Effect of silicates on the structure of Ni-containing catalysts obtained from hydrotalcite-type precursors

New nickel hydrotalcite-like compounds with silicates as interlayer anions used as catalyst precursors in the catalytic partial oxidation of methane were prepared by the coprecipitation method. The properties of these materials were compared with those of compounds obtained from carbonate-containing materials. The precursors and calcined samples were characterized by powder X-ray diffraction, FT-IR and Vis/UV/NIR spectroscopies, thermal analyses (DTA and TG), temperature programmed reduction (TPR) and N₂ adsorption/desorption at −196 °C. The results show that the incorporation of silicates in the lamellar compounds modifies the structural and textural properties of the precursors. After calcination, silicates – which are non-volatile anions – contribute to the final structure of the catalysts, which form a new forsterite-like phase, increasing their specific surface area but not altering the reducibility of the nickel species.     

LaMnO3 Perovskite Supported Noble Metal Catalysts for the Total Oxidation of Methane

Two series of LaMnO₃ supported noble metal (Pt, Pd, Rh) catalysts prepared by the citrate method and calcined in air at 600 and 800 °C, respectively, were investigated. The catalysts resulting from method A were prepared by simultaneous incorporation of the noble metals during perovskite preparation and those following method B were generated by impregnation of the calcined perovskites with the noble metal compounds. The noble metals form solid solutions with the perovskite lattice. Reduction of the catalysts with hydrogen prior to the catalytic reaction led to a significant enhancement of the catalytic activity. During the catalytic reaction, the noble metal clusters are partially transformed to highly dispersed noble metal oxides or nonstoichiometric noble metal oxide phases, which are the catalytically active phases for the total oxidation of methane. The best results were obtained with the Pd containing catalysts prepared by method B.     

Effects of Noble Metal Promoters on In Situ Reduced Low Loading Ni Catalysts for Methane Activation

The commercial potential for a given catalytic process may be influenced by requirements on metal loading, in particular where noble metals are used. In an effort to substantially decrease the amount of catalyst material used for methane activation and catalytic partial oxidation (CPO), the effect of 0.005 wt% noble metal (Rh, Ru, Pd or Pt) on 0.5 wt% Ni/γ-Al₂O ₃ catalysts have been studied at temperatures below 1,173 K and 1 atm. The successful catalysts were activated directly by in situ reduction, without a calcination step, to promote formation of a highly dispersed (supported) metal phase from nitrate precursors. The obtained metal particles were not observable by XRD (size <  2–3 nm). This activation procedure had a decisive effect on catalyst activity, as compared to a catalyst which was calcined ex situ before in situ reduction. Adding a noble metal caused a significant drop in the ignition temperature during temperature programmed catalytic partial oxidation (TPCPO). The ignition temperature for partial oxidation coincides well with the temperature for methane dissociation, and is likely correlated to the reducibility of the noble metal oxide. Methane partial oxidation over 0.5 wt% Ni catalysts, both with and without promoter, yielded high selectivity to synthesis gas (>93%) and stable performance for continued operation, but synthesis gas production at temperatures below 1,073 K required a promoter when the catalyst was ignited by TPCPO. Ignition of the CPO reactions by introducing the feed at a high furnace temperature (1,073 K) also enabled formation of synthesis gas, but the reaction was then less stable than obtained with the TPCPO procedure. A dual bed concept attempted to beneficially use the activation and combustion properties of the noble metal followed by the reforming properties of Ni. However, it was concluded that co-impregnated catalysts yielded as high, or even higher conversion of methane and selectivity to synthesis gas.     

Iron–ceria–zirconia fluorite catalysts for methane selective oxidation to formaldehyde

Ceria–zirconia catalysts modified by partial substitution of zirconium by iron were investigated for the reaction of the selective oxidation of methane to formaldehyde. The insertion of iron was ensured by the preparation method, based on decomposition of the mixed precursors. The crystalline structure was studied by XRD and the iron state was determined by Mössbauer spectroscopy. The insertion of Fe³⁺ into the lattice of the mixed oxide and the partial substitution of zirconium ions with iron cations up to 25% have not resulted in any phase rejection of the formed iron oxide. The reducibility of Ce⁴⁺ in the modified ternary oxide is enhanced by doping with iron. Despite their low specific surface area, the catalysts are efficient at activating the methane independently of the iron content. The selectivity to formaldehyde strongly depends on the amount of the iron inserted in the mixed oxide.     

Promoting effect of water vapor on catalytic oxidation of methane over cobalt/manganese mixed oxides

Methane oxidation was conducted in a fixed bed quartz tubular reactor on a series of mixed oxides of cobalt/manganese prepared by a sol–gel method. A unique promoting effect of water vapor on methane conversion was observed for the first time on these cobalt/manganese mixed oxides calcined at 450 or 600 °C. However, these mixed catalysts lost their catalytic activities after being calcined at 850 °C. The catalytic activity of methane oxidation was significantly improved by supporting the cobalt/manganese mixed species onto the high surface area SiO₂ or Al₂O₃–SiO₂ materials. It was noteworthy that the water enhancement effect was retained on these supported catalysts.     

Total Oxidation of Chlorinated Hydrocarbons on A1–xSrxMnO3 Perovskite‐Type Oxide Catalysts – Part II: Catalytic Activity

The influence of the kind of A‐site cation in A_1–xSr_xMnO₃ perovskites (A = La, Pr, Nd, Di [didymium]) on the catalytic activity in the total oxidation of methane, chloromethane, dichloromethane, and trichloroethylene has been studied. In contrast to methane, the total oxidation of chlorinated hydrocarbons (CHC) is connected with a reversible catalyst deactivation and the formation of byproducts at low reaction temperatures. For the catalysts calcined at 600 and 800 °C, resp., the catalytic activity is determined mainly by specific surface area, amount of oxide admixtures and crystallinity of the perovskite. DiMnO₃ showed the highest and PrMnO₃ catalysts the lowest catalytic activity in the total oxidation of methane and CHC. Partial substitution of A by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane, but not in the total oxidation of CHC.     

Isotopic Studies of Sn-Cr Binary Oxide Catalysts for Methane Total Oxidation

A series of Sn-Cr binary oxide catalysts were prepared by a co-current co-precipitation method and tested for methane total oxidation. The binary oxide catalysts have much higher surface areas and catalytic activities for methane oxidation than pure SnO₂. CrO _x /SnO₂ with a Cr/Sn atomic ratio of 3:7 displays the highest activity. Selected samples were subjected to temperature-programmed ¹⁸O isotope-exchange measurements. Both complete and partial heteromolecular ¹⁸O isotope exchange, as well as oxygen release, was observed for all catalysts. Reaction between CH₄ and ¹⁸O₂ under static conditions was performed to investigate the reaction mechanism and it was found that the total oxidation of methane over Sn-Cr binary oxide catalysts occurs via a redox cycle with the chromium ion in a high oxidation state as the active center. Oxygen mobility of the catalyst plays an important role in the total oxidation of methane, but too high a mobility leads to very high oxygen release and a reduction of the surface reoxidability. This causes a decrease in the catalytic oxidation activity.     

Investigation of the catalytic performance of Ni/MgO catalysts in partial oxidation,dry reforming and combined reforming of methane

Dry reforming, partial oxidation and combined reforming of methane (combination of partial oxidation and dry reforming) to synthesis gas over nickel catalysts supported on nanocrystalline magnesium oxide with various nickel loadings have been studied. Among the catalysts evaluated, catalyst with 15 wt.% nickel content revealed the most active catalytic performance toward dry reforming, partial oxidation and combined reforming reactions. In addition, catalyst with 5 wt.% nickel loading was employed in long term stability test and has shown stable catalytic performance up to 50 h time on stream without any decrease in methane conversion in these three processes.     

Catalytic Removal of Toluene in Air over Co–Mn–Al Nano-oxides Synthesized by Hydrotalcite Route

Co–Mn–Al hydrotalcite type solids were synthesized as precursors of catalysts for the total oxidation of toluene in air. For the as-prepared solids, XRD measurements indicate the coexistence of hydrotalcite and MnCO₃ phases. When calcination is performed at 500 °C, different mixed oxides are found as a function of Co:Mn molar ratio and preparation method, and very high specific surface areas were obtained for the Co–Mn solids. The comparison of catalytic activities in the presence of calcined hydrotalcites with those in the presence of calcined hydroxides evidences the superiority of the first oxides due to their higher reducibility. Co–Mn–Al nano-oxides synthesized using hydrotalcite type solids as precursors, are very promising candidates for the substitution of noble metal based solids.     

Selective Catalytic Oxidation (SCO) of Ammonia to Nitrogen over Hydrotalcite Originated Mg–Cu–Fe Mixed Metal Oxides

Abstract  

Mg–Cu–Fe oxide systems, obtained from hydrotalcite-like precursors, were tested as catalysts for the selective catalytic oxidation (SCO) of ammonia. Copper containing catalysts were active in low-temperature SCO processes; however, their selectivity to nitrogen significantly decreased at higher temperatures. The optimum composition of the catalyst to guarantee high activity and selectivity to N₂ was proposed. Temperature-programmed experiments, SCO catalytic tests performed with various contact times and additional tests on the samples in the selective catalytic reduction of NO with ammonia showed that the SCO process over the studied calcined hydrotalcites proceeds according to the internal SCR mechanism and oxidation of ammonia to NO is a rate-determining step in the low-temperature range.     

V2O5 catalysts supported on Al2O3–SiO2 mixed oxide: V, H MAS solid-state NMR, DRIFTS and methanol oxidation studies

Alumina–silica mixed oxide, synthesized by the sol–gel technique, was used as a support for dispersing and stabilizing the active vanadia phase. The catalysts were characterized employing ⁵¹V and ¹H solid-state MAS NMR, diffuse reflectance FT-IR, BET surface area measurements. The partial oxidation activities of the catalysts were tested using methanol oxidation as a model reaction. ⁵¹V solid-state NMR studies on the calcined catalysts showed the peaks corresponding to the presence of both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V₂O₅ content, the ⁵¹V chemical shifts corresponding to amorphous V₂O₅ like phases were observed. DRIFTS studies of the catalysts indicated the vibrations corresponding tetrahedral vanadia species at low and medium loadings and at high V₂O₅ contents the vibrations corresponding V=O bonds of V₂O₅ agglomerates were observed. The V/Al–Si catalysts exhibited high selectivity for the dehydration product dimethyl ether in the methanol partial oxidation studies showing the predominance of the acidic nature of the alumina–silica support over the redox properties of the active vanadia phase.     

Effect of Sol–gel Synthesis on Physical and Chemical Properties of V/SiO2 and V/MgO Catalysts

Catalytic properties of supported vanadium oxide catalysts are highly dependent on surface vanadia configuration and the nature of the support. In this study, the effects of using the same supports (silica and magnesia) from two different preparation procedures on the catalytic properties in methanol partial oxidation and on the vanadium surface structures were investigated. Both types of catalysts were prepared by the incipient-wetness method and characterized using BET, Raman spectroscopy, UV–Vis and H₂-TPR to determine the structures of vanadium on the supports’ surface. Both aerogel-prepared V/SiO₂ and V/MgO catalysts were more active than conventionally prepared samples in methanol partial oxidation. These differences in catalytic results are attributed to the configurations of active phase. It was also found that small vanadium phases were more active in methanol partial oxidation than larger phases.     

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Mixed LaNi_xFe_(1−x)O₃ perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi_0.3Fe_0.7O₃.     

Influence of gel composition in the synthesis of MoVTeNb catalysts over their catalytic performance in partial propane and propylene oxidation

MoVTeNb mixed oxides catalysts have been prepared by a slurry method with different molar compositions (Mo/Te ratio from 2 to 6 and Nb/(V + Nb) ratio from 0 to 0.7) in the synthesis gel leading to different crystalline phases distribution and catalytic behaviour in the partial oxidation of both propane and propylene to acrylic acid. Chemical analysis indicates that the composition of samples before and after the heat-treatment changes, especially the Te-content, since a significant amount of Te is lost during the heat-treatment step when the amount of oxalate (from niobium oxalate) increases in the synthesis gel. Thus, the nature of the crystalline phases and the catalytic performance of heat-treated materials will be related to the final chemical composition. On the other hand, only the catalysts presenting Te₂M₂₀O₅₇ (M = Mo, V, Nb) crystalline structure, the so-called M1 phase, were active and selective in the partial oxidation of propane to acrylic acid. Moreover, all catalysts were active and relatively selective to the formation of O-containing products, i.e. acrolein and/or acrylic acid, during the partial propylene oxidation although the more active ones were those presenting M1 phase.     

Simultaneous catalytic removal of NOx and soot particulate over Co–Al mixed oxide catalysts derived from hydrotalcites

Co–Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NO_x and diesel soot particulates by the temperature-programmed reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500 °C and 800 °C, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800 °C were higher than that at 500 °C. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.     

Total oxidation of selected mono-carbon VOCs over hydrotalcite originated metal oxide catalysts

Hydrotalcite originated Cu–Mg–Al, Co–Mg–Al and Cu–Co–Mg–Al oxide systems were tested as catalysts for the total oxidation of mono-carbon VOCs (methane, methanol, and formic acid). Both calcination temperature of the hydrotalcite precursors as well as doping of the catalysts with potassium promoter influenced their catalytic activity. Increased calcination temperature, which resulted in a decrease of the surface area of the samples and formation of the spinel phases, activated the Co–Mg–Al catalyst, while the opposite effect was observed for the Cu–Mg–Al and Cu–Co–Mg–Al catalysts. On the other hand doping of the catalysts with potassium promoter significantly activated the Cu–Mg–Al and Cu–Co–Mg–Al catalysts in the processes of methanol and formic acid conversion, while only slightly influenced the catalytic performance of the Co–Mg–Al sample.     

Olivine catalysts for methane- and tar-steam reforming

The removal of tar and lower hydrocarbons is a vital technological barrier hindering the development of biomass gasification. The present work evaluates four olivine catalysts (three untreated of different origin and one calcined) for lowering the amount of these compounds in biomass derived syngas by reforming model compounds (naphthalene, toluene, and methane). Treatments prior to reaction were shown to largely impact the catalytic activity and physiochemical properties of the olivine catalysts depending on its origin. The formation of free Fe phases following decomposition of a Fe-bearing serpentine phase ((Mg,Fe)₃Si₂O₅(OH)₄) near the surface of untreated olivine catalysts proved most important for facilitating higher activity compared to olivine catalysts with little or no serpentine phase initially. The most active catalyst was efficient at naphthalene removal (90% conversion at 800 °C), but more active catalysts are needed for applications where methane removal is required. Additionally, carbon deposition during naphthalene-steam reforming as well as Fe clustering during naphthalene-steam reforming and exposure to reducing conditions suggested stability may be a liability.     

Mn-containing catalytic materials for the total combustion of toluene: The role of Mn localisation in the structure of LDH precursor

Mg/Mn/Al mixed oxide systems of similar atomic ratios close to 2/0.5/1 were obtained by calcination of Mg,Al layered double hydroxide (LDH) type precursors containing Mn either in the cationic form within the brucite layer or as permanganate anions in the interlayer. The materials were characterised with PXRD, thermal analysis, XPS, ESR, TPR and BET. In mixed oxides derived from interlayer-doped precursor PXRD identified MgO–MnO solid solution and poorly crystalline Al-rich spinel phase, while those obtained from layer-doped LDH contained a better crystalline Mn-rich spinel. Surface of both materials was covered with poorly crystalline and/or amorphous Mn⁴⁺-containing phases. Higher reducibility of this surface coat in calcined layer-doped catalyst, as compared to interlayer-doped one, was attributed to the differences in the nature of underlying crystalline phases, and was considered the chief reason for the higher catalytic activity of this catalyst in the total oxidation of toluene.     

甲烷催化燃烧催化剂催化理论与应用研究进展

概述了甲烷催化燃烧催化剂的研究现状,从组成甲烷燃烧催化剂的3个部分（基体、活性组分、氧化物载体）分别加以论述。通过掺杂一些金属和金属氧化物,不但可以提高高活性贵金属催化剂的热分解温度,还可以提高高温催化剂（如钙钛矿和六铝酸盐材料等）的催化活性。最后简要综述了甲烷催化燃烧反应机理。     

Methane partial oxidation to synthesis gas using nickel on calcium aluminate catalysts

Nickel was supported on calcium aluminate carriers that were obtained with varying CaO to Al₂O₃ molar ratios and calcination temperatures. The variations of the supports lead to catalysts of different surface properties and catalytic performance. Metallic nickel (Ni⁰) was proven to be the active species for the methane partial oxidation reaction. The presence of filamentous carbon on used catalysts was also suggested. The differences in the catalytic activity and selectivity for the methane partial oxidation reaction was ascribed to a varying degree of reducibility of the surface nickel species.