|Title:||Synthesis and electrochemical studies of copper catalysts for nitrite reduction|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xv, 282 leaves : ill. ; 30 cm.|
|Abstract:||A number of water-soluble copper complexes with tetradentate N-donor ligands [CuN4]2+ [N4 = N,N'-bis(2-pyridylmethyl)ethane-l,2-diamine (pmen), N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine (Me2-pmen), N,N'-bis(6-methyl-2-pyridylmethyl)ethane-l,2-diamine (bispmen) and N,N'-dimethyl-N,N'-bis(6-methyl-2-pyridylmethyl)ethane-l,2-diamine (Me2-bispmen)] that are active towards the electrocatalytic reduction of nitrite and nitric oxide have been synthesized. The X-ray crystal structures of the copper-nitrite complexes show that the nitrite coordinates to the copper ion via one of its oxygen (i.e. the nitrito binding mode) in a cis-monodentate fashion. The structure of the Cu-ONO complexes can best be described as distorted square pyramidal. The degree of square pyramidal distortion decreases (with the trigonal distortion factor τ varies from 0.128 to 0.033) as the number of methyl group on the bis(pyridyl)-diamine ligand increases. The half-wave reduction potential (El/2) of the copper bis(pyridyl)-diamine complexes in pH 7.0 buffer solution increases in the order [Cu(pmen)]2+ < [Cu(Me2-pmen)]2+ < [Cu(bispmen)]2+ < [Cu(Me2-bispmen)]2+. Similar trend was also observed for [CuN4(ONO)]+ in N,N-dimethylformamide. The reversibility of the Cu(II)/Cu(I) couple decreases as the number of methyl group on the bis(pyridyl)-diamine ligand increases - the peak-to-peak separation (ΔEP) of the Cu(II)/Cu(I) couple is ~60 mV for [Cu(pmen)]2+ and -120 mV for [Cu(Me2-pmen)]2+ at a scan rate of 100 mVs-1. The catalytic activities of the copper complexes towards the reduction of NO2- and NO decrease in the order [Cu(pmen)]2+ > [Cu(Me2-pmen)]2+ > [Cu(bispmen)]2+ ~ [Cu(Me2-bispmen)]2+ Hence, the most active catalyst is the one with the most cathodic E1/2, the highest reversibility in the interconversion between the Cu(II) and Cu(I) state and the largest degree of trigonal distortion in the Cu(II) ion. The second order rate constants for the reduction of NO2- by [Cu(pmen)]2+ and [Cu(Me2-pmen)]2+ in pH 5.0 buffer solution estimated by cyclic voltammetry are 4.3 x 103 M-1S-1 and 2.4 x 103 M-1S-1 respectively. Controlled potential reductive electrolysis of NO2- in the presence of the copper catalysts generates NO and N2O, whereas some NH3 were also produced upon prolonged electrolysis. Copper complexes with tridentate N-donor ligands such as 2,6-bis(N-pyrazolyl)pyridine (bpp), 2,6-bis(pyrazol-l-ylmethyl)pyridine (pmp) and tris(pyrazol-l-yl)methane (tpm) have also been synthesized. These ligands can provide either a meridional or facial coordination geometry for the copper ion. The crystal structures of a number of copper(I) and copper(II) complexes with these ligands have been determined by X-ray crystallography. An interesting [Cu(tpm)(NO2)(ONO)] complex that exhibits both nitro- (N-bound) and nitrito (O-bound) coordination mode on the same copper ion has been isolated. These copper complexes with the fac- directed (tpm) and the mer-directed ligands (bpp and pmp) were found to be active in catalyzing the reduction of NO2- and NO. The catalytic efficiency of these copper complexes towards NO2- reduction decreases in the order [Cu(pmp)(OH2)]2+ > [Cu(tpm)(OH2)]2+ > [(tpm)2Cu]2+ > [Cu(bpp)(OH2)]2+ The reduction products are NO and N2O, whereas some NH3 were also detected upon prolonged electrolysis. The electrochemical behavior of adsorbed copper-bipyridine and copper-phenanthroline complexes on graphite electrode surface have been investigated. The mono-ligated adsorbed copper complexes [CuL2+]ads (L = bipyridine or phenanthroline) are the active catalysts for the reduction of NO2- and NO. At NO2- concentration below 5 mM, the cathodic current varies linearly with [NO2-].|
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