Yoshihisa Sakata

Carbon monoxide and carbon dioxide adsorption on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 1.—Formation of carbonate species on dehydroxylated CeO2, at room temperature

The adsorption of CO and CO2 on cerium oxide has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). For CO adsorption at room temperature, in addition to linearly adsorbed CO (2177 and 2156 cm–1), two kinds of carbonate (unidentate: 854, 1062, 1348 and 1454 cm–1 and bidentate: 854, 1028, 1286 and 1562 cm–1) and inorganic carboxylate (1310 and 15⊙0 cm–1) species were identified spectroscopically. As for CO2 adsorption, apart from weak bands at 1728, 1396, 1219, and 1132 cm–1 attributed to bridged carbonate species, bands due to unidentate carbonate, bidentate carbonate and inorganic carboxylate species, similar to those aising from CO adsorption, were observed. Except for the linearly adsorbed-CO, all species arising from CO and CO2 are stable at room temperature in vacuo. The desorption of these species at elevated temperatures shows that the order of thermal stability is bridged carbonate <bidentate carbonate < inorganic carboxylate < unidentate carbonate species, and the residual of unidentate carbonate species can remain on the surface up to 773 K under evacuation. Forming carbonte and inorganic carboxylate species proved that the CeO2 surface could be partially reduced by CO even at room temperature. No bands in the region 2300–800 cm–1 were detected below 373 K for CO adsorption on hydroxylated CeO2. This indicates that CO adsorption depends on the degree off dehydroxylation of the surface. The mechanism of CO adsorption is also discussed.

Adsorption of carbon monoxide and carbon dioxide on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 2.—Formation of formate species on partially reduced CeO2 at room temperature

The adsorption of CO on partially reduced CeO2[CeO2(573-H) and CeO2(673-H), CeO2 reduced in H2 at 573 and 673 K for 1 h, respectively] has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). The observation of formate species (771, 1329, 1369, 1558, 1587, 2852 and 2945 cm–1) formed by the reaction of CO with the surface of CeO2(673-H) at room temperature is reported. Two weak bands at 2796 and 2706 cm–1, tentatively attributed to formyl species, were also detected at room temperature when the bands of the formate species no longer appeared to grow. No band was observed when CO was dosed on CeO2(573-H) or CeO2(673-H2O)(CeO2 hydrated at 673 K for 1 h) at room temperature and 373 K. Two sharp bands at 3685 and 3643 cm–1, due to isolated hydroxyl groups, and a broad band centred at 3421 cm–1 were generated on CeO2(673-H) through H2 reduction. Among these only the OH groups giving the 3685 cm–1 band manifest activity to CO at room temperature. A mechanism involving a formyl intermediate was proposed to explain the formation of formate species on partially reduced CeO2. The partial reduction of CeO2 causes pronounced inhibition of the formation of carbonate-like species from CO at room temperature. This is consistent with the conclusions reported in Part 1 of this work.

Composites for hydrogen storage by mechanical grinding of graphite carbon and magnesium

Novel hydrogen storage Mg/G nano-composites obtained by mechanical grinding of magnesium (Mg) and graphite carbon (G) with organic additives (benzene, cyclohexane or tetrahydrofuran) have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and temperature programmed desorption (TPD) techniques. The occurrence of various effects as a result of the formation of Mg/G composites ground with benzene, cyclohexane or tetrahydrofuran (designated hereafter as (Mg/G)BN, (Mg/G)CH or (Mg/G)THF, respectively) is expected. Upon mechanical grinding with benzene or cyclohexane for 4–40 h, new hydrogen-storing sites, other than those due to the magnesium component, were formed in the Mg/G composites and they took up hydrogen reversibly. The cleavage-degraded graphite in the composites plays an important role in such hydrogen uptake and release. The formation of Mg/G composites upon grinding with the organic additives led to not only a drop in the onset temperature of MgH2 decomposition, but the formation of additional hydrogen uptake sites. In marked contrast to (Mg/G)BN and (Mg/G)CH, the composites ground without any additives (referred as (Mg/G)none) did not show such behavior. The effective nano-composites are those in which there are synergetic interactions between magnesium and graphite as a result of mechanical grinding with the organic additives.

Hydriding-dehydriding behavior of magnesium composites obtained by mechanical grinding with graphite carbon

Abstract Novel Mg/G composites were prepared by mechanical grinding of magnesium (Mg) and graphite carbon (G) with cyclohexadiene, cyclohexene, cyclohexane, benzene or tetrahydrofuran as an additive. The presence of organic additives during the grinding was very important in determining the composite structures and hydriding–dehydriding properties. The composites prepared without additives [designated hereafter as (Mg/G)none] showed negligible activity for hydriding, whereas the use of additives led to drastic changes in composite structures, leading to much improved hydriding and dehydriding behavior. The effectiveness of organic additives in the initial hydriding was in the order: cyclohexadiene≈tetrahydrofuran≈cyclohexene>benzene>cyclohexane. In the course of the composite formation in the presence of organic additives, the graphite was predominantly degraded by cleavages along graphite layers, resulting in the occurrence of synergetic interactions with magnesium. The graphite for (Mg/G)none was broken irregularly and disorderly to rapid amorphization with negligible interactions with magnesium. Various metal-doped Mg/G composites obtained by grinding of magnesium and graphite with organometallic solutions (Al(C2H5)3, Ti(OC3H7)4, Fe(C5H5)2, Ni(C5H5)2 or Zn(C2H5)2) in benzene have been further examined. Ti-doped Mg/G composites using Ti(OC3H7)4 among others showed an excellent activity; the initial hydriding activity increased above 10-fold relative to that for the metal-free composites.

Infrared studies of adsorbed species of H2, CO and CO2 over ZrO2

The adsorption and reaction of H2, CO, CO2, OH(a)+ CO(g) and CO2(a)+ H2(g) have been studied in detail by infrared spectroscopy. Hydrogen is dissociatively adsorbed to form the OH and the Zr—H species and CO is weakly adsorbed as the molecular form. The infrared spectrum of adsorbed species of CO2 over ZrO2 shows three main bands at ca. 1550, 1310 and 1060 cm–1 which can be assigned to the bidentate carbonate species. The reaction of OH(a)+ CO(g) at 373 K gave rise to formate and bidentate carbonate species, and after having introduced hydrogen onto this system at 473 K, methoxide species appeared. When hydrogen was introduced over CO2-preadsorbed ZrO2, formate and methoxide species also appeared. It is concluded that the formation of the formate and methoxide species results from the hydrogenation of bidentate carbonate species.

Infrared study of hydrogen adsorbed on ZrO2

The adsorption of H2 and D2 at various temperatures and the isotope effect have been studied in detail over a ZrO2 catalyst by means of FTIR. Four different types of adsorbed hydrogen were observed as follows: (1) molecularly adsorbed hydrogen, H2(a), was observed below 173 K and was easily desorbed by evacuation; (2) homolytic dissociative adsorption produces Zr[graphic omitted] species was observed below 373 K and was stable below 178 K; (3) heterolytic dissociative adsorption which forms OH and ZrH was observed in the temperature range 223–373 K and the isotope effect of adsorption between H2 and D2 was also found; (4) the adsorption which generates two OH bands at 3778 and 3668 cm–1 takes place above room temperature. For the heterolytic dissociative adsorption of H2 and D2, both the kinetic and the equilibrium isotope effects were observed. The magnitude of these isotope effects is in good agreement with the values calculated via IR data.

A Doping Technique that Suppresses Undesirable H2 Evolution Derived from Overall Water Splitting in the Highly Selective Photocatalytic Conversion of CO2 in and by Water

Photocatalytic conversion of CO2 to reduction products, such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive propositions for producing green energy by artificial photosynthesis. Herein, we found that Ga2O3 photocatalysts exhibit high conversion of CO2. Doping of Zn species into Ga2O3 suppresses the H2 evolution derived from overall water splitting and, consequently, Zn-doped, Ag-modified Ga2O3 exhibits higher selectivity toward CO evolution than bare, Ag-modified Ga2O3. We observed stoichiometric amounts of evolved O2 together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO2 on the catalyst surface.

Flux-mediated doping of SrTiO3 photocatalysts for efficient overall water splitting

SrTiO3 is a photocatalyst that is well known for its activity for the overall water splitting reaction under UV light irradiation. In this study, the effects of SrCl2 flux treatments and Al doping on the photocatalytic properties of SrTiO3 were investigated. The SrTiO3, which showed an apparent quantum efficiency of 30% at 360 nm in the overall water splitting reaction, the highest value reported so far, was prepared by SrCl2 flux treatments in alumina crucibles. Scanning electron microscopy and X-ray diffractometry revealed that the flux-treated SrTiO3 consisted of well-crystalline particles with a cubic shape reflecting the perovskite-type structure. Inductively coupled plasma optical emission spectroscopy revealed that Al ions from the alumina crucibles were incorporated into the SrTiO3 samples. The SrTiO3 that was treated with SrCl2 flux in Al-free conditions showed a marginal improvement in photocatalytic activity despite the high crystallinity and the clear crystal habit. Doping SrTiO3 with Al improved the photocatalytic activity even without SrCl2 treatment. These results suggested that Al doping was a principal factor in the dramatic improvement in the water splitting activity of the flux-treated SrTiO3. The effects of flux treatments and Al doping on the morphology and water splitting activity of SrTiO3 were discussed separately.

Characteristics of the charge transfer surface complex on titanium( iv ) dioxide for the visible light induced chemo-selective oxidation of benzyl alcohol

This paper deals with the oxidation of benzyl alcohol by O2 on pure TiO2 under visible light irradiation, and it was found that the benzyl alcohol is converted into benzaldehyde with high selectivity (>99%). In order to understand the origins of the visible light induced photocatalysis, surface characterizations of the charge transfer complex formed by the interaction of benzyl alcohol with the TiO2 was extensively performed by investigating the effect of heat-treatment on TiO2 or its chemical modification with hydrofluoric acid. Moreover, the study of the kinetic isotope effect (KIE) for the oxidation of benzyl alcohol showed that the α-deprotonation from benzyl alcohol is the rate determining step (RDS), the process of which is assisted by the terminal OH groups of TiO2. Photo-electrochemical investigations were also incorporated to demonstrate the reaction mechanism behind the visible light induced photocatalytic reaction.

Tuning the selectivity toward CO evolution in the photocatalytic conversion of CO2 with H2O through the modification of Ag-loaded Ga2O3 with a ZnGa2O4 layer

Stoichiometric evolutions of CO, H2, and O2 were achieved for the photocatalytic conversion of CO2 with H2O as an electron donor using Ag-loaded Zn-modified Ga2O3. The selectivity toward the evolution of CO over H2 can be controlled by varying the amount of Zn species added in the Ag-loaded Zn-modified Ga2O3 photocatalyst. The production of H2 gradually decreased with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The XRD, XAFS, and XPS measurements revealed that a ZnGa2O4 layer was generated on the surface of Ga2O3 by modification with Zn species. The formation of the ZnGa2O4 layer eliminated the proton reduction sites on Ga2O3, although the crystallinity, surface area, and morphology of Ga2O3 as well as the particle size and chemical state of Ag did not change. In conclusion, we designed a highly selective photocatalyst for the conversion of CO2 with H2O as an electron donor using Ag (the cocatalyst for the CO evolution), ZnGa2O4 (the inhibitor of the H2 production), and Ga2O3 (the photocatalyst).