Theoretical Study of Direct Carbon Dioxide Conversion to Formic Acid on Transition Metal-doped Subnanometer Palladium Clusters

We studied the direct conversion of CO2 to HCOOH through hydrogenation reaction without the presence of base additives on the transition metal-doped subnanometer palladium (Pd7) cluster (PdxM: M = Cu, Ni, Rh) by using a combination of density functional theory and microkinetic calculations. It was shown that the CO2 hydrogenation on Pd7 and Pd6M clusters are more selective towards the formate pathway to produce HCOOH than the reverse water gas shift pathway to produce CO. Inclusion of Ni and Rh doping in the subnanometer Pd7 cluster could successfully enhance the turnover frequency (TOF) for CO2 hydrogenation to formic acid at low temperature. The order of TOF for formic acid formation is as follows: Pd6Ni > Pd6Rh > Pd7 > Pd6Cu. This order can be explained by the trend of the activation energy of CO2 hydrogenation to formate (HCOO*). The Pd6Ni cluster has the highest TOF value because it has the lowest activation energy for the formate formation reaction. The Pd6Ni system also has a superior TOF profile for HCOOH formation compared to several metal surfaces in low and high-temperature regions. This finding suggests that the subnanometer PdxNi cluster is a promising catalyst candidate for direct CO2 hydrogenation to formic acid

Oxygen Reduction Reaction Mechanism on the Square Paddle-Wheel Cage Site of TM-BTC (TM=Mn, Fe, Cu) Metal Organic Framework

We study the oxygen reduction reaction (ORR) mechanism on the square paddle-wheel cage active site of TM-BTC (TM= Mn, Fe, Cu) metal organic framework by using a combination of DFT and microkinetic calculations. By using a small cluster for modeling the TM-BTC active site structure, we have successfully reproduced the experimental trend of ORR activity on the TM-BTC systems: Mn-BTC > Fe-BTC > Cu-BTC. We also show that the unusual ORR activity trend from experiments for Mn and Fe systems is originated from the strength of OH adsorption on these systems. Mn-BTC system has better ORR activity than the Fe-BTC system because it has weaker OH adsorption. A very strong OH adsorption makes the final OH reduction step become more sluggish, and hence hindering the ORR process

Oxygen Reduction Reaction Mechanism on the Square Paddle-Wheel Cage Site of TM-BTC (TM=Mn, Fe, Cu) Metal Organic Framework

We study the oxygen reduction reaction (ORR) mechanism on the square paddle-wheel cage active site of TM-BTC (TM= Mn, Fe, Cu) metal organic framework by using a combination of DFT and microkinetic calculations. By using a small cluster for modeling the TM-BTC active site structure, we have successfully reproduced the experimental trend of ORR activity on the TM-BTC systems: Mn-BTC > Fe-BTC > Cu-BTC. We also show that the unusual ORR activity trend from experiments for Mn and Fe systems is originated from the strength of OH adsorption on these systems. Mn-BTC system has better ORR activity than the Fe-BTC system because it has weaker OH adsorption. A very strong OH adsorption makes the final OH reduction step become more sluggish, and hence hindering the ORR process

Understanding the Properties of Gallium Implanted LGAD Timing Detectors

ATLAS is proposing a High Granularity Timing Detector (HGTD) to be installed in front of the end-cap calorimeters for the upgrade of High Luminosity LHC project with Low Gain Avalanche Detectors (LGAD) chosen as preferred timing detectors. A beam test campaign has been conducted in order to be able to study the properties of these new detectors under severe conditions in June 2018 with a high-energy pion beam of 120 GeV at the H6A line at the CERN SPS. This study is aimed to understand the properties of gallium implanted LGAD timing detectors which was also included in the latest beam test campaign. A simple time reconstruction method of Constant Fraction Discriminator (CFD) was carried out to calculate the time resolution of this sensor. Preliminary studies show that boron implanted sensor, W9LGA35, has a better time resolution ($32.88 \pm 0.08$ ps) than gallium implanted sensor, W6S1021 ($52.93 \pm 0.15$ ps)

Mekanisme Hidrogenasi CO2 pada Klaster Subnanometer Ni7 yang Disangga pada Graphene

We study the mechanism of carbon dioxide (CO2) hydrogenation to carbon monoxide (CO) and formic acid (HCOOH) on a graphene-supported subnanometer Ni7 cluster by means of density functional theory calculations. We find that this system has similar activation energies for the first CO2 hydrogenation step for the formate and RWGS pathways. However, the second hydrogenation step for these pathways has very distinct profiles. The HCOOH formation on the formate pathway has very large activation energy, while the CO formation on the RWGS pathway has negligible activation energy. We conclude that the CO2 hydrogenation process on this system is more selective towards the RWGS pathway to produce CO

Chemical and Structural Evolution of AgCu Catalysts in Electrochemical CO2 Reduction

Silver-copper (AgCu) bimetallic catalysts hold great potential for electrochemical carbon dioxide reduction reaction (CO2RR), which is a promising way to realize the goal of carbon neutrality. Although a wide variety of AgCu catalysts have been developed so far, it is relatively less explored how these AgCu catalysts evolve during CO2RR. The absence of insights into their stability makes the dynamic catalytic sites elusive and hampers the design of AgCu catalysts in a rational manner. Here, we synthesized intermixed and phase-separated AgCu nanoparticles on carbon paper electrodes and investigated their evolution behavior in CO2RR. Our time-sequential electron microscopy and elemental mapping studies show that Cu possesses high mobility in AgCu under CO2RR conditions, which can leach out from the catalysts by migrating to the bimetallic catalyst surface, detaching from the catalysts, and agglomerating as new particles. Besides, Ag and Cu manifest a trend to phase-separate into Cu-rich and Ag-rich grains, regardless of the starting catalyst structure. The composition of the Cu-rich and Ag-rich grains diverges during the reaction and eventually approaches thermodynamic values, i.e., Ag0.88Cu0.12 and Ag0.05Cu0.95. The separation between Ag and Cu has been observed in the bulk and on the surface of the catalysts, highlighting the importance of AgCu phase boundaries for CO2RR. In addition, an operando high-energy-resolution X-ray absorption spectroscopy study confirms the metallic state of Cu in AgCu as the catalytically active sites during CO2RR. Taken together, this work provides a comprehensive understanding of the chemical and structural evolution behavior of AgCu catalysts in CO2RR

Selectivity of CO2 reduction reaction to CO on the graphitic edge active sites of Fe-single-atom and dual-atom catalysts: A combined DFT and microkinetic modeling

We study the carbon dioxide reduction reaction (CO2RR) activity and selectivity of Fe single-atom catalyst (Fe-SAC) and Fe dual-atom catalyst (Fe-DAC) active sites at the interior of graphene and the edges of graphitic nanopore by using a combination of DFT calculations and microkinetic simulations. The trend of limiting potentials for CO2RR to produce CO can be described by using either the adsorption energy of COOH, CO, or their combination. CO2RR process with reasonable reaction rates can be achieved only on the active site configurations with weak tendencies toward CO poisoning. The efficiency of CO2RR on a catalyst depends on its ability to suppress the parasitic hydrogen evolution reaction (HER), which is directly related to the behavior of H adsorption on the catalyst’s active site. We find that the edges of the graphitic nanopore can act as potential adsorption sites for an H atom, and in some cases, the edge site can bind the H atom much stronger than the main Fe site. The linear scaling between CO and H adsorptions is broken if this condition is met. This condition also allows some edge active site configurations to have their CO2RR limiting potential lower than the HER process favoring CO production over H2 production

Understanding the Structural Evolution of IrFeCoNiCu High-Entropy Alloy Nanoparticles under the Acidic Oxygen Evolution Reaction

High-entropy alloy (HEA) nanoparticles are promising catalyst candidates for the acidic oxygen evolution reaction (OER). Herein, we report the synthesis of IrFeCoNiCu-HEA nanoparticles on a carbon paper substrate via a microwave-assisted shock synthesis method. Under OER conditions in 0.1 M HClO4, the HEA nanoparticles exhibit excellent activity with an overpotential of ∼302 mV measured at 10 mA cm-2 and improved stability over 12 h of operation compared to the monometallic Ir counterpart. Importantly, an active Ir-rich shell layer with nanodomain features was observed to form on the surface of IrFeCoNiCu-HEA nanoparticles immediately after undergoing electrochemical activation, mainly due to the dissolution of the constituent 3d metals. The core of the particles was able to preserve the characteristic homogeneous single-phase HEA structure without significant phase separation or elemental segregation. This work illustrates that under acidic operating conditions, the near-surface structure of HEA nanoparticles is susceptible to a certain degree of structural dynamics

Hollow zinc oxide microsphere-multiwalled carbon nanotubes composites for selective detection of sulfur dioxide

This work reports the first utilization of anthocyanin extracted from black rice (Oryza sativa L.) grains as a structure-directing agent for the synthesis of hollow zinc oxide (ZnO) spheres via a simple solvothermal reaction and their subsequent modifications with various amounts of multiwalled carbon nanotubes (MWCNTs). Following hybridization with MWCNTs, some MWCNTs are observed to penetrate into the inner cavities of the spheres, while ZnO nanoparticles are formed on the surface of some MWCNTs. When employed as a sulfur dioxide (SO2) sensor, the ZnO-MWCNT (15:1) composite displays a high response of 156 to 70 ppm of SO2 at an optimum temperature of 300 °C as well as good selectivity to SO2 with the response to 50 ppm of SO2 gas being 3 times higher than those to other gases, such as CO, CO2, methanol, toluene, hexane, and xylene. Interestingly, the sensing behavior of this composite is strongly influenced by the proportion of MWCNTs. Specifically, n-type sensing behavior is observed for both ZnO-MWCNT (10:1) and (15:1) composites, while p-type behavior is observed for the ZnO-MWCNT (5:1) composite. The switch in sensing behavior suggests the major contribution of p-type MWCNTs to the electronic and sensing properties of the ZnO/MWCNT composites. The density functional theory (DFT) simulations on the adsorption of SO2 on the ZnO/CNT system reveal that the SO2 molecule only chemically interacts with the O adatom of ZnO (i.e., oxygen atom adsorbed on the surface of ZnO) to form sulfur trioxide (SO3), and charge transfer is observed from ZnO to CNT, which enhances the change in resistance of the composite sensor upon exposure to SO2 gas. </p