Structural Features of Mixed La–Al Oxides and Their Catalytic Properties in the Oxidation of Methane

The oxidation of methane on the stoichiometric hydroxyapatite Ca10(PO4)6(OH)2(HAP) has been examined in the presence and absence of tetrachloromethane (TCM). In contrast to the previously observed effects of TCM on the oxidation of methane with other catalysts, the introduction of a small amount of TCM into the feedstream with stoichiometric HAP produced an increased selectivity to carbon monoxide and a decreased conversion of methane with increasing time-on-stream. The selectivity to C2 compounds changed relatively little on addition of TCM. X-Ray analysis showed that the chlorided species was formed not only on the surface but also in the bulk phase and, at least in the latter, was identified as chlorapatite Ca10(PO4)6Cl2. Pretreatment experiments, in which the hydroxyapatite was exposed to a mixture of O2 and TCM before use in the methane conversion process, showed that the chlorinated surface formed by this method possessed distinctly different catalytic properties from that produced during the methane conversion process in the presence of feedstream TCM.

Catalytic and redox properties of bimetallic Cu–Ni systems combined with CeO2 or Gd-doped CeO2 for methane oxidation and decomposition

Изучено влияние способа синтеза смешанных оксидных систем Ln—Al (Ln—La, Ce, Pr) с атомным соотношением Ln : Al = 1 : 1 на формирование их фазового состава и каталитические свойства в реакции окисления метана. Предшественники готовили пропиткой беззольной фильтровальной бумаги смешанными растворами нитратов соответствующих металлов по водопоглощающей способности с последующей сушкой и сжиганием полученной массы на воздухе. Дальнейшую обработку проводили, совмещая прокаливание при 600 и 900 °С с обработкой в среде водного или водно-аммиачного флюида (ВФ или ВАФ). Закономерности превращения аморфного предшественника оксидов Pr—Al в средах ВФ и ВАФ и при высокотемпературном синтезе аналогичны полученным для системы La—Al. В обоих случаях аморфный предшественник в содержащем воду флюиде превращается в LnAlO3 со структурой кубического перовскита с примесью фаз AlO(OH) (бёмита) и основных карбонатов РЗЭ. Последующая обработка при 900 °С на воздухе приводит к образованию смеси, содержащей алюминаты LnAlO3 и свободные оксиды La2O3 или PrO2. Однофазные образцы, содержащие только алюминаты La и Pr, были получены путем нагревания аморфных предшественников при 900 °С на воздухе. При обработке в ВФ или ВАФ системы Ce—Al вместо алюмината или Ce-содержащих гидроксидов образуется хорошо окристаллизованный оксид CeO2, а Al-содержащий компонент остается рентгеноаморфным. CeAlO3 был получен обработкой смеси предшественников оксидов Ce и Al в токе водорода. Установлено что из-за различий в значениях 4-го потенциала ионизации (IP 4) атомов La, Ce и Pr (49,9, 36,7 и 39,0 эВ соответственно) для образования алюминатов со структурой перовскита формулы LnAlO3, содержащих РЗЭ в степени окисления (3+), требуются совершенно разные условия синтеза. Изучены каталитические свойства синтезированных образцов в процессе окисления метана. Эффективность и устойчивость изоструктурных алюминатов LnAlO3 в реакции окислительной конденсации метана (ОКМ) снижается в ряду La > Pr ≫ Ce. Хотя наиболее активен среди них PrAlO3, но LaAlO3 проявляет наибольшую селективность в образовании продуктов ОКМ (этан + этилен). The effect of the synthesis method of mixed Ln—Al (Ln — La, Ce, Pr) oxide systems with the atomic ratio Ln : Al = 1 : 1 on the formation of their phase composition and catalytic properties in methane oxidation were studied. The precursors were prepared by impregnation of ashless filter paper with mixed solutions of nitrates of the corresponding metals according to the water absorption capacity, followed by drying and combustion of the obtained mass in air. Further processing was carried out by combining calcination at 600 and 900 °C with treatment in water or water-ammonia fluid (WF, WAF) media. The patterns of the amorphous precursor of Pr-Al oxides transformation in the WF and WAF media and during high-temperature synthesis are similar to those obtained for the La—Al system. In both cases, the amorphous precursor in the water-containing fluid transforms into LnAlO3 with a cubic perovskite structure with an admixture of AlO(OH) (boehmite) and basic REE carbonate phases. The subsequent treatment at 900 °C in air leads to the formation of mixture containing aluminates LnAlO3 and free oxides La2O3 or PrO2. Single-phase samples containing only La and Pr aluminates were obtained by heating the amorphous precursors at 900 °C in air. During the treatment in the watercontaining fluid medium of the Ce—Al system, well-crystallized oxide CeO2 instead of aluminate or Ce-containing hydroxides is formed, and the Al-containing component remains X-ray amorphous. CeAlO3 was obtained by treating the mixture of Ce and Al oxide precursors in a hydrogen stream. It has been established that due to the differences in the values of the 4th ionization potential (IP 4) of La, Ce and Pr atoms (49.9, 36.7 and 39.0 eV, respectively), very different synthesis conditions are required for the formation of perovskite-type aluminates of the LnAlO3 formula containing the REE in the (3+) oxidation state. The catalytic properties of the synthesized samples in methane oxidation have been studied. The efficiency and stability of isostructural aluminates LnAlO3 in the oxidative coupling of methane (OCM) decreases in the La > Pr ≫ Ce series. While PrAlO3 is the most active among them, LaAlO3 shows the highest selectivity in the formation of the OCM products (ethane + ethylene).

Surface and bulk properties, catalytic activities and selectivities in methane oxidation on near-stoichiometric calcium hydroxyapatites

Total Oxidation of Carbon Monoxide and Methane over Transition Metal Fluorite Oxide Composite Catalysts: I. Catalyst Composition and Activity

The catalytic properties of deposited metal ions in the reaction between methane and nitrous oxide were studied. On the basis of a previously proposed mechanism for the heterogeneous-catalytic reaction, an interpretation was given for the observed activity and selectivity series with respect to the formation of methanol, carbon monoxide and dioxide. The determining factor regulating the catalytic properties in this system is the bond energy of metal ions with the adsorbed oxygen anion radicals O∸.

This study investigates the effect of nitric acid (Pt:NA) treatment on the catalytic performance of Pt/TiO2 catalysts in methane oxidation. By varying the molar ratio of Pt to nitric acid (NA) from 1:0.1 to 1:10 of Pt:NA, a series of Pt/NA‐TiO2 catalysts are synthesized. The structural and catalytic properties are identified using X‐Ray photoelectron spectroscopy, transmission electron microscopy, and diffuse reflectance infrared Fourier transform spectroscopy. The NA treatment during the catalyst synthesis promotes the formation of acidic sites, enhancing the uniform adsorption of the Pt precursor, which in turn improves Pt dispersion and catalytic activity. However, excessive NA treatment above Pt:NA ratio of 1:5 induces the formation of larger Pt particles due to competitive nitrate adsorption, leading to decreased Pt dispersion and diminished catalytic performance. These findings provide a simple method for synthesizing active Pt supported on TiO2 catalysts with NA treatment having well‐dispersed Pt particles, thereby maximizing the catalytic activity of the methane oxidation.

Chemical looping of methane oxidation and water splitting is a promising method for cleaner production of syngas. This process depends on efficient oxygen carriers or catalysts to facilitate the reactions. Perovskite‐based oxygen carriers show great potential for chemical looping due to their excellent oxygen storage and catalytic properties. This study synthesized and investigated the reactivity of novel solid oxide‐based oxygen carrier Ba1−xSrxCo1−xAxO3−δ (A═Mn/Ni/Fe) to accelerate the surface‐active sites for methane oxidation. Among the catalyst Ba0.5Sr0.5CO0.8Mn0.2O3 achieves methane conversion up to 80% at 850 °C through effective selective methane oxidation through lattice oxygen migration. The addition of Mn at B‐site shows an enhanced performance when compared to the other materials improving redox activity. It balances the surface oxygen and facilitates the replenishment of lattice oxygen from the bulk enhancing the performance while reducing carbon deposition. This provides an enhanced durability and sustained syngas production for up to 40 min. Further during the water splitting stage, Mn doped perovskite shows a stable hydrogen production for up to 200 min by effectively replacing the lattice oxygen. The material exhibits a reduced CO oxidation when compared to other materials. These attribute makes Mn‐doped perovskite promising for advancing longevity and stability in a chemical loop reforming for clean energy production.

The bimetallic Pd–Pt/Al2O3 supported catalysts with different Pd/Pt ratios have been prepared and studied in methane oxidation reaction. Comparison of their catalytic properties with those of monometallic Pd/Al2O3 and Pt/Al2O3 samples has shown that the bimetallic catalysts with a relatively low Pt loading (less than 0.4 mol%) are more active than the monometallic ones indicating the synergy effect. An increase in Pt loading above 0.5 mol% decreases the catalytic activity, so that these catalysts become less active than the monometallic Pd/Al2O3 catalysts, but still keep an activity higher than that of the least active monometallic Pt/Al2O3 sample. The chemical states of the palladium and platinum for all the catalysts produced depending on the reaction conditions have been investigated with in situ XPS. It has been shown that even at room temperature the reaction mixture affects the surface composition of the catalysts, which does not change much at higher reaction temperatures. The data have led us to suggestion that activity in the total methane oxidation is promoted by formation of Pd2+, surface fraction of which depends on Pt content in the bimetallic catalysts.

Oxidation behaviour and catalytic properties of Pd/Al 2O 3 catalysts in the total oxidation of methane

Perovskite-type oxides synthesized by reactive grinding: Part IV. Catalytic properties of LaCo1−xFexO3 in methane oxidation

The effects of transition metal (Cu, Co, Ni) oxides and platinum-group metals (Pt, Pd, Rh) in composites based on Y(Sc)-stabilized zirconia on their catalytic properties (activity, selectivity, sulfur resistance) and service life characteristics (thermal stability, performance stability) in methane oxidation reactions have been investigated. The high activity of the composites (75–99% methane conversion at 600–800°C) correlates with the amount and mobility of surface oxygen in these materials. The promoting effect of the platinum-group metals depends on the composition of the reaction medium (on whether it corresponds to the partial or total methane oxidation and on whether it contains sulfur dioxide).

The ultimate objective of bioinspired catalysis is the development of efficient and clean chemical processes. Cytochrome P450 and soluble methane monooxygenase enzymes efficiently catalyze many challenging reactions. Extensive research has been performed to mimic their exciting chemistry, aiming to create efficient chemical catalysts for functionalization of strong C-H bonds. Two current biomimetic approaches are based on (i) mononuclear metal porphyrin-like complexes and (ii) iron and diiron non-heme complexes. However, biomimetic catalysts capable of oxidizing CH4 are still to be created. In the search for powerful oxidizing catalysts, we have recently proposed a new bioinspired strategy using N-bridged diiron phthalocyanine and porphyrin complexes. This platform is particularly suitable for stabilization of Fe(IV)Fe(IV) complexes and can be useful to generate high-valent oxidizing active species. Indeed, the possibility of charge delocalization on two iron centers, two macrocyclic ligands, and the nitrogen bridge makes possible the activation of H2O2 and peracids. The ultrahigh-valent diiron-oxo species (L)Fe(IV)-N-Fe(IV)(L(+•))═O (L = porphyrin or phthalocyanine) have been prepared at low temperatures and characterized by cryospray MS, UV-vis, EPR, and Mössbauer techniques. The highly electrophilic (L)Fe(IV)-N-Fe(IV)(L(+•))═O species exhibit remarkable reactivity. In this Account, we describe the catalytic applications of μ-nitrido diiron complexes in the oxidation of methane and benzene, in the transformation of aromatic C-F bonds under oxidative conditions, in oxidative dechlorination, and in the formation of C-C bonds. Importantly, all of these reactions can be performed under mild and clean conditions with high conversions and turnover numbers. μ-Nitrido diiron species retain their binuclear structure during catalysis and show the same mechanistic features (e.g., (18)O labeling, formation of benzene epoxide, and NIH shift in aromatic oxidation) as the enzymes operating via high-valent iron-oxo species. μ-Nitrido diiron complexes can react with perfluorinated aromatics under oxidative conditions, while the strongest oxidizing enzymes cannot. Advanced spectroscopic, labeling, and reactivity studies have confirmed the involvement of high-valent diiron-oxo species in these catalytic reactions. Computational studies have shed light on the origin of the remarkable catalytic properties, distinguishing the Fe-N-Fe scaffold from Fe-C-Fe and Fe-O-Fe analogues. X-ray absorption and emission spectroscopies assisted with DFT calculations allow deeper insight into the electronic structure of these particular complexes. Besides the novel chemistry involved, iron phthalocyanines are cheap and readily available in bulk quantities, suggesting high application potential. A variety of macrocyclic ligands can be used in combination with different transition metals to accommodate M-N-M platform and to tune their electronic and catalytic properties. The structural simplicity and flexibility of μ-nitrido dimers make them promising catalysts for many challenging reactions.

Oxide configurations with perovskite structures exhibit some interesting and controllable properties. BaTiO3 compounds have attracted considerable attention due to their potential use as electric and optical applications (1). Additionally, BaTiO3 has been studied for its catalytic properties in the oxidative coupling of methane (2), partial oxidation of methane (3) and for the total oxidation of methane (4) showing some promising results. Moreover, the presence of BaTiO3 on the anode material showed to suppress the C deposition for the low temperature dry methane reforming (5). In a similar way, BaTiO3 has been used as anode for solid oxide fuel cells (SOFCs) exhibiting quite low performance when methane was employed as fuel, but its performance increased in the presence of sulfur containing fuel (6). Although, this material showed some curious properties when used partially or totally as anode for SOFCs to date there are only speculations regarding its potential. In several occasions theoretical studies have been used to elucidate the phenomena behind some exciting properties such as catalytic behaviour. However, this is not any easy task since heterogeneous catalytic processes are complex to model due to the absence of information on the molecule-surface interaction, and the effect that the operating conditions and catalyst preparation have on its performance. In this work an idealized model will be used to describe the methane decomposition on the surfaces of BaTiO3(001) employing density functional theory (DFT) method. Preliminary results show that the TiO2-terminated surface is approximately four times more active for the total decomposition of methane than the BaO-terminated surface. References S. Saha, T. P. Sinha, and A. Mookerjee, Phy. Rev. B, 62, 8828, (2001). M. Teymouri, and E. Bagherzadeh, J. Mat. Sci., 30, 3005, (1995). C. Guo, J. Zhang, W. Li, P. Zhang, and Y. Wang, Cat. Today, 98, 583, (2004). I. Popescu, A. Urda, T. Yuzhakova, I. Marcu, J. Kovacs, and I. Sandulescu, C. R. Chimie, 12, 1072, (2009). X. Li, Q. Hu, Y. Yang, Y. Wang, and F. He, Appl. Catal. A: General, 413– 414, 163, (2012). J. H. Li, X. Z. Fu, J. L. Luo, K. T. Chuang and A. R. Sanger, J. Power Sources, 213, 69 (2012).

Author(s): Guan, E; Ciston, J; Bare, SR; Runnebaum, RC; Katz, A; Kulkarni, A; Kronawitter, CX; Gates, BC | Abstract: Complexes with neighboring metal centers in isolated pairs and their analogues on surfaces are drawing increasing attention as catalysts. These include molecular homogeneous catalysts incorporating various ligands, enzymes, and solids that include pairs of metal atoms mounted on supports. Catalysts in this broad class are active for numerous reactions and offer unexplored opportunities to address challenging reactions, such as oxidation of methane and oxidation of water in artificial photosynthesis. The subject of supported metal pair-site catalysts is in its infancy, facing challenges in (a) precise synthesis, (b) structure determination at the atomic scale, and (c) stabilization in reactive atmospheres. In this Perspective, we summarize key characteristics of molecular and enzymatic catalysts that incorporate neighboring metal centers and build on this foundation to assess the emerging literature of metal pair-site catalysts on various supports. The supported catalysts include those synthesized by anchoring molecular dinuclear precursors to support surfaces and those synthesized by selective formation of dinuclear surface species from mononuclear surface species. Examples of metals in this class are rhodium and iridium, and examples of supports are MgO and Fe2O3. We summarize characterization of these materials by electron microscopy and spectroscopy, emphasizing atomic-resolution aberration-corrected scanning transmission electron microscopy and spectroscopies that provide atomic-scale structural information and allow characterization of functioning catalysts, especially X-ray absorption spectroscopy. We list some opportunities for research, including suggestions that might lead to structurally well-defined supported metal pair-sites with new catalytic properties.