Gas-solid surface interaction with reactive and less reactive gases : a near ambient pressure photoelectron spectroscopy study

Abstract

Chapter 1 and 2, a brief introduction role of surface science, photoelectron spectroscopy and gas solid interaction in catalysis has been given. Photoelectron spectroscopy is a versatile technique that can be advantageously used for characterization of a number of surface properties of the solid surface and interfaces, like chemical composition, oxidation state, element mapping, solid-gas interactions etc. Conventionally, the photoelectron spectroscopy is an ultra-high vacuum technique. However, the photoelectrons cannot reach the detector due to inelastic collision with gas-phase molecules at high-pressure (>10-5 mbar) conditions. This obstacle has been overcome in the development of photoelectron spectrometers which can function at near ambient pressures. A new term ambient pressure (AP) or near ambient pressure (NAP) or high pressure (HP) has been introduced with photoelectron spectroscopy (PES) to distinguish it from the traditional UHV set ups. The NAP-PES can operate at near ambient pressure, by using the sophisticated electron energy analyzer and differentially pumped electro static lenses. Thus the NAP-PES, can bridge the “pressure gap” between the real world and ideal surface science studies. This thesis mainly focused on different studies to bridge the pressure gap between a real-world condition where the actual surface reaction happens and ideal surface science condition. All experiments are carried out in a custom built laboratory ambient pressure photoelectron spectrophotometer unit installed in our laboratory at CSIR-National Chemical Laboratory, Pune. The system is equipped with the differentially pumped Scienta R3000HP analyzer. Two sets of differential pumping are available in the electrostatic lens regime (ELR), and the third one is available in the electron energy analyzer (EEA). The distance between sample surface and aperture (of the cone, R = 0.4 mm) attached to ELR was maintained at 1.2 mm for all of the experiments reported. The main advantage with this design is a fast decrease in pressure with a steep pressure gradient from the aperture to the EEA. It is to be underscored that the first differential pumping records 2 × 10−4 mbar when the analysis chamber is at 1 mbar. This helps to minimize the inelastic scattering of low KE electrons. Further, R3000HP employs the advanced concept of electron converging with an aperture free ELR. In contrast to the conventional ELR, electrostatic voltages in the R3000HP model analyzer are applied in such a way that they converge all the electrons. The system also equipped Al-Mg dual anode, Al- monochromatic X-ray source, differentially pumped xvi discharge lamp to generate He I and He II UV radiation source The polycrystalline foils (Ni, Co, Si, Ag, Au, 99.999 pure) which are used in the experiments are purchased from MaTeck, Germany. These foils are cleaned by the several cycles of Ar sputtering and annealing in UHV up to 1000 K. A series of sputter-anneal cycles produced clean metal foil surfaces which is confirmed by the XPS as well as UVPES. In chapter 3a, Silver valence band was probed by PES at near ambient pressure of oxygen (up to 0.2 mbar) with He I radiation. Three distinct regimes have been identified in silver-oxygen interaction between 300 and 500 K, which are, (a) oxygen chemisorption between ambient and 390 K, (b) O-diffusion into the subsurface layers of Ag from 390 to 450 K, and (c) formation of metastable oxide on the silver surface above 450 K; the latter two regimes are dynamic in nature.. The trend in oxygen coverage on Ag 390 K and above 475 K is similar, but it decreases to the lowest in between 390 and 450 K, in the presence of large excess of molecular oxygen. Interaction with oxygen changes the work function of Ag from 4.95 (≤390 K) to 5.30 eV (400-450 K), and then to 5.7 eV (≥450 K). It is attributed to oxygen diffusion into the subsurface layers of Ag between 400 and 450 K and plays a key role for ethylene epoxidation reaction on Ag surfaces. Subsurface oxygen influences in two significant ways; it converts the Ag surface from metallic to electron deficient in nature, and facilitates the formation of space charge layer above the Ag surface. Oxygen when adsorbed on this electron deficient Ag surface, acts as electrophilic oxygen. The electrophilic oxygen can insert into the C=C double bond of an alkene, and hence forms an epoxide. Above 450 K, oxygen binds strongly and acts as nucleophilic oxygen. The nucleophilic oxygen favors complete combustion of alkene to carbon dioxide. Changes in the Ag-oxygen system are dynamic. The metallic surface reappears if oxygen supply is removed above 400 K. This emphasizes in situ and operando investigations are essential to understand the active structure of a catalyst. In chapter 3b, we have synthesized 5 wt % Fe2O3/support (support=Al2O3, CeO2, MgO, ZSM-5 and Nb2O5) catalysts by wet impregnation method. The synthesized catalysts subjected to different physico- chemical characterization techniques to understand the structure and morphology of the catalysts. These catalysts were screened for butane oxidative dehydrogenation (ODH) reaction in fixed bed reactor at different temperature between (450°C to 600°C) with varying butane: oxygen ratio (1:1, 1:0.5 and 1:0.25). Among all these catalyst xvii Fe2O3/Al2O3 shows best activity in terms of 1,3-butadiene yield (higher selectivity towards 1,3-butadiene) at all different temperatures. In order to understand the active site of the compared the results with another average catalyst i.e., Fe2O3/Nb2O5. The catalyst was screened at 0.2 mbar pressure (Argon: Butane: Oxygen is 2:1:0.5 respectively) under in situ condition from 298 to 500 K. We have concluded that the Fe on the Al2O3 support reduced to Fe+2 from Fe+3, whereas Fe on the Nb2O5 support remains in Fe+3 states and the reduced Fe+2 is responsible for the higher selectivity towards 1,3-butadiene Chapter 4 shows Valence band and core level photoelectron spectral measurements at near-ambient pressures (up to 0.5 mbar) were made in the presence of molecular oxygen to explore the various stages of silicon oxidation. Dangling bonds feature observed in NAP-UPS on clean Si-surfaces decreases due to adsorption of molecular oxygen between ambient temperature and up to 400 K at 0.1 mbar O2 pressure. The adsorption of oxygen on dangling bonds seems to be localized as islands and the same reflects as heterogeneous surface and responsible for the broadening in the oxygen gas phase vibrational features. This is further supported by an increase in the work function and can be correlated with the presence of Höfer (molecular) precursor. When the temperature increased to 500 K, molecular precursor species dissociates to –Si=O species and further supported by the change in the work function as well as by the oxidized silicon species from Si 2p core level. At 600 K the –Si=O species dissociates to form a uniform 2D oxide layer on the silicon surface, which is characterized by the sharp vibration features of gas-phase oxygen molecules. This layer is also quite stable up 800 K and without any further oxidation in bulk. However, when the temperature increased to 850 K at 0.2 mbar oxygen pressure, bulk Si oxidation begins and the work function increases drastically by 1 eV. An angle-dependent Si 2p core level spectra recorded map out the presence of all possible oxidation states (elemental Si0 to Si4+) from bulk to the surface. A continuous change in work function and electronic states observed due to gas-solid (O2-Si) interaction indicates the implications of heterogeneous catalysis and electrochemistry. In chapter 5 the gas phase vibrational spectra of reactive and inert gases have been studied by the in situ ultraviolet photoelectron spectroscopy up to 0.3 mbar. Results obtained is divided into two parts and discussed. In the first part, we have studied the molecular photoelectron spectra of monoatomic Argon gas and some homonuclear diatomic molecular gases like H2, O2, and N2 by using NAPUPS and the effect of pressure on their energy xviii position. In this study, we have demonstrated that NAPUPS can be an essential tool to determine the gaseous composition and their electronic configuration. In the second part, we have studied the influence of surface nature on the binding energy position and pattern of the vibrational features of Nitrogen and Argon gas. It has been observed that with changing the electronic nature of the surface, the binding energy of vibrational spectra also changes which reflects the change in the work function of the material. Further, if the solid surface undergoes any chemical/electronic changes due to gas-solid interaction, such as oxidation, the work function of the surface changes again and underscores the identification of in-situ changes. Therefore, the change in the binding energy of the gas phase can be used to determine the actual work function change of material during the the chemical reaction