Use of potassium conductors in the electrochemical promotion of environmental catalysis

This study summarizes the effect of the electrochemical promotion of catalysis (EPOC) by using potassium conductors in environmental catalytic reactions applied to the removal of several automotive pollutants, such us CO, C₃H₆ and nitrogen oxides (N₂O and NO_x). It has been shown the extraordinarily potential of using Pt/K–βAl₂O₃ in a wide variety of environmental reactions (oxidation, reduction), activating the catalyst at lower reaction temperatures and decreasing the inhibitory effect of poisons such as water in the reaction atmosphere. In addition, a new application of potassium conductors-based electrochemical catalysts has been developed for the NO_x storage/reduction process (NSR). The idea of coupling catalysis and solid-state electrochemistry on this process would allow to improve and simplify the current NSR technology. Finally, the results have been obtained under reaction conditions compatible with the treatment of automotive exhaust emissions. This demonstrates the potential for the practical use of the phenomenon of electrochemical promotion by using potassium conductors on this kind of process.     

Pt/K–βAl2O3 solid electrolyte cell as a “smart electrochemical catalyst” for the effective removal of NOx under wet reaction conditions

This study has shown that the phenomenon of electrochemical promotion can be used to activate a metal catalyst for the selective catalytic reduction of nitrogen oxides (NO_x) in the presence of water in the feed. The application of different potentials optimized the catalytic performance of the Pt catalyst-working electrode at each reaction temperature range. In addition, the measurement of the open circuit voltage in the cell gave useful information on the competitive adsorption between the reactants. Thus, very interestingly, the combined use of the Pt/K–βAl₂O₃ cell as a sensor and as an electrochemical catalyst allows anticipating and optimizing the catalytic behaviour of the system under changing reaction conditions, such as those found in the exhaust of an engine. Finally, characterization of the cell by Cyclic Voltammetry along with NO_x analysis and XRD provided useful information about the nature of the promoter species under wet reaction conditions.     

The effect of SO2 on the activity of Pd-based catalysts in methane combustion

The effect of SO₂ on Pd-based catalysts for the combustion of methane has been investigated. It is shown that while SO₂ poisons Al₂O₃- and SiO₂-supported catalysts, pre-treatment of Pd/ZrO₂ by SO₂ enhances the activity substantially.     

Electrochemical promotion of Pt impregnated catalyst for the treatment of automotive exhaust emissions

This study has shown that Pt/K-βAl₂O₃ electrochemical catalyst can reach high catalytic activity for the selective reduction of N₂O by C₃H₆. In addition it was also demonstrated that Electrochemical Promotion could be a solution to reduce the adverse effects of poisons (O₂ and H₂O) on the catalytic activity. For instance, in the presence of O₂ (1%) and H₂O (3%) in the reactive stream, electrochemical pumping of potassium ions to the Pt catalyst increased the N₂O reduction rate by a factor of 7.4 at 400 °C. We have also demonstrated that the wet impregnation procedure led to a very stable Pt film, with very good resistance to thermal sintering under real operation conditions. Therefore, the use of Pt impregnated films deposited on K-βAl₂O₃ solid electrolytes is a feasible solution for the treatment of automotive exhaust gases.     

Enhanced electrochemical response of carbamazepine at a nano-structured sensing film of fullerene-C60 and its analytical applications

In the present work, electrochemical behavior of carbamazepine (CBZ) at fullerene-C₆₀ modified glassy carbon electrode has been investigated. Cyclic voltammogram of CBZ showed two each of oxidation and reduction peaks at fullerene-C₆₀ modified glassy carbon electrode (GCE) in phosphate buffer of pH 7.2 at the scan rate of 100 mV s⁻¹. The fullerene film on GCE surface exhibited excellent enhancement effects on electrochemical response of CBZ. Marked negative shift in peak potential with enhanced peak current was noticed in the cyclic and differential pulse voltammograms of CBZ at fullerene-C₆₀ modified electrode. The effect of accumulation time, amount of fullerene-C₆₀ and pH on electrochemical behavior of CBZ has been investigated using differential pulse voltammetry (DPV). An analytical method was developed for the determination of CBZ employing DPV. The oxidation peak current of CBZ was observed to be linearly dependent on the concentration of CBZ in the range of 90 nM–10 μM. The values of limit of detection and limit of quantification were found to be 16.2 nM and 54.0 nM, respectively. The developed DPV method was satisfactorily applied to the determination of CBZ in pharmaceutical formulations, spiked human serum and urine samples.     

Catalytic activity and long-term stability of palladium oxide catalysts for natural gas combustion: Pd supported on LaMnO3-ZrO2

The catalytic activity and long-term stability of 2% Pd/LaMnO₃-ZrO₂ catalysts for natural gas combustion were deeply investigated. The catalyst, prepared via solution combustion synthesis, was completely characterized (XRD, BET, FESEM/EDS, TPC/TPD/TPR and FT-IR analysis) in the fresh status, and in the aged one, after prolonged treatment under hydro-thermal ageing and S-compounds poisoning (up to 3 weeks of hydro-thermal treatment at 800 °C under a flow of domestic boiler exhaust gases typical composition of 9% CO₂, 18% H₂O, 2% O₂ in N₂, including 200 ppmv of SO₂). An increased catalytic activity towards NG combustion with ageing was detected: the T₅₀, in fact, got lowered from 570 (fresh sample) to 465 °C (after 3 weeks ageing). Highly dispersed Pd centers were predominant on fresh catalyst. Upon ageing, oxygen covered Pd metal particles formed, at the expense of dispersed cationic and zerovalent Pd atoms. The increase in the catalytic activity was associated to the phase modification occurring in the bulk support, where Mn oxides, active towards CH₄ combustion, segregated. Moreover, bands due to sulfate species were detected in aged samples: IR analysis showed that Pd atoms did not interact significantly with these species. The bands of sulfate species decreased in intensity after 3 weeks ageing, likely mostly due to sintering of the catalyst, with the corresponding decrease in the surface area.     

Preparation of SnO2-CNTs supported Pt catalysts and their electrocatalytic properties for ethanol oxidation

SnO₂-carbon nanotubes (CNTs) composites were prepared by sol-gel method, and characterized by scanning electron microscopy and X-ray diffraction. Due to high stability in diluted acidic solution, SnO₂-CNTs composites were selected as the catalyst support and second catalyst for ethanol electrooxidation. The electrocatalytic properties of the SnO₂-CNTs supported platinum (Pt) catalyst (Pt/SnO₂-CNTs) for ethanol oxidation have been investigated by typical electrochemical methods. Under the same mass loading of Pt, the Pt/SnO₂-CNTs catalyst shows higher electrocatalytic activity and better long-term cycle stability than Pt/SnO₂ catalyst. Additionally, the effect of the mass ratio of CNTs to SnO₂ on the electrocatalytic activity of the electrode for ethanol oxidation was investigated, and the optimum mass ratio of CNTs to SnO₂ in the Pt/SnO₂-CNTs catalyst is 1/6.3.     

Organometallics derived (Pd + Ln)/SiO2 catalysts for the reactions of synthesis gas conversion

C₃H₆ hydroformylation and CH₃OH synthesis on organometallics derived (Pd + Ln)/ SiO₂ and Pd/SiO₂ catalysts have been studied. The activity and selectivity towards methanol in CO + H₂ reaction were observed to increase for all the modified catalysts while both the hydroformylation activity and selectivity towards oxygenates in C₃H₆ hydroformylation decreased for the catalysts in comparison to those of Pd/SiO₂. The FTIR, TPD data and characteristic catalytic properties of the catalysts studied allow to suggest that C₃H₆ hydroformylation on (Pd + Ln)/SiO₂ catalysts occurs on monometallic Pd clusters without participation of mixed active sites and CO complexes activated thereon.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

n-butane oxidation to maleic anhydride: effect of Co and Fe addition by the method of incipient wetness on vanadium phosphate catalysts prepared by the aqueous HC1 method

The effect of the addition of Co²⁺ and Fe³⁺ on the catalytic performance and structure of vanadium phosphate catalysts is described and discussed. The catalyst precursor VOHPO₄.0.5H₂O was prepared using the aqueous HC1 route and Co²⁺ and Fe³⁺ were incorporated using the incipient wetness method. Addition of Co²⁺ improves the selectivity to maleic anhydride, but is found to decrease the specific activity for the formation of maleic anhydride, whereas the addition of Fe³⁺ increases the specific activity up to a maximum level at ca. 2 mol% Fe. The effects are not due to changes in the specific surface area of the final catalyst, which remains 6.5 ±0.5 m² g^–1 for all the catalysts studied. Part of the effect is due to the formation of VOPO₄ phases in the catalyst precursor due to the method of addition of the Co²⁺ or Fe³⁺ and this is both related to the effect of the oxidation potential of the additive cation and to the pH of the impregnating solution. It is apparent that the use of the incipient wetness method for the addition of promoter compounds should be used with care for the VPO system at variance with the direct incorporation of the promoter during the preparation of the VOHPO₄.0.5H₂O precursor.     

SSITKA studies of the catalytic flameless combustion of methane

This paper presents results which were obtained for the flameless combustion of methane over the Pd(PdO)/Al₂O₃ catalyst by using the steady state isotopic transient kinetic analysis method. During the reaction switches between ¹⁶O₂/Ar/CH₄/He and ¹⁸O₂/CH₄/He were carried out. The obtained results indicate the presence of large amounts of oxygen as well as of intermediates leading to the formation of carbon dioxide on the surface of the palladium catalyst. Additionally, information was obtained proving that the complete oxidation of methane over Pd/Al₂O₃ catalyst proceeds according to the Mars and van Krevelen redox mechanism. With the increase of the reaction temperature there is an increase in the number of active centres on the Pd(PdO)/Al₂O₃ catalyst surface—a larger amount of oxygen from the lattice of the catalyst is accessible for the reaction of methane oxidation.     

Performance of potassium-promoted catalysts for NO x and soot removal from simulated diesel exhaust

In order to elucidate the effect of support in the catalytic performance, two selected potassium-promoted catalysts (K1Cu/beta and KCu2/Al₂O₃) were tested for the simultaneous NO _x /soot removal from a simulated diesel exhaust. For comparative purpose, the behaviour of a platinum catalyst (Pt/beta) was also studied. Isothermal experiments revealed that the potassium-promoted catalysts show a high activity for NO _x /soot removal in the 350–450 °C temperature range. In addition, the catalysts present the advantage that the main reaction products are N₂ and CO₂. Among the catalysts tested, KCu2/Al₂O₃ presents the best global performance at 450 °C: the highest soot consumption rate, even higher than the platinum catalysts, and a high NO _x reduction.     

Regeneration of S-poisoned Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts for the combustion of methane

This work investigates the regeneration of S-poisoned Pd/Al₂O₃ and Pd/CeO₂/Al₂O₃ catalysts under different CH₄ containing atmospheres. Under lean combustion conditions in the presence of excess O₂, partial regeneration took place for both systems only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Under alternate lean combustion/CH₄-reducing pulse regeneration is markedly anticipated down to 550–600 °C. Experiments evidenced an effective role of ceria in preventing PdO from sulphation and in promoting regeneration via sulphates decomposition under reducing conditions.     

The nature of surface species on modified Pt-based catalysts for the SCR of NO by C<Subscript>3</Subscript>H<Subscript>6</Subscript> under lean-burn condition

The catalytic activity behavior for the selective catalytic reduction of NO by C₃H₆ under excess oxygen and the nature of surface species on the active sites of Pt/Al₂O₃ catalyst after adding a second metal (Fe, Sn, Co, Cr or W) were investigated. It has been found that an important role of second metals is on TONs of C₃H₆ and NO conversions and the nature of surface species produced on the catalyst surface at low temperature instead of the catalytic activity behavior towards the temperature programmed reaction. Although the introduction of each second metal distinctly disturbs the characteristic of surface species, the reaction mechanism is presumably similar. The observation of few surface species and the investigation about their reactivity indicate that few mechanisms are simultaneously proceeding at the same reaction condition.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

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).     

Study of the BMIM-PF6: Acetonitrile binary mixture as a solvent for electrochemical studies involving CO2

A series of binary mixtures ranging from 0 vol.% to 100 vol.% CH₃CN in BMIM-PF₆ were investigated to identify an optimum ratio for use in electrochemical studies involving CO₂. Density, viscosity and conductivity were measured for the range of binary mixtures and compared to previously published data. The electrochemistry of a model compound, azobenzene, was studied as well. The data indicated that a binary mixture containing 15–20 vol.% (approximately 0.5 mol fraction) CH₃CN in BMIM-PF₆ was optimal for electrochemistry, and FTIR of CO₂ saturated solutions demonstrated that the solubility of CO₂ in the 15–20% CH₃CN mixtures was only about 10% lower than that seen in the neat BMIM-PF₆.     

Relationship between oxidation states of copper supported on alumina and activities for catalytic combustion of NH3

The relationship between the oxidation state of Cu supported on an alumina catalyst (Cu/Al₂O₃) and the activity for combustion of NH₃ was investigated. Combustion of NH₃ on the catalyst treated in hydrogen at 800°C occurred at lower temperature than on the catalyst treated in air. It was also much better than Pt and Rh catalysts for the conversion of NH₃ to N₂. Characteristics of the catalyst were investigated by XRD, XPS, and the N₂O pulse injection method to understand the reason of its high catalytic activity. The reason of the high activity of the catalyst treated in hydrogen at the high temperature was attributed to the lower oxidation state of Cu in the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

Influence of the Type of Reducing Agent (H2, CO, C3H6 and C3H8) on the Reduction of Stored NOX in a Pt/BaO/Al2O3 Model Catalyst

In this investigation, a comparative study for a NO _X storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H₂, CO, C₃H₆ and C₃H₈) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al₂O₃ catalyst. Transient experiments show that H₂ and CO are highly efficient reductants compared to C₃H₆ which is somewhat less efficient. H₂ shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C₃H₈does not show any NO _X reduction ability for NO _X stored in Pt/BaO/Al₂O₃ at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.