The adsorption and catalytic oxidation of carbon monoxide on evaporated palladium particles

Surface Science 0 North-Holland

102 (1981) Publishing

lSlL171 Company

THE ADSORPTION

AND CATALYTIC

ON EVAPORATED

PALLADIUM

OXIDATION

OF CARBON MONOXIDE

PARTICLES

S. LADAS, H. POPPA and M. BOUDART * Stan,ford/NASA Joint Institute fbr Surface and Microstructure Research, Stanford, California, USA Received

17 July 1980

Palladium crystallites, evaporated in UHV on a o1-A120s single crystal support and characterized by transmission electron microscopy (TEM), have been used as catalysts for the low pressure oxidation of carbon monoxide between 450 and 550 K. Average particle diameters varied between 1.5 and 8 nm. The turnover rate N, i.e. the number of molecules of CO2 made per second per surface palladium atom was measured. The number of surface palladium atoms was determined by combining TEM and temperature programmed desorption of CO. Values of N, under constant conditions, were practically identical on all samples. It is noted that the reaction studied is structure insensitive on palladium.

1. Introduction

Metal particles evaporated onto a flat support and characterized by electron microscopy can serve as a model for highly dispersed supported metal catalysts [ 11. Advantages are easy control of the average particle size, direct observability of the particles, and very little contamination, provided of course that the support is clean and inert, and that the evaporation is carried out under high vacuum. Thus, evaporated metal particles appear to be ideal for the study of the effect of particle size on the rate of a catalytic reaction. Changing particle size from 1 nm upward is one way to change the structure of the metal surface which may be important in heterogeneous catalysis [2]. The only reported case where a catalytic reaction was carried out on evaporated metal particles of varied size was the hydrogenolysis of various hydrocarbons on Pt [3]. The reactions were studied in the pressure range of a few kPa, in which experiments with supported metal catalysts are usually performed. Recently, catalytic reactions on metals have been studied on large single crystals with surface structure and cleanliness determined by LEED and AES [4]. The surfaces studied were either smooth or stepped single crystals of various orientations. * To whom inquiries should be sent: Department Stanford, California 94305, USA.

151

of Chemical

Engineering,

Stanford

University,

152

S. Ladas et al. /Adsorption

and oxidation

of CO on Pd

Changing orientation is a way to vary the surface structure in a well-characterized manner. Reactions were usually carried out at low pressure (below 10e3 Pa) although very recently single crystal surfaces have also been used in reactions at high pressure. The work presented here is to our knowledge the first to be reported in which evaporated small metal particles were successfully used in low-pressure catalytic experiments. The model catalyst system was Pd evaporated in UHV onto single crystal a-AlaO in vacuum. The key improvement over previous work [3] is the characterization of the catalyst by Transmission Electron Microscopy (TEM) and Transmission Electron Diffraction (TED) without having to thin the specimens after the experiments, or to remove the particles from the support for observation. Furthermore, o~-Al~Oa is an inert support and could be heated in UHV above 1400 K to clean off carbon contamination. The Pd particles ranged in average size from 1.5 to 8 nm. The use of an UHV chamber for the preparation of the particles and the subsequent experiments allowed the study of the chemisorption and the oxidation of CO on a nearly clean Pd surface. The nucleation and growth of metal particles evaporated in UHV have been investigated extensively in the past by one of us using, among other techniques, TEM and TED [5-71. In the most recent study [7], small Fe and Pd particles were grown in UHV in an in-situ electron microscope by vapor deposition onto different phases of electron transparent films of Al2O3. The results showed that the size, shape, and crystallographic orientation of the particles strongly depended on the cleanliness and crystallographic orientation of the support and could be reproducibly manipulated by adjusting the deposition conditions. The experiments with vapor deposited particles of Pd, Ni, and Fe were extended to include CO chemisorption measurements by temperature programmed desorption (TPD) [8,9], and steady state oxidation studies of amorphous carbon by in-situ TEM in UHV [lo]. The chemisorption of CO on Pd has been studied extensively in the past by various techniques on both single-crystal and polycrystalline surfaces. Early work on polycrystalline Pd has been reviewed [ 111. Recently Ertl and coworkers have published extensive data on the binding energy of CO, its kinetics of chemisorption, the bonding, and the geometrical arrangement and mutual interactions of adsorbed CO molecules on various Pd planes, especially the (11 l} face, as well as Pd polycrystalline wires [ 12 -141. The oxidation of CO has been also extensively studied over single-crystal and polycrystalline Pd at low pressure [ 12,1517], and over small Pd particles near atmospheric pressure [ 181. Very recently, Engel and Ertl unequivocably established the mechanism of the reaction on Pd(ll1) between 200 and 700 K and at pressures between 1Oe6 and 1.0m4Pa [ 171. They showed by means of modulated molecular beam experiments that COZ is formed by a surface reaction between adsorbed CO and adsorbed 0 in a LangmuirrHinshelwood mechanism. At the same time the rate constant of the Langmuir-Hinshelwood step was measured. The main objective of our investigation was to study the chemisorption and

S. Ladas et al. /Adsorption

153

and oxidation of CO on Pd

catalytic oxidation of CO at low pressures on Pd particles with known variable size, shape, and crystal habit characterized by TEM and TED.

2. Experimental

procedures

The preparation of the Pd particles, and the subsequent catalytic experiments were carried out in a dual-chamber ultrahigh vacuum system (fig. 1). The main chamber with a base pressure
SP

MAIN

CHAMBER

TEM

DISC

TO PUMPS

Fig. 1. UHV system: I ion pump, Q quadrupole mass spectrometer, L leak valve, Sh shutters, Sp specimen, Q’ quartz crystal, P liquid nitrogen cooled Ti sublimation pump, M mask, E multiple source electron gun evaporator, F electron gun filament.

154

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

sensitivity was calibrated for N, with a nude ionization gauge. The sensitivity for CO, O2 and CO2 was calculated according to Nakao [19] taking into account the ionization cross-section, transmission coefficient and electron multiplier gain for each ion relative to N,‘, and also the characteristic cracking pattern for each gas. The pumping speeds for CO and CO2 in the reaction chamber (4.9 and 4.2 1 s-l) were obtained from the manufacturer’s pumping speed calibration for the High-Q pump taking into account the impedance of the connecting vacuum line. Palladium was evaporated onto a 4 cm2 area near the center of a finely finished (iO12) o(-A120a slab, 19 X 38 X 0.5 mm, from Union Carbide Corporation. Each slab was resistively heated through a 1 pm thick film of fl-Ta sputtered onto the backside, as described by Chang [20]. Because of the relatively good thermal conductivity of wA1203 and the small thickness of the slab, the front surface temperature was approximately equal to the temperature of the Ta film as measured by an infrared pyrometer (above 410 K). The pyrometer was used to check the temperature uniformity over the area of Pd deposition (typically *2 K around 500 K and +4 K around 800 K), and to calibrate a Chromel-Alumel thermocouple attached on the slab so that it read the temperature at the center. The calibration was performed for both static and dynamic (TPD) heating conditions. Before Pd deposition each slab was heated in UHV up to 1420 K for 10 to 30 min. As shown by AES in agreement with earlier work [10,20], this treatment removed most of the surface carbon leaving less than a tenth of a monolayer of residual C and some Si and Ca (probably as oxides). After heating, the slab was cooled to the desired deposition temperature, T, (723 or 823 K), and then was exposed to a given uniform Pd vapor flux, NV (-2.5 X 1013 cme2 s-l), for a predetermined time, td, so that Pd particles of the desired size distribution were obtained. After deposition, the specimen was transferred in the reaction chamber where TPD and kinetic experiments were performed. The samples for transmission electron microscopy (TEM) were finely polished disks of (iO12)(w-A1203, 3 mm in diam. and 120pm thick. The disks were prethinned around the center (dimpled) from the backside only, and then were attached to the slabs so that they received identical heat-treatment and Pd deposition. This ensured that the Pd particles examined by TEM were representative of those on the slabs. The pre-thinning of the TEM specimens was carried out in two stages: first, mechanical grinding down to -10 pm thickness, and then ion-beam etching at a 1.5’ angle until a small hole was formed. The area near the edge of the hole was sufficiently thin for TEM. After the catalytic experiments (and sometimes before) the particles were examined by conventional bright-field TEM and by TED. Additional information about the vacuum system and the experimental techniques can be found elsewhere [2 1]

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

155

3. Results and discussion 3.1. Electron microscopy Bright field micrographs of two TEM specimens show typical large and small Pd particles (figs. 2a and 2b). Specimens prepared under identical conditions and examined before and after catalytic experiments yielded essentially identical micrographs, even for the smallest observable particles. Some qualitative observations could immediately be made from the micrographs. The interference of substrate background structure with the Pd particle images prevented the identification of particles below about 1 nm in these specimens (thinner single crystal substrates with even lower detection limits have been used successfully [22]). Most particles exhibited definite triangular or polygonal shapes, suggesting crystallinity and faceting. The crystallinity of particle deposits with average diameter larger than about 2 nm was proven by selected area TED patterns such as that shown in fig. 2a. The strong diffraction spots come from the (i012)a-A1203 whereas the faint ones originate from the particles. All Pd diffraction spots corresponded to the regular fee bulk Pd lattice spacings. Although preferred azimuthal orientations were often observed, no general support/particle epitaxial relationship was observed under the prevailing deposition conditions used for these studies. From the micrographs the particle number density, N, (cmm2), the average twodimensional particle diameter, D,, (nm), and the fractional area of the support covered, 0 = $71N,D&, were obtained. The average diameter was defined as the median of the size distribution, which was always bell-shaped with a half-width of ca. 2 nm. The results for the four Pd particle deposits investigated are given in table 1. Decreasing D,, with roughly the same IV, was achieved by decreasing the deposition time, td, while keeping the Pd vapor flux, NV, and the deposition temperature, T,, constant. To obtain an adequate Pd surface area with very small particles, we used a lower T, (leading to higher N,, and decreasing the probability of epitaxial order). Under the same deposition conditions, D,, decreased roughly as the 4 power of td. This behavior is consistent with a three-dimensional particle growth model whereby all impinging Pd atoms that stick to the support diffuse rapidly on the surface and attach themselves to growing particles [6]. If this model is correct, the average particle height, h, and the exposed Pd surface area, Apd, can be calculated from a Pd material balance: h = &wd APd

=&up

vale W

, + 4hlQJ

(1) >

(2)

where S, is the condensation coefficient (assumed equal to OS), V, is the atomic volume of bulk Pd (1.47 X 1O-23 cm3), and Asup is the slab deposition area (4 cm’). Eqs. (1) and (2) assume pill-box shaped particles. However the A Pd values

156

S. Ladas et al. / Adsorptim

and oxidatiorz of CO on Pd

Pig. 2. Pd on fi012)wA1~03. Average size D 3v (nm) is 8 (a) and 1.9 (b). Deposition conditions: support temperature Ts (K) is 823 (a) and 723 (b); deposition time tD (s) is 156 (a) and 12 (b); flux NV (cm-’ S-I) is 2.2 x 1013 (a) and 2.1 x iO13 (b).

S. Ladas et al. /Adsorption

Table 1 Electron microscopy ____ Number incident atoms,

of Pd

Nvtd (1015 cm-2)

and oxidation of CO on Pd

157

data ____

support temperature, TS (K)

Number density of visible particles,

Average particle diameter, Da,

Ns (10’ 1 cm-2)

(run)

1.8 2.2 1.2 9

8.0 4.9 2.8 1.5

Average particle height, h

Fractional area covered, i3

Palladium surface area, Apd /cm 5 )

(nm)

__I__. 3.4 0.9 0.2 0.2

823 823 823 723

0.09 0.04 0.008 0.016

2.9 1.5 1.1 0.8

0.88 0.37 0.08 0.19 + 0.04 __-

are, within 20%, valid for any regular polyhedral or hemispherical shape. Since particles smaller than about 1 nm are not visible in the micrographs, Ns for deposits with very small particles is probably underestimated; however, for the same reason, D, is overestimated, therefore 19, and consequently APd, should not be affected much. The main uncertainty in the Apd values arises from the assumed value of 0.5 for S,. Although S, values for our system could not be found in the literature, an S, value of 0.5 has been reported for Au on glass at 640 K [23]. Further support for the APd values in table 1 is provided by the CO chemisorption data.

5

2.5

O

’

L/

350 Fig. 3. TPD spectra from 4 x lob5 Pa, T < 350 K.

400

Pd/ 0012}ol-AIaOa.

450 Da, = 4.9

5oo nm;

T/K

exposure

conditions:

PC- =

158

S. Ladas et al. /Adsorption

3.2. Temperature

programmed

and oxidatiotl of CO OIZPd

desorption

Typical TPD spectra from Pd particles with 4.9 and 1.5 nm average diameter are shown in figs. 3 and 4. All spectra were taken within 30 to 40 min after Pd deposition. Blank runs showed negligible desorption from the support at temperatures for CO desorption. The qualitative features of the spectra in figs. 3 and 4 are characteristic of all four Pd particle deposits investigated. The desorption peaks shifted to lower temperature as the CO exposure increased, indicating first order desorption with activation energy decreasing with increasing coverage. For exposures less than a few Langmuirs, CO desorbed from a single adsorption state (state 1) leading to a single desorption peak. Higher exposures increasingly filled a second adsorption state (state 2) which desorbed at lower temperatures giving rise to a shoulder in the TPD spectrum. The relative population of the two states depended on particle size. As the exposure increased from 3 up to 45 L the shoulder grew only slightly for the larger particles, but for 1.5 nm particles the shoulder grew into the predominant peak in the

I 6 i P co 10e6Pa

0

Fig. 4. TPD spectra from 7 X 1O-6 Pa. Ti 350 K.

IO

20

30

350

400

450

Pd/ fiOl2}c~-A1203.

Da, = 1.5

40

50 5oo

nm;

t/s

T/K

exposure

conditions:

Pco

>

S. Ladas et al. /Adsorption

Pco au

and oxidation of CO on Pd

159

6

5

-350

400

450

5oo

T/K

Fig. 5. Comparison of TPD spectra. E for 4.9 nm particles; F for 1.5 nm particles; subscripts 1 and 2 pertain to states 1 and 2 respectively; spectra normalized with respect to state 1.

TPD spectrum. This is better seen in fig. 5, where we compare TPD spectra from 4.9 and 1.5 nm particles exposed to 45 L of CO at PC0 = 4 X lo-’ Pa and T * 3 10 K. The spectra have been simply deconvoluted into state 1 and state 2 components with state 1 spectra normalized to the same height. The spectra from the first adsorption state are almost the same for small and large particles, except for a slight (-15 K) shift of the peak to higher temperature for the small particles. For the same area under the state 1 spectrum, the area under the state 2 component for the 1.5 nm particles is about 9 times larger than for the 4.9 nm particles. The integrated area under the TPD spectra, Ades (Pa s) is directly proportional to the number of desorbed molecules, NcO. According to Redhead [24 1, NC-~ can be obtained from N c0 = 2.45 X 10” &Ad,,

,

(3)

where SC0 (1 s-l) is the pumping speed for CO. Eq. (3) is valid if the partial pressure of CO recorded during desorption by the calibrated mass spectrometer is the average partial pressure of CO in the reaction chamber. In our experiments, this was

160

S. Ladas et al. /Adsorption

and oxidation

of’C0

on Pd

Table 2 TPD data Exposure

Average particle diameter, D a” (nm) ~____

a)

‘@O/A I’d

CL)

(cm-*)

-1
t’rom eq. (4) by replacing tot

Area correction factor, a

CWZlge,

0 l (*203),

Coverage ,) 0 tot cl

w. (4)

__ 2-3 2-3

8.0 4.9 2.8 1.5

Density of adsorbed CO in first state,

necessary to saturate first state

7.3 7.1 5.0 4.5

x x x x

1014 1o14 lOI 1014

-1 -1 -1.25 -1,s

0.50 0.48 0.43 0.46

N&O by the number

of molecules

0.60 1.15 dcsorbed

from

both

states,

NC.0.

indeed the case as the mass spectrometer was not in line-of-sight with the specimen. The inverse proportionality between Ades and SC0 expected from (3) for the same specimen was also experimentally verified. Using NC0 values from (3) and the TEM particle surface area from table 1, we obtained the surface density of adsorbed CO molecules at saturation of state 1, N,&/Apd (cm-*), given in table 2. A more interesting quantity is the corresponding coverage 0’ (CO,d,/Pd surface atom), which was obtained from

where d, is the average density of surface Pd atoms, and a is an area correction factor. The value of d, was taken equal to 1.43 X 10” cmm2 [13] corresponding to an arithmetic average between the (111) and {loo} planes. The area correction factor takes into account the fact that in measuring the surface area of a faceted particle corner and edge surface atoms are included at-least twice. Large particles (>-4 nm) have a very small proportion of corner and edge atoms and, therefore, a - 1. As the particle size decreases, the proportion of corner and edge atoms and, hence, a increase: Each a value in table 2 is an average calculated for a number of equal-sized regular fee crystallites, as those used by Van Hardeveld and Hartog [25]. The 0’ values obtained from eq. (4) were near 0.45 regardless of particle size. However, the value of 0 at saturation for both states, et,,, increased from about 0.6, for 4.9 nm particles, to about 1.15, for 1.5 nm particles. The 9’ values in table 2 are supported by a comparison of the coverage dependence of the heat of adsorption, Ma&, on the particles with that on various bulk single-crystal Pd planes. The coverage dependent activation energy for desorption, E, which is equal to the differential heat of chemisorption, was estimated from TPD spectra, such as in fig. 4, by the analysis of Tokoro et al. [26] for each adsorption state. First, the initial value of E (at the limit of zero coverage) was calculated from

S. Ladas et al. /Adsorption

and oxidation ofC0 on Pd

161

the experimental heating rate and the peak temperature at very small exposure (T,,,,,,, in figs. 3 and 4). A conventional first-order desorption spectrum analysis was used [24], with v = 2.4 X 1Ol4 s-l, as recently measured for CO desorption from Pd (11 I} [ 141. To apply the analysis of Tokoro et al., the slightly exponential experimental heating rate was approximated by a linear rate (-4 K s-l), fi was assumed to decrease linearly with coverage. The coverage dependence of E, and therefore of Mads, is depicted in fig. 6. The same graph includes AHads versus 0 data for CO adsorption on bulk Pd{l ll}, Pd {IOO}, Pd 1110) and Pd (2 lo} planes [ 131. The main observation from fig. 6 is that the initial heat of adsorption, or the binding energy, of CO is approximately the same (-146 kJ mol-‘) on all Pd particles and all single-crystal planes with the exception of Pd (1 lo}. This is a clear demonstration of the remarkable structure insensitivity of CO chemisorption on Pd. Inspection of fig. 6 supports our earlier assumption that the particles exhibit predominantly {ll l} and {loo} faces, corroborates the 0’ value of 0.45, and suggests that saturation of the first adsorption state involves strongly chemisorbed and mutually interacting CO molecules on similar sites, regardless of particle size. It also appears that the TEM surface areas are reliable as well as the mass spectrometer and pumping speed calibrations. The slightly less steep decrease of AHHadswith coverage (up to 8 - 0.5) for the small particles indicates less pronounced interactions between CO molecules adsorbed on small particles. The same conclusion was reached earlier, based on IR spectroscopic observations [27]. This may be due to the fact that’ CO molecules

SATURATION FIRST STATE

0

l

Pd {Ill}

a

Pd {IOO}

0

Pd {IIO}

OF

ADSORPTION

0.5

0

Pd {ZIO}

A

Pd/{T012} a-Aiz03

,6nm

n

Pd/{iOl2} a-Ai*O,

,I 5nm

I.0

1.5 8

Fig. 6. Differential [lOI.

heat

of adsorption

AHco

versus

coverage

kO,d/Pd) 0. Single

crystal

data from ref.

S. Ladas et al. /Adsorption

162

and oxidation of CO on Pd

adsorbed near corners and edges are subject to less crowding than molecules adsorbed on extended low-index faces. The almost twofold increase of Btot, as the average particle size decreases from 4.9 to 1.5 nm, can be accounted for by multiple chemisorption of CO on corner and edge Pd atoms. This is consistent with the known ability of several transition metals to form metal cluster carbonyl complexes with CO molecules multiply coordinated on the same metal atom. Possible analogies between CO held in such a complex, Rh6(CO)r6, and CO chemisorption on Pd (111) have been recently discussed [28]. 3.3. Catalytic oxidation of CO The oxidation of CO was investigated under steady-state, flow-reactor conditions. The reactants were admitted in the continuously pumped reaction chamber at a rate sufficient to maintain the desired partial pressures, PO* and I’,,, measured by the calibrated mass spectrometer. The COZ produced at a given specimen temperature T,, reached a constant partial pressure, Pco2, at steady state. The rate of CO2 production, Ncoz (molecules per second) was obtained from a simple CO* balance in the reaction chamber NW,

=2.45X

10”s

C0,PCO~ 3

(5)

where P,,, is expressed in Pascal and ScoZ is the pumping speed for CO* (4.2 1 s-r). The partial pressure of COZ was corrected for a small CO2 production in blank experiments. The turnover rate, N (s-l), was then defined as N = 0 ‘NcoZ/N~O

.

(6)

Here 8l and N& are the coverage (-0.45) and the number of CO molecules adsorbed on each specimen at saturation of the first adsorption state, as discussed in the previous section. The turnover rate, as defined above, is the number of COZ molecules produced per surface Pd atom per-second, the number of surface atoms being titrated by CO chemisorption on the first state with an experimentally determined adsorption stoichiometry at saturation. The rate, Nco2, was measured as a function of T,, between 410 and 580 K, for fixed pairs of PO2 and PC,, in the range of 1.3 X lo-’ to 1.6 X 10s4 Pa, for fixed T,. The surface area of the particles was regularly checked during the kinetic measurement, by TPD. Successive TPD cycles as well as sheer exposure of the specimen to the chamber background gas caused a gradual loss of the area under the TPD spectrum. However, exposure of the specimen to excess oxygen (-low4 Pa) above 500 K during the early stages of deactivation could restore the surface area to almost its original value (measured 30 min after Pd deposition). Running the experiments within a few hours after Pd deposition and taking advantage of oxygencleaning phenomenon, we could work on Pd particles of reproducible activity.

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

163

N s-1 0.08

004

I

OL

Fig. 7. Turnover lo4 Pa.

400

rate N versus

temperature

T, / K T, on Pd/(i012}wA120~,

4.9 nm; PO* = 1.3 X

Accordingly, the 19~and IV’ values used in eq. (6) to obtain the turnover rate were taken from table 2 (using APd from table 1). The qualitative features of the N versus T, and N versus P curves shown in figs. t 0.096

Fig. 8. Pd/ fiOl2}ol-A1203, PO, (Pa): (A) 550,1.3

-

8 nm. Turnover

X 104;

rate N versus PCO,

(o) 448, 1.3 X 104;

at pairs of values of Z’, (K) and

(0) 433, 1.3 X 104;

(0) 448,4.8

X 1O-5.

164

S. Ladas et al. /Adsorption

3

6

and oxidation of CO on Pd

9

I2

15

PO, / 1 O+ Fig. 9. Pd/ fi012}a-A1203, 4.9 nm. Turnover rate versus PQ, PCO (Pa): (0) 522, 1.2 X lo+; (0) 560,1.2 X lo+; (0)522,6.0

Pa

at pairs of values X10-‘;(~)481,

of Tr (K) and 1.2 ~10~.

j 045-

N s-1 036

0

6

12

18

P/10m5 Fig. 10. Turnover rate versus total pressure 530 K, 1.5 nm; (a) 7’= 523 K, 2.8 nm.

at (Poz/P&

Pa

= 1.1 for Pd/ fi012}~-Al~O~;

(A) T =

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

165

445K n

0’ I:&. 11. Turnover

rate (a, 0) Pd/fi012}oI-A1203;

u I 2

I 4

-----_a

”

6

versus particle size at (PoJPco) (0, 0) Pd[lll}from ref. [lj].

t 8

= 1.1

/w+

DAv /nm and

PCO = 1.2 X 10e4 Pa:

7-10 were the same for all Pd particle deposits investjgated. For a given ratio Paz/ PCO, N increased rapidly with T,, passed through a maximum, and then decreased slowly. The temperature at the maximum, Tr,max, and the maximum turnover rate, N max, both increased as P&PC0 decreased from 4 to 1 .l . For a given P,-0, N initially increased almost linearly with PO* and then levelled off at PO* > Pco. For a given Pea, and Tr -=cTr,max, N decreased almost linearly with increasing Pco, whereas for T, > Tr,max, N first increased almost linearly and then levelled off at PC, - Po2. For a given ratio Po.JPco and T, > Tr,max, N increased linearly with total pressure, whereas below Tr,max, there was a temperature region where N was approximately independent of total pressure. The quantitative dependence of N on Pd particle size is shown in fig. 11 at two selected reaction temperatures 445 and 5 18 K, and for standard partial pressures PO, = 1.3 X 10m4 Pa and P,-0 = 1.2 X 10m4 Pa (Po,/Pco - 1.1). At 445 K, N was within experimental error independent of particle size between 1.5 and 8 nm. At 5 18 K, N remained approximately constant for particles larger than -4 nm, and then increased almost threefold for 1.5 nm particles.

4. Discussion The dependence of the rate on the reaction temperature of CO and 02 was the same on all Pd particle deposits reported by Engel and Ertl [ 171. This invariance of the gests that the reaction mechanism established in ref. [ 171

and the partial pressures as on Pd{ll l}, recently steady-state kinetics sugshould be valid regardless

166

S. Ladas et al /Adsorption

and oxidation of CO on Pd

of particle size in the studied range. This mechanism lowing elementary steps:

co+*

can be described by the fol-

kl

FCO*.

(7)

2

02+2*~~0*.

CO*tO*~~C02t2*.

At steady state all 3 steps should proceed at the same rate, which is equal to the measured reaction rate. Working with turnover rates N with subscripts corresponding to those of the rate constants in steps (7) to (9), we can write N=Nr

m-Nz=Na=NLH.

The individual NI = &o&o

(LO)

forward rates can be expressed as >

(11)

Ns = 2d$,,

(12)

NLH=~LF@co%~.

(13)

Here, 4 is the impingement rate of gas molecules per surface metal atom, S is an effective sticking coefficient taking into account adsorption and reaction, and 0 is the steady state coverage. The irreversibility of steps (8) and (9) is well established below 700 K. Therefore two extreme cases are conceivable with respect to step (7). In the first case, (7) is very close to equilibrium, that is N1 - Nz or N= NLFI = N3 Q Nr . Assuming #k. < &J, (PC0 2S0, and %co +z %CO,eq"il> whereas B. can be considerably higher than before. The first case can in principle be achieved if $J& and &, (i.e. the pressure) are very large, or if the surface reaction is very slow. The measured rate is then that of the LangmuirHinshelwood step. The second case can in principle be achieved if the pressure is very low or if the surface reaction is very fast. The measured rate is then that of CO adsorption and is directly proportional to P,o, for fixed Po,/Pco, as long as %CO remains much smaller than 8 co(equil). In the pressure range of our work as in ref. [ 171, the first case was obtained sufIn that region, NLH was small because kLH was small due to ficiently below T,,,,,. the low temperature, and B. was small because of the inhibition of 0 adsorption by the adsorbed CO. The second case was obtained near and above 7’r,max, because kLH increased due to the higher T, and %O increased due to desorption of the inhibiting CO. At 445 K, PC0 = 1.2 X 10m4 Pa and PO, = 1.3 X 10m4 Pa, for example, N was -0.012 s-r, much smaller than @;. which, for large particles, was

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

167

-0.24 s-l. We have assumed that the particle surface consisted predominantly of (11 l} and (100) flat faces, so that 4 ,& = $cO/ds, with @ being the rate of collisions of CO molecules per cm* and d, = 1.43 X 1015 cm?. Above Tr,max, N was shown before to be directly proportional to PC0 for fixed Po,/Pco. Since eCO,equit in that pressure range (5 X lo-’ to 5 X low4 Pa) is not directly proportional to The last inequality has also been proven Pco, this means that 8~0 <@,o,,,,ir. experimentally by Matsushima et al. [ 151 for the low pressure oxidation of CO on polycrystalline Pd foil above Tr,max. At 518 K, N for the large particles was 0.14 s-l. Since this is approximately equal to Nr as @&&o, ScO = 0.14/0.24 = 0.6. The value of 0.6 for SC0 at 5 18 K is approximately the same as that reported by Engel and Ertl [ 171 for the reaction probability, S,, of transient CO* production. That value of S, was obtained on an O-saturated Pd {l 1 I} surface (0, - 0.25) with a small coverage (6’,-0 G 0.06). The same value of S, is also valid for the steady-state COZ production above Tr,max, where tic0 is small, under conditions that N is independent of Po2 (excess 0,). Using S,, we could convert the arbitrary steadystate rate scale of Engel and Ertl [ 171 into turnover rates. The resulting N values for Pd{lll}at518and445K,andforP~o=1.2X10~4PaandP~~=1.3X10~4Pa, are included in fig. 11. They are practically identical to the N values for the larger particles. Summarizing the kinetic results below Tr,max, we have observed that the oxidation kinetics and the turnover rates were the same on Pd{ll l} and Pd/ {1012}a-Al*Oa, with Pd particles between 1.5 and 8 nm. Since, in this region, the observed rate is the rate of the Langmuir-Hinshelwood step, the surface reaction is truly structure insensitive. Near and above Tr,max, N began to rise as the particle size fell below -4 nm (see fig. 11). The observed increase of N below 4 nm cannot be accounted for by an intrinsic particle size effect (increase of kLH or N& as the reaction probability is already near unity. Under these conditions N measures only the CO adsorption kinetics which were already shown to be structure insensitive. Actually, in view of the same reaction kinetics, it is reasonable to assume that the reaction probability at 518 K is -0.6, regardless of particle size. In such a case, the threefold increase of N from 4.9 to 7.5 nm can only mean that, although PC0 is the same, $6. is -3 times larger for the small particles. This increase of $’ as particle size goes down is a novel observation. It suggests that the CO exposure necessary to reach a certain coverage on small particles should be -3 times less than on large particles. Large particles required 2-3 L of CO to saturate the first adsorption state (0 - 0.45), whereas small particles required only 0.8 to 1 L (table 2). Besides, the exposures to reach the same coverage of adsorbed CO are about the same on the large planes [13] and the larger particles (table 2) and the sticking probality is almost unity on the large planes. Hence 4’ must increase for the smaller particles. The increase of 4’ can be explained intuitively by the fact that the smaller particles contain a larger proportion of surface atoms at corners and edges rather than on flat faces. These are more accessible to impinging gas molecules. A more quanti-

168

S. Ladas et al. /Adsorption and oxidation of CO on Pd

tative explanation is based on a comparison of the values of 4’ derived from the kinetic theory of gases [29] in two extreme cases: first, of a single metal atom completely exposed to the gas phase, and second, of a flat extended metal surface with a density of atoms d,. In the first case, 4’ becomes the frequency of collisions in three dimensions ,

(14)

where u is the mean and 12is the number pressure of the gas). rate of collisions per

molecular-atomic diameter, u is the mean molecular velocity, density of gas molecules (which is proportional to the partial In the second case, as previously mentioned, G1 is equal to the unit surface area

2 = v%Av2

(s-l)

@=$Wl

(cm-" s-l),

divided by d,,

(15)

OI

@hat = Wd,

(s-l) .

(16)

The ratio of the two extreme values of $’ is Z&It

= 4&-ro2d,

=

17.702d,

(17)

If the gas is CO (molecular diameter -0.35 nm), the metal is Pd (atomic diameter -0.27 nm), and the extended surface is a mixture of Pd {l 1 l} and Pd (100) planes cm-2) then u = 0.27 + 0.35/2 = 0.31 nm = 3.1 X lo-’ cm and (d, = 1.43 X 10” Z/&rat N 24. This large difference can easily account for the observed increase of @’ as the particles become very small and their surface consists predominantly of partially exposed corner and edge atoms. RATE a.u. 30

(b) 0032

20-

j

0.016 ~

0

L P,,(A)

IO-/&y 16

8 or

I&, (01

24

/10e5 Pa

I

0

P,,(a)

3.

2

or F&(o)

/ kPa

Fig. 12. Comparison of pressure dependence of the oxidation rate of CO at low and high pressures. (a) Pd/ ~012}or-A1203; 8 nm, T = 448 K; (a) l’co = 6.7 X lo-’ Pa, (o) PO* = 3.6 X low5 Pa. (b) 5% Pd/SiOz (ref. [ 151); (A) T = 446 K, PCO = 1 kPa; (o) T = 427 K, PO* = 0.9 kPa.

S. Ladas et al. /Adsorption

Table 3 Comparison

of high- and low-pressure

and oxidation ofC0

CO oxidation

turnover

on Pd

rates at 450 K

Catalyst

5% Pd/SiOz (ref. [ 151)

PO* (Pa)

6.5 X lo2

3.6 x lo-’

PC0

1.3 x lo3

7.2 x 10-S

0.03 b)

0.012

(Pa)

Turnover

rate N (s-l) a)

169

Pd/{1012}n-A120~, (this work)

8 nm particles

c)

a) Per surface Pd atom. b, Estimated error *lOO%. c) Estimated error *40%.

The increase of 4’ for the small particles applies not only to CO but also to Oz. constant. This is why no As a result, the ratio &,/Q&O remains approximately effect is observed on N at 445 K. The increase of N at 518 K is a direct consequence of the experimental fact that above Tr,,,X the value of N is proportional to Pco for a fixed Po,/Pco ratio. We will last compare our kinetic data with those reported by Cant et al. [18] for the steady-state oxidation of CO over 5% Pd/SiO* near atmospheric pressure and below 450 K. The dependence of the rate on PO2 and PC0 is compared in figs. 12a and 12b. Although the total pressure in fig. 12b was -7 orders of magnitude higher than in fig. 12a, the behavior is remarkably similar. Table 3 shows a compari’son of turnover rates at 450 K for the same Po,/Pco. The value of N in the high pressure work [ 1S] is only a factor of two higher. This remarkable invariance of the turnover rate over a large pressure range must be related to the fact that, at -450 K, N depends on Po,lPco only and not on the total pressure. It also suggests that the same mechanism is applicable in both cases. Finally, it is safe to conclude from the equality of rates at low and high pressures, that the oxidation of CO below 450 K is also structure insensitive at high pressures.

5. Conclusion The striking structure insensitivity of the oxidation of carbon monoxide on palladium is not easy to explain quantitatively both below and above Tr,,,ax. Below Tr,max, the measured rate is the product of three quantities as shown in eq. (13): a Langmuir-Hinshelwood rate constant, a surface coverage by CO and a surface coverage by oxygen. All three are expected to change, if only slightly, with surface structure which changes values of bond energy. That the change in bond energies is not large is an empirical fact for the interaction of CO with various surface structures of Pd (fig. 6). Yet a change in binding energy or activation energy by only AE = 2 kcal mol-r (about 8 kJ mol-r) leads to a change of exp(AE/RT) equal

170

S. Ladas et al. /Adsorption

and oxidation of CO on Pd

to 10 at 400 K. Nevertheless, our turnover rates are certainly changing by considerably less than a factor of two from one surface structure to another. The same remarks apply to the situation above T,,,,,. Here, the measured rate is determined by the effective sticking coefficient of CO, taking into account adsorption and reaction, as seen in eq. (12). But the sticking coefficient must be affected, if only slightly, by coverage of the surface by carbon monoxide and by oxygen. Surface coverage must depend on surface structure. Yet the measured turnover rate does not. Of special interest is the fact that structure insensitivity has been found for a reaction at low pressure under conditions such that contamination of the surface by reagents is certainly minimized. Indeed, in previously found structure insensitive reactions involving hydrocarbons [2], the possibility arises that the working catalytic metal surface is largely covered with carbon compounds or residues which erase differences of surface structures. Particularly convincing is the agreement between our work and that of Engel and Ertl [17] where surface contamination can be definitely excluded. Furthermore, it is worthwhile to point out again that the direct measurement of Pd particle size and number density of TEM before and after the reactions contributed significantly to the interpretation of our experimental results, and it is important to recall the difficulty we encountered with very small particles, 2 nm or less of calculating correctly the number of surface atoms from surface area measurements. Similarly, it was found that very small particles are more accessible to gas phase molecules than extended flat surfaces. While the effect is easy to understand, it has not been noticed before but should be of general importance in catalysis by small clusters.

Acknowledgment This work was carried out as partial fulfillment for the PhD degree at Stanford University with funding by NASA Grant NCA2-OR745-908. One of us acknowledges partial support by NSF Grant No. NSF-ENG 79-09141 (M.B.).

References [l] J.R. Anderson, Structure of Metallic Catalysts (Academic Press, London, 1975). [2] M. Boudart, in: Proc. 6th Intern. Congr. on Catalysis, Eds. G.C. Bond, P.B. Wells and F.C. Tompkins (Chemical Society, London, 1977) pp. 1-9. (31 J.R. Anderson and Y. Shimoyama, in: Proc. 5th Intern. Congr. on Catalysis, Ed. J. Hightower (North-Holland, Amsterdam, 1973) p. 695. [4] H.P. Bonzel, Surface Sci. 68 (1977) 236. [5] H. Poppa, J. Appl. Phys. 38 (1967) 3883. [6] H. Poppa, in: Epitaxial Growth, Part A, Ed. J.W. Matthews (Academic Press, New York, 1975).

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and oxidation of CO OK Pd

171

[7] H. Poppa, E.H. Lee and R. Dale Moorhead. J. Vacuum Sci. Technol. 15 (1978) 1100. [8] M.E. Thomas, H. Poppa and G.M. Pound, Thin Solid Films 58 (1979) 273. [9] D.L. Doering, H. Poppa, J.T. Dickinson, J. Vacuum Sci. Technol. 17 (1980) 198. [lo] R.D. Moorhead, H. Poppa and K. Heinemann, J. Vacuum Sci. Technol. 17 (1980) 248. [ 1 l] J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 5 1 (1969) 4852. [12] G. Ertl and J. Koch, in: Proc. 5th Intern. Congr. on Catalysis, Ed. J. Hightower (NorthHolland, Amsterdam, 1973) p. 969. [ 131 H. Conrad, G. Ertl, J. Koch and E.E. Latta, Surface Sci. 43 (1974) 462. [14] T. Engel, J. Chern. Phys. 69 (1978) 373. [15] T. Matsushima, C.J. Mussett and J.M. White, J. Catalysis 41 (1976) 397. (161 H. Conrad, G. Ertl and J. Kuppers, Surface Sci. 76 (1978) 323. [17] T. Engel and G. Ertl, J. Chem. Phys. 69 (1978) 1267. [ 181 N.W. Cant, P.C. Hicks and B.S. Lennon, J. Catalysis 54 (1978) 372. [ 191 F. Nakao, Vacuum 25 (1975) 201. [20] C.C. Chang, J. Vacuum Sci. Technol. 8 (1971) 500. [21] S. Ladas, Ph.D. Dissertation, Stanford University (1979). [22] H. Poppa and K. Heinemann, Optik, in print. 1231 K.L. Chopra, Thin Film Phenomena (McGraw-Hill, New York, 1969) ch. IV. 1241 P.A. Redhead, Vacuum 12 (1962) 203. [25] R.V. Van Hardeveld and F. Hartog, Surface Sci. 15 (1969) 189. [26] V. Tokoro, M. Misono, T. Uchijima and Y. Yoneda, Bull. Chem. Sot. Japan 51 (1978) 85. [27] E. Kugler and M. Boudart, J. Catalysis 59 (1979) 201. [28] G. Ertl, J. Vacuum Sci. Technol. 14 (1977) 435. [29] E.H. Kennard, Kinetic Theory of Gases (McGraw-Hill, New York, 1938).