Surface “explosions” of acetate intermediates on Rh crystals and catalysts

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Surface

Science 307-309

(1994) 143-146

Surface “explosions” of acetate intermediates on Rh crystals and catalysts M. Bowker

*‘l, T.J. Cassidy

‘, M.D. Allen ‘, Y. Li

Leuerhulme Centre Department of Chemistry and I.R.C. f or S urf ace Science, University of Liverpool, Liverpool L69 3BX, UK (Received

20 August

1993)

Abstract

In recent years a new class of surface reactions has been reported on single crystals, reactions classified by others as “surface explosions”. We show for the first time that these reactions translate directly to catalyst surfaces consisting of finely dispersed metal particles, Rh on alumina in this case, under normal conditions of ambient pressure. These reactions appear to be classical second order autocatalytic decompositions, depending on the creation of active metal sites for the isothermal acceleration which is observed. The acetate on the Rh surface is proposed to be the pivotal intermediate in high pressure synthesis of ethanol from synthesis gas (CO + H,), a reaction of interest as a possible source of an important future feedstock chemical from natural gas.

In 1973 Professor leagues

discovered

Robert a new

Madix

phenomenon

and

his colon single

crystal surfaces [l]. They dosed a Ni(ll0) surface with formic acid and upon heating found an unusually rapid, autocatalytic, evolution of decomposition products (H, and CO,) which they labelled as a “surface explosion”. This was manifested as an extremely narrow peak in temperature programmed desorption (Fig. 11, whereas all the experiments to that date revealed “normal” spectra with widths of N 30 K. Upon detailed analysis in turn these kinds of curves yielded unreasonably high values for reaction A factors and activation energies [1,2]. The reason for this was that the normal kinetic representations were

* Corresponding author. I Now at: Department of Chemistry, Whiteknights, P.O. Box 224, Reading, 0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0798-Y

University of Reading, RG6 2AD, UK. Science

invalid for such an autocatalytic decomposition, which Madix and co-workers proposed was due to islands of clean surface appearing in an otherwise condensed phase of the formate [3]. Since that time there have been no other reports of surface “explosions” of this type and there was a feeling in the community that these studies represented an anomalous situation for surface reactions. However recently at Liverpool we have found these effects on several single crystal surfaces and consider that they may be far more widespread than previously thought. On Rh(llO1 we have found a similar surface “explosion” for the adsorbed acetate species (Fig. 2) with a very narrow peak width of 6 K [4,.5]. However, they are only found when a coadsorbed adatom is present with the carboxylate. In the case of Fig. 2 oxygen was predosed onto the surface. In the absence of an adatom the acetate is unstable and decomposes rapidly at 300 K, with

B.V. All rights reserved

was proposed that the decomposition then begins at initiation centres (defects or steps in the surface) and radiates outwards from them. This can be expressed in the following generalised kinetic form, where the half power arises from the relationship of the circumference of the growing island (where the “active” sites are located) to the area of the island. /++@,‘/Lk(l 330

340

3io

360

370

Temperature Fig. 1. Desorption reaction

profiles

from (a) a normal

Cb) an autocatalytic

for the evolution

sion” peak. characteristic

390

400

of products

of

1st order dest~rption process and

process,

given in the text. Note

380

(Kl

computed

using the equation

the very narrow

width of the “explo-

of such phenomena.

normal kinetics. In fact any adatom appears to have this effect, including carbon or nitrogen (we have not examined any others as yet). At that point it appeared that these reactions may be unique to two-fold symmetric (1 IO&type surfaces. However, we have now found explosions on Rh(lllI [2] and have also extended the work to another metal surface, Pd(ll0) [6]. It is likely to be a very general phenomenon and we predict that it will be seen on other precious metal surfaces (such as Pt) in the near future, and possibly on a wider range of transition metals, depending on the details of the decomposition mechanism. We are not working on Pt surfaces in our laboratory. Whether it is restricted only to intermediates like carboxylates, which are restricted in movement by multiple site bonding, is not yet clear. Several possibilities existed for the microscopic cause of these phenomena, but the most likely was considered to be that order in the adlayer was crucial to the reaction [2,51. It was known from diffraction experiments that the acetate on clean Rh(lll> forms a disordered layer, and atoms form ordered adsorbed layers. Thus the role of the coadsorbed atoms may have been to induce order in the carboxylate adlayer by mutual interactions and the diffraction experiment confirms order remaining after carboxylate adsorption. It

_Hf’;?

The autocatalytic nature of the reaction then derives from the (1 - 0)“’ term which increases with increasing extent of reaction. Up to this time obse~ations of these phenomena were restricted to the rather rarefied cnnditions of UHV and single crystal surfaces. It was

a

1

:

,

r

TEMPERATURE (K) Fig. 2. Experimental

desorption

profiles for acetate decompo-

sition on Rh(l IO) showing the narrow, tion of COz,

Hz

and H,O.

after such a decomposition

near coincident

C is left behind

rvc~iu-

on the surface

and some water is evolved because

a small amount of oxygen atoms were left coadsorbed with the acetate. The figures in brackets refer to relative multiplication of the signais.

M. Bowker et al. /Swface

of considerable interest to determine whether they could be extended to high area catalyst surfaces under the more realistic conditions of high pressure which occur in much of industrial and academic catalysis. It is also particularly relevant since acetates are seen by infra-red adsorption spectroscopy on such materials under conditions where ethanol can be selectively formed from synthesis gas (CO + Hz) [7,81. Furthermore, a complex Rh catalyst is used in an homogenous process for acetic acid production by carbonylation of methanol [9]. However, there were many reasons to believe, a priori, that surface “explosions” would not be seen on supported Rh. These include the fact that the catalyst we used consists of small particles of Rh of - 10 A radius, which means that (a) there is not an extensive surface and (b) the surface will not be homogeneous in structure. Thus it could be imagined that highly ordered structures would not be obtained with the acetate on such a surface. Also, it might be anticipated that the Rh particles, being of very high surface energy (low average coordination number of sur-

7,

145

face atoms), would be more reactive to decomposition of the acetic acid and might possibly decompose it at room temperature. Experiments were carried out using a microreactor in which we could dose the catalyst with specific amounts of oxygen prior to introducing small aliquots of acetic acid vapour. Subsequently the catalyst could be heated to decompose and desorb any species which might be present on the surface and the products were monitored using a multiplexed quadrupole mass spectrometer sampling the gas eluting from the microreactor. The results of such experiments are shown in Fig. 3 and show that, remarkably, the single crystal data do translate to a catalyst surface and the surface “explosion” is seen. Furthermore, isothermal experiments show a rate which initially increases with reaction time, an essential manifestation of the autocatalysis. The products observed are coincident CO, and H,O desorption, the peak width is - 7 K and the peak temperature varies between 488 and 537 K, depending on the dose of oxygen. After the decomposition, C is left adsorbed on the surface and can be identified by

I

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Science 307-309 (1994) 143-146

a

1: +

5-

l

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4-

l

+a

-

l

33-

Dioxide (x7)

l

q_

f + 1 500

I 520

*

I 540

'

Temperature

I 560

*

(K)

I 580

' 600

250

350

300

Time

400

450

(sew)

Fig. 3. (a) Desorption profiles from acetate on the Rh/AI,O, catalyst showing near coincident CO, and H,O evolution, the width of the CO, peak is - 7 K wide at half the peak height. Note that no hydrogen is seen because the oxygen precoverage of the surface is high and all the H atoms produced from the decomposition react with oxygen on the surface to produce H,O. The same effect is seen in the single crystal work. The broad peak of CO, and H,O evolution after the autocatalytic peak is due to the normal decomposition of acetates on the support component. (b) Isothermal CO, evolution from the same type of experiment as in (a) carried out at a temperature of 506 K. The autocatalysis is demonstrated by the increasing rate with time between 310 and 370 s.

flowing H, over the catalyst at elevated temperature when CH, is evolved. With no oxygen predosed there is no obvious formation of acetate on the Rh component of the catalyst at all, which we believe is due to the spillover of the acetate from the metal to favoured sites on the support and others have reported that formate species preferentially spillover to the support on a Rh/AI,O, catalyst [lo]. The observation of the autocatalytic reaction on the smail particles precludes the earlier island model and indicates a mechanism more closely related to classical second order autocatalytic reactions ill]. In this, a reaction product is involved in the rate equation and in our case the important product is a free Rh site, the coverage of which have a (1-0) dependence as required for the increasing isothermal rate with time in agreement with the earlier proposal of Madix et al. The line shapes for TPD and isothermal desorption curves derived from such a treatment (the rate equation being R, = k@l - 8)) are very similar to those shown in Fig. 3. The difference between this model and the earlier one described above is simply that the free sites are homogeneously distributed instead of clustered in islands. Note that it is the acetates themselves which block sites and not the predosed atoms. Careful experiments with varying oxygen and acetic acid doses on Rh(llO) showed that if the dosing of oxygen is done carefully, such that it is all removed by water formation and if the remaining acetate coverage is sufficiently high, the explosion is seen 151. The observation of acetate formation is important in relation to ethanol synthesis since previ-

ously no definite evidence of acetate on the metal component had been found. Acetates that were found were ascribed to “spectator” species (probably existing on the support component) by some authors [7,8], while others have proposed it to be an active intermediate directly involved in the selective reaction pathway [ 12-141. It has been shown here that a stable acetate can be formed on the Rh itself. We believe that this observation of a surface explosion on a dispersed surface is the first of its type in catalysis and we expect to see further examples appear in the literature in the near future.

1. References 111J. McCarthy,

J. Falconer and R.J. Madix. J. (‘atal. 30 (1973) 235; Surf. Sci. 42 119?41 239. [2] Y. Li and M. Bowker. Surf. Sci. 285 (1993) 219. [.?I J. Falconer and R.J. Madix, Surf. Sci. 46 (1974) 473. 141 M. Bowker and Y. Li, Catal. tett. 10 (1991) 249. iSI Y. Li and M. Bowker. J. Catal. 142 (1993) 630. Lhl N. Aas and M. Bowker. J. Chem. Sot. Faraday Trans. X9 (1993) 1249. [71 R. Underwood and A.T. Bell, J. C’atal. I I I (IYX8) 325. and M. Ichikawa, J. Chem. Sot.. (‘hem. [Xl T. Fukushima Commun. flY85) 72Y. PI T. Deklava and D. Forster. Adv. C’atal. 34 (1086) 81. [lOI F. Solymosi, A. Erdohelyi and T. Bansagi. J. C’atal. OX (lY81) 371. iill See, for instance, AA. Frost and R.G. Pearson. Kinetics and Mechanism, 2nd ed. (Wiley, New York. 1961) p.ic). 1121 M. Bowker, Catal. Today 77 (1992) IS. 1131 G. Van Der Lee, A. Bastein, J. Van Der Boogert, B. Schuller, H. Luo and V. Ponec. J. Chem. Sot. Faraday I, X3 119%‘) 2103. I141 A. Efstathiou and C. Bennet. J. Catal. 120 (1989) 118. 1.17.