Stability of supported metal and supported metal oxide combustion catalysts

Catalysts used for high-temperature combustion of light hydrocarbons must maintain high activity over long time intervals by avoiding excessive sintering and deactivation in the hot and corrosive combustion environment. The sintering resistance and chemical stability of catalytically active phases is a key technical problem that must be solved for the development of commercially viable combustion catalysts. All noble metals and transition metal oxides that are catalytically active rapidly sinter at temperatures required for high combustion rates. Advanced materials used in the development of stable catalysts include highly sintering-resistant hexaaluminate supports for dispersion of noble metals, chemically and thermally stable supporting oxides for active transition metal oxides, and single-phase, substitutionally activated, sintering-resistant complex metal oxides. This paper will review deactivating phenomena, such as sintering and vapor transport and assess recent progress in the development of durable combustion catalysts.     

NMR studies of supported metal catalysts

Oxide-supported metals constitute one of the most important classes of heterogeneous catalysts, and for this reason they have been investigated by many techniques: adsorption isotherms, IR of chemisorbed molecules, electron microscopy, EXAFS, etc. However, the fact that they have been studied by so many methods proves that no one technique is totally satisfactory. The physical and chemical properties of sufficiently small particles, in particular their magnetism, must depend on their size, whence the idea of using NMR to study them. Because of the theoretical and technical difficulties associated with the NMR detection of metals, and especially since only a few can be detected, NMR work has been mainly devoted to the indirect study of metallic properties, by the intermediary of easily detected chemisorbed phases (¹H, ¹³C) or of physically adsorbed probes (¹²⁹Xe). However, when the metal can be detected directly (mainly Pt), it is possible to relate the form of the NMR spectrum to the dispersion of the metal and to calculate the electron density of states at the Fermi level.     

Dibenzothiophene hydrodesulfurization by noble metal supported catalysts

The hydrodesulfurization of dibenzothiophene (DBT) is investigated at 240°C, under one atmosphere pressure of H₂ + DBT, in a DBT conversion range giving rise to a H₂S concentration between 30 and 200 ppm. The turnover number (TN) of platinum slightly decreases as the dispersion of platinum (Ptins) to Pt ratio = D (Pt)) increases from 0.01 to 0.7, whatever support (α-Al₂0₃, SiO₂, chlorinated γ-Al₂O₃, SiO₂-Al₂O₃, montmorillonite) was used. Furthermore, the TN is much lower for highly dispersed Pt/γ-Al₂O₃-C1 (D(Pt) = 0.9; ø (Pt) = 1.0 nm). Pt/TiO₂ (D(Pt) = 0.06 or D(Pt) = 0.34) reduced or re-reduced at 240°C has a higher TN than the preceding catalysts. Reduction or re-reduction of Pt/TiO₂, at 500°C results in only a relatively small decrease in the catalytic activity, in spite of the appearance of a strong metal-support interaction observed by several workers. For different supported metals on Al₂O₃, the TN follows the sequence: Pt > Ir > Ru > Re. Alloying Ir or Ru (25 to 30 at %) with Platinum results in a relatively small decrease in the TN (per one (Pt_s + Ir_s or Ru_s ) atom).The activity, per unit weight of catalyst, of an industrial Co-Mo/γ-Al₂O₃ sulfided catalyst (HR 306) is equivalent to that of 1 % Pt/γ-Al₃0₃ (D(Pt) = 0.7). The present platinum supported catalysts have a very low selectivity in hydrogenation (production of phenylcyclohexane, PCH < 1 %), except when supported by silica, SiO₂-Al₂O₃ or montmorillonite; the Co-Mo/γ-Al₂0₃ catalyst has a selectivity which is intermediate between Pt/γ-Al₂O₃ and Pt/SiO₂-Al₂O₃ or montmorillonite. The deactivation (decrease in activity versus the time en-stream) is lower for Co-Moγ-Al₃0₃ than for Pt/γ-Al₃O₃, but the addition of Ge to Pt/γ-Al₂O₃ gives rise to a more stable catalyst.     

Selective hydrogenation of cyclopentadiene over supported metal catalysts

The selective hydrogenation of cyclopentadiene to cyclopentene has been studied in the liquid phase using Pd and Pd Me/Al₂O₃ bimetallic catalysts (Me = Mn, Ni, Co, W). The highest activity was obtained with Pd Co and Pd W/Al₂O₃. For these catalysts, no hydrogen or CO chemisorption was detected although Pd could be seen by XPS at 335·8 eV; it is considered that new species, more active for the selective hydrogenation of cyclopentadiene, appeared at the catalyst surface. The sulfur resistance towards thiophene has also been studied. It was observed that the highest sulfur resistance is coincident with the highest activity. XPS analysis shows that the poisoning species is thiophene adsorbed on the catalyst surface.     

Hydrogenation of 2-methylindole using supported metal catalysts

The hydrogenation of 2-methylindole to 2-methylindoline in toluene was studied at 60 °C and 40 bar hydrogen pressure in a batch mode using a range of carbon, alumina, and zeolite supported metal catalysts were performed in toluene. The results were compared with a more conventional catalytic system containing besides 5% Pt/C also 1.2 eq. of p-toluenesulfonic acid in ethanol:water (50:50 v/v). The highest turnover frequency (TOF) was obtained a zeolite supported iridium catalyst.     

Oxidative decomposition of naphthalene by supported metal catalysts

A fixed-bed reactor was utilized to investigate the activities of six metal catalysts (1% Pt, 1% Pd, 1% Ru, 5% Co, 5% Mo and 5% W on γ-Al₂O₃ support) in decomposing naphthalene, based on the production of carbon dioxide and the disappearance of naphthalene. The Pt and Pd catalysts were found to exhibit higher naphthalene oxidizing activity than other catalysts tested. The Co catalysts, whose activity is similar to that of the Ru catalysts, are promising for naphthalene oxidation. The kinetic results of naphthalene oxidation over 1% Pt/γ-Al₂O₃ catalysts are reported for the first time. A first-order reaction with respect to P_naphthalene was found, while the reaction order with respect to P_O2 decreased with increasing reaction temperatures. A Langmuir–Hinshelwood model was used to describe the observed kinetic behavior. Oxygen adsorption dominates at higher reaction temperatures (>140 °C), and consequently the oxidation of naphthalene over the Pt catalysts appeared to be insensitive to P_O2.     

负载型金属配合物催化硅氢加成反应研究进展

硅氢加成反应是有机硅化学中制备含Si-C化合物的一类重要反应。综述了硅氢加成反应中负载型金属配合物催化剂的种类、催化性能及催化机理,并展望了此类反应的研究前景,表明探索价廉高效的硅氢加成催化体系将是这一领域的主要研究方向。     

Bio-ethanol catalytic steam reforming over supported metal catalysts

Considering both the influence of the nature of the metal (Rh, Pt, Ni, Cu, Zn, Fe) and the role of the support (γ-Al₂O₃, 12%CeO₂–Al₂O₃,CeO₂,Ce_0.63Zr_0.37O₂), CO₂ is presented as a primary product in the bio-ethanol steam-reforming catalytic reaction (SRR) over some supported metal catalysts. Based on this unexpected observation, a new strategy for maximizing the hydrogen production and minimizing the CO formation is proposed. Any highly selective catalytic formulation should be free of any promoter in the water gas shift reaction (WGSR) which tends to equilibrate the SRR gas towards higher CO concentrations.     

负载化/固载化酞菁金属催化剂研究进展

作为新材料及催化剂,酞菁金属化合物得到了广泛深入研究。综述了酞菁金属催化剂负载化/固载化的研究进展。以共价键或配位键将酞菁金属与高分子载体连接,使得酞菁金属催化剂固载化,从而减少催化剂活性组分的流失,提高催化剂性能,是目前该领域的重要研究方向。而在酞菁金属原子上引入种类不同、数目不同的轴向配体,则能够大大改善酞菁金属化合物的催化及光学性能,为酞菁金属材料新的应用奠定了基础。     

Catalytic combustion of benzene over supported metal oxides catalysts

Catalytic combustion of benzene over supported metal oxides has been investigated. The catalysts have been prepared by incipient wetness method and characterized by XRD, FT-Raman, ESR and TPR. Among supported metal oxides, CuO_x, supported on TiO₂ is found to have the highest activity for benzene oxidation. In addition, among the catalysts of copper oxide supported on TiO₂, A1₂O₃ and SiO₂, titania-supported catalyst (CuO_x/TiO₂) gives the highest catalytic activity. CuO_x/TiO₂ (Cu loading 5.5 wt%) shows the total oxidation of benzene at about 250 °C. From the ESR and FT-Raman results, the CuO dispersed on the TiO₂ surface acts as an active site of CuO_x/TiO₂ catalysts on the oxidative decomposition of benzene. The catalytic activity gradually increases with an increase of Cu loading on TiO₂. When Cu loading reaches 5.5 wt%, the total conversion temperature is lowered to 300 °C. However, the catalytic activity considerably decreases at 7 wt% Cu loading. The catalytic activity increased with an increase of oxygen concentration but the concentration of benzene showed no difference in the benzene conversion. This result suggests that the rate determining step is the adsorption of oxygen.     

负载型金属催化剂催化KBH4水解析氢性能研究

以高比表面积和规整性强的蜂窝陶瓷（2MgO₂·Al₂O₃·SiO₂）为载体，涂敷TiO₂后，用水热合成法分别负载铁、钴和镍盐，500 ℃焙烧及硼氢化钾（KBH₄）浸渍后，合成了负载型金属催化剂。负载不同金属盐的催化剂析氢性能比较结果为钴＞铁＞镍。实验研究表明，负载硝酸铁［Fe(NO₃)₃·9H₂O］和氯化钴(CoCl₂·6H₂O)的催化剂经KHB₄浸渍后析氢性能均明显改善。利用X射线衍射（XRD）和X射线光电子能谱（XPS）对催化剂的表征结果证实,负载硝酸铁催化剂表面真正起催化析氢性能的活性组分为单质铁，负载氯化钴催化剂表面生成了CoTiO₃，且该催化剂具有优良的催化碱性KBH₄溶液水解析氢性能，常温下该催化剂0.34 g于NaOH质量分数约为18%，KBH4质量分数约为10%的溶液中催化析氢速率可达160 mL·min^－1。     

Palladium-hydrogen interaction on supported palladium catalysts of different metal dispersions

Palladium-hydrogen interaction on supported Pd catalysts of different metal dispersions has been studied by hydrogen chemisorption and back-sorption, and by temperature-programmed desorption (TPD) of either deuterium or hydrogen from the Pd surface after different initial gas dosages. Pd interacts with three hydrogen species whose amounts vary with the metal dispersion and with the dosage pressure. The amount of hydrogen strongly adsorbed on the Pd surface increases with the metal dispersion but is unaffected by the hydrogen pressure. The amount of hydrogen absorbed in the bulk of Pd is significant with a poorly dispersed catalyst, particularly when the hydrogen pressure is higher than about 10 Torr, but decreases with the metal dispersion. Hydrogen weakly bound with Pd shows two characteristic TPD peaks, whose intensity depends on the metal dispersion and the initial gas dosage. The peak at 270-330 K, assigned to the recombination of absorbed hydrogen with surface hydrogen, is large with a poorly dispersed catalyst and grows with the initial hydrogen dosage. The intensity of the peak at 215-226 K is not affected by the metal dispersion or by the hydrogen dosage. It has been concluded that an analysis of the intrinsic properties of Pd catalysts should be based on an understanding of the hydrogen species associated with the catalysts.     

Synthesis of supported metal nanoparticle catalysts using ligand assisted methods

The synthesis and characterization methods of metal nanoparticles (NPs) have advanced greatly in the last few decades, allowing an increasing understanding of structure-property-performance relationships. However, the role played by the ligands used as stabilizers for metal NPs synthesis or for NPs immobilization on solid supports has been underestimated. Here, we highlight some recent progress in the preparation of supported metal NPs with the assistance of ligands in solution or grafted on solid supports, a modified deposition-reduction method, with special attention to the effects on NPs size, metal-support interactions and, more importantly, catalytic activities. After presenting the general strategies in metal NP synthesis assisted by ligands grafted on solid supports, we highlight some recent progress in the deposition of pre-formed colloidal NPs on functionalized solids. Another important aspect that will be reviewed is related to the separation and recovery of NPs. Finally, we will outline our personal understanding and perspectives on the use of supported metal NPs prepared through ligand-assisted methods.     

氧化物助剂对费托合成钴基催化剂的促进作用

氧化物助剂促进的钴基催化剂具有费托合成反应活性和长链烃选择性高等特点,是高选择性地获得馏分油的关键,具有良好的应用和研究价值。本文针对氧化物助剂对钴基催化剂的促进作用,综述了氧化物助剂对钴基催化剂的结构、稳定性以及费托合成反应性能的影响,详细分析了氧化物助剂对钴基催化剂的还原性能和分散度的影响, 同时介绍了影响氧化物助剂促进作用的因素,重点讨论了氧化物助剂的促进作用机理。并对如何更好地发挥氧化物助剂在钴基催化剂中的促进作用进行了展望：应加强氧化物助剂对钴基催化剂促进作用机理的基础研究,并且重视影响氧化物助剂促进作用的因素。     

Catalytic wet oxidation of p-chlorophenol over supported noble metal catalysts

Heterogeneous catalytic wet oxidation of aqueous p-chlorophenol (p-CP) solution was investigated at 453 K and 2.6 MPa in a slurry reactor. The performance of noble metal catalysts was compared with that of traditional manganese catalysts. Activated carbon supported catalysts showed significant higher activities for total organic carbon (TOC) reduction than those supported on alumina or cerium oxide. Pt was found to be the most active metal for p-chlorophenol oxidation. The activities of noble metal catalysts were found to correlate with heat of formation of metal oxides. CO₂ is the predominant oxidation product with formation of minor p-benzoquinone and acetic acid as intermediate compounds. Possible reaction pathway was also discussed.     

Production of supported metal catalysts by the decomposition of metal carbonyls (review)

The following text is a review of efforts made to produce highly dispersed metal catalysts via the thermal decomposition of metal carbonyls. It is shown that on hydroxylated supports the decomposition of all metal carbonyls follows a characteristic pattern. Low-temperature (<400 K) heating leads to the formation of partially decarbonylated metallic species (subcarbonyls) which are associated with hydroxyl groups. It is probable that these metallic subcarbonyls are monodispersed. Higher-temperature treatment (the precise temperature is a function of the particular metal carbonyl) leads to the interaction of the subcarbonyls with the oxygen of the hydroxyl groups to form completely decarbonylated, possibly monodispersed, metal oxide species. There is some evidence that these highly dispersed oxide species have unique catalytic properties. Only following high-temperature reduction (ca. 750 K) have stable metallic particles been produced on hydroxylated supports by metal carbonyl decomposition. The metal particles produced in this manner are not of an unusually high dispersion. The nature of the decomposition process on dehydroxylated supports has not been as thoroughly studied as the process on hydroxylated supports. However, preliminary results offer promise that metal carbonyl decomposition on dehydroxylated supports may lead to the formation of very highly dispersed, zero-valent metal particles.     

A new method to prepare highly dispersed supported metal catalysts

On a solid surface the effects of the implosion of the bubbles, created during a sonication run, are commonly used to clean or to erode the surface itself. In the present work, ultrasound was used in order to obtain a ruthenium on alumina catalyst distinguished by a very high value of the dispersion of the metal. The samples were characterized with different techniques in order to study the influence of the sonication on both the metal crystallites and the support.     

Carbon xerogel supported noble metal catalysts for fine chemical applications

Carbon xerogels are mesoporous materials obtained upon pyrolysis of the dried gels resulting from polycondensation of resorcinol and formaldehyde. Treatment with nitric acid under severe conditions introduces high amounts of oxygen containing functional groups onto the surface of the material, leading however to the collapse of its porous structure. The resulting material is then used to support 1 wt.% Pt, Ir and Ru monometallic catalysts by wet impregnation using organometallic precursors. The catalysts are characterized by N₂ adsorption–desorption isotherms at 77 K, temperature programmed desorption coupled with mass spectrometry, scanning and transmission electron microscopy, and H₂ chemisorption. The liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol is used in order to assess the catalytic performance of the prepared materials. Pt and Ru catalysts are initially very selective towards the hydrogenation of the olefinic double bond, while Ir is mostly selective towards the carbonyl group. After a thermal post-reduction treatment at 973 K, selectivity towards cinnamyl alcohol is significantly improved regardless of the metal nature. The Pt catalyst exhibits the best behavior, a complete shift in C=C to C=O hydrogenation being detected. The improvement in selectivity is rationalized in terms of both an increase in metal particle size and a modification in the surface chemistry of the catalyst after the post-reduction treatment.     

High-resolution scanning electron microscopy for the characterization of supported metal catalysts

A scanning electron microscope with a short-focal-length immersion lens and subnanometer resolution has been used to characterize several oxide-supported metal particle catalysts. Nanometer-sized metal particles in the Pt/TiO₂ and Pd/SiO₂ systems could be imaged with best clarity at the upper end of the operating voltage range (20–30 kV). However, visibility depended upon an adequate yield of secondary electrons relative to the support: small Pt particles on CeO₂ could not be located by secondary electron imaging. Best visibility of the surface topography of the support was obtained at lower accelerating voltages.     

Recent progress in diffuse reflectance spectroscopy of supported metal oxide catalysts

Diffuse reflectance spectroscopy is a suitable technique for studying heterogeneous catalysts, as both d–d and charge transfer transitions of supported transition metal ions can be probed. Within the past several years, new developments have resulted in a more detailed understanding of the surface chemistry of supported metal oxide catalysts. In this review, the fundamental advances of the use of diffuse reflectance spectroscopy in the field of heterogeneous catalysis will be discussed. It will be shown that (1) oxidation states can be spectroscopically quantified in well-defined conditions; (2) chemometrical techniques are crucial for an unbiased analysis of the spectra; (3) in situ spectroscopy, in combination with experimental design and on line catalytic measurements, allows the development of relevant structure–activity relationships, and (4) the reduction kinetics of supported metal oxides can be evaluated.