Doped Catalysts Research Articles

Reverse micelle strategy for effective substitutional Fe-doping in small-sized CeO2 nanocrystals: Assessment of adsorption and photodegradation efficiency of ibuprofen under visible light

Reverse micelle nanoreactors were successfully designed to synthesize small-sized ceria nanocrystals (3.5–4.2 nm) with a sizeable amount of substitutional iron. Undoped and doped CeO2 catalysts with an iron content (0.50–10 mol %) compliant with the nominal value were prepared and tested for the first time for the removal of ibuprofen both in the dark and under UV or visible light irradiation.The effective inclusion and distribution of iron in the ceria lattice were ascertained through in-depth physicochemical characterization. In particular, X-ray diffraction suggested the formation of an F-type crystal structure, ruling out the formation of separate iron-containing crystalline phases. On the other hand, substitutional doping of CeO2 with Fe atoms favoured the formation of Ce3+ defects and vacancy sites (VOs) with a maximum for the sample with 2.5 mol % iron (Fe2.5), as evidenced by X-ray photoelectron spectroscopy (XPS) measurements and Raman spectroscopy. UV–Vis spectroscopy showed that the optical properties were successfully modified by the presence of iron, which causes a gradual decrease in band gap as iron content increases. The experimental evidence was further verified and supported by density functional theory calculations. DFT calculations also revealed that the surface iron and oxygen vacancies are the preferential sites for ibuprofen adsorption. Nevertheless, it was found under dark conditions that adsorption capacity does not monotonically increase with iron content, revealing contrasting roles of surface characteristics. Indeed, catalytic experiments have identified a trade-off between adsorption and photodegradation, identifying Fe2.5 as the best-performing catalyst for ibuprofen removal under visible light irradiation. These results were discussed by considering the key properties of the catalysts as well as their different surface charge determined by ζ potential measurements. The best catalyst was tested through reuse experiments that proved its stability over 4 cycles. Finally, an attempt was made to identify the photodegradation by-products, allowing the detection of 1-ethenyl-4-(2-methylpropyl)benzene as the main by-product.

Promoting mechanism of Mn doping on the NH3-SCR reaction over the Fe/γ-Al2O3 catalyst surface

To verify the superiority of Mn-Fe dual-component doped catalysts in terms of deNOx performance, the differences of mechanisms in selective catalytic reduction with ammonia (NH3-SCR) between Fe/γ-Al2O3 and Mn-Fe/γ-Al2O3 catalysts were investigated by using the density functional theory (DFT). On the surface of the Fe/γ-Al2O3 catalyst, the NH2 group generated by NH3 dehydrogenation can react with NO and NO2 to form NH2NO and NH2NO2, respectively. The decomposition process of NH2NO is established as the rate-determining step in the reaction between NH3 and NO, while the rate-determining step in the reaction between NH3 and NO2 is the decomposition of NHNO2. Mn doping significantly enhances the catalyst's adsorption performance for O2 and oxidation ability toward NO, thereby promoting the production of NO2. Additionally, Mn doping markedly reduces the reaction barriers, facilitating the dissociation of NH2NO and NHNO2 on the Mn-Fe/γ-Al2O3 catalysts. These findings may elucidate the reasons of the effective enhancement of deNOx performance in the Mn-doped Fe/γ-Al2O3 catalysts, which establishes a foundation for the subsequent development and application of Mn-Fe bimetallic-doped SCR deNOx catalysts.

Study on the mechanisms of DMP degradation by electro-Fenton method modified by single atomic-Co doped carbon-based catalyst

Electro-Fenton technology is an advanced oxidation process for phthalic acid esters (PAEs) that builds upon traditional Fenton technology, enhancing its efficiency, selectivity, and flexibility. Thus, cobalt-doped carbon-based catalyst (Co@NSC) were prapered via melamine doped by cobalt (Co) and calcinated at high temperature, and showed excent mesoporous structure (mainly range from 1.3 to 16 nm), high surface area (223.26 m2/g) and big pore volume (0.392 cm3/g). And the result of electrochemical performance test showed that the two-electron oxygen reduction ability of the material can be effectively improved in the case of Co doping. DMP can be degraded within 30 min by using Co@NSC as an electro-Fenton cathode, with excellent degradation efficiency (100 %) and total organic carbon (TOC) mineralization rate (about 40 %) even under the optimal conditions. The exploration of the mechanism demonstrated that OH and 1O2 were the main contributors. According to density functional theory (DFT) theoretical calculation and liquid chromatograph-mass spectrometer(LC-MS) results, the vulnerable site and degradation path of DMP were reasonably speculated. The ecotoxicity evaluation of the degradation products showed that the the toxicity of DMP has been significantly reduced during degradation process by Co@NSC electro-Fenton. Thus, Co@NSC is a cost-effective and promising catalyst, in which all the metal sites were utilized while the cost of metal were significantly minimized.

Amorphization-Induced Cation Exchange in Indium Oxide Nanosheets for CO2-to-Ethanol Conversion.

Cation exchange (CE) in metal oxides under mild conditions remains an imperative yet challenging goal to tailor their composition and enable practical applications. Herein, we first develop an amorphization-induced strategy to achieve room-temperature CE for universally synthesizing single-atom doped In2O3 nanosheets (NSs). Density functional theory (DFT) calculations elucidate that the abundant coordination-unsaturated sites present in a-In2O3 NSs are instrumental in surmounting the energy barriers of CE reactions. Empirically, a-In2O3 NSs as the host materials successfully undergo exchange with unary cations (Cu2+, Co2+, Mn2+, Ni2+), binary cations (Co2+Mn2+, Co2+Ni2+, Mn2+Ni2+), and ternary cations (Co2+Mn2+Ni2+). Impressively, high-loading single-atom doped (over 10 atom %) In2O3 NSs were obtained. Additionally, Cu/a-In2O3 NSs exhibit an excellent ethanol yield (798.7 μmol g-1 h-1) with a high selectivity of 99.5% for the CO2 photoreduction. This work offers a new approach to induce CE reactions in metal oxides under mild conditions and constructs scalable single-atom doped catalysts for critical applications.

Insights into the enhanced activity and SO2 resistance of air oxidation treated Mn-Fe doped biochar catalyst in the low-temperature catalytic reduction of NOx with NH3

In this study, a sustainable two-stage air oxidation (TAO) was employed to prepare Fe modified MnOx-biochar catalyst (C-O2-MF-O2) instead of high temperature treatment and/or chemical activation for selective catalytic reduction of NO with NH3 (NH3-SCR). The collaborative effect of Fe modification and TAO on the physicochemical properties and catalytic performance of catalysts was systematically studied. Compared with counterpart catalysts treated with two-stage N2 (C-N2-MF-N2) or one-stage O2 (C-O2-MF-N2, C-N2-MF-O2), C-O2-MF-O2 showed obvious enhanced physicochemical properties (SBET, pore properties, high-valance metal ratio, chemisorption oxygen (Oα) content), superior catalytic performance, achieving 91.3% NO conversion with nearly 100% N2 selectivity at 150 °C, and much better H2O and SO2 resistance. It is worth noting that TAO facilitated the redox performance of MnOx after Fe modification to generate more dispersed high-valance metal (Mn4+ and Fe3+), thereby favoring the low-temperature SCR activity of the catalyst. Further, the in situ DRIFTS studies revealed TAO produced remarkable effects on the adsorption behaviors of NH3, NO and SO2, and the corresponding mechanisms of catalyst activity enhancement and SO2 resistance were proposed. Generally, TAO could be a favorable avenue to prepare catalysts with binary active metals for improving performance of NH3-SCR.

Toward the Rational Design of More Efficient Mo2C Catalysts for Hydrodeoxygenation-Mechanism and Descriptor Identification.

Viable alternatives to scarce and expensive noble-metal-based catalysts are transition-metal carbides such as Mo and W carbides. It has been shown that these are active and selective catalysts in the hydrodeoxygenation of renewable lipid-based feedstocks. However, the reaction mechanism and the structure-activity relationship of these transition-metal carbides have not yet been fully clarified. In this work, the reaction mechanism of butyric acid hydrodeoxygenation (HDO) over molybdenum carbide (Mo2C) has been studied comprehensively by means of density functional theory coupled with microkinetic modeling. We identified the rate-determining step to be butanol dissociation: C4H9*OH + * → C4H9* + *OH. Then we further explored the possibility to facilitate this step upon heteroatom doping and found that Zr- and Nb-doped Mo2C are the most promising catalysts with enhanced HDO catalytic activity. Linear-scaling relationships were established between the electronic and geometrical descriptors of the dopants and the catalytic performance of various doped Mo2C catalysts. It was demonstrated that descriptors such as dopants' d-band filling and atomic radius play key roles in governing the catalytic activity. This fundamental understanding delivers practical strategies for the rational design of Mo2C-based transition-metal carbide catalysts with improved HDO performance.

Enhanced photocatalytic degradation of Rhodamine B by (Mg/Ta): doped Bi4Ti3O12 catalyst under sunlight irradiation

Enhanced photocatalytic degradation of Rhodamine B by (Mg/Ta): doped Bi4Ti3O12 catalyst under sunlight irradiation

Computational design and experimental validation of monometallic-doped SnO2 catalysts for selective catalytic oxidation of ammonia

Due to the intricate structure of catalysts, traditional catalyst design relies on iterative trial-and-error experiments. We have systematically established a catalyst design strategy to evaluate the performance of NH3-SCO reactions from the perspective of DFT calculations. Specifically, we used the catalyst formation energy as a stability descriptor and the adsorption energies of NH3, NH2, and O p-band center as performance descriptors, we identified the four most promising catalysts among 45 kinds of doped SnO2 catalysts. Subsequently, experimental validation was performed to demonstrate the outstanding consistency between the DFT-driven descriptors and the stability and catalytic performance of the catalyst. Particularly, the Ce doping resulted in a 175 °C reduction in the T90 compared to the SnO2 catalyst. It is noteworthy that Ce doping promotes the cycling between oxygen vacancies and lattice oxygen, which contributes primarily to the enhancement of O2 activation capability and, consequently, the improvement in catalytic activity.

Designing Al-SiO2 nanospheres with co-doped Ce/Cd for the removal of 4,6-dimethyldibenzothiophene of fuel oil

Catalytic oxidative coupled in-situ hydrodesulfurization technology as an emerging desulfurization process is applied to the removal of organic sulfides from fuels. In this study, Al-SiO2@Ce/Cd core-shell nanosphere catalysts were prepared by a sol-gel method and a co-impregnation method using silica as the carrier, and were in turn applied for the ultra-deep desulfurization of highly resilient 4,6-DMDBT from diesel oil under visible light irradiation. The prepared photocatalyst were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Brauner-Emmette-Teller (BET). Characterization results revealed that Al-SiO2@Ce/Cd had a nanosphere structure, and Ce/Cd were successfully loaded on the surface of Al-SiO2. Bimetallic loading helped increase the specific surface area of the SiO2 material, which could anchor more effective metal active sites. The effects of metal doping amount, doping ratio, catalyst dosage, and reaction temperature on 4,6-DMDBT desulfurization were studied. The results showed that 20% Al-SiO2@Ce/Cd achieved 100% desulfurization performance within 30 min at 60 ℃, thus surpassing many state of the art catalysts reported in literature. Al-SiO2@Ce/Cd remained highly active after 10 times successive reuses with rapid recovery by washing and filtering. Organic nitride had a strong inhibitory effect on diesel desulfurization, which was derived from the competitive adsorption of nitride and sulfide on the effective active site of the catalyst. Based on characterization results and desulfurization results, the appropriate reaction mechanism of the catalytic oxidative coupling in-situ hydrodesulfurization of Al-SiO2@Ce/Cd catalysts was speculated.

In-situ surface reconstructed NiOOH/Ni-ZnSe composite electrocatalysts with high activity toward oxygen evolution reaction in alkaline media

Fabricating long-durability and high-activity electrodes for oxygen evolution reaction (OER) in alkaline electrolyte is of great importance for practical water electrolysis. In this work, we develop a composite catalyst composed of Ni-doped zinc selenide nanoparticles embedded in nitrogen-doped amorphous carbon framework (denoted as Ni-ZnSe composite). Concretely, the systematic investigations disclose that the amorphous nickel (oxy)-hydroxide (NiOOH) in-situ formed on the surface of the hybrid catalyst during the OER process is considered as the genuine active centers. This self-reconstructed electrocatalyst (denoted as NiOOH/Ni-ZnSe) only demands small overpotentials of 218, 242 and 292 mV at the 10, 20 and 50 mA cm-2, respectively, as well as low Tafel slope of 56 mV dec-1 in alkaline media. XPS characterization reveals that performance enhancement is attributed to the electronic interaction between Ni-ZnSe and nitrogen-doped carbon (NC) that facilitate interfacial electron transfer, which in turn favors improve the oxygen evolution activity. This work not only enhances the electrocatalytic activity of selenide-based catalyst, but also provides a judicious avenue for developing advanced heterometallic doped catalysts toward OER and beyond.

The promotion of NH3-SCR performance and its mechanism on Sm modified birnessite

To develop novel NH3-SCR catalysts with low-temperature and high-efficency, a series of birnessite (δ-MnO2) catalysts with different Sm doping by methanol reduction are synthesized. The results showed that the introduction of Sm successfully inhibited the crystallization of MnOX, promoted the specific surface area, the contents of chemisorbed oxygen species and the formation of surface acid sites. It also increased the relative content of (Mn4+ + Mn3+)/Mn, which was beneficial to low-temperature SCR activity. The catalytic performance of xSm/Mn (δ-MnO2 based with xSm doping) catalysts was evaluated by NH3-SCR performances. Among them, 0.05 Sm/Mn showed 100% NOX conversion and high N2 selectivity compared to the other Sm addition catalysts within the operating low-temperature window (25–200 °C), at a high GHSV (gas hourly space velocity) of 60,000 h−1. The modified 0.05 Sm/Mn catalysts followed the L-H and E-R reaction mechanisms and were dominated by L-H, as revealed by in-situ DRIFTs analysis.

First principles study on the structural properties of 13 transition metal doped CdS and the reaction mechanism of ammonia synthesis

Single-atom catalysts with ultra-high atomic utilization and selectivity show great potential in the production of NH3 through electrocatalytic nitrogen reduction reaction (NRR). In this paper, 13 single-atom cadmium sulfide (TM − CdS) doped with different transition metals (TM) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Ag) were constructed separately using a density functional theory approach to calculate the structural stability and the reaction mechanism of catalytic ammonia synthesis. hree systems, Fe − CdS, Mo − CdS, and Ru − CdS, were screened out from the 13 doped metal 0structures on the basis of the computational results. The above three systems have more stable structures, can transfer more charge to activate N2 molecules, and exhibit better NRR catalytic activity. The limiting potential NRR on the Mo − CdS structure is only − 0.261 V, which is lower than that of Fe − CdS (−0.353 V), Ru − CdS (−0.461 V) and CdS (−2.124 V). This work will be of great value of designing and employing the doped single-atom catalysts for efficient electrolytic nitrogen reduction reaction.

Enhancing deoxygenation of waste cooking palm oil over CaO-MgO catalyst modified by K2O for green bio-fuel

There are challenges to replacing traditional fossil fuels completely, so producing efficient green biofuels is crucial. In this work, waste cooking palm oil (WCPO) was used as raw material to produce biofuel through pyrolytic catalysis cracking (PCC) over alkaline catalysts as CaO-MgO with various dopants at 0, 3, 6 and 9 wt% K2O contents. The WCPO reacted under atmospheric pressure without purging gases at 500, 525 and 550 °C, respectively. Catalysts were characterized by various techniques in terms of physical and chemical properties. The pyrolytic oil (PO) was separated from the crude oil product. Then, PO was characterized by TGA/DTG, CHNO, FTIR and GC–MS. The PO was distilled following an ASTM D86 method. The properties of biofuels, such as density, kinematic viscosity and acidity, were measured. The basicity of the catalyst increased with increasing K2O contents. K2O not only enhanced the pyrolytic-liquid product yield but also significantly reduced the oxygenated compounds in biofuel through the deoxygenation process to improve biofuel quality. The different reaction temperatures were observed in which low-temperature reaction provided incompletely cracking heavy molecules, and more gas product occurred at the high reaction temperature. Surprisingly, the fatty acid compounds of WCPO were transferred to hydrocarbon compounds by deoxygenation reactions confirmed by GC–MS and FTIR results. The highest pyrolytic oil yield was approximately 92 vol% compared with crude oil product, obtained from 3 wt% K2O-MgO/CaO and 525 °C. The paraffinic and olefinic compounds were the main hydrocarbon products with K2O doping catalysts, while ketone compounds were found in all catalysts via the ketonic decarboxylation mechanism. In this condition, pyrolytic oil was completely distilled to get gasoline, kerosene and diesel. The kinematic viscosity and density were under ASTM standards except for acidity. Remarkably, the acidity of biofuel with K2O doping was reduced about 2–3 times compared to MgO/CaO alone.

Efficient in situ conversion of captured CO2 into fuels enabled by direct solar driven multifunctional calcium looping

Carbon neutrality requires credible action to reduce greenhouse gas emissions and develop clean and renewable energy. The solar driven multifunctional calcium looping (CaL), which integrates CO2 capture, CO2 conversion into fuels, and thermochemical energy storage into a solar driven cycle, tends to be one of the most promising pathways to establish a zero-carbon energy system. However, the progress of this concept remains in the proof-of-principle stage and the research into its magnified operations and underlying fundamental scientific problems is seldom reported. In fact, the thermodynamic and kinetic features of this integrated cycle are intertwined during the solar harvesting, CO2 capture, calcination, and CO2 reduction processes, so that the synergetic manipulation of the reactions becomes vital and challenging. Here, attempts are made to develop the design and regulation scheme for implementing full-scale application according to the research into a preliminarily enlarged operation carried out in a stirred bed reaction system. The kinetic matching relationships between different chemical reactions involving intense heat and mass transfer across the entire solar energy conversion chain are investigated. The average solar absorption of CaCO3 based composite particle material is promoted to 90.9% through doping catalysts into CaCO3. The solar-to-chemical conversion efficiency reaches up to 51.6%. More than 88.2% of the CO2 is converted into syngas in-situ. The high energy conversion efficiency indicates the bright prospect of such a direct solar driven multifunctional CaL system, and the proposed design and enhancement methods are expected to provide theoretical and practical references for its industrialization process.

Synergistic Effect of Dual-Doped Carbon on Mo2C Nanocrystals Facilitates Alkaline Hydrogen Evolution

HighlightsThe B and N dual-doped carbon layer that encapsulated on Mo2C nanocrystals (Mo2C@BNC) were fabricated for accelerating HER under alkaline condition.Theoretical calculations reveal that the H2O could be decomposed spontaneously over the introduced B sites, and the defective C atoms in the dual-doped carbon layer provide the best H binding sites.The optimized dual doped Mo2C catalyst with synergistic effect of non-metal sites delivers superior HER performances.

Hydrangea-like NiO@KNbO3 as catalyst doping for MgH2: Boosting hydrogen storage performance

MgH2 is regarded as one of the most favorable materials for hydrogen storage caused by its safety, high efficiency, considerable capacity for storing hydrogen, and low cost. However, the application of this material is limited owing to subpar thermodynamics and kinetics, which can be improved via combination with other materials. A hydrangea-like NiO@KNbO3 was prepared by solvent thermal, hydrothermal, and calcination methods. Then, different amounts of NiO@KNbO3 (3, 5, 7, and 10 wt %) were added to MgH2 using ball-milling. MgH2 composite material doped with 5 wt % NiO@KNbO3 exhibits the most superior hydrogen storage performance, with a hydrogen absorption capacity of 4.95 wt % H2 within 5 min at 150 ℃ and 1.5 MPa H2 pressure. Furthermore, H2 release started approximately at 175 °C with 5.96 wt % H2 at 300 °C. The composite's H2 desorption activation energy was decreased to 71.75 kJ/mol. The Mg2NiH4/Mg2Ni generated in situ can serve as a "hydrogen pump" by creating numerous activation sites and pathways for hydrogen diffusion, thereby facilitating H2 dissociation during the process of hydrogen absorption. Additionally, the NiO@KNbO3-doped MgH2 samples exhibited excellent integrated hydrogen storage properties due to the combined effect of KNbO3 and the in situ formed NbO. The study provides clear evidence that NiO@KNbO3, a bimetallic oxide, can effectively enhance the hydrogen storage performance of MgH2 as an additive. Furthermore, the straightforward synthesis of NiO@KNbO3 offers guidance for the development of high-performance oxide catalysts that can improve the properties of MgH2.

Built-in electric field accelerates photogenerated carrier separation in vanadium-oxygen doped C3N4 for flash degradation of methylene blue: Experimental and molecular simulations

Flash photodegradation of organic pollutants at high concentrations is an urgent need for industrial organic wastewater treatment. Situ high-energy wet ball milling is adopted to fabricate the vanadium-oxygen doped carbon nitride catalysts to implement the flash degradation of methylene blue (MB). The experimental results indicated that the photocatalyst degradation ratio of MB was higher than 90% in 3 min under visible-light irradiation 300 W, catalyst dosage 0.2 g/L and MB concentration of 100 mg/L. MB removal rate was nearly 20 times higher than that of primitive carbon nitride. Vanadium-oxygen doping was beneficial to the reduction of band gap less than 2.0 eV and the formation of built-in electric field. It is notable notice that both valence band and conduction band of catalyst were tuned to be in positive potential region, benefiting for holes (h+) formation to boost the flash degradation of contaminants. Strong electrophilic attack of h+ caused the bonds of C-S+=C and CN-C broken, triggering the aromatic ring opened to decompose into smaller molecules.

Engineering Cooperative Catalysis in Li–S Batteries

AbstractLithium–sulfur (Li–S) batteries are regarded to be one of the most promising next‐generation batteries owing to the merits of high theoretical capacity and low cost. However, the aprotic S electrochemistry is hampered by the shuttling effect and sluggish conversion of soluble lithium polysulfides (LiPSs). Various electrocatalysts have been designed to optimize the conversion kinetics of the LiPSs. Heteroatom doping or polar catalyst incorporation plays an important role to remedy these shortcomings. Here, the cooperative catalytic effects engendered by heteroatom codoping engineering, catalyst/heteroatom doping composite strategy, and integrating polar‐metal‐based material design are summarized. First, the design principles of the cooperative catalytic effect on the conductive interface are described in detail. Moreover, a systematic investigation of electrocatalytic kinetics characterization techniques that establish a bridge between the cooperative solid–liquid interface and the macroscopic electrochemical performance is highlighted, providing more scientific guidance for the catalyst design. In conclusion, stemming from the current achievements, future directions targeting the high‐energy‐density Li–S batteries for commercialization are proposed.

Synergistic catalytic effects of Ni MOF catalyst and RE alloying on the hydrogen absorption/desorption kinetics of MgH2

The Mg90Ce5Y5 + x wt. % Ni MOF (x = 0, 3) composites were prepared successfully and we investigated the synergistic catalytic effects of Ni MOF catalyst doping and RE alloying for the hydrogen storage properties of pure Mg. The N3 sample absorbs about 3.05 wt. % H2 at 473 K within 2 min, while the pure Mg almost does not initiates reaction at the same condition. The Edes of N3 only is calculated as 69.0 ± 3.9 kJ/mol and much lower than pure MgH2 (160 kJ/mol). The improved hydrogen storage kinetics is considered that the CeH2.73 and YH2 phases provide nucleation sites and channels of diffusion inside the particle and the extra Ni active positions, defects and cracks facilitate H2 dissociation/recombination, generated by Ni MOF on the particle surface.

Effect of Transition Metals Addition on the Nickel Oxide Catalyst toward Reduction Behaviour in Carbon Monoxide Atmosphere

The chemical reduction progression behaviour of transition metals (Mo, Zr, W, Ce, and Co) doped on NiO was studied using temperature programmed reduction (TPR) analysis. A wet impregnation method was applied to synthesise the doped NiO series catalysts. The reduction progress of the catalysts was attained by using a reductant gas at the concentration of 40% v/v CO/N2. X-ray diffraction (XRD) was employed to determine the composition of the reduced phases. Undoped NiO was reduced at 384℃ to obtain a cubic phase of NiO. It was observed that Ce/NiO exhibited the lowest reduction temperature of 370℃ among all catalysts. This phenomenon might be due to a higher surface area of Ce/NiO compared to undoped NiO, which facilitated a faster reduction reaction. The rest of the doped NiO series catalysts (Co/NiO, Mo/NiO, W/NiO and Zr/NiO) demonstrated a higher reduction temperature compared to undoped NiO. New peaks in the XRD pattern were observed only for the reduced catalysts of Mo/NiO and W/NiO, which were associated with monoclinic MoO2 and WO2.72 phases, respectively. The formation of new compounds or more stable nickel alloys led to a slower reduction reaction than undoped NiO. Therefore, Ce/NiO was the most efficient catalyst in promoting the formation of Ni under the CO atmosphere.