Pre-reforming of liquefied petroleum gas over nickel catalysts supported on magnesium aluminum mixed oxides

A series of Ni/Mg_xAl catalysts with different Mg/Al molar ratios were prepared by impregnating Mg-Al mixed oxides with nickel nitrate aqueous solution and used for the pre-reforming of LPG in the temperature range of 400-500 °C. XRD and H₂-TPR results showed that the Ni/Mg_xAl catalysts calcined at 800 °C mainly consisted of γ-Al₂O₃, Mg(Ni)Al₂O₄ and Mg(Ni)O phases varying with Mg/Al molar ratio without free NiO species observed. The effects of Mg/Al molar ratio, S/C molar ratio and reaction temperature on the catalytic behavior of the Ni/Mg_xAl catalysts were investigated in detail. The results revealed that the catalyst with Mg/Al molar ratio of 1.25 had the highest catalytic activity and stability. The increase in S/C molar ratio promoted both the steam reforming of LPG and the methanation of carbon oxides and hydrogen. The stability tests of 15%Ni/Mg_1.25Al catalyst showed that the catalyst was stable for the pre-reforming of LPG, and the stability decreased with elevating the reaction temperature due to more coke deposition.     

Hydrogen production by steam reforming of liquefied natural gas over a nickel catalyst supported on mesoporous alumina xerogel

Mesoporous alumina xerogel (A-SG) is prepared by a sol–gel method for use as a support for a nickel catalyst. The Ni/A-SG catalyst is then prepared by an impregnation method, and is applied to hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of the mesoporous alumina xerogel support on the catalytic performance of Ni/A-SG catalyst is investigated. For the purpose of comparison, a nickel catalyst supported on commercial alumina (A-C) is also prepared by an impregnation method (Ni/A-C). Both the hydroxyl-rich surface and the electron-deficient sites of the A-SG support enhance the dispersion of the nickel species on the support during the calcination step. The formation of the surface nickel aluminate phase in the Ni/A-SG catalyst remarkably increases the reducibility and stability of the catalyst. Furthermore, the high-surface area and the well-developed mesoporosity of the Ni/A-SG catalyst enhance the gasification of surface hydrocarbons that are adsorbed in the reaction. In the steam reforming of LNG, the Ni/A-SG catalyst exhibits a better catalytic performance than the Ni/A-C catalyst in terms of LNG conversion and hydrogen production. Moreover, the Ni/A-SG catalyst shows strong resistance toward catalyst deactivation.     

Hydrogen production by steam reforming of liquefied natural gas (LNG) over nickel catalysts supported on cationic surfactant-templated mesoporous aluminas

Two types of mesoporous γ-aluminas (denoted as A-A and A-S) are prepared by a hydrothermal method under different basic conditions using cationic surfactant (cetyltrimethylammonium bromide, CTAB) as a templating agent. A-A and A-S are synthesized in a medium of ammonia solution and sodium hydroxide solution, respectively. Ni/γ-Al₂O₃ catalysts (Ni/A-A and Ni/A-S) are then prepared by an impregnation method, and are applied to hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of a mesoporous γ-Al₂O₃ support on the catalytic performance of Ni/γ-Al₂O₃ is investigated. The identity of basic solution strongly affects the physical properties of the A-A and A-S supports. The high surface-area of the mesoporous γ-aluminas and the strong metal–support interaction of supported catalysts greatly enhance the dispersion of nickel species on the catalyst surface. The well-developed mesopores of the Ni/A-A and Ni/A-S catalysts prohibit the polymerization of carbon species on the catalyst surface during the reaction. In the steam reforming of LNG, both Ni/A-A and Ni/A-S catalysts give better catalytic performance than the nickel catalyst supported on commercial γ-Al₂O₃ (Ni/A-C). In addition, the Ni/A-A catalyst is superior to the Ni/A-S catalyst. The relatively strong metal–support interaction of Ni/A-A catalyst effectively suppresses the sintering of metallic nickel and the carbon deposition in the steam reforming of LNG. The large pores of the Ni/A-A catalyst also play an important role in enhancing internal mass transfer during the reaction.     

Hydrogen production by steam reforming of LPG using supported perovskite type precursors

Perovskites supported on Al₂O₃ (LaNi/AL) and CeO₂–SiO₂ (LaNi/CS) were tested as catalyst precursors for steam reforming of liquefied petroleum gas (SRLPG). The role of different supports in perovskites-derived catalysts performances was evaluated. A non-supported LaNiO₃ (LaNi) was also synthesized for comparison. According to the in-situ X-Ray Diffraction analyses, a reduction temperature of 700 °C was enough to generate Ni⁰ and La₂O₃, except for LaNi/AL. X-ray absorption Near Edge Spectroscopy (XANES) showed a direct influence of the supports on the reducibility of the samples. At 600 °C, the supported precursors LaNi/AL and LaNi/CS were more stable than LaNi during the SRLPG reaction. The alumina support provided a higher activity to LaNi/AL mainly due to the strong metal-support interaction. At 700 °C, the LPG conversion of LaNi/CS was near 100%. At both temperatures, the lowest carbon accumulation rate was obtained by LaNi/CS likely due to ceria oxygen mobility.     

Steam reforming of liquefied petroleum gas using catalysts supported on ceria-silica

Hydrogen production from steam reforming of liquefied petroleum gas (SR-LPG) process was studied over nickel, platinum, rhodium, and ruthenium-based catalysts supported on CeO₂–SiO₂ (CS). BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy (XANES) techniques were used to characterize the samples. The catalytic activity and stability of the samples during SR-LPG were measured at 400 °C and 600 °C, respectively. XRD data were used to estimate the average crystallite size of ceria, which was small in all samples. For Pt and Rh containing samples, no diffraction peaks related to the metals were identified, which suggests high dispersion of these metals on the support. TPR and XANES experimental results showed that the addition of metals promoted the reduction of ceria. The order of catalyst activity for during SR-LPG at 400 °C was: Rh/CS » Pt/CS > Ru/CS » Ni/CS. Pt/CS and Rh/CS samples exhibited lower deactivation than nickel and ruthenium catalysts for SR-LPG at 600 °C during 24 h. Catalysts deactivation was mainly due to carbon deposition. In situ XRD data collected during SR-LPG at 600 °C revealed small increases in the mean particle sizes of Ni(0) and Ru (0), which could be pointed out as an additional cause for faster deactivation of the Ru/CS and Ni/CS catalysts.     

Effect of calcination temperature of mesoporous alumina xerogel (AX) supports on hydrogen production by steam reforming of liquefied natural gas (LNG) over Ni/AX catalysts

Mesoporous alumina xerogel (AX) supports prepared by a sol–gel method were calcined at various temperatures. Ni/mesoporous alumina xerogel (Ni/AX) catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of calcination temperature of AX supports on the catalytic performance of Ni/AX catalysts in the steam reforming of LNG was investigated. Physical and chemical properties of AX supports and Ni/AX catalysts were strongly influenced by the calcination temperature of AX supports. Crystalline structure of AX supports was transformed in the sequence of γ-alumina → (γ + θ)-alumina → θ-alumina → (θ + α)-alumina with increasing calcination temperature from 700 to 1000 °C. Nickel species were strongly bonded to the divalent vacancy of γ-alumina, (γ + θ)-alumina, and θ-alumina through the formation of nickel aluminate phase. In the steam reforming of LNG, both LNG conversion and hydrogen composition in dry gas showed volcano-shaped curves with respect to calcination temperature of AX supports. Among the catalysts tested, Ni/AX-900 (nickel catalyst supported on AX that had been calcined at 900 °C) showed the best catalytic performance. The smallest nickel crystalline size and the strongest nickel–alumina interaction were responsible for high catalytic performance of Ni/AX-900 catalyst in the steam reforming of LNG.     

Cu,Mg and Co effect on nickel-ceria supported catalysts for ethanol steam reforming reaction

Ethanol steam reforming is a promising reaction which produces hydrogen from bio and synthetic ethanol. In this study, the nano-structured Ni-based bimetallic supported catalysts containing Cu, Co and Mg were synthesized through impregnation method and characterized by XRD, BET, SEM, TPR and TPD analysis. The prepared catalysts were tested in steam reforming of ethanol in the S/C = 6, GHSV of 20,000 mL/(g_cat h) at the temperature range of 450–600 °C. Among the xNi/CeO₂ (x = 10, 13, 15 wt%) catalyst, the sample containing 13 wt% Ni with surface area of 64 m²/g showed the best performance with 89% ethanol conversion and 71% H₂ selectivity as well as low CO selectivity of 8% at 600 °C and The addition of Cu, Mg, and Co to catalyst structure were evaluated and it was found that the nature of second metal has a strong influence on the catalyst selectivity for H₂ production. Considering to results of TPR analysis, the 13Ni–4Cu/CeO₂ catalyst showed proper reduction which caused in better activity. On the other side based on TPD analysis, the more basic property of 13Ni–4Mg/CeO₂ bimetallic catalyst provided a better condition to methane steam reforming, leading to lower CH₄ selectivity and consequently more H₂ production. The 13Ni–4Cu/CeO₂ exhibited the highest activity and lowest selectivity towards ethanol conversion and CO production about 99% and 4%, while the 13Ni–4Mg/CeO₂ catalyst possessed the highest H₂ selectivity and lowest CH₄ selectivity about 74% and 1% respectively at 600 °C. The Ni–Cu and Ni–Mg bimetallic catalysts shows good stability with time on stream.     

Estimating the agricultural demand for natural gas and liquefied petroleum gas in the presence of measurement error in the data

The paper begins by discussing the importance of accurate estimates of the price elasticity of demand and some of the problems frequently encountered in obtaining these estimates. To these problems is added that associated with inaccuracy in the measurement of the dependent variable and one or more of the independent variables that affect the quantity demanded. Two diagnostics, i.e. the regression coefficient bounds and the bias correction factor, have been introduced to assess the effect that such measurement error has on the estimated coefficients of demand relationships. The use of these diagnostics will aid in assessing the integrity of the estimates obtained. In considering the demand for natural gas and the demand for liquefied petroleum gas by farmers in the USA, both the quantity demanded and the price data available for demand model estimation purposes contain measurement error. The regression coefficient bounds diagnostic was used to indicate a range over which the true price responsiveness of farmers to changes in energy prices lies. The results suggest that each 1% increase (decrease) in the price of energy will result in a decrease (increase) of between 0.41 and 0.17% in the quantity of natural gas demanded and a decrease (increase) of between 0.48 and 0.07% in the quantity of liquefied petroleum gas demanded. The bias correction factor was computed to evaluate the magnitude of the underestimation of the responsiveness of the quantity of natural gas and liquefied petroleum gas demanded to a change in the number of acres irrigated. For natural gas, the under-estimation was 26.5%, whereas, for liquefied petroleum gas, it was 9.5%.     

Catalyst light-off behavior of a spark-ignition LPG (liquefied petroleum gas) engine during cold start

The effects of the length of the gas flow path from the exhaust outlet in the cylinder head to the catalyst inlet in the exhaust line, the ignition timing and the engine idle speed on the three-way catalyst light-off behavior in an electronically controlled inlet port LPG (liquefied petroleum gas) injection SI (spark-ignition) engine during cold start were investigated experimentally. The results showed that these factors affect the catalyst light-off behavior significantly during cold start. The reduction of the gas flow path length upstream the catalyst reduces the heat loss from the exhaust gases, increases the temperature of the catalyst, and results in faster catalyst light-off. Retarding the ignition timing from 0 to 15°CA ATDC decreases 22 and 8 s catalyst light-off time for HC and CO respectively. Increasing the engine idle speed from 1400 to 1800 rpm decreases 19 and 15 s the light-off time for HC and CO respectively.     

Hydrogen production by steam reforming of liquefied natural gas (LNG) over nickel catalyst supported on mesoporous alumina prepared by a non-ionic surfactant-templating method

A mesoporous alumina (A-NS) support was prepared by a non-ionic surfactant-templating method. A nickel catalyst supported on mesoporous alumina (Ni/A-NS) was then prepared by an impregnation method for use in hydrogen production by steam reforming of liquefied natural gas (LNG). For comparison, a nickel catalyst supported on commercial alumina (Ni/A-C) was also prepared by an impregnation method. Well-developed mesoporosity of A-NS support and strong metal-support interaction of Ni/A-NS catalyst greatly enhanced the nickel dispersion and nickel surface area through the formation of surface nickel aluminate. In the steam reforming of LNG, Ni/A-NS catalyst showed a better catalytic performance than Ni/A-C catalyst. High nickel surface area, high nickel dispersion, and well-developed mesoporosity of Ni/A-NS catalyst not only provided a large number of active nickel sites, but also suppressed the carbon deposition and nickel sintering during the reaction. Furthermore, Ni/A-NS catalyst exhibited a better catalytic performance than nickel catalyst supported on mesoporous alumina prepared by either an anionic surfactant-templating method or a cationic surfactant-templating method.     

Effect of SiO2-ZrO2 supports prepared by a grafting method on hydrogen production by steam reforming of liquefied natural gas over Ni/SiO2-ZrO2 catalysts

SiO₂-ZrO₂ supports with various zirconium contents are prepared by grafting a zirconium precursor onto the surface of commercial Carbosil silica. Ni(20 wt.%)/SiO₂-ZrO₂ catalysts are then prepared by an impregnation method, and are applied to hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of SiO₂-ZrO₂ supports on the performance of the Ni(20 wt.%)/SiO₂-ZrO₂ catalysts is investigated. SiO₂-ZrO₂ prepared by a grafting method serves as an efficient support for the nickel catalyst in the steam reforming of LNG. Zirconia enhances the resistance of silica to steam significantly and increases the interaction between nickel and the support, and furthermore, prevents the growth of nickel oxide species during the calcination process through the formation of a ZrO₂-SiO₂ composite structure. The crystalline structures and catalytic activities of the Ni(20 wt.%)/SiO₂-ZrO₂ catalysts are strongly influenced by the amount of zirconium grafted. The conversion of LNG and the yield of hydrogen show volcano-shaped curves with respect to zirconium content. Among the catalysts tested, the Ni(20 wt.%)/SiO₂-ZrO₂ (Zr/Si = 0.54) sample shows the best catalytic performance in terms of both LNG conversion and hydrogen yield. The well-developed and pure tetragonal phase of ZrO₂-SiO₂ (Zr/Si = 0.54) appears to play an important role in the adsorption of steam and subsequent spillover of steam from the support to the active nickel. The small particle size of the metallic nickel in the Ni(20 wt.%)/SiO₂-ZrO₂ (Zr/Si = 0.54) catalyst is also responsible for its high performance.     

Improved configuration of supported nickel catalysts in a steam reformer for effective hydrogen production from methane

The heat and mass transfer characteristics in a steam reformer are investigated via experimental and numerical approaches and a new configuration of packed catalysts is proposed for effective hydrogen production. Prior to the numerical investigation, parametric studies are carried for the furnace temperature, steam-to-carbon (S:C) ratio, and gas flow rate. After validation of the developed code, numerical work is undertaken to determine the relationship of the operating parameters. Based on the experimental and numerical results, and with the goal of obtaining optimum heat transfer characteristics and an efficient catalyst array, a new configuration for the packed bed is proposed and numerically investigated taking into account the endothermicity of the steam reforming reaction. A bed packed repeatedly with inert and active catalysts is found to be an efficient means to obtain the same, or better, hydrogen production with small amounts of the active catalysts compared with a typical steam reformer.     

Correlations for laminar burning velocities of liquefied petroleum gas–air mixtures

Spherically expanding flames have been employed to determine the laminar flame speeds of liquefied petroleum gas–air mixtures, diluted or not by the combustion exhaust gas, over equivalence ratios from 0.7 to 1.4. The effect of the stretch imposed at the flame front has been explored experimentally, and Markstein lengths are estimated to characterize the flame stretch. After omitting the stretch effect, one has obtained the unstretched laminar burning velocities of liquefied petroleum gas–air flames with or without diluent. Explicit formulas have been obtained to express the laminar burning velocity dependencies on the equivalence ratio and diluent rate.     

Laminar burning velocities,Markstein lengths,and flame thickness of liquefied petroleum gas with hydrogen enrichment

In this paper, experimental data of laminar burning velocity, Markstein length, and flame thickness of LPG flames with various percentages of hydrogen (H₂) enrichments have been presented. The experiments were conducted under the conditions of 0.1 MPa, 300 K in a constant volume chamber. The tested equivalence ratios of air/fuel mixture range from 0.6 to 1.5, and the examined LPG contains 10%–90% of hydrogen in volume. Experimental results show that hydrogen addition significantly increase the laminar burning velocity of LPG, and the accelerating effectiveness is substantial when the percentage of hydrogen is larger than 60%. Effect of hydrogen addition on diffusion thermal instability, as indicated by Markstein length, was analyzed at various equivalence ratios. Hydrogen addition decreases the flame thickness. Equivalence ratio has more dominating effect on flame thickness than hydrogen does. For the fuel with 10% LPG and 90% hydrogen, the flame thickness values are close for all equivalence ratios.     

Reactivity of Ce-ZrO2 (doped with La-, Gd-, Nb-, and Sm-) toward partial oxidation of liquefied petroleum gas: Its application for sequential partial oxidation/steam reforming

Ce-ZrO₂ was found to have useful partial oxidation activity under moderate temperatures. It converted liquefied petroleum gas (LPG) to H₂, CH₄, CO and CO₂ with small amounts of C₂H₆ and C₂H₄ formations depending on the operating temperature and provided significantly greater resistance toward carbon deposition compared to conventional Ni/Al₂O₃. The doping of La, Sm and Gd over Ce-ZrO₂ considerably improved catalytic reactivity, whereas Nb-doping reduced its reactivity. It was found that the impact of doping element is strongly related to the degrees of oxygen storage capacity (OSC) and/or lattice oxygen (O_O^x) of materials. Among all catalysts, La-doped Ce-ZrO₂ was observed to have highest OSC value and was the most active catalyst. Above 850 °C with inlet LPG/O₂ molar ratio of 1.0/1.0, the main products from the reaction over La-doped Ce-ZrO₂ were H₂, CH₄, CO, and CO₂.     

Nanocrystalline MgO supported nickel-based bimetallic catalysts for carbon dioxide reforming of methane

Nanocrystalline magnesium oxide with high surface area and plate-like shape was employed as catalyst support for preparation of nickel-based bimetallic catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), Temperature programmed oxidation and desorption (TPO–TPD), Thermal gravimetric and differential thermal gravimetric (TGA–DTG), H₂ chemisorption and Transmission and electron microscopies (TEM and SEM) analyses. CO₂–TPD data showed the high CO₂ adsorption capacity of catalysts which improves the resistance of catalysts against the carbon formation. The H₂ chemisorption results also indicated that the addition of Pt to nickel catalyst improved the nickel dispersion. The obtained results revealed that the prepared catalysts showed a high activity and stability during the reaction with a low amount of deposited carbon. Addition of Pt to nickel catalyst improved both the activity and resistivity against carbon formation.     

Structured catalysts for the conversion of liquefied petroleum gas to hydrogen-rich gas and for anode off-gas afterburning

A number of mixed oxide Ce_0.75Zr_0.25O₂ supports were prepared, tested for reproducibility, and characterized by physicochemical methods. The most reproducible preparation method was adapted for depositing the mixed oxide on a FeCrAlloy mesh substrate coated by a protective alumina layer. Based on the obtained structured Ce_0.75Zr_0.25O₂/θ-Al₂O₃/FeCrAl support, the Rh/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl and Pt/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl catalysts were prepared and tested in the reactions of partial oxidation of LPG and deep oxidation of anode gases, respectively. Rh/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl provided complete conversion of LPG into synthesis gas of a composition close to the equilibrium one. Pt/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl provided complete conversion of all components of the anode gases at GHSV = 20,000–40,000 h⁻¹; however, at higher GHSV values, methane conversion decreased. The studies on the effect of methane content on deep oxidation of anode off gases showed that methane conversion began to decrease at a 1.5-fold excess of methane and dropped to 50% at a 10-fold excess of methane.     

Structure-dependent activity and stability of Al2O3 supported Rh catalysts for the steam reforming of n-dodecane

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al₂O₃ catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N₂ adsorption, TEM, HAADF-STEM, H₂-TPR, CO chemisorption, NH₃-TPD, CO₂-TPD, XPS, Al²⁷ NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al₂O₃-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh⁰ active species, which due to the unique properties of Al₂O₃ nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al₂O₃-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al₂O₃-NR catalyst showed the highest hydrogen production rate of 87,600 mmol g_fuel^?1 g_Rh^?1min^?1 among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al₂O₃–NF and Rh/Al₂O₃-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al₂O₃-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.     

Zirconia supported nickel catalysts for glycerol steam reforming: Effect of zirconia structure on the catalytic performance

The effect of the zirconia structure in Ni/ZrO₂ catalysts on the glycerol steam reforming (GSR) reaction was studied. A tetragonal zirconia support was synthesized via a hydrolysis technique and loaded with 5 wt% Ni via a wet-impregnation method. Similarly, a commercial monoclinic zirconia support was also impregnated with 5 wt% Ni. Following calcination at 600 °C, physico-chemical properties of the prepared catalysts were investigated by X-Ray Diffraction (XRD), H₂-Temperature Programmed Reduction (H₂-TPR) and CO₂-Temperature Programmed Desorption (CO₂-TPD) techniques. The catalysts were then tested in the GSR reaction in the 400–700 °C range with a steam to glycerol molar ratio of 9:1 and a flow rate of 0.025 mL/min. The monoclinic catalyst exhibited a better performance giving higher hydrogen yields and glycerol conversions. This was attributed to an improved reducibility of Ni in this catalyst. Stability tests at 600 °C revealed the deactivation of the tetragonal catalyst during 6 h as a result of the formation of encapsulating coke which blocked active Ni metal sites. The monoclinic catalyst, exhibiting the formation of only filamentous coke, remained relatively stable for 24 h.     

Ni/Al2O3 catalysts for CO2 methanation: Effect of silica and nickel loading

Samples containing from 1 to 33 wt.% of NiO on silica and alumina doped with silica (1 and 20 wt.% silica in the support) have been prepared and characterized by BET, XRD, FT-IR, UV–vis–NIR, FE-SEM, EDXS, and TPR techniques. Catalysts have been pre-reduced in situ before catalytic experiments and data have been compared with Ni/Al₂O₃ reference sample. Characterization results showed that SiO₂ support has a low Ni dispersion ability mainly producing segregated NiO particles and a small amount of dispersed Ni²⁺ in exchange sites. Instead, for the Si-doped alumina a “surface spinel monolayer phase” is formed by increasing Ni loading and, only when the support surface is completely covered by this layer, NiO is formed. Moreover, H₂-TPR results indicated that NiO particles are more easily reduced compared to Ni species. Low loading Ni/SiO₂ catalysts show high selectivity and moderate activity for RWGS (reverse Water Gas Shift) reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. High loading Ni/SiO₂ catalysts show both methanation and RWGS but evident short-term deactivation for methanation, attributed to large, segregated Ni metal particles, covered by a carbon veil. Ni on alumina -rich carriers, where nickel disperses forming a surface spinel phase, show high activity and selectivity for methanation, and short-term catalyst stability as well. This activity is attributed to small nickel clusters or metal particles interacting with alumina, formed upon reaction. The addition of SiO₂ in Al₂O₃ support decreases the activity of Ni catalysts in CO₂ methanation, because it reduces the ability of the support to disperse nickel in form of the surface spinel phase, thus reducing the amount of Ni clusters in the reduced catalysts.