XPS study of the chemisorption induced surface segregation in LaNi5 and ThNi5

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XPS study of the chemisorption induced surface segregation in LaNi5 and ThNi5

L. Schlapbach, C.R. Brundle

To cite this version:

L. Schlapbach, C.R. Brundle. XPS study of the chemisorption induced surface segregation in LaNi5 and ThNi5. Journal de Physique, 1981, 42 (7), pp.1025-1028. �10.1051/jphys:019810042070102500�.

�jpa-00209078�

XPS study ^{of the} chemisorption induced surface segregation ⁱⁿ LaNi5

and ThNi5

L. Schlapbach

Laboratorium für Festkörperphysik ETH, ^CH-8093 Zürich, Switzerland

and C. R. Brundle

IBM Research Laboratory, 5600 Cottle Road, San Jose, Ca. 95193, ^U.S.A.

(Reçu ^le 27janvier 1981, accepté ^{le 5}

1981 )

Résumé.

La ségrégation de surface induite _par l’oxydation ^{du La et du Th} accompagnée de la formation des

précipités ^{de Ni} joue ^{un rôle} important pour l’absorption ^de l’hydrogène par LaNi5 ^et pour les propriétés ^cata- lytiques ^du ThNi5. ^{Par la} spectroscopie ^de photoélectrons des niveaux de c0153ur nous montrons que la chimisorp-

tion de O2, H2 ^et H2O ^sur LaNi5 ^induit respectivement ^{une forte} ségrégation, ^{une faible} ségrégation ^et pas de

ségrégation. L’adsorption préalable ^de SO2 bloque ^la ségrégation ^alors qu’un ^tel bloquage n’est pas observé

avec H2S. ^La ségrégation ^induite par O2 ^sur ThNi5 ^est plus faible que pour LaNi5 ^à température ambiante, ^mais

considérablement plus ^{forte à 200 °C.}

Abstract.

Surface segregation ^{with the} disproportionation into La and Th oxide plus precipitations ^{of ele-}

mental Ni play ^an important role in the understanding of the easy hydrogen absorption ^of LaNi5 ^{and of the cata-} lytic properties ^of ThNi5. By ^means of core-level X-ray photoelectron spectroscopy ^we show that the chemisorp-

tion of O2, H2O, ^and H2 ^on LaNi5 surfaces induces a strong, ^{weak and no surface} segregation, ^resp. Precoverage

with SO2 blocks further segregation, whereas exposure ^to H2S ^{does not.} The oxygen induced segregation ^of ThNi5 ^{at room} temperature is much weaker than in LaNi5, ^but is very extensive ^at 200 °C.

Classification

Physics ^Abstracts

86.40K - 82.65 - 81.60B

1. Introduction. - There is an increasing ^interest

in the application ^of* intermetallics between lantha- nides or actinides and d-transition metals for hydro-

gen storage [1] ^and catalysis [2, 3]. Most of the com-

pounds ^{react at room} temperature readily ^with hydrogen ^{and form} ternary hydrides, ^{in contrast to}

many hydride forming ^elemental metals, ^{which have}

to be activated. Based on surface analysis and magne- tic investigations ^we have shown [4-6] that surface

segregation ^and decomposition ^{accounts for this}

favourable behaviour of _e.g. LaNis : ^{In a surface} layer ^{La diffuses to the} top ^{and reacts with} impurities

in the hydrogen gas forming e.g. La2o3. The remain-

ing ^Ni precipitates ⁱⁿ superparamagnetic particles ^of

elemental Ni. Dissociative chemisorption ^of hydro-

gen, which is often the rate limiting step ⁱⁿ hydrogen absorption, ^{can occur at} the metallic Ni precipita-

tions and on the metallic subsurface of the interme- tallic compound. ^The large catalytic activity ^of e.g.

ThNis ⁱⁿ hydrogenation ^reactions [2] around 300 OC is probably ^{related to the} precipitation of elemental Ni in a similar disproportionation ^reaction [7].

Von Waldkirch and Zürcher [8] ^showed by ^means

of Auger ^electron spectroscopy (AES) ^{that in} LaNis

surface segregation ^can be induced by oxygen. The exposure of the _oxygen precovered ^{surface to} hydro-

gen had no further influence on the La to Ni ratio

[4, 8]. ^{From the AES} experiment ^no analysis ^{of the}

chemical state of La and Ni could be obtained. In the meantime surface segregation ^induced by ^selec-

tive oxidation has been found ^on many other hydride forming intermetallics and has been related to their

catalytic activity [5, 9]. ^The driving force for the sur-

face segregation ^is generally ^the difference in surface energy of the constituents [10]. The selective oxida- tion of La at the surface further lowers the surface energy of La and thus increases the driving ^force

for the segregation. ^The chemisorption ^of e.g. hydro-

gen and the formation of surface La hydride ^could possibly ^{have a} similar effect.

We have investigated ^the influence of the chemi-

sorption of 02, H20, H2, S02 ^and H2S ^on the surface

segregation ^of LaNis ^{and on} the chemical state of La and Ni by ^{means of} X-ray photoelectron spectros-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019810042070102500

1026

copy (XPS) ^{of the core levels.} S02 ^{is known to} poi-

son the hydrogen absorption ^and desorption [11].

Results of surface segregation ^of ThNi5 ^{are also} briefly reported and will be compared ^{with recent}

studies [2, 7] ^{on the} catalytic properties ^of ThNis.

2. Experimental.

^The polycrystalline LaNi5 ^and ThNis samples ^were prepared ^from 99.9 % ^La (Research Chemicals), ^99.7 % Th and 99.998 % ^Ni (both ^Koch Light products) by levitation melting.

Details of the preparation and characterization are

given ^elsewhere [4]. ^{Caution :} Handling ^{of Th is}

hazardous ! The photoelectron energy distribution

curves were measured at room temperature ^{on a} VG spectrometer (1 ^{x 10-1°} mbar, Mg Ka ^radia- ton, ^{1.0 eV} line width of the Au 4f peak) ^{which was} equipped ^{with a} scraper (hardmetal) ^to produce

UHV clean surfaces. For LaNis ^{the core levels}

La 3d3/2,s/2, ^Ni 2pl/2,3/2 ^and 0 ISI/2 ^{were inves-} tigated. ^The sampling depth ^is of the order of 20 A.

To evaluate the La to Ni atomic ratio the peaks ^were integrated and divided by the theoretical cross sec-

tions [12]. As the kinetic _{energy of} the photoemitted

La 3d and Ni 2p ^{electrons is} almost the _same, no

corrections for different _escape depth ^or for transmis- sion function of the analyser ^were necessary.

To study the oxidation of Th in ThNi5 ^{the stron-} gest ^Th peaks (Th 4f7/2,5/2) ^{around 340 eV} binding

energy and the Ni 2P3/2 ^{at 853 eV were} investigated.

To avoid corrections because of the different _escape

depths (different ^kinetic energy of the photoemitted electrons) ^{we also} analysed ^{the Th} 5ds/2 ^{(88 eV) and}

Ni 3PI/2,3/2 (68 eV) peaks and evaluated the Th : Ni atomic ratio from there.

3. Results.

Freshly scraped LaNi5 (Fig. 1) ^shows

very little _oxygen (oxygen ^to metal ratio ~ 1 : 30).

Fig. ^1.

XPS-spectra ^of scraped LaNis exposed ^to 02 (20°C,

1

10-10 mbar).

The La 3d5/2 ^{and Ni} 2pl/2 (870.3 eV) signals ^corres- pond ^{to metallic} [4,13, 14] ^La and metallic [15, 16] ^Ni.

The La 3d3/2 peak ^is completely ^covered by ^the

Ni 2p3/2 peak ^{at 852.9 eV} (metallic ^[15-17] Ni). ^The

low energy shoulder on the La 3d 5/2 peak ^was explain-

ed by Crecelius et al. [13] ^{as a} shake down satellite.

The La to Ni ratio is 1 : 4 ; i.e. close to the bulk concentration. A carbon Is signal ^{at 286} eV, ^which may result from contamination by ^the scraper, ^was

always present ^after scraping.

After exposure of the scraped ^{surface to} 1 L oxygen

(1 ^L

¹ Langmuir

^{10 - 6} torr.s) ^{the La} 3d5/2 ^level already ^{exhibits a double} peak characteristic of the oxidized light ^{rare earth metals} [4, 14, 181. ^{The Ni} 2p peaks ^still correspond ^to metallic Ni. The La to Ni ratio increases. Further ¹⁰ and 100 L 02 doses oxi- dize La completely. ^{The Ni} 2p signal ^{decreases so that}

the weaker underlying ^La 3d3/2 level also becomes visible. The La to Ni ratio increases ^to 1 : 1. Most of the Ni (about 2/3) is still in the metallic ^state. The

2p1/2,3/2 ^{peaks of oxidized Ni would appear at some}

volts higher binding energy than of metallic ^Ni and

are accompagned by high binding energy satel- lites [17, ^19, 20].

In a second and third series of the experiment ^the freshly scraped sample (not completely oxygen free)

was exposed ^{up to 104 L H2 and 103 L} H20 (Fig. 2).

Fig. ^2.

XPS-spectra ^of scraped LaNis exposed ^to H2 ^and H20 (20 OC, ¹

^{10- 10} mbar).

The _exposure to hydrogen ^{did not} affect the La to Ni ratio nor the chemical state of Ni. A small part ^{of the} La oxidized, probably because of the _oxygen impu- rity ^{in the} hydrogen gas. Upon ^the exposure ^{to water} vapour, however, ^{La formed a} hydroxide (0 Is ^at

532.5 eV La 3d5/2,3/2 ^{separated by 3.7 eV} [4]) ^{and the}

La to Ni ratio increased. From the unchanged posi-

tion of the Ni 2PI/2,3/2 peaks ^we conclude that Ni

remained largely ^{in the metallic state.}

In a fourth series (Fig. 3) ^the freshly scraped ^sur-

face was precovered ^{with 10 L} S02 ^followed by ^the

exposure up ^to 2 x 104 L 02. ^The precoverage with

S02 oxidized the La completely and decreased the Ni 2p signal considerably ^without altering ^the posi-

tion of the Ni peaks. The sulfur S 2P3/2 ^electrons

appear ^{as two} peaks, ^{about one third at 163.4 eV}

and two thirds at 167.7 eV. The main 0 ls peak ^is

at 532.1 eV. The S02 ^oxidized [21] ^{the La to LaS} (1/3) ^and LaS04 (2/3). After the precoverage with 10 L S02 ^{the La to Ni ratio is 1 : 2.} Astonishingly ^the

exposure of the precovered sample up ^to 2 x 104 L 02

did not result in an increase of the 0 its signal. ^Neither

the La to Ni ratio nor the chemical state of Ni and S varied.

Fig. ^3.

XPS-spectra ^of scraped LaNis exposed ^to SO, ^and 0, (20°C, ¹

^{10 - 10} mbar).

Contrary ^{to these} S02 results, ^after precovering

the freshly scraped sample ^{with 15 L} H2S (Fig. 4)

the _exposure to oxygen increases the 0 Is signal ^and

Fig. ^4.

XPS-spectra ^of scraped LaNis exposed ^to H2S ^and 02 (20 oC, ¹

^10-10 mbar).

induces a further increase of the La to Ni ratio _up

to 1 : 1 after 105 L 02. ^{All La seen} within the _escape

depth ^is oxidized and so is Ni to a increasing ^extent.

Sulfur is present ^{as sulfide} (S 2p3/2 ^{at 163} eV), pro-

bably ^LaS.

The freshly scraped ThNi5 sample (Fig. 5) ^exhibits

metallic Ni (2p) ^and largely ^metallic Th (4f ). ^The

Th to Ni ratio, ^as calculated from the Th 5d and Ni 3p peaks, ^{is 1 : 5.1.} After the _exposure to 100 L O2

about one third of the Th is oxidized, ^Ni remaining

metallic. The Th to Ni ratio increased to 1 : 3.9. The exposure of the sample ^{to 1 mbar} 02 for 20 min.

(~ ^{109 L} 02) ended with the complete ^oxidation

of Th and partial oxidation of Ni. The Th to Ni ratio had only ^{increased to} 1 : 2.8. The heating ^{of the} freshly scraped sample for 25 min. to 200 oC at

10-7 mbar 02 results in a much stronger segregation.

Fig. ^5.

XPS-spectra ^of scraped ThNis exposed ^to 02 (20 oC).

The Th to Ni ratio increased to 1 : 0.9, ^{Th oxidized} completely, but Ni remained metallic. A similar heat treatment without the _oxygen atmosphere (the

pressure ^{rose to} 5 ^x 10-8 mbar _upon heating ^the sample) caused the Th to Ni ratio to increase only

to 1 : 1.8.

4. Discussion.

The La to Ni ratio (1 : 4) ^{of the} freshly scraped LaNis surface deviates from the bulk ratio. We cannot decide whether this is due to

errors in the background subtraction of the Ni 2p peaks and the theoretical cross sections, ^whether there is some segregation ^{from 1 : 5 to 1 : 4 within}

the few minutes between the scraping ^{of the} sample

and the XPS analysis, ^or whether thz.C ^contamina-

tion present ^{causes some} segregation.

At room temperature ^{the reaction} with 02 ^results

in a strong ^surface segregation ^of LaNis ^whereas

the chemisorption ^of H2 has no effect on the La to Ni ratio and H20 only ^{a small one.} The heat of for- mation of La hydride (208 kJ/mol ^LaH2 [23]) ^which

is much smaller than the heat of the oxide formation

1028

(1860 kJ/mol La203 [24]) ^is apparently ^{too small to}

induce surface segregation.

The surface segregation ^{and the} disproportiona-

tion of LaNis ^{into La oxide and} Ni, ^which accounts for the excellent kinetics of hydrogen absorption,

has important implications [25] ^{on the} cyclic ^life-

time if LaNis ^{is used as a} reversible hydrogen ^sto-

rage material with impure hydrogen. The fact that

ThNis segregates much less than LaNis ^{at room} temperature indicates the possibility ^of developing hydrogen storage materials where the segregation

and selective oxidation properties ^{are tailored to the} application required.

Our results suggest ^{that the} poisoning ^of LaNis

for hydrogen absorption ^or desorption by ^sulfur might ^{occur in two different} ways. First, sulfur blocks

some how [26] the active dissociation cites which

were created by the surface segregation ^and secondly, as found after S02 coverage, it might ^{block or dece-}

lerate further segregation. ^From the above results we

would expect ^that poisoning by H2S is much less

severe than by S02, because of the self restoring [4]

mechanism of the further segregation.

The kinetically limited surface segregation ⁱⁿ ThNi5

’

at room temperature becomes extensive at 200 OC

indicating ^{that the} disproportionation into Th oxide and precipitations of elemental Ni _may account for the high catalytic activity ^of ThNi5 ⁱⁿ hydrogena-

tion reactions around 300 oC [2, 7].

5. Conclusions.

The chemisorption ^of O2, H20

and H2 ^{induces a strong, weak and no surface} segre-

gation, resp., in LaNis. Under ultra high ^vacuum

conditions S02 precoverage of the surface forms La sulfide and La sulfate overlayers which block or

decelerate further segregation, whereas after H2S

precoverage only ^{sulfide is} formed and the _segrega- tion continues if the sample ^is exposed subsequently

to oxygen. At low _coverages Ni remains metallic.

Acknowledgments.

^{We are} grateful ^{to H. C.}

Siegmann ^for stimulating discussions, ^R. Haefeli, EIR, ^{for the} preparation ^{of the} ThNis sample ^and

P. Brack and H.R. Scherrer for drawing.

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Properties ^and

Applications ^{of Metal} Hydrides », J. Less-Common Met.

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