Laser Induced Breakdown Spectrometry of Vanadium in Titania Supported Silica Catalysts

8/12/2019 Laser Induced Breakdown Spectrometry of Vanadium in Titania Supported Silica Catalysts

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Talanta 47 (1998) 143151

Laser induced breakdown spectrometry of vanadium in titania

supported silica catalysts

P. Lucena a, L.M. Cabaln a, E. Pardo b, F. Martn b, L.J. Alemany b, J.J. Laserna a,*a Department of Analytical Chemistry, Faculty of Sciences, Uniersity of M alaga, E-29071, Malaga, Spainb Department of Chemical Engineering, Faculty of Sciences, Uniersity of Malaga, E-29071, Malaga,Spain

Received 17 September 1997; received in revised form 4 February 1998; accepted 10 February 1998

Abstract

The capability of laser induced breakdown spectrometry (LIBS) for vanadium determination in a x V2TiO2SiO2catalyst is presented. The microplasma was generated onto the sample surface using a pulsed Nd:YAG laser operating

in the second harmonic (532 nm). Laser produced plasmas were collected and detected using a charge-coupled device

(CCD). In order to minimize the complex spectral interferences of emission lines and matrix effects a wide spectral

range (210 660 nm) was studied. The focusing of the laser beam on the surface was optimized to improve the

signal-to-background ratio, and consequently the limit of detection. The analytical lines selected were used to evaluate

the calibration curve. The detection limit for V was estimated to be 38 g g1 in 2TiO2 SiO2. The method precision

expressed as relative standard deviation (RSD) was better than 6% in the concentration range 2001000 g g1.

1998 Elsevier Science B.V. All rights reserved.

Keywords: LIBS; Vanadium analysis; Catalysts; Quantitative determination

1. Introduction

TiO2-supported vanadium oxides have been ex-

tensively studied and used due to their high cata-

lytic activity and selectivity in many chemical

reactions [17]. For instance, V/TiO2is one of the

most effective catalysts in the selective catalytic

reduction (SCR) of NOx

by NH3 [7,8]. Industri-

ally, in these catalysts the anatase, a polymorph

form of TiO2, is used as support for vanadium

oxide. Titania is the support more widely used for

this purpose, although alumina, and to a lesser

extent silica are also used. In spite of its wide use,

titania suffers several drawbacks, including lim-

ited surface area, poor mechanical strength and a

low sintering resistance. On the other hand, the

interaction of vanadia with silica is weak and,

consequently it results in a higher tendency to

thermally induced aggregation with a poor disper-

sion of the active phase, while alumina-supported

vanadia catalysts are susceptible to sulfation. The

binary TiO2 SiO2 system, in principle, seems an

ideal candidate to overcome the above disadvan-

tages [9].* Corresponding author. Tel.: +34 5 2131881; fax: +34 5

2132000; e-mail: [email protected]

0039-9140/98/$19.00 1998 Elsevier Science B.V. All rights reserved.

PIIS0039-9140(98)00063-0


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P. Lucena et al./Talanta 47 (1998) 143151144

It is envisaged that ternary V Ti Si systems

display similar characteristics to those of binary

V Ti catalysts. In addition to the economic

benefits, the presence of silica grants much better

mechanical properties to the system, which may

allow its application in fluidized bed reactors or

extrusion into monolith reactors. The bulk, the

surface physico-chemical properties and the reac-tivity of VTiSi systems are strongly dependent

on the method used for their preparation and the

precursors used in the vanadia distribution. Vana-

dium loading determines the nature and distribu-

tion of vanadia species on the titania support.

Therefore the structure and morphology of the

TiO2 SiO2 substrate will determine to a large

extent the dispersion of vanadia. In contrast to

binary systems, ternary catalysts are still not fully

understood.

For VTiSi systems, it is very interesting to

evaluate the dispersion and possible diffusion of

vanadium into the support. Knowledge of thequantitative microstructure and composition are

of great importance in understanding the possible

correlations of distribution and composition with

catalytic properties. Consequently, the develop-

ment of analytical methods for these materials is

of great practical interest.

The capability of laser induced breakdown

spectrometry (LIBS) for materials characteriza-

tion has been widely demonstrated [10 14]. Re-

cent analytical applications of LIBS include the

determination of aluminum in zinc alloy [15], the

simultaneous determination of aluminum, copper,

iron, nickel, and zinc in alloys [16], the determina-tion of copper in steel [17], and the detection of

lead in concrete [18]. Surface analysis of photonic-

grade silicon has been demonstrated [19,20]. How-

ever, no applications of LIBS for vanadium

determination in catalysts have been previously

reported. For this purpose, other surface analysis

techniques (such as secondary ion mass spec-

trometry; X-ray photoelectron spectroscopy) can

be used [21]. In comparison with those techniques,

LIBS presents a number of advantages including

the need for little or no sample preparation, the

minute sample quantities needed, the possibility

of work without controlled atmospheres and the

rapid analysis time. In this paper, LIBS has been

evaluated for the quantitative determination of

vanadium in xV2TiO2 SiO2 catalysts.

2. Experimental section

2.1. Apparatus

The LIBS system has been described in previ-

ous works [22,23]. Briefly, it consisted of a pulsed

Nd:YAG laser operating in the second harmonic

(Continuum, model Surelite SLI-20, =532 nm,

pulse width 5 ns), which was used to generate the

microplasma. The laser energy at the sample was

4 mJ pulse1. The laser beam was focused at

normal incidence onto the sample surface using a

planoconvex glass lens with a focal length of 100

mm and f-number of 4. The plasma image was

collected by a planoconvex quartz lens with focal

length of 100 mm and dispersed by an imaging

spectrograph (Chromex, model 500 IS, fitted withthree indexable gratings of 300, 1200 and 2400

grooves mm1). Two of the three gratings (300

and 2400 grooves mm1) were employed in this

study. The reciprocal linear dispersion was 20 and

2.5 nm mm1, respectively. These values gave

spectral coverages of 120 nm for the 300 grooves

mm1 grating and 15 nm for the 2400 grooves

mm1 grating with the detector used. The en-

trance slit width was 10 m and the height was 10

mm.

The spectrally resolved light was detected with

a solid-state two-dimensional charge-coupled

device (CCD) system (Stanford Computer Optics,model 4 Quik 05). The CCD consists of 752(h)

582(v) elements. The photoactive area is 64.5

mm2. The spectral resolution of the system was

0.16 and 0.02 nm pixel1 using the 300 grooves

mm1 and the 2400 grooves mm1 gratings,

respectively. The CCD is equipped with an S 20 Q

photocatode (spectral response from 180 to 820

nm) and an intensifier system (microchannel plate,

MCP). Operation of the detector was controlled

by 4 Spec 1.20 software. Shutter and delay times

can be selected in 50 ns steps. A fast photodiode

was used as external trigger for exact synchroniza-

tion of the incident laser pulse and opening of the

camera shutter. The emission signal was corrected


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P. Lucena et al./Talanta 47 (1998) 143151 145

by subtraction of the dark current of the detector,

which was separately measured for the same expo-

sure time. Calibration of the detector system was

conducted by using a mercury emission lamp and

several lines emitted from a laser induced titanium

plasma. Samples were placed on a manual XY

Z translation stage to be moved with respect to

the laser beam.

2.2. Samples

Catalysts were prepared using 2TiO2 SiO2 as a

carrier for the ternary systems xV2TiO2 SiO2.

Silica microspheres (Aerosil 200 from Degussa)

with size ca. 13 nm diameter and surface area of

200 m2 g1 were used as starting material. The

resulting xV2TiO2 SiO2 material was obtained

by co-deposition of titania and vanadia on the

silica surface by incipient wetness impregnation,

as described by Geuss et al. [24] and others [25

28]. The impregnation was made by adding amethanolic solution containing variable amounts

of titanium tetraisopropoxide and vanadium

acetil-acetonate to the support particles. The sus-

pension was ultrasonically dispersed to ensure a

good homogeneity. The superficial precipitation

on silica occurs in a few minutes at room temper-

ature. Powders were then washed and dried in air

at 373 K overnight, and afterwards they were

calcined at 773 K in air for 2 h. This procedure

allows deposition of oxo-hydrated titanium

vanadium, which yields dispersed TiO2, with

vanadium incorporated into the TiO2 crystal lat-

tice. Several samples with different vanadiumloads were prepared, and they are labelled as

xV2TiO2 SiO2, where x denotes the theoreti-

cal vanadium loading in g g1 and 2TiO2, two

theoretical monolayers incorporated onto the sil-

ica surface. A monolayer was considered as the

loading of titania for completed covering of silica

surface by a 0.38 nm thick [26] film of TiO2,

which corresponds to the longest axis of the rutile

unit cell. The titanium oxide incorporated on

silica remains as small crystals of anatase covering

the silica surface, as previously reported by

Galan-Fereres et al. [26].

A set of six sample pellets were grounded with

a mortar and pelletized at a pressure of 7.5 Ton

cm2 for 15 min. Approximately 0.15 g of mix-

ture was pressed leading to samples of 13 mm in

diameter and about 0.5 mm thick. Concentration

of the calibration standards was in the range

20010000 g g1 vanadium in the 2TiO2 SiO2support. In addition, for qualitative analysis other

series of three pellets was prepared: one pellet

with a 2TiO2 SiO2 support, the second one withvanadium in form of V2O5 and the third one with

a 1:1 (w/w) V2O5 TiO2 mixture. As precision and

accuracy of LIBS are highly dependent on sample

composition, homogeneity, and surface condition,

samples were carefully prepared according to the

described methodology.

3. Results and discussion

3.1. Spectral analysis

In the analysis of solid samples by LIBS, theresulting plasma includes lines corresponding to

the sample elements and matrix constituents.

Thus, the goal in the quantitative determination

of an element by LIBS is to find a well-resolved

line for the element of interest, free of matrix

interferences. In the determination of V in xV

2TiO2 SiO2 samples, silica can be easily iden-

tified. A simple study of spectral range permits the

choice of the optimal region for observing vana-

dium without spectral interferences of Si and O

emission lines. The main problem in this kind of

sample is the presence of TiO2. The large number

of Ti lines along the UV-VIS region and theirhigh intensities can complicate the qualitative and

quantitative analysis. Five different spectral re-

gions were studied for the most important emis-

sion lines of Ti and V in the range 210660 nm.

The spectral window covering the range 404418

nm was chosen since it contains the most intense

vanadium peak (411.18 nm) free from interference

of neighbouring spectral lines. Fig. 1(ac) shows,

respectively, the LIBS spectra corresponding to

the 2TiO2 SiO2 matrix, to neat V2O5 and to a 1:1

(w/w) V2O5 TiO2 mixture. As shown in Fig. 1(c),

the main peaks for Ti and V are clearly distin-

guished. In these figures, only the most intense

emission lines were labelled [29]. Spectra were


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obtained using a single laser shot. Integration

time and delay time were 1 s and 300 ns,

respectively.

3.2. Effect of laser beam focusing on the

signal-to-background ratio

It is well-known that performance of LIBS for

quantitative analysis is related among other fac-

tors to the signal-to-background ratio (S/B) [30].

Since the background in LIBS depends on the

laser fluence used, a study on the effect of laser

focusing conditions on the S/B was performed.

Fig. 2 shows the variation of the average signal-

to-background values and their precision (in

terms of relative standard deviation, RSD) as a

function of the relative lens-target distance. The

values were calculated taking the background as

the mean background signal along 30 pixels in an

interference free region and close to the peak of

interest. In Fig. 2, the distance 0 indicates that

the sample was placed at the lens focal length.

Positive values of the relative focusing lens-target

distance refer to the beam focused at a distance

above the sample surface, while negative values

refer to the beam focal position placed inside the

material.From this figure, it is interesting to note that

the S/B precision appeared approximately con-

stant for relative lens-target distances from 2 to

+4 mm. However, the S/B reached a maximum

when the focal point was placed 12 mm above

the target surface. This value decreases drastically

when the laser beam was defocused a few millime-

ters onto the sample surface because the laser

fluence decreases. Scanning electron micrographs

of the craters produced by laser ablation in the

10000V2TiO2 SiO2 pellet surface at two differ-

Fig. 1. Single-shot LIB spectra corresponding to (a) 2TiO2SiO2support, (b) V2O5and (c) 1:1 (w/w) V2O5TiO2mixture. The delay

time was 300 ns. Acquisition time was of 1 s. MCP gain: 700 V.


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Fig. 1. (Continued)


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Fig. 2. Signal-to-background ratio and its RSD (%) vs relative focusing lens-target distance. The V(I) line at 411.18 nm was used

for the measurements and the results were obtained using the pellet 10000V2TiO 2SiO2 support. Other conditions as in Fig. 1.

ent focusing distances are shown in Fig. 3. Micro-

graphs A and B were taken respectively at the

focal point and at a relative lens-target distance of

+2 mm, respectively. Two cumulative laser shots

were used. Micrographs confirm that the 0 posi-

tion presents the smallest irradiated surface. How-

ever, the efficiency in ablating the sample, and

consequently the S/B, can be increased when the

laser beam is focused above the focal point (+2

mm) because the laser energy is still enough to

allow ablation and the beam has a larger area ofcontact with the target surface. When further

defocused, the laser fluence becomes lower and it

is less efficient in ablating the sample. Conse-

quently, the optimum focal condition for both

maximum ablation and higher signal-to-back-

ground ratio is at a focusing lens-target distance

of+2 mm.

The irradiated areas and the laser fluences for

each focusing distance are summarized in Table 1.

The ablated areas were calculated assuming an

elliptical shape of the crater. As shown, a fluence

of 2.6 J cm2 with ablated area of 18.6102

mm2 were found for the optimum focusing condi-

tions. It should be to noted that although the

plasma is formed at atmospheric pressure and

above the focal point, the laser fluence used is

below the threshold fluence for breakdown of air,

and hence no lines corresponding to its several

components are observed.

3.3. Quantitatie analysis

It is well known that at early times following

plasma formation, the LIBS spectrum is domi-

nated by an intense radiation continuum andionic emissions. Emission lines are broadened by

the Stark effect. Temporal resolution was found

to improve both linearity and signal reproducibil-

ity of the catalyst analysis. In this case, to com-

pensate for the decreased signal at delayed

integration, ten laser shots were accumulated. The

optimal delay was estimated to be 1.3 s after the

laser shot. The use of internal standardization was

necessary to compensate for the pulse-to-pulse

variability and to minimize matrix effects. At

delayed integration, continuum emission was re-

duced, but the intensity of the lines of the internal

standard were also affected. For instance, the

ionic Ti (II) line intensity at 416.37 nm decreased


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Fig. 3. Scanning electron micrographs of 10000V2TiO2SiO2 sample, showning the craters produced after two cummulative laser

shots. (A) Sample placed at the focusing lens focal position. (B) Sample placed 2 mm below the focusing lens focal position.

significantly, while the intensity of the neutral Ti

line at 407.85 nm increased. For this reason, the

atomic emission line of Ti was then chosen for

internal standardization. In addition, this line sa-

tisfies the regular requirements of an internal stan-

dard, i.e. proximity to the analyte line and

freedom from interference with the neighbouring

spectral lines.

Several lines of V were evaluated to construct

the calibration graph. The V(I) line at 411.18 nm

was finally selected as it provides the largest sensi-tivity with the smallest standard deviation, thus


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Table 1

Irradiated areas and laser fluences at different focusing lens-to-

sample distances

Focusing lens-target Fluence (J cm2)Crater area

(mm2, 102)distance (mm)

3 42.1 1.0

2.615.72

6.5 6.215.0 8.00

+1 15.6 2.7

+2 18.6 2.6

+3 30.8 1.3

The sample was 10 000 g g1 V in a TiO2SiO2 support.

presents good linearity, with correlation coeffi-

cient R2=0.9966. Deviation from linearity was

found at concentrations above 1000g g1, prob-

ably due to self-absorption [31,32].

The detection limit (CL) was calculated from

the formula:

CL=3s/S

where s is the standard deviation of the V to Tisignal ratio at low concentration andS represents

the method sensitivity calculated from the slope of

the linear section of the calibration curve. The

LOD from the time-resolved calibration graph

using thes value at 200 g g1 V was 38 g g1

V. This value is well below the V level expected in

xV2TiO2 SiO2 catalysts. The method precision

was better than 6% RSD in the concentrate range

2001000 g g1.

To check for the accuracy of the proposed

method, a recovery experiment was performed.

The results are summarized in Table 2. As shown,

recovery values are satisfactory, better than 90%

leading to the best limit of detection. Fig. 4 shows

the calibration graph for V in the 2TiO2 SiO2support. This figure presents the ratio between the

net line intensities of vanadium and internal stan-

dard as a function of V concentration. The net

peak signal was obtained by subtraction of the

background signal. As shown in Fig. 4, the curve

Fig. 4. Time-resolved calibration curve of V in a 2TiO2SiO2 support. The intensity of the 411.18 nm V(I) line was ratioed to that

of the 407.85 nm Ti(I) line. For construction of the calibration curve, mean values were calculated from six repeated measurements,each measurement representing an average of ten shots, from different target locations. The Inset shows the linear portion of

calibration curve where each point represents the mean value.


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Laser Induced Breakdown Spectrometry of Vanadium in Titania Supported Silica Catalysts