Number of metallic clusters in Y zeolites obtained from129Xe NMR

We present a simple model which permits to obtain the number of metallic clusters entrapped in Y zeolites from the curves of the chemical shifts of¹²⁹Xe NMR against the pressure of Xe. From the metal loading the average number of atoms per cluster can also be calculated. We apply the model to Pt/NaY and the bimetallic Pt-Cu/NaY.     

129Xe NMR to probe microscopic mixing state of composite catalysts

¹²⁹Xe NMR spectra obtained from microporous solid mixtures such as NaY-Pt/NaY and NaY-Pt/Al₂O₃ have indicated that the NMR peak pattern can depend greatly on the mixing method.¹²⁹Xe NMR can thus be a very simple and useful technique to probe the physical mixing state of many composite catalysts.     

129Xe NMR study of 12-tungstophosphoric heteropoly acid supported on silica

The structure of 12-tungstophosphoric heteropoly acid (H₃PW₁₂O₄₀) supported on silica has been studied by¹²⁹Xe NMR of adsorbed xenon. The¹²⁹Xe NMR spectra are found to depend on the surface HPA concentration. The¹²⁹Xe NMR data provide evidence for the presence of an organized microporous structure within HPA overlayers. These results are in agreement with nitrogen adsorption and capillary condensation (77 K) measurements. The high sensitivity of the¹²⁹Xe NMR method and its applicability for testing of silica-based HPA catalysts are demonstrated.A preliminary report of this work was presented to the Second European Congress on Catalysis EUROPACAT-II, Maastricht, The Netherlands, 3–8 September 1995.Deceased.     

Catalytic reduction of N2O by H2 over well-characterized Pt surfaces

A 0.65% Pt/SiO₂ catalyst has been prepared using an ion exchange technique and extensively characterized prior to being used for continuous catalytic N₂O reduction by H₂ at very low temperatures, such as 363 K. The supported Pt with a high dispersion of 92% gave no presence of O atoms remaining on an H-covered Pt _s , based on in situ DRIFTS spectra of CO adsorbed on Pt _s after either N₂O decomposition at 363 K or subsequent exposure to H₂ for more than 1 h; thus the residual uptake gravimetrically observed even after the hydrogen titration on an O-covered surface is associated with H₂O produced by introducing H₂ at 363 K onto the oxidized Pt _s . Dissociative N₂O adsorption at 363 K on Pt _s was not inhibited by the H₂O_(ad) on the silica surface but not on Pt _s , as acquired by IR peaks at 3,437 and 1,641 cm⁻¹, in very consistent with the same hydrogen coverage, established via H₂-N₂O titration on a reduced Pt _s , as that revealed upon the titration reaction with a fully wet surface on which all bands and their position in IR spectra for CO are very similar to that obtained after H2 titration on a reduced Pt _s . Based on the characterization using chemisorption and in situ DRIFTS and TPD measurements, the complete loss in the rate of N₂O decomposition at 363 K after a certain on-stream hour, depending significantly on N₂O concentrations used, is due to self-poisoning by the strong chemisorption of O atoms on Pt _s , while the presence of H₂ as a reductant could readily catalyze continuous N₂O reduction at 363 K that is a greatly lower temperature than that reported earlier in the literature.     

Hyperpolarized 129Xe NMR and N2 sorption cross-investigations of the crystallization of Al-SBA-15 amorphous walls into ZSM-5 type materials

The present work deals with the study of the parameters which govern the partial conversion of the amorphous walls of Al-SBA-15 supports into ZSM-5 type deposits under alkaline aqueous conditions. The influence of the mode of insertion of aluminium into Al-SBA-15 as well as of the quantities of ammonium fluoride and ZSM-5 template (TPABr) has been investigated. XRD and nitrogen sorption data indicate that the mesopore network is very fragile under the conditions required for the crystallization of ZSM-5. A compromise was found by using a TPABr/Al molar ratio lower than 1, thus affording materials containing both mesopores and zeolite-type micropores. The combined use of hyperpolarized (HP) ¹²⁹Xe NMR and α-plot studies gave evidence for the ZSM-5 nature of the micropores as well as some indications in favour of the interconnection of the different pore networks. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Mixed protein-templated luminescent metal clusters (Au and Pt) for H2O2 sensing

A simple and cost-effective method to synthesize the luminescent noble metal clusters (Au and Pt) in chicken egg white aqueous solution at room temperature is reported. The red-emitting Au cluster is used as fluorescent probe for sensitive detection of H₂O₂.     

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

129Xe NMR spectroscopy on molecular sieves SAPO-37, AIPO4-5, SAPO-5 and SSZ-24

Xe NMR spectroscopy is used to study SAPO-37, NaY, AlPO₄-5, SAPO-5, and SSZ-24. Of these samples, those with FAU topology show that the¹²⁹Xe chemical shift is dependent upon framework composition while the materials with API topology do not. Values of the¹²⁹Xe NMR chemical shifts at zero xenon pressure cannot be correlated with the size of the framework void space.     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

Preparation of nanosize Pt clusters using ion exchange of Pt(NH3) 4 2+ inside mesoporous channel of MCM-41

Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO₂0.1 Al₂O₃1 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH₄)₂O1.5 Na₂O300 H₂O. The MCM-41 sample was calcined in O₂ flow at 813 K and subsequently ion exchanged with Ca²⁺. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH₃) ₄ ²⁺ . The Pt(NH₃) ₄ ²⁺ ion supported on MCM-41 has been activated in O₂ flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve.     

Comparison of (non)-sulfided NiNaY zeolite catalysts prepared by ion-exchange and impregnation by xenon adsorption and129Xe NMR

Ni²⁺ exchanged and NiCl₂ impregnated non-sulfided and sulfided NaY zeolites were characterized by xenon adsorption isotherms,¹²⁹ Xe NMR and thiophene hydrodesulfurization. The nickel species are located mainly inside the micropores of the zeolites in all samples. Ion-exchange results in a more homogeneous distribution of these species than impregnation. This explains their lower hydrodesulfurization activity compared to the ion-exchanged samples.On leave of absence from the Institute of Isotopes, Budapest, Hungary.     

Catalytic Performance of Pt/ZrO2 and Pt/Ce-ZrO2 Catalysts on CO2 Reforming of CH4 Coupled with Steam Reforming or Under High Pressure

CO₂ reforming of methane was performed on Pt/ZrO₂ and Pt/Ce-ZrO₂ catalysts at 1073K under different reactions conditions: (i) atmospheric pressure and CH₄:CO₂ ratio of 1:1 and 2:1; (ii) in the presence of water and CH₄:CO₂ ratio of 2:1; (iii) under pressure (105 and 190 psig) and CH₄:CO₂ ratio of 2:1. The Pt supported on ceria-promoted ZrO₂ catalyst was more stable than the Pt/ZrO₂ catalyst under all reaction conditions. We ascribe this higher stability to the higher density of oxygen vacancies on the promoted support, which favors the cleaning mechanism of the metal particle. The increase of either the CH₄:CO₂ ratio or total pressure causes a decrease in activity for both catalysts, because under either case the rate of methane decomposition becomes higher than the rate of oxygen transfer. The Pt/Ce-ZrO₂ catalyst was always more stable than the Pt/ZrO₂ catalyst, demonstrating the important role of the support on this reaction.     

Analysis of gas transport properties of PPO/PS blends by Xe NMR spectroscopy

Relationships among variations of microvoids and gas transport properties for miscible poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)/polystyrene (PS) blends in the glassy state have been investigated by Xe sorption, Xe permeation, and ¹²⁹Xe NMR measurements. Xe sorption isotherms of the blends can be interpreted successfully on the basis of the dual-mode sorption model. Decrease in the permeability of Xe is attributed to the decrease in the diffusivity of that in the Langmuir site. ¹²⁹Xe NMR spectra of ¹²⁹Xe in the blends show non-linear low-field shift with increasing sorption amount of Xe because of a fast exchange of Xe atoms between Henry and Langmuir sites. From the analysis of ¹²⁹Xe NMR chemical shifts, it is found that the mean volume of individual microvoids varies with a negative deviation against volume fraction of PPO in the blend. For PPO/PS blends, it has been clarified that the contraction of individual microvoids occurs by blending and highly affects gas transport properties.     

Improvement of Pt/ZrO2 by CeO2 for high pressure CH4/CO2 reforming

CH₄/CO₂ reforming over Pt/ZrO₂, Pt/CeO₂ and Pt/ZrO₂ with CeO₂ was investigated at 2 MPa. Pt/ZrO₂, which shows stable activity under 0.1 MPa, and Pt/CeO₂ showed gradual deactivation with time at the high pressure. The deactivation was suppressed drastically on Pt/ZrO₂ with CeO₂ prepared by different impregnation order (co-impregnation of Pt and CeO₂ on ZrO₂, and consecutive impregnation of Pt and CeO₂ on ZrO₂). The amount of coke deposition was found insignificant and similar among all the catalysts (including Pt/ZrO₂ and Pt/CeO₂). Catalytic activity after the reaction for 24 h was in agreement with Pt particle size after the reaction for same period, indicating that the difference of the catalytic stability is mainly dependent on the extent of Pt aggregation through catalyst preparation, H₂ reduction, and the CH₄/CO₂ reforming. Pt aggregation and the amount of coke deposition were least pronounced on (Pt–Ce)/ZrO₂ prepared by impregnation of CeO₂ on Pt/ZrO₂ and the catalyst showed highest stability.     

Model catalysts fabricated by electron beam lithography: AFM and TPD surface studies and hydrogenation/dehydrogenation of cyclohexene + H2 on a Pt nanoparticle array supported by silica

Pt nanoparticle model catalysts with 28 ± 2 nm diameters and 100 ± 2 nm square periodicity have been fabricated with electron beam lithography on silica substrates. The reactivity of the Pt/SiO₂ arrays was compared to a Pt foil for cyclohexene + H₂ at 100°C. The overall reactivity of the Pt particle arrays was higher by a factor of two, the selectivity towards dehydrogenation was three times higher, and the rate of deactivation was about the same as for the Pt foil. Since the primary difference between the nanoparticle array and the Pt foil was the interface between the Pt and the SiO₂, the interfacial region was most likely responsible for the changes in reactivity on the arrays. Using AFM, SEM, and TPD, the arrays were characterized before and after being exposed to reaction conditions. AFM images of a sample cleaned by ion sputtering showed that the pattern of the Pt nanoparticle array was replicated in the silica during the sputtering process. This revised version was published online in June 2006 with corrections to the Cover Date.     

EXPERIMENTAL STUDY OF CHARACTERISTICS OF Pt/TiO2 CATALYST

Chemisorption of hydrogen, carbon monoxide, and oxygen on 2 wt% Pt/TiO₂ was investigated to determine a satisfactory method for measuring the platinum dispersion on titania. The reduction of Pt/TiO₂ at high temperature results in sharp decrease in hydrogen and carbon monoxide chemisorption capacities and catalytic activity which cannot be accounted for by metal sintering. Oxidation-reduction treatments restore the usual properties of the catalyst. The TPD spectrum of hydrogen on Pt/TiO₃ shifts to higher temperature upon reduction at high temperature and shifts again to its original position upon oxidation. Shift of TPD spectrum does not appear in other catalysts. Oxygen adsorption method is inaccurate to measure crystallite diameter of platinum because oxygen uptake is enhanced not only due to reduced nonstoichiometric oxide support but also to penetration of oxygen into bulk platinum. Hydrogen chemisorption method, however, is a satisfactory technique to measure the platinum dispersion only if the strong metal-support bonding has been destroyed completely. Sintering rate in oxygen is greater than that in hydrogen regardless of the nature of the supports, and sintering resistance of Pt/TiO₂ in hydrogen is much greater than that of other supported platinum catalysts whereas stability of the platinum in oxygen is not dependent on the supports. The influence of chlorine in the presence of oxygen on the redispersion phenomena is found to be very important.     

Oscillations of CO and CO + H2 oxidation in air over supported and unsupported Pt catalysts

Oscillation behaviour of the oxidation of CO (0.2–2.2%) in air over Pt wire coils and over Pt/Al₂O₃ catalysts deposited onto the coils (pellistors) has been investigated. The waveforms differ considerably between the two catalytic systems. Over unsupported Pt at 240–260° C, regular oscillations were accompanied by slowly declining activity and by deposition of carbon. Over supported Pt at 110–180°C, relatively complex but sustained oscillation occurred by a different mechanism. This oscillation was greatly enhanced by H₂ (0.25–1.0%), and may involve fluctuations in the concentrations of CO and H₂ around the supported catalyst.     

The effects of a potassium overlayer on the reaction pathway of CH2 and C2H5 on Rh(111)

The effect of potassium on the reaction pathways of adsorbed CH₂ and C₂H₅ species on Rh(111) was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TDS). Hydrocarbon fragments were produced by thermal and photo-induced dissociation of the corresponding iodo compounds. Potassium adatoms markedly stabilized the adsorbed CH₂ and converted it into C₂H₄, the formation of which was not observed for K-free Rh(111). New routes of the surface reactions of C₂H₅ have been also opened in the presence of potassium, namely its transformation into butane and butene.