Three-dimensional distribution analysis of platinum,
palladium and rhodium in auto catalytic converters
using imaging-mode laser-induced breakdown
spectrometry
Patricia Lucena, J. Javier Laserna
UDepartment of Analytical Chemistry, Faculty of Sciences, Uni¨ersity of Malaga, E29071 Malaga, Spain´ ´
Received 4 August 2000; accepted 9 November 2000
Abstract
Ž .
Laser-induced breakdown spectrometry LIBS is reported here as an effective technique to describe the volume distribution of platinum, rhodium and palladium in catalytic converters installed in motor vehicles. Using the second harmonic output of a Nd:YAG laser and a CCD-based atomic emission spectrometer, LIBS is used in multielemen-tal, imaging-mode to permit the simultaneous analysis of the several elements present in the converter, including the internal standard. The data are reported with a lateral resolution of 1.75 mm over a fresh catalytic structure which is
Ž .
128 mm long. The concentrational variability of the platinum group metals PGMs varies in the range ;3᎐23% relative standard deviation depending on the element, the substrate and the direction investigated. The causes of the dispersion observed are discussed.䊚2001 Elsevier Science B.V. All rights reserved.
Keywords: Platinum; Palladium; Rhodium; Auto catalytic converters; Laser-induced breakdown spectrometry
1. Introduction
Air contamination by emissions from the oper-ation of car engines has been of concern in many countries for over 30 years. Massive automobile traffic in highly populated areas leads to the release of vast amounts of carbon monoxide, nitrogen oxides and uncombusted hydrocarbons
UCorresponding author. Fax:
q34-5-213-2000.
into the atmosphere. A range of mature analytical technologies are available for monitoring their primary emissions and atmospheric
transforma-w x
tions 1,2 , and a legal framework exists to cope with emission standards in most industrialized countries.
Car manufacturers claim that emissions of these contaminants can be reduced to almost negligible levels by improvements in the combustion engi-neering of the fuel᎐air mixture. However, it is also recognized that some time is still needed for
0584-8547r01r$ - see front matter 䊚2001 Elsevier Science B.V. All rights reserved.
Ž .
the technology to mature and to comply with the current emission legislation without significantly penalizing the fuel consumption and the drive-ability of the vehicle. In the interim, the best known remedial action is to fit cars with catalytic converters. These devices are capable of catalyz-ing the transformation of these compounds into less harmful chemicals such as carbon dioxide, nitrogen and water. The active components in charge of the catalytic action belong to the
plat-Ž .
inum group metals PGMs , namely platinum, palladium and rhodium, which under optimal engine and catalyst operation, are capable of re-moving more than 90% of the pollutants from the
w x exhaust gases 3 .
Unfortunately, evidence now exists that sig-nificant amounts of PGMs are being released continuously to the environment adsorbed on small particles as a result of surface abrasion of the catalytic structure during car operation. At the beginning there was only a vague knowledge of the release levels, but with the advance in surveillance programs and monitoring techniques, the situation is becoming increasingly of concern. Release trends for platinum of the order of sev-eral tens of ng per km are high enough to create
w x
a new environmental challenge 4 . Indeed, solu-tions are being sought for understanding the emission pathways, for assessment of the associ-ated risks to the human health, and for develop-ing reliable analytical techniques for gaindevelop-ing in-formation from the relevant environmental loca-tions. A carefully planned effort to accumulate the relevant information available has been
re-w x cently published 5 .
Perhaps the most neglected aspect in the effort to assess the environmental contamination by PGMs has been the technical basis for the release of these elements from their origin, i.e. the cat-alytic converter. The development of an ancat-alytical technology capable of describing the distribution of the PGMs in the converter before and after engine operation is required for understanding both functional issues and release trends. Unfor-tunately, the measurement capabilities for the PGMs in the converters are limited as a result of the complexity of the converter structure. The
configuration of a catalytic converter involves a monolith prepared by extrusion of cordierite to form a three-dimensional honeycomb structure. This material exhibits a large number of chan-nels, through which the engine exhaust gases flow and where fast oxidation᎐reduction reactions cat-alyzed by the PGMs occur. PGMs are incor-porated into the cordierite walls by impregnation with a washcoat to form a thin coating on the exposed internal surfaces through the whole structure. The particular chemical composition and the PGM load vary depending on the manu-facturer, but in all instances a homogeneous dis-tribution in the PGM load is pursued to ensure consistent catalytic conversion. From this descrip-tion it is clear that the technical challenges to meet the analytical description of the PGMs in a converter are substantial. Surface analysis tech-niques have been used for examining the catalyst surface including transmission electron micros-copy, X-ray diffraction, X-ray photoelectron spec-troscopy and low-energy ion scattering
spectro-w x
scopy 6,7 . However, when trying to characterize the catalyst converters these techniques are not practical because of several experimental prob-lems due to the extensive sample preparation needed, to the small sampling areas examined, to the presence of charging effects and to the dif-ficulties with handling large samples in high vac-uum environments.
This paper addresses some of the important considerations in designing and implementing a monitoring technique for converters based on laser ablation, with particular attention to surface and volume distribution of the PGMs. The capa-bilities of LIBS for surface-oriented analytical tasks have been documented in a number of
w x
reports 8᎐13 . Depth profiling applications of LIBS for characterization of layered materials, w14᎐16 and LIBS surface imaging of impuritiesx w x on silicon substrates have been reported 17,18 . LIBS is used here in multielemental, imaging-mode to permit the simultaneous analysis of the several elements present in commercial convert-ers, and to provide a general picture of the dis-tribution of the PGM load across the catalyst structure.
2. Experimental section
The experimental set up used for LIBS study consisted of a pulsed Q-switch Nd:YAG laser
Ž
working at 532 nm Continuum, model Surelite .
SLI-20, pulse width 5 ns focused on the sample at normal incidence at room temperature, in air at 760 torr, using a planoconvex glass lens with a focal length of 100 mm and f噛4. Laser plasma emission was collected with a planoconvex quartz lens with a focal length of 100 mm and f噛4 onto the entrance slit of a triple indexable
Ž
Czerny᎐Turner spectrograph Chromex, model .
500 IS, focal length 0.5 m fitted with classically ruled gratings of 300, 1200 and 2400 grooves mmy1. The reciprocal linear dispersion of the spectrograph was 2.5 nm mmy1 using the 2400 grooves mmy1 grating, giving a spectral coverage of approximately 15 nm for the detector used. The spectral resolution of the system was 0.02 nm pixely1. The spectrograph entrance slit was 10 mm high and 10 m wide. The plasma axis was normal to the entrance slit.
The plasma-dispersed light was detected with a solid-state two-dimensional charge-coupled
de-Ž
vice CCD; Stanford Computer Optics, model .
4Quik 05 equipped with an intensifier system. Ž . Ž .
The CCD consists of 768 h =512 v elements. The photoactive area is 6.4=4.5 mm2. The sync output signal of the laser was used as trigger signal to control the delay time. The dispersed image of the plasma was then integrated and binned in the vertical direction to produce a single spectrum. The spectra were corrected by subtraction of the dark current of the detector, which was separately measured for the same ex-posure time. Operation of the detector was con-trolled by 4 Spec 1.20 software.
Samples were placed on a manual x᎐y᎐z trans-lation stage to be moved with respect to the laser beam. The catalyst reported in this paper corre-sponds to a gasoline fueled engine supplied by Ford Espana S.A. It consists of two monolithic
˜
substrates, the front and the rear substrates in the gas flow direction. The front substrate con-tains Pt and Rh and has a precious metal loading of 2472 g my3 with a weight ratio of 5Pt:1Rh. The rear substrate contains Pd and Rh and has a precious metal loading of 2119 g my3 with a weight ratio of 9Pd:1Rh. Each substrate had an
Fig. 1. Infographic representation of a quarter of a catalytic substrate cut in five pieces. The five areas mapped and the coordinate origin used for image reconstruction are indicated.
ellipsoidal shape with 80 mm=1700 mm for the minor and major axes and a length of 64 mm. Other characteristic data of the converter are: maximum specific area 22500 m2; minimum speci-fic area 7900 m2; 62 cells cmy2; shell material-stainless steel. A Teflon blanket is used as a thermal insulator of the front and rear sub-strates. Once the exhaust pipe was opened, the substrates were carefully extracted. Both subs-trates were cut in quarters, and only one quarter in each substrate was analyzed. Then, each quar-ter was cut in five sections with one of the faces in each section being analyzed for imaging pur-poses. Fig. 1 shows a picture of the resulting sections with the analyzed faces labeled as map 1᎐5 and the coordinate origin used to reconstruct the spatial distribution of the PGMs within the samples. When cutting the substrates care was taken to avoid mechanical stress that would remove the washcoat from the cordierite matrix.
3. Results and discussion
3.1. Spectral description
Due to the dispersive characteristics of the spectrometer used the analysis of the converter required investigation of emission in two spectral regions, namely, 209᎐225 nm used for Pt, and 336᎐351 nm used for Pd and Rh. Fig. 2 shows the spectra corresponding to the front substrate in both regions. The spectra shown correspond to a single laser shot in the washcoat and in the cordierite matrix. Fig. 3 shows the corresponding spectra in the rear substrate. The most important lines are denoted in the spectra. In Fig. 2, the Pt line at 214.42 nm is observed in the front wash-coat, while there is no evidence of platinum in the
Ž .
rear substrate Fig. 3 . Concerning palladium, emission lines were observed in both regions only
Ž .
from the rear substrate Fig. 3 . The Pd line at 340.46 nm was used for analytical purposes. Sev-eral Rh emission lines were observed in the Pd᎐Rh region from both the front and rear sub-strates. Rh lines at 339.68 nm and at 343.49 nm were used as analytical lines. It is apparent that different additives were used in the washcoat of
Fig. 2. Single-shot LIB spectra from the front substrate of the converter. The plasma light was separated in two spectral regions corresponding to Pt and Pd-Rh. The spectra belong to
Ž . Ž .
the washcoat top and cordierite bottom . The pulse energy was;40 mJ. The integration time was 1s.
the front and rear substrates. The spectral studies in the Pt and Pd᎐Rh regions reveal that Zr is present in the front washcoat, characterized by a metallic gray color, while Ce is the basic additive in the rear substrate, which shows a yellow color. By comparison with the LIBS spectra of metal oxides of the corresponding elements, most lines in the washcoat spectra shown in Figs. 2 and 3
Ž .
could be assigned to these additives Zr and Ce with only the lines used for internal
standardiza-Ž .
tion see below being labeled. No lines corre-sponding to Zr were observed in the rear subs-trate. The spectra of the front and rear cordierite matrix are identical and only emission lines from
Ž .
the matrix elements Si, Al, Mg, . . . were observed. Some lines appearing in the spectra could not be assigned to elements known to be present in the converter.
Experimental conditions for the spectral study were selected according to the ablation behavior of the material. Due to the loose nature of the
Ž y1
washcoat high laser energy approx. 40 mJ pulse y2.
equivalent to ;41 J cm was required. How-ever, the laser could not be focused on the
sam-Fig. 3. Single-shot LIB spectra from the rear substrate of the catalytic converter. The plasma light was separated in two spectral regions, Pt and Pd-Rh. The spectra belong to the
Ž . Ž .
washcoat top and cordierite bottom . The pulse energy was
;40 mJ. The integration time was 1s.
ple surface as this energy would cause the drilling of the converter wall with a single laser shot. Instead, the beam was focused 6 mm into the sample surface to maintain the sampling depth within the dimensions of washcoat. The timing of the data acquisition had also to be adapted to the
Ž spectral region studied. A short delay time 300
. Ž
ns in the Pt region and a larger delay time 5500 .
ns in the Pd᎐Rh region were found priority conditions for the analysis of the washcoat PGMs.
3.2. Distribution maps and three-dimensional analysis
To attain a general view of the PGMs distribu-tion in the converters, several distribudistribu-tion maps were obtained at selected substrate locations. Each substrate was longitudinally cut in quarters. If the assumption is made that the resulting four pieces had representative elemental composition, the analysis of one quarter would suffice for de-scribing the PGMs distribution in the entire sub-strate. Fig. 1 shows a schematic view of a quarter
of a substrate that has been physically sectioned in five pieces for mapping purposes. LIBS maps from the washcoat in the top part of each piece were obtained for platinum and rhodium in the front substrate, and for palladium and rhodium in the rear substrate. Consequently, twenty maps were obtained from the converter.
To reduce statistical fluctuations of the LIBS signals derived from variations in the amplitude of the laser pulse and in the coupling efficiency with the surface, internal standardization had to be used for developing the distribution maps. In principle, aluminum was considered an excellent candidate internal standard as this element is a major component of the washcoat. Unfortunately, Al shows no sensitive emission lines in the Pt and Pd-Rh spectral regions and alternative internal standards had to be used. The additives Zr and Ce were then evaluated first checking their subs-trate homogeneity using in this case Al as the standard. The emission lines used were the
fol-Ž . Ž .
lowing: Zr II at 267.86 nm to Al I at 266.03 nm;
Ž . Ž .
and Ce II at 311.63 nm to Al I at 309.28 nm. The ZrrAl and CerAl intensity ratios in selected sampling locations exhibited relative standard de-viation values of ;5%, a level compatible with the precision expected in LIBS. Consequently, Ce and Zr can be considered to be homogeneously distributed, at least to the level of signal variabil-ity observed in LIBS.
The intensity ratios of the lines Pt 214.42rZr 209.70, Rh 343.49rZr 339.20, Pd 340.46rCe 348.51, and Rh 339.68rCe 348.51 were monitored for mapping Pt and Rh in the front substrate and Pd and Rh in the rear substrate, respectively.
Fig. 4 is provided as an example of the location map 1 in Fig. 1. Surface distributions of Pt and
Ž Rh in the front substrate are shown in Fig. 4 top
.
panel . The surface is analyzed in alternate sets of three channels each three or four channels. Thirty channels were analyzed for Pt and 29 for Rh. In each channel 36 uniformly spaced locations were sampled in the longitudinal axis with a lateral resolution of 1.75 mm. To integrate the PGM contribution from the full washcoat, the signal from four laser shots was accumulated in each sampling location. A total of 8496 shots were necessary to generate the maps. This large
num-Fig. 4. LIBS maps of Pt, Pd and Rh in a converter corresponding to map 1 in num-Fig. 1. The Pt 214.42rZr 209.70, Rh 343.49rZr 339.20, Pd 340.46rCe 348.51, Rh 339.68rCe 348.51 intensity ratios were used for mapping the Pt and Rh distribution in the front substrate and the Pd and Rh distributions in the rear substrate, respectively.
Fig. 5. Three-dimensional LIBS image of the spatial distribution of palladium in the substrate of a fresh gasoline catalyst. The Pd 342.13rCe 348.51 intensity ratio was used for mapping the Pd distribution in the rear substrate.
Table 1
Ž .
Precision as relative standard deviation, R.S.D. of LIBS signals of Pt and Rh in the front substrate
Ž . Ž .
Map Longitudinal R.S.D. % Radial R.S.D. %
Ž . Ž . Ž . Ž .
Pt 214.42 nm Rh 343.49 nm Pt 214.42 nm Rh 343.49 nm
Žns36. Žns36. Žns30. Žns29.
1 3.3 15.8 4.4 18.9
2 4.7 16.8 8.1 18.7
3 4.1 17.5 4.7 18.9
4 4.7 18.2 5.7 22.1
5 4.0 16.1 4.3 17.4
ber of laser shots was needed since Pt and Rh are analyzed in different spectral regions. For display purposes, the limiting values in the color scale correspond to the intensity range for each image. The remaining values span the complete scale in equal intervals.
Ž .
Fig. 4 bottom panel shows the distributions of Pd and Rh in the rear substrate. Since palladium and rhodium are analyzed in the same spectral region, only thirty channels were needed to image the rear substrate. Consequently, the resulting data set is half the size of that in the front substrate, while it retains the same level of lateral resolution.
As shown in the maps, Pt seems to be regularly spaced and it also appears the most uniform of the three metals. In the front substrate Rh seems to be heterogeneously distributed in the surface without any trend in the longitudinal or radial direction. In the rear substrate, the distribution of both PGMs is heterogeneous. Only where the gas flow contact the substrate, i.e. in the first few millimeters of the longitudinal axis, the Pd and
Rh content is larger than at the end of the rear substrate i.e. close to the end of the channels. This effect may result from the converter manu-facturing, in which the washcoat is stuck on the monolith by soaking the structure in vertical
posi-w x tion 19 .
In order to have a quantitative evaluation of the PGMs distribution along the substrates, the signal precision was computed from the map data sets. Tables 1 and 2 summarize the numeric val-ues of the longitudinal and radial PGM distribu-tions in terms of mean relative standard deviation ŽRSD% . The data in Table 1 confirm the results. in the maps and indicate that while Pt is
dis-Ž .
tributed homogeneously R.S.D. ;5% , the dis-Ž
tribution of Rh is less homogeneous R.S.D. in .
the range 15.8᎐22.1% . In the rear substrate, R.S.D. values for Pd range between 12.9% and 18.5% and for Rh between 16.6% and 22.7%. It should be noted that the level of signal variability for the three PGMs is of the same order when computed from the longitudinal axis or from the radial direction. The precision data quoted above
Table 2
Ž .
Precision as relative standard deviation, R.S.D. of LIBS signals of Pd and Rh in the rear substrate
Ž . Ž .
Map Longitudinal R.S.D. % Radial R.S.D. %
Ž . Ž . Ž . Ž .
Pd 340.46 nm Rh 339.68 nm Pd 340.46 nm Rh 339.68 nm
Žns33. Žns33. Žns30. Žns30.
1 12.9 16.6 13.2 18.7
2 16.6 21.1 17.4 22.7
3 14.6 18.8 15.2 20.4
4 15.9 21.1 18.5 22.4
reflect both the spatial distribution of the compo-nents and the LIBS response to experimental variables. The levels observed for Pt are compati-ble with the precision expected in LIBS, so this element can be considered homogeneously dis-tributed in the substrates, while the excess vari-ability observed for Pd and Rh can be ascribed to a heterogeneous distribution.
Once the five location maps were processed, a 3D image for each PGM in the converter was reconstructed. Fig. 5 shows the image obtained for palladium in the rear substrate. The five maps arranged as they are located in the space within the converter are shown. In this figure the cat-alytic structure has been given the property of transparency to show the distribution maps that lie inside the converter. The distance between the planes is 10 mm. Interpolation between the planes could be done linearly to increase the visual acceptability of the image. However, no interpola-tion between secinterpola-tions was applied to avoid creat-ing an apparent distribution. The lateral resolu-tion in the gas flow direcresolu-tion was 1.75 mm. Con-sidering that the size of the converter is quite large, this level of lateral resolution is adequate for its purpose. The images obtained for platinum and rhodium are similar in appearance to that shown in Fig. 5 for palladium and the figures show a fairly realistic picture of the PGM dis-tribution in the converter. The morphological information of the converter, the analysis proce-dure and the visualized images have been ani-mated in avi video format for personal computers by the authors.
4. Conclusions
Catalytic converters are periodic structures whose functional activity depends critically on the spatial distribution of PGMs across the full speci-men. Due to the complex structure of the con-verters attempts to describe this distribution us-ing conventional wet chemistry techniques seems unpracticable. Also, due to the non-conducting nature of the ceramic materials involved and the large size of the converter this sample seems not amenable for ion spectroscopies which are prone
to charging effects and work in a high vacuum environment. However, LIBS has been demon-strated to work comfortably with these materials and it is capable to provide a viable solution for these difficulties. The converter is described in full using an analytical protocol which involves physical sectioning the converter in pieces of ap-propriate size, obtaining the spectral information from defined locations in the exposed surface, and reconstructing selective chemical images of the PGMs using a suitable software package. The images represent only relative concentration information and provide no data on the absolute concentration levels found in the converters. However, the main advantage of the approach consists of a process capability compatible with the sample size. Thus the complete structure can be mapped to real dimensions. Furthermore, the capability of working at atmospheric pressure allows much better access to the sample during analysis, which is particularly useful when large samples such as catalytic converters need to be analyzed.
Acknowledgements
Research supported by the European Union under the Environment & Climate Program ŽCEPLACA Project, contract ENV4-CT97-0518 ..
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