Chemical Speciation of Platinum and Vanadium in Mobile Source Emissions and Urban Atmospheres

MARTIN SHAFER (1), Brandy Toner (2), Joel Overdier (1), James Schauer (1)

(1) University of Wisconsin-Madison, Madison, (2) University of Minnesota, Minneapolis

     Abstract Number: 153
     Last modified: November 6, 2009

Abstract
Controlling emissions from mobile sources are critical for continued reduction in health impacts of air pollution, and for addressing regional and global climate impacts. Reduction of regulated pollutants in vehicle exhausts relies on the use of catalytically active metals, e.g. Pt in three-way catalysts (TWC) in gasoline vehicles. Current and proposed strategies for diesel engines also employ metal catalysts to reduce tailpipe emissions of regulated species. The use of Pt in fuel-borne catalysts (FBC) and Pt-catalyzed diesel particulate filters, and the use of V in selective-catalytic reactors are key examples. However, the use of these metals raises concerns about potential environmental contamination and health implications of widespread trace metal dissemination.

The toxicological responses of many metals (e.g. Cr, Mn, Ni, Pt, and V) are determined by the specific chemical/physical speciation in the emissions. For Pt, the primary focus is on soluble and halogenated species, where the threshold exposure limits are 500 times lower than metallic species. For V, the pentoxide species is considerably more toxic than the lower oxidation state species. Unfortunately extant methodologies provide little relevant speciation information, and most analytical tools that are speciation capable lack the required sensitivity, particularly in the context of the lower emissions from vehicles operated with modern control devices.

We have been developing and applying novel tools for the chemical speciation of these trace metals in mobile source PM emissions and urban aerosols, employing a strategy of parallel development of species selective wet-chemical approaches and direct solids speciation using synchrotron x-ray absorption (XAS) techniques. The wet-chemical extraction approaches for Pt and V address the regulatory construct of “solubility”, and further advance our understanding by providing information on specific chemical species (e.g. soluble halogenated Pt and soluble V+5 species). The synchrotron tools complement the wet-chemistry techniques in that they can provide information on the total pool of a specific species directly from the PM solids.

The method developed for speciation of soluble V is based upon oxidation state selective retention of V-species on an immobilized ligand (Chelex). The method exhibits good selectivity for the target redox species (V(IV) and V(V)), and we demonstrated robust performance at exceptionally low total vanadium masses (50-100 pg). Significant V(V) fractions were observed in engine PM, with higher V(V) fractions measured in PM from an engine fitted with a vanadium-based SCR. Relatively small V(V) fractions were measured in extracts of roadway dusts and ambient aerosols. Applying a suite of speciation tools (e.g. ultrafiltration, ion chromatography) to extracts of PM from diesel engines burning a Pt-FBC, revealed small, but potentially relevant levels of soluble anionic Pt species.

High spatial resolution synchrotron-XRF mapping, coupled with XANES/EXAFS spectroscopy has enabled us to identify several of the species of concern in samples of engine PM. Particles mapped as enriched in total V were shown to be primarily V2O5. Significant fractions of oxidized Pt were determined in samples of used TWCs and in PM from engines running with a Pt-FBC.