Role Of Nitrogen Species Research Articles

Nitrogen-doped hydrochars from shrimp waste as visible-light photocatalysts: Roles of nitrogen species

The increasing shrimp waste production has caused severe environmental problems. In this study, nitrogen-doped hydrochars (NDHCs) were facilely synthesized from shrimp waste and glucose by one-pot hydrothermal carbonization (HTC). The characterizations showed that NDHCs had large surface areas of up to 30.5 m2 g−1 with numerous functional groups on their porous surfaces. The nitrogen content (1.3–2.8%) and species distribution in NDHCs were associated with the amount of added glucose. These NDHCs were applied as visible-light-induced photocatalysts, and their photocatalytic performances were evaluated by methylene blue (MB) degradation. The removal rate of MB reached 88.9% after 1 h of visible light radiation by NDHC-1, which was 2.3 times higher than that of glucose-derived hydrochar (GHC). The mechanism study showed that the improved photoactivity of NDHCs was attributed to the increased adsorption capacity by porous surface and the promoted formation of hydroxyl radicals by synergistic effects of quaternary N and pyrrolic N during photocatalysis. This study offered a green approach to preparing tunable, efficient, and low-cost photocatalyst from waste biomass and insight into the photocatalytic mechanism of hydrochar materials.

Nitrogen doped carbon for Pd-catalyzed hydropurification of crude terephthalic acid: roles of nitrogen species.

The purification of crude terephthalic acid was performed by the hydrogenation of 4-carboxybenzaldehyde (4-CBA) over activated carbon (AC) supported Pd catalysts in industry. However, traditional Pd/AC catalysts usually suffer from low hydrogenation activity and poor thermal stability. Herein, nitrogen was incorporated into AC via a simple hydrothermal treatment of AC with urea as the nitrogen resource. The N doped AC contained pyridinic N, pyrrolic N, graphitic N and oxidized N. Wide characterizations revealed that N doping not only effectively improved the dispersion of Pd NPs but also increased the proportion of Pd0. In addition, N doping also enhanced the dissociative adsorption capacity of molecular hydrogen. More importantly, the resistance to sintering of Pd NPs was efficiently suppressed after N doping. As a result, N doped AC supported Pd showed both higher activity and better thermal stability than the N-free one.

The Intrinsic Nature of Persulfate Activation and N-Doping in Carbocatalysis.

Persulfates activation by carbon nanotubes (CNT) has been evidenced as nonradical systems for oxidation of organic pollutants. Peroxymonosulfate (PMS) and peroxydisulfate (PDS) possess discrepant atomic structures and redox potentials, while the nature of their distinct behaviors in carbocatalytic activation has not been investigated. Herein, we illustrated that the roles of nitrogen species in CNT-based persulfate systems are intrinsically different. In PMS activation mediated by nitrogen-doped CNT (N-CNT), surface chemical modification (N-doping) can profoundly promote the adsorption quantity of PMS, consequently elevate potential of derived nonradical N-CNT-PMS* complexes, and boost organic oxidation efficiency via an electron-transfer regime. In contrast, PDS adsorption was not enhanced upon incorporating N into CNT due to the limited equilibrium adsorption quantity of PDS, leading to a relatively lower oxidative potential of PDS/N-CNT system and a mediocre degradation rate. However, with equivalent persulfate adsorption on N-CNT at a low quantity, PDS/N-CNT exhibited a stronger oxidizing capacity than PMS/N-CNT because of the intrinsic higher redox potential of PDS than PMS. The oxidation rates of the two systems were in great linearity with the potentials of carbon-persulfate* complexes, suggesting N-CNT activation of PMS and PDS shared the similar electron-transfer oxidation mechanism. Therefore, this study provides new insights into the intrinsic roles of heteroatom doping in nanocarbons for persulfates activation and unveils the principles for a rational design of reaction-oriented carbocatalysts for persulfate-based advanced oxidation processes.

The roles of nitrogen species on graphene aerogel supported Cu-Zn as efficient catalysts for CO2 hydrogenation to methanol

The nitrogen heteroatom has been recently doped into carbon materials such as carbon nanotubes and graphene for enhance the catalytic activity of CO2 hydrogenation to methanol. However, the role in each of the nitrogen species are not yet fully understood. In this work, the nitrogen has been doped to the graphene aerogel by using different nitrogen sources (i.e., ammonia, hydrazine hydrate, and urea) via a hydrothermal reduction method, and the role of each nitrogen species has been investigated. It is found that 15%CuZn loaded on nitrogen doped graphene aerogel catalyst using urea as a precursor provided the highest space-time yield of methanol (405.49 mg gcat−1 h−1). Remarkably, the increase of pyridinic nitrogen can greatly promote the catalytic performance and enhance the methanol production. This work is the first systematic study on the influent of nitrogen species doping on graphene aerogel support and also proves that pyridinic-N plays an important role to enhance the catalytic performance.

Roles of nitrogen species on nitrogen-doped CNTs supported Cu-ZrO2 system for carbon dioxide hydrogenation to methanol

Cu-ZrO2 catalysts supported on nitrogen-doped CNTs were tested for CO2 hydrogenation to methanol in a fixed-bed plug flow reactor. The characterization results indicate that the presence of pyridinic nitrogen increases the copper oxide dispersion, promotes their reduction, decreases the Cu particle size and enhances H2 adsorption capability; the activated CO2 adsorption predominantly controls CO2 conversion. In addition, pyridinic nitrogen contributes to the strong adsorption of CO2, stimulating methanol formation, while pyrrolic nitrogen contributes to the medium adsorption of CO2, mainly boosting CO formation. Thus, pyridinic nitrogen dominant CNTs supported catalyst (CZ/CNT-N) possesses more active sites compared with the other three catalysts, leading to maximum methanol yield (8.64%) and space time yield (102.21mgg−1cath−1); pyrrolic nitrogen dominant CNTs supported catalyst (CZ/CNT-NH2) obtains more highly active structure, giving the highest intrinsic activity (TOF values).