Calcination Pretreatment Research Articles

Mechanism of calcination modification of phosphogypsum and its effect on the hydration properties of phosphogypsum-based supersulfated cement

Supersulfated cement (SSC) is a kind of environment-friendly and energy-saving cement, which is almost entirely made of industrial waste and could consume a lot of phosphogypsum (PG), which has great potential for industrial application. The calcination treatment at the temperature of 150 °C, 350 °C, 600 °C, 800 °C with calcination time of 0.5 h, 1 h, 1.5 h, 2 h is used to improve the performance of PG. The phase transformation of PG, the removal of soluble phosphorus impurities, and the hydration properties of SSC containing PG with or without modification are investigated, including the water requirement of normal consistency and setting time, the compressive strength, the pH value of pore solution and the microstructure performance of pastes. The relationship between these properties and calcination pretreatment processes are established as well. The results show that the calcination pretreatment of PG reduced the adverse effects of impurities in PG and changed the gypsum phase to prepare SSCs with higher performance in all aspects. While the modification effect varied with calcination temperature and calcination time, 90% of the setting time was shortened after calcination temperature of 350 °C for 2 h, which makes the PG-incorporated SSC meet the requirements of existing standard. And high temperature treatment showed the best modification effect on strength, especially after calcination at 600 °C for 2 h, both early and later strength was significantly increased, which was almost twice of the reference specimen. Besides, more formed hydration products and the denser microstructure were observed in the specimen with high temperature treated PG. Moreover, the slight decreases of 90-day compressive strength in some specimens were observed, which could be due to the carbonation of the specimen.

Calcination pretreatment effects on acid leaching characteristics of rare earth elements from middlings and coarse refuse material associated with a bituminous coal source

The pretreatment of middlings and coarse refuse material collected from a Pocahontas No.3 coal source using calcination was investigated for the potential of improving the leaching recovery of rare earth elements. Calcination at 600 °C significantly and preferentially improved light REE (LREE) recovery to values in the range of 80–90% using 1.2 M HCl. The positive effect was due to the increased pore diameter and volume as well as thermal decomposition of the minerals associated with the LREEs. Heavy REE recovery was not increased after calcination at 600 °C due to the need for higher temperatures to achieve thermal decomposition of HREE minerals. Furthermore, some HREEs were also associated with sulfide minerals (e.g., pyrite) which were transformed into low-solubility oxides (e.g., hematite) after roasting at 600 °C. Based on the leaching characteristics of the major elements (Fe, Al, Ca and Mg) and REEs along with SEM-EDX data, it was concluded that REEs in the two samples mainly occurred as phosphate minerals. In the middlings, some of the minerals were completely encapsulated by kaolinite, which was less apparent in the coarse refuse. In addition, it was found that a portion of the REEs (10–25%) in the samples were associated with the organic matter and micro-dispersed minerals in the organic matrix, was released after calcination and easily recovered using 0.1 M (NH4)2SO4 solution at pH 5. Scandium primarily occurred in mineral forms with minimum amounts associated with ion-adsorbed clays and organic association.

Advanced purification of industrial quartz using calcination pretreatment combined with ultrasound-assisted leaching

Advanced purification of industrial quartz using calcination pretreatment combined with ultrasound-assisted leaching

Preparation of an Aluminum and Iron‐Based Coagulant From Fly Ash for Industrial Wastewater Treatment

Fly ash was investigated to produce a composite coagulant containing aluminum (Al) and iron (Fe) salts through the calcination and leaching processes. The extraction of Al and Fe oxides from fly ash demonstrated similar results for HCl and H2SO4 leaching. Besides, the calcination pretreatment of fly ash (without any additive) was found to be less effective in extraction. However, the conversion efficiencies of Al and Fe were affected greatly by calcination pretreatment with addition of Na2CO3, and leaching conditions of ratio of solution to fly ash (L/S) and H+ concentration. The maximum concentrations of Al and Fe solubilized from fly ash were 0.21 and 0.04 mol/L, respectively, which were attained by calcination pretreatment using an optimal mass ratio (ca. 1:16.7) of Na2CO3 to fly ash at 805°C. These concentrations correspond to conversion efficiencies of 31.2% for Fe and 18.3% for Al in the fly ash. The slurry was filtrated to obtain a solution containing Al‐ and Fe‐sulfates and silicates. The Al/Fe/Si mole ratio indicated the feasibility of further production of inorganic polymer coagulant. The produced coagulant was determined to be effective for treating soybean wastewater and dairy wastewater in terms of COD and SS removal.

The effect of pre-treatment parameters on the quality of glass-ceramic wasteforms for plutonium immobilisation, consolidated by hot isostatic pressing

Glass-ceramics with high glass fractions (70 wt%) were fabricated in stainless steel canisters by hot isostatic pressing (HIP), at laboratory scale. High (600 °C) and low (300 °C) temperature pre-treatments were investigated to reduce the canister evacuation time and to understand the effect on the phase assemblage and microstructure of the hot isostatically pressed product. Characterisation of the HIPed materials was performed using scanning electron microscopy (SEM), coupled with energy dispersive X-ray analysis (EDX) and powder X-ray diffraction (XRD). This analysis showed the microstructure and phase assemblage was independent of the variation in pre-treatment parameters. It was demonstrated that a high temperature pre-treatment of batch reagents, prior to the HIP cycle, is beneficial when using oxide precursors, in order to remove volatiles and achieve high quality dense materials. Sample throughput can be increased significantly by utilising a high temperature ex-situ calcination prior to the HIP cycle. Investigation of glass-ceramic wasteform processing utilising a glass frit precursor, produced a phase assemblage and microstructure comparable to that obtained using oxide precursors. The use of a glass frit precursor should allow optimised throughput of waste packages in a production facility, avoiding the need for a calcination pre-treatment required to remove volatiles from oxide precursors.

Potassium recovery from the fly ash from a grate boiler firing agro‐residues: effects of unburnt carbon and calcination pretreatment

AbstractBACKGROUNDThe biomass‐fired power in China is mainly fueled by agro‐residue, and is suffering the bottlenecks of feedstock supply, super heater corrosion and nutrient cycle. How to recycle the ash nutrient to farmland is crucial for ecological sustainability. This work investigated potassium recovery from the fly ash of a grate boiler firing agro‐residue using leaching methods.RESULTSThe studied ash contains a massive amount of fixed carbon (158.92 mg g−1) and little volatile matter (39.44 mg g−1). The unburnt carbon strongly decreases the concentration (CK) in the leachate and recovery percentage (RPK) of potassium nutrient by 17.6–24.1% and 9.0–18.8%, respectively. Calcination pretreatment in air at 500 °C for 30 min should be employed before leaching to remove the unburnt carbon. Mineralogy analysis shows that low‐temperature pre‐calcination (≤575 °C) does not affect potassium solubility. Under the optimal leaching of calcined ash in water with liquid‐to‐solid ratio 3 for 30 min at room temperature, CK in the leachate, dissolution percentage (DPK) and recovery percentage (RPK) of potassium element reached 17.21 g L−1, 82.0%, and 62.6%, respectively.CONCLUSIONSThe fly ash from grate boilers firing agro‐residue is much better than other marginal potash sources. This work is helpful to improve the technology of agro‐residue fired power. © 2016 Society of Chemical Industry

Modified natural halloysite nanotube solely employed as an efficient and low-cost solid acid catalyst for alpha-arylstyrenes production via direct alkenylation

Owing to unique mesoporous/macroporous hollow nanotubular structure, relatively high specific surface area, abundant surface hydroxyl groups, ecological and biocompatibility as well as low cost and large availability, the natural halloysite nanotube (HNT) has attracted great attention in drug delivery, catalysis, adsorption, energy storage, etc. HNT was generally used as support in catalysis, but rare report for HNT being solely used as catalyst. In this work, we firstly reported that HNTS were solely employed as a solid acid catalyst for alkenylation of diverse aromatics with phenylacetylene to their α-arylstyrenes. It was found that the HNT with a facile ion exchange and calcination pretreatment demonstrated outstanding catalytic performance in alkenylation of p-xylene with phenylacetylene, and 95.6% of phenylacetylene conversion with 95.8% of selectivity toward α-(2,5-dimenthylphenyl) styrene was achieved. The HNT were characterized by N2 physisorption, transmission electron microscopy (TEM), NH3 temperature-programmed desorption (NH3-TPD), pyridine infrared spectroscopy (Py-IR), and Inductively coupled plasma (ICP), and thermogravimetric analysis (TGA) techniques. Correlated the nature of HNTs to reaction results, it was revealed that the catalytic performance of HNT was significantly dependent on their acidic properties and textural feature notably affected by ion exchange times (NIE) and calcination temperature (TC). Especially, besides acidic properties, the nature of the formed mesopores on the walls of HNTs strongly affects the catalytic activity, selectivity and coke-deposition behavior. The appropriate acidic sites and their accessibility are essential for obtaining satisfactory catalytic performance. The modified HNT catalyzed alkenylation can be extended to diverse substrates with good catalytic properties for diverse α-arylstyrenes production. The combination of excellent catalytic properties and the facile regeneration of deactivated HNT by calcination process allows it to be a promising candidate for, but limited to alkenylation of aromatics with phenylacetylene to their α-arylstyrenes.

Enhanced Ru/Alumina catalyst via the adsorption-precipitation (AP) method for the hydrogenation of dimethyl maleate

Two methods, incipient wetness impregnation (IM) and adsorption-precipitation (AP) were used to prepare alumina-supported Ru catalysts. Using the selective hydrogenation of dimethyl maleate as a probe reaction, the obtained results demonstrated an enhanced catalytic activity with the AP method as compared to the IM technique. It was also found that higher hydrogenation activity was obtained with samples that were directly activated in H2 flow. In contrast, the activity decreased seriously when the catalyst samples were subjected to a calcination pretreatment prior to activation with H2 reduction, especially for the IM technique. Additional catalytic testings on Ru/Al(AP)-R with chlorine addition demonstrated that the catalytic activity was greatly decreased, suggesting that chlorine has a negative effect on the hydrogenation behaviors of Ru. Combined with the characterization results of XRD, H2-TPR, H2-TPD and XPS, the advantage of the AP method can be summarized as the following: this technique effectively eliminates chlorine, thus resulting in a better dispersion of Ru species and decreasing the negative influence of chlorine on Ru. A superior Ru/alumina catalyst can be obtained with the AP followed by direct activation with H2 reduction.

Understanding the role of TiO2crystal structure on the enhanced activity and stability of Ru/TiO2catalysts for the conversion of lignin-derived oxygenates

Although Ru catalysts supported on reducible oxides such as TiO2 hold significant promise for the deoxygenation of biomass derived oxygenates, a significant drawback is their instability under oxidation conditions necessary for catalyst regeneration. In this contribution, the role of TiO2 crystal structure on resistance to metal particle sintering during calcination treatments at 400 and 500 °C is investigated. The resulting impact of the calcination temperature and TiO2 support phase for the conversion of guaiacol at 400 °C under atmospheric pressure of hydrogen over supported Ru catalysts is presented. Results suggest that the rutile TiO2 phase plays an important role in stabilizing Ru particles during calcination pretreatment in comparison with anatase supported Ru catalysts. Furthermore, rates normalized to the area of the support and the Ru suggest that the high activity of Ru/TiO2 systems for guaiacol conversion is attributed to defect sites created by hydrogen spillover from the Ru metal to the reducible TiO2 as opposed to only the sites located at the Ru/TiO2 interface.

Glymes as benign co-solvents for CaO-catalyzed transesterification of soybean oil to biodiesel

The base (such as CaO)-catalyzed heterogeneous preparation of biodiesel encounters a number of obstacles including the need for CaO pretreatment and the reactions being incomplete (typically 90-95% yields). In this study, a number of glymes were investigated as benign solvents for the CaO-catalyzed transesterification of soybean oil into biodiesel with a high substrate loading (typically soybean oil >50% v/v). The triglyceride-dissolving capability of glymes led to a much faster reaction rate (>98% conversions in 4h) than in methanol alone (typically 24h) and minimized the saponification reaction when catalyzed by anhydrous CaO or commercial lime without pre-activation. The use of glyme (e.g. P2) as co-solvent also activates commercial lime to become an effective catalyst without calcination pretreatment. The SEM images suggest a dissolution-agglomeration process of CaO surface in the presence of P2, which could remove the CaCO3 and Ca(OH)2 layer coated on the surface of lime.

Fluidized bed desulfurization using lime obtained after slow calcination of limestone particles

In this work we have tested the fluidized bed desulfurization performance of lime particles obtained by means of a limestone slow calcination pre-treatment technique. This performance was compared to that of the parent untreated limestone particles. The occurrence of particle fragmentation and attrition during the fluidized bed operation was also investigated with a specific test protocol for both raw limestone and pre-treated lime sorbent. Two particle size ranges were tested under typical fluidized bed coal combustion conditions (T=850°C; SO2=1800ppm). The experiments were complemented by porosimetric and morphological (SEM) analyses of the sorbent. Results showed that limestone pre-treatment was able to preserve the high mechanical strength of the parent particles as opposed to the fast in situ calcination typically active in fluidized beds. In addition, a high calcium reactivity and final conversion were observed for the pre-treated lime particles, leading to a SO2 capture capacity per unit mass of sorbent much higher than that obtained with the untreated limestone. Simple economic evaluations suggest that the use of the pre-treated lime in place of limestone can involve significant economies for fluidized bed coal combustor operators.

Effects of preparation conditions of Au/FeO x/Al 2O 3 catalysts prepared by a modified two-step method on the stability for CO oxidation

Abstract Composite oxide FeO x/Al 2O 3-supported gold catalysts were prepared by a modified two-step method. The effects of preparation conditions on the initial catalytic activity and long-time stability were studied for CO oxidation. XRD, XPS and in situ FTIR were employed to investigate the state of FeO x and the species on the catalyst surface. The results showed that Au/FeO x/Al 2O 3 catalysts prepared by this method exhibited high activity and high stability in a wide pH value range. Calcination pretreatment was proved to be beneficial to improving the activity and stability. The beneficial effects of FeO x acting as a structural promoter could be ascribed to the ability to supply active oxygen species. As the precursor of FeO x, Fe(NO 3) 3 is superior to FeCl 3 for obtaining higher stability.

The OSRM reaction over gold promoted copper zinc catalyst

Hydrogen production by oxidative steam reforming of methanol (OSRM) was investigated over CuOZnO-based catalysts promoted by Au, which were prepared by using co-precipitation and deposition–precipitation and were characterized by X-ray powder diffraction (XRD), H2-temperature programmed reduction (TPR), induced coupled plasma mass analyzer (ICP-MS), and transmission electron microscopy (TEM). We found that the reducibility of CuOZnO-based catalyst could be improved with increasing Au loading, which induced better performance at temperatures lower than 200°C. In addition, through calcination pretreatment, the residual chloride on the catalyst could be removed, which might expose more Au on the surface and result in a decrement of CO selectivity.

Steam Reforming of Methanol with Sm2O3−CeO2-Supported Palladium Catalysts: Influence of the Thermal Treatments of Catalyst and Support

The design of new anode materials to promote the reforming of methanol is important for the development of intermediate temperature direct methanol solid oxide fuel cells. A previous study showed Pd/CeO2−Sm2O3 to be a good candidate. Here, the influence of the calcination pretreatment of the support and the reduction temperature of the catalyst on the steam reforming of methanol are investigated, as they can become key factors in the performance of the catalyst. Conversion, H2 yield, and TOF were considerably higher when the support was calcined at 800 °C (Pd/CS-800) instead of 1000 °C (Pd/CS-1000). Characterization results suggest a stronger interaction of Pd particles with the support in Pd/CS-1000, which hinders its accessibility to the gas atmosphere, and a less homogeneous distribution of Pd particles. In both cases, the activity increases on increasing the reduction temperature from 400 to 500 °C. In addition, these catalysts were highly resistant to deactivation.

H2 Production via the Steam Reforming of Methanol Over NiAl-layered Double Hydroxide Derived Catalysts

This article reviews our recent results on the steam reforming of methanol over a series of NiAl-layered double hydroxide catalysts prepared by the co-precipitation method. The influence of calcination temperature, reaction temperature, pretreatment temperature and atmospheres, inorganic salt ions and steam to methanol ratio on the catalytic performance was studied. The major products for many of the catalysts were H2, CO, CO2 and CH4. However, the product composition varies significantly with the experimental parameters and high selectivity for CO2 and H2 was observed under various conditions, showing the potential of Ni based catalysts for the production of highly pure hydrogen.

HMS mesoporous silica as cobalt support for the Fischer–Tropsch Synthesis: Pretreatment, cobalt loading and particle size effects

The use of mesostructurated materials as cobalt support for the Fischer–Tropsch Synthesis (FTS) has taken increasing importance in the last decade. However, little use has been made of the hexagonal mesoporous silica (HMS), as most of the work has been reported for Co/MCM-41 and Co/SBA-15 catalytic systems. The effect of cobalt loading, impregnation method, particle size and pretreatment, on the catalytic properties of cobalt supported on HMS catalysts for the Fischer–Tropsch Synthesis have been studied. Catalysts were synthesized by incipient wetness impregnation, using an aqueous solution of hexahydrated cobalt nitrate as precursor salt. Additionally, a series of catalysts was prepared using an ethylene diamine cobalt complex which enabled cobalt nanoparticles formation. All catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), chemical analysis and nitrogen adsorption. Calcination pretreatment has great influence in the catalytic behavior of the cobalt catalysts. All the catalysts prepared showed high C 5 + fraction selectivity, specifically towards the diesel fraction. The use of HMS as cobalt catalyst support for the Fischer–Tropsch Synthesis seems to be very promising. When SiO 2 is used as support, C 5 + hydrocarbons distribution shows a marked tendency towards C 19 + fraction, while when HMS is used, the diesel fraction is favored. This result leads to the conclusion that the HMS pore structure produced a chain growth hindrance, tailoring the product distribution towards the diesel fraction.

Ni/SiO2 catalysts prepared with ethylenediamine nickel precursors: Influence of the pretreatment on the catalytic properties in glucose hydrogenation

Nickel catalysts prepared with different metal loadings (5, 10, and 20 wt% Ni) by impregnation with nickel ethylenediamine complexes revealed small nickel particles (mean diameter: 2–3 nm) after reduction, even for high metal loadings (20 wt%). When used in the hydrogenation of glucose, these catalysts show almost no nickel leaching compared to a commercial Ni/SiO2 catalyst (prepared by precipitation, 67 wt% Ni), but they are slightly less active (TOF: 2 to 10 × 10−3 s−1) than a commercial nickel/silica catalyst (TOF: 14 × 10−3 s−1) and revealed lower yields to sorbitol (3–42% compared to 60%). The catalytic results depend on the pretreatment conditions performed. When the catalysts are calcined before reduction, higher conversion (19–45%) and selectivity to sorbitol (81–92%) are obtained than when they are directly reduced without calcination pretreatment (conversion: 10–16% and selectivity to sorbitol: 21–59%) after 5 h of reaction time, respectively. The difference was ascribed to the fact that complete decomposition of the nickel ethylenediamine precursor was achieved only when calcination pretreatment was performed. Indeed, when the catalysts were directly reduced, X-ray photoelectron spectroscopy (XPS) revealed a nitrogen-compound probably responsible for lower conversion and higher selectivities to fructose and mannose, indicating Lobry de Bruyn-Alberda van Ekenstein rearrangement. However, no nickel sintering was observed in contrast to the catalyst calcined before reduction.

Influence of oxychlorination treatment on the surface and bulk properties of a Pt–Sn/Al 2O 3 catalyst

A Pt(3%)–Sn(4.5%)/Al 2O 3 catalyst has been studied after treatments prior to reduction involving calcination in air at 673 K or 823 K, or oxychlorination in air+1,2-dichloropropane at 823 K. Bulk phases present were characterised by XRD and surface character was probed by IR of adsorbed CO and by CO chemisorption. Comparative IR and chemisorption results for Pt(3%)/Al 2O 3 are also presented. Pt–Sn/Al 2O 3 was used to catalyse heptane reforming reactions. Catalyst prepared from a chlorine-free precursor and calcined at 673 K contained well-dispersed SnO 2 and PtO 2 but no Pt°, whereas calcination at 823 K gave Pt° particles covered with a layer of O-adatoms. After reduction both catalysts contained Pt° particles, the surfaces of which, although exposing arrays of Pt° atoms, were partly covered in Sn°. No alloy was formed. Tin, probably as Sn 2+, was also spread over the alumina support surface. The spreading of Sn over both Pt and alumina was greater after the higher temperature calcination pretreatment followed by reduction. Oxychlorination also gave Pt° particles with surface O-adatoms and probably surface Cl together with segregated crystalline SnO 2. However, in marked contrast to the Cl-free catalysts, subsequent reduction gave 1:1 PtSn alloy particles with excess Sn spread over the alumina. Alloy surfaces did not exhibit large ensembles of exposed Pt° atoms. Alloy formation reduced catalyst activity but induced stability with respect to both activity and selectivity. Both alloy formation, and coking with time for catalysts with arrays of exposed Pt, promoted isomerisation reactions, but decreased aromatisation, hydrogenolysis and C 5 cyclisation. Selectivity changes accompanying alloy formation are discussed and the results are primarily attributed to geometric ensemble effects.

A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au—TiO 2 catalysts

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated Au TiO 2 is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated Au TiO 2 and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO x overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of Au TiO 2 catalysts and argues for the formation of active sites at the Au TiO x interface produced by the mobility of TiO x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated Au TiO 2 reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO 2; however, a band for adsorbed CO is observed on the pure TiO 2. Kinetic studies with a 1.0 wt.-% impregnated Au TiO 2 sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O 2 between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O 2 adsorbing on TiO 2, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and car☐ylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO 2 (near 2340 cm −1) which may be competing with CO for adsorption sites.

Influence of Pretreatment on Lanthanum Nitrate, Carbonate, and Oxide Powders

Diffuse reflectance FTIR spectra, X-ray diffraction patterns, and BET surface areas of La(NO3)3, La2(CO3)3, and La2O3 have been obtained after various stages of thermal decomposition in the presence and absence of O2. In situ DRIFTS provided information about the surface chemistry taking place during the adsorption of NO and CO2 on the La oxide surfaces obtained. Decomposition of La(NO3)3 under flowing Ar at 773 K resulted in a mixture of La2O3 and a nitrate phase with ionic (noncoordinated) nitrate groups, i.e., those not directly coordinated to a La cation. However, when the decomposition was performed under flowing O2, LaONO3 was the principal compound. NO adsorption on the oxide surface after decomposition enhanced the peak intensity of residual nitrate surface speciesno new peaks appeared. La2(CO3)3 was more stable and was only partly transformed into La2O2CO3 during thermal treatment at 773 K. The commercial La2O3 samples contained mainly hydroxide if exposed to, or stored under, ambient air. The La(OH)3 decomposes when heated to 773 K, but rehydroxylation occurs rapidly if the samples are exposed to ambient air. Both the temperature and the gaseous medium of the calcination pretreatment determine the final state of the material. High temperatures (>1173 K) and a CO2-free medium must be used to guarantee transformation into principally La2O3, which still contains variable amounts of surface or subsurface carbonate groups. However, these high calcination temperatures result in considerable loss of surface area. Bidentate and unidentate carbonates were formed on La2O3 by adsorbing CO2 at either 298 or 773 K, thus revealing the surface sites have medium Lewis basicity. NO adsorption at 298 and 773 K leads to an exchange reaction with carbonate ions to disproportionate NO and form nitrate and nitrite groups, N2 and CO2. These results provide comprehensive references for the preparation and characterization of catalytic La2O3 surfaces.