NiMgAl Catalysts Research Articles

Hydrogen production by aqueous phase reforming over stable La-promoted Ni-based hydrotalcite catalysts

Methanol, a common product of biomass pyrolysis or gasification, contains a high hydrogen content and serves as a valuable feedstock for hydrogen production through catalytic aqueous phase reforming (APR). However, poor hydrothermal stability of metal oxides support of the APR catalyst in liquid phase presents a challenge for the continuous and effective production of H2 over Ni-based hydrotalcite catalysts. A series of La-promoted NiMgAl hydrotalcite catalysts were prepared and their hydrogen production performances during methanol APR were evaluated. Promoted 5La–NiMgAl catalyst shows superior methanol APR reactivity due to the higher concentration of medium-strength basic sites than unpromoted catalyst. The La species adsorbs CO2 to form LaCO3OH and maintains La–NiMgAl catalyst structural integrity, which limits Ni leaching during reaction and promotes catalyst stability.

Roles of metal oxide overlayers on hydrotalcite-structured Ni-MgAlOx for dry reforming of CH4 with CO2

The thermal stability of Ni nanoparticles on the coprecipitated hydrotalcite-like Ni-MgAl2O4 (NiMgAl) was enhanced by simply introducing an overlayer coating method with four different metal oxides (M) such as CeO2, ZrO2, SiO2 and Al2O3 (NiMgAl@M) for a harsh dry reforming reaction of CH4 with CO2 (DRM). Among the catalysts, the superior catalytic properties on the NiMgAl@Al such as less sintering and coke deposition were originated from the much stronger metal support interaction (SMSI), which were originated from the formed spatially confined Ni nanoparticles on the MgAl2O4 frameworks through the possible exsolution phenomena with the help of the porous Al2O3 overlayers. The unmodified NiMgAl catalyst revealed a relatively lower catalytic activity due to the facile coke depositions, which was mainly caused by the selective formation of larger Ni nanoparticles above 25 nm with its heterogeneous size distribution. The much simpler overlayer coating method with the porous overlayer structures, especially for Al2O3 coating layer, was found to be an effective way to achieve a superior catalytic activity and stability by preserving the thermally stable smaller Ni nanoparticles on the hydrotalcite-like Ni-MgAl2O4 surfaces.

Role of phase in NiMgAl mixed oxide catalysts for CO2 dry methane reforming (DRM)

Supported Ni catalysts are considered promising for dry methane reforming (DRM) because of their low cost and high activity. However, they suffer from coke formation at elevated reaction temperatures, leading to rapid catalyst deactivation. Here, we developed a synthesis route for highly active and stable nickel-based catalysts for DRM by changing the Mg/Al ratio. Manipulation of the Mg/Al ratio in NiMgAl catalysts induces a change in the phases present, which subsequently leads to enhancement in activity and stability. The highest activity and stability were observed when both spinel and periclase phases were present in the catalysts at Mg/Al ratio of 1.5 (Ni-Mg1.5AlOx). Based on X-ray diffraction (XRD), H2 temperature-programmed reduction (TPR), chemisorption, and UV-Vis-NIR characterizations, as well as CO2 temperature-programmed desorption (TPD), we found that the co-existence of spinel and periclase phases is associated with changes in Ni reducibility, basicity, and the location of Ni2+ species. This work demonstrates how a change in phase in catalysts influences key factors determining activity and stability for the design of active and stable catalysts.

Improving robustness of kinetic models for steam reforming based on artificial neural networks and ab initio calculations

• Kinetic modeling for steam reforming of naphtha surrogates on NiMgAl catalyst. • The model discrimination was conducted based on artificial neural networks. • Ab initio (DFT) calculations allow further model refinements and corroborations. • The selected kinetic model satisfactorily captures catalyst activity and selectivity. • Kinetic model was validated for a wider set of operating conditions and feedstocks. Steam reforming of hydrocarbons is and will be, in the short-medium run, the leading technology for producing hydrogen. At the same time, steam reforming has a large carbon footprint that can be decreased by implementing better kinetic models for process intensification. In this work, we have developed a methodology based on artificial neural networks to fit and improve the robustness of the kinetic model for steam reforming of naphtha surrogates (hexane and heptane) using a NiMgAl catalyst derived from hydrotalcite precursors. We analyze several strategies to obtain the fittest kinetic model and discuss the robustness of each. These models include hydrocarbon steam reforming, water gas shift and methanation reactions, and differ mainly in the type of adsorption term in the Langmuir-Hinshelwood formalism. The adsorption energies calculated by ab initio (DFT) provide insights on the different adsorption mechanisms of hydrocarbons and water on the catalyst surface sites. At the same time, validation of the kinetic model was conducted using wider range of experimental conditions and different model and real feeds (methane, naphtha, diesel and vegetable oil). In this way, the versatility of the model proposed and the strengths and weaknesses of using a data-driven approach for the kinetic model selection were proven.

Highly stable and coke-resistant Zn-modified Ni-Mg-Al hydrotalcite derived catalyst for dry reforming of methane: Synergistic effect of Ni and Zn

Considering Zinc’s basic nature and its propensity to alloy with transition metals, the improvement in the activity and carbon deposition-resistance of hydrotalcite-derived NiMgAl for the dry reforming of methane (DRM) is investigated with various Zn loadings. The prepared catalysts are characterized by various spectrometric, microscopic, and temperature-programmed techniques. Compared to the unmodified catalyst, the NiMgAl catalyst modified containing 3 wt% Zn exhibited excellent DRM activity and stability for 100 h on stream without any observable deactivation. Characterization results indicate that adding Zn to NiMgAl significantly increases the metal-support interaction, stabilizes smaller Ni particles, and minimizes carbon nanotube (CNT) growth. These advantages are a result of enhanced catalyst basicity and Ni-Zn alloy formation. Further analysis via temperature-programmed surface reaction with CH4 indicates a decrease in CH4 decomposition rate and relatively facile carbon gasification, thus increasing resistance to deactivation by coking.

Enhanced low-temperature catalytic performance in CO2 hydrogenation over Mn-promoted NiMgAl catalysts derived from quaternary hydrotalcite-like compounds

Mn-promoted NiMgAl mixed-oxide (NiMnx-LDO, x = 0, 5, 10, 15) catalysts derived from hydrotalcite were synthesized using co-precipitation for CO2 hydrogenation to synthetic natural gas. By regulating Mn contents, NiMn5-LDO delivered the most excellent catalytic performance, being about 2 times higher than that of undoped NiMn0-LDO catalyst (TOF of NiMn5-LDO and NiMn0-LDO: 0.61 s−1 vs 0.31 s−1 @ 240 °C). Through extensive characterization, it was found that Mn dopants promoted the reduction of bulk NiO through tuning the interaction between Ni and Mg(Mn)AlOx support. A high surface ratio of Ni0/Ni2+ was achieved over NiMn5-LDO. Furthermore, the surface basicity strength was tailored by Mn dopants. With 5 wt% of Mn, NiMn5-LDO catalyst showed a stronger medium-strength basicity and higher capacity of CO2 adsorption than others. Particularly, TOF indicates a good correlation with medium-strength basicity over NiMnx-LDO catalysts. The strong metal-support interaction originated from the hydrotalcite structure kept nickel uniformly dispersed, endowing to the improved catalytic performance.

Anti-coking freeze-dried NiMgAl catalysts for dry and steam reforming of methane

Finding supported nickel catalysts with high activity and stability is yet a challenging aim for industrial applications. In this work, we synthesized a surface defect-promoted Ni catalyst supported on Mg/Al hydrotalcite via a freeze-dried method instead of calcination. This approach leads to the increase in oxygen vacancies, which is attributed to the high dispersion of active sites after adding samarium. X-ray diffraction (XRD) measurements demonstrate a homogeneous layered double hydroxide (LDH) structure without the formation of any oxides. High-resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy images (FE-SEM) illustrated that the samarium-promoted NiMgAl catalyst possesses a scaffold structure with surface defects and oxygen vacancies compared to the unpromoted NiMgAl catalyst which confirmed by X-ray photoelectron spectroscopy (XPS). Moreover, the impact of the samarium incorporation on the physicochemical features of NiMgAl catalysts was investigated for catalytic activity in dry and steam reforming of methane at 700 °C. NiMgAl-Sm catalyst showed the highest conversion of CH4 (72%) and stability without any carbon formation during 20 h of time on stream in dry reforming process. because the strong metal–support interaction inhibits the sintering of nanocatalysts at 700 °C and the scaffold structure increases the mass transportation of feedstock and products.

Measurement of elastic modulus of CNT composites: a nondestructive study

CNT based nanocomposites are known to have superior mechanical properties, such as elastic modulus (E), tensile strength (σ), etc., compared to other nanocomposite materials. Traditionally E of the nanocomposites is measured by direct methods like the three-point bend test, four-point bend test, etc., which are destructive. However, it is possible to determine their E value by an indirect method, namely Raman spectroscopy, which is nondestructive. In this paper, four different CNT composites have been prepared, such as CNT-SiO2, CNT-NiO, CNT-NiMgAl, and CNT-NiOFe by pyrolyzing waste plastic with SiO2, NiO, NiMgAl, and NiOFe catalysts, respectively. The prepared CNTs are multiwall types with diameters and the number of walls are in the range of 4.88–108 nm and 5–133, respectively. The E of the prepared CNT composites have been determined using Raman spectra and found to be in the range of 4.99-28.63 GPa, which are within the range of other reported CNT composites (∼1.5-95.40 GPa). The present work will help in the study of the measurement of mechanical properties of CNT based nanocomposites by nondestructive methods.

Promotion of CO2 methanation at low temperature over hydrotalcite-derived catalysts-effect of the tunable metal species and basicity

The CO2 methanation is an effective strategy for making full use of waste gases and converting them into valuable chemicals. Nowadays, the key challenge is the design of efficient catalysts to enhance low-temperature catalytic performance. A series of hydrotalcite-derived catalysts with tunable metal species were prepared by hydrothermal synthesis method for CO2 methanation at low-temperature. It was found that suitable synergistic effect between metallic Ni and basic sites in the support could be achieved by regulating the metal composition of hydrotalcite-derived catalysts. The NiMgAl catalyst exhibited the highest catalytic activity with CO2 conversion of 91.8% at 250 °C. The in situ DRIFTS measurements and DFT calculation further revealed that the alkaline metal oxide MgO and more Ni(111) active sites in the NiMgAl catalyst could promote the activation of CO2 and the formation of active intermediates, which actively participated in improving low-temperature activity. Therefore, the ternary mixed oxides catalyst derived from LDHs precursors shows a potential strategy in achieving excellent catalytic properties for CO2 methanation at low-temperature.

Nickel-magnesium mixed oxide catalyst for low temperature methane oxidation

A series of nickel-magnesium mixed oxides (NiMg) of varying molar ratios, Ni9Mg, Ni2Mg, NiMg, NiMg2 and NiMg9, were synthesized using the co-precipitation technique. These materials were prepared as possible lower cost alternatives to Pd-based methane oxidation catalysts. The NiMg catalysts were characterized by means of X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR). Their activity for complete methane oxidation was investigated using temperature programmed oxidation. Ni9Mg was found to be the most active catalyst of the series. Characterization results showed that most of the materials formed solid solutions of the NixMg1-xO type. The activity of the Ni9Mg was attributed to its highest surface area and highest oxygen mobility. It was further tested and aged in simulated natural gas engine exhaust that contained 10 vol% water vapor. The results were compared to a reference 1 wt% Pd/Al2O3 methane oxidation catalyst tested under the same conditions. Ni9Mg exhibited approximately 94% methane conversion after 40 h of aging which was higher than that of the Pd-based reference catalyst with methane conversion below 80% after only 16 h of aging. Ni9Mg is thus proposed as a promising alternative to PGM-based catalysts for complete methane oxidation.

Stability and activity of a co-precipitated Mg promoted Ni/Al2O3 catalyst for supercritical water gasification of biomass

We have investigated the stability and activity of a co-precipitated Mg promoted Ni/Al2O3 catalyst (Ni-Mg-Al) for supercritical water gasification (SCWG) of various biomass model compounds and real biomass. Phase stability and activity recovery of the Ni-Mg-Al catalyst were first compared with a catalyst prepared by impregnation method. It was found that the co-participated catalyst showed higher activity recoveries than the impregnated catalyst due to the stable Ni crystal size. Then, effects of SCWG variables including heating up rate, gasification temperature, catalyst loading amount and feedstock concentration, on the non-catalytic and catalytic gas yields and gasification efficiencies of glucose and phenol were evaluated. Results demonstrated that the presence of sufficient amount of Ni catalyst could realize complete carbon gasification of different organics, including phenol and real biomass. Catalyzed by Ni, CH4 was the more favored produced gas at 400–500 °C while H2 yields were more abundant at 500–600 °C. Without catalyst, carbon gasification efficiencies of SCWG of different feedstock were in the order: glycerol > glucose > cellulose ≈ corncob ≈ poplar leaf ≈ sawdust > phenol, while those catalyzed by Ni were in the order: glycerol ≈ glucose ≈ cellulose ≈ phenol > corncob ≈ poplar leaf ≈ sawdust, illustrating that the co-precipitated NiMgAl catalyst is more active on catalyzing the gasification of water-soluble organics than real biomass.

Effect of impregnation sequence of Mg on performance of mesoporous alumina supported Ni catalyst in dry reforming of methane

In this study, effect of Mg impregnation sequence on the activity of the mesoporous alumina supported Ni catalysts was investigated in dry reforming of methane. Characterization and activity test results showed that Mg incorporation sequence significantly influenced the physicochemical properties and the activity of the catalyst as well as coke deposition during reforming reaction. The synthesized catalysts were characterized by x-ray diffraction, N2 adsorption, temperature programmed reduction, scanning electron microscopy, CO2-temperature programmed desorption, inductively coupled plasma optical emission spectrometry, x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and pyridine adsorbed diffuse reflectance FTIR spectroscopy techniques. Mg incorporation altered the reduction profile of the monometallic catalyst and increased the reduction temperature of the nickel particles. XRD diffraction peaks corresponding to γ-A2O3 and α-Al2O3, as well as nickel-magnesium spinels and metallic Ni were observed depending on Mg incorporation sequence. The monometallic Ni catalyst showed higher activity than the bimetallic NiMg catalysts. However, coke formation was significantly influenced as a result of synthesis route. Total organic carbon, thermogravimetric analysis and SEM images exhibited that the highest coke formation was obtained over the catalyst which was prepared by sequential impregnation of Mg and then Ni. Almost no coke formation was observed on the spent catalyst, which was synthesized by simultaneously impregnation of Mg and Ni, due to the high interaction between Ni and Mg with the formation of a NiOMgO solid solution during the high calcination temperature.

Insight into the phase evolution of a NiMgAl catalyst from the reduction stage to the post-reaction stage during the dry reforming of methane.

Herein, phase evolution of a NiMgAl oxide catalyst at the reduction stage was qualitatively analysed and quantitatively determined by employing the continuous changes in its XRD intensity and TPR information. The stable crystallite size of both the active metal and spinel support was responsible for the long stability of the NiMgAl catalyst without carbon deposition during the DRM reaction.

CO2 reforming of CH4 over a highly active and stable Ni[sbnd]Mg[sbnd]Al catalyst

Although the catalytic systems containing the NiOMgO solid solution possess excellent catalytic performance in CO2 reforming of CH4 reaction, a serious drawback was that the higher than 10 wt% Ni loadings would increase the burden on the environment and finance. A NiMgAl catalyst with 3 wt% Ni loadings derived from a hydrotalcite precursor was prepared in order to study the CO2 reforming of CH4 reaction. For comparative purpose, a NiOMgO catalyst and a Ni/γ-Al2O3 catalyst containing NiAl2O4, were also synthesized. Analysis showed that the NiMgAl catalysts had high catalytic activity due to the improved reducibility which was caused by the presence of Al3+ ions in the NiOMgO cubic lattice. Moreover, the NiMgAl catalyst showed ultra-stability without carbon formation for 320 h period due to the small Ni particles and the suitable Ni-support interaction which effectively prevented the sintering of Ni particles.

Promotional effects of rare earth elements (Sc, Y, Ce, and Pr) on NiMgAl catalysts for dry reforming of methane

The promoting effects of rare earth elements on NiMgAl catalysts for dry reforming of methane were clarified.

Ni-based catalyst derived from Ni/Mg/Al hydrotalcite-like compounds and its activity in the methanation of carbon monoxide

The supported Ni-based catalyst is widely used in the methanation process. Nevertheless, the major disadvantages of this catalyst are a poor behavior in the water-gas-shift (WGS) reaction and the deactivation at higher temperatures. A new kind of catalyst, nickel-containing oxides catalyst (NiMgAl), obtained from thermal treatment of hydrotalcite-like compounds (HTlcs) was prepared using the co-precipitation method. The performance of this catalyst was systematically investigated and compared with that of the Ni/Al2O3 catalyst. It was found that the NiMgAl catalyst shows an enhanced methanation activity compared to that of the Ni/Al2O3 catalyst and the former catalyst shows a better performance for the methanation especially at temperature over 550°C. Three NiMgAl catalysts with different nickel content were prepared and tested in the methanation operated at a GHSV of 15000 h−1 and n(H2)/n(CO) of 1.5. The results indicate that with the NiMg8 catalyst a higher activity and stability could be achieved than with the NiMg5 and NiMg6 samples, the effect mainly attributed to a higher extent of Ni dispersion was confirmed by XRD results.

Effects of Promoter on NiMgAl Catalyst Structure and Performance for Carbon Dioxide Reforming of Methane

Catalysts were prepared by adding different types of promoter (Co, Ir, or Pt) to the supported nickel catalyst NiMgAl samples. These catalysts were characterized by H2 temperature-programmed reduction (H2- TPR), CO2/CH4 temperature-programmed surface reactions (CO2/CH4-TPSR), and CO2 temperature- programmed desorption (CO2-TPD). The effects of the catalyst structure on catalytic performance in the methane dry reforming reaction with carbon dioxide were investigated. The addition of a small amount of promoter (Pt or Ir) can lower the reduction temperature of the nickel active component, and enhance performance in the methane dry reforming reaction. The catalysts with Co or Ir promoter feature lower activation energies than the unmodified NiMgAl catalyst. The activation energy was 51.8 kJ∙mol - 1 for the NiMgAl sample, decreasing to 26.4 kJ∙mol - 1 for the NiPtMgAl catalyst, which showed overall better catalytic performance. Results of CH4-TPSR and CO2-TPSR demonstrate that the NiPtMgAl catalyst can generate more active carbon species on the catalyst surface. The CO2-TPD results show that adding a promoter can increase the CO2 adsorbed/desorbed amount compared with the unmodified NiMgAl catalyst over the same reaction temperature range.

Reforming of a model sulfur-free biogas on Ni catalysts supported on Mg(Al)O derived from hydrotalcite precursors: Effect of La and Rh addition

Ni catalysts supported on calcined Mg–Al hydrotalcite, Mg(Al)O, were prepared and the effect of the addition of La and/or Rh was tested in the performance of the catalysts in the dry reforming of methane with excess of methane in the feed, simulating a model sulfur-free biogas. The effect of adding synthetic air was assessed. The catalysts were characterized by surface area (BET), XRD, TPR and XPD. The results showed the reconstruction of the hydrotalcite structure during the Ni(NO3) impregnation, with the segregation of the lanthanum. In the catalyst without Rh and La, Ni showed a strong interaction with the support Mg(Al)O, showing high reduction temperatures in TPR test. The addition of Rh and La increased the amount of reducible Ni species and facilitated the reduction of the species interacting strongly with the support which resulted in high rates of carbon deposition. The NiMgAl catalyst presented the strong Ni-support interactions and the best performance with low carbon deposition at both conditions of reaction. The NiMgAl catalyst did not present deactivation during 24 h of stability testing in the oxidative reforming of a model biogas.

Promoting effect of Ce and Mg cations in Ni/Al catalysts prepared from hydrotalcites for the dry reforming of methane

Several catalytic systems containing Ni/Mg/Al/Ce were synthesized from nitrates of Ni2+, Mg2+, Al3+ and Ce3+ cations with M2+/M3+ = 2 ratios by means of the carbonate co-precipitation method and subsequent calcination at 800 °C. Atomic absorption spectroscopy, X-ray diffraction (XRD), FT-IR spectroscopy, BET, temperature programmed reduction and scanning electron microscopy were used in order to describe the structural, morphological and surface characteristics of the solids completely. The effect of substitution/incorporation of Al by Ce and/or Mg on NiAl sample was studied. XRD analyses confirm that on Al-containing samples (NiAl, NiMgAl), the formation of the precursors layered double hydroxide structure. On the other hand, on cerium containing samples (NiCe, NiMgCe), poorly resolved diffractograms were observed what can be explained by the large radius of cerium. The catalysts were evaluated in the reaction of CO2 reforming of methane at 750 °C. NiCe and NiMgAl catalysts exhibit higher activity and a H2/CO ratio of almost 1. NiAl and NiMgCe samples showed lower conversions and a CH4/CO2 ratio <1, indicating the occurrence of reverse water gas shift reaction.

Stability of Ni and Rh–Ni catalysts derived from hydrotalcite-like precursors for the partial oxidation of methane

In this work, NiMgAl and RhNiMgAl catalysts prepared from HTLCs precursors were investigated for the Partial Oxidation of Methane (POM) at 550 and 750 °C. Samples have been characterized by XRD, TPR, H2 chemisorption, TPSR analyses, XPS, field emission scanning electron microscopy and Raman spectroscopy. NiMgAl catalysts with high Ni content (40 and 16 wt%) showed high stability and high methane conversion for POM. On the other hand those with lower Ni content (NiHT15 and NiHT25, with 6 and 4 wt%) exhibited low catalytic activity with low H2/CO ratio (<2) and fast deactivation. In RhNiHT25 (0.6 wt. % Rh), the Ni reducibility was improved, increasing the methane conversion and hydrogen selectivity. In addition, the noticeable increase in stability was related to the absence of carbon deposition after 30 h on stream at 550 °C. These results show that RhNiHT25 is promising for application in membrane reactors to produce high purity hydrogen.