Novel carbon nitride/hydroxyapatite composite material for effective CO2 electroreduction into C1-based product

The reduction of CO2 using different techniques represents a promising technology in the ongoing efforts to manage carbon dioxide emissions. One of the most intriguing technologies is CO2 electrochemical reduction (CO2-ER) into carbon-based products, which is currently undergoing deep investigation. Although CO2-ER has great potential for environmental applications, many holdbacks are present regarding energy efficiency, reaction selectivity, and overall conversion rate. Electrocatalytic materials must be developed to address the aforementioned drawbacks in the CO2 electrochemical process. Here, we present an innovative composite material composed of graphitic carbon nitride (CN) properly functionalized with metal nanoparticles, such as copper and hydroxyapatite nanorods (HAP). Carbon nitride is a conductive and supportive material, and thanks to its porous structure, it can be easily functionalized. HAP, a calcium phosphate mineral with highly versatile properties, has recently gained attention in material science and environmental applications both as an adsorbent and as an acid-base catalyst. An innovative and effective electrocatalyst can be obtained by combining different materials with different properties and features. It may be employed in CO2-ER in order to decrease the total overpotential and to increase faradic efficiency towards added value molecules, particularly C1 molecules such as methane and formic acid. Cu@CN and HAP_Cu@CN are materials that exploit the singular features of their components to promote a more efficient reduction of carbon dioxide towards methane and formic acid, compared with other copper-carbon based catalysts such as multi-walled carbon nanotubes (MWCNT) or copper-zinc alumina material (CZA). Electrochemical investigation were performed using a three-electrode cell, in a 0.1 M KHCO3 aqueous solution, comprising a working electrode (carbon paper polished with the catalyst), a reference electrode (saturated Ag ׀AgCl) and a counter electrode (Pt wire) (Fig.1, a). Linear sweep voltammetry (LSV) curves show a net increase in the current density slope at potentials lower than −1.5 V (vs Ag ׀AgCl) when HAP is present in the catalyst, indicating that admixing with HAP results in a slight increase in the activity of the systems. To highlight the products formed in the presence of catalysts, a 1-hour chronoamperometric (CA) test was performed. Despite the inevitable presence of the parasitic hydrogen evolution reaction (HER), the catalysts showed CO2ER activity. The relative faradic efficiencies (FE) of HER and CO2ER are consistent with LSV data, showing a predominant HER at a lower cathodic potential than -1.5 V (vs Ag׀AgCl). For Cu@CN and HAP_Cu@CN, H2 FE decreased significantly by increasing cathodic potential, while other copper-based catalysts showed low catalytic activity and hydrogen production. Focusing on the performance of the composite with HAP and CN, the catalyst showed significant FE towards formic acid (Fig.1, b): at cathodic potential such as -1.8 V (vs Ag׀AgCl) selectivity reached 55%, proving that the apatite may influence the reaction mechanism through the presence of different acidic and basic sites. The composite material showed promising results in CO2RR towards methane or formic acid, important C1 added-value molecules, that have applications in the energy field or in chemical transformation

Functional and innovative carbon nitride/hydroxyapatite composite material for efficient carbon dioxide electrocatalytic reduction to formic acid

CO2 conversion is one of the most encouraging approaches among carbon capture and utilization technologies (CCUs) to recycle carbon dioxide emissions on a large scale. Among CCUs, CO2 electrochemical reduction (CO2-ER) represents a promising strategy in CO2 emission recycling. However, CO2-ER faces various challenges, such as energy efficiency, reaction selectivity, and conversion rate that can be overcome with the proper design of an electrocatalyst1. Here, we present an innovative composite material(Fig.1) composed of graphitic carbon nitride (CN), obtained by pyrolysis of a covalent organic framework as a precursor2, decorated by copper nanoparticles(Cu) and hydroxyapatite nanorods. The latter is introduced as a key-component to decrease overpotential and increase efficiency towards the production of value-added molecules, such as formic acid3. Morphological, spectroscopic and thermal analyses were conducted on the obtained material HAP_Cu@CN. The performance of HAP_Cu@CN in CO2-ER were evaluated and compared with those of other copper-carbon based catalysts. Electrochemical investigations were carried out using a three-electrode cell in a 0.1 M KHCO3 aqueous solution(Fig.2,a). Linear sweep voltammetry curves showed that admixing with HAP resulted in a slight increase in system activity. The chronoamperometric tests revealed that the HAP-based catalyst showed higher CO2-ER activity than the other catalysts. Specifically, at -1.8 V applied potential (vs Ag-AgCl), parasitic H2 faradic efficiency (FE) decreased by 40% and HCOOH became the main product with a 50% FE, while other Cu-based materials were more efficient for hydrogen production without a positive effect on product distribution and selectivity(Fig.2,b). The composite material showed promising results in CO2ER, demonstrating that combining different surfaces can pave the way for more efficient and selective electrocatalysts

Tuning zeolite properties for highly efficient synthesis of propylene from methanol

Series of nanosized ZSM-5 samples is synthesized at 170 °C, 150 °C, 120 °C and 100 °C. Experimental data show that the decrease of crystallization temperature leads to significant changes in zeolite properties. Crystals synthesized at 100 °C exhibit many framework defects with lower acid sites density, strength and larger external surface area. The selectivity to light olefins and the propylene-to-ethylene ratio increases as the crystallization temperature decreases. Propylene-to-ethylene ratio above 6 with the highest selectivity to propylene of 53 % is obtained over ZSM-5 catalyst prepared at 100 °C. Stability of the nanosized zeolite in MTO is also improved compared to industrial sample with similar Si/Al ratio. This catalytic performance is a result of the decrease in the acid sites density, strength and the crystals’ size, providing shorter diffusion path and larger external surface area. The presence of structural defects and different external surface are of the crystals has been shown to play an important role in the MTO catalyst performance

Stoichiometric methane conversion to ethane using photochemical looping at ambient temperature

Methane activation and utilization are among the major challenges of modern science. Methane is potentially an important feedstock for manufacturing value-added fuels and chemicals. However, most known processes require excessive operating temperatures and exhibit insufficient selectivity. Here, we demonstrate a photochemical looping strategy for highly selective stoichiometric conversion of methane to ethane at ambient temperature over silver–heteropolyacid–titania nanocomposites. The process involves a stoichiometric reaction of methane with highly dispersed cationic silver under illumination, which results in the formation of methyl radicals. Recombination of the generated methyl radicals leads to the selective, and almost quantitative, formation of ethane. Cationic silver species are simultaneously reduced to metallic silver. The silver–heteropolyacid–titania nanocomposites can be reversibly regenerated in air under illumination at ambient temperature. The photochemical looping process achieves a methane coupling selectivity of over 90%, a quantitative yield of ethane of over 9%, high quantum efficiency (3.5% at 362 nm) and excellent stability

Desilication of highly siliceous zeolite ZSM-5 with NaOH and NaOH/tetrabutylamine hydroxide

The results of both chemical and XPS analysis pointed out that desilication of highly siliceous ZSM-5 of Si/Al = 164 was more effective in the surface zone than in the bulk, contrary to zeolite ZSM-5 of Si/Al = 31.6. According to the IR studies in parent zeolite the concentration of protonic sites was very close to the concentration of Al indicating that all Al atoms can form Si-OH-Al. The results of our quantitative IR studies strongly support the realumination thesis, i.e. some Al atoms extracted in basic solutions are subsequently reinserted forming new acidic hydroxyls. In desilicated zeolites all Al atoms were able to form protonic sites, however part of them dehydroxylated during the activation of zeolite producing Lewis acid sites according to the stoichiometry: one protonic site was transformed into one Lewis site. Low temperature nitrogen adsorption revealed that the alkaline treatment of highly siliceous zeolite with 0.2 M NaOH/TBAOH mixture produced mesopores of smaller diameter and narrower pore size distribution than in the case of zeolite of medium Si/Al ratio. This result can be explained by low concentration of Al which similarly as TBA(+) cations plays the role of pore directing agents (PDA). Contrary to TEA(+), the presence of Al in desilication mixture, led to the formation of larger pores. Therefore, in highly siliceous zeolite TBA(+) played dominant role as PDA producing narrower pores. Highly siliceous zeolite with uniform distribution of relatively narrow pores may be useful catalyst or catalyst support. The influence of desilication temperature on porosity development was also investigated. The increase of desilication temperature from 338 to 353 K resulted in both more extensive demetalation (more Si and Al is extracted) and the distinct increase of the volume and surface of mesopores. Both lower concentration of protonic sites and higher concentration of Lewis sites confirmed partial zeolite destruction upon desilication at elevated temperature. The experiments of pivalonitrile sorption followed by IR spectroscopy showed a significant increase of accessibility of acid sites to bulky pivalonitrile molecules. (C) 2012 Elsevier Inc. All rights reserved.The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.00-12-023/08).Sadowska, K.; Góra-Marek, K.; Drozdek, M.; Kustrowski, P.; Datka, J.; Martínez-Triguero, J.; Rey Garcia, F. (2013). Desilication of highly siliceous zeolite ZSM-5 with NaOH and NaOH/tetrabutylamine hydroxide. Microporous and Mesoporous Materials. 168:195-205. https://doi.org/10.1016/j.micromeso.2012.09.033S19520516

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