High Metal Dispersion Research Articles

Size-dependent selectivity and activity of highly dispersed sub-nanometer Pt clusters integrated with P25 for CO2 photoreduction into methane fuel

Integration of sub-nanometer–sized Pt cocatalysts on semiconductor materials is a promising approach to improve the activity and selectivity of CO2 photoreduction. However, the practical realization of such an integrated catalyst is a challenging task. We rationally integrated sub-nanometer Pt nanoclusters (Pt NCs) on the surface of commercial P25 catalyst by adjusting the amount of Pt loading. The lateral reduction in the size of Pt NCs resulted in high metal dispersion, increased active metal surface area, better CO2 adsorption, and hindered photoexcited charge-carrier recombination. The sub-nanometer Pt NCs integrated with P25 with a weight percentage of 0.5% (Pt-NC/P25-0.5) exhibited exceptional CO2 photoreduction activity for CH4 production and excellent stability during successive test runs. This catalyst outperformed its counterparts with larger Pt NCs and several previously reported state-of-the-art photocatalysts in terms of CH4 generation. Importantly, compared to its counterparts, Pt-NC/P25-0.5 had a higher proportion of edge and corner sites, which have a strong affinity for CO molecules, resulting in a high CH4 selectivity of 95% against H2. The size-dependent selectivity and activity of Pt NCs for CO2 reduction demonstrated in this study offer insights into the development of sub-nanometer metal-based catalysts for photocatalytic energy applications.

Selective Hydrogenation of Glycolic Acid to Renewable Ethylene Glycol over Supported Ruthenium Catalysts

AbstractThe production of industrially demanded polymer precursor ethylene glycol (EG) from biomass‐accessible aqueous glycolic acid (GA) was investigated in this study. The systematic investigation of different active metals as well as, for the first time, various support materials revealed Ru/TiO2 and Ru/ZrO2 to be the most active catalysts in the heterogeneously catalyzed reduction of GA with molecular H2. Catalytic activity is enhanced by facilitated reduction of the active Ru species due to metal‐support interactions. An assessment of several other catalyst properties (metal surface area, acid properties, porosity) revealed that high metal dispersion is also beneficial. Up to 94 % GA conversion and 95 % selectivity for EG were obtained under optimized and mild reaction conditions (105 °C, 60 bar H2 pressure, 23 h, Ru/TiO2 as catalyst). This shows that the heterogeneously catalyzed reduction of biomass‐derived aqueous GA solutions can be a promising alternative for the renewable production of EG.

Tailoring the properties of calcium modified fibrous mesoporous silica KCC-1 for optimized sulfur dioxide removal

Dry regenerative flue gas desulfurization (FGD) is a promising method to tackle industrial issues regarding SO 2 emission into the atmosphere due to its sorbent being highly accessible, the lack of water dependency and reduction in waste management. This study examined the feasibility of using fibrous mesoporous silica KCC-1 which has been reported to possess better properties than several other predecessor mesoporous silica as alternative sorbents for dry FGD. Calcium metal was introduced to overcome the lack of active sites available on KCC-1 while simultaneously providing sufficient basicity to counter the increase in acidity brought by SO 2 adsorption. Three sorbent modification parameters were analyzed: metal loading (5–15 wt %), calcination temperature (823–973 K) and calcination time (5.5–7 h), and the prepared samples were characterized using BET surface area and pore analyzer, FESEM-EDX, XRD and H 2 -TPR. The breakthrough experiment was conducted using a lab scale fixed bed reactor system with 1500 ppm SO 2 /N 2 at 200 mL/min. SO 2 removal was optimized by sorbent prepared with calcium loading of 5 wt %, calcination temperature of 923 K and calcination time of 6.5 h with adsorption capacity of 3241.94 mg SO 2 /g KCC-1. The optimized sorbent demonstrated highest surface area, good pore development, high dispersion of calcium metal, appropriate impregnation of calcium oxide which caused only minor distortion to the silica framework of KCC-1. Subjecting the optimized sample to five consecutive regeneration cycles by heating at 773 K while simultaneously flowing N 2 gas for an hour shows good regeneration performance with a total final reduction of only 25% from the initial adsorption capacity obtained from a fresh sample. • Dry desulfurization using calcium modified KCC-1 mesoporous silica as sorbent. • Optimized calcium metal loading, calcination temperature and calcination time. • Adsorption capacity of 3241.94 mg/g attained by optimized Ca/KCC-1. • SO 2 removal controlled by surface area, porosity, basicity and crystallite size. • Feasible for regeneration by simple heating at 773 K and nitrogen flow.

Synthesis gas production by dry reforming of methane over Neodymium-modified hydrotalcite-derived nickel catalysts

Neodymium-modified hydrotalcite-derived nickel catalysts (NixNdMgAlO) by adjusting the Nd content (x) at a range of 2.5–10% were synthesized via co-precipitation method. The catalytic properties of these catalysts were evaluated in the reaction of dry reforming of methane (DRM) at 750 °C. The initial CH4 conversion decreased with the order of Ni7.5NdMgAlO > Ni5NdMgAlO > NiMgAlO > Ni2.5NdMgAlO > Ni10NdMgAlO. The Ni7.5NdMgAlO catalyst with a compromised proportion of Nd at 7.5% had the smallest nickel crystallite size (8.1 nm), highest metal dispersion (11.8%), the highest exposed active sites on the surface (6.3 m2/g) as well as the relatively intimate interaction between small Ni2+ species and the support. Moreover, Ni7.5NdMgAlO with the largest amount of CO2 desorption (736.6 μmol/g-cat) effectively oxidized the coke on the catalyst surface, and manifested the excellent coke resistance (2.4%) in addition to the highest CH4 and CO2 conversion in the DRM reaction. In addition, the optimal addition (7.5%) of Nd to NiMgAlO resulted in a lowering of the activation energy of CO2 by 15%. At excessive Nd content, Ni10NdMgAlO exhibited larger nickel crystallite size (10.5 nm) and was subject to severe accumulation of encapsulating carbon leading to pronounced deactivation.

Photo-thermal catalytic degradation of organophosphate simulant over Cu, Co, and Fe on titania

Herein we demonstrate the degradation of organophosphate pesticide malathion in aerated alcohol over a tandem photo-thermal catalyst. TiO2-supported Cu, Co, and Fe was prepared by impregnation of metal-EDTA chelates or metal nitrates. Under UV irradiation, TiO2 generates H2O2 in situ (up to 1.8 ± 0.4 mmol/l) from dissolved O2 and alcohol, while the metals promote Fenton-like degradation reactions. The TOF of malathion degradation is maximum (11.9 ± 1.1 ks−1) for Cu/TiO2 at the highest metal dispersion, achieved by Cu-EDTA impregnation at low surface density (2.0 ± 0.3 atom/nm2). TOF of catalysts prepared from nitrates were at least 45% lower than those of catalysts prepared from chelates.

Excellent behaviors of highly dispersed Ni-based catalyst in CO methanation synthesized by in-situ hydrothermal method with carbon quantum dots assisted

Confining the active metal particles inside the channel of mesoporous support is an effective way to improve the catalytic activity and stability, as well as enhance the metal usage thus reducing the required amount of supported metal. In our research, a novel highly efficient Nickel-based catalyst with the metal particles confined in the pores of mesoporous support was obtained by in-situ hydrothermal synthesis with carbon quantum dots (CQDs) assisted. This catalyst is of scientific significance for low metal content (as low as 0.9 wt%), except for its large specific surface area, high metal dispersion and small metal particle size. And the catalyst exhibits satisfying CO methanation catalytic activity with 95% CO conversion and 90% CH4 selectivity at 400 °C and 15000 mlg−1h−1 when the metal loading is 0.9 wt%. Meanwhile, the catalyst presents excellent thermal and long-term stability for that the metal particles are confined inside the pores of support, which could strengthen the metal-support interaction and thus inhibiting the metal agglomeration and carbon deposition effectively.

Combined steam and CO2 reforming of methane over Co–Ce/AC-N catalyst: Effect of preparation methods on catalyst activity and stability

An appropriate preparation method is the essential to improve the catalyst performance. In this study, Co–Ce/AC-N catalysts were fabricated on N-doped activated carbon supports by impregnation, sol-gel, precipitation and mix methods, respectively. It was used to catalyze the combined steam and dry reforming of methane (CSDRM). The effects of different preparation methods on the catalyst performance were investigated by means of N2 adsorption-desorption, XRD, H2-TPR, TEM, CO2-TPD, FTIR and XPS. Compare with the catalysts prepared by other methods, the catalyst prepared by impregnation exhibits a large surface area, high active metal dispersion, and strong metal-support interaction. Meanwhile, it also has strong basic sites and abundant oxygen vacancies. These greatly improve the activity and stability of the catalyst. The conversions of CH4 and CO2 at 650 °C were achieved 71.6% and 64.4%, and H2/CO was retained at 1.5.

Small Reduced Graphene Oxides for Highly Efficient Oxygen Reduction Catalysts.

We demonstrated highly efficient oxygen reduction catalysts composed of uniform Pt nanoparticles on small, reduced graphene oxides (srGO). The reduced graphene oxide (rGO) size was controlled by applying ultrasonication, and the resultant srGO enabled the morphological control of the Pt nanoparticles. The prepared catalysts provided efficient surface reactions and exhibited large surface areas and high metal dispersions. The resulting Pt/srGO samples exhibited excellent oxygen reduction performance and high stability over 1000 cycles of accelerated durability tests, especially the sample treated with 2 h of sonication. Detailed investigations of the structural and electrochemical properties of the resulting catalysts suggested that both the chemical functionality and electrical conductivity of these samples greatly influence their enhanced oxygen reduction efficiency.

Comparison of preparation methods for improving coke resistance of Ni-Ru/MgAl2O4 catalysts in dry reforming of methane for syngas production

ABSTRACT A small loading of highly dispersed Ru on Ni/MgAl2O4 catalyst was investigated in the dry reforming of methane (DRM) process. The catalysts were synthesized by impregnation and deposition-precipitation methods; the DRM reaction was performed at CH4/CO2/N2 = 1/1/1 and 600‒750°C. The bimetallic Ni‒Ru catalysts exhibited improved reducibility and metal dispersion compared with that of the Ni catalyst. Among the synthesized catalysts, the Ru(D)/Ni(I)/MgAl2O4 catalyst had the highest metal dispersion (7.23%) and largest amount of CO2 desorption (135 μmol/g). Thus, this catalyst exhibited the highest CH4 conversion (85.6%) at 750°C. Additionally, coke deposition of only 2.69% occurred during the stability test.

An overview of the obtaining of biomass-derived gamma-valerolactone from levulinic acid or esters without H2 supply

Gamma-valerolactone (GVL) is a highly reactive keto-lactone and a promising platform biomolecule, used as an additive for food and fuels, green solvent, and fuels precursor, among others. Its production from biomass usually involves hydrogenation and subsequent cyclization of levulinic acid or its esters. The process of conventional hydrogenation requires high pressures and temperatures, an external hydrogen source, and scarce noble/precious materials as catalysts. However, it could be produced under mild conditions, using bifunctional metal-acid catalysts with high metal dispersion and meso or microporosity, high surface area, temperatures lower than 200 °C, pressures ≤ 1MPa, and secondary alcohols (such as isopropanol) as hydrogen donors. The catalytic transfer hydrogenation followed by cyclization (CTHC) of levulinic acid (LA) and its esters (LE) to produce GVL using secondary alcohols as H donor is a great alternative. Variables involved in CTHC such as raw material, time, temperature, and type of catalyst, mainly transition metals and their combinations, are reviewed in this work.

An overview of the obtaining of biomass-derived gamma-valerolactone from levulinic acid or esters without H2 supply

Gamma-valerolactone (GVL) is a highly reactive keto-lactone and a promising platform biomolecule, used as an additive for food and fuels, green solvent, and fuels precursor, among others. Its production from biomass usually involves hydrogenation and subsequent cyclization of levulinic acid or its esters. The process of conventional hydrogenation requires high pressures and temperatures, an external hydrogen source, and scarce noble/precious materials as catalysts. However, it could be produced under mild conditions, using bifunctional metal-acid catalysts with high metal dispersion and meso or microporosity, high surface area, temperatures lower than 200 °C, pressures ≤ 1MPa, and secondary alcohols (such as isopropanol) as hydrogen donors. The catalytic transfer hydrogenation followed by cyclization (CTHC) of levulinic acid (LA) and its esters (LE) to produce GVL using secondary alcohols as H donor is a great alternative. Variables involved in CTHC such as raw material, time, temperature, and type of catalyst, mainly transition metals and their combinations, are reviewed in this work.

Very high loading oxidized copper supported on ceria to catalyze the water-gas shift reaction

To prepare supported catalysts with high loading and high dispersion of active metals is a crucial challenge in heterogeneous catalysis. Here we report the fabrication of very reactive catalyst that contains finely dispersed copper with 20 wt% content supported on high-surface area CeO2 for the water–gas shift (WGS) reaction. This copper-ceria catalyst shows superb activity with a CO consumption rate of 116.7 μmolCO gcat-1 s−1 at 300 ℃, nearly one order of magnitude more active than other copper-based catalysts ever reported. Based on the precise and integratedcharacterizations, the oxidized not the metallic Cu are proved to be active centers where the adsorbed CO directly reacted with bridged —OH groups to finish the WGS reaction. The oxidized Cu sites are confirmed to be originated from the activation effect from Cu to CeO2, which promotes the oxygen of CeO2 diffuse from the bulk to surface, making Cu oxidized even under reductive atmosphere.

Hydrodeoxygenation of lignin-derived phenolic compounds over Ru/TiO2–CeO2 catalyst prepared by photochemical reduction method

Hydrodeoxygenation (HDO) is an important pathway for lignin-oil upgradation to produce high quality liquid fuels, in which the high-active catalyst is the key point to achieve a high HDO efficiency. In this study, Ru/TiO2, Ru/CeO2 and Ru/TiO2–CeO2 catalysts were synthesized by a novel photochemical method with a high Ru dispersion and used for the HDO of lignin-derived phenolic compounds. Among these three catalysts, Ru/TiO2–CeO2 showed a better performance than Ru/TiO2 and Ru/CeO2. And guaiacol was transformed into cyclohexane with conversion and product selectivity close to 100 % at 230 °C under 1 MPa hydrogen atmosphere. Ru/TiO2–CeO2 took the advantages of Ru/TiO2 and Ru/CeO2, namely high Ru metal dispersion, plenty of medium strength acid sites and abundance of oxygen vacancy, which promoted the guaiacol adsorption and the HDO catalytic activity. Moreover, Ru/TiO2–CeO2 catalyst showed an effective upgrading of other lignin-derived phenolic compounds. And the lignin-oil also enabled to be upgraded efficiently, producing a high content of hydrocarbons and alkylphenols.

Robust Hydrogen Production via Pickering Interfacial Catalytic Photoreforming of n-Octanol-Water Biphasic System.

Pickering emulsion offers a promising platform for conducting interfacial reactions between immiscible reagents; it is particularly suitable for hydrogen production by photoreforming of non-water soluble biomass liquid and water. Herein, Pt-promoted (001)-facet-dominated anatase TiO2 nanosheets were synthesized by a hydrothermal route associated with microfluidic technology for high activity and metal dispersion, and selective surface modification was carried out for preparing Janus particles. Photoreforming hydrogen production through n-octanol and water that formed O/W microemulsion with an average diameter of 540 µm was achieved to obtain amphiphilic catalyst. The as-prepared 2D Janus-type catalysts exhibited remarkably stable emulsification performance as well as photocatalytic activity. This finding indicates that triethoxyfluorosilane had negligible impact on the catalytic performance, yet provided a remarkable benefit to large specific surface area at microemulsion interface, thereby enhancing the H2 yield up to 2003 μmol/g. The cyclic experiments indicate that the decrease in cyclic performance was more likely to be caused by the coalescence of the microemulsion rather than the decrease in catalytic activity, and the microemulsion could be easily recovered by simply hand shaking to more than 96% of the initial performance.

Ni supported on MgO modified attapulgite as catalysts for hydrogen production from glycerol steam reforming

A series of Mg-modified Ni/Attapulgite (ATP) catalysts have been prepared by impregnation method for glycerol steam reforming to produce hydrogen. The physicochemical properties of catalysts were characterized using various techniques including N2 physical adsorption analysis, XRD, H2-TPR, SEM, TEM and NH3-TPD. The results of N2 physical adsorption indicated MgO modified Ni-based catalysts had unique mesostructure, resulting in the high metal dispersion and interaction between active metal and support as proven by XRD, TEM and H2-TPR. Results of glycerin reforming experiments showed that Ni/10MgO/ATP catalyst had the highest activity than that of the other catalysts. Ni/10MgO/ATP catalyst had the smallest Ni average crystal size (10.1 nm) and the highest surface area (110.31 m2/g). These excellent properties made it show the enhanced glycerol conversion (94.71%) and a higher H2 yield (88.45%) and the longest stability (30 h) during glycerol steam reforming (GSR) at 600 °C, W/G = 3, and WHSV = 1 h−1. The used catalysts after 60 h of glycerin reforming experiments were also investigated by XRD, SEM, TEM, Roman and TG-DTG. The results indicated that the addition of Mg significantly inhibited the sintering of nickel grains and the formation of amorphous carbon. Therefore, Ni/10MgO/ATP catalyst increased the activity of the catalyst and extended the life of the catalyst.

High Loading of Transition Metal Single Atoms on Chalcogenide Catalysts.

Transition metal doped chalcogenides are one of the most important classes of catalysts that have been attracting increasing attention for petrochemical and energy related chemical transformations due to their unique physiochemical properties. For practical applications, achieving maximum atom utilization by homogeneous dispersion of metals on the surface of chalcogenides is essential. Herein, we report a detailed study of a deposition method using thiourea coordinated transition metal complexes. This method allows the preparation of a library of a wide range of single atoms including both noble and non-noble transition metals (Fe, Co, Ni, Cu, Pt, Pd, Ru) with a metal loading as high as 10 wt % on various ultrathin 2D chalcogenides (MoS2, MoSe2, WS2 and WSe2). As demonstrated by the state-of-the-art characterization, the doped single transition metal atoms interact strongly with surface anions and anion vacancies in the exfoliated 2D materials, leading to high metal dispersion in the absence of agglomeration. Taking Fe on MoS2 as a benchmark, it has been found that Fe is atomically dispersed until 10 wt %, and beyond this loading, formation of coplanar Fe clusters is evident. Atomic Fe, with a high electron density at its conduction band, exhibits a superior intrinsic activity and stability in CO2 hydrogenation to CO per Fe compared to corresponding surface Fe clusters and other Fe catalysts reported for reverse water-gas-shift reactions.

Tuning the interactions among Ce, Pd and Rh over Ce-modified Pd-Rh three-way catalyst for exhaust treatment of natural gas vehicles

Three Ce-modified Pd-Rh bimetallic catalysts for exhaust purification of natural gas vehicles (NGVs) under stoichiometric conditions were prepared by deposition-precipitation (DP), stepwise impregnation (SI) and co-impregnation (CI) methods, respectively. It demonstrated that before and after aging the catalyst prepared by DP technique displayed much higher activity for the conversions of CH4 and NO than the other two catalysts, especially at high temperatures (about 410–550 °C). X-ray diffraction, CO-chemisorption, hydrogen temperature-programmed reduction and X-ray photoelectron spectroscopy analyzes demonstrated that there were smaller CeO2 particles, higher metal dispersion, stronger reducibility and more surface metallic Pd species over the catalyst prepared by DP method. This might be due to the stronger Pd-Ce but weaker Pd-Rh interactions over the catalyst prepared by DP mean. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption analysis implied that the electronic structures of Pd in the three catalysts were different, which may be also induced by the different interactions among Rh, Ce and Pd in these catalysts. The results indicated that the performance of Pd-Rh three-way catalyst for NGVs exhaust purification can be enhanced by modulating the interactions of Pd-Rh and Pd-Ce. Our work may provide a new strategy for the preparation of three-way catalyst with high efficiency for NGVs.

Study of the effect of the LDH cations precursors in the removal of arsenic in aqueous solution.

Layered double hydroxides (LDH) are anionic clays, mainly used as adsorbents, ion exchange material, and catalysts. Generally, they present high specific surface areas, alkaline character, high metallic dispersion, and high thermal stability. If they contain a transitional element in their structure, the solid may present redox properties. LDH were synthesized with the following combinations: MgAl, MgFe, and ZnMgFe, aiming to determine the effect of cationic nature in the structure and the functionality of the synthesized clay as adsorbents in polluted aqueous effluents. The textural properties were determined by nitrogen adsorption isotherms. Crystalline structure was studied by XRD and the presence of the anions of the interlayer was determined by FTIR spectroscopy. The studies of removal of As(III) and As(V) from aqueous solutions, using the LDH, show that after 24 h the solids reach a high removal efficiency. ZnMgFe solid removed both species of arsenic with values of 95 and 98% for As(II) and As(V), respectively. The MgFe solid showed some selectivity to the uptake of As(V), while the MgAl only removed As(V). This selective behaviour can be beneficial in studies of arsenic speciation.

Tailoring physicochemical and electrical properties of Ni/CeZrOx doped catalysts for high efficiency of plasma catalytic CO2 methanation

Promoters covering alkaline earth metals, lanthanides, transition and some of the typical rare earth metals, were added to Ni/CeZrOx based catalysts and examined in non-thermal plasma (NTP)-assisted CO2 methanation. The influence of physicochemical properties was examined by X-Ray Diffraction, N2 adsorption/desorption, H2-TPR, CO2-TPD, TPD-MS and FTIR spectroscopy. Moreover, the measurement of the dielectric constant of each catalyst was also performed. The promotion of Ni/CeZrOx with 4 wt% Gd significantly increases the energy efficiency. Furthermore, NiCZ-Gd resulted in the highest activity with 85 % conversion of CO2 fully converted to methane at 7 W. The enhancement of energy efficiency was attributed to electrical and physicochemical properties in particular: a low dielectric constant, a high metal dispersion, a high percentage of medium basic sites and finally an adequate promoter’s cation radii. This study allowed to set up a new methodology to select the most appropriate catalysts in terms of energy efficiency for plasma catalytic CO2 methanation.

Sub-nanometer ceria-promoted Ni 13X zeolite catalyst for CO2 methanation

Sub-nanometer zeolite 13X-supported Ni-ceria catalysts were synthesized for CO2 methanation. XRD and SEM results show the structure and morphology of 13X zeolite after impregnation and calcination. Ce loading affected the catalysts’ metal dispersion, reducibility, basicity and acidity, and thence their activity and selectivity. STEM-EDX elemental mappings showed that Ce and Ni are predominantly highly dispersed. Ce has a positive effect on the reduction of NiO and leads to a relatively high number of medium basic sites with a low Ce loading. Highly stable 5%Ni2.5%Ce13X had high activity and nearly 100% CH4 selectivity in CO2 methanation at 360 °C, which is mainly due to the high dispersion of metals and relatively high amount of medium basic sites. It can be inferred that this catalyst synthesis strategy has great potential for good catalyst dispersion, since metal uptake by the zeolite is selective for the metal citrate complexes in solution.