Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

Effect of thermal pretreatment on the surface structure of PtSn/SiO<Subscript>2</Subscript> catalyst and its performance in acetic acid hydrogenation

The effect of thermal pretreatment on the active sites and catalytic performances of PtSn/SiO₂ catalyst in acetic acid (AcOH) hydrogenation was investigated in this article. The catalysts were characterized by N₂ physical adsorption, X-ray diffraction, transmission electron microscopy, pyridine Fourier-transform infrared spectra, and H₂-O₂ titration on its physicochemical properties. The results showed that Pt species were formed primarily in crystalline structure and no PtSn _x alloy was observed. Meanwhile, with the increment of thermal pretreatment temperature, Pt dispersion showed a decreasing trend due to the aggregation of Pt particles. Simultaneously, the amount of Lewis acid sites was remarkably influenced by such thermal pretreatment owning to the consequent physicochemical property variation of Sn species. Interestingly, the catalytic activity showed the similar variation trend with that of Lewis acid sites, confirming the important roles of Lewis acid sites in AcOH hydrogenation. Moreover, a balancing effect between exposed Pt and Lewis acid sites was obtained, resulting in the superior catalytic performance in AcOH hydrogenation.

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The Characterization of Tungsten-Oxide-Modified Zirconia Supports for Dual Functional Catalysis

The results of catalytic titration measurements indicated that WO _x /ZrO₂ catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WO _x /ZrO₂ contains 0.004 meq-H⁺/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Brønsted acid sites on the surface of these catalysts. The concentration of strong Brønsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Brønsted–Lewis site. Pt/WO _x /ZrO₂ undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WO _x /ZrO₂ (16% W) catalyst, consistent with the presence of bulk WO₃ in this material as observed by TPR.     

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.     

Studies of platinum electroplating baths Part IV: Deposits on copper from Q bath

The deposition of platinum on copper from a modern commercial electroplating bath (Pt 5Q bath containing 26mm Pt(NH₃)₄HPO₄ 30 mm sodium phosphate buffer, pH 10.6 at 368 K) has been studied using voltammetry and potential step methods. In addition, polished Cu panels (area 3.4 cm²) have been electroplated using both constant potential and constant current conditions; current efficiencies have been determined and scanning electron microscopy has been used to show that the morphology of the platinum layers depends strongly on the plating conditions, particularly the potential at which deposition occurs. It is shown that good quality electroplates can be obtained with high current efficiency but a high rate deposition is more readily achieved using controlled potential.     

Heterogeneous Asymmetric Reactions 20. Effect of Ultrasonic Variables on the Enantiodifferentiation in Cinchona-Modified Platinum-Catalyzed Sonochemical Hydrogenations

The effect of sonochemical variables on a cinchona alkaloid modified Pt/Al₂O₃ catalyst system and its application in -ketoester hydrogenation are described. The sonochemical pretreatment of these commercial Pt/Al₂O₃-cinchonidine catalysts resulted in excellent ee values (up to 92-98% ee) under mild experimental conditions. To gain more insight into the nature of the ultrasonic effect the reactions were screened under widely varied conditions (ultrasound source, frequency, insonation time). Besides investigating the reactions, the catalyst-modifier system was also studied. The changes in metal particle size were determined by transmission electron microscopy, while the alteration of modifier concentration in the solvent upon sonication was followed by UV-vis spectroscopy. The transformation of the modifier during the pretreatment was detected by GC-MS and verified by NMR. Summarizing the results, the major effects of the sonochemical activation on the cinchona-modified supported Pt catalyst system can be described. The ultrasonic pretreatment increased the quantity of adsorbed cinchona and blocked its hydrogenation to provide more and highly stable chiral active sites for enantioselection.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.     

The dissolution of iron from the negative material in pocket plate nickel-cadmium batteries

A common mode of failure of nickel-cadmium flooded pocket plate cells is iron poisoning of the positive plate due to transfer of iron into the active material from active materials and materials of construction. Nickel plated steel pockets are sometimes used to minimize iron dissolution, particularly on the positive electrode. Sometimes-Fe₂O₃ is used as an additive to the cadmium electrode. This paper assesses the extent of dissolution of iron from-Fe₂O₃ by using electron microscopy, X-ray crystallography, cyclic voltammetry, coulometric and atomic absorption measurements.     

Role of Promoters on Tungstated Zirconia Catalysts

The isomerization of straight-chain alkanes is effectively catalyzed by tungstated zirconia promoted by Pt or Pt and Fe. In the present paper, it is attempted to better understand the role of these promoters. The materials were characterized by CO chemisorption and by transmission electron microscopy (TEM) in combination with EDX, and their catalytic performance was tested by the isomerization of n-pentane to isopentane in the presence of H₂. At low calcination temperature, very high Pt dispersion is observed with Pt atoms/clusters being embedded in or decorated by oxide material. The highly dispersed Pt moieties appear to catalyze the hydrogenolysis reaction, thus leading to low isoalkane selectivities. Higher calcination temperatures yield larger Pt particles (lower dispersion), which results in high catalytic activity and selectivity toward the isoalkane target product. It is inferred that the dissociative H₂ chemisorption occurs on the Pt particles and facilitates the reduction of the surface tungstate, which is responsible for the dehydrogenation and isomerization of n-alkane. The role of the additional promoter iron oxide still remains unresolved.     

Nanosized Pt-Perovskite Catalyst for the Regeneration of a Wall-Flow Filter for Soot Removal from Diesel Exhaust Gases

Perovskite-type catalysts have been investigated for diesel soot combustion: (i) the LaCr_0.9O_3– substoichiometric perovskite, (ii) K–La partially substituted chromites; (iii) Pt added ii-type perovskites. The catalysts prepared showed a progressively higher activity and potential for practical application in diesel particulate traps. Engine bench tests performed on a SiC wall-flow trap (Ibiden) lined with the La_0.9K_0.1Cr_0.9O_3– + 1 wt%Pt catalyst showed that the catalyst not only speeds up soot combustion on occasional trap heating (regeneration phase) but also prolongs the trap loading phase (soot accumulation during normal operation) as Pt active sites promote NO–NO₂ oxidation, followed by the non-catalytic reaction of NO₂ with the trapped soot.     

The mechanism of the ethane hydrogenolysis—a reconsideration of some kinetic data

In order to justify the deduction made previously from isokinetic effects of ethane hydrogenolysis, i.e., that the rate determining step of this reaction on Pt and Ni is the C-C bond breaking of the chemisorbed C₂H₅ unit, some kinetic data from literature are scrutinized. It turns out that the variation of the observed rate with ethane and/or hydrogen pressure is such as predicted by the simple mechanism C₂H₅* + H* +n* CH₃* + CH₃* +n*, where n means the number of free sites (*) involved. From an analysis of the kinetic data it is shown thatn=1.     

Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 m thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution at rotation frequencies 0 < f _rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.     

Heptane Dehydrocyclization Over Pt/KL Catalysts Doped with Barium or Lanthanum

Several 1 wt% Pt/KL catalysts doped with different concentrations of either BaO or La₂O₃ were prepared by successive impregnation of a zeolite KL and then characterized by H₂-O₂ titration, TPR, CO–FTIR, TEM-XEDS and XPS. Catalytic activity measurements in the hydroconversion of n-heptane showed that barium highly enhances the aromatization activity of Pt/KL, the more so the higher the barium concentration. The yield to aromatics increases to a lesser degree by adding 1 wt% La, but it is little modified at higher La concentrations. The CO–FTIR results suggest that the promoter effect of barium is related to an electron enrichment of Pt produced by the BaO Pt⁰ interaction which, according to the TEM–XEDS and XPS results, is more favored than the La₂O₃ Pt interaction.     

Electro-oxidation of methanol on co-deposited Pt-MoO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> prepared by cyclic voltammetry with different scanning potential ranges

Pt-MoO _x supported on glassy carbon was co-deposited by cyclic voltammetry (CV). The lower limit of potential was fixed at −0.25 V (vs. SCE), whereas the upper limit was adjusted to be 0.0, 0.10, 0.40, 0.60 and 1.0 V. The as-prepared catalysts were characterized by X-ray photoelectron microscopy, scanning electron microscopy and transmission electron microscopy. The results show that Pt-MoO _x particles are uniformly dispersed on the substrate and the agglomerated microparticles are composed of numerous nanoparticles with a size of several nanometers. The catalytic capabilities of Pt-MoO _x for methanol oxidation were examined by CV and chronoamperometry. Electrochemical measurements demonstrate that the catalytic activities and stabilities of Pt-MoO _x prepared in the potential ranges from −0.25 to both 0.60 and 1.0 V were higher than the others, which may due to the higher active surface area, more appropriate Pt/Mo ratio and more preferred Pt crystallographic orientation.     

Mechanistic insights into the structure‐dependent selectivity of catalytic furfural conversion on platinum catalysts

The effects of surface structures on the selectivity of catalytic furfural conversion over platinum (Pt) catalysts in the presence of hydrogen have been studied using first principles density functional theory (DFT) calculations and microkinetic modeling. Three Pt model surface structures, that is, flat Pt(111), stepped Pt(211), and Pt₅₅ cluster are chosen to represent the terrace, step, and corner sites of Pt nanoparticle. DFT results show that the dominant reaction route (hydrogenation or decarbonylation) in furfural conversion depends strongly on the structures (or reactive sites). Using the size‐dependent site distribution rule, our microkinetic modeling results indicate the decarbonylation route prevails over smaller Pt particles less than 1.4 nm while the hydrogenation is the dominant reaction route over larger Pt catalyst particles at T = 473 K and = 93 kPa. This is in good agreement with the reported experimental observations. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3812–3824, 2015     

Enhanced activity of PtSn/C anodic electrocatalyst prepared by formic acid reduction for direct ethanol fuel cells

The Pt₃Sn/C catalyst with high electrochemical activity was synthesized under optimizing preparation conditions. The surface of carbon support pretreated by strong acid contains many O-H and CO groups, which will increase the active sites of PtSn/C catalysts. The catalyst structure was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and temperature programmed reduction (TPR). The co-reduction of Pt⁴⁺ and Sn²⁺ ions causes Sn to enter Pt crystal lattice to form PtSn alloy whose surface, however, contains tin oxides with Sn⁴⁺ and Sn²⁺ valences, which can promote the ethanol oxidation. The crystallinity of PtSn decreases with the reduction of the atomic ratio of Pt:Sn. By prolonging the reaction time of formic acid, the forward anodic peak current of ethanol oxidation reaches 16.2 mA on the Pt₃Sn/C catalyst with 0.025 mg Pt loading.     

ESR Observation of W5+ and Zr3+ States in Pt/WOx/ZrO2 Catalysts

Electron spin resonance (ESR) studies of Pt/WO _x /ZrO₂ catalysts over the temperature range of 5–330 K are reported. Two sets of ESR signals, one with g =1.98 and g =1.96 and the other with g _x =1.39, g _y =1.70 and g _z =1.54 are identified as intrinsic signals from the W⁵⁺ and Zr³⁺ states, respectively, formed in the sample. These signals are absent in samples without Pt. Studies on samples annealed at 773 K showed possible electron transfer between the W⁵⁺ and Zr³⁺ states in which the concentration of one state increases at the expense of the other.     

Active sites on Pt containing sulfated zirconia

The influence of the Pt and sulfate concentration on the activity of Pt containing sulfated zirconia for n-heptane conversion was investigated. Pt was deposited on the support by impregnation and by photocatalytic deposition. The amount deposited was 2.5 and 0.4 wt% respectively. For comparison a hybrid catalyst consisting of sulfated zirconia and Pt on SiO₂ was prepared. As supports a commercial sulfated zirconia with a fixed sulfate concentration, a commercial and self synthesized Zr(OH)₄ were used. The sulfate content varied between 20 and 60% of a monolayer. The shifts to higher frequency in the IR spectra of CO adsorbed on Pt correlate with the increasing amounts of sulfates on zirconia and are attributable to the changes in the electron density of the supported metal, i.e. the electron deficiency of Pt increases with increasing concentration of acid sites. After activation in air and reduction in hydrogen two SO₂ peaks were detected by a temperature programmed heating procedure (TPE—temperature programmed evolution). The lower the desorption temperature of the first SO₂ peak, the higher the activity. The shift to lower temperature is connected with a higher Pt and sulfate concentration, furthermore with the proximity of the metal to acid sites. The catalysts with a low sulfate concentration possess only Lewis acid sites and are inactive for n-heptane conversion. At higher sulfate concentration, Br?nsted acid sites are present and the catalysts are active. The concentration of these acid sites is related to the concentration of sulfates, which desorb at lower temperature. Dedicated to Professor Konrad Hayek.     

Comparison of n-Pentane Reforming Over Pt Supported on Amorphous and γ-Al2O3

The n-pentane reforming activity of Pt supported on nonhydrolytic amorphous Al₂O₃ (Pt/NH–Al₂O₃), was investigated and compared to the catalytic activity of Pt supported on crystalline -Al₂O₃. The Pt was introduced by (a) impregnation with either a solution of H₂PtCl₆ in water or a solution of platinum acetylacetonate (PtAcac) in toluene; (b) in situ introduction of a Pt precursor, either PtBr₄ or cis-bis(benzonitrile)platinum dichloride, before gelation of the NH alumina. The rate-controlling step in the reforming of n-pentane for both amorphous and crystalline aluminas was found to be the reaction on the alumina acidic sites. The Pt/-Al₂O₃ catalysts exhibit higher conversions of n-pentane and higher selectivity to isopentane, than the corresponding amorphous alumina samples. After 1.5 h at 400 °C, the highest conversion of the -Al₂O₃-based catalysts was 47% with 20.3% selectivity to isopentane. The highest conversion of the NH–Al₂O₃-based catalysts under the same conditions was only 26% with 13.6% selectivity to isopentane. The high intrinsic Cl content (2.6wt%) of the amorphous alumina was found to have a minor effect on the activity of the alumina, compared to the activity of the more ordered -alumina. Catalysts prepared by impregnation of the NH alumina with aqueous chloroplatinic acid, exhibited higher conversions compared to catalysts prepared by impregnation of the NH alumina with a solution of PtAcac in toluene. This result occurred in spite of the lower surface area and lower Pt dispersions of the chloroplatinic acid-impregnated catalysts, and is explained by the formation of microcrystalline surface structures and existence of surface chlorine.