Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

The Bromine Electrode. Part I: Adsorption Phenomena at Polycrystalline Platinum Electrodes

A cyclic voltammetric study of the behaviour of Br^– and Br^– ₃ at Pt electrodes, in the potential range between hydrogen and oxygen evolution, is described. Different experiments were carried out, in the presence of Br^– and Br^– ₃, in which the ratio between the species has been kept constant and equal to 1. The halide concentration was varied between 4 × 10^–6 and 1 × 10^–3 and mol dm^–3, at constant ionic strength, in 1 M HclO₄ as well as in 1 M NaClO₄ adjusted to a pH of 2. Underpotential deposition of Br is observed at potentials as low as –0.125 V vs SCE. The adsorption parameters of Br species were determined from the adsorption/desorption peak pair in the hydrogen adsorption/desorption region, and from the oxide reduction peak data. In the absence of oxygen adsorption, a relatively high coverage of the electrode surface is attained. A Langmuir-type adsorption is observed under the different experimental conditions.     

Electron Microscopy (HREM, EELS) Study of the Reoxidation Conditions for Recovery of NM/CeO2 (NM: Rh, Pt) Catalysts from Decoration or Alloying Phenomena

The reversibility of metal–support interaction effects in NM/CeO₂ catalysts (NM: Rh, Pt) is investigated using high-resolution electron microscopy and electron energy loss spectroscopy. Reoxidation treatments at 773 K followed by a mild reduction at 473 K are not effective in recovering ceria-based systems from the decorated or alloyed states observed upon high-temperature reduction (T _red>973 K).     

Synthesis,Characterization and Properties of One-dimensional Complexes of <Emphasis Type="Italic">cis-syn-cis</Emphasis>-Dicyclohexyl-18-crown-6 with K<Subscript>2</Subscript>[M(mnt)<Subscript>2</Subscript>] (M==Ni,Pd, Pt)

Three complexes [K(DC18C6-A)]₂[M(mnt)₂] (DC18C6-A=cis-syn-cis-dicyclohexyl-18-crown-6, isomer A; M=Ni, 1; Pd, 2; Pt, 3; mnt = 1,2-dicyanoethene-1,2-dithiolate, maleonitriledithiolate, [C₂S₂(CN)₂]²⁻) have been synthesized and characterized by elementary analysis, UV–vis, FT-IR spectroscopy and X-ray single crystal diffraction. They are isomorphous and all display infinite one-dimensional chain-like structure formed by [K(DC18C6-A)]⁺ complex cations and [M(mnt)₂]²⁻ (M=Ni, Pd, Pt) complex anions through K–N interactions. Thermal analysis indicates that three complexes are all thermal stable under 260 °C and experience the same decomposition process of dissociation and evaporation of crown ether molecules. Their electrochemical behaviors have been studied by cyclic voltammetry.     

Oscillatory behaviour of the reduction of NO by H2 over Rh

The NO reduction by H₂ on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andP _NO = –1.5 ×10^–1 Torr andP _H2 =1×10^-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H₂ and NO-NH₃ reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.     

Synthesis and characterization of per-substituted spermine-bridged cyclophosphazene derivatives: the first example of syn/anti conformational polymorphism in a bridged cyclophosphazene

The synthesis and characterization of a series of per-substituted spermine-bridged cyclophosphazene derivatives are reported based on the known compound [N₃P₃X₄(NHCH₂CH₂CH₂N)CH₂CH₂]₂, (1a) (where X = Cl), to give a number of new compounds (1b–1h) in which X = OPh, [spiro-O(CH₂)₃O]_0.5, OCH₂CF₃, NHPh, NC₄H₈, Ph and NHBu^t, respectively. Two synthetic routes were utilized: (i) using the chloro-precursor 1a in nucleophilic substitution reactions with a variety of anionic and neutral nucleophiles to give compounds 1b–1f and (ii) reaction of spermine with the appropriate di-gem tetrasubstituted cyclophosphazene to give compounds 1g and 1h. Bridged compounds such as 1a–1h may exist as syn or anti conformers in the solid state and the first example of syn and anti conformational polymorphism is reported for a bridged cyclophosphazene, viz. for compound 1a.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media. Part I: Bulk electrodes

We studied the borohydride oxidation reaction (BOR) by voltammetry for BH₄⁻ concentrations between 10⁻³ M and 0.1 M NaBH₄ in 0.1–1 M NaOH for bulk polycrystalline Pt, Ag and alloyed Pt–Ag electrocatalysts. In order to compare the different electrocatalysts, we measured the kinetic parameters and the number of electrons exchanged (faradic efficiency). BOR on bulk Pt is more efficient when the concentration of NaBH₄ increases (3e⁻ in 1 mM and 6e⁻ in 10 mM BH₄⁻/0.1 M NaOH). BOR on Pt can occur both in a direct pathway and in an indirect pathway including hydrogen generation via heterogeneous hydrolysis of BH₄⁻ and subsequent oxidation of its by-products (e.g. BH₃OH⁻ and H₂). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH (2e⁻ at pH 12.6 and 6e⁻ at pH 13.9 at 25 °C). The BOR kinetics is faster for Pt than for Ag (i_Pt=0.02 A cm⁻², i_Ag=1.4 10⁻⁷ A cm⁻² at E=−0.65 V vs. NHE in 1 mM NaBH₄/0.1 M NaOH, 25 °C) both as a result from Pt high activity regarding the BH₄⁻ heterogeneous hydrolysis and subsequent HOR, above −0.83 V vs. NHE and following direct oxidation of BH₄⁻ or BH₃OH⁻ below −0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH₄⁻ concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect.     

Electrocoating of iron and other metals with polypyrrole

Galvanostatic electrodeposition of polypyrrole from more than 20 nonaqueous and aqueous electrolytes containing anions such as BF ₄ ^– , ClO ₄ ^– , HSO ₄ ^– , Tos^–, HCO ₃ ^– , H₂PO ₄ ^– , HPO ₄ ^2– , H₂BO ₃ ^– and NO ₃ ^– at the substrates Pt, Au, Cu, Ti, stainless steel/V2A and Fe was investigated. The passivating substrates Pt, Au, Ti and V2A could be electrocoated under a broad variety of conditions. Cu dissolved anodically. The same was true for Fe with only one important exception, namely aqueous nitrate. Smooth, well-adhering polypyrrole layers were obtained on iron at NO ₃ ^– concentrations of 0.01–1 M and at current densities of 0.5–10 mA cm^–2. The degree of insertion for nitrate ions was abouty=0.25, confirmed by elemental analysis. Current efficiencies with regard to thisy were about 90%. The layers were stable in air for a long time.     

Application of high-energy X-rays and Pair-Distribution-Function analysis to nano-scale structural studies in catalysis

We investigate the structure of supported Pt catalysts using high-energy X-ray scattering coupled with Pair-Distribution-Function (PDF) analysis. Recently, experimental approaches that enable the collection of PDF data in situ have been developed with time-resolution sufficient to study the structure of Pt nano-particles as they form. The differential PDF approach is utilized which allows the atom–atom correlations involving only Pt to be selectively recovered, enabling structural investigation of the supported particles and the mechanism of their formation. In parallel to the in situ analysis, we have examined samples prepared ex situ. Data collected on the ex situ samples show that the initial deposition of Pt⁴⁺ occurs as the PtCl₆²⁻ species which are retained even when annealed in an oxygen atmosphere. The Pt differential PDFs of the samples reduced in hydrogen at 200 and 500 °C indicated nano-crystalline face-centered-cubic (fcc) metallic Pt particles. The ex situ reduced samples also contain a weak correlations at 2.1 Å, which we assign to Pt–O interactions between the particles and the support surface. The in situ experiments, following the reduction of Pt⁴⁺ from 0 to 227 °C, indicate that the initial Pt nano-particles formed are ca. 1 nm in size, and become larger and more crystalline by 200 °C. The data suggest a particle growth mechanism where the initial particles that form are small (<1 nm), then agglomerate into ensembles of many small particles and lastly anneal to form larger well-ordered particles. Lastly, we discus potential future developments in operando PDF studies, and identify opportunities for synchronous application of complementary methods.     

Deep desulphurization of diesel fuels on bifunctional monolithic nanostructured Pt-zeolite catalysts

The preparation of Pt-zeolite catalysts, including choice of the noble metal precursor and loading (1.0–1.8 wt.%), was optimized for maximizing the catalytic activity in thiophene hydrodesulphurization (HDS) and benzene hydrogenation (HYD). According to data obtained by HRTEM, XPS, EXAFS and FTIR spectroscopy of adsorbed CO, the catalysts contained finely dispersed Pt nanoparticles (2–5 nm) located on montmorillonite and zeolite surfaces as: Pt⁰ (main, ν_CO = 2070–2095 cm⁻¹), Pt^δ+ (ν_CO = 2128 cm⁻¹) and Pt²⁺ (ν_CO = 2149–2155 cm⁻¹). It was shown that the state of Pt depended on the Si/Al zeolite ratio, montmorillonite presence and Pt precursor. The use of H₂PtCl₆ as the precursor (impregnation) promoted stabilization of an oxidized Pt state, most likely Pt(OH)_xCl_y. When Pt(NH₃)₄Cl₂ (ion-exchange) was used, the Pt⁰ and hydroxo- or oxy-complexes Pt(OH)₆²⁻ or PtO₂ were formed. The addition of the Ca-montmorillonite favoured stabilization of Pt^+δ. The Cl⁻ ions inhibit reduction of oxidized Pt state to Pt particles. The Pt-zeolite catalyst demonstrated high efficiency in ultra-deep desulphurization of DLCO. The good catalyst performance in hydrogenation activity and sulphur resistance can be explained by the favourable pore space architecture and the location and the state of the Pt clusters. The bimodal texture of the developed zeolite substrates allows realizing a concept for design of sulphur-resistant noble metal hydrotreating catalyst proposed by Song [C. Song, Shape-Selective Catalysis, Chemicals Synthesis and Hydrocarbon Processing (ACS Symposium Series 738), Washington, 1999, p. 381; Chemtech 29(3) (1999) 26].     

Performance characteristics of a commercial controlled-release dispenser of sex pheromone for control of codling moth (Cydia pomonella) by mating disruption

Performance characteristics of polyethylene tube dispensers containing a mixture of (E,E)-8,10-dodecadien-1-ol (1), dodecan-1-ol (2), and tetradecan-1-ol (3) were evaluated for suitability as a mating disruptant for codling moth control. The rate of loss of pheromone component from a dispenser at any time was found to be described by the equation: –dP/dt=(k ₁ k ₂ +k _d)P whereP is the amount of pheromone component in the dispenser well;t is time;k ₁ is the ratio of the amount dissolved in the dispenser wall to the amount in the dispenser well;k ₁ is the ratio of the evaporation rate to the amount dissolved in the dispenser wall;k _d is the rate constant for chemical decomposition. The evaporation rate,E, of a pheromone component at any time was given by:E=k ₁ k ₂ P For all three components during the first three weeks,k ₁ decreased from ca. 0.25 to ca. 0.10 and was approximately constant thereafter. The decrease ofk ₁ with time may have been caused by weather-induced cross-linking of the polyethylene. Over timek ₂ was constant and was 1.27 ± 0.26 × 10^–3/hr for1, 1.96 ± 0.33 × 10su–3/hr for 2, and 0.31± 0.05 × 10^–3/hr for 3. Thek ₂ was zero for2 and3 and 6.96 × 10^–4 for1. After 150 days in an orchard in 1991, 95% of1 was lost from the dispensers (61% of the loss was by chemical decomposition and 39% by evaporation). The heat summation units in a Yakima valley orchard during 1991 were 4.7% above the average for the 1980–1991 period, while during 1990 they were the highest for this period (26% above average). After the first three weeks of dispenser aging, the regression line half-lives for1 for 1990 and 1991 were 31.0 and 35.1 days, respectively. The difference in temperature between 1990 and 1991 did not affect the half-life of1 very much because so much of the loss was from photochemically induced decomposition. Based on an estimate of the required minimum evaporation rate for mating disruption of 2 mg/ha-hr and a half-life of 35 days for1, 2345 dispensers/ha would be required for one application per season; 944 dispensers/ha for two applications per season; and 734 dispensers/ha for three applications per season. If a different emission rate of1 is required for reliable mating disruption, then the number of dispensers required would be changed proportionately.     

Synthesis and characterization of soluble (porphyrinato)iron(II) polymers

The use of (tetrakis(4-hexylphenyl)porphyrinato)Fe(II) in polymerization reactions with bidentate ligands such as 9,10-diisocyanoanthracene and 1,4-diisocyanobenzene led to well-defined stacked polymers1 and2 which are still soluble in common organic solvents such as chloroform, dichloromethane, and tetrahydrofurane. They have been completely characterized by¹H-NMR and UV/vis spectropscopy in solution, even allowing end-group analysis for determination of the average degree of polymerization, yieldingn=10 andn=5 for1 and2, respectively. Mößbauer and IR spectroscopy further established the strong Fe-CN bonding reflected by very small isomer shifts and quadrupole splittings (E _Q0.2 mm s^–1) and a large decrease in the IR stretching frequency (v _CN60 cm^–1). The axially stacked polymers exhibit semiconducting properties only upon doping.Presented at the 5th International Symposium on Macromolecule-Metal Complexes (MMV), Summer 1993 in Bremen, Germany.     

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with¹³C in CH₃ or C-O groups (t-BuOH[2–¹³C₂] andt–BuOH[1-¹³C]), as well as selectively deuterated in methyl groups (t-BuOH[2-²H₉]), was studied on H-ZSM-5 zeolite simultaneously with¹³ C CP/MAS and²H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2–¹³C₁] andt-BuOH[1–¹³C] give rise to identical₁₃C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the side reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.     

Kinetics of ethylene oxidation on plane Pt/SiO2 catalysts in the viscous pressure regime: evidence of support activity

C₂H₄ oxidation on plane Pt/SiO₂ model catalysts with various Pt loadings was studied atT = 373–473 K and in the pressure ranges 10^–6–10² Torr C₂H₄ and 0.3–1500 Torr 02 (1 Torr = 133.3 Pa). Mass spectrometry combined with spatially resolved gas sampling enabled kinetic data to be collected far into the viscous pressure regime. Reaction orders and activation energies were similar to those of a macroscopic Pt surface. However, under fuel-lean conditions the global reaction rate decreases faster than the decrease in metal area. On the other hand, the global rate wasindependent of Pt loading and metal surface area in fuel-rich gas mixtures. This is interpreted in terms of a spillover effect.     

Selective Modification of Pt Active Sites on Pt–Au/C Catalysts Prepared by Surface Redox Reactions

This study focused on the selective deposition of Au⁰ onto (111), (100) faces and (111)/(100) edges of cuboctahedral Pt particles present on the Pt/C(graphite) model system. The Pt–Au/C catalysts were prepared by novel surface redox methods involving the direct reduction (DR) of AuCl ₄ ^– species onto the Pt particles or reducing these species on the Pt–H interface, i.e., the refilling (RE) method. The presence of Au on the Pt particles was verified by means of high-resolution energy dispersive spectroscopy (EDS), and, after treatment at 300°C in H₂, the formation of crystalline Au⁰ aggregates was verified by X-ray wide-angle diffraction; further treatments at 500°C in H₂ led to a true Pt–Au solid solution. The Monte Carlo simulation methods indicated the selective deposition of Au⁰ onto the (111)/(100) edges of the Pt cuboctahedral particles when the relative Au concentration varied from 10 to 50 wt% Au. The catalytic conversion of n-heptane on the Pt–Au/C (DR and RE solids) catalysts presented an oscillatory behavior with respect to Pt/C, indicating modification of the active Pt ensembles, driven by the energy released during the exothermic n-C₇ dehydrogenation and cracking reactions, which should enhance the Au⁰ mobility at the Pt particle surface level.     

Electrochemical treatment of methyl parathion based on the implementation of a factorial design

Commercial methyl parathion was treated by an electrochemical method using Ti/Pt as anode, Stainless Steel 304 as cathode and sodium chloride as electrolyte. Based on a number of preliminary experiments, a factorial experimental procedure was designed in order to optimize the electrolysis efficiency, in terms of removed COD and energy consumed kW h per kg of removed COD. The parameters examined were the temperature, the stirring rate of the brine solution, the input rate of the organic material, the current density, the electrolyte concentration and the concentration of Fe ²⁺ ions added. In the experimental range studied, the lower energy consumption measured was 6.61 kW h ( kg COD _r) ^–1 and the higher COD reduction measured was 86.3. From a mathematical model, the optimum conditions for the electrochemical treatment of MeP for 2.03 kW h ( kg COD _r) ^–1 were found to be Input rate of MeP 4300 mg COD min^–1, NaCl concentration 4.5, 4 g l^–1 of added FeSO₄, current density 0.47 A cm^–2, temperature 45 °C and stirring rate 400 rpm. An experiment was conducted under these optimum conditions which resulted in a satisfactory removal of the organic load (in terms of COD, BOD ₅). Furthermore, a significant improvement in the COD/BOD₅ ratio was achieved, rendering the effluent amenable to further biological treatment.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.     

Interactions oftrans-cinnamic acid,its related phenolic allelochemicals,and abscisic acid in seedling growth and seed germination of lettuce

Phenolic compounds have been identified as the most common allelochemicals produced by higher plants. Inhibitions of cinnamic acid, its related phenolic derivatives, and abscisic acid (ABA) on seedling growth and seed germination of lettuce were studied.trans-Cinnamic acid, ando-,m-, andp-coumaric acids inhibited the growth of etiolated seedlings of lettuce at concentrations higher than 10^–4 M and seed germination above 10^–3 M. Coumarin inhibited seedling growth and seed germination at 10^–5 M or above. Chlorogenic acid inhibited seedling growth above 10^–4 M, but did not inhibit seed germination at 10^–5–5×10^–3 M. Low concentrations (below 10^–3 M) of caffeic and ferulic acids promoted the elongation of hypocotyls, but higher concentrations (over 10^–3 M) inhibited seedling growth and seed germination. These phenolic compounds and abscisic acid had additive inhibitory effects both on seedling growth and seed germination. The inhibition on lettuce was reversed by caffeic and ferulic acids at concentrations lower than 10^–3 M except for the inhibition of germination by coumarin. These results suggest that in naturetrans-cinnamic acid,o-, m-, p-coumaric acids, coumarin, and chlorogenic acid inhibit plant growth regardless of their concentration. However, caffeic and ferulic acids can either promote or inhibit plant growth according to their concentration.     

Preparation and electrochemical characteristics of a new Li-Mn-V-O system formed from heat-treatment of a MnO2, NH4VO3 and LiNO3 mixture

New quarternary oxides (Li₂O) _x · MnO₂ · yV₂O₅ (x = 0.125 0.25, y = 0.125 0.25), formed by heating mixtures of MnO₂, NH₄VO₃ and LiNO₃ at various Li/Mn and V/Mn atomic ratios and at different temperatures (300 400 °C in air, have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy. The quarternary oxide with x = 0.25 and y = 0.25 showed a discharge capacity of 220 A h (kg oxide)^–1 and an energy density of ca. 600 W h (kg oxide)^–1 at a current density of 0.20 mA cm^–1 in 1 m LiClO₄-propylene carbonate at 25 °C. When charge-discharge cycling with the (Li₂O) · MnO₂ · 0.25V₂O₅ electrode was performed at a constant capacity of 30 A h (kg oxide)^–1 and at a constant current density of 0.10 0.20 mA cm^–2, the electrode sustained over 100 cycles at a high mean discharge potential of ca. 3 V vs Li/Li⁺.This paper was originally presented at the 183rd meeting of the Electrochemical Society (Honolulu, Hawaii, 1993).     

A model of Pt-Rh catalyst prepared by electro-chemical deposition of Rh ions on Pt(100) surface

When Pt-Rh(100) alloy surface was exposed to NO or O₂ at temperatures higher than 400 K, a characteristic p(3 × 1) LEED pattern appeared with segregation of Rh atoms on the surface. It was shown that when a Rh-deposited Pt(100) surface is heated in NO or O₂ the same 5 × 10^–8 Torr of H₂ at room temperature, the p(3 × 1) pattern disappears but is readily recovered 5 × 10^–8 Torr of H₂ at room temperature, the p(3 × 1) pattern disappears but is readily recovered by exposing to O₂ of 1 × 10^–7 Torr at room temperature. The growth of a Rh-O overlayer on Pt layer is responsible for the formation of the p(3 × 1) structure on either the Pt-Rh(100) alloy or Pt(100) surface, and this peculiar surface may correspond to the active surface of the Pt-Rh catalyst for NO _x reduction