Structure–Function Relations in Supported Ni–W Sulfide Hydrogenation Catalysts

Ni–W catalysts were prepared by impregnation of commercial -alumina and silica supports. The sulfidation, performed directly after drying at 100°C, yielded fully sulfided Ni–W species on both supports (SEM-EDAX, XPS, XRD). At optimal metals loading (50 wt% NiO + WO₃, Ni/W = 2), the sulfided catalysts had similar texture (N₂ adsorption) and displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni–W/SiO₂ catalyst in toluene hydrogenation (HYD) was six times higher than that of Ni–W/Al₂O₃. This is due to the more than two times higher WS₂ slabs stacking number in Ni–W/SiO₂ compared with Ni–W/Al₂O₃ (XRD, HR-TEM), yielding stronger adsorption of toluene (TPD).     

Modeling and simulation of a smart catalytic converter combining NOx storage, ammonia production and SCR

Dynamic simulation of the smart catalytic converter, proposed by Daimler AG, is presented. The smart catalytic converter combines NOx storage, on-board ammonia production and selective catalytic reduction (SCR) and functions in a dual-mode operation, alternating between lean burn and rich burn. It relies on intrinsic dynamic operation and synchronization of all units and its development demands a reliable dynamic simulator. A platform capable of simulating the dynamic behavior of multiple-unit aftertreatment system was developed based on COMSOL package. Predictive kinetic models were developed for NOx storage unit that includes ammonia formation function and for NH₃-SCR unit. Using these kinetic models, two-unit smart catalytic converter was simulated on the developed simulator. The results of the simulator were validated using two-unit experimental data. The simulator was also employed to control and optimize the performance of smart catalytic converter. It was shown that the simulator is vital for optimization of lean and rich periods in order to ensure stable lean–rich cycles.     

NO oxidation kinetics on iron zeolites: influence of framework type and iron speciation

Topics in Catalysis - Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized...     

Kinetic experiments and modeling of NO oxidation and SCR of NOx with decane over Cu- and Fe-MFI catalysts

The kinetic model of the reduction of NO to N₂ with decane, developed based on the experimental data over Fe-MFI catalyst, has been applied for the oxidation of NO to NO₂ and reduction of NO₂ to N₂ with decane over Cu-MFI catalyst. The model fits well the experimental data of oxidation of NO as well as reduction of NO to N₂. Remarkable differences have been found in performance of Cu-MFI and Fe-MFI catalysts. While Fe-MFI is more active in oxidation of NO to NO₂, Cu-MFI exhibits much higher activity in the reduction of NO with decane. The kinetic model indicates that the significantly lower activity of Fe-MFI in comparison with Cu-MFI in transformation of NO_x to nitrogen is due to higher rate of transformation of NO₂, formed in the first step by the oxidation of NO, back to NO instead to molecular nitrogen.     

Surface Energetics of Nanoscale LaMnO3+δ Perovskite

Perovskite type LaMnO₃ and related materials are important compounds with many useful and unique physical and chemical properties. There is a lack of experimental thermochemical data on the energetics of LaMnO₃ nanoparticles. In this work, a series of LaMnO_3+δ samples were synthesized via the citrate method and calcined at 700°C–1050°C. All samples displayed rhombohedral structure (X‐ray diffraction) with similar oxygen stoichiometry 3+δ = 3.16–3.18 (iodometric titration coupled with gravimetric analysis). The BET surface area varied from null for bulk sample to 6.88 ± 0.08 × 10³ m²/mol for the sample calcined at 700°C. The water content varied linearly with the surface area with the highest value being 2.34 wt%. The chemisorbed water adsorption enthalpy was ?63.0 ± 4.1 kJ/mol with the chemisorbed water coverage of 8 H₂O/nm². High‐temperature oxide melt drop solution calorimetry, performed in sodium molybdate at 702°C, yielded enthalpy of formation from La₂O₃, Mn₂O₃, and O₂ of bulk LaMnO_3.16 of ?77.85 ± 1.94 kJ/mol. After correction of drop solution enthalpies of nanometric samples for water content, the calorimetric data were used to calculate the surface energy of LaMnO_3+δ. The energy of the anhydrous surface was 2.27 ± 0.29 J/m², and that of the hydrous surface was 2.02 ± 0.27 J/m². These values are higher than the surface energies of LaMnO_3.00 predicted elsewhere by theoretical methods, probably due to the different oxygen content and possibly more complex surface structure and exposed surface planes. The measured surface energy of LaMnO_3+δ lies between the values reported recently for BaTiO₃ and PbTiO₃ and close to the reported values for MnO₂. This suggests that LaMnO_3+δ surface is predominantly MnO₂‐terminated, in line with the trends predicted by theoretical calculations.     

Application of Cs salt of 12-tungstophosphoric acid supported on SBA-15 mesoporous silica in NO x storage

Cs salt of 12-tungstophosphoric acid (HPW) was deposited simultaneously at the external surface of the SBA-15 silica microcrystals and inside its mesoporous channels at loading of 60 wt% and Cs/W ratio in the range between 0.9 and 2, followed by impregnation of 1 wt% Pt. The performance of the Pt/CsHPW/SBA-15 composite materials was tested in the NO _x storage. The optimal NO _x storage capacity and efficiency were achieved at Cs/W of 1.5. The dispersion of CsHPW on SBA-15 led to a significant decrease of its crystal size (5–13 nm) compared with bulk HPW and HPW supported on titania (28–29 nm). Pt/CsHPW/SBA-15 displayed lower NO _x absorption capacity but much higher absorption and desorption efficiency than the reference Pt/HPW and Pt/HPW/TiO₂ materials. Consequently, Pt/CsHPW/SBA-15 displayed a better performance in short lean (2 min)—rich (1 min) absorption-desorption cycles. The novel Pt/CsHPW/SBA-15 nanocomposites presents the basis for improved storage material for NO _x removal from lean exhaust gases in highly dynamic aftertreatment technologies.     

Deep desulfurization of diesel fuels: kinetic modeling of model compounds in trickle-bed

Five catalysts with different hydrodesulfurization (HDS) and hydrogenation activity were tested in HDS of fresh crude heavy atmospheric gas oil (HAGO) (1.33 wt% S), two partially hydrotreated HAGO (1100 and 115 ppm S) and two model compounds, dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT), dissolved in model solvents and HAGO. Aromatic compounds in the liquid decreased significantly the HDS rate of 4,6-DMDBT, especially for catalysts with high hydrogenation activity. H₂S displayed a similar inhibition effect with all catalysts. These effects were extremely pronounced in HAGO where the DBT HDS rate decreased by a factor of 10 while 4,6-DMDBT – of 20 relative to paraffinic solvent. The feasibility of using a highly active hydrogenation catalyst for deep HDS of HAGO is diminished by the strong impact of aromatics.     

Hydrodearomatization of petroleum fuel fractions on silica supported Ni-W sulphide with increased stacking number of the WS2 phase

Ni-W catalysts supported on commercial γ-alumina and silica displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni-W/SiO₂ catalyst in toluene hydrogenation (HYD) was 6 times higher compared with Ni-W/Al₂O₃. The dearomatization performance of Ni-W/SiO₂ catalyst was tested over a wide range of operation conditions with naphtha and middle distillates. 90% saturation of aromatics in FCC naphtha (340 °C, LHSV of 1 h⁻¹) and 50% in Light cyclic oil (LCO) (360 °C, LHSV of 1 h⁻¹) was achieved at 5.4 MPa. In a two stage process with the same Ni-W/SiO₂ and intermediate separation of hydrogen sulphide 90% saturation of aromatics in LCO was achieved at 320 °C and total LHSV of 0.5 h⁻¹. At equal conditions, Ni-W/Al₂O₃ catalyst yielded 1.5-4 times lower total aromatics saturation.     

NO Oxidation Kinetics on Iron Zeolites: Influence of Framework Type and Iron Speciation

Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized using nitrogen adsorption, FTIR and DR UV–Vis–NIR spectroscopy. The catalytic activity in NO oxidation and the occurrence of NO _x adsorption was determined in a fixed-bed mini reactor using gas mixtures containing oxygen and water in addition to NO and NO₂ and temperatures of 200–350 °C. Under these reaction conditions, the NO _x adsorption capacity of these iron zeolites was negligible. The kinetic data could be fitted with a LHHW rate expression assuming a surface reaction between adsorbed NO and adsorbed O₂. The kinetic analysis revealed the occurrence of strong reaction inhibition by adsorbed NO₂. FER and MFI zeolites were more active than *BEA type zeolite. MFI zeolite is most active but suffers most from NO₂ inhibition of the reaction rate. FTIR and UV–Vis spectra suggest that isolated Fe³⁺ cations and binuclear Fe³⁺ complexes are active NO oxidation sites. Compared to the isolated Fe³⁺ species, the binuclear complexes abundantly present in the MFI zeolite seem to be most sensitive to poisoning by NO₂.     

Effect of SBA-15 microporosity on the inserted TiO2 crystal size determined by Raman spectroscopy

The effect of SBA-15 microporosity on the crystal size of TiO₂ was investigated employing SBA-15 materials with high (SBA-15-HM) and low (SBA-15-LM) microporosities (14.2% and 4.7% of microporous volume, respectively). TiO₂ phase was incorporated inside SBA-15 using internal hydrolysis method over a wide range of loadings (7–63 wt%). At all loadings, TiO₂ inside SBA-15 pores was in the form of anatase nanocrystals as found in characteristic Raman spectra. The crystal size of TiO₂ anatase phase was determined by Raman spectroscopy using a correlation between Raman peak position and peak width and TiO₂ crystal size. The correlation was established based on the set of unsupported TiO₂ samples with the crystals size in the range 5–120 nm (BET and XRD). Using this correlation, it was found that the crystal size of TiO₂ inside SBA-15 with high microporosity was lower than inside SBA-15 with low microporosity. This is a direct proof of the effect of wall microporosity on the dispersion of TiO₂ inside SBA-15. Due to the higher TiO₂ dispersion, TiO₂/SBA-15-HM adsorbed more vanadia than TiO₂/SBA-15-LM at the same TiO₂ loadings. As a result, V₂O₅/TiO₂/SBA-15-HM displayed higher activity than V₂O₅/TiO₂/SBA-15-LM in NO SCR with ammonia.