Low Temperature CO2 Reforming with Methane Reaction over CeO2-Modified Ni@SiO2 Catalysts

Ni/MSS@CeO2 sandwich catalysts for methane dry reforming: the role of reduction on oxygen vacancies

In-depth exploration of the key role of reduction temperature in the Ni-SiO2-CeO2 system DRM catalysts remains a huge challenge. This work deeply analyzes the role of reduction temperature on the oxygen vacancies and structure of the Ni/MSS@CeO2 (MSS refers to mesoporous SiO2 spheres with inverted cone pores) sandwich structure catalyst. Studies have shown that appropriate reduction temperature can significantly facilitate the Ni distribution, boost the Ni-CeO2 interaction, and thus decrease the CO2 and CH4 activation energy. As the reduction temperature rises, the crystallinity of Ce9.33(SiO4)6O2 increases, and catalyst's oxygen vacancy concentration increases. However, the Ce9.33(SiO4)6O2 weakens the interaction between Ni and Ce. Its excellent thermal stability and weak redox ability result in its oxygen vacancies being unable to effectively activate CO2. Moreover, the Ce9.33(SiO4)6O2 with good crystallinity will cover some Ni active sites. When the reduction temperature is 600 °C, its strong Ni-CeO2 interface promoted the cracking of CH4, and the active oxygen species provided by the adsorbed CO2 significantly inhibited carbon deposition. The carbon deposits after 30 h of high space velocity and low temperature reaction are minimal. In situ DRIFT experiments found that the active oxygen species may promptly react with methane cracking products in a timely manner, preventing CH4 from deeply breaking and reducing carbon deposits and graphitization. This work offers some guidelines for designing appropriate DRM catalysts for the Ni-SiO2-CeO2 system.

Cu surface plasmon resonance promoted charge transfer in S-scheme system enhanced visible light photocatalytic hydrogen evolution

Reasonably constructing nanocomposite photocatalysts with fast charge transfer and broad solar response capabilities is significant for efficiently converting solar energy into chemical energy. Cu modifies P25/CeO2 heterojunctions prepared by photodeposition (P25 is commercial TiO2). The local surface plasmon resonance (LSPR) effect caused by Cu nanoparticles broadens the spectral response range and generates significant photothermal effects. After 90 s of irradiation, the temperature of 9.5 %Cu-P25/CeO2 increases to 148.1 °C. The photocatalytic hydrogen evolution rate (HER) of 9.5 %Cu-P25/CeO2 under visible light (λ = 400 nm) reaches 1538.2 μmol h-1 g-1, which is 158.6 times, 17.7 times, and 2.5 times higher than that of Cerium dioxide (CeO2), P25, and P25/CeO2, respectively. This catalyst has stronger light absorption, easier carrier transfer, and separation. This study guides the construction of efficient hydrogen evolution photocatalysts.

Construction of robust Ni-based catalysts for low-temperature Sabatier reaction

CO2 hydrogenation to methane, namely, CO2 methanation or Sabatier reaction, is a significant approach to convert CO2 and H2 to storable and transportable CH4. Low reaction temperature is the key to industrialization and has attracted plenty of research interest. Ni-based catalysts are commonly utilized owing to their favorable properties of excellent activity and economical price. However, it is still challenging to perform the Sabatier reaction under temperatures lower than 300 °C owing to the inertness of CO2. Hence, in this article, we summarize the advances of four important design principles of the Ni-based catalysts for low-temperature Sabatier reaction, namely, optimizing Ni active sites, tuning support properties, considering metal-support interactions, and choosing a suitable preparation method, which provides deep insights for the design of low-temperature CO2 methanation catalysts. Additionally, typical low-temperature CO2 methanation reaction mechanisms with *CO or *HCOO as the main intermediate and perspectives on this topic have been provided. We highlight that the rare-earth oxide-supported Ni-based catalysts with the potential reaction mechanism and corresponding reactor design would be promising for low-temperature Sabatier reaction.

Activating lattice oxygen of single-layer ZnO for the catalytic oxidation reaction

Tuning an oxide/metal interface is of critical importance for the performance enhancement of many heterogeneous catalytic reactions. However, catalytic oxidation occurring at the interface between non-reducible oxide and metal has been challenging, since non-reducible oxides hardly lose their lattice oxygen (O_L) or dissociate O₂ from the gas phase. In this work, a ZnO monolayer film on Au(111) is used as an inverse catalyst to investigate CO oxidation occurring at the ZnO/Au(111) interface via high pressure scanning tunneling microscopy. Surface science experiments indicate that oxygen intercalation under the ZnO monolayer film, termed ZnO/O/Au(111), can be achieved via a surface reaction with 1 × 10^-6 mbar O₃. Subsequent exposure of the formed ZnO/O/Au(111) surface to mbar CO at room temperature leads to the recovery of the pristine ZnO/Au(111) surface. Theoretical calculations reveal that O_L adjacent to intercalated oxygen (O_int) is activated due to the O_L-Zn-O_int bonding and surface corrugation, which can be directly involved in CO oxidation. Subsequently, O_int migrates to the formed oxygen vacancy from the subsurface resuming the pristine ZnO structure. These results thus reveal that oxygen intercalation underneath single-layer ZnO will strongly boost the oxidation reaction via activating adjacent lattice oxygen atoms.

Ni-Cu Alloy Nanoparticles Confined by Physical Encapsulation with SiO2 and Chemical Metal-Support Interaction with CeO2 for Methane Dry Reforming

Fabrication of sintering- and carbon-free Ni catalysts for methane dry reforming (MDR), which is attractive to upgrade greenhouse gases CH₄ and CO₂, is challenging. In this work, we innovatively synthesized Ni-Cu alloy nanoparticles confined by physical encapsulation and chemical metal-support interaction (MSI); the synergism of alloy effect, size effect, MSI, and confinement effect in the catalysts gave high rates of CH₄ and CO₂ of 6.98 and 7.16 mmol/(g_Nis), respectively, at 1023 K for 50 h. The rates were 2-3 times enhanced compared to those in the literature. XRD, TEM, H₂-TPR, and so forth revealed that the alloy effect, size effect, and MSI of Ni-Cu and CeO₂ enhanced the MDR activity; MSI promoted the ceria surface lattice oxygen mobility and generated more oxygen vacancies, almost completely gasifying carbon deposits; chemical confinement from MSI and physical confinement from SiO₂ nanospheres realized sintering-free alloys and CeO₂ nanoparticles. The synergistic approach provides a universal strategy for sintering- and carbon-free Ni catalyst design for MDR reaction.

Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production

Relentless endeavors have been committed to seeking simple structure-directing agents for synthesizing hierarchical mesoporous silica (HMS) materials but remaining challenges. In this contribution, we offered an improved one-pot hydrothermal route to prepare HMS materials using a single non-ionic triblock copolymer (F127) structure-directing agent under a mild polycarboxylic (citric acid) mediated condition. Via studies of key synthetic parameters including acid concentration, crystallization temperature and aging time, it was found that citric acid medium presents an important bridging effect under the optimal concentration from 0.018 M (pH = 2.57) to 1.82 M (pH = 1.09), contributing to the self-assemblage of partially protonated non-ionic triblock copolymer and tetraethyl orthosilicate (TEOS) into a high-quality multistage structure of silica materials. The specific surface area (SSA) of HMS shows a volcanic trend and is closely associated with the concentration of citric acid while the highest SSA of 739.9 m2/g can be achieved at the citric concentration of 0.28 M. Moreover, the as-synthesized HMS-CTA supported Ni/CeO2 catalysts indicate an excellent production of hydrogen through dry reforming of methane (DRM) reaction over 172 h stability. The improved, facile synthesis strategy under polycarboxylic medium displays an expanded perspective for synthesizing other mesoporous materials in a wide range of applications such as catalytic material carriers and drug inhibitors.

Impact of preparation method on nickel speciation and methane dry reforming performance of Ni/SiO2 catalysts

The methane dry reforming reaction can simultaneously convert two greenhouse gases (CH₄ and CO₂), which has significantly environmental and economic benefits. Nickel-based catalysts have been widely used in methane dry reforming in past decade due to their low cost and high activity. However, the sintering and coke deposition of catalysts severely limit their industrial applications. In this paper, three Ni/SiO₂ catalysts prepared by different methods were systematically studied, and the samples obtained by the ammonia evaporation method exhibited excellent catalytic performance. The characterization results such as H₂-TPR, XPS and TEM confirmed that the excellent performance was mainly attributed to the catalyst with smaller Ni particles, stronger metal-support interactions, and abundant Ni-O-Si units on the catalyst surface. The anti-sintering/-coking properties of the catalyst were significantly improved. However, the Ni/SiO₂-IM catalyst prepared by impregnation method had uneven distribution of nickel species and large particles, and weak metal-support interactions, showing poor catalytic performance in methane dry reforming. Since the nickel species were encapsulated by the SiO₄ tetrahedral network, the Ni/SiO₂-SG catalyst prepared by sol-gel method could not expose more effective active sites even if the nickel species were uniformly dispersed, resulting in poor dry reforming performance. This study provides guidance for the preparation of novel anti-sintering/-coking nickel-based catalysts.

Ni-CeO2/SBA-15 Catalyst Prepared by Glycine-Assisted Impregnation Method for Low-Temperature Dry Reforming of Methane

Developing low-temperature nickel-based catalysts with good resistance to coking and sintering for dry reforming of methane (DRM) is of great significance. In this work, Ni (5 wt%) and CeO2 (5 wt%) were supported on SBA-15 porous material by glycine-assisted impregnation method to obtain Ni-CeO2/SBA-15-G catalyst. XRD and TEM results showed that the addition of glycine can effectively promote the dispersion of NiO and CeO2 in the pores of SBA-15. H2-TPR and XPS results confirmed the formation of stronger metal-support interaction. In addition, after the addition of glycine, the NixCe1−xOy solid solution content was increased significantly, meanwhile, the Ce3+ concentration was increased from 31% to 49%, accompanied by more oxygen vacancies and generation of active oxygen species. For the above reasons, Ni-CeO2/SBA-15-G had better catalytic performance in the low-temperature DRM test (20 h, 600 °C) with high GHSV (600,000 mL/gcat/h), its CH4 conversion after reaction of 20 h was 2 times that of Ni-CeO2/SBA-15-C catalyst prepared by a conventional impregnation method. TGA-DTA test also proved that Ni-CeO2/SBA-15-G almost completely eliminated carbon deposition. The above advantages of the Ni-CeO2/SBA-15-G catalyst may have originated from the complexation of glycine with metal cations and can prevent them from gathering.

Barium-Promoted Yttria-Zirconia-Supported Ni Catalyst for Hydrogen Production via the Dry Reforming of Methane: Role of Barium in the Phase Stabilization of Cubic ZrO2

Developing cost-effective nonprecious active metal-based catalysts for syngas (H₂/CO) production via the dry reforming of methane (DRM) for industrial applications has remained a challenge. Herein, we utilized a facile and scalable mechanochemical method to develop Ba-promoted (1-5 wt %) zirconia and yttria-zirconia-supported Ni-based DRM catalysts. BET surface area and porosity measurements, infrared, ultraviolet-visible, and Raman spectroscopy, transmission electron microscopy, and temperature-programmed cyclic (reduction-oxidation-reduction) experiments were performed to characterize and elucidate the catalytic performance of the synthesized materials. Among different catalysts tested, the inferior catalytic performance of 5Ni/Zr was attributed to the unstable monoclinic ZrO₂ support and weakly interacting NiO species whereas the 5Ni/YZr system performed better because of the stable cubic ZrO₂ phase and stronger metal-support interaction. It is established that the addition of Ba to the catalysts improves the oxygen-endowing capacity and stabilization of the cubic ZrO₂ and BaZrO₃ phases. Among the Ba-promoted catalysts, owing to the optimal active metal particle size and excess ionic CO₃ ^2- species, the 5Ni4Ba/YZr catalyst demonstrated a high, stable H₂ yield (i.e., 79% with a 0.94 H₂/CO ratio) for up to 7 h of time on stream. The 5Ni4Ba/YZr catalyst had the highest H₂ formation rate, 1.14 mol g^-1 h^-1 and lowest apparent activation energy, 20.07 kJ/mol, among all zirconia-supported Ni catalyst systems.

Alloying Ni-Cu Nanoparticles Encapsulated in SiO2 Nanospheres for Synergistic Catalysts in CO2 Reforming with Methane Reaction

In this work, we studied CO2 reforming with the methane (CRM) reaction over Ni-Cu alloy nanoparticles encapsulated in SiO2 nanospheres, for which combinational functions of alloy effect, size effect, metal-support interaction, and confinement effect exhibited high performance, good sintering resistance, and trace carbon deposition in CRM. The appropriate Cu-addition catalysts 0.2Cu-Ni@SiO2 and 0.5Cu-Ni@SiO2 had smaller NiCu alloy nanoparticles and a stronger metal-support interaction, exhibiting a better performance than the excessive Cu-addition catalysts 1.5Cu-Ni@SiO2 and 3Cu-Ni@SiO2 having Cu clusters and a weaker metal-support interaction. The best synergy of alloy effect, size effect, confinement effect, and metal-support interaction in the 0.5Cu-Ni@SiO2 catalyst contributed to the highest rates of CH4 and CO2 in CRM reported so far. This work demonstrates the importance of appropriate Cu addition in Ni-Cu@SiO2 catalysts, and the synergy for perfectly resolving sintering and carbon deposition in CRM.

Sustainable Synthesis of a Highly Stable and Coke-Free Ni@CeO2 Catalyst for the Efficient Carbon Dioxide Reforming of Methane

A facile and green synthetic strategy is developed in this paper for the construction of an efficient catalyst for the industrially important carbon dioxide reforming of methane, which is also named the dry reforming of methane (DRM). Through controlling the synthetic strategy and Ni content, a high-performance Ni@CeO2 catalyst was successfully fabricated. The catalyst showed superb efficiency for producing the syngas with high and stable conversions at prolonged operating conditions. Incorporating Ni during the ceria (CeO2) crystallization resulted in a more stable structure and smaller nanoparticle (NP) size with a more robust interaction with the support than loading Ni on CeO2 supports by the conventional impregnation method. The H2/CO ratio was almost 1.0, indicating the promising applicability of utilizing the obtained syngas for the Fischer–Tropsch process to produce worthy chemicals. No carbon deposits were observed over the as-synthesized catalyst after operating the DRM reaction for 50.0 h, even at a more coke-favoring temperature (700 ∘C). Owing to the superb resistance to coke and sintering, control of the size of the Ni-NPs, uniform dispersion of the active phase, and potent metal interaction with the support, the synthesized catalyst achieved a magnificent catalytic activity and durability during serving for the DRM reaction for extended operating periods.

The Role of Strontium in CeNiO3 Nano-Crystalline Perovskites for Greenhouse Gas Mitigation to Produce Syngas

The transition metal-based catalysts for the elimination of greenhouse gases via methane reforming using carbon dioxide are directly or indirectly associated with their distinguishing characteristics such as well-dispersed metal nanoparticles, a higher number of reducible species, suitable metal–support interaction, and high specific surface area. This work presents the insight into catalytic performance as well as catalyst stability of Ce_xSr_1−xNiO₃ (x = 0.6–1) nanocrystalline perovskites for the production of hydrogen via methane reforming using carbon dioxide. Strontium incorporation enhances specific surface area, the number of reducible species, and nickel dispersion. The catalytic performance results show that CeNiO₃ demonstrated higher initial CH₄ (54.3%) and CO₂ (64.8%) conversions, which dropped down to 13.1 and 19.2% (CH₄ conversions) and 26.3 and 32.5% (CO₂ conversions) for Ce_0.8Sr_0.2NiO₃ and Ce_0.6Sr_0.4NiO₃, respectively. This drop in catalytic conversions post strontium addition is concomitant with strontium carbonate covering nickel active sites. Moreover, from the durability results, it is obvious that CeNiO₃ exhibited deactivation, whereas no deactivation was observed for Ce_0.8Sr_0.2NiO₃ and Ce_0.6Sr_0.4NiO₃. Carbon deposition during the reaction is mainly responsible for catalyst deactivation, and this is further established by characterizing spent catalysts.

Highly active and stable cobalt catalysts with a tungsten carbide-activated carbon support for dry reforming of methane: effect of the different promoters

A series of cobalt catalysts decorated with different transition metals were synthesized. The introduction of Y improves the dispersibility of the active metal and its oxygen vacancy content, thereby enhancing its activity and anti-coking ability.

Coke-resistant Ni-based bimetallic catalysts for the dry reforming of methane: effects of indium on the Ni/Al2O3 catalyst

In the quest for highly efficient coke-resistant catalysts for the dry reforming of methane (DRM) to produce syngas, a series of Ni–In/γ-Al2O3 catalysts with various Ni contents were prepared via a “two-solvent” method and used for the DRM reaction.

Bismuth single atom supported CeO2 nanosheets for oxidation resistant photothermal reverse water gas shift reaction

Bi single atoms supported on CeO2 nanosheets combined with a Ti2O3 based photothermal device showed oxidation resistance and outperforming weak solar driven RWGS with a CO production rate of 31.00 mmol g−1 h−1 under 3 sun units of irradiation.

Spherical Ni Nanoparticles Supported by Nanosheet-Assembled Al2O3 for Dry Reforming of CH4: Elucidating the Induction Period and Its Excellent Resistance to Coking

The design and preparation of efficient coking-resistant catalysts for dry reforming of methane (DRM) is significant for industrial applications but a challenge for supported Ni catalysts. Nanosheet-assembled Al₂O₃ (NA-Al₂O₃) with hierarchical hollow microspheres was used to support Ni nanoparticles, which exhibits superior long-time stability and coking resistance for the DRM reaction from 700 to 800 °C without coke deposition. Active Ni species, exsolved from NiAl₂O₄ spinel, are aggregated into Ni nanoparticles and finally stabilize as spherical Ni nanoparticles of 18.0 nm due to the spatial confinement of hierarchical hollow microspheres of the NA-Al₂O₃ support after the DRM reaction for 60 h. The catalytic activity in the induction period of the Ni/(NA-Al₂O₃) catalyst increases because of the enhancement of the surface Ni⁰/(Ni⁰+Ni²⁺) ratio, that is, the increment of the amount of active Ni sites. The spherical Ni nanoparticles embedded in the NA-Al₂O₃ support, superior CO₂ adsorption ability, and more surface hydroxyl groups on the Ni/(NA-Al₂O₃) catalyst are the determining factors for its long-time stability and excellent anti-coking for the DRM reaction.

Methane Dry Reforming by Ni-Cu Nanoalloys Anchored on Periclase-Phase MgAlOx Nanosheets for Enhanced Syngas Production

Stable and efficient syngas production via methane dry reforming is highly desirable as it utilizes two greenhouse gases simultaneously. In this work, active Ni-Cu nanoalloys stably anchored on periclase-phase MgAlO_x nanosheets were successfully synthesized by a hydrothermal method. These highly dispersed small Ni-Cu alloys strongly interacted with the periclase-phase MgAlO_x nanosheets, on which abundant base sites were accessible. On the optimal catalyst (6Ni6CuMgAl-S), methane and carbon dioxide conversion always reached 85 and 90% at 700 °C under a gas hour speed velocity of 40,000 mL/g_cat h for more than 70 h. The hydrogen production rate was maintained at 1.8 mmol/min, and the ratio of H₂/CO was kept at approximately 0.96 under a CH₄ and CO₂ flow rate of 25 mL/min. Coke deposition and Ni sintering were effectively suppressed by the formation of a Ni-Cu alloy, the laminar structure, and the periclase phase of the MgAlO_x support. Moreover, the alloy nanoparticles were reconstructed into a segregated Ni-Cu alloy structure in response to the reaction environment, and this structure was more stable and still active. Density functional theory calculations showed that carbon adsorption was inhibited on the segregated Ni-Cu alloy. Furthermore, the experimental thermogravimetric and O₂-TPO results confirmed the significant decrease in carbon deposition on the Ni-Cu alloy catalysts.

Fast synthesis of porous iron doped CeO2 with oxygen vacancy for effective CO2 photoreduction

The activity of photocatalytic CO₂ conversion to carbon-containing products is determined by the adsorption and activation of CO₂ molecules on the surface of catalyst. Here, iron doped porous CeO₂ with oxygen vacancy (PFeCe) was prepared by one-step combustion method. The amount of CO₂ adsorbed via using the porous structure has been significantly increased in the case of a relatively small specific surface area and CO₂ molecules are more easily captured and undergo a reduction reaction with photoinduced carriers. In addition, oxygen vacancies are formed in the iron doped CeO₂ lattice as the active sites for CO₂ reduction, which can form strong interactions with CO₂ molecules, thereby effectively activating CO₂ molecules. The reduction products of CO₂ over PFeCe composite are CO and CH₄, which is approximately 9.0 and 7.7 folds than that of CeO₂. This work offers insights for the construction of efficient ceria-based photocatalysts to further achieve robust solar CO₂ conversion.

Syngas Production via CO2 Reforming of Methane over SrNiO3 and CeNiO3 Perovskites

The development of a transition-metal-based catalyst with concomitant high activity and stability due to its distinguishing characteristics, yielding an abundance of active sites, is considered to be the bottleneck for the dry reforming of methane (DRM). This work presents the catalytic activity and durability of SrNiO3 and CeNiO3 perovskites for syngas production via DRM. CeNiO3 exhibits a higher specific surface area, pore volume, number of reducible species, and nickel dispersion when compared to SrNiO3. The catalytic activity results demonstrate higher CH4 (54.3%) and CO2 (64.8%) conversions for CeNiO3, compared to 22% (CH4 conversion) and 34.7% (CO2 conversion) for SrNiO3. The decrease in catalytic activity after replacing cerium with strontium is attributed to a decrease in specific surface area and pore volume, and nickel active sites covered with strontium carbonate. The stability results reveal the deactivation of both the catalysts (SrNiO3 and CeNiO3) but SrNiO3 showed more deactivation than CeNiO3, as demonstrated by deactivation factors. The catalyst deactivation is mainly attributed to carbon deposition and these findings are verified by characterizing the spent catalysts.

Acid etching induced defective Co3O4 as an efficient catalyst for methane combustion reaction

Development of an effective Co₃O₄ material as an advanced non-noble metal catalyst for methane combustion has great economic and environmental significance.

Constructing Ni-based confinement catalysts with advanced performances toward the CO2 reforming of CH4: state-of-the-art review and perspectives

The concept of Ni-based confinement catalysts has been proposed and developed to address the challenge of the thermal sintering of metallic Ni active sites during CRM by the space and/or lattice confinement effects.

One-pot synthesis of a highly mesoporous Ni/MgAl2O4 spinel catalyst for efficient steam methane reforming: influence of inert annealing

The inert annealing step during the synthesis of the Ni/MgAl₂O₄ catalyst induces positive changes in the catalyst substructure. The obtained catalyst displayed high catalytic activity towards steam methane reforming with low carbon deposition.

Ultra-small CeO2 nanoparticles supported on SiO2 for indoor formaldehyde oxidation at low temperature

Ultra-small CeO₂ nanocrystals with a size of 2.5 nm were synthesized for high efficiency HCHO oxidation at low temperature.