Bimetallic Pd–Pt catalysts are used industrially to saturate aromatics in industrial feedstocks under mild reaction conditions to bypass the thermodynamic limitations. A considerable amount of research effort has been focused on elucidating the structural and electronic properties of bimetallic Pd–Pt particles, generally supported on acidic supports, to correlate their properties with their sulfur tolerance as well as with their catalytic activity/selectivity. However, the properties of bimetallic Pd–Pt particles under these characterization conditions are still partly unknown, particularly what happens during hydrotreating. We therefore prepared bimetallic Pd–Pt catalysts (Pd/Pt atomic ratio of 4/1) using the same precursors of noble metals and various supports, such as acidic and non-acidic ultra-stable Y-type (USY) zeolites, SiO2–Al2O3, SiO2 and Al2O3, and then investigated the structural and electronic properties of the supported bimetallic Pd–Pt particles. These properties that appeared under liquid-phase hydrotreating conditions were correlated with reaction selectivity for tetralin hydrogenation and in 4,6-dimethyldibenzothiophene hydrodesulfurization, as well as with Fourier-transform analyses of adsorbed CO, dispersion and EXAFS data of reduced/sulfided catalysts. The effects of several parameters involved – such as calcination/activation conditions and the presence of extra-framework alumina and chlorine in the zeolite supports – on the sulfur tolerance of the bimetallic Pd–Pt catalysts were also investigated. In addition to sulfur poisoning, agglomeration of the Pd–Pt particles and inhibitory effects caused by nitrogen-containing compounds and aromatics were also investigated to develop measures to minimize the agglomeration of Pd–Pt particles under hydrotreating conditions for real feedstocks.

Supported Pd–Pt catalysts are efficient for hydrodesulfurization (HDS) and hydrodearomatization (HDA) reactions of diesel fuel and their activity varied with the kinds of supports. Concerning HDA, alumina supported catalysts showed four times higher TOF (turn over frequency) than silica supported one. In order to elucidate the difference in activity, the structural analysis of the active phase was performed. After reduction pretreatment, relatively uniform and large metallic alloy Pd–Pt particles were formed on SiO2, whereas, Pd and Pt atoms formed rather segregated particles on Al2O3. Subsequent X-ray absorption of fine structure (XAFS) analysis under HDS conditions showed no contribution of sulfur for SiO2 supported catalyst, whereas, formation of sulfided metal species was observed in XAFS spectra for the Al2O3 supported catalyst. It is suggested that on Pd–Pt/SiO2, thin sulfide layer on the metal cluster surface blocked the active sites and lowered the HDA activity. Presence of partially sulfided phase originated from rather segregated structure like Pd–Pt/Al2O3 is thought to be requisite for high HDA activity.

In-situ XAFS observation of Pd-Pt catalysts supported on SiO2, Al2O3 and USY was carried out during reduction and sulfidation processes. Under sulfidation conditions, Pd-Pt species on USY was the most seriously sulfided, whereas, no contribution of sulfur was observed in Pt LIII-edge EXAFS of sulfided Pd-Pt/SiO2. Pd-Pt species on Al2O3 showed intermediate properties for sulfidation. It is found that Pd and Pt metal species formed relatively uniform alloy particles on SiO2 and Al2O3, whereas, on USY, Pd and Pt atoms were segregated in the alloy particles.

Ultrafine Pd nanoparticles with an extremely narrow distribution were prepared by sonochemical reduction of [Pd(NH3)(4)](2+) in the presence of I-propanol. The average diameter of the particles was approximately 5 nm. These nanoparticles were extremely stable in the colloidal state, even in the absence of stabilizers. After vacuum drying, the primary particles connected to each other and formed radially arranged nanostructures of about 30-50 nm.

The effects of coexistence of nitrogen compounds on the catalytic activity of the Pd and Pt monometallic catalysts supported on amorphous silica and high-silica USY zeolite (SiO2/Al2O3=390) were investigated using a high-pressure fixed-bed continuous flow reactor operating at 3.9MPa and 553K. Types of catalytic activity studied included the tetralin hydrogenation activity and the 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization activity. The Pd/USY zeolite catalyst showed the highest tetralin hydrogenation activity with feedstock containing only 4,6-DMDBT (S=300ppm) without nitrogen compounds. However, its hydrogenation activity was strongly inhibited by the addition of n-butylamine (N=20ppm). Significant inhibitory effects by nitrogen compounds were also observed for the Pd/SiO2 catalyst. The Pt/USY zeolite catalyst showed lower hydrogenation activity than the Pd/USY zeolite, while the Pt/SiO2 catalyst showed higher activity than the Pd/SiO2 catalysts. The inhibitory effects of nitrogen compounds on these Pt catalysts were less pronounced than on Pd catalysts. On the other hand, with respect to hydrodesulfurization, Pt catalysts showed higher activity in 4,6-DMDBT hydrodesulfurization than Pd catalysts, irrespective of the support species, due to the latter's higher hydrogenolytic ability to cleave the CS bond even in the presence of nitrogen compounds. On the other hand, Pd catalysts were subject to a decrease in 4,6-DMDBT hydrodesulfurization resulting from a loss in hydrogenation activity in the presence of nitrogen compounds. We conclude that Pt catalysts are superior to Pd catalysts with respect to both hydrogenation and hydrodesulfurization for the hydrotreatment of industrial feedstocks containing sulfur and nitrogen compounds.

The sulfur tolerance of monometallic Pd, Pt and bimetallic Pd–Pt catalysts supported on slightly acidic ultra-stable Y (USY) zeolite (SiO2/Al2O3=390) and on non-acidic silica, having mesopores with a pore diameter of 3 or 10nm, were investigated using the CO adsorption method and the extended X-ray adsorption fine structure (EXAFS) method. Well-dispersed noble metal particles supported on USY zeolite and silica with an average pore diameter of 3nm showed high surface sulfur tolerance and high catalytic hydrogenation activity, although bulk phase sulfidation simultaneously occurred. The synergistic effects of sulfur tolerance were significant in the bimetallic Pd–Pt particles supported on USY zeolite and silica with an average pore diameter of 3nm. On the other hand, on silica with an average pore diameter of 10nm, the surface sulfur tolerance of low dispersed noble metals was the lowest, although its bulk phase sulfur tolerance was the highest. The Pd K-edge and Pt LIII-edge EXAFS spectra indicated a strong interaction between the well-dispersed noble metal particles and the supports of the USY zeolite and silica with an average pore diameter of 3nm. This distorted structure may increase the sulfur tolerance of noble metals, though some surface and bulk phase sulfidation simultaneously occurred.

The local structure around Ni in a nickel phosphide catalyst supported on SiO2 was investigated by an in situ X-ray absorption. ne structure (XAFS) method during the reduction process of the catalyst and the hydrodesulfurization (HDS) reaction of thiophene. In the passivated sample, the Ni phosphide was partially oxidized but during the reduction process Ni2P particles were regenerated. The particles had Ni-P and Ni-Ni distances of 0.220 and 0.259 nm, respectively, corresponding closely to those of bulk Ni2P. In situ XAFS clearly revealed that Ni2P structure was stable under the reaction conditions at atmospheric pressure and was an active structure for the HDS process.

The size effect of monometallic Pd and Pt clusters was investigated for the sulfur tolerance of these noble metal catalysts supported on high-silica USY zeolite, using the CO adsorption method and the extended X-ray adsorption fine structure (EXAFS) method. Both Pd and Pt particles show the highest surface sulfur tolerance in the form of <20 Å-diameter particles, although the bulk phase of these clusters is subject to penetration by adsorbed sulfur atoms. For noble metal particles >25 Å in diameter, the bulk phases of the Pd and the Pt particles approach that of the metallic structure. With increased cluster size, the surface sulfur tolerance of Pt particles decreases gradually, while that of Pd particles decreases rapidly. For large noble metal particles >50 Å in diameter, EXAFS spectra show that sulfidation in the bulk phase of the noble metal particles is suppressed for Pt but promoted for Pd.

The crystal structure of an open-tunnel oxide, alpha-MnO2, free from any large stabilizing cations was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). Rietveld refinement from neutron powder diffraction data for a partially deuterated specimen of MnO2 . 0.1 (D0.34H0.66)(2)O showed it to have a hollandite-type structure (tetragonal; space group I4/m: a = 9.777(2) and c = 2.8548(5) Angstrom; Z = 8; R-wp = 4.56%, R-P = 3.67% R-B = 1.52%, and R-F = 0.77%; S = 1.23). The bond valence sum of Mn was calculated at +4.04. The quadratic elongation and bond angle variance for the MnO6 octahedron proved that its distortion is relatively small even if small H2O molecules are contained in tunnels instead of large stabilizing cations. Electron-density distribution (EDD) in MnO2 . 0.15H(2)O was visualized by MEM-based pattern fitting from both synchrotron and conventional X-ray powder diffraction data. The resulting EDD images showed that the inner effective diameters of a cage in alpha-MnO2 are about 2.6 Angstrom for a bottleneck on the (002) plane and about 4.8 Angstrom for an inner space on the (001) plane. Thus, H2O molecules (2.2 Angstrom) can be trapped in the narrow tunnels of alpha-MnO2 whereas N-2 molecules (4.3 Angstrom) cannot penetrate the tunnel cavity. Elongation of electron densities for tunnel water along the tunnel direction was observed in the EDD images. Further, to obtain a reasonable isotropic atomic displacement parameter for the O-3 site in the tunnel cavity, O-3 had to be split into two pieces at the 4e site in the Rietveld refinement from the neutron diffraction data. These findings provide evidence that H2O molecules are not only vibrating markedly but also highly disordered, particularly along the [001] direction, near the center of the cage. (C) 2003 Elsevier Inc. All rights reserved.

Effect of pore structure of silica support on sulfur tolerance and tetralin hydrogenation activity of Pd, Pt and Pd-Pt catalysts was investigated. Pore diameter of SiO2 supports affected the sulfur tolerance of noble metals and resulting hydrogenation activity. High sulfur tolerance and tetralin hydrogenation activity were observed for the Pd-Pt and Pt catalysts supported on SiO2 having the average pore diameter of 3 nm. This sulfur tolerance was comparable to those supported on ultra stable Y (USY) zeolite having the SiO2/Al2O3 ratio of 390.

Deep hydrodesulfurization (HDS) of sterically hindered sulfur compounds in gas oils will require enhanced hydrogenation activity to hydrogenate the aromatic rings of the sulfur compounds. Although H2S is known to inhibit the direct HDS route for most of the sulfided catalysts, its promotion to the hydrogenation and subsequent HDS was newly observed for unsupported MoS2. This promotion suggests that ultra deep HDS over sulfide catalysts can be achieved along with high metal loading, minimal support-metal interactions and optimal dependence on the Ni species. On the other hand, the strong hydrogenation activity of sulfur-tolerant noble metal catalysts suggests that ultra deep HDS as well as deep aromatics saturation can be achieved. This paper discusses recent catalytic approaches for ultra deep HDS using conventional sulfide catalysts and/or noble metal catalysts, such as the newly developed Pd-Pt/Yb-USY zeolite catalyst.

Mesoporous titanium oxide with a high specific surface area of 120m2/g prepared by a novel method developed by Chiyoda was used for supporting molybdenum sulfide. In order to examine the influence of the surface area on the properties of the molybdenum sulfide phase, two different samples of titanium oxide were studied, a commercial one with a surface area of 72m2/g and that prepared by Chiyoda. Molybdenum was deposited on the TiO2 supports by incipient wetness impregnation with ammonium heptamolybdate in one or two steps depending on the Mo loading. Some samples were also prepared by impregnation of ammonium heptamolybdate basified by ammonia. Raman spectroscopy and XPS were used to examine the nature of the molybdate phase and its dispersion in the oxidic state. HREM and XPS were used for studying the sulfided state. As expected, the maximum amount of well-dispersed molybdenum is higher on the Chiyoda support than on the reference support with a lower surface area. The catalytic properties of the catalysts were studied in dibenzothiophene conversion. For the Chiyoda support, the catalytic activity varied linearly with the Mo loading up to 6–7 Mo/nm2 then became nearly constant for the higher loadings. Much higher activities (six times, expressed per gram of catalyst) were obtained compared to molybdenum sulfide supported on alumina.

Local structure around Ni in a nickel phosphide catalyst supported on K-USY was investigated by an in situ X-ray absorption fine structure (XAFS) method during the reduction process of the catalyst and the hydrodesulfurization (HDS) reaction of thiophene. In the passivated sample, Ni phosphide was partially oxidized but after the reduction, 1.1 nm diameter Ni2P particles were formed with Ni-P and Ni-Ni distances at 0.218 and 0.261 nm, respectively, corresponding to those of bulk Ni2P. In situ XAFS clearly revealed that the Ni2P structure was stable under reaction conditions and was an active structure for the HDS process.

Effects of lanthanum (La) modification on the surface properties and hydrodesulfurization (HDS) activity of NiMo/Al2O3 catalysts were studied. In the La-modified catalysts, La inhibited the lateral growth of MoS2 layers to form stacked MoS2 layers. The La-modified catalysts slightly enhanced the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) via the hydrogenation (HYD) pathway. The La loading influences the surface concentration of active components, and causes a decrease in surface Ni concentration of the sulfided catalysts. A low La loading (0.7wt.%) increased the HDS reaction rate constant of the NiMoLa/Al2O3 catalysts by more than 80%, whereas a further increase in the La loading decreased the HDS activity, due to a decrease in the surface Ni concentration.

Bimetallic palladium-platinum catalyst supported on yttrium-modified ultrastable Y (USY) zeolite, Pd-Pt/Y-USY, showed high activity and stability for deep hydrodesulfurization (HDS) and hydrodearomatization (HDA) of diesel fuel. In model reactions, the HDS activity based on rate constant increased as much as 5.3 times and HDA activity increased as much as 1.9 times after yttrium modification. The deeper hydrogenation of 4, 6-dimethyldibenzothiophene (4, 6-DMDBT) allowed the following HDS reactions to be promoted over the Pd-Pt/Y-USY catalyst. The adsorptive interaction between Pd-Pt/Y-USY and basic tetralin was weaker than that between Pd-Pt/USY and tetralin. Under reaction conditions of P = 4.9 MPa, WHSV = 4 h-1 and T = 280°C for hydrotreating desulfurized gas oil feedstock, Pd-Pt/Y-USY successfully removed a large part of 4, 6-DMDBT and also almost all refractory alkyl-substituted sulfur compounds, which are supposed to be more difficult to hydrodesulfurize. The hydrotreated product after 216 h on stream contained 28 wtppm sulfur and 8 wt% aromatics. NH3 adsorption analyses showed that yttrium modification decreased the number of strong acidic sites with little change in the total acidic sites. This modification in acidity could help to minimize the excessive hydrocracking which causes carbonaceous deposits, and to increase the nitrogen tolerance. Scanning transmission electron microscopy (STEM) analyses clearly showed that yttrium modification suppressed the agglomeration of Pd-Pt phases, which might be linked to the high stability of the Pd-Pt/Y-USY catalyst. Therefore, Pd-Pt/Y-USY catalyst is quite promising as a second stage catalyst for the integrated two-stage reformulation of gas oils.

In-situ XAFS observation of reduction process of USY zeolite supported Pd and Pd-Pt catalysts was performed to elucidate the structure of active Pd and Pd-Pt metal particles in connection with catalytic activity. Some of the metal particles were supposed to be located in mesopors ranging from 10 to 50 nm in diameter. Pt was found to promote reduction of Pd. The average size of Pd-Pt metal particles was similar to that of Pd. Pt atoms formed fine particles and were dissolved in Pd particles.

Dehydrogenative cracking reaction of n -butane was studied using HZSM-5 catalyst modified with various metal oxides. Alkaline earth (magnesium), transition metal (cobalt) and rare earth (lanthanum) elements are used for the modification. The selectivity of the products was studied at low conversion (≲20%). Methane, ethane, ethylene, propylene, butenes and butadiene were the main products. With the use of the cobalt- or magnesium-containing HZSM-5, dehydrogenative cracking was observed and the selectivity of ethylene was much larger than that of ethane. On the other hand, the selectivity of ethylene and ethane were almost the same in the reaction using the lanthanum-containing HZSM-5. It is considered that the cobalt- and magnesium-loaded sites on HZSM-5 played an important role in the dehydrogenative cracking.

The effect of alkaline earth modification on HZSM-5 was investigated by catalytic cracking of n-butane under non-oxidative and oxidative conditions. The yields of aromatic products were low compared with that obtained using the non-modified HZSM-5, and higher yields of ethylene and propylene were observed with Mg-, Ca-, and Ba-ZSM-5. The NH 3 -TPD spectra of these catalysts show that the strong acid sites were transformed to weak acid sites. The dehydrogenation cracking was considered to occur at the acid sites modified with the alkaline earth elements because the ethylene/ethane ratio and the C 2 /other products ratio were high using the alkaline earth-containing HZSM-5. It is suggested that the suppression of hydrogen transfer reaction and the stimulation of dehydrogenation cracking were the major cause of the improvement of olefin yield in the cracking.