Surface Science 164 (1985) 417 424
North-Holland, Amsterdam
417
T H E R E A C T I O N OF A T O M I C C O P P E R W I T H C H E M I S O R B E D
HYDROGEN ON RUTHENIUM *
D.W. GOODMAN,
J.T. Y A T E S , Jr. ** a n d C . H . F . P E D E N
Sandia National Laboratories, Surface Science Dicision, Albuquerque, New Me~ico 87185, USA
Received 29 April 1985: accepted for publication 31 July 1985
The effects of adding copper to a hydrogen-covered Ru(0001) surface at 100 K have been
studied by temperature programmed desorption (TPD). These results show that after completion
of the deposition of about one monolayer of copper, adsorbed hydrogen on the Ru is displaced
completely to the topside of the copper film. This hydrogen spillover to the Cu surface is observed
to occur even after completion of many monolayers of copper deposition on top of cbemisorbed H.
In addition, hydrogen bound to thin copper layers on Ru ( < 3 monolayers) shows decidedly
different desorption kinetics, consistent with a weaker copper hydrogen bond, than hydrogen
bound to a muhilayer ( > 3 monolayers) of copper or to bulk copper.
1. Introduction
In c o n t r a s t to t h e t r a n s i t i o n m e t a l s , little i n f o r m a t i o n is a v a i l a b l e w h i c h
d e s c r i b e s t h e i n t e r a c t i o n o f h y d r o g e n w i t h n o n - t r a n s i t i o n m e t a l s [1 4]. G e n e r ally t h e d i s s o c i a t i o n of H~ o n n o n - t r a n s i t i o n m e t a l s is a n a c t i v a t e d p r o c e s s .
T h e b a r r i e r to d i s s o c i a t i o n of H , o n t h e low, i n d e x s u r f a c e s o f c o p p e r h a s b e e n
d e t e r m i n e d b y m o l e c u l a r b e a m t e c h n i q u e s to b e in t h e r a n g e o f 3 - 6 k c a l / m o l
[1]. A p h y s i s o r b e d m o l e c u l a r s p e c i e s h a s b e e n o b s e r v e d [5,6] for t e m p e r a t u r e s
less t h a n 10 K w h i c h s u g g e s t s a v e r y s h a l l o w p h y s i s o r p t i o n well. A l t h o u g h n o
s p o n t a n e o u s H~ d i s s o c i a t i o n h a s b e e n o b s e r v e d for t h e low i n d e x c o p p e r
s u r f a c e s , a t o m i c h y d r o g e n h a s b e e n s h o w n to r e a d i l y c h e m i s o r b o n C u ( 111 ) [4].
B o t h p h o t o e m i s s i o n ( U P S ) a n d t h e r m a l d e s o r p t i o n ( T P D ) d a t a for a t o m i c a l l y
a d s o r b e d h y d r o g e n o n t h i s s u r f a c e a r e q u a l i t a t i v e l y s i m i l a r to d a t a for a t o m i c
h y d r o g e n o n d e n s e p a c k e d t r a n s i t i o n m e t a l s u r f a c e s [4]. T h e o n l y m a j o r
d i f f e r e n c e in t h e t w o s y s t e m s t h e n is t h e a c t i v a t i o n b a r r i e r for d i s s o c i a t i o n o f
H~ o n c o p p e r .
* This work performed at Sandia National Laboratories supported by the US Department of
Energy under contract No. DE-AC04-76DP00789.
** Visiting summer faculty from the Surface Science Center. University of Pittsburgh, Pittsburgh.
Pennsylvania 15260, USA.
0039-6028/85/$03.30
+) E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
(North-Holland Physics Publishing Division)
418
D. 1/I] G o o d m a n ¢>t a/. / R e a c t i o n <~/copper wilit t{vdro<e,en on r u t h e n i u m
The c o p p e r / r u t h e n i u m system has received extensive attention over the
years as a bimetallic catalyst which exhibits enhanced activity and selectivity
for certain reactions c o m p a r c d to unnlodified ruthenium. This bimetallic
system has been modelled by depositing Cu in vacuo onto a Ru(0001) single
crystal surface [7 14]. Its behavior toward the adsorption of H , (I)~) [8,9], ('()
[10], ()2 [11]. and N O [11] and its activity toward methanation of CO and tile
hydrogenolysis of ethane have been studied [12] as a function of ('u coverage.
In addition, Ru site blocking by Cu has been studied, and, for H e chemisorption. 1 monolayer of Cu blocks essentially all Ru adsorption sites [14]. At
submonohiyer ('u coverages, 1 Cu atom blocks 1 H adsorption site [14].
Of critical importance to the interpretation of the adsorption and reaction
results is a detailed knowledge of the geometrical and electronic structure of
the C u / R u surface. Ultraviolet photoemission [13] and work function restllts
[7] suggest that for a preparation temperature of 1080 K. the Cu is prcdominantly atomically dispersed: at coverages below 0.1 monolayers (ME1 ('u is
chemically bonded to the Ru surface as isolated atoms of Cu. As the coppcr
coverage increases, well-ordered two-dimensional islands form, and the desorplion energy and the pre-exponenlial factor for desorption rise due to lateral
binding between the copper atoms. The UPS results [13] suggest that these
copper islands are probably smooth and flat and have a two dimensional band
structure charactcristic for copper. At higher copper coveragcs a clear layerby-layer grow.'th occurs [7,13].
Here we have studied the effects of adding copper by vapor deposition in
ultrahigh vacuum to a Ru(0001) surface at 100 K which has been pre~b).ved with
a st(ljTcient exposure ~)/ fl, to saturate the surfilce with atomic/0,drogen, tt(adx).
The addition of atomic copper to this surface results in the displacement of thc
H(ads) to the topside of the metallic film for submonolayer to multilaycr
copper coverages. Marked changes in the H, T P D as a function of copper
coverage suggest that the copper morphology is decisively affected by the
presence of H(ads) during deposition. The T P D results also show that H(ads)
b o u n d to very thin copper layers on Ru ( < 3 monolayers) has a bond strength
decidedly different from hydrogen bound to multilayer ( > 3 monolaycrs)
copper or to bulk copper. Together these data suggest that thin copper layers
on Ru likely will exhibit a surface chemistry uniquely different from pure
copper.
2. Experimental
A conventional ultrahigh vacuum system (base pressure < 2 x 10 > Torr)
was used in this work. The apparatus is equipped with two collimated
molecular beam dosers which face the front surface of a Ru(0001) single
crystal disk. The crystal can be rotated to either doser for adsorption of gases.
D. ~ Goodman
el
al. / Reaction ~*f('opper wilt hydrogen on ruHwnium
419
Temperature p r o g r a m m e d desorption can be carried out with the crystal in
front of a UTI 100C multiplexing quadrupole mass spectrometer (QMS) using
a linear ramp of 10 K s ~. The mass spectrometer samples 4 mass peaks every
0.7 s with a sampling time of 0,1 s / a m u .
Prior to cleaning, the backside and edges of the Ru(0001) crystal were
masked by a 10 L exposure of H~S with the sample at 600 K. This sulfiding
procedure was shown to attenuate hydrogen chemisorption to less than 10% of
that found for the clean surface. The front face of the Ru(0001) was then
cleaned using an O, beam of flux ~ 5 x 10 ~4 O, cm 2 s ~ at a crystal
temperature of 1450 K for 300 s. Final cleaning was achieved by heating to
1500 K in vacuum, yielding an Auger spectrum for the front face of the crystal
free of O, S, and C contamination. Hydrogen desorption following a saturation
exposure gave a T P D spectrum with an integrated area approximately equal to
1 / 2 that for the unmasked sample.
C o p p e r was evaporated onto the clean or H-covered Ru(0001) sample from
a resistively heated tungsten wire wrapped with high-purity Cu wire. The Cu
source was thoroughly outgassed prior to Cu evaporation, and the deposition
rate was accurately controlled by monitoring the voltage drop across the
tungsten filament. The Cu flux from the evaporator was also checked routinely
via a quartz micro balance mounted off-axis to the Ru sample. T P D and Auger
analysis following Cu dosing showed no measurable contamination of the Ru
surface during evaporation, and Auger studies of the Cu overlayer indicated
that it too was atomically clean.
Copper coverages on the Ru(0001) surface could be measured by means of
line-of-sight T P D into the QMS [7-10,14]. These measurements give distinctive
indications of copper multilayer formation. At the onset of copper multilayer
formation, a low temperature copper desorption peak at - 1100 K appears as
a distinct shoulder on the monolayer copper desorption peak. This permits us
to accurately calibrate copper coverages in units of one copper monolayer.
3. Results and discussion
Fig. 1 shows the results of the evaporation of less than one monolayer of
c o p p e r onto a Ru(0001) surface at 100 K previously, saturated with H~. For
reference, fig. la is included indicating the H 2 T P D from a clean Ru(0001)
surface. Results for the clean surface have been discussed elsewhere [8,14]. In
fig. 2 are shown the H 2 T P D traces for copper deposition to coverages between
approximately 1 and 6 rnonolayers onto a Ru(0001 ) surface previously saturated
with H~ as in fig. 1. Finally, fig. 3 shows similar data for copper deposition of
between 13 and 200 monolayers onto chemisorbed H layers.
The first point to note is that the areas under the T P D traces remain
virtually constant during the course of the experiments described by figs. 1 3.
420
D. H/. Goodman et al. / Rea( tion ~[ copper with hydrogen on ruthenium
¢q
"1"
n
<:1
100
200
300
T
400
500
(K)
Fig. 1. T P D s p e c t r a ~f }t~ f r o m C u ( s u b m o n o l a y e r ) overla~ers d e p o s i t e d o n top of a n II
m o n o l a y c r on Ru(0001): (a) 0 ( ~ , = 0 0 (b) 0 ~ . = ( I . 4 1 , (c) 0 ~ . = 0 . 6 5 . (d) 0( = ) 8 8 (c) 0 ( . = 1 .
o,
100
200
300
400
500
T (K)
Fig. 2. T P D ~,pectra of H~ f r o m ( ' u (l 8 monolayer~,) o v e r l a y e r s d e p o s i t e d on lop of an II
m o n o l a y e r o n R u ( 0 0 0 1 ) : (a) 0~. = 1.4. (b) 0~L, = 1 . 7 , (c) 0C~, = 2.6. (d) 0~. = 3.6. (e) 0~ . ~ 6.1.
D. ~
Goodman et al. / Reaction of copper with hydrogen on ruthenium
421
04
q-
a.
i
100
200
I
300
i
400
500
T (K)
Fig. 3. TPD spectra of H 2 from Cu ( > 8 monolayers) overlayers deposited on top of an H
monolayer on Ru (0001): (a) 8c, = 13, (b) 0Cu = 39, (c) 0Cu = 108, (d) Ocu ~ 200.
T h a t is, the total a m o u n t of h y d r o g e n present originally on the clean r u t h e n i u m
surface is still present following large a m o u n t s of c o p p e r d e p o s i t i o n on top of
the H layer. N o m e a s u r e a b l e loss of H ~ is observed even during the sequential
d e p o s i t i o n of a p p r o x i m a t e l y 200 m o n o l a y e r s of copper. This clearly shows that
facile m i g r a t i o n of H a t o m s from the r u t h e n i u m to the c o p p e r overlayers has
occurred. A n c i l l a r y e x p e r i m e n t s described elsewhere [15] show that a similar
spillover of H a t o m s from r u t h e n i u m to c o p p e r can occur during the a d s o r p tion of H 2 o n t o a Ru(0001) surface p r e d o s e d with s u b m o n o l a y e r quantities of
Cu. These latter results have d e m o n s t r a t e d that this spillover of h y d r o g e n can
be a source of error in the titration by h y d r o g e n c h e m i s o r p t i o n techniques of
active metal (i.e. Ru) in a matrix of inactive (i.e. Cu) metal.
W i t h c o p p e r a d d i t i o n s of less than one m o n o l a y e r (figs. l a - l e ) distinct
changes in the H 2 T P D are observed in the t e m p e r a t u r e region 100 to 300 K.
N e w H 2 T P D states are s u p e r i m p o s e d at lower t e m p e r a t u r e s on an a t t e n u a t e d
H - R u ( 0 0 0 1 ) d e s o r p t i o n spectrum. A feature at - 1 9 0 K develops early and
d o m i n a t e s at - 1 m o n o l a y e r . A second lesser feature is also seen at - 220 K.
These two features can not be a t t r i b u t e d to H 2 d e s o r p t i o n from essentially
p u r e c o p p e r islands by c o m p a r i n g these results with H~ d e s o r p t i o n from bulk
c o p p e r [4]. P l u m m e r and coworkers [4] o b t a i n e d a T P D s p e c t r u m with features
at 230 and 300 K after dosing a C u ( l 11) crystal with a t o m i c hydrogen. Results
similar to those of P l u m m e r and coworkers [4] were o b t a i n e d in our study
using H - a t o m dosing techniques onto a m u l t i l a y e r c o p p e r film on Ru(0001).
T h e m a r k e d differences in the T P D profiles and d e s o r p t i o n peak t e m p e r a t u r e s
for the C u / R u a n d pure Cu systems therefore rules out the d e s o r p t i o n of H~
from a bulk c o p p e r surface in fig. 1. W e believe instead that the H 2 d e s o r p t i o n
422
D. W. Good#nan e/al. / Reaction o/copper wilh t vdro~e# ot rutheniul~l
H
•
•
•
,
°
•
•
•
°
.
¢X'X3CX-X'k'hr'I"~
(d)
H
H
OCu > 1 2 0 ML
H
•
•
,
•
°
•
•
•
,
•
•
•
•
o
.
(C) OCu> 5 ML
W2/W/2/J_I
H
H H
H
.,. 6 : ~ ; ~
V2/2A
H H
(b)
®Cu ~ 1 ML
(a)
OCu< 1 ML
H
..~;eb ;
Fig. 4. Schematic diagram of H adsorption ~n ,,arious thickness ('u/Ru (0001) deposits. The
actual bonding stoichiometr~ of H to Cu surl'ace sites is not known: (d) also shov,s schematically
trapped 11 in bulk defects or micro,~oids.
in fig. 1 arises from a highly dispersed Cu on the Ru(0001) substrate. A
schematic of the proposed Cu Ru structure at this copper coverage is shown in
figs. 4a and 4b. It should be noted that the overlayer growth as shown in fig. 4
contrasts with the type of growth believed to occur [7] oil the hydrogen-free
Ru(0001) surface. O n the clean Ru(0001) surface, the copper growth is thought
to be d o m i n a t e d by two d i m e n s i o n a l island formation. Evaporation onto tile
hydrogen covered substrate a p p a r e n t l y leads to a significant e n h a n c e m e n t of
the copper atom dispersion, due possibly to Cu--H b o n d formation which
suppresses Cu- Cu b o n d formation.
As the copper deposition on top of chemisorbed H increases from 1 to 6
monolayers in figs. 2a 2e, desorption features at 230 and - 2 7 0 K begin to
grow in at the expense of the 190 and 220 K peaks and are clearly established
in fig. 2c. These T P D features a p p r o x i m a t e those found for hydrogen on
C u ( l l l ) by P l u m m e t a n d coworkers [4]. This similarity suggests that at the
copper coverage of - 3 monolayers shown in fig. 2c, the overlayer is complete
a n d free of exposed r u t h e n i u m sites. There is no indication at this copper
coverage of any p e r t u r b i n g influence on the copper by the u n d e r l y i n g substrate. That is, the copper layer at a coverage of - 3 monolayers behaves as if
it were bulk copper. The close similarity between fig. 2c and the results of
D. W. Goodman et al. / Reaction oJ copper with hydroyen on ruthenium
423
Plummer and coworkers [4] also suggests that the copper growth is likely
epitaxial and thus morphologically very similar if not identical to a C u ( l l l )
surface. Such a growth process at this coverage has been shown to take place
for Cu/Ru(0001) by Christmann and coworkers [7].
Additional deposition of copper to coverages greater than 3 monolayers (fig.
2c and beyond) is accompanied by the development of a single H2-desorption
feature at - 310 K (figs. 2d and 2e) which broadens substantially (figs. 3a and
3b) with an increase in the metal coverage. Finally, at copper coverages greater
than 50 monolayers, a two peak TPD structure is apparent with peak maxima
at - 310 and 390 K. The origin of the multiple TPD features at high copper
coverages is not apparent. The 390 K feature may arise from surface defects in
the copper overlayer as depicted schematically in figs. 4c and 4d. Another
possibility is that the feature at 390 K corresponds to occluded or trapped
hydrogen in copper microvoids or defects, as schematically shown in fig. 4d.
In any case, deposition of copper onto a hydrogen covered Ru(0001) surface
leads to complete spillover of hydrogen to the copper. This technique provides
a convenient preparation procedure for atomic hydrogen on an evaporated
copper overlayer. Further studies are underway to characterize the nature of
the bonding of this hydrogen to copper as well as its activity toward coadsorbates.
4. Summary
(1) Copper deposition onto a hydrogen covered Ru(0001) substrate leads to
complete migration or spillover of the hydrogen from the ruthenium to the
copper.
(2) The morphology of the deposited copper is markedly' affected by' the
H-atom coverage and leads to highly dispersed copper in contrast to copper
deposition onto a hydrogen-free surface.
(3) Copper intimate to Ru (i.e. < 3 monolayers) exhibits a characteristic H,
TPD spectrum significantly different from that observed for bulk copper.
(4) At a copper coverage of - 3 monolayers the H : TPD from Cu/Ru(0001 )
shows a striking similarity to the TPD of H, from pure C u ( l l l ) .
(5) At very high Cu coverages, evidence for H adsorption or H trapping at
internal defect sites is observed.
Acknowledgements
We acknowledge with pleasure the partial support of this work by the
Department of Energy, Office of Basic Energy Sciences, Division of Chemical
Sciences. We also acknowledge the skilled technical assistance of Kent Hoffman throughout the course of this work.
424
D. Hi Goodman et al. / / R e a c t i o n o f copper wtl]l hydrogen on ruthenium
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