SO2 abatement over nanocrystalline MgAl2O4 spinel-supported catalysts

Thermally stable magnesium-rich MgAl₂O₄ spinel with mesoporous nanostructures and high surface area has been prepared by co-precipitation and post hydrothermal treatment, using glucose as organic template. Physical and chemical properties were characterized by XRD, N₂ sorption, TG, FTIR, SEM, and TEM. The synthesized MgAl₂O₄ showed a surface area of 324 m² g^?1 and centralized mesopore distribution (ca. 3.3 nm pore width) after calcination at 700 °C for 3 h. The prepared MgAl₂O₄ were impregnated with metal oxides as sulfur transfer catalysts for high-temperature SO₂ adsorption reaction. The results showed that ferric doped MgAl₂O₄ had the highest SO₂ pick-up capacity up to 58 % and best regeneration up to 81 %. These results showed that thermally stable nanostructured MgAl₂O₄ are a promising candidate as catalyst for desulfurization in fluid catalytic cracking process.     

Soft-templating pathway to create nanostructured Mg–Al spinel as high-temperature absorbent for SO2

Interest in reducing SO₂ emission from the fluid catalytic cracking (FCC) crude oil has been encouraging the development of new materials to achieve such goal. The nanostructured Mg–Al spinel (MgAl₂O₄) was prepared by co-precipitation and post hydrothermal treatment in the presence of glucose and followed by elimination of the organic components by calcination at 700 °C for 3 h. Physical and chemical properties were characterized by XRD, N₂ sorption, TG, FTIR, SEM, and TEM methods. Mesoporous nanostructured MgAl₂O₄ with a high surface area of 324 m² g^?1 were obtained. The organic components contributed to the development of mesoporosity, functioning as a soft template. SO₂ adsorption tests showed that the nanostructured MgAl₂O₄ had a 51.58 % increase of SO₂ sorption capacity than MgAl₂O₄ prepared without glucose. These results showed that the nanostructured MgAl₂O₄ is a promising candidate as catalyst for flue gas desulfurization in FCC process. Three kinetic models were also applied to analyze the SO₂ adsorption kinetics; the pseudo-second order kinetic model fit well with a correlation coefficient (R²) of 0.991 for nanostructured MgAl₂O₄.     

The promotional effect of K on the catalytic activity of Ni/MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> for the combined H<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> reforming of coke oven gas for syngas production

A K-promoted 10Ni-(x)K/MgAl₂O₄ catalyst was investigated for the combined H₂O and CO₂ reforming (CSCR) of coke oven gas (COG) for syngas production. The 10Ni-(x)K/MgAl₂O₄ catalyst was prepared by co-impregnation, and the K content was varied from 0 to 5 wt%. The BET, XRD, H₂-chemisorption, H₂-TPR, and CO₂-TPD were performed for determining the physicochemical properties of prepared catalysts. Except under the condition of a K/Ni=0.1 (wt%/wt%), the Ni crystal size and dispersion decreased with increasing K/Ni. The coke resistance of the catalyst was investigated under conditions of CH₄: CO₂: H₂: CO:N₂=1 : 1 : 2 : 0.3 : 0.3, 800 °C, 5 atm. The coke formation on the used catalyst was examined by SEM and TG analysis. As compared to the 10Ni/MgAl₂O₄ catalyst, the Kpromoted catalyst exhibited superior activity and coke resistance, attributed to its strong interaction with Ni and support, and the improved CO₂ adsorption characteristic. The 10Ni-1K/MgAl₂O₄ catalyst exhibited optimum activity and coke resistance with only 1wt% of K.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

LaCo1−x Pd x O3 Perovskite-Type Oxides: Synthesis, Characterization and Simultaneous Removal of NO x and Diesel Soot

A series of naometric perovskite catalysts LaCo_1?x Pd _x O₃ (x = 0, 0.01, 0.03) were prepared via a solution combustion synthesis route using metal nitrates as oxidizers and urea as fuel. It is essential to add a certain amount of ammonia aqueous solution to Pd²⁺ ions solution in the catalyst preparation process. Homogeneous nanoparticles LaCo_1?x Pd _x O₃ catalysts with the sizes in the range of 68–122 nm were obtained and characterized by using of XRD, BET, H₂-TPR, XPS, SEM and TEM. Pd was successfully introduced into the LaCoO₃ perovskite lattices. Further information was obtained by using XPS upon the LaCo_0.97Pd_0.03O₃ (with NH₄OH) sample after H₂-TPR. The results revealed that surface Pd was reduced to the metallic state at the end of the first step in the H₂-TPR experiment, and some surface Co could be reduced to metallic Co simultaneously. The catalytic properties were investigated for simultaneous NO _x -soot removal reaction. The performance of LaCo_1?x Pd _x O₃ catalysts were greatly improved by the partial substitution of Pd. The maximum NO conversion into N₂ and the ignition temperature of soot are 32.8% and 265 °C, respectively.     

Alumina and Alumina–Baria Supported Cobalt Catalysts for DeNO x : Influence of the Support and Cobalt Content on the Catalytic Performance

Co/Al₂O₃ and Co/Al₂O₃–BaO catalysts with low cobalt loading (0.1, 0.3 and 1 wt%) for the selective catalytic reduction (SCR) of NO _x by C₃H₆ were prepared. The distribution of cobalt species was investigated by UV–vis diffuse reflectance spectroscopy and by H₂-TPR in order to identify the active cobalt species in hydrocarbons (HC)-selective catalytic reduction (SCR). It was found that the nature of cobalt species strongly depends on the cobalt loading as well as on the properties of the support. The barium addition to the alumina slows down solid state diffusion processes, improving the thermal stability of the support and preventing diffusion of cobalt into the bulk. Highly dispersed surface Co²⁺ species over alumina were identified as active sites in the NO-SCR process. Accordingly, a high concentration of surface Co²⁺ sites in Co 1 wt%/Al₂O₃ improves the catalytic performance in NO-SCR, the long term stability as well as the water tolerance. On the contrary, the formation of Co₃O₄ particles in Co 1 wt%/Al₂O₃–BaO promotes the propylene oxidation by oxygen, decreasing the activity and selectivity of the catalyst in NO reduction.     

Selective Production of H2 from Ethanol at Low Temperatures over Rh/ZrO2–CeO2 Catalysts

A series of Rh catalysts on various supports (Al₂O₃, MgAl₂O₄, ZrO₂, and ZrO₂–CeO₂) have been applied to H₂ production from the ethanol steam reforming reaction. In terms of ethanol conversion at low temperatures (below 450 °C) with 1wt% Rh catalysts, the activity decreases in the order: Rh/ZrO₂–CeO₂ > Rh/Al₂O₃ > Rh/MgAl₂O₄ > Rh/ZrO₂. Support plays a very important role on product selectivity at low temperatures (below 450 °C). Acidic or basic supports favor ethanol dehydration, while ethanol dehydrogenation is favored over neutral supports at low temperatures. The Rh/ZrO₂–CeO₂ catalyst exhibits the highest CO₂ selectivity up to 550 °C, which is due to the highest water gas shift (WGS) activity at low temperatures. Among the catalysts evaluated in this study, the 2wt% Rh/ZrO₂–CeO₂ catalyst exhibited the highest H₂ yield at 450 °C, which is possibly due to the high oxygen storage capacity of ZrO₂–CeO₂ resulting in efficient transfer of mobile oxygen species from the H₂O molecule to the reaction intermediate.     

Enhanced Stability and Propene Yield in Propane Dehydrogenation on PtIn/Mg(Al)O Catalysts with Various In Loadings

The dehydrogenation of propane on In-promoted Pt (0.3 wt% Pt) supported on hydrotalcite Mg(Al)O with different In loadings (0.2–1.0 wt% In) was investigated at 550 °C atmospheric pressure. All the bimetallic PtIn/Mg(Al)O showed higher propane conversion and propene selectivity than the Pt/Mg(Al)O with Pt0.8In exhibited the best catalytic performances with 97.5% propylene selectivity and 27.5% yield after 5 h time-on-stream. The addition of In to the monometallic Pt catalyst could reduce the acidity strength especially the strong acid site. As revealed by the H₂-TPR and XPS results, addition of In by impregnation on Pt/Mg(Al)O also led to the formation of metallic In and PtIn alloy, which greatly enhanced the catalyst activity and reduced coke formation on the support. Nevertheless, excessive In loading (i.e., Pt1.0In) resulted in a descending trend of catalyst activity compared to the Pt0.8In, due probably to the large amount of metallic In being formed, which was disadvantageous in propane dehydrogenation.     

Rate-determining Step of Selective Catalytic Reduction of NO by Acetylene Over HZSM-5

Nitrate species is crucial intermediate of selective catalytic reduction of NO by acetylene (C₂H₂-SCR) over HZSM-5. It is proposed that formation of nitrate species on the zeolite is rate-determining step of C₂H₂-SCR. The proposition is supported by the experimental results: (1) No nitrate species could be detected by in situ FTIR in steady C₂H₂-SCR over HZSM-5 at 350 °C, while the bands due to nitrate species were clearly observed on the zeolite both in situ in NO + O₂ and after a brief evacuation at the temperature; (2) Yttrium incorporation into HZSM-5 zeolite considerably enhanced nitrate species formation on the catalyst and correspondingly the activity of the catalyst for C₂H₂-SCR was significantly increased. Both the promotional effect of yttrium on C₂H₂-SCR and the rate-determining step of C₂H₂-SCR being nitrate species formation over the catalyst were first reported herein.     

Ceramic matrix composite production by directed melt oxidation of pure aluminium metal using spinel dopants

Abstract

The production of Al₂O₃ matrix composites by directed melt oxidation of pure Al externally doped with spinel type dopants has been investigated. The presence of any one of MgAl₂O₄ , LiAlO₂, and ZnAl₂O₄ resulted in oxide growths in a similar fashion to the growths produced using elemental Mg, Li, and Zn. Rapid growth was achieved at 1180°C with MgAl₂O₄ and at 900°C with LiAlO₂ . The growth rates at 1180°C of the samples doped with ZnAl₂O₄ were less rapid than the growth rates of the samples doped with MgAl₂O₄ . The fact that growth is initiated by mixed oxide spinel dopants is further evidence in support of the cyclic reaction sequences that have been proposed for directed melt oxidation.     

Understanding the correlation between calcination temperature and performance in low-temperature methanation over Ni-Zr/Al2O3 catalysts

Low-temperature methanation of CO in the continuous stirred tank reactor (CSTR) over Zr doped Ni/Al₂O₃ catalyst calcined at different temperatures (673, 723, and 823 K) was investigated. XRD, TPR, XPS, ICP, SEM, and S-TPR techniques were employed to characterize the fresh and spent catalysts. Based on the characterization results, it was found that low-temperature (673 K) calcination could effectively prohibit the formation of NiAl₂O₄ spinel, thereby resulting in more reducible NiO particles, which were the essential precursor of methanation active sites over the catalyst surface. Thus, the highest CO conversion of 93.6% was achieved over the 25N3ZA-673 catalyst. In addition, the deactivation rate of 25N3ZA-673 was relatively slow in comparison to 25N3ZA-823 due to the presence of more reducible NiO. The formed nickel carbonyl species (Ni[CO]_x), which quickly decomposed at a higher reaction temperature, was closely related to the catalyst deactivation. Therefore, 25N3ZA-673 possessed much better stability at 593 K than that at 553 K.     

The deposition of coke from methane on a Ni/MgAl2O4 catalyst

Temperature-programmed reaction techniques and Raman spectroscopy were used to characterize coke species deposited on a 5% Ni/MgAl₂O₄ catalyst for dry reforming of methane. The CH₄ temperature-programmed decomposition profiles showed that the ignited decomposition temperatures of CH₄ increased from 273 to 378 °C when MgAl₂O₄ replaced the catalyst support γ-Al₂O₃. The temperature-programmed oxidation, temperature-programmed hydrogenation and temperature-programmed CO₂ reaction profiles showed that there were three carbon species (i.e. C_α, C_β and C_γ) on the catalyst surface. Raman spectroscopy showed that C_γ was graphite-like carbon species, which was responsible for catalyst deactivation. The C_γ species was the most inactive species toward H₂ and O₂, while it was unexpectedly more active toward CO₂. The unique reactivity of CO₂ with different coke specie could be ascribed to the carbonate, bidentate and formate species formation on MgAl₂O₄ surface. These surface species enhanced the oxidation of C_γ species and thus contributed to the high stability of Ni/MgAl₂O₄ catalyst.     

Effect of La2O3 on Ru/CeO2-La2O3 Catalyst for Ammonia Synthesis

A series of CeO₂-La₂O₃ supported ruthenium catalysts were prepared by co-precipitation method and the as-obtained samples were characterized by N₂ physisorption, X-ray diffraction, CO chemisorption, H₂-TPR, H₂-TPD and XPS. The activity test shows that ammonia concentration of the catalyst with 10% La is 13.9% at 10 MPa, 10,000 h^?1, 450 °C, which is 17% higher than that of Ru/CeO₂. La doping can improve the activity of Ru-ceria catalyst for ammonia synthesis by facilitating the reduction of oxygen which subsists in the cerium oxide surface. In addition, it can be realized that the test of catalyst stability proves the stability performance of Ru/CeO₂-La₂O₃ catalyst within the reaction time of 55 h.     

Pd-Integrated Perovskite as Effective Catalyst for Selective Catalytic Reduction of NOx by Propene

Perovskite based Pd catalysts were prepared by a modified citrate route and analyzed with SEM, XRD, TEM and XPS techniques regarding the state of palladium. Integration of Pd into the perovskite lattice was compared with impregnation onto the support. Clear indications of Pd-substitution in the La-based perovskites were found by XPS and SEM. The integrated Pd-ions diffuse out of the perovskite lattice under reductive conditions forming metallic palladium nano-particles (less than 15 nm size) while the Pd particles obtained via the impregnation route were in the order of 80 nm. In the selective catalytic reduction of NO by propene, up to 35% NO conversion at 250 °C were obtained at a very low W/F of 0.015 g s mL^?1, with decreasing tendency at increasing oxygen content. Differences between impregnated and Pd-integrated catalysts were obvious only at high O₂ content (5 vol.%) where the Pd-integrated catalyst exhibited a lower tendency to oxidize the propene reductant.     

Selective catalytic reduction of NO by propane on copper containing alumina pillared α-zirconium phosphates

Copper containing alumina pillared α-zirconium phosphates with different copper loadings, where copper was incorporated by ion exchange and by impregnation, have been synthesized and characterized by using techniques such as XPS, UV–VIS spectroscopy, H₂-TPR and NO-TPD. The catalytic activity of the synthesized materials in the selective catalytic reduction of NO using propane as a reducing agent in the presence of oxygen at 623–823 K has been evaluated. In the exchanged sample, copper is mainly present as isolated Cu²⁺ species, whereas the impregnated materials show copper to be present as small CuO clusters (not detectable by XRD) and a fraction as spinel-like structures within the alumina pillars. NO-TPD displayed a stronger interaction of NO molecules with the copper exchanged materials, because N₂O and O₂ are detected at temperatures higher than 773 K. Under the selected experimental conditions, the most active catalyst in the SCR of NO is the copper exchanged catalyst, attaining conversions of NO close to 30% at 823 K. Moreover, the competitiveness factor is high and the TOF number is 1.57×10⁻⁴ molecules_NO per second at_Cu for the copper exchanged catalyst, this being comparable to that obtained with a Cu-ZSM-5 used as a reference.     

Preparation and characterization of CeOx@MnOx core–shell structure catalyst for catalytic oxidation of NO

The CeOx@MnOx catalyst with a core–shell structure was prepared and used for catalytic oxidation of NO. It was found that CeOx@MnOx catalyst showed higher intrinsic catalytic activity than CeMnOx catalyst prepared by citric acid method. Based on the characterization results of N₂ adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS), we may conclude that the excellent catalytic performance of CeOx@MnOx catalyst is related to its low crystallinity, good reducibility, and high concentrations of Mn^4 + and active oxygen species.     

Uniform and fine Mg-γ-AlON powders prepared from MgAl2O4: A promising precursor material for highly-transparent Mg-γ-AlON ceramics

High-purity and sinterability Mg-γ-AlON (Mg_0.1Al_1.53O_1.89N_0.27) powders were synthesized by gas pressure sintering (GPS) of mixed powders of commercial Al₂O₃ and AlN, and lab-made MgAl₂O₄. The Mg-γ-AlON powders exhibited a uniform particle morphology and a small particle size of d₅₀ = 3.4 μm, owing to the use of MgAl₂O₄ as the Mg source. Highly-transparent Mg-γ-AlON ceramics were fabricated using the synthesized Mg-γ-AlON powders by spark plasma sintering (SPS) at 1800 °C for 5 min under an axial pressure of 80 MPa, followed by hot isostatic pressing (HIP) at 1800 °C for 2 h under a nitrogen gas pressure of 190 MPa. The ceramics showed a high in-line transmittance of ～ 80.5% at 450 nm, ascribed to the high sinterability of the MgAl₂O₄ raw powder that leads to a pore-free and fully densified microstructure. This indicates that MgAl₂O₄ as sintering additive is superior over MgO and MgF₂ in the fabrication of Mg-γ-AlON transparent ceramic.     

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al₂O₃ catalyst for removing NO_x in diesel engine exhaust gas was developed. The influence of SO₂ on the reduction of lean NO_x by ethanol over the Ag/Al₂O₃ catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al₂O₃ catalyst was highly active for the reduction of NO_x with ethanol in the presence of SO₂ although the reduction of NO_x is suppressed at lower temperatures. The activity for NO_x reduction is high even on the Ag/Al₂O₃ catalyst exposed to a SO₂ (200 ppm)/O₂ (10%)/H₂O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al₂O₃ catalyst. No crystallized Ag metal and Ag compounds were formed by the SO₂/O₂/H₂O exposure. On the other hand, crystallized Ag₂SO₄ was easily formed when the Ag/Al₂O₃ catalyst was exposed to a SO₂ (200 ppm)/O₂ (10%)/NO (800 ppm)/H₂O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag₂SO₄ results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al₂O₃ catalyst decreases from 221 to 193 m²/g. Although the dispersion of Ag particles was decreased by the formation of Ag₂SO₄, the activity for the reduction of lean NO_x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag₂SO₄ formation are still active for the lean catalytic reduction of NO_x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combined Steam and Carbon Dioxide Reforming of Methane on Ni/MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: Effect of CeO<Subscript>2</Subscript> Promoter to Catalytic Performance

Abstract  

The catalytic performance during combined steam and carbon dioxide reforming of methane (SCR) was investigated on Ni/MgAl₂O₄ catalyst promoted with CeO₂. The SCR catalyst was prepared by co-impregnation method using nickel and cerium metal precursors on hydrotalcite-like MgAl₂O₄ support. In terms of catalytic activity and stability, CeO₂-promoted Ni/MgAl₂O₄ catalyst is superior to Ni–CeO₂/Al₂O₃ or Ni/MgAl₂O₄ catalysts because of high resistance to coke formation and suppressed aggregation of nickel particles. The role of CeO₂ on Ni/MgAl₂O₄ catalyst was elucidated by carrying out the various characterization methods in the viewpoint of the aggregation of nickel particles and metal-support interactions. The observed superior catalytic performance on CeO₂-promoted Ni/MgAl₂O₄ catalyst at the weight ratio of MgO/Al₂O₃ of 3/7 seems to be closely related to high dispersion and low aggregation of active metals due to their strong interaction with the MgAl₂O₄ support and the adjacent contact of Ni and CeO₂ species. The CeO₂ promoter also plays an important role to suppress particle aggregation by forming an appropriate interaction of NiO–CeO₂ as well as Ni–MgAl₂O₄ with the concomitant enhancement of mobile oxygen content.     

Investigating the Influence of Fe Speciation on N<Subscript>2</Subscript>O Decomposition Over Fe–ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N₂O over Fe–ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe–ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99?×?10³ s^?1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60?×?10³ s^?1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe–ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.