Investigation of multi-component surface reactions by SIMS: The interaction between hydrogen and oxygen on polycrystalline nickel

Surface Science 89 (1979) 701-709 © North-Holland Publishing Company

INVESTIGATION OF MULTI-COMPONENT SURFACE REACTIONS BY SIMS: THE INTERACTION BETWEEN HYDROGEN AND OXYGEN ON POLYCRYSTALLINE NICKEL A. BENNINGHOVEN, P. BECKMANN, K.H. MOLLER and M. SCHEMMER Physikalisches lnstitut der Universitdt Mfmster, Schlossplatz 7, D-4400 Miinster, W-Germany Received 1 March 1979; manuscript received in final form 20 April 1979

Based on corresponding studies of H2 and 02 interaction with polycrystalline nickel, SIMS and TDMS investigations of 02 and H2 interaction with hydrogen, respectively oxygen covered polycrystalline Ni surfaces were carried out. 02 exposure of a hydrogen covered surface only results in a removal of the hydrogen adsorption layer accompanied by the usual oxygen incorporation into the surface. H2 exposure of nickel surfaces covered by various amounts of oxygen however results in the formation of OH groups on the surface with different binding energies. Heating of these hydroxide covered surfaces results in the disappearance of OH groups and simultaneous desorption of H20 molecules.

1. Introduction In this paper we present the mair results of an investigation of the interaction between oxygen and hydrogen on a nickel surface. This investigation is based on a careful study of the interaction between the single species H2 and 02 and a polycrystalline Ni surface. In the first part of the paper the main features of these two "basic" reactions are reported. Surface analytical techniques are of increasing iml~ortance in the investigation of surface reactions. Within these analytical techniques secondary ion mass spectrometry (SIMS) has a unique position concerning the investigation of hydrogen containing surface structures: It is the only surface sensitive technique which is capable to detect hydrogen and its compounds [1]. In addition, its isotope sensitivity as ... absol'.ite sensitivity . . . . . . . . . . are . . . . .extrernaly . . . . . . . . . . . . ~,e.~',,1 . well as its hiP.h for +u~,.....,~.~LA~a~l~, h . . ; ..... +;,..,,;..-- of stirface reactions. A disadvantage of SIMS, the difficulties in quantitative analysis, can partially be compensated by appropriate additional surface techniques. This was the reason for us to combine in one instrument SIMS with thermal desorption mass spectrometry (TDMS), a technique which is also sensitive to hydrogen and its compounds. The reason for selecti~g nickel was twofold: Firstly nickel is a very important catalyst, especially for hydrogen transfer reactions, and secondly it is one of the 701

702

A. Benninghoven et al. /Investigation o f surface reactions by SIMS

most extensively investigated metal surfaces. This is true also for its interaction with H2 and 02.

2. Basic reactions

The ~rwestigation of hydrogen-oxygen interaction on a Ni surface can only be carried out on the basis of an exact knov, ledge of the secondary ion emission b e h a v i m - f oxygen and hydrogen covered Ni surfaces. We recently carried out these inv~*~gations [2-4]. The main results may be summarized as follows. 2.1. Hydrogen on nickel

The most important secondary ions emitted from a hydrogen saturated nickel surface are, beside H -+, especially the molecular ion species NiH -+ and Ni2H÷. The behaviour of some of these ions during H2 exposure is plotted in fig. 1. Any influence of ion bombardment and residual H2 partial pressure could be excluded by the experimental conditions. Both of the well known bonding states/31 and f12 [5] produce the above mentioned secondary ions, with different intensities, however [2]. The main results of our SIMS and TDMS investigations are in good agreement with the results obtained by other techniqaes [5], and can be summarized as follows: - The t31 state is depleted at room temperature within some minutes. The/32 state needs a desorption temperature of about 350 K.

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A. Benninghoven et al. / Investigation of surface reactions by SIMS

703

- The sticking probability, s, of H2 is near 1 for a clean Ni surface. - The secondary ion emission can be explained qualitatively by the simple model of Ni*-H - dipole sputtering. - The smallest hydrogen coverage detectable by SIMS is in the range of 10 ppm of one monolayer. 2. 2. Oxygen on nickel

The interaction of O~ with a Ni surface is much more complex. This reaction has carefully been investigated mainly by electron spectroscopical methods [6]. On the basis of these results we recently carried out a careful combined SIMS-TDMS investigation of this reaction with the following main results (fig. 2) [3,4] : Different bonding states of oxygen on nickel can be distinguished: rel intenstty

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704

A. Benninghoven et at. /Investigation of surface reactions by SIMS

- Chemisorption I and II. - Surface oxide (NiO). Adsorbed oxygen on top of the surface oxide. Bulk sJlved oxygen. These different oxygen bonding states are formed successively during O2 exposure of a polycrystalline Ni surface at 300 K (fig. 2). Heating of these different nickel oxygen ~tructures shows: Desorption of adsorbed oxygen at 400 K (02 flash peak). Disap7 r a~.-ance of the surface oxide at 700 K. The lack of an 02 flash peak indicates oxy !,,en solution in the bulk. - A second 02 flash peak at 1100 K indicating the evaporation of oxygen from the chemisorption layer which is in thermal equilibrium with the bulk solved oxygen. Isotope experiments with ~SO2 give further information on the surface reaction between Ni and oxygen, and the behaviour of the resulting N i - O structures: - During further oxygen exposure the adsorbed oxygen on top of an oxide layer is incorporated into the underlying oxide layer. -Originally chemisorbed oxygen is incorporated into the oxide layer during further O2 exposure.

3. Experimental results for the

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The following investigation of the interaction between hydrogen and oxygen on a nickel surface by SIMS is based on the well known fact that directly Ni-bonded hydrogen is indicated quantitatively by the ratios Ni2H+/Ni~ and NiH÷/Ni+, and hydro.vide groups on the surface by OH- ions. In addition it can be assumed that H20 molecules on the surface are indicated by H20 + and related ions like Ni (H20) + [7-9]. 3 . 1 . 0 2 exposure o f a hydrogen covered nickel surface

02 exposure of a Ni surface covered by/32 hydrogen (corresponding to the final values in fig. 1) results in an increase and following decrease of NiH + and Ni2H + intensities [2], but in a continuous decrease of the ratio Ni2H÷/Ni~, indicating a decreasing concentration of metal bonded hydrogen (fig. 3). No emission of OHr~ n~o,~ .i .r.~. . .and . . ao • ~2,-, -,~,~ were observed. The increase of oxygen containing secondary ions as wel| as Ni÷ and Ni2 during O= exposure is the same as it was found for a cleaq avrface (see fig. 2). Apparently the O= exposure results in the formation of the same Ni-O structures as for a clean Ni surface. The only difference is that during the formation of the chemisorption layer the hydrogen layer on the Ni surface is removed. Our experiments could not answer the q'~estion, in which state hydrogen leaves the nickel surface (H2 or H20).

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3. 2. H2 exposure o f oxygen covered nickel surfaces

In contrast to 02 exposure of a hydrogen covered Ni surface, a change in the order of succession of gas exposure results in the formation of OH groups on the surface. These hydroxide groups are indicated by a strong emission of OH-, NiOH+, NiO2H-, and Ni2OH ÷. In the following, the intensity of OH- secondary ion emission will be applied as an indicator for the hydroxide concentration on the surface. - H2 exposure of a 1 L 02 exposed Ni surface (chemisorption I): H2 exposure of this surface results in the formation of OH- as well as Ni2H÷ ions with increasing intensity during H~ exposure (fig. 4). The sticking probability s for H2 on the chemisorbed oxygen surface is much below 1 in the range of 0.1 [10]. At 350 K, the OH- emission, as well as the emission of Ni2H + disappears. At the same temperature, a weak H20 as well as a H~. flash peak appear. These results demonstrate the formation of hydroxide groups (OH-), and a simultaneous formation of nickelhydrogen complexes (e.g. Ni2H÷). At 350 K hydroxide groups are transformed to desorbing H~O molecffies. By one hydrogen exposure up to saturation of OH- emission (5 L), about 50% of the originally chemisorbed oxygen is removed from the Ni surface by the following H20 desorption.

A. Benninghoven et aL / Investigation o f surface reactions by SIMS

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Fig. 4. (a) Change of O- emission during 1 L 09 exposure of a clean nickel target. (b) Influence of subsequent 5 L H2 exposure on +:he emission of characteristic secondary ions. (c) Thermal behaviour of characteristic secondary ions and related TDMS signals during temperature increase (5 K s-1 ) subsequent to (b).

-Hz exposure of a 5 L 02 exposed Ni surface (chemisorption II): This surface shows the same behaviour as described for chemisorption I. There is only a difference in the intensity of the OH- emission, which is larger for a chemiso, ption II layer. - H2 exposure of a Ni surface covered by a closed oxide (NiO) layer: Also for this surface a strong hydroxide formation (OH- emission) takes place during H2 exposure. No directly bonded hydrogen can be observed (no appearance of Ni2H ÷ e.g.). Sma!! amounts of directly Ni bonded hydrogen are still observed, however, for only fractionally oxide covered Ni surfaces. A great difference could be observed in the desorption temperature of rs r~ c~-,,e,,,~u . . . . . . ~ with mat .,--. on the chemisorbed oxygen • -2,-, layer: Now the desorption temperature is in the range of 700 K (fig. 5), which is equal to the temperature when oxygen solution from the oxide layer into the bulk takes place (fig. 2). No H2 flash signal is observed. - H2 exposure of physisorbed oxygen on top of an oxide covered Ni surface: The behaviour of such a surface (last surface state in fig. 2) during 02 exposure is very similar to the behaviour of the closed oxide layer. There is only one difference: At a temperature of 400 K, an 02 flash peak appears, indicating that adsorbed oxygen

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is not attacked by H2 exposure (fig. 5). The adsorbed oxygen is still present also on a hydroxide covered oxide layer. 02 exposure of hydroxide covered Ni and Ni oxide surfaces always results in a strong decrease of OH- emission.

4. Discussion This investigation clearly demonstrates the analytical capacities of SIMS especially for hydrogen containing surface structures. For the investigated reac~tion a clear differentiation was possible between Ni bonded hydrogen on one hand, and hydrogen in hydroxide groups on the other. For the two hydrogen bonding states, an extremely high sensitivity in the range of 10 ppm could be established. H20 could not be detected on the surface (no emission of H20 ÷ and related secondary ions). Our investigation further demonstrates the great importance of the order of succession of gas exposme for this :;urface reaction. Only in the case of an oxygen covered surface, the interaction of oxygen and hydrogen on the surface results in

708

A. Benninghoven et aL /Investigation of surface reactions by SIMS

the formation of OH groups, followed by the desorption of H20. The 02 exposure of a hydrogen covered surface only results in the removal of this hydrogen, and not in the formation of surface hydroxide complexes. There ¢;xists a great difference in the binding energy for hydroxide complexes depending on the underlying surface structure: The binding energy is very low for a directly Ni bonded OH group. This results from the very low desorption temperature (350 K) for H20 molecules from a Ni sl-rface which originally was covered by a chemisorbed oxygen layer before H2 exposure. On the other hand, hydroxide groups on a Ni oxide surface disappear only at a temperat~,,e in the range of 700 K, indicating a much higher bonding energy between n)droxide complexes and the surface. The hydroxide disappears at the same temperature at which the underlying Ni oxide is destroyed by solving the oxide incorporated oxygen in the bulk. Besides the formation of hydroxide groups (OH- emission), H2 exposure of 02preexposed Ni surfaces results in directly Ni bonded hydrogen (Ni2H+/Ni~ e.g.). Its maximum surface coverage depends on the oxygen precoverage of the Ni surface and varies between 50% for the chemisorption I layer (1.5 L), and 2% for a partially oxide covered surface (15 L), compared with the oxygen free, hydrogen covered surface. For a Ni surface covered by a closed oxide layer, no directly Ni bonded hydrogen can be detected. In addition, this investigation gives some insight into the mechanism of oxygen removal from Ni surfaces by H2. First step of this oxygen removal obviously is the formation of hydroxide groups. Then, at higher temperatures, a desorpt;.qn of H20 molecules takes place. As a result, one oxylgen atom is removed (in a H20 mglecule) when two hydroxide groups disappear frora the surface. This assumption is strongly supported by the results of hydrogen exposure of a 1 L (chemisorption I) oxygen layer on Ni" 50% of the originally observed O- emission (oxygen coverage [3,4])disappears after H2 exposure up to OH- saturation, and subsequent heating for !t20 desorption. The removal of bulk solved oxygen can be explained by the same mechanism. First step is the diffusion of bulk solved oxygen into the surface, which mainly takes place above 700 K. This oxygen is present on the surface in a chemisorbed state. By its interaction with H2 from the gas phase, hydroxide complexes are produced Thermal decomposition of these complexes results in H20 emission. This m,.~del clearly reveals the importance of oxygen bulk mobility, which is in general the fimiting factor for the removal rate of bulk solved ox~gen durLng H2 exposure.

References [1] A. Benninghoven, Surface Sci. 53 (1975) 596. [2] P. Beckmann, A. Benninghoven K.H. Mfiller and M. Schemmer, Surface Sck, to be published.

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[31 A. Benninghoven, K.H. Miiller, M. Schemmer and P. Beckmann, Appl. Phys. 16 (1978) 367.

[41 K.H. Miiller, P. Beckmann, M. Schemmer and A. Benninghoven, Surface Sci. 80 (1979) 325.

[51 See, for example: G. Ertl and D. Kiippers, Ber. Bunsen-Ges. Physik. Chem. 75 No. 10, (1971) 1017; T.N. Taylor and P.J. Estrup, J. Vacuum Sci. Technol. 11 (1974) 244; K. Christmann, O. Schober, G. Ertl and M. Neumann, J. Chem. Phys. 60 (1974) 4528. See, for example: [61 D.F. Mitchell, P.B. Sewell and M. Cohen, Surface Sci. 61 (1976) 355; D.F. Mitchell, P.B. Sewell and M. Cohen, Surface Sci. 69 (1977) 310; S. Evans, J. Pielaszek and J.M. Thomas, Surface Sci. 56 (i976)644. [71 A. Benninghoven, K.H. Miiller, C. Plog, M. Schemmer and P. Steffens, Surface Sci. 63 (1977)403. 181 J. Estel, H. Hoinkes, H. Kaarmann, H. Nahr and H. Wilsch, Surface Sci. 54 (1976) 393. [91 M. Cavallini and G. Nencini, in: Rarefied Gas Dynamics Vol. II (1974) pp. E 10.1E 10.10. [10l K.D. Rendulic and A. Winkler, Surface Sci. 74 (1978) 318.