Catalytic activity of copper and palladium based catalysts for toluene total oxidation

In this study, catalysts containing 0.5 wt.% of palladium or 5 wt.% of copper were compared towards toluene total oxidation using FAU Zeolite and ZrO₂ supports. A 0.5%Pd/NaFAU and 5%Cu/ZrO₂ were found to be promising catalysts for this reaction. Palladium presented then a better affinity for FAU zeolite and copper oxide had a better affinity for zirconia. The performances of Pd based catalysts were correlated to interaction between the active phase and the support whereas the activity of copper oxide was related to oxygen mobility property of the support leading to copper oxide particles easily reducible. Support modifications, yttrium addition for ZrO₂ and cation exchange for the zeolite FAU, still enhanced the catalytic activity. Therefore, 0.5%Pd/CsFAU and 5%Cu/Zr₉₅Y₅ samples were found to be interesting catalysts for total VOC oxidation.     

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.     

Kinetics of Pd/alumina catalysed 1,2-dichloroethane gas-phase oxidation

The complete catalytic oxidation of 1,2-dichloroethane (DCE) over palladium supported on alumina was evaluated. The effect of temperature, inlet DCE concentration, and the space time on the reaction rate and selectivity was examined with the aim of better understanding of the reaction pathway. DCE oxidation reactions were carried out in a conventional fixed-bed reactor at atmospheric pressure under conditions of lean DCE concentration in air. The outlet composition was analysed by a gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). The reaction scheme resulted in a first dehydrochlorination step of DCE, leading to the formation of vinyl chloride (VC), followed by direct oxidation of VC to CO and CO₂. CO is finally oxidized to CO₂. The developed kinetic model provided an accurate correlation of the experimental data, determined by a non-linear least-squares regression.     

Improvement of toluene catalytic combustion by addition of cesium in copper exchanged zeolites

HY and HMFI zeolites exchanged with copper and cesium have been studied for the catalytic combustion of toluene (800 ppm) with air. The catalysts activity has been analyzed by comparison of light-off curves and in both Cu zeolites, the addition of Cs leads to a decrease of the light-off temperature by 50 °C. Temperature-programmed reduction (H₂-TPR) and EPR studies have evidenced clear differences in the characteristics of the copper species both in the presence and absence of Cs co-cations. These differences account for the nature of the active centers in the Cu zeolites for the toluene oxidation. The position and geometry of the copper ions in the zeolite matrix are of great significance for the redox behavior and activity for toluene oxidation. In both MFI and FAU structures, the bulky Cs co-cations are located in the more accessible main zeolite pores, forcing the copper ions to occupy the most stable, but less accessible positions within each structure. In the case of the MFI zeolite, the EPR study revealed that the Cs exchange resulted in an increased abundance in the number of square pyramidal Cu²⁺ ions relative to the other Cu environments. Cs co-cations also lead to an increase in the reducibility of the copper ions mainly due the reduction of protons in Cu, Cs-containing samples.     

Polydopamine modified Au/FAU catalytic membrane for CO preferential oxidation

A hollow-fiber-supported stable Au/FAU catalytic membrane was successfully synthesized through a polydopamine coating modification-removal strategy and used as a flow-through catalytic membrane reactor for preferential oxidation of CO. Small Au nanoparticles can be efficiently isolated by dopamine and the dopamine-derived carbon shells. The interactions between Au nanoparticles and zeolite layer support are enhanced during annealing at high temperature under an inert atmosphere. A zeolite membrane supported Au nanoparticle catalyst was obtained after the removal of carbon shells, which showed high catalytic activity and stability for the removal of CO from hydrogen.     

A new synthesis procedure for titanium-containing zeolites under strong alkaline conditions and the catalytic activity for partial oxidation and photocatalytic decomposition

A new synthesis procedure for titanium-containing aluminosilicate zeolites has been obtained using a clear colloidal dispersion (sol) which was prepared with titanium isopropoxide, water, hydrochloric acid and colloidal silica, Ludox. When NaY, KL, offretite, mordenite and ZSM-5 zeolites were crystallized from the sol following conventional hydrothermal crystallization procedures, all the aluminosilicate zeolites showed the same X-ray absorption fine structure at the Ti K edge indicating framework Ti. The zeolites showed remarkable catalytic activity for the partial oxidation of cyclohexene using H₂O₂ as an oxidant and the photocatalytic decomposition of trichloroethene using water.     

Stability of protonic zeolites in the catalytic oxidation of chlorinated VOCs (1,2-dichloroethane)

The deactivation of protonic zeolites in the catalytic oxidation of 1,2-dichloroethane (DCA) was evaluated. DCA oxidation reactions were carried out in a conventional fixed-bed reactor at atmospheric pressure under conditions of lean DCA concentration in air (1000 ppm). The outlet composition was analysed by a gas chromatograph, an IR spectroscopy-based analyser and another UV analyser. The effect of the zeolite crystalline structure was examined in order to track the catalytic stability of H-ZSM-5, H-MOR and H-BEA under typical reaction environment and conditions (1000 ppm DCA, 300 °C, 13,500 h⁻¹). With the aim of a better understanding of the deactivation pathway, the influence of the space velocity and temperature on the durability of protonic zeolites was analysed. Since some products formed during reaction could also cause deactivation, H₂O and HCl were introduced in the feed stream along with the DCA itself, so as to evaluate their effect. In general terms, coke formation was concluded to be the main reason for zeolite catalyst deactivation. Coke was formed from the intermediate vinyl chloride (VC), which resulted from a first dehydrochlorination step of DCA.     

Promotional effect of gold added to palladium supported on a new mesoporous TiO2 for total oxidation of volatile organic compounds

Gold and palladium were supported on a mesoporous TiO₂ for total oxidation of volatile organic compounds (VOCs). Mesoporous high surface area titania support was synthesised using of Ti(OC₂H₅)₂ in the presence of CTMABr surfactant. After removing the surfactant molecules, 0.5 or 1.5 wt% of palladium and 1 wt% of gold were precipitated on the support by, respectively, wet impregnation and deposition–precipitation methods. The activity for toluene and propene total oxidation of the prereduced samples follows the same order: 0.5%Pd-1%Au/TiO₂ > 1.5%Pd/TiO₂ > 0.5%Pd/TiO₂ > 1%Au-0.5%Pd/TiO₂ > 1%Au/TiO₂ > TiO₂. Moreover, a catalytic comparison with samples based on a conventional TiO₂, shows the catalytic advantage of the mesoporous TiO₂ support. The promotional effect of gold added to palladium could be partly explained by small metallic particles (TEM), but meanly by metallic particles made up of Au-rich core with a Pd-rich shell. Moreover, the hydrogen TPR profile of 0.5%Pd-1%Au/TiO₂ shows only the signal attributed to small PdO particles. Gold also implies a protecting effect of the support under reduction atmosphere. Operando diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was carried on and allowed to follow the VOCs oxidation and the formation of coke molecules, but also a metallic electrodonor effect to the adsorbed molecule which increases in the same order as the activity for oxidation reaction. The presence of coke after test was also shown by DTA–TGA by exothermic signals between 300 and 500 °C and by EPR (g = 2.003).     

Palladium-exchanged MFI-type zeolites in the catalytic reduction of nitrogen monoxide by methane. Influence of the Si/Al ratio on the activity and the hydrothermal stability

Pd-exchanged MFI-type zeolites containing 3.7 and 0.7 framework aluminium atoms per unit cell (corresponding to Si/Al ratios of 25 and 131) were found active in the selective reduction of nitrogen monoxide in the presence of excess oxygen. Upon steaming at 800°C, both catalysts exhibited the total loss of their catalytic activity in the reduction of NO. Such a behaviour was ascribed to the complete aggregation of Pd ions into large metal particles on the external surface of the zeolite crystals. Both supports, although maintaining their crystallinity, are shown to experience extended dealumination upon steaming. Although the loss of Pd exchange capacity could partially explain the Pd migration and sintering, a mechanism involving the formation of mobile Pd hydroxyl entities condensing into PdO particles outside the zeolite crystallites is preferred.     

Size-controlled synthesis and impedance-based mechanistic understanding of Pd/C nanoparticles for formic acid oxidation

This work provides a detailed electrochemical impedance study for formic acid electro-oxidation on size-controlled Pd/C nanoparticles, the synthesis of which was done by a simple protocol using ethylene glycol as a reducing agent. By controlling KOH concentration, this strategy provides a synthesis method for Pd nanoparticles with a selective size range of 3.9–7.5 nm. The as-prepared Pd nanoparticles exhibited size-dependent electrochemical property and electrochemical characterizations of four different Pd/C nanocatalysts (3.9, 5.2, 6.1, and 7.5 nm) showed that Pd particle with average size of 6.1 nm has the highest formic acid oxidation activity. Electrochemical impedance-based characterizations of formic acid oxidation on Pd/C suggested that at high potentials the adsorbed oxygen species could block the catalyst surface and inhibit the oxidation reaction, as reflected by the negative polarization resistance. Unlike Pd/C, the intermediate adsorbed CO species (CO_ads) plays a critical role for formic oxidation on Pt/C and thus the impedance spectra of Pd/C and Pt/C appear different potential-dependent patterns in the second quadrant. The issue of CO was investigated by an impedance investigation of Pd/C in a mixture of formic acid containing dissolved CO.     

Evaluation of the performance of catalytic oxidation of VOCs by a mixed oxide at a semi‐pilot scale†

Volatile organic compounds (VOCs) are one of the main contributors to air pollution. To reduce anthropogenic emissions, it is necessary to improve existing techniques such as catalytic oxidation through the development of new cost‐effective catalysts. Although many studies deal with the development and testing of new materials, most are performed at laboratory scale, of which only a few study mixtures of VOCs. To assess their viability for industrial applications, further tests are required, namely, mixture tests at intermediate scale in relevant environment and extrapolated on an industrial scale. In this work, the catalytic performance of a new mixed oxide Co‐Al‐Ce was investigated towards the oxidation of the n‐butanol and toluene on a semi‐pilot scale (TRL 4). Single component and mixture experiments were performed for several concentrations at a fixed flow rate. A commercial catalyst Pd/γ‐Al₂O₃ was used as the benchmark to evaluate the performance of the mixed oxide. The Co‐Al‐Ce catalyst enables complete oxidation of n‐butanol at the same temperature as the reference catalyst. Moreover, it provides a better selectivity for n‐butanol, while providing an equivalent one for the oxidation of toluene. In mixtures, the presence of n‐butanol promotes the oxidation of toluene for both catalysts but more significantly for the Co‐Al‐Ce catalyst. The presence of toluene inhibits the oxidation of n‐butanol for the Co‐Al‐Ce and promotes it for high conversions of n‐butanol for the Pd/γ‐Al₂O₃ catalyst.     

Catalytic oxidation of aromatic VOCs with Cr or Pd-impregnated Al-pillared bentonite: Byproduct formation and deactivation studies

The catalytic behavior of chromium and palladium-impregnated Al-pillared bentonite for the oxidation of aromatic VOCs, i.e. chlorobenzene or xylene, was investigated. The Cr-impregnated bentonite showed high activity for the total oxidation of cholorobenzene and xylene but the materials were completely deactivated during the reaction at 600 °C. Atomic absorption, XPS, XRD and TG analyses suggested three main causes for the deactivation, i.e. the loss of Cr due to the formation of volatile CrO₂Cl₂, a strong decrease on the surface area due to the collapse of the pillars and the formation of coke. For the Pd supported pillared bentonite, the impregnation procedure completely destroyed the Al-pillars but produced a very active and stable catalyst to oxidize aromatic contaminants. However, in the case of chlorobenzene almost 20% yield of the hazardous hexachlorobenzene was obtained likely by an oxychlorination process.     

Alkali and alkaline-earth cation exchanged chabazite zeolites for adsorption based CO2 capture

In this study chabazite zeolites were prepared and exchanged with alkali cations – Li, Na, K and alkaline-earth cations – Mg, Ca, Ba and were studied to assess their potential for CO₂ capture from flue gas by vacuum swing adsorption for temperatures below 120 °C. Isotherm measurements (CO₂ and N₂) were made for all samples at 273 K, 303 K and 333 K using a volumetric apparatus and represented with the Dual-site Langmuir model for CO₂ and N₂. Henry’s constants and isosteric heats of adsorption were calculated and qualitative analyses performed for all samples. Adiabatic separation factor (ASF) and capture figure of merit (CFM) were proposed and used as indices for assessing adsorbent performance and compared with a commercial NaX-zeolite sample. It was found that NaCHA and CaCHA hold comparative advantages for high temperature CO₂ separation whilst NaX shows superior performance at relatively low temperatures.     

Bimetallic Pt/Pd diesel oxidation catalysts: Structural characterisation and catalytic behaviour

Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) studies on supported bimetallic Pt/Pd diesel oxidation catalyst (Pt:Pd weight ratio 2:1) after various treatments were compared with those of monometallic Pd and Pt catalysts prepared under similar conditions. After calcination and thermal ageing, the coexistence of alloyed bimetallic Pt/Pd particles and of tetragonal PdO has been found in the bimetallic Pt/Pd catalyst. PdO is present in form of crystals at the surface of the Pt/Pd particles or as isolated PdO crystals on the support oxide. Bimetallic Pt/Pd nanoparticles were already formed in the Pt/Pd catalyst after calcination. Hydrogen treatment causes the formation of randomly alloyed Pt/Pd nanoparticles. In the thermally aged catalyst, a strong indication for an enrichment of Pt in the interior of the particle and of Pd at its outer shell was found. In the monometallic catalyst, the Pd is found to be completely oxidised already after calcination and to consist of metallic Pd in zero-valent state exclusively after reductive treatment. Ageing under hydrothermal oxidative atmosphere leads to complete oxidation of the Pd species. After calcinations, the catalytic activity of the Pt/Pd catalyst studied is comparable to those of monometallic Pt catalysts. In contrast to monometallic Pt catalysts, the alloyed system show significant stabilisation against sintering and a much higher activity after the thermal ageing step. This stabilisation of dispersion and the presence of Pt atoms on the surface of the Pt/Pd particles are considered to cause the higher catalytic activity of metallic particles for the oxidation of carbon monoxide and propene after ageing.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

Identification of cationic and oxidic caesium species in basic Cs-overloaded BEA zeolites

A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs⁺ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion-exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH₃ reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of carbonates by adsorption of CO₂, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H⁺ with Cs⁺ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs₂O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX measurements). IR spectroscopy of adsorbed CO shows that Cs⁺ as countercations or as surface sites of occluded Cs₂O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs⁺ sites (formed by two countercations and/or one countercation and a Cs⁺ at the surface of Cs₂O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct ν_CO band at 2145 cm⁻¹.     

Nanostructured macro-mesoporous zirconia impregnated by noble metal for catalytic total oxidation of toluene

Hierarchical bimodal macro-mesoporous zirconia oxide has been synthesized by a simple method in the presence of CTMABr surfactant. The synthesized zirconia having uniform macropores of 300–600 nm in diameter with wormhole-like mesoporous walls and high surface area was calcined at 400 and 600 °C and impregnated with 0.5 wt.% of palladium and compared with classical 0.5 wt.% Pd/ZrO₂ catalyst for toluene oxidation. The highest activity of 0.5 wt.%/macro-mesoporous zirconia calcined at 600 °C was mainly explained by a rather high Pd dispersion and by H₂-TPR measurements showing a higher quantity of PdO species easily reducible at 0 °C.     

Superior catalytic behaviour of Pt-doped Pd catalysts in the complete oxidation of methane at low temperature

The catalytic combustion of methane at low temperature under lean conditions was investigated over bimetallic palladium-platinum catalysts supported on alumina. Pd-Pt catalysts with constant 2 wt.% metal loading and varying compositions in Pt and Pd were prepared by successive impregnations of the metal salts. The catalysts were characterised by powder X-ray diffraction, transmission electron microscopy/electron dispersion X-ray spectroscopy (TEM/EDX), volumetry of H₂ chemisorption, FTIR study of CO adsorption and temperature-programmed oxidation (TPO). In the absence of water added to the feed, the methane conversion over Pd-rich bimetallic catalysts (Pt/Pt + Pd molar ratios less than 0.3) was found to be the same as that of the reference Pd/Al₂O₃ catalyst. Interestingly, under wet conditions, these bimetallic catalysts exhibited an improved performance with respect to Pd/Al₂O₃. This effect was found to be maintained upon mild steam ageing. An interaction between both metals was suggested to explain the enhanced activity of bimetallic catalysts. This was confirmed by TPO experiments indicating that formation and decomposition of PdO is affected upon Pt addition even for very low amounts of Pt. The adsorption of CO on reduced catalysts studied by FTIR revealed new types of adsorbed CO species, suggesting again an interaction between two metals.