Ni-CeO2 Catalyst Research Articles

Exploring enhanced syngas production via the catalytic performance of metal‐doped X‐Ni/CeO2 (X = Zr, La, Sr) in the dry reforming of methane

AbstractBackgroundThe quest to manufacture large amounts of syngas to bridge fossil fuels and the renewable energy ecosystem stimulates the creation of efficient and stable heterogeneous catalysts. The NiCeO2 catalysts, synthesized via ultrasonic‐assisted citric acid complexation, are highly efficient for the dry reforming of methane (DRM) reaction. Different promoter metals (Zr, La and Sr) were tested for catalytic performance and syngas production. A range of analyses, including X‐ray diffraction (XRD), N2 physisorption, H2 temperature‐programmed reduction, CO2 temperature‐programmed desorption, field emission scanning electron microscopy (FESEM), transmission electron microscopy and X‐ray photoelectron spectroscopy, were employed to characterize the physicochemical properties of the catalysts.ResultsXRD results indicated the formation of NiO, CeO2, solid solution ceria–zirconia, perovskite LaNiO3 and SrNiO3 crystalline phases. FESEM results showed the promoted catalysts (Zr, La, Sr) produce large pores to facilitate reactant diffusion, with zirconia specifically creating a spiderweb morphology. At 800 °C, the CH4 and CO2 conversions follow the sequence of NiCeO2 catalyst (CH4 = 54%, CO2 = 45%) < Sr/NiCeO2 (CH4 = 60%, CO2 = 67%) < La/NiCeO2 (CH4 = 85%, CO2 = 84%) < Zr/NiCeO2 (CH4 = 95%, CO2 = 87%). The integration of promoters in DRM catalysts has notably improved carbon formation resistance, as evidenced by the following ranking: Zr/NiCeO2 (5.1 wt%) < commercial catalyst (6.0 wt%) < La/NiCeO2 (7.85 wt%) < Sr/NiCeO2 (10.9 wt%) < NiCeO2 (11.3 wt%).ConclusionThis study demonstrates that incorporating promoters, particularly Zr, in NiCeO2 significantly enhances resistance to carbon formation. It offers valuable insights into selecting metal catalyst promoters for excellent catalytic performance in DRM. © 2024 Society of Chemical Industry (SCI).

Three-Dimensional Mesoporous Ni-CeO2 Catalyst for Dry Reforming of Methane

Using the colloidal solution combustion approach, a three-dimensional mesoporous 5%Ni-CeO2-M catalyst was developed, with Ni incorporated into the pores, and applied in the dry reforming of methane. Comprehensive characterization revealed that the 5%Ni-CeO2-M catalyst had a large specific surface area and a three-dimensional mesoporous structure. A rich Ni-CeO2 interface was formed by closely spaced tiny CeO2 and NiO nanoparticles within the spherical pore wall. With very little carbon deposition over a 100 h period at 700 °C, the catalyst showed excellent activity and stability. The tiny Ni nanoparticles, along with the substantial Ni-CeO2 interfaces that make up this three-dimensional in-form mesoporous catalyst, are responsible for the outstanding effectiveness of this 5%Ni-CeO2-M catalyst.

Molybdenum-doping promoted surface oxygen vacancy of CeO2 for enhanced low-temperature CO2 methanation over Ni-CeO2 catalysts

It is challenging to improve the low-temperature activity of Ni-based catalyst for the CO2 methanation reaction. In this study, a molybdenum-doped cerium oxide supported Ni catalyst (Ni/MoCe) was constructed, and it showed remarkable CO2 methanation performance at low temperatures (<300 °C). Structural analysis revealed that the incorporation of Mo into the CeO2 lattice could enrich the surface oxygen vacancies, reduce the particle size of Ni species and enhance the metal-support interaction. The Ni/MoCe demonstrated a faster HCOO* pathway for the conversion of CO2 to CH4 compared with Ni/CeO2. Besides, the rich surface oxygen vacancies in Ni/MoCe facilitated the adsorption form of bidentate carbonates and the enhanced Ni-CeO2 interaction reduced the approach resistance of active H species from Ni for subsequent hydrogenation process. This work proves that Mo doping is an effective vacancy engineering strategy to design high-performance heterogeneous catalysts for CO2 hydrogenation.

Effects of catalyst morphology on oxygen defects at Ni–CeO2 interfaces for CO2 methanation

Inverse Ni–CeO2 catalysts contain dispersed reducible CeO2 islands supported on Ni particles that exhibit oxygen defect sites favoring CO2 methanation.

CO oxidation over three-dimensional transition metal-doped CeO2 catalysts: A density functional calculation study

Carbon monoxide (CO) oxidation is considered one of the effective methods to address environmental pollution caused by vehicle emissions and incomplete combustion of fuels. However, designing efficient and affordable catalysts has always been a major challenge. In this study, a series of transition metals (TMs)-doped CeO2(111) catalysts were designed based on density functional theory (DFT) to catalyze the oxidation reaction of CO. According to the results, TMs-CeO2(111) catalysts exhibited excellent thermal stability and adsorption performance. In particular, Fe-CeO2(111) and Ni-CeO2(111) catalysts displayed good catalytic activity, with their respective rate-determining step energy barriers being 0.57 and 0.43 eV. Of particular note, Fe-CeO2(111) demonstrated high reaction rates at low to medium temperatures, making it a promising candidate for CO oxidation reaction catalyst. This study provides new insights into designing efficient, low-cost, and readily accessible CO oxidation catalysts, and establishes a solid foundation for the development of catalytic combustion kinetic models.

Catalytic upgrading of bio-oil from halophyte seeds into transportation fuels

Because of socioeconomic considerations, wide-scale production of biofuel necessitates the utilization of nonedible biomass feedstock that does not compete for land and fresh water resources. In this regard, Salicornia bigelovii (SB) is the most investigated halophyte species. The high oil content in SB seeds has sparked mounting research that aims to utilize SB as an industrial crop in the production of bio-oil, particularly in coastal areas where these plants thrive. However, the oil extracted from the pyrolysis of raw SB seeds is largely dominated by oxygenated fatty acids, most notably 9,12-octadecadienoic acid and 9,17-octadecadienal, typical to that of other crops. The pyrolysate bio-oil of the raw SB seeds exhibited a relative yield of oxygenated compounds that decreased from 57.05 % at 200 °C to 9.81 % at 500 °C, and the relative yield of nitrogenated compounds increased from 4.86 % at 200 °C to 21.97 % at 500 °C. To improve the quality of the produced bio-oil, herein we investigated the catalytic hydrodeoxygenation (HDO) of the fragments that were produced from the thermal degradation of SB seeds. A 5 %Ni–CeO2 catalyst was prepared and characterized by a wide array of methods X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, scanning electron microscope, Brunauer-Emmett-Teller analysis, and thermogravimetric analyzer. The catalytic run was executed between 200 and 500 °C in a flow reactor. The deployed catalytic methodology displayed a profound HDO capacity. At 400 °C, for instance, the gas chromatography mass spectroscopy (GC–MS) detected loads of paraffin and aromatic compounds exists at appreciable values of 48.0 % and 28.5 %, respectively. With a total relative yield of 43.2 % (at 400 °C), C8–C15 species (i.e., jet fuel fractions) were the most abundant species in the upgraded SB bio-oil. The release of H2, CO, CO2, and CH4 was analyzed qualitatively and quantitatively using gas chromatography thermal conductivity detector and Fourier infrared spectroscopic analysis. When the Ni–CeO2 catalyst was utilized, a complete deoxygenated bio-oil was obtained from SB seeds using the surface-assisted HDO reaction. On the basis of the elemental analysis, the biochar's hydrogen and oxygen contents were found to decrease significantly. Density functional theory computations showed mechanisms for reactions that underpinned the experimentally observed hydrodeoxygenation process. Outcomes presented herein shall be instrumental toward the effective utilization of halophyte in the production of commercial transportation fuels.

Catalytic partial oxidation of methane over bimetallic Ru–Ni supported on CeO2 for syngas production

Methane (CH4) is the second most abundant greenhouse gas (GHG) after carbon dioxide (CO2) and is widely recognized as one of the most destructive GHGs, exerting negative impacts on the Earth's atmosphere. In order to effectively reduce CH4 emissions, the conversion of methane into synthesis gas, which is a mixture of hydrogen (H2) and carbon monoxide (CO), has emerged as an appealing approach. Recent advancements have demonstrated that catalytic partial oxidation of methane (POM) holds great promise as a reaction pathway for syngas production. Here we study a series of catalysts consisting of monometallic Ru and Ni, and bimetallic Ru–Ni supported on CeO2. These catalysts were synthesized using both mechanochemical and conventional incipient wetness impregnation methods. Various preparation parameters were investigated, including the Ru:Ni metal ratio, the order of metal addition, and the milling energy and time for the mechanochemically prepared samples. The catalysts were subjected to low-temperature POM (350–600 °C) experiments to evaluate their performance. The experimental results reveal that the bimetallic Ru–Ni/CeO2 catalysts exhibit superior catalytic activity and stability compared to the monometallic Ru–CeO2 and Ni–CeO2 catalysts. Notably, the bimetallic Ru–Ni/CeO2 catalysts prepared through ball milling demonstrate a significantly lower temperature requirement for syngas production compared to the conventional catalysts prepared via incipient wetness impregnation.

Transformation of levoglucosan into liquid fuel via catalytic upgrading over Ni-CeO2 catalysts

This contribution presents a viable gaseous hydrodeoxygenation (HDO) route for Levoglucosan (LG) that leads to the formation of non-oxygenated hydrocarbon cuts that make commercial transportation fuels, namely gasoline, diesel, and jet fuels. The outlined process encompasses HDO of an evaporated stream of dissolved LG over 5% Ni-CeO2 catalysts between 100 °C–500 °C. It is found that the load of the aliphatic compounds attains values between 68.1% and 75.3% across the investigated temperature window. Similarly, fractions of aromatic compounds remain within 8.1%–13.9%. Major observed aliphatic compounds include tetradecane, dodecane, octane, and decane. Alkylated benzenes appear in appreciable quantities. Governing HDO's mechanisms were mapped out by density functional theory (DFT) calculations. Utilizing a 10% load of Ni has slightly reduced the relative area of aliphatic compounds. The combined area of the oxygenated compounds remains less than 10% at all temperatures. This finding entails a profound HDO's capacity of the deployed catalyst and opens a direct venue for the effective utilization of LG in fuel production.

CO2 Methanation on Ni Catalysts Supported over Activated Carbons Derived from Cork Waste

Activated carbons (ACs) derived from cork wastes were for the first time used as supports for CO2 methanation catalysts. Cork wastes (natural-N and processed-P) were physically activated under N2/H2O(v) atmosphere at 800 °C to obtain the corresponding AC supports (ACN and ACP, respectively). Ni and Ni-CeO2 catalysts were prepared by loading the ACs with 15 wt % Ni (Ni/AC) or 15 wt % Ni and 15 wt % Ce (NiCe/AC) by incipient wetness impregnation or co-impregnation, respectively, using 2-propanol as the solvent. For comparison purposes, a commercial activated carbon (ACC) was impregnated with the same metals and loadings. The catalysts were characterized by N2 and CO2 sorption, XRD, TGA, SEM, and TEM, and finally tested at atmospheric pressure under CO2 methanation conditions (86 100 mL·h–1·gcat–1, H2/CO2 = 4:1). Microporous activated carbons with high surface areas (757–903 m2·g–1) were obtained from natural and processed cork wastes. While nickel was incorporated as Ni0 over the materials, CeO2 phases were detected on Ni-CeO2 catalysts. The incorporation of these metal species over the activated carbons induced a decrease of the textural properties, while the affinity toward CO2 was promoted by CeO2. In terms of average Ni0 particle sizes, while the smallest ones were detected on Ni/ACC (8 nm) for Ni/AC samples, the addition of CeO2 led to a remarkable decrease of this parameter on cork-derived materials (from 20 to 7 nm and from 33 to 16 nm on ACN and ACP, respectively). Higher CH4 yields were exhibited by CeO2-containing catalysts, with relative CH4 yield increases of 210–670% using Ni samples as the basis being observed. This was ascribed to CeO2’s ability to activate CO2 and promote Ni0 dispersion. The best results (CH4 yield of 73% at 400 °C) were obtained for the NiCe/ACN catalyst, its performance being similar to or higher than that reported in the literature for similar systems, thus motivating further development and optimization of these materials for CO2 methanation.

MvK mechanism dominated methane combustion over Ni-CeO2 derived from MOF by flame pyrolysis

Methane catalytic combustion is a significant technique in the fields of enhanced combustion, methane removal, and methane detection with the development of the natural gas industry and the demand for low-carbon combustion. Here, using a facile flame pyrolysis technique, two different types of Ni-CeO2 catalysts for methane combustion were synthesized from metal-organic framework (MOF) and metal salt (MS). Moreover, the Mars–van Krevelen (MvK) mechanism was investigated by combining kinetics with density functional theory (DFT) calculation. The performance shows that Ni-CeO2(BTC) catalysts give high activity owing to the large surface area, abundant Ni cations, and adsorbed oxygen. The high intrinsic activity is also related to a large number of oxygen vacancies which are essential for the MvK mechanism. In terms of kinetic analysis, the methane concentration determines the catalysis, and a redox MvK mechanism shows a good description of the experimental results. DFT calculations further clarify that, according to the MvK mechanism, the reduction of the active site is via dissociative adsorption of CH of CH4 on the catalytic site NiOCe, resulting in the formation of an oxygen vacancy, followed by dissociation of gas-phase oxygen on the oxygen vacancy to reoxidize the active site. Summarily, the MvK mechanism dominates the methane heterogeneous combustion over Ni-CeO2 and provides insight into the catalyst design for strengthening combustion.

Enhanced Conversion of Ethanol into n-Butanol over NiCeO2@CNTs Catalysts with Pore Enrichment Effects

The determining role of the pore structure in controlling the activity of porous catalysts has been observed in a variety of reactions. However, it is still a challenge to unveil the underlying reasons for the enhancements, especially for ethanol conversions. Herein, carbon nanotubes (CNTs) with different outer diameters were employed to prepare NiCeO2 catalysts for upgrading ethanol to n-butanol. According to physicochemical characterizations and reactant adsorption results, Ni and CeO2 species were encapsulated in CNTs of 4–6 nm outer diameter with confined micropores of 1.8 nm, which enriched the adsorption of acetaldehyde and correspondingly accelerated ethanol conversion. Moreover, the highly dispersed CeO2 in CNTs produced abundant acid and strong base pairs, which condensed the local concentrated acetaldehyde into n-butanol with the assistance of hydrogenation sites. By coupling the pore enrichment effect and improved acid–base pairs, the ethanol conversion was nearly doubled with improved n-butanol selectivity over NiCeO2@CNTs catalysts compared to that of the counterpart NiCeO2/CNTs catalysts.

Novel nanowire self-assembled hierarchical CeO2 microspheres loaded with nickel-based catalysts for hydrogen production from steam reforming of glycerol

The nickel-based catalysts with shape-specific CeO2 nanocrystals (hierarchical and spheres) and adjustable impregnation solvents were successfully synthesized. The synthesized Ni-CeO2 catalysts were used as the model catalyst to study the structure-dependent catalytic behavior and catalytic reaction mechanism for hydrogen production from glycerol steam reforming (GSR). Ni/CeO2-HE with the hierarchical structure and ethanol impregnation solvent obtained the strongest metal-support interaction. Electron effects because of the strong interaction between nickel and cerium dioxide promoted the formation of reducible nickel species. High nickel reduction and nickel dispersion contributed to the maximum active nickel exposure surface area, thus Ni/CeO2-HE exhibited superior glycerol conversion than other catalysts. The abundant oxygen vacancies generated due to strong Ni-CeO2 interaction facilitated the dissociation of water, thereby achieving a high yield of H2 for Ni/CeO2-HE. As a comparison, Ni/CeO2 and Ni/CeO2-E showed relatively poor catalytic activity due to lower exposed specific surface area and oxygen vacancy concentration. Moreover, the strong oxygen storage-release ability (CO-OSCC) promoted the oxidation of carbon deposition on Ni/CeO2-HE. As a result, Ni/CeO2-HE obtained good catalytic stability with no inactivation within 25 h. The study of the modulation role of Ni-CeO2 interaction on catalytic behavior can provide valuable guidelines for designing efficient catalysts for GSR.

The synergy of Ni doping and oxygen vacancies over CeO2 in visible light-assisted thermal catalytic methanation reaction

The photo-assisted thermal catalytic system has provided a new alternative way to drive highly efficient CO2 methanation reaction. In this work, we investigated the role of visible light for thermal catalytic methane synthesis, emphasizing the synergy effect of Ni doping and oxygen vacancies for the hydrogenation of CO2 over Ni-CeO2 catalysts. It was found that the optimal 7.5 wt% Ni-CeO2 catalysts exhibited the best visible light-promoted CO2 methanation performance. At 250 °C, the light-promoted performance increased from 0.67 to 0.93 mmol−1·g−1·h which increased by 39 %. Extensive characterization demonstrates that heteroatom Ni doping increases the content of oxygen vacancies which narrows the band gap and improves the electron density of the catalyst, resulting in accelerating the transformation of carbonate and CO intermediate species which is beneficial to yield CH4. Furthermore, the visible-light-induced regeneration of oxygen vacancies also provides more active site to facilitate the adsorption and activation of CO2 which co-promoted the photo-assisted thermal catalytic CO2 methanation reaction. The synergy effect of Ni doping and oxygen vacancies provide a new strategy to design a high-efficiency photo-assisted thermal catalyst.

A high-performance Ni-CeO2/Ni/Ni-Y2O3·ZrO2 three-layer anode for direct iso-octane feeding of solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) directly fed with iso-octane are expected to be power sources for mobile devices and automobiles. However, the conventional anode catalysts nickel (Ni) or cerium oxide (CeO2) used for direct feeding of iso-octane do not suppress carbon deposition or generate high power. In this study, we investigated the Ni-CeO2/Ni/Ni-yttria-stabilized-zirconia (YSZ) three-layer anode to establish the suppression of carbon deposition and high-power generation in the SOFC. The anode consists of a Ni-CeO2 catalyst layer as the top layer, an Ni catalyst layer as the second layer, and a Ni-YSZ catalyst layer as the third layer on top of the electrolyte. The concept of the three-layer anode is as follows: fuel reforming occurs in the Ni-CeO2 layer, the reformed H2 or CO is electrochemically oxidized in the Ni-YSZ catalyst layer, and the Ni catalyst middle layer prevents the reaction between YSZ and CeO2. Scanning electron microscopy and electrochemical characterization confirmed carbon deposition suppression and improved power generation. The anode showed no carbon deposition and generated high-power, 600 mA cm−2 and 150 mW cm−2, at 950°C and a steam/carbon ratio of 3.0. Additionally, we discuss the fuel reforming reactions on the three-layer electrode by the results of exhaust gas analysis.

Synthesis of Ni-doped ceria nanoparticles and their unusual surface reduction in hydrogen

This work presents an original approach to preparing pure and Ni-doped CeO2 nanoparticles (NPs) that can be directly drop-casted on a substrate or calcined to form powders. The reduction of the NPs in H2 is very different than the one usually anticipated for supported Ni–CeO2 catalysts. In situ soft X-ray absorption and infrared spectroscopies revealed that the reduction of Ce4+ into Ce3+ in H2 proceeds via simultaneous oxidation of Ni2+ ions into Niδ+ (2<δ<3). Comparison with reference samples indicates that Ce4+ ions reduction is promoted over Ni-doped CeO2 NPs, whereas that of Ni2+ is hindered. Theoretical simulation of Ni L-edge spectra suggested that Ni dopant into ceria is in a square planar four-coordinate environment, in contrast to the familiar octahedral symmetry of bulk nickel oxides. Our results reveal that the surface chemistry of Ni-doped CeO2 is quite distinct as compared to that of the individual bulk oxides, which potentially can lead to a different performance of this material, notably in catalytic applications.

Ni-CeO2 Catalyst with High Ni Loading Prepared by Salt-Assisted Solution Combustion for CO2 Methanation

An Ni-CeO2 catalyst with high Ni loading (50 wt.%) prepared by a salt-assisted solution combustion method was characterized by different methods and used for CO2 methanation. The specific surface area of the Ni-CeO2 catalyst prepared by salt-assisted solution combustion is 7 times that of the catalyst prepared by conventional solution combustion. The Ni-CeO2 catalyst prepared by salt-assisted solution combustion has smaller particle sizes of Ni and exhibits excellent activity at low temperatures. The high Ni loading and small Ni particle size can provide more metal Ni site and Ni-CeO2 interface, which help to improve the CO2 methanation performance.

Promotion of methanation suppression by alkali and alkaline earth metals in Ni-CeO2 catalysts for water–gas shift reaction using waste-derived synthesis gas

In this study, alkali and alkaline earth metal-promoted Ni-CeO2 catalysts were developed for the high-temperature water–gas shift reaction of waste-derived synthesis gas in which hydrogen is produced. The elements K, Ca, Mg, and Ba were selected to repress methanation, which is a major side reaction of Ni-based catalysts. The prepared catalysts were characterized using different techniques including the Brunauer–Emmett–Teller method, X-ray diffraction, H2-temperature programmed reduction, CO2-temperature programmed desorption, H2-chemisorption, and X-ray photoelectron spectroscopy to determine the correlations between the catalyst activity and their properties. The results show that the alkali and alkaline earth metal-promoted Ni-CeO2 catalyst effectively suppressed methanation. Among the promoted catalysts, the Ca/Ni-CeO2 catalyst exhibited the highest activity and stability for 18 h at a very high gas hourly space velocity of 1,050,957 h−1, which is mainly due to the high oxygen storage capacity.

Wheat-Straw-Derived Activated Biochar as a Renewable Support of Ni-CeO2 Catalysts for CO2 Methanation

Ceria- and urea-doped activated biochars were used as support for Ni-based catalysts for CO2 methanation purposes. Different materials were prepared and tested to find the best catalytic formulation. After several CO2 methanation experiments—carried out at 0.35–1.0 MPa and 300–500 °C—it was found that the most suitable catalyst was a wheat-straw-derived activated biochar loaded with 30 wt.% of CeO2 and 20 wt.% of Ni. Using this catalyst, a CO2 conversion of 65% with a CH4 selectivity of 95% was reached at 1.0 MPa, 400 °C, and 13,200 h−1. From the study of the influence of the gas hourly space velocity, it was deduced that the most likely reaction mechanism was a reverse water–gas shift reaction, followed by CO hydrogenation. N-doping of the carbon support as an alternative to the use of ceria was also investigated. However, both CO2 conversion and selectivity toward CH4 values were clearly lower than those obtained for the ceria-containing catalyst cited above. The outcomes of this work indicate that a renewable biomass-derived support can be effectively employed in the catalytic conversion of CO2 to methane.

Boosting hydrogen production from steam reforming of ethanol on nickel by lanthanum doped ceria

Stable and efficient hydrogen production has always been the focus of industry, and steam reforming of ethanol (SRE) is one of the most robust, renewable and cheap technologies. To develop excellent SRE catalysts highly resisting coke formation and metal sintering, La, Tb, and Zr have been doped as a third metal into Ni-CeO2 catalyst using a sol-gel method. The size and dispersion of Ni nanoparticles, surface oxygen vacancies and metal-supported interaction have been finely tuned, with which the catalytic performance is closely associated. La-doped ceria supported Ni (Ni-CeLa0.20) exhibited superior catalytic performance, which is ascribed to its large Ni active surface area, abundant oxygen vacancies and suitable metal-supported interaction. DFT calculations showed that the doping of La into Ni-CeO2 can promote the generation of oxygen vacancies and facilitate the dissociation of water. On Ni-CeLa0.20, complete conversion of ethanol and stable hydrogen production have been maintained in the 3000-min test. The coke deposition is only 0.48 mgc/gcat h at 600 °C under the GHSV of 55920 mL/gcat∙h.

CO2 methanation on Ni-Ce0.8M0.2O2 (M=Zr, Sn or Ti) catalyst: Suppression of CO via formation of bridging carbonyls on nickel

The effect of different cations (M=Zr, Sn or Ti) doping on ceria supported nickel (Ni-Ce0.8M0.2O2) catalysts and their performance in the CO2 methanation reaction has been investigated. Among the prepared catalysts, the Ni-CeO2 (CN) catalyst exhibited the best catalytic performance with a CO2 conversion of 62.9% at 400 °C. The selectivity on Ni-Ce0.8Zr0.2O2 (CZN) remained above 98.4% from 200 to 400 °C. As confirmed by in-situ DRIFTS, the dissociative adsorption of H2 mainly proceeds on Ni, by-product CO is mainly assigned to the releasing of weak-binding linearly adsorbed CO on nickel, which is from the hydrogenation of formates with adsorbed H. The CO-suppressing effect is attributed to the strong adsorption of carbon monoxide on nickel via the forming of bridging carbonyls. It was found that CO2 is adsorbed on the catalysts to form carbonate, bicarbonate and formate species. The formate species could be further hydrogenated by dissociated H and then finally release methane.