Surface chemistry of glycine on Pt{111} in different aqueous environments

doi:10.1016/j.susc.2012.08.015

Surface Science

Volume 607, January 2013, Pages 10-19

https://doi.org/10.1016/j.susc.2012.08.015 Get rights and content

Abstract

Adsorption of glycine on Pt{111} under UHV conditions and in different aqueous environments was studied by XPS (UHV and ambient pressure) and NEXAFS. Under UHV conditions, glycine adsorbs in its neutral molecular state up to about 0.15 ML. Further deposition leads to the formation of an additional zwitterionic species, which is in direct contact with the substrate surface, followed by the growth of multilayers, which also consist of zwitterions. The neutral surface species is most stable and decomposes at 360 K through a multi-step process which includes the formation of methylamine and carbon monoxide.

When glycine and water are co-adsorbed in UHV at low temperatures (< 170 K) inter-layer diffusion is inhibited and the surface composition depends on the adsorption sequence. Water adsorbed on top of a glycine layer does not lead to significant changes in its chemical state. When glycine is adsorbed on top of a pre-adsorbed chemisorbed water layer or thick ice layer, however, it is found in its zwitterionic state, even at low coverage. No difference is seen in the chemical state of glycine when the layers are exposed to ambient water vapor pressure up to 0.2 Torr at temperatures above 300 K. Also the decomposition temperature stays the same, 360 K, irrespective of the water vapor pressure. Only the reaction path of the decomposition products is affected by ambient water vapor.

Introduction

One of the great challenges in studying model surface systems related to bio-molecular heterogeneous catalysts lies in the fact that practically all relevant reactions take place in aqueous solutions near room temperature. Conventional surface science techniques cannot accurately study such adsorption/reaction complexes as the coadsorption of the relevant surface species cannot be modeled under reaction conditions in ultra-high vacuum (UHV). In UHV water only forms stable condensed layers below150 K [1], [2], [3], where the kinetic barriers are too high for many elementary steps of heterogeneous reactions, such as exchange of molecules between surface layer and solution and chemical reactions between the surface species. Reaction conditions for an aqueous solution/catalyst interface require temperatures where the vapor pressure is in the mbar/Torr range. This pressure range has recently become accessible for photoemission and X-ray absorption experiments, such as XPS and NEXAFS through new beam-line and detector designs [4], [5], [6].

Since amino acids are essential building blocks of living matter, their interaction with metal surfaces is an important area in the study of bio-inorganic interfaces with many applications in heterogeneous catalysis. To date, a number of studies of amino acid adsorption on copper surfaces (to some extent on gold and nickel surfaces) were conducted with an emphasis on enantiomeric effects in the adsorption of chiral amino acids [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Less attention was paid to the chemical composition of organic overlayers and their chemical interaction with metal surfaces, partly due to the low reactivity of the metals used in these studies at room temperature and partly because the application of copper–amino acid systems is very limited in heterogeneous catalysis. It has been shown in these studies that simple amino acids adsorb on Cu{110} in their anionic (de-protonated) form [10], [11], [15], [17], [22], [24], [28], [31] and are stable up to 450–500 K. Above this temperature complete desorption/decomposition occurs within a relatively small temperature window. The exact decomposition mechanism depends on the specific amino acid. Our most recent work has shown that the decomposition process of alanine and glycine is dramatically affected by the presence of ambient water at pressures above 10^− 5 Torr, making the anion less stable [34].

Adsorption of small amino acids on Pt-group metal surfaces, which are commonly used as active components for heterogeneous catalysts, was studied experimentally with temperature-programmed desorption (TPD) and laboratory-based X-ray photoelectron spectroscopy (XPS) and theoretically using density functional theory (DFT) [35], [36], [37], [38], [39], [40], [41]. Particular attention was paid to the chemical composition at different temperatures. The experimental studies report that adsorption of glycine and alanine on Pt{111} and Pd{111} leads to the formation of zwitterionic molecules, while DFT modeling favors neutral and anionic species [41]. Annealing of amino acid overlayers on these metals leads to the decomposition of molecules via C single bond C bond scission [35], [36], [37], [38].

The current work aims at studying the influence of water on the surface chemistry of the smallest amino acid, glycine, with Pt{111}. Our particular emphasis is on comparing the dissociation behavior of glycine when the surface is exposed to water under UHV conditions and under ambient pressure conditions up to the Torr range. Surprisingly, the differences are very small, which is in stark contrast to earlier findings concerning the surface chemistry of glycine and alanine on Cu{110} [34].

Section snippets

Experiment

Most of the UHV experiments with Pt{111} were carried out at beamline I311 of the MAX-lab synchrotron facility in Lund, Sweden. The ambient pressure experiments were performed at beamline 11.0.2 of the Advanced Light Source (ALS) in Berkeley, USA. Both endstations consist of two ultrahigh-vacuum (UHV) chambers with base pressures in the 10^− 10 Torr range, which are described in detail elsewhere [42], [5], [43]. The preparation chambers are equipped with standard instruments for sample preparation

Adsorption on clean Pt{111}

Glycine adsorbs in its neutral molecular form on clean Pt{111} when the surface is held at 200 K. Fig. 1 shows a series of C 1s, N 1s and O 1s XP spectra measured after glycine was deposited for the indicated times. The N 1s spectrum for the lowest coverage in the series, after 5 min dosing, in Fig. 1(b) shows a single narrow peak with symmetric shape at BE 399.6 eV. This is a characteristic for a neutral amino group (H₂N) where the nitrogen atom forms a bond with the substrate, as it has been

Discussion

Our experiments clearly show that glycine adsorbs on Pt{111} in its neutral molecular state (H₂NC₂COOH) at low coverage, both in UHV and under ambient water vapor pressure up to 0.2 Torr (the highest pressure for which XPS measurements were conducted). This is in contrast to the adsorption states that were observed experimentally for glycine and other small amino acids adsorbed on other late transition metals, such as Cu [11], [10], [12], [22], [17], [24], [51], [28], [15], [52], [30], [31], [32]

Summary and conclusions

Under UHV conditions, glycine adsorbs in its neutral form up to about 0.15 ML (rel. coverage 1.0). This state has not been reported in earlier spectroscopic studies of this adsorption system [36]. Additional molecules adsorb as zwitterions. Up to a relative coverage of 2.5 these are strongly bound to the substrate surface, either directly or via the neutral molecules. Multilayer growth is observed for higher coverages. These multilayers desorb intact below 330 K, the more strongly bound

Acknowledgment

This work was supported by the European Community through the Marie Curie Early Stage Training Network “MONET” (MEST-CT-2005-020908) and by the European Community Research Infrastructure Action under the FP6 “Structuring the European Research Area” program (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S.

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Surface chemistry of glycine on Pt{111} in different aqueous environments

Abstract

Introduction

Section snippets

Experiment

Adsorption on clean Pt{111}

Discussion

Summary and conclusions

Acknowledgment

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