Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Hydrogen production by steam reforming of LNG over Ni/Al2O3-ZrO2 catalysts: Effect of ZrO2 and preparation method of Al2O3-ZrO2

An Al₂O₃-ZrO₂ support was prepared by grafting a zirconium precursor onto the surface of commercial γ-Al₂O₃. A physical mixture of Al₂O₃-ZrO₂ was also prepared for the purpose of comparison. Ni/Al₂O₃-ZrO₂ catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of liquefied natural gas (LNG). The effect ZrO₂ and preparation method of Al₂O₃-ZrO₂ on the performance of supported nickel catalysts in the steam reforming of LNG was investigated. The Al₂O₃-ZrO₂ prepared by a grafting method was more efficient as a support for nickel catalyst than the physical mixture of Al₂O₃-ZrO₂ in the hydrogen production by steam reforming of LNG. The well-developed tetragonal phase of ZrO₂ and the high dispersion of ZrO₂ on the surface of γ-Al₂O₃ were responsible for the enhanced catalytic performance of Ni/Al₂O₃-ZrO₂ prepared by way of a grafting method.     

Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts

Supported nickel catalysts of composition Ni/Y₂O₃–ZrO₂ were synthesized in one step by the polymerization method and compared with a nickel catalyst prepared by wet impregnation. Stronger interactions were observed in the formed catalysts between NiO species and the oxygen vacancies of the Y₂O₃–ZrO₂ in the catalysts made by polymerization, and these were attributed to less agglomeration of the NiO during the synthesis of the catalysts in one step. The dry reforming of ethanol was catalyzed with a maximum CO₂ conversion of 61% on the 5NiYZ catalyst at 800 °C, representing a better response than for the catalyst of the same composition prepared by wet impregnation.     

Hydrodesulphurization performance of NiW/TiO2-Al2O3 catalyst for ultra clean diesel

TiO₂-Al₂O₃ binary oxide supports were obtained by sol–gel methods from Tetra-n-butyl-titanate and pseudoboehmite/aluminium chloride resources. The typical physico-chemical properties of NiW/TiO₂-Al₂O₃ catalysts with different TiO₂ loadings and their supports were characterized by means of BET, XRD and UV–vis DRS, etc. The BET results indicated that the specific surface areas of NiW/TiO₂-Al₂O₃ catalysts were as higher as that over pure γ-Al₂O₃ support, and the pore diameters were also large. The XRD and UV–vis DRS analyzing results showed that the Ti-containing supported catalysts existed as anatase TiO₂ species and the incorporation of TiO₂ could adjust the interaction between support and active metal, and impelled the higher reducibility of tungsten. The hydrodesulphurization (HDS) performance of the series catalysts were evaluated with diesel feedstock in a micro-reactor unit, and the HDS results showed that NiW/TiO₂-Al₂O₃ catalysts exhibited higher activities of ultra deep hydrodesulphurization of diesel oil than that of NiW/Al₂O₃ catalyst. The optimal TiO₂ content of NiW/TiO₂-Al₂O₃ catalysts was about 15 m%, and the corresponding HDS efficiency could reach to 100%. The sulphur contents of diesel products over NiW/TiO₂-Al₂O₃ (from pseudoboehmite/AlCl₃) catalysts with suitable TiO₂ content could be less than 15 ppmw, which met the sulphur regulation of Euro IV specification of ultra clean diesel fuel.     

Influence of composition of MgAl2O4 supported NiCeO2ZrO2 catalysts on coke formation and catalyst stability for dry reforming of methane

Dry reforming of methane was studied over Ni catalysts supported on γAl₂O₃, CeO₂, ZrO₂ and MgAl₂O₄ (670 °C, 1.5 bar, 16–20 l CH₄ ml_catalyst⁻¹ h⁻¹). It is shown that MgAl₂O₄ supported Ni catalysts promoted with both CeO₂ and ZrO₂ are promising catalysts for dry reforming of methane with carbon dioxide. Within a certain composition range, the simultaneous promotion with CeO₂ and ZrO₂ has great influence on the amount of coke and the catalyst service time. XRD analyses indicate that formation of crystalline Ce_xZr_1−xO₂ mixed oxide phases occurs on double promotion. In particular, incorporation of low amounts of Zr in the CeO₂ fluorite structure provides stable dry reforming catalysis. As shown with TPR, promotion leads to a higher reduced state of Ni. SEM, XRD and TPR analyses demonstrate that highly dispersed, doubly promoted Ni catalysts with a strong metal-support interaction are essential for stable dry reforming and suppression of the formation of carbon filaments.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

SELECTIVE CARBON MONOXIDE OXIDATION IN H2-RICH GAS STREAM OVER Co3O4/CeO2/ZrO2, Ag/CeO2/ZrO2, AND MnO2/CeO2/ZrO2 CATALYSTS

Activity and selectivity of selective CO oxidation in an H₂-rich gas stream over Co₃O₄/CeO₂/ZrO₂, Ag/CeO₂/ZrO₂, and MnO₂/CeO₂/ZrO₂ catalysts were studied. Effects of the metaloxide types and metaloxide molar ratios were investigated. XRD, SEM, and N₂ physisorption techniques were used to characterize the catalysts. All catalysts showed mesoporous structure. The best activity was obtained from 80/10/10 Co₃O₄/CeO₂/ZrO₂ catalyst, which resulted in 90% CO conversion at 200°C and selectivity greater than 80% at 125°C. Activity of the Co₃O₄/CeO₂/ZrO₂ catalyst increased with increase in Co₃O₄ molar ratio.     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe₂O₃ is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH₄, CO and H₂ respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe₂O₃ (20 wt.%)/ZrO₂ was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH₄, CO and H₂, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH₄, but the phenomenon was not remarkable for the reduction by CO and H₂. The reduction rate achieved using CO and H₂ was relatively faster than achieved using CH₄. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe₂O₃ to FeO by CH₄, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH₄. The reduction of Fe₂O₃ to Fe by CO and H₂ can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe₃O₄ by water and FeO to Fe₂O₃ by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.     

Study of ruthenium supported on Ta2O5–ZrO2 and Nb2O5–ZrO2 as catalysts for the partial oxidation of methane

Ru-based catalysts supported on Ta₂O₅–ZrO₂ and Nb₂O₅–ZrO₂ are studied in the partial oxidation of methane at 673–873 K. Supports with different Ta₂O₅ or Nb₂O₅ content were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as precursors to prepare the catalysts (ca. 2 wt.% Ru). At 673 K high selectivity to CO₂ was found. An increase of temperature up to 773 K produced an increase in the selectivity to syngas (H₂/CO = 2.2–3.1), and this is related with the transformation of RuO₂ to metallic Ru as was determined from XRD and XPS results. At 873 K and with co-fed CO₂ an increase of the catalytic activity and CO selectivity was found. A TOF value of 5.7 s⁻¹ and H₂/CO ratio ca. 1 was achieved over Ru(Cl)/6TaZr. Catalytic results are discussed as a function of the support composition and characteristics of Ru-based phases.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Synthesis, characterization and performance of CuO/La2O3 composite catalyst for ammonia catalytic oxidation

This work considers the oxidation of ammonia (NH₃) by selective catalytic oxidation (SCO) over a CuO/La₂O₃ composite catalyst at temperatures between 150 and 400 °C. A CuO/La₂O₃ composite catalyst was prepared by co-precipitation of copper nitrate and lanthanum nitrate at various molar concentrations. This study also considers how the concentration of influent NH₃ (C₀ = 1000 ppm), the space velocity (GHSV = 92,000 l/h), the relative humidity (RH = 12%) and the concentration of oxygen (O₂ = 4%) affect the operational stability and the capacity for removing NH₃. The catalysts that were characterized using FTIR, XRD, UV-Vis, BET and PSA, have shown that the catalytic behavior is related to the copper (II) oxide, while lanthanum (III) oxide may serve only to provide active sites for the reaction during a catalyzed oxidation run. The experimental results show that the extent of conversion of ammonia by SCO in the presence of the CuO/La₂O₃ composite catalyst was a function of the molar ratio. The ammonia was removed by oxidation in the absence of CuO/La₂O₃ composite catalyst, and around 93.0% NH₃ reduction was achieved during catalytic oxidation over the CuO/La₂O₃ (8:2, molar/molar) catalyst at 400 °C with an oxygen content of 4.0%. Moreover, the effect of the reaction temperature on the removal of NH₃ in the gaseous phase was also monitored at a gas hourly space velocity of under 92,000 h^− 1.     

NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using γ-Al₂O₃ and α-Al₂O₃ as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO₂ and H₂O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl₂O₄ phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl₂O₄ that affected negatively to the OC reactivity and methane combustion selectivity towards CO₂ and H₂O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on γ-Al₂O₃.The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on γ-Al₂O₃ with NiO content lower than 30 wt%, and OC prepared by the DP method on α-Al₂O₃ with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using γ-Al₂O₃ as support were higher than the ones prepared using α-Al₂O₃ as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on α-Al₂O₃ as support had appropriated characteristics to be used in the chemical-looping combustion process.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Molten salt synthesis of α-Al2O3 platelets using NaAlO2 as raw material

α-Al₂O₃ platelets were prepared by a molten salt synthesis method when NaAlO₂ was used as raw material. The effects of the stirring rate during the gel preparation, heating temperature, type and addition amount of molten salts, addition of plate-like α-Al₂O₃ seeds, additives such as TiOSO₄ and Na₃PO₄·12H₂O on the morphology of α-Al₂O₃ were studied. High stirring rate during the gel preparation and high heating temperature not only help to restrain the overlapping of α-Al₂O₃ platelets, but also improve the size distribution. When the heating temperature increases to 1200 °C, most of α-Al₂O₃ platelets are hexagonal in its morphology, and the size of platelets becomes relatively uniform. When Na₂SO₄-K₂SO₄ flux is used instead of NaCl-KCl flux, it is easy to obtain α-Al₂O₃ platelets with a big size. When the molar ratio of salt to final Al₂O₃ powders increases to 4:1, most of α-Al₂O₃ platelets are hexagonal, and the overlapping of powders is inhibited. The addition of a small amount of plate-like seeds has a significant effect on the size of α-Al₂O₃ platelets. With the increase of seed amount, the diameter of α-Al₂O₃ platelets tends to decrease. The addition of 5.45 wt.% TiOSO₄ results in the formation of hexagonal α-Al₂O₃ platelets with an average diameter of 5.1 μm and an average thickness of 1.4 μm. Thin α-Al₂O₃ platelets with a discal shape are obtained owing to the co-addition of 0.51 wt.% Na₃PO₄·12H₂O and 3 wt.% TiOSO₄.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.