Surface-modified carbon materials for CO2 electroreduction

The electrochemical reduction of CO2 to produce sustainable fuels and chemicals has attracted great attention in recent years. It is shown that surface-modified carbons catalyze the CO2RR. This study reports a strategy to modify the surface of commercially available carbon materials by adding oxygen and nitrogen surface groups without modifying its graphitic structure. Clear differences in CO2RR activity, selectivity and the turnover frequency between the surface-modified carbons were observed, and these differences were ascribed to the nature of the surface groups chemistry and the point of zero charge (PZC). The results show that nitrogen-containing surface groups are highly selective towards the formation of CO from the electroreduction of CO2 in comparison with the oxygen-containing surface groups, and the carbon without surface groups. This demonstrates that the selectivity of carbon for CO2RR can be rationally tuned by simply altering the surface chemistry via surface functionalization

Oxide-derived Silver Nanowires for CO₂ Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.</p

Ligand-Free Silver Nanoparticles for CO2 Electrocatalytic Reduction to CO

Silver-based catalysts are attractive for electroreduction of CO2 to CO. To understand the electrocatalyst properties, a good control over the nanoparticle size is necessary. Herein, we report a strategy to synthesize highly dispersed, ligand-free silver Ag nanoparticles supported on carbon. We demonstrate that the heat treatment atmosphere and carbon surface chemistry are crucial to control the Ag particle size in the 10–30 nm range. Even at low silver loadings (0.099 m2Ag m−2), Ag nanoparticles outperforms the bulk silver at low overpotentials, leading to a 23.5 % CO Faradaic efficiency at −1.2 V vs RHE. The Ag weight-based activity of the catalysts scales with the inverse particle size, while the Ag surface-specific activity is independent of the particle size in this range. The supported silver nanoparticles can produce a H2 to CO ratio of 2.9 to 1, interesting for further exploration of this type of catalysts for syngas synthesis

Oxide-derived Silver Nanowires for CO₂ Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.</p

Surface-Modified Carbon Materials for CO2 Electroreduction

The electrochemical reduction of CO2 to produce sustainable fuels and chemicals has attracted great attention in recent years. It is shown that surface-modified carbons catalyze the CO2RR. This study reports a strategy to modify the surface of commercially available carbon materials by adding oxygen and nitrogen surface groups without modifying its graphitic structure. Clear differences in CO2RR activity, selectivity and the turnover frequency between the surface-modified carbons were observed, and these differences were ascribed to the nature of the surface groups chemistry and the point of zero charge (PZC). The results show that nitrogen-containing surface groups are highly selective towards the formation of CO from the electroreduction of CO2 in comparison with the oxygen-containing surface groups, and the carbon without surface groups. This demonstrates that the selectivity of carbon for CO2RR can be rationally tuned by simply altering the surface chemistry via surface functionalization

Oxide-derived Silver Nanowires for CO2 Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires

Alkylamine-Functionalized Carbon Supports to Enhance the Silver Nanoparticles Electrocatalytic Reduction of CO2 to CO

Silver electrocatalysts enable the conversion of CO2 to CO, thereby facilitating the transition to a carbon neutral society. To lower the cost of the expensive metal, silver nanostructures are often supported on carbon. This substrate offers great electrical conductivity, but it enhances the selectivity towards the competing hydrogen evolution reaction. In this work, carbon supports were functionalized with linear alkylamines of different chain lengths, to understand its effect on electrochemical performance. Alkylamines interact with the carbon surface and confer hydrophobic properties to the carbon support as well as making the local environment less acidic. These properties led not only to a suppression of the hydrogen evolution, but also to a remarkable enhancement in CO production. Despite the low silver weight loading (0.0016 mgAg cm−2), hexylamine-functionalized carbon-based catalysts achieved a CO to H2 ratio of 2.0, while the same material without the alkylamine functionalization only reached a ratio of 0.3, at −1.3 V vs RHE. This demonstrates the potential of hydrophobic functionalization for enhancing the CO selectivity of carbon-supported catalysts

Influence of carbon support surface modification on the performance of nickel catalysts in carbon dioxide hydrogenation

The interaction between metal nanoparticles and a support is of key importance in catalysis. In this study, we demonstrate that the introduction of oxygen- or nitrogen-containing support surface groups on a graphite nanoplatelet support influence the performance of nickel supported catalysts during CO2 hydrogenation. By careful design of the synthesis conditions, the Ni nanoparticle size of the fresh catalysts was not affected by the type of support surface groups. A combination of H2 chemisorption and high resolution TEM demonstrates that the available metal surface depends on the interaction with the carbon support. The amination treatment results in the weakest interaction between the Ni and the support, showing the highest initial Ni weight-based activity, although at the expense of nanoparticle stability. Hence initial enhancement in activity is not always optimal for long term catalysis. The use of carbon with a higher density of oxygen functional groups that are stable above 350 °C, is beneficial for preventing deactivation due to particle growth. Furthermore, small amounts of contaminants can have a substantial influence on the CH4 selectivity at low conversions

Influence of carbon support surface modification on the performance of nickel catalysts in carbon dioxide hydrogenation

The interaction between metal nanoparticles and a support is of key importance in catalysis. In this study, we demonstrate that the introduction of oxygen- or nitrogen-containing surface groups on a graphite nanoplatelet support influences the performance of nickel supported catalysts during CO2 hydrogenation. By careful design of the synthesis conditions, the Ni nanoparticle size of the fresh catalysts was not affected by the type of support surface groups. A combination of H2 chemisorption and high resolution TEM demonstrates that the available metal surface depends on the interaction with the carbon support. The amination treatment to introduce nitrogen-containing groups results in the weakest interaction between the Ni and the support, showing the highest initial Ni weight-based activity, although at the expense of nanoparticle stability. Hence initial enhancement in activity is not always optimal for long term catalysis. The use of carbon with a higher density of oxygen functional groups that are stable above 350 °C, is beneficial for preventing deactivation due to particle growth. Furthermore, small amounts of contaminants can have a substantial influence on the CH4 selectivity at low conversions

Carbon-Supported Silver Catalysts for Electrocatalytic Reduction of CO2 to CO

The work described in this thesis was aimed at understanding the influence of structural properties of silver catalysts supported on carbon for the electrocatalytic reduction of CO2 to CO. This goal was achieved by rationally designing, characterizing and testing cathode materials. This enabled a correlation between material properties and the catalytic performance. Chapter 1 describes the potential benefits of electrochemistry and electrocatalysis in the context of global warming. The CO2 electrocatalytic reduction to value-added chemicals was described, including the effect of different metal electrodes and buffer electrolytes. Specifically, a background is given on the CO2RR to CO over silver electrocatalysts, and the properties of carbon electrodes, based on literature. In chapter 2, the effect of surface-modification of carbon-based electrodes on the reduction of CO2 to CO is systematically treated. The surface chemistry of the electrodes was characterized with acid-base titration, potentiometric titration and XPS. The basic surface properties (high point of zero charge) of the N functionalized carbon catalyst led to an increased CO production with respect to the O-functionalized and reduced carbon materials. The CO turnover frequency per surface group for pyridinic groups was higher than for O-containing groups. This study demonstrated the possibility to tune the surface properties of carbon materials to enhance the ability of the electrocatalyst to reduce CO2 to CO. In chapter 3, the effect of silver nanoparticle size on the CO2 reduction to CO is discussed. Using the surface modification methods described in chapter 2, control over the ligand-free silver particle size was achieved by tuning the surface properties of the carbon supports. It was demonstrated that the silver particle size, in the range of 10 to 30 nm, decreased by increasing the density of O-containing group on the support. The small nanoparticles (11 nm diameter) effectively steered the selectivity towards CO, even greater than the selectivity achieved by bulk silver electrodes under the same conditions. In chapter 4, the aim was to suppress the hydrogen formation over the high surface area carbon support by functionalizing the surface of the support with alkylamines. The effect of the number of carbon atoms in the alkyl chain on the HER suppression and CO selectivity was investigated. Alkylamine functionalization successfully suppressed H2 evolution, while at the same time promoting CO production. This resulted in a 1 to 2 H2 to CO ratio for the catalyst functionalized with hexylamine, more favorable than for the pristine carbon-based catalyst (benchmark), able to generate only a 3.3 to 1 H2 to CO ratio. In chapter 5, the catalytic properties of oxide-derived silver nanowires, are explored. XRD and XPS analysis confirmed that by selecting the final potential during the oxidation procedure, both different silver oxidations states and different nanowires roughness were achieved as a function of the oxidation potential. This surface-modification procedure enhanced the catalytic properties of the nanowires. The active surface-normalized CO partial current density increased 3.7-times when the pristine nanowires were oxidized to 1.0 V vs Ag/AgCl in basic electrolyte solution