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Mn and Li promoted Rh catalysts supported on SiO2 with a thin TiO2 layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO2 layer on the surface of SiO2 was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh0 and Rhδ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H2/CO ratio in the feed is recommended. [ABSTRACT FROM AUTHOR]

The commercial potential for a given catalytic process may be influenced by requirements on metal loading, in particular where noble metals are used. In an effort to substantially decrease the amount of catalyst material used for methane activation and catalytic partial oxidation (CPO), the effect of 0.005 wt% noble metal (Rh, Ru, Pd or Pt) on 0.5 wt% Ni/γ-Al2O 3 catalysts have been studied at temperatures below 1,173 K and 1 atm. The successful catalysts were activated directly by in situ reduction, without a calcination step, to promote formation of a highly dispersed (supported) metal phase from nitrate precursors. The obtained metal particles were not observable by XRD (size < 2–3 nm). This activation procedure had a decisive effect on catalyst activity, as compared to a catalyst which was calcined ex situ before in situ reduction. Adding a noble metal caused a significant drop in the ignition temperature during temperature programmed catalytic partial oxidation (TPCPO). The ignition temperature for partial oxidation coincides well with the temperature for methane dissociation, and is likely correlated to the reducibility of the noble metal oxide. Methane partial oxidation over 0.5 wt% Ni catalysts, both with and without promoter, yielded high selectivity to synthesis gas (>93%) and stable performance for continued operation, but synthesis gas production at temperatures below 1,073 K required a promoter when the catalyst was ignited by TPCPO. Ignition of the CPO reactions by introducing the feed at a high furnace temperature (1,073 K) also enabled formation of synthesis gas, but the reaction was then less stable than obtained with the TPCPO procedure. A dual bed concept attempted to beneficially use the activation and combustion properties of the noble metal followed by the reforming properties of Ni. However, it was concluded that co-impregnated catalysts yielded as high, or even higher conversion of methane and selectivity to synthesis gas. [ABSTRACT FROM AUTHOR]

Autothermal reforming of methane was studied over La-doped ceria–zirconia-supported Rh catalysts. The CH4 conversion increased from 49 to 60% on increasing the content of La3+ from 5 to 15%, while further increase in the La3+ content led to a slight decrease on both CH4 conversion and H2 selectivity. H2-TPR and UV–vis DRS spectrum showed that the interaction between Rh and the support was enhanced by increasing the content of La. We speculated that a so-called “Rh–La interfacial species” was formed on the surface of the support, which played an important role in catalytic activity. The balance between exposed Rh and the “Rh–La interfacial species” was necessary to improve the catalytic activity. Upon increasing the Rh loading on 15% La-doped ceria–zirconia support, the balance was built, i.e., CH4 conversion increased from ~60 to 69% by increasing Rh loading from 0.1 to 0.5 wt% and only 2% conversion was elevated by doubling the Rh loading from 0.5 to 1.0 wt%. [ABSTRACT FROM AUTHOR]

Pt–Rh three-way catalysts (TWCs) of two formulations were examined by electron probe microanalysis (EPMA) and cross sectional transmission electron microscopy (TEM), in order to study the effect of accelerated aging on microstructure. Both formulations have Pt and Rh applied in separate washcoat layers, Rh in a zirconia-rich outer layer and Pt in an alumina-rich inner layer(s). Microstructural analyses show that in spite of the initial separation, the precious metals inter-diffuse between the washcoat layers and alloy during aging. Following dynamometer aging the two formulations exhibited significant differences in the precious metal (PM) particle size distributions and the correlation between PM particle size and composition. These differences are attributed to the increased transport of Pt due to the presence of ceria and zirconia in the alumina-rich inner washcoat of one formulation, compared to other formulation. The presence or absence of porosity > 1 μm in diameter and differences in initial Pt distributions in the inner washcoat were also noted as possibly contributing to the PM particle growth. The transition alumina of one formulation, containing Ba, partially transformed to corundum during aging. The other formulation utilized a La-stabilized alumina which did not transform to corundum after aging. [ABSTRACT FROM AUTHOR]

Supported Rh catalysts have been developed for selective H2 production at low temperatures. Ethanol dehydration is favorable over either acidic or basic supports such as γ-Al2O3 and MgAl2O4, while ethanol dehydrogenation is more favorable over neutral supports. CeO2–ZrO2-supported Rh catalysts were found to be especially effective for hydrogen production. We focused on a support prepared by a co-precipitation method having composition Ce0.8Zr0.2O2. A 2%Rh/Ce0.8Zr0.2O2 catalyst, prepared via impregnation without pre-calcination of support, exhibited the highest H2 yield at 450 °C among various supported Rh catalysts evaluated in this study. This may be due to both the strong interaction between Rh and Ce0.8Zr0.2O2 and the high oxygen transfer rate favoring reforming of acetaldehyde instead of methane production. [ABSTRACT FROM AUTHOR]

High selectivities and conversions in the preferential oxidation of CO in the presence of large quantities of H2, H2O and CO2 are demonstrated on noble metal catalysts at millisecond contact times (~10–15 ms) for temperatures between 150 and 500 °C. With a simulated water-gas shift product stream containing 0.5% CO and varying amounts of H2, H2O and CO2, we are able to achieve ~90% CO conversions on a Ru catalyst at temperatures of ~300 °C using a stoichiometric amount of O2 (0.25%). Experiments with and without O2 and with varying H2O reveal that significant water-gas shift occurs on Pt and Pt-ceria catalysts at temperatures between 250 and 400 °C, while significant CH4 is formed on Ru and Rh catalysts at temperatures greater than 250 and 350 °C, respectively. The presence of H2O blocks H2 adsorption and allows preferential CO oxidation at higher temperatures where rates are high. We propose that a multistage preferential oxidation reactor using these catalysts can be used to bring down CO content from 5000 ppm at the reactor entrance to less than 100 ppm at very short contact-times. [ABSTRACT FROM AUTHOR]

This paper deals with an experimental investigation concerning steam reforming of methanol at 280, 340 and 380 °C over NiAl and Ni (Au, Rh or Ir)Al layered double hydroxides (LDHs) derived catalysts. Incorporation of noble metal ions into the NiAl-LDH framework was evidenced by XRD, TGA and TEM techniques. High selectivity to H2 and CO2 with less than 5% (volume) CO and trace CH4 was observed over the NiAl-LDH catalyst. Whereas CO and H2 are major products at lower temperatures after addition of Au, Rh and Ir to the NiAl-LDH system. They are significantly reduced with the concomitant increase in CH4 and CO2 as the temperature increased. [ABSTRACT FROM AUTHOR]

Comparative studies of supported metal catalysts are facilitated if the catalysts consist of well-faceted metal particles whose structural changes under reaction conditions are known. We describe the use of regular noble metal crystallites (Ir, Rh and Pt), obtained by epitaxial growth on a single crystal (NaCl) substrate and subsequently contacted with the support by reactive high-vacuum deposition of the oxide, to study the inherent catalytic activity of the metals. Most of these metal particles exhibit defined zone axes parallel to the electron beam direction, which allows a characterization by selected area electron diffraction and lattice plane imaging in the electron microscope. The microstructural changes during activation processes (oxidation and reduction at temperatures up to 723 K) have been studied and correlated with corresponding changes in the conversion of neopentane with excess hydrogen. Particular emphasis was placed on the dependence of the reaction rates on hydrogen pressure. The structural changes at the metal–oxide interface upon reduction above ∼673 K may be interpreted as a prestage to alloy formation. [ABSTRACT FROM AUTHOR]

The role of Fe promoters has been investigated on Pd/ceria, Pt/ceria and Rh/ceria catalysts for the water–gas shift (WGS) reaction in 25 torr of CO and H2O under differential reaction conditions. While no enhancement was observed with Pt and Rh, the activity of Pd/ceria increased by as much as an order of magnitude upon the addition of an optimal amount of Fe. Similarly, the addition of 1 wt% Pd to an Fe2O3 catalyst increased the WGS rate at 453 K by a factor of 10 over that measured on Fe2O3 alone, while the addition of Pt or Rh to Fe2O3 had no effect on rates. The amount of Fe that was necessary to optimize the rates increased with Pd loading but was independent of the order in which Fe and Pd were added to the ceria. Increased WGS activity was also observed upon the addition of Fe to Pd supported on Ce0.5Zr0.5O2. XRD measurements, performed after running the catalyst under WGS conditions, show the formation of a Fe–Pd alloy, even though similar measurements on an Fe/ceria catalyst showed that Fe3O4 was the stable phase for Fe in the absence of Pd. Possible implications of these results on the development of new WGS catalysts are discussed. [ABSTRACT FROM AUTHOR]

Small amounts of Rh-promoted Ni/α-Al2O3 catalysts possessed higher activity than pure Ni/α-Al2O3, Rhα-Al2O3 catalysts and exhibited excellent coke resistance ability in methane reforming with CO2. XRD, H2-TPR, CO2-TPD and coking reaction (via CH4 temperature-programmed decomposition) indicated that Rh improved the dispersion of Ni, retarded the sintering of Ni and increased the activation of CO2 and CH4 on the surface of catalyst. [ABSTRACT FROM AUTHOR]

Small amounts of Rh-promoted Ni/α-Al2O3 catalysts possessed higher activity than pure Ni/α-Al2O3, Rhα-Al2O3 catalysts and exhibited excellent coke resistance ability in methane reforming with CO2. XRD, H2-TPR, CO2-TPD and coking reaction (via CH4 temperature-programmed decomposition) indicated that Rh improved the dispersion of Ni, retarded the sintering of Ni and increased the activation of CO2 and CH4 on the surface of catalyst. [ABSTRACT FROM AUTHOR]

The catalytic conversion of N2O to N2 in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al2O3, SiO2, TiO2, ZrO2 and calcined hydrotalcite MgAl2(OH)8·H2O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on γ-alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH4, C3H6, C3H8) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N2O to N2 was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO2 and H2O in the feed. A shift of the N2O conversion versus temperature curve to higher temperatures was observed when SO2 and H2O were added, separately or simultaneously, to the feed. The inhibition caused by SO2 was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H2O and N2O on the same active sites. [ABSTRACT FROM AUTHOR]

Steady state N2O decomposition reaction on polycrystalline Pt and Rh surfaces has been studied using a supersonic free jet molecular beam (2.1 × 1018 molecules/cm2 s). The energy of the incident N2O beam was controlled by a nozzle heating technique in conjunction with a seeding technique. The decomposition rate shows both translational and vibrational energy dependence on the Pt surface. However, there is also the surface temperature dependence of the decomposition rate even varying the incident beam energy, indicating precursor mediated dissociation of N2O on the Pt surface. On the other hand, no energy dependence was observed on the Rh surface, suggesting that the decomposition dynamics are different between Pt and Rh surfaces. [ABSTRACT FROM AUTHOR]

The selective catalytic reduction (SCR) of NO with C3H6 was studied over three noble metal based catalysts: 2% Pt/γ Al2O3, 2% Rh/γ Al2O3 and 1.5% Rh/TiO2(4% WO3). The SO2 effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO2 containing feeds. Temperature programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C3H6, respectively, in the absence or the presence of SO2. The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO2 presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates. [ABSTRACT FROM AUTHOR]

The reaction of ethylene on a model automotive emissions control catalyst consisting of Rh particles on an epitaxial ceria film grown on the (100) surface of an yttria stabilized zirconia single crystal was studied using TPD. The TPD results showed that ethylene undergoes dehydrogenation on the model catalyst at temperatures below 500 K depositing carbon on the Rh particles. Surface carbon formed in this manner was oxidized to CO upon heating to above 550 K using oxygen supplied by the ceria lattice. Heating to 900 K was found to induce reduction of the ceria film resulting in a marked decrease in oxidation activity. [ABSTRACT FROM AUTHOR]

The redox properties of praseodymia have been compared to those of ceria using temperature programmed desorption (TPD) of O2 from films prepared by vapor deposition onto an α Al2O3(0001) support, in order to assess the potential for praseodymia as an oxygen storage component in three way catalysts. Desorption of O2 was observed between 700 and 1000 K, and diffraction measurements confirmed that the films change from Pr6O11 to Pr2O3 in this temperature range. However, once reduced, the films could not be reoxidized by exposures to O2 in the vacuum system. Evidence was found for oxygen transfer from Pr6O11 to supported Rh particles. Following CO adsorption on Rh particles deposited onto Pr6O11 film, significant amounts of CO2 were observed in TPD until the film was completely reduced to Pr2O3. Following deposition of Rh particles onto Pr6O11, the oxygen desorption curves became identical to those normally observed for Rh. These results are compared with similar measurements performed on ceria films and the implications for automotive, emission control catalysis are discussed. [ABSTRACT FROM AUTHOR]

The catalytic reduction of NO with H2 or CO and the O2+H2 reaction have been investigated at low pressure (p < 10-3 mbar) on microstructured bimetallic Pt(100)/Rh and Pt(100)/Ti surfaces prepared by lithographic techniques. Photoemission electron microscopy (PEEM) was the spatially resolving technique used. It is shown that diffusional coupling leads to dynamic effects which are size-dependent and thus can be controlled through the design of the surface microstructure. In connection with periodic parameter forcing these dynamic effects can potentially be exploited to improve the yield and selectivity of catalytic reactions. [ABSTRACT FROM AUTHOR]

Using X-ray absorption near-edge spectroscopy (XANES) at the Ce LIII edge, we have measured the extent of reduction of Rh-loaded and Rh-free, mixed Ce-Zr oxides under hydrogen as a function of temperature. The high surface area, mixed oxides were synthesized by sol-gel techniques and hypercritical drying. Using a simple spectrum subtraction method, the degree of reduction has been measured and compared with previous results for CeO2 and (Ce0.5Zr0.5)O2. Addition of Zr lowers the temperature of reduction and increases the extent of Ce reduction. Rh catalyzes the reduction process at low temperatures but does not substantially affect the extent of reduction achieved at high temperature. A synergism between Rh and Zr is found which leads to very high reducibility in the range of 400–600 K. [ABSTRACT FROM AUTHOR]

The catalytic reduction of NO by hydrogen is investigated at ( T = 650 K and ( p≈10-6 mbar on a microstructured Rh/Pt(100) surface consisting of Pt(100) domains surrounded by a 600 Åthick Rh film. Synchrotron radiation scanning photoemission microscopy (SPEM), using photons focused into a spot of less than 0.2 μm diameter, is employed as a spatially and chemically resolving in situ technique. The chemical waves which arise in the bistable system NO+H2/Rh are imaged with SPEM monitoring the N 1s and O 1s photoelectrons. The reaction fronts initiate transitions from an inactive oxygen-covered surface (ΘO≈0.25 ML) to a reactive nitrogen-covered surface (ΘN≈0.06 ML). At the Pt/Rh interface, synergetic effects can be observed: the chemical waves on the Rh film nucleate preferentially at the Pt/Rh interface. This nucleation is poisoned by carbon contamination on the Pt area but is prevented in the vicinity of the Pt/Rh interface by the adjacent clean Rh film. No segregation of Pt to the surface was observed for the 600 Å thick Rh film. [ABSTRACT FROM AUTHOR]

Steady-state, steam-reforming rates for methane were measured on model and high-surface-area, ceria-supported Pd catalysts and compared to rates on Pd/silica between 600 and 825 K. Specific rates on Pd/ceria were higher than those on Pd/silica by a factor of 10 5 and were higher even than rates reported in the literature for Ni catalysts. Model catalysts prepared with Pt or Rh on ceria also showed very high rates, essentially identical to rates for Pd/ceria. However, catalysts prepared by addition of Pd to ceria which had been calcined to high temperatures (>1175 K) before the addition of Pd were not active. Based on these results, we suggest that steam reforming on ceria-supported catalysts occurs through a bifunctional mechanism in which oxygen from ceria reacts with dissociated methane on the precious metal. High-temperature calcination of ceria significantly changes its reducibility, so that the reaction is not enhanced. [ABSTRACT FROM AUTHOR]