Applied Catalysis A, General (v.295, #1)
The catalytic active sites in partially reduced MoO3 for the hydroisomerization of 1-pentene and n-pentane by H. Al-Kandari; F. Al-Kharafi; N. Al-Awadi; O.M. El-Dusouqui; S.A. Ali; A. Katrib (1-10).
In situ characterization by XPS and UPS of bulk commercial MoO3 and the equivalent five atomic layers of MoO3 deposited on TiO2 before and following the reduction by hydrogen at different temperatures led to the identification of the chemical species on the surface. These spectra reveal that MoO3 is the only state present prior to reduction. However, a predominant Mo5+ state on the sample surface is formed at reduction temperatures between 573 and 623 K. Continued reduction leads to the formation of MoO2, which reaches a stable state at temperatures between 653 and 673 K. The metallic character of MoO2 is observed as a density of states (DOS) at the Fermi level. This metallic function dissociates hydrogen molecules to atoms. Bonding of these active (H) atoms with surface oxygen leads to the formation of Brönsted acid (Mo-OH) group(s) as characterized by O 1s and catalytic properties. As a result, a bifunctional MoO2(H x )ac phase is formed on the outermost sample surface layer in these systems. More sensitive UPS measurements established the presence of MoO2 in the upper four to five surface monoatomic layers during the reduction process. Combination of XPS–UPS provides valuable information on the chemical composition of the upper surface layers. Apparently, a stable sandwich-like structure is formed in which the composition from bulk to surface consists of MoO3, Mo2O5 and MoO2. The outermost surface layer in this structure is a bifunctional MoO2(H x )ac phase. The catalytic performances of the different Mo suboxides towards the hydroisomerization reactions of 1-pentene and n-pentane were studied. The MoO3 phase has no catalytic activity due to its insulating properties. Double-bond and skeletal isomerization reactions of 1-pentene take place on the acid function(s) of Mo5+ state, while n-pentane and isopentane are the only products observed in the case of MoO2(H x )ac. Both metallic and acid functions are required for the isomerization of n-pentane to isopentane. Consequently, it is concluded that the dehydrogenation process is the rate-determining step in the isomerization reaction of n-pentane. Hydrogenation of 1-pentene is the dominant reaction when the bifunctional MoO2(H x )ac phase is used.
Keywords: XPS–UPS of MoO3 and MoO2; TiO2; Hydroisomerization of 1-pentene and n-pentane;
Ultra deep hydrodesulfurization of dibenzothiophene derivatives over NiMo/TiO2-Al2O3 catalysts by Y. Saih; M. Nagata; T. Funamoto; Y. Masuyama; K. Segawa (11-22).
We have investigated the catalytic activity of NiMo sulfide catalysts supported on TiO2-coated Al2O3 carriers during the HDS of dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene at 603 K and 3 MPa. TiO2-Al2O3 supports were prepared by chemical vapor deposition of TiCl4 on the surface of γ-Al2O3 (240 m2 g−1, 0.84 cm3 g−1). Using XPS, we showed that TiO2 is highly dispersed on the surface of Al2O3 for TiO2 loadings up to ca. 14 wt%. Moreover, TiO2-Al2O3 (14 wt% TiO2) support showed textural properties very similar to those of the parent alumina support. The composite support with TiO2 loading around 14 wt% was used as support for NiMo sulfide catalysts. The sulfidation degree of Mo, as determined by XPS, was higher for NiMo/TiO2-Al2O3 than for NiMo/Al2O3. This can be attributed to the lower interaction of molybdenum species with the TiO2-Al2O3 composite support. The hydrodesulfurization (HDS) tests conducted here showed that NiMo/TiO2-Al2O3 catalysts are more active than NiMo/Al2O3 ones for the HDS of dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Comparison of product selectivity of cyclohexylbiphenyl (CHB)/biphenyl (BP) values showed that NiMo/TiO2-Al2O3 catalysts exhibit higher hydrogenation activity than NiMo/Al2O3.
Keywords: TiO2-Al2O3 composite; Hydrodesulfurization; Dibenzothiophene derivatives; X-ray photoelectron spectroscopy;
Role of a co-metal in bimetallic Ni–Pt catalyst for hydrogenation of m-dinitrobenzene to m-phenylenediamine by M.M. Telkar; J.M. Nadgeri; C.V. Rode; R.V. Chaudhari (23-30).
Bimetallic Ni–Pt catalysts supported on carbon were found to give very high turn over frequency numbers and almost complete selectivity to m-phenylenediamine in m-dinitrobenzene hydrogenation as compared to the monometallic nickel catalysts. The XRD and XPS characterization revealed that most of the nickel remains as Ni2+ in a monometallic catalyst while, the addition of platinum leads to the stabilization of Ni0 state, in case of bimetallic catalysts.
Keywords: Bimetallic catalyst; m-Dinitrobenzene; m-Phenylenediamine; Catalytic hydrogenation; XPS and XRD of nickel; Selective hydrogenation; Turn over number;
Effect of rare earths (La, Pr, Nd, Sm and Y) on the methane combustion over Pd/Ce–Zr/Al2O3 catalysts by Baohua Yue; Renxian Zhou; Yuejuan Wang; Xiaoming Zheng (31-39).
The effects of rare earths (La, Pr, Nd, Sm and Y) addition to Pd/Ce–Zr/Al2O3 catalyst have been investigated. The supported Pd catalysts are characterized by BET, XRD, XPS, TEM, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that the addition of Y or Sm obviously improves the catalytic activity of Pd/Ce–Zr/Al2O3 catalyst, and the Pd/Ce–Zr–Y/Al2O3 shows the highest catalytic activity and thermal stability. The addition of Y to Pd/Ce–Zr/Al2O3 inhibits the site growth and decomposition of PdO particles and improves the reduction–reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al2O3 catalyst.
Keywords: Palladium; Ce–Zr/Al2O3; Rare earths; Thermal stabilization; Methane combustion;
Methane reforming kinetics within a Ni–YSZ SOFC anode support by Ethan S. Hecht; Gaurav K. Gupta; Huayang Zhu; Anthony M. Dean; Robert J. Kee; Luba Maier; Olaf Deutschmann (40-51).
This paper reports experimental and modeling investigations of thermal methane reforming chemistry within porous Ni–YSZ anode materials. Because the reforming chemistry is difficult to observe directly in an operating fuel cell, a specially designed experiment is developed. In the experiment a 0.75 mm-thick anode is sandwiched between two small co-flowing gas channels. One channel represents the fuel channel of a solid-oxide fuel cell (SOFC). The composition in the other channel carries the species that would be produced in an operating fuel cell by the electrochemical charge-transfer reactions in the thin three-phase regions near the interface between the anode and the dense electrolyte membrane (i.e., H2O and CO2). Because the anode structure is porous (and there is no dense electrolyte or cathode applied), there is convective and diffusive species flux between the two flow channels. The entire assembly is maintained at approximately 800 ° C in a furnace. The results of heterogeneous reforming kinetics are determined by using mass spectrometry to measure the species composition at the outlet of both channels. Experimental results are interpreted using a computational model that incorporates channel gas flow, porous-media transport, and elementary heterogeneous chemical kinetics. The overall objective is to develop quantitative models of non-electrochemical heterogeneous reforming chemistry within a Ni–YSZ anode.
Keywords: Methane reforming; Solid-oxide fuel cell; Heterogeneous kinetics; Reaction mechanism;
Selective hydrogenation of myrcene catalyzed by sol–gel Pd/SiO2 by Patricia A. Robles-Dutenhefner; Marcelo G. Speziali; Edésia M.B. Sousa; Eduardo N. dos Santos; Elena V. Gusevskaya (52-58).
Sol–gel palladium silica composites are active catalysts for the selective hydrogenation of myrcene. The temperature of the thermal treatment of the catalyst is essential for its selectivity. Catalysts treated at 1100 °C give monohydrogenated products with excellent combined selectivity of 90–95% at almost complete conversion of the substrate. The obtained products arise from the σ-alkyl and/or η3-allyl intermediates formed by the interaction of the palladium catalyst with both terminal C＝C bonds of myrcene and the main products (ca. 70%) result from the reaction with the less substituted one. The selective monohydrogenation of myrcene could be useful to produce a mixture of diolefins of different reactivity, which could be further transformed to oxygenated derivatives with selectivities better than myrcene itself.
Keywords: Myrcene; Sol–gel Pd/SiO2 catalysts; Hydrogenation;
Investigation of the behaviour of a Pd/γ-Al2O3 catalyst during methane combustion reaction using in situ DRIFT spectroscopy by O. Demoulin; M. Navez; P. Ruiz (59-70).
In situ and/or operando DRIFT spectroscopy was applied to the study of the behaviour of a Pd (2 wt.%)/γ-Al2O3 catalyst, used in methane combustion, under various conditions. Under conventional O2-rich reaction conditions, spectator species (carbonates and/or formates) could be observed, but no surface intermediate species. No differences were observed under reaction conditions in function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (<400 °C) and disappeared when CO2 production started. They are not involved in the overall reaction pathway of methane oxidation and are thus suggested to be spectator species. Their production requires a continuous source of CO and would come from the interaction of CO with OH-groups of the γ-alumina support. Under O2-lean (O2/CH4 = 2) conditions, CO bridging on metallic palladium ( P d x 0 – CO ) is observed at high temperature (>400 °C), evidencing incomplete oxidation of methane. Based on previous operando Raman studies, a mechanistic scheme is proposed, which takes into account the various surface phenomena observed.
Keywords: Palladium; Catalytic combustion; Methane oxidation; Operando spectroscopy; In situ DRIFTS;
Photocatalytic activity of CdS nanoparticles incorporated in titanium silicate molecular sieves of ETS-4 and ETS-10 by Guoqing Guan; Tetsuya Kida; Katsuki Kusakabe; Kunio Kimura; Eiichi Abe; Akira Yoshida (71-78).
Nanoscopic CdS particles were encapsulated into microchannels of Engelhard titanosilicates, ETS-4 as well as ETS-10, with the photocatalytically active –Ti–O–Ti– quantum wires in their frameworks. The photocatalytic activities for hydrogen production from water under visible light irradiation (λ > 420 nm) were investigated in the CdS/ETS system. A stable photocatalytic activity under visible light irradiation was found in an aqueous solution containing Na2S and Na2SO3 as electron donors. The results suggest that the encapsulation of CdS in ETS-zeolites is effective for separating charge-carriers photogenerated in CdS and for improving the activity as well as the stability.
Keywords: Photocatalysis; Hydrogen production; CdS; Titanosilicate; ETS-4; ETS-10; Reaction stability;
Methane dehydroaromatization over Mo/HZSM-5 catalysts: The reactivity of MoC x species formed from MoO x associated and non-associated with Brönsted acid sites by Hongmei Liu; Wenjie Shen; Xinhe Bao; Yide Xu (79-88).
The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773 K have been investigated in combination with ex situ 1H MAS NMR characterization. Prolongation of the calcination time at 773 K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo species into the channels, resulting in a further decrease in the number of Brönsted acid sites, while causing only a slight change in the Mo contents of the bulk and in the framework structure of the HZSM-5 zeolite. The MoO x species associated and non-associated with the Brönsted acid sites can be estimated quantitatively based on the 1H MAS NMR measurements as well as on the assumption of a stoichiometry ratio of 1:1 between the Mo species and the Brönsted acid sites. Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the MoO x species to associate with the Brönsted acid sites, while a 6 wt.% Mo/SiO2 sample can be taken as a catalyst in which all MoO x species are non-associated with the Brönsted acid sites. The TOF data at different times on stream on the 6 wt.% Mo/HZSM-5 catalyst calcined at 773 K for 18 h and on the 6 wt.% Mo/SiO2 catalyst reveal that the MoC x species formed from MoO x associated with the Brönsted acid sites are more active and stable than those formed from MoO x non-associated with the Brönsted acid sites. An analysis of the TPO profiles recorded on the used 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the more of the MoC x species formed from MoO x species associated with the Brönsted acid sites, the lower the amount of coke that will be deposited on it. The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature.
Keywords: Methane dehydroaromatization; Mo/HZSM-5; 1H MAS NMR; Brönsted acid sites;
Effect of asphaltenes on hydroprocessing of heavy oils and residua by Irena Gawel; Dagmara Bociarska; Piotr Biskupski (89-94).
Asphaltenes raise trouble in petroleum processing. During the hydroprocessing of heavy feedstock, asphaltenes limit the efficiency of conversion and refining, act as coke precursors leading to catalyst deactivation. The effect of asphaltenes and their solubility in oil on the formation of solids have been extensively studied in the last decades. In this paper, the state of the art is reviewed, covering the changes in the chemical structure of asphaltenes upon hydrotreatment, leading to the reduction in product stability. As a result, asphaltenes might precipitate, causing coke deposition on the catalyst and the formation of deposits in the refinery equipment. This is the main reason for the limitation of conversion resulting in the reduction in the product yield.
Keywords: Asphaltenes; Coke formation on catalyst; Hydroprocessing; Sediment formation;