Reactions of C5 and C6-sugars, cellulose, and lignocellulose under hot compressed water (HCW) in the presence of heterogeneous acid catalysts

The benefit of TiO₂, ZrO₂ and SO₄-ZrO₂ on the reactions of C₅-sugar (xylose), C₆-sugar (glucose), cellulose, and lignocellulose was studied in hot compressed water (HCW) at 473-673 K with an aim to produce furfural and 5-hydroxymethylfurfural (HMF). TiO₂ and SO₄-ZrO₂ were found to active for hydrolysis and dehydration reactions producing high furfural and HMF yields with less by-products (i.e. glucose, fructose, xylose, and 1,6-anhydroglucose (AHG)) formation, whereas ZrO₂ was highly active for isomerization reaction; thus significant amount of fructose was observed in the liquid product.Importantly, it was also found that the starting salt precursor, the sulfur-doping content (for SO₄-ZrO₂) and the calcination temperature strongly affected the catalyst reactivity. Catalysts prepared from the chloride-based precursors (i.e. ZrOCl₂ and TiCl₄) gained higher reactivity compared to those prepared from nitrate-based precursors (i.e. ZrO(NO₃)₂ and TiO(NO₃)₂) due to their greater acidity, according to the NH₃- and CO₂-TPD studies. For SO₄-ZrO₂, among the catalyst with sulfur contents of 0.75%, 1.8% and 2.5%, SO₄-ZrO₂ with 1.8% sulfur content presented the highest acidity and reactivity toward hydrolysis and dehydration reactions. It is noted that the suitable calcination temperature for all catalysts was at 773 K; the XRD patterns revealed that different portions of phase formation was observed over catalysts with different calcination temperatures i.e. anatase/rutile for TiO₂ and monoclinic/tetragonal for ZrO₂ and SO₄-ZrO₂; the portion of these phase formations obviously affected the acidity-basicity of catalyst and thus the catalyst reactivity.     

Influence of reaction products of K-getter fuel additives on commercial vanadia-based SCR catalysts: Part II. Simultaneous addition of KCl, Ca(OH)2, H3PO4 and H2SO4 in a hot flue gas at a SCR pilot-scale setup

A commercial V₂O₅–WO₃–TiO₂ corrugated-type SCR monolith has been exposed for 1000 h in a pilot-scale setup to a flue gas doped with KCl, Ca(OH)₂, H₃PO₄ and H₂SO₄ by spraying a water solution of the components into the hot flue gas. The mixture composition has been adjusted in order to have P/K and P/Ca ratios equal to 2 and 0.8, respectively. At these conditions, it is suggested that all the K released during biomass combustion gets captured in P–K–Ca particles and the Cl is released in the gas phase as HCl, thus limiting deposition and corrosion problems at the superheater exchangers during biomass combustion. Aerosol measurements carried out by using a SMPS and a low pressure cascade impactor have shown two distinct particle populations with volume-based mean diameters equal to 12 and 300 nm, respectively. The small particles have been associated to polyphosphoric acids formed by condensation of H₃PO₄, whereas the larger particles are due to P–K–Ca salts formed during evaporation of the water solution. No Cl has been found in the collected particles. During the initial 240 h of exposure, the catalyst element lost about 20% of its original activity. The deactivation then proceeded at slower rates, and after 1000 h the relative activity loss had increased to 25%. Different samples of the spent catalyst have been characterized after 453 h and at the end of the experiment by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH₃-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monolith have also been carried out. From the characterization, it was found that neither K nor Ca were able to penetrate the catalyst walls, but only accumulated on the outer surface. Poisoning by K has then been limited to the most outer catalyst surface and did not proceed at the fast rates known for KCl. This fact indicates that binding K in P–K–Ca compounds is an effective way to reduce the negative influence of alkali metals on the lifetime of the vanadia-based SCR catalysts. On the other hand, P-deposition was favoured by the formation of the polyphosphoric acids, and up to 1.8 wt% P was accumulated in the catalyst walls. Deactivation by polyphosphoric acids proceeded at about 0.2% day⁻¹. About 6–7% of the initial activity was lost due to the accumulation of these species. However, the measured relative activity reached a steady-state level during the last 240 h of exposure indicating that the P-concentration in the bulk reached a steady-state level due to the simultaneous hydrolysis of the polyphosphoric acids.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media. Part I: Bulk electrodes

We studied the borohydride oxidation reaction (BOR) by voltammetry for BH₄⁻ concentrations between 10⁻³ M and 0.1 M NaBH₄ in 0.1–1 M NaOH for bulk polycrystalline Pt, Ag and alloyed Pt–Ag electrocatalysts. In order to compare the different electrocatalysts, we measured the kinetic parameters and the number of electrons exchanged (faradic efficiency). BOR on bulk Pt is more efficient when the concentration of NaBH₄ increases (3e⁻ in 1 mM and 6e⁻ in 10 mM BH₄⁻/0.1 M NaOH). BOR on Pt can occur both in a direct pathway and in an indirect pathway including hydrogen generation via heterogeneous hydrolysis of BH₄⁻ and subsequent oxidation of its by-products (e.g. BH₃OH⁻ and H₂). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH (2e⁻ at pH 12.6 and 6e⁻ at pH 13.9 at 25 °C). The BOR kinetics is faster for Pt than for Ag (i_Pt=0.02 A cm⁻², i_Ag=1.4 10⁻⁷ A cm⁻² at E=−0.65 V vs. NHE in 1 mM NaBH₄/0.1 M NaOH, 25 °C) both as a result from Pt high activity regarding the BH₄⁻ heterogeneous hydrolysis and subsequent HOR, above −0.83 V vs. NHE and following direct oxidation of BH₄⁻ or BH₃OH⁻ below −0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH₄⁻ concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

Highly dispersed and active Pd nanoparticles over titania support through engineering oxygen vacancies and their anchoring effect

Rational synthesis of highly dispersed and active metal nanoparticles (NP) for application in catalysis has been a hot but challenging research topic. Herein, we report a facile strategy for dispersing and stabilizing noble metal NP in a reducible metal oxide support (e.g., TiO₂) through engineering oxygen vacancies and their anchoring effect. The pre-reduction of TiO₂ support produces oxygen vacancies, and Pd NP prefer to occupy these vacancies with a strong meal-support interaction and near metallic state. A near monatomic Pd NP distribution over anatase TiO₂ support is obtained with this strategy. Significant activity enhancement is demonstrated in both oxygen activation and formaldehyde oxidation reactions by this catalyst design method. H₂, NaBH₄ and HCHO are suitable reducing agent for TiO₂, the pre-reduction catalysts by H₂, NaBH₄ and HCHO shows highly activity for formaldehyde oxidation. Such a work demonstrates support pre-reduction strategy is a facile way to prepare powerful supported noble metal catalysts.     

Use of a nickel-boride–silica nanocomposite catalyst prepared by in-situ reduction for hydrogen production from hydrolysis of sodium borohydride

Hydrogen was produced by hydrolysis of sodium borohydride (NaBH₄) using nickel-boride–silica nanocomposite catalyst. The catalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometry (EDX). The Ni-B–silica nanocomposite catalyst was found to consist of amorphous Ni-B nanoparticles attached to the surface of amine-modified silica nanosphere. The kinetics of hydrolysis of NaBH₄ by Ni-B–silica composite catalyst was investigated. The effects of temperature, NaBH₄ concentration, and catalyst concentration on hydrogen generation were also investigated. A rate of hydrogen generation as high as 1916 ml H₂/min/g Ni was achieved by catalytic hydrolysis of NaBH₄. The stability of the composite catalyst was also explored.     

Preparation of hollow TiO2 nanoparticles through TiO2 deposition on polystyrene latex particles and characterizations of their structure and photocatalytic activity

In a mixed solvent of water and ethanol, polystyrene/titanium dioxide (PSt/TiO₂) composite particles of core-shell structure were prepared by hydrolysis of tetrabutyl titanate in the presence of cationic PSt particles or anionic PSt particles surface-treated using γ-aminopropyl triethoxysilane. Hollow TiO₂ particles were obtained through calcination of the PSt/TiO₂ core-shell particles to burn off the PSt core or through dissolution of the core by tetrahydrofuran (THF). An alternative process constituted of preheating the PSt/TiO₂ particles at 200°C to allow partial crystallization followed by calcination or PSt dissolution by THF. The outcome TiO₂ particles thus prepared were examined by TEM, and hollow TiO₂ particles were observed. The crystalline phase structure and phase transformation were characterized, which revealed that preheating before the removal of the PSt core was useful to achieve the desired hollow TiO₂ particles, and the calcination process was beneficial to the formation of anatase and rutile structures. The tests of TiO₂ particles as catalyst in the photodegradation of Rhodamine B demonstrated that a much higher catalytic activity was observed with the TiO₂ hollow particles prepared through calcination combined with preheating.     

Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems

This paper introduces and discusses the latest research on the use of H₂ generated via the NaBH₄ hydrolysis reaction for proton exchange membrane fuel cells (PEMFCs). To realize the NaBH₄–PEMFC system, many hydrolysis catalysts such as Ru/anion-exchange resins, Pt/LiCoO₂, Co powder/Ni foam, PtRu/LiCoO₂ and Ru/carbon have been proposed. Through these efforts, the hydrolysis reaction conversion approached 100%. In addition, the average H₂ generation rate based on most of the reports generally ranged from 0.1 to 2.8 H₂ l min^− 1 g^− 1 (catalyst), which produced a level of PEMFC performance equivalent to 0.1–0.3 kW g^− 1 (catalyst). However, it was also reported that the H₂ generation rate was 28 H₂ l min^− 1 g^− 1 (catalyst) with the catalyst of Pt/carbon (acetylene black).Considering these reports and the advantageous features of NaBH₄ hydrolysis, the NaBH₄–PEMFC system seems to be technologically feasible and would constitute an alternative system of supplying H₂ in fuel cells.However, some challenges remain, such as the deactivation of the catalyst, the treatment of the by-products, and the proper control of the reaction rate. In addition, if the price of NaBH₄ were to be further reduced, this system could become the most powerful competitor in portable application fields of PEMFC.     

One-step synthesis of platinum nanoparticles loaded in alginate bubbles

Composite particles with multifunctions have been extensively utilized for various applications. Bubble particles can be applied for ultrasound-mediated imaging, drug delivery, absorbers, cell culture, etc. This study proposes a one-step strategy to obtain Pt nanoparticles loaded in alginate bubbles. A needle-based droplet formation was used to generate uniform alginate particles about 2 mm in diameter. The hydrolysis reaction of NaBH₄ was utilized to produce gaseous hydrogen and then trapped within alginate particles to form bubbles. The Pt⁴⁺ mixed with alginate solution was dropped into the reservoir to react with reducing NaBH₄ and hardening CaCl₂ to form Pt nanoparticles-alginate composite bubbles. Results indicate that the size of bubbles decreases with the CaCl₂ concentration (1% ~ 20%), and size of bubbles increases with the NaBH₄ concentration (1 ~ 20 mM). The advantages for the present approach include low cost, easy operation, and effective production of Pt nanoparticles-alginate composite bubbles.     

A comparative study on the selective hydrogenation of α,β unsaturated aldehyde and ketone to unsaturated alcohols on Au supported catalysts

The hydrogenation of trans,4-phenyl,3-buten,2-one (benzalacetone) and trans,3-phenyl, propenal (cinnamaldehyde) was carried out on Au supported on iron oxides catalysts. Commercial goethite (FeOOH), maghemite (γFe₂O₃) and hematite (αFe₂O₃) were used as supports. The catalytic activity of Au/Fe₂O₃ reference catalyst, supplied by the World Gold Council, was also investigated. Gold catalysts and the parent supports were characterized by BET, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of ammonia (NH₃-TPD) and high resolution transmission electron microscopy (HRTEM).Among the catalysts investigated Au supported on FeOOH shows the highest activity and selectivity to UA in the hydrogenation of unsaturated carbonyl compounds whereas Au supported on αFe₂O₃ are the less active and selective catalysts.The catalytic activity and selectivity to unsaturated alcohols (UA) in the hydrogenation of benzalacetone and cinnamaldehyde are less influenced by the morphology of gold particles and are mainly influenced by the nature of the support.A correlation between the reducibility of the catalysts and the activity and selectivity to UA has been found. Increasing the reducibility of the catalysts both the activity and selectivity to UA increase. These results let us to argue that active and selective sites are formed by negative gold particles formed through the electron transfer from the reduced support to the metal.     

FeVO4 nanorods supported TiO2 as a superior catalyst for NH3–SCR reaction in a broad temperature range

In this work, a catalyst with FeVO₄ nanorods supported on TiO₂ was prepared and applied for NH₃–SCR reaction. A significantly enhanced low temperature catalytic activity has been achieved in the presence of 10% H₂O with the active window shifting by 15 °C to lower temperatures, compared to the classical catalyst with FeVO₄ nanoparticles supported TiO₂. For the catalyst containing FeVO₄ nanorods, enhanced redox ability and enriched surface active oxygen species originated from its unique crystal structure and predominantly exposed reactive crystal facets (− 2 1 0) on the catalyst surface are responsible for its improved low temperature catalytic activity.     

Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells

The electrochemical behaviour of sodium borohydride on a platinum electrode in the absence and presence of thiourea (TU) was investigated by cyclic voltammetry. In the absence of thiourea, several overlapping peaks associated with the hydrolysis of BH₄⁻ appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 to −0.50 V versus Ag/AgCl. As a consequence of secondary reactions, the borohydride oxidation in 3 M NaOH solution shows a four to six-electron process, according to its concentration, in direct fuel cells. A conveyable TU/NaBH₄ concentration ratio of 0.6 inhibits the delivery of hydrogen simultaneously with catalytic hydrolysis of BH₄⁻. Thus, the coulombic efficiency in direct fuel cell discharge was increased showing an about eight-electron process for the oxidation of BH₄⁻.     

Catalytic wet air oxidation of acetic acid over different ruthenium catalysts

Ruthenium catalysts were prepared by impregnation of different supports: ZrO₂, CeO₂, TiO₂, ZrO₂–CeO₂ and TiO₂–CeO₂. Their activities for acetic acid oxidation in aqueous solution were investigated in a stirred reactor at a reaction temperature of 200 °C and total pressure of 4 MPa. The order of the catalyst activity obtained was RuO₂/ZrO₂–CeO₂ > RuO₂/CeO₂ > RuO₂/TiO₂–CeO₂ > RuO₂/ZrO₂ > RuO₂/TiO₂, which corresponds to surface concentration of non-lattice oxygen (defect-oxide or hydroxyl-like group) of these catalysts. The non-lattice oxygen on the catalyst surface plays an important role in the catalytic activity.     

Electrochemical oxidation of boron containing compounds on titanium carbide and its implications to direct fuel cells

Electrochemical oxidation of sodium borohydride (NaBH₄) and ammonia borane (NH₃BH₃) (AB) have been studied on titanium carbide electrode. The oxidation is followed by using cyclic voltammetry, chronoamperometry and polarization measurements. A fuel cell with TiC as anode and 40 wt% Pt/C as cathode is constructed and the polarization behaviour is studied with NaBH₄ as anodic fuel and hydrogen peroxide as catholyte. A maximum power density of 65 mW cm⁻² at a load current density of 83 mA cm⁻² is obtained at 343 K in the case of borhydride-based fuel cell and a value of 85 mW cm⁻² at 105 mA cm⁻² is obtained in the case of AB-based fuel cell at 353 K.     

The synthesis and hydrolysis of dimethyl acetals catalyzed by sulfated metal oxides. An efficient method for protecting carbonyl groups

Sulfated metal oxides including SO₄ ^2–/ZrO₂, SO₄ ^2–/TiO₂, SO₄ ^2–/HfO₂, SO₄ ^2–/Fe₂O₃, SO₄ ^2–/SnO₂, and SO₄ ^2–/Al₂O₃ were highly efficient catalysts for the reaction of aldehydes and ketones with trimethyl orthoformate producing dimethyl acetals under mild reaction conditions. At room temperature, dimethyl acetal yields of 83–100% were obtained for the five carbonyl compounds chosen. These mesoporous solid acids also effectively catalyzed the hydrolysis of dimethyl acetal to regenerate the original carbonyl compounds in aqueous acetone. They not only provided an effective method for synthesizing dimethyl acetals of larger molecular size but also acted as a versatile catalyst for protecting and deprotecting carbonyl groups during organic synthesis.     

Mechanism of sulfur poisoning on supported noble metal catalyst — The adsorption and transformation of sulfur on palladium catalysts with different supports

A series of supported palladium catalysts (Pd/Al₂O₃, Pd/MgO and Pd/TiO₂) were prepared by the impregnating method and treated with H₂S, H₂ +O₂ or O₂, among which H₂S is used as a poison and H₂ +O₂ or O₂ are as purging atmospheres. The S^2– species in the supports was introduced by means of mechanically mixing Na₂S with the supports or catalysts. X-ray photoelectron spectroscopy (XPS) was employed to determine the changes in the chemical states of oxygen, palladium and sulfur in the catalysts before and after the treatment, while infrared (IR) spectroscopy was used to measure the SO^2– ₄ group produced in the catalysts and supports. The results show that on MgO and TiO₂ carriers whose acidities are weak, there exist two kinds of oxygen species, one is the lattice oxygen, the other one is the active species of oxygen. The latter can oxidize the S^2– into SO^2– ₄ even at room temperature in air. Because of the weak acidities and smaller specific surface area of MgO and TiO₂, the S^2– is liable to adsorb on the catalysts and to transform into SO^2– ₄. But for the case of Al₂O₃ support its acidity is rather strong, and its surface oxygen species under the experimental conditions is not so active as that in MgO and TiO₂ carries. The poison H₂S on the Al₂O₃ support only experiences a process of physical adsorption-desorption. In Pd/Al₂O₃ catalyst, the negatively charged sulfur ions are not so easily adsorbed and transformed as those in Pd/MgO and Pd/TiO₂. It is also implied that the properties of the carriers are related to the ability of self-regeneration of the corresponding catalysts. Pd/Al₂O₃ catalyst is more able to self-regenerate than Pd/MgO and Pd/TiO₂ catalyst.     

Hydrogen Production by Catalytic Hydrolysis of Sodium Borohydride with a Bimetallic Solid-State Co-Fe Complex Catalyst

The addition of the active non-noble metal species on a ligand can influence the catalytic performance of catalyst. In the present work, a new bi-metallic solid-state complex catalyst system, including 4-4’-methylene bis(2,6-diethyl) aniline-3,5-di-tert-butylsalisilaldimin ligand and Fe and Co metal salts are prepared for hydrogen generation by catalytic hydrolysis of NaBH₄. It was found that the Co/Fe mixture ratio, temperature, NaBH₄, and NaOH concentrations, all exert considerable influence on the catalytic effectiveness of Co–Fe complex catalyst towards the hydrolysis reaction of NaBH₄. The results suggested that the optimal mixture percentage of Co–Fe complex catalyst is 80:20. The obtained complex catalysts are characterized by XRD, FT-IR, and SEM techniques.     

Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods

The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO₂ by ammonia at low temperature was investigated by transient reactive experiments over a commercial V₂O₅/WO₃/TiO₂ catalyst for diesel exhaust aftertreatment. NO + NH₃ temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO₂ and HNO₃ were able to reoxidize the V catalyst at much lower temperature than gaseous O₂: furthermore, they significantly enhanced the NO + NH₃ reactivity below 250 °C via the buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which were not observed in comparative experiments over a V-free WO₃/TiO₂ catalyst, point out a key catalytic role of the vanadium redox properties and can explain the greater deNO_x efficiency of the “fast” SCR (NO + NH₃ + NO₂) compared with the “standard” SCR (NO + NH₃ + O₂) reaction. A unifying redox approach is proposed to interpret the overall NO/NO₂–NH₃ SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH₃ and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO₂ disproportion over other nonreducible oxide catalyst components.     

Preparation of porous PVDF-NiB capsules as catalytic adsorbents for hydrogen generation from sodium borohydride

Porous poly(vinylidene fluoride) (PVDF)-NiB capsules were prepared by phase inversion of PVDF, nickel salt and sodium borohydride (NaBH₄) mixtures in water. During the process, nickel salt was reduced to NiB particles by NaBH₄, and the excess NaBH₄ was hydrolyzed to produce hydrogen gas bubbles which aided the formation of the large holes inside the porous capsules. It was proved that the porous capsules can adsorb about 33 wt.% NaBH₄ in THF/NaBH₄ solution for about 4 h. Most of NiB particles were attached on the surface of the porous capsules with inside holes according to the observation of scanning electron microscopy (SEM) and EDS. The special structure of the capsules was successfully used as catalysts and container simultaneously for hydrogen production via catalytic hydrolysis of NaBH₄ in aqueous solution.