Catalysis and deactivation of montmorillonite K10 in the aryl <Emphasis Type="Italic">O</Emphasis>-glycosylation of glycosyl trichloroacetoimidates

The catalysis of montmorillonite K10 (MK10) for aryl O-glycosylation of glycosyl trichloroacetimidates was investigated. It was found that the catalyst MK10 is deactivated gradually in the recycle glycosylation. The fresh and the deactivated catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetric analysis (TGA), and N₂ adsorption-desorption. The results show that the eliminated trichloroacetamide molecule deposits on the MK10, which blocks and poisons the active sites, resulting in the deactivation of the catalyst. The regeneration of the deactivated MK10 by calcination was studied preliminarily.     

Deactivation of Cu/ZnO catalyst during dehydrogenation of methanol

The deactivation of Cu/ZnO catalyst during methanol dehydrogenation to form methyl formate has been studied. The Cu/ZnO catalyst was seriously deactivated under the reaction conditions: various temperatures of 493, 523 and 553 K, atmospheric pressure and methanol GHSV of 3000 ml (STP)/g-cat h. The weight loss due to reduction of ZnO in the Cu/ ZnO catalyst was monitored by a microbalance. X-ray induced Auger spectroscopy of Zn(L₃M_4,5M_4,5) showed the increase in the concentration of metallic Zn on the catalyst surface after the reaction. Temperature-programmed reduction (TPR) of the Cu/ZnO catalyst with methanol demonstrated that the reduction of ZnO in Cu/ ZnO was suppressed by introduction of CO₂ into the stream of helium-methanol. As the concentration of CO₂ in the feed gas increased, the weight loss of the Cu/ZnO catalyst due to the reduction of ZnO decreased. The deactivation of the Cu/ZnO catalyst in the methanol dehydrogenation was also retarded by the addition of CO₂. In particular, oxygen injection into the reactant feed regenerated the Cu/ ZnO catalyst deactivated during the reaction. Based on these observations, the cause of deactivation of the Cu/ZnO catalyst has been discussed.     

Regeneration of Lamina TS-1 Catalyst in the Epoxidation of Propylene with Hydrogen Peroxide

The deactivation and regeneration of the lamina titanium silicalite (TS-1) catalyst for the epoxidation of propylene with dilute H₂O₂was investigated in a fixed-bed reactor. In the scale-up experiment, the dosage of the lamina TS-1 catalyst is 2. 5 kg, after 1000 h reaction the catalyst still exhibits good performance and further increases the reaction time, the conversion of H₂O₂begins to decrease. TG and BET analyses of the deactivated catalysts show that the main species occluded within the zeolite pore are propylene oxide oligomers, and these species occupying the active Ti site and blocking the pores of the lamina TS-1 are the main reason for the deactivation of catalyst. The deactivated catalyst can be regenerated by different regeneration methods. The activity of deactivated catalysts regenerated by dilute H₂O₂or heat treatment by using air or nitrogen as a calcination media can be fully recovered, but a decline in propylene oxide (PO) selectivity of the regenerated catalyst has been observed during the first hours of reaction. However, water vapor treatment of the deactivated catalyst can improve the PO selectivity with the same activity as that of the fresh lamina TS-1 catalyst.     

Study on deactivation of Cu–Zn–Al catalysts in the synthesis of N-ethylethylenediamine

The performance of a Cu–Zn–Al catalyst employed in the synthesis of N-ethylethylenediamine from ethylenediamine and ethanol was studied. The results showed that the activity of the Cu–Zn–Al catalyst decreased with time-on-stream. Fresh and deactivated catalysts were characterized by XRD, XPS, N₂ adsorption–desorption and TEM. It was found that the crystallite size of Cu and ZnO in the deactivated catalyst were much bigger than those for the fresh catalyst. In addition, channels in the deactivated catalyst were blocked by carbonaceous deposits, so the surface area and pore volumes of the deactivated catalyst were much smaller than in the fresh catalyst. Therefore, it was concluded that the deactivation of the Cu–Zn–Al catalyst was mainly caused by the growth in the Cu and ZnO crystallite sizes and carbonaceous deposits.     

Environmentally benign catalytic synthesis and hydro‐refining of linear alkylbenzenes

The synthesis technology of linear alkylbenzenes (LABs) was studied through the preparation of an Al‐SBA‐15 catalyst, the optimization of alkylation conditions, and the regeneration of deactivated catalyst. The hydro‐refining over the Pd/Al₂O₃ catalyst was carried out to remove unsaturated hydrocarbon impurities from the LAB. The results of the alkylation reactions over the Al‐SBA‐15 catalyst in a liquid fixed bed reactor showed that the olefin conversion remained above 98 % for time on stream of 3000 h, and the LAB selectivity was above 93 % under the following conditions: temperature of 260 °C; pressure of 5.0 MPa; weight hourly space velocity (WHSV) of 1.0 h^?1; and the molar ratio of benzene to olefin of 25:1. Through the burning coke regeneration, the catalytic performance of the deactivated alkylation catalyst was satisfactorily restored. The quality of the LAB synthesized through alkylation and hydro‐refining was better than that of the industrial LAB produced using the hydrofluoric acid catalytic process.     

New Approaches to Immobilization of Aluminum Chloride on γ-Alumina and Its Regeneration After Deactivation

The preparation of an AlCl₃ catalyst immobilized on -Al₂O₃ and its regeneration after deactivation have been studied. AlCl₃, generated by reacting CCl₄ with -Al₂O₃, was carried by N₂ to a reactor containing the -Al₂O₃ support. The immobilized AlCl₃ catalyst with meso- and macro-pore bimodal structure was shown to be suitable for isobutene oligomerization. The amount of AlCl₃ immobilized on the support in terms of AlCl _x (x = 2.2) was 7.5 wt%. The catalyst exhibited excellent catalytic properties for isobutene polymerization under mild conditions. The average molecular weight of the product was 1000–2500, and its distribution was narrow, around 2.0 in the reaction temperature range 10–40 °C. This catalyst showed nearly perfect reactive specificity to isobutene polymerization and remarkable stability. After 2000 h of continuous running, the conversion dropped from 99 to 57%, the selectivity was maintained with little change at about 90%, and the average molecular weight was within the range 1000–1200 under the conditions T = 32 ± 1 °C, LHSV = 2.0 h^-1, and P = 1.0 MPa.Reneration of the deactivated catalyst was satisfactorily accomplished by treating the used catalyst with a saturated solution of AlCl₃ in CCl₄ either in situ or ex situ. The activity recovery can be as high as 96%, and the deactivated catalyst can be regenerated repeatedly.     

Rh<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Pd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Nanoparticle Composition Dependence in CO Oxidation by NO

Abstract  

Bimetallic 15 nm Pd-core Rh-shell Rh_1−x Pd _x nanoparticle catalysts have been synthesized and studied in CO oxidation by NO. The catalysts exhibited composition-dependent activity enhancement (synergy) in CO oxidation in high NO pressures. The observed synergetic effect is attributed to the favorable adsorption of CO on Pd in NO-rich conditions. The Pd-rich bimetallic catalysts deactivated after many hours of oxidation of CO by NO. After catalyst deactivation, product formation was proportional to the Rh molar fraction within the bimetallic nanoparticles. The deactivated catalysts were regenerated by heating the sample in UHV. This regeneration suggests that the deactivation was caused by the adsorption of nitrogen atoms on Pd sites.     

Hydrocarbon steam reforming using Silicalite‐1 zeolite encapsulated Ni‐based catalyst

A Silicalite‐1 zeolite membrane encapsulated 1.6 wt % Ni–1.2 wt % Mg/Ce_0.6Zr_0.4O₂ steam reforming composite catalyst synthesized by a physical coating method was used to investigate effect of encapsulation on size selective steam reforming, using methane (CH₄) and toluene (C₇H₈) as representative species. Characterization methods (scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Physisorption Analysis, and x‐ray diffraction (XRD)) were used to analyze pre‐ and post‐reaction samples. SEM, EDS, and XRD analyses showed that Silicalite‐1 was coated successfully onto the core catalyst. Weisz‐Prater Criteria and Thiele moduli calculations indicated internal diffusion limitations. Combined reforming of CH₄ and C₇H₈ at 800°C on the composite catalyst demonstrated stability during the 10 h time on stream while the uncoated SR catalyst deactivated. The non‐acidic Silicalite‐1 encapsulated catalyst showed decreases (～2–7%) in both CH₄ and C₇H₈ conversions compared to acidic H‐β zeolite confirming that shell acidity did contribute to conversion and suggested that shell defects/grain boundaries were responsible for the C₇H₈ conversion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 200–207, 2017     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

The effect of the change of the structure on oxidative dehydrogenation of propane over cerium–zirconium catalysts

A series of pure CeO₂, ZrO₂, and CeZrO_x mixed metal oxide catalysts were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 500°C and 0.1 MPa. The prepared catalysts were characterized by thermal gravimetric analysis (TGA), Brunauer, Emmett, and Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopes (TEM), Raman spectroscopy, and H₂-TPR. It was observed that the zirconium content of the solid solution of the mixed metal oxide catalyst was 5%–25%, while the zirconium content of the material with phase segregation was higher (50%). The addition of zirconium was proven to decrease the oxygen vacancy concentration on the catalyst surface and change the intensity of (111) crystal of cerium oxide in the catalysts. Among the prepared catalysts, the Ce_0.90Zr_0.10O_x catalyst with the maximum strength of the (111) crystal plane of cerium oxide exhibited the better catalytic oxidation performance for the dehydrogenation of propane to propylene. Compared with ZrO₂ in the blank experiment, the average propane conversion and propylene selectivity of the Ce_0.90Zr_0.10O_x catalyst were increased by 10.78% and 17.95%, respectively.     

Efficient In-Situ Regeneration Method of the Catalytic Activity of Aged TWC

The expansion of durability of deactivated “three-way” catalysts (TWCs) used in gasoline-driven cars by applying efficient, economically viable and environmentally friendly methods for the in␣situ regeneration of their performance to acceptable levels was investigated. New experimental results on the use of a weak oxalic acid washing solution as a means of an efficient regeneration method of a severely aged (83,000 km mileage) commercial TWC are presented. Oxalic acid is shown to be the most efficient extracting agent of phosphorus, a severe poison of TWCs, among acetic acid, citric acid, NTA and EDTA investigated. X-ray diffraction studies provided strong evidence that washing of the aged TWC results in the removal of CePO₄, AlPO₄ and (Mg,Ca,Zn)₃(PO₄)₂ type phosphates leading to a significant increase of BET area and pore volume, as well as of CO and NO conversions (catalytic activity tests). The latter is strongly related with the increase in the number of active catalytic sites, as illustrated by in␣situ DRIFTS studies, after opening closed pores and uncovering additional catalyst surface.     

NO Reduction with Hydrogen over Cobalt Molybdenum Nitride and Molybdenum Nitride: A Comparison Study

Cobalt molybdenum nitride (Co₃Mo₃N) and molybdenum nitride (Mo₂N) were investigated for the catalytic reduction of NO with H₂. The latter deactivated rapidly with time on stream, whereas the former remained active and stable over a test period of 30 h. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H₂-temprature-programmed reduction (H₂-TPR) characterization indicated that the deactivation of Mo₂N was due to the bulk oxidation of Mo₂N to MoO₂. As for the Co₃Mo₃N catalyst, despite partial decomposition into Mo₂N and Co, it remained resistant to oxidation. The results suggest that compared to the monometallic nitride, the bimetallic one is more suitable for NO reduction with H₂.     

Electrochemical promotion of Rh catalyst in gas-phase reduction of NO by propylene

The concept of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been applied for the in situ control of catalytic activity of a rhodium film deposited on YSZ (yttria stabilized zirconia) solid electrolyte towards reduction of 1000 ppm NO by 1000 ppm C₃H₆ in presence of excess (5000 ppm) O₂ at 300 °C. A temporary heating at this feed composition results in a long-lasting deactivation of the catalyst under open circuit conditions due to partial oxidation of the rhodium surface. Positive current application (5 A) over both the active and the deactivated catalysts gives rise to an enhancement of N₂ and CO₂ production, the latter exceeding several hundred times the faradaic rate. While active rhodium exhibits a reversible behaviour, electrochemical promotion on the deactivated catalyst is composed of a reversible and an irreversible part. The reversible promotion results from the steady-state accumulation of current-generated active species at the gas exposed catalyst surface whereas the irreversible effect is due to the progressive reduction of the catalyst resulting in an increased recovery rate of lost catalytic activity. The results are encouraging with respect to application of rhodium for the catalytic removal of NO from auto-exhaust gases under lean-burn conditions.     

NO oxidation over supported cobalt oxide catalysts

NO oxidation was conducted over cobalt oxides supported on various supports such as SiO₂, ZrO₂, TiO₂, and CeO₂. The N₂ physisorption, an inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), NO chemisorptions, the temperature-programmed desorption (TPD) with a mass spectroscopy after NO or CO chemisorptions were conducted to characterize catalysts. Among tested catalysts, Co₃O₄ supported on ceria with a high surface area showed the highest catalytic activity. This catalyst showed superior catalytic activity to unsupported Co₃O₄ with a high surface area and 1 wt% Pt/γ-Al₂O₃. For ceria-supported Co₃O₄, the catalytic activity, the NO uptake at 298 K and the dispersion of Co₃O₄ increased with increasing the surface area of CeO₂. The active participation of the lattice oxygen in NO oxidation could not be observed. On the other hand, the lattice oxygen participated in the CO oxidation over the same catalyst. The deactivation was observed over Co₃O₄/CeO₂ and 1 wt% Pt/γ-Al₂O₃ in the presence of SO₂ in a feed. 1 wt% Pt/γ-Al₂O₃ was deactivated by SO₂ more rapidly compared with Co₃O₄/CeO₂.     

Application of Ru exchanged zeolite-Y in ammonia synthesis

Na-Y zeolite was cation exchanged with Ru(NH₃)₆Cl₃ yielding at 25% exchange level a light-purple solid which was active in ammonia synthesis at atmospheric pressure. Pulse conversion experiments show that the catalyst stores nitrogen as it was observed with the conventional iron catalyst. At 810 K the conversion reached about 20% of the maximum conversion of the iron catalyst. The catalyst deactivated reversibly within 30 h due to agglomeration. The active species in the catalyst is most likely a cluster-like Ru metal particle prevented from sintering under the reducing conditions of catalysis by the zeolite framework.     

Fe–Nx/C electrocatalysts synthesized by pyrolysis of Fe(II)–2,3,5,6-tetra(2-pyridyl)pyrazine complex for PEM fuel cell oxygen reduction reaction

2,3,5,6-Tetra(2-pyridyl)pyrazine (TPPZ) was employed as a ligand to prepare an iron(II) complex (Fe–TPPZ) that served as a precursor to synthesize carbon-supported catalysts (Fe–N_x/C) through heat-treatment at 600, 700, 800 and 900 °C under N₂ atmosphere. Both the structure and composition of the synthesized Fe–N_x/C were analyzed by X-ray diffraction and energy-dispersive X-ray microanalysis, respectively. The rotating disk and ring-disk electrode measurements showed that these catalysts have strong ORR activity with an overall 4-electron transfer process through a (2 + 2)-electron transfer mechanism, which was assigned to the catalytic function of the Fe–N_x center. A study on the heat-treatment temperature on the ORR activity showed that 800 °C is the optimal temperature for the synthesized catalysts. Furthermore, the effect of both catalyst and Nafion^® ionomer loadings in the catalyst layer on the corresponding ORR activity was also investigated. The kinetic parameters such as the chemical reaction rate between O₂ and Fe–N_x/C (adduct formation reaction), the rate constant for the rate-determining step (RDS), and the electron numbers in the ORR, were obtained. The methanol tolerance of the catalyst was also tested. To validate the ORR activity, a membrane electrode assembly in which the cathode catalyst layer contained Fe–N_x/C was constructed and tested in a real fuel cell. The results obtained are encouraging when compared with similar non-noble catalysts.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part II: preparation and characterization of MEAs

Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe _x catalysts. Preparation parameters varied were: catalyst loading (0.5–2 mg RuSe _x cm⁻²), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe _x in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe _x catalysts investigated, i.e. ∼40 mW cm⁻² at 80 °C under ambient pressure and λ_MeOH = λ_air = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe _x catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe _x particle size (∼5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe _x catalyst layers.     

Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds

Catalytic oxidations of volatile organic compounds (VOCs) (benzene, toluene and o-xylene) over 1 wt% Pd/γ-Al₂O₃ catalyst were carried out to assess the properties and performance of the Pd based catalyst. The properties of the prepared catalysts were characterized by the Brunauer Emmett Teller (BET) surface area, H₂ chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The experimental results revealed a significant increase in VOCs conversion with the lapse of the reaction time at certain reaction temperatures. On the other hand, the hydrogen pretreated 1 wt% Pd/γ-Al₂O₃ catalyst, whose shape of conversion curve is similar to the non pretreated catalyst, led the conversion curves for the total oxidation of VOCs to be shifted to lower temperature. It was also found that such increases in VOCs conversion were highly dependent on the oxidation state of Pd and the growth of Pd particles in the catalyst. In addition, in the case of the catalyst consisting of the same oxidation state (PdO/Pd²⁺ or Pd⁰), the particle sizes possibly play a more important role in the catalytic activity. The activity order of 1 wt% Pd/γ-Al₂O₃ catalyst with respect to the VOC molecule was o-xylene > toluene > benzene.     

Guaiacol HDO on La-modified Pt/Al2O3: Influence of rare-earth loading

The stability of the catalyst used in hydrodeoxygenation (HDO) of biomass-derived oils needs improvement. La has been applied in delaying Al₂O₃ phase-change under reaction conditions. Lanthanum (0.5–8 wt.%)-γ-alumina was studied as Pt (1 wt.%) carrier aimed at guaiacol (GUA) HDO. Materials characterization included N₂ physisorption, X-ray diffraction (XRD), thermal analysis, FTIR, UV–vis, and TPR. Solids pore size (~8–10 nm) was suitable for GUA (kinetic diameter~0.668 nm) hydrotreating. Mixed carriers were amorphous (XRD), suggesting well-dispersed La domains; meanwhile, carbonates/bicarbonates were formed (from CO₂) due to the basic surface properties of modified supports (FTIR). That could impart catalyst stability by inhibiting coking through the passivation of Lewis acidity on Al₂O₃. Pt reducibility increased with La loading in various formulations. However, that was not reflected in enhanced GUA HDO (T = 488 K and P = 3.2 MPa, batch reactor), presumably due to the strong metal–support interaction (SMSI), where LaO_x covered the metallic Pt particle surface. GUA HDO on various catalysts was approximated by pseudo-first-order kinetics (integral regime, k), where deviations were observed as La loading increased, presumably by an SMSI state that could affect the rate-determining step of the reaction mechanism. Basic sites provided by rare-earth could contribute to altering HDO reaction pathways as well. At 1 wt.% rare-earth, GUA HDO was maximized (k~25% higher than that on Pt/Al₂O₃), with that material also exhibiting similar deoxygenation (85%–90% at total GUA conversion) to the latter Pt over pristine alumina. Conversely, both parameters significantly diminished over the catalyst of the highest La content. Materials at low rare-earth concentrations deserve further studies focused on catalyst stability under HDO conditions.     

Effect of reducing agents on microstructure and catalytic performance of precipitated iron-manganese catalyst for Fischer–Tropsch synthesis

Effects of reducing agents on the textural properties and bulk/surface phase compositions of a precipitated iron-manganese catalyst were investigated by N₂-physisorption, X-ray photoelectron spectroscopy (XRD), Mössbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (LRS). Fischer–Tropsch synthesis (FTS) was performed in a slurry-phase continuously stirred tank reactor. The characterization results indicated that the hematite in the fresh catalyst was converted mainly to magnetite in H₂ atmosphere without the formation of intermediate metallic iron. Large amounts of Fe₃O₄ and small amounts of ε′-Fe_2.2C and χ-Fe_2.5C were formed after syngas pretreatment. In contrast, CO activation led to the formation of large amounts of χ-Fe_2.5C and carbonaceous species on the surface of magnetite. In the FTS reaction, the CO-activated catalyst presented the highest initial activity compared to the H₂ and syngas-reduced catalysts, and remained unchanged in the activity following the transformation of iron carbides to Fe₃O₄. Furthermore, the FTS activity of the H₂-reduced catalyst increased gradually accompanied with the conversion of magnetite to iron carbides. All of the results suggested that the formation of iron carbides (especially for χ-Fe_2.5C) on the surface layers provides probably the active sites for FTS, whereas the Fe₃O₄ formed plays a negligible role in the FTS activity.