The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (-0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed COOH and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.

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The practical recycling of carbon dioxide (CO2) by the electrochemical reduction route requires an active, stable, and affordable catalyst system. Although noble metals such as gold and silver have been demonstrated to reduce CO2 into carbon monoxide (CO) efficiently, they suffer from poor durability and scarcity. Here we report three-dimensional (3D) graphene foam incorporated with nitrogen defects as a metal-free catalyst for CO2 reduction. The nitrogen-doped 3D graphene foam requires negligible onset overpotential (-0.19 V) for CO formation, and it exhibits superior activity over Au and Ag, achieving similar maximum Faradaic efficiency for CO production (∼85%) at a lower overpotential (-0.47 V) and better stability for at least 5 h. The dependence of catalytic activity on N-defect structures is unraveled by systematic experimental investigations. Indeed, the density functional theory calculations confirm pyridinic N as the most active site for CO2 reduction, consistent with experimental results.

With the fast development of nanotechnology, the nanomaterials start to cause people's attention for potential toxic effect. In this paper, the cytotoxicity and oxidative stress caused by 20-nm cerium oxide (CeO2) nanoparticles in cultured human lung cancer cells was investigated. The sulforhodamine B method was employed to assess cell viability after exposure to 3.5, 10.5, and 23.3 microg/ml of CeO2 nanoparticles for 24, 48, and 72 h. Cell viability decreased significantly as a function of nanoparticle dose and exposure time. Indicators of oxidative stress and cytotoxicity, including total reactive oxygen species, glutathione, malondialdehyde, alpha-tocopherol, and lactate dehydrogenase, were quantitatively assessed. It is concluded from the results that free radicals generated by exposure to 3.5 to 23.3 microg/ml CeO2 nanoparticles produce significant oxidative stress in the cells, as reflected by reduced glutathione and alpha-tocopherol levels; the toxic effects of CeO2 nanoparticles are dose dependent and time dependent; elevated oxidative stress increases the production of malondialdehyde and lactate dehydrogenase, which are indicators of lipid peroxidation and cell membrane damage, respectively.

Nitrogen-doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon-dioxide-reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. -0.18 V) and increases the selectivity (ca. 80%) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO2. However, for graphitic-like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO2 binding.

Nitrogen‐doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon‐dioxide‐reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. −0.18 V) and increases the selectivity (ca. 80 %) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO2. However, for graphitic‐like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO2 binding.

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The effects of electrolyte on selectivity and activity were investigated in the electrochemical reduction of CO 2 on the Sn electrode. The production of formate, the primary product in our three-electrode cell was quantitatively characterized by solution phase 1 H NMR spectroscopy. Both SO 4 2− and Na + favor higher faradaic and energy efficiencies, while HCO 3 − and K + enable a higher rate of formate production. The faradaic efficiency was as high as ∼95% for 0.1 M Na 2 SO 4 at a potential of −1.7 V vs. a Saturated Calomel Electrode (SCE). 0.5 M KHCO 3 was an optimal electrolyte for obtaining a high production rate of formate which can reach over 3.8 μmol min −1 cm −2 at a potential of −2.0 V vs. SCE while maintaining a faradaic efficiency of ∼63%. A trend we observed was that the faradaic efficiency increases as the concentration of electrolyte is diluted. Our studies also show the degradation of electrocatalytic activity of the Sn electrode in this three-electrode cell can be attributed to the electrodeposition of trace amounts of Zn onto the surface of the Sn electrode, which reduces the active surface area.