Tadahiro Fujitani

Development of copper/zinc oxide-based multicomponent catalysts for methanol synthesis from carbon dioxide and hydrogen

Abstract The role of metal oxides such as Ga 2 O 3 , Al 2 O 3 , ZrO 2 and Cr 2 O 3 contained in Cu/ZnO-based ternary catalysts for methanol synthesis from CO 2 and H 2 was classified into two categories: to improve the Cu dispersion and to increase the specific activity. The Cu/ZnO-based multicomponent catalysts developed on the basis of the role of metal oxides were highly active and stable for a long period in a continuous methanol synthesis operation.

Mechanism and Active Sites of the Oxidation of CO over Au/TiO2†

Gold nanoparticles supported on titanium oxides are highly active catalysts for the oxidation of CO even at low temperature. Despite extensive research to determine the reaction mechanism and the active sites on a molecular scale, there is still no consensus about 1) the the active sites and the mechanism of activation of O2 molecules and 2) the role of moisture and its influence on the activity of the catalysts. For instance, although most research groups agree that small Au nanoparticles are the predominant catalytic species, other groups have proposed that cationic gold species, undercoordinated sites on the gold nanoparticles, or electron-rich gold nanoparticles play an essential role in the reaction. Furthermore, dissociation of O2 has been reported to occur when undercoordinated gold atoms are present on extended gold surfaces under model conditions. 14] On the basis of high-intensity in situ X-ray absorption near-edge structure analysis, van Bokhoven and coworkers indicated that O2 molecules can dissociate on gold nanoparticles supported on Al2O3 and TiO2 substrates. [15] In contrast, Liu et al. calculated that the barrier to dissociation of O2 on unsupported gold is > 2 eV and that even at the Au/ TiO2 interface, the dissociation barrier is still 0.52 eV. [16] These values indicate that O2 interacts weakly with gold, and thus spontaneous dissociation of O2 molecules on the gold surface is not energetically favorable. Recently, Behm and co-workers found that the amount of active oxygen species on the Au/ TiO2 surface is linearly related to the number of perimeter sites at the interface between the oxide support and the gold nanoparticles, indicating that the gold-support interface plays a dominant role in O2 activation. [17]

The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity

The effect of ZnO in Cu/ZnO catalysts prepared by the coprecipitation method has been studied using measurements of the surface area of Cu, the specific activity for the methanol synthesis by hydrogenation of CO2, and XRD. Although the Cu surface area increases with increasing ZnO content (0–50 wt%) as is generally known, the specific activity of the Cu/ZnO catalysts with various weight ratios of Cu:ZnO is greater than that of a ZnO-free Cu catalyst. These facts clearly indicate that the role of ZnO in Cu/ZnO catalysts can be ascribed to both increases in the Cu dispersion and the specific activity. The XRD results indicate the formation of a Cu–Zn alloy in the Cu particles of the Cu/ZnO catalysts, leading to the increase in specific activity. It is thus considered that the Cu–Zn surface alloy or a Cu–Zn site is the active site for methanol synthesis in addition to metallic copper atoms that catalyze several hydrogenation steps during the methanol synthesis. Furthermore, the advantage of the coprecipitation method through a precursor of aurichalcite is ascribed to both improvements in the Cu surface area and the specific activity.

On the issue of the active site and the role of ZnO in Cu/ZnO methanol synthesis catalysts

The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO2 and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO2, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu+ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.

The role of metal oxides in promoting a copper catalyst for methanol synthesis

The coverage of oxygen formed on the surface of catalysts during methanol synthesis from CO2 has been measured for copper-based catalysts including various metal oxides using a method called reactive frontal chromatography (RFC). An excellent correlation between the specific activity for methanol synthesis and the oxygen coverage (θ) was obtained, where the activity increased linearly with oxygen coverage atθ<0.16 and then decreased atθ>0.18. The results strongly indicate that the support effect or addition of metal oxides revealed in methanol synthesis over copper catalysts is ascribed to the ratio of Cu+ to Cu0 on the surface of copper particles.

The kinetics and mechanism of methanol synthesis by hydrogenation of CO2 over a Zn-deposited Cu(111) surface

Abstract The hydrogenation of CO 2 over a Zn-deposited Cu(111) surface has been studied using an X-ray photoelectron spectroscopy (XPS) apparatus combined with a high-pressure flow reactor. It was shown that the turnover frequency (TOF) for methanol formation linearly increased with Zn coverage below ϑ Zn =0.19 and decreased above ϑ Zn =0.20. The optimum TOF obtained at ϑ Zn =0.19 was thirteen-fold larger than that of the Zn-free Cu(111) surface. On the other hand, the TOF for CO formation started to decrease at ϑ Zn =0.10 and approached zero at ϑ Zn =0.5. No promotional effect of Zn was thus observed for the reverse water-gas shift (RWGS) reaction on Cu(111). Post-reaction surface analysis by XPS showed the formation of formate species (HCOO a ) on the Cu(111) surfaces. The formate coverage linearly increased with the Zn coverage below ϑ Zn =0.15, suggesting that the formation of the formate species was stabilized by the Zn species. The relation between ϑ HCOO and ϑ Zn is similar to that between TOF and ϑ Zn ; thus, the formate species is considered to be the reaction intermediates during methanol formation, and the amount of the formate species should determine the rate of the reaction. It was found that the surface chemistry of the Zn-deposited Cu surface drastically changed at ϑ Zn =0.15. At higher Zn coverages ( ϑ Zn >0.15), Zn on Cu(111) was readily oxidized to ZnO during the CO 2 hydrogenation reaction. On the other hand, at low Zn coverages below ϑ Zn =0.15, Zn was partially oxidized in the absence of oxygen in ZnO or O a on the Cu surface under the reaction conditions. It was suggested that the Zn on Cu(111) was directly bound to the oxygen in the surface formate species as the role of the active sites.

The role of ZnO in Cu/ZnO methanol synthesis catalysts

The methanol synthesis by the hydrogenation of CO2 over Cu-based catalysts and Zn-deposited Cu(111) model catalysts was studied using XRD, TEM-EDX, reactive frontal chromatography, and surface science techniques such as XPS and AES. For the powder catalysts, a volcano-shaped relation between the oxygen coverage on the Cu surface and the specific activity for methanol formation was obtained, suggesting that a Cu+/Cu0 ratio on the surface control the catalytic activity. Experiments using a physical mixture of Cu/SiO2 and ZnO/SiO2 showed that ZnOχ species migrated from the ZnO particles onto the Cu surface upon reduction with H2, leading to the formation of the Cu+ active species in the vicinity of the ZnOχ species on Cu. This model was proven by the surface science studies using partially Zn-deposited Cu(111), where the ZnOχ species on the Cu(111) surface promoted the catalytic activity of methanol formation, and a volcano-shaped relation between the Zn coverage on the Cu surface and the catalytic activity was obtained. The results definitely contradict the model of single Cu0 active sites for methanol formation because the activity increased with decreasing Cu0 surface area. On the other hand, the activity for the reverse water-gas shift reaction decreased with increasing Zn coverage.

The synergy between Cu and ZnO in methanol synthesis catalysts

The hydrogenation of CO2 over physically-mixed Cu/SiO2 and ZnO/SiO2 was carried out to clarify the synergetic effect between Cu and ZnO in Cu/ZnO methanol synthesis catalysts. The activity of the physical mixtures significantly increased with increasing reduction temperature in the range of 573–723 K. TEM-EDX results definitely showed that ZnOx moieties migrated from ZnO/SiO2 particles onto the surface of Cu particles when the physical mixtures were reduced at high temperatures above 573 K. Upon the migration of the ZnOx species, the oxygen coverage on the surface of Cu, measured after the hydrogenation of CO2, increased with the reduction temperature. The results clearly showed that the synergetic effect of ZnO in the physical mixtures can be ascribed to the creation of active sites such as Cu+ which the ZnOx moieties stabilize on the Cu surface. Further, XRD results showed that the migrated ZnOx species partly dissolved into the Cu particles to form a Cu—Zn alloy.

Evidence for the migration of ZnOx in a Cu/ZnO methanol synthesis catalyst

The behavior and role of ZnO in Cu/ZnO catalysts for the hydrogenations of CO and CO2 were studied using XRD, TEM coupled with EDX, TPD and FT-IR. As the reduction temperature increased, the specific activity for the hydrogenation of CO2 increased, whereas the activity for the hydrogenation of CO decreased. The EDX and XRD results definitely showed that ZnOx (x = 0–1) moieties migrate onto the Cu surface and dissolve into the Cu particle forming a Cu-Zn alloy when the Cu/ZnO catalysts were reduced at high temperatures above 600 K. The content of Zn dissolved in the Cu particles increased with reduction temperature and reached ∼ 18% at a reduction temperature of 723 K. The CO-TPD and FT-IR results suggested the presence of Cu+ sites formed in the vicinity of ZnOx on the Cu surface, where the Cu+ species were regarded as an active catalytic component for methanol synthesis.

Development of an active Ga2O3 supported palladium catalyst for the synthesis of methanol from carbon dioxide and hydrogen

Abstract A significant effect of the support on the catalytic activity of palladium-based catalysts for methanol synthesis from carbon dioxide and hydrogen was observed, where Pd/Ga 2 O 3 was more active than Cu/ZnO by a factor of 2 in yield and 20 in turnover frequency