Oxygen Storage Behavior of Ceria–Zirconia-Based Catalysts in the Presence of SO2

This paper investigates the effect of the presence of SO₂ in the dynamics of oxygen storage on ceria and ceria–zirconia. The introduction of SO₂ 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 SO₂, 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 O₂ (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.     

Effect of Gas Atmosphere on Catalytic Behaviour of Zirconia, Ceria and Ceria–Zirconia Catalysts in Valeric Acid Ketonization

Ketonization of valeric acid, which can be obtained by lignocellulosic biomass conversion, was carried out in a fixed bed flow reactor over ZrO₂, 5–20 % CeO₂/ZrO₂ and CeO₂ both under hydrogen and nitrogen stream at 628 K and atmospheric pressure. Regardless gas-carrier 10 wt% CeO₂/ZrO₂ was found to show higher catalytic activity compared to zirconia per se as well as other ceria modified zirconia while ceria per se exhibited very low catalytic activity. All catalysts provided higher acid conversion in H₂ than in N₂ whereas selectivity to 5-nonanone was insensitive to gas atmosphere. XRD, FTIR, UV–Vis DRS, XPS, HRTEM methods were applied to characterize catalysts in reduced and unreduced states simulating corresponding reaction conditions during acid ketonization. XRD did not reveal any changes in zirconia and ceria/zirconia lattice parameters as well as crystalline phase depending on gas atmosphere while insertion of ceria in zirconia caused notable increase in lattice parameter indicating some distortion of crystalline structure. According to XPS, FTIR and UV–Vis methods, the carrier gas was found to affect catalyst surface composition leading to alteration in Lewis acid sites ratio. Appearance of Zr³⁺ cations was observed on the ZrO₂ surface after hydrogen pretreatment whereas only Zr⁴⁺ cations were determined using nitrogen as a gas-carrier. These changes of catalyst’s surface cation composition affected corresponding activity in ketonization probably being crucial for reaction mechanism involving metal cations catalytic centers for acid adsorption and COO^? stabilization at the initial step.     

Study of the Reducibility of Pt or Pd on Ceria–Zirconia Catalysts by XANES Measured at the Ce LIII Edge and Magnetic Susceptibility Measurements

The reduction behaviour of Ce_xZr_1–xO₂ and Pt or Pd supported on Ce_xZr_1–xO₂ was investigated by means of XANES measured at the Ce L_III edge and magnetic susceptibility measurements. Both techniques agree quite closely for the materials studied. The presence of a noble metal greatly improves the reduction of the Ce_xZr_1–xO₂ support at low temperatures. The reoxidation that occurred after evacuation of the reduced solids and heating to 500°C suggests that it is linked to a reversible surface reduction process of the sample.     

Ceria and Gold/Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons: An In Situ DRIFTS Study

Gold catalysts supported on nano-crystalline ceria prepared by deposition precipitation have been characterised and tested for the total oxidation of naphthalene. Two different precipitation methods were used to prepare the nano-crystalline ceria supports and it was observed that although both supports were active materials for naphthalene oxidation, ceria synthesized by homogeneous precipitation with urea was markedly more active than CeO₂ precipitated by carbonate. The addition of gold to both active CeO₂ catalysts resulted in different effects for the total oxidation of naphthalene. Gold addition promotes the naphthalene conversion to CO₂ when ceria is prepared by precipitation with carbonates, whilst the light off temperature is shifted towards higher temperatures when gold is added to ceria synthesized by the urea method. This behaviour has been related to a change in the support characteristics and a removal of the carbonate surface species, when gold is deposited onto the ceria support.     

Fischer–Tropsch Synthesis: Studies on the Effect of Support Doping with Si,Mn and Cr on the Selectivity to Alcohols in Ceria Supported Cobalt Catalysts

Mn and Cr doped CeSi mixed oxides were used as supports for Co and tested for CO hydrogenation. Co/CeSi was found to be more active and significantly more selective to n-alcohols/olefins. An increasing selectivity to n-alcohols and decreasing selectivity to olefins as a function of time on stream was also observed, suggesting a trade-off between those two products. Addition of Mn led to similar behavior, although at slightly lower conversions. Addition of Cr, however, considerably suppressed n-alcohol formation, while it kept selectivities to olefins within a 20–30 % range over more than 250 h of testing, indicating either higher alcohol dehydration activity, or that the presence of Cr ions lowered the hydrogenating activity of Co. The present work indicates that enhanced contact area between Co and the reducible support is likely a key factor for enhancing selectivity to alcohols.     

Comparative Study of Approaches based on the Taguchi and ANOVA for Optimising the Leaching of Copper–Cobalt Flotation Tailings

Approaches based on Taguchi and analysis of variance (ANOVA) methods have been proposed for studying the leaching of copper–cobalt flotation tailings. Taguchi method was used to determine the optimum leaching conditions as well the most influential operating parameters. ANOVA was used to determine the relationship between the experimental conditions and yield levels, as well as to define the significances of parameters on the leaching yields. An L₂₅(5⁵) orthogonal array experimental plan was used to assess the effects of initial acidity (25, 50 , 75, 100, and 125?g/L), leaching time (30, 60, 90, 120, and 150?min), temperature (25, 35, 45, 55, and 65°C), pulp density (S/L?=?10, 15, 20, 25, and 30 solid percent), and ferrous iron concentration (0.0, 1.0, 2.5, 5.0, and 7.5?g/L) on individual dissolution of Cu and Co. Under the optimum conditions (50?g/L, 60?min, 45°C, 15%, 2.5?g/L for Cu; and 100?g/L, 60?min, 65°C, 15% and 2.5?g/L for Co), leaching yields were 95.98% Cu and 97.74% Co. pH and Fe²⁺ were found to be key influential parameters of Cu and Co, respectively.     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.     

How Far is the Concept of Isolated Active Sites Valid in Solid Catalysts?

Oxide catalysts that have open structures and contain cations exhibiting usually just one or a few oxidation states, and those that are close-packed and contain transition metal ions in many different oxidation states are contrasted from the viewpoint of the concept of site isolation in heterogeneous catalysts. Much valuable progress can be made in this way to design new solid catalysts that are viable for the production of a wide range of compounds such as bulk or commodity chemicals (adipic acid and caprolactam), builder molecules (epoxides and lactones), as well as some fine chemicals and vitamins. The close-packed, multiple-valence oxide structures are also of enormous importance in the context of commercial manufacture of such vital commodities as acrolein, acrylic acid and acrylonitrile. It is thought that new techniques are still needed (to supplement the powerful insights gained from electron microscopy and other approaches) to unravel the mechanistic details involved in the operation of the Mars–van Krevelen (MVK) way of effecting catalytic selective oxidations. It has recently transpired that the MVK mechanism operates when certain metal catalysts (e.g. Pt, Pd) effect selective oxidations at pressures and temperatures under which they are used commercially.     

XAFS and XRD Analysis of Ceria–Zirconia Oxygen Storage Promoters for Automotive Catalysts

Three types of CeO₂–ZrO₂ (Ce:Zr = 1:1 molar ratio) compounds with different oxygen storage/release capacity (OSC) were characterized by means of the Ce K-edge and Zr K-edge XAFS. In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO₂–ZrO₂ solid solution, the OSC performance increased. Additionally, from the XRD analysis, the homogeneous CeO₂–ZrO₂ solid solution has an ordered cation arrangement, and exhibits the highest OSC. The crystal structure of this CeO₂–ZrO₂ solid solution is usually termed as “κ-CeZrO₄ phase”. However, the OSC performance of κ-CeZrO₄ degrades upon a high-temperature treatment under an oxidative atmosphere. The fresh κ-CeZrO₄ was aged at 973, 1,273 and 1,473 K under an oxidative atmosphere, respectively. The OSC performance deteriorated as: the fresh sample ≈973 > 1,273 > 1,473 K-aged samples. We also found that, if the temperature was beyond 1,273 K, the Ce/Zr ordered arrangement would collapse and the local structure around Ce and Zr ions would also changed remarkably. These results indicated that OSC was strongly dependent on its atomic structure.     

Study on the reactivity of ultrafine cerium–molybdenum oxide particles in the absence of molecular oxygen

The cracking, hydrocracking and hydrodesulfurization activities of gallium–aluminum mixed oxide catalysts, with fluoride added as promoter, were investigated. The addition of fluoride to the Ni–Mo catalyst supported on the mixed oxides leads to a significant enhancement in the cracking (no metals) and the hydrocracking (Ni–Mo) reactions of the catalysts, respectively. This enhancement of activity is attributed to the promotional effect of fluoride and the presence of gallium in the support, which leads to catalysts that are more resistant to deactivation. The results of the hydrodesulfurization experiments are not so encouraging. The activity of the catalysts was depressed by the addition of fluoride and with increasing gallium oxide content of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Influence of Copper–Copper Interaction on the Structure and Applications of a Metal–Organic Framework Based on Cyanide and 3-Chloropyridine

The structure of the new metal–organic framework, [(CuCN)₂·(3-Clpy)], 1, was characterized by IR, UV–visible, TGA and X-ray single crystal analysis. The structure of 1 consists of CuCN building blocks, which are connected by CN group to form two different chains; one puckered chain with TP-3 geometry around Cu(1) and a linear Cu(2)(CN)₂ chain. The two kinds of chains are bonded by cuprophilic interactions creating a 3D-network. The network structure of 1 is further close-backed by π–π stacking and hydrogen bonds. The electronic absorption and emission spectra as well as the thermodynamic parameters from TGA of the MOF 1 are discussed. MOF 1 was used as an effective heterogeneous catalyst for the oxidative discoloration of methylene blue dye by dilute solution of hydrogen peroxide as the oxidant. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was used to determine the in vitro antitumor activity of MOF 1 on human breast cancer cell line, MCF7.