Where oxide and metals meet: The activation of an efficient associative mechanistic pathway for the water–gas shift reaction by an oxide–metal interface leads to an increase in the catalytic activity ...of nanoparticles of ceria deposited on Cu(111) or Au(111) by more than an order of magnitude (see graph). In situ experiments demonstrated that a carboxy species formed at the metal–oxide interface is the critical intermediate in the reaction.
Reducibility is an essential characteristic of oxide catalysts in oxidation reactions following the Mars–van Krevelen mechanism. A typical descriptor of the reducibility of an oxide is the cost of ...formation of an oxygen vacancy, which measures the tendency of the oxide to lose oxygen or to donate it to an adsorbed species with consequent change in the surface composition, from M n O m to M n O m–x . The oxide reducibility, however, can be modified in various ways: for instance, by doping and/or nanostructuring. In this review we consider an additional aspect, related to the formation of a metal/oxide interface. This can be realized when small metal nanoparticles are deposited on the surface of an oxide support or when a nanostructured oxide, either a nanoparticle or a thin film, is grown on a metal. In the past decade, both theory and experiment indicate a particularly high reactivity of the oxygen atoms at the boundary region between a metal and an oxide. Oxygen atoms can be removed from interface sites at much lower cost than in other regions of the surface. This can alter completely the reactivity of a solid catalyst. In this respect, reducibility of the bulk material may differ completely from that of the metal/oxide surface. The atomistic study of CO oxidation and water-gas shift reactions are used as examples to provide compelling evidence that the oxidation occurs at specific interface sites, the actual active sites in the complex catalyst. Combining oxide nanostructuring with metal/oxide interfaces opens promising perspectives to turn hardly reducible oxides into reactive materials in oxidation reactions based on the Mars–van Krevelen mechanism.
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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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•in situ/operando spectroscopic evidence on associative redox mechanism for water–gas shift.•two Ce4+–OH species adjacent to Au nanoparticles are reduced by CO to yield the ...Ce3+-□-Ce3+ species and the products (CO2, H2) via carboxyl intermediates.•acidic Ce-OH species exhibit the highest reactivity during the reduction half-cycle.•the reduction half-cycle has a higher apparent barrier than the oxidation half-cycle.
Cyclic and repeatable CO2/H2 formation under periodic CO ↔ H2O feeds, that is unsteady-state water–gas shift reaction (uss-WGS), was found to be catalyzed by a gold nanoparticles-loaded CeO2 (Au/CeO2) catalyst. Kinetics of the CO-reduction of Ce4+ to Ce3+ and Ce3+ reoxidation by H2O in combination with transient CO2/H2 formation over Au/CeO2 were studied by operando ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies at 175 °C. The Ce4+–OH species were reduced by CO to give Ce3+-□-Ce3+ (□: oxygen vacancy) and gas phase products (H2, CO2). The Ce3+-□-Ce3+ species were oxidized by H2O to give H2 and Ce4+–OH species. The reduction and reoxidation rates of Ce4+/Ce3+ redox couple were close to the rates of transient CO2/H2 formation. The X-ray absorption spectroscopy results showed that the oxidation states of Au remained unchanged during the redox reactions. These results indicate that the uss-WGS is primary driven by the Ce4+/Ce3+ redox couple. Combined with the observation of adsorbed carboxylate intermediates as a precursor of H2 and CO2, associative redox mechanism is proposed as the main pathway for the unsteady-state WGS reaction on Au/CeO2 at 175 °C.
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•Recent advances in three primary ethanol reforming routes for H2 production are reviewed.•75 % hydrogen selectivity is attained at 400 °C in all three techniques.•Small-sized ...(3–10 nm) particles have high dispersion and catalytic activity.•Hydrogen selectivity is higher for ESR than POX and ATR.•The addition of oxygen reduces the relative selectivity of hydrogen.
The emerging study of hydrogen energy is receiving substantial attention in the scientific community due to its efficiency in approaching net zero and environmental sustainability. Meanwhile, bioethanol is a sustainable and carbon–neutral fuel for hydrogen production. This research aims to assess various ethanol reforming routes, including ethanol steam reforming, partial oxidation, and autothermal reforming, and evaluate the differences in hydrogen production as a function of catalyst physicochemistry and experimental parameters. For all three techniques, 75 % hydrogen selectivity is attained at 400 °C. In the ethanol steam reforming, non-noble metals (Co and Ni) are more reactive than noble metals (Rh and Ru). However, the sequence of hydrogen selectivity is featured by Rh > Ir > Ru > Pt > Ni > Co in autothermal reforming of ethanol. The partially filled d-orbitals of various transition metals can uptake or provide electrons to various reagents, thereby controlling reaction activity. Non-noble metals are inexpensive, making these catalysts appealing for a variety of reforming processes. The small crystal size <10 nm and the large Brunauer-Emmett-Teller surface area of the metal-support particles regulate the dispersion and reactivity of the catalyst. Hydrogen selectivity is lower in partial oxidation and autothermal reforming, while CO and CO2 exhibit no specific selectivity trend. The reactivity of intermediate reactions such as dehydrogenation and decarbonylation positively correlated with the reaction temperature and the steam/oxygen/ethanol ratio, which regulates syngas product distributions. Overall, this review provides a vision for sustainable hydrogen production and decarbonization to achieve the net zero target.
Vapour phase decomposition of formic acid has been studied systematically over a range of catalysts: 1.0 and 10
wt.% Pd/C, 0.8
wt.% Au/C and 1.0
wt.% Au/TiO
2. The mean metal particle size of these ...materials was estimated by HRTEM and turnover frequencies were calculated using these data. The Au/C catalyst was the least active and the Pd/C catalysts were the most active for the formic acid decomposition reaction. At about 400
K, these Pd catalysts gave up to 0.04 moles of H
2 per minute per gram of Pd, with a selectivity of 95–99%. The H
2 selectivity for these catalysts was found to be only weakly dependent on the reaction temperature and the formic acid conversion. The Au/TiO
2 catalyst showed only a moderate selectivity to H
2 formation (<70%). The selectivity of this catalyst was improved considerably by the introduction of water vapour. This improvement derived from the high activity of the catalyst for the water–gas shift reaction.
The modulation of strong metal–support interaction (SMSI) plays a key role and remains a challenge in achieving the desired catalytic performance in many important chemical reactions. Herein, we ...report a TiO2–x -modified Ni nanocatalyst with tunable Ni–TiO2–x interaction via a two-step procedure: preparation of Ni/Ti mixed metal oxide (NiTi–MMO) from NiTi-layered double hydroxide (NiTi–LDH) precursor, followed by a further reduction treatment at different temperatures. A combination study (XRD, TEM, H2-TPR, XPS, and in situ EXAFS) verifies that a high reduction temperature enhances the Ni–TiO2–x interaction, which results in an increased coverage degree of Ni nanoparticles by TiO2–x as well as electron density of interfacial Ni (Niδ−). Moreover, the creation of a Niδ−–O v –Ti3+ interface site (O v denotes oxygen vacancy) induced by strong Ni–TiO2–x interaction serves as dual-active site to efficiently catalyze the water–gas shift reaction (WGSR). The optimized catalyst (Ni@TiO2–x (450)) via tuning Ni–TiO2–x interaction gives a TOF value of 3.8 s–1, which is ∼7 times larger than the conventional 15%Ni/TiO2(450) catalyst. Such a high catalytic efficiency is attributed to the interfacial site (Niδ−–O v –Ti3+) with medium strength of metal–support interaction, as revealed by in situ diffuse reflectance Fourier transform infrared spectroscopy (in situ DRIFTS), which promotes the synergic catalysis between Niδ− and oxygen vacancy toward WGSR.
Summary
A comprehensive model is developed to predict and optimize the hydrogen production via integrated configuration of steam gasification process of biomass and water‐gas shift reaction by taking ...advantage of the ASPEN plus software and sensitivity analysis techniques. The steam gasification process of three different generations of biomass, including corn cob as the first generation, wood residue and rice husk as second generation, and Spirulina algae as the third, are investigated and evaluated to achieve maximum hydrogen production. This model is successfully validated with experimental data reported in the literature about steam gasification of rice husk in the fluidized bed reactor. The impact of main operating parameters is considered in terms of products composition, hydrogen yield, CO conversion, H2/CO ratio in the gas products stream, and cold gas efficiency (CGE). The results indicated that the maximum hydrogen concentration is achieved at the highest steam to biomass (S/B) ratio in the gasifier and water‐gas shift (WGS) reactor and at the lowest WGS reaction temperature, whereas there is an optimum value about the gasification temperature. The highest CO conversion and H2/CO molar ratio belong to rice husk biomass at all considered range of temperature, whereas they are almost the same for wood residue and Spirulina. The predicted results confirmed that CGE of all feedstocks improves with increasing gasification temperatures and S/B ratio. The steam gasification performance of different feedstocks at 750°C is ranked as wood residue (83.56%) > spirulina (83.47%) > corn cob (74.94%) > rice husk (69.86%). The presented configuration can be applied as a novel approach for process evaluation and optimization through the downdraft biomass gasification integrated with water‐gas shift reaction to intensify the hydrogen production.
Hydrogen production via integrated configuration of steam gasification process of biomass and water‐gas shift reaction was investigated at a wide range of operating condition to industrialize this new configuration. A comprehensive simulation model was developed to consider the steam gasification of three generation of biomass feedstock such as corn cob, wood residue, rice husk and spirulina. The effect of main parameters on the gas composition, H2/CO ratio, CO conversion and cold gas efficiency (CGE) was analyzed to determine the optimum condition.
Biogas is comprised of two major compounds (i.e., CH4 and CO2) derived from fermentation of organic wastes. Therefore, biogas can be used as a source for the generation of syngas (H2 and CO: through ...dry reforming of methane). Given that the dominant fraction of biogas is consumed as a feedstock for lower-end products, such as heat and power, dry reforming can be used as an effective option for the valorization of biogas. In this review, we offer up-to-date knowledge on the development of biogas dry reforming in the context of the effects of the composition of the biogas, reaction conditions, and impurities in the biogas. Theoretical estimations of biogas compositions were made along with the compositional matrix of organic substrates. The thermodynamic calculations of dry reforming were also described with other side reactions. In conclusion, the challenges and the potential future directions of this research field were given to help open up new paths toward hybrid biological/chemical processes for H2 production.
•Theoretical estimations and practical results of biogas compositions were discussed.•Up-to-date knowledge on biogas reforming and technical challenges were addressed.•A hybrid platform for H2 production from organic wastes was discussed.
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