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 21,810 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Characteristics of carbon deposition by CH4 and carbon elimination by CO2 over conventional and nanoscale Ni/gamma-Al2O3 catalysts were investigated by using a pulse reaction, as well as by TGA, TEM, TPO-MS, H2-TPR and H2-chemisorption techniques. It was found that the behaviors of carbon deposition by CH4 decomposition and carbon elimination by CO2 depend on the active metal dispersion and the metal-support interaction. The filamentous carbon was formed on the conventional Ni/gamma-Al2O3 catalyst with low metal dispersion and relatively large particles, this type of filamentous carbon was far from the active centers and difficult to eliminate by CO2. On the other hand, the carbon deposition originated from CH4 decomposition on the nanoscale Ni/gamma-Al2O3 catalyst would mainly cover the surface of active centers, this type of highly active carbon was easily eliminated by CO2 because it is close to the active center Ni atoms. As a result, the improvement of coking-resistance was ascribed to the high metal dispersion and strong metal-support interaction, a model of CH4 decomposition carbon deposition on Ni/gamma-Al2O3 catalyst was proposed.
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PMID:Influence of the nanoscale support on carbon deposition and carbon elimination over Ni/gamma-Al2O3 catalyst for CH4 conversion. 1557 Sep 78

A series of oxygen-deficient perovskite-supported palladium catalysts were prepared by the "solid phase crystallization" (spc) method and investigated with XRD, TPR, TPD, TEM, XPS, BET analysis and CO oxidation. It was found that Pd/perovskite catalysts synthesized by the spc method were more active for CO oxidation than the calcined LaCo0.95 Pd0.05 O3, where Pd dispersed in the solid solution. H2-reducing treatment in the spc method could yield not only high-dispersed fine Pd particles on the perovskite surface but also oxygen-deficient structure. In these perovskite-supported Pd catalysts, oxygen vacancies adsorbed, activated and supplied oxygen to the active Pd sites, where the oxidation occurred with adsorbed CO. The high activities were due to the cooperative action of Pd and oxygen vacancies.
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PMID:Synergistic effect of palladium and oxygen vacancies in the Pd/perovskite catalysts synthesized by the spc method. 1590 Jul 51

Nickel nanoparticles were obtained by the reduction in hydrazine aqueous media of nickel acetate as a precursor supported on activated carbon of high surface area. Classical catalysts using nickel acetate or nitrate were prepared for comparison. The catalysts were characterized by N(2) physisorption, H(2)-TPR, H(2)-adsorption, TPD, TEM, and XRD, and tested in the gas phase hydrogenation of benzene. Hydrazine catalysts were found much more active in benzene hydrogenation than corresponding classically prepared catalysts. Remarkably, their reactivity is comparable (turn-over frequency of 0.2001-0.2539 s(-1) at 393 K) to that of Pt classical catalysts supported on activated carbon in the same conditions. Evidence is given for the existence of the hydrogen spillover effect in benzene hydrogenation, not reported before in the literature. As a result of the hydrogen spillover effect, catalysts performances can be explained by a combination of surface metal atom reactivity, metal-support interaction strength, and specific surface area extent. Maximum effect is observed with hydrazine preparation method, for 1% Ni content and nickel acetate as a precursor. Unexpectedly, it was also found that hydrazine preparation increases the specific area of the catalysts.
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PMID:Study of nickel nanoparticles supported on activated carbon prepared by aqueous hydrazine reduction. 1656 18

The evolution of nickel speciation during the successive preparation steps of Ni-SiO(2) catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)(x)(H(2)O)((6-2x))](2+) (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(4)](2+) onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)(2)(SiO)(2)], resulting from the grafting of [Ni(en)(2)(H(2)O)(2)](2+) onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.
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PMID:Evolution of nickel speciation during preparation of Ni-SiO(2) catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6-2x]2+ precursor complexes. 1663 58

Nanostructured ceria doped with other rare earth elements is a good oxygen ion conductor, which gives rise to various catalytic applications such as the construction of membranes for syngas production by partial oxidation of methane. This article focuses on the Gd-doped cerium dioxides, which can be modified with Pt or Pd to enhance the reactivity of the lattice oxygen in interaction with methane. The aim of the work is the elucidation of correlations between the structural, electronic, and chemical properties of these nanomaterials. Detailed studies were performed for a series of samples with and without surface modification by noble metals using a complex combination of physicochemical methods: XRD, TEM, CH(4) TPR, XPS, SIMS, and FTIR spectroscopy of adsorbed CO. XPS and TPR data revealed that surface modification with noble metals enhances the reducibility of the doped ceria support, where the effect is more pronounced for Pd than for Pt. The formation of highly cationic Pd species due to strong metal support interactions provides a possible explanation for this behavior. Furthermore, the results obtained in the present work for the Gd-doped ceria system are compared to those obtained previously for the Pr-doped ceria system.
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PMID:Nanostructured, Gd-doped ceria promoted by Pt or Pd: investigation of the electronic and surface structures and relations to chemical properties. 1685 96

We report the preparation of highly ordered mesoporous Fe-Al-SBA-15 with isolated extraframework Fe species under acidic conditions. The materials were characterized by means of UV resonance Raman spectroscopy, in conjunction with BET, XRD, TEM, UV-vis, H2-TPR, FT-IR, and 27Al MAS NMR spectroscopy. The addition of both Fe and Al to the synthesis gel of SBA-15 results in the formation of isolated extraframework Fe species located close to the framework Al ions and the Fe content an order of magnitude higher than that in Fe-SBA-15 synthesized without Al. The existence of anchored extraframework Fe species was confirmed by the presence of a strong absorption band at 270 nm, hydrogen reduction at relatively low temperature, and the presence of a resonance Raman band at 1140 cm(-1). The location of Fe in close proximity to framework Al nuclei is further supported by 27Al MAS NMR measurements. Two characteristic UV Raman bands at 510 cm(-1) and 1090 cm(-1) excited by 244-nm laser are assigned to Fe-O-Si symmetric and asymmetric stretching modes of isolated tetrahedral Fe ions in the silica framework for Fe-SBA-15. The resonance Raman band at 1140 cm(-1) excited by 325-nm laser is attributed to the asymmetric stretching mode of the isolated extraframework iron species in Fe-Al-SBA-15. The isolated Fe species close to framework Al species are stable in acidic HCl solution, whereas the majority of Fe species in Fe-SBA-15 can be easily removed.
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PMID:Effect of aluminum on the nature of the iron species in Fe-SBA-15. 1718 Dec 65

Gold catalysts modified by Fe and Ni and supported on different zeolite matrixes have been studied by TEM, TPR, and catalytic testing. The presence of a metal oxide additive allows stabilizing small gold particles, particularly in the case of Fe. The shape of light-off curves shows two temperature regions of the catalyst activity, a low-temperature range below 250 degrees C and a high-temperature range above 300 degrees C. This situation is explained considering the existence of at least two types of catalytically active sites of gold assigned to gold clusters and gold nanoparticles, respectively, while the ionic state of gold (Au3+) remains inactive. It is shown that interaction of gold with Fe promoter leads to activation of catalysts at low temperature due to a change of electronic state and redox properties of gold. NiO additive cause a similar, but less pronounced effect.
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PMID:Catalytic activity of gold nanoparticles incorporated into modified zeolites. 1765 59

Hydrotalcite-like catalysts were synthesized by co-precipitation and then these were promoted by the addition of noble metals, alkaline earth metals and ceria. Reaction tests were conducted using a feed of H2O/C/O2 = 3/1/0.37 at a temperature range from 300 degrees C to 700 degrees C. Catalysts were characterized by XRD, TEM, FESEM, TPR, and BET. Reaction test results confirmed an enhancement of the catalytic activity of the promotor-modified catalysts due to low carbon deposition. Among the alkaline earth metals tested, those with larger atomic number exhibited higher activity at a lower temperature range.
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PMID:Autothermal reforming of propane over hydrotalcite-like catalysts containing promotor. 1804 6

Ni3V2O8 catalyst was prepared by oxalate co-precipitation method with microwave heating in this paper. In order to study the relationship between the catalytic performance and the surface species, the catalyst was characterized by XRD, BET, H2-TPR, XPS, TEM and conductivity measurement. The surface property of Ni3V2O8 was studied by XPS and the catalytic performance of the oxidative dehydrogenation of propane to propylene was also investigated. The results of XRD showedthat pure Ni3V2O8 with nice structure was obtained. TEM experiments results demonstrated that the prepared Ni3V2O8 catalyst at 700 degrees C calcination showed uniform particle with the mean particle size of 30 nm. The surface area of the catalyst was 8.623 m2 x g(-1). The diagram of the relationship between electrical conductivity and oxygen partial pressure of Ni3V2O8 showed dsigma/dPO2, >0, implying that Ni3V2O8 catalyst was a p-type semiconductor. H2-TPR results showed that only one unsymmetrical reduction peak appeared at 663.5 degreesC within 300-900 degrees C region over Ni3V2O8 catalyst and no obvious shoulder peak was observed. It could also be found that the ratio of non complete reduction oxygen species was about 33.59% (O(-) 27.55%, O2(2-) 6.04%) from the O(1s) XPS result and more V4+ species existed on the Ni3V2O8 catalyst surface. The TPR and XPS results illustrated that the transformation of the lattice oxygen to non-complete reduction oxygen in NiV2O8 catalyst might promote the oxidation-reduction reaction between different valence vanadium and promoted the oxygen vacancy formation. This then led to abundant non-complete reduction oxygen O(-) and V4+ species formation on the surface of Ni3V2O8 catalyst. The active result of oxidative dehydrogenation of propane to propylene showed that the 60.02% propylene selectivity could be reached at 18.60% propane conversion. Compared with the reported results over the coexistent NiO and Ni3V2O8 system from the literature, pure Ni3V2O8 catalyst system in this present paper showed higher propylene selectivity than the coexistent NiO and Ni3V2O8 system under the same propane conversion condition, suggesting that the performance of propane to propene is correlated to the oxidation-reduction of V4+ / V5+ couple and non complete reduction oxygen species (O(-) or O2(2-)). This result further illustrated that NiV2O8 was active phase for oxidative dehydrogenation of propane to propylene. Combining the active and characterization results, it was found that catalytic activity was correlated to the surface non-complete reduction O(-) and V4+ species, which was beneficial to improving the propylene selectivity.
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PMID:[Study on performance of Ni3 V2O8 catalyst and analysis of X-ray photoelectron spectroscopy]. 1830 14

The effect of sodium hydroxide on the-liquid phase hydrodechlorination (LPHDC) of polychlorinated dibenzo- p-dioxins/polychlorinated dibenzofurans (PCDD/Fs) over 2% Pd/gamma-Al 2O 3 was evaluated. Reactions were carried out using 2-propanol both as a hydrogen donor and as a solvent. Fresh and used catalyst samples were characterized by BET, hydrogen chemisorption, TEM/EDS, XPS, and TPR. When the reaction mixture contained no NaOH, active-phase leaching and Pd-C formation were observed even after 10 min of reaction. Therefore, sodium hydroxide appears to be required to maintain surface metal clusters on the support and avoid binding of carbon species to the active metal. On the other hand, excess NaOH in the reaction mixture led to deposition of organic and inorganic solid residues on the catalyst surface, blocking the active sites. Under the conditions of this study, the addition of 30 mg of NaOH maintained the basicity of the system and diminished deposition of solid residues on the catalyst samples, and almost 100% detoxification was reached after a 3 h reaction.
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PMID:The effect of NaOH on the liquid-phase hydrodechlorination of dioxins over Pd/gamma-Al2O3. 1872 32


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