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Adsorption of Oxygen on Dy, Cu and DymCun Bimetallic Surfaces
:112
ADSORPTION OF OXYGEN ON Dy, eu AND JYmCun
3I~ETALLIC
SUn?ACES
J. Kopestansky J. Heyrovsky Institute of ?hysical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2, Czechoslovakia
Rare earth metals, alloys and compounds and their interactions with gases have been subject to extensive investigations [1],
[2].
The objective of the present work was the study of the properties of dysprosium-copper bimetallic systems in relation to their interaction with oxygen (and partly with carbon monoxide, too). The gas adsorption was examined via work function changes measurements, using a Kelvin's vibrational capacitor method [3]. Changes in the work function 6
Chemisorption of oxygen on metals results largely in an in-
:31;)
crease in the work function of the metal. This is usually ascribed to the effect of the formed negatively oriented dipole layer constitutedbY the oxygen atoms (oxygen adsorbs dissociativelly on the majority of metals at room temperature). Occasionally, a decrease in the work function has been observed, followed by an increase at higher amounts of interacting oxygen; this has been ~e lated to the incorporation of oxygen atoms in the sub-surface layers and subsequent formation of oxide [4], [5J. The observed work function changes for Dy, eu and DyCu films due to their interaction with oxygen at room temperature are shown in Figs 1 and 2. From these results, in conjuction with published data, it can be inferred that oxYgen does not form the bulk oxide with dysprosium at this temperature; the surface layer formed on the adsorption is constituted by two different types of surface species with different dipole moments (Fig. 1). For copper, on the other hand, the results give unambiguous evidence of the formation of a sub-surface bulk oxide even at room temperature (Fig. 2). For DyCu films, the work-function-change patterns in the initial stage closely resemble those obtained with pure copper (see Fig. 2); this is true at degrees of coverage not exceeding values of 0= 0.4 - 0.5, which may approximately correspond to the amount of Cu atoms on the surfaces. This implies that interacting with surfaces of DyCu films, oxygen first adsorbs on copper and only then on dysprosium. In other words, in the initial stage of adsorption, Cu surfaces are more reactive with respect to oxygen than Dy surfaces. This may be in relation to the readiness of oxygen incorporation in Cu. After the ~~ value has passed a maximum, the work function of the DyCu system exhibits an additional decrease at higher amounts of interacting oxygen, which may be associated with the formation of addi tional Cu and/or Dy oxides with a subsequent enhanced incorporation of oxygen in the sub-surface layers. This can be deduced from the fact that for some metals, oxygen chemisorption and its incorporation in the sub-surface layers have been observed to proceed simultaneously even at room temperature [6]. Moreover, the composition of the oxide layers formed depends heavily on the amount of interacting oxygen [7], different oxides and even different oxide mixtures being formed.
:1l4
1.0 0
~'f
6'P
0.4
-0.2
o
ot. Fig. 1
8
-0.4 1.0 0
'-------'-_....l.---.L_----l....-----l
0.4
8
1.0
Fig. 2
Fig. 1 : Work function changes 6~(eV) for dysprosium films on the degree of coverage 0 during adsorption of oxygen. Fig. 2: Work function changes 6
[4]
Handbook of physics and Chemistry of Rare Earths (K.A. Gschneider and L. Eyring, Eds). Elsevier, Amsterdam 1983 and references thereino Netzer F.P., oertel E. in: Handbook of the Physics and Chemistry of Rare Earths (K.A. Gschneider and L. Eyring, Eds), Vol. 5, Chapter 3. Elsevier, Amsterdam 1983 and references therein. Kope s Lansky J.: Collect. Czech. Chem.::;ommun. (in press). Hofmann P., Wyrobisch W., 3radshaw A.M.: Surface Sci. 80,
[5] [6J [7]
Strasser G., Bertel E., Netzer F.P.: J. Catal. 79, 420(1983). Foord J. S., Lambert R.M.: Surface Sci. 161, 513 (1985). Sanz J.lr.., Hofmann S.: Surf. Interface Anal. 8, 147 (1986).
[2]
[3]
334, (1979).
ADSORPTION OF OXYGEN ON Dy, eu AND JYmCun
3I~ETALLIC
SUn?ACES
J. Kopestansky J. Heyrovsky Institute of ?hysical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2, Czechoslovakia
Rare earth metals, alloys and compounds and their interactions with gases have been subject to extensive investigations [1],
[2].
The objective of the present work was the study of the properties of dysprosium-copper bimetallic systems in relation to their interaction with oxygen (and partly with carbon monoxide, too). The gas adsorption was examined via work function changes measurements, using a Kelvin's vibrational capacitor method [3]. Changes in the work function 6
Chemisorption of oxygen on metals results largely in an in-
:31;)
crease in the work function of the metal. This is usually ascribed to the effect of the formed negatively oriented dipole layer constitutedbY the oxygen atoms (oxygen adsorbs dissociativelly on the majority of metals at room temperature). Occasionally, a decrease in the work function has been observed, followed by an increase at higher amounts of interacting oxygen; this has been ~e lated to the incorporation of oxygen atoms in the sub-surface layers and subsequent formation of oxide [4], [5J. The observed work function changes for Dy, eu and DyCu films due to their interaction with oxygen at room temperature are shown in Figs 1 and 2. From these results, in conjuction with published data, it can be inferred that oxYgen does not form the bulk oxide with dysprosium at this temperature; the surface layer formed on the adsorption is constituted by two different types of surface species with different dipole moments (Fig. 1). For copper, on the other hand, the results give unambiguous evidence of the formation of a sub-surface bulk oxide even at room temperature (Fig. 2). For DyCu films, the work-function-change patterns in the initial stage closely resemble those obtained with pure copper (see Fig. 2); this is true at degrees of coverage not exceeding values of 0= 0.4 - 0.5, which may approximately correspond to the amount of Cu atoms on the surfaces. This implies that interacting with surfaces of DyCu films, oxygen first adsorbs on copper and only then on dysprosium. In other words, in the initial stage of adsorption, Cu surfaces are more reactive with respect to oxygen than Dy surfaces. This may be in relation to the readiness of oxygen incorporation in Cu. After the ~~ value has passed a maximum, the work function of the DyCu system exhibits an additional decrease at higher amounts of interacting oxygen, which may be associated with the formation of addi tional Cu and/or Dy oxides with a subsequent enhanced incorporation of oxygen in the sub-surface layers. This can be deduced from the fact that for some metals, oxygen chemisorption and its incorporation in the sub-surface layers have been observed to proceed simultaneously even at room temperature [6]. Moreover, the composition of the oxide layers formed depends heavily on the amount of interacting oxygen [7], different oxides and even different oxide mixtures being formed.
:1l4
1.0 0
~'f
6'P
0.4
-0.2
o
ot. Fig. 1
8
-0.4 1.0 0
'-------'-_....l.---.L_----l....-----l
0.4
8
1.0
Fig. 2
Fig. 1 : Work function changes 6~(eV) for dysprosium films on the degree of coverage 0 during adsorption of oxygen. Fig. 2: Work function changes 6
[4]
Handbook of physics and Chemistry of Rare Earths (K.A. Gschneider and L. Eyring, Eds). Elsevier, Amsterdam 1983 and references thereino Netzer F.P., oertel E. in: Handbook of the Physics and Chemistry of Rare Earths (K.A. Gschneider and L. Eyring, Eds), Vol. 5, Chapter 3. Elsevier, Amsterdam 1983 and references therein. Kope s Lansky J.: Collect. Czech. Chem.::;ommun. (in press). Hofmann P., Wyrobisch W., 3radshaw A.M.: Surface Sci. 80,
[5] [6J [7]
Strasser G., Bertel E., Netzer F.P.: J. Catal. 79, 420(1983). Foord J. S., Lambert R.M.: Surface Sci. 161, 513 (1985). Sanz J.lr.., Hofmann S.: Surf. Interface Anal. 8, 147 (1986).
[2]
[3]
334, (1979).