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Cyanogels Project

Cyanogels are inorganic gels with backbones formed by a network of metal centers bridged by cyanide ligands. Gels, in general, are a class of materials that consist of a continuous solid phase that encloses a continuous liquid phase. They have been known for quite a while and some famous examples include silica gel, alumina gel, or gelatin.

Cyanogels were discovered serendipitously by Bocarsly's student Brian Pfennig. He was working on another project, expecting to make a tri-nuclear water-soluble complex for the study of charge transfer. He mixed solutions of Na2PdCl4 and K3Fe(CN)6 and in about 20 minutes the initially transparent mixture turned opaque and gelled.

Brian's gel was originally studied by UV-VIS, FTIR, by electrochemistry, by rheometry, and by thermal analysis. Infrared spectroscopy proved especially useful in determining the structure of these extended molecules. The studies showed that the new gel is an inorganic coordination polymer, in which Pd and Fe centers are linked by bridging cyanide ligand. The gel contains up to 99% water.

Gels are commonly used in materials science in the so-called "sol-gel processing" depicted to the right. Sol-gel processing has been known to produce ultrahomogeneous materials under relatively mild conditions. Precursor materials prone to gelling are intimately mixed in solution. They react and form a suspension of small particles – a sol phase. During the sol-gel transition, the sol cross-links further transforming into a gel. When the liquid phase is present, the gel is called a hydrogel. The liquid can be removed by regular oven or room temperature drying to form xerogels or under supercritical conditions to form more porous aerogels. During subsequent heat treatment, the gel is sintered and densified. Most of the gel-forming materials, such as metal hydroxides and alkoxides, contain oxygen, which stays in the system after heat treatment. So traditionally, the sol-gel processing has been limited to ceramic materials, such as silica, alumina, titania, etc. gel gif
In our group, the possibility of using cyanogels as sol-gel precursors was explored. It was shown that cyanogels could be processed by heat treatment into oxides and nitrides, as well as metal alloys, depending on the processing environment (oxygen, nitrogen, argon). The possibility to process cyanogels into both metals and ceramics, makes them quite unique sol-gel precursors.

Bulk gelling has been achieved for mixtures of square planar tetrachlorometalates and with a variety of cyanometalates containing V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, Pd, Os, and Pt. The scheme for a typical synthetic route is:

Gelation


In addition, gels have been made using SnCl4/K4Fe(CN)6, and RuCl3/K4Ru(CN)6.

Even though the crystal structure of a cyanogel has never been established due to the impossibility to grow crystals of the gel, other experiments suggest that cyanogels are 3-D bridged networks, as in the following scheme of a Pd-Co cyanogel.

Gaussian Image of a Gel


During heat processing under an inert atmosphere, the cyanide ligands reduce the metal cations in the gel into metals in their zero valent state, while they are oxidized to cyanogen, (CN)2. Due to the intimate mixing of the metals in the gel phase, the resulting metal alloy is very homogeneous.

The stability studies showed that the most stable gel was the Pd-Co gel with Pd:Co ration equal to 2:1. The Pd-Co based cyanogel has been the most intensively studied gel in the Bocarsly lab. The gas adsorption properties and porosity of the gels were studied using CO2, CO and N2. The Pd-Co based cyanogel was also processed by spin coating and subsequent heat treatment into Pd-Co nanoparticles. Currently, other methods aimed at preparing metal alloy nanoparticles using cyanogels are being explored.

Some of the important instruments for this project are the Thermogravametric Analyzer, Super Critical Fluid Extractor, X-ray powder deffractometer, UV-Visible and Infrared Spectrometers and the microscopes down in PRISM.


Here are some of the important papers that have come out of this project:

147. "The Morphology and Gas Adsorption Properties of the Palladium-Cobalt Based Cyanogels," Rahul S. Deshpande, Stefanie L. Sharp-Goldman, Jennifer L. Willson, Andrew B. Bocarsly, Joachim Gross, Adam C. Finnefrock and Sol M. Gruner, Chem. Mat., 2003, 15, 4239-4246.

137. "Thermodynamics and Kinetics of CO2 Adsorption on Dehydrated Palladium Cobalt Based Cyanogels:  A Highly Selective, Fully Reversible System for CO2 Storage," Rahul S. Deshpande, Stefanie L. Sharp-Goldman, and Andrew B. Bocarsly, Langmuir, 2002, 18, 7694.

133. "Photochemical Image Generation in a Cyanogel System Synthesized from Tetrachloropalladate(II) and the Trimetallic Mixed-Valence Complex, [(NC)5FeII-CN-PtIV(NH3)4-NC-FeII(CN)5]4-: Consideration of Photochemical and Dark Mechanistic Pathways of Prussian Blue Formation," David F. Watson, Jennifer L. Willson, and Andrew B. Bocarsly, Inorganic Chemistry, 2002, 41, 2408.

112. "Formation and Structure of a Tin-Iron Oxide Solid-State System with Potential Applications in Carbon Monoxide Sensing through the Use of Cyanogel Chemistry," Stefanie L. Sharp, Gireesh Kumar, Edward P. Vicenzi, Andrew B. Bocarsly, and Marija Heibel, Chem. Mat. 1998, 10, 880.

109. "Bulk Properties of a Cyanogel Network: Toward an Understanding of the Elastic, Mechanical, and Physical Processes Associated with Sol-Gel Processing of Cyanide Bridged Gel Systems," Stefanie L. Sharp, Andrew B. Bocarsly, and George W. Scherer, Chem. Mat. 1998, 10, 825.

98. "Use of Sol-Gel Chemistry for the Preparation of Cyanogels as Ceramic and Alloy Precursors," Marija Heibel, Gireesh Kumar, Carrie Wyse, Peter Bukovec, and Andrew B. Bocarsly, Chem. Mat., 1996, 8, 1504.

97. "Sol-Gel Processing of Inorganic Cyanogels," Marija Heibel and Andrew B. Bocarsly, ISMANAM-95, The 1995 International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials Proceedings, Quebec, Canada, 1995.

68. "Synthesis of a Novel Hydrogel Based on a Coordinate Covalent Polymer Network," B. W. Pfennig, A. B. Bocarsly, and R. K. Prud'homme, J. Am. Chem Soc., 1993, 115, 2661.