Selectivity Of Pt Research Articles

Extraordinarily effective promotion by sodium in emission control catalysis: NO reduction by propene over Na-promoted Pt/γ-Al 2O 3

The catalytic activity and selectivity of Pt dispersed on γ-Al 2O 3, for the reduction of NO by propene is promoted extremely strongly by sodium in the temperature range 470–770 K. Remarkable and unprecedented effects on both activity and selectivity are found. Rate increases by two orders of magnitude are achievable, while the selectivity towards to N 2 is improved from ∼20% over the Na-free unpromoted Pt catalyst, to >95% over the optimally Na-promoted catalyst. The Na-free Pt is almost ineffective for the NO + C 3H 6 reaction over the gas composition and temperature range of this investigation. The promoting effect is due to a sodium-induced increase in the strength of Pt–NO chemisorption relative to propene on a catalyst surface predominantly covered by propene and its fragments. This is accompanied by weakening of the N–O bond, thus facilitating NO dissociation, which process is proposed as the critical reaction initiating step.

Anisotropic etching characteristics of platinum electrode for ferroelectric capacitor

The effect of etch parameters of platinum etching using Cl/sub 2//CO plasma on the etching properties and the etch profiles was investigated. The etching characteristics with respect to substrate temperature are different in two temperature regions below and above 210/spl deg/C and significantly depended on Cl/sub 2/ concentration in each temperature region. The etch rates of Pt were enhanced suddenly at the substrate temperature of around 210/spl deg/C when Cl/sub 2/ concentration is 50-80%. The etch rates of Pt below 210/spl deg/C did not change much with increasing temperature. The selectivity of Pt over SiO/sub 2/ was governed by the etch rate of SiO/sub 2/ in the lower temperature region but determined by the etch rates of Pt in the higher temperature region. The anisotropy of etch profiles was high enough to achieve vertical pattern without etch residues in the lower temperature region for the application in fabricating 1-Gbit era. In the higher temperature region, however, the slopes of etch profiles due to the volatile products of Pt were found. XPS was used to analyze the surface atomic compositions after various etching treatments.

Skeletal isomerization of n-pentane over Pt-promoted cesium hydrogen salts of 12-tungstophosphoric acid

Two kinds of Pt-promoted Cs 2.5H 0.5PW 12O 40 (Cs 2.5H 0.5PW 12O 40 will be denoted by Cs2.5), Pt directly impregnated on Cs2.5 (Pt/Cs2.5) and a physical mixture of Cs2.5 and Pt/Al 2O 3 (Pt+Cs2.5), were used to catalyze the isomerization of n-pentane in the presence of hydrogen at 453–573 K. Cs2.5 showed a high initial activity but deactivated rapidly. Addition of Pt greatly suppressed the deactivation and increased the selectivity to isopentane. High stationary conversion (34.8%) and selectivity (96.9%) were obtained by using Pt+Cs2.5 at a relatively low temperature (453 K) and a low hydrogen pressure (0.05 atm, hydrogen/pentane=1). Under these reaction conditions, the stationary activity and selectivity of Pt+Cs2.5 were significantly higher than those of Pt-promoted H–ZSM-5 or SO 4 2−/ZrO 2. It was deduced that the remarkable effect of Pt in suppressing the catalyst deactivation was brought about by activated hydrogen, which were formed on Pt, transferred to Cs2.5, and utilized to remove carbonaceous deposits or their precursors. Increase in the hydrogen pressure decreased the initial activity probably due to a decrease in the concentration of pentenes or pentyl carbenium ion.

Benzaldehyde Hydrogenation over Titania-Covered Pt Powder

Titania when used as a support has been found to have a significant effect on the activity and selectivity of Pt during the hydrogenation of aldehydes and ketones. Turnover frequencies based on hydrogen adsorption sites are markedly enhanced, and rates per gram Pt (at similar dispersions) are also often increased. There are several explanations to account for this performance, at least partially. In an effort to determine the validity of these explanations, and hopefully to eliminate them as possibilities, a Pt powder was studied before and after the addition of varying amounts of TiO{sub 2} on its surface, and a physical mixture of this powder plus TiO{sub 2} was also examined and compared both to these catalysts and to Pt dispersed on TiO{sub 2}. The results follow.

Selectivity in heterogeneous catalytic processes

The selectivity of several catalytic systems was studied. Shape selectivity of Pt on carbon-fiber catalysts was demonstrated in the competitive hydrogenation of 1-hexene and cyclohexene and in the parallel dehydrogenation of cyclohexanol to cyclohexanone and phenol. Both reactions were carried out in a gas-phase fixed-bed reactor. Catalysts prepared on carbon fibers, containing pores with small constrictions (5 Å) yielded significantly higher rates of hydrogenation of 1-hexene compared to those of cyclohexene and selectively produced cyclohexanone from cyclohexanol. Other catalysts, supported on carbon fibers with large constrictions (7 Å) or activated carbon, displayed comparable rates of hydrogenation for both reactants and yielded cyclohexanone as well as phenol from cyclohexanol. Nitration of o-xylene with nitrogen dioxide was carried out in the gas phase over a series of solid acid catalysts packed in a fixed bed. Several zeolites, supported sulfuric acid, and sulfated zirconia were tested. Zeolite H-β was found to be the most active and selective catalyst for the production of 4-nitro- o-xylene. A preliminary kinetic model indicated that the selectivity to 4-nitro- o-xylene increased with decreasing concentration of nitrogen dioxide. Alkylation of phenol with methanol was performed on zeolites, supported sulfuric and phosphoric acids, and sulfated zirconia packed in a fixed-bed. The ratio of o- to c-alkylation, measured at 180°C and methanol to phenol feed molar ratio of unity, ranged from 4 with the supported acids to 2 with zeolite H-β. This ratio decreased with temperature. The ratio of o- to p-cresol changed from about 2 in zeolites in supported sulfuric acid and to 0.5 in phosphoric acid supported on carbon fibers.

In SituControlled Promotion of Catalyst Surfaces via NEMCA: The Effect of Na on the Pt-Catalyzed NO Reduction by H2

It was found that the catalytic activity and selectivity of Pt for NO reduction by H2can be markedly and reversibly affected by depositing polycrystalline Pt films on β′′-Al2O3, a Na+conductor, and applying external currents or potentials to supply or remove Na to or from the Pt catalyst surface. The increase in the rate of NO reduction to N2is typically 103–105times larger than the rate of supply of Na and up to 30 times larger than the umpromoted rate. The use of the β′′-Al2O3support permits precisein situcontrol of the Na coverage on the Pt surface. Sodium coverages of 0.06 cause up to 1450% increases in the total rate of NO reduction and enhance the selectivity to N2from 30 to 75%. The promotional effect of Na is due to enhanced NO chemisorption and dissociation on the Pt surface.

Platinum–Tin Catalysts Supported on Silica Highly Selective forn-Hexane Dehydrogenation

Silica-supported Pt–Sn catalysts were prepared by two-step impregnation from [PtCl2(PPh3)2] and SnCl2solutions of appropriate concentrations to yield Pt/Sn atomic ratios ranging from 0.2 to 5.0. In these systems, the presence of true Pt–Sn alloys was confirmed by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis and electron nanodiffraction. Pt and PtSn alloy phases were found on catalysts with Pt/Sn>1, PtSn alloy alone on the catalyst with Pt/Sn=1 and PtSn and PtSn2alloys, together with Sn in the catalysts with Pt/Sn<1. All these catalysts were tested in the skeletal reactions ofn-hexane at 753 K and atmospheric pressure. The selectivity of Pt changed significantly when alloyed with tin. For Sn-rich compositions a segregation of tin toward the catalyst surface was shown by photoelectron spectroscopy, and high hydrogenolysis selectivity and fast deactivation were observed. In contrast, Pt-rich catalysts, in which a well defined PtSn alloy was observed, were much more stable and exhibited high selectivity to dehydrogenation reaction while maintaining low conversions to benzene and hydrogenolysis products. This selectivity pattern can be interpreted in terms of a change in adsorption properties due to differences in the number of adjacent Pt atoms required for the various reaction pathways.

Modification of supported Pt catalysts by preadsorbed phosphines: enhanced selectivity in the oxidation of l-sorbose

The influence of phosphine and aminophosphine modifiers on the performance of 5 wt.-% Pt/C and Pt/alumina catalysts was studied in the partial oxidation of l-sorbose with molecular oxygen. The molar ratio of modifier to surface Pt atoms (Pt s) was varied between 0.1 and 1.0. In general, higher modifier/Pt s ratios resulted in higher selectivity to 2-keto- l-gulonic acid and lower reaction rate. The activity and selectivity of Pt strongly depended on the chemical structure of phosphine. Tributhylphosphine in trace amounts (modifier: Pt s = 0.1 mol/mol ; modifier: sorbose = 600 ppm) proved to be an outstanding modifier, which improved the selectivity of Pt/C by 28–37% in the whole investigated conversion range. Monitoring the catalyst potential during the reaction revealed that the oxidation state of Pt decreased by promotion, which is an indication that some side reactions leading to catalyst poisoning are suppressed.

Sulfur adsorbed on Pt catalyst: its chemical state and effect on catalytic properties as studied by electron spectroscopy and n-hexane test reactions

Pt black samples in a fresh state, deactivated by coking and treated with H 2S were studied with electron spectroscopy. Their catalytic properties were tested in skeletal reactions of n-hexane. The Pt 4f XPS peaks show that Pt is predominantly in the metallic state after the treatments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amounts on sulfided samples. The S 2p band shows 4–8 at.-% S with respect to the total surface in two valence states: sulfate and sulfide, the latter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt—S interaction even after O 2/H 2 regeneration when the Fermi-edge Pt intensity rose to a height almost equal to that of a regenerated sulfur-free Pt. ISS shows that sulfur — as opposed to carbon and oxygen — is a surface component. Sputtering effect of He + ions used in ISS can remove sulfate while a large part of sulfide is removed during O 2/H 2 regeneration and/or during n-hexane test runs. Both sulfidation and carbonization strongly decrease the overall catalytic activity. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containing sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O 2/H 2 treatment at 600 K but the same treatment — even repeated — restored the overall activity of a sulfided Pt only partially. We believe that sulfide acts mainly as a bonding modifier whereas sulfate can be a structural modifier influencing the selectivity similarly as coke or coke precursors.

Platinum Catalysts Modified by Adsorbed Amines: A New Method of Enhancing Rate and Selectivity ofl-Sorbose Oxidation

The Pt-catalyzed oxidation at C-1 of unprotectedl-sorbose to 2-keto-l-gulonic acid, an intermediate in vitamin C synthesis, has been studied using molecular oxygen and neutral aqueous solution. The performance of Pt/alumina and Pt/C catalysts could be improved by addition of small quantities of tetraalkylammonium hydroxides, trialkyl-, triaryl-, and cycloaliphatic amines. The optimum modifier : Ptsmolar ratio is around 0.1, which corresponds to an amine : sorbose molar ratio of 1 : 1700. Rate enhancement (by a factor of up to 4.6) can be obtained when the pKais around 10 or higher, and this correlation depends very little on the chemical structure of the amine. Rate acceleration is proposed to be connected with the hydration of the intermediate aldehyde (bifunctional catalysis). The influence of supported N-bases on the selectivity of Pt is a function of their chemical structure. The best results, 95% selecti-vity at 30% conversion and about 40% selectivity enhancement in the whole investigated conversion range, was obtained with Pt/C and hexamethylenetetramine. Electrochemical model studies revealed that hexamethylenetetramine is adsorbed on Pt and not oxidized during reaction. Molecular modeling suggests that the preferential oxidation at C-1 is due to complex formation by H-bonding between hexamethylenetetramine and sorbose. The adsorption of this complex on Pt results in a tilted position of the reactant in which only C-1 is exposed to oxidative dehydrogenation.

Hydrogenation of 3-Methyl-crotonaldehyde on the Pt(553) Stepped Surface: Influence of the Structure and of Preadsorbed Tin

The hydrogenation of 3-methyl-crotonaldehyde was studied, in the gas phase, on a well-defined Pt(553) or Pt(S)-‘5(111)×(111)’ stepped surface and the results are compared with those of similar experiments on Pt(111). The selectivity is governed mainly by structural factors. Low-coordination step atoms create favorable hydrogenation sites for the production of saturated aldehyde, whereas (111) flat terraces lead to the formation of unsaturated alcohol. The influence of a preadsorbed submonolayer of tin was investigated on Pt(111) and Pt(553). The effect of this metallic additive depends on its local concentration and arrangement. Tin drastically changes the selectivity of Pt(553). At low coverage, tin, located in the vicinity of the steps, changes the selectivity to the benefit of the saturated alcohol. Al higher coverage (θ>0.3), tin grows in islands on the terraces and the expected improvement in selectivity toward unsaturated alcohol was observed, suggesting a combination of electronic and geometric effects.

A study of platinum—dysprosium—KL zeolite reforming catalyst

The activity and selectivity of Pt—KL and Pt—Dy—KL zeolite catalysts were studied with a pulsed micro-catalytic reactor. The activity for the aromatization of either n -hexane or methylcyclopentane (MCP) on Pt—Dy—KL zeolite catalysts containing dysprosium ranging from 0.1 to 0.5 wt.% is found to be higher than that with Pt—KL catalyst. The stability of the Pt—KL and Pt—Dy—KL catalysts has been studied using a poisoning method with carbon disulphide.

Electrochemical modification of methanol oxidation selectivity and activity on a platinum single-pellet catalytic reactor

It was found that the catalytic activity and selectivity of Pt for the oxidation of methanol to formaldehyde and CO 2 can be altered significantly and reversibly by depositing a Pt catalyst film on an yttria-stabilized zirconia (YSZ) disc and by applying current or potential between the catalyst film and a Ag film deposited on the other side of the O 2 -conducting YSZ disc. Both the catalyst film and the Ag counter and reference electrodes are exposed to the reacting CH 3 OH-O 2 mixture. The observed increase in the rate of H 2 CO production was typically a factor of 100 higher than the rate of O 2 -supply to the catalyst with a concomitant 2-fold increase in selectivity

Selective Labeling, Cross-Linking, and Cleavage of Proteins with Transition-Metal Complexes

Abstract Because the substitution reactions of transition-metal complexes can be controlled precisely by the choice of the metal and the ligands, and because these complexes have various spectroscopic and electrochemical properties, they are well-suited for many applications in biochemical and biophysical laboratories. Complex [Pt(trpy)Cl)+ reacts, under mild conditions and noninvasively, with the exposed side chains of His, Cys, and even Arg residues in proteins. The derivatives can be separated chromatographically. The Pt(trpy)2+ tags are stable. They are easily detected and quantitated owing to the strong UV-vis bands, whose positions and relative intensities depend on the identity and the environment of the tagged residue. The selectivity of Pt(II) chloro complexes as labeling reagents can be controlled by purposeful changes of the other three ancillary ligands. Treatment of Cys residues with (η5-C5H5)Fe(η5-C5H4HgCl)results in attachment of the redox-active ferrocenyl tag to the protein. Proteins can ...

Elimination of the influence of interfering ions in the voltammetry of catalytic hydrogen evolution in the presence of platinum salts

To improve the selectivity of Pt(IV) determination by voltammetry, a square-wave potential is used during the depostition step. The interfering elements are stripped from the surface of the electrode during the positive half-cycle. Pt(IV) may be determined in the presence of a 2 × 10 6-fold amount of copper and in the presence of a large excess of other ions (Fe, Ni, Pb, Zn, Ca). The dependence of the catalytic hydrogen evolution half-peak potential on log C pt, remains linear in the range 2.5 × 10 −9 to 2 × 10 −7 M. The procedure proposed may be used to improve the selectivity of other determinations.

A transition-metal chromophore as a new, sensitive spectroscopic tag for proteins. Selective covalent labeling of histidine residues in cytochromes c with chloro(2,2':6'2"-terpyridine)platinum(II) chloride

Reactivity and selectivity of Pt(trpy)Cl/sup +/ toward proteins are studied with cytochromes c from horse and tuna as examples. The new transition-metal reagent is specific for histidine residues at pH 5. The reaction, facile one-step displacement of the Cl/sup -/ ligand by imidazole, produces good yield. The binding sites, His 26 and His 33 in the horse protein and His 26 in the tuna protein, are identified by UV-vis spectrophotometry and by peptide-mapping experiments. Model complexes with imidazole, histidine, histidine derivatives, and histidine-containing peptides are prepared and characterized. The covalently attached Pt(trpy)/sup 2 +/ labels allow easy separation of the protein derivatives by cation-exchange chromatography. The labels do not perturb the conformation and reduction potential of cytochrome c, as shown by UV-vis spectrophotometry, cyclic voltammetry, differential-pulse voltammetry, EPR spectroscopy, and /sup 1/H NMR spectroscopy. The selectivity of Pt(trpy)Cl/sup +/ is entirely opposite from that of PtCl/sub 4//sup 2 -/ although both of them are platinum(II)-chloro complexes. Owing to an interplay between the steric and electronic effects of the terpyridyl ligand, the new reagent is unreactive toward methionine (a thio ether) and cystine (a disulfide), which are otherwise highly nucleophilic ligands, but very reactive toward imidazole, which is otherwise a relativelymore » weak ligand. Unusual and useful selectivity of preformed transition-metal complexes toward proteins evidently can be achieved by a judicious choice of ancillary ligands.« less

Activity and selectivity of Pt−Fe/C alloys for the liquid phase hydrogenation of cinnamaldehyde to cinnamyl alcohol

Activity and selectivity of Pt−Fe/C alloys for the liquid phase hydrogenation of cinnamaldehyde to cinnamyl alcohol

Mdiecuiar Die Catmalysis: Hexane Ackmzation Over Pt/KL

AbstractIt is proposed that the high aromatization selectivity of Pt supported in zeolite L is caused by the ability of the channels to collimate the flux of hexane molecules, leading to end-on adsorption at the Pt surface.

Toxicity of cisplatin and hydroxymalonatodiammine platinum (II) towards mouse bone marrow and B16 melanoma in relation to DNA binding in vivo

The antitumour selectivity of cisplatin and hydroxymalonatodiammine platinum II (Pt[OHmal(NH 3) 2]), a second generation platinum drug. was evaluated in C57Bl/Cbi mice bearing advanced intramuscular B16 melanoma. At maximally tolerated doses, Pt[OHmal(NH 3) 2] produced greater inhibition of growth of the advanced B16 melanoma than did cisplatin. Comparison of dose-response curves for survival of B16 lung colony-forming cells and bone marrow stem cells treated in vivo indicated that selective killing of B16 cells was achieved with Pt[OHmal(NH 3) 2], whereas cisplatin was relatively nonselective. The extent of reaction of platinum with DNA at doses producing measurable levels of survival in tumour and marrow in vivo was similar to values previously observed in cultured cells treated with cisplatin in vitro. Studies of the amount of platinum bound to DNA in tumour and marrow following administration of the two drugs revealed that the improved selectivity of Pt[OHmal(NH 3) 2] was associated with a selective increase in the amount of platinum bound to tumour DNA, relative to cisplatin. In addition, platinum lesions produced in DNA by Pt[OHmal(NH 3) 2] appeared to be more effective in producing tumour cell killing than those produced by cisplatin. No significant excision of total DNA-bound platinum from tumour was observed up to 48 hr after administration of either drug.

Reactions of C4−C6 alkanes on platinum and platium-gold films

Results are reported on the reactions of n-butane, n-pentane and n-hexane on Pt and Pt/Au alloy films. The results are compared with those obtained for Pt−Au alloys on a SiO2 carrier. It is concluded that alloying can change the selectivity of the catalyst. No indication has been found for the possible role of oxygen in the high selectivity of Pt in non-destructive reactions.