Selective catalytic reduction of NOx to nitrogen over Co-Pt/ZSM-5: Part A. Characterization and kinetic studies

The selective catalytic reduction of NO by propene in the presence of excess oxygen has been studied over catalysts based on Co-Pt supported on ZSM-5. Pure Pt based catalysts are highly active, but produce large amounts of N₂O. Bimetallic Co-Pt/ZSM-5 catalysts with low Pt contents (0.1 wt.%) show a synergistic effect by combining high stability and activity of Pt catalysts with the high N₂ selectivity of Co catalysts. The lower selectivity to N₂O is attributed to its selective conversion over Co. The catalysts also showed high water and sulfur tolerance above 350°C.     

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.     

Co–ZSM-5 catalysts in the decomposition of N2O and the SCR of NO with CH4: Influence of preparation method and cobalt loading

Co–ZSM-5 prepared via different methods with Co/Al ratios ranging from 0.03 to 0.83 are investigated in both the direct N₂O decomposition and the selective catalytic reduction (SCR) of NO with CH₄. UV–vis and H₂-TPR are used to get an insight in the active species in these reactions. It is observed that in catalysts with low Co loadings (Co/Al < 0.3) Co is predominantly present as mono-atomic Co species, located at ion exchange positions in ZSM-5. Higher Co loadings result in the formation of different kinds of Co-oxides, which constitute the majority of species in the over-exchanged catalysts (Co/Al > 0.5). The mono-atomic species show the highest activity in the direct decomposition of N₂O, whereas the oxidic Co species do not seem to contribute much to the overall decomposition. In the SCR, the Co-oxide species catalyze the combustion of CH₄ whereas the selectivity towards NO reduction is much increased at low Co loadings. Therefore, over-exchange of Co–ZSM-5 does not seem to be favorable for both the direct N₂O decomposition and the SCR of NO with CH₄. Co/Al ratios <0.3 give the best results both in terms of conversion and activity per Co atom in both reactions.     

Screening of Catalysts for Acrylonitrile Decomposition

The catalytic decomposition of acrylonitrile over various metal components (Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Ga, Pd, Ag, and Pt) supported on several metal oxides (Al₂O₃, SiO₂, TiO₂, ZrO₂, and MgO) and ZSM-5 was studied. The most promising catalyst was Cu-ZSM-5, which exhibited 100% conversion and at least 80% N₂ selectivity above 350 °C.     

Decomposition of nitrous oxide in excess oxygen over Co- and Cu-exchanged MFI zeolites

The decomposition of nitrous oxide on several Co- and Cu-ZSM-5 zeolite catalysts was studied in the absence and presence of excess oxygen. Also, the effect of methane addition, as well as catalyst steaming in dry and wet feeds is reported. N₂O decomposition with no oxygen in the feed was proportional to metal loading on both catalysts. Co-ZSM-5 was much more resistant than Cu-ZSM-5 in excess oxygen. The tolerance of Co-ZSM-5 catalysts to excessive amounts of oxygen is high when Co²⁺ is stabilized in the zeolite framework and depends on the catalyst method of preparation. The presence of methane with no oxygen in the feed enhanced N₂O decomposition while the addition of both methane and oxygen to the feed decreased N₂O conversion on all catalysts tested. Co²⁺ ions stabilized by ZSM-5 framework have high hydrothermal stability in comparison to Cu²⁺ -exchanged ZSM-5.     

Preparation and Characterization of Pt/Al-ZSM-5 Catalysts and their Reactivities for the Oxidation of CO with N2O at Low Temperatures

Al-ZSM-5 was prepared by treating H-ZSM-5 with an aqueous solution of Al(NO₃)₃ and used as a support for Pt catalysts. The Pt-loaded Al-ZSM-5 acts as an efficient catalyst for CO oxidation with N₂O at 273 K. TEM investigations revealed that Pt clusters with an average particle size of around 1–1.5 nm were homogeneously dispersed within Al-ZSM-5. Moreover, FT-IR and XPS analyses indicated that the small Al₂O₃ clusters formed within Al-ZSM-5 plays a significant role in the formation of highly dispersed Pt clusters within the pore structure of the ZSM-5 zeolite, leading to the high catalytic activity of Pt/Al-ZSM-5 as compared to Pt/ZSM-5.     

The Effect of Different Active Sites on the Catalytic Activity of Fe-ZSM-5 Zeolite for N2O Direct Decomposition

Fe-modified ZSM-5 zeolites (Si/Al = 25) were prepared by adopting the liquid ion-exchange method with nitrate and oxalate of iron as Fe precursors and their catalytic performance was studied in the N₂O decomposition reaction. The results of FT-IR and H₂-TPR investigations indicated that (i) part of the iron ions could replace Brönsted acid protons at the straight channel wall (α sites), intersection of straight and sinusoidal channels (β sites), and sinusoidal channel wall (γ sites) within the ZSM-5 zeolite; and (ii) different Fe precursors gave rise to various distributions of α, β, and γ sites. We observed that the Fe-ZSM-5 catalyst prepared with iron oxalate as Fe precursor outperformed the ones prepared with iron nitrate as Fe precursor in the direct decomposition of N₂O. Furthermore, the catalytic activity of iron ions located at the α sites was higher than those of iron ions located at the β and γ sites.     

Isothermal “light-off” during catalytic N2O decomposition over Fe/ZSM-5

The catalytic decomposition of N₂O over Fe/ZSM-5 was studied at pressures up to 1 atm. As the partial pressure of N₂O in the feed to a packed bed reactor was varied, the catalytic activity was observed to abruptly increase at inlet partial pressures above some critical value. This abrupt increase in activity was not due to the reaction exotherm, but is believed to be a kinetic phenomenon. In similar experiments where the temperature was varied the activity did not jump to a higher level. The dual levels of activity were observed for several Fe/ZSM-5 catalysts that spanned a range of SiO₂/Al₂O₃ ratios from 30 to 280, irrespective of whether the iron was introduced into the zeolite by ion exchange or by sublimation of iron chloride. When the inlet partial pressure of N₂O to the packed bed reactor was sufficiently high to cause the catalyst to operate in the high activity regime, the high activity state was sustained throughout the catalyst bed, even though the N₂O partial pressure in the latter part of the bed had dropped below the critical level. Microkinetic modeling shows that this kind of behavior is possible in an isothermal catalytic system. The microkinetic model includes two redox cycles. In one cycle the cation oxidation state alternates between Fe²⁺ and Fe³⁺. In the second, more active redox cycle there is alternation between a surface nitrite and nitrate. The former redox cycle predominates at low partial pressures and the latter at high N₂O partial pressures.     

Preparation and Pretreatment Temperature Influence on Iron Species Distribution and N2O Decomposition in Fe–ZSM-5

Fe–ZSM-5 catalysts are prepared by FeCl₃ sublimation between 320 and 850 °C. The catalysts are characterised by XRD, H₂–TPR, NH₃–TPD, NO adsorption by DRIFTs, and catalytic activity is evaluated for N₂O decomposition. The influence of high temperature (850 °C) and pretreatment environment (air, He, He+H₂O and H₂) on the nature of iron species in Fe–ZSM-5 is further investigated by DRIFTs. High temperature FeCl₃ sublimation results in decreased FeO_x formation, easily reducible and narrow distribution of iron species in close proximity to alumina in Fe–ZSM-5. High temperature FeCl₃ sublimation or pretreatment results in isolated hydroxylated iron species, –Fe(OH)₂, which are not significant in Fe–ZSM-5 prepared by 320 °C FeCl₃ sublimation followed by calcination below 600 °C. Fe–ZSM-5 prepared by high temperature FeCl₃ sublimation show high N₂O decomposition activity and the improved performance can be correlated to –Fe(OH)₂ species in close proximity to alumina.     

Characterization of the Active Sites on Pt-Loaded ZSM-5 (Pt/ZSM-5) Prepared by an Ion-Exchange Method for the Oxidation of CO at Low Temperatures

Highly dispersed Pt-loaded ZSM-5 (Pt/ZSM-5) catalysts were prepared by a combination of ion-exchange and thermal pretreatment in different temperatures under vacuum. Highly dispersed ion-exchanged Pt²⁺ ions were reduced into Pt⁺ and then Pt⁰, sustaining their high dispersion state with an increase in the thermal pretreatment temperatures up to 773 K. Thus, prepared Pt⁰ highly dispersed in the cavities of ZSM-5 exhibited high catalytic activity for the oxidation of CO with N₂O at 273?K. However, pretreatment at temperatures higher than 973 K led to the aggregation of highly dispersed Pt⁰ clusters, resulting in a decrease in the catalytic activity for low-temperature oxidation.     

Highly Selective and Active Niobia-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

The performance of Co/Nb₂O₅ was compared to that of Co/γ-Al₂O₃ for the Fischer–Tropsch synthesis at 20 bar and over the temperature range of 220–260 °C. The C₅₊ selectivity of Nb₂O₅-supported cobalt catalysts was found to be very high, i.e. up to 90 wt% C₅₊ at 220 °C. The activity per unit weight cobalt was found to be similar for Nb₂O₅ and γ-Al₂O₃-supported catalysts at identical reaction temperature. However, due to the low porosity of crystalline Nb₂O₅, the cobalt loading was limited to 5 wt% and consequently the activity per unit weight of catalyst was lower than of Co/γ-Al₂O₃ catalysts with higher cobalt loadings. This low activity was largely compensated by increasing the reaction temperature, although the C₅₊ selectivity decreased upon increasing reaction temperature. Due to the high intrinsic C₅₊ selectivity, Nb₂O₅-supported catalysts could be operated up to ~250 °C at a target C₅₊ selectivity of 80 wt%, whereas γ-Al₂O₃-supported catalysts called for an operation temperature of ~210 °C. At this target C₅₊ selectivity, the activity per unit weight of catalyst was found to be identical for 5 wt% Co/Nb₂O₅ and 25 wt% Co/Al₂O₃, while the activity per unit weight of cobalt was a factor of four higher for the niobia-supported catalyst.     

Rh–Sr/Al2O3 Catalyst for N2O Decomposition in the Presence of O2

The improving effect of Sr in the catalytic activity of Rh for N₂O decomposition has been studied under 1,000 ppm N₂O/He and 1,000 ppm N₂O/5% O₂/He (GHSV = 10,000 h^?1). Different techniques have been used for catalysts characterization: TEM, SEM-EDX, XRD, N₂ adsorption at ?196 °C and in situ XPS. Sr favours the Rh dispersion and reduction under reaction conditions, and allows the low temperature removal of N₂O in the presence of O₂ (100% decomposition at 350 °C).     

An investigation of the mechanism of the selective catalytic reduction of NO on various metal/ZSM-5 catalysts: reactions of H2/NO mixtures

Several metal/ZSM-5 catalysts (Pt, Rh, Co and Cu/ZSM-5) were studied for the reduction of nitric oxide by a reducing agent which does not contain carbon, such as H₂, in the absence of oxygen. It has been found that these catalysts are very active towards the above reaction with H₂ even at relatively low temperatures, with a reactivity in the order: Pt/ZSM-5>Rh/ZSM 5>Co/ZSM-5>Cu/ZSM-5. Between the above catalysts Co/ZSM-5 was the most selective to nitrogen with a formation of ammonia much lower than that observed on Pt/ZSM-5, Rh/ZSM-5 and Cu/ZSM-5 even at higher hydrogen partial pressures. A comparison between hydrogen, methane and ethane as the reducing agent has been made. In all cases the catalytic activity is higher using hydrogen rather than ethane or methane, the latter being the least active reductant. All data are consistent with a simple redox mechanism of NO reduction, and exclude the participation of carbonaceous deposits or carbon-containing species in the reaction mechanism.     

Observations on the Aging Environment Dependent NO Oxidation Activity of Model Pt/Al2O3 Diesel Oxidation Catalyst

The influence of aging environment of model diesel oxidation catalyst Pt/Al₂O₃ on the NO oxidation activity is studied. The fresh catalyst Pt/Al/F (calcined in air at 500 °C) is aged with or without phosphorus (P) poisoning (7.5 wt%) at 800 °C either in air (P/Pt/Al/O or Pt/Al/O) or in simulated diesel exhaust (P/Pt/Al/R or Pt/Al/R). Catalyst aged under diesel exhaust environment (Pt/Al/R) surprisingly presents the best NO oxidation activity under excess of O₂ followed by the fresh (Pt/Al/F) and thermally aged (Pt/Al/O) catalysts. The activity difference between the catalysts is quite large, especially between Pt/Al/R and Pt/Al/O that are aged at the same temperatures but under different environments suggesting the importance of the aging environment for the catalytic activity. The NO oxidation activity of P poisoned catalysts P/Pt/Al/R and P/Pt/Al/O is minute as compared to their P free counter parts indicating that chemical aging is more detrimental for catalytic efficiency than thermal aging.     

Quantification and redox property of the oxygen-bridged Cu<Superscript>2+</Superscript> dimers as the active sites for the NO decomposition over Cu-ZSM-5 catalysts

For a range of Cu-ZSM-5 catalysts with different Cu-exchange levels on the two kinds of ZSM-5 with different Si/A1 ratios, temperature programmed reduction using CO (CO-TPR) followed by H₂ (H₂-TPR), and temperature programmed desorption of oxygen (O₂-TPD) were conducted using an online mass spectrometer to characterize and quantify the copper species on the catalysts in the calcined state. Copper species on the ZSM-5 were quantitatively characterized as Cu²⁺, (Cu-O-Cu)²⁺ and CuO after calcination in oxygen environment. The N₂ formation activities of the catalysts in the decomposition of NO were well correlated with the quantified catalytic amounts of the Cu²⁺ ions involved in the Cu-dimers, (Cu-O-Cu)²⁺. The mol fraction of the Cu ions present as the Cu-dimers increased at the sacrifice of the isolated Cu²⁺ with increasing Cu ion exchange level, suggesting that the species could be formed between the two Cu²⁺ in close proximity. Oxygen that could be thermally desorbed from the oxidized catalysts in the O₂-TPD was responsible for the reduction of the Cu-dimers. It was concluded that the decomposition of NO over Cu-ZSM-5 catalyst proceeded by the redox of (Cu-O-Cu)²⁺, as active centers. With the temperature programmed surface reaction using N₂O or NO over an oxidized catalyst sample as well as the O₂-TPD, it was possible to estimate the change of the oxidation state of the Cu ions engaged in the Cu-dimers.     

Hydrogen Production with a Microchannel Reactor by Tri-Reforming; Reaction System Comparison and Catalyst Development

In this work, Pt based mono and bimetallic catalysts were tested under conditions of tri-reforming (TR). All the catalysts contained 25% of CeO₂ and a metal loading of 2.5 or 5.0% (wt%). The bimetallic catalysts contained 2.5% Pt and 2.5% of Me, where Me?=?Ni, Co, Mo, Pd, Fe, Re, Y, Cu or Zn. For all the experiments, a synthetic biogas which consisted of 60% CH₄ and 40% CO₂ (vol.) was mixed with water, S/C?=?1.0, and oxygen, O₂/CH₄?=?0.25, and fed to a fixed bed reactor (FBR) system or a microreactor. The 2.5Pt catalyst was used in order to compare the performance of each reaction system. The tests were performed at reaction temperatures between 700 and 800?°C, and at volume hourly space velocities (VHSV) between 100 L_N/(h g_cat) and 200 L_N/(h g_cat) for the FBR system and between 1000 L_N/(h g_cat) and 2000 L_N/(h g_cat) for the microreactor, at atmospheric pressure. Then, all catalysts were deposited into microchannel reactors and tested at a constant VHSV of 2000 L_N/(h g_cat) and reaction temperatures between 700 and 800?°C. Catalysts under investigation were characterized applying the following techniques: inductively coupled plasma optical emission spectroscopy (ICP-OES), N₂ Physisorption, Temperature Programmed Reduction (TPR), CO chemisorption, Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The microreactor was identified as the most efficient and promising reaction system, and the 2.5(Pt–Pd) catalyst as the bimetallic formulation with the highest activity. Therefore its activity and stability was compared with the reference 5.0Pt catalyst at 700?°C and VHSV of 2000 L_N/(h g_cat) for more than 100 h. Although slightly lower activity was measured operating with the 2.5(Pt–Pd) catalyst, a significant reduction of the Pt content compared to the reference 5.0Pt catalyst was achieved through the incorporation of Pd.     

Transesterification of Refined Sunflower Oil to Biodiesel Using a CaO/ZSM-5 Powder Catalyst

The production of biodiesel from refined sunflower vegetable oil over basic CaO/ZSM-5 catalysts was investigated. Several catalysts with various loadings of CaO on ZSM-5 were prepared, calcined at 800 °C, and characterized by N₂ adsorption-desorption, X-ray diffraction, Fourier transform infrared spectroscopy, and CO₂-temperature-programmed desorption techniques. Calcined catalysts were tested in the transesterification reaction and reaction conditions were optimized by varying the catalyst-to-oil ratio and reaction time. The most active catalyst was the CaO/ZSM-5 catalyst with a 35 wt % loading which gave the highest fatty acid methyl ester yield. The high catalytic activity was attributed to the active basic sites generated following CaO addition. Furthermore, the catalyst demonstrated stability against the leaching process.     

Effect of Si/Al ratio on performance of Pt–Sn-based catalyst supported on ZSM-5 zeolite for n-butane conversion to light olefins

The performance of Pt–Sn-based catalyst, supported on ZSM-5 of different Si/Al ratios were investigated for simultaneous dehydrogenation and cracking of n-butane to produce light olefins. The catalysts were characterized by number of physio-chemical techniques including XRF, TEM, IR spectra, NH₃-TPD and O₂-pulse analysis. Increase in Si/Al ratio of zeolite support ZSM-5 significantly increased light olefin's selectivity, while feed conversion decreases due to lower acidity of support. The results indicated that both the n-butane cracking and dehydrogenation activity to light olefin's over Pt–Sn/ZSM-5 samples with increasing Si/Al ratios greatly enhanced catalytic performance. The catalysts were deactivated with time-on-stream due to the formation of carbon-containing deposits. A coke deposition was significantly related to catalyst activity, while at higher Si/Al ratio catalyst the coke precursors were depressed. These results suggested that the Pt–Sn/ZSM-5 catalyst of Si/Al ratio 300 is superior in achieving high total olefins selectivity (above 90 wt.%). The Pt–Sn/ZSM-5 also demonstrates resistance towards hydrothermal treatment, as analyzed through the three successive reaction-regeneration cycles.     

Fischer–Tropsch Synthesis on SiC-Supported Cobalt Catalysts

Seven SiC supports provided by SICAT with different surface areas and pore volumes were impregnated with 12,5 wt% Co. H₂-chemisorption, N₂-adsorption, temperature programmed reduction and Fischer–Tropsch synthesis in a fixed-bed reactor at 483 K, 20 bar and H₂/CO = 2.1 were performed in order to characterize and test the samples. The performances were compared with well characterized Co/Al₂O₃ and Co-Re/Al₂O₃ reference catalysts. The selectivity towards heavier hydrocarbons (C₅₊) was found to be moderately higher for the SiC supported catalysts while the site-time yields was 20 to 66 % lower than the Co-Re/Al₂O₃ catalyst. Elemental analysis showed the presence of several impurities in the SiC material. Alkali and alkaline earth elements, such as Na, K and Ca, are all known to lower the catalytic activity and also to influence the selectivity. It is proposed that these impurities in addition to sulfur and phosphorus known to be present in SiC, are responsible for the significantly lower catalytic activity of the SiC supported catalysts.     

Enhancing the Production of Light Olefins by Catalytic Cracking of FCC Naphtha over Mesoporous ZSM-5 Catalyst

The enhanced production of light olefins from the catalytic cracking of FCC naphtha was investigated over a mesoporous ZSM-5 (Meso-Z) catalyst. The effects of acidity and pore structure on conversion, yields and selectivity to light olefins were studied in microactivity test (MAT) unit at 600 °C and different catalyst-to-naphtha (C/N) ratios. The catalytic performance of Meso-Z catalyst was compared with three conventional ZSM-5 catalysts having different SiO₂/Al₂O₃ (Si/Al) ratios of 22 (Z-22), 27 (Z-27) and 150 (Z-150). The yields of propylene (16 wt%) and ethylene (10 wt%) were significantly higher for Meso-Z compared with the conventional ZSM-5 catalysts. Almost 90% of the olefins in the FCC naphtha feed were converted to lighter olefins, mostly propylene. The aromatics fraction in cracked naphtha almost doubled in all catalysts indicating some level of aromatization activity. The enhanced production of light olefins for Meso-Z is attributed to its small crystals that suppressed secondary and hydrogen transfer reactions and to its mesopores that offered easier transport and access to active sites.