Temperature-programmed Hydrogenation Research Articles

Highly dispersed Pt boosts active FexN formation in ammonia decomposition

Ammonia decomposition plays an important role in hydrogen production, especially in the context of a hydrogen economy. Fe-based catalysts are a popular choice due to their affordability and moderate activity, making them attractive for large-scale applications. However, the transformation of Fe-based catalysts into more active FexN species can be slow, resulting in a prolonged induction period. To address this issue, we investigated the effects of Pt addition on FexN formation in ammonia decomposition. Our results show that even a slight Pt addition significantly enhances the FexN formation rate, increasing it over threefold. Pt aids in H2 desorption via reverse spillover, which, in turn, exposes more Fe surface sites where nitridation occurs, leading to the formation of iron nitride. Characterization via High-angle annular dark-field imaging scanning transmission electron microscopy, X-ray diffraction, and in situ X-ray absorption spectroscopy (XAS) revealed the formation of Fe2N as active species, whereas temperature-programmed hydrogen desorption, temperature-programmed reduction by hydrogen, and in situ XAS supported the existence of H reverse spillover and spillover effects. Overall, our study provides an improved understanding of the active species formation mechanism of Fe catalysts in ammonia decomposition and offers a simple strategy for improving their catalytic performance.

Hydrogen Production from Biogas: Development of an Efficient Nickel Catalyst by the Exsolution Approach

Hydrogen production from biogas over alumina-supported Ce1−xNixO2−x catalysts was studied in a temperature range of 600–850 °C with an initial gas composition of CH4/CO2/H2O of 1/0.8/0.4. To achieve a high and stable hydrogen yield, highly dispersed Ni catalysts were prepared through the exsolution approach. A solid solution of Ce1−xNixO2−x was firstly formed on the surface of Al2O3 and then activated in H2/Ar at 800 °C. The genesis and properties of the Ce1−xNixO2−x/Al2O3 catalysts were established using X-ray fluorescence analysis, thermal analysis, N2 adsorption, ex situ and in situ X-ray diffraction, Raman spectroscopy, electron microscopy, EDX analysis, and temperature-programmed hydrogen reduction. The performance of Ce1−xNixO2−x/Al2O3 catalysts in biogas conversion was tuned by regulation of the dispersion and reducibility of the active component through variation of content (5–20 wt.%) and composition (x = 0.2, 0.5, 0.8) of Ce1−xNixO2−x as well as the mode of its loading (co-impregnation (CI), citrate sol–gel method (SG)). For the 20 wt.% Ce1−xNixO2−x/Al2O3 catalyst, the rate of the coke formation decreased by a factor of 10 as x increased from 0.2 to 0.8. The optimal catalyst composition (20 wt.% Ce0.2Ni0.8O1.8/80 wt.% Al2O3) and preparation mode (citrate sol–gel method) were determined. At 850 °C, the 20 wt.% Ce0.2Ni0.8O1.8/Al2O3-SG catalyst provides 100% hydrogen yield at full CH4 conversion and 85% CO2 utilization.

Supported Ni2P catalysts derived from nickel phyllosilicate with enhanced hydrodesulfurization performance

Highly dispersed Ni2P catalysts were prepared from nickel phyllosilicate precursor, which was generated on the support by the deposition–precipitation (DP) method. Si-containing materials, including SiO2, MCM-41, S-1, and TS-1, were used as the supports. The strong interaction between Ni and Si species in nickel phyllosilicate effectively suppressed the agglomeration of nickel species. Highly dispersed Ni2P catalyst (Ni2P/SiO2-DP) was obtained by subsequent temperature-programmed hydrogen reduction, and the size of Ni2P crystallites was smaller than that of Ni2P/SiO2-IM prepared from conventional impregnation precursor (7.7 nm vs 14.2 nm). HRTEM observation demonstrated that a large fraction of (201) planes were present in Ni2P/SiO2-DP whereas (111) planes were predominant in Ni2P/SiO2-IM. The strong metal-support interaction and the smaller particle size, along with the higher degree of coordinately unsaturated sites facilitated the hydrogen spillover and the hydrogenation activity. As a result, Ni2P/SiO2-DP exhibited significantly higher performance than Ni2P/SiO2-IM in hydrodesulfurization of 4,6-dimethyldibenzothiophene. Similarly, highly dispersed Ni2P catalysts supported on Si-containing molecular sieves (MCM-41, S-1, and TS-1) were obtained from the in situ generated nickel phyllosilicate as well. It seems that the strong interaction between Ni and Si species in nickel phyllosilicate precursor played a decisive role in the formation of highly dispersed and (201) plane-exposed Ni2P catalysts.

Effect of Preparation Method on the Activity of Fe2O3-NiO/γ-Al2O3 Catalyst in Decomposition of Methane

The effect of method preparation on the activity of Fe2O3-NiO/γ-Al2O3 catalyst was investigated in process decomposition of methane. Fe2O3-NiO/γ-Al2O3 catalyst was prepared by impregnation and solution combustion methods. The samples were characterized by X-ray phase analysis (XRD), temperature-programmed hydrogen reduction (TPR-H2), BET and Raman spectroscopy. It has been shown that the method of preparation plays an important role in regulating the textural and morphological properties of catalysts and provides a difference in their catalytic activity. The synthesis of the Fe2O3-NiO/γ-Al2O3 catalyst by the solution combustion method, in comparison with the capillary impregnation method, leads to the formation of a large amount of FeNi and FeAl2O4 solid solutions, which ensured good catalytic activity at high temperatures. The Fe2O3-NiO/γ-Al2O3 catalyst synthesized by the solution combustion method demonstrated good activity with a hydrogen yield of 52% within 150 min of the reaction without any deactivation. According to the results of Raman spectroscopy, graphene-like carbon was obtained on the surface of the catalysts. On the catalyst of Fe2O3-NiO/γ-Al2O3 (СI) synthesized by capillary impregnation, 4‒5 layer graphene on Fe2O3-NiO/γ-Al2O3 (SC)-6-7 layer graphene is formed.

Investigations of mechanism, surface species and support effects in CO hydrogenation over Rh

The Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates over Rh/ZrO2 and Rh/SiO2 was studied. Catalytic reaction tests show a support effect with a faster steady state reaction rate over Rh/ZrO2 compared to Rh/SiO2. Temperature programmed hydrogenation (TPH) experiments reveal that the CO dissociation on the metal surface is rate limiting, and the support effect thus accelerates the CO dissociation on the metal. Combined TPH and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies after low-temperature CO pre-adsorption reveal a H-assisted C-O bond breakage through CH3O species on the Rh surface. Transient measurements indicate that this mechanism is also likely to be in operation during the steady state reaction at higher temperatures, although here the methoxide is too short-lived to be detected at steady state. This hydrogen-assisted CO activation can help to explain that previous studies have observed an inverse H/D isotope effect for Rh despite CO dissociation being the rate limiting step. TPH studies show that both CO pre-adsorption at 30 ℃ and CO/H2 exposure at 250 ℃ lead to so high CO coverages that it restricts the CO activation, which only starts once part of the CO has desorbed. The near-complete CO coverage on the working Rh surface is restricting the rate, but is essential for the selectivity towards oxygenates. Studies of acetaldehyde conversion in various atmospheres over Rh/SiO2 and Rh/ZrO2 catalysts show that acetaldehyde decomposes over a bare Rh surface with a rate that greatly exceeds the oxygenate formation rate in CO hydrogenation. In the presence of CO the acetaldehyde decomposition is strongly inhibited. The high CO coverage on the surface of the working Rh catalysts thus prevents oxygenate decomposition and is therefore essential for the ability to produce oxygenated products. The reaction temperature is observed to play a role for the establishment of the high coverage. During exposure to CO/H2 at reaction temperatures (>200 ℃) an activated process occurs whereby both the stability and coverage of the CO adlayer increases. This activated stabilization of the CO adlayer shifts the CO activation up in temperature in a subsequent TPH. The results contribute to a fundamental understanding of the reaction mechanism and support effects in the Rh-catalyzed CO hydrogenation, which can assist the formulation of improved Rh-based catalysts. The results, such as the identification of a H-assisted mechanism of CO hydrogenenation via methoxide, could also be of general relevance for the understanding of CO hydrogenation over other metals.

Strong Metal-Support Interactions of Ni-CeO2 Effectively Improve the Performance of a Molten Hydroxide Direct Carbon Fuel Cell.

A strong metal–support interaction (SMSI) type catalyst has been synthesized and applied to a molten hydroxide direct carbon fuel cell (MHDCFC) to enhance the reaction activity of the anode carbon fuel through the interaction between the metal Ni and the support CeO2. Two catalysts have been prepared by a direct precipitation method (denoted NiO@CeO2) and a hydrothermal method (denoted NiO-CeO2), which are reduced by H2 to obtain Ni@CeO2 and Ni-CeO2, respectively. X-ray photoelectron spectroscopy (XPS), Raman, and temperature-programmed hydrogen reduction (H2-TPR) analysis results show that there are obvious oxygen vacancies and a Ni-O-Ce interface structure in NiO-CeO2 and Ni-CeO2, which is induced by the interaction between Ni and CeO2. The calculation results of current density and power density show that the performance of the MHDCFC is significantly improved in the presence of Ni-CeO2. The function fitting curves of the logarithm of the reaction rate constant (ln k) and the reciprocal of the temperature (1/T) show that the slope of the curve is decreased significantly after the addition of Ni-CeO2. In combination with density functional theory (DFT), the anode carbon reaction path is simulated in the MHDCFC, and the calculation results show that the reaction energy for the anodic carbon to generate carbon dioxide is decreased by 1.03 eV in the presence of Ni-CeO2.

Effect of MgO Promoter on the Structure, Performance and Carbon Deposition of Ni‐Al2O3 Catalyst for CO Methanation Based‐on Slurry‐bed Reactor

AbstractA series of Ni/MgO‐Al2O3 catalysts with different MgO abundance were prepared by mechanochemical method for CO methanation based‐on slurry‐bed reactor. The prepared catalysts were characterized by X‐ray diffraction (XRD), H2 temperature‐programmed reduction (H2‐TPR), N2 adsorption‐desorption, thermogravimetric analysis (TG‐DTG) and temperature‐programmed hydrogenation (TPH) techniques, and CO methanation performance evaluation was carried out for the samples. Results showed that the catalyst with MgO loading of 2 wt.% (Ni/MgO(2)‐Al2O3) presented smaller Ni grain size of 9.64 nm and larger specific surface area of 329 m2 ⋅ g−1, indicating that a well dispersion of active component on the catalyst. Evaluation results showed that Ni/MgO(2)‐Al2O3 could improve the activity of catalyst for CO methanation. Carbon deposition analysis, from TG‐DTG characterization, found that MgO could significantly inhibit the formation of carbon deposition in the reaction process. The amount of carbon deposition on Ni/MgO(2)‐Al2O3 was the least, only 6.5 %, and the deposited carbon was consisted with the results obtained from TPH analysis. Kinetic analysis showed that the carbon removal reaction mainly occurred at the stage of 250–550 °C. And the activation energy from carbon removal reaction for Ni/MgO(2)‐Al2O3 was lower to 13.12 kJ/mol, demonstrating that the catalyst with MgO as additive is more beneficial to anti‐carbon ability.

Isotopic Exchange Study on the Kinetics of Fe Carburization and the Mechanism of the Fischer–Tropsch Reaction

The kinetics of the transformation of metallic Fe to the active Fe carbide phase at the start of the Fischer–Tropsch (FT) reaction were studied. The diffusion rates of C atoms going in or out of the lattice were determined using 13C-labeled synthesis gas in combination with measurements of the transient 12C and 13C contents in the carbide by temperature-programmed hydrogenation. In the initial 20 min, C diffuses rapidly into the lattice occupying thermodynamically very stable interstitial sites. The FT reaction starts already during these early stages of carburization. When reaching steady state, the diffusion rates of C in and out of the lattice converge and the FT reaction continues via two parallel reaction mechanisms. It appears that the two outer layers of the Fe carbide are involved in hydrocarbon formation via a slow Mars–Van Krevelen-like reaction contributing to ∼10% of the total activity, while the remainder of the activity stems from a fast Langmuir–Hinshelwood reaction occurring over a minor part of the catalyst surface.

Effect of the Nickel Source on the Structure, Performance, and Carbon Deposition of the Ni/Al2O3 Catalyst for CO2-CH4 Reforming.

Ni/Al2O3 catalysts were prepared with Ni(NO3)2·6H2O, NiSO4·6H2O, NiCl2·6H2O, and NiC4H6O4·4H2O as nickel sources by the solution combustion method. The catalysts were characterized by X-ray diffraction, H2 temperature-programmed hydrogenation, TG-DTG, TPH, and transmission electron microscopy methods, and the effect of the nickel source on performance of the Ni/Al2O3 catalyst was investigated via the CO2–CH4 reforming experiment. Results showed that Ni dispersion, Ni size, and the metal–support interaction between active component Ni and the support were influenced significantly by anion in nickel sources, resulting in that the performance of each catalyst was different. Highly dispersed Ni species, small Ni crystallite size, and strong metal–support interaction were presented in the Ni/Al2O3 catalysts with Ni(NO3)2·6H2O and NiSO4·6H2O as nickel sources. Evaluation results showed that the catalyst prepared with Ni(NO3)2·6H2O exhibited higher activity and stability, with CH4 and CO2 conversions of 31.21 and 48.97%. Carbon deposition analysis demonstrated that the catalyst prepared with NiSO4·6H2O contained more graphite carbon.

Effects of Promoters on the Structure, Performance, and Carbon Deposition of Ni-Al2O3 Catalysts for CO2-CH4 Reforming.

Modified Ni-Al2O3 catalysts with Ca, Co, and Ce species as promoters were prepared by the combustion method, and the structure, morphology, reduction characteristic, and CO2–CH4 reforming of the catalysts were discussed by X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), energy-dispersive X-ray (EDX) mapping, NH3-temperature-programmed desorption (NH3-TPD), N2 adsorption–desorption, thermogravimetric-differential thermal analysis (TG-DTG), and temperature-programmed hydrogenation (TPH) methods. The crystal size of Ni on Ca-Ni-Al2O3 was 16.97 nm, and the active component and additive were distributed well in the catalyst. Co-Ni-Al2O3 presented a surface area of 65.70 m2·g–1 and a pore diameter of 161.60 nm. Ce-Ni-Al2O3 showed relatively stable nickel–aluminum spinel (NiAl2O4), which could not be easily reduced to the active component Ni. Evaluation results demonstrated that the performance of the catalysts followed the order Co-Ni-Al2O3 > Ca-Ni-Al2O3 > Ni-Al2O3 > Ce-Ni-Al2O3. Carbon deposition analysis showed that the carbon resistance of Ca-Ni-Al2O3 was poor and graphitic carbon was generated on the catalyst. However, Ce-Ni-Al2O3 showed less carbon deposition, which might have resulted from the lower activity of the catalyst.

Influence of Ni Precursors on the Structure, Performance, and Carbon Deposition of Ni-Al2O3 Catalysts for CO Methanation.

Three Ni-Al2O3 catalysts were prepared, in planetary ball-milling machine, by the mechanochemical method with Al(NO3)3·9H2O as the aluminum precursor, (NH4)2CO3 as the precipitant, and Ni(NO3)2·6H2O, NiCl2·6H2O, and Ni(CH3COO)2·4H2O as nickel precursors (the corresponding catalysts were labeled as Ni-NO, Ni-Cl, and Ni-Ac). The prepared catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (H2-TPR), and N2 adsorption–desorption technologies, and CO methanation performance evaluation was carried out for the catalysts. Results showed that the catalyst with Ni(NO3)2·6H2O as the precursor presented good Ni dispersibility and a small Ni grain size of 6.80 nm. CO conversion, CH4 selectivity, and yield of the catalyst were as high as 78.8, 87.9, and 69.8%, respectively. Carbon deposition analysis from temperature-programmed hydrogenation (TPH) characterization showed that the H2 consumption peak area of the three samples followed the order: Ni-NO (2886.66 au) < Ni-Cl (4389.97 au) < Ni-Ac (5721.65 au), indicating that the Ni-NO catalyst showed higher resistance to carbon deposition, which might be due to its small Ni grain size.

A low temperature organic synthesis of monodispersed NiRu nanocrystals for CO2 methanation.

In this study, monodispersed NiRu nanocrystals with a diameter of 3 nm were synthesized at 90 °C via a tuning hot-inject method to lower the temperature of the organic phase synthesis of monodispersed nanomaterials. The key factor for the nanocrystalline formation of NiRu alloy nanocrystals was summarized in detail. Simultaneously, the synergistic effect of Ni and Ru in CO2 methanation was explored. Doping trace Ru can significantly improve the conversion rate of CO2 methanation and CH4 selectivity. The underlying mechanism was studied in detail via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed hydrogen reduction (H2-TPR) and desorption (H2-TPD) tests, and temperature-programmed desorption of CO2 (CO2-TPD). This study gives out a new way for the general synthesis of monodisperse nickel-based nanocrystals and provides a reference for the development and application of monodispersed nanoparticles for CO2 methanation.

Fabrication of Ni-Based Bimodal Porous Catalyst for Dry Reforming of Methane

Dry reforming of methane (DRM) can effectively convert two greenhouse gases into high-valued chemicals, in which the syngas produced by the reaction can be directly used as raw gases for Fischer–Tropsch synthesis and methanol synthesis. Ni-based catalysts for the DRM reaction with comparable initial activity to noble metals are the focus of most researchers, but their poor carbon deposition resistance easily causes their low stability. More importantly, the nickel loading will affect the catalytic activity and carbon deposition resistance of the catalyst. Herein, a series of Ni/Al2O3 catalysts with bimodal pores was prepared and characterized by X-ray diffraction (XRD), N2 physical adsorption–desorption, H2-temperature programmed reduction (H2-TPR), temperature programmed hydrogenation (TPH), Raman, and thermogravimetric analysis (TG). The results show that the interesting bimodal structure catalysts could provide a high surface area and contribute to the mass transfer. Besides, the catalytic performance of the DRM reaction is sensitive to nickel loadings. In this study, the Ni/Al2O3 catalyst with nickel loadings of 6% and 8% exhibited excellent catalytic activity and carbon deposition resistance. These findings will provide a new strategy to design a highly efficient and stable heterogeneous catalyst for industry.

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

SSITKA coupled with transient isothermal (TIH) and temperature-programmed hydrogenation (TPH) were used to investigate the influence of Co particle size (dCo = 11–21 nm) and time-on-stream, TOS (up to 25 h) on important kinetic parameters of CO methanation at 230 °C on Co/γ-Al2O3. TOFCO and TOFCH4 were independent of dCo and decreased with TOS. The θCO and θCHx (active CO-s and CHx-s) slightly increased with dCo as opposed to θCxHy (inactive species), which remained practically constant. On the contrary, θCHx and θCxHy largely decreased and increased, respectively, while θCO stayed practically constant with TOS. An inverse D-KIE independent of dCo was estimated by a novel 13CO/D2-SSITKA experiment, reported for the first time to the best of our knowledge. The reason of catalyst deactivation was the decrease of kCHx (hydrogenation of -CHx) likely because of the influence of -CxHy on the bonding of active species and on θH (a decrease was determined).

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.

INFLUENCE OF MANGANESE ON FORMATION OF ACTIVE PHASE OF COBALT CATALYST OF HYDROCARBON SYNTHESIS

The active phase and catalytic properties of cobalt catalysts of Fischer-Tropsch synthesis promoted with manganese and prepared by impregnation were studied. KSKG silica gel having a monodisperse structure with an average pore diameter of 12–16 nm was used as a support. The cobalt content in the obtained samples was 20.8-21.8 wt.%, manganese content was 0.5–2 wt.%. The structural characteristics of the catalysts were investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and temperature-programmed hydrogen desorption. The catalytic activity and selectivity of the samples in the process of hydrocarbon synthesis were researched in the flow mode at a pressure of 0.1 MPa, at a Gas Hourly Space Velocity (GHSV) of 100 h–1, at the temperatures of 150–220 °C and a ratio of CO to hydrogen in synthesis gas of 2:1. Preliminary reduction of catalysts was carried out with hydrogen at a temperature of 400 °C. In the composition of the studied samples with manganese the formation of solid solutions of metals such as xCo3O4–(1–x)Mn3O4 was confirmed. It is shown that the reduction process of Co3O4 is determined by the composition and structure of solid solutions of components that are hard-to-recover compounds. The features of the promoting action of manganese depending on the concentration are determined. The introduction of 0.5–2 wt.% manganese allows to regulate the composition and microstructure of cobalt oxide compounds, the properties of metallic Co, increases the catalytic characteristics: activity, selectivity for C5+ and reduces the yield of methane. It is shown how, depending on the content of the metal additive, the manifestations of the structural (at a concentration of 0–1 wt.%) or electronic (at a concentration of 2 wt.%) promoting action of manganese increase. The optimum concentration of manganese is 1 wt.%. The catalyst is characterized by the minimum average size of crystallites, the highest value of the specific surface of cobalt metal and the maximum catalytic indicators.

Effect of the promoters on oxidation behavior of Fe-based Fischer-Tropsch catalyst: Deciphering the role of H2O

The effect of carburization and reduction degree on H2O oxidation behaviour for the iron carbides in Fe-based FTS catalyst were firstly investigated using a combination method including X-ray diffraction (XRD), Raman and temperature-programmed-hydrogenation (TPH). The relationship between carbon species transformation and H2O oxidation behaviour of iron carbides was investigated simultaneously. Based on these observations, the influence of typical promoters like K and SiO2 on the structure and H2O oxidation behaviour of Fe-based FTS catalysts was further studied. The results indicated that, for the iron catalyst, the stability of iron carbides against H2O oxidation was increased with the increase of iron carbides content, and the H2O oxidation process led to the formation of more graphitic carbon. The carburization ability was effectively enhanced when certain amount of K promoter was incorporated. Addition of K into Fe-based FTS catalyst increased the number of graphitic carbons, which increased the stability of iron carbides toward H2O oxidation ultimately. It was also found that promotion of SiO2 greatly degraded the carburization degree of the catalyst, while their tendency to be oxidized to form Fe3O4 in the H2O atmosphere was obviously hindered.

CO2 methanation on Mg-promoted Fe catalysts

This paper deals with the hydrogenation of CO2 to CH4 and H2O on Fe catalysts. An extensive screening of various promoters for an α-Fe2O3 precursor was carried out providing Mg as most effective. Thus, a systematic series of Mg/Fe2O3 catalysts with different Mg contents was prepared and thoroughly characterized by X-ray diffraction, N2 physisorption, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Additionally, the carbonaceous species formed in CO2 hydrogenation were analyzed by temperature-programmed hydrogenation. From characterization and catalytic results, a structure-activity correlation was derived showing that the methanation was predominately referred to surface carbon and Fe carbide species. Consequently, the catalyst with a Mg load of 2 wt.% exhibited the highest CH4 production.Finally, the best catalyst was evaluated under practical operation conditions (8 bar, 10,000 h−1). In these tests, CH4 was formed as the major product revealing highest yield of 32% (400 °C).

The effect of preparation route of commercial Co/γ-Al2O3 catalyst on important Fischer-Tropsch kinetic parameters studied by SSITKA and CO-DRIFTS transient hydrogenation techniques

Two commercial Co-based catalysts supported on carbon-coated and uncoated γ-Al2O3 (during Co deposition) of different Co particle size (∼7 and 10 nm) and surface structure were investigated to address the effect of preparation route on important kinetic parameters of the methanation reaction (5 vol% CO, H2/CO = 2, P = 1.1 bar) by advanced transient isotopic and operando DRIFTS experiments. 13CO-SSITKA-Mass Spectrometry experiments performed at 230 °C and after 32 h on TOS, showed that the surface coverage (θCO) and mean residence time (τCO, s) of reversibly adsorbed CO-s are not influenced by the different preparation route, while θCHx, TOFCH4 (s−1) and the τCHx (s) related to the active carbonaceous species were slightly increased with increasing Co particle size (in the 7–10 nm range). On the other hand, the kinetic rates of CO conversion and CH4 production (μmol g−1 s−1) were found to increase with decreasing Co particle size. The TOFCH4,ITK (s−1) estimated on the basis of the measured active CHx-s and CO-s species was found very similar for the two Co particle sizes. The effective rate constant of hydrogenation of CHx-s (keff, s−1) after short time on stream (1 h) was found to increase with increasing Co particle size as the result of the different preparation route applied. Transient isothermal hydrogenation (TIH) experiments following the 13CO-SSITKA-MS gas switch measured the concentration of inactive CHx (Cβ) species formed after 32 h of reaction and which were found to be readily hydrogenated at 230 °C. Temperature-programmed hydrogenation (TPH) experiments estimated the concentration of other types of inactive carbonaceous species (Cγ), which were hydrogenated at elevated temperatures (250–600 °C). The amounts of Cβ and Cγ species were found to significantly increase with increasing Co particle size, and their kinetics of hydrogenation was dependent of Co particle size. These results provided evidence for the lower deactivation rate observed under industrial FTS conditions over the Co/γ-Al2O3 (carbon-coated) catalyst. Operando SSITKA-MS-DRIFTS studies followed by transient isothermal hydrogenation coupled with kinetic modeling highlighted the influence of Co particle size on the relative reactivity (keff) of two different types of adsorbed CO-s towards hydrogen. Catalyst characterization was performed by HAADF-STEM, H2-TPR and transient hydrogen chemisorption at 100 °C followed by TPD.

Water-saving dry methanation for direct conversion of syngas to synthetic natural gas over robust Ni0.1Mg0.9Al2O4 catalyst

Production of synthetic natural gas (SNG) from coal is typically water intensive due to the need of increasing H2/CO ratio from 1 to 3 via water gas shift reaction. Here we report a water-saving dry methanation reaction that allows direct conversion of low H2 content syngas (H2/CO = 1) to methane without consumption of water for adjusting H2/CO to ≥ 3 and without production of waste water. Conventional Ni/Al2O3 catalyst is active for the dry methanation reaction but deactivates very rapidly due to carbon deposition and metal sintering. A Ni0.1Mg0.9Al2O4 catalyst exhibits much higher activity and better stability during steady state catalytic reaction and cyclic reaction-regeneration operations. Using a suite of characterization tools including BET, XRD, STEM, TEM, H2-TPR, Raman, TG-DSC, CO2 and NH3 TPD, and temperature-programmed hydrogenation, we reveal that carbon deposition did not occur on the surface of Ni particles in Ni0.1Mg0.9Al2O4 catalyst. Moreover, the population of small Ni nanoparticles (ca. 2 nm) increased after multiple reaction-regeneration operations possibly due to Ni redispersing via reversible diffusing among the framework of spinel. The exposure of active Ni surface and the formation of small-sized Ni nanoparticles together contributed to the stable and improved activity for dry methanation during the reaction-regeneration operations.