (pH 10). By changing the Mg²⁺/Al³⁺ ratio (1.5–4.5), the charge density on the (NO₃)–MgAl–LDH sheets was varied. After pillaring with Fe(CN)³⁻₆, which was based on an anion exchange process, the interlayer space became accessible. This was reflected in the large created surface areas and micropore volumes. The applied models for the calculation of the micropore size distributions (Maes–Zhu–Vansant and Horvath–Kawazoe) gave matching results, revealing narrow distributions for all the samples, with the majority of the pores smaller than 0.71 nm. A correlation was found between the Mg²⁺/Al³⁺ ratio and the resulting microporosity after pillaring. The optimal ratio was situated around 3.3, resulting in a pillared [Fe(CN)₆]–MgAl–LDH with a Langmuir surface area of 499 m²/g and a micropore volume between 0.158 ml/g (μPV_min) and 0.177 ml/g (μPV_max). As an alternative, direct coprecipitation of the pillared LDHs was evaluated. This one-step mechanism proved to be a method producing similar results. Taking all this into consideration, one can conclude that hexacyanoferrate(III) complexes form ideal anionic pillars for the creation of microporous layered double hydroxides.     

Preparation of supported Pt-M catalysts (M = Mo and W) from anion-exchanged hydrotalcites and their catalytic activity for low temperature NO-H2-O2 reaction

The NO-H₂-O₂ reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO₂)₄²⁻, MoO₄²⁻, and/or WO₄²⁻ and subsequent heating at 600 °C in H₂. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H₂ in the absence of O₂. The Pt-HT catalyst showed the almost complete NO conversion at 70 °C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O₂, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30 °C, compared to ≥60 °C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO₃ and promote the catalytic conversion of NO to N₂O at low temperatures.     

Redox behavior of palladium at start-up in the Perovskite-type LaFePdOx automotive catalysts showing a self-regenerative function

In order to elucidate the superior start-up activity of LaFePdO_x catalysts in practical automotive emission control, the redox property of Pd species in a Perovskite-type LaFe_0.95Pd_0.05O₃ catalyst was studied at temperatures ranging from 100 to 400 °C using X-ray spectroscopic techniques. In a reductive atmosphere, and even at temperatures as low as 100 °C, Pd⁰ species is partially segregated out onto the catalyst surface from the B-site of the Perovskite-type matrix of LaFe_0.95Pd_0.05O₃. Passing through successive oxidizing atmospheres, the segregated Pd⁰ species is re-oxidized into Pd²⁺ at 200–300 °C. The formation of a solid solution between the re-oxidized Pd species and the Perovskite-type matrix begins to be seen at around 400 °C and accelerates at higher temperatures. Thus a quasi-reversible redox reaction between the surface Pd⁰ and the cationic Pd in the LaFe_0.95Pd_0.05O₃ matrix takes place. The start-up activity of LaFePd_xO_x catalysts can be attributed to Pd⁰ that segregates under the reductive atmosphere which is a natural part of the redox fluctuation in automotive exhaust gases at 100–200 °C.     

Dielectric and ferroelectric properties of lead indium niobate ceramic prepared by wolframite method

The dielectric and ferroelectric properties of lead indium niobate (Pb(In_1/2Nb_1/2)O₃, PIN) ceramic prepared by an oxide-mixing method via wolframite route were investigated. The 98.5% perovskite fine-grained PIN ceramics with average grain sizes of 1–2 μm were obtained by sintering at 1050 °C for 2 h. The dielectric properties of the PIN were of relaxor ferroelectric behavior with temperature of dielectric maximum (T_m) 53 °C and dielectric constant (_r) 4300 (at 1 kHz). The P–E hysteresis loop measurements at various temperatures showed that the ferroelectric properties of the PIN ceramic changed gradually from the paraelectric behavior at temperature above T_m to slim-loop type relaxor behavior at temperature below T_m. Moreover, the P–E loop became more open at temperatures much lower than T_m. At −25 °C, the maximum polarization is found to be 8 μm/cm² at a field of 30 kV/cm, with P_r value of 2.5 μm/cm² and E_c of +7.5 kV/cm.     

Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N₂, increased as Cu loading increased. N₂ selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H₂ (H₂-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu²⁺ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H₂-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu⁺ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO₂ suggested that CuO could form N₂, but, independent of the CuO dispersion, nitrogen oxides (NO_x) were formed above 350 °C. Cu⁺ and the square-pyramidal and distorted square-pyramidal forms of Cu²⁺ showed low activity for AN decomposition. Temperature-programmed desorption of NH₃ suggested that N₂ formation from NH₃ proceeded on Cu²⁺, resulting in the formation of Cu⁺. The Cu⁺ ions were oxidized to Cu²⁺ at around 300 °C. Thus, high N₂ selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu²⁺, which can be reversibly reduced and oxidized.     

In situ FTIR characterization of the adsorption of CO and its reaction with NO on Pd-based FCC low NOx combustion promoters

The adsorption of CO and its reaction with NO in the 400–600 °C temperature range on Ceⁿ⁺/Na⁺/γ-Al₂O₃ and Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃ type materials used commercially as FCC additives were monitored by FTIR spectroscopy. Exposure of both types of samples to CO leads to the formation of carboxylates and carbonates. The concentration of these species was higher in samples containing Pd, indicating that palladium catalyzes their formation. The Pdⁿ⁺ cations initially present in these samples undergo partial reduction to form metallic Pd in the presence of CO even at room temperature. More complete reduction of Pd, along with some aggregation, was observed after exposure to CO at elevated temperatures. Exposure of both types of samples to NO/CO mixtures in the 400–600 °C temperature range leads to the formation of surface isocyanate species. Both Na⁺ and Ceⁿ⁺ promote the formation of such NCO species. However, surface isocyanate species were formed with substantially higher rates in the presence of palladium. The formation of the isocyanate species strongly correlates with changes observed in the ν_OH region, indicating that hydroxyls actively participate in the surface chemistry involved and are capable of protonating the NCO species. The isocyanates are also reactive towards O₂ and NO yielding CO₂ and N₂. These results suggest that isocyanates are possibly involved as intermediates in the CO–NO reaction over the materials examined.