Investigation of chemical and adsorption properties of carbon nanotubes: building a bridge for technological applications of carbon nanotubes.

In the present work, the results of investigations of the chemical, adsorption and optical properties of carbon nanotubes (CNTs) will be presented. A brief introduction describes CNTs, how they are produced, and how they are purified. Experimental investigations of the effect of air/HCl purification on the introduction of oxygen functionalities will be reported. It was established that air/HCl purification results in the introduction of oxygen containing functionalities to single wall carbon nanotubes (SWCNTs) produced by the HiPco (high pressure carbon monoxide) method. The introduced oxygen functionalities decompose at ~670 K detected as masses 18 (H2O), 28 (CO) and 44 amu (CO2) in the mass spectrum. The exact chemical nature of those functionalities requires more detailed investigation.Low-temperature (100 K) adsorption of acetone on carbon black, as-produced and air/HCl purified SWCNTs allowed the accessibility of different adsorption sites in SWCNTs to be established. A key variable was the vacuum-annealing temperature. The energetics of interaction of acetone with different adsorption sites was determined. The most energetic adsorption sites were found to be endohedral adsorption sites.The interaction of solvents with carbonaceous materials was studied under different conditions: sonication, reflux, and exposure to solvent vapors over a range of pressures. It was shown that the binding energy of molecules with SWCNTs depends on the interaction conditions: the higher the temperature and pressure during the contact of molecules with SWCNTs, the higher the adsorption energy of molecules on/in SWCNTs. This finding suggests a "pressure gap" effect for nanoporous carbonaceous materials.Infrared studies of CNTs suggest that molecules adsorbed inside of endohedral channels are invisible to IR. This result is in contrast with experimental findings by other authors. Additional research, both experimental and theoretical, must be done to identify factors responsible for the screening of molecules adsorbed inside SWCNTs

ENSEMBLE SIZE EFFECT IN CATALYSIS BY PLATINUM-COPPER SILICA SUPPORTED BIMETALLIC CATALYSTS

The present study is of great importance both from industrial application side and from fundamental point of view. The work considers ecologically significant industrial processe - utilization of chlorinated hydrocarbons. Particularly, 1,2-dichloroethane utilization is chosen as a model reaction to study the kinetics of hydrogenation process in fixed batch flow reactor over Pt-Cu silica supported bimetallic catalysts. The fundamental part of the study includes correlation search between kinetics performance and electronic-structural properties of Pt-Cu bimetallic catalyst. Establishing correlation between electronic-structural properties and performance in chemical reactions are of prime significance for understanding of chemical nature of particular chemical systems. The understanding means the ability to govern the process that is of huge interest for industrial applications.The kinetic performance is determined directly by testing the selected catalyst in fixed bed reactor. The main characteristics derived from kinetics testing that are interesting for current study - selectivity and activity. The product of interest for presented process is ethylene, C2H4. The electronic-structural properties were derived mainly from the infrared-red (FTIR) study of carbon monoxide (CO) test molecule. The correlation between electronic-structural properties and kinetics performance are related to freshly pretreated catalyst.It was established that the selectivity toward C2H4 is a strong function of Cu/Pt atomic ratio that depends on the size of Pt ensembles: the smaller the size of Pt ensembles the higher selectivity toward C2H4. Activity slightly decreases as Cu/Pt ratio is increasing. The observed kinetics performance is rationalized based on knowledge derived from FTIR study and knowledge from previously published works.Structural sensitivity of C-Cl bond cleavage reaction is established for 1,2-dichloroethane hydrodechlorination. Elementary step of C-Cl bond cleavage is require 2-6 platinum atoms. Besides, the role of Cu as a main component responsible for C2H4 formation is shown experimentally. The structural dependence is rationalized in terms of ensemble size effect.The mechanism of 1,2-dichloroethane hydrodechlorination reaction for highly selective catalysts is explained in terms of independent role of Pt and Cu

Evaluating the effects of carbon nanoreactor diameter and internal structure on the pathways of the catalytic hydrosilylation reaction

Three different types of carbon nanoreactors, double-walled nanotubes (DWNT), multi-walled nanotubes (MWNT) and graphitised carbon nanofibers (GNF) have been appraised for the first time as containers for the reactions of phenylacetylene hydrosilylation catalysed by a confined molecular catalyst [Rh4(CO)12]. Interactions of [Rh4(CO)12] with carbon nanoreactors determining the ratio of β-addition products are unchanged for all nanoreactors and are virtually unaffected by the confinement of [Rh4(CO)12] inside carbon nanostructures. Conversely, the relative concentrations of reactants affecting the ratio of addition and dehydrogenative silylation products is very sensitive to nanoscale confinement, with all nanoreactors demonstrating significant effects on the distribution of reaction products as compared to control experiments with the catalyst in bulk solution or adsorbed on the outer surface of nanoreactors. Surprisingly, the widest nanoreactors (GNF) change the reaction pathway most significantly, which is attributed to the graphitic step-edges inside GNF providing effective anchoring points for the catalyst and creating local environments with greatly altered concentrations of reactants as compared to bulk solution. Possessing diameters significantly wider than molecules, GNF impose no restrictions on the transfer of reactants while providing the strongest confinement effects for the reaction. Furthermore, GNF facilitate the effective recyclability of the catalyst and thus represents a superior nanoreactor system to carbon nanotubes

Chemical reactions at the graphitic step-edge: changes in product distribution of catalytic reactions as a tool to explore the environment within carbon nanoreactors

A series of explorative cross-coupling reactions have been developed to investigate the local nanoscale environment around catalytically active Pd(II)complexes encapsulated within hollow graphitised nanofiber (GNF). Two new fullerene-containing and fullerene-free Pd(II)Salen catalysts have been synthesised, and their activity and selectivity towards different substrates has been explored in nanoreactors. The catalysts not only show a significant increase in activity and stability upon heterogenisation at the graphitic step-edges inside the GNF channel, but also exhibit a change in selectivity affected by the confinement which alters the distribution of isomeric products of the reaction. Furthermore, the observed selectivity changes reveal unprecedented details regarding the location and orientation of the catalyst molecules inside the GNF nanoreactor, inaccessible by any spectroscopic or microscopic techniques, thus shedding light on the precise reaction environment inside the molecular catalyst-GNF nanoreactor. Keywords: nanoreactor, catalysis, fullerene, salen, cross-couplin

Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review

The search for effective materials for environmental cleanup is a scientific and technological issue of paramount importance. Among various materials, carbon nanotubes (CNTs) possess unique physicochemical, electrical, and mechanical properties that make them suitable for potential applications as environmental adsorbents, sensors, membranes, and catalysts. Depending on the intended application and the chemical nature of the target contaminants, CNTs can be designed through specific functionalization or modification processes. Designer CNTs can remarkably enhance contaminant removal efficiency and facilitate nanomaterial recovery and regeneration. An increasing number of CNT-based materials have been used to treat diverse organic, inorganic, and biological contaminants. These success stories demonstrate their strong potential in practical applications, including wastewater purification and desalination. However, CNT-based technologies have not been broadly accepted for commercial use due to their prohibitive cost and the complex interactions of CNTs with other abiotic and biotic environmental components. This paper presents a critical review of the existing literature on the interaction of various contaminants with CNTs in water and soil environments. The preparation methods of various designer CNTs (surface functionalized and/or modified) and the functional relationships between their physicochemical characteristics and environmental uses are discussed. This review will also help to identify the research gaps that must be addressed for enhancing the commercial acceptance of CNTs in the environmental remediation industry

Hydrogen-Assisted 1,2-Dichloroethane Dechlorination Catalyzed by Pt-Cu/SiO2: Insights into the Nature of Ethylene-Selective Active Sites

Differently pretreated silica-supported Pt, Cu, and Pt-Cu catalysts with Cu to Pt atomic ratio of 1 to 6 have been investigated by a combination of reaction kinetics and FTIR spectroscopic studies in order to understand the factors that control the selectivity toward ethylene and ethane in the CH2ClCH2Cl+H2 reaction. Carbon monoxide adsorption was used to probe the electronic modification of Pt and Cu as well as the nature of ethylene-selective active sites. It was shown that there is a very limited, if any, electronic interaction between Pt and Cu in the bimetallic catalysts reduced at 493 K. However, the Pt-Cu catalysts, for which no dipole-dipole coupling shift was observed in the IR spectra of adsorbed CO suggesting extremely small Pt ensembles on the catalyst surface, demonstrated high ethylene selectivity in the 1,2-dichloroethane dechlorination. Electronic interactions between Pt and Cu have been discovered for the Pt-Cu/SiO2 catalysts reduced at 773 K. The interactions manifested themselves by a higher stability of Cu0-CO adsorption complexes in vacuum and by an increase in intensity of the Pt-CO band in the FTIR spectra upon evacuation of CO from the gas phase suggesting the formation of Pt-Cu solid solutions. The higher temperature reduction resulted in the dipole-dipole coupling shift of 6 to 19 cm-1 in the FTIR spectra of adsorbed CO. The initial ethylene selectivity of the catalysts was inversely proportional to the dipole-dipole coupling shift. The observations are consistent with the idea that the nature of the Pt-Cu species, viz., alloy particles as opposed to Cu/Pt overlayers, does not control the reaction selectivity, which is a function of the Pt ensemble size on the surface of Pt-Cu moieties

Hydrogen-Assisted 1,2-Dichloroethane Dechlorination Catalyzed by Pt-Cu/SiO2: Insights into the Nature of Ethylene-Selective Active Sites

Differently pretreated silica-supported Pt, Cu, and Pt-Cu catalysts with Cu to Pt atomic ratio of 1 to 6 have been investigated by a combination of reaction kinetics and FTIR spectroscopic studies in order to understand the factors that control the selectivity toward ethylene and ethane in the CH2ClCH2Cl+H2 reaction. Carbon monoxide adsorption was used to probe the electronic modification of Pt and Cu as well as the nature of ethylene-selective active sites. It was shown that there is a very limited, if any, electronic interaction between Pt and Cu in the bimetallic catalysts reduced at 493 K. However, the Pt-Cu catalysts, for which no dipole-dipole coupling shift was observed in the IR spectra of adsorbed CO suggesting extremely small Pt ensembles on the catalyst surface, demonstrated high ethylene selectivity in the 1,2-dichloroethane dechlorination. Electronic interactions between Pt and Cu have been discovered for the Pt-Cu/SiO2 catalysts reduced at 773 K. The interactions manifested themselves by a higher stability of Cu0-CO adsorption complexes in vacuum and by an increase in intensity of the Pt-CO band in the FTIR spectra upon evacuation of CO from the gas phase suggesting the formation of Pt-Cu solid solutions. The higher temperature reduction resulted in the dipole-dipole coupling shift of 6 to 19 cm-1 in the FTIR spectra of adsorbed CO. The initial ethylene selectivity of the catalysts was inversely proportional to the dipole-dipole coupling shift. The observations are consistent with the idea that the nature of the Pt-Cu species, viz., alloy particles as opposed to Cu/Pt overlayers, does not control the reaction selectivity, which is a function of the Pt ensemble size on the surface of Pt-Cu moieties