Elsevier

Surface Science

Volume 574, Issues 2–3, 10 January 2005, Pages 166-174
Surface Science

The structure of formate species on Pd(1 1 1) calculated by density functional theory and determined using low energy electron diffraction

https://doi.org/10.1016/j.susc.2004.10.027Get rights and content

Abstract

The structure of formate species adsorbed on Pd(1 1 1) has been determined using low-energy electron diffraction (LEED). The presence of formate species on the Pd(1 1 1) surface was established using reflection-absorption infrared spectroscopy (RAIRS). The oxygen atoms of the formate species were found to be adsorbed over surface palladium atoms with the OCO plane perpendicular to the surface. The CO bond length was found to be 1.26 ± 0.05 Å, the palladium–oxygen distance was 2.16 ± 0.06 Å, and the OCO angle 130 ± 5°. The experimentally determined values were in excellent agreement with those calculated using density functional theory (DFT).

Introduction

The participation of carboxylate intermediates has been invoked in a number of catalytic reactions including the water-gas shift reaction [1], methanol synthesis and decomposition [2], [3], [4], [5], [6], and the methanation of CO and CO2 [7], [8], [9], [10], [11], [12]. As an example, the reaction pathway for the synthesis of oxygenates from CO and hydrogen invokes the formation of a stable carboxylate that hydrogenates to yield alcohols over supported group-VIII transition-metal catalysts [13], [14], [15]. Carboxylates (in this case acetates) are also proposed to be intermediates in the palladium-catalyzed synthesis of vinyl acetate monomers from ethylene, acetic acid and oxygen [16], [17]. The decomposition of formates (HCOO) [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and acetates (CH3COO) [33], [34], [35], [36], [37] has been studied on a number of single crystal surfaces. In spite of the importance of these species as catalytic intermediates, there have been remarkably few structural studies of carboxylates on single-crystal, transition-metal surfaces. This is partly due to the fact that they do not tend to form ordered structures and are therefore not amenable to analysis by standard low-energy electron diffraction (LEED) methods. Photoelectron diffraction has been used to measure the structure of formate [38], [39] and acetate [40] species on copper where the oxygen atoms are found to be located above copper atoms with the OCO plane oriented perpendicular to the surface with an OCO bond angle of ∼124°.

We have recently shown that it is possible to use conventional LEED methods to measure the structures of disordered overlayers by interrogating the way in which the intensities of the substrate (1 × 1) Bragg diffraction spots are modified by the presence of an overlayer [41], [42], [43]. This strategy is applied in the following to measuring the structure of formate species on Pd(1 1 1) where no ordered LEED patterns are observed. An additional advantage of this method is that the adsorbate coverage appears as a variable in the analysis and thus provides an additional method of determining surface coverages.

Finally, we have recently shown that amino acids can act as chiral templates for the adsorption of propylene oxide on Pd(1 1 1) [44]. That is, precovering a Pd(1 1 1) surface with either an R- or S-amino acid preferentially adsorbs propylene oxide of the same chirality. A detailed knowledge of the structure of the amino acids on surfaces is central to understanding this behavior. Since the amino acids are more complex than the carboxylic acids, it is not feasible to vary all geometrical parameters to find the best fit to the LEED intensity versus beam energy (I/E) curves. In such cases, it would be extremely beneficial to have structures calculated by density functional theory (DFT) as inputs into the LEED calculations. We have therefore compared the results of structural measurements by LEED with the results of DFT on Pd(1 1 1). One goal of this work, therefore, is to test how well DFT can predict the measured structure for carboxylates on Pd(1 1 1).

Section snippets

Experimental methods

LEED measurements were carried out in a doubly μ-metal shielded ultrahigh vacuum chamber operating at a base pressure of 5 × 10−11 Torr, and containing a Pd(1 1 1) single crystal, which could be cooled to 80 K and resistively heated to 1200 K. The sample was cleaned using standard procedures. LEED patterns of the clean surface were photographed as a function of incident energy using a Nikon Coolpix digital camera (5.0 MBytes resolution) and the images stored on an IBM Smartcard memory (1 GByte). The

Results

The RAIRS spectrum of formic acid adsorbed on Pd(1 1 1) at 150 K is displayed in Fig. 1 as a function of formic acid exposure, where the exposures (in Langmuirs, 1 L = 1 × 10−6 Torr) are marked adjacent to the corresponding spectrum. At low exposures, the spectrum consists of three relatively sharp features at 792, 1342 and 2904 cm−1. These features persist as the exposure increases, and additional peaks are detected at 947, 1243, 1630 and 1704 cm−1 after an exposure of 0.6 L of formic acid. These features

Discussion

The vibrational frequencies of formate species formed on Pd(1 1 1) are in good agreement with those found on Pt(1 1 1) [20] (Table 1, Fig. 1, Fig. 2). The symmetric OCO mode appears at 1342 cm−1 on Pd(1 1 1), while no corresponding asymmetric mode is observed indicating that the OCO plane of the formate is oriented perpendicular to the surface. Some formate species form on the surface immediately following formic acid exposure at 150 K (Fig. 1), while further exposure leads to the adsorption of

Conclusions

The structure of ∼0.23 monolayers of formate species has been determined on a Pd(1 1 1) surface using low-energy electron diffraction. The molecular plane is oriented perpendicular to the surface with a vector passing through the oxygen atoms being oriented along the short bridge on the (1 1 1) surface. The distance between the oxygen and palladium atoms is 2.16 ± 0.06 Å, the C–O bond length is 1.26 ± 0.05 Å and the OCO bond angle is 130 ± 5°. The structure determined by low-energy electron diffraction is

Acknowledgment

We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant Nos. DE-FG02-00ER11501 and DE-FG02-03ER15474.

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