CO2 Methanation Research Articles

Facilitating photocatalytic CO2 methanation via synergetic feed of photon-induced carriers and reactants over S-scheme BiVO4@TiO2 nanograss/needle arrays

Herein, a unique structure BiVO4@TiO2 nanograss/needle arrays S-scheme heterojunction photocatalyst for CO2 photoreduction reaction was created. The photocatalyst can drive electrons to the reactive spots spontaneously and continuously, which effectively tackles the problem of severe recombination and disordered migration of photogenerated carriers. Theoretical calculations and experimental results demonstrate that adding BMIM-BF4 ionic liquid to generate BMIM-*CO2 intermediate promotes CO2 activation and enrichment of reactants concentration around the catalytic sites. Benefiting from the sufficient electron supply of catalytic sites and reinforced reactants supply in the interfacial microenvironment, the BiVO4@TiO2 photocatalyst shows high selectivity and activity of CO2-to-CH4 conversion. The CH4 selectivity increased to 57.3 % (132.7 μmol m-2h−1) and the methanation activity was increased by 8.6 times in the 5.0 % BMIM-BF4 aqueous solution. Significantly, the BiVO4@TiO2 NNAs catalyst obtains a solar-to-fuels energy efficiency of ∼ 0.46 ‰ under ultraviolet-enhanced light sources without any sacrificial agent or co-catalyst. This work displays the relationship between reaction process factors (electron supply, CO2 molecule, and proton supply) and CO2 photoreduction activity and selectivity, giving new insight into achieving efficient CO2 photoreduction conversion.

Understanding the Self‐Reconfiguration Properties of a Novel Spinel‐Type High‐Entropy Catalyst

AbstractHigh‐entropy oxides (HEOs), known for their excellent thermodynamic stability due to their high‐entropy effect. In this study, high‐entropy (NixAlCoCrMn)3O4 oxides of spinel structure were synthesized by sol‐gel method. This HEO has a self‐reconfiguration properties in the reaction, and its internal Ni and Co ions exsolve to form a uniformly dispersed NiCo alloy on the surface. The increase of Ni content promotes the exsolution of Ni and Co ions from the interior of the spinel. The high‐entropy with a large number of oxygen vacancies not only possesses a stable structure to limit the migration of NiCo alloys, but also enhances the hydrogen overflow. In the CO methanation reaction, it has 99 % CO conversion with 87 % methane selectivity within 400 °C, and it is stable in the long‐term test. This work provides a new idea for exploring the use of high‐entropy materials to improve thermal catalytic activity and stability.

Study on the arrangement of CO2 sorbent and catalyst for integrated CO2 capture and methanation

Integrated CO2 capture and utilization (ICCU) is an emerging technology to reduce CO2 emissions by greatly simplifying the processes of conventional CO2 capture and utilization. In particular, ICCU-methanation could directly convert the low-concentration CO2 into CH4 using hydrogen from new energy electricity, representing an efficient Power-to-Gas route. CO2 adsorption/catalytic materials play a crucial role for the operation of ICCU-methanation, and it is more feasible to scale up material production and avoid components interference by directly combining the sorbent and catalyst. However, it is necessary to reveal the effect of different arrangements of sorbent and catalyst on reaction characteristics of ICCU-methanation. In the work, the matching of K2CO3-doped Li4SiO4 sorbent with various supported Ni-based catalyst was tested and four arrangements of sorbent and catalyst particles in a fixed-bed were investigated to understand the influences on ICCU-methanation. Results show that the presence of catalyst accelerates CO2 supply by sorbent and achieves quicker methanation, and the promotion effect becomes more obvious with a closer contact between the sorbent and catalyst. Under optimized conditions, ICCU-methanation of uniformly mixed K-Li4SiO4 and Ni/Al2O3 shows an excellent performance with very stable CO2 conversion of 95.58% and CH4 selectivity of 93.29% during cyclic reactions.

Rare earth oxide promoted Ru/Al2O3 dual function materials for CO2 capture and methanation: An operando DRIFTS and TGA study

Dual-function materials (DFMs) combine sorbent and catalytic components to perform selective CO2 capture and subsequent hydrogenation. This study explores the performance of rare-earth oxides (REOs) as CO2 adsorption sites on Ru/Al2O3. REOs increase CO2 uptake by upwards of +60 % by enhancing the overall catalyst surface basicity and favoring metal–support interactions. Thermogravimetric analysis during CO2 adsorption-hydrogenation cycles exhibited significant catalytic activity and enhanced stability of Ru-REO/Al2O3 at temperatures as low as 200 °C. This leads to methane production of 50–85 µmol g−1, surpassing recently reported values obtained for alkali and alkali-earth promoted Ru-based materials operated at 250 °C. The highest performing studied DFM, RuNd2O3/Al2O3, achieved 85 % CO2 capture efficiency and steadily produced methane in cyclic operation (+120 % CO2 uptake relative to Ru/Al2O3). Operando DRIFTS revealed that the dominant mechanism for methane formation is the hydrogenation of ruthenium carbonyls, which are stabilized by REOs. Upon CO2 exposure, surface carbonates and bicarbonate species form more abundantly on DFMs than on Ru/Al2O3. This confirms that REOs enhance the adsorption and retention of carbonates, which generate additional promoter-related reaction pathways during low-temperature hydrogenation. These findings are crucial in the advancement of sustainable, wider operation range carbon capture and utilization technologies.

Boosting Photocatalytic CO2 Methanation through Interface Fusion over CdS Quantum Dot Aerogels.

In the field of photocatalytic CO2 reduction, quantum dot (QD) assemblies have emerged as promising candidate photocatalysts due to their superior light absorption and better substrate adsorption. However, the poor contacts within QD assemblies lead to low interfacial charge transfer efficiency, making QD assemblies suffer from unsatisfactory photocatalytic performance. Herein, a novel approach is presented involving the construction of strongly interfacial fused CdS QD assemblies (CdS QD gel) for CO2 reduction. The novel CdS QD gel demonstrates outstanding photocatalytic performance for CO2 methanation, achieving a CH4 generation rate of ≈296 µmol g-1 h-1, with a selectivity surpassing 76% and an apparent quantum yield (AQY) of 1.4%. Further investigations reveal that the robust interfacial fusion in these CdS QDs not only boosts their ability to absorb visible light but also significantly promotes charge separation. The present work paves the way for utilizing QD gel photocatalysts in realizing efficient CO2 reduction and highlights the critical role of interfacial engineering in photocatalysts.

Enhancing CO2 methanation via doping CeO2 to Ni/Al2O3 and stacking catalyst beds

This work synthesized a series of Ni/CeO2/Al2O3 catalysts with varying CeO2 doping amounts to enhance low-temperature CO2 methanation. The introduction of CeO2 weakens the interaction between Ni and Al2O3, leading to the formation of Ni-CeO2 active sites. This results in a high dispersion of Ni and CeO2, improved catalyst reducibility, increased number of active sites, and enhanced the CO2 methanation. This work further investigated the impact of WHSV and catalyst stacking configuration to enhance the reaction. When the catalyst is stacked into three segments with a temperature gradient of 330 °C, 300 °C, and 250 °C under WHSV = 9000 ml·h–1·(g cat)−1, the CO2 conversion significantly increases to 95%, which is remarkably close to the thermodynamic equilibrium (96%).

Performance of supported Co3O4 catalysts in the preferential oxidation of carbon monoxide: Effect of support type and synthesis method

This study investigated the performance of supported Co3O4 catalysts during the preferential oxidation of carbon monoxide (CO-PrOx) in a H2-rich environment, focusing on the effects of different catalyst synthesis methods, namely, wetness impregnation (WI) and solution combustion synthesis (SCS), and different support materials, namely, Al2O3 and SiC. During CO-PrOx, the SiC-supported Co3O4 catalysts attained higher CO2 yields when compared with the Al2O3-supported Co3O4 catalysts possibly because of the existence of weaker interactions between Co3O4 and SiC. Moreover, the catalysts prepared via SCS achieved higher CO2 yields than the catalysts prepared via WI likely due to the presence of smaller and well-dispersed Co3O4 particles in the SCS-prepared catalysts. Significantly high amounts of unwanted CH4 were produced over the SiC-supported catalysts between 225 and 250 °C. The high CO methanation activity was also attributed to the weaker Co3O4-SiC interactions, which enabled the easier reduction of Co3O4 to methanation active metallic Co.

Optically selective catalyst design with minimized thermal emission for facilitating photothermal catalysis

Converting solar energy into fuels is pursued as an attractive route to reduce dependence on fossil fuel. In this context, photothermal catalysis is a very promising approach through converting photons into heat to drive catalytic reactions. There are mainly three key factors that govern the photothermal catalysis performance: maximized solar absorption, minimized thermal emission and excellent catalytic property of catalyst. However, the previous research has focused on improving solar absorption and catalytic performance of catalyst, largely neglected the optimization of thermal emission. Here, we demonstrate an optically selective catalyst based Ti3C2Tx Janus design, that enables minimized thermal emission, maximized solar absorption and good catalytic activity simultaneously, thereby achieving excellent photothermal catalytic performance. When applied to Sabatier reaction and reverse water-gas shift (RWGS) as demonstrations, we obtain an approximately 300% increase in catalytic yield through reducing the thermal emission of catalyst by ~70% under the same irradiation intensity. It is worth noting that the CO2 methanation yield reaches 3317.2 mmol gRu−1 h−1 at light power of 2 W cm−2, setting a performance record among catalysts without active supports. We expect that this design opens up a new pathway for the development of high-performance photothermal catalysts.

Enhancement of magnesium aluminate-based nickel and cobalt nanostructured catalysts with iron for improved performance in carbon dioxide methanation

This study investigates the performance of Ni and Co catalysts based on Fe-promoted MgAl2O4 for CO2 methanation, which is a crucial step in mitigating environmental carbon dioxide levels. The MgAl2O4 support was modified with various Fe loading (5, 10, and 15 ​wt%) and fabricated via a novel coprecipitation technique with the help of ultrasonic waves and chosen as support for 15 ​wt% Ni and Co active phases. Examination of the BET surface properties of the catalysts showed an increase in surface area in the range of 54–82 ​m2/g and 73–85 ​m2/g with an increasing Fe loading for Ni and Co catalysts, respectively. Among the Ni-based catalysts, the 15Ni/10FeMgAl2O4 specimen exhibited the best performance (with a 73.31 ​% CO2 conversion and 95.61 ​% selectivity rate) and remarkable lifetime during 10 ​h at 400 ​°C due to the better reducibility and the increase in hydrogen consumption. However, a rise in Fe amount to 15 ​wt% led to a reduction in the CO2 conversion to 34.43 ​%. The catalytic outcomes also demonstrated that the presence of Fe in Co/MgAl2O4 catalysts negatively affects catalytic performance. The unpromoted Co/MgAl2O4 sample demonstrated the best performance, achieving a conversion rate of 52.41 ​% at 350 ​°C.

CO2 methanation on Ni/Y2O3 Catalysts: The effects of preparation procedure and cerium oxide promotion

Present paper reports on the CO2 methanation (CME) performance of novel Ni catalysts supported on yttria, ceria, and ceria-doped yttria carriers. Catalysts were synthesized using incipient wetness impregnation and precursor nitrate co-decomposition. These systems outperformed on a mass-specific basis an industrial Ni/alumina benchmark catalyst. In addition, this work examines in details the impact of both the preparation procedure and the Ni-content on the CME activity both in terms of the reaction rate and the apparent activation energy. Our research demonstrates that Y2O3 is an effective catalytic support promoting stability and activity of the catalysts. CeO2–doping further improves activity, remarkably with little to no penalty to the thermal stability of the catalysts. Comparison of the performance of a conventional IWI-synthesized Ni-catalyst with a counterpart obtained via a co-decomposition route, demonstrated an advantage of the latter. Our findings suggest Ni-CeO2/Y2O3 prepared via co-decomposition may be advised as the active and stable catalysts for CME applications.

Preparation of Sustainable Ni/SiO2 Catalysts from Rice Husk by Different Methods for CO2 Methanation.

Silicon dioxide (SiO2) from rice husk can be extracted and be used as support for Ni-based catalysts. The impregnation method (IM) is usually used for preparing Ni/SiO2 catalysts, but its catalytic activity in CO2 hydrogenation to CH4 remains unsatisfactory. In this work, we explored alternative preparation methods, using ammonia evaporation method (AEM) and hydrothermal method (HM) to prepare the catalysts. The results showed that the catalysts prepared by AEM and HM were significantly superior to that prepared by IM. Notably, the catalyst synthesized by AEM from sustainable silica exhibited the best performance, achieving 81.69% CO2 conversion and over 99% methane selectivity at low reaction temperature of 300 °C. The characterization techniques indicate that the Ni/SiO2-AEM catalyst can form nickel phyllosilicate with lamellar structure, leading to better Ni dispersion and higher specific surface area. Furthermore, the results of in-situ DRIFTS have revealed the potential catalytic mechanism over Ni/SiO2 catalysts, indicating that it involves pathways with both the CO* and HCOO* as the key intermediates.

Improvement in the activity of Ru/ZrO2 for CO2 methanation by the enhanced hydrophilicity of zirconia

Increasing interests in the supported Ru catalysts for CO2 methanation can be recently found in the literature. In this work, we demonstrated that the enhanced surface hydrophilicity of ZrO2 via air plasma treatment has significant effects on the properties of Ru/ZrO2 catalyst for CO2 methanation. At the same CO2 conversions, the reaction temperature over the catalysts on ZrO2 with enhanced hydrophilicity is 20–70 °C lower than those on untreated ZrO2. The catalyst characterization confirms that the enhanced hydrophilicity leads to more hydroxyl groups and oxygen vacancies on the support, which further promotes CO2 adsorption and activation, facilitating the conversion of CO2 to HCO3* and HCOO* in formate pathway. The enhanced hydrophilicity also causes a high Ru dispersion with stronger electronic interaction between Ru and ZrO2, which forms more interfacial active sites and improves the adsorption and dissociation of H2, promoting the linear-CO-Ru0 adsorption in CO* pathway.

Engineering the interfaces in MgO-modified Ni/Al2O3 for CO2 methanation

The interaction between metallic active phases and oxides is of great significance for catalytic properties, but still remains elusive. Here, two reverse interfaces are constructed (Ni/MgO and MgO/Ni on Al2O3) for a typical CO2 methanation catalyst, Ni/Al2O3, by depositing Ni and MgO in opposite sequences on Al2O3. Enhanced performance was found on both interface structures, however, distinct turnover frequencies indicate different mechanisms. With MgO present in between Ni and Al2O3, the formation of an inactive NiAl2O4 spinel phase is mitigated, making more NiO available for reduction. When MgO is added on top of Ni, the new MgO/Ni interface exhibits high reactivity in CO2 methanation. CO2 temperature programmed desorption, in situ quick X-ray adsorption spectroscopy (QXAS) and modulation excitation XAS (MEXAS) demonstrate that a Ni-NiO redox mechanism occurs with enhanced CO2 activation at the MgO/Ni interface. The subsequent hydrogenation of adsorbed carbon monoxide and carbonate species requires nearby Ni to provide H spillover and occurs preferably at the interface sites, where adsorbed species are more easily activated. Hence, interfaces between the same compounds, but with reverse structures, result in different phenomena, illustrating the role an interface structure can play in catalytic systems.

Green synthesis of Mn3O4@CoO nanocomposites using Rosmarinus officinalis L. extract for enhanced photocatalytic hydrogen production and CO2 conversion

In response to the urgent need for sustainable environmental remediation technologies and clean energy production, this study introduces an eco-friendly synthesized Mn3O4@CoO nanocomposite (MCNC) leveraging the natural reducing capabilities of Rosmarinus officinalis leaves (L.) extract. Focused on the hydrogen evolution reaction (HER) and CO2 methanation, our findings reveal significant enhancements in photocatalytic activity and CO2 methanation efficiency. The MCNC exhibited a promising hydrogen production rate of up to 823 μmol g−1 h−1 and a CO2 conversion efficiency exceeding 84 % under optimal conditions. The investigation highlights the synergistic interfacial effect between Mn3O4 and CoO, contributing to notable performance improvements. Additionally, the study explores the role of catalyst quantity in optimizing reaction efficiencies, presenting a scalable and environmentally benign approach to address global energy and environmental challenges. The apparent quantum efficiency (AQE%) was calculated to be 1.92, 2.39, 2.73, 3.21, 3.44, and 3.56 % for mass values of 15, 25, 35, 45, 55, and 65 mg, respectively. Furthermore, the CO2 methanation also shows a rise with the increase in catalyst quantity. A mass of 0.35 mg achieved a remarkable CO2 conversion efficiency of 71.9 %, while its 0.5 mg counterpart showcased an even more striking CO2 conversion efficiency of 84.4 %. The green synthesis method, characterized by its low energy requirement and the absence of toxic chemicals, aligns with the sustainable practices sought in environmental chemistry and nanotechnology. This work not only opens new avenues for developing efficient photocatalysts but also contributes to the broader application of green synthesized nanomaterials in environmental remediation and clean energy generation.

Bimetallic MOF@CdS nanorod composite for highly efficient piezo-photocatalytic CO2 methanation under visible light

CO2 methanation is leading progress in both dwindling the emitted greenhouse gas and taking advantage of CO2 conversion to a worthwhile fuel. Various types of catalysts have gained researchers' attention. On the other hand, those catalysts chiefly suffer from being uneconomical, owning laborious processes, and having low efficiency. Particularly in the photocatalytic process, electron-hole recombination, charge separation efficiency, and the photocorrosion are the most remarkable obstacles in the path of gaining high efficiency. To conquer the aforementioned hindrances, Cu/Zr-MOF@CdS had been designed in order to not only do elevate CH4 selectivity but also increase CO2 conversion by altering the electron transfer mechanism. Doping Cu in Zr-MOF structure restrains C-C coupling and ameliorates the viability of protonation of *CO to *HCO during methane production. CdS and Zr-MOF both grant piezoelectricity trait to the catalyst in a way that by merging it with the photocatalytic process the mechanism of process converted from type (II) scheme to Z-scheme, culminating in thwarting recombination and increase of charge separation efficiency. The photocatalytic process achieved 23.6 μmol. g−1. h−1 CH4 reaction rate and 80 % CO2 conversion, hereafter applying the piezo-photocatalytic process, these two factors reached 52.2 μmol. g−1. h−1 and 99 %, respectively. This work unveils the viable reaction routes along with their several quotas in piezo-photocatalytic CO2 methanation process by scrutinizing the intricate mechanisms via in-situ analyses.

Ba2+ doping into Ni/Al2O3 nanofibers promotes CO2 methanation via alkaline modulation

Ni-based catalysts are promising catalysts for CO2 methanation due to low lost. However, the activity and selectivity of Ni-based catalysts in CO2 methanation at low temperatures still need to be improved. Here, Ni4Al2BamOx (m = 0–0.5) nanofibers were prepared. Doping Ba2+ would increase alkaline sites and facilitate generating oxygen vacancies. Especially, Ni4Al2Ba0.2Ox exhibited the high specific surface area with 127.1 m2 g−1, being potential for exposing more active sites. Indeed, compared with undoped Ni4Al2Ox catalysts (CO2 conv. = 45 %, CH4 select. = 92 % at 300 °C), Ba2+ doping significantly improved activity (CO2 conv. = 74 %, CH4 select. = 99 % at 300 °C) and stability within 200 h for Ni4Al2Ba0.2Ox. Both EPR and O1S XPS confirmed that Ni4Al2Ba0.2Ox can form more oxygen vacancies and CO2-TPD confirmed that Ni4Al2Ba0.2Ox had stronger CO2 adsorption capacity compared to Ni4Al2Ox. In-situ infrared spectroscopy and DFT calculations both indicated that Ba2+ doping can promote generating surface hydroxyl groups and formate pathways.

Rational H2 Partial Pressure over Nickel/Ceria Crystal Enables Efficient and Durable Wide-Temperature-Zone Air-Level CO2 Methanation.

On the way to carbon neutrality, directly catalyzing atmospheric CO2 into high-value chemicals might be an effective approach to mitigate the negative impacts of rising airborne CO2 concentrations. Here, we pioneer the investigation of the influence of the H2/CO2 partial pressure ratio (PPR) on air-level CO2 methanation. Using Ni/CeO2 as a case catalyst, increasing H2/CO2 PPR significantly improves low-temperature CO2 conversion and high-temperature CH4 selectivity, i.e., from 10 of H2/CO2 PPR on, CO2 is completely methanized at 250 °C, and nearly 100% CH4 selectivity is achieved at 400 °C. 100-hour stability tests demonstrate the practical application potential of Ni/CeO2 at 250 °C and 400 °C. In-situ DRIFTS reveal that reinforced formate pathway by increasing H2/CO2 PPR is responsible for the high CH4 yield. In contrast, even though the CO pathway dominated CO2 conversion on Ni is enhanced by rising H2/CO2 PPR, but at a high reaction temperature, the promoted CO desorption still leads to lower CH4 selectivity. This work offers deep insights into the direct air-level CO2 resourceization, contributing to the achievement of airborne CO2 reductions.

Enhancing low-temperature CO2 methanation performance by selectively covering Ni with CeO2 to form Ni-O-Ce interface active sites

The methanation of CO2 is crucial for resource utilization, however, achieving excellent CO2 methanation performance with Ni-based catalysts at low temperatures remains highly challenging. In this study, a high-performance 10Ni-0.1Ce/Al catalyst was synthesized using a simple and efficient one-pot combustion method. Unlike other catalysts with the same components, this catalyst exhibits a unique feature where a small amount of the CeO2 promoter is dispersed on the support, while the majority selectively binds to and coats the Ni nanoparticles. This distinctive structure allows it to achieve approximately 80 % CO2 conversion and 99.9 % CH4 selectivity under conditions of 260 °C, 0.1 MPa, and a GHSV of 48,000 mL·g−1·h−1. The high catalytic performance is attributed to the unique structure of Ni nanoparticles and CeO2, which promotes the formation of numerous Ni-O-Ce interface active sites. These Ni-O-Ce interfaces enhance the reducibility and increase the content of weak basic sites in the catalyst. The combination of in-situ DRIFTS and TPSR reveals that the decomposition of bridged HCOO– to *CO is the rate-determining step. The presence of Ni-O-Ce interface active sites in the 10Ni-0.1Ce/Al catalyst facilitates the generation of abundant bridged HCOO– intermediates with higher activity than the 10Ni/Al catalyst, leading to a substantial enhancement in catalytic performance.

Impact of nickel and alkaline earth metals interaction on sustainable carbon nanotubes generation from plastic face masks via catalytic pyrolysis

The one-pot synthesis of carbon nanotubes (CNTs) emerges as a promising approach for plastic valorization, owing to its simplicity and scalability. However, limited attention has been given to catalyst design for this method, particularly regarding the influence of Group 2 alkaline earth metal elements (X) on the Ni-based catalyst activity. This study investigates the impact of X on the catalytic activity of Ni towards sustainable CNTs synthesis using plastic face masks. The findings revealed that X influenced the carbon yield of NiMo-X catalysts and the morphology of the resultant carbon nanomaterials. Notably, NiMo-Ca demonstrated the highest carbon yield, whereas NiMo-Mg and NiMo-Ba exhibited the lowest. Furthermore, NiMo-Mg tends to produce clean and thin CNTs, while NiMo-Ba tends to yield carbon flakes with numerous irregular cokes. In addition to the effect of Ni crystallite size, which influences the transition from tubular to flake-like carbon structures, environmental factors are also considered. Specifically, NiMo-Mg generated a high CO2/CH4 environment, while NiMo-Ba exhibited slower catalytic cracking of polymeric chains from face masks, both of which were unfavorable for CNTs growth. Conversely, NiMo-Ca facilitated higher carbon recovery, likely due to its creation of a low CO2/CH4 environment via CO2 methanation. Optimization studies indicated that increasing pyrolysis temperatures and durations lead to reduced carbon yield, while cyclic pyrolysis of face masks up to five iterations enhances the reusability of NiMo-X catalysts with improved carbon recovery. Overall, this study provides valuable insights into the early pyrolytic gas formation and carbon nucleation phase, which are influenced by the surface Ni fraction and Ni polarization, as well as the later CNTs growth phase, which is related to the Ni crystallite size.

Revisiting the influence of Ni particle size on the hydrogenation of CO2 to CH4 over Ni/CeO2

Nickel nanoparticles were supported on CeO2 and evaluated in the hydrogenation of CO2 to CH4 at 538 K and 1 atm. Chemisorption of H2 was used to estimate the average Ni particle size, which ranged from 1.3 to 17 nm. The apparent activation energy for CH4 formation and turnover frequency for CO2 conversion was independent of Ni particle size. The smallest Ni particles (1.3 nm) were less selective to CH4. Comparisons to alumina-supported Ni reveal the same performance as Ni/CeO2 when normalized to active Ni atoms counted by H2 chemisorption. A beneficial effect of ceria relative to alumina is the enhanced reducibility of Ni as evaluated by H2 temperature-programmed reduction. The constancy of the turnover frequency on Ni particles suggests the acid-base and/or redox properties of the support do not play a significant role in the CO2 methanation reaction under the conditions of study, which contrasts many earlier works.