Estructura De Rutilo Dopado

Chem. Mater. 2009, 21, 1627–1635

1627

Tungsten-Doped Titanium Dioxide in the Rutile Structure: Theoretical Considerations
Masoud Aryanpour,†,‡ Roald Hoffmann,*,‡ and Francis J. DiSalvo‡
Department of Geosciences, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York14850 ReceiVed February 4, 2009. ReVised Manuscript ReceiVed March 5, 2009

Tungsten-doped titanium dioxide has the potential to replace conventional carbon black in catalyst support applications. In this paper, structural and electronic properties of W-doped rutile are theoretically studied. Lattice parameters as well as W-W pairing in models for a range of doping are calculated and match wellwith the experimental results. W doping leads to in-plane expansion and c-axis contraction in the rutile structure. The pairing of tungsten atoms is a Peierls-type distortion, resulting in less cluttering of bands around the Fermi energy. Our computational finding of paired structures as energetically more stable is in agreement with the literature on similar systems. W-doped rutile is conducting atboth high and low doping levels; the states involved near the Fermi level are predominantly W(5d).

1. Introduction Our unabated demand for energy has motivated much research and industrial investment to improve and/or replace conventional combustive power sources. The shift to alternative sources is accelerated by pollution issues and environmental considerations.1,2 As noncombustive means ofpower generation already exist, the grand current task is to reduce the overall cost of both power production and maintenance with these methods. Most promising alternative energy sources, such as fuel cells and batteries, aim to directly convert chemical energy into electrical energy. At the heart of the challenges facing us lies the need for better materials that could increase overall energyproduction through improved chemistry based on their superior functionality. Focusing on the electrodes of proton-exchange membrane (PEM) fuel cells, there is room for improvement of both the catalyst and its support. For the catalyst, one wishes to reduce the overpotential of the oxygen reduction reaction, and to use materials cheaper than platinum-based catalysts. The catalyst support must fulfillother requirements:2 it must (i) be conductive by more than 1 S/cm, (ii) be stable up to at least 1.5 V with respect to the standard hydrogen electrode, and (iii) be stable in the acidic, low pH environment of PEM fuel cells. At least in automotive applications of PEM fuel cells, carbon black is currently the main material for catalyst support. Although showing the primary features of a goodsupport, carbon is only kinetically persistent, not thermo* Corresponding author. E-mail: rh34@cornell.edu. † The Pennsylvania State University. ‡ Cornell University.

dynamically stable, under fuel cell operating conditions. It corrodes at low pH and high electrode potential according to the following redox reaction C + 2H2O f CO2 + 4H+ + 4eE0 ) 0.2 V (1)

(1) Carrette, L.; Friedrich, K. A.;Stimming, U. Fuel Cells 2001, 1 (1), 320–324. (2) Mathias, M. F.; Makharia, R.; Gasteiger, H. A.; Conley, J. J.; Fuller, T. J.; Gittleman, C. J.; Kocha, S. S.; Miller, D. P.; Mittelsteadt, C. K.; Xie, T.; Yan, S. G.; Yu, P. T. J. Electrochem. Soc. Interface 2005, 14 (3), A970-A977.

On the basis of experimental measurements and a simple model, Mathias et al.2 showed that at 0.9 V and after severalthousand hours, 5% of carbon is lost in a standard Pt/C electrode. Similar studies estimate 15% carbon weight loss at 1.2 V and after only 20 hours of operation. This much weight loss is not tolerable in automotive applications, because more than 5% loss leads to an unacceptable decrease in the cell performance. The same authors also studied a sample of 30% Pt alloy on graphitized carbon as the...