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Nonconventional Technologies Review 2013 Romanian Association of Nonconventional Technologies Romania, June, 2013

INVESTIGATING THE QUALITY OF CATALYTIC CONVERTERS MICROSTRUCTURE BY X-RAY DIFFRACTION AND X-RAY FLUORESCENCE

SPECTROMETRY

Ilie Sorin 1, Miuţescu Adrian, Nicolae Viorel 2 and Crivac Gheorghe 3 1 University of Piteşti, Romania, sorin.ilie@upit.ro

2 University of Piteşti, Romania, viorel.nicolae@upit.ro 3 University of Piteşti, Romania, gheorghe.crivac@upit.ro

ABSTRACT: In order to estimate the deterioration level of a catalytic converter, is of great importance to know the quality of its microstructure. In this paper, we propose to study the quality of this microstructure by X-ray diffraction and X-ray fluorescence spectrometry, being presented a case study also. After carrying out the elemental chemical and microstructure analysis of the samples prelevated from three catalysts (standard, deteriorated and functional) through mentioned methods, it is concluded that by coupling the two complementary techniques (X-ray fluorescence spectrometry and X-ray diffraction) is obtained a logical, comprehensive and effective method for catalyst analysis, especially in cases in which the goal is the recovery of precious metals from used catalytic converters. KEY WORDS: catalytic converter, X-ray diffraction, X-ray fluorescence spectrometry, elemental chemical analysis, microstructure analysis, qualitative phase analysis, quantitative analysis

1. INTRODUCTION

From the need to protect the environment by reducing pollution from automobile engine emissions, were enacted more stringent regulatory limits on the quantities of pollutants discharged into the atmosphere, and manufacturers were obliged to improve the post-combustion treatment systems of exhaust gases. Multifunctional catalyst systems have been developed for the simultaneous oxidation and reduction of the three types of pollutants: carbon monoxide, unburnt hydrocarbons and oxides of nitrogen.

The deterioration level of the catalytic converter directly contributes to the worsening depollution performance of its and has direct influence on engine operation. Knowing catalyst deterioration mechanisms can help manufacturers to design and produce more efficient catalytic converters, with longer life and from which, subsequently decommissioning, can be recovered the precious metals deposited onto the catalyst support (rhodium, platinum, palladium).

2. THE DETERIORATION OF CATALYTIC CONVERTERS

The deterioration of catalytic converters and worsening of its operation represents a very important issue because emission standards require that performances to be maintained for at least 80000 km.The causes of catalysts deteriorations are thermal effects combined with poisoning with

exhaust gases contaminants. In the specialized literature, there are provided a series of researches carried out in order to substantially reduce the deterioration of the catalyst. If the causes of deteriorations are eliminated, it is used a smaller amount of noble metals in manufacturing process of catalysts, and the engines of vehicles may be calibrated for a greater fuel economy.

Except in cases of malfunctioning of catalytic converter (non-functioning of oxygen sensor, faulty ignition, excessive oil consumption, etc.), there are several mechanisms underlying the decrease and even loss of performances of the catalyst in time [1], [2]:

• Chemical mechanisms: poisoning (adsorption or reversible reaction on/with catalyst surface); inhibition (selective and reversible adsorption of partial harmful compounds); restructuring of the active surface of the catalyst due to its poisoning; physical and chemical blockage of the porous structure of the catalyst;

• Thermal mechanisms: surface sintering and decrease of active species dispersion; synthesis of alloys in the active area; support modification; interaction between base metals and precious metals; interaction between metals or oxides and secondary support; surface orientation of precious metals, metals volatilization;

• Polluting mechanisms: catalyst coking; • Mechanical mechanisms: thermal coking;

physical destruction.

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3. X-RAY DIFFRACTION AND X-RAY FLUORESCENCE SPECTROMETRY

X-rays are electromagnetic radiations with wavelengths ranging from~10 to 0.1 nm and associated energies of 0.125 to 125 keV produced by the rapid deceleration of high-energy charged particles (electrons, protons, ions, etc.) and/or by the transitions of electrons from the inner orbits of atoms. X-radiation used in diffractometry and fluorescence spectrometry arises in X-ray generating tubes through one of following mechanisms:

• rapid deceleration (instantaneous) of some high energy charged particles (electrons, protons, ions, etc.);

• following the de-excitation of atoms of a target, whose electrons from electron-classes K, L or M were initially excited (collision with charged particles or photons X or ϒ of high energy).

3.1 X-ray diffractometry

The diffraction of X-radiation on crystals has been explained by Bragg in terms of a "reflection" of X-radiation on a set of parallel atomic planes. Figure 1 presents a sectional view through a series of parallel atomic planes of indexes ( )hkl , separated by interplanar distance hkld . On this atomic planes falls, at an angle of incidence θ , a beam of X-radiation perfectly parallel and perfect monochromatic.

Figure 1. The reflection of X-radiation on the crystalline

planes (hkl) [3]

Incident X-rays are coherently scattered by atoms in all directions; for certain directions, coherently scattered X-rays are in phase and they will give birth through their constructive interference, to a diffracted X-radiation. Diffraction occurs when Bragg's law is satisfied:

λ⋅=θ⋅⋅ nsind2 hkl (1)

The recording of the diffraction phenomenon is carried out in diffractometers by the help of the X-radiation detectors. The dependence of the intensity

of recorded radiation by the detector on the diffraction angle is called diffraction spectrum, and the spectrum acquisition can be performed in a continuous or step-by-step manner. The main advantage of step-by-step acquisition consists into the precise determination of the angular positions of diffraction lines (diffraction maxima) and measuring the intensity for any angle. The disadvantage of this method is due to the fact that the acquisition of the diffraction spectrum is not carried out simultaneously for all the angles, but in sequence, in step-by-step regime. This requires a very good stabilization of the diffractometer parameters, for the intensity of the incident X-radiation beam to be constant throughout the entire acquisition, and also the counting and recording installation of diffracted X-radiation to provide a very high stability in operation. 3.2 X-ray fluorescence spectrometry

The analysis by X-ray fluorescence spectrometry is one of the most advanced instrumental methods for determining the chemical composition of a sample. This is a non-destructive, fast and accurate analysis method.

The physical principle of the method consists of: an X-ray beam of high intensity (primary beam) that reaches on the sample produces an excitation of its atoms; the return of excited atoms to the fundamental state is achieved by characteristic X-ray emission; this characteristic X-ray emission is called fluorescence or secondary emission of X-rays and it is characteristic for each element.XRF (X-Ray Fluorescence) allows the determination of chemical elements in a sample, with a detection limit of tenths of ppm. 4. THE CORRELATION BETWEEN THE

DEGRADATION STATE OF CATALYTIC CONVERTER AND THE CONTENT OF CONTAMINANT ELEMENTS - EXPERIMENTAL INVESTIGATION

The samples were analyzed using the spectrometer Spectro Midex M, in the following configuration: 30 W-Molybdenum X-ray tube, generating a spot of 2 mm diameter, and SDD type detector, with a resolution of 160 eV on the line Mn-Kα [3], [4]. Spectro Midex M (Figure 2) is a multi-functional spectrometer that allows nondestructive analysis of very small liquids or solids samples (or very small portions of the sample), by X-Ray Micro-Fluorescence (XRMF).This represents an ideal tool for the study of inclusions in various materials or in the cases in which is desired the study the

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distribution of the various elements on the surface of the sample.

Figure 2. General view of the spectrometerSpectro Midex M

There were analyzed three samples of catalytic converters (Table 1) [3], [4]. For each scan were acquisitioned two fluorescence spectra [3], [4]. The acquisition conditions were as follows:

a) direct excitation in Helium controlled atmosphere, slot of 1 mm, operating voltage: U=44.70 kV, current intensity: I=0.30 mA, acquisition time: 180 s, the number of energetic channels used: 2048;

b) direct excitation in Helium controlled atmosphere, slot of 2 mm, operating voltage: U=18.87 kV, current intensity: I=0.30 mA, acquisition time: 300 s, the number of energetic channels used: 1024.

Table 1. Analyzed samples [3], [4] Number Sample code Description

1. standard catalytic converter NS 247 2. deteriorated catalytic converter NS 247, 17000 km 3. functional catalytic converter NS 247, 17235 km

X-ray diffraction measurements were performed on a Rigaku Ultima IV system (Cu-Kα radiation), equipped with parallel optics and plane graphite monochromator in the diffracted beam (Figure 3). The divergence angle of the beam emitted through the multilayer mirror is about 0.05°. This type of optics is preferred due to the strong intensity of the beam and significant reduction of instrumental errors (the position of diffraction lines is not out of phase, the shape and the width of diffraction line profiles are preserved even at high inclination angles).

Figure 3. General view of the diffractometerRigaku Ultima IV

For qualitative phase analysis, X-ray diffraction spectra were acquired in Bragg-Brentano geometry,

in the angle range 2θ of 15°- 53°, with the step of 0.05° and counting time of 5 s per step.

4.1 The results of analysis by X-ray fluorescence spectrometry

Following overlapping the spectra of the three types of samples, it is noticed the presence of elements Zn and Mn only on the fluorescence spectrum of deteriorated catalyst (the 4_deteriorated_(1) spectrum) (Figure 4). The quantitative analysis confirmed a concentration of 0.097% Zn only for deteriorated catalyst, as well as an increase in the concentration of Mn of from 0.04% in the standard sample to 0.21% in the deteriorated catalyst. The two elements unique identified on fluorescence spectrum of deteriorated catalyst can act as poisoning elements [3], [4].

The catalytic converters are poisoned by impurities coming from the fuel and motor oil. Even very low amounts of impurities, 0.1 wt-% or less, are sufficient to cover the active sites and decrease the performance of the converter. Phosphorus, Zinc, Calcium and Magnesium are contaminants commonly found in lubricating oils. These elements or their compounds accumulate on the catalyst surface, inhibiting their activity. A number of studies have demonstrated the combined effect of Zinc and Phosphorus on catalyst deactivation, at low temperatures of exhaust gasses

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is generally formed the Zinc Pyrophosphate (Zn2P2O7).

The quantitative results are shown in Table 2 [3], [4].

Figure 4. Overlapping the energy dispersive x-ray fluorescence spectra for the three types of catalysts [3], [4]

Table 2. The quantitative results [3], [4] aStandarda ADeteriorateda aFunctionala

4.2 The results of analysis by X-ray diffraction

Figure 5 presents the X-ray diffraction spectra corresponding to the three samples analyzed.

Qualitative phase analysis of catalytic converters indicated the presence of hexagonal phase of cordierite - indialite in all analyzed samples.

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This crystalline phase forms the basic ceramic material of catalytic converter, with honeycomb shape, being a material with high mechanical and chemical resistance, low coefficient of thermal expansion and low dielectric constant.

In the case of deteriorated converter (placed upper in Figure 5), can be observed diffraction lines

corresponding to zirconium oxide (marked with *). This structural variation of the catalytic converter can be explained by sintering and coalescence of the metal oxide particles at high temperature of the operation. Zirconium oxides are used to improve catalyst efficiency due to their ability to retain oxygen [3], [4].

Figure 5. The qualitative phase analysis of catalytic converters [3], [4]

There weren't identified any diffraction lines corresponding to some crystalline phase to show catalytic converter poisoning or impairment of structure (e.g. phosphates or sulphates, lead compounds etc.). 5. CONCLUSIONS

The use of two complementary techniques (X-ray fluorescence spectrometry and X-ray diffraction) is an effective way to approach the elemental chemical and microstructural analysis of catalysts.

While the X-ray fluorescence (XRF) provides information on the impurities present (minor elemental concentrations that can reach up to ppm level), the X-ray diffraction (XRD) can not quantify minor phases (below 1%). The covering area of elements analyzed by X-ray fluorescence spectrometry varies between Sodium to Uranium, even if they are organized in crystalline or amorphous phases. Conversely, X-ray diffraction can provide additional information about the type of phases in which the sample compounds crystallizes. By coupling the two techniques, there is obtained a logical, comprehensive and effective method to approach the elemental chemical and microstructural

analysis of catalysts, especially in cases aiming to recover certain metals from used catalytic converters [3], [4]. 6. REFERENCES 1. Carol, L., Newman, N., Mann, G., High

Temperature Deactivation of Three-Way Catalyst, SAE Technical Paper 892040, (1989).

2. Gulati, S. and Sweet, R., Strength and Deformation Behavior of Cordierite Substrates from 70° to 2550°F, SAE Technical Paper 900268, (1990).

3. Miuţescu, A., Contributions to optimize quality maintenance of de-pollution catalytic equipments of spark ignition engine and to recover precious metals from catalytic converter components, PhD Thesis, Piteşti, (2011).

4. Negrea, D., David, E., Malinovschi, V., Moga, S., Ducu, C., X-Ray analysis of spent catalysts and recovered metals, Environmental Engineering and Management Journal, Vol. 9, No. 9, 1235-1241, (2010).