Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation

The Co/ZrO₂ catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO₂ nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO₂. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO₂ which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO₂ crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO₂ prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO₂ catalysts were increased with increasing Co loading and precursor feed rate. According to H₂ chemisorption and H₂ temperature program reduction results, the improvement of catalytic activity and C₂–C₆ selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO₂ support obtained via the FSP synthesis.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

Characteristics and Catalytic Properties of Pd/SiO2 Synthesized by One-step Flame Spray Pyrolysis in Liquid-phase Hydrogenation of 1-Heptyne

In this study, Pd/SiO₂ catalysts with 0.5–10 wt.% Pd loadings were prepared by one-step flame spray pyrolysis (FSP) and characterized by N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption, and X-ray photoelectron spectroscopy (XPS). The average cluster/particles size of Pd as revealed by TEM were ca. 0.5–3 nm. The turnover frequencies (TOFs) of the flame-made catalysts decreased from 66.2 to 4.3 per s as Pd loading increased from 0.5 to 10 wt.%, suggesting that the catalytic activity was dependent on Pd particle/cluster size. However, there were no appreciable influences on 1-heptene selectivity. The flame-made Pd/SiO₂ showed better properties than the conventional prepared catalysts. Their advantages are not only the presence of large pores that facilitates diffusion of the reactants and products, but also the high-catalytic activity of as-synthesized catalysts so that further pretreatment is not necessary.     

Geometrical confinement effect in the liquid-phase semihydrogenation of phenylacetylene over mesostructured silica supported Pd catalysts

Three types of mesostructured silica with different pore sizes (MCM-41, SBA-15, and MCF) were employed as supports for deposition of colloidal Pd nanoparticles obtained by the solvent reduction method. As determined by transmission electron microscopy (TEM), average Pd particle sizes on the various supports were quite similar and were not significantly different from the colloidal particles (~ 2.3–2.5 nm). There was limited access of the reactants to Pd active sites and suppression of CO chemisorption for the Pd/SBA-15, probably because most of the Pd particles were located inside the pores. This geometrical confinement effect in the case of Pd/SBA-15, however, resulted in an improved selectivity towards styrene at complete conversion of phenylacetylene. Such effect was similar to those reported in the literature for the Pd nanoparticles encapsulated in support matrices (simultaneous synthesis) and the presence of strong metal-support interaction effect in Pd/TiO₂.     

Structural and luminescence properties of undoped,Nd3+ and Er3+ doped TiO2 nanoparticles synthesized by flame spray pyrolysis method

Bulk and thin film forms of titanium dioxide (TiO₂) have been studied many times due to its very promising optical properties. In this study, low-cost flame spray pyrolysis (FSP) synthesis of Nd³⁺/Er³⁺doped TiO₂ nanoparticles has been reported for the first time. The produced particles were post-annealed after FSP process at 550 °C in order to obtain crystalline structure. The phase and elemental analysis of the produced materials were performed by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), respectively. The surface morphology, accurate size and specific surface area of the primary particles were identified using scanning electron microscopy (SEM) and particle size analyser. Luminescent properties of the produced nanoparticles were investigated by steady state and time resolved fluorescence spectra. Doping of TiO₂ nanoparticles with the rare earths of Nd³⁺and Er³⁺resulted in visible and near-infrared light emission when excited at 364 nm. The utilized nanoparticles yielded bi-and tri-exponential decay curves. Additionally, they exhibited typical upconversion luminescence when radiated by 810 nm.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Characteristics and catalytic properties of Pt–Sn/Al2O3 nanoparticles synthesized by one-step flame spray pyrolysis in the dehydrogenation of propane

The Pt–Sn/Al₂O₃ catalysts with 0.3 wt% Pt and 0.5–1.5 wt% Sn loading were prepared by one-step flame spray pyrolysis (FSP). Unlike the catalysts prepared by conventional impregnation method, the FSP-derived catalysts were composed of single-crystalline γ-alumina particles with the as-prepared primary particle size of 10–18 nm and contained only large pores. The FSP catalysts exhibited superior catalytic activity and better stability than the ones made by impregnation in the dehydrogenation of propane, while they did not alter the selectivity to propylene (in all cases, propylene selectivity ≥96%). The presence of large pores in the flame-made catalysts not only facilitated diffusion of the reactants and products but could also lessen the amount of carbon deposited during reactions. As revealed by CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), the metal particles appeared to be partially covered by the alumina matrix (Al–O) due to the simultaneous formation of particles during FSP synthesis. Such phenomena, however, were shown to result in the formation of active Pt–Sn ensembles for propane dehydrogenation as shown by higher turnover frequencies (TOFs).     

Flame sprayed tri-metallic Pt–Sn–X/Al2O3 catalysts (X = Ce,Zn, and K) for propane dehydration

Trimetallic nanocrystalline Pt–Sn–X/Al₂O₃ catalysts (X = Ce, Zn, and K) consisting of 0.3 wt.% Pt, 1 wt.% Sn, and 0.5 wt.% X have been prepared by one-step flame spray pyrolysis (FSP). As shown by the X-ray diffraction (XRD) and the transmission electron microscopy (TEM) results, the as-synthesized FSP-made catalysts were consisted of single-crystalline γ-alumina particles with average primary particle sizes 8 to 9 nm. The N₂ physisorption results revealed that all the catalysts contained only the macropore structure. The catalytic properties of the FSP-made catalysts were investigated in the dehydration of propane. Addition of Ce during FSP synthesis resulted in higher Pt dispersion as well as improved catalytic activity and stability than the non-promoted Pt–Sn/Al₂O₃. An opposite trend was found with the ones doped with Zn and K in which high surface coverage of Zn and K resulted in a significant loss of Pt active sites. The mechanism for the formation of the trimetallic nanoparticles during one-step FSP synthesis appeared to depend strongly on the differences in the vapor pressure of the metals and the alumina support in flame.     

Pd nanoparticles supported on carbon-modified rutile TiO2 as a highly efficient catalyst for formic acid electrooxidation

In this article, Pd nanoparticles supported on carbon-modified rutile TiO₂ (CMRT) as a highly efficient catalyst for formic acid electrooxidation were investigated. Pd/CMRT catalyst was synthesized by using liquid phase reduction method in which Pd nanoparticles was loaded on the surface of CMRT obtained through a chemical vapor deposition (CVD) process. Pd/CMRT shows three times the catalytic activity of Pd/C, as well as better catalytic stability towards formic acid electrooxidation. The enhanced catalytic property of Pd/CMRT mainly arises from the improved electronic conductivity of carbon-modified rutile TiO₂, the dilated lattice constant of Pd nanoparticles, an increasing of surface steps and kinks in the microstructure of Pd nanoparticles and slightly better tolerance to the adsorption of poisonous intermediates.     

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO₂ catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k₁ of the CO oxidation reaction on PdO, and that of k ₂ ^app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO₂ pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO₂. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.     

Low-temperature preferential oxidation of CO in a hydrogen rich stream (PROX) over Au/TiO2: Thermodynamic study and effect of gold-colloid pH adjustment time on catalytic activity

By simulating CO and H₂ oxidations at thermodynamic equilibrium and studying the catalytic oxidations over Au/TiO₂, preferential oxidation of CO in a H₂ rich stream (PROX) was investigated. During the simulation, at least two cases under different gaseous feeds, H₂/CO/O₂/N₂ = 50/1/0.5/48.5 or 50/1/1/48 (vol.%) were examined under the assumption of an ideal gas and one atmosphere pressure in the reactor. It was found that the addition of 1% O₂ (the latter case) effectively reduced CO concentration to less than 100 ppm in the temperature range between 0 and 90 °C. This range narrowed to between 0 and 50 °C with the addition of 3% H₂O and 15% CO₂ in the feed. The thermodynamic study suggests that 1% CO in a H₂ rich system can be decreased to below 100 ppm within those low temperature ranges, if there is no substantial adsorptions onto the catalyst surface and the reactions rapidly reach equilibrium. During the catalysis reaction study, a well-pH adjusted Au/TiO₂ catalyst was found very active for PROX. CO conversions at the reactor outlet were close to those at equilibrium. Au/TiO₂ used in this work was prepared via deposition-precipitation (DP) method. The influence of gold colloid pH (at 6) adjustment time on gold loading, gold particle size and chloride residue on TiO₂ surface was detected by atomic absorption (AA), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A pH adjustment time of at least 6 h for the preparation of gold colloids at room temperature was demonstrated to be essential for the high catalytic activity of Au/TiO₂. This was attributed to the smaller gold particle and the less chloride residue on the catalyst surface.     

High-temperature close coupled catalysts Pd/Ce-Zr-M/Al2O3 (M = Y, Ca or Ba) used for the total oxidation of propane

A series of high-temperature close coupled catalysts Pd/Ce-Zr-M/Al₂O₃ (M = Y, Ca or Ba) were prepared by ultrasonic-assisted successive impregnation. The catalysts were subjected to a series of characterization measurements. The results of activity evaluation show that Y is the best promoter for propane total oxidation, especially at the calcination temperature of 1100 °C. It is interesting that although the BET specific surface areas and the dispersion of Pd species decrease, the Y-promoted catalyst calcined at 1100 °C shows higher catalytic activity than the corresponding one calcined at 900 °C and better sulfur-resisting performance. The results of TEM, TPHD and CO chemisorption indicate that Y can remarkably increase the dispersion of Pd species. However, the dispersion is hard to be connected with the activity increase as the calcination temperature is elevated from 900 to 1100 °C. The change of active phases and the interaction between Pd species and the supports may account for the activity enhancement. Combined with XRD, H₂-TPR and O₂-TPD results, it is deduced that the coexistence of metallic Pd and PdO species in the catalysts calcined at 1100 °C may be also favorable to C₃H₈ oxidation. In a word, Pd/Ce-Zr-Y/Al₂O₃ is indeed a promising high-temperature close coupled catalyst applicable to high temperature.     

Investigation of surface modification of rutile TiO2 nanoparticles with SiO2/Al2O3 on the properties of polyacrylic composite coating

Surface modification and characterization of TiO₂ nanoparticles as an additive in a polyacrylic clear coating were investigated. For the improvement of nanoparticles dispersion and the decreasing of photocatalytic activity, the surface of nanoparticles was modified with binary SiO₂/Al₂O₃. The surface treatment of TiO₂ nanoparticles was characterized with FTIR. Microstructural analysis was done by AFM. The size, particle size distribution and zeta potential of TiO₂ nanoparticles in water dispersion was measured by DLS method. For the evaluation of particle size and the stability of nanoparticles in water dispersions with higher solid content the electroacoustic spectroscopy was made. To determine the applicability and evaluate the transmittance of the nano-TiO₂ composite coatings UV–VIS spectroscopy in the wavelength range of 200–800 nm was employed. The results showed that surface treatment of TiO₂ nanoparticles with SiO₂/Al₂O₃ improves nanoparticles dispersion and UV protection of the clear polyacrylic composite coating.     

Structure and Size of Poly-Domain Pd Nanoparticles Supported on Silica

A polydomain internal structure was evidenced by means of High Resolution Transmission Electron Microscopy (HRTEM) in palladium nanoparticles of two 5% Pd/SiO₂ catalysts prepared by impregnation technique. Such poly-domain structure biased the line broadening analysis of the XRPD patterns, from which highly underestimated average diameters were obtained in comparison to those measured by means of Transmission Electron Microscopy (TEM).In both the investigated catalysts, a Pd/CO average chemisorption stoichiometry close to 2 was found, in good agreement with previous results obtained for Pd/C and Pd/-Al₂O₃ catalysts. The consistency of this result, in cases with different particle sizes and different support materials, suggests that such average stoichiometry is of general validity for palladium, when chemisorption measurements of CO are made using pulse flow technique.     

Atomically dispersed Pd on nanostructured TiO2 for NO removal by solar light

Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use. Here Pd clusters onto nanostructured TiO₂ particles are prepared in one step by scalable flame aerosol technology while controlling the Pd cluster size from a few nanometers to that of single atoms. Annealing such materials at appropriate temperatures leads to solar photocatalytic NO_x removal in a standard ISO reactor up to 10 times faster than that of commercial TiO₂ (P25, Evonik). Such superior performance can be attained by only 0.1 wt.% Pd loading on TiO₂. Annealing these flame‐made powders in air up to 600 °C decreases the amorphous TiO₂ fraction and increases its crystal and particle sizes as observed by x‐ray diffraction (XRD) and N₂ adsorption. The growth of single Pd atoms to Pd clusters on TiO₂ prepared at different Pd loading and annealing conditions was investigated by scanning transmission electron microscopy and XRD. The single Pd atoms and clusters on TiO₂ are stable up to, at least, 600 °C for 2 h in air but at 800 °C they grow into PdO nanoparticles whose fraction is comparable with the nominal Pd loading. Hence, most of Pd atoms are on the TiO₂ surface where at 800 °C they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy reveals NO adsorption on single, double, three and fourfold coordinated Pd atoms depending on their synthesis and annealing conditions. The peak intensity of NO adsorption sites involving multiple Pd atoms is substantially lower in TiO₂ containing 0.1 wt.% than 1 wt.% Pd but that intensity from single Pd atoms is comparable. This indicates the dominance of isolated Pd atoms compared to clusters in Pd/TiO₂ containing 0.1 wt.% Pd that match or exceed the photocatalytic NO_x removal of Pd/TiO₂ of higher Pd contents. © 2016 American Institute of Chemical Engineers AIChE J, 63: 139–146, 2017     

The influence of the crystal structure of TiO2 support material on Pd catalysts for formic acid electrooxidation

The influence of the crystal structure of TiO₂ support material on Pd catalyst-mediated formic acid electrooxidation was investigated. Pd/TiO₂ catalysts were synthesized by loading Pd on TiO₂ with different crystal structures obtained by calcinations at different temperatures. Electrochemical tests showed that TiO₂ with the rutile structure improved the catalytic activity of Pd nanoparticles toward formic acid electrooxidation. Physicochemical and electrochemical characterizations revealed that the enhancement of Pd/TiO₂ (rutile) catalytic activity arose from uniform dispersion of Pd nanoparticles, an increase in surface-active sites, and good tolerance to the adsorption of poisonous intermediates (such as CO_ad, COOH_ad and so on).     

Photocatalytic degradation of methylene blue with TiO2 nanoparticles prepared by a thermal decomposition process

The generation of TiO₂ nanoparticles by the thermal decomposition of titanium tetraisopropoxide (TTIP) was carried out experimentally using a tubular electric furnace at various synthesis temperatures (700-1300 °C) and TTIP heating temperatures (80-110 °C). The photocatalytic activity of the resulting TiO₂ nanoparticles was examined by measuring the rate of methylene blue decomposition. The TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurements and transmission electron microscopy (TEM). The crystallite size and crystallinity increased with increasing synthesis temperature and TTIP heating temperature. A TTIP heating temperature and synthesis temperature of 95 °C and 900 °C, respectively, were found to be the optimal synthesis conditions. The primary particle diameter obtained under optimum synthesis conditions was considerably smaller than the commercial photocatalyst (Degussa, P25). The specific surface areas were more than 134.4 m² g^− 1. Under the optimal conditions, the photocatalytic activity for methylene blue was higher than that of the commercial photocatalyst.     

Ultra-high temperature ablation behavior of Ti2AlC ceramics under an oxyacetylene flame

The linear and mass ablation rates of Ti₂AlC ceramics under an oxyacetylene flame at a temperature up to 3000 °C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14 μm s⁻¹ for the first 30 s of the ablation to 0.08 μm s⁻¹ after 180 s. Ti₂AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of α-Al₂O₃ and TiO₂ and some Al₂TiO₅, and a porous sub-surface layer containing Ti₂Al_1−xC and TiC_xO_y. With increasing ablation time, the content of TiO₂ and Al₂TiO₅ in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti₂AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.     

Dopant-free, polymorphic design of TiO2 nanocrystals by flame aerosol synthesis

A dopant-free aerosol synthesis of highly crystalline TiO₂ nanoparticles (20–35 nm) with tunable polymorphic content is demonstrated by rapid flame spray pyrolysis. By controlling precisely the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, anatase content as high as 96 wt% (4 wt% rutile) was obtained at high oxic flame conditions, while rutile content as high as 94 wt% (6 wt% anatase) was obtained under anoxic flames. The polymorphic variability lies within a narrow range of combustion equivalence ratios, that is, 1.0<Φ<1.5. Unlike any other flame aerosol syntheses, the anatase and rutile crystallite sizes were similar within each sample. Under highly oxic flame conditions (Φ<1.0), twinnings between anatase {0 1 1} planes could be observed, inferring oriented attachment taking place. Such mechanism could not, however, be seen under anoxic flame (Φ>1.0) possibly due to physical hindrance by surface carbonaceous content (typically <2 wt%). The carbon content can be easily removed by short calcination without significantly affecting the surface areas and crystallite properties of the original TiO₂ nanocrystals, preserving hence its pristine state.