Redox chemistry over CeO2-based catalysts: SO2 reduction by CO or CH4 (original) (raw)
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Ceria-based catalysts for the recovery of elemental sulfur from SO2-laden gas streams
Catalysis Today, 2000
The catalytic reduction of SO 2 to elemental sulfur by CO or CH 4 over ceria-based catalysts under demanding operating conditions is reported in this paper. Cu-or Ni-containing ceria catalysts have been shown before to be highly active and selective for SO 2 reduction by CO in dry gas streams or in the presence of low amounts of H 2 O. In this work, the activity/selectivity of 10 at.%La-doped ceria, Ce(La)O x , and Cu-or Ni-containing Ce(La)O x for the (SO 2 + CO) reaction was tested in gas streams containing 10-45 mol%H 2 O at high space velocities (>80,000 h −1 ). The addition of 5 at.% (∼2.5 wt.%) copper or nickel significantly improved the low-temperature (<500 • C) reactivity of Ce(La)O x . This was correlated with the improved reducibility of ceria in the metal-ceria catalysts and the ensuing low-temperature activity for the water-gas-shift (WGS) reaction. The combined reduction of SO 2 and NO by CO was also studied in this work over the same catalysts. In dilute gas mixtures containing 0.1-1.0 mol%SO 2 and NO, stoichiometric amount of CO and in the presence of 40%H 2 O, the NO presence in the feed gas enhances both the SO 2 conversion and the elemental sulfur yield. At 550 • C, in the presence of NO, SO 2 conversion and sulfur yield over the 5%Ni-Ce(La)O x catalyst were 0.94 and 0.77, respectively, the NO conversion to N 2 was complete, and the CO 2 produced was the sum of the SO 2 and NO reduction reactions by CO. Ceria-based materials are also active for SO 2 reduction by methane to elemental sulfur at temperatures higher than 550 • C. The addition of Cu or Ni has a different effect on the sulfur selectivity of ceria under fuel-rich conditions. The Cu-ceria system is a complete oxidation catalyst to a much higher temperature than Ni-ceria. Over Ni-CeO 2 , dissociation of methane begins at <550 • C, and side reactions favor H 2 S production over elemental sulfur. However, the 5 at.%Ni-Ce(La)O x catalyst showed remarkable resistance to carbon deposition, both in the SO 2 -methane reaction as well as in partial oxidation of methane by O 2 to synthesis gas with a 2:1 H 2 :CO ratio. This is attributed to the high dispersion of nickel in this catalyst and the fast rate of oxygen supply from ceria to the nickel interface. Catalysts were characterized by temperature-programmed-reduction, XPS and STEM/EDS.
E-journal of Surface Science and Nanotechnology, 2019
In this work, various ternary cerium oxide/lanthanum oxide/cobalt oxide (Ce/La/Co) nanocatalysts were synthesized by co-precipitation method based on response surface methodology (RSM). The optimum predicted surface area was found to be 67.6 m 2 g −1 at calcination temperature of 650℃, La content of 10.0 wt%, and Co content of 8.0 wt%. Average crystal size of optimum ternary Ce/La/Co catalyst was estimated 11.4 nm. The confirmation tests revealed that experimental data can be predicted well by the model. Furthermore, the prepared catalysts were evaluated by Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) and NH3 temperature programmed desorption (NH3-TPD) analyses. The characterization results confirmed that ternary Ce/La/Co catalysts were successfully synthesized. Also, the NH3-TPD result showed that total active sites of optimum ternary Ce/La(10)/Co(8) catalyst with La content of 10.0 wt% and Co content of 8.0 wt% was greater than that of single cerium oxide catalyst. The optimum synthesized catalyst was tested for SO2 reaction by methane to sulfur. SO2 conversion and selectivity of catalysts at various temperatures were determined. The better performance of Ce/La/Co optimum catalyst at different temperatures was obtained for SO2 reduction. Also the selectivity of the optimum catalyst for production of sulfur is better than other catalysts.
Oxygen storage behavior of ceria–zirconia-based catalysts in the presence of SO2
This paper investigates the effect of the presence of SO 2 in the dynamics of oxygen storage on ceria and ceria-zirconia. The introduction of SO 2 under reaction conditions at T < 873 K negatively affects CO conversion under oscillating conditions on all the supports studied, owing to the formation of sulfate species. Deactivation is observed on all supports and activity is recovered only after desorption of SO 2 , which occurs at 950 < T < 1000 K, depending on catalyst composition (Ce/Zr ratio) and treatment atmosphere. The amount of sulfur adsorbed is higher over solid solutions, reaching a maximum with Ce x Zr 1−x O 2 (0.5 < x < 0.68). However this does not adversely affect activity compared to ceria. In the presence of Rh and Pd, reactivation is favored under reaction conditions. More generally, it appears that the removal of sulfates is facilitated in reductive atmospheres (both hydrogen and CO) over mixed oxides. No differences are observed following regeneration under oxidizing conditions.
Industrial & Engineering Chemistry Research, 2012
CeO 2 catalysts modified with earth alkaline metals (M = Mg, Ca, and Sr) were prepared by the citrate method in a M:Ce molar ratio of 1:1. Different analytical techniques including diffuse reflectance Ultraviolet−Visible (DR UV−vis) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature programmed desorption of CO 2 (TPD-CO 2) were used for the characterization of the catalysts. The materials were tested in the oxidative coupling of methane (OCM). The results of calcined samples revealed that the charge transfer from cerium to oxygen ions is shifted when CeO 2 is doped, indicating defects on the catalyst surface. A linear correlation was found between the amount of surface basic sites and the ratio of the oxygen species O 2 − and O 2 2− to lattice oxygen. This behavior is crucial for the CH 4 conversion and selectivity to C 2 H 6 and C 2 H 4 at 700°C. Ca-doped CeO 2 catalyst revealed the best performance in the OCM, which can be attributed to the similar ionic radii of Ca 2+ and Ce 4+ .
Appl Catal a Gen, 2005
Three series of CoMo catalysts (MoO 3 : 6, 11, 16 wt.%) with various Co/Mo molar ratios were prepared by successive incipient wetness impregnations of a titania sample previously prepared by the pH swing method, which provides a TiO 2 carrier with a high SSA (134 m 2 g À1) and excellent mechanical properties. DBT HDS activity of the catalysts increased with addition of cobalt up to Co/Mo = 0.4 and then decreased for higher ratios, irrespective of the Mo loading. The results of a [ 35 S]DBT HDS method showed that S 0 , the amount of labile sulfur atoms, increased in parallel with the activity when adding Co up to a molar ratio of 0.4. In contrast, unlike on CoMo/Al 2 O 3 catalysts, only a slight increase in k RE , the H 2 S release rate constant, was observed upon Co addition. This was due to formation of the TiMoS phase: while formation of Ti-S *-Mo bonds favorably induces an increase in sulfur mobility on Mo/TiO 2 catalysts, electronic density on Mo atoms increases, which limits the promoting effect of Co on mobility of S a sulfur atoms bridged between Mo and Co. Further, while the increase in HDS activity upon Co addition on a uniform MoS 2 monolayer on TiO 2 was in rather good agreement with the quantitative and kinetics data of the 35 S tracer method, over MoS 2 slabs on TiO 2 this increase was larger than that expected from the results of the 35 S tracer method. This was attributed to the presence of & * and & a CUS, i.e. catalytic sites that are not replenished with sulfur, at the steady state. A larger number of & * CUS was present at low Mo wt.% or for low Co/Mo. As these particular catalytic sites are in the form of CUS at the steady state, they are not accounted by the [ 35 S] radiotracer method. This explains the differences observed between the experimental DBT HDS promotion and the promotion deduced from the increase in k RE and S 0 .
NO reduction by C3H6 in excess oxygen over fresh and sulfated Pt‐ and Rh‐based catalysts
2000
The selective catalytic reduction (SCR) of NO with C 3 H 6 was studied over three noble-metal-based catalysts: 2% Pt/γ-Al 2 O 3 , 2% Rh/γ-Al 2 O 3 and 1.5% Rh/TiO 2 (4% WO 3 ). The SO 2 effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO 2 -containing feeds. Temperature-programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C 3 H 6 , respectively, in the absence or the presence of SO 2 . The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO 2 presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates.