Vibrational study of ammonia adsorption on Pt/SiO₂

Abstract

Vibrational properties of surface species formed upon NH₃ adsorption on Pt/SiO₂, model system for the gas sensitive part in ammonia sensors based on field effect devices, have been investigated with in situ DRIFT spectroscopy. Experiments have been performed for a series of samples with different Pt loading at three temperatures, 50, 150 and 300 °C, and in the absence and presence of oxygen. In addition, electronic structure calculations and vibrational analysis have been performed within the density functional theory (DFT) for NH₃ and NH₂ species adsorbed on platinum and hydroxylated silica model systems. Observations from both DRIFT spectra and DFT calculations indicate that NH₃ is more strongly bound to platinum than to silanol groups on the SiO₂ support. Vibrational modes assigned to NH₂ appeared in the DRIFT experiments, indicative of NH₃ dissociation, an interpretation supported by the calculations. Exposure of O₂ was found to have minor effect on the vibrational spectrum at 50 °C. However, at 150 °C an increase of the vibration band assigned to the NH₂ surface species was observed together with formation of gas phase N₂O for samples with high platinum content. Thus, ammonia is oxidised over Pt at this temperature and oxygen is most likely facilitating ammonia dissociation.

Introduction

Owing to a lower fuel consumption, diesel and lean-burn engines are important alternatives to the conventional gasoline engines, which operate close to stoichiometric air-to fuel ratios [1]. The exhausts from diesel and lean-burn engines contain, in contrast to the exhausts from conventional gasoline engines, large excess of oxygen. The high oxygen content prevents efficient reduction of harmful nitrogen oxides, NO_x, produced in the combustion process using the conventional three-way-catalyst. One approach to overcome this problem is selective catalytic reduction of NO_x with ammonia as reducing agent, NH₃-SCR. In this technique ammonia, or ammonia releasing substances like urea, are injected to the exhaust gases and reacts with NO_x and oxygen over a catalyst to form harmless nitrogen and water. In such an exhaust after-treatment system it is essential that excess ammonia is not emitted to the environment. Thus, reliable ammonia injection and detection methods are crucial to prevent any slip of ammonia after the SCR system.

Metal insulator silicon carbide field effect transistors (MISiCFET) have recently been reported to be suitable as chemical gas sensors to monitor ammonia in SCR systems [2], [3]. The high band gap and the inertness of SiC enable high temperature operation in harsh environments like vehicle exhaust gases. The structure of a MISiCFET device may schematically be described as SiC bottom layer covered with a thin insulating SiO₂ film and a top gate layer of a catalytic active metal like platinum. A similar type of device, with silicon as bottom layer, has been commercialised for hydrogen detection [4], [5]. The proposed mechanism for hydrogen detection involves dissociative adsorption of H₂ on the catalytic metal surface followed by atomic diffusion of H to the metal/SiO₂ interface. At the interface, hydrogen form a dipole layer which shifts the capacitance–voltage characteristics of the device [6]. The shift was shown to be proportional to the concentration of hydrogen in the gas phase [6]. Several studies indicate that, in contrast to the response to hydrogen, these type of sensors are only sensitive to ammonia if the platinum film is porous [7], [8], [9], [10]. Hence, exposure of the SiO₂ layer to the gas phase seems to be crucial for the ammonia response. Moreover, it has been speculated that steps in the mechanism could include ammonia reacting with oxygen and spill over of surface species from the metal to the (hydroxylated) SiO₂ surface [2], [7]. A detailed knowledge of the ammonia detection mechanism is important to achieve the high sensitivity, selectivity, speed of response and long-term stability required for application of the sensors in vehicles.

The aim of the present study is to reveal details in the NH₃ detection mechanism by applying in situ DRIFT spectroscopy to study the evolution of surface species during ammonia exposure on Pt/SiO₂ samples at the temperatures corresponding to, as well as under and above, the temperature interval where the MISiCFET type sensor have shown high NH₃ response. The active surface of a platinum based MISiCFET sensor consists of a discontinuous Pt film deposited on a SiO₂ layer. An appropriate model system for the Pt–SiO₂–gas interface is dispersed platinum supported on silica with high surface area. In the present study this system was used to represent the top layers of the MISiCFET sensor, which are available for gas phase adsorption. The adsorption of ammonia was studied both in the presence and absence of oxygen. Previously, in situ diffuse reflectance FTIR (DRIFT) spectroscopy has shown to be a powerful technique to study surface species under reaction conditions [11], [12], [13], [14], [15], [16]. In the context of metal oxide based sensors, DRIFT spectroscopy has recently been used by Emirogluo et al. to follow the adsorption of CO on SnO₂ powder surfaces [17].

In addition to the DRIFT experiments, we have performed electronic structure calculations within the density functional theory (DFT) for NH₃, NH₂ and H adsorbed on Pt₄ and Pt(1 1 1), and Si(OH)₄ and hydroxylated β-cristobalite SiO₂(1 1 1) surface. The purpose with the calculations is twofold: i) to establish fundamental energy relations between the surface species within one and the same computational approach and (ii) evaluate vibrational properties of the NH₃ and NH₂ adsorbates on platinum and SiO₂ to support the interpretation of the experimentally obtained vibrational data.