Article pubs.acs.org/est
Local Structure and Speciation of Platinum in Fresh and Road-Aged North American Sourced Vehicle Emissions Catalysts: An X‑ray Absorption Spectroscopic Study Peter W. Ash,† David A. Boyd,† Timothy I. Hyde,*,† Jonathan L. Keating,‡ Gabriele Randlshofer,⊥ Klaus Rothenbacher,# Gopinathan Sankar,*,‡ James J. Schauer,§ Martin M. Shafer,§ and Brandy M. Toner∥ †
Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, United Kingdom Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom § Water Science and Engineering Laboratory, University of Wisconsin-Madison, 660 North Park Street, Madison, Wisconsin 53706-1484, United States ∥ University of MinnesotaTwin Cities, St. Paul, Minnesota 55108, United States ⊥ International Platinum Group Metals Association, Schiess-Staett-Strasse 30, D-80339 Munich, Germany # European Precious Metals Federation, Avenue de Broqueville 12, B-1150 Bruxelles, Belgium ‡
S Supporting Information *
ABSTRACT: Given emerging concerns about the bioavailability and toxicity of anthropogenic platinum compounds emitted into the environment from sources including vehicle emission catalysts (VEC), the platinum species present in selected North American sourced fresh and road-aged VEC were determined by Pt and Cl X-ray absorption spectroscopy. Detailed analysis of the Extended X-ray Absorption Fine Structure at the Pt L3 and L2 edges of the solid phase catalysts revealed mainly oxidic species in the fresh catalysts and metallic components dominant in the road-aged catalysts. In addition, some bimetallic components (Pt−Ni, Pt−Pd, Pt− Rh) were observed in the road-aged catalysts from supporting Ni-, Pd-, and Rh-K edge XAS studies. These detailed analyses allow for the significant conclusion that this study did not find any evidence for the presence of chloroplatinate species in the investigated solid phase of a Three Way Catalyst or Diesel Oxidation Catalysts.
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particulate.13−15 A recent study examined the solid phase metallic and oxidic platinum species on both VECs and from road dust emitted into the environment.16 However, a full speciation including assay for chloroplatinates,17 (a known sensitizer) was not undertaken. Hence, we focused this investigation on the speciation of platinum in authentic vehicle exhaust catalysts operated in real world conditions.18,19 A number of techniques have been used in the literature to determine the structure and speciation of model and industrially relevant catalysts. Among these, X-ray absorption spectroscopy (XAS) has been identified since it is atom-specific and does not depend on long-range order that challenges X-ray Diffraction (XRD).20−22 X-ray photoelectron spectroscopy is also widely used, however, it is limited to the observable near surface species and furthermore studied under high-vacuum conditions; but it has nevertheless provided some useful
INTRODUCTION Platinum group metals (PGM), and in particular, Pt, Pd, and Rh dispersed on high surface area supports are widely used as the active components of emission control catalysts to convert gases such as nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons to N2, CO2, and H2O.1−6 Considerable development of PGM-based catalysts to meet ever more stringent levels demanded by legislation1−6 has brought benefit to the environment from the conversion to more benign gaseous species. Contrastingly, a number of studies have focused on the quantification, distribution, and chemical form of anthropogenic PGMs in the urban and rural environment. Emitted PGM-containing particulates have been observed, probably via mechanisms including mechanical attrition of the vehicle exhaust and other industrial chemical processing, and their presence has led to investigation of impact on human health and living organisms.7−12 While it is highly relevant to investigate occurrence and solubility, it is equally important to precisely characterize the solid phase of the catalysts, which may be responsible for the emitted particles, and determine the likely chemical nature of attrited © 2014 American Chemical Society
Received: Revised: Accepted: Published: 3658
July 25, 2013 January 31, 2014 February 25, 2014 February 25, 2014 dx.doi.org/10.1021/es404974e | Environ. Sci. Technol. 2014, 48, 3658−3665
Environmental Science & Technology
Article
information.23 Similarly, microscopy has been used extensively to determine particle morphology and element mapping.16,24,25 Here we utilized all these techniques and show the structure of fresh and road-aged (driven >30 000 miles) catalysts employed in both diesel and gasoline engines. As catalyst formulations are known to vary regionally to meet local emissions standards, we studied catalytic materials sourced from different North American geographic regions; one predominantly from the central continental area, the other from a coastal environment (higher chloride availability). Both gasoline Three-way Catalysts (TWC) and light and heavy duty Diesel Oxidation Catalysts (DOC) vehicle emission catalysts were studied for this purpose using XAS as the main tool to determine the structure of the unused (fresh) and used (road-aged) catalysts.
Chlorine K-edge XANES measurements were carried out at the Canadian Double Crystal Monochromator beamline of the Synchrotron Radiation Source (SRS), Madison, WI, USA. The ring energy operated at 800 MeV or 1 GeV. Sample powders were ground up and spread thinly and uniformly across a piece of conducting carbon tape which was mounted onto an aluminum plate and placed in front of the beam under ultrahigh vacuum conditions. Cl K-edge XAS measurements were performed in fluorescence mode and using the electron yield technique. In addition, all these materials were also characterized by XPS (Thermo Escalab 250 instrument), transmission electron microscopy (FEI Tecnai F20), and X-ray diffraction (Cu radiation on a Bruker D8 Advance Diffractometer with Göbel mirrors). Details of these measurements are given in the SI. Thermodynamic Modeling calculations were performed using a quad precision build (2898) of the National Physical Laboratory (NPL) MTDATA thermodynamics and phase equilibrium code (02DAV/DIN).29 MTDATA is designed to solve the chemical and phase equilibrium in a very general way using a reliable Gibbs energy minimization algorithm. The thermodynamic data used in calculations were supplied to NPL by Johnson Matthey Technology Centre.
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EXPERIMENTAL SECTION To maintain manufacturer anonymity, catalyst samples were supplied via the Manufactures of Emission Control Association (MECA), Arlington, VA, U.S. Three-way Catalyst samples are denoted as “TWC AC” (coastal) and “TWC ANC” (noncoastal). Road aged diesel samples, both Heavy Duty Diesel (HDD) and Light Duty Diesel (LDD) were sourced similarly and are referred to as “AC” for road aged of coastal origin and “ANC” for road aged of noncoastal origin. Replacement original equipment manufacturer (OEM) parts were sourced for the respective vehicles and utilized as control samples for this characterization study and are referred to as “Fresh”. Total chloride analysis was carried out by Combustion−Ion Chromatography (CIC) on a Mitsubishi AQF-2100H anion analyzer connected to a Metrohm modular ion chromatograph with a Metrohm A Supp 5 250 anion column; amounts of chloride determined are given in in the Supporting Information (SI) Table S2. Samples were digested by performing a lithium tetraborate fusion for full chemical assay. The leach solution was diluted and analyzed on an Agilent 7700 ICP-MS for the full suite of elements. Elements that could not be identified using the ICP-MS, due to interferences, including Li, B, and Pt (a Pt tipped sample interface was used), were analyzed using an Optima 3300RL ICP-OES. Pt L3 and L2 edge X-ray absorption spectroscopic (XAS) experiments were carried out beamline 20-BM at the Advanced Photon Source, IL, U.S. (APS), which operates at a 7 GeV ring energy. A 25-μm Pt foil was used to calibrate the monochromator. The samples were also measured at the Pd K, Ni K, and Rh K edges, for samples where Pd, Rh, and Ni were present in appreciable amounts. These XAS experiments were carried out on beamline B18 at the Diamond Light Source, UK, which operates at 3 GeV ring energy.26 In a typical XAS experiment, part of the monolith was crushed into a power (note that all TWC supplied by MECA were in the form powders) 150 mg of sample was pelletized to a 13 mm diameter disc. The data were collected at ambient temperature in fluorescence mode since the Pt, Pd, Rh, and Ni concentrations are fairly low, using a multielement Canberra solid-state detector XAS data were processed using “Athena” and “Artemis” software for background subtraction and detailed analysis of the EXAFS data, respectively. We analyzed the data taken27,28 from all the possible edges of a given sample and simultaneously refined to obtain a unique best fit between experimental and calculated valuesdetails of the analysis procedure are given in SI.
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RESULTS AND DISCUSSION Laboratory analysis, including XPS, XRD, TEM, and Assay, has been undertaken in support of synchrotron-based analysis to determine chemical speciation in vehicle emissions catalysts (VEC). In Table 1, we show the amounts of selected elements present in the ground catalysts obtained from chemical analysis. Table 1. Selected Elemental Assay Concentrations for Fresh and Road-Aged VEC crushed monolith sample
Pt (ppm)
Pd (ppm)
Rh (ppm)
Ni (ppm)
Cl (ppm)
TWC fresh TWC AC TWC ANC LDD fresh LDD AC HDD fresh HDD ANC
800 700 700 2600 2600 1800 1800
30 25
