Experimental research on acetic acid steam reforming over Co–Fe catalysts and subsequent density functional theory studies

An experimental study on the catalytic steam reforming of acetic acid was initially performed over a series of co-precipitated Co–Fe unsupported catalysts at relatively low temperatures. It was found that the catalyst activity increased with increasing cobalt content, and the highest performance, with an acetic acid conversion of 100% and an H₂ yield of 96% was obtained over pure cobalt catalyst at 400 °C. The catalysts have been systematically characterized by BET, XRD, and HRTEM. The results revealed that the superior activity and stability of pure cobalt catalyst can be ascribed to small particle size, coexistence of metallic cobalt and CoO, and stable H₂O adsorption. Furthermore, the mechanism route of acetic acid decomposition on cobalt surface was proposed via DFT calculations.     

Pathways of ethanol steam reforming over ceria-supported catalysts

Ceria-supported Pt, Ir and Co catalysts are prepared herein by the deposition–precipitation method and investigated for their suitability in the steam reforming of ethanol (SRE) at a temperature range of 250–500 °C. SRE is tested in a fixed-bed reactor under an H₂O/EtOH molar ratio of 13 and 20,000 h⁻¹ GHSV. Possible pathways are proposed according to the assigned temperature window to understand the different catalysts attributed to specific reaction pathways. The Pt/CeO₂ catalyst shows the best carbon–carbon bond-breaking ability and the lowest complete ethanol conversion temperature of 300 °C. Acetone steam reforming over the Ir/CeO₂ catalyst at 400 °C promotes a hydrogen yield of up to 5.3. Lower reaction temperatures for the water–gas shift and acetone steam reforming are in evidence for the Co/CeO₂ catalyst, whereas the carbon deposition causes its deactivation at temperature over 500 °C.     

Ethanol steam reforming over rhodium and cobalt-based catalysts: Effect of the support

The effect of support on the properties of rhodium and cobalt-based catalysts for ethanol steam reforming was studied in this work, by comparing the use of magnesia, alumina and Mg–Al oxide (obtained from hydrotalcite) as supports. It was found that metallic rhodium particles with around 2.4–2.6 nm were formed on all supports, but Mg–Al oxide led to the narrowest particles size distribution; cobalt was supposed to be located on the support, affecting its acidity. Rhodium interacts strongly with the support in the order: alumina> Mg–Al oxide > magnesia. The magnesium-containing catalysts showed low ethene selectivity and high hydrogen selectivity while the alumina-based ones showed high ethene selectivity, assigned to the Lewis sites of alumina. The Mg–Al oxide-supported rhodium and cobalt catalyst was the most promising sample to produce hydrogen by ethanol reforming, showing the highest hydrogen yield, low ethene selectivity and high specific surface area during reaction.     

Hydrogen production through methanol steam reforming: Effect of synthesis parameters on Ni–Cu/CaO–SiO2 catalysts activity

The objectives of this study were to prepare Ni–Cu/CaO–SiO₂ catalysts by a modified polyol process with different preparation conditions and evaluate the feasibility of hydrogen production from methanol steam reforming. CaO–SiO₂ materials possess high specific surface areas and CO₂ absorption capacities which were synthesized through the sol–gel method to serve as supports. The experimental results of the methanol steam reforming indicated that the highest catalytic activity was achieved when the Ni–Cu/CaO–SiO₂ catalyst was prepared under Ar atmosphere at a reduction temperature of 160 °C (160-Ar). The 160-Ar catalyst synthesized by this method has a large pore volume and a high mesoporosity. These physical properties contribute to the effective dispersion of metal particles in the 160-Ar catalyst. Increasing the MeOH/H₂O ratio was found to promote the water–gas shift reaction and direct methanol decomposition to produce more H₂.     

Characterization and activity test of commercial Ni/Al2O3, Cu/ZnO/Al2O3 and prepared Ni–Cu/Al2O3 catalysts for hydrogen production from methane and methanol fuels

In this study, methane and methanol steam reforming reactions over commercial Ni/Al₂O₃, commercial Cu/ZnO/Al₂O₃ and prepared Ni–Cu/Al₂O₃ catalysts were investigated. Methane and methanol steam reforming reactions catalysts were characterized using various techniques. The results of characterization showed that Cu particles increase the active particle size of Ni (19.3 nm) in Ni–Cu/Al₂O₃ catalyst with respect to the commercial Ni/Al₂O₃ (17.9). On the other hand, Ni improves Cu dispersion in the same catalyst (1.74%) in comparison with commercial Cu/ZnO/Al₂O₃ (0.21%). A comprehensive comparison between these two fuels is established in terms of reaction conditions, fuel conversion, H₂ selectivity, CO₂ and CO selectivity. The prepared catalyst showed low selectivity for CO in both fuels and it was more selective to H₂, with H₂ selectivities of 99% in methane and 89% in methanol reforming reactions. A significant objective is to develop catalysts which can operate at lower temperatures and resist deactivation. Methanol steam reforming is carried out at a much lower temperature than methane steam reforming in prepared and commercial catalyst (275–325 °C). However, methane steam reforming can be carried out at a relatively low temperature on Ni–Cu catalyst (600–650 °C) and at higher temperature in commercial methane reforming catalyst (700–800 °C). Commercial Ni/Al₂O₃ catalyst resulted in high coke formation (28.3% loss in mass) compared to prepared Ni–Cu/Al₂O₃ (8.9%) and commercial Cu/ZnO/Al₂O₃ catalysts (3.5%).     

Effect of calcination conditions on time on-stream performance of Ni/La2O3–ZrO2 in low-temperature dry reforming of methane

A series of catalysts based on Ni supported on mesoporous La₂O₃–ZrO₂ was prepared and tested in low-temperature (400 °C) dry reforming of methane for 100 h on stream. The catalysts were obtained from the same precursor by calcining in either flowing air or Ar at different temperatures. Both the temperature and the atmosphere had an effect on the catalytic activity and on-stream stability. With increasing calcination temperature, the dispersion of Ni decreased. Surprisingly, this resulted not in the lower, but in the higher intrinsic activity of Ni species. This increase can be rationalized by assuming that the rate-determining step is not CH₄ decomposition, but the removal of carbon deposits from Ni particle by reaction with CO₂. The catalysts calcined at 800 °C in Ar and air showed the strongest and the second strongest deactivation, respectively, caused by the formation of crystalline carbon coatings due to a lower number of CO₂ adsorption sites. The size of Ni particles favoring the formation of layered carbon species was found to be the main origin of the catalysts deactivation in the low-temperature dry reforming of methane.     

Renewable hydrogen production from steam reforming of glycerol by Ni–Cu–Al,Ni–Cu–Mg,Ni–Mg catalysts

H₂ production from glycerol steam reforming by the Ni–Cu–Al, Ni–Cu–Mg, Ni–Mg catalysts was evaluated experimentally in a continuous flow fixed-bed reactor under atmospheric pressure within a temperature range from 450 to 650 °C. The catalysts were synthesized by the co-precipitation methods, and characterized by the elemental analysis, BET, XRD and SEM. The GC and FTIR were applied to analyze the products from steam reforming of glycerol. The coke deposited on the catalysts was measured by TGA experiments during medium temperature oxidation. The results showed that glycerol conversion and H₂ production were increased with increasing temperatures, and glycerol decomposition was favored over its steam reforming at low temperatures. The Ni–Cu–Al catalyst containing NiO of 29.2 wt%, CuO of 31.1 wt%, Al₂O₃ of 39.7 wt% performed high catalytic activity, and the H₂ selectivity was found to be 92.9% and conversion of glycerol was up to 90.9% at 650 °C. The deactivation of catalysts due to the formation and deposition of coke was observed. An improved iterative Coats–Redfern method was used to evaluate the non-isothermal kinetic parameters of coke removal from catalysts, and the results showed the reaction order of n = 1 and 2 in the Fn nth order reaction model predicted accurately the main phase in the coke removal for the regeneration of Ni–Mg and Ni–Cu–Al catalysts, respectively.     

Cu–Zn–Al based catalysts for low temperature bioethanol steam reforming by solar energy

Bioethanol steam reforming is one of the most promising route to produce hydrogen from a renewable liquid biofuel. Activity of two Cu–Zn–Al based catalysts was investigated at low temperatures, ranging from 420 to 500 °C, in view of temperature limitations associated with solar energy supply by parabolic trough technology. At 450 °C the space velocity effect was also investigated, by varying the weight hourly space velocity (WHSV) from 1.67 to 3.32 h⁻¹. In each experimental conditions, together with the expected hydrogen and carbon dioxide, also methane, ethylene, acetaldehyde and diethylether were detect as products, so indicating the presence of several parallel reaction pathways. A good selectivity to ethanol reforming was obtained only at 500 °C (with values of the H₂/CO₂ mol ratio of 3.4 and 4.5) with both catalysts, while at lower temperatures alcohol dehydration into acetaldehyde seemed to be the main reaction.     

Mild temperature hydrogen production by methane decomposition over cobalt catalysts prepared with different precipitating agents

Hydrogen production by methane decomposition has been studied using different cobalt catalysts obtained by reduction of cobalt oxide precursors synthesized in ethylene glycol and using three different precipitating agents: sodium carbonate, ammonium hydroxide and urea. The physicochemical properties of the catalysts precursors vary with the precipitating agent, which shows a significant influence in their catalytic performance. Thus, the catalysts obtained from precursors precipitated with Na₂CO₃ or CO(NH₂)₂ show remarkable catalytic activity at lower temperatures, which in both cases has been assigned to the lower particle size and aggregation degree of the final metallic Co phase. Accordingly, the use of urea as precipitating agent led to the catalyst with the highest H₂ production at 600 °C after 12 h of time on stream. Likewise, it is worth mentioning that the catalyst prepared using Na₂CO₃ shows significant activity in this reaction even at temperatures as low as 400 °C.     

Effect of CaO addition on acid properties of Ni–Ca/Al2O3 catalysts applied to ethanol steam reforming

Ni/Al₂O₃ catalysts containing 5 wt% of Ni and modified by addition of CaO (0–5 wt%) were tested in ethanol steam reforming reaction in order to reduce the dehydration ethanol reaction, which produces ethylene that may polymerize and produce coke. The catalysts were prepared by impregnation (I) and co-precipitation (C) methods. All catalysts were investigated for ethanol steam reforming and the catalytic performance was compared in terms of additive addition. The catalysts 5Ni–5Ca/Al (I) and 5Ni–5Ca/Al (C) were less selective to ethylene production and therefore were characterized by the following techniques: energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption near edge structure (XANES), specific surface area by the BET method, scanning electron microcopy (SEM) and isopropanol decomposition reaction. By comparing the catalysts, the 5Ni–5Ca/Al (I) catalyst presented the lowest acidity and carbon deposition, and also presented no deactivation in 24 h of catalytic test.     

Activity and selectivity of CO oxidation in H2 rich stream over the Ag/Co/Ce mixed oxide catalysts

Silver, cobalt and ceria mixed oxide catalysts were prepared at different metal/metal oxide molar ratios by the co-precipitation method, calcined at different temperatures (200 °C, 450 °C) and tested for the selective CO oxidation reaction in H₂ rich gas stream. XRD, XPS, N₂ physisorption, SEM and TPR-H₂ techniques were used to characterize the catalysts. Catalysts have an average pore diameter in the mesoporous range. Catalysts which were calcined at 200 °C had amorphous phase structure. After calcination at 450 °C, not only the crystal phase structure but also decrease in BET surface areas of the catalysts and the shift at light off temperatures of the catalysts to the higher temperatures were obtained. The highest activity and selectivity was obtained from the catalysts calcined 200 °C which were 50/50 Ag–Co and 50/50 Co–Ce mixed oxide catalysts, respectively, which did not loose their activity and selectivity in a reaction period of 5800 min.     

Catalysts for hydrogen production in a multifuel processor by methanol,dimethyl ether and bioethanol steam reforming for fuel cell applications

Methanol, dimethyl ether and bioethanol steam reforming to hydrogen-rich gas were studied over CuO/CeO₂ and CuO–CeO₂/γ-Al₂O₃ catalysts. Both catalysts were found to provide complete conversion of methanol to hydrogen-rich gas at 300–350 °C. Complete conversion of dimethyl ether to hydrogen-rich gas occurred over CuO–CeO₂/γ-Al₂O₃ at 350–370 °C. Complete conversion of ethanol to hydrogen-rich gas occurred over CuO/CeO₂ at 350 °C. In both cases, the CO content in the obtained gas mixture was low (<2 vol.%). This hydrogen-rich gas can be used directly for fuelling high-temperature PEM FC. For fuelling low-temperature PEM FC, it is needed only to clean up the hydrogen-rich gas from CO to the level of 10 ppm. CuO/CeO₂ catalyst can be used for this purpose as well. Since no individual WGS stage, that is necessary in most other hydrogen production processes, is involved here, the miniaturization of the multifuel processor for hydrogen production by methanol, ethanol or DME SR is quite feasible.     

Bioethanol/glycerol mixture steam reforming over Pt and PtNi supported on lanthana or ceria doped alumina catalysts

The catalytic activity of Pt and PtNi catalysts supported on γ-Al₂O₃ modified by La and Ce oxides was investigated in the steam reforming of ethanol/glycerol mixtures. In general, all the catalysts fully converted the glycerol at the temperatures tested. However, the conversion of ethanol depended on the reaction temperature and catalyst type. The conversion into gaseous products operating at 500 °C and 450 °C was 100% using the most active catalysts (PtNiAl6La and PtNiAl10Ce). These two bimetallic catalysts gave H₂ yields close to those predicted by thermodynamic equilibrium at these temperatures. However, when the reaction temperature was lowered to 400 °C, these catalytic systems and the PtNiAl one recorded a significant decrease in ethanol conversion and H₂ yield, which moved away from the thermodynamic equilibrium value. This deviation was due to intermediate liquid products (acetaldehyde, acrolein, etc.) not being further reformed and the formation of other gaseous ones (light alkanes and ethylene). PtNiAl10Ce catalyst presented the highest conversion into gas at 400 °C, resulting in the largest H₂ yield, followed by PtNiAl6La and PtNiAl catalysts. This order is in agreement with the Ni/Al surface atomic ratio measured by XPS technique in reduced samples. However, filamentous carbon nanotubes were detected but this carbon type maintained the active sites accessible for reactants, since TEM and TGA results showed that the density of this carbon was lower for PtNiAl₁₀Ce catalyst. Pt catalysts presented lower activity than PtNi catalysts possibly due to the formation of carbon nanotubes, which covered some metallic active sites.     

CNT-based catalysts for H2 production by ethanol reforming

Hydrogen production by steam reforming of ethanol (SRE) was studied using steam-to-ethanol ratio of 3:1, between the temperature range of 150–450 °C over metal and metal oxide nanoparticle catalysts (Ni, Co, Pt and Rh) supported on carbon nanotubes (CNTs) and compared to a commercial catalyst (Ni/Al₂O₃). The aim was to find out the suitability of CNTs supports with metal nanoparticles for the SRE reactions at low temperatures. The idea to develop CNT-based catalysts that have high selectivity for H₂ is one of the driving forces for this study. The catalytic performance was evaluated in terms of ethanol conversion, product gas composition, hydrogen yield and selectivity to hydrogen. The Co/CNT and Ni/CNT catalysts were found to have the highest activity and selectivity towards hydrogen formation among the catalysts studied. Almost complete ethanol conversion is achieved over the Ni/CNT catalyst at 400 °C. The highest hydrogen yield of 2.5 is, however, obtained over the Co/CNT catalyst at 450 °C. The formation of CO and CH₄ was very low over the Co/CNT catalyst compared to all the other tested catalysts. The Pt and Rh CNT-based catalysts were found to have low activity and selectivity in the SRE reaction. Hydrogen production via steam reforming of ethanol at low temperatures using especially Co/CNT catalyst has thus potential in the future in e.g. the fuel cell applications.     

Coke deactivation of Ni and Co catalysts in ethanol steam reforming at mild temperatures in a fluidized bed reactor

The deactivation by coke deposition of Ni and Co catalysts in the steam reforming of ethanol has been studied in a fluidized bed reactor under the following conditions: 500 and 700 °C; steam/ethanol molar ratio, 6; space time, 0.14 g_catalyst h/g_ethanol, partial pressure of ethanol in the feed, 0.11 bar, and time on stream up to 20 h. The decrease in activity depends mainly on the nature of the coke deposited on the catalysts, as well as on the physical–chemical properties (BET surface area, pore volume, metal surface area) of the catalysts. At 500 °C (suitable temperature for enhancing the WGS reaction, decreasing energy requirements and avoiding Ni sintering), the main cause of deactivation is the encapsulating coke fraction (monoatomic and polymeric carbon) that blocks metallic sites, whereas the fibrous coke fraction (filamentous carbon) coats catalyst particles and increases their size with time on stream with a low effect on deactivation, especially for catalysts with high surface area. The catalyst with 10 wt% Ni supported on SiO₂ strikes a suitable balance between reforming activity and stability, given that both the capability of Ni for dehydrogenation and C–C breakage and the porous structure of SiO₂ support enhance the formation of filamentous coke with low deactivation. This catalyst is suitable for use at 500 °C in a fluidized bed, in which the collision among particles causes the removal of the external filamentous coke, thus minimizing the pore blockage of the SiO₂. At 700 °C, the coke content in the catalyst is low, with the coke being of filamentous nature and with a highly graphitic structure.     

Steam reforming of ethanol on zinc containing catalysts with spinel structure

Hydrogen production by ethanol steam reforming (SRE) was studied on zinc based catalysts with spinel-like structure. ZnM₂O₄ (M = Al, Co, Fe) mixed oxides were prepared by sol–gel technique with citric acid and characterized by XRD, TPR-H₂, S_BET, TPD-NH₃ and decomposition of cyclohexanol. The XRD patterns confirm presence of well defined spinel-like structure of prepared materials. The highest catalytic activity towards hydrogen production was found for zinc cobaltate: the hydrogen yield on this catalyst is 2.8 mol H₂/mol EtOH. The ZnO promoting the reduction of cobalt cations to Co⁰ that is more active in SRE.     

Valorization of alcoholic wastes from the vinery industry to produce H2

This paper focuses on the development of active and stable catalysts for the steam reforming of alcoholic wastes. Two catalysts with high activity in the steam reforming of ethanol have been studied in the steam reforming of an alcoholic waste from the vinery industry, as previous step to their industrial scale up. The catalysts are based on cobalt supported on Zn-hydrotalcite-derived material and natural sepiolite. At laboratory level, the catalytic material based on natural sepiolite showed the best catalytic performance maintaining its catalytic activity for more than 160 h in presence of 50 ppm of sulfur contained in the alcoholic waste. Thus, sepiolite based catalyst was scaled up to prepare a first generation of catalytic monoliths. The reforming activity of this first generation of monoliths was found lower than their corresponding powdered catalyst. The high calcination temperature (1573 K) used in the manufacture of the first generation of monoliths was the responsible of their low performance, which was related to the sintering of cobalt catalyst in a larger crystallite size of metallic cobalt. A second generation of monoliths was prepared using a lower calcination temperature (873 K). Now, the monoliths exhibited a high catalytic performance, similar to the powdered catalyst. The excellent results obtained with the second generation of monoliths have been protected under an invention patent (E201731077). This is the first time that catalytic monoliths based on natural sepiolite promoted with Co are successfully manufactured and tested in the steam reforming of alcoholic wastes from the distillery industry to produce hydrogen.     

Steam reforming of ethanol over Co3O4–Fe2O3 mixed oxides

Co₃O₄, Fe₂O₃ and a mixture of the two oxides Co–Fe (molar ratio of Co₃O₄/Fe₂O₃ = 0.67 and atomic ratio of Co/Fe = 1) were prepared by the calcination of cobalt oxalate and/or iron oxalate salts at 500 °C for 2 h in static air using water as a solvent/dispersing agent. The catalysts were studied in the steam reforming of ethanol to investigate the effect of the partial substitution of Co₃O₄ with Fe₂O₃ on the catalytic behaviour. The reforming activity over Fe₂O₃, while initially high, underwent fast deactivation. In comparison, over the Co–Fe catalyst both the H₂ yield and stability were higher than that found over the pure Co₃O₄ or Fe₂O₃ catalysts. DRIFTS-MS studies under the reaction feed highlighted that the Co–Fe catalyst had increased amounts of adsorbed OH/water; similar to Fe₂O₃. Increasing the amount of reactive species (water/OH species) adsorbed on the Co–Fe catalyst surface is proposed to facilitate the steam reforming reaction rather than decomposition reactions reducing by-product formation and providing a higher H₂ yield.     

K (Na)-promoted Ni,Al layered double hydroxide catalysts for the steam reforming of methanol

Production of hydrogen by methanol steam reforming has been studied over a series of Ni/Al layered double hydroxide catalysts prepared by the co-precipitation method, with the aim to develop a stable catalyst that can be used in a membrane-joint performer at temperatures greater than 300 °C. H₂, CO and CO₂ are generally the major products together with trace amounts of CH₄. The presence of potassium and/or sodium cations was found to improve the activity of methanol conversion. The selectivity for CO₂ rather than CO was better with K ions than Na ions, especially at higher temperatures (e.g. 390–400 °C). Methanol steam reforming over a K-promoted Ni/Al layered double hydroxide catalyst resulted in better activity and similar stability compared to a commercial Cu catalyst.     

Steam reforming of methanol over Cu/ZnO/ZrO2/Al2O3 catalyst

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.