Bimetallic catalysts as dissipative structures: stationary concentration patterns in the O2 + H2 reaction on a composite Rh(110)/Pt surface

The O₂ + H₂ reaction has been studied under low pressure conditions (10^-5 mbar) employing a microstructured Rh(110)/Pt surface as catalyst. Photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM) were used as spatially resolving in situ methods. Under reaction conditions stationary concentration patterns (Turing‐like structures) of the adsorbates develop inside the Pt domains which are associated with a compositional change of the metallic substrate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microchemical engineering of catalytic reactions

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.     

Two-dimensional surface properties of 2-methoxy ethyl oleate at the air/water interface

Methoxy ethyl oleate, a nonionic surfactant, has been investigated at the air/water interface for various surface properties by employing the Langmuir film balance technique. The ester forms an expanded isotherm at the air/water interface. The minimum area of packing (A ₀), initial area of increase of surface pressure (A _i ), collapse pressure (π _c ), and area/molecule at collapse pressure (A _c ) have been estimated from the isotherm curve.The higher compressibility coefficient (K) suggests that the ester forms a more expanded liquid film than the parent oleic acid. Besides, the ester film is fairly stable as suggested by only about 30% loss in area over a period of 20 min. The relaxation rates of the ester film at different surface pressures of 10, 15, and 20 mN·m^?1 have been estimated from changes in the surface area/molecule with time. Interestingly, the surface area (54.2 Ų/molecule) that corresponds to a minimized structure projected for the ester, calculated theoretically, agrees reasonably well with the experimental value (57.2 Ų/molecule).     

Correlating Low-temperature Hydrogenation Activity of Co/Pt(111) Bimetallic Surfaces to Supported Co/Pt/γ-Al2O3 Catalysts

The low-temperature self-hydrogenation (disproportionation) of cyclohexene was used as a probe reaction to correlate the reactivity of Co/Pt(111) bimetallic surfaces with supported Co/Pt/γ-Al₂O₃ catalysts. Temperature-programmed desorption (TPD) experiments show that cyclohexene undergoes self-hydrogenation on the ~1 ML Co/Pt(111) surface at ~219 K, which does not occur on either pure Pt(111) or a thick Co film on Pt(111). Supported catalysts with a 1:1 atomic ratio of Co:Pt were synthesized on a high surface area γ-Al₂O₃ to verify the bimetallic effect on the self-hydrogenation of cyclohexene. EXAFS experiments confirmed the presence of Co–Pt bonds in the catalyst. Using FTIR in a batch reactor configuration, the bimetallic catalyst showed a higher activity toward the self-hydrogenation of cyclohexene at room temperature than either Pt/γ-Al₂O₃ or Co/γ-Al₂O₃ catalysts. The comparison of Co/Pt(111) and Co/Pt/γ-Al₂O₃ provided an excellent example of correlating the self-hydrogenation activity of cyclohexene on bimetallic model surfaces and supported catalysts.     

Enhanced Low Temperature NO x Reduction Performance Over Bimetallic Pt/Rh–BaO Lean NO x Trap Catalysts

The overall NSR operation was tested over a bimetallic Pt/Rh–BaO lean NO _x trap (LNT) catalyst in the range of 473–673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/γ-Al₂O₃ and 0.5 wt% Rh/BaO/γ-Al₂O₃ samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/γ-Al₂O₃ and Rh/BaO/γ-Al₂O₃, which both were mildly aged, have limited NO _x reduction capacity at 473 K. However, combining Pt and Rh in the NO _x storage catalyst assisted the NO _x reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO₂-TPD data showed that the presence of Rh into the Pt/BaO/γ-Al₂O₃ system has a considerable effect on the spill-over process of NO _x , accelerating the release of NO _x at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/γ-Al₂O₃ catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NO _x reduction efficiency in comparison with that of the monometallic Pt and Rh–BaO LNT catalysts.     

Imaging and chemical probing on the atomic scale: reconstruction and dynamics of the systems O2/Rh and NO2–H2/Pt

This paper presents two case studies of adsorbate-induced surface reconstruction, on the one hand, and dynamical reaction imaging along with local chemical probing, on the other hand. The first one deals with the oxygen-induced reshaping of 3D Rh crystals. Field ion microscopy (FIM) was applied to image in real-space the change from a nearly hemispherical shape in the absence of oxygen toward a polyhedral one in the presence of oxygen. Shape transformation occurs at temperatures of 380–550 K and is associated with the appearance of facets with {111} and {001} orientation. The only high-index planes present in the polyhedral form are of {137} symmetry. (1×2) and (1×3) missing-row reconstructions appear in the {113} and {011} planes. The polyhedral form has also been imaged under in situ conditions of the oxygen–hydrogen reaction on Rh at 505 K. The second case study deals with kinetic non-linearities occurring in the NO₂ reaction with hydrogen on the surface of a 3D Pt crystal reconstructed to a top- and edge-truncated pyramid. The reaction was found to ignite in the {012} corner planes of the crystal. One-dimensional wavefronts were subsequently observed to move along the 211 zone lines. These studies were performed by video-FIM and could be correlated with a local chemical analysis by time-of-flight mass spectrometry of ionised species. The mass spectrum provided information on water product (H₂O⁺ and H₃O⁺) and NO intermediate formation. Strong fluctuations in the NO ₂ ⁺ current indicated the occurrence of NO₂ surface diffusion. These species are most likely responsible for the field ion image formation.     

Effect of high temperature reduction in hydrogen on Pt deposited on the TiO2(100) surface: An angle resolved x-ray photoemission study

The effect of high temperature reduction in hydrogen was studied by angle resolved XPS in a system formed of platinum deposited on the (100) surface of a TiO₂ single crystal sample. The analysis of the variation of the Pt/Ti XPS signal ratio as a function of the electron collection angle showed that partially reduced titanium oxide is present on the Pt film after the reduction treatment. The amount of oxide on the Pt surface is of the order of one monolayer equivalent.     

Preparation and electrochemistry of cadmium pentacyanonitrosylferrate film modified glassy carbon electrode

A glassy carbon (GC) electrode surface was modified with a cadmium pentacyanonitrosylferrate (CdPCNF) film as a novel electrode material. The modification procedure of the GC surface includes two consecutive procedures: (i) the electrodeposition of metallic cadmium on the GC electrode surface from a CdCl₂ solution and (ii) the chemical transformation of the deposited cadmium to the CdPCNF films in 0.05 M Na₂[Fe(CN)₅NO] + 0.5 M KNO₃ solution. The modified GC electrode showed a well-defined redox couple due to [Cd^IIFe^III/II(CN)₅NO]^0/−1 system. The effects of supporting electrolytes and solution pH were studied on the electrochemical behavior of the modified electrode. The diffusion coefficients of alkali-metal cations in the film (D), the transfer coefficient (α) and the charge transfer rate constant at the modifying film | electrode interface (k_s), were calculated in the presence of various alkali-metal cations. The stability of the modified electrode was investigated under various experimental conditions.     

Induced slim ferroelectric hysteresis loops and enhanced energy-storage properties of Mn-doped (Pb0·93La0.07)(Zr0·82Ti0.18)O3 anti-ferroelectric thick films by aerosol deposition

In this study, the high ferroelectric hysteresis loss of (Pb_0·93La_0.07)(Zr_0·82Ti_0.18)O₃ (PLZT 7/82/18) antiferroelectric (AFE) ceramics was reduced by employing a combinatorial approach of Mn acceptor doping followed by thick film fabrication via an aerosol deposition (AD) process. The grains of the as-deposited PLZT 7/82/18 AFE AD thick films were grown by thermal annealing at 550 °C to enhance their electrical properties. Investigation of the electrical properties revealed that Mn-doping results in improved dielectric and ferroelectric properties, increased dielectric breakdown strength (DBS), and energy-storage properties. The Mn-doped PLZT AFE AD films possess a frequency-independent high dielectric constant (ε_r ≈ 660) with low dielectric loss (tan δ ≈ 0.0146), making them suitable candidates for storage capacitor applications. The bipolar and unipolar polarization vs. electric field (P-E) hysteresis loops of PLZT 7/82/18 AFE AD thick films were found to be slimmer than those of their bulk form (double hysteresis) with significantly reduced ferroelectric hysteresis loss, which is attributed to the AD-induced mixed grain structure. The Mn-doped PLZT 7/82/18 AFE AD thick films exhibited a low remnant polarization (P_r ≈ 9.22 μC/cm²) at a high applied electric field (~2750 kV/cm). The energy-storage density and energy efficiency of the Mn-doped PLZT AFE AD thick films were calculated from unipolar P-E hysteresis loops and found to be ~38.33 J/cm³ and ~74%, respectively.     

Changes in the oxidation state of transition metal containing MCM-41 during deNOx reactions

The oxidation state of Co, Pt and Rh containing MCM-41 after co-adsorption of NO, CH₄, and O₂ in the presence of water vapor was studied by in situ XANES experiments. All catalysts were oxidized after the adsorption of NO and reduced after the co-adsorption of NO and CH₄. The presence of water vapor did not influence the oxidation/reduction of Rh/MCM-41 and Pt/MCM-41, while Co/MCM-41 was oxidized by the presence of H₂O at reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Catalyst Composition on NOX Trap Performances

In the present work the influence of the type of noble metals present in NO_X storage catalysts was investigated regarding the NO oxidation, NO_X storage and NO reduction performances. Monolith samples with combinations of platinum, palladium and rhodium were tested in a flow-reactor. From this study, it was concluded that platinum is necessary for NO oxidation while palladium shows a better ability to oxidize NO to NO₂ at high temperature. The presence of Rh on Pt/Rh and Pt/Pd/Rh was found to have a negative effect on NO oxidation reaction. Results suggest also that the storage capacity is partially linked to the oxidation function: Surface area developed by the storage material and the Pt–Ba interaction have an important influence on NO_X storage function. The rich step was not efficient to purge completely the trap and leads to accumulation of NO_X and so to performances degradation which was assigned to accumulation of NO_X after each cycle and storage on of Ba-bulk sites. In presence of Rh, a high oxygen storage capacity leads to high exothermic process between reducing agent and stored oxygen which improves the ability of catalyst to release NO_X.     

A new NOx storage-reduction electrochemical catalyst

A new NO_x storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NO_x storage capacity of Pt-BaO/YSZ system was investigated at 350 °C and 400 °C under lean conditions. Results have shown that the electrochemical catalyst was effective for NO_x storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO₂ desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NO_x storage occurs but only the reaction of NO oxidation into NO₂. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NO_x storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NO_x storage process was explained in terms of oxygen coverage modifications on the Pt.     

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. NO _x 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.     

Incorporation of bromo-octadecane into long-chain ester monolayers at the air/water interface

Surface compatibility of 2-methoxy ethyl stearate and oleate esters with 1-bromo-octadecane (RBr) has been investigated at the air/water interface with a Langmuir film balance. The methoxy ethyl head group in the esters promotes flat conformation at the air/water interface as observed from its higher surface area (stearate, 22.41 Ų/molecule; oleate, 57.20 Ų/molecule) in comparison to the corresponding acids. The stearate ester forms a homogeneous mixed monolayer with maximal incorporation of 0.5 mole fraction of RBr. This is indicated by the retention of liquid condensed and solid condensed phases of stearate ester, and the positive deviation of the mean molecular area of the mixed film from the calculated additive areas. When the mole fraction of RBr (x ₂) exceeds 0.5, the onset of formation of heterogeneous mixed film is indicated by the appearance of initial and final collapse pressures. On the contrary, oleate ester shows the least compatibility with RBr, which is indicated by the progressive decrease in mean molecular area with x ₂. The more liquid expanded-phase structure of oleate ester probably does not promote compatibility with RBr at the air/water interface.     

Characterization of Pt?CAu and Ni?CAu Clusters on TiO2(110)

The surface composition and properties of Pt?CAu and Ni?CAu clusters on TiO₂(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS) and soft X-ray photoelectron spectroscopy (sXPS). STM studies show that bimetallic clusters are formed during sequential deposition of the two metals, regardless of the order of deposition. At the 2?ML of Au/2?ML of Pt or Ni coverages studied here, the second metal contributes to the growth of existing clusters rather than forming new pure metal clusters. LEIS experiments demonstrate that the surfaces of the bimetallic clusters are almost 100% Au when 2?ML of Au is deposited on top of 2?ML of Pt or Ni. However, a much larger fraction of Pt or Ni (50 and 20%, respectively) remains at the surface when 2?ML of Pt or Ni is deposited on 2?ML of Au, most likely due to limited diffusion of atoms within the clusters at room temperature. According to sXPS investigations, the binding energies of the metals in the bimetallic clusters are shifted from those observed for pure metal clusters; the Pt(4f_7/2) and Ni(3p_3/2) peaks are shifted to lower binding energies while the position of the Au(4f_7/2) peak is dominated by surface core level shifts. Pure Pt clusters as well as 0.4?ML of Au on 2 ML of Pt clusters reduce the titania support upon encapsulation after annealing to 800?K, whereas 2?ML of Au on 2?ML of Pt clusters do not reduce titania, presumably because there is no Pt at the surface of the clusters. Pure Ni clusters are also known to become encapsulated upon heating, but the reduction of titania is much less extensive compared to that of pure Pt clusters.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Effect of chemisorbed carbon monoxide on Pt/C electrode on the mechanism of the hydrogen oxidation reaction

The influence of poisoning of Pt catalyst by CO on the kinetics and mechanism of H₂ oxidation reaction (HOR) at Pt/C electrode in 0.5 mol dm⁻³ HClO₄, saturated with H₂ containing 100 ppm CO, was examined with rotating disc electrode (RDE) at 22 °C. Commercial carbon black, Vulcan XC-72 was used as support, while Pt/C catalyst was prepared by modified polyol synthesis method in an ethylene glycol (EG) solution. The kinetically controlled current (I_k) for the HOR at Pt/C decreases significantly at CO coverage (Θ_CO) > 0.6. For Θ_CO < 0.6 the HOR takes place through Tafel-Volmer mechanism with Tafel reaction as rate-determining step at the low CO coverage, while Volmer step controls the overall reaction rate at the medium CO coverage. When CO coverage is higher then 0.6, Heyrovsky-Volmer mechanism is operative for the HOR with Heyrovsky as the rate-determining step (rds).