Model studies on bimetallic CuRh catalysts

487

Surface Science 152/153 (1985) 487-495 North-Holland, Amsterdam

MODEL

STUDIES

J.S. FOORD Department Received

ON BIMETALLIC

Cu-Rh

CATALYSTS

and P.D. JONES

of Chemistry,

The University. Southampton

SO9 5NH. UK

1 April 1984

Cu deposition on Rh(ll1) has been investigated and the surface reactivity of the Cu-Rh(ll1) interface with regard to CO and C,H, chemisorption examined. Growth of (111) oriented Cu films is shown to take place via a 3D island (Volmer-Weber) growth mechanism, with island coalescence occurring at a total Cu coverage of around 2 monolayers. Two binding states are observed in thermal desorption studies. The more weakly adsorbed state (desorption energy 330 kJ molt ‘) is associated with bulk copper while a more strongly adsorbed state (desorption energy 355 kJ mol-‘) is observed for Cu in contact with Rh. Alloying of Cu with the underlying Rh takes place slowly at 1000 K. Cu efficiently blocks the strong chemisorption of CO on Rh(ll1) and a small ligand effect is observed whereby the RI-CO bond is weakened in the presence of the second metal. Cu has very little effect on the sticking probability of CO, suggesting “spill over” from Cu to Rh takes place in a mobile precursor state. The chemisorption of C,H, on the bimetallic surface is essentially similar to that observed on the separate metals. Using conventional ensemble effects are observed. This is attributed to log eada versus log S,, p lots, no pronounced the presence of attractive interactions within the adlayer which result in the formation of Cu clusters.

1. Introduction

The catalytic activity and selectivity of a metal is generally altered when it is alloyed with a second component and research into the surface reactivity of metal alloys has made a fundamental contribution to the understanding of the general field of catalysis [l]. An important group of such catalysts consists of the Group VIII transition metals alloyed with inert Group 1B elements; interesting model studies of such systems can be carried out using surfaces prepared by vacuum deposition of the Group 1B elements onto single crystal Group VIII metal substrates [2-61. We have studied the Rh(lll)-Cu surface in this way and here report results concerning CO and C,H, chemisorption.

2. Experimental All experiments were carried out in the stainless steel UHV system which we have described in detail elsewhere [7]. The chamber was operated at a base

0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

pressure of around 10 ’ Pa and contained facilities for LEED, a CMA for AES and a quadrupole mass filter for thermal desorption studies. The Rh(ll1) crystal was cleaned by heating in lO-i Pa OL at 900 K. Ar ’ ion etching and annealing in vacua at 1450 K. No surface impurities as measured by AES were present following this treatment. Cu deposition was carried out by evaporation from a copper coated tungsten filament operated at a constant temperature of around 1250 K, while dosing with CO and C,H, was by admission of high purity gases through a bakeable metal leak valve.

3. Results 3. I. Cu depositim The first part of the work involved the characterisation of the Cu deposit on the Rh(lll) surface and this was carried out using LEED, AES, and thermal desorption measurements. Copper was evaporated from the copper source at a constant rate and the intensities of the Cu M,,M,M, (62 eV) and the Rh M,N4,5N4,s (302 eV) AES transitions determined as a function of exposure time of the copper beam to the Rh(ll1) crystal at 300 K. The results are shown in fig. 1 and it is apparent that both the Cu and the Rh signals show a smooth variation as the copper deposit builds up on the crystal. The ratio of the Cu(62 eV) : Rh(302 eV) AES signal intensities as measured by peak-to-peak heights in the second derivative mode serves as a useful monitor of the amount of copper deposited, provided that the crystal temperature is kept at 300 K during deposition and measurement, and this ratio I will be used to define the surface coverage of copper throughout this paper. LEED measurements were also made during Cu deposition at 300 K. Little change in the LEED pattern of the Rh( 111) surface was visible, apart from a

Rh(302eV)

\

Cu DEPOSITION Fig.

1. Auger

signal-time

plots

TIME durmg

c‘u deposition

on Rh( 111).

J.S. Foord. P.D. Jones / Bimetallic

489

Cu- Rh catalysts

gradual increase in the diffuseness of the spots throughout the deposition process, up to the point at which the AES signal from the underlying Rh(ll1) substrate disappeared. However, the diffuseness was removed if the crystal was annealed at 700 K and a small displacement of the observed beams away from the (0, 0) beam was noted as the Cu growth thickened. The data is consistent with the growth of the copper film according to the relationships (111)Cu II (lll)Rh, [llO]Cu II [llO]Rh, the slight displacement of the two hexagonal spot systems from Rh and Cu arising due to the slightly different lattice parameters (3.80 A Rh, 3.62 A Cu) of the bulk metals. No other information was obtainable from LEED. Thermal desorption results are shown in fig. 2. At low Cu coverages (x < 0.4) a single peak at 1210 K is visible corresponding to Cu atoms in a single state (/?) on the surface. The peak temperature for this state increases with copper coverage and the traces very nearly exhibit common leading edges. At copper coverages x > 0.43, a second peak at lower temperatures (a state) becomes visible and both peaks develop as x increases up to 1.1. At higher Cu coverages, exclusive growth of the (Ystate peak occurs with characteristic zero order kinetics. Arrhenius plots using the leading edge of the desorption traces (where the Cu coverage remains relatively constant) yielded an activation energy of 330 f 10 kJ mol-’ for desorption from the (Y state and a value of 355 & 15 kJ mol-’ for the j3 state. The value for the cr state is in good

I

1030

I

1210

I

1390 TEMPEG :ATURE(K)

Fig. 2. Thermal desorption spectra of Cu from Rh(ll1).

I

1030

I

1210

I

1390

agreement with the sublimation energy of Cu and the continuous increase in the area of the a: state peak with increase in the copper coverage, as well as the zero order desorption kinetics, all point to the cy state representing evaporation of bulk copper from the Rh-Cu surface. The ,B state represents Cu in contact with Rh; the binding energy conseyuentiy differs from that of the LYstate. Flash heating of the sample (25 K/s) to 1000 K and cooling to 300 K showed that the Cu(62 eV) : Kh(302 eV) AES intensity ratio dropped by fess than 105: during this process. However. if the sample was annealed at 1000 K for 300 x the ratio dropped roughly by a factor of two for copper deposits s < 2. Using evaporation rates calculated from thermal desorption data, this apparent loss of copper from the surface is roughly two orders of magnitude greater than could be accounted for by evaporation alone. This suggests that the Cu diffuses into the Rh at elevated temperatures.

Cu was evaporated onto the crystal at 300 K in amounts determined by AES and the interface was then exposed to 20 L CO. which experiment showed was sufficient to saturate all the sites available for population at 300 K.

36J

8 620

300

i 300

620

TEMPERATURE(K) Fig. 3. CO thermal

desorption

spectra

from Rh( 11 I )-Cu.

for varying xurfacc concentrations

of Cu.

491

.I. S. Fwrd. P. D. Jones / BtmetalIic Cu- Rh rata&s

Thermal desorption spectra of the CO were then monitored and the results are shown in fig. 3. At this temperature the Rh substrate accommodates approximately half a monolayer of CO and exhibits a desorption spectrum with two overlapping peaks from terminally bonded and a more weakly bonded bridging CO [S]. As the copper contribution rises, the nett amount of CO adsorbed decreases and some changes occur in the desorption profile, commensurate with an increase in the relative concentration of weakly bound CO. The CO desorption yield, plotted as a function of AES intensity ratio X, is shown in fig. 4. The initial sticking probability of CO on the copper covered surface was estimated by measuring the amount of CO adsorbed in a 0.1 L dose by TDS (calibrated by assuming 0.5 monolayers to be the saturation coverage of CO at 300 K [S]). A value of 0.6 was calculated and it was particularly interesting to note that this remained the same within experimental error over a wide range of Cu coverages. Similar experiments were carried out using C2H, and relevant thermal desorption spectra monitoring H, and C,H, are shown in fig. 5. Acetylene converts to ethylidyne species on Rh(lll) at 300 K, decomposing into CH_, fragments and H,, which is evolved as a sharp peak at 420 K. Subsequent decomposition of the CH, fragments is then responsible for the high temperature tail which is visible in the H, spectra [9]. In contrast, acetylene adsorbs into a molecular state on Cu and gives rise to exclusive C,HZ desorption (lo]. As the Cu coverage increases up to x = 1.1 the H, desorption yield drops to

Fig. 4. Desorption

yields plotted

against

s, for CO and C,H,

adsorption

on Rh(lll)-Cu.

zero and the acetylene yield increases to a maximum value: the variation in yields is shown in fig. 4. If C,H, is adsorbed on clean Rh, and the crystal heated to X00 K to desorb yield of H, is measured after the bound H,, a reduced thermal desorption subsequent resaturation of the surface with C,H2 since deposited carbon poisons the surface against C,H2 chemisorption. Very similar results were obtained from the Rh-Cu bimetallic interface, the H, desorption yield falling with each C2H, ads~~rption-desorption cycle. However. the C,H, desorption yield remained unchanged during this procedure. A limited number of experiments were carried out to examine the possible surface to bulk transport properties of Cu suggested by the results in section 3.1. Cu was deposited on the surface to produce a value of x = 1.1. The sample was then annealed at 1000 K for 5 min during which the Cu: Rh AES ratio dropped by about a factor of two. The surface was then exposed to acetylene at 300 K and the C,H2 and H, desorption yields determined. The H, yield was very close to that found for Cu-free Rh, while no CzH2 desorption was observed. This is in marked contrast to the behaviour observed on the Cu-Rh interface prepared at 300 K for the corresponding value of .x.

I’,C2H2 /i

1

300 Fig. 5. Thermal coverage.

I

570

760

I

,

300

570

TEMPERATURE (K) desorption spectra of H,

and

C2 H2 from

Rh(1 1 I)-C’u

as ;I function

of C‘u

J. S. Foord, P. D. Jones / Bimetallic

Cu - Rh catalysts

493

4. Discussion 4. I. Cu deposition The results all indicate that Cu deposition on Rh(ll1) at 300 K follows a 3D crystallite island growth mechanism, with island coalescence at higher Cu coverages resulting in the formation of (111) oriented epitaxial films. The conclusion regarding the orientation of the film is verified by LEED while AES, TDS and chemisorption measurements show that clustering of the Cu atoms take place at low Cu coverages. The two peaks visible in the thermal desorption spectra arise from the desorption of bulk Cu (a state) and Cu in contact with the Rh (p state) and the desorption yields can be used to monitor the development of the first Cu layer. The spectra clearly show the onset of desorption from the (Y state before the j3 state is saturated. Any thermal conversion process that takes place is likely to be (Y+ p, and therefore the results are in line with a 3D island growth mechanism. The spectra suggest that at least two Cu monolayers need to be deposited before the Rh surface is completely covered in Cu. The amount of copper deposited can also be estimated crudely using AES by determining the damping of the Rh(302 eV) peak as a function of exposure to the Cu beam. Taking a value for the mean free path in Cu of electrons with a kinetic energy of 302 eV (Rh AES signal), as 7 A [6], then a monolayer of Cu should reduce the Rh signal by about 25% which corresponds to a value of x = 0.2. Such an estimate is likely to be inaccurate but the observation that chemisorption of CO can still take place at Cu coverages well above this value is a good pointer towards island growth behaviour. Finally, it is fairly well established that Auger signal-time plots recorded during metal deposition (fig. 1) which show no abrupt changes in slope are characteristic of a Volmer-Weber (island growth) mechanism [ll]. At low Cu coverages (x < 0.4) the thermal desorption results suggest that the Cu is present as a single adlayer, at least after the crystal has been heated to near the desorption temperature. At low coverage the Cu could be dispersed randomly at the surface, or else be clustered in two-dimensional islands. This is important in the quantitative evaluation of ensemble effects [4]. The desorption profile of the /3 state shifts to higher temperatures as the copper coverage rises, indicating (pseudo) zero order or fractional order kinetics. This is characteristic of the presence of attractive interactions within the adlayer and 2D island growth [12,13]. It therefore seems likely that the Cu is not distributed randomly over the surface, but is clustered into islands, even at very low Cu coverages (2 X 1O’a atoms mm2). As described in section 3.1, prolonged annealing of the Rl-Cu interface resulted in the apparent diffusion of Cu into the bulk of the crystal. This is confirmed by the experiments described in section 3.2, which illustrated that the annealed surface with a Cu : Rh ratio of 0.5 behaved essentially the same as

pure Rh, in contrast to interfaces maintained at 300 K with a value of I = 0.5. Presumably the Cu AES signal arises from the presence of sub-surface Cu which apparently has very little effect on the chemical reactivity of the surface Rh. Bulk diffusion is unsurprising in view of the fact that copper shows appreciable miscibility with Rh. and forms several discrete bulk alloy phases [14]. Since bulk diffusion can occur below the desorption temperature it might have been expected that no thermal desorption of copper would be observed. The fact that it is must mean the desorption process has a larger activation energy than bulk diffusion, but a smaller pre-exponential factor: by using a large heating rate the crystal is heated to temperatures where evaporation can take place and this dominates over diffusion into the bulk. 4.2. Chemisorption

of CO und C,H,

Chemisorption and catalysis on bimetallic surfaces is normally described in terms of ligand (electronic structure) and ensemble (geometric arrangement) effects and we discuss the results in this same framework here. The most obvious conclusion is that no dramatic alterations in surface chemistry from that of the two separate components are brought about by alloying, emphasising the view that chemisorption on metals is a localised process. No differences in the Hz and C,H, desorption profiles, characteristic respectively of Rh and Cu, were observed from the alloy, pointing to the absence of ligand effects for C2H, adsorption on Rh-Cu. The low coverage CO chemisorption energy on Rh estimated from the desorption spectra using the Redhead equation and an assumed pre-exponential factor of lOI s ‘, drops from 130 to 110 kJ mol. ’ as B,,, increases. Similar reductions have been observed on Ku-Cu surfaces [4]. and they may be ascribed to a ligand effect, whereby Rh (Ru) --+ c’u charge transfer reduces back-donation of electrons into the CO 7~* orbital and weakens the chemisorption bond. However, changes in CO bonding geometry and steric repulsions in the presence of Cu, rather than a simple electronic effect, could also account for the observations. The most clear cut behaviour is the variation in extent of chen~~s~~rption with Cu coverage. Both the CO and the Hz (from C,Hz) desorption yields drop to zero at a value of s = 1.1, indicating that the Rh substrate becomes completely masked at this coverage. The Cu thermal desorption spectra agree nicely with this conclusion, assuming that saturation of the fl state represents completion of the Cu layer in contact with the Rh. If the Cu was randomly distributed over the Rh surface, the linear plots shown in fig. 5 would imply that both CO chemisorption and ethylidyne formation require only isolated Rh atoms [15]. This conclusion is scarcely tenable in the case of ethylidyne where an ensemble size of at least 3 would be required [19]. The discrepancy arises because the deposited Cu exhibits a high degree of clustering; conventional log-log plots which determine critical ensemble sizes do not work under such

J.S. Foord

P.D. Jones / Bimetallic

Cu- Rh catalysts

495

circumstances. In actual fact we have also measured ensemble effects for these adsorbates on Rh(lll)/S surfaces, where C2 H, adsorption was shown to require an ensemble of 6 Rh atoms, although the value for CO was again found to be less than 2 atoms [7]. CO adsorbs preferentially on the “on-top” positions on Rh(lll), so the ensemble size for this molecule is unsurprising. The amount of CO adsorbed on Ru-Cu surfaces [4] at high 0,., also shows a linear variation with Cu coverage although it exhibits a sharper drop when small amounts of Cu are deposited. This feature was attributed to a disruption of the long range forces required to stabilise the CO compression phase on Ru. In the case of Rh, LEED showed that CO compression was still observable at comparatively high Cu coverages (x = OS), so it is this feature that is responsible for the differences between the Rh and Ru surfaces. It is interesting that Cu has little effect on the sticking probability of CO. This implies that the incoming CO molecules can be captured into a mobile weakly bound state on Cu. and then “spill over” into chemisorption sites on Rh before desorbing. It is difficult to estimate if spill-over effects also occur for C,H, adsorption since this molecule forms stable chemisorption states on both components in the bimetallic interface, However, the selective poisoning of the Rb function by carbon deposits during C,H, adsorption-desorption cycling illustrates that the CH, species formed on Rh show little tendency to migrate to cu.

Acknowledgement

The authors

are grateful

to Dr. R.N. Lambert

for loan of a Rh(lll)

crystal.

References [I] J.H. Sinfelt, Act. Chem. Res. 10 (1977) 15. [ZJ K. Christmann, G. Ertl and H. Shim&, J. Catalysis 61 (1980) 397. [3] H. Shim&, K. Christmann and G. Erti, J. Catalysis 61 (1981) 412. [4] J.C. Vickerman, K. Christmann and G. Ertl, J. Catalysis 71 (1981) 175. I.51 J.C. Vickerman and K. Christmann, Surface Sci. 120 (1982) 1. [6] W.M. Daniel, Y. Kim, H.C. Peebles and J.M. White, Surface Sci. 111 (1981) 189. [7] J.S. Foord and A.E. Reynolds, Surface Sci. 152/153 (1985) 426. [8] L.H. Dubois and G.A. Somorjai, Surface Sci. 91 (1980) 514. [9] L.H. Dubois, D.G. Castner and G.A. Somorjai, J. Chem. Phys. 72 (1980) 5234. [lo] KY. Lu, W.E. Spicer, I. Landau, P. Pianetta and SF. Lin. Surface Sci. 57 (1978) 157. [ll] G.R. Rhead, M.G. Barthes and C. Argile, ‘Thin Solid Films 82 (1981) 201. 112) R.G. Jones and D.L. Perry, Surface Sci. 82 (1979) 540. (131 R.G. Jones and D.L. Perry, Surface Sci. 71 (1978) 59. 1141 M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). [15] K.Y. Lu, D.T. Ling and W.E. Spicer, J. Catalysis 44 (1973) 373.