Theoretical investigation of the reactivity of flat Ni (111) and stepped Ni (211) surfaces for acetic acid hydrogenation to ethanol

Reactivity of two types of Ni surfaces-flat (111) and stepped (211) surfaces for acetic acid hydrogenation to ethanol was investigated using density functional theory method. The most stable configurations of the reactants, intermediates and products were obtained by investigating all the possible adsorption sites. Results showed that the adsorption of all the studied molecules on the Ni (211) surface are stronger than that on the Ni (111) surface, except for H atom (similar adsorption strength of H atom on the both surfaces was found). In addition, most of the molecules on the Ni (211) surface preferred to adsorb at the step edge, indicating that different coordination numbers of Ni atoms could result in different adsorption strength. Moreover, the elementary reactions with energy barriers related to ethanol and ethyl acetate formations were studied. The most favorable pathways for ethanol formation on the Ni (111) and (211) surfaces are CH₃COOH → CH₃CO → CH₃CHO → CH₃CHOH→ CH3CH₂OH and CH₃COOH → CH₃CO → CH₃COH → CH₃CHOH → CH₃CH₂OH, respectively. The direct decomposition of acetic acid molecule to form acetyl species was the rate-determining step on the both surfaces. Slight difference for the rate-determining step barriers was observed (1.04 eV vs. 1.13 eV). However, the elementary step of ethyl acetate formation by CH₃CO and CH₃CH₂O became much more difficult on the Ni (211) surface than that on the Ni (111) surface (1.06 eV vs. 0.67 eV). These results suggests that the Ni (211) surface is more likely to inhibit ethyl acetate formation compared with the Ni (111) surface. Meanwhile, the results of the rate constants and the effective barriers indicates that the Ni (211) surface presents a higher probability for higher ethanol selectivity.     

Insight into the role of Fe on catalytic performance over the hydrotalcite-derived Ni-based catalysts for CO2 methanation reaction

A series of Fe modified hydrotalcite-derived Ni-based catalysts (Ni₃Fe_x-calc) were synthesized to evaluate the effect of Fe on CO₂ methanation performance over Ni₃-calc catalyst. The results showed that Ni₃–Fe_0.5-calc had superior catalytic activity with 78% CO₂ conversion rate at 200 °C. The addition of moderate amount of Fe can effectively improve the reducibility, enrich the medium basic sites of Ni₃-calc catalyst, and further facilitate the adsorption and activation of CO₂. This resulted in the outstanding low-temperature CO₂ methanation activity, as well as the enhanced resistance of carbon deposition. In-situ DRIFTS results indicated that the CO₂ methanation reaction mechanism involved a progressive hydrogenation of carbonate and formate species to methane route. The formate species was the main intermediates during CO₂ methanation. The introduction of Fe could significantly accelerate the hydrogenation rate of carbonates and formate species.     

Computational study of CO2 methanation over two-dimensional molybdenum carbide catalysts

Two-dimensional molybdenum carbide (2D-Mo₂C) is thought to be promising for catalytic hydrogenation of CO₂ to CH₄, but little is known about its catalytic reaction mechanism. In this work, we investigate the hydrogenation of CO₂ to CH₄ on 2D-Mo₂C using density functional theory. Our calculations show that Mo on the surface can efficiently decompose CO₂ to CO and O, and also H₂ to H. The hydrogenation of CO produces CHO that is readily deoxygenated to CH, and CH is selectively hydrogenated to produce CH₄. Interestingly, the embedded Ir₁ on 2D-Mo₂C can act as a single-atom promoter to improve the performance of CO₂ methanation, while on the other hand maintaining its high selectivity for CH₄. This work provides insight into the mechanism of 2D-Mo₂C-catalyzed CO₂ methanation reactions and suggests a strategy to improve the performance of such catalysts through single-atom promoters.     

The influence of CO,CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO₂ and H₂O content on the performance of chlorinated NiCeO₂ catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO₂ increases with the increasing H₂ concentration, is independent of CO₂ concentration and decreases with increasing CO and H₂O concentrations; the rate of CO₂ methanation is weakly sensitive to H₂ and CO₂ concentrations and decreases with increasing CO and H₂O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO₂ hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO₂ was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO₂ (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO₂-η-Al₂O₃/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.     

Methanation of CO2 on bulk Co–Fe catalysts

The efficiency of CO₂ methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO₂ methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO₂ molecule on the ensemble and detailed reaction mechanism of CO₂→CH₄ conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H₂O molecule was considered as the rate-limiting step.     

A comprehensive DFT study of CO2 catalytic conversion by H2 over Pt-doped Ni catalysts

PtNi bimetallic catalysts show superior performance for CO₂ catalytic conversion by hydrogen, but the underlying mechanism and the key elementary steps in controlling the activity and selectivity of CO₂ hydrogenation remain unclear. In present work, the complete reaction network for CO₂ hydrogenation has been investigated systematically over Pt/Ni (111) surface based on periodic density functional theory, and active sites and reaction mechanism have been determined. It is found that HCOOH is mainly produced by undergoing the HCOO pathways while synthesis of CH₃OH and CH₄ via RWGS+CO hydrogenation is the dominant reaction pathway, and their selectivity are determined by the competitive reaction between hydrogenation and CO bond scission of H₂COH species. The dissociation of COOH is regarded as the rate-determining step as it has the highest barrier (2.07 eV) in RWGS+CO hydrogenation. Moreover, it is observed that the doping of Pt on Ni surface can promote the transformation of CO₂ into chemisorbed CO₂^δ− and reduce the barrier in H₂ dissociation, which further facilitate the activation and hydrogenation of CO₂. More importantly, the doped Pt atom could promote H_xCO hydrogenation to H_xCOH, meanwhile, suppress H_xCOH dissociation into CH_x. Especially, the activation barrier and reaction energy for C formation is markedly enhanced, and the ability for C hydrogenation is promoted over Pt/Ni (111) surface, which could lower the possibility of coke formation. These results provide helpful information in understanding the process of CO₂ hydrogenation at atomic scale, and could benefit for the synthesis of Ni-based bimetallic catalysts.     

Effect of active site and charge transfer on methane dehydrogenation over different Co doped Ni surfaces by density functional theory

To uncover the effects and the underlying mechanisms of Co content on CH₄ dehydrogenation over Ni–Co bimetal catalyst, the CH₄ successive dehydrogenation process over Ni (111) and different Co doped Ni (111) surface has been systematically studied via DFT calculation. Active sites and electronic properties have been obtained. CH₄ physically located at the top site of Ni or Co, while other CH_x species preferably occupied the threefold site. Besides, the charge transferred from surface to absorbates and the p-band center of absorbates could well describe the adsorption strength of CH_x and the activation barrier of CH dehydrogenation on different surfaces. More importantly, the addition of small Co could improve the resistance to carbon deposition by weakening the adsorption of C, suppressing the activity of CH₄ dehydrogenation and promoting C hydrogenation process.     

Impacts of metal loading in Ni/attapulgite on distribution of the alkalinity sites and reaction intermediates in CO2 methanation reaction

Both metal sites and alkaline sites are essential parameters for a catalyst used in methanation of CO₂. This study investigated the impacts of the relative abundance of metal sites and alkaline sites on the catalytic performances of nickel-based catalyst with attapulgite, a natural mineral, as the support. The results showed that the increase of nickel loading to attapulgite significantly decreased the abundance of alkaline sites, remarkably enhanced the catalytic activity, and suppressed the formation of CO. The in situ DRIFTS characterization of the CO₂ methanation indicated that the alkaline sites favored formation of the oxygen-containing reaction intermediates such as CO1, –OH, 1CO₂, formate, carbonate and bicarbonate species. In comparison, metallic nickel species promoted their further hydrogenation to form CH₄. Besides the absorption/activation of 1CO₂ was more preferable on surface of metallic nickel, but not on the alkaline sites. The availability of the alkaline sites was not as important as the metallic nickel species for preparation of an efficient catalyst for CO₂ methanation.     

Solid-state synthesis method for the preparation of cobalt doped Ni–Al2O3 mesoporous catalysts for CO2 methanation

In this study, a simple solid-state synthesis method was employed for the preparation of the Ni–Co–Al₂O₃ catalysts with various Co loadings, and the prepared catalysts were used in CO₂ methanation reaction. The results demonstrated that the incorporation of cobalt in nickel-based catalysts enhanced the activity of the catalyst. The results showed that the 15 wt%Ni-12.5 wt%Co–Al₂O₃ sample with a specific surface area of 129.96 m²/g possessed the highest catalytic performance in CO₂ methanation (76.2% CO₂ conversion and 96.39% CH₄ selectivity at 400 °C) and this catalyst presented high stability over 10 h time-on-stream. Also, CO methanation was investigated and the results showed a complete CO conversion at 300 °C.     

K-doped LaNiO3 perovskite for high-temperature water-gas shift of reformate gas: Role of potassium on suppressing methanation

LaNiO₃ perovskite has been successfully used as a catalyst precursor for high temperature water-gas shift (HT-WGS) reaction of reformate gas to produce additional hydrogen from the hydrocarbon reforming. The Ni⁰ nanoparticles with the particle size of ca. 21 nm obtained after reduction of LaNiO₃ perovskite can effectively suppress CO methanation during HT-WGS reaction using pure CO/H₂O gas. However, for HT-WGS reaction of reformate gas (including CO, H₂O, CO₂ and H₂), LaNiO₃ perovskite exhibits lower catalytic activity with significant CH₄ formation predominantly via CO₂ methanation. In this work, the CO₂ methanation during HT-WGS reaction of reformate gas was suppressed by the addition of potassium onto LaNiO₃ perovskite. This is due to the adsorption of H₂O on the potassium which is located at the interface between La₂O₃ and Ni⁰ nanoparticle (as deduced from XPS and HRTEM results) that forms stable KOH, blocking the methanation of CO₂ adsorbed on the La₂O₃ with H₂ adsorbed on the Ni⁰ nanoparticles. Moreover, the formation of stable KOH also promotes the formation of formate (HCOO) – a key intermediate for WGS reaction over the reduced LaNiO₃ perovskite – even at high reaction temperature by continuously supplying hydroxyl group to react with CO adsorbed on the Ni⁰ nanoparticle, which helps to maintain the catalytic activity for WGS reaction at high reaction temperature.     

The CO removal performances of Cr-free Fe/Ni catalysts for high temperature WGSR under LNG reformate condition without additional steam

The goal of this study was to investigate Cr-free, Fe/Ni, metal oxide catalysts for the high temperature shift (HTS) reaction of a fuel processor using liquefied natural gas (LNG). As hexavalent chromium (Cr⁶⁺) in commercial HTS catalyst is a hazardous material, we selected Ni as a substitute for chromium in the Fe-based HTS catalyst and investigated the HTS activities of these Cr-free, metal oxide catalysts under the LNG reformate condition. Cr-free, Fe/Ni-based catalysts containing Ni instead of Cr were prepared by coprecipitation and their performance was evaluated under a gas mixture condition (56.7% H₂, 10% CO, 26.7% H₂O, and 6.7% CO₂) that simulated the gas composition from a steam methane reformer (SMR, at H₂O/CH₄ ratio = 3 with 100% CH₄ conversion). Under this condition, the Fe/Ni catalysts showed higher CO removal activities than Fe-only and Cr-containing catalysts, but the methanation was promoted when the Ni content in the catalyst exceeded 50 wt%. Brunner-Emmett-Teller (BET), X-ray diffraction (XRD), inductively coupled plasma (ICP) and X-ray photoelectron spectroscopy (XPS) analyses were performed to explain the HTS activity of the Fe/Ni catalysts based on the catalyst structure.     

Coal char supported Ni catalysts prepared for CO2 methanation by hydrogenation

Catalytic CO₂ methanation is a potential solution for conversion of CO₂ into valuable products, and the catalyst plays a crucial role on the CO₂ conversion and CH₄ selectivity. However, some details involved in the CO₂ methanation over the carbon supported Ni catalysts are not yet fully understood. In this work, commercial coal char (CC) supported Ni catalysts were designed and prepared by two different methods (impregnation-thermal treatment method and thermal treatment-impregnation method) for CO₂ methanation. Effects of the preparation conditions (including the thermal treatment temperature and time, the mass ratio of CC:Ni and the preparation method), as well as the reaction temperature of CO₂ methanation, were investigated on the catalyst morphology, reducibility, structure and catalytic performance. Fibrous Ni-CC catalyst is achieved and shows high CO₂ conversion (72.9%–100%) and CH₄ selectivity (>99.0%) during the 600-min methanation process. Adverse changes of the catalyst surface and textural properties, reducibility, particle size and morphology are the potential factors leading to the catalyst deactivation, and possible solutions resistant to the deactivation were analyzed and discussed. The CO₂ methanation mechanism with the CO route was proposed based on the oxidation-reduction cycle of Ni in this work.     

CO2 methanation over Ni and Rh based catalysts: Process optimization at moderate temperature

H₂ was produced from aluminum/water reaction and reacted with CO₂ over Ni and Rh based catalysts to optimize the process conditions for CO₂ methanation at moderate temperature. Monometallic catalysts were prepared by incorporating Ni and Rh using nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and rhodium(III) chloride trihydrate (RhCl₃·3H₂O)as a precursor chemical. The preliminary study of the catalysts revealed higher activity and CH₄ selectivity for Rh based catalyst compared to that of Ni based catalyst. Further, Rh based catalyst was investigated using response surface methodology (RSM) involving central composite design. The quadratic model was employed to correlate the effects of variable parameters including methanation temperature, %humidity, and catalyst weight with the %CO₂ conversion, %CH₄ selectivity, and CH₄ production capacity. Analysis of variance revealed that methanation temperature and humidity play an important role in CO₂ methanation. Higher response values of CO₂ conversion (54.4%), CH₄ selectivity (73.5%) and CH₄ production capacity (8.4 μmol g^?1 min^?1) were noted at optimum conditions of 206.7°C of methanation temperature, 12.5% humidity and 100 mg of the catalyst. The results demonstrated the ability of Rh catalyst supported on palm shell activated carbon (PSAC) for CO₂ methanation at low temperature and atmospheric pressure.     

Effect of sulfur–carbon interaction on sulfur poisoning of Ni/Al2O3 catalysts for hydrogenation

The poisoning effects of two types of carbon-containing sulfides (CS₂ and CH₃SSCH₃) on Ni/Al₂O₃ catalysts for the hydrogenation of benzene and cyclohexene were systematically investigated via experiments and DFT calculations. The toxicity of CH₃SSCH₃ is two and three times greater than that of CS₂ for the hydrogenation of cyclohexene and benzene, respectively. The characterization and DFT results reveal that CH₃SSCH₃ dissociates easily during hydrogenation and releases CH₄, allowing sulfur atoms to poison the Ni sites. However, the presence of CS₂ in the hydrogenation step slows the decline in the catalytic performance, because of resistance to the direct dissociation of the strong CS bond of CS₂. The chemisorbed CS₂ molecules and their incomplete dissociation weaken the strength of NiS bond and decrease the poisoning effect of sulfur. The poisoning processes of two sulfides are also discussed following a DFT study. This work opens up promising possibilities for the industrial study of S-poisoning resistance in supported Ni catalysts.     

Water enhanced methanol decomposition for simultaneous heat recovery and ready-to-use synthesis gas production over CO2 capture enhanced Ni/zeolite 4A catalyst

Methanol decomposition is considered as a “one stone two birds” approach for simultaneously recovering waste heat and affording synthesis gas. However, this approach requires efficient catalysts with high CO selectivity and low selectivity to byproducts. Herein, a rational design of CO₂ capture enhanced Ni/zeolite 4 A catalyst for synthesis gas production by water enhanced methanol decomposition is reported. 5%-Ni/NaA-500 catalyst achieves the Y_H2 of 80.6%, Y_CO of 76.2%, H₂/CO molar ratio of 2.11, high stability, low selectivity to CO₂ and CH₄, and no coke at 325 °C. Ni atoms highly disperse on the surface and microporous of zeolite 4 A, and the strong interaction between Ni atoms and zeolite 4 A inhibits the reduction of Ni atoms. Consequently, Ni³⁺, Ni²⁺ and Ni⁰ coexist in 5%-Ni/NaA-500, and the redox couples of Ni³⁺↔Ni²⁺, Ni²⁺↔Ni⁰, and Ni³⁺↔Ni⁰ will enhance the redox processes during methanol decomposition. CO₂ capture capacity of x%-Ni/NaA-Y below 350 °C promotes the reverse water gas shift reaction by concentrating CO₂ molecules, which hence increases CO selectivity and declines the selectivity to byproducts. The reaction path follows CH₃OH→CH₃O→CH₂O→CHO→CO. This work will pave the way to industrial applications that combine ready-to-use synthesis gas production and heat recovery.     

Impacts of nickel loading on properties,catalytic behaviors of Ni/γ–Al2O3 catalysts and the reaction intermediates formed in methanation of CO2

In this study, methanation of CO₂ over Ni/Al₂O₃ with varied nickel loading (from 0 to 50 wt%) was evaluated, striving to explore the effects of nickel loading on catalytic behaviors and the reaction intermediates formed. The results showed that agglomeration of nickel particles were closely related to interaction between nickel and alumina. Increasing nickel loading resulted in the increased proportion of nickel having medium strong interaction with alumina, the reduced reduction degree of NiO, the increase of medium to strong basic sites, the enhanced activity for methanation and the competition between reverse water gas shift (RWGS) reaction and methanation. Lower nickel loading promoted RWGS reaction while methanation of CO₂ dominated at higher nickel loading. The catalyst with a nickel loading around 25% achieved the best activity for methanation. The in–situ DRIFTS studies of methanation of CO₂ showed that CO₂ could be absorbed on surface of metallic Ni, NiO or alumina. More metallic nickel species on alumina suppressed formation of carbonate species while promoted further conversion of HCOO¹ species and ¹CH₃ species, achieving a higher catalytic efficiency. Moreover, more metallic nickel species was crucial for gasifying the carbonaceous intermediates, prevented aggregation of the intermediates to coke and achieving a higher catalytic stability.     

Formation of AlNi phase and its influence on hydrogen absorption kinetics of Mg77Ni23-xAlx alloys at intermediate temperatures

In order to reduce the obstacle influence of coarse Mg₂Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg₇₇Ni_23-xAl_x (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg₂Ni phase decreases with increasing Al content till 6 at.%, while primary Mg₂Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg₇₇Ni_23-xAl_x alloys were increased, which is resulted from the refined primary Mg₂Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg₇₇Ni₁₇Al₆ alloy. The apparent activation energy of the Mg₇₇Ni₁₇Al₆ alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg₇₇Ni₂₃ alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg₂Ni phase, the solid dissolution of Al in Mg₂Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties.     

Carbon dioxide methanation over Ni catalysts prepared by reduction of NixMg3‒xAl hydrotalcite-like compounds: Influence of Ni:Mg molar ratio

A series of supported Ni catalysts have been prepared from Ni_xMg_3?xAl hydrotalcite-like compounds (HTlcs) and the influence of Ni:Mg molar ratio on the structural property and catalytic activity for CO₂ methanation is investigated. The catalysts were characterized by N₂ physical adsorption, X-ray powder diffraction (XRD), temperature-programmed reduction (H₂-TPR), temperature-programmed desorption (CO₂-TPD), H₂ chemisorption, scanning electronic microscopy (SEM), scanning transmission electronic microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). By reducing HTlcs at 800 °C, well dispersed Ni particles with average size of 5–10 nm are formed. The Ni crystal size decreases with the decrease of Ni:Mg ratio, attributable to the strong interaction between nickel and magnesium oxides. Among the catalysts, Ni₂Mg₁Al-HT shows the highest activity, giving ～93% CO₂ conversion and >99% CH₄ selectivity at 275 °C and SV = 5000 mL g^?1 h^?1. Meanwhile, this catalyst exhibits good stability without obvious sintering and coking. The high activity is related to the large amount of surface Ni⁰ species and medium basic sites. From CO₂-TPD and DRIFTS, it is inferred that CO₂ adsorbs on the medium basic sites, i.e., Ni–Mg(Al)O interface, forming monodentate carbonate. In situ DRIFTS reveals that monodentate carbonate, monodentate formate, and adsorbed CO are the main intermediate species, suggesting that the reaction may proceed via the formate formation route.     

Methanation of CO2 over Ni/Al2O3 modified with alkaline earth metals: Impacts of oxygen vacancies on catalytic activity

This study investigates the impacts of the alkaline earth metal (Mg, Ca, Sr, Ba) additives on properties and performances of nickel catalysts for CO₂ methanation. The results show that addition of Mg, Sr, and Ba creates more pores while Ca addition leads to merge of small pores. The alkalinity of the catalyst increases with the addition of Mg, Ca, Sr or Ba, however, it does not necessarily enhance the catalytic activity. The degree of reduction of nickel species is another important factor affecting catalyst activity. Mg or Ca addition promotes the reverse water gas shift reaction to form more CO but not the methanation. In converse, with the addition of Sr or Ba, the activities for methanation increased drastically, especially in the low temperature region. In situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies show that *OH, *CO₃, *CO₂, CH_x, HCOO*, *CO and H₂CO* species are main reaction intermediates. Mg or Ca promotes the carbonate formation. Sr or Ba promotes *CO and H₂CO* formation, which are the important reaction intermediates in the conversion of CO₂ to CH₄. In addition, the Electron Paramagnetic Resonance (EPR) characterization shows that the catalyst modified with Sr species generates the oxygen vacancies that prevent electrons from being paired, forming a Lewis basic position. The oxygen vacancies generated are crucial for enhancing the catalytic activities for methanation at the low reaction temperatures.     

The role of oxygen vacancies in the CO2 methanation employing Ni/ZrO2 doped with Ca

The Ni/ZrO₂ catalyst doped with Ca and Ni/ZrO₂ were employed in the CO₂ methanation, a reaction which will possibly be used for storing intermittent energy in the future. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, reduction in situ), X-ray diffraction (XRD, reduction in situ and Rietveld refinement), electron paramagnetic resonance (EPR), temperature-programmed surface reaction, cyclohexane dehydrogenation model reaction, temperature-programmed desorption of CO₂ and chemical analysis. The catalytic behavior of these catalysts in the CO₂ methanation was analyzed employing a conventional catalytic test. Adding Ca to Ni/ZrO₂, the metallic surface area did not change whereas the CO₂ consumption rate almost tripled. The XRD, XPS and EPR analyses showed that Ca⁺² but also some Ni²⁺ are on the ZrO₂ surface lattice of the Ni/CaZrO₂ catalyst. These cations form pairs which are composed of oxygen vacancies and coordinatively unsaturated sites (cus). By increasing the number of these pairs, the CO₂ methanation rate increases. Moreover, the number of active sites of the CO₂ methanation rate limiting step (CO and/or formate species decomposition, rls) is enhanced as well, showing that the rls occurs on the vacancies-cus sites pairs.