Technological approaches which enable the effective utilization of CO2 for manufacturing value-added chemicals and fuels can help to solve environmental problems derived from large CO2 emissions ...associated with the use of fossil fuels. One of the most interesting products that can be synthesized from CO2 is methanol, since it is an industrial commodity used in several chemical products and also an efficient transportation fuel. In this review, we highlight the recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to methanol. The main efforts focused on the improvement of conventional Cu/ZnO based catalysts and the development of new catalytic systems targeting the specific needs for CO2 to methanol reactions (unfavourable thermodynamics, production of high amount of water and high methanol selectivity under high or full CO2 conversion). Major studies on the development of active and selective catalysts based on thermodynamics, mechanisms, nano-synthesis and catalyst design (active phase, promoters, supports, etc.) are highlighted in this review. Finally, a summary concerning future perspectives on the research and development of efficient heterogeneous catalysts for methanol synthesis from CO2 will be presented.
A catalytic comparative study of COx-free hydrogen production by methane decomposition was carried out. Catalytic performances of bulk Ni-mixed oxides derived from Ni/Mg/Al-hydrotalcites (ex-HTs-Ni) ...were compared with those obtained with Ni supported on mixed oxides derived from Mg/Al-hydrotalcites (Ni/ex-HTs), or on commercial supports (γ-Al2O3, MgO and MgO-modified γ-Al2O3). Catalyst characterization and their catalytic performance showed both ex-HTs-Ni and Ni/ex-HTs appear to be a similar regardless of their method of preparation. Ni/γ-Al2O3 was the best supported catalyst, although the catalytic performances of the ex-HTs catalysts were better. Higher NiMg interaction in ex-HTs provides higher resistance to deactivation. Characterization by TG, Raman spectroscopy and TEM of spent catalysts in the reaction suggest the degree of ordering of the graphitic layers of the carbon deposit onto the catalyst surface is the key factor in the catalyst deactivation. The higher degree of ordering or graphitization of the carbon produced with the higher concentration of sp2 carbons on the surface of the Ni/γ-Al2O3 favours its faster deactivation by Ni-coverage than the bulk catalyst (ex-HT-Ni), in which the MWNT type carbon is mainly obtained.
•Ni/Mg/Al bulk ex-hydrotalcites were the most stable and active catalysts of the study.•Ni is incorporated into mixed oxides MgAl matrix to support it on ex-hydrotalcites.•Higher stability of ex-hydrotalcite materials allowed reach higher H2 production level.•Ni into Mg/Al matrix is better catalyst in the pyrolysis of CH4 than acid Ni/Al2O3.•The type and order of carbon formed have influence on the catalytic deactivation.
A wide variety of carbon materials (ordered mesoporous carbons, carbon blacks, activated carbon, carbon nanotubes, coke and graphite) have been investigated as catalysts for hydrogen production by ...methane decomposition, with the aims of identifying the carbon properties which control in a greater extension the catalytic activity and determine the nature of the active sites involved in the reaction.
The catalytic activity of the different carbon materials was determined and compared using temperature-programmed experiments in a thermobalance. The initial activity was followed through the threshold temperature, defined as the temperature at which hydrogen production starts being detected, whereas the average reaction rate was also calculated and compared. The lowest threshold temperature was observed with ordered mesoporous carbons (CMK materials), followed by activated carbon and carbon blacks. On the other hand, at long reaction times activated carbon was quickly deactivated yielding a relatively low average reaction rate. The deactivation process seems to be greatly linked to the presence of micropores while the long-term activity is retained in those materials with ordered mesoporosity (CMKs) or formed by nanoparticles (carbon blacks), which make them more resistant to deactivation by the formation of carbonaceous deposits.
Whereas no clear dependence is observed between the threshold temperature and the surface area neither with the presence of polar groups in the carbon catalysts, characterization of these materials by XPS shows that a direct relationship exists with the amount of defects present on the graphene layers. This fact strongly supports that these defects are the main active sites for methane decomposition over carbon catalysts.
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•Co-modification with Al and Ga improves the activity of Cu/ZnO.•Activity of Cu/ZnO depends the Al/Ga ratio.•Better intrinsic activity with co-loading with equal amount of Al and Ga.
...This contribution describes the modification in the structure and activity of Cu/ZnO catalyst in the methanol synthesis reaction induced by the co-modification with a low amount of Al and Ga (Al + Ga = 3% atom). To elucidate the effect of the co-modification, the catalysts were characterized by means of variety of techniques (XRD, N2 physisorption, FTIR, TGA, TEM, TPR and N2O chemisorption) and tested in the methanol synthesis reaction carried out in a flow reactor at 250 °C and total pressure of 3 MPa using syngas. The combined characterization and activity tests demonstrated that the co-doping with Al and Ga affects the structure and activity of Cu/ZnO catalysts in a different manner depending on the Al/Ga ratio used. The best catalyst co-loaded with Ga and Al in equal amount shows an improvement in the activity for methanol synthesis respect to the activity obtained on catalysts individually loaded with Al and Ga. The difference in the activity of the samples co-loaded with Al and Ga normalized by N2O chemisorption point to a different participation of the copper surfaces in combination with the defect sites of ZnO depending on the Al and Ga loading.
Methane decomposition offers an interesting route for the CO
2-free hydrogen production. The use of carbon catalysts, in addition to lowering the reaction temperature, presents a number of ...advantages, such as low cost, possibility of operating under autocatalytic conditions and feasibility of using the produced carbons in non-energy applications. In this work, a novel class of carbonaceous materials, having an ordered mesoporous structure (CMK-3 and CMK-5), has been checked as catalysts for methane decomposition, the results obtained being compared to those corresponding to a carbon black sample (CB-bp) and two activated carbons, presenting micro- (AC-mic) and mesoporosity (AC-mes), respectively. Ordered mesoporous carbons, and especially CMK-5, possess a remarkable activity and stability for the hydrogen production through that reaction. Under both temperature programmed and isothermal experiments, CMK-5 has shown to be a superior catalyst for methane decomposition than the AC-mic and CB-bp materials. Likewise, the catalytic activity of CMK-5 is superior to that of AC-mes in spite of the presence of mesoporosity and a high surface area in the latter. The remarkable stability of the CMK-5 catalyst is demonstrated by the high amount of carbon deposits that can be formed on this sample. This result has been assigned to the growth of the carbon deposits from methane decomposition towards the outer part of the catalyst particles, avoiding the blockage of the uniform mesopores present in CMK-5. Thus, up to 25
g of carbon deposits have been formed per gram of CMK-5, while the latter still retains a significant catalytic activity.
Molybdenum carbide has been prepared according to the carbothermal reduction method. Carbon black substrate was used as C-source whereas a H2-flow was the reducing agent. Two different H2 consumption ...steps were identified during the carburization treatment. The low temperature step is related to the reduction of Mo6+-to-Mo4+, the higher temperature process accounts for the deep reduction of Mo4+-to-metal Mo0 and its subsequent reaction with C to form the Mo-carbide. The influences of the maximum carburization temperature, carburization time, gas hourly space velocity regarding Mo-loading, heating rate and temperature of Ar pre-treatment were analyzed.
All these conditions are interrelated to each other. Thus, the carburization process ends at 700°C when Mo-loading is 10wt%, however Mo-loading higher than 10wt% requires higher temperatures. Carburization temperatures up to 800°C are needed to fulfill Mo-carbide formation with samples containing 50wt% Mo. Nevertheless, Ar pre-treatment at 550°C and slow heating rates favor the carburization, thus requiring lower carburization temperatures to reach the same carburization level.
H2-consumption profile (TPR) during the molybdenum carburization process, XRD patterns of the reduced Mo-samples after carburization and TEM-micrographs with two different enlargement of the samples with 5, 20 and 50wt% Mo. Display omitted
► Control of carburization variables: tailor the reduced/carbide Mo-phases (single/mixture). ► Mo carburization in two stages: (1) Mo6+–Mo4+; (2) Mo4+–Mo0 and, at once, MoC. ► The carburization process is faster than Mo4+ reduction. ► XPS probed: reduced Mo particles show core–shell structure. ► Core: reduced Mo (Mo2C, MoO2 and/or Mo0); Shell: 2–3nm of MoO3.
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•Ni-mixed oxides derived from hydrotalcite precursors are active for COx-free hydrogen production from methane decomposition.•Ni-Mg-Al mixed oxide matrix is the main phase detected ...and more active than Ni segregated particles from Ni-Mg-Al matrix.•The hydrogen production enhances upon increasing the Ni content with exception of 58 Ni % at sample due to the NiO segregated.•Low quality nanotubes and nanofibers are formed from isolated metal Ni, increasing their crystallinity with Ni particles.
Hydrogen production free of CO and CO2 was carried out by methane decomposition process using Ni-mixed oxides from hydrotalcite like-materials (ex-HTs) as catalysts. The chemical composition was changed in order to evaluate the influence of the Ni-loading on the main physicochemical (ICP-AES, XRD, XPS, TPR, adsorption/desorption of N2 at 77K and CO2-TPD) properties and on the catalytic performance. For all Ni-loadings (from 7.5 to 58 atomic%), the hydrotalcite structure was the only precursor. After calcination at 850°C, different mixed oxides were formed as a function of Ni-loading, although Ni-Mg-Al mixed oxide matrix or non-stoichiometric spinel phase was the main phase for all catalysts, with the exception of the 58 Ni atomic% sample, in which the NiO segregated from the Mg-Al matrix was the main phase.
Methane conversion close to 55% was achieved, hydrogen was the only gaseous product. The most active catalyst was that containing 46% Ni at. Transmission Electron Microscopy (TEM) images of the spent catalysts showed the appearance of multiwall carbon nanotubes (MWCNTs) although under severe reaction conditions, e.g. high temperatures for thermo-programmed temperature experiments, and the long reaction times for isothermal experiments produced low quality MWCNTs.
Five Co, Ni and Cu oxides derived from hydrotalcite-like precursors (
ex-LDHs) were prepared and tested in the oxidative steam reforming reaction of ethanol under autothermal conditions. Highly ...crystalline LDH-precursors were obtained using urea hydrolysis method and both the precursors and the calcined
ex-LDH oxides were characterized with several physical and chemical techniques. It has been shown that the particle size of the segregated active metal oxide decreases upon increasing the crystallinity of the LDH-precursor. Moreover, these small particle sizes favour the strong interactions between active metals and the amorphous matrix of Al-modifying cations, which cause a high stabilization of the active metal phases.
All the
ex-LDH catalysts (Co–Zn–Al, Co–Mg–Al, Co–Al, Ni–Mg–Al and Cu–Mg–Al) were tested in the oxidative steam reforming of ethanol with EtOH/H
2O molar ratio
(
n
H
2
O
/
n
EtOH
)
of 2.28 and O
2/EtOH molar ratio
(
n
O
2
/
n
EtOH
)
of 0.36, at temperatures of 848–898–948 K. All
ex-LDH catalysts, apart from Cu-catalyst, reached the full ethanol conversion in the temperature range, and H
2 and CO
2 were the main reaction products. Thus, high absolute H
2 production values of 14.5 L(STP) h
−1 g
cat
−1 at 848 K with CoZnAl
ex-LDH catalyst and nearly 18 L(STP) h
−1 g
cat
−1 at 948 K with CoAl and CoMgAl catalysts were reached, which means H
2 selectivity values of 85% at 848 K and 89% at 948 K, respectively.
► Highly crystalline LDH-precursors determine the stabilization of the active metal oxide phases. ► Hydrogen production rates as high as 14.5
L(STP)·h
−1g
−1
cat at 848
K are reached using a CoZnAl
ex-LDH catalyst. ► The highest H
2 production rate (nearly of 18
L(STP)·h
−1g
−1
cat) is obtained at 948
K on the CoMgAl
ex-LDH catalyst. ► H
2 selectivities values of 85% at 848
K and 89% at 948
K are reached on CoZnAl and CoMgAl catalysts, respectively.
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