NCER Assistance Agreement Final Report Executive Summary

Date of Final Report: January 1, 2009

EPA Agreement Number: X-83254101-1

Center: Center for Environmental and Energy Research (CEER)

Project Title: Tunneled Titanate Photocatalysts for Environmental Remediation and Hydrogen Generation

Investigator(s): Doreen Edwards and Scott Misture

Institution(s) of PI(s): Alfred University

Research Category: Congressionally Mandated Center

Project Period: September 1, 2006 to February 28, 2008

Description and Objective of Project:

Photocatalytic processes are being used for environmental remediation and have shown promise for the production of clean-burning hydrogen fuel. The continued development of these technologies, particularly those which use solar energy for photoexcitation, will require new and improved photocatalysts. In studies of BaTi₄O₉ and M₂Ti₆O₁₃ (M = Na, K and Rb), other researchers have suggested that structural features such as distorted TiO₆ octahedra and tunnel sites may be responsible for enhanced photocatalytic activity. In this project, six titanate materials with different tunneled structures were investigated in an attempt to understand the structural features that affect photocatalytic activity.

The main technical objectives of this project were 1) to prepare tunneled titanates with different structures (tunnel size and shape) and composition, and 2) to assess their photocatalytic activity. Sample powders of each composition (Na_0.7Ga_4.7Ti_0.3O₈, Na_0.8Ga_4.8Ti_1.2O₁₀, Na_0.8Ga_4.8Ti_2.2O₁₂, K₁Ga₁₇Ti₁₅O_56, K_1.5Ga_1.5Ti_6.5O₁₆, and BaTi₄O₉) were prepared by solid state reaction and characterized to determine their particle size and morphology, surface area, phase purity, and optical properties. Ruthenium dioxide (1 weight percent) was deposited onto the surface of each powder sample using a wet chemical method in an attempt to enhance photocatalytic activity. (Ruthenium dioxide is thought to act as an electron acceptor for photoexcited electrodes and thereby serve as a reduction site for the photocatalytic reactions.) The photocatalytic activity of the powders with and without RuO₂ additions was assessed using two different methods. The first method used optical spectroscopy to test the powders’ activity for decomposing a model organic molecule, i.e. methylene blue. The second test used gas-chromatography to measure the powders’ photocatalytic ability to generate hydrogen from a water-methanol solution.

Summary of Findings:

Six tunneled titanate powders were prepared using solid state methods which involved reacting oxide and carbonate starting powders at 1050-1350 ^oC for up to 48 hours. The particle size and morphology of the powders were highly dependent on processing temperature. Powders processed at the lower temperatures (BaTi₄O₉ and Na_0.7Ga_4.7Ti_0.3O₈) had relatively small (1-2 mm average), spherically shaped particles whereas those prepared at higher temperatures (Na_0.8Ga_4.8Ti_1.2O₁₀, Na_0.8Ga_4.2Ti_2.2O₁₂, K_1.5Ga_1.5Ti_6.5O₁₆, and KGa₁₇Ti₁₅O₅₆) had larger, angular particles (7-25 µm average) with internal pores. As a point of comparison, the surface area of the powders, which ranged from 0.40 to 1.58 m²/g, is relatively low compared to that of commercial photocatalysts like Degussa P25 (>55m²/g). Ruthenium dioxide additions (1 wt%) were made to portions of the six powders in an attempt to enhance photocatalytic activity.

Most of the powders contained some minor impurity phases, which were estimated to be less than 5 weight % of the total sample based on the height of the most intense peak in the primary and minority phases. The one exception to this generalization is the sample prepared as Na_0.8Ga_4.8Ti_1.2O₁₀, which contained significant amounts of Na_0.7Ga_4.7Ti_0.3O₈. Attempts to prepare phase-pure Na_0.8Ga_4.8Ti_1.2O₁₀ using different reaction temperatures and repeated heating cycles were unsuccessful.

Most of the samples were white in color and exhibited distinct transitions from absorption to reflection, allowing the estimation of the band gap. The K_1.5Ga_1.5Ti_6.5O₁₆ sample was gray and has a spectrum significantly different from the other samples, which prevented an estimation of the band gap. With the exception of the hollandite sample, the band gaps of the materials were similar, around 2.9 - 3.0 eV.

All prepared sample powders exhibited some degree of photocatalytic activity. The BaTi₄O₉ powder exhibited the highest activity, decomposing 98% of the methylene blue after 6 hours of irradiation. The K_1.5Ga_1.5Ti_6.5O₁₆, Na_0.8Ga_4.8Ti_2.2O₁₂, and KGa₁₇Ti₁₅O₅₆ sample showed moderate activity, decomposing over 90% of the methylene blue after 8 hours. The Na_0.7Ga_4.7Ti_0.3O₈ and Na_0.8Ga_4.8Ti_1.2O₁₀ sample powders showed marginal activity in that the concentration of methylene blue measured after 8 hours was similar to that measured in the solution tested in the absence of a photocatalyst. As points of comparison, the decomposition of methylene blue in the presence of a commercial photocatalyst was over 99% complete within 30 minutes for a sample of equivalent weight and within 2 hours for a sample of comparable surface area. Reducing the particle size of the K_1.5Ga_1.5Ti_6.5O₁₆ sample from 25 µm to 4 µm improved its photocatalytic activity. For most powders, the addition of RuO₂ had little effect or even decreased their photocatalytic activity. The one exception was found for the Na_0.7Ga_4.7Ti_0.3O₈ powders which showed a notable improvement upon the addition of 1 weight percent (nominal) RuO₂.

A reaction cell was designed and constructed to test the photocatalytic activity of the powders for generating hydrogen from a methanol-water solution under illumination. The RuO₂-loaded BaTi₄O₉ had the highest activity, producing 48 µmol per 0.1 gram of photocatalyst over a 4 hour period. The results measured for BaTi₄O₉ in this study are comparable to those reported previously by other researchers. As a point of reference, an equivalent weight of the commercial photocatalyst, Degussa P25 produced 4.3 µmol after four hours of irradiation. The RuO₂-free and RuO₂-loaded K_1.5Ga_1.5Ti_6.5O₁₆ samples produced 1.4 µmol and 2.4 µmol of hydrogen, respectively. The other tunneled titanate powders produced less than 1 µmol during the 4 hour tests.

Conclusions:

The tunneled titanates showed some promise as photocatalysts for decomposing a model organic molecule and for generating hydrogen from a methanol-water solution. Continued investigations of the materials, and their compositional variations, are warranted. Specifically, different powder processing methods aimed at achieving submicron (high surface area) powders is recommended. Additionally, improved coating methods for achieving controlled distribution of RuO₂ on the surface should be explored.

Publications/Presentations:

S. B. Sanford, J. Ovenstone, S. T. Misture, and D. D. Edwards, Tunneled titanate photocatalysts for environmental remediation and hydrogen generation, Annual Meeting of the American Ceramic Society at Materials Science and Technology, Detroit, MI, September 16-20, 2007

S. B. Sanford, J. W. Amoroso, S. T. Misture, and D. D. Edwards, Screening of Tunneled Titanate Photocatalysts for Environmental Remediation and Hydrogen Generation, Fall Meeting of the Materials Research Society, Boston, MA, 2006

Supplemental Key Words:

tunneled titanates, photocatalysts, photocatalysis, titanium dioxide, hollandite, beta-gallia rutile intergrowths, hydrogen generation

Relevant Web Sites:

http://ceer.alfred.edu