Applied Catalysis B, Environmental (v.105, #3-4)

Contents (CO4).

Effective catalytic reduction of Cr(VI) over TiO2 nanotube supported Pd catalysts by Huan Chen; Yun Shao; Zhaoyi Xu; Haiqin Wan; Yuqiu Wan; Shourong Zheng; Dongqiang Zhu (255-262).
Display Omitted► Cr(VI) can be effectively reduced over supported Pd catalysts by catalytic hydrogenation. ► TiO2 nanotube (TNT) has higher surface acidity than TiO2. ► Pd particles are effectively dispersed on TNT. ► Pd/TNT exhibits substantially higher catalytic activity than Pd/TiO2. ► Cr(VI) reduction rate decreases with the increase of initial Cr(VI) concentration and pH.Cr(VI) is a commonly identified heavy metal pollutant in the aquatic environment. In the present study, TiO2 nanotube (TNT) supported Pd (Pd/TNT) and commercial TiO2 (P25) supported Pd (Pd/TiO2) catalysts were prepared using the photo-deposition method and Cr(VI) reduction by catalytic hydrogenation over the catalysts were investigated. Compared with P25, TNT had much higher specific surface area and surface acidity, resulting in higher Pd dispersion on the catalyst support. Given a Pd loading amount of approximately 2.0 wt.%, complete removal of Cr(VI) at an initial concentration of 1.92 mM by catalytic hydrogenation over Pd/TNT and Pd/TiO2 was achieved in 30 min and 50 min, respectively. The higher catalytic activity of Pd/TNT was attributed to the combined effect of higher Pd dispersion and higher surface acidity on TNT. Increasing the initial concentration of Cr(VI) or pH suppressed the hydrogenation reduction due to the lowered availability of the support acid sites by Cr(VI) adsorption or deprotonation. The findings indicate that catalytic hydrogenation over Pd/TNT is a promising technique to reduce Cr(VI) in aqueous solutions.
Keywords: Catalytic hydrogenation; Cr(VI) reduction; TiO2 nanotube; Supported Pd catalyst;

Design of active and stable NiCeO2ZrO2MgAl2O4 dry reforming catalysts by Steven Corthals; Joris Van Nederkassel; Hendrik De Winne; Jan Geboers; Pierre Jacobs; Bert Sels (263-275).
. Active NiCeO2ZrO2MgAl2O4 catalysts are developed for dry reforming in severe coking conditions using a D-optimal design. Experimentally verified models (activity, selectivity, coke amounts) were used to predict optimal catalyst compositions.Display Omitted► Discovery of new NiCeO2ZrO2MgAl2O4 catalyst compositions for stable dry reforming of methane in industrially relevant conditions. ► D-optimal design was used to model the catalytic performance (in terms of coke formation, selectivity and conversion) as function of the catalyst composition. ► Composition areas were predicted for stable and active reforming of methane with CO2 using a well-considered objective function. ► The importance of the selection of the proper objective function and unit for the amount of coke is emphasized.An active and stable NiCeO2ZrO2MgAl2O4 catalyst for dry reforming in severe coking conditions (CO2/CH4  = 1.1, GHSV = 32,000 h−1, 670 °C and 1.5 bar) was developed by combining conventional and combinatorial research strategies using high-throughput experimentation. A D-optimal design was applied to setup statistically verified models for catalytic activity ( X C H 4 , X C O 2 ) , selectivity (H2/CO molar ratio) and stability (coke resistance) as function of the selected input factors, viz. the amount of active element and amount of promoters. The importance of selecting the proper objective function R and appropriate unit to express the amount of coke deposited are discussed in detail. Via the most reliable objective function R an optimal catalyst composition area is obtained, where catalysts combine high activity with a long service lifetime. It was recognized that stable dry reforming occurs only with a subtle balance of the catalyst composition. Best dry reforming performances are achieved with catalyst compositions having low Ni (2 wt%), ZrO2 (<1 wt%) and high CeO2 (>3 wt%) content.
Keywords: Dry reforming; CH4/CO2 reforming; Carbon deposition; NiCeO2ZrO2MgAl2O4; D-optimal design;

Oxidative reforming of diesel fuel over LaCoO3 perovskite derived catalysts: Influence of perovskite synthesis method on catalyst properties and performance by J.A. Villoria; M.C. Alvarez-Galvan; S.M. Al-Zahrani; P. Palmisano; S. Specchia; V. Specchia; J.L.G. Fierro; R.M. Navarro (276-288).
Display Omitted► Different physicochemical properties of LaCoO3 catalyst precursors depending on the synthesis method. ► Physicochemical properties of LaCoO3 precursors affect the structure of the catalysts derived after thermal reduction. ► Higher initial activity in diesel reforming on catalysts with La2CoO4 after thermal reduction of LaCoO3. ► Long-term activity depends on the extension of Co0 and La2O2CO3 phases developed under reaction.Oxidative reforming of diesel fuel was studied over Co/La2O3 catalysts derived from LaCoO3 perovskite precursors synthesized by co-precipitation (COP), sol–gel (PEC) and combustion (SCS) methods. Physical–chemical characterization of perovskite precursors by N2-adsorption isotherms, Hg-intrusion porosimetry, XRD, XPS, TPR and SEM showed that the method of preparation produced changes in the porosity and homogeneity both at bulk and surface levels of the LaCoO3 perovskite precursors. The perovskite prepared by the SCS method achieved a higher development of the porous network as well as higher homogeneity in bulk and surface compared to COP and PEC counterparts. By contrast, the PEC and COP methods produced perovskites with lower porosity and with the presence of some secondary phases such as Co3O4 and La(OH)3. The modifications of the characteristics of LaCoO3 perovskites directly affected the structure and morphology of the catalytic materials derived from the thermal pre-treatment of perovskites before the activity tests. The differences in catalyst characteristics resulted from a different reduction and interaction between the gaseous reducing stream and the initial LaCoO3 perovskites which occurred during the thermal pre-treatment. The activity of the samples at the beginning of the reaction test followed the order: PEC > SCS > COP. The latter could be related to the different interaction of the catalysts with the reactants as well as with the larger presence of the La2CoO4 phase produced during the thermal pre-treatment. The evolution of the catalysts for long times on stream resulted in the activity order: SCS > PEC > COP. Characterization of used samples disclosed the key role of the Co0 exposure on the catalyst surface as concerns the achieved catalytic activities and the extent of the La2O2CO3 phase on catalysts as regards the inhibition of coke deposition.
Keywords: Hydrogen production; Diesel fuel; Oxidative reforming; Perovskites; LaCoO3;

Novel quantum dots (QDs) sensitized CdS-Bi2WO6 photocatalysts were prepared by chemical hydrothermal approach for the first time. The DRS results revealed that CdS-Bi2WO6 samples have a red shift and stronger absorption in the visible light region. After being sensitized by CdS QDs, the Bi2WO6 samples showed the high efficiency for the degradation of methyl orange. The enhanced photocatalytic performance could be attributed to the synergic effect between CdS QDs and Bi2WO6, which promoted the migration efficiency of photo-generated carriers.Display Omitted► The novel CdS QDs sensitized Bi2WO6 photocatalyst were prepared for the first time. ► The CdS-Bi2WO6 composites showed enhanced photocatalytic activity. ► The arrangement of CdS QDs on the surface of Bi2WO6 was revealed by HRTEM. ► Photodegraded mechanism of CdS-Bi2WO6 was proposed based on energy band positions. ► The mechanism was confirmed by PL spectra and photocatalytic results.Photocatalytic active Bi2WO6 sensitized by CdS quantum dots (QDs) was synthesized via a chemical hydrothermal approach for the first time. The QDs sensitized CdS-Bi2WO6 photocatalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis absorption spectroscopy (UV–vis) and photoluminescence spectra (PL). The photocatalytic activities of QDs sensitized CdS-Bi2WO6 samples were investigated based on the decomposition of methyl orange and phenol in aqueous solution under visible light irradiation (λ  > 420 nm). The DRS results revealed that CdS-Bi2WO6 samples have a red shift and stronger absorption in the visible light region. After being sensitized by CdS QDs, the Bi2WO6 samples showed the high efficiency for the degradation of methyl orange. The enhanced photocatalytic performance could be attributed to the synergic effect between CdS QDs and Bi2WO6, which promoted the migration efficiency of photo-generated carriers. According to experimental results, the possible photocatalytic mechanism of the CdS-Bi2WO6 photocatalyst was proposed in order to guide the further improvement of its photocatalytic activity.
Keywords: Quantum dots; Photocatalysis; CdS-Bi2WO6; Functional; Semiconductors;

. The schematic figure shows the relation between effects generated by crystal growth and surface hydroxylation in TiO2 photoactivity synthesized by oxidant peroxo-based method.Display Omitted► Changes in synthesis pH causes morphological variation in TiO2 nanocrystals. ► Photoactivity of TiO2 is directly related to the crystal morphology. ► Surface hydroxylation has a positive effect on photocatalysis. ► Crystal growth by oriented attachment has a negative effect on photocatalysis. ► Synthesis pH around 8 generates materials with higher photoactivity by OPM route.Morphologically different TiO2 nanocrystals were prepared by oxidant-peroxo method (OPM) synthesis. Morphological characterization was performed with the use of field emission gun-scanning electron microscope, X-ray diffraction and low temperature N2 adsorption. The photocatalytic behavior of synthesized TiO2 powders was studied in the reaction of Rhodamine B photodegradation in water. The trials showed that synthesis performed at pH value around 8 generated materials with higher photoactivities than at other pH values. Zeta potential, Fourier transform infrared spectroscopy, X-ray absorption near edge structure part of the absorption spectra, photoluminescence spectroscopy and high resolution-transmission electron microscopy measurements were performed to investigate why the materials had different photoactivities. Two dominant particle characteristics, which came from synthesis environment, interfered in the photoactivity of materials. These characteristics were surface hydroxylation, which had a positive effect on photocatalysis, and crystalline defects due to crystal growth, which had a deleterious effect on photocatalysis. Both characteristics increased with an increase in synthesis pH value. Dye degradation mechanism in water, an oxidation by hydroxyl radicals, was used to explain the different photoactivities and the optimum synthesis pH.
Keywords: Titanium dioxide; Morphology; Photocatalytic activity;

Role of metal components in Pd–Cu bimetallic catalysts supported on CeO2 for the oxygen-enhanced water gas shift by Junichiro Kugai; Jeffrey T. Miller; Neng Guo; Chunshan Song (306-316).
Display Omitted► Oxygen-enhanced water gas shift (OWGS) is more effective than WGS. ► Among various bimetallic catalysts, Pd–Cu on CeO2 showed uniquely high activity. ► H2 production over Pd–Cu is enhanced more by increasing Pd loading. ► Cu in Pd–Cu suppresses H2 oxidation in OWGS and facilitates CO2 desorption. ► Pd and Cu form alloy on CeO2 and act in concert to promote CO shift with H2O.Catalytic hydrogen production and CO removal in a post-reforming process are critical for low-temperature fuel cell applications. The present study aims at clarifying the role of metal components in bimetallic catalysts for oxygen-enhanced water gas shift (OWGS), wherein a small amount of O2 is added to H2-rich reformate gas to enhance CO shift. Among CeO2-supported bimetallic catalysts, Pd–Cu and Pt–Cu combinations were found to show strong synergetic promoting effect in OWGS, which leads to much higher CO conversion and higher H2 yield than WGS at low temperature around 250 °C. Temperature programmed reduction (TPR) showed strong interaction between Pd and Cu in Pd–Cu/CeO2 by a single reduction peak in contrast to multiple peaks on monometallic Cu/CeO2. Extended X-ray absorption fine structure (EXAFS) analysis revealed that such bimetallic Pd–Cu and Pt–Cu form alloy nanoparticles, where noble metal is mainly surrounded by Cu atoms. Oxygen storage capacity (OSC) measurements point to higher resistance of Pd–Cu to oxidation indicating that Pd keeps Cu in reduced state in air pulse condition. From kinetic study, Pd in Pd–Cu was found to promote CO shift, rather than CO oxidation by increasing the number of active sites and by suppressing H2 activation (that is inherent to monometallic Pd), which minimizes both the inhibition effect of H2 and the loss of H2 by oxidation in OWGS. Transient response technique revealed that Cu in Pd–Cu enhances desorption of strongly chemisorbed CO2 on catalyst surface in contrast to very slow CO2 desorption from surface of monometallic Pd. Thus, the excellent OWGS activity of Pd–Cu catalyst has been attributed to the complementary roles of the two metals for enhancing CO shift, which is realized by its alloy structure and the accompanying strong interaction between metal components.
Keywords: Oxygen-enhanced water gas shift (OWGS); Water gas shift (WGS); Pd; Cu; Metal catalyst; CeO2-supported Pd–Cu; Bimetallic catalysts;

Display Omitted► Photoreduction of 4-Nitrophenol was carried out under the visible irradiation. This involves metal-free heterogeneous photocatalyst: resin supported Eosin Y. Catalyst is highly light responsive, can be reused for this photoreduction process. Interestingly, the borohydride used here does not act as a usual hydride donor. Here, borohydride acts as a single electron-hydrogen atom transferring agent.Reduction of 4-nitrophenol (4-NP) with NaBH4 in the aqueous medium is a kinetically inert reaction, and generally the reaction occurs in the presence of metal/metal oxide nanoparticles. In the present study, the reduction of 4-nitrophenol (4-NP) has been carried out photocatalytically using the visible irradiation under a metal-free (eco-friendly) condition. By systematic screening, Eosin Y loaded resin is identified to be efficient in the reduction of 4-NP under the visible light. The effectiveness of the catalytic process depends upon the adsorption and the electron transfer. The adsorption favours the reaction molecules to come closer on the resin surface, and then reduction occurs by transfer of electrons. Gradual decrease in the absorbance at 400 nm confirms the decrease in concentration of the 4-NP. Simultaneously, there is an increase in absorbance at 300 nm infers the formation of 4-aminophenol (4-AP) during the photolysis. Results obtained from the irradiation on–off experiment suggests that the visible light irradiation is essential for the above processes, and also it indicates the sensitivity of the photocatalyst. The optimized reaction condition had been arrived from the results obtained after a series of experiments, including variation of catalytic dose, concentrations of 4-NP and the reducing agent (NaBH4). The rate of the reaction is found to be pseudo first order kinetics, and the rate constant (k) value is 6.90 × 10−2  min−1, under the experimental condition. From the control experiments, it is identified that the photocatalyst is primarily serving as an electron carrier in the reaction mechanism.
Keywords: Photocatalysis; Dyes; Metal-free reduction; 4-Nitrophenol; Visible irradiation;

Monoclinic BiVO4 with multiple morphologies and/or porous structures derived alcoho-hydrothermally with dodecylamine, oleylamine or oleic acid as surfactant show excellent visible-light-driven photocatalytic performance for phenol removal.Display Omitted► BiVO4 with various shapes and/or porous structures are obtained alcoho-hydrothermally. ► pH value and surfactant have a great effect on the shape and pore structure of BiVO4. ► Porous olive-like monoclinic BiVO4 performs the best for phenol photodegradation. ► Surface area, oxygen vacancy, bandgap energy, and pore structure govern photocatalytic activity.Bismuth vanadates with multiple morphologies and/or porous structures were prepared using the alcoho-hydrothermal strategy with bismuth nitrate and ammonium metavanadate as metal source, NaOH as pH adjustor, ethanol and ethylene glycol as solvent, and/or dodecylamine (DA), oleylamine (OL) or oleic acid (OA) as surfactant. The materials were characterized by means of the XRD, Raman, TGA/DSC, FT-IR, BET, SEM, TEM, XPS, and UV–vis techniques. The photocatalytic performance of the as-obtained samples was evaluated for the degradation of phenol in the presence of a small amount of H2O2 under visible-light irradiation, and the effect of phenol concentration on the photocatalytic activity was also examined. It is found that the surfactant and pH value had a significant influence on the particle morphology and even the crystalline structure of the product. Porous olive-like monoclinic BiVO4 samples could be prepared with DA, OL or OA as surfactant at pH = 1.5 or 3.0 and alcoho-hydrothermal temperature = 100 °C. With DA as surfactant at an alcoho-hydrothermal temperature of 100 °C, short-rod-like monoclinic BiVO4 and porous sheet-layered spherical orthorhombic Bi4V2O11 were obtained when the pH value of the precursor solution was raised to 7.0 and 11.0, respectively. Among the BiVO4 samples, the porous olive-like one with a surface area of 12.7 m2/g exhibited the best visible-light-driven photocatalytic performance for phenol degradation. It is concluded that the excellent photocatalytic activity of the porous olive-like BiVO4 sample was associated with its higher surface area and surface oxygen vacancy density, porous structure, lower bandgap energy, and unique morphology.
Keywords: Visible-light-driven photocatalyst; Porous bismuth vanadate; Surfactant-assisted alcoho-hydrothermal synthesis; Olive-like morphology; Phenol photodegradation;

Display Omitted► The flower-like ZnO was successfully prepared with or without ultrasonic assistance. ► A special dual-template directing growth mechanism for such ZnO is proposed. ► The novel photocatalytic activity of such ZnO is highly related to oxygen vacancies. ► Ultrasonic suppresses green emission and decreases the catalytic activity of ZnO.The flower-like ZnO materials were hydrothermally synthesized with or without ultrasonic assistance. When low dosage of citric acid is used, ultrasonic treatment can enhance the distribution and complexation of Zn ions; the formed complicated citrates can serve as soft template directing the formation of seaweed-like ZnO/Zn(OH)2 precursor; this precursor with citrates attaching on it serves as hard template for the formation of regular flower-like ZnO in the hydrothermal condition. The photoluminescence spectra have identified several kinds of defects in as-synthesized ZnO materials, such as oxygen vacancy (V o), zinc vacancy (V Zn) and interstitial zinc (Zni). The activity evaluation and kinetic calculation show that the samples with higher relative content of oxygen vacancy possess larger reaction rate constant and higher catalytic activity for Rhodamine B photodegradation. The employment of ultrasonic treatment during preparation decreases the relative content of oxygen vacancy, thus suppressing green emission and reducing the photocatalytic activity of such flower-like ZnO.
Keywords: Zinc oxide; Morphology; Ultrasonic; Photoluminescence; Photocatalysis; Mechanism;

Structure and redox properties of Co promoted Ni/Al2O3 catalysts for oxidative steam reforming of ethanol by S. Andonova; C.N. de Ávila; K. Arishtirova; J.M.C. Bueno; S. Damyanova (346-360).
Display Omitted► Reducibility of Co and Ni oxide species in alumina-supported CoNi catalysts increases with increasing the Co content. ► CoNi catalyst exhibits the best performance in oxidative steam reforming of ethanol. ► CoNi catalyst has a high resistance to oxidation of metallic Ni and Co particles during reaction.The effect of the Co addition on the structure and redox properties of Ni/γ-Al2O3 catalyst for oxidative steam reforming of ethanol (OSRE) was studied as a function of the CoO content (1–6 wt%). Systematically investigations during reduction process of the oxide samples under H2 atmosphere and the oxi-reduction behaviour of the reduced catalysts under reaction conditions of OSRE were performed via in situ temperature resolved X-ray absorption near structure (XANES) spectroscopy. The composition-dependent and temperature-dependent structural changes were analyzed by means of N2 adsorption–desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (TPR). The TPR and in situ XANES-TPR analysis of the samples clearly indicated that the Co addition into the Ni/Al2O3 system has a considerable effect on the metal–support interaction, by assisting the formation of NiO and Co3O4 species weakly interacting with the alumina surface and decreasing the reduction temperature compared to that of the monometallic system. It was found that the reducibility of the metallic oxide species in bimetallic CoNi/Al2O3 catalysts increases with increasing the Co content. The bimetallic CoNi/Al2O3 catalysts exhibited a higher resistance to oxidation compared to that of the monometallic system in the reaction of OSRE by hindering the oxidation of the metallic Ni and Co particles on the surface. It was suggested that the synergetic effect between the Ni and Co components reflects to a significant extent on the stability and ethanol conversion of the bimetallic CoNi/Al2O3 catalysts toward higher H2 yields in the OSRE with time on stream.
Keywords: CoNi/γ-Al2O3 catalysts; Oxidative steam reforming of ethanol; Characterization; XANES;

Performance of alkali modified Pd/Mg(Al)O catalysts for hydrodechlorination of 1,2,4-trichlorobenzene by B.T. Meshesha; R.J. Chimentão; A.M. Segarra; J. Llorca; F. Medina; B. Coq; J.E. Sueiras (361-372).
TCB reaction rate expressed by TCB consumed (mmol min−1) per exposed Pd surface (m2) vs. number of basic sites (mmol of CO2 per g of the catalyst). The HDC activity is highly dependent in the intrinsic property of alkali metal species.Display Omitted► Alkali-Pd/HT catalysts are very active for the HDC reaction of 1,2,4-trichlorobenzene. ► The basic property and metal area of the catalysts control the catalytic behaviour. ► Hydrodechlorination reaction was strongly dependent on the type of alkaline used. ► The catalysts can be regenerated by reduction treatments.Alkali-modified Pd (wt%)/HT catalysts, where HT is calcined Mg/Al hydrotalcite (Mg(Al)O), were studied for the HDC reaction of 1,2,4-trichlorobenzene at mild conditions (atmospheric pressure and 373 K) using a continuous reactor. Characterization of the alkali-modified Pd/HT catalysts by H2-chemisorption, N2-physisorption, SEM, HRTEM and CO2-TPD analysis revealed different textural, metallic and basic properties depending on the type of alkaline metal and precursor. The hydrodechlorination reaction of 1,2,4-trichlorobenzene was strongly dependent on the type of alkaline additives (Li, Na or Cs) and alkaline precursors (OH or NO3 ). The basic properties of the catalysts showed a predominant role in activity, selectivity and stability of the catalysts. Furthermore the HDC catalytic activity is highly dependent in the intrinsic property of alkali metal species. The affinity of the chloride produced during the reaction to the strong basic alkali surface contributes for the higher stability of the alkali-modified catalyst. Calcination–reduction and reduction treatments were employed to regenerate the catalyst. The initial catalytic activity was recovered when the used sample was regenerated by reduction treatment.
Keywords: Hydrodechlorination; Pd; Pd/MgAlO x ; Mg/Al mixed oxide; Basic additives; Alkali metal; Alkali metal precursor; 1,2,4-Trichlorobenzene; Deactivation and regeneration; Chloride species;

Multi-component zirconia–titania mixed oxides: Catalytic materials with unprecedented performance in the selective catalytic reduction of NO x with NH3 after harsh hydrothermal ageing by Nathalie Marcotte; Bernard Coq; Claire Savill-Jovitt; Philippe Bichon; Romain Cavalier; Robert Durand; Virginie Harle; Rui Marques; Emmanuel Rohart (373-376).
Display Omitted► DeNO x catalysts of tomorrow in diesel exhaust gas treatment ► A new multi-component CeZrTiSiW mixed oxide catalyst for NO x abatement ► High efficiency of the catalyst for the selective catalytic reduction of NO x with NH3. ► A new SCR catalyst with the highest performances after harsh hydrothermal treatment.Standing harsh hydrothermal treatment (∼10% H2O, ∼1050 K) is a prerequisite for deNO x catalysts of tomorrow in diesel exhaust gas treatment. New catalytic materials composed of Mn-, Fe- and Ce-ZrTiSiW mixed oxide were evaluated in the selective catalytic reduction NO by NH3 at high gas hourly space velocity (330,000 mL g−1  h−1) and after high temperature hydrothermal treatment, 16 h at 1023 K in H2O/air (10/90). They were compared with a Fe-ZSM5 SCR catalyst treated in the same manner. CeZrTiSiW exhibits the best performance and keeps high NO conversion (>80%) from 630 to 800 K. It is stable and fully selective to N2 up to 823 K. It is more active and selective than Fe-ZSM5. The presence of NO2 in the feed (NO2/NO = 0.85) boosts the NO conversion by shifting the light-off temperature (50% NO conversion) from 560 K to 460 K.
Keywords: DeNO x ; Nitrogen oxide; Mixed oxide; Cerium; Zirconia; Selective catalytic reduction;