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•The difference in PEC activity and stability of the Ta3N5 photoanode was studied in basic and acidic electrolyte.•The surface passivation of the Ta3N5 surface leads to deactivating ...the PEC activity of photoanode.•The suppression of oxide layer formation on the Ta3N5 surface was investigated under an acidic electrolyte.•Potentiodynamic polarization reveals that the corrosion mechanism of Ta3N5 is transpassivation.
The unexpectedly improved photoelectrochemical activity and stability of the Ta3N5 photoelectrode in acidic electrolytes is first reported. The oxidation mechanism of the Ta3N5 surface is revealed as transpassivation, that the passive layer (TaNxOy) continuously grows from the film surface to the bulk side. The surface oxidation rate of Ta3N5 was suppressed in acidic electrolytes compared to basic pH conditions. The less formation of the passive layer at the Ta3N5 surface leads to improved photoelectrochemical activity and stability of Ta3N5 photoanode. Experiments on the TaNxOy layer formation at the Ta3N5/electrolyte interface revealed that interaction of the hydoroxonium ion with the N atom in Ta3N5 leads to the different behavior of the Ta3N5 photoelectrode. With suppressed TaNxOy formation under acidic pH conditions, the fast hole extraction from Ta3N5 photoanode by VO2+ ion enhanced the photoelectrochemical activity. This result indicates that Ta3N5 photoelectrode can be utilized under wide pH range conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•CO and CO2 methanation was investigated over supported Ni catalysts.•Ni/CeO2 catalysts showed the highest activity.•The high-surface-area ceria provides the high Ni dispersion and ...activity.•Co-precipitated Ni0.8Ce0.2Ox catalyst showed the highest specific activity.
CO and CO2 methanation was investigated over Ni catalysts supported on different supports such as γ-Al2O3, SiO2, TiO2, CeO2, and ZrO2. Among them, Ni/CeO2 was determined to be the most active for CO and CO2 methanation. These catalytic activities increased with increasing surface area of CeO2. To increase the specific catalytic activity for CO and CO2 methanation, various Ni-CeO2 catalysts with different Ni contents were prepared using co-precipitation method. The optimum Ni content was determined for both reactions. The prepared catalysts were characterized with inductively coupled plasma-atomic emission spectroscopy, N2 physisorption, temperature-programmed reduction, temperature-programmed desorption, and X-ray diffraction. The high Ni dispersion and strong CO2 adsorption appeared to be responsible for the high catalytic activity for CO and CO2 methanation. This Ni-CeO2 can be applied to the low-temperature CO and CO2 methanation reactor to achieve high single-pass conversions of CO and CO2.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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•The dehydration of d-xylose was examined over dealuminated and desilicated H-ZSM-5.•The total amount of acid sites decreased significantly for desilicated H-ZSM-5.•The fraction of ...Brønsted acid sites increased after dealumination of H-ZSM-5.•The fraction of Brønsted acid sites decreased after desilication of H-ZSM-5.•The furfural selectivity increased with increasing dealumination in H-ZSM-5.
The continuous liquid-phase dehydration of d-xylose into furfural was examined over various zeolites (H-ferrierite, H-ZSM-5, H-mordenite, H-β, and H-Y) having a similar surface area, external surface area, and amount of acid sites to determine the effects of zeolite structure on catalytic performance. Xylose conversion was proportional to the zeolite channel size, whereas the H-ZSM-5 zeolite showed the highest furfural selectivity due to shape-selectivity. In order to improve the catalytic activity, H-ZSM-5 zeolite was dealuminated or desilicated using an aqueous solution of HCl or NaOH, respectively. The prepared catalysts were characterized using N2 physisorption, X-ray diffraction (XRD), solid-state 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS/NMR) spectroscopy, temperature-programmed desorption of ammonia (NH3-TPD) and isopropylamine (IPA-TPD). Dealumination of H-ZSM-5 appeared to remove extra-framework Al species without a collapse of the crystalline zeolite framework, while desilication generated extra-framework Al species accompanied by destruction of the zeolite framework. Among the tested catalysts, dealuminated H-ZSM-5 showed the highest furfural selectivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Ni/SiC and Ni/SiO
2
catalysts prepared by both wet impregnation (WI) and deposition–precipitation (DP) methods were compared for CO and CO
2
methanation. The prepared catalysts were characterized ...using N
2
physisorption, temperature-programmed reduction with H
2
(H
2
-TPR), H
2
chemisorption, pulsed CO
2
chemisorption, temperature-programmed desorption of CO
2
(CO
2
-TPD), transmission electron microscopy, and X-ray diffraction. H
2
-TPR analysis revealed that the catalysts prepared by DP exhibit stronger interaction between the nickel oxides and support than those prepared by WI. The former catalysts exhibit higher Ni dispersions than the latter. The catalytic activities for both reactions over Ni/SiC and Ni/SiO
2
catalysts prepared by WI increase on increasing the Ni content from 10 to 20 wt%. The Ni/SiC catalyst prepared by DP shows higher catalytic activity for CO and CO
2
methanation than that of the Ni/SiC catalyst prepared by WI. Furthermore, it exhibits the highest catalytic activity for CO methanation among the tested catalysts. The high Ni dispersion achieved by the DP method and the high thermal conductivity enabled by SiC are beneficial for both CO and CO
2
methanation.
CO + 3H2 → CH4 + H2O.
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•A Ni@SiO2core–shell catalyst was prepared by a sol–gel method.•Nano-sized Ni was stabilized even with a high Ni content.•This was far superior for CO ...methanation than the conventional Ni/SiO2 catalyst.•This was selective for selective CO methanation in CO2.
The specific catalytic activity of a supported metal catalyst increases with an increase in the number of active sites per mass of catalyst, which can be accomplished by increasing the metal content and/or decreasing the particle size of the metal. However, this leads to sintering of metal particles during the reaction, especially in highly exothermic reactions such as CO methanation. In this study, we prepared different SiO2-supported Ni catalysts by wet impregnation and sol–gel methods, and applied them to CO methanation. The prepared catalysts were characterized with N2 physisorption, X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), temperature-programmed reduction with H2 (H2-TPR), and transmission electron microscopy (TEM). Some problems associated with the wet impregnation method, such as sintering of Ni and the inability to load the silica with large amounts of Ni, were avoided by using the sol–gel method, in which size-controlled NiO was first synthesized using a polymer stabilizing agent, and then coated with a mesoporous silica shell through a polymerization approach. The prepared 55wt% Ni@SiO2 catalyst exhibited the co-presence of Ni nanoparticles (mean size=8.0±4.4nm) and nanorods (mean length=15.5±13nm, mean width=8.1±4.4nm). This catalyst was far superior for CO methanation than the conventional 33wt% Ni/SiO2 catalyst prepared by wet impregnation, in which the Ni particle size was 24.5nm. The 55wt% Ni@SiO2 catalyst also exhibited excellent catalytic performance for selective CO methanation in the presence of an excessive amount of CO2.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•CO methanation activity decreases with Na content over Na/Ni/SiO2 and Na/Ni/CeO2.•CO2 methanation activity decreases with Na content over Na/Ni/CeO2.•The promotional effect of Na on ...CO2 methanation was confirmed over Na/Ni/SiO2.•The addition of Na decreases the surface area of Na/Ni/SiO2 and Na/Ni/CeO2.•CO2 methanation activity is related to the amount of CO2 chemisorbed on the catalyst.
CO and CO2 methanation over Na/Ni/SiO2 and Na/Ni/CeO2 catalysts with different Na contents (0, 0.1, and 1wt%) were studied. N2 physisorption, H2 chemisorption, temperature-programmed reduction with H2, CO2 chemisorption, temperature-programmed desorption of CO2, temperature-programmed oxidation, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy were employed to characterize the catalysts. Even just 0.1wt% Na was observed to have a negative effect on CO methanation for the Na/Ni/SiO2 and Na/Ni/CeO2 catalysts owing to surface blockage of the Ni metal. The negative effect of Na on CO2 methanation was also observed for the Na/Ni/CeO2 catalysts. Conversely, Na exhibited a positive effect upon CO2 methanation over the Na/Ni/SiO2 catalysts. The different effect of Na on CO2 methanation is closely related to the amount of CO2 chemisorbed on the catalysts. Stable catalytic activity for CO and CO2 methanation was observed for Ni/SiO2, Na/Ni/SiO2, and Ni/CeO2. However, the Na/Ni/CeO2 catalyst was deactivated during CO methanation owing to coke formation following olefin production. However, this catalyst was stable for CO2 methanation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Steam reforming of methanol was investigated over Cu–ZnO–ZrO2–Al2O3 catalysts at 473 and 573 K. The Cu:Zn:(Al + Zr) molar ratio was 3:3:4; however, the Zr:Al molar ratio was varied and the catalysts ...were pretreated at different calcination and reduction temperatures. The synthesized catalysts were characterized by N2 physisorption, temperature-programmed reduction with H2 (H2-TPR), X-ray diffraction, oxidized surface TPR, and infrared spectroscopy after carbon monoxide chemisorption. The crystalline size of Cu decreased on increasing the calcination temperatures from 573 to 623 K and increased on increasing the reduction temperatures from 523 to 573 K. Among the tested catalysts, the Cu–ZnO–ZrO2 catalyst exhibited the highest and lowest hydrogen-formation rates at 473 and 573 K, respectively. After the reaction at 573 K, all the tested catalysts exhibited an increase in the Cu crystalline size, causing the catalyst deactivation. Among the tested catalysts, the Cu–ZnO–ZrO2–Al2O3 catalyst, where the Cu:Zn:Al:Zr molar ratio was 3:3:2:2, showed the highest and most stable catalytic activity at 573 K. Cu dispersion and catalyst composition affected the catalytic performance for steam reforming of methanol.
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•The Cu/ZnO/ZrO2/Al2O3 catalysts were applied to steam reforming of methanol (SRM).•The Cu dispersion was affected by the pretreatment conditions.•The beneficial effect of ZrO2 was observed for low-temperature SRM.•Coexistence of Al2O3 and ZrO2 was beneficial for high-temperature SRM.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Direct conversion of methane into value‐added chemicals is a challenging but worthwhile subject. In this communication, the direct conversion of methane into methane oxygenates was achieved in an ...aqueous solution at room temperature using iron salts and Pd/C as catalysts and hydrogen peroxide as an oxidant. The hydrogen peroxide can be directly added or generated in situ from hydrogen and oxygen. The Pd/C catalyst greatly enhanced the reaction rate together with the iron salts. The effect of some parameters such as reaction temperature, reaction time, pH, acid type, and catalyst amount on the yields of the methane oxygenates was also investigated. When hydrogen peroxide was directly added, the turnover frequency (TOF), defined as the moles of methane oxygenates per moles of Fe per unit time, at 293 K was 29 h−1 at pH=2.3. The TOF at 293 K was 42 h−1 at pH=1.3 with in situ generated hydrogen peroxide from hydrogen and oxygen.
Methane‐to‐Oxygenates: Aqueous‐phase selective oxidation of methane with molecular oxygen under mild conditions was successfully demonstrated using iron salts and Pd/C in the presence of molecular hydrogen
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FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Green hydrogen is increasingly recognized as one of the most promising options to decarbonize the energy and transport sector to achieve the goal of climate neutrality, but its low volumetric density ...presents significant challenges that need to be addressed. Because of their ease of handling and transportability, liquid carriers such as liquid organic hydrogen carriers (LOHCs) and ammonia are particularly appealing. This work presents a comparative sustainability analysis of three liquid carriers: liquid hydrogen, liquid ammonia, and synthetic natural gas, and the three promising LOHCs (methanol, di-benzyl toluene, and methylcyclohexane), based on rigorous economic and environmental evaluations. The overall results indicate that by 2050, the promising carriers for small- to medium-level market penetrations ( i.e. , 50 000 kg d −1 ) would be ammonia and methanol, with levelized costs of 3.60 and 3.78 $ per kg H 2 , respectively, while that for high-level market penetrations ( i.e. , 57.1 Mt per year) is LH 2 with a levelized cost of 2.12 $ per kg H 2 . LH 2 is a promising carrier from an environmental perspective because it does not have an energy-intensive dehydrogenation stage. Finally, policy implications are presented as these necessitate the development of a strategic policy framework around the globe to comply with the legal requirements set forth by the United Nations, such as the Paris Agreement and Sustainable Development Goals.
