211086448
Effect of Metal Content on Ethanol Decomposition over Ni-Co Catalysts Supported on La-Ce Oxides
The search for catalysts with features that can improve coke resistance and decrease byproduct formation is a current goal in H2 production from renewable sources. In this work, the effect of the presence of Ni nanoparticles over Co/La-Ce oxides on the ethanol decomposition reaction was studied. Catalysts were synthetized using as precursor a La0.8Ce0.2NixCo1-xO3 perovskite-type material to ensure a low segregation of phases and a high dispersion of metals. After reduction at 873 K, the perovskite structure was destroyed, and metal Co-Ni particles were supported over a lanthanum-cerium oxide. The materials were characterized by different techniques before and after reaction. Solids exhibited metal particle sizes between 5 and 15 nm demonstrating the advantages of the preparation method to obtain Ni-Co alloys. Although the results of adsorption of ethanol followed by diffuse reflectance infrared fourier transformed spectroscopy (DRIFTS) showed acetate species strongly adsorbed on the catalyst’s surface, the material (Ni0.7Co0.3/La0.8Ce0.2) with the lowest particle size was the most stable system leading to the lowest amount of carbon deposits during ethanol decomposition. This catalyst showed the better performance, with a higher ethanol conversion (98.4%) and hydrogen selectivity (75%). All catalysts exhibited carbonaceous deposits, which were an ordered and disordered carbon phase mixture.
138694854
Ethanol Production from Biomass
In the New Stone Age, alcoholic drinks were produced by natural fermentation of fruits. As principles of ethanol fermentation have been elucidated, industrial production of ethanol has been developed for applications in beverages, cosmetics, industrial solvents, and pharmaceuticals. Presently, bioethanol is produced primarily from corn starch or sugar cane. As these raw materials are in competition with food and animal feed, ethanol production from lignocellulosic biomass has been eagerly investigated worldwide. Although there are various microorganisms producing ethanol, the best microorganisms for efficient conversion of lignocellulosic biomass into ethanol have not yet been discovered. Genetic engineering of microorganisms and fermentation technology for achieving high ethanol productivities and yields have been studied enthusiastically worldwide. This chapter provides an overview of the principles of ethanol fermentation, ethanol-producing microorganisms, and fermentation technology.
39554506
Production of hydrogen from ethanol: review of reaction mechanism and catalyst deactivation.
Mechanism and Catalyst Deactivation Lisiane V. Mattos,† Gary Jacobs,‡ Burtron H. Davis,‡ and Fab́io B. Noronha* †Departamento de Engenharia Química e de Petroĺeo, Universidade Federal Fluminense (UFF), Rua Passo da Pat́ria, 156-CEP 24210-240, Niteroí, RJ, Brazil ‡Center for Applied Energy Research, The University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States Instituto Nacional de Tecnologia−INT, Av. Venezuela 82, CEP 20081-312, Rio de Janeiro, Brazil
93465177
Study of catalyst deactivation and reaction mechanism of steam reforming, partial oxidation, and oxidative steam reforming of ethanol over Co/CeO2 catalyst
The mechanisms of Co/ceria catalyst deactivation during steam reforming, oxidative steam reforming, and partial oxidation of ethanol were explored by comparing the results from different characterization techniques with those obtained from catalytic testing in a fixed-bed reactor. The nature of carbon deposition and the reaction conditions played critical roles in determining the extent of a catalyst deactivation. To shed light on the modes of carbon deposition under different reaction conditions, the mechanisms by which the adsorbed surface species turned over on the catalyst surface were evaluated using diffuse reflectance infrared spectroscopy under reaction conditions and temperature-programed desorption of adsorbed ethanol. In steam reforming, ethoxy species were converted to acetate and steam promoted forward acetate demethanation. The resulting methane decomposed on Co metal particles. In this case, carbon diffused through the Co particle, nucleating growth sites for filamentous carbon behind it, with the resulting filaments lifting Co from the support. High H2O/ethanol ratios and oxygen promoted cleaning of the cobalt surface.
107985097
Enhanced bimetallic Rh-Ni supported catalysts on alumina doped with mixed lanthanum-cerium oxides for ethanol steam reforming
Abstract Enhanced bimetallic Rh (0.25, 0.5, 0.75, 1.0 wt%)-Ni 10 wt%) promoted with La2O3(15 wt%) and CeO2(10 wt%)/alumina supported catalysts for ethanol steam reforming compared to their monometallic counterparts is reported. The larger stability of bimetallic Rh-Ni supported catalysts can be due to a: i) cooperative effect of Rh and Ni activities; ii) larger capacity to gasify the methyl group produced from the decomposition of the intermediaries, iii) non-accumulation of acetate species and, iv) favoring the WGS reaction. The 1%Rh-10%Ni bimetallic catalyst shows the best catalytic performance towards H2 selectivity, operating stability over 144 h of reaction and almost non-carbon formation after 24 h on steam. A significant decrease in the carbon-deposition of the 1%Rh-10 wt%Ni bimetallic catalyst, displaying a 70- and 560-times larger stability than their respective monometallic 1%Rh and 10%Ni counterparts was detected. The study of deactivation/regeneration process of the 1%Rh-10%Ni catalytic system indicates that a reactivation treatment fully recovered the catalytic activity of the deactivated catalyst at least in two regeneration cycles. In both cycles, the bimetallic 1%Rh-10%Ni catalyst was stable until 72 h of reaction appearing as a promising catalyst for the reaction of reforming ethanol at mild temperature conditions.
95475315
SULFUR-PASSIVATED NICKEL CATALYSTS FOR CARBON-FREE STEAM REFORMING OF METHANE
Abstract It has been observed that carbon-free steam reforming of methane can be obtained on a partly sulfur-passivated nickel catalyst under conditions which, without the presence of sulfur, would result in formation of whisker carbon. This effect has been studied by means of kinetic experiments and thermogravimetry. The kinetic data can be explained by simple blockage of the surface as reflected in the observed kinetic orders and activation energy. The studies of carbon formation confirm a threshold coverage of about 70% of full coverage below which the inhibition of carbon is not effective. Above this coverage, amorphous carbon structures may be formed at very high carbon potentials. The retarding effect of sulfur on carbon formation is a dynamic phenomenon. Sulfur inhibits the rate of carbon formation more than the rate of the reforming reactions. The effects are explained by assuming that a large ensemble is involved in the nucleation of carbon, whereas the reforming reaction can proceed on the small ensembles left at high sulfur coverages.
5696895
Layered double hydroxide-based catalysts: nanostructure design and catalytic performance.
Layered double hydroxides (LDHs) are a class of clays with brucite-like layers and intercalated anions which have attracted increasing interest in the field of catalysis. Benefiting from the atomic-scale uniform distribution of metal cations in the brucite-like layers and the ability to intercalate a diverse range of interlayer anions, LDHs display great potential as precursors/supports to prepare catalysts, in that the catalytic sites can be preferentially orientated, highly dispersed, and firmly stabilized to afford excellent catalytic performance and recyclability. The approaches to prepare catalysts based on LDH materials include, but are not limited to, exfoliation of the brucite-like layers, lattice orientation/lattice confinement by the brucite-like layers, and intercalation. This Feature Article summarizes the latest developments in the design and preparation of nanocatalysts by using LDHs as precursors/supports.
155921601
Effect of Ni incorporation in cobalt oxide lattice on carbon formation during ethanol decomposition reaction
Abstract In this work, we report the results of catalytic decomposition of ethanol over a porous mixed metal oxide of Ni-Co (NiCoO2) and analyze the effect of Ni incorporation in Co3O4 lattice on the activity and hydrogen selectivity from ethanol decomposition reaction. In-situ FTIR analysis was conducted between 50 °C to 400 °C for both, Co3O4 and NiCoO2, catalysts to understand the reaction mechanism leading to gas phase product distribution. On cobalt surface the reaction pathway proceeds via the formation of surface ethoxy intermediate that subsequently transforms to aldehyde and acetate intermediates before decomposing to release CO2, H2 and CH4 gases at high temperature. A similar pathway is followed on NiCoO2 surface, leading to aldehyde intermediate, which is relatively unstable and decomposes to release CO along with CH4 and H2 at 400 °C. The NiCoO2 catalyst shows relatively low selectivity for CO2, possibly due to the unstable aldehyde species providing little acetate intermediate that decomposes to release gaseous CO2. The addition of Ni improves the activity for ethanol decomposition by achieving a complete ethanol conversion at 350 °C as compared to 420 °C for cobalt alone. The crystallinity, morphology and particle analysis of the spent catalyst after the reaction was studied using XRD, SEM, and TEM respectively. The XRD shows a phase change of porous NiCoO2 to NiCo alloy, whereas SEM indicates the presence of fibrous structure on the surface with 91.7% of carbon while keeping 1:1 ratio of Ni and Co after the reaction. The detailed analysis of carbon structure using HRTEM-STEM shows the simultaneous growth of carbon nanofibers (CNFs) and multiwalled carbon nanotubes (MWCNTs) that were favored by larger and smaller crystallites respectively. A detailed mechanism of carbon deposition is also proposed.
99807426
Stable reduced Ni catalysts for xylose hydrogenation in aqueous medium
Abstract Precursor materials with 20 wt.% nickel content and formula mater Nd1-xCexAl0.162Ni0.838O3 (x = 0.0, 0.1, 0.5, 0.7) were successfully used to prepare reduced Ni nanoclusters to be used as catalysts in the catalytic hydrogenation of xylose to xylitol. Reduction at 500 °C led to large reduced nickel content with almost no changes in the structure of the started mixed oxide precursors. The catalytic performance of the Ni-reduced catalysts did not show a clear relation with Ce content while a close dependence of the hydrogenated products with Ni° crystal size and the isomerization product with acidity was obtained. The well-known Ni leaching in aqueous medium was absent in the prepared Ni-reduced catalysts corroborating the promising use of crystalline mixed oxides as precursors to prepare stabilized Ni-reduced catalysts for aqueous-phase processing of biomass-derived compounds.
94773283
In situ XAS and FTIR studies of a multi-component Ni/Fe/Cu catalyst for hydrogen production from ethanol
Abstract Multicomponent catalysts containing Ni, Fe, Cu active for ethanol reforming reactions, prepared by solution combustion synthesis are characterized by multiple techniques such as ex situ XRD, XPS, and in situ XAFS and FTIR. XRD results indicate copper to be present in the reduced state as Cu Ni bimetal while nickel and iron are observed to be partially in a spinel NiFe 2 O 4 structure. In situ XANES and XAFS analysis show a change in Ni, Fe and Cu oxidation states during reaction. Cu, which was fully reduced before reaction, became partly oxidized upon exposure to ethanol and oxygen. Ni is mostly (75%) reduced and does not seem to change its oxidation state during the reaction. Fe is not present in metallic form after reduction and during the reaction, but some change in the oxidation state from Fe(II) to Fe(III) occurred during the reaction. XPS and SEM images indicate the formation of carbon filament on the spent catalyst. XPS results also indicate the enrichment of surface by Fe and Cu during the reduction of the catalyst. Based on the activity and characterization results obtained, and literature review, the role of predominant phases during ethanol decomposition reaction is proposed.
97210401
Ethanol decomposition and steam reforming of ethanol over CeZrO2 and Pt/CeZrO2 catalyst: Reaction mechanism and deactivation
Abstract The catalyst performance and the deactivation profiles of CeZrO 2 and Pt/CeZrO 2 were evaluated for the ethanol decomposition and steam reforming of ethanol reactions at 773 K. Unpromoted CeZrO 2 was quite stable whereas Pt/CeZrO 2 catalyst deactivated for all feed compositions studied. A reaction mechanism was proposed based on diffuse reflectance infrared spectroscopy analyses carried out under reaction conditions. The decomposition of the dehydrogenated and the acetate species is facilitated by the presence of the metal and resulted in high selectivity to hydrogen, methane, CO and CO 2 over Pt/CeZrO 2 . Diffuse reflectance infrared spectroscopy, transmission electron microscopy and temperature programmed oxidation and desorption analyses indicate that the loss of the Pt-support synergy leads to a buildup of carbonaceous residue, which is the likely reason of the deactivation of Pt/CeZrO 2 . In addition, once the interfacial boundary is lost, the demethanation of acetate becomes hindered, resulting in an increase in acetaldehyde selectivity with time onstream.
210939850
Decomposition of ethanol over Ni-Al catalysts: effect of copper addition
A series of Ni-Al hydrotalcite-type samples modified with copper was prepared and evaluated for use in the decomposition of ethanol. The partial substitution of Ni by Cu has small influence on the specific surface area and on the structure (XRD). However, it has a strong influence on the degree of reducibility (shifts to lower temperatures) and on the selectivity of reaction. The selectivity towards H2 and CO increases for reactions conducted at a higher temperature, which is followed by a decrease in the amount of carbon formed. Catalysts with a high Cu content exhibit lower selectivity for H2 and CO. SEM images show that carbon nanotubes with greater and more variable diameters and lengths were formed by Cu-loaded catalysts, possibly due to the sintering of copper. Nevertheless, the Raman spectra suggest that the nanotubes produced by the Cu-containing samples have higher purities and fewer defects. © 2012 Published by Elsevier Ltd. Selection under responsibility of the Congress Scientific Committee (Petr Kluson)
96661547
Production of hydrogen-rich gas and multi-walled carbon nanotubes from ethanol decomposition over molybdenum modified Ni/MgO catalysts
Abstract A series of molybdenum modified Ni/MgO catalysts (Ni-Mo/MgO) with different loading ratios of Ni : Mo were prepared by impregnation method. Ethanol decomposition to co-produce multi-walled carbon nanotubes and hydrogen-rich gas at temperatures of 600–800 °C was investigated over the synthesized Ni-Mo/MgO catalysts. The results showed that the catalytic activity depended strongly on the reaction temperature and loading ratio of Ni : Mo. According to the gaseous and solid products obtained, the reaction pathways for ethanol decomposition were suggested.
97567550
Reactivity of high surface area CeO2 synthesized by surfactant-assisted method to ethanol decomposition with and without steam
Abstract High surface area CeO2 (CeO2 (HSA)) synthesized by surfactant-assisted method was found to have useful ethanol decomposition activity producing H2, CO, CO2, and CH4 even when operated without the presence of steam. The catalyst provides high resistance toward carbon deposition compared to Ni/Al2O3 and the conventional low surface area CeO2 (CeO2 (LSA)). The reactivity toward ethanol decomposition for CeO2 is due to its high oxygen storage capacity (OSC). During the decomposition process, the gas–solid reactions between the gaseous components, which are homogenously generated from the ethanol decomposition, i.e. C2H6, C2H4, CH4, CO2, CO, H2O, and H2, and the lattice oxygen (OOx) on CeO2 surface take place. The reactions of surface adsorbed hydrocarbons with the lattice oxygen (CnHm + OOx → nCO + m/2(H2) + VO  + 2e′) can produce synthesis gas (CO and H2) and prevent the formation of carbon species from hydrocarbon decomposition reactions (CnHm → nC + m/2H2). In particular, it was observed that the ethanol decomposition rate over CeO2 (HSA) is proportional to the inlet ethanol partial pressure but independent of the inlet steam partial pressure. This result suggests that the rate of ethanol decomposition is governed by the slow reaction of adsorbed ethanol or surface hydrocarbon species with lattice oxygen in CeO2, and a rapid gas–solid reaction between oxygen source in the system and the reduced ceria to replenish the oxygen.
103457092
Structural transformations of highly porous nickel catalysts during ethanol conversion towards hydrogen
Abstract In this work, we report a liquid-phase reduction method to prepare porous non-supported amorphous nickel catalysts with high surface areas (65–250 m 2 /g). A highly crystalline face center cubic Ni (fcc-Ni) catalyst with 110 m 2 /g surface area was also prepared by frontal crystallization of the amorphous nickel catalyst. The catalytic activity and stability of these catalysts for ethanol decomposition was investigated at different time on stream (TOS) to understand structural transformations occurring at the early stages of catalyst activation-deactivation. Activity vs. TOS results obtained at 473 K show that on the amorphous catalysts the conversion increases from about 50% to 60–75% reaching a steady value at ∼30 h TOS, which remains constant during the observed 96 h of TOS. The fcc-Ni catalyst initially exhibits a higher conversion (∼85%), however, it quickly deactivates to a conversion in the similar range as the amorphous catalysts. It is also shown that BET surface areas of amorphous catalysts decreases during hydrogen pretreatment at 473 K due to crystallization, grain growth, and sintering. The structure of amorphous catalysts continuously refines to form a combination of fcc-Ni and hexagonal close-packed nickel (hpc-Ni) phases, as well as nickel carbide (Ni 3 C) and carbon layers that stabilize catalytic activity. The structure of the fcc-Ni catalyst remains unchanged during the 96 h TOS experiment indicating that carbon deposition might cause its initial deactivation. At 523 K, the amorphous catalyst shows 100% conversion, which remains constant during 96 h of TOS, while the fcc-Ni crystalline catalyst initially exhibits 95% conversion and then slowly deactivates to ∼80% at 96 h TOS. Thus at 523K the stabilized amorphous catalyst does not deactivate under the same TOS compared to the crystalline fcc-Ni catalyst, showing that the active sites on these catalysts are different. The findings of this work suggest that the liquid-phase reduction method can be used to prepare active and stable catalysts for reactions involving decomposition of alcohols and hydrocarbons to produce hydrogen.
95706987
Co–Ni bimetal catalyst supported on perovskite-type oxide for steam reforming of ethanol to produce hydrogen
Abstract Perovskite-type oxide (PTO) of LaFeO3 supported Ni–Co bimetallic catalysts were prepared by citric acid complexation-impregnation method and were used for the steam reforming of ethanol (SRE) to produce hydrogen. The anti-sintering and anti-coking properties of the catalysts for the reaction have been investigated and compared with the monometal catalysts. The catalysts were characterized by using temperature programmed reduction, X-ray diffraction, transmission electron microscopy and thermal analysis techniques. The results indicate that the catalyst was both highly selective to hydrogen and very stable for SRE reaction. Characterization results indicated that Ni–Co was in the state of solid solution alloy. Comparing with corresponding monometal catalysts, the bimetal catalyst exhibited better anti-sintering ability and similar anti-carbon deposition ability. The valuable information indicated in this work is that based on the special characters of PTO, bimetal nano-particles can be supported on metal oxides, which should be a new and promising method for preparing supported bimetal catalysts.
14894781
Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)
Mise au point comportant des definitions generales et la terminologie, la methodologie utilisee, les procedes experimentaux, les interpretations des donnees d'adsorption, les determinations de l'aire superficielle, et les donnees sur la mesoporosite et la microporosite
208734018
Transfer hydrogenation from glycerol over a Ni-Co/CeO2 catalyst: A highly efficient and sustainable route to produce lactic acid
Abstract Bimetallic Ni-Co catalysts supported on nanosized CeO2 were prepared and investigated as heterogeneous catalysts for the transfer hydrogenation between glycerol and various H2 acceptors (levulinic acid, benzene, nitrobenzene, 1-decene, cyclohexene) to selectively produce lactic acid (salt) and the target hydrogenated compound. The bimetallic NiCo/CeO2 catalyst showed much higher activity than the monometallic Ni or Co counterparts (with equal total metal mass), thus indicating strong synergetic effects. The interaction between the metallic sites and the CeO2 support was thoroughly characterised by means of transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX) mapping, X-ray photoelectron spectroscopy (XPS), hydrogen-temperature programmed reduction (H2-TPR) and X-ray diffraction (XRD). Combining characterisation and catalytic results proved that the Ni species are intrinsically more active than Co species, but that incorporating Co into the catalyst formulation prevented the formation of large Ni particles and led to highly dispersed metal nanoparticles on CeO2, thus leading to the observed enhanced activity for the bimetallic system. The highest yield of lactic acid (salt) achieved in this work was 93% at 97% glycerol conversion (160 °C, 6.5 h at 20 bar N2, NaOH: glycerol = 1.5). The NiCo/CeO2 catalyst also exhibited high activity and selectivity towards the target hydrogenated products in the transfer hydrogenation reactions between glycerol and various H2 acceptors. Batch recycle experiments showed good reusability, with retention of 80% of the original activity after 5 runs.
8632561
X-ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems
Quantitative chemical state X-ray photoelectron spectroscopic analysis of mixed nickel metal, oxide, hydroxide and oxyhydroxide systems is challenging due to the complexity of the Ni 2p peak shapes resulting from multiplet splitting, shake-up and plasmon loss structures. Quantification of mixed nickel chemical states and the qualitative determination of low concentrations of Ni(III) species are demonstrated via an approach based on standard spectra from quality reference samples (Ni, NiO, Ni(OH)2, NiOOH), subtraction of these spectra, and data analysis that integrates information from the Ni 2p spectrum and the O 1s spectra. Quantification of a commercial nickel powder and a thin nickel oxide film grown at 1-Torr O2 and 300 °C for 20 min is demonstrated. The effect of uncertain relative sensitivity factors (e.g. Ni 2.67 ± 0.54) is discussed, as is the depth of measurement for thin film analysis based on calculated inelastic mean free paths. Copyright © 2009 John Wiley & Sons, Ltd.
40705171
New interpretations of XPS spectra of nickel metal and oxides
Abstract A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d 9 4s 2 (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd 9 L and the unscreened cd 8 final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH) 2 , NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni 2+ and Ni 3+ simulate the main line and satellite structures for Ni(OH) 2 and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.
94600774
Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces
Abstract CoO and Co3O4 have been studied by high-resolution X-ray photoemission. The characteristic binding energies in the Co 2p 3 2 , 2p 1 2 , and 3s regions, their band shapes and widths, the associated shake-up structure, the O(1s) and O(2s) BE's, and the valence band spectra have been examined. The two oxides are readily distinguished from their spectra though it is shown that the O(1s) BE's are identical at 529.50 ± 0.14 eV. Argon and oxygen ion sputtered surfaces were examined to establish the integrity of the oxides. A higher BE O(1s) component (530.7–531.6 eV), the intensity and BE of which vary with the treatments mentioned above, corresponds to non-stoichiometric surface oxygen. The results are discussed with respect to the electronic structures of the oxides and the often conflicting earlier studies of these oxides.
104571238
Unraveling the Chemical State of Cobalt in Co-Based Catalysts during Ethanol Steam Reforming: an in Situ Study by Near Ambient Pressure XPS and XANES
The steam reforming of ethanol (ESR) has been studied by near ambient pressure XPS (NAP-XPS), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) under in situ conditions in the ALBA synchrotron facility at 200–580 °C and S/C = 3 over different cobalt-based catalysts that showed different catalytic performances: Co3[Si2O5]2(OH)2 (Co-talc), [Co2Mg4Al2(OH)16]CO3·4H2O (Co-hydrotalcite shortened to Co-HT) calcined at 550 °C, and Co3O4 (Co-spinel). Both Co-spinel and Co-talc yield to a greater or lesser degree metallic cobalt under ESR conditions. While Co-spinel shows a complete reduction to metallic cobalt under the conditions used for the XANES measurements, the more bulk-sensitive Co-talc sample exhibits only a partial reduction. On the other hand, under the ESR conditions used with the NAP-XPS, a more surface sensitive technique, the results indicate a higher reduction degree for the Co-talc sample in comparison to the Co-spinel sample. In contrast, the catal...
95447403
XPS studies of damage induced by X-ray irradiation on CeO2 surfaces
Core-level photoemission spectra show that CeO2 is partially reduced to Ce2O3-like phases after prolonged AlKα X-ray exposures. It is suggested that the damage is a result of a locally intense heating of the first few layers of ceria. Experimental evidence is given that some of the Ce 3d XPS features, commonly ascribed in the literature to excited electronic states of CeO2, are actually X-ray induced damage peaks.
97506917
Steam reforming, partial oxidation, and oxidative steam reforming of ethanol over Pt/CeZrO2 catalyst
Abstract The catalytic performance of a Pt/CeZrO 2 catalyst was tested for ethanol decomposition, steam reforming, partial oxidation, and oxidative steam reforming. At low temperature, the catalyst underwent significant deactivation during ethanol decomposition and steam reforming reactions. Co-feeding oxygen decreased the deactivation rate of the catalyst but adversely affected the selectivity to hydrogen. Increasing the reaction temperature greatly improved the stability of the catalyst. A reaction mechanism was proposed based on results obtained from in situ diffuse reflectance infrared spectroscopy analyses carried out under reaction conditions. Ethanol adsorbs as ethoxy species, which may follow one of two distinct pathways: (i) decomposition and production of CO, CH 4 , and H 2 or (ii) dehydrogenation to acetaldehyde and acetyl species. The dehydrogenated species may undergo oxidation to acetate species. The addition of water to the feed promoted the formation of acetate species. Water also facilitated the decomposition of acetaldehyde and acetate reactions, resulting in the formation of methane, CO, and carbonate.
96255875
Hydrogen production for fuel cell applications by ethanol partial oxidation on Pt/CeO2 catalysts: the effect of the reaction conditions and reaction mechanism
This work studied the effect of reaction conditions and catalyst reducibility on the performance of the Pt/CeO2 catalyst in the partial oxidation of ethanol. Oxygen storage capacity (OSC) measurements allowed evaluation of the oxygen transfer capacity of the catalysts. Metal dispersion was determined through cyclohexane dehydrogenation, a structure-insensitive reaction. Infrared spectroscopy of adsorbed ethanol, temperature-programmed desorption, and temperature-programmed surface reaction of ethanol was performed to establish the reaction mechanism. The low dispersion and low oxygen transfer capacity led to a decrease in both the activity and stability of Pt/CeO2 catalysts on partial oxidation of ethanol. The higher amount of oxygen near the metal particles promotes the mechanism of carbon removal from the metallic surface, which takes place at the metal–support interfacial perimeter. Moreover, the increase in particle size decreases the metal–support interfacial area, reducing the effectiveness of the mechanism of carbon removal from the metallic surface. Furthermore, the results obtained on partial oxidation of ethanol showed that an increase in the residence time or reaction temperature increased the ethanol conversion and H2 yield. Regarding selectivity for hydrocarbons and oxygenated products, the production of methane increased and the selectivity for acetaldehyde decreased as the residence time or reaction temperature was increased. At low conversions, the ethanol dehydrogenation dominates, forming acetaldehyde, whereas at high conversions the decomposition of ethanol is favored, producing CH4, H2, and CO. A reaction mechanism is proposed to explain the catalytic tests.
95654510
Hydrogen formation in ethanol reforming on supported noble metal catalysts
Abstract The formation and stability of surface species generated in the interaction of ethanol and ethanol–water mixture with Al2O3 and CeO2-supported noble metal catalysts were studied by FT-IR, TPD and TPR methods. It was found that water enhanced the stability of ethoxide surface species formed in the dissociation of ethanol. Dehydrogenation of molecularly adsorbed ethanol was proposed as a key reaction step. The TPD spectra of ethanol adsorbed on supported noble metals exhibited a high temperature desorption stage which was explained by the formation and decomposition of surface acetate species. Ethylene (product of the dehydration of ethanol) was mainly formed on Al2O3-supported noble metals, while on CeO2-supported noble metals significant amount of acetaldehyde (originated from the dehydrogenation of ethanol) was also formed. In the steam reforming of ethanol the selectivity of H2 formation decreased but that of C2H4 increased in time, while the conversions were stable on alumina-supported noble metals. These observations were explained by the inhibiting effect of surface acetate species.
98023549
Data underpinning:Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution
DN thanks the European Project METSAPP (FCH JU-GA 278257) for funding. We also thank NSF and EPSRC for Materials World Network funding ref EP/J018414/1.
106131304
LaMnO3-based perovskite with in-situ exsolved Ni nanoparticles: a highly active, performance stable and coking resistant catalyst for CO2 dry reforming of CH4
Abstract Wet impregnation method has been widely adopted to synthesize the oxide substrate supported catalysts for CO2 dry reforming of CH4. However, their particle size and distribution are not stable and uniform, causing rapid particle growth and carbon deposition. Here, we show that these problems can be overcome by using a LaMnO3 (LM)-based perovskite in which 20 mol% Ni is doped (La0.9Mn0.8Ni0.2O3, LMN) and then exsolved (R-LMN) by in-situ reduction in 5%H2-N2 stream at 800 °C. The perfomance of such catalyst is compared with that of the Ni-impregnated LM (NLM) reduced under the same conditions (R-NLM). The SEM and TEM analyses reveal that the exsolved Ni nanoparticles in R-LMN are uniform in size, evenly distributed and partially embedded into and hence bonded strongly with the substrate; but the Ni particles in R-NLM are variable in size and weakly bonded with the substrate. Therefore, the particle growth (coalescence) is largely suppressed for the exsolved Ni nanoparticles in R-LMN but not for the Ni nanoparticles in R-NLM. It is also comfirmed that the Ni particles in R-NLM are lifted off from the substrate by carbon fibers formed during test. Consequently, R-LMN demonstrates high and stable conversion (above 80%) and selectivity (above 90%) with a strong resistance to carbon deposition at 700 °C for 24 h. But, a large amount of carbon fiber is formed with a Ni particle at the tip in tested R-NLM, resulting in rapid performance degradation. The high performance of R-LMN for CO2 dry reforming of CH4 is attributed to the stable nanosized Ni particles and LMN substrate that provides more oxygen vacancies for CO2 activation and in turn carbon oxidation.
98605030
A combined in situ time-resolved UV–Vis, Raman and high-energy resolution X-ray absorption spectroscopy study on the deactivation behavior of Pt and Pt-Sn propane dehydrogenation catalysts under industrial reaction conditions
The catalytic performances of Pt/Al2O3 and Pt–Sn/Al2O3 catalysts for the dehydrogenation of propane through consecutive reaction–regeneration cycles have been studied under realistic reaction conditions. A 10-fold successive dehydrogenation–regeneration cycling study, similar to that employed in an industrial propane dehydrogenation reactor, was performed in order to examine the catalyst activity and stability as well as the propene selectivity. Combined in situ UV–Vis/Raman spectroscopy measurements were taken in order to follow the coke formation processes during propane dehydrogenation. This approach allowed correlating propane conversion and propene formation with the on-line determined Raman D over G band intensity ratio and amount of coke formed. These in situ measurements on coke formation and related catalyst deactivation were supplemented by in situ high-energy resolution fluorescence detected (HERFD) XANES measurements in order to characterize the structural and electronic properties of the supported Pt and PtSn nanoparticles during the successive dehydrogenation–regeneration cycles. This combination of powerful spectroscopic techniques revealed unique information regarding the activity behavior and deactivation mechanism of Pt- and PtSn-based propane dehydrogenation catalysts, enabling us to identify important structure-electronic-performance relationships as well as fundamental insight into the dynamics of PtSn alloy formation processes in Pt nanoparticles at elevated temperatures.
102511273
Operando Raman spectroscopy on CO2 methanation over alumina-supported Ni, Ni3Fe and NiRh0.1 catalysts: Role of carbon formation as possible deactivation pathway
Abstract The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt.% Ni/Al2O3, 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300–350 °C). At 400 °C, all three tested catalysts showed high conversion of CO2 (67–75%; Ni > Ni3Fe > NiRh0.1) and selectivity towards CH4 (95–98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high-resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere.