M. Konsolakis

Redox properties and VOC oxidation activity of Cu catalysts supported on Ce1−xSmxOδ mixed oxides

A series of Cu catalysts supported on Ce1-xSmxOδ mixed oxides with different molar contents (x=0, 0.25, 0.5, 0.75 and 1), was prepared by wet impregnation and evaluated for volatile organic compounds (VOC) abatement, employing ethyl acetate as model molecule. An extensive characterization study was undertaken in order to correlate the morphological, structural and surface properties of catalysts with their oxidation activity. The optimum performance was obtained with Cu/CeO2 catalyst, which offers complete conversion of ethyl acetate into CO2 at temperatures as low as 260°C. The catalytic performance of Cu/Ce1-xSmxOδ was interpreted on the basis of characterization studies, showing that incorporation of samarium in ceria has a detrimental effect on the textural characteristics and reducibility of catalysts. Moreover, high Sm/Ce atomic ratios (from 1 to 3) resulted in a more reduced copper species, compared to CeO2-rich supports, suggesting the inability of these species to take part in the redox mechanism of VOC abatement. Sm/Ce surface atomic ratios are always much higher than the nominal ratios indicating an impoverishment of catalyst surface in cerium oxide, which is detrimental for VOC activity.

Effect of preparation method on the solid state properties and the deN2O performance of CuO–CeO2 oxides

The present work aims at investigating the catalytic decomposition of N2O over CuO–CeO2 single or mixed oxides prepared by different synthesis routes, i.e., impregnation, precipitation and exotemplating. To gain insight into the particular role of CeO2 as well as of CuO–CeO2 interactions, three different types of materials were prepared and tested for N2O decomposition both in the absence and presence of excess O2: (i) bare CeO2 prepared by precipitation and exotemplating, (ii) CuO/CeO2 oxides synthesized by the impregnation of CeO2 samples prepared in (i) with CuO, and iii) single stage CuO–CeO2 mixed oxides synthesized employing the co-precipitation and exotemplating methods. The corresponding commercial samples were also examined for comparison purposes. All materials were characterized by N2 adsorption at −196 °C, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy (micro-Raman) and scanning electron microscopy (SEM). The results demonstrated the key role of the preparation procedure on the direct catalytic decomposition of N2O. Among the bare CeO2 samples, the best performance was obtained with the samples prepared by the precipitation method, followed by exotemplating, while commercial CeO2 showed the lowest performance. All bare oxides demonstrated low N2O conversion, never exceeding 40% at 600 °C. Amongst the CuO–CeO2 oxides, the optimum performance was observed for those prepared by co-precipitation, which achieved complete N2O conversion at 550 °C. In the presence of excess oxygen in the feed stream, a slight degradation is observed, with the sequence of deN2O performance remaining unchanged. The superiority of the Cu–Ce mixed oxides prepared by precipitation compared to all of the other materials can be mainly ascribed to their excellent redox properties, linked to Ce4+/Ce3+ and Cu2+/Cu+ redox pairs. A redox mechanism for the N2O catalytic decomposition is proposed, involving N2O adsorption on Cu+ sites and their regeneration through Cu–ceria interactions.

The effect of sodium on the Pd-catalyzed reduction of NO by methane

The kinetics of NO reduction by methane over Pd catalysts supported on 8 mol% yttria-stabilised zirconia (YSZ) has been studied at atmospheric pressure in the 620‐770 K temperature range. Langmuir‐Hinshelwood type kinetics are found with characteristic rate maxima reflecting competitive adsorption of NO and methane: NO adsorption is much more pronounced than that of methane within the temperature range of this investigation. Pd is an effective catalyst: 100% selectivity towards N2 can be achieved at 100% conversion of NO over this wide temperature range. Sodium causes strong poisoning of the reaction. The response of the system to variations in NO and methane concentrations, temperature, and sodium loading indicate that this is due to the Na-induced enhancement of NO chemisorption and dissociation relative to methane adsorption, i.e. sodium enhances oxygen poisoning of the catalyst. These results stand in revealing contrast to the strong promotional effect of sodium in the reduction of NO by propene over the same catalysts. The very different response of the two hydrocarbon reductants to Na doping of the Pd catalyst receives a consistent explanation. # 1998 Elsevier Science B.V. All rights reserved.

Optimal promotion by rubidium of the CO + NO reaction over Pt/γ-Al2O3 catalysts

The reduction of NO by CO over Rb-promoted Pt/-Al2O3 catalysts has been investigated over a wide range of temperature (ca. 200–500 ◦ C), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/-Al2O3 catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N 2selectivity. Rate increases by factors as high as 110 and 45 for the production of N 2 and CO2, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N2-selectivity (e.g. from 24 to 82% at 350 ◦ C and [CO] = 0.5%, [NO] = 1%). Under stoichiometric conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N2 at T ∼ 350 ◦ C and at an effective reactant contact time of only ∼0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N2-selectivity (approximately <40%); even at 480 ◦ C the conversion was only ∼60%. The observed promotional effects are ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products which lead to the observed activity enhancement and dependence of N2-selectivity on promoter loading. The effects of K-promotion mirror those of Rb-promotion, but are significantly less pronounced. Rb is the best alkali promoter.

Effect of carbon type on the performance of a direct or hybrid carbon solid oxide fuel cell

The impact of carbon type on the performance of the direct carbon fuel cell (DCFC) or hybrid carbon fuel cell (HCFC) is investigated by utilizing bare carbon or carbon/carbonate mixtures as feedstock, respectively. In this regard, four different types of carbons, i.e. bituminous coal (BC), demineralised bituminous coal (DBC), anthracite coal (AC) and pine charcoal (PCC), are employed as fuels in a SOFC of the type: carbon (carbonate)|Cu–CeO2/YSZ/Ag|Air. The results reveal that in the absence of carbonates (DCFC configuration) the optimum performance, in terms of maximum power density (Pmax), is obtained for the charcoal sample, which demonstrated a power output of ∼12 mW cm−2 at 800 °C, compared to 3.4 and 4.6 mW cm−2 with the anthracite and bituminous samples, respectively. Demineralization treatment of bituminous coal is found to improve the DCFC performance resulting in a maximum power density of 5.5 mW cm−2. A similar trend in terms of maximum power density, i.e., PCC > DBC > BC > AC, is obtained in the hybrid carbon fuel cell (HCFC) employing a eutectic mixture of lithium and potassium carbonates (62 mol% Li2CO3 + 38 mol% K2CO3) in the anode compartment at a carbon/carbonate weight ratio of 4:1. An enhancement of up to 185% in the maximum power density is achieved by admixing molten carbonates with carbon feedstock, with its extent being dependent on carbon type and temperature. The obtained results are interpreted on the basis of carbon physicochemical characteristics and their impact on DCFC performance. It is found that the observed trend in volatile matter, porosity and structure disorder is perfectly correlated with the achieved power output. In contrast, high ash and sulfur contents notably inhibit the electrochemical performance. The superior performance demonstrated by pine charcoal in conjunction with its availability and renewable nature, reveals the potential of biomass as feedstock in both DCFCs and HCFCs.

Ultrasound-assisted removal of Acid Red 17 using nanosized Fe3O4-loaded coffee waste hydrochar.

The Fe3O4-loaded coffee waste hydrochar (Fe3O4-CHC) was synthesized using a simple precipitation method. The as-prepared adsorbent was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and Fourier transform infrared spectroscopy (FT-IR). The EDX analysis indicated the presence of Fe in the structure of Fe3O4-CHC. The specific surface area of hydrochar increased from 17.2 to 34.7m2/g after loading of Fe3O4 nanoparticles onto it. The prepared Fe3O4-CHC was used for removal of Acid Red 17 (AR17) through ultrasound-assisted process. The decolorization efficiency decreased from 100 to 74% with the increase in initial dye concentration and from 100 to 91 and 85% in the presence of NaCl and Na2SO4, respectively. The synthesized Fe3O4-CHC exhibited good stability in the repeated adsorption-desorption cycles. The high correlation coefficient (R2=0.997) obtained from Langmuir model indicated that physical and monolayer adsorption of dye molecules occurred on the Fe3O4-CHC surface. Furthermore, the by-products generated through the degradation of AR17 was identified by gas chromatography-mass spectrometry analysis.

Successful application of electrochemical promotion to the design of effective conventional catalyst formulations

Electrochemical promotion (EP), discovered and developed by Vayenas and co-workers provides a novel in situ reversible and highly controllable means of catalyst promotion. We found that Pt-group metal catalysts exhibit strong EP by sodium during reactions related to emission control catalysis, such as NO reduction by hydrocarbons. Close similarities are found between the performance of Pt-film catalyst promoted electrochemically with Pt highly dispersed on large surface area carriers (e.g. g-Al O ) promoted by conventional means (impregnation). These similarities include (i) the overall kinetic 23 behaviour and (ii) the dependence of the activity and selectivity on Na loading. Using both methods of Na-promotion, the catalytic reduction of NO by propene over Pt exhibited rate enhancements as high as two orders of magnitude accompanied by very pronounced increases of the system selectivity towards N. The results serve to validate further the interpretation 2 offered for the EP (or NEMCA) phenomenon. More importantly, they demonstrate that the insight obtained from EP studies can be used to design conventional type effective catalyst formulations that were previously untried, thus opening up new areas for investigation in the frontiers between catalysis and electrochemistry.

Ethyl Acetate Abatement on Copper Catalysts Supported on Ceria Doped with Rare Earth Oxides

Different lanthanide (Ln)-doped cerium oxides (Ce0.5Ln0.5O1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N2 adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H2. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO2 ≈ Ce0.5Pr0.5O1.75 > Ce0.5Sm0.5O1.75 > Ce0.5Gd0.5O1.75 > Ce0.5Nd0.5O1.75 > Ce0.5La0.5O1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO2 and Cu/Ce0.5Pr0.5O1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen.

Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides

Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N2 adsorption at -196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure-activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO2 and Co3O4, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.

Impact of the synthesis parameters on the solid state properties and the CO oxidation performance of ceria nanoparticles

Ceria-based materials have received considerable attention in catalysis field due to their unique physicochemical characteristics. Compared to bulk ceria, nanosized ceria received particular interest, due to its high surface to volume ratio, improved reducibility and optimal morphological features. Hence, the fine-tuning of ceria properties by means of advanced synthesis routes is of particular importance. In this regard, the present work aims at investigating the impact of synthesis parameters on the solid state properties of CeO2 materials. Four different time- and cost-effective preparation methods were followed, i.e. thermal decomposition, co-precipitation and hydrothermal method of low and high NaOH concentration, employing in all cases Ce(NO3)3·6H2O as cerium precursor. A complementary characterization study, involving N2 adsorption at −196 °C (BET method), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and scanning/transmission electron microscopy (SEM/TEM), was carried out to gain insight into the impact of synthesis route on the textural, structural, morphological and redox properties. The results revealed the superiority of the hydrothermal method towards the development of ceria nanoparticles of high specific surface area (>90 m2 g−1), well-defined geometry (nanorods) and improved redox properties. CO oxidation was employed as a probe reaction to gain insight into the structure–activity correlation. Ceria nanorods prepared by hydrothermal method of high NaOH concentration demonstrated the optimum CO oxidation performance. A direct quantitative correlation between the catalytic activity and the abundance of easily reduced, loosely bound oxygen species, was revealed. Hence, this particular reactivity descriptor can potentially be used for the rational design of ceria-based materials.