CAN. J . CHEM. VOL. 5 1 . 1973

The Surface Analysis of Nickel Chromia Catalysts by X-Ray Photoelectron Spectroscopy1

Clzeinistty ntld Mnterinls Science Division, Alornic Energy of Cnt~ndn Limited, Whiteshell N~rclenr Research E.srril~li.slzt~~et~t, Pitlc~rvci, Mcrtzitobn ROE ILO

A N D

WARREN GEORGE PROCTOR Pl~ysictrl Metr.srrret~letl~s App1icntior1.s It~str~rt?zetzt Divisiot~, Vnrint~ Associntes,

611 Htzt~setr W a y , Polo Alto, Cotifortiin 94303

Received January 25, 1973

The surface composition of a series of nickelxhromia catalysts has been examined by X-ray photoelec- tron spectroscopy. Correlation of the amount of available surface nickel with the activity of the particular catalyst for hydrogen water deuterium isotope exchange is very good. No significant alterations are observed in the valence band spectra as the su~face composition changes.

Lacomposition de la surface d'une serie de catalyseurs nickel-chrome a ete examinee par spectroscopie photoelectronique o b t e n ~ ~ e par rayon X. La corr6lation de la surface de nickel disponible avec I'activite du catalyseur concernant I'echange isotopique hydrogtne4eutCrium pour I'eau, est tres bonne. II n'y a pas de changements notables dans les spectres de bande de valence avec les changements de composition de la su~face. [Traduit par le journal] Can. J . Chem.. 51. 1670 (1973)

The use of photoelectron spectroscopy (XPS) for catalyst characterization has been recently reviewed (1). From this, it is clear that the ability of XPS measurements to probe the chemical nature as well as the composition of the first few atomic surface layers is potentially very useful in catalyst research. A number of studies (2-4) have already reported chemical shifts in elemental binding energies and changes in line shapes between "active" and "non-active" catalyst samples. This investigation has concentrated on the XPS of a number of "nickel-chromia" catalysts which are known to catalyze the deu- terium isotope exchange between hydrogen and water vapor (5, 6). This particular system was chosen because there is a reasonably well defined optimum concentration of nickel (95%) for which catalytic activity reaches a maximum. It was expected that changes in photoelectron line shapes and intensities would be observable in the region of this maximum.

Catalysts with [Ni]/([Ni] + [Cr]) = 100, 99, 98, 97, 95, 90 and 81% w/w were prepared from 2 M solutions of the mixed nickel and chromium nitrates. The carbonates were precipitated by slow addition of (NH,),CO, and were filtered,

-. - -

'AECL No. 4413.

dried, crushed, and sieved to 40-60 mesh. The samples were then reduced in purified hydrogen at 300°C and were stored under high purity nitrogen. Chromium and nickel concentrations were determined by atomic absorption spectro- photometric analysis of the dissolved carbonates.

Core and valence electron spectra of the catalysts were studied using a Varian VIEE-15 electron spectrometer fitted with a high intensity MgKcr X-ray source (1253.6 eV). Samples were attached to the probe by cellophane tape and were transferred to the spectrometer, under a nitrogen atmosphere. Energy values reported are those directly obtained from the spectrometer which had been calibrated using the core 4f,,, level of deposited gold and a secondary voltage standard. The sample surface charging was noted by measuring the shift of the C (1s) impurity line.

When the intensities of the Ni(3d) and (3s) bands are plotted against bulk nickel composi- tion (see Fig. I ) a maximuin occurs at 95 % nickel catalyst composition. The Cr(3p) intensity also rises sharply up to the 95% sample, past which little further variation is observed.

The increasing nickel intensity in the coniposi- tion range 99-95% probably results from the decrease in nickel particle size and the resultant increase in nickel surface area known to occur in

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COMMUNICATIONS --- "\

I - 15.0 , \. * ~

CATALYTIC ACTIVITY

Cr ( 3 p )

Ni ( 3 s ) . . . . . . . . Ni ( 3 d )

Y U-

1000-

I 9 0 8 5 8 0

% W/W BULK NICKEL

FIG. 1. Elemental surface concentrations and catalytic activity as a function of bulk nickel concentration; catalytic activity (see ref. 6); A, Cr(3p) line intensity; Ni(3s) line intensity: +, Ni(3d) line intensity.

BINDING ENERGY ( e V )

FIG. 2. Nickel valence band shape as a function of bulk nickel composition. Dashed line in (d) ~ndicates the position and shape of the Cr203 bands.

this composition range (6). It was previously proposed (6) that as chromia is added to the system, it is first deposited in a very thin layer, or possibly a Ni-Cr compound of undetermined structure gives way to a bulk chromia phase at a composition of around 95% nickel. These sugges- tions are supported by our observation of an initial drop in nickel line intensity at the 99% composition as chromia is first introduced to the system, and by the constant value of the Cr(3p) intensity at compositions lower than 95% nickel.

Catalytic activities for hydrogen-water deu- terium exchange are taken from Tarina et al. (6). They represent moles of HD formed per g-s at 130 "C. The parallel variation of the Ni(3p) and

(3d) line intensities with the reported catalytic activity in the nickel composition range 95-80% suggests that the exchange rate in this range may depend mainly on the availability of surface nickel and is consequently a function of the physical structure. Conclusions regarding a simi- lar correspondence in the composition range 100-95% require further kinetic data for inter- mediate points in this area.

The Ni(3d) band shapes for four catalyst compositions are shown in Fig. 2. In the pure nickel sample (a), the band maximum occurs at a binding energy 1.4 eV higher than the Ferrni level compared with the 1.0 eV found by previous authors (7). This difference and the slightly

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1672 CAN. J. CHEM. VOL. 5 1 . 1973

broader band may be due to the powdered nature of our samples. Superimposed on the pure nickel band shape, two new features gradually appear at binding energies of 0.7 and 4.0 eV above the original band maximum as the chromia content increases. A spectrum of pure Cr203 in the same energy region (see Fig. 2(d)) has two d electron bands whose positions closely match those appearing in the catalyst spectrum using the O(2s) peak as energy reference. Thus the observed changes occurring in the catalyst valence spectra probably result from the presence of Cr203.

Examination of the Ni(2p3/,) line for this catalyst series suggests that some oxygen con- tamination of the surface occurred during sample transfer. However, the Ni(3p3/,, ,/,) line for those samples low in Cr,03 is a clean gaus- sian singlet with only a small shoulder on the high binding energy side indicating that only a very small fraction of the nickel is present as oxide. The apparent difference in surface com- position suggested by the two nickel lines is explained by the fact that escaping Ni(3p) elec- trons have a kinetic energy of about 1200 eV while those from the 2p level have only an energy of about 400 eV. Recent studies of electron escape depths (8) suggest that 1200 eV electrons have at least twice the escape depth of 400 eV electrons. The Ni(3p) line is thus less sensitive to contamination of the very outermost surface since it samples a somewhat greater depth than the 2p line.

A clear correlation between XPS peak intensity

and catalytic activity is shown in the present study, but no indication of electronic promotion is seen in the valence band spectra. It is not clear, at present, whether catalytically significant changes in valence band structure can be resolved bv current XPS techniaues. However. such inter- action obviously does occur, since the available surface nickel in the 100% nickel catalyst (Fig. 1) is almost as great as that in the 95% catalyst yet the former is completely inactive. Electronic differences may be occurring in unoccupied nickel orbitals and thus may be observable in the "shake-up" electron spectrum associated with the Ni(2p) lines (9).

1. W. N. DELGASS, T . R. HUGHES, and C. S. FADLEY. Catal. Rev. 4, 179 (1970).

2. A. W. MILLER, W. ATKINSON, M. BARBER, and P. SWIFT. J. Catal. 22, 140 (1971).

3. D. A. WHAN, M. BARBER, and P. SWIFT. J.C.S. i ' l em. Commun. 198 (1972).

4. J. S. EP.INEN and A. MELERA. J. Phys. Chem. 76,2525 (19721.

5. P. M A R G I N E A N U and A. OLARIU. J. Catal. 8, 359 (1967).

6. D. TARINA, E. WEISSMANN and D. BARB. J. Catal. 11, 348 (1968).

7. Y. BAER, P. F. HEDEN, J. HEDMAN, M. KLASSON, C. NORDLING, and K. SIEGBAHN. Physica Scripta, 1, 55 (1970).

8. M. KLASSON, J. HEDMAN, A. BERDTSSON, R. NILS- SON, C. NORDLING, and P. NELICK. Uppsala Univer- sity, Institute of Physics, Rept. UUIP-757 (1972).

9. T. NOVAKOV. Phys. Rev. N, 3, 2693 (1971); T . NOVAKOV and R. PRINS. I I I Electron spectroscopy. Edited by D. A. Shirley. North-Holland, London. 1972.

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