Reductive precipitation of metals photosensitized by tin and antimony porphyrins

This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the Department of Energy. The Government has certain rights in the invention.

The invention relates to a method for reducing metals and more particularly, to a method of reducing and precipitating a group of nanostructured metal materials by tin and antimony porphyrins.

A porphyrin is a cyclic tetrapyrrolic system consisting of a 20-carbon skeleton and has been used in a variety of electrical, optical, structural, and catalytic applications. Metal ions can covalently bond within the porphyrin structure. Various peripheral groups, both inorganic and organic, can be attached to the 20-carbon skeleton to provide desired physical, chemical, and optical characteristics.

Metalloporphyrin complexes exhibit a wide range of biological functions in proteins. For instance, the iron porphyrin (heme) in cytochrome c 3 , a well studied protein found in iron-reducing bacteria ( Shewanella putrefaciens ) or sulfate-reducing bacteria (e.g., Desulfovibrio vulgaris ) is likely involved in electron transport. Recently, it was recognized that cytochrome C 3 also catalyzes the non-biological reduction of metals such as U, Cr and Se.

Photoinduced redox reactions of a three-component system containing a photosensitizer, an electron donor, and an electron acceptor have been studied by several authors. Metallo-porphyrins are well-studied photosensitizers for the reduction of various acceptor molecules, usually methylviologen. For example, photoreduced tin porphyrins act as strong reductants in solution, in micelles, and at water-organic solvent interfaces, upon excitation by visible light and reduction by an electron donor such as a tertiary amine.

Shelnutt (Shelnutt, J., J. Amer. Chem. Soc., 1983, 105, 7179-7180; U.S. Pat. No. 4,568,435, issued on Feb. 4, 1986; both herein incorporated by reference) studied the ternary system comprised of Sn(IV) protoporphyrin IX (SnPP), TEA, and methylviologen (MV 2+ ), where SnPP is the photosensitizer, TEA is the electron donor, and MV 2+ is the electron acceptor. The photoinduced oxidation-reduction reaction is illustrated in FIG. 1 . Irradiation of SnPP by visible light leads to excitation of the porphyrin to its lowest-lying triplet π—π state (SnPP*). Because the redox potential of the couple SnPP*/SnPP (+1.1 V) is higher than that of TEA/TEA ox (+0.82 V), excited SnPP* accepts an electron from TEA resulting in the radical porphyrin anion (FIG. 1 ). The low redox potential of the SnPP − /SnPP couple (−0.66 V) allows the reduction of MV 2+ to MV + (MV 2+ /MV + ; −0.45 V). 11 The quantum yield for the reaction is near 0.8. This photochemical cycle is reductive, a feature that is to-date unique to Sn(IV) and Sb(V) porphyrins. What makes the cycle reductive is that reduction of the porphyrin, rather than oxidation, is the initial step following photoexcitation. Several metalloporphyrins such as Zn porphyrins are known to follow an oxidative cycle.

FIG. 1 shows an illustration of the tin porphyrin mediated photocycle for the reduction of metal salts.

FIG. 2 shows an illustration of the structure of a tin porphyrin compound.

In the method of the present invention, a tin porphyrin (SnP) or antimony porphyrin (SbP) is used to reduce metal ions in a photoinduced reduction-oxidation (redox) reaction, where the metals include uranium (U), mercury (Hg), copper (Cu), lead (Pb), gold (Au), silver (Ag), and platinum (Pt). The metal ions that can be reduced depend on the redox potential. The potential can be controlled by suitable selection of the porphyrin's substituents, particularly by electron-withdrawing/donating substituents. Au, Ag and Pt are precious metals commonly used in microelectronic fabrication, and the method of the present invention can be utilized in nanoscale materials processing. Metals such as U, Hg, Cu and Pb are common contaminants in wastewater and groundwater and the method of the present invention can be utilized to remediate contaminated waters and in metal refining.

In one embodiment, a tin or antimony porphyrin, such as illustrated in FIG. 2, in the presence of an electron donor, such as ethylenediaminetetraaceticacid (EDTA), triethylamine, sodium nitrite, or triethanolamine (TEA), is exposed to a solution containing at least one metal selected from uranium, mercury, copper, lead, silver, gold and platinum. The solution is irradiated, such as by sunlight or an artificial light source, to reductively precipitate the metal. The time to precipitate the metal depends on the porphyrin and the metal and can range from minutes to days.

Typical reduction reactions of the metals are listed in Equations (1) to (7), where the metals are in typically-encountered compounds, such as nitrates and halides. Similar redox reactions would occur with the metals encountered as metal salts and like compounds.

2SnPP − +UO 2 (NO 3 ) 2 .6H 2 O=2SnPP+UO 2 +2NO 3 − +6H 2 O  (1)

2SnPP − +HgCl 2 =2SnPP+Hg 0 +2Cl −   (2)

2SnPP − +CuCl 2 .2H 2 O=2SnPP+Cu 0 +2Cl − +2H 2 O  (3)

2SnPP − +Pb(NO 3 ) 2 =2SnPP+Pb 0 +2NO 3 −   (4)

2SnPP − +Ag(NO 3 ) 2 =2SnPP+Ag 0 +2NO 3 −   (5)

3SnPP − +HAuCl 4 .xH 2 O=3SnPP+Au 0 +4Cl − +H + +xH 2 O  (6)

4SnPP − +H 2 PtCl 6 .6H 2 O=4SnPP+Pt 0 +6Cl − +2H + +6H 2 O  (7)

The solubility of uraninite (2×10 −8 M), Hg 0 (10 −29 M), Cu 0 (10 −44 M) and Pb 0 (10 −60 M) in water at 25° C. is very low. Therefore, their precipitation mediated by the tin protoporphyrin could be used to clean up contaminated waters. Following precipitation, these metal phases may be filtered for reuse or disposed of. Using SnPP for water remediation can be an efficient technique because it uses sunlight as the source of energy and an inexpensive electron donor, such as sodium nitrite or TEA.

Precipitation of precious metals photosensitized by SnP can be a practical way to extract and concentrate them from oxidizing waters. Furthermore, in the case of Au and Ag, formation of nanoparticles can find application in a variety of fields due to their optical, electrical, and catalytic properties. Growth of nanostructures such as nanowires and nano-networks can also be accomplished using the SnP photocycle to regenerate the SnP radical anion as shown in the reactions described by Equations (1)-(7).

The photoinduced reactions of the present invention can be induced by sunlight as the source of energy, lamps (such as a tungsten lamp) or any other energy source that produces light with wavelengths of 600 to 300 nm or even shorter or longer wavelengths, depending on the specific porphyrin used.

Stock 10-mM solutions of each of the metals were prepared by dissolving their respective salts in deionized water. In aqueous solution, Sn(IV)Cl 2 protoporphyrin (SnPP) or Sn(IV)Cl 2 tetra(N-methylpyridinium)porphyrin (SnTNMPP) exists as the dihydroxyl complex following replacement of the chloride ligands in aqueous solution.

For each experiment, the final concentration of the different components were: [metal]=10 −2 M; [SnP]=10 −6 M; [TEA]=4×10 −1 M (or [EDTA]or [NaNO 2 ]=8×10 −2 M). Control experiments using the SnPP were performed with methylviologen (MV 2+ ) as the electron acceptors; these showed that the reduction reaction, as demonstrated by the blue color of MV + , can occur under exposure to sunlight, although the reactions were faster under an intense artificial tungsten light. Oxygenation of the solution led to the re-oxidation of MV + to MV 2+ although it did not affect the stability of SnPP. In fact, following oxygenation, evidenced by the loss of blue color due to oxidation of MV + to MV 2+ , the oxidation of reduced MV 2+ by O 2 occurred and this reduction/oxidation cycle was repeated several times. Precipitation of metals was very fast (a few minutes) for certain metals (Ag, Hg), but took from a few hours to a few days for a noticeable precipitate to form for the rest of metals.

To analyze the resulting reduced metal precipitate, a few drops of the metal suspension were deposited onto a carbon-coated grid and rinsed with de-ionized water to remove soluble salts. The grids were placed into a JEOL transmission electron microscope (TEM), equipped with an energy dispersive spectrometry (EDS) system. The microscope was operated at 200 keV. The precipitates were analyzed for chemical composition and morphology. Crystal structure information was obtained using selected area electron diffraction (SAED).