Electrochemical promotion of deep oxidation of methane on Pd/YSZ

Electrochemical catalysts based on Pd deposited by Physical Vapour Deposition on YSZ were used for methane deep oxidation. Different thicknesses of Pd films varying from 11 to 75 nm were catalytically characterized between 150 and 750 °C. The Pd loadings were extremely low. Catalytic and EPOC experiments were carried out on those electrochemical catalysts. Their catalytic activities were compared with the performances of a reference catalyst. It was found that the catalytic activity can be in situ tuned by applying an anodic polarization thus supplying oxygen ions at the surface of the catalyst. Faradaic efficiency values up to 258 were observed and the induced modifications of the catalytic rate were typically 100 times higher than the corresponding ionic current. The influence of the polarization on the temperature of decomposition of the palladium oxide was also examined. The polarization was found to enhance the thermal stability of the oxide and turn palladium oxide into metallic palladium at higher temperatures.     

Electrochemical promotion of YSZ monolith honeycomb for deep oxidation of methane

A honeycomb monolith of YSZ (yttria stabilized zirconia) was used, for the first time, as a solid electrolyte to perform electrochemical promotion of the deep oxidation of methane. The goal of this study was to validate the concept of Electrochemical Promotion of Catalysis (EPOC) by using Pd particles dispersed on the channels surface of a YSZ dense honeycomb monolith. The Pd catalyst was an effective material as methane conversion reached 20% at about 320 °C. The catalytic properties of the monolithic electrochemical catalyst were investigated upon electrical polarization at 400 °C. Non faradaic effects were observed both under positive and negative polarizations with a maximum of the faradaic efficiency of 47.     

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO₂-promoted γ-Al₂O₃ with various CeO₂ (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO₂ structure and dispersion on the γ-Al₂O₃ support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO₂ structure. Pd is present on the oxidized CeO₂-promoted catalysts as Pd⁰, Pd⁺ and Pd²⁺, at ratios strongly dependent on CeO₂ structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO₂ and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO₂. FTIR spectra of adsorbed CO revealed that after pre-treatment under H₂ or in the presence of amorphous CeO₂, partial encapsulation of Pd particles occurs. CeO₂ structure influences the CH₄ steam reforming reaction rates. Crystalline CeO₂ and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO₂ as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al₂O₃ or on amorphous (reduced) CeO₂/Al₂O₃ catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO₂/γ-Al₂O₃ than on Pd/γ-Al₂O₃, which is attributed to a catalytic synergism between Pd and CeO₂. The low rates on the reduced Pd/CeO₂/Al₂O₃ catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.     

Low temperature oxidation of methane over Pd/SnO2 catalyst

Catalytic activities of supported Pd were investigated for low temperature oxidation of methane. Pd/SnO₂ catalysts demonstrated excellent activity for methane oxidation in spite of their low surface area. The catalytic activity of Pd/SnO₂ was strongly affected by the preparation procedure. Impregnation of Pd on SnO₂ using aqueous solution of Pd(CH₃COO)₂ was most effective in enhancing the catalytic activity. The catalytic activity was also improved when well-crystallized SnO₂ was employed as a support material. TEM observations revealed that catalytic activity is strongly influenced by the dispersion state of Pd. For the active catalysts, strong interaction between Pd and SnO₂ support was observed in the adsorption of oxygen.     

Influence of the reaction conditions on the electrochemical promotion by potassium for the selective catalytic reduction of N2O by C3H6 on platinum

In this work, we have investigated for the first time the selective catalytic reduction of N₂O by C₃H₆ over an electrochemical catalyst (Pt/K-βAl₂O₃). It was evaluated the influence of the reaction conditions (temperature, oxygen concentration, water vapour presence and time on stream treatment under reaction conditions) on the catalytic performance of the electrochemical catalyst. Electrochemical pumping of potassium ions to the Pt catalyst working electrode strongly increased the N₂O reduction rate, activating the catalyst at lower temperatures. However, it was found that the efficiency of the electrochemical promotion decreased as the oxygen concentration increased because of a strong inhibition of propene adsorption and a relative increase of the oxygen coverage. On the contrary, the presence of potassium ions on the Pt catalyst strongly decreased the inhibiting effect of water vapour, increasing the catalytic activity of the catalyst. In addition, the catalyst stability was confirmed by a deactivation study. It was found that a long term treatment at high temperature under operating conditions had a positive effect on the efficiency of the Pt/K-βAl₂O₃ electrochemical catalyst.     

Transformation of methane formation sites into methanol formation ones during CO---H2 reaction over Pd/CeO2 in its SMSI state

The phenomena of induction period of methanol formation, observed in CO---H₂ reaction over Pd/CeO₂ catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H₂ reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during CO---H₂ reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during CO---H₂ reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.     

Low-temperature catalytic combustion of methane over Pd/CeO2 prepared by deposition–precipitation method

Ceria supported 2 wt% Pd catalysts for low-temperature methane combustion were prepared by the impregnation (IM) and deposition–precipitation (DP) methods, which are denoted as Pd–IM and Pd–DP, respectively. DP was found to be an available method for achieving high activity and stability of the Pd/CeO₂ catalyst. The temperatures for methane ignition (T_10%) and total conversion (T_100%) over Pd–DP are 224 and 300 °C at GHSV of 50,000 h⁻¹, which are 83 and 110 °C lower than the corresponding temperatures of Pd/Al₂O₃. X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses show that palladium species in Pd–DP is highly dispersed, positively charged and difficultly reduced. Raman spectra disclosed that the largest concentration of defects and/or oxygen vacancies was formed in Pd–DP catalyst. A kind of cationic PdO^δ+ sites with higher binding energies than PdO are in close vicinity to the oxygen vacancies in the CeO₂ support and might act as the active centers for methane oxidation. Furthermore, the deactivation and steam aging tests for Pd–DP showed that the performance of this type of palladium was very stable and could be repeatedly recovered after several long time aging tests.     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.     

Oxygen storage in O2/Pt/YSZ cell

The phenomenon of electrochemical promotion of catalysis (EPOC) was initially characterized as fully reversible, i.e. the catalyst restores its initial activity after current interruption. However, it has been recently demonstrated that after prolonged anodic polarization an unusual promoted activity is observed for a certain time after current interruption. This phenomenon has been reported as permanent electrochemical promotion of catalysis (P-EPOC).In this work the oxygen storage reported as responsible of P-EPOC has been investigated by transient electrochemical techniques using an O₂(g)Pt/YSZ cell. A model has been proposed involving place interchange of Pt and O species in Pt/YSZ system. This seems to be induced by the strong lateral interaction of Pt–O surface dipoles and by increasing electric field at the Pt/YSZ interface. Such a rearranged oxide, so-called “phase oxide” can have a lower free energy than the initial monolayer oxide. This cooperative interaction of Pt and O species can lead to further thickening of this “phase oxide” especially at high temperature and potentials (currents). Furthermore, as the charge involved in this oxide thickening shows a t^1/2 dependency, the process seems to be diffusion controlled.     

Electrochemical promotion of the oxidation of propane on Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of electrochemical promotion (EP) or non-faradaic electrochemical modification of catalytic activity (NEMCA) was studied in the catalytic reaction of the total oxidation of propane on Pt and Rh films deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, in the temperature range 420–520 °C. In the case of Pt/YSZ and for oxygen to propane ratios lower than the stoichiometric ratio it was found that the rate of propane oxidation could be reversibly enhanced by application of both positive and negative overpotentials (“inverted volcano” behavior), by up to a factor of 1350 and 1130, respectively. The induced rate increase Δr exceeded the corresponding electrochemically controlled rate I/2F of O²⁻ transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency Λ=Δr/(I/2F) up to 2330. The Rh/YSZ system exhibited similar EP behavior. Abrupt changes in the oxidation state of the rhodium catalyst, accompanied by changes in the catalytic rate, were observed by changing the O₂ to propane ratio and catalyst potential. The highest rate increases, by up to a factor of 6, were observed for positive overpotentials with corresponding absolute values of faradaic efficiency Λ up to 830. Rate increases by up to a factor of 1.7 were observed for negative overpotentials. The observed EP behavior is explained by taking into account the mechanism of the reaction and the effect of catalyst potential on the binding strength of chemisorbed reactants and intermediates and on the oxidative state of the catalyst surface.     

Methane oxidation over vanadium-modified Pd/Al2O3 catalysts

Three different vanadium-modified Pd/Al₂O₃ catalysts were prepared and tested as catalysts for the deep oxidation of methane. Vanadium was added to the palladium catalyst by incipient wetness of palladium catalyst in order to modify its properties and improve its thermal stability and thioresistance. The behaviour of vanadium-modified catalysts depends on the concentration of this compound, being 0.5 wt.% the optimum amount. However, when strong catalyst poisons are present in the gas (SO₂), these modified catalysts do not show a better performance than unmodified catalyst. Bimetallic catalysts were tested with and without further reduction, being observed that reduced bimetallic catalysts perform worse than the non-reduced ones.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

Catalytic combustion of methane in the presence of organic and inorganic compounds over La0.9Ce0.1CoO3 catalyst

The catalytic combustion of the emission from coke ovens containing various volatile organic compounds (VOCs) and inorganic species over a La_0.9Ce_0.1CoO₃ catalyst is investigated in an integral fixed reactor through several steps. (1) Combustion of a mixture of VOC reveals that the kinetics of total oxidation of methane determines the total VOC conversion. (2) The conversion of methane, in the case of sulfur-free feed is inhibited by H₂O and CO₂.     

Non-steady activity during methane combustion over Pd/Al2O3 and the influences of Pt and CeO2 additives

Methane combustion over Pd/Al₂O₃ catalysts with and without added Pt and CeO₂ in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al₂O₃, which was greatly enhanced over Pd–Pt/Al₂O₃. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al₂O₃ and Pd/Al₂O₃. Such non-steady behaviour was almost eliminated over Pd/Al₂O₃–CeO₂. Under a very narrow range of conditions and over a Pd/Al₂O₃ packed bed, oscillation of methane combustion was observed.     

CO oxidation at low temperature over Pd supported on CeO2-TiO2 composite oxide

Low temperature CO oxidation was carried out over CeO₂-TiO₂ composite oxide and thereon supported Pd catalysts. The effects of Ce/Ti ratio and pre-treatments of calcination and reduction on the catalytic behaviour were investigated. The CO oxidation starts at about 220 °C over CeO₂-TiO₂ and the pre-reduction treatment has little influence on the catalytic activity. Pd supported on CeO₂-TiO₂ (Pd/CeO₂-TiO₂) exhibits high activity for CO oxidation and a complete conversion of CO to CO₂ can be achieved even at ambient temperature, which suggests a synergistic effect between Pd and CeO₂-TiO₂. The activity and stability of Pd/CeO₂-TiO₂ can be further improved by the pre-reduction treatment. Ce/Ti ratio influences the catalytic behaviour significantly; the catalyst Pd/CeO₂-TiO₂ with a Ce/Ti mole ratio of 0.20 (Pd/Ce20Ti) owns the highest activity and stability, which suggests an optimization of the Pd-Ce-Ti interaction in Pd/Ce20Ti. The calcined Pd/CeO₂-TiO₂ with a Ce/Ti mole ratio higher than 0.10 shows a distorted light-off profile with the temperature, which implies an alternation of the reaction mechanism with increasing temperature.     

Effect of Pd precursor salt on the activity and stability of Pd-doped hexaaluminate catalysts for the CH4 catalytic combustion

This study reports the influence of palladium salt precursor on the catalytic activity of palladium-doped hexaaluminate catalysts for the combustion of 1 vol% CH₄ in the presence of CO₂ and H₂O as inhibitors. Thermal stability of the catalysts is evaluated in long-term catalytic test at 700 °C. The hexaaluminate supports were synthesized using two different procedures: conventional coprecipitation and solid/solid diffusion procedure. Palladium impregnation was carried out by two different routes using Pd(NO₃)₂ in water or Pd(acac)₂ in toluene as impregnation solution. It was observed that using Pd(acac)₂ as precursor allows to attain higher dispersion of the active phase (Pd particles size <3 nm). Compared to the catalysts obtained by impregnation of Pd(NO₃)₂, higher catalytic activities are then obtained. Nevertheless, a deactivation of the samples obtained using Pd(acac)₂ is observed. At the end of the stability test, almost similar catalytic activity is obtained whatever the palladium precursor. Reduction–reoxidation experiment showed that this deactivation is irreversible, and TEM analysis suggest that this deactivation is related to the sintering of Pd particles under reaction over samples synthesized using Pd(acac)₂ as precursor.     

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

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

Toluene oxidation on Pd catalysts supported by CeO2–Y2O3 washcoated cordierite honeycomb

A novel CeO₂–Y₂O₃ (CY) washcoat on cordierite honeycomb was prepared by an impregnation method, which was used as a support to prepare a Pd catalyst. A model reaction of the complete combustion of toluene was conducted to evaluate the performance of the developed Pd/CY catalyst. The CY washcoat support and the Pd/CY catalyst were characterized by XRD, Raman spectroscopy, H₂-TPR and SEM techniques. The results show that compared with conventional washcoat the CY washcoat has better adhesion and higher vibration- and heat-resistance. The CY washcoat can anchor well Pd onto the cordierite honeycomb substrate. The formation of a CeO₂–Y₂O₃ solid solution and the steady present of PdO occur at high calcination temperatures, resulting in a better thermal stability. On a Pd/CY catalyst calcined at 500 °C, a 99% of toluene conversion was obtained at 210 °C, and it was stable for reaction time up to 30 h.     

Efficient photocatalysts by hydrothermal treatment of TiO2

The aim of this work was to identify the optimum synthesis conditions and the most effective technique for noble metal deposition in a perovskite/palladium-based catalyst for natural gas combustion. The solution combustion synthesis (SCS) of perovskite/zirconia-based materials was investigated, by starting from metal nitrates/glycine mixtures. Characterization and catalytic activity tests were performed on as-prepared powders and then repeated after calcination for 2 h at 900 °C in calm air. Calcination appeared to be beneficial in that, despite lowering the specific surface area, it promoted the simultaneous crystallization of both LaMnO₃ and ZrO₂ and the half-conversion temperature (T₅₀), regarded as an index of catalytic activity, was lowered. Two phases, both active towards methane oxidation – lanthanum manganate and palladium oxide – were combined so as to evaluate their synergism in terms of catalytic activity. Pd was therefore added either via incipient wetness impregnation on LaMnO₃·2ZrO₂ or through a one-step SCS-based route. Characterization and catalytic activity tests followed suit. Optimal composition and preparation routes were found: T₅₀ was lowered from 507 °C – pure LaMnO₃ prepared via SCS – to 432 °C attained with a 2% (w/w) Pd load on pre-calcined LaMnO₃·2ZrO₂.     

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.