Effect Of Noble Metals Research Articles

Photocatalytic activity of NH4F-doped TiO2 modified by noble metal nanoparticle deposition

The effect of noble metal (Pt and Au) nanoparticle photodeposition on a series of NH4F-doped TiO2 photocatalysts calcined at 700 °C was investigated both in a thermodynamically down-hill reaction, i.e. the degradation of formic acid in aqueous suspension, and in an up-hill reaction, i.e. hydrogen production from methanol-water vapour mixtures. All photocatalysts were characterized by BET, XRD, UV-vis absorption and HRTEM analysis. Intriguing synergistic effects of simultaneous bulk and surface TiO2 modification were evidenced in both photocatalytic reactions, which can be interpreted in relation to the structural features of the materials. On one hand NH4F doping guarantees that the most active TiO2 anatase phase is stabilised up to high calcination temperature, ensuring high crystallinity and good photoinduced charge carriers production, on the other hand noble metal nanoparticles contribute in increasing the separation of photoproduced charge carriers, resulting in enhanced photocatalytic performances of the surface- and bulk-modified photocatalyst systems.

Oxidative steam reforming of bio-butanol for hydrogen production: effects of noble metals on bimetallic CoM/ZnO catalysts (M = Ru, Rh, Ir, Pd)

The influence of the addition of a noble metal (Ru, Rh, Ir or Pd) on the catalytic performance of Co/ZnO was studied with respect to oxidative steam reforming of the bio-butanol raw mixture (butanol/acetone/ethanol=6/3/1, mass ratio). Bimetallic catalysts, CoM/ZnO (M=Ru, Rh, Ir, Pd) had higher conversion values, decreased deactivation and higher H2 concentration in the outlet gas than monometallic Co/ZnO. However, significant differences were found in both raw mixture conversion and H2 concentrations for the different CoM/ZnO, depending on M. CoRh/ZnO presented the best catalytic behavior (65.1mol% hydrogen in the outlet after 100h of reaction at 500°C with 84% conversion). Characterization of catalysts post-reaction by Raman spectroscopy and TPO showed that less ordered and less abundant carbonaceous deposits were formed over bimetallic catalysts than over the monometallic one; CoRh/ZnO had the lowest amount of such coke.

Effect of Noble Metals on the Strength of Brønsted Acid Sites in Bifunctional Zeolites

Abstract1H magic angle spinning (MAS) NMR and FTIR spectroscopy with deuterated acetonitrile and pyridine as probe molecules, respectively, were used to study the strength of Brønsted acid sites in Pt‐ and Ir‐modified zeolites Y and Beta in comparison with the materials in their H,Na forms. For both types of zeolites, the following sequence of the acid strengths was found: zeolites in their H,Na form>Pt‐modified zeolites>Ir‐modified zeolites. As the 1H MAS NMR signals of the bridging OH groups (Si(OH)Al) in the noble‐metal‐modified zeolites have the same spectroscopic properties as those observed in the materials in the H,Na forms, no direct influence of the noble metals on the nature of the hydroxyl protons or on the local structure of Si(OH)Al groups is expected. However, it is suggested that noble metals in these bifunctional zeolite catalysts are involved in charge transfer with neighboring framework atoms, which affects the mean framework electronegativity of the zeolites under study. This effect causes a variation of the acid strength of the Si(OH)Al groups similar to that observed for zeolites with different framework Al contents.

Noble Metal Catalysts for Methane Reforming in Material Application Engineering

Reforming of methane is an important route to produce sygas. In this paper, recent progresses of noble metals (Rh, Ru, Ir, Pt, Pd) catalysts for methane reforming in material application engineering is reviewed. The discussion mainly focuses on catalytic performance of noble metal catalysts or noble metal promoted Ni catalysts in methane reforming reaction. Effects of noble metals, supports and preparation methods on the catalytic activity, selectivity, coke deposition and stability of catalysts have been briefly summarized. In conclusion, Rh as active component, Pd as material for membrane reactor, Pt or Rh as promoters for Ni catalysts, all gave high CH4 conversion, improving catalytic performance.

Effect of noble metals on activity of MnOx/γ-alumina catalyst in catalytic ozonation of toluene

This work addresses effect of addition of two noble metals, platinum or palladium (1wt.%), on performance of alumina-supported manganese oxides (10wt.%) in catalytic ozonation of toluene in the range of 22–100°C. Catalysts were characterized by ICP-MS, BET, X-ray diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR) and X-ray absorption spectroscopy (XAS) including quantitative X-ray absorption near edge structure (XANES) and qualitative extended X-ray absorption fine structure (EXAFS). It was found that addition of platinum improves activity of manganese oxides leading to complete conversion of toluene at 70°C. Palladium was found to be ineffective in enhancing activity of manganese oxides mainly because of lack of atomic interaction between Pd and Mn. Pt and Mn XANES and EXAFS spectra changed after addition of Pt to manganese oxides implying an atomic interaction between platinum and manganese oxides. It is proposed that the interaction between Pt and Mn occurs via surface oxygen of manganese oxide clusters increasing dispersion of Pt atoms in the presence of Mn. This interaction could increase electron occupancy of Mn 3d orbital which is more favorable in decomposition of ozone and consequently oxidation of toluene.

Effect of noble metal coated on magnetic nanoparticles on horseradish peroxidase structure

Effect of noble metal coated on magnetic nanoparticles on horseradish peroxidase structure

One pot preparation of plasmonic photocatalyst at low temperature

Plasmonic photocatalysts have attracted intensive interests both with respect to applied and fundamental points of views because their photocatalytic performance can benefit from both plasmon resonance and charge separation effect of noble metals. In this paper, the AgCl-Ag plasmonic photocatalysts with different Ag content were prepared through a facile one-pot chemical route at low temperature. All the samples were structurally characterized by X-ray diffraction, field emission scanning electron microscope, and transmission electron microscope. It is shown that the Ag clusters can form on the surface of AgCl nanoparticles with the assistance of ascorbic acid as a reducing agent. This study may provide a new insight into the synthesis and structure manipulation of metal-semiconductor composites.

Effect of Noble Metal in CdS/M/TiO2 for Photocatalytic Degradation of Methylene Blue under Visible Light

ABSTRACT The CdS/M/TiO2 (M = Ag, Ru, Au, Pd, and Pt) three-component nanojunction systems were constructed using a two-step photodeposition method, and evaluated for their photocatalytic activities through the degradation of methylene blue in aqueous solution under visible light irradiation. The authors found that the photocatalytic activity of CdS/M/TiO2 (M = Ag, Ru, Au, Pd, Pt) three-component nanojunctions was superior to that of CdS/TiO2 two-component system. Moreover, the photocatalytic activity of the three-component nanojunction system was found to be dependent significantly on the type of the noble metals. The results can be explained by the influence of charge transfer on the basis of the work functions of different noble metals.

Visible light active TiO2 films prepared by electron beam deposition of noble metals

Abstract TiO 2 films prepared by sol–gel method were modified by electron beam deposition of noble metals (Pt, Pd, and Ag). Effects of noble metals on the chemical and surface characteristics of the films were studied using XPS, TEM and UV–Vis spectroscopy techniques. Photocatalytic activity of modified TiO 2 films was evaluated by studying the degradation of methyl orange dye solution under visible light UV irradiation. The result of TEM reveals that most of the surface area of TiO 2 is covered by tiny particles of noble metals with diameter less than 1 nm. Broad red shift of UV–Visible absorption band of modified photocatalysts was observed. The catalytic degradation of methyl orange in aqueous solutions under visible light illumination demonstrates a significant enhancement of photocatalytic activity of these films compared with the un-loaded films. The photocatalytic efficiency of modified TiO 2 films by this method is affected by the concentration of impregnating solution.

Effect of platinum, palladium and rhodium on the activity and sulfur resistance of catalysts based on ZrO2 in the oxidative conversion of methane

A study was carried out on the effect of noble metals (Pd, Pt, and Rh) on the activity and sulfur resistance of a Cu-Ni-Ce oxide composite on yttrium-stabilized zirconium in the oxidation of methane by oxygen. The enhancement in activity in the deep oxidation of methane, which is greatest for the pallaldium sample, is related to the oxidation-reduction properties of the composites. The platinum catalyst displayed greatest sulfur resistance, which may be attributed to the mechanism for the oxidation of SO2 on platinum-group metals.

Structural and electronic properties of NM-doped ceria(NM = Pt,Rh): a first-principles study

The effects of noble metal (NM) dopants(NM = Pt,Rh) on the structural and electronic properties of ceria are studied using a densityfunctional theory (DFT) method, with the inclusion of on-site Coulomb interaction(DFT+U). It is found that these NM dopants give rise to large perturbations of the atomic andelectronic structures of ceria and induce metal-induced gap states at the Fermi levelsuitable for accommodating extra electrons, thereby lowering the reduction energy of ceriaand making the NM-doped cerias more reducible than pure ceria. This mechanism forfacilitating the reduction of ceria is different from that of Zr-doped ceria where theincreased reducibility is largely due to the structural distortions and not to electronicmodifications.

Performance of Sulfur Tolerant Reforming Catalysts for Production of Hydrogen from Jet Fuel Simulants

The development of robust desulfurizers and new reforming catalysts is a critical path for the use of jet fuels in powering the commercial growth of fuel cell systems for air and military applications. The presence of high concentrations of sulfur-containing organic compounds leads to rapid deactivation of traditional reforming catalysts, and removal of the sulfur components from the fuel through adsorptive methods is not practical for long term operations. The current work describes the use of several ceria-based catalyst compositions that were studied to assess their performance based on the formation of hydrogen and product yield from a fuel consisting of toluene and thiophene. The effect of noble metals, metal oxide additives, and stabilized ceria supports on the performance of the catalyst was studied. The addition of selected components led to higher yields or greater stability; combinations of these additives were not necessarily synergistic. Interestingly, the presence of sulfur in the fuel was shown to enhance the initial activity of catalysts containing rhodium. Analysis in terms of the kinetic rates of reaction and deactivation illustrated the effects of these additives and provided insight into the design of a more highly stable steam reforming catalyst for production of hydrogen from jet fuel.

Effect of noble metals in the decomposition of nitrous oxide over Fe-ferrierites

The decomposition of nitrous oxide was studied over Fe-ferrierite, Me-ferrierites and Fe/Me-ferrierites (Me: Pt, Rh and Ru). Flow as well as batch experiments were carried out and showed a synergy between Fe and Me ions. Ions of noble metals in Fe-ferrierite increased the catalytic activity in the sequence Pt < Rh ≅ Ru. Addition of NO substantially decreased the decomposition of N2O over Rh/ferrierite and Ru/ferrierite, but not over bimetallic ferrierites. NOx species created during the decomposition of nitrous oxide alone as well as with addition of NO, and employment of nitrous oxide labeled with 18O allowed us to assume a changing decomposition mechanism in the presence of Me ions in Fe-ferrierites.

Autothermal reforming of methane over Ni/γ-Al2O3 catalysts: the enhancement effect of small quantities of noble metals

The effect of introducing small amounts of Pt, Pd and Ir (<0.3% by weight) into Ni/γAl 2 O 3 catalysts (15% Ni w/w) for the autothermal reforming of methane (ATR) was investigated. While the unpromoted catalyst took the partial oxidation of methane to equilibrium, the promoted ones increased the methane conversion in ATR. No electronic modifications of nickel sites were observed with the addition of noble metals, but they did cause an increase in metal surface area. The effect of noble metals on this reaction, under these conditions, was assigned to this expansion of the metal surface.

Modification of H2-sensitive breakdown voltages of SnO2 varistors with noble metals

Effects of noble metals on the H2-sensitive nonlinear I–V characteristics of SnO2 varistors have been investigated. All the SnO2 varistors exhibited nonlinear I–V characteristics in air at 300–600°C. The breakdown voltage shifted to a lower electric field upon exposure to 1.0% H2 at elevated temperatures, thus exhibiting H2 sensitivity. It was found that SnO2 varistors loaded with a noble metal such as Au, Pt, and Pd exhibited a higher breakdown voltage than a pure SnO2 varistor in air. The increase in the breakdown voltage in air arose mainly from the electronic interaction based on the difference between the electron affinity of SnO2 and the work function of the metal loaded. Effects of noble metals on the H2 sensitivity were strongly dependent on the kind of noble metals. Addition of Au resulted in an increase in sensitivity owing to ‘chemical sensitization’, but Pt and Pd caused a decrease in sensitivity due to ‘configurational effect’ associated with their high catalytic activities. Among the varistors tested, SnO2 loaded with 0.1 wt% Au exhibited the largest H2-sensitive breakdown voltage shift at 300°C.

Effect of noble metals on selective detection of liquid petroleum gas by SnO2

The present investigation deals with the electrical response of doped SnO2 to improve the selectivity for liquid petroleum gas (LPG) in the presence of CO and CH4, by utilizing noble metal sensitizers such as Pd, Pt, and Rh. SnO2 with the addition of Pd (1.5 wt. %) or Pt (1.5 wt. %) sintered at 800 °C which have shown high sensitivity toward LPG with no cross interference of CO and CH4 at an operating temperature of 350 °C. The results suggest the possibility of utilizing the sensor for the detection of this hydrocarbon gaseous mixture. X-ray diffraction studies have been carried out to evaluate the crystallite size as a function of sintering temperature; x-ray photoelectron spectroscopy studies have been carried out to define the possible chemical species involved in the gas-solid interaction and the sensitivity enhancing mechanism of the SnO2/Pd sensor element toward LPG.

Effect of Noble Metals on Selective Detection of LPG Based on SnO2

ABSTRACTThe present investigation deals with the electrical response of noble metal doped SnO2 to improve the selectivity for Liquid Petroleum Gas (LPG) in the presence of CO and CH4. Addition of small amounts of nobel metals (Pd, Pt and Rh) to the base material SnO2 is carried out by co-precipitation method. X-ray diffraction and X-ray photoelectron spectroscopy studies are carried out to find out the crystalline phase and chemical composition of the SnO2. The sensor element has been tested for cross selectivity to reducing gases by measuring sensitivity versus sintering temperatures and sensitivity versus operating temperatures. The sensor elements with the composition of Pd (1.5 wt%) andPt (1.5 wt%) in the base material SnO2 sintered at 800°C showed high sensitivity towards LPG at an operating temperature of 350°C suggestingthe possibility to utilize the sensor for the detection of LPG.

Vitrification of Fission Product Solutions: Investigation of the Effects of Noble Metals on the Fabrication and Properties of R7T7 Glass

The effects of incorporating additional qualities of insoluble dissolution fines containing noble metals together with the fission product feed solutions were investigated for application to the French T7 vitrification facility at La Hague. Three types of tests were conducted: nonradioactive laboratory tests, radioactive laboratory tests, and industrial-scale tests in a prototype vitrification unit. The laboratory test results showed that the quality of R7T7 glass containing from 1.5 to 4 wt% of platinoids is fully equivalent to that of standard R7T7 glass without platinoids. These findings were confirmed on glass casting samples containing 0 to 3 wt% of platinoids from a full-scale industrial vitrification prototype facility. Recent tests in which video cameras have been used to visualize the molten glass and model simulations of glass properties and of melting pot behavior suggest that industrial operating conditions can be optimized to produce glass with platinoid concentrations approaching this limit value.

Oxygen availability in mixed cerium/praseodymium oxides and the effect of noble metals

Oxyreduction studies of mixed Ce/Pr oxides have been carried out. Temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) were used to study the uptake and release of the oxygen. Large amounts of oxygen, exceeding those in ceria, are accessible in the mixed metal oxides at moderate temperatures. The addition of small amounts of noble metals to the mixed oxides shifts the accessibility of the “stored” oxygen to still lower temperatures with the effect of Pd being more pronounced than that of Pt. In a sample containing 45 mol % ceria and 55 mol % praseodymia, a small addition of Pd (0.24 mol %) was found to lower the reduction temperature by more than 100 °C. The addition of Pt had a lesser effect. Similarly, in pure praseodymia (Pr6O11) Pd influences the reduction much more strongly than Pt. In the mixed samples, whether doped with a noble metal or not, the whole oxyreduction effect can be accounted for by the change in oxidation state of the praseodymium ions solely. This notwithstanding, the reduction of the mixed oxides, without noble metals or doped by Pt, is more facile than that of praseodymia. Only the incorporation of Pd makes the reduction of praseodymia proceed at a temperature below that registered for a mixed ceriapraseodymia sample.

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide, and nitrogen oxide emissions

The paper investigates several features of the operation of noble metal catalysts in automobile exhaust near the stoichiometric air:fuel ratio ( A F ). These features include the extent of intrapellet diffusion limitations as a function of feedstream stoichiometry, the mechanism of poisoning, the effects of noble metal (Pt, Rh, Pd) impregnation profiles on activity and poison resistance, and the effects of A F oscillations on catalyst performance. A pellet configuration, containing an external shell of Pt and internal rings of Rh and Pd, was found to have improved poison resistance and lightoff characteristics. The addition of Ce improved the catalyst's initial performance in transient operation.