Origin of rate bistability in Mn–O/Al2O3 catalysts for carbon monoxide oxidation: role of the Jahn–Teller effect

Peculiarities in catalytic activity in carbon monoxide oxidation as well as some structure, electronic and magnetic properties of the three oxide catalysts, Mn³⁺–O/Al₂O₃ (1), Mn³⁺–O–Fe/Al₂O₃ (Mn-substituted spinel, 2) and -Fe₂O₃/Al₂O₃ (3), were studied by kinetic measurements and by Mössbauer spectroscopy. The catalysts 1 and 2 showed a kinetic bistability with a sharp transition towards more reactive state at 200°C (ignition point). In contrast, for catalyst 3, at 200–250°C, the behavior of reaction rate against temperature did not display noticeable hysteresis. On cooling the catalysts 1 and 2, extinction was observed at about 170 and 120°C, respectively, i.e., at 30–80°C lower than the corresponding ignition points. Proximity of activation energy for the high and low activity (15–19 kJ/mol) for both Mn-containing catalysts suggests an increase in the number of active sites at high temperature with no changes in the reaction mechanism. The considerable difference between Mn-containing catalysts 1, 2 and Fe-containing catalyst 3 may be caused by Jahn–Teller (JT) type distortions of the oxygen polyhedron around Mn³⁺. A significant spontaneous axial bond stretching within the local polyhedron seems to diminish Mn–O binding energy, facilitate the participation of surface oxygen species, O_S, in the oxidation of CO by a redox mechanism and promote oxygen vacancies at the surface that would cause considerable effect on the activity. An increase in the width of the counterclockwise hysteresis loop for the catalyst 2 compared to the catalyst 1 indicates that clusters of mixed spinel provide more active sites and more labile O_S species than clusters of the binary Mn oxide.     

Supported Cu Catalysts with Yttria-Doped Ceria for Steam Reforming of Methanol

The steam reforming of methanol was studied over Cu/Al₂O₃ catalysts with the addition of yttria-doped ceria (YDC). The YDC-modified catalysts were prepared by impregnating a -Al₂O₃ support with Y and Ce then with Cu. The addition of YDC drastically enhanced the activity of Cu/Al₂O₃ in the methanol reforming reaction. The enhanced activity was attributed to the increase of Cu⁺ species by YDC in the methanol reforming environment. However, the addition of YDC decreased the copper dispersion. The Cu dispersion could be enhanced by adding chromium oxide. The addition of YDC and Cr where Al₂O₃ was first impregnated with Cr then with YDC showed the most pronounced enhancement of the catalyst activity. At reaction temperatures of 200250 °C, the CO concentration in the products was smaller than 0.1%.     

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O₂ ^– and O₂ ^2– adspecies), most probably O₃ ^2– formed from reversible redox coupling of an O₂ adspecies at an anionic vacancy with a neighboring O^2– in the surface lattice. This active precursor may initiate H abstraction from CH₄ and be itself converted to OH^–+O₂ ^–, or it may abstract an electron from the oxide lattice and be converted to O₂ ^2–+O^–. The prospect of developing this type of OCM catalysts is discussed.     

Propane oxidative dehydrogenation on silica based oxide catalysts

The oxidative dehydrogenation of propane to propylene with molecular O₂ (PPD) has been investigated on commercial bare SiO₂ and medium loaded V₂O₅ and MoO₃ catalysts at 450–525°C. The direct relationship between the density of reduced sites (, 10¹⁶ s_r g _cat– ¹ ), evaluated in steady-state conditions by O₂ chemisorption, and the POD reaction rate proves the occurrence of a concerted reaction mechanism involving the activation of gas-phase O₂ on the reduced sites of the catalyst surface. A straight-line correlation between the activity of such silica based catalysts in POD and methane partial oxidation to formaldehyde (MPO) at 525°C has been disclosed.     

Methanol: A “Smart” Chemical Probe Molecule

A novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts (MgO, CaO, SrO, BaO, Y₂O₃, La₂O₃, CeO₂, TiO₂, ZrO₂, HfO₂, V₂O₅, Nb₂O₅, Ta₂O₅, Cr₂O₃, MoO₃, WO₃, Mn₂O₃, Fe₂O₃, Co₃O₄, Rh₂O₃, NiO, PdO, PtO, CuO, Ag₂O, Au₂O₃, ZnO, Al₂O₃, Ga₂O₃, In₂O₃, SiO₂, GeO₂, SnO₂, P₂O₅, Sb₂O₃, Bi₂O₃, SeO₂ and TeO₂). The number of surface active sites for methanol dissociative adsorption corresponds to 3 mol/m² on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites on metal oxides since redox sites yield H₂CO, acidic sites yield CH₃OCH₃ and basic sites yield CO₂. The distribution of the different types of surface active sites was found to vary widely for the different metal oxide catalysts. In addition, the commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover frequency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10^–3 to 10⁴ s^–1). An inverse relationship (for metal oxide catalysts displaying high (>85%) selectivity to either redox or acidic products) was found between the methanol oxidation TOF values and the decomposition temperatures of the surface M–OCH₃ intermediates reflecting that the decomposition of the surface M–OCH₃ species is the rate-determining step during methanol oxidation over the metal oxide catalysts.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

IR studies of Re2O7 metathesis catalysts supported on alumina and phosphated alumina

FTIR data from Re₂O₇/-Al₂O₃ metathesis catalysts indicate that a low Re contents ReO₄ ions have predominantly reacted with basic surface OH groups, while at higher Re₂O₇ loadings they have also reacted with neutral and more acidic OH groups.On phosphated -Al₂O₃ the phosphate has reacted with the more acidic OH groups on the -Al₂O₃; in catalysts based on this support, the ReO₄ ^– ions have also reacted with the phosphorus-bonded OH groups already at low Re₂O₇ loadings, resulting in more active catalysts.     

Highly active polymer-alumina dually supported palladium catalysts for the carbonylation of allyl halides under atmospheric pressure

Polymer-alumina dually supported palladium catalysts (PVP-Pd/Al₂O₃, PVP = poly(N-vinyl-2-pyrrolidone); PPPO-Pd/ Al₂O₃, PPPO = phosphinated poly(phenylene oxide)) were used to catalyze the carbonylation of allyl halides to form butenoic acid under the mild conditions of 40C and 0.1 MPa. These catalysts exhibit high activity and stability towards the carbonylation of allyl halides. The maximum turnovers (mol CO/mol Pd) of PPPO-Pd/Al₂O₃ are up to 8.2×10⁵ and 4.6×10⁵ for the carbonylation of allyl bromide and allyl chloride respectively in an aqueous NaOH/benzene medium.     

Development of Monolithic Catalysts with Low Noble Metal Content for Diesel Vehicle Emission Control

Monolith washcoated catalysts with potential for diesel emission control have been developed. Two types of catalysts have been prepared for further study: (1) MnO _x supported on granulated -Al₂O₃, (2) MnO _x supported on cordierite monolith washcoated with -Al₂O₃. Both catalysts have been calcined at 500 and 900 °C and subsequently modified by doping with 0.1–1.0 wt% of Pt or Pd. The influence of the concentration of both manganese oxide (0–10 wt%) and noble metals Pt and Pd in the range 0–1.0 wt% on the catalytic activity in methane oxidation has been studied. Comparison of the catalytic activity of MnO _x /Al₂O₃ and MnO _x + Pt(Pd)/Al₂O₃ with that of a standard 1 wt%Pt/Al₂O₃ catalyst shows the existence of a synergetic effect. This effect is more pronounced for the samples calcined at 900 °C. The developed monolithic catalysts MnO _x + Pt(Pd)/Al₂O₃ demonstrate higher activity and thermal stability (up to 900 °C) compared to the commercial monolithic catalyst (TWC's).     

High‐activity catalyst of SO 4 2− /ZrO2 supported on γ‐Al2O3 for n‐butane isomerization

A series of Al₂O₃supported SO ₄ ^2– /ZrO₂ superacid catalysts (named SZ/Al₂O₃) were prepared by a precipitation method and their catalytic behavior for nbutane isomerization at low temperature in the absence of H₂ and at high temperature in the presence of H₂ was studied in this paper. The catalytic activities of some of these catalysts were enhanced significantly at both low and high temperatures. At 250°C after 6 h on stream, the steady activity of the most active sample, 60%SZ/Al₂O₃, is about two times higher than that of conventional SZ. The texture properties of catalysts were studied by the methods of XRD and the adsorption of N₂. Experimental evidence of IR of adsorbed pyridine indicates that the significant activity enhancements of SZ/Al₂O₃ catalysts are caused by the increasing of the amount of strong acid sites.     

A novel alkaline air electrode based on a combined use of cobalt hexadecafluoro-phthalocyanine and manganese oxide

Electrocatalytic reduction of O₂ with dual catalysts of cobalt 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24, 25-hexadecafluoro-29 H, 31H-phthalocyanine (CoPcF₁₆) and MnOOH was studied in alkaline media with cyclic and rotating ring-disk electrode (RRDE) voltammetry. Cyclic voltammetric results show that CoPcF₁₆ possesses a good catalytic activity for redox-catalyzing an apparent two-electron reduction of O₂ with superoxide (O₂⁻) as an intermediate. The combined use of CoPcF₁₆ with MnOOH which shows a bifunctional catalytic activity toward the sequential disproportionations of the reduction intermediate and product, i.e. O₂⁻ and peroxide (HO₂⁻), eventually enables an apparent four-electron reduction of O₂ to be achieved at a positively-shifted potential in alkaline media. The possibility of utilizing the dual catalysts for the development of practical alkaline air electrodes was further explored by confining the catalysts in active carbon (AC) and carbon black (CB) matrices that are generally used as the substrate for constructing air electrodes. The RRDE voltammetric results suggest that an apparent four-electron reduction of O₂ reduction can be obtained at the as-prepared carbon-based air electrode at a potential close to that at the Pt-based air electrode, and that the as-prepared electrode shows a high tolerance against methanol and glucose crossover.     

Highly visible-light active C- and V-doped TiO2 for degradation of acetaldehyde

C-doped and C- and V-doped TiO₂ photocatalysts were prepared by a sol–gel process. Both catalysts showed high activity for the degradation of acetaldehyde under visible irradiation (>420 nm). The co-doped TiO₂ catalysts also were highly active in the dark; 2.0% V-containing co-doped TiO₂ had the highest activity, comparable with the activity under visible light irradiation. The catalysts were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and N₂ adsorption–desorption. The results suggest that vanadium ions were introduced both on the surface and into the bulk of TiO₂. A free electron, induced by the formation of V⁵⁺ in the sublayers of TiO₂ during calcination at 500 °C in air, was delocalized and promoted into the conduction band by thermal energy and further transferred to O₂, generating a superoxide radical anion (O⁻₂) that is responsible for degradation of acetaldehyde in the dark. In addition to functioning as a photosensitizer that shifts the optical response of TiO₂ from the ultraviolet (UV) to the visible light region, the doped elemental carbon increased the surface area and improved the dispersion of vanadium.     

Preparation and electrochemical studies of metal–carbon composite catalysts for small-scale electrosynthesis of H2O2

Numerous transition metal–carbon composite catalysts (M = V, Zn, Ni, Sn, Ce, Ba, Fe, Cu) have been synthesized and tested for electroreduction of O₂ to H₂O₂, The activity and selectivity of all synthesized catalysts for electrosynthesis of H₂O₂ were determined by the rotating ring-disk electrode method in acidic and neutral electrolytes. The Co-based catalysts in general showed the highest activity towards H₂O₂ formation. Experiments with different loading contents of Co showed that the activation overpotential losses of oxygen reduction to H₂O₂ reduces as loading increases to about 4 wt% Co. Addition of Co beyond this level did not seem to impact the overpotential losses. The cobalt-based catalysts, were spray-coated onto 120 μm thick Toray^® graphite substrates, and were studied in bulk electrolysis cells for up to 100 h at potentiostatic conditions (0.25 V vs. RHE) in pH 0, 3, and 7 electrolytes. At (25 °C and 1 bar) with a catalysts loading of about and using dissolved O₂ in 0.5 M H₂SO₄, typical H₂O₂ electrosynthesis rates of about were reached with current efficiencies of about 85 ± 5% at 0.25 V (vs. RHE).     

Amination of Polyethylene Glycol to Polyetheramine over the Supported Nickel Catalysts

Surface nickel (NiO _x ) species, surface NiAl _x O _y compound, and NiO crystallites are present on the Ni/Al₂O₃ catalysts, and the ratio of these nickel species is dependent on the nickel loading. Surface nickel interacts with the TiO₂ support to form a surface nickel titanate compound (NiTiO _x ) which has a lower reducibility. The weak interaction between the surface nickel and the silica support results in the formation of NiO crystallites on the SiO₂ surface. The Ni/Al₂O₃ and Ni/TiO₂ catalysts contain new surface Lewis acid sites and the amount of surface Lewis acid sites increases with increasing nickel concentration. The Ni/SiO₂ catalysts have no sign of the presence of the surface Lewis acid sites. Only the Ni/Al₂O₃ catalysts have shown the ammonia adsorption at temperature of 200°C. Supported nickel on alumina catalysts possess the highest amination conversion, and the amine yield increases with increasing nickel loading up to 15% and starts to level off. By comparing amination catalysis with quantitatively TPR studies of the H₂ consumed of the Ni/Al₂O₃ catalysts, it appears that the dispersed nickel species are the active sites for amination. In addition, the amination product is mainly the secondary amine due to the presence of water.     

Effect of Sulfur or Nitrogen Poisoning on the Activity and Selectivity of Y-Zeolite-Supported Pt–Pd Catalysts in the Hydrogenation of Tetralin

The partial oxidation of methane (POM) to synthesis gas over SiO₂-supported Rh and Ru catalysts was studied by in situ microprobe Raman and in situ time-resolved FTIR (TR-FTIR) spectroscopies. The results of in situ microprobe Raman spectroscopic characterization indicated that no Raman band of Rh₂O₃ was detected at 500°C over the Rh/SiO₂ catalyst under a flow of a CH₄/O₂/Ar (2/1/45, molar ratio) mixture, while the Raman bands of RuO₂ can even be detected at 600°C over the Ru/SiO₂ catalyst under the same atmosphere. The experiments of in situ TR-FTIR spectroscopic characterizations on the reactions of CH₄ over O₂ pre-treated Rh/SiO₂ and Ru/SiO₂ catalysts indicated that the products of CH₄ oxidation over Rh/SiO₂ and Ru/SiO₂ greatly depend on the concentration of O²– species over the catalysts. On the catalysts with high concentration of O²–, CH₄ will be completely oxidized to CO₂. However, if the concentration of O²– species over the catalysts is low enough, CH₄ can be selectively converted to CO without the formation of CO₂. The parallel experiments using in situ TR-FTIR spectroscopy to monitor the reaction of the CH₄/O₂/Ar (2/1/45, molar ratio) mixture over Rh/SiO₂ and Ru/SiO₂ catalysts show that the mechanisms of synthesis gas formation over the two catalysts are quite different. On the Rh/SiO₂ catalyst, synthesis gas is mainly formed by the direct oxidation of CH₄, while on the Ru/SiO₂ catalyst, the dominant pathway of synthesis gas formation is via the sequence of total oxidation of CH₄ followed by reforming of unconverted CH₄ with CO₂ and H₂O. The significant difference in the mechanisms of partial oxidation of CH₄ to synthesis gas over Rh/SiO₂ and Ru/SiO₂ catalysts can be well related to the difference in the concentration of O²– species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the two metals.     

With different structure ligands heterogeneous Ziegler-Natta catalysts for the preparation of copolymer of ethylene and 1-octene with high comonomer incorporation

Comparison with the conventional Ziegler-Natta catalyst TiCl₄/MgCl₂ (I), the modified supported Ziegler-Natta catalysts (iso-PentylO)TiCl₃/MgCl₂ (II) and (BzO)TiCl₃/MgCl₂ (III) were prepared as efficient catalysts for copolymerization of ethylene with 1-octene. The complexes (II) and (III) were desirable for the production of random ethylene/1-octene copolymers coupled with higher molecular weight, higher comonomer incorporation within copolymer chain and good yield even at high temperature 80 ^°C and fairly low Al/Ti molar ratio of 100. The effects of catalysts ligands, Al/Ti molar ratio, polymerization temperature, as well as concentration of 1-octene on the catalytic activity, molecular weight and microstructure of the copolymer were investigated in detail. The structure and properties of the copolymers were characterized with ¹³C NMR, GPC, DSC and WAXD. The kinetic results also indicate that these catalysts (II) and (III) show higher catalytic activity and the produced polymers feature higher molecular weight, because of lower ratio of K_trm/K_p and K_tra/K_p, and higher ratio of K_tra/K_trm which indicates that chain transfer to cocatalyst is predominant.     

Drastic ligand electronic effect on anilido-imino nickel catalysts toward ethylene polymerization

A novel nitro-anilido-imino nickel complex (Ar¹NCHC₆H₃(-5-NO₂)NAr²)NiBr (Ar¹ = Ar² = 2,6-dimethylphenyl) was designed, synthesized, and characterized to investigate ligand electronic effect on late transition metal olefin polymerization catalysts. As a catalyst for ethylene polymerization, neutral anilido-imino nickel complex with an electron-withdrawing nitro group showed good activity (442.1 kg (mol Ni h)⁻¹) with MAO as cocatalyst. The catalytic activity and molecular weight of the obtained products were significantly affected by electronic effect of the anilido-imine ligand. Theoretical calculations suggested that ligand electronic effect led to different charge distribution on the nickel metal atom, and the catalytic activity predominantly increased with an increase in electrophilicity of the nickel metal center.     

Influence of the nature of the platinum precursor on the surface properties and catalytic activity of alumina-supported catalysts

The influence of the nature of the metal precursor — platinum acetylacetonates and chloroplatinic acid — on the surface properties and catalytic activity of Pt/Al₂O₃ catalysts is reported. The obtained results indicate that the catalysts prepared by the organometallic route present higher metal dispersion and lower acidity compared with those prepared from H₂PtCl₆. On the other hand, XPS results showed that the state of platinum is essentially Pt⁰ in the catalysts obtained from Pt(acac)₂ while in the solids prepared by impregnation of H₂PtCl₆ there exists an important contribution of Pt⁺ species which plays a positive role in the hydrogenation of toluene. An additional hydrogen spillover due to the presence of more acidic support is also suggested as an explanation of the observed catalytic results.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.