Active Phase Of Catalyst Research Articles

In Situ Transformed CoOOH@Co3S4 Heterostructured Catalyst for Highly Efficient Catalytic OER Application.

The deprived electrochemical kinetics of the oxygen evolution reaction (OER) catalyst is the prime bottleneck and remains the major obstacle in the water electrolysis processes. Herein, a facile hydrothermal technique was implemented to form a freestanding polyhedron-like Co3O4 on the microporous architecture of Ni foam, its reaction kinetics enhanced through sulfide counterpart transformation in the presence of Na2S, and their catalytic OER performances comparatively investigated in 1 M KOH medium. The formed Co3S4 catalyst shows outstanding catalytic OER activity at a current density of 100 mA cm-2 by achieving a relatively low overpotential of 292 mV compared to the pure Co3O4 catalyst and the commercial IrO2 catalyst. This enhancement results from the improved active centers and conductivity, which boost the intrinsic reaction kinetics. Further, the optimized Co3S4 catalyst exhibits admirable prolonged durability up to 72 h at varied current rates with insignificant selectivity decay. The energy dispersive X-ray spectroscopy (EDX) and Raman spectra measured after the prolonged OER stability test reveal a partial transformation of the active catalyst into an oxyhydroxide phase (i.e., CoOOH@Co3S4), which acts as an active catalyst phase during the electrolysis process.

Effects of surface hydrophobization on the phase evolution behavior of iron-based catalyst during Fischer–Tropsch synthesis

Iron-based Fischer–Tropsch synthesis (FTS) catalyst is widely used for syngas conversion, but its iron carbide active phase is easily oxidized into Fe3O4 by the water produced during reaction, leading to the deterioration of catalytic performance. Here, we show an efficient strategy for protecting the iron carbide active phase of FTS catalyst by surface hydrophobization. The hydrophobic surface can reduce the water concentration in the core vicinity of catalyst during syngas conversion, and thus inhibit the oxidation of iron species by water, which enhances the C − C coupling ability of catalyst and promotes the formation of long-chain olefins. More significantly, it is unraveled that appropriate shell thickness plays a crucial role in stabilizing the iron carbide active phase without Fe3O4 formation and achieving good catalytic performance.

Direct conversion of biogas to syngas over bimetallic nickel–cobalt supported on α-alumina catalysts

Syngas is an essential feedstock for many valuable chemicals, produced primarily by steam reformation of methane. Recently, dry reformation (reaction between methane and carbon dioxide) has gained importance as both the reactants are greenhouse gases (GHGs). However, dry reformation (DRM) produces syngas with a low H2/CO (<1.0). The DRM requires catalyst such as nickel supported on alumina, which results in higher coke. In this study, 6Ni–6Co/α-Al2O3 and 12Ni/α-Al2O3 catalysts were prepared by wetness impregnation. The catalysts were characterized using FESEM, XRD, XPS, BET, H2-TPR, NH3-TPD and H2-Chemisorption. The catalyst performance under DRM conditions were carried out in a fixed bed reactor at 20,000 ml/gcat.hr GHSV, between 600 °C and 700 °C, at atmospheric pressure. The performance of the catalysts was compared based on conversion of methane and carbon dioxide, hydrogen to carbon monoxide and coke deposition. The rates of consumption of methane carbon dioxide were found to be equal, and higher rates were observed for 6Ni–6Co/α-Al2O3, which is attributed to NiAl2O4/CoAl2O4 phase of active metals, higher surface area, smaller crystallite size and higher dispersion. This active phase of catalyst also responsible for higher rate of CO2 adsorption and reduced coke deposition. Highest H2/CO was found to be 1.26 for 6Ni–6Co/α-Al2O3.

ICP-MS Assisted EDX Tomography: A Robust Method for Studying Electrolyte Penetration Phenomena in Gas Diffusion Electrodes Applied to CO2 Electrolysis.

A carbon paper-based gas diffusion electrode (GDE) is used with a bismuth(III) subcarbonate active catalyst phase for the electrochemical reduction of CO2 in a gas/electrolyte flow-by configuration electrolyser at high current density. It is demonstrated that in this configuration, the gas and catholyte phases recombine to form K2CO3/KHCO3 precipitates to an extent that after electrolyses, vast amount of K+ ions is found by EDX mapping in the entire GDE structure. The fact that the entirety of the GDE gets wetted during electrolysis should, however, not be interpreted as a sign of flooding of the catalyst layer, since electrolyte perspiring through the GDE can largely be removed with the outflow gas, and the efficiency of electrolysis (toward the selective production of formate) can thus be maintained high for several hours. For a full spatial scale quantitative monitoring of electrolyte penetration into the GDE, (relying on K+ ions as tracer) the method of inductively coupled plasma-mass spectrometry (ICP-MS) assisted energy dispersive X-ray (EDX) tomography is introduced. This new, cheap and robust tomography of non-uniform aspect ratio has a large planar span that comprises the entire GDE surface area and a submicrometer depth resolution, hence it can provide quantitative information about the amount and distribution of K+ remnants inside the GDE structure, in threedimensions.

Synthesis and Catalytic Performance of Bimetallic Oxide-Derived CuO-ZnO Electrocatalysts for CO2 Reduction.

Steering the selectivity of electrocatalysts toward the desired product is crucial in the electrochemical reduction of CO2. A promising approach is the electronic modification of the catalyst's active phase. In this work, we report on the electronic modification effects on CuO-ZnO-derived electrocatalysts synthesized via hydrothermal synthesis. Although the synthesis method yields spatially separated ZnO nanorods and distinct CuO particles, strong restructuring and intimate atomic mixing occur under the reaction conditions. This leads to interactions that have a profound effect on the catalytic performance. Specifically, all of the bimetallic electrodes outperformed the monometallic ones (ZnO and CuO) in terms of activity for CO production. Surprisingly, on the other hand, the presence of ZnO suppresses the formation of ethylene on Cu, while the presence of Cu improves CO production of ZnO. In situ X-ray absorption spectroscopy studies revealed that this catalytic effect is due to enhanced reducibility of ZnO by Cu and stabilization of cationic Cu species by the intimate contact with partially reduced ZnO. This suppresses ethylene formation while favoring the production of H2 and CO on Cu. These results show that using mixed metal oxides with different reducibilities is a promising approach to alter the electronic properties of electrocatalysts (via stabilization of cationic species), thereby tuning the electrocatalytic CO2 reduction reaction performance.

Enhancing Cu-ligand interaction for efficient CO2 reduction towards multi-carbon products.

Electrochemical CO2 reduction (CO2R) to valuable products provides a promising strategy to enable CO2 utilization sustainably. Here, we report the strategy of using Cu-DAT (3,5-diamino-1,2,4-triazole) as a catalyst precursor for efficient CO2 reduction, demonstrating over 80% selectivity towards multicarbon products at 400 mA cm-2, with intrinsic activity over 19 times higher than that of Cu nanoparticles. The catalyst's active phase is determined to be metallic copper wrapped with the DAT ligand. We attribute this enhanced CO2R performance to the accelerated steps of *CO adsorption and C-C coupling induced by the closely cooperated DAT ligand.

Tailoring the active phase of CoO-based thin-film catalysts in order to tune selectivity in CO2 hydrogenation.

In this study, we prepared CoO-based thin films deposited on Kanthal steel wire gauze meshes by plasma-enhanced chemical vapor deposition. X-ray photoelectron spectroscopy (XPS) analysis revealed a structure characterized by a combination of cobalt oxide and metallic cobalt embedded within a carbon matrix. Our primary objective was to gain insights into the roles of Co0 and CoO in CO2 hydrogenation reactions. To achieve this, the performance of the thin-film CoO-based catalyst with an initial atomic ratio of CoO/Co0 at 10.2 was compared with two series of the thin-film catalysts that underwent pre-reduction processes at 350 °C for durations of 30 and 60 minutes, resulting in atomic ratios of CoO/Co0 at 3.1 and 1.1, respectively. Subsequently, catalytic tests were conducted in a continuous flow stirred tank reactor operating at temperatures ranging from 250 °C to 400 °C. Our findings indicate that CoO plays a significant role in activating the CO2 methanation reaction which can be due to the high hydrogen coverage of CoO, while Co0 is the active phase in the reverse water-gas shift reaction. Results highlight the importance of oxidized cobalt for hydrogen adsorption and dissociation in CO2 hydrogenation for CH4 formation.

Intelligent method for analysis of the active phase of hydrogenation catalysts: stripe phase identification of UC-S/SC-S

In order to realize the intelligent recognition and statistics of hydrogenation catalyst image information, different from traditional methods, this paper presents a new method to judge the active phase by image recognition.

A model study of ceria-Pt electrocatalysts: stability, redox properties and hydrogen intercalation.

The electrocatalytic properties of advanced metal-oxide catalysts are often related to a synergistic interplay between multiple active catalyst phases. The structure and chemical nature of these active phases are typically established under reaction conditions, i.e. upon interaction of the catalyst with the electrolyte. Here, we present a fundamental surface science (scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction) and electrochemical (cyclic voltammetry) study of CeO2(111) nanoislands on Pt(111) in blank alkaline electrolyte (0.1 M KOH) in a potential window between -0.05 and 0.9 VRHE. We observe a size- and preparation-dependent behavior. Large ceria nanoislands prepared at high temperatures exhibit stable redox behavior with Ce3+/Ce4+ electrooxidation/reduction limited to the surface only. In contrast, ceria nanoislands, smaller than ∼5 nm prepared at a lower temperature, undergo conversion into a fully hydrated phase with Ce3+/Ce4+ redox transitions, which are extended to the subsurface region. While the formation of adsorbed OH species on Pt depends strongly on the ceria coverage, the formation of adsorbed Hads on Pt is independent of the ceria coverage. We assign this observation to intercalation of Hads at the Pt/ceria interface. The intercalated Hads cannot participate in the hydrogen evolution reaction, resulting in the moderation of this reaction by ceria nanoparticles on Pt.

Prediction of Higher Heating Values in Bio-Oil from Solvothermal Biomass Conversion and Bio-Oil Upgrading Given Discontinuous Experimental Conditions.

Both the conversion of lignocellulosic biomass to bio-oil (BO) and the upgrading of BO have been the targets of many studies. Due to the large diversity and discontinuity seen in terms of reaction conditions, catalysts, solvents, and feedstock properties that have been used, a comparison across different publications is difficult. In this study, machine learning modeling is used for the prediction of final higher heating value (HHV) and ΔHHV for the conversion of lignocellulosic feedstocks to BO, and BO upgrading. The models achieved coefficient of determination (R2) scores ranging from 0.77 to 0.86, and the SHapley Additive exPlanations (SHAP) values were used to obtain model explainability, revealing that only a few experimental parameters are largely responsible for the outcome of the experiments. In particular, process temperature and reaction time were overwhelmingly responsible for the majority of the predictions, for both final HHV and ΔHHV. Elemental composition of the starting feedstock or BO dictated the upper possible HHV value obtained after the experiment, which is in line with what is known from previous methodologies for calculating HHV for fuels. Solvent used, initial moisture concentration in BO, and catalyst active phase showed low predicting power, within the context of the data set used. The results of this study highlight experimental conditions and variables that could be candidates for the creation of minimum reporting guidelines for future studies in such a way that machine learning can be fully harnessed.

MgAl2O4 with CaO in supported Ni and Ni–Co catalysts – impact on CO2 reforming of CH4

ABSTRACT To obtain insight into the effect of promoter–support interactions in CO2 reforming of CH4, a series of Ni and Ni–Co (3:1) catalysts were prepared over modified-MgAl2O4 and MgAl2O4. The prepared samples were characterised and investigated for dry reforming of methane reactions at 873 K and 1 atm. From XRD, it appears that CaO, in a specified amount, is an appropriate modifier to retain the active metallic phase of catalyst and also prevents the formation of a solid solution of itself with the metal oxides during catalyst synthesis. From activity results, enhancement in catalytic activity was observed for Ni and Ni–Co catalysts supported over modified-MgAl2O4 support due to the presence of new and more number of active metal sites. For 100Ni–2.5CaO/MgAl2O4 catalyst, initial CH4 conversion was 1.3 times greater than 100Ni/MgAl2O4. For Ni–Co catalysts comparable results were observed over modified-MgAl2O4 and MgAl2O4 supports. Moderately active carbon species were formed during dry reforming of methane reaction and the presence of new and more number of basic sites helped to minimise carbon deposition. Highlights Incorporation of a metal oxide in appropriate composition proves to be more chemically active than MgAl2O4 Ni–Co synergy exhibited higher catalytic activity Formation of new adsorption site due to support modification Improvement in H2: CO ratio was observed after addition of CaO Similar kind of carbon formation over both the series and Methane decomposition reaction is the source

Novel synthesis strategy of SAPO-11 molecular sieve with superior performance for hydroisomerization of long-chain n-alkanes: Supergravity-cyclone assisted hydrothermal crystallization method

In this work, we successfully synthesized high performance SAPO-11 molecular sieve via the novel supergravity-cyclone assisted hydrothermal crystallization method for the first time. The initial gel was subjected to cyclone treatment and supergravity shearing treatment to obtain the secondary molecular sieve precursor mixture with high viscosity and micro-bubble distribution. The effects of supergravity shear-cyclone operation on the physicochemical properties of SAPO-11 molecular sieve were investigated. The supergravity-cyclone treatment improved the textural properties and acidity of SAPO-11 molecular sieve. The effects of support properties on the properties of active phase of NiWS-support catalyst and its catalytic performance for hydroisomerization of n-hexadecane were further investigated. NiW/SAPO-11-B showed more active phase with hydroisomerization kinetics advantage and nearly doubled yield of i-hexadecane compared with conventional SAPO-11-based catalyst. The process and mechanism of synthesizing high performance SAPO-11 molecular sieve via the supergravity-cyclone technology were explored. It was found that the supergravity-cyclone assisted crystallization method shortened the crystallization time of SAPO-11 and improved the crystallization efficiency. In addition, the hydroisomerization kinetics of n-hexadecane on SAPO-11-based NiWS supported catalyst was studied. The structure–activity relationship between the structure of active phase and the formation of CUS was investigated, and the influence of support properties on the properties of NiWS active phase and their synergistic mechanism in the hydroisomerization of n-hexadecane were revealed. This is expected to provide a new strategy for the synthesis of high performance SAPO-11 molecular sieve and the preparation of non-noble metal supported catalyst with superior hydroisomerization performance for long-chain alkanes.

Construction of high content CoMoS active sites through the combination of Co5Mo10 molecular precursors and inert carbon matrix support for deep hydrodesulfurization

The rational design of high sulfurization and abundant CoMoS active phases catalysts is the key to drive the industrialization of hydrodesulfurization (HDS). Herein, we prepared a nano-catalyst with well-dispersed and numerous CoMoS active sites through confining Co3(II)[Co2(III)Mo10O38H4] (Co5Mo10) polyoxometalates (POMs) molecules with high Co/Mo ratio in hollow mesoporous carbon spheres (HMCS). On the one hand, POMs acted as pre-assembled molecular platform can build and regulate the structure of active phases by precisely adjusting the Co/Mo atomic ratio at molecular level. Meanwhile, the intimate interaction of Co-Mo on the POMs precursor also ensures that the Co species mainly exists as Co-promoted MoS2 active phase after the sulfidation process, thus the highly efficient synergism of the bimetals can be fully utilized in the reaction. On the other hand, the mesoporous carbon sphere with homogeneous pore size and high specific surface area can effectively confine the growth of the catalyst to disperse the active metals. The weakening metal–metal interaction property of such inert support enables the S-Co5Mo10/HMCS-10 catalyst to realize higher sulfurization, shorter slab length and higher stacking number. In addition, the abundant mesopores and high pore capacity of support greatly facilitate the mass transfer process and provide sufficient space for the HDS reaction as the nanoreactor. The unique combination of POMs and HMCS enabled the S-Co5Mo10/H10 catalyst to achieve up to 99.2% conversion of dibenzothiophene (DBT) after 4 h of HDS reaction, while obtaining a TOF value of 1.3 × 10−3 s−1 (calculated when keeping the low conversion at 42% after 0.5 h of reaction). The structural orientation of the POMs precursors to the active phases of HDS catalysts was investigated using various characterization methods, and the structure–activity relationship between the catalytic performance and the sulfide active phases was also revealed in this work.

In situ rapid and deep self-reconstruction of Fe-doped hydrate NiMoO4 for stable water oxidation at high current densities

The rational design of efficient electrodes that can withstand high current densities is urgent for electrolytic hydrogen generation, but it is a significant challenge to develop high activity and stability electrocatalysts with simple methods. Herein, we report an electrocatalyst that is obtained by in situ rapid depth self-reconstruction of Fe-doped hydrate NiMoO4 (denoted as NiFeMoO). Experimental and theoretical calculations show that the co-leaching of crystal water and MoO42− from the original structure induces the structure crack and electrolyte penetration, as well as synergistic Fe incorporation, leading to the generation of active catalyst phase of Fe doped γ-NiOOH (denoted as SR-NiFeMoO) during electro-oxidation. In 1 M KOH, the resulting SR-NiFeMoO, with unique poly-/low-crystalline features, possesses ultralow overpotential of 168 mV at 10 mA cm−2 and 226 mV at 100 mA cm−2, and performs stable catalysis for over 106 h at a high current density of 1000 mA cm−2. In addition, the presence of dissolved Mo leads to the change of electrolyte properties over time, which leads to the continuous reduction of potential for water oxidation. The discovery of deep self-reconstruction may provide a new methodology for the development of robust and efficient electrocatalysts for electrolytic hydrogen generation.

The influence of activation atmosphere on the active phase and hydrotreating activity of LDH-based NiW pre-sulfurized catalysts

Activation atmosphere and temperature determined active phases of NiW pre-sulfurized hydrotreating catalysts prepared via a tetrathiotungstate intercalated NiAl layered double hydroxide.

Solvent-Free Synthesis of Nickel Nanoparticles as Catalysts for CO2 Hydrogenation to Methane

The solid-state combustion method was used to prepare nickel-based catalysts for CO2 hydrogenation from [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2. These complexes were synthesized by adding nickel nitrate and perchlorate to melted imidazole. The composition and structure of the obtained complexes was confirmed by ATR FTIR, powder XRD, and elemental analysis. The stages of thermal decomposition of the complexes and their kinetic parameters were established. It was found that incomplete gasification of more thermostable Ni(C3H4N2)6](ClO4)2 led to the formation of carbon, nitrogen, and chlorine impurities. According to powder XRD and XPS, the solid products of gasification of both complexes consist of NiO and Ni0 covered with nickel hydroxide and/or a carbonate layer. In the case of the sample prepared from [Ni(C3H4N2)6](ClO4)2, this layer was pronounced. Therefore, it limits the nickel reduction in the reaction medium of CO2 hydrogenation, even at 450 °C. The surface of the sample prepared from [Ni(C3H4N2)6](NO3)2 contains nickel oxide, which is easily reduced. So, the catalyst active phase is already formed at 250 °C in the presence of CO2 and efficiently catalyzes CO2 hydrogenation as the temperature increases. Therefore, [Ni(C3H4N2)6](NO3)2 is a promising precursor for the CO2 hydrogenation catalyst, and its solvent-free synthesis follows Green Chemistry principles.

Fundamental analysis for active phase of rutile-based IrO2 catalyst: Comparison study of CH4 oxidation on IrO2(1 0 0) and IrO2(1 1 0)

This study examined the reactivity of IrO2(100) toward CH4 oxidation and compared the reactivity with the IrO2(110) surface to obtain fundamental insights of the active phase for rutile structure-based IrO2 catalyst. Our DFT simulation predicted that stoichiometric IrO2(100) surface stabilizes and easily activates the adsorbed CH4. The subsequent reactions toward complete oxidation are also facile with the predicted rate-limiting steps from CH2O to CO. Similar kinetic behavior was predicted from CH4 oxidation on an oxygen-rich IrO2(100) surface, but it is more reactive than the stoichiometric IrO2(100) surface. The simulated TPRS results confirmed that the (110) facet is more active than (100); CH4 conversions on the (100) surface are predicted to be ∼ 35 % lower than on the (110) surface. Based on these results, the IrO2(100) surface is expected to be active toward CH4 oxidation but not as active as IrO2(110). Overall, the (110) surface is not the only active facet determining the reactivity of the rutile IrO2 catalyst; the (100) surface is also active. The comparison results from the current study would provide fundamental insights into the active phase of rutile structure-based IrO2 catalyst.

Oxygen-functionalized activated carbon supported vanadia catalysts: Unexpected improvement in low-temperature NH3-SCR performance

To regulate the active phase of carbon-supported catalysts, we report a synthetic protocol for oxygen-functionalized activated carbon (AC) supported vanadium catalysts for low-temperature selective catalytic reduction (LT-SCR) of NOx with NH3 via wet oxidation method using ammonium persulfate ((NH4)2S2O8, APS) as an oxidant. Optimum APS oxidative modification promoted the acid and redox properties of V/OAC2-60-3 and produced abundant reactive oxygen species, which enhance the catalytic activity significantly. Especially, APS activation promoted a high V4+/V5+ ratio on V/OAC2-60-3. The in-situ DRIFTs experiments demonstrated that V/OAC2-60-3 could absorb more NH3 species on Lewis acid sites and both the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms were present on V/OAC2-60-3, and then a promotional mechanism for LT-SCR was proposed over V/OAC2-60-3. Enhancement of redox and acid potential cycle proved that this APS oxygen-functionalization is a viable strategy for carbon-based catalysts in NH3-SCR applications at low temperatures.

Synthetic natural gas production in a 1 kW reactor using Ni–Ce/Al2O3 and Ru–Ce/Al2O3: Kinetics, catalyst degradation and process design

Nickel and ruthenium promoted by ceria were compared as catalyst active phase for the production of synthetic natural gas in a 1 kW fixed-bed reactor (49 cm in length and 3 cm in diameter). Both catalysts were tested in a broad range of conditions: 250–450 °C, 8000–16000 h−1 GHSV, 3.5–5.5H2/CO2 ratio, and atmospheric pressure. The Ni-based catalyst presented higher CO2 conversion and lower reduction of BET area and metal dispersion after utilization. Two kinetic models were elaborated according to the performance of the catalysts in this reactor. The minimum mean squared error between the kinetic model and experiments was 3.0% for the Ru-based catalyst and 1.4% for the Ni-based catalyst. The kinetic model of the Ni-based catalyst was implemented in Aspen Plus to evaluate potential methanation plant designs. The simulation minimized the amount of catalyst required to reach 95 vol% of CH4, within proper technical limits (GHSV ≤ 5000 h−1 and temperature ≥ 300 °C). The 3-reactor plant was the most adequate configuration. The total catalyst mass was 3.26 kg per kg/h of SNG, the heat removed from the reactors was 10.8 MJ/kgSNG, and the preheating necessity was 4.90 MJ/kgSNG. The electrolysis energy consumption was 86.4 MJ/kgSNG.

Role of Nanoscale Inhomogeneities in Co2FeO4 Catalysts during the Oxygen Evolution Reaction.

Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co2FeO4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm2. In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co2+-rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co2+-rich domains transform during OER to structurally different Co3+-rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.