Quantification of photocatalytic hydrogen evolution

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A new photoreactor with defined irradiation geometry was developed and tested for the water reduction reaction using carbon nitride (“C3N4”) as a photocatalyst. The hydrogen evolution rate was investigated with a sun simulator (I = 1000 W m−2) in two different operation modes: circulation and stirring of the catalyst dispersion. Only in the stirred mode, where shear stress is lower, a stable hydrogen evolution rate of about 0.41 L m−2 h−1 is obtained. It is confirmed by experiments with D2O that hydrogen is obtained from the water splitting process and not by dehydrogenation of the sacrificial agent. The obtained rate results in an efficiency of <0.1% based on a reference experiment with a photovoltaic-powered electrolysis setup. The change from distilled water to tap or simulated sea water results in a lower hydrogen evolution rate of about 50%.BMBF, 03IS2071D, Light2Hydroge

Electrochemical activation of small molecules by the example of the CO2 reduction and the methane oxidation

Die Verwendung von überschüssigem Strom zur Umwandlung von „Niedrig-Energie-Molekülen“, wie Kohlenstoffdioxid (CO2), in nützliche Chemikalien oder Kraftstoffe, wie Kohlenwasserstoffe und Methanol, wird zu einem immer wichtigeren Thema in der Forschung und Industrie. Es ist bekannt, dass CO2 an Kupferkatalysatoren bei guten Stromausbeuten elektrochemisch zu Methan und Ethylen reduziert werden kann. Die zielgerichtete Lenkung der CO2 Reduktion zu diesen gewünschten Produkten bleibt weiterhin eine große Herausforderung. Dazu ist es nötig, den Reaktionsmechanismus und die Kinetik der elektrokatalytischen CO2 Reduktion besser aufzuklären. Die Erforschung und Entwicklung neuer elektrochemischer Katalysatoren und Verfahren sind für eine wirtschaftliche Nutzung nötig. Methanol wird als Ausgangsstoff zur Synthese vieler organischer Verbindungen verwendet. Die großtechnische Herstellung erfolgt aus Methan, welches zu Synthesegas reformiert wird, um es chemisch zu aktivieren. Diese Reformierung erfolgt heutzutage immer noch bei hohen Temperaturen und Drücken, was zu einem enormen Energieverbrauch führt. Die Produktion von Methanol ist in den letzten Jahren stetig angestiegen und auch das Umdenken zu klimafreundlicheren Herstellungsprozessen veranlasst die Forschung und Industrie, neue katalytische Verfahren zu entwickeln, um sich Methan energieeffizienter zu Nutze zu machen. In dieser Doktorarbeit wurde eine Kombination aus Linear Sweep Voltammatrie (LSV), Chronoamperometrie (AMP), DEMS (differentielle elektrochemische Massenspektrometrie) und Gaschromatographie (GC) verwendet. Die Produktverteilung und Selektivität für Methan und Ethylen wurden während der elektrochemischen CO2 Reduktion an polykristallinen Kupferelektroden in Abhängigkeit des Reduktionspotentials, der Temperatur, und der Oberflächenrauigkeit sowie der Einfluss von Halogeniden auf die Reaktion untersucht. Weiterhin wurden katalytische Effekte von nanoskalierten Kupferschichten und Nanopartikeln auf die Selektivität der CO2 Umsetzung aufgedeckt. Intermetallisches Gallium-Palladium wurde als neues Katalysatormaterial für die elektrochemische CO2 Reduktion erforscht und die resultierenden Aktivitäten und Selektivitäten mit anderen polykristallinen Metallen verglichen. Weiterhin ließ sich CO2 an katalytisch aktiven Gold-Platin-Nanopartikeln zu Synthesegas (CO und H2) reduzieren, wobei das H2/CO-Verhältnis über die Variation des Goldanteils einstellbar ist. Die elektrochemische Erzeugung von reaktiven Sauerstoffspezies, wie Superoxide durch Sauerstoffreduktion an Platin und Gold in ionischen Flüssigkeiten und Hydroxylradikale durch Wasseroxidation in wässrigen Elektrolyten an bordotierten Diamantelektroden, wurde für die direkte Oxidation von Methan bei niederen Temperaturen untersucht. Während die Erzeugung von hochreaktiven Sauerstoffspezies gelang, stellt bei den gewählten Versuchsparametern die Methanoxidation noch eine große Herausforderung dar. Die Erkenntnisse dieser Doktorarbeit können genutzt werden, um den Mechanismus und die selektivitätskontrollierenden Parameter der elektrochemischen CO2 Reduktion besser zu verstehen und um Strategien zu entwickeln, die Produktverteilung gezielt zu steuern.The utilization of excess electricity for the conversion of "low-energy" molecules, such as carbon dioxide (CO2) into useful chemicals or fuels such as hydrocarbons and methanol, is becoming an increasingly important topic in science and industry. It is well known that CO2 can be electrochemically reduced to methane and ethylene on copper catalysts with good current yields. Controlling the selectivity of the CO2 reduction towards the desired products still remains a major challenge. For this purpose, it is necessary to elucidate the reaction mechanism and kinetics of the electrocatalytic CO2 reduction. To this end, new catalysts and electrochemical methods are developed and tested. Methanol is used as a chemical precursor for the synthesis of numerous organic compounds. The large-scale synthesis of methanol is based on methane which is activated synthesis gas by steam reforming. This reformation process requires high temperatures and pressures, leading to huge consumption of energy. In the recent years, the production of methanol has steadily risen and the rethinking of alternatively climate-friendly production processes has prompted the academia and industry to develop new catalytic processes in order to take advantage of methane much more energy-efficiently. In this thesis, a combination of linear sweep voltammetry (LSV), chrono amperometric measurements (AMP), DEMS (Differential Electrochemical Mass Spectrometer) and gas chromatography (GC) was used. The product distribution and selectivity towards methane and ethylene during the electrochemical reduction of CO2 on polycrystalline copper electrodes were studied depending on the reduction potential, temperature and surface roughness as well as influence of halides on the reaction. Furthermore, the catalytic effects of nanoscaled copper layers and nanoparticles on the selectivity of CO2 conversion were analyzed. In this work, an intermetallic gallium-palladium catalyst was tested as a new material for the electrochemical reduction of CO2 and the resulting activity and selectivity were compared to other polycrystalline metals. As well, the formation of synthesis gas (CO and H2) produced by CO2 reduction on catalytically active gold and platinum nanoparticles is shown in this thesis. Tuning of H2/CO ratio was realized by varying the gold content in bimetallic nanoparticles mixtures. Beside electrochemical CO2 reduction, the electrochemical generation of reactive oxygen-containing species such as super oxides produced by oxygen reduction on platinum and gold in ionic liquids, and hydroxyl radicals formed by oxidation of water in aqueous electrolytes on boron-doped diamond electrodes, were studied for the direct oxidation of methane at low temperatures. However, under the chosen conditions, the detected oxygen-containing species showed no direct oxidation of methane. The new insights, acquired from this research helps to provide a better understanding of the mechanism and the parameters controlling the selectivity of the electrochemical CO2 reduction. They also aid in developing practical strategies into how the product distribution and selectivities can be controlled

Size-Dependent Reactivity Of Gold-Copper Bimetallic Nanoparticles During Co2 Electroreduction

New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5 nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying

Particle Size Effects In The Catalytic Electroreduction Of Co2 On Cu Nanoparticles

A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2-15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity-selectivity-size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification. © 2014 American Chemical Society

Exceptional Size-Dependent Activity Enhancement In The Electroreduction Of Co2 Over Au Nanoparticles

The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1-8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles

Particle Size Effects in the Catalytic Electroreduction of CO₂ on Cu Nanoparticles

A study of particle size effects during the catalytic CO₂ electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2–15 nm mean size range were prepared, and their catalytic activity and selectivity during CO₂ electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H₂ and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H₂ and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity–selectivity–size relations provide novel insights in the CO₂ electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification

Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions

The selectivity of heterogeneously catalyzed chemical reactions is well-known to be dependent on nanoscale determinants, such as surface atomic geometry and composition. However, principles to control the selectivity of nanoparticle (NP) catalysts by means of mesoscopic descriptors, such as the interparticle distance, have remained largely unexplored. We used well-defined copper catalysts to deconvolute the effect of NP size and distance on product selectivity during CO₂ electroreduction. Corroborated by reaction-diffusion modeling, our results reveal that mesoscale phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity. More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the production of desired reaction products

Exceptional Size-Dependent Activity Enhancement in the Electroreduction of CO₂ over Au Nanoparticles

The electrocatalytic reduction of CO₂ to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1–8 nm for the electroreduction of CO₂ to CO in 0.1 M KHCO₃. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H₂ over CO₂ reduction to CO. We show here that the H₂/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles

Controlling Catalytic Selectivities during CO₂ Electroreduction on Thin Cu Metal Overlayers

The catalytic activity and selectivity of the electrochemical CO₂ reduction on Cu overlayers with varying atomic-scale thickness on Pt was investigated. Hydrogen, methane, and ethylene were the main products. Beyond an activity improvement with increasing copper layer thickness, we observed that the thickest 15 nm Cu layer behaved bulk-like and resulted in high relative faradaic selectivities for hydrocarbons. With decreasing Cu layer thickness, the formation of methane decreased much faster than that of ethylene. As a result, the relative faradaic selectivity of the technologically useful product ethylene increased sharply. The selectivity ratios between methane and ethylene were independent of electrode potential on a Cu monolayer. A combination of geometric tensile strain effects and electronic effects is believed to control the surface reactivity and product distribution on the copper surfaces. This study highlights the general strategy to tune product distributions on thin metal overlayers