Carbon oxidation with platinum supported catalysts

The effect of the support oxide, Pt precursor and reactant gas composition on the catalysis of soot oxidation was investigated using carbon black as a model soot and simulated exhaust gases. The Pt precursors used were Pt(NH₃)₄(OH)₂, H₂PtCl₆·6H₂O, Pt(NH₃)₄(NO₃)₂, and Pt(NH₃)₄Cl₂. The support metal oxides used were SiO₂, Al₂O₃, and ZrO₂. Pt/SiO₂ prepared from Pt(NH₃)₄(OH)₂ showed the highest carbon oxidation activity. It had much higher activity in the condition of N₂+O₂+H₂O+NO+SO₂ than without NO and SO₂.     

The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

The selective catalytic reduction of NO_x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH₄–O₂ reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO_x reduction, while for the NO₂–CH₄–O₂ reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO_x conversion in NO₂–CH₄–O₂ reaction was significantly higher than that in NO–CH₄–O₂ reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO₂]/([NO]+[NO₂]) in the effluent gases in NO–O₂ and NO₂–O₂ reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO_x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO₂, and high activity for NO₂ reduction to N₂ (Pd, Rh); (ii) high activity for NO oxidation to NO₂, and low activity for NO₂ reduction to N₂ (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO_x by methane.     

Hydrodechlorination of CCl4 over Pt/Al2O3: effects of platinum particle size on product distribution

The impregnated platinum catalysts showed various platinum particle sizes depending on the nature of the platinum precursors (Pt(NH₃)₂(NO₂)₂ versus H₂PtCl₆) and on the pH of the Al₂O₃ suspension. The average platinum particle size increased with decrease in pH of the suspension in case of Pt(NH₃)₂(NO₂)₂, but this trend was vice versa for H₂PtCl₆. The product distribution in hydrodechlorination (HDC) of CCl₄ varied greatly with the platinum particle size; the larger the platinum particle size was, the higher was the selectivity to CHCl₃. To elucidate the origin of this platinum particle size effect on product distribution, CO chemisorption, NH₃ and CO₂ temperature-programmed desorption (TPD), high resolution transmission electron microscopy (HRTEM), temperature-programmed surface reaction (TPSR), Fourier-transformed-infrared spectra (FT-IR) and X-ray absorption fine structure (XAFS) experiments were carried out. The formation of completely dechlorinated CH₄ was favorable owing to the strong chemisorption of CCl₄ on the small platinum particles characterized by low surface coordination numbers and by an electron-deficient property. The nature of carbonaceous species formed on platinum surface at the beginning of reaction also varied greatly with platinum particle sizes and changes of electronic state of platinum particles affected catalytic activity and products’ distribution.     

Performance of alumina-supported noble metal catalysts for the combustion of trichloroethene at dry and wet conditions

The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h⁻¹) in air was studied in this work. Experiments were carried out at both dry and wet (20,000 ppm of H₂O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO₂, HCl, Cl₂, C₂Cl₄, CCl₄ and CHCl₃), and stability at reaction conditions.

As general trend, the activity of the catalysts decreases in the order Ru  Pd > Rh > Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C₂Cl₄ the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl₄ and CHCl₃ being the main organic by-products.

Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts.

Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes.     

Degradation of olive oil mill effluents by catalytic wet air oxidation: 2-Oxidation of p-hydroxyphenylacetic and p-hydroxybenzoic acids over Pt and Ru supported catalysts

Catalytic wet air oxidation of p-hydroxyphenylacetic acid and p-hydroxybenzoic acid, two important pollutants present in the olive oil mill wastewaters, was studied in a batch reactor using platinum and ruthenium catalysts supported on titanium and zirconium oxides at 140 °C and 50 bar of total air pressure. Reaction pathways for the oxidation of these two substrates were proposed, with formation of different aromatic compounds and short-chain organic acids through hydroxylation and decarboxylation reactions.

It was observed that the conversion and the mineralization of these two substrates were markedly affected by the nature of the ruthenium precursor (RuCl₃ or Ru(NO)(NO₃)₃), with the non-chlorine containing salt giving the best performances. Calcination of the catalyst precursor before reduction was detrimental. The nature of the metallic precursor (H₂PtCl₆ or Pt(NH₃)₄(NO₃)₂) had little influence on the catalytic properties of platinum catalysts, whereas the textural properties of the support were an important factor.     

前驱体结构对多环芳烃选择性开环Pt/Beta催化剂的影响

以海绵铂为原料合成出［Pt（NH₃）₆］Cl₄络合物,采用热重分析（TG）、扫描电镜-能谱（SEM-EDS）、紫外-可见分光光度计（UV-Vis）、质谱（MS）、X射线光电子能谱（XPS）等手段确定了［Pt（NH₃）₆］Cl₄的结构组成;以H₂PtCl₆、Pt（NH₃）₄Cl₂和［Pt（NH₃）₆］Cl₄为前驱体,采用等体积浸渍法制得Pt/Beta催化剂,采用X射线衍射（XRD）、X射线荧光光谱（XRF）、氨程序升温脱附（NH₃-TPD）、氢氧滴定（H₂-O₂）、透射电镜（TEM）、氢气程序升温脱附（H₂-TPD）等表征了Pt/Beta催化剂的物化性质,并考察了Pt/Beta催化剂的多环芳烃选择性开环性能。结果表明,［Pt（NH₃）₆］Cl₄络合物具有更高的“抗自还原”能力,可从前驱体结构上降低铂氨前驱体受热分解时的自还原现象。前驱体结构对铂纳米颗粒的几何尺寸及分布有较大影响,一方面络合物的价态显著影响前驱体与分子筛间的静电作用,进而影响铂纳米颗粒的落位与尺寸;另一方面络合物的空间结构影响前驱体在分子筛微孔中的分布,影响铂纳米颗粒的Ostwald熟化速率。前驱体结构可调变Pt/Beta催化剂的双功能匹配关系,显著影响Pt/Beta催化剂转化甲基萘的活性、稳定性,采用［Pt（NH₃）₆］Cl₄前驱体制备的Pt/Beta催化剂具有更优的活性及长周期稳定性。     

Modified iron-molybdate catalysts with various metal oxides by a mechanochemical method: enhanced formaldehyde yield in methanol partial oxidation

A mechanochemical method was employed to prepare modified iron molybdate catalysts with various metal salts as precursors. The physicochemical properties of the iron molybdate catalysts were characterized, and their performances in catalyzing the reaction from methanol to formaldehyde (HCHO) were evaluated. Iron molybdate catalysts doped with Co(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O resulted in high HCHO yields. Compared with a commercial catalyst, the HCHO yields in the reaction with the modified catalyst at an optimal Co/Mo molar ratio reached 97.37%. According to chemical state analysis, the formation of CoO and the efficient decrease in the MoO₃ sublimation rate could be important factors enhancing the HCHO yield in reactions catalyzed with iron molybdate doped with different Co/Mo mole ratios.     

The reduction phase in NOx storage catalysis: Effect of type of precious metal and reducing agent

The effect of different reducing agents (H₂, CO, C₃H₆ and C₃H₈) on the reduction of stored NO_x over PM/BaO/Al₂O₃ catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO₂-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H₂ and CO have a relatively high NO_x reduction efficiency compared to C₃H₆ and especially C₃H₈ that does not show any NO_x reduction ability except at 350 °C over Pd/BaO/Al₂O₃. The type of precious metals affects the NO_x storage-reduction properties, where the Pd/BaO/Al₂O₃ catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al₂O₃ catalyst shows a high reduction ability but a relatively low NO_x storage capacity.     

Conversion of CO into CO2 by high active and stable PdNi nanoparticles supported on a metal-organic framework

The solubility of Pd(NO₃)₂ in water is moderate whereas it is completely soluble in diluted HNO₃ solution. Pd/MIL-101(Cr) and Pd/MIL-101-NH₂(Cr) were synthesized by aqueous solution of Pd(NO₃)₂ and Pd(NO₃)₂ solution in dilute HNO₃ and used for CO oxidation reaction. The catalysts synthesized with Pd(NO₃)₂ solution in dilute HNO₃ showed lower activity. The aqueous solution of Pd(NO₃)₂ was used for synthesis of mono-metal Ni, Pd and bimetallic PdNi nanoparticles with various molar ratios supported on MOF. Pd₇₀Ni₃₀/MIL-101(Cr) catalyst showed higher activity than monometallic counterparts and Pd+ Ni physical mixture due to the strong synergistic effect of PdNi nanoparticles, high distribution of PdNi nanoparticles, and lower dissociation and desorption barriers. Comparison of the catalysts synthesized by MIL-101(Cr) and MIL-101-NH₂(Cr) as the supports of metals showed that Pd/MIL-101-NH₂(Cr) outperforms Pd/MIL-101-(Cr) because of the higher electron density of Pd resulting from the electron donor ability of the NH₂ functional group. However, the same activities were observed for Pd₇₀Ni₃₀/MIL-101(Cr) and Pd₇₀Ni₃₀/MIL-101-NH₂(Cr), which is due to a less uniform distribution of Pd nanoparticles in Pd₇₀Ni₃₀/MIL-101-NH₂(Cr) originated from amorphization of MIL-101-NH₂(Cr) structure during the reduction process. In contrast, Pd₇₀Ni₃₀/MIL-101(Cr) revealed the stable structure and activity during reduction and CO oxidation for a long time.     

Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction

The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, and Pd) for the WGS reaction is investigated with respect to the physichochemical properties of the metallic phase and the support. It has been found that, for all metal-support combinations investigated, Pt is much more active than Pd, while Rh and Ru exhibit intermediate activity. The turnover frequency (TOF) of CO conversion does not depend on metal loading, dispersion or crystallite size, but depends strongly on the nature of the metal oxide carrier. In particular, catalytic activity of Pt and Ru catalysts, is 1-2 orders of magnitude higher when supported on “reducible” (TiO₂, CeO₂, La₂O₃, and YSZ) rather than on “irreducible” (Al₂O₃, MgO, and SiO₂) metal oxides. In contrast to what has been found in our previous study over Pt/TiO₂ catalysts, catalytic activity of dispersed Pt does not depend on the structural and morphological characteristics of CeO₂, such as specific surface area or primary crystallite size.     

Platinum group metals as catalysts in enantioselective 1-phenylpropane-1,2-dione hydrogenation

Different γ-Al₂O₃ supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C₁O₁ carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al₂O₃ (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation.     

NH3-TPD and XPS studies of Ru/Al2O3 catalyst and HDS activity

The effects of pretreatment of catalyst on its surface properties and the HDS activity of a 0.49% Ru/Al₂O₃ catalyst were studied in a single-pass, differential microreactor. The surface properties of the catalyst were measured by NH₃-TPD and XPS analysis. The Ru/Al₂O₃ catalyst was pretreated in three ways: reduced in H₂ (Ru-R catalyst), oxidized in air and subsequently reduced in H₂ (Ru-OR catalyst), or sulfided in H₂S/H₂ (Ru-S catalyst). Three types of peaks (low, middle, and high temperatures) were observed in the NH₃-TPD study. The predominant high-temperature peak was observed for both the Ru-OR and Ru-S catalyst, pretreated at 300°C. Mass spectrometry showed that the high-temperature peak in NH₃-TPD consisted of N₂ and H₂ formed from the decomposition of NH₃ on the ruthenium sites. NO adsorption of unsaturated Ru species was related to the low-temperature peak in the NH₃-TPD. The XPS analysis showed that the peaks at 279.9 eV, 280.6 eV, and 282.5 eV were ascribed to metallic ruthenium, RuO₂, and RuO₃, respectively. The low-, middle-, and high-temperature peaks were assigned to RuO₂, acid sites on alumina, and metallic Ru, respectively. Metallic ruthenium was effective in the HDS of thiophene and the decomposition of NH₃.     

Effect of additives on redox behavior of iron oxide for chemical hydrogen storage

The redox behaviors of iron oxides, which were modified with Pd, Pt, Rh, Ru, Al, Ce, Ti and Zr as additives, were investigated using temperature-programmed reaction (TPR) technique. The modified iron oxides were prepared by co-precipitation method using urea precipitant. The role of additives was also examined using XRD and SEM analysis in detail. As a result, Pd, Pt, Rh and Ru additives have an effect on promoting the reduction and lowering the re-oxidation temperature of iron oxide. Especially, it is revealed that the effect of Rh species on lowering the reduction temperature is attributed to decrease of activation energy for H₂ reduction according to Fe₂O₃ → Fe₃O₄ course. Meanwhile, Al, Ce, Ti and Zr additives played an important role in prevention of deactivation of iron oxide by repeated redox cycles. Redox performances of iron oxides were also enhanced due to cooperative effects by co-addition of Rh and another species such as Al, Ce and Zr. Finally, Fe–O/(Rh, Ce, Zr) sample exhibited good performance for H₂ evolution by water-splitting through synergistic effect of component additives.     

氢气高温还原制备的负载型镍基催化剂用于苯酚加氢的性能

通过等体积浸渍法分别将Ni(NO₃)₂、NiCl₂、NiSO₄ 3种镍前体浸渍于A1₂O₃或SiO₂载体上,然后通过H₂高温还原法制备了负载型镍基催化剂,考察了镍前体、载体种类、镍负载量、反应条件等对镍基催化剂苯酚加氢性能的影响。结果表明,对比3种镍前体,在H₂高温还原体系中Ni(NO₃)₂最容易被还原,制备的镍基催化剂苯酚加氢活性最高。SiO₂负载的镍基催化剂活性远高于γ-Al₂O₃催化剂。适宜的Ni负载量有助于活性组分的分散和催化活性的提高。镍基催化剂的苯酚加氢产物以环己醇为主,相对缓和的反应条件更容易生成环己酮。在非极性溶剂正庚烷或环己烷存在下,苯酚加氢反应速率远远高于极性溶剂水或乙醇存在下的结果,而且环己酮的选择性更高。     

The visible light induced photocatalytic activity of tungsten trioxide powders

The preparation, characterization and photoreactivity of tungsten trioxide powders are presented. Tungsten trioxide powders were prepared by air annealing of various W precursors. The effects of W precursor type and pretreatment conditions on the physical properties, and photocatalytic performance of the obtained WO₃ powders were examined. The photooxidation of water to oxygen and protons in the presence of reducible additives Ce⁴⁺, using the luminous and near IR illumination was used as a test reaction to evaluate the activity of the powders. Increasing annealing temperatures gave materials with a high degree of crystallinity and red-shifted the onset of light absorption. The light absorption of the obtained powders in the long wavelength region versus the type of the W precursor increased in the order: H₂WO₄<(NH₄)₆W₁₂O₃₉<(NH₄)₁₀W₁₂O₄₁. The level of crystallinity of the obtained powders increased in the order: (NH₄)₁₀W₁₂O₄₁≈(NH₄)₆W₁₂O₃₉2WO₄. The activity of the WO₃ powders depended on the type of W precursor used, annealing conditions, and the physico-chemical characteristics of the resulting powders. The activity according to the types of the W precursor increased in the order: (NH₄)₁₀W₁₂O₄₁<(NH₄)₆W₁₂O₃₉2WO₄. The activity as a function of annealing temperature and duration of W precursors goes through a maximum at 700–800°C and 4–8 h, respectively. Increasing the specific surface area of WO₃ powders, did not alter the activity significantly. Addition of 0.1–1% Pt and RuO₂ as co-catalysts improved the initial rates and long-term activity by about 1.3–1.5 times. Small amounts of hydrogen were also produced from photochemical reactions involving the photoexcitation of Ce³⁺.     

Effects of iron precursors on the structure and catalytic performance of iron molybdate prepared by mechanochemical route for methanol to formaldehyde

Mechanochemical synthesis has been applied for many novel material preparations and gained more and more attention due to green and high-efficiency recently. In order to explore the influences of iron precursors on structure and performance of iron molybdate catalyst prepared by mechanochemical route, three typical and cheap iron precursors have been used in preparation of iron molybdate catalyst. Many characterization methods have been employed to obtain the physical and chemical properties of iron molybdate catalyst. Results indicate that iron precursors have the significant impact on the phase composition, crystal morphology and catalytic performance in the conversion of methanol to formaldehyde. It is hard to regulate the phase composition by changing Mo/Fe mole ratios for Fe_2(SO_4)_3 as iron precursor. In addition, as for Fe_2(SO_4)_3, the formaldehyde yield is lower than that from iron molybdate catalyst prepared with Fe(NO_3)_3·9H_2O due to the reduction in Fe_2(MoO_4)_3 phase as active phase. Based on mechanochemical and coprecipitation method, the solvent water could be a key factor for the formation of MoO_3 and Fe_2(MoO_4) for FeCl_3·6H_2O and Fe_2(SO_4)_3 as precursors. Iron molybdate catalyst prepared with Fe(NO_3)_3·9H_2O by mechanochemical route, shows the best methanol conversion and formaldehyde yield in this reaction.     

Investigation of the oxygen storage process on ceria- and ceria–zirconia-supported catalysts

A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO₂ or Ce_0.63Zr_0.37O₂, were prepared. Catalysts were fully characterized using XRD, N₂ adsorption at −196 °C, TEM and H₂ chemisorption. Oxygen storage processes were carefully investigated. The influence of temperature was checked and a key role of oxygen diffusion was further demonstrated. A review of the reactions involved in the CO transient oxidation reaction is finally proposed.     

Two conversion maxima at 373 and 573 K in the reduction of nitrogen monoxide with hydrogen over Pd/TiO2 catalyst

Supporting Pt and Pd catalysts have been examined for the reduction of NO with H₂ in the presence of oxygen and moisture. All catalysts showed a conversion maximum in the NO reduction at around 373 K. An additional conversion maximum was found to appear at around 573 K over several metal oxides supporting Pd, and Pd/TiO₂ gave the highest conversion at around 573 K among the catalysts tested. In the reaction at 373 K, NO might be reduced directly by H₂ both on Pt and Pd catalysts to give N₂ and N₂O. At the conversion maximum of the Pd/TiO₂ catalyst at 575 K, however, in situ generated NO₂ seems to react with H₂.     

Preparation of Ru metal nanoparticles in mesoporous materials: influence of sulfur on the hydrogenating activity

Hydrogenating catalysts were prepared by inserting Ru into the pores of mesoporous Al-MCM-41 materials by selective adsorption of [Ru(NH₃)₆]³⁺. Ru/support catalysts were obtained after reduction with H₂. The activities of these catalysts in hydrogenation reactions were compared to those of Ru/HY and Ru/SiO₂. The catalytic properties in the absence of sulfur were tested in benzene hydrogenation, and the intrinsic activities of all the catalysts (either supported on mesoporous materials or on zeolites) were identical. It was concluded from this result that the dispersion of the Ru metallic phase was similar for all these catalysts. These samples were tested in the tetralin hydrogenation in pure H₂ and in the presence of H₂S (330 ppm of H₂S in H₂). They were found to be much less active than the zeolite-supported catalysts in the presence of H₂S. It is proposed that the lower activity of the catalysts supported on mesoporous materials is either due to their milder acidity, as evidenced by NH₃-TPD, cumene cracking and pyridine desorption experiments, or to the localization of the Ru nanoparticles on alumina islands.     

Influence of CO2 and H2O on NOx storage and reduction on a Pt–Ba/γ-Al2O3 catalyst

In this paper, the effect of CO₂ and H₂O on NO_x storage and reduction over a Pt–Ba/γ-Al₂O₃ (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO₂ and H₂O, NO_x is stored on BaCO₃ sites only. Moreover, H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N₂ as well as complete regeneration of stored NO. In the presence of CO₂, NO is oxidized into NO₂ and more NO_x is stored as in the presence of H₂O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO_x trapping in the presence of CO₂·NH₃ formation is seen in the rich phase and the selectivity towards N₂ is 83%. Ba(NO₃)₂ is always completely regenerated during the subsequent rich phase. In the absence of CO₂ and H₂O, both surface and bulk barium sites are active in NO_x storage. As lean/rich cycling proceeds, the selectivity towards N₂ in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO_x storage.