Hreels Study of Formic Acid Adsorption on Gold (110) and (111) Surfaces.

Journal of Electron Spectroscopy and Related Phenomena, 29 (1983) 293-299 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

293

HREELS STUDY OF FORMIC ACID ADSORPTION ON GOLD (110) AND (111) SURFACES. M. CHTArB, P.A. THIRY, J.P. DELRUE, J.J. PIREAUX t and R. CAUDANO Laboratoire de Spectroscopie Electronique - IRIS FNDP, 61, rue de Bruxelles, B-5000 Namur (Belgium).

ABSTRACT The adsorption at 100 K and the temperature decomposition of formic acid were investigated on (110) and (111) gold single crystal surfaces by high resolution electron energy loss spectroscopy. A multilayer build-up of physisorbed HCOOH with intense hydrogen bondings was observed at increasing coverages for the two gold surface orientations. Above room temperature, formic acid decomposed and desorbed from the (110) crystal, whereas it evolved into an intermediate formic anhydride on the (111) face. Further heating produced on the surfaces species similar to those observed on oxygen treated metals. INTRODUCTI ON High resolution electron energy loss spectroscopy (HREELS) is one of the techniques that have already been successfully used to study the adsorption and temperature decomposition of formic acid (HCOOH)on clean and oxygen dosed metallic single crystals [Cu (100) - ref. 1 ; Ag (110) - ref. 2; Pt (111) - ref. 3] , and on a semiconductor [GaAs (110) - ref. 4] • These measurements together with results obtained by other spectroscopies (ref. 5-7, for example) vlere explained by two reaction mechanisms for formic acid on a solid surface: dehydration on a surface oxide, or dehydrogenation on a transition metal surface, both reactions being consecutive to the formation of a formate species. Gold is known to behave as a relatively poor catalytic surface, compared to other metals (ref. 6), and for example, its chemical activity towards oxygen is very moderate (ref. 8). Moreover, it has been suggested (ref. 7) that gold is unable to dissociate formic acid on the clean surface, as it is also the case for clean silver (ref. 2). Therefore, HREELS measurements of the formic acid interaction with gold were undertaken for a wide range of substrate temperature. And, for the first time, two different surface orientations of the same metal, namely Au (110) and Au (111), were studied in order to discover some possible crystallographic effect. In this short communication, we will report briefly on our first results. More details, a complete interpretation and results obtained on oxygen precovered tResearch Associate of the National Fund for Scientific Research (Belgium). 0368-2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company

294

crystals will be published later (ref. 9,10). It must be added that these HREELS data have also been complemented with X-Ray Photoelectron and Thermal Desorption Spectroscopic measurements (ref. 10,11). EXPERIMENT The gold (110) and (111) crystals (5 N purity, Metal Crystals Ltd) were prepared as follows. After mechanical polishing and an electrochemical treatment in a solution containing mainly potassium cyanide and phosphoric acid (ref. 12), the samples were cleaned in the spectrometer UHV vessel (2.10- 11 Torr base pressure) by argon ion etching and thermal treatment cycles (600 K, with flashes up to 900 K), until no oxygen or carbon contamination could be detected by Auger Spectroscopy. Moreover, no other impurity like silicon, calcium or sulphur has been detected. . The surface orientation was checked by LEED : the (110) crystal showed the expected 2x1 reconstruction when sufficiently cleaned; and for the clean (111) sample, it showed the regular hexagonal pattern. Formic acid (99 %) was purified by freezing and pumping cycles before its introduction into the spectrometer via a bakeable UHV stainless steel valve. The gas purity was checked by mass spectrometry. The gas exposures given here are uncorrected gauge readings. HREELS data were collected from a substrate kept at 100 K with a RIBER-SEDRA spectrometer that will be described elsewhere (ref. 13). For an electron impact energy of 6 or 5 electron volt (for the (110) and the (111) crystals respectively) the elastic peak intensity in the specular direction was currently around 105 counts/s with a resolution in the 7 to 10 meV range (60 - 80 cm- 1). ADSORPTION OF HCOOH ON Au (110) AND Au (111) AT 100 K When letting the gaseous formic acid interact with the cleaned gold (110) or (111) surface, no significant HREELS vibrational band could be detected before the exposures were cumulated up to the 5 to 10 Langmuir (1 L = 1.10 -6 Torr.s) range. Even for those spectra, the magnification factor used (x 512 or 256 relatively to the elastic peak) attests the low sticking coefficient for HCOOH towards gold. Still, with increasing gas doses, formic acid was observed to condense physically and form multilayers. We will comment later (ref. 9) on an evolution in this build-up, as we noted some drastic changes in the low coverage regime, and a different behavior for the two gold surfaces. Shown at the bottom of Fig. 1 is the HREELS spectrum recorded for a 100 L exposure on the Au (110) surface. It reveals numerous excitation bands, that can be attributed to vibrations of the formic acid, whose energies are compiled in Table 1. The carboxylate (COO) group is identified by the two v(C-O) and v(C=Q) modes, at 1230 and 1705 cm- 1 respectively, whereas further confirmation

295

Au1110'· HCOOH Tmeas.

= 100K

100L

oL

Fig. 1. HREELS spectra from formic acid on a Au (110) surface. The measurement is performed at 100 K, after a surface treatment indicated respectively on the right hand side of each spectrum. of the presence of the acid is deduced From the observation of the v(C-H) band at 2960 cm- 1 and the v(O-H) contribution extending in the range between 2600 and 3000 em-I, This last observation confirms the existence of hydrogen bondings in the condensed molecular solid, as it has been observed for crystalline formic acid (ref. 14) and also for adsorption on Cu, Ag and Pt (ref. 1-3). The other vibrational bands are attributed in accordance with other authors, and especially with the new considerations developed in ref. 3.

296

a

b AUI111'+ HCOOH Tmeas. = 100K

Fig. 2. HREELS spectra from formic acid on a Au (111) surface. The measurement is performed at 100 K, after a surface treatment indicated respectively on the right hand side of each spectrum (a). Part b is an enlarged view of the v(C=O) band (peak h) evolution during the temperature treatment. The last unassigned band is a vibration at 490 cm- 1; we will tentatively consider it as a fingerprint of the metal-oxygen vibration band v(Au-O). Apart from a different intensity distribution between the various vibrational bands, the HREELS spectrum recorded for the Au (111) surface + 100 L HCOOH presents the same features as the Au (110) one (see Fig. 2 and Table 1). The major differences are : - a lower energy of the v{O-H) band at 2650 cm -1 ; - the sensibly lower energy of the v(C=O) band, with a different shape.

297

Those facts suggest the existence on Au (111) of condensed formic acid in a crystalline form, as it was evidenced also on Pt (111) (ref. 3) : with strong hydrogen bonds, planar zig-zag chains of formic acid are formed. This crystallinity induces a splitting of the carbonyl (C;O) and hydroxyl vibration bands (ref. 14). TEMPERATURE TREAT!1ENT OF THE GOLD+HCOOH SURFACES The thermal treatment of a surface previously exposed to 100 L HCOOH at lOOK was an exciting experiment, as it could check if the formic acid molecule is so weakly bound to the gold surface that it will desorb without dissociating, and more precisely without passing through the formate species intermediate. It was observed that warming the crystal did affect the HREELS spectra, first by attenuating the vibration intensities, and then by modifying profoundly the band shapes. This is interpreted as a progressive thermal desorption of the condensed layers with a final decomposition of the first monolayer. We present and discuss here two significant steps in the heating process, inducing sensible modifications of the HREEL5 data. Au (110) + 100 L HCOOH. The Au (110) crystal face exposed at 100 Kwas warmed up to 273 K and then cooled back to its initial temperature. Fig. 1 presents the vibration spectrum after this thermal treatment. The major differences with the non heat-treated sample are: - the disappearance of the O-H bending and stretching vibrations; - the drastic intensity decrease of the 1230 and 1700 cm- 1 bands; - the appearance of a new band at 740 cm- 1, in place of the 220 and 490 cm- 1 vibrations. The complete absence of any band due to the O-H group suggests that the physisorbed HCOOH has converted to another species, after the removal of the hydroxyl hydrogen. But surprisingly enough both v(C-O) and v(C;O) have also almost completely disappeared. So, according to the assignment given on Table 1, the sole intense bands are attributed to remaining hydrocarbon on the surface, with the n(C-H), a(C-H) and v(C-H) at 1070, 1400 and 2930 cm- 1 respectively. Peak losses (f,g,h) are attributed to non-desorbed formic acid during the temperature treatment or to re-adsorbed HCOOH during the sample cooling. The wide band at lowest energy (740 em-I) is tentatively attributed to water librations [as they have been observed with a high intensity and with no v(O-H) companion band on Ag (110) (ref. 2)J; H20 was probably readsorbed on the cooled sample during the spectrum recording. Further heating of the same surface to 500 K (Fig. 1) produced species similar to those observed for Au (110) dosed with oxygen at 600 K. These data and a careful investigation of possible impurity diffusion effect will be discussed

298

in another Paper (ref. 9). We conclude that no formate species is formed on the annealed Au (110) surface : the adsorbed formic acid desorbs in molecular form, or after a dissociation without any intermediate formate species stable on the surface. Previous TDS measurements, which showed a HCOOH peak at 165 K and a CO 2 one in the range of 170 - 190 K (ref. 11). Au (Ill) + 100 L HCOOH. The spectrum recorded for the dosed Au (111) surface heated up to 245 K presents also differences with the untreated one (Fig. 2 We note that the disappearance of the v(O-H) and ~(O-H) bands enhances the v(C-O) and v(C=O) peaks at 1230 and 1720 cm- 1 respectively, and allows vibrations to be detected at 620 and 840 em-I. This is consistent with the disappearance of the hydroxyl protons but as the vsymmetric (COO) and Vasymmetric (COO) bands expected around 1330 and 15501600 cm- 1 respectively (ref. 15) are not detected, the new species cannot be identified as a formate with two equivalent C-O bonds. The only way to accommodate a HCOO-like species on the surface, is to invoke the formation of another intermediate, formic anhydride (HCOOOCH).

TABLE 1 Vibrational band energies (in cm- 1) and assignments for formic acid on two gold single crystal surfaces. Vibrational mode lattice mode v(Au-O) X

o(OCO) X X

~(O-H)

~(C-H~

v(C-O o(C-H) v(C=O) v(O-H) v(C-H) sh

X

Au (110) + 100 L at 100 K

Au (110) Au (111) + 100 L at 100 K at 273 K

220 490

240 480

700

710

940 (1060)sh 1230 1400 1705 2730,3160 2960

950 (1070)sh 1220 1390 1680 2650,3100 2950

shoulder or very weak peak a tentative ass i gnment is given in the text

Au (111) at 245 K

620 740 1070 (1230)sh 1400 (1700)sh 2935

840

a b c

d (1060)sh e 1230 f (1395) sh g 1720 h i j 2940

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To su~port this interpretation, one should recall that on the Au (111) crystal, formic acid condenses probably in a crystalline phase, where the planar chain structure is a first step to favour an interaction between two adjacent adsorbed species, -to allow the anhydride formation. Further support is given by part b of Fig. 2, showing the evolution of the carbonyl band during the tem~erature treat~ent: two structures separated by = 60 c~-l are seen, fingerprinting an anhydride (ref. 18). Two bands clearly observed at 620 and 840 c~-l are then tentatively assigned to the 6(OCO) bending vibrations: it is thought that in the same formic anhydride molecule two such excitations ~ight couple into in-phase (620 em-I) and out-of-rhase (840 em-I) vibrations. To our knowledge, this see~s to be the first HREELS evidence of formic anhydride, whose existence has been suggested by other techniques on Ni(110) (ref. 16) and on RU(10IO) (ref. 17).

Another step in the heating of the dosed Au (111) surface has been recorded above 400 K. It will be discussed later (ref. 9). AC KriOWL EDGEi-1ENT This work is supported by the Fund for Joint Basic Research (Belgium).

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