CO2 methanation reaction mechanism over Ni/CeO2-ZrO2 catalyst.
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•Ni/CZ-AE prepared via ammonia evaporation method gave the best catalytic activity.•Better activity is due to enhanced ...metal-support interaction.•Methane formation does not go through CO formation route.•CO2 is hydrogenated into formates on the support.•Formation of methane was observed from 150 to 350°C on Ni/CZ-AE catalyst.
A series of nickel catalysts supported on CexZr1-xO2(CZ) were prepared by ammonia evaporation (AE), impregnation (IMP) and deposition–precipitation (DP) methods. Their performances for CO2 hydrogenation to methane were investigated at reaction temperatures between 200 and 350°C. Among the catalysts tested, the Ni/CZ (Ni/CZ-AE) catalyst prepared via AE method gave superior catalytic performance at comparatively lower reaction temperatures. At 275°C, it can attain a maximum CO2 conversion and methane selectivity of 55% and 99.8%, respectively and it is stable for nearly 70h reaction time. The better catalytic performance of Ni/CZ-AE is due to the ability of the catalyst to be activated at low temperatures. XPS results for Ni/CZ-AE catalyst shows that some of Ni species are well incorporated into CeO2 lattice of CZ support, resulting in the imbalance of electric charge and lattice distortion of CeO2. Thus, oxygen vacancies can be generated, allowing for adsorption of oxygen species on these vacancies. This finding correlates well with the H2-TPR and CO-TPR results, which demonstrated the shifting of the reduction temperature towards lower temperature ranges than bare CZ support with the incorporation of Ni in Ni/CZ-AE catalyst. DRIFTS experiments for CO2 hydrogenation reaction revealed that the methane formation is via CO free mechanism i.e, formation of carbonates and formate species which are further hydrogenated and decomposed directly to release methane.
Steam reforming of toluene as a biomass tar model compound was performed over Ni supported CaO–Al2O3 (Ca–Al) and CeO2 promoted CaO–Al2O3 (Ca–Al–Ce) catalysts to explore promotional effect of CeO2 on ...Ca–Al support. Among all the catalysts tested, Ni/Ca–Al–Ce(0.2) catalyst gave superior catalytic performance over other catalysts. The basic strength of catalytic supports measured by CO2 TPD and Hammett indicator methods indicates Ca–Al–Ce(0.2) support has higher surface basicity and base strength compared to Ca–Al and other Ca–Al–Ce(x) supports. Furthermore, CO pulse chemisorption results showed that Ni/Ca–Al–Ce(0.2) catalyst has a higher amount of surface metallic nickel compared to other Ni/Ca–Al–Ce catalysts. TPR analysis reveals that the redox property of CeO2 can enhance the reducibility of supported nickel species, which is further confirmed using XPS analysis, where addition of CeO2 enhanced the interaction of Ni species with Ce by reducing the interaction of Ni species with the Al support, resulting in the formation of Ni° rich surface. However, formation of bulk NiO species was also observed for the catalyst having higher amount of CeO2. TGA analysis on spent catalysts reveals that all CeO2-containing catalysts generally result in lower carbon formation rates as compared to Ni/Ca–Al catalyst.
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•Steam reforming of toluene over CeO2-promoted Ni/Ca–Al supported catalysts.•Ni/Ca–Al–Ce(0.2) gave superior catalytic performance over others.•Ni/Ca–Al–Ce(0.2) catalyst has higher basicity and amount of surface Ni species.•Promotional effect of CeO2 to enhance the reducibility of surface Ni species.•All Ni/Ca–Al–Ce catalysts have less carbon deposition than Ni/Ca–Al catalyst.
Iron–alumina-supported nickel–iron alloy catalysts were tested in a fixed-bed reactor for steam reforming of toluene as a biomass tar model compound. The influence of the calcination temperature of ...the iron–alumina support was also explored for the steam reforming reaction. Ni supported on an Fe2O3–Al2O3 support calcined at 500 °C NFA(500) gave superior catalytic performance in terms of activity and stability over other catalysts. NFA(500) gave a toluene conversion of more than 90% for a period of 26 h with a H2/CO value of 4.5. According to XRD analysis, the Ni–Fe alloys were formed and stable throughout the reforming reaction. It was observed from XPS results that the surface of the reduced NFA(500) catalyst was enriched with Fe species, where the other catalysts were enriched with Ni species. These surface Fe species play the role of cocatalysts by increasing the coverage of oxygen species during the reforming reaction to enhance the reaction of toluene and suppresses coke formation. The presence of oxygen species in the reduced catalysts was confirmed by temperature-programmed surface reaction (TPSR) with toluene and water over NFA catalysts. A temperature-programmed oxidation (TPO) study on spent catalysts revealed that the NFA(500) and NFA(700) catalysts have an additional low-temperature oxidation peak at around 525 and 535 °C, respectively, suggesting the presence of a higher amount of amorphous carbon compared with the NFA(900) catalyst. The presence of a low-temperature oxidation peak at 525 °C for the NFA(500) catalyst is one of the reasons for its stable catalytic performance compared with other catalysts.
•SRT over CaO doped Ni–Fe alloy supported Al2O3–Fe2O3 catalysts at S/C=2.•Ni/Ca(1.5)–Fe–Al catalyst gave >80% of toluene conversion for 22h runtime.•Fe plays a co-catalyst role by forming Ni–Fe alloy ...particles.•Ni/Ca(1.5)–Fe–Al catalyst can be able to activate H2O at lower temperature than others.•Ni–Fe alloy was confirmed in Ni/Ca(1.5)–Fe–Al catalyst and stable even after 22h.
CaO doped iron–alumina-supported nickel–iron alloy catalysts were tested in a fixed-bed reactor for steam reforming of toluene as a biomass tar model compound at a relatively low steam-to-carbon (S/C) ratio of 2. The influence of doping CaO to iron–alumina support was also explored for the steam reforming reaction. Ni supported on a CaO(1.5)–Fe2O3–Al2O3 support (Ni/Ca(1.5)–Fe–Al) gave superior catalytic performance in terms of activity and stability over other catalysts. Ni/Ca(1.5)–Fe–Al gave a toluene conversion of more than 80% for a period of 22h testing at a S/C ratio of 2. XRD analysis showed that the Ni–Fe alloys formed were stable throughout the reforming reaction. It was observed from XPS results that the surface of the reduced Ni/Ca(1.5)–Fe–Al catalyst was enriched with Fe species compared to other catalysts. These enriched surface Fe species play the role of co-catalysts by increasing the coverage of oxygen species during the reforming reaction to enhance the reaction of toluene and to suppress coke formation. The temperature-programmed surface reaction (TPSR) with water reveals that the Ni/Ca(1.5)–Fe–Al catalyst can activate water molecule at relatively lower temperature over other CaO doped catalysts. TGA analysis on spent catalysts reveals that all CaO-containing catalysts generally result in lower carbon formation rates as compared to Ni/Fe–Al catalyst.
La0.8Sr0.2Ni0.8M0.2O3 (LSNMO) (where M = Bi, Co, Cr, Cu and Fe) perovskite catalyst precursors have been successfully developed for CO2 dry-reforming of methane (DRM). Among all the catalysts, ...Cu-substituted Ni catalyst precursor showed the highest initial catalytic activity due to the highest amount of accessible Ni and the presence of mobile lattice oxygen species which can activate C–H bond, resulting in a significant improvement of catalytic activity even at the initial stage of reaction. However, these Ni particles can agglomerate to form bigger Ni particle size, thereby causing lower catalytic stability. As compared to Cu-substituted Ni catalyst, Fe-substituted Ni catalyst has low initial activity due to the lower reducibility of Ni–Fe and the less mobility of lattice oxygen species. However, Fe-substituted Ni catalyst showed the highest catalytic stability due to: (1) strong metal–support interaction which hinders thermal agglomeration of the Ni particles; and (2) the presence of the abundant lattice oxygen species which are not very active for C–H bond activation but active to react with CO2 to form La2O2CO3, hence minimizing carbon formation by reacting with surface carbon to form CO.
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► La0.8Sr0.2Ni0.8M0.2O3 (LSNMO) (where M = Bi, Co, Cr, Cu and Fe) perovskite acts as catalyst precursors for H2 production via dry CO2 reforming of methane. ► Cu-substituted Ni catalyst shows the highest initial catalytic activity and slowly decreases with time due to carbon deposition. ► Fe-substituted Ni catalyst shows slowly increasing activity but no carbon formation. ► Involvement of metal–support interaction and surface lattice oxygen species play crucial roles in catalytic DRM performance.
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•Ni–Cu alloy phase exhibits high activity and selectivity toward WGSR.•Ni–Cu alloy enhances CO adsorption at high temperature, preventing CO dissociation.•Carboxyl associative ...mechanism is found as a dominant reaction pathway.•Carboxyl species is the main intermediate with formate species as a spectator.
The formation of methane as the undesired side product is one of the major issues in the water–gas shift (WGS) reaction, particularly for nickel-based catalysts. A detailed study of Ni–Cu bimetallic catalyst supported on nanopowder CeO2 is extensively investigated to suppress the methanation reaction as well as maintain high WGS reaction rate. XRD, EXAFS, H2-TPR and XPS reveal the formation of Ni–Cu alloy, while CO-TPR-MS, CO-TPD-MS and in situ DRIFTS show the enhancement of CO adsorption on Ni–Cu alloy at high temperature. The Ni–Cu/CeO2 catalyst with Ni/Cu ratio of 1 exhibits high reaction rate with the least methane formation due to the formation of Ni–Cu alloy phase. The Ni–Cu alloy phase is found to be the active site for WGS reaction with methane suppression as Ni–Cu alloy can enhance CO adsorption which prevents CO dissociation during high-temperature WGS reaction. Kinetic studies performed reveal that one-site carboxyl mechanism could be the main reaction pathway with formate as spectator. However, there could be other possibilities for the real reaction mechanism on Ni–Cu/CeO2 catalyst.
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•Formation of bimetallic Pd-Ni particles is important for high catalytic activity.•Small metal particle size is also important for high catalyst activity.•Presence of chloride forms ...metal-support compound between Pd and Y2O3 support.•Metal-support compound Pd-Y2O3 plays an important role in high catalyst stability.
The effect of chloride in Pd precursors on catalyst activity and stability of Pd-Ni/Y2O3 catalyst was investigated for oxy-CO2 reforming of methane. A series of Pd-Ni/Y2O3 catalysts with different Pd precursors (PdCl2 and Pd(NO3)2) was prepared using wet-impregnation method over Y2O3 support. The presence of chloride in the Pd(Cl)-Ni/Y2O3 catalyst (PdCl2 as precursor) aids in formation of Pd-Y2O3 compound which can interact with Ni to form bilayer Pd-Ni and prevent agglomeration at high temperature reaction condition. The presence of Pd-Y2O3 compound is of very importance for superior catalytic stability of Pd(Cl)-Ni/Y2O3 catalyst. The catalytic activity of Pd(Cl)-Ni/Y2O3 catalyst is much better than Pd(N)-Ni/Y2O3 catalyst (Pd(NO3)2 as precursor) due to smaller metal particle size and synergy between Pd and Ni to form bimetallic particles on the Pd(Cl)-Ni/Y2O3 catalyst. The effect of metal particle size on catalytic activities is clearly shown in Pd(C)-Ni/Y2O3 catalyst with various Pd/Ni ratios. A formation mechanism of bilayer Pd-Ni formed by interfacial chloride is proposed for Pd(Cl)-Ni/Y2O3 catalyst.
The effect of Fe addition on catalytic activity and stability of LaNixFe1−xO3 perovskite catalyst was investigated for hydrogen production via steam reforming of tar using toluene as a model ...compound. The addition of Fe to LaNiO3 catalyst at the optimum amount enhanced the catalytic performance in steam reforming of toluene. LaNi0.8Fe0.2O3 catalyst shows the best performance in terms of catalytic activity and stability for 8 h of reaction time. The catalyst characterization indicates the presence of Ni-rich Ni–Fe smaller bimetallic particles, strong metal support interaction, and lower carbon deposition rate on LaNi0.8Fe0.2O3 catalyst. The synergy between Ni and Fe atoms on the small Ni–Fe bimetallic particles is crucial for high activity of the LaNi0.8Fe0.2O3 catalyst. In addition, the strong interaction between metal and support on the LaNi0.8Fe0.2O3 catalyst can prevent metal sintering, thus, achieving high catalytic stability.
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•Addition of Fe to Ni catalyst at optimum amount enhances activity and stability.•High activity of LaNi0.8Fe0.2O3 catalyst due to smaller bimetallic Ni–Fe particles.•Strong metal–support interaction on LaNi0.8Fe0.2O3 catalyst prevents metal sintering.•High metal sintering resistance leads to high catalytic stability.
The effect of Na loading on the water–gas shift (WGS) activity of Ni/xNa/CeO2 (with x = 0, 0.5, 1, 2, 5, and 10 wt %) catalysts has been investigated. Ni/2Na/CeO2 exhibited the highest performance in ...terms of WGS activity and methane suppression. Through H2-TPR and XRD, the solubility limit of Na+ in CeO2 was found to be 2 wt %. At low loadings of Na (0.5 to 2 wt %), Na+ was incorporated into the CeO2 lattice, generating a lattice strain and activating the lattice O2, thereby increasing the reducibility of the catalyst. However, beyond the solubility limit of 2 wt %, Na deposited on the CeO2 surface, retarding the reducibility of the catalyst. XPS spectra reveal greater surface concentration of adsorbed oxygen species with the introduction of Na. This can be attributed to the generation of more oxide vacancies for oxygen adsorption due to Na substitution into the ceria lattice. By in situ DRIFTS, methanation was found to be inhibited by the interaction between Na and Ni, leading to the absence of subcarbonyl species which are responsible for this undesirable side reaction.
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