Mixed Catalysts Research Articles

Influence of MnO2 crystal existing form on its catalytic performance for toluene oxidation

A series of MnO2 catalysts with different crystal forms were synthesized by in situ self-doping of Mn induced by bridging interaction between Mn and carboxyl group under moderate medium, and applied to catalytic oxidation of toluene. XRD, SEM and TEM results show that the physical structure of the prepared catalyst is extremely sensitive to the ratio of Mn/CA. Only MnCA-4 catalyst has a special morphology with double crystal and a uniform mixture of rod-like and spherical structures, and some of them are hollow rod-like structures. Due to the interaction between different phase structures, the mixed MnCA-4 catalyst with dual crystal has the best catalytic performance of toluene oxidation, which can oxidize 90 % toluene at 228 °C. The characterization of H2-TPR, O2-TPD, and XPS proved that the strong interaction between different crystalline forms of manganese oxide led to the generation of more oxygen vacancies and Mn3+ species. In situ DRIFTS results show thatMnCA-4 catalyst has stronger ability to adsorb and activate toluene compared with MnCA-12 catalyst, which may be the main reason for its excellent catalytic oxidation performance of toluene. This study provides an effective strategy for designing high efficiency non-noble metal catalysts to construct interface engineering of different crystal types.

Enhancing selectivity of aromatics in direct conversion of syngas over K/FeMn and HZSM-5 bifunctional catalysts

Mixed Fischer-Tropsch synthesis (FTS) catalysts and HZSM-5 (HZ5) have received attention as bifunctional catalysts that directly convert syngas into value-added aromatic monomers. However, both low selectivity of aromatics and severe deactivation remain challenges. Here we report a simple method for obtaining selective aromatics from syngas by tuning the distribution of hydrocarbon (HC) intermediates on K/FeMn. We prepared K/FeMn catalysts with a varying Mn/(Fe+Mn) wt% to demonstrate the effect of the HC distribution as intermediates of aromatization on HZ5. The C6-C10 distribution was increased by addition of up to 60 wt% of Mn promoter due to the improved dispersion of Fe7C3 and alkyl insertion of MnO. At 340 °C under 20 bar, 97.2% CO conversion and 39.8% aromatics selectivity were achieved by using a physically mixed catalyst of K/FeMn with Mn/(Fe+Mn) 60 wt% and HZ5 with an optimization of the FTS-derived C6-C10 olefin yield as efficient intermediates in Syngas to Aromatics (STA).

Fast and continuous conversion of xylose to furfural in micropacked bed reactors

In this work, the continuous conversion of xylose to furfural in water aqueous based on micropacked bed reactors (μPBRs) with mixed solid acid catalyst of SO42−/γ-Al2O3 and HND-580 was studied. For liquid-solid process, the best conversion and yield were 96.86% and 44.65% with the reaction time of only 1.6 min at 170 °C and 2 MPa, respectively. The reaction rate constant, apparent activation energy, and space–time yield (STY) were 1.37 min−1, 47.41 kJ·mol−1, and 5.00 × 10−3 g·mL−3·min−1, respectively. Also, the CO2 can enhance the conversion of xylose to furfural in μPBRs to achieve a high selectively and yield. Compared with the batch operation, the μPBRs can greatly increase the reaction rate and STY of 1–3 order magnitude. Micropacked bed reactor could be an alternative of the batch reactor to achieve a high efficient, green, continuous, and safe dehydration process of biomass.

Fast Catalyst Ratio Estimate in Gas Phase Polyethylene Dual Catalyst System

AbstractOne approach used in the industry to improve the properties of polyethylene is to use multi‐reactor with a single catalyst or multiple catalysts in a single reactor. In the latter case, two catalysts with distinct kinetics are selected to achieve the desired product properties. Such mixed catalyst systems enable tailored and advantageous properties at the cost of more challenging process control, because the ratio of the two catalysts serves as an additional manipulated variable. A fast method to estimate the ratio of active catalysts using headspace gas chromatography measurements is proposed here. In this method, a small perturbation in the feed rate is introduced to induce transient responses in the gas phase concentration. Ideally, with known responses from each individual catalyst, the active catalyst ratio can be estimated. To demonstrate this concept, a process model is developed in Aspen Plus. A set of dynamic simulation is performed to understand the responses of each catalyst and the mixed catalyst system, to changes in feed comonomer concentration. The results demonstrate that this method has significantly faster responses compared to feedback from bulk polymer properties and induces minimal process upset or product off‐spec due to small perturbations in a short period of time.

Experimental investigation on biodiesel production through simultaneous esterification and transesterification using mixed rare earth catalysts

In this study, biodiesel production through simultaneous esterification and transesterification of palm oil with 10 wt% of oleic acid using the mixed rare earth catalyst was investigated. The mixed rare earth catalysts were prepared via the co-precipitation method. The effects of the precipitating parameters such as temperature, stirring speed and pH on the physicochemical and morphology of the catalysts were studied. All catalysts were thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), fourier transform-infrared spectroscopy (FTIR), N2 adsorption/desorption, CO2 temperature programmed desorption (CO2-TPD) and NH3 temperature programmed desorption. (NH3-TPD). The results indicated that the mixed rare earth catalyst prepared under the precipitation conditions: at pH 9, a stirring of 400 rpm and temperature of 30 °C showed the highest catalytic of 90% FAME content. High surface area of the catalyst, a significant larger amount of Ce and La contents in the catalyst and an appropriate amount of acid and basic sites on the catalyst led to the high catalytic activity. The catalyst could also accelerate the initial reaction rate to achieve the high FAME content of 50% within 30 min.

Selectivity control of furfuryl alcohol upgrading to 1,5-pentanediol over hydrotalcite-derived Ni-Co-Al catalyst

1,5-pentanediol (1,5-PeD) is a value-added chemical that can be produced from furfuryl alcohol (FFA) via selective scission of CO bond coupled with following hydrogenation reaction. In this work, a series of hydrotalcite-derived Ni-Co-Al catalysts with different Ni/Co ratios were evaluated for FFA upgrading to 1,5-PeD at a temperature range of 413–473 K and hydrogen pressure from 2 to 5 MPa. The catalytic performance and reaction products distribution were found to be highly sensitive to the Ni/Co ratio in the catalysts as well as reaction conditions. The highest catalytic activity was achieved with the Ni-Co-Al catalyst at a Ni/Co atomic ratio of 1:11 (Ni1Co11Al), which attained a 1,5-PeD yield of 42.5 % after 4 h of reaction. The presence of Ni in the Ni1Co11Al catalyst could significantly improve the BET surface area and suppress the growth of metal nanoparticles, leading to its superior catalytic stability in 5 runs. According to the characterization results, it was supposed that hydrogen spillover occurred from the surface of Ni to that of Co species, which was expected to enhance the hydrogen adsorption/activation capability and surface H concentration. The rapid H transfer was beneficial to the hydrogenolysis of furan ring and in situ hydrogenation of reaction intermediates, thus greatly facilitating the activity of the catalyst. Based on the experimental and characterization results, a plausible reaction mechanism was proposed for the selective generation of 1,5-PeD from FFA over the Ni1Co11Al catalyst. The results attained in this paper demonstrated the possibility of using non-precious mixed metal catalyst for FFA upgrading, and could provide useful insights for the design of efficient catalysts.

Chemical Catalytic Applications of Nanocomposites in Photo Reactors: Review

Nanocomposites as a photocatalysts are chosen for the photocatalytic degradation of organic pollutant molecules. Noble metal decorated mixed metal oxides composite and reduced graphene oxide deposited metal oxide nanocomposites are of cost-effective photocatalysts. Nanocomposites are act as better photocatalysts due to their tunable surface characteristics as acidic and basic properties, advanced oxidation or reduction abilities makes them unique approach for heterogeneous photocatalysts. Development of cheaper semiconductor photocatalysts, which are highly effective and reusable photocatalytic applications of nanocomposites under visible light, is very challenging materials. We prepared cost-effective ternary mixed metal oxide nanocomposites chemical catalysts with rGO with heterojunctions of rGO-ZnO-TiO2 -Nb2 O5 labelled as rGZTNb for efficient photo catalysis in photo reactor under visible light radiations for chemical catalysis. Author described about eh development effect of heterogeneous photocatalysts. Heterogeneous visible light photocatalysts of semiconductor mediated materials at the nanometer scale have paid much attention towards the past decade due to their beneficial strategies in the photocatalytic usage. ZnO and TiO2 along with Nb2 O5 nanocomposite combinations as solid powders can significantly enhance light absorption, then separation pairs of electron-hole by photogeneraton

XANES characterizations of amorphous mixed metal oxides derived from Zn/Al layered double hydroxides through thermal treatment

The amorphous phase obtained by thermal treatment of Zn/Al-layered double hydroxide (Zn/Al-LDH) was characterized by X-ray absorption near-edge Spectroscopy (XANES) and Ultraviolet–Visible Diffuse Reflection (UV–vis DR). The highly crystalline Zn/Al-LDH was synthesized through homogeneous precipitation with urea as precursor, and then calcined up to 900 ​°C. X-ray Diffraction (XRD) and Thermogravimetry-Differential Thermal Analysis (TG-DTA) results showed that the crystalline Zn/Al-LDH was changed to amorphous phase through dehydroxylation by 300 ​°C. The amorphous phase decomposed to wurtzite-ZnO and spinel (Zn/Al2O4) above 600 ​°C. The amorphous phase was characterized with Zn L-edge, O K-edge and Al K-edge spectra. Those spectra determined the formation of amorphous Zinc/Aluminium mixed metal oxide (ZAO) with characteristic electron. The ZAO showed absorption of UV light which is higher energy band gap than pure zinc oxide. The analysis indicated changes in coordination due to the formation of a metal oxide phase by dehydration of the hydroxide layer of Zn/Al-LDH, and the formation of ZAO. This research is believed to be the first to apply Zn L-edge, along with O K-edge XANES spectra to analyses and clarify the thermal behavior of Zn/Al-LDH and is demonstrated to be a powerful tool for probing the structure of a variety of mixed metal oxides and catalysts based on Zn based LDH materials.

Non-thermal plasma synthesis of supported Cu-Mn-Ce mixed oxide catalyst towards highly improved catalytic performance for volatile organic compound oxidation.

Compared with that of the transition metal mixed oxide pellet catalyst, the catalytic activity of the supported mixed oxide catalyst was significantly reduced, which was limited in practical industrial applications. In this work, supported Cu-Mn-Ce mixed oxide catalysts were prepared by non-thermal plasma. Catalyst characterization result demonstrated that plasma treatment could promote the proportion of oxygen vacancy and enhance the adsorptive strength of VOCs on the surface of catalyst. Meanwhile, plasma treatment process exerted a slight influence on the pore structure and morphological property of the catalyst. Consequently, CMC/SiO2-P exhibited much higher catalytic activity than CMC/SiO2 prepared by the incipient wetness impregnation method for the catalytic oxidation of toluene and n-hexane. Among the catalysts prepared, the 15%CMC/SiO2-P catalyst even exhibited a high catalytic activity comparable to that of the supported noble metal catalyst for the oxidation of the inert hexane. The T98 of toluene and n-hexane over 15%CMC/SiO2-P was 260 and 280°C under the conditions of VOC concentration at 1000 ppm and WHSV at 20,000 mL·g-1·h-1, respectively. This work provided a novel method for the preparation of the supported transition metal mixed oxide catalyst.

Understanding the effect of polymer concentrations on the phase formation and activity of electrospun nanofibrous photocatalyst

One-dimensional titanium dioxide electrospun nanofiber have been extensively studied, it provides channels for quick charge transfer due to their long axial ratio, but as per the experiment reports the phase formation is not conclusive about the origin of the crystallinity of titanium oxide, nor about role of polymer amount. In this study, the effect of sacrificial polymer in the electrospun solution on Ti-O bonds breaking and on the arrangement of TiO6 octahedra was obtained for different titanium oxide phase formations. In particular, polymer-metal precursor plays the role of Magnéli phase formation at low temperature. The nanofibers were prepared at different titanium butoxide and polymer concentrations with the optimization of electrospinning parameters such as electric field strength, the flow rate of the electrospinning solution, the distance between the nozzle and the receiving drum collector, and the speed of the rotating collector. The effect of polymer concentration on the crystallinity and phase compositions was examined in detail. Surprisingly, the percentage of anatase phase was found to decrease with increasing the polymer amount with a fixed annealing temperature. The experimental results show that carrier polymer polyvinylpyrrolidone is not only involved in the fiber formation through chain entanglements but plays a significant role to control TiO6 octahedral configurations and connectivity. More polymers promote edge-sharing between TiO6 rather than corner-sharing. We demonstrate the efficient photocatalytic activity of the mixed phase TiO2 nanofiber in the photocatalytic degradation of Rhodamine B dye. A significant increase in the reaction rate was observed with the increase of the Titanium butoxide:polyvinylpyrrolidone ratio. It was found that 70% anatase and 30% rutile mixed phase TiO2 nanofiber catalysts exhibit optimum photocatalytic activity with rate constant values of 0.345 min−1. The mechanism relates to the interfacial charge carrier at the junction of anatase and rutile band alignment.

Heterogeneous Metal-Activated Persulfate and Electrochemically Activated Persulfate: A Review

The problem of organic pollution in wastewater is an important challenge due to its negative impact on the aquatic environment and human health. This review provides an outline of the research status for a sulfate-based advanced oxidation process in the removal of organic pollutants from water. The progress for metal catalyst activation and electrochemical activation is summarized including the use of catalyst-activated peroxymonosulfate (PMS) and peroxydisulfate (PDS) to generate hydroxyl radicals and sulfate radicals to degrade pollutants in water. This review covers mainly single metal (e.g., cobalt, copper, iron and manganese) and mixed metal catalyst activation as well as electrochemical activation in recent years. The leaching of metal ions in transition metal catalysts, the application of mixed metals, and the combination with the electrochemical process are summarized. The research and development process of the electrochemical activation process for the degradation of the main pollutants is also described in detail.

Application of Co/Co9S8@N, S doped porous carbon composites prepared by ball milling for zinc-air battery

• Metal sulfides were prepared by a unique strategy to achieve sulfidation and double doping with ammonium bisulfate. • The amount of ammonium bisulfate has a great effect on the synthesis of Co/Co 9 S 8 @N, S-PC. • Co/Co 9 S 8 @N, S-PC-N exhibits a unique porous morphology and optimized structure and composition. • Co/Co 9 S 8 @N, S-PC-N possesses outstanding exhibits the performance of double-effect oxygen catalysis. With the advantage of high electrocatalytic activity, cobalt sulfides has become an efficient electrocatalyst for oxygen evolution and oxygen reduction reaction (OER/ORR). Selecting an appropriate strategy to construct a optimal structure for cobalt sulfides to explore the potential of their application in zinc-air battery is one of the meaningful topics. Here we propose a facile strategy to synthesize Co/Co 9 S 8 @N, S doped porous carbon composite material with mesoporous structure by ball milling. The Co/Co 9 S 8 @N, S doped porous carbon composite material exhibits excellent dual function catalytic activity for OER and ORR. Also, the material exhibits excellent power density and cycle stability when applied to the electrode catalytic material of normal zinc-air battery. Among them, the optimal sample present better power density than Pt-Ru-C mixed catalyst used in the normal zinc-air battery, and the limit value is 202.5 mW cm -2 . Connecting two solid-state zinc-air batteries in series can meet the normal operation of 53 LED light-emitting diodes. This work can provide a powerful reference for the further development of cobalt sulfides in oxygen electrocatalysis and zinc-air battery.

A Dual-Bed Strategy for Direct Conversion of Syngas to Light Paraffins

The authors studied the direct conversion of syngas to light paraffins in a dual-bed fixed-bed reactor. A dual-bed catalyst composed of three catalysts, a physically mixed methanol synthesis catalyst (CZA), and a methanol dehydration to dimethyl ether (DME) catalyst (Al2O3(C)) were put in the upper bed for direct conversion of syngas to DME, while the SAPO-34 (SP34-C) zeolite was put in the lower bed for methanol and DME conversion. The effects of the mass ratio of CZA to Al2O3(C), the H2/CO molar ratio, and the space velocity on catalytic performance of syngas to DME were studied in the upper bed. Moreover, a feed gas with a CO/CO2/DME/N2/H2 molar ratio of 9/6/4/5 balanced with H2 was simulated and studied in the lower bed over SP34-C; after optimizing the reaction conditions, the selectivity of light paraffins reached 90.8%, and the selectivity of propane was as high as 76.7%. For the direct conversion of syngas to light paraffins in a dual bed, 88.9% light paraffins selectivity in hydrocarbons was obtained at a CO conversion of 33.4%. This dual-bed strategy offers a potential route for the direct conversion of syngas to valuable chemicals.

Process optimization and kinetic studies for the production of biodiesel from Artocarpus heterophyllus oil using modified mixed quail waste catalyst

Biodiesel production from renewable resources is emerging as a future energy solution that is both environmentally friendly and energy efficient. The current study looks into the use of calcined mixed quail wastes, such as quail bones and egg shells, as novel heterogeneous catalysts for biodiesel production from Artocarpus heterophyllus oil. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), and Fourier Transform Infrared spectroscopy (FTIR) were used to characterize the calcined MQW catalysts. Seed to solvent ratio of 1:10, temperature of 70 °C and extraction time of 180 min was observed to be optimal parameters for the extraction of oil from Artocarpus heterophyllus seeds. The extraction of oil from Artocarpus heterophyllus seeds was found to be maximum with a seed to solvent ratio of 1:10, a temperature of 70 °C, and an extraction time of 180 min. The physico-chemical properties of extraction oil were investigated. A maximum biodiesel yield of 64 % was attained with optimal process parameters - methanol to oil ratio of 6:1, catalyst concentration of 3.5 %, temperature of 55 °C and reaction time of 120 min. Kinetic studies of transesterification process revealed the pseudo-first order model as the best fitting model with an activation energy of 38.864 kJ/mol.

Front Cover: Molecular Metallocorrole–Nanorod Photocatalytic System for Sustainable Hydrogen Production (ChemSusChem 17/2022)

The Front Cover shows the solar-driven photocatalytic generation of hydrogen from water. It is accomplished via a successful combination of nanorod photocatalysts that have proven record activity for the water reduction half-reaction, with molecular metallocorroles suitable for catalyzing the accompanying oxidation reactions. The utilization of redox species as charge transfer shuttle between freely mixed molecular catalysts and rods afford a simple and elegant alternative to a chemical linkage, which results in high quantum efficiency for hydrogen production. In place of a sacrificial scavenger capable of extracting but a single hole, the molecular metallocorrole catalyst is found to successfully handle nearly 300,000 holes during its lifespan. More information can be found in the Research Article by K. Dong, T.-A. Le et al.

Emerging natural and tailored perovskite-type mixed oxides-based catalysts for CO2 conversions.

The rapid economic and societal development have led to unprecedented energy demand and consumption resulting in the harmful emission of pollutants. Hence, the conversion of greenhouse gases into valuable chemicals and fuels has become an urgent challenge for the scientific community. In recent decades, perovskite-type mixed oxide-based catalysts have attracted significant attention as efficient CO2 conversion catalysts due to the characteristics of both reversible oxygen storage capacity and stable structure compared to traditional oxide-supported catalysts. In this review, we hand over a comprehensive overview of the research for CO2 conversion by these emerging perovskite-type mixed oxide-based catalysts. Three main CO2 conversions, namely reverse water gas shift reaction, CO2 methanation, and CO2 reforming of methane have been introduced over perovskite-type mixed oxide-based catalysts and their reaction mechanisms. Different approaches for promoting activity and resisting carbon deposition have also been discussed, involving increased oxygen vacancies, enhanced dispersion of active metal, and fine-tuning strong metal-support interactions. Finally, the current challenges are mooted, and we have proposed future research prospects in this field to inspire more sensational breakthroughs in the material and environment fields.

A new method for the synthesis of 4-aminobenzoic acid – an intermediate for the production of procaine

Procaine is one of the oldest local anesthetics used in medicine. When absorbed and entering the systemic circulation reduces the excitability of peripheral cholinoreactive systems. Has a blocking effect on the autonomic ganglia, reduces smooth muscle spasms, and reduces the excitability of the myocardium and motor areas of the cerebral cortex. It is synthesized by oxidizing 4-nitrotoluene to 4-nitrobenzoic acid, which is subsequently reacted with thionyl chloride, the resulting acid chloride is then esterified with 2-diethylaminoethanol to give nitrocaine. Finally, the nitro group is reduced by hydrogenation over a Raney nickel catalyst. The oxidation stage is characterized by the formation of toxic, difficult to dispose of wastewater, valuable mineral oxidants, or high temperatures and excess pressure, in the case of using oxygen as an oxidant. Therefore, the search for new environmentally friendly and low-temperature methods of obtaining 4-nitrobenzoic acid is an urgent task. The aim of the work is to study the products, conditions, and study kinetics of the reaction of ozone with 4-nitrotoluene in acetic acid solution to develop a new method for the synthesis of 4-nitrobenzoic acid. Materials and methods. The glacial acetic acid qualification “P. F. A.” before use was purified by distillation under vacuum in the presence of potassium permanganate. Salts of metals of qualification “P. F. A.” and potassium bromide qualification “Ch. P.” were used without prior purification. A gas-phase gradient-free catalytic duck reactor was used for kinetic studies. The mixing of gas and liquid phases in the reactor was achieved by shaking the reactor at a speed that allowed it to work in the kinetic region. The kinetics of the reaction was studied by changing the concentration of ozone in the gas phase at the outlet of the reactor by spectrophotometric method on a spectrophotometer “SF-46 LOMO”. Results. The products, conditions, and kinetics of ozone reaction with 4-nitrotoluene were studied. It was shown that at temperatures of 20–90 °C is mainly ozonolysis of the aromatic ring, and the total yield of oxidation products by methyl group does not exceed 24.2 %, among which identified in the early stages of 4-nitrobenzyl alcohol and 4-nitrobenzaldehyde, and at deeper – 4-nitrobenzoic acid. The introduction of cobalt (II) acetate into the catalyst system almost completely was prevented ozonolysis and the main reaction product is 4-nitrobenzoic acid with a yield of 86.5 %. The addition of potassium bromide to the solution reduced the concentration of the catalyst by seven times and increased the reaction rate and yield of 4-nitrobenzoic acid to 95.6 %. Conclusions. A new environmentally friendly, low-temperature method for the synthesis of 4-nitrobenzoic acid by conducting the process of ozonation of 4-nitrotoluene in a solution of glacial acetic acid in the presence of a mixed cobalt bromide catalyst was developed.

Carbothermal Shock Synthesis of High Entropy Oxide Catalysts: Dynamic Structural and Chemical Reconstruction Boosting the Catalytic Activity and Stability toward Oxygen Evolution Reaction

AbstractMixed transition‐metals (TM) based catalysts have shown huge promise for water splitting. Conventional synthesis of nanomaterials is strongly constrained by room‐temperature equilibria and Ostwald ripening. Ultra‐fast temperature cycling enables the synthesis of metastable metallic phases of high entropy alloy nanoparticles, which later transform to oxide/hydroxide nanoparticles upon use in aqueous electrolytes. Herein, an in situ synthesis of non‐noble metal high entropy oxide (HEO) catalysts on carbon fibers by rapid Joule heating and quenching is reported. Different compositions of ternary to senary (FeNiCoCrMnV) HEO nanoparticles show higher activity towards catalyzing the oxygen evolution reaction (OER) compared to a noble metal IrO2 catalyst. The synthesized HEO also show two orders of magnitude higher stability than IrO2, due to stronger carbide‐mediated intimacy with the substrate, activated through the OER process. Alloying elements Cr, Mn and V affect OER activity by promoting different oxidation states of the catalytically active TM (Fe, Ni and Co). Dissolution of less stable elements (Mn, V and Cr) leads to enhancements of OER activity. Dynamic structural and chemical perturbations of HEO oxide nanoparticles activate under OER conditions, leading to enlargement in ECSA by forming mixed single atom catalysts and ultra‐fine oxyhydroxide nanoparticles HEOs.

Tandem Electrochemical Conversion of CO2 to Liquid Fuels and Chemical Feedstocks

Increasing demand for energy has led to a high dependence on fossil fuels, which are limited and have been closely associated with major environmental issues such as groundwater pollution and imbalances in the carbon cycle. A more sustainable and cleaner method of producing chemicals and fuels is the electrochemical CO2 reduction reaction (CO2RR). When coupled with renewable electricity, CO2 can be converted to high energy density fuels and commodity chemicals, such as ethanol, propanol, and ethylene, in an environmentally friendly way.Recently, the use of CO feedstocks, instead of CO2, was shown to enhance the selectivity toward multi-carbon products on copper-based electrocatalysts. Technoeconomic analyses have also demonstrated that CO can be produced from CO2RR cost-effectively. This has led to increased efforts in developing tandem electrocatalytic systems. However, state-of-the-art CO2-to-CO electrocatalysts are based on expensive noble metals such as Ag and Au, while earth-abundant Zn displays relatively poorer selectivity and activity. Herein, we show that oxide-derived Zn with high surface area can reduce CO2 to CO with a Faradaic efficiency of 86% and a partial current density (j CO) of −201 mA cm-2. While oxygen vacancies were previously implicated for CO2RR to CO, we pinpointed by detailed experiments and density functional theory calculations that highly undercoordinated Zn sites provide even higher activity, in view of their nearly optimal *COOH adsorption energies. These findings indicate that suitably engineered ZnO-derived materials can potentially be an alternative to the more costly Ag and Au electrocatalysts, and that the main guideline for their design is to increase the undercoordination of the catalytic sites.We also investigate the electrochemical CO reduction reaction (CORR) to liquid fuels and industrial feedstocks on copper-based and mixed copper-silver catalysts. To further enhance the CORR activity and product selectivities, a systematic optimization of the experimental environment, such as the use of various supports, catalyst binder and electrolytes, was done. We develop a set of experimental conditions for optimal CORR performance to value-added products.

Uncovering Activity-Stability Relationships in Mixed Ir-Based Catalysts Toward Improved Water Electrolysis

The role of proton exchange membrane water electrolysers (PEMWE) in providing clean and reliable green hydrogen is undeniable in a fully decarbonized energy system. Indeed, the ambitious “Hydrogen Shot” initiative by the U.S. DoE seeks to decrease in a decade green hydrogen pricing down to 1$/Kg H2.(1) Such endeavour will require a mass-scale implementation of PEMWEs, which is currently bottlenecked by the use of scarce precious metal catalysts, particularly at the anode where loadings of the state-of-the-art iridium (Ir) catalyst typically range between 1-3mgIr cm-2.(2) Thus, recent strategies in the literature have aimed to reduce Ir loading with several approaches such as higher catalyst dispersion and preparation of mixed metal oxides (IrxM1-xOy). The latter is particularly promising if the cation substituent M is an earth-abundant metal highly active to the oxygen evolution reaction (OER), which could lead to drastic catalyst cost reductions retaining high activities. The activity-stability relationships of various Ir phases are well-reported,(3) but hardly studied for IrxM1-xOy where any preferential dissolution of M under OER operation could result in misleading conclusions.The work presented here aims to bridge this knowledge gap by online quantification of catalyst dissolution products with our electrochemical flow cell coupled to inductively-coupled plasma mass spectrometry setup. First, we evaluated these relationships in cationic-substituted IrxM1-xO2 (100) epitaxial thin films, model systems which will allow us to clearly uncover any cation-dependent trends. It was observed that the Ir stability in such model systems directly correlated with the OER enhancement effect of the cation substituent: higher OER activities induced by the cation yielded lower Ir stabilities and vice versa. In addition, the IrO2 matrix does not seem to affect the thermodynamic dissolution trends of the cation M. In conjunction with ex-situ XPS characterization, we successfully identified two promising cation substituents considering the experimental activity-stability trends: Mo and W. Next, we moved to the industrially-promising IrxRu1-xO2 nanoparticle (NP) systems, with different relative compositions and degrees of crystallinity. Although Ru generally presents high dissolution rates under OER potentials, preliminary studies on sputtered thin films revealed a drastic stabilization of the RuO2 matrix upon incorporation of less than 20 at. % Ir.(4) The one-step synthetic approach used here, flame spray pyrolysis, allowed to easily tune the NPs Ir:Ru ratio by the relative content of the metal precursors in the pyrolized solution. Additionally, a post-calcination step enabled to convert the NPs phase from hydrous to rutile-type oxide. Using as start-up/shut-down OER protocol, we observed at 1000-fold lower stability for hydrous vs. rutile-type IrxRu1-xO2 regardless of the composition after selective Ru leaching. For rutile-type IrxRu1-xO2 nanoparticles, the sequential on/off OER operation revealed a ca. 10-fold decrease in Ru dissolution upon Ir incorporation, as well as the key role of surface Ru in OER activity.(5) Minimal Ru losses (<1 at. %) led to the formation of an Ir-rich protective shell, which for Ir0.2Ru0.8O2 resulted in an OER performance shift equivalent to that of pristine Ir0.8Ru0.2O2. These results showcase, contrary to previous reports, that Ru-rich compositions can indeed be implemented in PEMWE devices.