V. I. Bukhtiyarov

Observation of Parahydrogen-Induced Polarization in Heterogeneous Hydrogenation on Supported Metal Catalysts†

For homogeneous hydrogenation reactions catalyzed by transition-metal complexes in solution, utilization of the nuclear spin isomers of molecular hydrogen has become an established tool for studies on reaction mechanisms and kinetics. Parahydrogen-induced polarization (PHIP) can enhance the NMR spectroscopy signals of reaction intermediates and products by several orders of magnitude and provides the high sensitivity essential for such studies. It was demonstrated recently that PHIP effects can also be observed in hydrogenation reactions catalyzed by metal complexes immobilized on a solid support. Industrial hydrogenation processes are predominantly heterogeneous and utilize supported metal catalysts. Such catalysts are not expected to produce PHIP effects, since the reaction mechanism involved should destroy the original correlation of the two nuclear spins of parahydrogen. Herein we demonstrate for the first time that, contrary to these expectations, supported metal catalysts such as Pt/Al2O3 and Pd/ Al2O3 do exhibit PHIP effects. This fact can be used for the production of spin-polarized fluids for MRI applications and for developing new research tools for mechanistic and kinetic studies on heterogeneous hydrogenation processes. Homogeneous hydrogenation of unsaturated compounds in solution is often performed with transition metal complexes (e.g., Wilkinson2s catalyst, [RhCl(PPh3)3]). [6] The detailed mechanism of the reaction is fairly well understood. The catalytic cycle (Scheme S1 in the Supporting Information) starts with oxidative addition of an H2 molecule to the metal center to give a metal dihydride species and ends with reductive elimination of the product. Molecular hydrogen is known to be a mixture of two nuclear spin isomers: orthohydrogen with a total nuclear spin of I= 1, and parahydrogen with I= 0. If one of them (usually para-H2) is used in the hydrogenation reaction, pairwise addition of the two hydrogen atoms from the same H2 molecule, ensured by the reaction mechanism, preserves their correlated nuclear spin state. Furthermore, this correlation can strongly enhance NMR signals of the reaction intermediates and products. If hydrogenation is performed in the probe of an NMR spectrometer (i.e., in the high magnetic field of the NMR instrument), two strongly enhanced antiphase multiplets are commonly observed in the H NMR spectrum of the reaction product (Figure S1a). This experimental scheme is known as PASADENA (parahydrogen and synthesis allow dramatic enhancement of nuclear alignment). If hydrogenation is carried out in a low magnetic field and the reaction products are then adiabatically transferred to the NMR magnet for detection, the two multiplets show net signal enhancement of the opposite sign (Figure S1b). This experimental scheme is termed ALTADENA (adiabatic longitudinal transport after dissociation engenders net alignment). The observation of both ALTADENA and PASADENA requires that the two H atoms from the same para-H2 molecule travel as a pair throughout the entire catalytic cycle all the way to the product, and that the time elapsed between initial dissociation of the H2 molecule and formation of the product molecule is not much longer than the nuclear spin relaxation time of the intermediates involved. All this is favored by the fact that all processes take place on a single metal atom of the complex in solution. Since the NMR spectroscopy signal-enhancement factors observed can be as large as several orders of magnitude, hydrogenation with parahydrogen has become a powerful tool for studying the mechanisms and kinetics of homogeneous hydrogenation reactions. Heterogeneous hydrogenation processes often use highly dispersed supported metals (e.g., Pt/Al2O3, Pd/Al2O3) as catalysts. Unlike homogeneous hydrogenation, which takes place on a well-defined single metal center, heterogeneous hydrogenation proceeds over a vast surface of a metal cluster. This gives rise to a large number of interaction possibilities and a variety of relevant and irrelevant species present on the surface during the reaction. As a result, despite a great deal of effort devoted to studying the mechanisms of heterogeneous hydrogenation of simple alkenes such as ethylene, conclusions regarding the reaction mechanism are still controversial. By combining the use of parahydrogen with heterogeneous hydrogenation processes, it may be possible to develop new fundamental and practical applications which rely on the substantial amplification of the NMR signals, such as mechanistic studies of heterogeneous hydrogenation and production of polarized fluids for advanced MRI studies. However, the use of parahydrogen in combination with supported metal catalysts has been postulated to be pointless, [*] K. V. Kovtunov, Prof. Dr. I. V. Koptyug International Tomography Center SB RAS 3A Institutskaya St., Novosibirsk 630090 (Russia) Fax: (+7)383-333-1399 E-mail: koptyug@tomo.nsc.ru

Propane-d6 Heterogeneously Hyperpolarized by Parahydrogen.

Long-lived spin states of hyperpolarized propane-d6 gas were demonstrated following pairwise addition of parahydrogen gas to propene-d6 using heterogeneous parahydrogen-induced polarization (HET-PHIP). Hyperpolarized molecules were synthesized using Rh/TiO2 solid catalyst with 1.6 nm Rh nanoparticles. Hyperpolarized (PH ∼ 1%) propane-d6 was detected at high magnetic field (9.4 T) spectroscopically and by high-resolution 3D gradient-echo MRI (4.7 T) as the gas flowed through the radiofrequency coil with a spatial and temporal resolution of 0.5 × 0.5 × 0.5 mm3 and 17.7 s, respectively. Stopped-flow hyperpolarized propane-d6 gas was also detected at 0.0475 T with an observed nuclear spin polarization of PH ∼ 0.1% and a relatively long lifetime with T1,eff = 6.0 ± 0.3 s. Importantly, it was shown that the hyperpolarized protons of the deuterated product obtained via pairwise parahydrogen addition could be detected directly at low magnetic field. Importantly, the relatively long low-field T1,eff of HP propane-d6 gas is not susceptible to paramagnetic impurities as tested by exposure to ∼0.2 atm oxygen. This long lifetime and nontoxic nature of propane gas could be useful for bioimaging applications including potentially pulmonary low-field MRI. The feasibility of high-resolution low-field 2D gradient-echo MRI was demonstrated with 0.88 × 0.88 mm2 spatial and ∼0.7 s temporal resolution, respectively, at 0.0475 T.

High‐Resolution 3D Proton MRI of Hyperpolarized Gas Enabled by Parahydrogen and Rh/TiO2 Heterogeneous Catalyst

Several supported metal catalysts were synthesized, characterized, and tested in heterogeneous hydrogenation of propene with parahydrogen to maximize nuclear spin hyperpolarization of propane gas using parahydrogen induced polarization (PHIP). The Rh/TiO2 catalyst with a metal particle size of 1.6 nm was found to be the most active and effective in the pairwise hydrogen addition and robust, demonstrating reproducible results with multiple hydrogenation experiments and stability for ≥1.5 years. 3D (1) H magnetic resonance imaging (MRI) of 1 % hyperpolarized flowing gas with microscale spatial resolution (625×625×625 μm(3) ) and large imaging matrix (128×128×32) was demonstrated by using a preclinical 4.7 T scanner and 17.4 s imaging scan time.

Platinum nanoparticle size effect on specific catalytic activity in n-alkane deep oxidation: Dependence on the chain length of the paraffin

The specific activity of 0.8% Pt/Al2O3 catalysts in the deep oxidation of C1–C6n-alkanes increases with an increase in the Pt particle size from 1 to 3–4 nm. Further coarsening of the particles insignificantly changes the specific activity. The size effect was studied for a series of catalysts containing platinum nanoparticles 1 to 11 nm in diameter. The specific catalytic activity variation range depends on the size of the reacting hydrocarbon molecules. As the platinum particle size increases, the specific catalytic activity increases 3–4 times for the oxidation of CH4 and C2H6 and by a factor of 20–30 for the oxidation of n-C4H10 and n-C6H14.

XPS study of the silica-supported Fe-containing catalysts for deep or partial H2S oxidation

Abstract Catalytic properties of silica-supported Fe-containing catalysts in H 2 S oxidation have been correlated with their surface composition characterized by XPS. This allows us to show that iron sulfate supported on silica is active in sulfur production with 100% selectivity, whereas the decrease in selectivity to sulfur is accompanied by the appearance of iron disulfide phase in the catalyst.

The Nature of Electrophilic and Nucleophilic Oxygen Adsorbed on Silver

Results of a spectroscopic study of two forms of adsorbed atomic oxygen on a silver surface, which participate in ethylene epoxidation reaction, are presented. The possibility of the combined use of the methods of photoelectron spectroscopy and X-ray absorption for a detailed analysis of adsorbate electron structure on solid surfaces is demonstrated. It is found that a significant difference in the position of O 1s lines for nucleophilic (528.3 eV) and electrophilic (530.4 eV) oxygen is determined by the effects of the initial state, that is, by the difference in the charge state of oxygen anions. The use of the well-know correlation of the Auger line splitting with a Pauling charge at an oxygen atom showed a substantial difference (∼1 electron charge unit) in charge transfer from metal to the nucleophilic or electrophilic adsorbed oxygen atom. Based on the X-ray absorption data of the oxygen K-edge, it is found that there is a substantial overlap of the 4d- and 5sp orbitals of silver with oxygen 2p orbitals in the nucleophilic state in the formation of an Ag–O bond and there is only an overlap of 5sp orbitals of silver with oxygen 2p orbitals in the electrophilic state. Structural models of the adsorption site are presented for both states. The conclusion is drawn that the charge state of oxygen in oxide systems may depend substantially on its binding to metal atoms.

Study of reactivity of oxygen states adsorbed at a silver surface towards C2H4 by XPS, TPD and TPR

Reactivity of oxygen states absorbed at the surface of a silver foil towards ethylene has been studied by XPS, TPD and TPR. Isotope experiments with “ionic” oxygen (BE9(C 1s)=528.4 eV) labelled by 18O2 and the “covalent” oxygen (BE(O 1s)=530.5 eV) labelled by 16O2 have been carried out. It has been revealed that both oxygen states are desorbed separately. Following the ethylene experiment indicated that the “ionic” oxygen is active in total oxidation only, while the “covalent” oxygen takes part in the ethylene epoxidation.

An XPS study of the composition of iridium films obtained by MO CVD

Abstract Iridium films were deposited on flat quartz substrates by MO CVD from iridium tris-acetilacetonate; these processes were performed at atmospheric pressure in the presence of hydrogen, and substrate temperatures were varied in the range 350–550°C. X-ray photoelectron spectroscopy was used to study film composition: carbon- and oxygen-containing impurities (up to one monolayer) are present on the surface of all the films. During etching by argon ions up to the film-substrate boundary only iridium lines were observed in the spectra. Subsequent etching results in the decrease of the intensity of the iridium lines and the appearance of lines of the substrate material — Si2p, Si2s and O 1s. In the region of the IrSiO 2 boundary there is a transition layer where a compound of the type IrSi x O y is formed. The thickness of this layer increases with increasing deposition temperature. For films deposited at T > 500°C differential charging effects were found, i.e. the samples contain two phases - conductive and nonconductive ones. This might be due to the crystallization of two phases - metallic and “silicate”. The existence of differential charging can also be explained by grain size effects which depend on the deposition temperature.

XPS STUDY OF THE SIZE EFFECT IN ETHENE EPOXIDATION ON SUPPORTED SILVER CATALYSTS

Supported silver catalysts (Ag/α-Al 2 O 3 ) with different particle sizes (100–1000 A) have been prepared and studied by XPS. It has been shown that the increase in the ethene epoxidation rate with silver particle size (size effect) is accompanied by a decrease in the Ag 3d 5/2 binding energy value. The variation in E b (Ag 3d 5/2 ) cannot be explained in terms of a change in the metallic character of the silver (initial state effect), but seems to be determined by the differential charging of the supported silver. The latter effect originates from the easier screening of surface charge induced by photoemission on the silver particles owing to their internal conductivity, rather than that on the dielectric support. The correlation between the internal conductivity of supported silver particles and their catalytic activity is discussed in the context of possible mechanisms for ethene epoxidation.

Size effect in the oxidation of platinum nanoparticles on graphite with nitrogen dioxide: An XPS and STM study

The interaction of NO2 with model catalysts prepared by platinum evaporation onto the surface of highly oriented pyrolytic graphite has been investigated at room temperature and a pressure of 3 × 10−6 Torr by X-ray photoelectron spectroscopy and scanning tunneling microscopy. In the catalyst containing only small (<2.5 nm) platinum particles, these particles oxidize to PtO and PtO2. The action of NO2 on the graphite support and on the graphite-supported Pt catalyst causes graphite oxidation. The oxygen concentration in the model catalyst is higher than on the support. This is supposed to be due to the spillover of oxygen atoms from platinum particles to graphite.