Elsevier

Surface Science

Volume 415, Issues 1–2, 30 September 1998, Pages 183-193
Surface Science

Chemical adsorption of acetic acid and deuterated acetic acid on Ru(0001), by RAIRS

https://doi.org/10.1016/S0039-6028(98)00590-1Get rights and content

Abstract

The adsorption and thermal decomposition of acetic acid and deuterated acetic acid on clean Ru(0001) were studied by reflection absorption infrared spectroscopy. Acetic acid adsorbs molecularly, with dimer formation, at 100 K. At 113 K, the molecular form is still predominant, although some dissociative adsorption already occurs. Dehydrogenation is complete at 123 K, leaving adsorbed acetate as a symmetric bidentate species. Depending on coverage, two bonding configurations are proposed: a bidentate bridging [μ2-η2(O,O)-CH3COO], with each oxygen bonded to a neighbour Ru atom, at low coverages, and a bidentate chelating [η2(O,O)-CH3COO], with the two oxygen atoms bonded to the same Ru, at higher coverages. The decomposition temperature increases with coverage, giving evidence in favour of a stabilizing effect of acetate ions and decomposition products on neighbour adsorbed acetates. For high enough coverages, adsorbed acetate was still detected at 500 K, proving the high stability of this surface species on Ru(0001). The adsorption of deuterated acetic acid gave consistent results, although dissociative adsorption starts occurring at 123 K and complete dehydrogenation occurs at 133 K, ∼10 K above the respective temperatures for acetic acid.

Introduction

Two main motivations are responsible for the interest developed, in the last 20 years, on the study of small carboxylic acids adsorbed on transition metal surfaces and metal oxides. On one hand, these studies may contribute to a better understanding of catalytic oxidation reactions, which have carboxylic acids as reaction intermediates or products [1]. On the other, the relative simplicity of the carboxylate species formed upon chemical adsorption of those acids, by O–H bond cleavage, turns them into ideal molecules for fundamental surface studies and theoretical simulations of adsorbed polyatomic molecules 2, 3.

A wide range of surface techniques has been used, including high resolution electron energy loss spectroscopy (HREELS), reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), molecular beam measurements, X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), Auger spectroscopy and photoelectron diffraction 4, 5, 6, 7, 8, 9, 10. In particular, formic acid adsorption has been extensively studied on a variety of transition metals, having been shown that it generally decomposes into formate species below room temperature. These studies have allowed to distinguish between two decomposition mechanisms of adsorbed formate, one involving only C–H bond cleavage and the other both C–H and C–O bond cleavage. In the first case only H2 and CO2 are formed [on surfaces such as Cu(001), Cu(110) and Pt(111)], and in the second one also CO and H2O are produced [on surfaces such as Ni(110), Ni(100), Ni(111), Fe(100) and Ru(0001)] [11]. The influence of coverage 11, 12and the effect of surface modifications by oxygen [13]and hydrogen [14]have also been analysed. The co-adsorption of alkali metals, namely K, on Ru(0001) 15, 16, Co(101̄0) [17], Pd(100) [18]and Rh(111) [19]has been the subject of a number of studies. These have shown that the alkali metal lowers the decomposition temperature of formate bound to the substrate, but a more stable formate species is formed, which is bound directly to the alkali metal. Depending on the coverage of co-adsorbates, “explosion” reactions of formate have been observed [20].

On clean Ru(0001), the surface in which we are particularly interested, a detailed understanding of the adsorption and decomposition of formic acid has already been published 11, 14, 15, 16, 21, 22. It was concluded that this acid adsorbs dissociatively on the perfect Ru(0001) surface, via O–H bond cleavage, leaving formate and a hydrogen adatom; on defect sites, it decomposes via C–O bond cleavage, leaving carbon monoxide, a hydrogen adatom and a hydroxyl group [22]. The thermal decomposition of adsorbed formate over the temperature range 340–360 K occurs by a coupled mechanism first proposed by Sun and Weinberg [22]: the hydrogen resulting from C–H bond cleavage in one formate (which produces CO2) reacts with a neighbouring formate to yield formyl and hydroxyl on the surface, resulting equal amounts of CO and CO2 in the overall thermal decomposition reaction.

Contrarily to formic acid, and to the best of our knowledge, no studies on the acetic acid/Ru(0001) system have been published. On Pt(111), it has been shown, by HREELS, that the adsorption of acetic acid at 168 K is extremely dependent on coverage: it is mainly molecular (in a hydrogen bonded dimer form) for θ≥0.5, and dissociative for lower coverages (θ<0.3), with formation of an acetate species adsorbed in the bidentate configuration, η2(O,O)-CH3COO, of Cs symmetry; further dissociative adsorption as CO, atomic oxygen and CHx fragments (x=1, 2) is observed at even lower coverages (θ≤0.2), occurring probably on surface defects. Complementary TPD spectra showed that, by annealing an adsorbed multilayer (θ≈0.6), molecular acetic acid desorbs at ∼223 K, leaving acetate on the surface. At 303 K, linear and bridged adsorbed CO were observed by EELS. The surface limited desorption of CO occurs at 486 K. The other product formed upon thermal decomposition is H2, which is observed at 230, 370 and 498 K [10].

When adsorption occurred from an acid solution in contact with the Pt(111) surface, a splitting of the band at ∼1420 cm−1, assigned to the symmetric stretch of the OCO group, was observed with increasing coverage. It was attributed to the existence of two different conformations of acetate species [23]. This effect was not observed on less compact platinum surfaces, such as Pt(100) or Pt(110) [23].

On Cu(100) at 400 K, the observation of a strong symmetric stretch of the OCO group, in HREEL spectra, lead also to the proposition of a bidentate structure for adsorbed acetate species, bound to the copper surface via the oxygen atoms. Acetate was stable up to 450 K [24]. Identical conclusion was recently drawn from a combined LEED, XPS and X-ray photoelectron diffraction (XPD) study of acetic acid adsorption on clean Cu(110) at ∼300 K: a well ordered acetate layer is formed, bound to the surface through both oxygen atoms, the unit cell being occupied by three acetate species, one of them tilted [25].

On clean Rh(111) [26]and Pd(110) [27], acetic acid also dissociates into hydrogen and adsorbed acetate, at 300 K. At ∼400 K, CO2 and H2 were identified as decomposition products by TPD, leaving only adsorbed carbon on the surface. The same decomposition products were observed on clean Rh(110) [28]. A rather different decomposition path has been proposed for acetic acid on Ni(111), involving as major intermediate acetic anhydride, (CH3CO)2O [29].

All these studies prove that, on clean metallic surfaces, the decomposition temperatures of adsorbed acetate and the decomposition products are dependent on the metal and on coverage 10, 24, 25, 26, 27. Furthermore, the stability of the adsorbed acetate is very sensitive to the surface state, especially to the presence of co-adsorbed atoms. For instance, co-adsorbed atomic oxygen, nitrogen and carbon usually have a stabilizing effect on the surface acetate. The proposed cause was an induced ordering of the adsorbate layer by the co-adsorbates 26, 27, 28. This effect is usually accompanied by a “surface explosion” when the stabilized acetate decomposes autocatalytically 27, 28, 30, 31.

In this work, we have studied the adsorption of acetic acid on Ru(0001) at different temperatures and exposures by RAIRS. Complete dehydrogenation into acetate has been observed at 123 K. The RAIR spectra are consistent with the acetate adsorbed symmetrically, with a Cs symmetry, as two bidentate surface configurations, respectively bridged and chelating. The first one was predominant at lower coverages. The stability of surface acetate upon heating was coverage dependent. Adsorption of deuterated acetic acid was used to confirm the proposed interpretation.

Section snippets

Experimental

Experiments were performed in a ultra-high vacuum (UHV) chamber described in detail elsewhere [32], with a base pressure of 1×10−10 Torr, after baking for 48 h at 380 K.

The ruthenium single crystal, 1 mm thick and 10 mm diameter with a diamond polished (0001) surface, was cleaned by several cycles of Ar+ sputtering followed by annealing to ∼1170 K. The cleanliness and flatness of the surface were checked with a RAIR spectrum of CO adsorbed at 100 K. On a regular and clean Ru(0001) surface, a

Physical adsorption

In Fig. 1A, the transmission IR spectrum of solid acetic acid is compared to the RAIR spectrum of acid adsorbed on the Ru(0001) surface, after exposure to 10 L (1 L=10−6 Torr s−1) at 100 K. The same comparison is made for deuterated acetic acid in Fig. 1B.

The solid phase spectra (Fig. 1A-aFig. 1B-a) compare well with the corresponding ones obtained for the crystalline acetic acids at low temperature [34]. We may infer that the solid phase samples have a crystal-like structure, with the acid

Conclusions

The first conclusion from this RAIRS study of acetic acid and deuterated acetic acid on clean Ru(0001) is that, at temperatures below 113 and 123 K, respectively, they adsorb molecularly, mainly as dimers. Beyond these temperatures, both acids decompose on the surface to leave the corresponding acetate. A discussion on whether these species were adsorbed on different sites (such as defects) or as two different conformations lead us to the conclusion that, on the Ru(0001) surface as prepared in

Acknowledgements

Ana Rosa Garcia wishes to express her gratitude for a F.C.T./PRAXIS XXI Ph.D. grant.

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