Characterization Of Catalysts Research Articles

Impact of chelating agents on precursor formation and efficiency of Ni/MgO-Al2O3 catalysts in dry reforming of methane

Using cheap and non-toxic chelating agents during catalyst preparation is considered as an effective tool to control the particle size and to strengthen the metal-support interactions (MSI), while keeping the simplicity of the selected preparation method. The previous properties are useful to improve the catalyst activity, stability, and selectivity in dry reforming of methane (DRM). Therefore, we prepared a series of low Ni loading (2.5 wt%) catalysts supported over MgO-Al2O3 (Mg/Al = 1.3) mixed oxide using the classic wet impregnation method modified with chelating agents (citric acid (CA), tartaric acid (TA) and diethylenetriaminepentaacetic acid (DTPA)) and subsequently evaluated these in DRM. Intensive catalyst characterization (ICP, BET, XRD, XPS, STEM, H2-TPR and O2-TPO) suggested the presence of different structure-activity relationships. Particularly, using chelating agents during catalyst preparation controlled the final particle size range, which in turn played a pivotal role in shaping the strength of the metal support interactions (MSI). These impacts were crucial to improve the activity, stability and to control the formation of coke deposits. Moreover, special focus was put on the catalytic testing of the outperforming sample (Ni(TA)/Mg1.3AlOx) prepared using tartaric acid as chelating agent under severe regimes prone to catalyst deactivation to demonstrate the benefits of the proposed preparation technique.

New multi-functional catalyst coated membrane structure for improved water electrolysis

The state of proton exchange membrane water electrolysis (PEMWE) technology is largely dependent on conventional iridium catalyst layers with a lack of material and structural alternatives. For the first time, a catalyst coated membrane (CCM) structure is demonstrated to improve iridium catalyst utilization and decrease hydrogen crossover simultaneously. The anode catalyst layer is composed of an electrolessly-deposited Pt layer (<130 μg cm−2) at the membrane surface with a lower loading of iridium oxide catalyst (∼1.2 mg cm−2 IrOx) sprayed on the platinized membrane with variable thicknesses. These samples were subjected to rigorous catalyst characterization and crossover evaluation via a benchtop modified rotating disk electrode (MRDE) and an electrolysis cell (50 cm2), respectively. The MRDE results show that the CCM structure with the electroless Pt layer decreases the potential by as much as 100–200 mV at 1 A cm−2. At the unit cell level, the performance enhancement was in the range of 400–600 mV at 2 A cm−2. The parasitic hydrogen crossover current density is reduced by about 40 % when comparing this novel structure approach to baseline CCMs. This multi-functional CCM structure can potentially reshape the standards for PEM and for alkaline electrolyte membrane water electrolysis.

Ultra-low Pt loading catalyst on (Nb–Ti)2AlC support as advanced material for low-temperature fuel cell application

ABSTRACT The oxygen reduction reaction was investigated at 10 monolayers (MLs) of Pt electrodeposited on (Nb–Ti)2AlC substrate. Following the discussion of detailed kinetics and electrodeposition optimisation in the authors’ previous paper, the focus of this research was on stability testing. Previously performed optimisation results showed the best activity shown by 10 monolayers of Pt. Catalyst characterisation was performed by scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. SEM, EDS and XPS analysis showed that the surface was covered with a homogeneous layer of Pt. TEM analysis of a cross-section confirmed the presence of Pt layer of thickness ∼3.5 nm, corresponding to the 10 MLs of Pt. Although the mass activity for the ORR at 0.8 V vs. RHE of 37.4 A g−1 was comparable with the best one for the Pt/C benchmark, US Department of Energy Protocols revealed excellent catalyst stability – the loss of electrochemically active surface area (EASA) was found to be only 9.3%.

MgO impregnation to Al2O3 supported Ni catalyst for SYNGAS production using greenhouse gases: Some aspects of chemical state of Ni species

Al2O3 was prepared by sol–gel method and stabilized at 700 °C, then impregnated with 5, 15, and 30 wt% MgO to obtain the mixed oxide and stabilized again at 700 °C. The catalytic Ni/Al2O3 and Ni/Al2O3–MgO materials were prepared by impregnation method. The catalytic properties were studied on the dry reforming of methane (DRM) reaction using greenhouse gases (CO2+ CH4) to SYNGAS (Carbon Oxide/Hydrogen: CO/H2) at 700 °C for 70 h. The catalysts characterization includes TPR-H2, X-RD, BET, SEM, TPD-CO2, TPO-MS and TEM techniques. The CH4 and CO2 conversion was more stable in the Ni-base catalysts supported on mixed Al2O3–MgO oxides than in the Ni/Al2O3 sample. The Ni/Al2O3–MgO(5%) catalyst showed a lower presence of C (carbon) as evidenced by TPO-MS, XRD, SEM and TEM characterization. This finding reveals that a low quantity of MgO in the catalyst, delays the carbon development and may be associated with the basic character and a greater dispersion of this oxide on the surface of the alumina. In addition to the Ni–Mg–Al–O spinel formation, which enhanced the strong metal-support interaction (SMSI) effect between the support and the active phase. The Ni sintering of the catalysts after catalytic reaction is likely at the expenses of Ni–Al–O and MgNiO2 phases as was evidenced by XRD technique. The effect of the oxidation state of the active phase on the Ni/Al2O3–MgO (5%) sample had influence on the CO2 transformation, H2/CO ratio, basic sites, and stability in the C species.

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition.

The structural and morphological characterization of individual catalyst particles for olefin polymerization, as well as for the reverse process of polyolefin decomposition, can provide an improved understanding for how these catalyst materials operate under relevant reaction conditions. In this review, we discuss an emerging analytical toolbox of 2D and 3D chemical imaging techniques that is suitable for investigating the chemistry and reactivity of related catalyst systems. While synchrotron-based X-ray microscopy still provides unparalleled spatial resolutions in 2D and 3D, a number of laboratory-based techniques, most notably focused ion beam-scanning electron microscopy, confocal fluorescence microscopy, infrared photoinduced force microscopy and laboratory-based X-ray nano-computed tomography, have helped to significantly expand the arsenal of analytical tools available to scientists in heterogeneous catalysis and polymer science. In terms of future research, the review outlines the role and impact of in situ and operando (spectro-)microscopy experiments, involving sophisticated reactors as well as online reactant and product analysis, to obtain real-time information on the formation, decomposition, and mobility of polymer phases within single catalyst particles. Furthermore, the potential of fluorescence microscopy, X-ray microscopy and optical microscopy is highlighted for the high-throughput characterization of olefin polymerization and polyolefin decomposition catalysts. By combining these chemical imaging techniques with, for example, chemical staining methodologies, selective probe molecules as well as particle sorting approaches, representative structure-activity relationships can be derived at the level of single catalyst particles.

Multiple active sites for glucose conversion identified in Sn-Beta catalysts by CPMG MAS NMR and operando UV-Vis spectroscopy

Sn-Beta is an emerging heterogeneous zeolite catalyst that displays high activity for the catalytic conversion of glucose to desirable bio-based chemicals. Due to its promising catalytic properties, the synthesis and characterisation of Sn-Beta catalysts of different composition and preparation method have been the subject of a great number of studies. However, development of structure-activity-lifetime relationships for glucose conversion chemistry has been obstructed by several challenges, making the tailored preparation of high performance Sn-Beta catalysts harder to achieve. In this manuscript, we undertake continuous kinetic studies of four different Sn-Beta catalysts for glucose conversion, and pair the kinetic insights obtained with operando UV-Vis spectroscopy and quantitative 119Sn CPMG MAS NMR. In doing so, we reveal that at least two types of Sn species that are active for glucose conversion can exist in Sn-Beta catalysts, but that the two Sn species exhibit different levels of activity, selectivity and stability. In particular, we find that a Sn species resonating at − 690 ppm is characterised by high activity and is primarily selective towards the isomerisation of glucose to fructose, whereas a second Sn species resonating at − 658 ppm is characterised by lower activity for glucose conversion, and preferential selectivity towards alkylation and β-dehydration. Our studies also demonstrate that both Sn sites also possess different levels of stability during continuous operation. Consequently, we reveal that it is the ratio amongst these species that governs the overall catalytic performances of Sn-Beta catalysts for biomass conversion processes.

Biodiesel production from waste cat fish oil using heterogeneous catalyst from cat fish born: A viable waste management approach, and ANN modeling of biodiesel yield

Biodiesel from vegetable or animal feedstock can serve as a suitable source for renewable energy. This study utilized heterogeneous catalyst obtained from fish bone for biodiesel production from raw cat fish waste (CFW) oil. The CFW oil yield obtained using wet reduction extraction method was 96.85 %. The heterogeneous catalyst was prepared by calcination at 600 °C, for 4 h and characterized using SEM-EDX. Catalyst characterization showed suitable elements that can improve the catalytic activity. Physiochemical characterization results showed that viscosity of CFW oil (48.782 mm2/s), reduced to 9.391 mm2/s in the produced biodiesel. Similarly, after transesterification free fatty acid (3.593 mg/KOH/Kg) and acid value (7.186 mg/KOH/Kg) of CFW oil reduced to 1.48 mg/KOH/Kg and 2.96 mg/KOH/Kg, respectively, in the biodiesel. Finding from the Fourier Transform Infrared (FTIR) Spectrum of the raw CFW oil showed the presence of –OH and was assigned to the axial deformation of water molecule. However, the –OH group disappeared in the FTIR spectrum of biodiesel due to the transesterification process. This observation is consistent with the findings for moisture contents of raw CFW oil (0.073 %) and biodiesel (0.00 %), which showed disappearance of water after transesterification. The biodiesel yield was further modeled using three algorithms (Scaled conjugate gradient, Bayesian regularization and Levenberg maraquardt) of the artificial neural network (ANN). It was evident that the predictions from the Scaled conjugate gradient algorithm were comparable to the experimental responses. Finally, transesterification of raw CFW oil using heterogeneous catalyst from fish bone, was a cost-effective approach for biodiesel production.

Production of Oximes Directly from Sustainable Lignocellulose-Derived Aldehydes and Ammonia over HTS-1 Catalyst.

Oxime chemicals are the building blocks of many anticancer drugs and widely used in industry and laboratory. A simple but robust hierarchically porous zeolite (HTS-1) catalyst was prepared by hydrothermal methods and used for the preparation of vanillin oxime from vanillin in NH3 ⋅ H2 O/DIO (v/v 1/10) system. The results of the catalyst characterization showed that the larger pore size and more framework Ti were conducive to promote the transformation of the substrates. The conversion of vanillin and the yield of vanillin oxime were both higher than 99 % under optimized reaction conditions. It was found that the reaction proceeded by oxidation of NH3 to hydroxylamine (NH2 OH), and oximation of hydroxylamine with vanillin to obtain vanillin oxime, where the rate-controlling step was the hydroxylamine formation, and the apparent activation energy was 26.22 kJ/mol. The corresponding oximation products could also be obtained by extending this method to other compounds derived from lignin. Furthermore, the catalytic system was used directly to the conversion of birch biomass to obtain oxime products such as vanillin oxime, syringaldehyde oxime, and furfural oxime etc. This work might give insights into the sustainable production of N-containing high-value products from lignocellulose.

Effect of carbon modifier on characteristics and catalytic properties of zeolite–carbon hybrid supported Zr towards γ-valerolactone production

Developing efficient and stable solid functional materials for the catalytic upgrading of biomass-based platform furanic molecules is of great significance to produce value-added chemicals and biofuels. We herein report β-zeolite (β)/biobased carbon hybrids supported Zr species towards the cascade conversion of furfural into γ-valerolactone (GVL) with high activity and favorable stability. The role and importance of carbon modifier were highlighted. The cellulose (CEL) modified Zr/β-CEL catalyst was found to be uniquely effective, leading to an increase of GVL yield from 49.3% to 81.1%. Catalyst characterizations revealed that the structural integrity of β was largely preserved using CEL as the carbon modifier, accompanied by the increased mesopore volume and pore size for better accessibility of active sites and diffusion of reactants. Simultaneously, the presence of CEL offered an appropriate ratio of Lewis and Brønsted acid sites due to the well incorporation and dispersion of Zr species, which ensured the successive transfer hydrogenation and ring-opening reactions. Kinetic studies demonstrated the positive effect of CEL modification on reducing the apparent activation energy and thereby facilitating the GVL formation. More importantly, Zr/β-CEL with enhanced structural stability also exhibited markedly higher recyclability compared with the pristine Zr/β. The understanding on the roles of carbon modifier can be employed to optimize the physicochemical properties of zeolite-based catalysts for roundly improving the catalytic reactivity of biomass-related compounds.

Thermally activated natural chalcopyrite for Fenton-like degradation of Rhodamine B: Catalyst characterization, performance evaluation, and catalytic mechanism

In this work, catalytic activity of natural chalcopyrite (CuFeS2) was improved by thermal activation. The modified chalcopyrite was used as efficient catalyst for degradation of organic dye Rhodamine B (RhB) through advanced oxidation process (AOP). Effects of catalyst dosage, H2O2 concentration, reaction temperature, solution pH, anions, and natural organic matter on the degradation efficiency of RhB were investigated. This study revealed that thermal activation at 300 °C changed the chemical valency of surface elements rather than transforming the major chemical phase of natural chalcopyrite. The Fenton-like degradation of RhB was significantly improved by thermally activated chalcopyrite. RhB degradation could be obtained under broad pH and showed high resistance to natural organic matter and anions. Under optimal conditions of H2O2 43.0 mM, catalyst 0.75 g/L, initial pH 5.1, and reaction temperature 25 °C, the degradation of RhB reached 96.7% at 50 min. Based on the rate constant of reaction kinetics, the activation energy for RhB degradation was calculated to be 9 kJ/mol. Radical scavenging experiments and electron paramagnetic resonance (EPR) technique demonstrated that RhB degradation was dominated by the generated hydroxyl radicals in activated chalcopyrite/H2O2 system. The formation of surface sulfates resulted from thermal activation induced the dissolved copper or iron ions, and promoted H2O2 activation and facilitated RhB degradation by reactive hydroxyl radicals. This work provides an in-depth understanding of the mechanism behind thermal activation to enhance the activity of natural chalcopyrite, offering a theoretical basis for utilizing natural minerals for Fenton-like treatment of organic wastewater towards cleaner production.

Longevity increase of an impregnated Ni/CeO2-Al2O3 dry reforming catalyst by indium

The catalytic and structural changes due to the presence of 0.25 wt% indium on a 3% Ni/CeO2-Al2O3 catalyst prepared by impregnation method were investigated. Catalyst characterizations by XRD, TPR, XPS, TEM and DRIFTS (in presence of CO/CO2/CH4 +CO2) were carried out. In the short (6 h) catalytic methane dry reforming tests at 650 °C both samples were stable, produced essentially no carbon, but indium lowered the activity. In the 4 day longevity tests at 675 °C, stable activity was achieved by the In-promoted catalyst in contrast with the continuous deactivation of the unpromoted Ni sample due to coking and sintering. Indium, partially alloyed with nickel, could better keep nickel in metallic state, and the increased CO2 activation ability, the intimate Ni(In)-CeO2 interface inside the support pores resulted in practically no coking.

In Situ X-ray Diffraction as a Basic Tool to Study Oxide and Metal Oxide Catalysts

X-ray diffraction (XRD) is a standard technique that is widely applied in heterogeneous catalysis to determine phase composition, atomic structure, and size of crystallites. This review is focused on the application of in situ XRD for studying the catalysts during their “lifetime” (under synthesis, activation, operation, and deactivation conditions), limiting the objects of research to oxide and metal oxide catalysts. Also included is a brief overview of modern techniques and instruments and the latest works illustrating different aspects of this technique in catalyst research. The main conclusion is that the field of heterogeneous catalysis research would benefit substantially from the application of in situ XRD for the structural, phase, and morphological characterization of solid catalysts. Even more useful information can be obtained if XRD is combined with other techniques that are more sensitive at length scales different from that of XRD.

Ti3+/Ti4+ and Co2+/Co3+ redox couples in Ce-doped Co-Ce/TiO2 for enhancing photothermocatalytic toluene oxidation

Cerium and cobalt loaded Co-Ce/TiO2 catalyst prepared by impregnation method was investigated for photothermal catalytic toluene oxidation. Based on catalyst characterizations (XPS, EPR and H2-TPR), redox cycle between Co and TiO2 (Co2+ + Ti4+ ↔ Co3+ + Ti3+) results in the formation of Co3+, Ti3+ and oxygen vacancies, which play important roles in toluene catalytic oxidation reaction. The introduction of Ce brings in the dual redox cycles (Co2+ + Ti4+ ↔ Co3+ + Ti3+, Co2+ + Ce4+ ↔ Co3+ + Ce3+), further promoting the elevation of reaction sites amount. Under full spectrum irradiation with light intensity of 580 mW/cm2, Co-Ce/TiO2 catalyst achieved 96% of toluene conversion and 73% of CO2 yield, obviously higher than Co/P25 and Co/TiO2. Co-Ce/TiO2 efficiently maintains 10-hour stability test under water vapor conditions and exhibits better photothermal catalytic performance than counterparts under different wavelengths illumination. Photothermal catalytic reaction displays improved activities compared with thermal catalysis, which is attributed to the promotional effect of light including photocatalysis and light activation of reactive oxygen species.

Polysaccharide-Stabilized PdAg Nanocatalysts for Hydrogenation of 2-Hexyn-1-ol

We used a new one-pot green technique to prepare polysaccharide-based Pd and PdAg nanocatalysts. Catalysts were obtained using a sequentially supporting natural polymer (2-hydroxyethyl cellulose (HEC), chitosan (Chit), pectin (Pec)) and metals on zinc oxide. Nanocatalysts based on a polysaccharide were studied using various physicochemical methods (IR spectroscopy, transmission electron microscopy, X-ray powder diffraction, etc.). The catalyst characterization results indicated the complete adsorption of polysaccharides and metal ions onto the inorganic support (ZnO). We demonstrated the formation of polysaccharide-stabilized Pd nanoparticles with a size of ~2 nm. Metal nanoparticles were uniformly located on the surface of polysaccharide-modified zinc oxide. The synthesized catalysts were tested using liquid-phase hydrogenation of 2-hexyn-1-ol under mild conditions (0.1 MPa, 40 °C). Close conversion values of 2-hexyn1-ol were obtained for all the developed catalysts. The selectivity for cis-hexen-1-ol of the polysaccharide-based PdAg nanocatalysts varied as follows: PdAg-HEC/ZnO &gt; PdAg-Pec/ZnO &gt; PdAg-Chit/ZnO. A similar correlation was obtained in the presence of monometallic Pd-polysaccharide/ZnO catalysts. We determined the optimum reaction temperature and catalyst loading for PdAg catalysts modified using HEC and Chit (40 °C, 0.05 g).

NH3 decomposition for H2 production by thermal and plasma catalysis using bimetallic catalysts

Plasma catalysis has emerged as a promising approach for driving thermodynamically unfavorable chemical reactions. Nevertheless, comprehending the mechanisms involved remains a challenge, leading to uncertainty about whether the optimal catalyst in plasma catalysis aligns with that in thermal catalysis. In this research, we explore this question by studying monometallic catalysts (Fe, Co, Ni and Mo) and bimetallic catalysts (Fe-Co, Mo-Co, Fe-Ni and Mo-Ni) in both thermal catalytic and plasma catalytic NH3 decomposition. Our findings reveal that the Fe-Co bimetallic catalyst exhibits the highest activity in thermal catalysis, the Fe-Ni bimetallic catalyst outperforms others in plasma catalysis, indicating a discrepancy between the optimal catalysts for the two catalytic modes in NH3 decomposition. Comprehensive catalyst characterization, kinetic analysis, temperature program surface reaction experiments and plasma diagnosis are employed to discuss the key factors influencing NH3 decomposition performance.

Efficient catalytic degradation of VOCs using Mn-based hydrotalcite-like compounds coupled with non-thermal plasma

Aiming to discover a novel catalyst that exhibits a high synergistic effect with plasma technology for the degradation of volatile organic compounds (VOCs), we employed manganese-based layered double hydroxides (LDHs), which are referred to as MMn-LDHs (M=Ni, Co, Fe). Comprehensive physicochemical characterization via catalyst characterization techniques such as XRD, SEM, XPS, BET, confirmed the successful synthesis of the catalysts. These analyses revealed excellent crystallinity, distinctive lamellar structure, and remarkable thermal stability for three synthesized catalysts. The experimental results demonstrate that post non-thermal plasma (post-NTP) catalytic system has significantly improved the degradation efficiency, CO2 selectivity, and carbon balance for toluene, while notably suppressing the formation of by-products (e.g., NOx and O3) compared to the NTP alone system. Among the three MMn-LDHs, NiMn-LDHs exhibited the best performance in the post-NTP-catalytic system, achieving a toluene degradation rate of 95.87%, a CO2 selectivity of 57.41%, a carbon balance of 79.86%, and an energy efficiency of 3.19 g·kWh−1 at a specific energy density (SED) of 930.45 J/L. The catalytic degradation mechanism of toluene was proposed based on a combination of catalyst characterization techniques, plasma emission spectroscopy, and gas chromatography-mass spectrometry (GC-MS). The findings revealed that plasma discharge generated a substantial amount of free radicals and active species, thereby promoting the decomposition of toluene. Subsequently, the residual toluene and intermediate products were adsorbed onto the catalysts, where they underwent further reacted with the abundant hydroxyl radicals and activate metal species on the catalyst surface, ultimately decomposing into CO2 and H2O. Additionally, the interaction of ozone with LDHs facilitated the generation of reactive oxygen species, consequently accelerating the decomposition of organic intermediates, formation of CO2, and the enhancement of toluene mineralization. The finding in this work offer valuable theoretical support for the plasma-catalytic degradation of VOCs.

Synthesis and characterization of eggshell-based catalysts for the treatment of gaseous pollutants

The intensification of production in the poultry industry causes the generation and accumulation of solid waste. Efforts are being made to reduce the pollutant load of these materials by using them as a productive input. In this sense, the development of catalysts is an alternative. In this work, powdered and monolithic cobalt and calcium catalysts were synthesized using waste eggshells by means of a simple, environmentally friendly, and low-cost method. The catalysts were tested in the toluene oxidation reaction. The best catalytic performance was obtained with a catalyst prepared with Co (16 wt%) on eggshell calcined at 450 ºC (CoC-450). The combination and interaction between Co and Ca species and the highest Co/Ca and Olatt/Oads surface ratios improved the adsorption and activation of toluene. For application under real conditions, a structured catalytic material M-CoC-450 was developed, which reached 100% conversion at 260 °C.

Oxygen vacancy-driven strong metal-support interactions on AuPd/TiO2 catalysts for high-efficient air-oxidation of 5-hydroxymethylfurfural

The regulation ofstrong metal-support interactions (SMSI) has emerged as a powerful strategy in boosting catalytic activity and controlling product selectivity. However, directly tuning the metal-supportinteractions in oxide supported metal nanocatalysts remains challenging. Herein, citric acid (CA)‑assisted synthesized TiO2 layer on halloysite nanotubes (HNTs) with varied oxygen vacancies (Ov) concentrations was designed for loading AuPd nanoparticles. SMSI between the AuPd nanoparticles and the TiO2-xCA@HNTs support was successfully induced by adjustable Ov concentrations. The synthesized catalyst was employed for air-oxidation of bio-based 5-hydroxymethylfurfural (HMF) to bioplastic monomer 2,5-furandicarboxylic acid (FDCA) in water. A satisfactory FDCA yield of 98.4 %, accompany with an outstanding FDCA formation rate of 900.9 mmol·g−1·h−1 and an excellent TOF value at 441.2 h−1 was afforded. Based on catalysts characterizations and experimental studies, the catalytic performance was significantly affected by Ov concentrations. The kinetic study showed that the supported gold nanoparticles in alkaline environment had strong aldehyde oxidation activity but weak alcohol oxidation ability. On the contrary, supported palladium nanoparticles were more conducive to alcohol oxidation. This imbalance was overcome by the loading of AuPd nanoparticles, which enhanced the activity for air-oxidation of HMF. Mechanistic studies demonstrated that the stronginteractions between noble metal nanocatalysts and supports were regulated by Ov, which not only increased the adsorption capacity of the substrate and crucial intermediate, but also facilitated the adsorption and activation of molecular oxygen to generate oxygen active species of superoxide radicals. The possible catalytic mechanism for air-oxidation of HMF over Ov-driven SMSI on the obtained catalysts was proposed. This work sheds lights on the design of supported metal catalysts with SMSI for the catalytic upgrading of biomass-derived platform chemicals.

Performance of nickel-manganese and nickel-cobalt-manganese mixed oxide catalysts in ethanol total oxidation

The mixed oxide catalysts containing Ni, Co, and Mn components in various molar ratios were examined in the total oxidation of ethanol and the effect of their composition on physical chemical properties and catalytic performance was studied. The catalysts were obtained by calcination (4 h at 500 °C in air) of the coprecipitated precursors. The chemical analysis (AAS), powder XRD, Raman spectroscopy, TEM, N2 adsorption, H2-TPR, and XPS were used for the catalysts characterization. Formation of mixed oxides was indicated by powder XRD and Raman spectroscopy: NiCo2O4 spinel together with NiO were detected in the Ni-Co catalysts, MnIV-containing mixed oxides Ni6MnO8 (murdochite) and NiMnO3 (ilmenite) were identified in the Ni-Mn samples, and Ni-Co-Mn mixed oxides with spinel structure were found in the Ni-Co-Mn catalysts. Combination of transition metal components in the catalysts resulted in a synergistic effect; easier reduction in H2-TPR measurements and enhanced catalytic performance compared to single-component Ni, Co, and Mn oxides were observed. Addition of Mn to the Ni and Ni-Co oxides increased their catalytic activity in the ethanol oxidation, which was higher than that of the Ni-Co catalysts. The Ni-Mn catalyst with Ni:Mn molar ratio of 1:1 showed the highest activity (50 % conversion of ethanol was achieved at 91 °C) and good selectivity (acetaldehyde and carbon monoxide were detected as main byproducts of the ethanol oxidation). High activity of the Ni-Mn catalysts can be ascribed to the presence of MnIV-containing mixed oxides, namely NiMnO3. Activity of the Ni-Co-Mn catalysts, especially those with high (Co+Mn) contents, was comparable with that of the most active Ni-Mn catalyst with Ni:Mn molar ratio of 1:1. The high activity of Ni-Co-Mn catalysts can be explained by the formation of spinel-type mixed oxides with rather poor structure ordering and large surface area.

Structure-activity relationships of ZrO2 crystalline phases in the catalytic transfer hydrogenation of methyl levulinate with ethanol

The depletion of fossil resources is driving the research towards renewable alternatives, like lignocellulosic biomass. Therefore, the development of efficient continuous-flow processes, allowing to achieve better productivity compared to batch processes, will play a crucial role in promoting a sustainable transition. In this context, we report on the continuous-flow, gas-phase, catalytic transfer hydrogenation (CTH) of methyl levulinate and ethanol over zirconia catalysts, in particular focusing on the effect of two different crystalline phases (i.e. monoclinic, m-ZrO2, and tetragonal, t-ZrO2) on catalyst performance. An in-depth catalyst characterisation was coupled with both computational and 1H NMR relaxation studies to assess the structure-activity relationship, providing fundamental insights into the catalytic process and future catalyst optimization. The results, indicate that the higher Lewis acidity and basicity along with the lower affinity with ethanol of m-ZrO2 with respect to t-ZrO2 are responsible for the promotion of undesired oligomerisation reactions of angelica lactones responsible for catalyst deactivation.