Effect of Ag addition on the properties of Pd–Ag/TiO2 catalysts containing different TiO2 crystalline phases

Anatase and rutile TiO₂ were used for preparation of the TiO₂ supported Pd and Pd–Ag catalysts for selective hydrogenation of acetylene. It was found that Pd/TiO₂-anatase exhibited higher acetylene conversion and ethylene selectivity than rutile TiO₂ supported ones. However, addition of Ag to Pd/TiO₂-anatase catalyst resulted in lower ethylene selectivity while that of Pd/TiO₂-rutile increased. It is suggested that Ag addition suppressed the beneficial effect of the Ti³⁺ sites presented on the anatase TiO₂ during selective acetylene hydrogenation whereas without Ti³⁺, Ag promoted ethylene selectivity by blocking sites for over-hydrogenation of ethylene to ethane.     

Pd/Pt on Ti-containing Mixed Oxides as Dearomatization Catalysts: Physico-chemical Characterization and Activity

Pd/Pt supported on pure and doped TiO₂ (TiO₂–WO₃ and TiO₂–WO₃–SiO₂) were prepared and characterized by different techniques (XPS, TEM, XRD, H₂-TPR and TPD of ammonia). These catalysts were investigated in the hydrogenation of tetralin at 6.0 MPa, checking also their thio-tolerance by feeding increasing amounts of dibenzothiophene (DBT, 300 and 1000 wt ppm). The catalytic activity followed the order: Pd/Pt–TiO₂ > Pd/Pt–TiO₂–WO₃–SiO₂ > Pd/Pt–TiO₂–WO₃, evidencing a negative role of a second oxide inside TiO₂. The Pd/Pt–TiO₂ catalyst showed high activity regardless of reaction conditions (temperature, contact time, H₂/tetralin ratio) together with a good thio-tolerance up to 300 wt ppm of DBT.     

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.     

Synthesis of HFC-134a by isomerization and hydrogenation

For the preparation of HFC-134a, the isomerization of CFC-114 and the hydrogenation of CFC-114a were investigated. Both reactions were catalyzed by AlC₃ and supported Pd catalysts, respectively. For the comparison purpose, the isomerization of CFC-113 was carried out also. With virgin AICI₃ catalyst, both isomerization reactions proceeded after a certain induction period probably because the catalyst needed the activation by the halogen exchange. The catalyst deactivated gradually with the time on stream. However, the deactivation rate could be reduced by removing impurities from the reactants. Isomerization rate of CFC-114 was much slower than that of CFC-113. Palladium supported on carbon catalyzed the hydrogenation reaction quite selectively while the selectivity declined when the support was replaced with different supports. The catalytic activity and selectivity to desired products increased in the following order. Pd/kieselguhr Pd/silica-alumina ≅Pd/silica gel Pd/TiO₂Pd/Al₂O₃Pd/C     

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).     

Regeneration of Pd/TiO2 catalyst deactivated in reductive CCl4 transformations by the treatment with supercritical CO2, ozone in supercritical CO2 or oxygen plasma

Carbonaceous deposits formation was established as the primary reason of Pd/TiO₂ catalyst deactivation during reductive processing of CCl₄ to form hydrodechlorination and oligomerization products. Three methods of carbonaceous deposits elimination were tested: (1) extraction by supercritical CO₂, (2) oxidation by ozone in supercritical CO₂, and (3) low-temperature glow-discharge oxygen plasma treatment. Synchronic thermal analysis confirms effective carbonaceous deposits removal during regeneration by ozone or low temperature glow-discharge oxygen plasma; by XPS deep oxidation of surface Pd after oxidative treatment (by ozone or oxygen plasma) was found. Thus H₂ reduction was proposed as the second step making possible full regeneration of initial catalytic activity of Pd/TiO₂.     

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO₂ nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO₂ were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O₂ dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO₂ particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO₂ nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti-O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO₂.     

Wet oxidation of wastewater containing hydrocarbons by novel supported Pd catalysts

Pd catalysts supported on TiO₂, ZrO₂, ZSM-5, MCM-41 and activated carbon were used in catalytic wet oxidation of hydrocarbons such as phenol, m-cresol and m-xylene. It was found that the Pd/TiO₂ catalyst was highly effective in the wet oxidation of hydrocarbon. The activities of catalysts with various hydrocarbon species, catalyst support, oxidation state of catalyst performed in a 3-phase slurry reactor show that reaction on Pd surface is more favorable than that in aqueous phase and that the active site is oxidized Pd in catalytic wet air oxidation of hydrocarbons. Based on the experimental results, a plausible reaction mechanism of wet oxidation of hydrocarbons catalyzed over Pd/TiO₂ catalyst was proposed. This catalyst is superior to other oxide catalysts because it suppressed the formation of hardly-degradable organic intermediates and polymer.     

Selective hydrogenation of o-chloronitrobenzene over palladium supported nanotubular titanium dioxide derived catalysts

Titania derived nanotubes were synthesized by treating P-25 Degussa TiO₂ with a concentrated (18 M) KOH solution. Ageing the material in KOH solution for 2 days resulted in formation of tubular titania and Raman analysis revealed that the material has a titanate structure. The synthesized material was used as a catalyst support for the hydrogenation of ortho-chloronitrobenzene (o-CNB) with Pd as the active phase. The vapour-phase hydrogenation of o-CNB was carried out in ethanol at 523 K and atmospheric pressure over a Pd/TiO₂ derived nanotube catalyst (Pd/TiO₂-M). Pd/TiO₂-M gave complete conversion (100%) of o-CNB with the selectivity to ortho-chloroaniline (o-CAN) of 86%. The stability of the Pd/TiO₂-M catalyst was tested over 5 h during which time the conversion slowly dropped to 80% (selectivity 93%) due to catalyst poisoning. TPR analysis revealed the existence of a strong palladium-support interaction and this was found to be crucial to the overall activity of the catalyst.     

Gas-phase hydrogenation of maleic anhydride to butyric acid over Cu/TiO2/γ-Al2O3 catalyst promoted by Pd

The promoter effect of palladium on the Cu/TiO₂/γ-Al₂O₃ catalyst was investigated for the gas-phase selective hydrogenation of maleic anhydride to butyric acid at atmospheric pressure. The results show that Pd is added rarely into the Cu/TiO₂/γ-Al₂O₃ catalyst for the hydrogenation of maleic anhydride, the higher selectivity to butyric acid can be obtained. In the absence of Pd (or Cu) in the Cu–Pd/TiO₂/γ-Al₂O₃ catalyst, the selectivity to butyric acid (BA) is nearly zero. Using the Cu–Pd/TiO₂/γ-Al₂O₃ (Pd/Cu=3/100 (atom)) catalyst, 56.2% selectivity to BA and 100% conversion of maleic anhydride were obtained at 280 °C.     

A novel catalyst for CO oxidation at low temperature

Supported catalysts of palladium over ceria–titania mixed oxides (Pd/CeO₂–TiO₂) were prepared and tested for carbon monoxide oxidation. The catalysts exhibited high catalytic activity at room temperature. The Pd/CeO₂–TiO₂ catalyst was more active than Pd/CeO₂, Pd/SnO₂–TiO₂, Pd/ZrO₂–TiO₂, Pd/Al₂O₂–TiO₂ and Pd/TiO₂ catalysts under the same conditions examined. The effects of preparation methods of the support, the mole ratio of ceria and titania in mixed supports as well as Pd loading upon the catalytic activity of CO oxidation were investigated. Among the Pd/CeO₂–TiO₂ catalysts, the best one corresponds to the Pd loading of 1.0 wt% or above, and the mole ratio of ceria and titania ranging from 1 : 7 to 1 : 5. The steady-state catalytic performance of such catalyst was recorded without any deactivation over 8 h time-on-stream in the present study. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ligandless palladium supported on SiO2–TiO2 as effective catalyst for Suzuki reaction

TiO₂–SiO₂ mixed oxides were prepared by the homogeneous precipitation method, and the supports were characterized by XRD, IR and BET. Ligandless palladium supported on SiO₂–TiO₂ was prepared by the deposition–precipitation method and used as recoverable catalyst for Suzuki reactions. The results revealed that the supported catalyst Pd/SiO₂–TiO₂ exhibited excellent catalytic activity for the coupling of aryl bromides with arylboronic acid, and also exhibited moderate catalytic activity for the coupling of aryl chlorides. The catalyst was recycled by simple filtration and reused without further disposal, no significant Pd leaching and loss of catalytic activity was observed except the first reuse.     

Selective Catalytic Reduction of NOx with CH4 over the In/Sulfated TiO2 Catalyst

Tungstated TiO₂ (WTi), tungstated Fe₂O₃ (WFe), tungstated SnO₂ (WSn), sulfated TiO₂ (STi), sulfated Fe₂O₃ (SFe), and sulfated SnO₂ (SSn) are used as the support for the In and Pd catalysts for the SCR of NO with methane. It was found that the In/STi catalyst with a 2% indium loading showed the highest NO_x conversion (39% at 450 °C and 12,000 h⁻¹) among all of the catalysts studied. The acid strength of the STi support was very important for the formation of the InO⁺ site, which was thought to be the active site for NO reduction. The activity of the In/STi catalyst can be improved by increasing the surface area of the STi support. The most attractive feature of the In/STi catalyst is its high resistance for SO₂.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

Acetylene Hydrogenation on Au-Based Catalysts

Hydrogenation of acetylene has been investigated on Au/TiO₂, Pd/TiO₂ and Au-Pd/TiO₂ catalysts at high acetylene conversion levels. The Au/TiO₂ catalyst (avg. particle size: 4.6 nm) synthesized by the temperature-programmed reduction-oxidation of an Au-phosphine complex on TiO₂ showed a remarkably high selectivity to ethylene formation even at 100% acetylene conversion. Au/TiO₂ prepared by the conventional incipient wet impregnation method (avg. particle size: 30 nm), on the other hand, showed negligible activity for acetylene hydrogenation. Although the Au catalysts showed a high selectivity for ethylene, the acetylene conversion activity and catalyst stability were inferior to the Pd-based catalysts. Au-Pd catalysts prepared by the redox method showed high acetylene conversions as well as high selectivity for ethylene. Interestingly Au-Pd catalysts prepared by depositing Pd via the incipient wetness method on Au/TiO₂ showed very poor selectivity (comparable to mono-metallic Pd catalysts) for ethylene. High-resolution transmission electron microscopy (TEM) studies coupled with energy dispersive X-ray spectroscopy (EDS) showed that while the redox method produced bimetallic Au-Pd catalysts, the latter method produced individual Pd and Au particles on the support.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

Total oxidation of volatile organic compounds by vanadium promoted palladium-titania catalysts: Comparison of aromatic and polyaromatic compounds

Vanadium oxide, palladium oxide and mixed Pd/V-supported on titania catalysts have been prepared and tested in the total oxidation of volatile organic compounds (VOCs). A comparative study with two different aromatic VOCs (benzene and naphthalene) has been carried out. For benzene, the mixed Pd/V-catalysts presented the highest catalytic activity. However, whilst studies with benzene led to the formation of CO₂ only, the total conversion of naphthalene to CO₂ was not achieved throughout the full temperature range for naphthalene conversion. A naphthalene conversion to CO₂ of 99% was obtained over Pd/TiO₂, V/TiO₂ and Pd/V/TiO₂ catalysts at 275, 325 and 300 °C, respectively. Therefore, the requirements for an effective benzene total oxidation catalyst cannot be readily extrapolated to larger polycyclic aromatic compounds, as in the naphthalene oxidation the most active catalyst from an environmental point of view is Pd supported on TiO₂.     

The effect of TiO2 particle size on the characteristics of Au–Pd/TiO2 catalysts

The nanocrystalline TiO₂ materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO₂ catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO₂ crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H₂ 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO₂ crystallite size in which it was more pronounced for Au/Pd on the larger TiO₂ (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst.     

An enhancement in the photocatalytic activity of TiO2 by the use of Pd: the question of layer sequence in the resulting hierarchical structure

TiO₂ and Pd-modified TiO₂ photocatalysts were prepared by sol–gel method. X-ray diffraction (XRD) data showed that the presence of Pd in TiO₂ catalyst decreases the crystalline size of TiO₂ and stabilizes anatase phase. The study of the photocatalytic activity of the films via Linear Sweep Voltametry (LSV) plots and antibacterial test against Escherichia coli ATCC 25922—a gram negative bacterium—showed that Pd increases the photocatalytic activity of the coatings. Besides, the sequence of layer deposition (TiO₂ and Pd-modified TiO₂) influences the photocatalytic properties. In other words, more photocatalytic activity is obtained when Pd-modified layer is deposited first.