Ni–Fe/Mg(Al)O alloy catalyst for carbon dioxide reforming of methane: Influence of reduction temperature and Ni–Fe alloying on coking

Various Ni–Fe/Mg(Al)O alloy catalysts were obtained by calcination of Ni–Fe–Mg–Al hydrotalcite-like compounds, followed by reduction at different temperatures (973–1173 K). The characterizations of XRD and STEM-EDX suggest that the resulting Ni–Fe alloy particles are composition-uniform and size-controllable. The alloy composition is little affected by the reduction temperature, whereas the particle size (5.8–8.2 nm) increases with the increase of reduction temperature. This property is ascribed to the homogeneous distribution of nickel and iron species during the catalyst preparation. All of the Ni–Fe/Mg(Al)O alloy catalysts show relatively high and stable activity for CH₄–CO₂ reforming during 25 h of investigation at 773–1073 K. Particularly, the 973 K-reduced catalyst exhibits higher coke-resistance due to its smaller particle size. E_a-CH₄ and CH₄-TPSR measurements indicate that Ni–Fe alloying inhibits CH₄ dissociation. It is considered that during DRM CH₄ is dissociated at the Ni sites and CO₂ may be activated at the metal-support interface as well as the Fe sites. Ni–Fe alloying may inhibit CH₄ dissociation and/or promote CO₂ activation, thus contributing to the suppression of coke deposition.     

Dry reforming of methane to hydrogen-rich syngas over robust fibrous KCC-1 stabilized nickel catalyst with high activity and coke resistance

A remarkably stable and active nickel (Ni) catalyst, supported on fibrous KCC-1 silica particles, was prepared by in situ one-pot hydrothermal method to produce a hydrogen-rich syngas. The catalysts’ properties were characterized by BET, XRD, FESEM-mapping, FTIR-pyrrole, FTIR-KBr, and XPS, while coke deposition was evaluated using Raman spectra, TEM and TGA/DTA. The high dispersion of Ni crystallites, enhanced basicity, strong Ni-KCC-1 interaction, and encapsulation of Ni particles contributed to the enhanced catalyst stability and activity. The one-pot catalyst produced high CH₄ and CO₂ conversions at 92% and 88% respectively, with high H₂/CO ratio, and an extended stability over 72 h at 750 °C. There was limited coke deposition, predominantly of the amorphous type, owing to its synthesis method and support morphology.     

The influence of shell thickness on coke resistance of core-shell catalyst in CO2 catalytic reforming of biomass tar

The development of catalysts with resistance to sintering and coke deposition is the key to tar cracking during biomass gasification technology. In this work, the core-shell catalysts with mesoporous and microporous silica-coated nickel nanoparticles were prepared for the CO₂ reforming of toluene as a model compound for biomass tar. The influence of thickness of SiO₂ shell layer on catalyst activity, coke and sintering resistance of the catalysts in the CO₂ reforming of toluene was explored. Appropriate increasing in the thickness of the silica shell can significantly increase the specific surface area, pore volume and the interaction between core and shell of the catalysts, which can further improve the reactivity and coke resistance ability. However, excessive increase in shell thickness can lead to a drastic decrease in the specific surface area and pore volume of the catalyst, resulting in significant coke deposition. The Ni@SiO₂-4 catalyst showed the highest catalytic activity of toluene conversion of around 50% within 300 min, stability H₂/CO ratio of 0.25～0.3 and durability of 26 h lifetime in the CO₂ reforming of toluene. Overall, the optimization of the silica shell thickness can improve the reactivity and coke resistance ability of Ni@SiO₂.     

EDTA impregnation assisted synthesis of nickel nanoparticles embedded in activated carbon and its application for dry reforming of methane

In this paper, the catalysts with nanoscale nickel particles embedded in activated carbon were synthesized by Ethylenediaminetetraacetic acid (EDTA) assisted impregnation method. The experimental results revealed that EDTA addition could promote Ni particle dispersion and decrease Ni particle size. Ni particle size was significantly decreased from 18.23 to 4.57 nm when Ni/EDTA ratio was increased from 1:0 to 1:3. Catalytic performance of Ni₁₅-AC-E₃ demonstrated that CH₄ and CO₂ conversion rate under 850 °C were 90.05% and 96.28%, which were decreased by 1.08% and 1.27% after 72 h dry reforming methane (DRM) reaction. Moreover, the characteristic of the used catalyst showed that Ni particle size was only increased from 4.57 to 5.94 nm after the stability test, and NiO was not appeared. Meanwhile, the deposited carbon in the used catalyst was 5.02%. The above results meant that the catalyst prepared by EDTA assisted impregnation method had the ability of suppressing carbon deposition, Ni particle sintering and oxidation during DRM reaction.     

Upgrading biogas into syngas via bi-reforming of model biogas over ruthenium-based nano-catalysts synthesized via mechanochemical method

The upgrading of biogas to value-added syngas via reforming of biogas is of great significance from the perspective of green carbon science. A series of Ru based nano-catalysts (Ru/MgO, Ru/La₂O₂CO₃, Ru/CeO₂) with relatively low Ru loading and high dispersion were successfully prepared by mechanochemical method, and applied in the bi-reforming of model biogas reaction. The representative catalysts were comprehensively characterized by XRD, N₂ physical adsorption, ICP-OES, SEM, XPS, H₂-TPR, CO₂-TPD, HRTEM, FTIR of CO adsorption, TPSR, TG, and Raman spectroscopy, etc. Ascribed to the appropriate basicity of MgO, homogeneous Ru dispersion with ultra-small particle size, and strong interaction between Ru ultra-small nano particles and MgO support, Ru/MgO exhibit higher CO₂ adsorption and activation, higher CH₄ activation and dissociation, compared with Ru/La₂O₂CO₃, Ru/CeO₂, which facilitate the formation of active oxygen species and intermediate coke removal, thus enhance the resistance to coke deposition and sintering. At reaction conditions of T = 750 °C and weight hourly space velocity = 32,400 mLg_Cat⁻¹ h⁻¹, 0.5Ru/MgO catalyst exhibit satisfying catalytic activity (initial CH₄/CO₂ conversation rate of 87/48%) and superior stability (stable for 150 h) with negligible deactivation rates.     

Biogas dry reforming over Ni-Al catalyst: Suppression of carbon deposition by catalyst preparation and activation

Biogas dry reforming is as an alternative renewable route for the hydrogen production. However, the major drawback of this process is the catalyst deactivation by carbon deposition and sintering. In this work, Ni-Al catalysts were studied aiming to suppress the carbon deposition in the dry reforming of biogas. The catalysts were prepared by coprecipitation and evaluated the washing step. The reactions were carried out with unreduced and reduced catalysts in a fixed bed tubular reactor using a synthetic biogas (60% CH₄ and 40% CO₂). The washing and activation steps influenced the characteristics of the catalysts and the catalytic properties in the biogas reforming. The unwashed sample resulted in an oxide containing potassium nickelate with high basicity and low surface area. Both washed samples, reduced and unreduced, showed a high amount of carbon formation, whereas no carbon formation was observed in the unwashed samples for the reactions in the temperature range of 500–750 °C. The unwashed and unreduced sample was the only one that maintained the activity during all the reaction time at 700 °C (40% CH₄ conversion and 75% CO₂ conversion), low coke amount and no evidence of sintering, which was confirmed by XRD, TPO, and SEM analyses. The carbon suppression was related to the nickelate phase and to the Ni carbide formation in the unwashed and unreduced catalyst. In summary, the carbon deposition in biogas dry reforming was completely controlled between 600 and 750 °C using the unwashed and unreduced Ni-Al catalyst.     

Steam reforming of methane over unsupported nickel catalysts

This paper describes a study of steam reforming of methane using unsupported nickel powder catalysts. The reaction yields were measured and the unsupported nickel powder surface was studied to explore its potential as a catalyst in internal or external reforming solid oxide fuel cells. The unsupported nickel catalyst used and presented in this paper is a pure micrometric nickel powder with an open filamentary structure, irregular ‘fractal-like’ surface and high external/internal surface ratio. CH₄ conversion increases and coke deposition decreases significantly with the decrease of CH₄:H₂O ratio. At a CH₄:H₂O ratio of 1:2 thermodynamic equilibrium is achieved, even with methane residence times of only ∼0.5 s. The CH₄ conversion is 98 ± 2% at 700 °C and no coke is generated during steam reforming which compares favorably with supported Ni catalyst systems. This ratio was used in further investigations to measure the hydrogen production, the CH₄ conversion, the H₂ yield and the selectivity of the CO, and CO₂ formation. Methane-rich fuel ratios cause significant deviations of the experimental results from the theoretical model, which has been partially correlated to the adsorption of carbon on the surface according to TEM, XPS and elemental analysis. At the fuel: water ratio of 1:2, the unsupported Ni catalyst exhibited high catalytic activity and stability during the steam reforming of methane at low-medium temperature range.     

Phosphorus-tuned nickel as high coke-resistant catalyst with high reforming activity

Here we demonstrate that phosphorus (P) can affect both reforming and coking of alumina-supported nickel (10 wt% Ni with respect to alumina) at 750 °C and 1 atm. The P-containing catalysts were prepared by incipient wetness co-impregnation. Compared with the P-free nickel catalyst, the reforming activity is enhanced but the coke content decreases 8-fold at Ni:P atomic ratio of 4; the latter further reduces 55-fold at a ratio of 2, without a substantial decline in the reforming activity. Phosphorus not only improves nickel dispersion but also reduces the surface acidity and facilitates CO₂ activation. At high P loadings, a portion of nickel transforms into nickel oxide during reforming. Besides tuning the reforming and coking activity, phosphorus changes the behavior of carbon crystallization. In methane pyrolysis and dry reforming, filamentous carbon occurs on the P-free catalyst but it is absent on the P-containing catalyst.     

Modification of silica supported nickel catalysts with lanthanum for stability improvement in methane reforming with CO2

Dry reforming of methane (DRM) with excessive methane composition at CH₄/CO₂ = 1.2:1 was studied over lanthanum modified silica supported nickel catalysts (Ni-xLa-SiO₂, x: 1, 2, 4, and 6% in the target weight percent of La). The catalysts were prepared by ammonia evaporation method. Nickel phyllosilicate and La₂O₃ were the main phases in calcined catalysts. The modification of La enhanced the formation of 1:1 and Tran-2:1 nickel-phyllosilicate. There existed an optimum content of La loading at 1.50 wt% in Ni–2La–SiO₂ which resulted in its highest reduction degree (95.3%). The catalysts with appropriate amounts of La exhibited higher amount of CO₂ adsorption and created more medium and strong base centers. The sufficient number of exposed metallic nickel sites to catalyze the reforming reaction, as well as enough medium and strong basic sites in Ni–La–SiO₂ interface to accomplish the carbon removal were two important factors to attenuate catalyst deactivation. The catalyst stability evaluated at 750 °C for 10 h followed the order: Ni–2La–SiO₂ > Ni–4La–SiO₂ > Ni–1La–SiO₂ ≈ Ni–6La–SiO₂ > Ni–SiO₂. Ni–2La–SiO₂ catalyst possessed the lowest deactivation behavior, whose CH₄ conversion dropped from 60.2 to 55.9% after 30 h operation at 750 °C, indicating its high resistance against carbon deposition and sintering.     

Methane decomposition and carbon deposition over Ni/ZrO2 catalysts: Comparison of amorphous,tetragonal, and monoclinic zirconia phase

The influence of crystal phase of ZrO₂ on the catalytic performance of methane decomposition and the properties of deposited carbon on Ni/ZrO₂ catalysts was investigated. Ni/ZrO₂ catalysts were prepared by incipient-wetness impregnation method with 5% Ni loading, using amorphous, monoclinic, and tetragonal ZrO₂ as supports. It was found that Ni/am-ZrO₂ exhibited high activity and superior stability for carbon dioxide reforming of methane during 50 h of TOS, which could be attributed to the smaller Ni particle size and the lower coking rate. Additionally, the results of O₂-TPO/TPH/CO₂-TPO showed that the nature of carbon deposition via CH₄ decomposition on Ni/ZrO₂ catalysts was strongly influenced by both the Ni particle size and the crystal phase of zirconia. The lowest coking rate on Ni/am-ZrO₂ for carbon dioxide reforming of methane was due to the lower CH₄ decomposition rate and the higher gasification rate of carbon species by CO₂. Carbon deposited on Ni/am-ZrO₂ could be greatly removed by CO₂, due to the presence of large amount of adsorbed oxygen species on the surface of amorphous ZrO₂. For comparison, the imbalance of CH₄ cracking and removal of coke by CO₂ resulted in the accumulation of coke, leading to the deactivation of catalysts.     

Investigation of effects of sulfur on dry reforming of biogas over nickel–iron based catalysts

The aim of this study is to maintain and increase the activity of the catalyst in the presence of H₂S with the addition of iron to the Ni catalyst. Alumina-supported monometallic iron and bimetallic nickel-iron catalysts with different weight percentages (8% Fe, 3% Ni – 3% Fe and 8% Ni – 8% Fe) were synthesized using the wet impregnation method in this study. Alumina was prepared by the sol-gel method. The activities of these synthesized catalysts in the methane dry reforming reaction were investigated at different H₂S concentrations (0 ppm, 2 ppm, and 50 ppm) with a total flow rate of 60 mL/min containing an equimolar ratio of CH₄, CO₂, and Ar at 750 °C and atmospheric pressure. To investigate the effect of sulfur compounds on the catalytic activity, the catalysts were also exposed to different gas compositions such as the mixture of H₂S + He, H₂S + CO₂ + He, and H₂S + CO₂ + CH₄ + He. In this case, FT-IR with a gas cell was used to determine the components in the gas stream at the reactor outlet. To explain catalytic performance, characterization studies were carried out using XRD, N₂ adsorption/desorption, DRIFT, SEM, TGA, and XPS analysis. All-synthesized materials showed Type-IV isotherm with a hysteresis loop corresponding to an ordered mesoporous structure. The DRIFT analysis showed a decrease in the Lewis acid sites after the addition of iron into the Ni-catalysts. In the activity test carried out in the presence of 50 ppm H₂S, it was observed that the iron-containing 8Ni–8Fe@SGA catalyst increased the sulfur resistance slightly, compared to the monometallic 8Ni@SGA catalyst. TGA analysis showed that Fe addition reduced coke deposition, as the Ni–Fe catalyst had a lower nickel crystal size than the Ni-based catalyst. FTIR analysis with a gas cell showed that sulfur in H₂S transformed to other sulfur compounds such as COS and/or SO₂ during dry reforming of biogas over alumina-supported Ni–Fe catalysts.     

Metal precursor impregnation sequence effect on the structure and performance of NiCo/MgO catalyst

The influence of metal precursor impregnation sequence has been analyzed in terms of catalytic activity and stability of NiCo/MgO catalysts for coke oven gas reforming of carbon dioxide. It is found that the metal precursor impregnation sequence overwhelmingly affected the interaction among Ni, Co and MgO and resulted in different CO₂ sorption capacity. Compared to the catalysts prepared by first Ni precursor impregnation (Co/Ni/MgO) or by simultaneous Ni and Co precursor impregnation (NiCo/MgO), the catalysts prepared by first Co precursor impregnation (Ni/Co/MgO) obtained a stronger interaction among Ni, Co and MgO, leading to strong CO₂ adsorption, smaller Ni particle size (9.6 nm), higher metal dispersion (10.6%), lower carbon deposition (1.5 wt%) and finally resulted in a superior catalytic activity and stability for coke oven gas reforming of carbon dioxide (CH₄ and CO₂ conversion were 55 ± 1% and 80 ± 2%, respectively). We also proposed a model for the effect of metal precursor impregnation sequence on the particle distribution of Ni and Co in NiCo/MgO catalyst.     

Influence of reduction temperature on Ni particle size and catalytic performance of Ni/Mg(Al)O catalyst for CO2 reforming of CH4

Hydrotalcite-derived Ni/Mg(Al)O is promising for CH₄–CO₂ reforming. However, the catalysts reported so far suffer from sever coking at low temperatures. In this work, we demonstrate that a significant improvement of coke-resistance of Ni/Mg(Al)O can be achieved by fine tuning the Ni particle size through adjusting the reduction condition of catalyst. Ni particles having average size within 4.0–7.1 nm are in situ generated by reducing the catalyst at a selected temperature within 923–1073 K. Controllability of Ni particle size is related to the formation of Mg(Ni,Al)O solid solution upon hydrotalcite decomposition. It is found that the catalyst reduced at 973 K exhibits high activity, stability, and coke-resistance even at reaction temperature as low as 773 K. In contrast, the catalyst reduced at 923 K has low activity and deactivates due to Ni oxidation, while those reduced at 1023 and 1073 K suffer from sintering and severe coking. STEM and O₂-TPO reveal that coke deposition is directly proportional to the Ni particle size but becomes negligible at a size below 6.2 nm. It is evidenced that a critical size of about 6 nm is required to inhibit coking effectively. CO₂ temperature-programmed surface reaction indicates that the deposited carbon on small Ni particles can be easily removed by the CO₂ activated at the Ni–Mg(Al)O interfaces, accounting for the better resistance to coking.     

Various calcium loading and plasma-treatment leading to controlled surface segregation of high coke-resistant Ca-promoted NiCo–NiAl2O4 nanocatalysts employed in CH4/CO2/O2 reforming to H2

High coke-resistant Ca-promoted NiCo–NiAl₂O₄ nanocatalysts with different calcium loading (0, 0.25, 0.5 and 1.5 wt%) were synthesized via hybrid sol-gel-plasma method. Synthesized samples were applied in the CH₄/CO₂/O₂ reforming to H₂ reactions. Analyses revealed that the calcium addition caused the lower surface area, non-uniform distribution and larger particle size. Therefore, higher activity and yield were found for the nanocatalyst without calcium while catalytic activity and yield were descended for the other ones. This trend was according to the covering effect of calcium and undesirable effect of calcium over the surface area and particle size distribution. But owing to the enhanced coke gasification rate of Ca-rich samples, by increasing of Ca amount, coke deposition was descended and stable performance was promoted. Due to the time on stream performance (TOS) during the 48 h and at 750 °C, 1.5 wt% Ca-promoted NiCo–NiAl₂O₄ (NCCa1.5A (SGP)) was stable, but demonstrated the lower yield. Moreover, 0.25 wt% Ca-promoted NiCo–NiAl₂O₄ nanocatalyst (NCCa0.25A (SGP)) was illustrated the higher yield in comparison with NCCa1.5A (SGP). It must be noted that just 2.7% deactivation for H₂ yield was detected for NCCa0.25A (SGP) during the 48 h TOS performance. H₂ yield of the NCCa0.25A (SGP) at 850 °C was 84%. Based on the reverse trends of the yield and TOS performance, NCCa0.25A (SGP) was able to present as a promising nanocatalyst for CH₄/CO₂/O₂ reforming to H₂. Moreover, this offer was ascribed to the excellent coke resistance and superior catalytic performance of NCCa0.25A (SGP).     

Boron-doped Ni/SBA-15 catalysts with enhanced coke resistance and catalytic performance for dry reforming of methane

Nickel-based heterogeneous catalysts have shown promising results in many industrial-scale catalytic reforming processes and hydrocarbon reforming reactions such as dry reforming of methane (DRM). However, it is also reported that Ni-based catalysts generally show less resistance to the carbonaceous deposition, which ultimately causes their rapid deactivation during the reaction. One possible solution to improve the coke resistance is the addition of a promoter to the catalyst, which has shown successful results to reduce the coke formation. Therefore, this study also aimed to prepare boron-promoted Ni-based catalysts and investigate their efficiency for DRM reactions. A series of different catalysts with 10% nickel and x% boron (x: 1%, 2%, 3%, and 5%) were prepared by using an ordered mesoporous silica as a support and tested in DRM. The results demonstrated that boron-promoted Ni/SBA-15 catalysts obtained significant catalytic activity for CH₄ and CO₂ conversions. Meanwhile, it was noticed that a lower concentration of boron (1 and 2%) was more favourable to achieve higher catalytic activity, whereas the higher concentration (3% and 5%) resulted in a comparatively lower conversion for CH₄ and CO₂. Evidently, the higher activity of 2% B-promoted catalyst was ascribed to the synergistic effect of high surface area and lower crystallite size that greatly improved the active sites accessibility. Moreover, the results confirmed 14% carbon deposition on unpromoted (NS) catalyst and it was reduced to 1.3% for 2% boron-promoted catalyst owing to the presence of B-OH species on catalyst surface.     

Hydrogen production from carbon dioxide reforming of methane over highly active and stable MgO promoted Co–Ni/γ-Al2O3 catalyst

CoNi/Al₂O₃ and MgCoNi/Al₂O₃ catalysts are investigated for hydrogen production from CO₂ reforming of CH₄ reaction at the gas hourly space velocity of 40,000 mL g⁻¹ h⁻¹. The MgO promoted CoNi/Al₂O₃ catalyst shows much higher conversions (97% for CO₂ and 95% for CH₄ at 850 °C) than the CoNi/Al₂O₃ catalyst. In addition, the stability is maintained for 200 h in CO₂ reforming of CH₄. The outstanding catalytic activity and stability of the MgO promoted CoNi/Al₂O₃ catalyst is mainly due to the basic nature of MgO, an intimate interaction between Ni and the support, and rapid decomposition/dissociation of CH₄ and CO₂, resulting in preventing coke formation in CO₂ reforming of CH₄.     

Highly coke resistant Ni–Co/KCC-1 catalysts for dry reforming of methane

Nanofibrous KCC-1 supported Ni–Co bimetallic catalysts were investigated for dry reforming of methane for syngas generation. Monometallic catalysts such as Ni/KCC-1 and Co/KCC-1, and a series of bimetallic Ni–Co/KCC-1 catalysts were prepared by impregnation and co-impregnation method, respectively. All the catalysts were characterized by XRD, FT-IR, HR-SEM, FE-SEM, XPS, FT-Raman, BET, UV–Visible DRS and AAS techniques. Monometallic nickel supported catalyst contains NiO as an active phase, whereas bimetallic nickel catalysts contain Ni₂O₃, and NiCo₂O₄ on the surface. In the case of cobalt loaded catalysts, spinel Co₃O₄ is the dominant active species, apart from NiCo₂O₄. The addition of cobalt in Ni/KCC-1 has a pronounced effect on the crystallite size, surface area and active species. The hydrogen pretreatment of the catalyst produces bimetallic Ni–Co alloy on the surface. The catalytic activities of the bimetallic catalysts towards dry reforming of methane are better than monometallic catalysts. Mesoporous silica-based KCC-1 offers easy accessibility to the entire surface moieties due to its fibrous nature and the presence of channels, instead of pores. The 2.5%Ni-7.5%Co/KCC-1 showed the maximum CH₄ and CO₂ conversion along with a remarkably low H₂/CO ratio. The life-time test confirms the high thermal stability of the catalysts at 700 °C for 8 h, with less deactivation due to coke formation. The spent catalysts were characterized by XRD, TGA, FT-Raman, and FE-SEM to understand the structural and chemical changes during the reaction. The insignificant D band and G band of graphitic carbon in FT-Raman spectra for the highly active 2.5%Ni-7.5%Co/KCC-1 and 5%Ni–5%Co/KCC-1 catalysts along with TGA results containing 12% weight loss confirms the minimum coke deposition, formation of amorphous carbon and highest coke resistance. The fibrous support restricts the sintering and aggregation of nickel particles as well the deposition of coke. The addition of amphoteric cobalt increases the activity and stability of the catalysts. Ni–Co/KCC-1 with high coke resistance seems to be a promising catalyst for dry reforming of methane.     

Ni/MgAl2O4 catalyst for low-temperature oxidative dry methane reforming with CO2

Oxidative dry reforming of methane has been performed for 100 h on stream using Ni supported on MgAl₂O₄ spinel at different loadings at 500–700 °C, CO₂/CH₄ molar ratio of 0.76, and variable O₂/CH₄ molar ratio (0.12–0.47). Syngas with an H₂/CO ratio of 1.5–2.1 has been produced by manipulating reforming feed composition and temperature. The developed oxidative dry reforming process allowed high CH₄ conversion at all conditions, while CO₂ conversion decreased significantly with the lowering of the reforming temperature and increasing O₂ concentration. When considering both greenhouse gas conversions and H₂/CO ratio enhancement, the optimal reforming condition should be assigned to 550 °C and O₂/CH₄ molar ratio of 0.47, which delivered syngas with H₂/CO ratio of 1.5. Coke-free operation was also achieved, due to the combustion of surface carbon species by oxygen. The 3.4 wt% Ni/MgAl₂O₄ catalyst with a mean Ni nanoparticle diameter of 9.8 nm showed stable performance during oxidative dry reforming for 100 h on stream without deactivation by sintering or coke deposition. The superior activity and stability of MgAl₂O₄ supported Ni catalyst shown during reaction trials is consistent with characterization results from XRD, TPR, STEM, HR-STEM, XPS, and CHNS analysis.     

Effect of synthesis temperature on catalytic activity and coke resistance of Ni/bio-char during CO2 reforming of tar

Understanding how the synthesis parameters affect nickel particle distribution is critical to the synthesis of Ni/bio-char with excellent catalytic performance. In this work, the influence of synthesis temperature on catalytic activity and coke resistance of Ni/bio-char during CO₂ reforming of tar was explored. With the increase of synthesis temperature from 200 °C to 250 °C, the dispersion of nickel precursor into bio-char was promoted, resulting in an increase in crystallite size of metallic nickel particle from 51.98 nm to 62.45 nm. Besides, parts of the metallic nickel particles were oxidized to nickel oxides, providing more lattice oxygen to oxidize the coke deposited on the catalyst. However, further increasing the synthesis temperature to 300 °C would aggravate the oxidation of active nickel particles. The increase in crystallite size of nickel oxide particle from 23.25 nm to 43.38 nm could block the pore structure and hinder the access of reactants, resulting in a drop in the tar conversion rate from 40-51% to 13–27%.     

Enhanced coke suppression by using phosphate-zirconia supported nickel catalysts under dry methane reforming conditions

Ni based phosphate zirconium catalysts were prepared by impregnation technique and used under CH₄ dry reforming conditions. Catalysts (x%Ni/8%PO₄–Zr, where x = 5, 10, 15 or 20) were characterized by nitrogen physical adsorption-desorption, X-ray diffraction, temperature programmed reduction, CO₂ and NH₃ temperature programmed desorption, thermal gravimetric analysis and transmission electron microscopy (TEM-EDAX). Catalysts displayed a typical mesoporous structure and different reducibility grade as a function of Ni loading, diagnostic of a different extent of metal-support interaction. Activity and stability strongly depend upon Ni loading while the best performance was observed for catalyst characterized by a Ni loading of 10 wt%. The CO₂-TPD profiles of spent catalysts indicated that such catalyst had more tendency to gasify coke formed over the catalyst surface. TGA analysis of used catalysts quantitatively showed that catalysts at higher Ni loading deactivated as result of huge graphitic carbon formation on catalyst surface. On the contrary, system 10%Ni8%PO₄/ZrO₂ turns out to be an excellent candidate to conduct the methane reforming reaction with CO₂ without coke formation at high CH₄ and CO₂ conversions. Phosphate play a fundamental role in promoting Ni–ZrO₂ interaction which reflects in the stabilization of catalytic system against metal sintering and coke formation.