CuxCeMgAlO mixed oxide catalysts derived from multicationic LDH precursors for methane total oxidation

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Exploring the current state of play for cost-effective water treatment by membranes

This article presents a perspective on the current development and application of membranes for the treatment of water. We examine how membranes contribute to the global challenge of sustainable supply of clean water. The main theme is on desalination and how innovative science and emerging technology is being applied. Thus, we appraise how techniques such as advanced membrane materials, biomimetic membranes, hybrid systems, forward osmosis, and membrane distillation are being used to improve production to meet the increasing global demand for water

Oxizi micşti ex-LDH pe bază de metale tranziţionale, catalizatori pentru combustia metanului

In an attempt to converge the benefits of preparation of mixed oxides from layered double hydroxide (LDH) precursors with the high activity of Cu-based systems in methane combustion and taking simultaneously into consideration an expected Cu-Ce synergistic effect, the first part of this work is focused on new LDH-derived Cu-Ce-MgAlO mixed oxides catalysts. Thus, a series of five Cu(x)CeMgAlO mixed oxides with different copper contents (x) ranging from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic ratio of 3, was prepared by thermal decomposition at 750 °C of precursors consisting of poorly crystallized LDH and boehmite (AlOOH) phases. Powder X-ray diffraction (XRD) was used to characterize the crystalline structure and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) was used to monitor the morphology and chemical composition of both as prepared and calcined materials. Additionally, the textural properties and the reducibility of the mixed oxide catalysts were studied by nitrogen adsorption/desorption and temperature programmed reduction with hydrogen (H2-TPR) techniques, respectively. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the elements on the catalyst surface and the diffuse reflectance UV-Vis spectroscopy (DR UV-Vis), to obtain information about the stereochemistry and aggregation of copper in the Cu-containing mixed oxides. It has been shown that the mixed oxide catalysts have slit-like bimodal mesopores and relatively high surface areas, which regularly decrease from 169 to 108 m2 g-1 with increasing the Cu content, and consist of periclase-like Mg(Al)O mixed oxide and CeO2 fluorite phases, except for Cu(18)CeMgAlO, which also contains well developed CuO crystallites. At low Cu content, highly dispersed Cu-doped ceria crystallites coexist with oligomeric (Cu2+– O2-– Cu2+)n2+ species in the Mg(Al)O matrix, while at high Cu content larger ceria crystallites less exposed on the catalyst surface coexist with separate CuO particles enriching the surface. Their catalytic properties in the total oxidation of methane, used as a volatile organic compound (VOC) model molecule, were evaluated and compared with those of an industrial Pd/Al2O3 catalyst. Their catalytic behavior was explained in correlation with their physicochemical properties. The Cu(II)/Cu surface atomic ratio was shown to be a key factor controlling the catalytic activity of the Cu(x)CeMgAlO catalysts. Indeed, with the highest Cu(II)/Cu surface atomic ratio, the Cu(15)CeMgAlO mixed oxide was shown to be the most active catalyst in this series, with a T50 (temperature corresponding to 50 % methane conversion) value of only ca. 45 °C higher than that of an industrial Pd/Al2O3 catalyst. Its enhanced catalytic activity was attributed to an excellent dispersion of CuO, oligomeric (Cu2+– O2-– Cu2+)n2+ species coexisting with tiny not XRD-visible CuO crystallites strongly interacting with ceria, which leads to a strong synergy effect between Cu and Ce. The catalytic sites are located at the CuO-CeO2 particles interface. Decreasing the calcination temperature of the Cu(15)CeMgAl precursor from 750 to 650 and 550 °C resulted in mixed oxides with higher surface areas and, hence, higher catalytic activities in terms of T50, which for the Cu(15)CeMgAlO-550 system is only ca. 25 °C higher than that of the industrial Pd/Al2O3 catalyst. However, in terms of intrinsic reaction rates the most active catalyst was that calcined at 750 °C accounting for a strengthened Cu-Ce interaction with increasing calcination temperature. The influences of the contact time and of the methane concentration in the feed gas on the catalytic performances of the Cu(15)CeMgAlO catalyst calcined at 750 °C have also been investigated and its good stability on stream was evidenced.In the second part of this work, in order to improve the catalytic performance of the Cu(15)CeMgAlO mixed oxide catalyst calcined at 750 °C in the total oxidation of methane reaction, it was promoted with different transition-metal cations M (M = Mn, Fe, Co and Ni), an enhanced synergy effect between Cu and Ce in their presence being expected. Thus, two series of multicationic mixed oxides were prepared by the controlled thermal decomposition at 750 °C of their corresponding LDH precursors synthesized by coprecipitation at constant pH of 10 under ambient atmosphere. The first series of catalysts consisted of four M(3)CuCeMgAlO mixed oxides containing 3 at. % M (M = Mn, Fe, Co, Ni), 15 at. % Cu, 10 at. % Ce (at. % with respect to cations) and with Mg/Al atomic ratio fixed to 3. Taking into consideration that Co was found to be the best promoter and that the content of the transition-metal promoter is known to strongly influence the trimetallic synergistic interaction and, hence, the catalytic performance of the mixed oxide, the second series consisted of four Co(x)CuCeMgAlO mixed oxides with x = 1, 3, 6 and 9 at. % Co, while keeping constant the Cu and Ce contents and the Mg/Al atomic ratio. All the mixed oxides were characterized using a battery of techniques including XRD, transmission electron microscopy (TEM), SEM-EDX, XPS, nitrogen adsorption/desorption at –196 °C, H2-TPR, and DR UV-Vis, while thermogravimetric and differential thermal analyses together with XRD were used for the LDH precursors. The latter consisted of well-crystallized LDH phase together with poorly-crystallized boehmite (AlOOH) side phase. All the mixed oxides calcined at 750 °C were mesoporous materials with surface areas lower than that of the parent CuCeMgAlO mixed oxide, in the range from 66 to 78 m2 g-1, and consisted of periclase-like Mg(Al)O mixed oxide, CeO2 fluorite, and CuO tenorite phases. The segregation of the CuO tenorite phase was shown to increase with the Co content in the Co(x)CuCeMgAlO series. The nature of the transition-metal M for the M(3)CuCeMgAlO catalysts series and the Co content for the Co(x)CuCeMgAlO series strongly influence, in a complex manner, their physicochemical characteristics and, hence, their catalytic performance in the complete oxidation of methane. No clear correlations could be observed between the catalytic performance and the physicochemical characteristics of the mixed oxide in the M(3)CuCeMgAlO series. However, their enhanced activity compared to the M-free CuCeMgAlO system was attributed to an enhanced synergistic interaction between Cu and Ce in the presence of transition-metal cations M. On the other hand, the catalytic activity in the Co(x)CuCeMgAlO series was shown to increase with increasing of both the Co3+, Cu2+ and Ce4+ surface concentrations and the catalyst reducibility, unambiguously demonstrating that the synergistic interaction between these species is a key factor controlling their catalytic behavior in the complete oxidation of methane. The Co-promoted CuCeMgAlO mixed oxide with 3 at. % Co was shown to be the most active catalyst in both series, with a T50 value of 438 °C, which is only 19 °C higher than that of a reference Pd/Al2O3 catalyst. Notably, this T50 value is ca. 25 °C lower than that observed for the unpromoted CuCeMgAlO system, accounting for the improved performance of the Co-promoted catalyst. Its enhanced catalytic activity was attributed to an excellent Cu-Co-Ce synergistic interaction. A good stability on stream of the Co(3)CuCeMgAlO catalyst was also noticed.Dans une tentative de faire converger les avantages de la préparation d'oxydes mixtes à partir de précurseurs hydroxydes doubles lamellaires (HDL) avec l’activité élevée des systèmes à base de Cu dans la combustion du méthane et en prenant simultanément en considération un effet synergique Cu-Ce attendu, la première partie de ce travail est axée sur des nouveaux catalyseurs oxydes mixtes Cu-Ce-MgAlO issus des HDL. Ainsi, une série de cinq oxydes mixtes Cu(x)CeMgAlO avec différentes teneurs en cuivre (x) allant de 6 à 18 at. % par rapport aux cations, mais avec une teneur en Ce fixée à 10 at. % et un rapport atomique Mg/Al fixé à 3, a été préparée par décomposition thermique à 750 °C des précurseurs constitués de phases HDL et boehmite (AlOOH) mal cristallisées. La diffraction des rayons X sur poudre (XRD) a été utilisée pour caractériser la structure cristalline et la microscopie électronique à balayage couplée à la spectroscopie de rayons X à dispersion d'énergie (SEM-EDX) a été utilisée pour surveiller la morphologie et la composition chimique des matériaux fraichement préparés et calcinés. En plus, les propriétés texturales ainsi que la réductibilité des catalyseurs oxydes mixtes ont été étudiées respectivement par les techniques d'adsorption/désorption d'azote et de réduction programmée en température avec de l'hydrogène (H2-TPR). La spectroscopie photoélectronique à rayons X (XPS) a été utilisée pour déterminer l'état chimique des éléments à la surface du catalyseur et la spectroscopie UV-Vis à réflectance diffuse (DR UV-Vis), pour obtenir des informations sur la stéréochimie et l'agrégation du cuivre dans les oxydes mixtes contenant du Cu. Il a été démontré que les catalyseurs oxydes mixtes ont des mésopores bimodaux en forme de fente et des surfaces spécifiques relativement élevées, qui diminuent régulièrement de 169 à 108 m2 g-1 avec l'augmentation de la teneur en Cu, et sont constitués d’un mélange de phases oxyde mixte Mg(Al)O périclase et CeO2 fluorite, à l'exception de Cu(18)CeMgAlO, qui contient également des cristallites CuO bien développées. À faible teneur en Cu, des cristallites de cérine dopée au Cu bien dispersées coexistent avec des espèces oligomères (Cu2+ – O2- – Cu2+)n2+ dans la matrice Mg(Al)O, tandis qu'à forte teneur en Cu, des cristallites de cérine plus larges et moins exposées à la surface du catalyseur coexistent avec des particules de CuO séparées enrichissant la surface. Leurs propriétés catalytiques dans l'oxydation totale du méthane, utilisé comme molécule modèle de composé organique volatil (COV), ont été évaluées et comparées à celles d'un catalyseur industriel Pd/Al2O3. Leur comportement catalytique a été expliqué en corrélation avec leurs propriétés physico-chimiques. Il a été démontré que le rapport atomique de surface Cu(II)/Cu était un facteur clé contrôlant l'activité catalytique des catalyseurs Cu(x)CeMgAlO. En effet, avec le rapport atomique de surface Cu(II)/Cu le plus élevé, l'oxyde mixte Cu(15)CeMgAlO s'est avéré être le catalyseur le plus actif de cette série, avec une valeur T50 (température correspondant à une conversion de 50 % du méthane) de seulement 45 °C environ supérieure à celle d'un catalyseur industriel Pd/Al2O3. Son activité catalytique accrue a été attribuée à une excellente dispersion de CuO, des espèces oligomères (Cu2+ – O2- – Cu2+)n2+ coexistant avec de minuscules cristallites de CuO non visibles aux rayons X et interagissant fortement avec l'oxyde de cérium, ce qui entraîne un fort effet de synergie entre Cu et Ce. Les sites catalytiques sont situés à l'interface entre les particules de CuO et de CeO2. La diminution de la température de calcination du précurseur Cu(15)CeMgAl de 750 à 650 et 550 °C a permis d'obtenir des oxydes mixtes de plus grande surface spécifique et, par conséquent, des activités catalytiques plus importantes en termes de T50, qui, pour le système Cu(15)CeMgAlO-550, est de seulement 25 °C environ supérieure à celle du catalyseur industriel Pd/Al2O3. Toutefois, en termes de vitesse de réaction intrinsèque, le catalyseur le plus actif est celui qui est calciné à 750 °C, ce qui rends compte d’une interaction renforcée entre le Cu-Ce avec l'augmentation de la température de calcination. Les influences du temps de contact et de la concentration de méthane dans le gaz d'alimentation sur les performances catalytiques du catalyseur Cu(15)CeMgAlO calciné à 750 °C ont également été étudiées et sa bonne stabilité sous flux réactionnel a été mise en évidence.Dans la deuxième partie de ce travail, afin d'améliorer les performances catalytiques du catalyseur oxyde mixte Cu(15)CeMgAlO calciné à 750 °C dans la réaction d'oxydation totale du méthane, il a été promu par différents cations de métaux de transition M (M = Mn, Fe, Co et Ni), un effet de synergie accru entre Cu et Ce en leur présence étant attendu. Ainsi, deux séries d'oxydes mixtes multicationiques ont été préparées par décomposition thermique contrôlée à 750 °C de leurs précurseurs HDL correspondants synthétisés par coprécipitation à pH constant de 10 sous atmosphère ambiante. La première série de catalyseurs était composée de quatre oxydes mixtes M(3)CuCeMgAlO contenant 3 at. % M (M = Mn, Fe, Co, Ni), 15 at. % Cu, 10 at. % Ce (at. % par rapport aux cations) et avec un rapport atomique Mg/Al fixé à 3. Compte tenu du fait que le Co s'est avéré être le meilleur promoteur et que la teneur en promoteur de métal de transition est connue pour influencer fortement l’interaction synergique trimétallique et, par conséquent, les performances catalytiques de l'oxyde mixte, la deuxième série se composait de quatre oxydes mixtes Co(x)CuCeMgAlO avec x = 1, 3, 6 et 9 at. % Co, tout en maintenant constants les teneurs en Cu et Ce et le rapport atomique Mg/Al. Tous les oxydes mixtes ont été caractérisés à l'aide d'une batterie de techniques comprenant XRD, microscopie électronique à transmission (TEM), SEM-EDX, XPS, adsorption/désorption d'azote à –196 °C, H2-TPR et DR UV-Vis, tandis que l’analyse thermogravimétrique couplée à l’analyse thermique différentielle ainsi que la XRD ont été utilisées pour les précurseurs HDL. Ces derniers étaient constitués d'une phase HDL bien cristallisée avec une phase impureté de boehmite (AlOOH) mal cristallisée. Tous les oxydes mixtes calcinés à 750 °C étaient des matériaux mésoporeux avec des surfaces spécifiques inférieures à celle de l'oxyde mixte CuCeMgAlO parent, allant de 66 à 78 m2 g-1, et consistaient en un mélange de phases oxyde mixte Mg(Al)O périclase, CeO2 fluorite et CuO ténorite. Il a été démontré que la ségrégation de la phase CuO ténorite augmentait avec la teneur en Co dans la série Co(x)CuCeMgAlO. La nature du métal de transition M pour la série de catalyseurs M(3)CuCeMgAlO et la teneur en Co pour la série Co(x)CuCeMgAlO influencent fortement, de manière complexe, leurs caractéristiques physico-chimiques et, par conséquent, leurs performances catalytiques dans l’oxydation totale du méthane. Aucune corrélation claire n'a pu être observée entre les performances catalytiques et les caractéristiques physico-chimiques de l'oxyde mixte de la série M(3)CuCeMgAlO. Cependant, leur activité accrue par rapport au système CuCeMgAlO sans M a été attribuée à une interaction synergique améliorée entre Cu et Ce en présence de cations de métaux de transition M. D'autre part, l'activité catalytique dans la série Co(x)CuCeMgAlO s'est avéré augmenter avec l'augmentation des concentrations de Co3+, Cu2+ et Ce4+ en surface et la réductibilité du catalyseur, démontrant sans ambiguïté que l'interaction synergique entre ces espèces est un facteur clé contrôlant leur comportement catalytique dans l'oxydation complète du méthane. L'oxyde mixte CuCeMgAlO promu par 3 at. % Co s'est avéré être le catalyseur le plus actif des deux séries, avec une valeur T50 de 438 °C, qui n'est que de 19 °C supérieure à celle d'un catalyseur de référence Pd/Al2O3. A noter que cette valeur T50 est de 25 °C environ inférieure à celle observée pour le système CuCeMgAlO non promu, ce qui explique les performances améliorées du catalyseur promu par Co. Son activité catalytique améliorée a été attribuée à une excellente interaction synergique Cu-Co-Ce. Une bonne stabilité sous flux réactionnel du catalyseur Co(3)CuCeMgAlO a également été constatée.Într-o ȋncercare de a converge beneficiile preparării oxizilor micşti din precursori hidroxizi dubli lamelari (HDL) cu activitatea mare ȋn combustia metanului a sistemelor pe bază de cupru şi ţinând seama, ȋn acelaşi timp, de un posibil efect de sinergie Cu-Ce, prima parte a acestei lucrări este dedicată unor noi catalizatori oxizi micşti Cu-Ce-MgAlO ex-LDH. Astfel, a fost preparată o serie de cinci oxizi micşti Cu(x)CeMgAlO cu conţinut diferit de cupru (x) ȋn domeniul 6-18 % (% atomi faţă de cationi), dar cu conţinut fix de 10 % Ce şi raport atomic Mg/Al = 3, prin descompunerea termică la 750 °C a unor precursori constând ȋntr-un amestec de faze LDH şi boehmit (AlOOH) slab cristalizate. Caracterizarea sructurii cristaline atât a precursorilor cât şi a materialelor calcinate s-a realizat prin difracţie de raze X (DRX) ȋn pulbere, iar morfologia şi compoziţia chimică a acestora au fost determinate cu ajutorul microscopiei electronice cu baleiaj cuplată cu spectroscopia de raze X dispersivă ȋn energie (MEB-EDXS). Proprietăţile texturale şi reductibilitatea catalizatorilor oxizi micşti au fost studiate prin adsorbţie/desorbţie de azot şi, respectiv, termoreducere programată cu hidrogen (H2-TPR). Compoziţia şi starea de valenţă a elementelor la suprafaţa catalizatorului au fost determinate prin spectroscopie fotoelectronică de raze X (XPS), ȋn timp ce spectroscopia UV-Vis cu reflexie difuză (DR UV-Vis) a fost utilizată pentru a obţine informaţii despre stereochimia şi agregarea cuprului ȋn oxizii micşti conţinând acest element. S-a arătat că oxizii micşti conţin un sistem bimodal de pori sub forma de fantă şi au suprafeţe specifice relativ mari care scad continuu de la 169 la 108 m2 g-1 cu creşterea conţinutului de Cu. Aceştia constau ȋntr-un amestec de faze Mg(Al)O periclaz şi CeO2 fluorină, cu excepţia Cu(18)CeMgAlO care mai conţine şi cristalite de CuO bine dezvoltate. La conţinut scăzut de Cu, cristalite ȋnalt disperaste de dioxid de ceriu dopat cu Cu coexistă in matricea Mg(Al)O cu specii oligomerice (Cu2+– O2-– Cu2+)n2+, ȋn timp ce la conţinut mare de cupru, cristalite mai dezvoltate de dioxid de ceriu dar mai puţin expuse la suprafaţa catalizatorului coexistă cu particule separate de CuO care ȋmbogăţesc suprafaţa. Proprietăţile lor catalitice ȋn oxidarea totală a metanului, utilizat ca moleculă model pentru compuşii organici volatili (COV), au fost evaluate şi comparate cu cele ale unui catalizator industrial Pd/Al2O3. Comportamentul lor catalitic a fost explicat pe baza proprietăţilor lor fizico-chimice. S-a arătat că raportul atomic Cu(II)/Cu superficial este un factor cheie care controlează activitatea catalitică a catalizatorilor Cu(x)CeMgAlO. Într-adevăr, oxidul mixt Cu(15)CeMgAlO, având cel mai ridicat raport atomic Cu(II)/Cu superficial, este cel mai activ catalizator din această serie cu o valoare T50 (temperatura corespunzătoare unui grad de conversie a metanului de 50 %) cu doar cca. 45 °C mai ridicată decât cea a unui catalizator industrial Pd/Al2O3. Activitatea mai mare a acestui catalizator a fost atribuită unei dispersii excelente a oxidului de cupru, specii oligomerice (Cu2+– O2-– Cu2+)n2+ coexistând cu cristalite mici de CuO, invizibile la DRX, aflate ȋn interacţie puternică cu dioxidul de ceriu, ceea ce conduce la un puternic efect de sinergie ȋntre Cu şi Ce. Centrii catalitici sunt situaţi la interfaţa particulelor CuO-CeO2. Scăzând temperatura de calcinare a precursorului Cu(15)CeMgAl de la 750 la 650 şi 550 °C s-au obţinut oxizi micşti cu suprafaţă specifică mai ridicată şi, deci, activitate catalitică ȋn termeni de T50 mai mare care pentru sistemul Cu(15)CeMgAlO-550 este cu doar cca. 25 °C mai ridicată decât cea a catalizatorului industrial Pd/Al2O3. Totuşi, ȋn termeni de viteză intrinsecă cel mai activ catalizator a fost cel calcinat la 750 °C dând seama de o mai puternică interacţie Cu-Ce cu creşterea temperaturii de calcinare. Au fost, de asemenea, studiate influenţele timpului de contact şi a concentraţiei de metan ȋn gazul de alimentare asupra performanţei catalitice a sistemului Cu(15)CeMgAlO calcinat la 750 °C şi a fost pusă ȋn evidenţă stabilitatea lui ridicată ȋn flux.În partea a doua a lucrării, cu scopul de a ȋmbunătăţi performanţa catalitică a oxidului mixt Cu(15)CeMgAlO calcinat la 750 °C ȋn reacţia de oxidare totală a metanului, acesta a fost promotat cu diferiţi cationi de metale tranziţionale M (M = Mn, Fe, Co and Ni), fiind aşteptat un efect de sinergie mai intens ȋntre Cu şi Ce ȋn prezenţa acestora. Astfel, au fost preparate două serii de oxizi micşti multicationici prin descompunerea termică controlată la 750 °C a precursorilor HDL corespunzători obţinuţi prin coprecipitarea la pH constant egal cu 10 ȋn aer. Prima serie de catalizatori este constituită din patru oxizi micşti M(3)CuCeMgAlO având un conţinut de 3 % M (M = Mn, Fe, Co, Ni), 15 % Cu, 10 % Ce (% atomi faţă de cationi) şi un raport Mg/Al fixat la 3. Întrucât Co s-a dovedit a fi cel mai bun promotor şi ştiind că conţinutul de promotor metal tranziţional influenţează puternic interacţiunea sinergetică trimetalică şi, deci, performanţa catalitică a oxidului mixt, a doua serie este constituită din patru oxizi micşti Co(x)CuCeMgAlO cu x = 1, 3, 6 şi 9 % Co şi menţinând constante concentraţiile de Cu şi Ce şi raportul atomic Mg/Al. Toţi oxizii micşti au fost caracterizaţi utilizând o baterie de tehnici cum ar fi DRX, microscopie electronică cu transmisie, MEB-EDXS, XPS, adsorbţia/desorbţia azotului la –196 °C, H2-TPR şi DR UV-Vis, ȋn timp ce pentru precursorii HDL s-au folosit analiza termică diferenţială şi termogravimetrică şi DRX. Cei din urmă sunt constituiţi dintr-o fază HDL bine cristalizată şi o fază boehmit (AlOOH) secundară slab cristalizată. Toţi oxizii micşti calcinaţi la 750 °C sunt materiale mezoporoase cu suprafeţe specifice mai mici decât a oxidului mixt CuCeMgAlO de bază, ȋn domeniul 66-78 m2 g-1, şi sunt constituiţi dintr-un amestec de faze Mg(Al)O periclaz, CeO2 fluorină şi CuO tenorit. S-a observat că segregarea fazei CuO ten

Highly Active Transition Metal-Promoted CuCeMgAlO Mixed Oxide Catalysts Obtained from Multicationic LDH Precursors for the Total Oxidation of Methane

To improve the catalytic performance of an active layered double hydroxide (LDH)-derived CuCeMgAlO mixed oxide catalyst in the total oxidation of methane, it was promoted with different transition-metal cations. Thus, two series of multicationic mixed oxides were prepared by the thermal decomposition at 750 °C of their corresponding LDH precursors synthesized by coprecipitation at constant pH of 10 under ambient atmosphere. The first series of catalysts consisted of four M(3)CuCeMgAlO mixed oxides containing 3 at.% M (M = Mn, Fe, Co, Ni), 15 at.% Cu, 10 at.% Ce (at.% with respect to cations), and with Mg/Al atomic ratio fixed to 3. The second series consisted of four Co(x)CuCeMgAlO mixed oxides with x = 1, 3, 6, and 9 at.% Co, while keeping constant the Cu and Ce contents and the Mg/Al atomic ratio. All the mixed oxides were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with X-ray energy dispersion analysis (EDX), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption at −196 °C, temperature-programmed reduction under hydrogen (H2-TPR), and diffuse reflectance UV-VIS spectroscopy (DR UV-VIS), while thermogravimetric and differential thermal analyses (TG-DTG-DTA) together with XRD were used for the LDH precursors. The catalysts were evaluated in the total oxidation of methane, a test reaction for volatile organic compounds (VOC) abatement. Their catalytic performance was explained in correlation with their physicochemical properties and was compared with that of a reference Pd/Al2O3 catalyst. Among the mixed oxides studied, Co(3)CuCeMgAlO was found to be the most active catalyst, with a temperature corresponding to 50% methane conversion (T50) of 438 °C, which was only 19 °C higher than that of a reference Pd/Al2O3 catalyst. On the other hand, this T50 value was ca. 25 °C lower than that observed for the unpromoted CuCeMgAlO system, accounting for the improved performance of the Co-promoted catalyst, which also showed a good stability on stream

Prolonged Intragastric Drug Delivery Mediated by Eudragit®E-Carrageenan Polyelectrolyte Matrix Tablets

Interpolyelectrolyte (IPE) complexation between carrageenan (CG) and Eudragit E (EE) was studied in 0.1 M HCl and was used to develop floating matrix tablets aimed to prolong gastric-residence time and sustain delivery of the loaded drug. The optimum EE/CG IPE complexation weight ratio (0.6) was determined in 0.1 M HCl using apparent viscosity measurements. The IPE complex was characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry. Metronidazole matrix tablets were prepared by direct compression using EE, CG, or hybrid EE/CG with ratio optimal for IPE complexation. Corresponding effervescent tablets were prepared by including Na bicarbonate as an effervescent agent. Tablets were evaluated for in vitro buoyancy and drug release in 0.1 M HCl. Both CG and EE–CG effervescent matrices (1:2 drug to polymer weight ratio, 60 mg Na bicarbonate) achieved fast and prolonged floating with floating lag times less than 30 s and floating duration of more than 10 h. The corresponding EE effervescent matrices showed delayed floating and rapid drug release, and completely dissolved after 3 h of dissolution. CG matrices showed an initial burst drug release (48.3 ± 5.0% at 1 h) followed by slow drug release over 8 h. EE–CG matrices exhibited sustained drug release in almost zero-order manner for 10 h (68.2 ± 6.6%). The dissolution data of these matrices were fitted to different dissolution models. It was found that drug release followed zero-order kinetics and was controlled by the superposition of the diffusion and erosion