Recent Developments in Near Ambient
Pressure Photoelectron Spectroscopy
By
I
M.Sc. Chemistry
Analytical Sciences
Literature Thesis
Recent Developments in Near Ambient
Pressure Photoelectron Spectroscopy
By
Ahmed A. H. S. Ali
April 2016
Supervisor:
III
Foreword:
This literature study is a review of the recent developments in near ambient pressure X-ray photoelectron spectroscopy. This is not confidential and can be used by any one with interest in XPS. The intention is not to go deep in the physics theories behind since is out the scope of this study; however, the reader is referred to read the scientific papers for more details.
I would you like to thank my shell’s line-mangers HP Calis/ Jan van Schijndel who gave me the chance to do this master program and my direct colleagues who supported me across this journey Sander van Bavel, Constant Guedon and Heiko Oosterbeek. Also I thank my coordinator W. Th. Kok for his patient and helping me during this study and during the whole master study.
IV
Summary
Across this literature review, we have seen that developments in NAP-XPS, both in lab-based and in synchrotron-based high pressure XPS, have been progressing. The main obstacle to achieve high pressure XPS is the scattering of the ejected photoelectrons by the gas environment. To overcome this obstacle, differential systems with adjustable apertures were developed. Due to the distortion of the heterogeneous reaction on the surface when placing the aperture too close to the surface, the ideal position was agreed to be twice the aperture diameter. To achieve a pressure as high as possible, the aperture diameter must be as small as possible, which in turn means a very short distance between the surface and the aperture in the front of the first differential pump. In this way, the travelling distance for the ejected photoelectron to enter the aperture becomes short and accordingly, the pressure can be increased significantly. Based on these criteria, the spot size of the incident beam becomes of extreme importance. By using a very small spot size, the aperture diameter can be significantly reduced till ideally the spot size diameter; one can imagine the great benefit of the synchrotron where a very small spot size can be produced. In the differential pumping system, different approaches were adapted, namely, a standard energy analyzer with pre-lens zone, where adjustable apertures are used, while another approach was using an electrostatic pre-lens in front of the analyzer to increase the photoelectron collection efficiency, and finally a totally modified energy analyzer.
Exposing the surface to gases in the NAP-XPS was carried out in different ways. Back ground dossing where the ample is placed in the XPS chamber and the pressure in the whole chamber can be increased, beam dossing where the gas is provided by a stainless steel tube of very small diameter directly positioned to the surface of the sample, finally using a high pressure reactor cell in the batch or flowing mode. In case of using a high pressure reactor, an additionally analytical technique can be used such as GC to investigate the performance of the catalyst. This combination can give a correlation between the dynamics of the surface changes and the real catalyst performance. A few real studies were mentioned to highlight how NAP-XPS was applied and to illustrate the additional great insight value that can be provided compared to the conventional XPS. A good example of NAP-XPS was the investigation of the active site in the bimetallic interface catalyst surface of Ru/Co in the reduction of CO2, where
NAP-XPS in combination with GC could give a clear insight that the active site of the catalyst is on the metal-metal interface, where Ru facilities the reduction of Co.
I believe that the development of high pressure XPS will continue to achieve much more realistic conditions. This expectation is mainly based on the need of a surface-sensitive technique at more realistic conditions in the catalysis world. That will make the investigation of dynamic changes on the outmost surface layer under realistic conditions possible. This demand will be the drive force for further developments.
V
Abbreviations
ALS Advanced Light Source
AP-XPS Ambient Pressure X-ray Photoelectron Spectroscopy
ASF
Atomic Sensitivity Factor
BE Binding Energy
CVD Chemical vapor Deposition
EEA Electron Energy Analyzer
ESCA
Electron Spectroscopy for Chemical Analysis
ESR Ethanol Steam Reforming
FT Fischer-Tropsch
FWHM Full width halve maximum
GC Gas Chromatography
IMFP Inelastic Mean Free Path
IRRAS Infra-Red Reflection Absorption Spectroscopy
KE Kinetic Energy
LEED Low Energy Electron Diffraction
ML Mono layer
NAP-XPS Near Ambient Pressure X-ray Photoelectron Spectroscopy
NEXAFS Near-Edge X-ray Adsorption Spectroscopy
PES
Photo Electron Spectroscopy
STM Scanning tunneling Microscopy
TEM Transition Electron Microscopy
UHV Ultra High Vacuum
WGSR Water Gas Shift Reaction
XPS X-ray Photoelectron Spectroscopy
VI
Contents
Foreword: ... III Summary ... IV Abbreviations ... V 1 Introduction ... 12 Basic theory of X-ray photoelectron spectroscopy: ... 3
Design and considerations for AP-XPS ... 7
2.1 X-Ray source ... 7
2.2 Aperture size and differential pumps ... 8
2.3 Analysis chamber design: ... 10
3 Laboratory-based ambient pressure XPS ... 13
3.1 Advantages ... 13
3.2 Disadvantages: ... 13
3.3 Examples: ... 14
3.3.1 Water-induced formation of cobalt oxides: ... 14
3.3.2 Catalytic Conversion of Carbon Dioxide to Methane on Ruthenium−Cobalt Bimetallic under reaction conditions: ... 15
4 Synchrotron- based in situ ambient pressure XPS ... 18
4.1 Advantages ... 18
4.2 Disadvantages ... 18
4.1 Applications ... 19
4.1.1 Water adsorption on polycrystalline vanadium from ultra-high vacuum to ambient relative humidity .... 19
4.1.2 Study the importance of the Metal–Oxide Interface in the Water–Gas Shift Reaction by Near Ambient-Pressure X-ray Photoelectron Spectroscopy ... 21
5 Conclusion ... 23 6 References ... 24 7 Appendices ... 27 7.1 Appendix 1 ... 27 7.2 Appendix 2 ... 28 7.3 Appendix 3: ... 29
1
1 Introduction
A heterogeneous catalyst is in general a supported, metal-based catalyst. A reduction of the catalyst cost, and an extended catalyst life, are often required to make the heterogeneous catalytic process economically feasible. Understanding of the mechanism is of crucial importance as a first step in improving catalyst performance [11]. In heterogeneous catalysis, the surface plays the crucial role in the catalytic reactions. To investigate the interface of the active metal surface and the adsorbed gas molecules, and accordingly, following the dynamic changes on the surface, sensitive surface science techniques are required. A powerful very sensitive surface technique is in-situ X-ray photoelectron spectroscopy (XPS), with which the elemental composition and chemical state of the surface can be determined.
X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis
(ESCA), is a widely used technique to investigate the chemical composition of surfaces. This technique is
based on the photoelectric effect which was explained by Einstein (Nobel Prize 1921) and brought into
analytical technique by Kai Siegbahn in 1981.
In XPS, the information is encoded in the kinetic energy of the ejected electron from the surface. Photoelectrons with kinetic energies between 100-1400 eV have an inelastic mean free path (IMFP) of about 20Å[12], that makes the technique surface very sensitive. In modern XPS(synchrotron-like), a tunable monochromatic X-Ray source with high resolution can be used to adjust the incident photon energy
which in turn can control the kinetic energy of the ejected electrons, and in this way a high resolution surface sensitive analysis of the outermost surface layers can be achieved. Due to the strong scattering effect of the ejected electrons by the gas surrounding at high pressure and the high vacuum requirements for the electron energy analyzer (EEA) and the X-ray source , XPS is conventionally operated under ultra-high vacuum conditions (UHV)[13].
For many heterogeneous catalytic reactions high adsorbate coverages are required for the reaction to take place. With the conventional UHV conditions, high adsorbate coverage can be achieved only by using low temperature. However, decreasing the temperature lowers the kinetics of the reaction and that makes reaching the thermodynamic equilibrium impossible[8]. To overcome the so called “pressure gap”, the difference between UHV conditions and the real catalytic conditions (from minimal 1to 1000 mbar), in terms of surface morphology, adsorbate-adsorbate interaction and the electronic dynamic changes on the surface, a high pressure surface sensitive spectroscopic technique is required. Infrared spectroscopy and Raman spectroscopy are directly used at high pressure; however, the major advantages of XPS as direct quantification and chemical sensitivity make it very desirable to extend the operating pressure rangeMangolini and Carpick [14].
Near ambient pressure –XPS (NAP-XPS) was first carried out by Siegbahn and his colleagues in the mid-1960s, when they investigated the gas phase at approximately 0.1 Torr[14]. Since then continuous progress has been made. The challenge with XPS is the short mean free path of the photoelectrons with kinetic energies below 1500 eV at high gas pressure. These electrons suffer from scattering by the gas environment. The solution for that problem was to separate the analysis chamber where the sample is positioned from the analyzer and the X-ray source, where the high vacuum is required. By using a differentially pumped electrostatic lens, the ejected photoelectrons can be focused and detected. The entrance of the analyzer must be close to the mean free path of the photoelectron and the distance between the analyzer and the sample must be in the same order of magnitude. The X-Ray gun is protected with an aluminum coated window (44 mm in diameter and in the order of 1 µm thick) to maintain the vacuum in the X-Ray source[15].
This literature study is meant to give a summary of the recent developments in NAP-XPS, as a relevant tool for heterogeneous catalysis in general. Two directions will be followed.
2
1) Laboratory-based AP-XPS:
A modified conventional X-ray source is used (dual anode Al Kα and Mg Kα) in the routine surface science lab. The focus of this overview will be on the design considerations: the differential pumping system, analysis chambers (the high pressure cell), and ultimately some real research examples will be given to demonstrate how successful the technique is to investigate the surface under elevated (near realistic conditions) pressure.
2) Synchrotron –based AP-XPS:
When using a "harder" X-Ray source, the photoelectrons have much longer mean free path due to their higher kinetic energy; accordingly, the effect of scattering by the gas phase is considerably lower. Additionally, the high photon flux reduces the measuring time needed per spectrum to the domain of seconds or even less. A monochromatic focused light source with high-intensity tunable beam can be used and high resolution XPS spectra can be obtained. Additionally, by changing the beam energy, depth analysis can be achieved. This also can be done by angle-resolved XPS by tilting the sample rather than attenuating the photon incident energy. Also a view of the design considerations will be given, and the performance of the technique will be demonstrated with examples.
3 0
2 Basic theory of X-ray photoelectron spectroscopy:
X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis
(ESCA) is a widely used technique to investigate the chemical composition of surfaces. In this technique,
by exposing the sample to soft
X-ray (with 200-2000 eV), the core electron-levels can be examined. Photoelectrons are ejected from the surface after absorbing sufficient photon energy.Figure1 shows schematically how a core electron can be ejected by absorbing X-ray energy. The kinetic energy of the ejected electron KE can be measured by the electron energy analyzer (EEA) according to equation 1, where hν is the incident X-Ray source energy, usually the source is dual Al Kα and Mg Kα anode with energies 1486.7 eV and 1253.6 eV respectively. BE is the binding energy of the core electron and it is unique for each element and the environmental chemical composition. KE is the gained kinetic energy which is the difference between the incident X-ray energy and the binding energy of the core electron. Equation 1 includes also spectrometer specific parameter which is called work function (Φ spectrometer).
BE = hν − KE − ΦSpectrometer
1
Figure1 Schematic of the photoelectron process. Reproduced from ref. [1]
4 In the XPS working energy range, where the ejected photoelectrons have KE in the range 10-1000 eV, the expected detection depth is about 1-3.5 nm, as shown in the universal curve in Figure2. To explain the universal curve, we need to describe what can happen for the ejected core electron. There are there processes can be occurred as illustrated below and decreasing the photoelectron energy:
o Scattering process.
o The ejected electron can excite another electron, e.g. excitation of the valance electron. o Ionizing another core level and producing another photoelectron or auger electron.
If we try to understand the universal curve, we can divide it into three sections; at very low energy, which simply means the ejected electron does not have enough energy to undergo the mentioned processes which results in its mean free path is too long. At very high energy, the ejected photoelectron spends less time passing through a given thickness surface, and it is not likely to suffer energy loss, accordingly, has long mean free path in the solid surface. However, between these two regions, the ejected photoelectron suffers from the mentioned processes and accordingly, its mean free path passes through the minimum.
Based on the Lambert-Beer-like law in XPS (equation2) to determine the contribution of specific surface element in the intensity of the measured XPS,
Is 𝐼0
⁄ = e
(−d| λcos θ)2 Where Is is the intensity of the collected ejected photoelectron of the element on the surface, I0 the intensity from
infinite depth, cosθ is the takeoff angle of the ejected photoelectron (normally is zero) and λ is the IMFP (IMFP is average distance between inelastic collisions (Å)) of the photoelectron.
When d=λ, where d is the depth the electron travels to escape the surface, that means Is /I0=63.3% and that is the
contribution of the first monolayer (of the adsorbate) on the surface in the total XPS intensity (assuming that λ is equal to the monolayer, which is almost the case)
Electron Energy (eV)
Figure2 ”universal curve” of electron inelastic mean free path (IMFP) versus KE (eV), reproduced from [1].
5 The ejected photoelectrons are collected by an electron energy analyzer. The most popular analyzer is the concentric hemispherical analyzer (CHA) working in constant analyzer energy mode (CAE). In this mode the resolution of the XPS peak across the spectrum remains the same.
To explain how that works, we need to remember the definition of the resolution: 𝑅 =∆𝐸
𝐸 3 ∆𝐸 ≈ 𝐹𝑊𝐻𝑀 (𝑒𝑉) 4
𝐸 ≈ 𝐾𝐸 𝑜𝑓 𝑝𝑒𝑎𝑘 (𝑒𝑉) 5
If we want to achieve uniform resolution of 0.5 eV across the entire XPS spectrum, it is clear that is not possible, because E is variable across the XPS spectrum.
The easiest way to achieve the constant resolution is, the photoelectrons are retarded to the same KE (pass energy) before entering the spectrometer. By varying the retardation potential, the spectrometer will be able to collect photoelectrons of different pre-retarded KE. In this way, the resolution remains the same across the entire XSP spectrum.
Ultimately, XPS data will be presented as the photoelectron intensity as a function of binding energy (BE). A typically XPS survey is given in figure 3[1, 16-18].
It is worthwhile to mention that the XPS peak shape depends on the orbital type that was occupied by the ejected electron, by so-called spin-orbit splitting. For any electron in an orbital with orbital angular momentum, coupling between the magnetic fields of spin (s) and angular momentum (l) occurs. The S value is ± ½ and L values are 0, 1, 2 and 3 for s, p, d and f orbitals. For this reason an electron from S orbital gives always a singlet peak, while for p, d and f doublet peaks are observed based on total angular momentum j = |l ± s|. However, spin-orbit split levels in the doublet can be derived? From the so called degeneracy = 2j + 1 as in Table 1.
Table1 Spin-Orbit Split Ratio's
Subshell L value j values Ratio
s
0 1/2 −p
0,1 1/2, 3/2 1: 2d
1,2 3/2, 5/2 2: 3f
2,3 5/2, 7/2 3: 4The relative concentrations of elements can be determined from the photoelectron intensities. The detection limit of XPS is about 0.1-1 atomic %.
For quantitative analysis in XPS, most analyses use empirical calibration constants (called atomic sensitivity factors ASF) derived from standards:
6
7
Design and considerations for AP-XPS
The whole principle of the design is based on decreasing the attenuation of the electrons due to the scattering effects in the high pressure gas zones by minimizing their path length. That has been achieved by introducing a differential pump system to isolate the analysis chamber ( high pressure parts) by small apertures from the analyzer optic and the X-ray source [14].
There are three aspects have to be considered to achieve photoelectron emission spectroscopy(PES) measurements at high pressure, the scattering effect of the ejected electrons by the gas phase, protection of the X-ray source and the high vacuum needed to protect the electron multiplier( normally <10-7 mbar).
2.1 X-Ray source
In the Laboratory-based AP-XPS, A modified Al/Mg twin anode X-ray source is used, which is placed at 15 mm (the specified working distance) from the sample surface at 59.8° with respect to the surface normal, which is called magic angle, some deviations from this angle can be adapted depending on the setup orientation. The X-ray source is differential pumping to keep the anode under vacuum. Additionally, the X-ray source is equipped with Aluminum window of thickness 2-µm to protect the source from the high pressure at the surface.
As discussed previously, the count rate of the collected ejected photoelectrons under the reaction conditions is to some extend determined by the flux of the incident radiation beam and the size of the aperture to the analyzer. As the pressure increases the molecular density of the gases increases in the reaction cell and accordingly the count rate dramatically decreases, as previously mentioned, this attributed to the strong inelastic scattering of the ejected photoelectron by the gas molecules under reaction conditions. To partially overcome this, a beam with high flux density is required to eject the sub-shell electrons. In Synchrotron based facilities that is not an issue, due to the high flux beam used, however; in Lab-based NAP-XPS is a challenge, an example of high flux beam (monochromated AlKα source, XRF 1000MC, from SPECS Surface Nano Analysis GmbH) was illustrated by Franklin et al. [9, 19].
8
2.2 Aperture size and differential pumps
In this review, we describe the design principle of the differential pumping and the criteria of the aperture size, for more details the reader is appointed to the following references [4, 8, 10, 12, 15, 19-34]. The basic principle hampers performing high pressure XPS is the scattering of the ejected photo electron by the gas molecules. When the electrons have kinetic energy above 100 eV (this is typically the energy range in XPS), the inelastic scattering dominates, and this kind of scattering has the main contribution in the signal attenuation.
The attenuation of the signal I at high pressure P compared to the signal I0 at UHV condition P0 can be expressed
in exp(-σdp)
,
where d is the distance the photo electron travels in the gas at pressure P and σ is the scattering cross section which depends on the type of the gas. The aim of the NAP-XPS is P to be high as possible, which implies that the distance d must be as short as possible to limit the reduction of the signal intensity. To illustrate that for instance, a photo electron with kinetic energy 100 eV at 1 mbar H2O vapor has inelastic mean free path about 1mm, which is much shorter than the distance between the sample surface and the entrance of the electrostatic lens of the energy analyzer which is in order of magnitudes higher (cm). That means under this condition these ejected photo electron cannot be detected.
To overcome this problem, the differential pump system was adapted in lab-based XPS since 1970 by Kai Siegbahn’s group at Uppsala University. The effort was continued by Joyner, Roberts and shortly thereafter Grunze, where three setups were built with differential pumps in Cardiff, Novosibirsk and Maine in the mid- 1980s.
The first synchrotron was built up at Lawrence Berkeley National Laboratory (LBNL) in 1990s. Appendix 2 shows the near ambient pressure XPS timeline, illustrating the number of publications and the number of the new instruments around the world, indicating that NAP-XPS measurements have large contribution in surface science.
Figure4 shows scheme of the different pumping stages (3) and the distances from the sample surface till the energy analyzer. from[4]
Figure5 illustrates various differential pump systems and the electron trajectories, a using a different differential-pumping apertures in front of a standard energy analyzer, b using a pre-lens section in front of standard energy analyzer and c shows the using of modified analyzer, reproduced from [10]
9 Each differential stage can create 10-2-10-5 mbar (that is depending on the aperture conductance, pump speed and
type of the gas), accordingly to measure mbar range, several stages are required. Figure 5 demonstrates three different differential pump systems combined with the energy analyzer, a) using apertures before the analyzer input lens of a standard analyzer, b) using pre-lens section in front of the analyzer input lens to focus the electron trajectories and c) shows a modified energy analyzer.
Due to the pressure difference between the front of the aperture “differential pumping” and the background of the surface in the high pressure zone, the pressure distribution in the front of the aperture is not homogenous. To prevent this pressure interruption, the aperture must be placed at distance twice the aperture diameters. The aperture diameter is the critical parameter to achieve as high as possible pressure. To decrease the distance between the surface and the entrance aperture, the aperture diameter must be decreased. A very important parameter comes to play, which is the spot size of the incident photon on the surface sample. One can imagine the great advantage of using the small spot size of the synchrotron as mentioned under the advantages of the synchrotron. Reducing the photon spot size leads to decreasing the aperture diameter, this in turn decreases the surface-aperture distance travelled by the ejected photoelectron, accordingly, high pressure can be obtained. An additionally benefit by decreasing the aperture size is decreasing the gas flow to the second pumping stage, which allows to use larger aperture in the second differential pump which has the positive effect on the electron collection efficiency which in turn can be translated in a good signal/noise ratio. It is worthwhile to mention that, reducing the aperture size will obtain larger differential pressure, however, will reduce the acceptance angle of the electrons. Therefore, it is a matter of compromise between the aperture shape/size and the differential pressure to get good signal intensity.
10
2.3 Analysis chamber design:
The sample either can be positioned in the analysis chamber on a sample holder, where the surface is entirely exposed to the chamber environmental or placed in reactor-like holder which is separated from the detection system. Kraus et.al demonstrated using ultra-thin graphene membrane as alternative economically approach to achieve photoelectron spectroscopy at elevated pressure instead of using the very expensive differential electronic lens [31].The graphene has relatively high electron transparency for low kinetic energy( less than 1000 eV), gas impermeability and can sustain the pressure difference between UHV and the high pressure zone. The used graphene layers (4ML) was prepared by chemical vapor deposition (CVD) afterwards was transported to the XPS synchrotron facility ELETTRA. It is demonstrated that the transparency of 1ML graphene (3.35 Å thicknesses) can be greater than 50% for electrons with kinetic energy more than 300 eV. The beneficial of this approach would be applying NAP-XPS for liquid samples as demonstrated for water underneath the graphene membrane and even for more toxic sample. However, transporting the prepared graphene to the facility might increase the contaminations layers upon the original graphene, as consequence, the transparency would dramatically decrease.
Dosing the gases can be carried out by three ways [4]:
1. Background dosing:
The valve to the turbo-pump in the analysis chamber (where the sample is positioned) is closed. The pressure in the whole chamber can be increased; the only minor pumping can be achieved through aperture a1 to the first differential pump in figure6.
2. Beam dosing:
The gas is provided by stain steel tube of very small diameter (about 0.18 mm) and directly positioned to the surface of the sample. The gas can be dossed by using high precision valve. This directed doing to the surface provides “effective pressure” on the surface of the sample which is about 100 times more than in case of background dosing. Advantage of this way is the valve between the turbo-pump and analysis chamber remains open, which decreases the pressure in the whole analysis chamber significantly comparable with the background dosing. For more gases, separate tubes are used for each individual gas. Figure 6 shows the beam characterization
Figure6 shows the beam characterization of 1000 mbar gas outlet, the pressure with respect to the distance X from the beam axis; z is the distance between the outlet of the gas and the sample surface at 72°with respect to the sample normal [4]
11 of gas of pressure 1000 mbar at angle 72° with respect to the surface normal. As the distance between the surface and the gas outlet(z) becomes 1.5 mm, the effective pressure on the sample surface increases to about 5 mbar.
3. Contineously flow dossing reactor:
This mode is different from NAP-XPS, where gases are introduced into a UHV chamber through background dosing or a direct dose(beam dossing). In a typical microreactor, the volume is about 10-30 ml, however; the volume of a typical UHV chamber is a few litres. Dossing in the UHV chamber “ Static mode”, the volume is so large that would have some undiserble consequences such as
o Possibly leads to re-adsorption of the products on the surface.
o Limits the temperature of the sample which can be heated to by the thermal conduction of the hot sample surface with the cold UHV chamber walls through the huge gas volume.
o possibly leads to contaminate the surface by deadsorption of the gas molecules on the wall by heating the wall.
One way to limit these undesirbale effects is using conteniously flow reatctor cell. Using integrated flow cell (analysis chamber), the surface chemistry becomes more comparable with the kinetic studies and the performance of the catalytic studies in research groups for fix-bed and microreactor which is of great interest [9].
Figure7 schematic and photos of the high pressure contentiously flow reactor. a) is the front side of the reactor with two windows, one for the video camera, the other is for an alternative X-ray source. b) Illustrates the two windows at the back side of the reactor, one for the x-ray source and the other for the video camera. c) presents a photo of the integrated high pressure cell in the UHV system, re-produced from [9]
12 As a prove of principle of a flowing high pressure XPS reactor prototype was demonstrated by Franklin et al [9]. The reactor can be heated up till 500-550°C and the pressure can be till 25-50 Torr. The reactor is integrated in the UHV system, where the gases can be introduced to the reactor and the outlet can be mixed with carrier gas to be sent to GC to monitor the performance of the catalyst. This is an elegant way to combine the real catalyst performance with the dynamic reaction changes by NAP-XPS. Figure 8 illustrates a) the front side where two windows for video camera and possibly alternative X-source, b) the back side at the reactor where video camera and X-ray source are present and c) where the reactor is place in the UHV system. It is worthwhile to mention that the heating element has to be critical chosen, owning that some materials have some activity under the reaction conditions, that was the reason to keep the heating element outside the reactive environment. The pressure at the inlet and the outlet can be measured simultaneously; the average is the pressure on the reactor. The flow through the reactor can be controlled by a leak valve. The reactor was demonstrated by measuring the Ag 3d peak under N2
up till 25 Torr. It was found that even under 25 Torr the count rate of the Ag 3d still at an acceptable level. The reader is referred to references [9, 19] for more details.
13
3 Laboratory-based ambient pressure XPS
3.1 Advantages
The big advantage comparable to the synchrotron-base facilities is the accessibility, which can facilitate the spreading of the technique to other fields. Additionally, more research on the same topic can be achieved, as consequence, more deep valuable information can be obtained. Another benefit is easier to modify the setup, e.g. gas feeding, reactor design and probably transfer the sample in-situ to another chamber for further more specific analysis. Furthermore, building model sample is near the setup, which minimizes the probability of contamination. Building a lab-base NAP-XPS is economically affordable comparable with synchrotron-based XPS, that last is limited to the national facilities.
3.2 Disadvantages:
Since the spot size of the incident X-ray beam is of huge importance to control the aperture diameter in the front of the differential pump system, it is to be expected that the high pressure which can be achieved by using lab-based NAP-XPS will be significantly lower that with the comparable synchrotron based NAP-XPS. Owing to the low incident photon flux, the time needed to get reasonable peak is likely a matter of hours, which limits the possibility to follow the dynamic change on the surface.
14
3.3 Examples:
3.3.1 Water-induced formation of cobalt oxides:
A classical use of in-situ XPS was performed by Lin et al [7], where the catalyst is treated in an isolated chamber separated from the XPS chamber. After evacuation, the sample is transported to the XPS to be measured without exposing the surface to the ambient atmosphere. In this way the surface is kept under the vacuum and what investigated is the post-exposure remained changing. This example was given as alternative way for NAP-XPS Water induced formation of cobalt oxide is of great importance in Fischer-Tropsch (FT) reaction where a based cobalt catalyst is exposed to the produced water at elevated temperature (usually around 220°) [35, 36]. An explanation of decreasing the activity was formation of inactive metal oxide. Furthermore, cobalt based catalyst is used in hydrogen production process in Ethanol steam reforming (ESR). It is believed that in ESR the C-C bond cleavage takes place on the metal Co at around 400°C, while the carrier (CeO2–ZrO2) plays dual role; stabilizes the
cobalt nanocrystallites and facilitates the oxidation process by activating the water. It is not clear whether the cobalt oxide has a role in this process. Investigation of the cobalt surface after (preferentially during) the exposure to water is needed to answer this question. Figure 9 illustrates exposing the catalyst 10% Co/CeO2– ZrO2 to different conditions (a) after calcination, mainly Co3O4, (b) after reduction by exposing the catalyst to
110 Torr at 450°C, which form the active phase (Co0) (c) shows the effect of pure water(20 Torr at 450°C for
1h) and (d) demonstrated the effect of 20 Torr ethanol/5 Torr water at 450°C, which is more or less the real condition of the reaction for 2.5 h. Clearly, water has effect on the oxidation state of the cobalt(c), where forming the peak at 781.0 eV is corresponding to the formation of Co+2. However, less water with ethanol
shows more oxidation degree , Lin et al [7] explained that as effect of the presence of ethanol and ignored the exposure time effect (2.5 h). This application shows how powerful XPS is to distinguish between different
Figure8 In-situ XPS of Co based catalyst 10% Co/CeO2–ZrO2(a) calcinated at 500°C for 4h, (b) exposed to 110 torr at 450°C, (c) exposed to water(20 torr ) at 450°C for 1h and (d) exposed to water (5 torr)/ethanol (20 torr) at 450 °C for 2.5 h, reproduced from [7]
15 cobalt oxidation states on the surface [37]. In this study some important factors were ignored like the exposure time and the particle size effect, which might play role in the oxidation degree formation.
3.3.2 Catalytic Conversion of Carbon Dioxide to Methane on Ruthenium−Cobalt Bimetallic under reaction conditions:
This is an interesting application of NAP-XPS, where Zhu et.al was able to figure out why a bimetallic alloy of Co/Ru was more active and selective than only Co or Ru catalyst in the conversion of carbon dioxide to methane reaction (CO2 + 4H2 → CH4 +2H2O) [3]. CO2 capture and chemical utilization have attracted a lot of attention in
the energy and environmental communities in the last decades owning to the greenhouse effect. Chemical utilization of CO2 is based on the catalysis reaction of conversion of CO2 to chemicals. One of the most important
of these reactions is the reduction of CO2 to methane. Noble metals like Ru are the most active catalyst, however;
on large industrial scale that would not be economically feasible. Economically, Co can be used as alternative catalyst in this process; however, Co is not as active as Ru. Additionally, Ru can be used to tune the activity and selectivity.
Under reaction condition, restructuring of the surface takes place, as consequence, the surface before the reaction and after the reaction is most likely not representative to the real surface under reaction condition. To overcome what so called, the previously mentioned, pressure gap and the dynamic changes on the catalytic surface during the reaction conditions, an operando/in-situ technique is required. In this study NAP-XPS could proof that it is a powerful technique to investigate the surface under reaction conditions and ultimately could give a clear insight what is the active phase in the reduction of CO2 to methane.
In this study, NAP-XPS at different temperature was combined with measuring the catalytic performance of the catalyst by GC.
Figure9 shows the catalytic performance of Co3O4 and (Co 0.95Ru0.05)3O4, a) illustrates the activity at different temperatures, b) illustrates the selectivity expressed in % of CH4 formation, reproduced from [3]
16
Binding Energy eV
To study the performance, the catalyst was loaded in a fixed-bed microreactor at 1 bar and at operating temperature window 100-420°C. To study the influence of Ru as promoter in general and the effect of Co-Ru interface in particularly, a comparison was made between Co3O4, (Co 0.95Ru0.05)3O4, Co/SiO2 and Ru/SiO2.
Figure10 shows the catalytic performance of two catalysts Co3O4, (Co 0.95Ru0.05)3O4, figure10a shows the measured
conversion rate of CO2 to methane at different temperature. The starting activity of Co 0.95Ru0.05)3O4 is around
220°C while the starting activity of Co3O4 catalyst is around 300°C, a significantly higher than the bimetallic catalyst.
In the temperature range 220°C-340°C, the activity of the Ru promoted Co was about 20% higher than unpromoted Co. Figure10b presents the selectivity of the two catalysts to production of CH4. For Co3O4 the
selectivity is 0 at 260°C and increased to 50% at 300°C. For the Ru promoted catalyst, the selectivity reached 99% at 260°C.
Additionally, catalytic performance measurements were carried out for Ru/SiO2 and for Co/SiO2, in general, (Co 0.95Ru0.05)3O4 has better catalytic performance than Ru/SiO2 and Co/SiO2( not shown here, the reader is referred to
the reference [3] for more details).
From all above, the active phase in the catalyst (Co 0.95Ru0.05)3O4 is most likely Ru-Co bimetallic interface instead of
pure Co or pure Ru.
To figure out the promoter effect and answer the question, why (Co 0.95Ru0.05)3O4 catalyst is more active and
selective, NAP-XPS measurements were carried out under 0.1 Torr CO and 0.4 Torr H2 at different temperatures
for two catalysts Co3O4 and (Co 0.95Ru0.05)3O4. Figure11 presents the results of NAP-XPS, figure11c shows the Co
2P XPS peaks of Co3O4. It is clear that up to 220°C, Co3O4 remains unchanged as the started peak form and
position(not shown here). Correlation of this observation with figure11 indicates clearly that Co3O4 not active in
reduction of CO2 to methane. At 260°C the Co 2P peak positions and peak form changed to the characteristic Co+2
peaks [7, 37]. At 300°C, a new Co 2P peak appears, notably, 778.9 eV, which is characteristic for Co0, which
indicates the reduction of CoO to Co0. Figure11a shows NAP-XPS for Co 2P spectra in the catalyst (Co
Figure 10 shows the NAP-XPS spectra under 0.1 Torr CO2/0.4 Torr H2 and under the mentioned temperatures, a) shows the XPS spectra of Co 2P in the bimetallic catalyst ((Co 0.95Ru0.05)3O4), b) shows the Ru 3P spectra in the bimetallic catalyst ((Co 0.95Ru0.05)3O4), while c) presents the Co 2P spectra in only Co3O4, reproduced from [3]
17
0.95Ru0.05)3O4 under the same conditions. Clearly, the reduction of the Co takes place at significantly lower
temperature than only Co3O4,as consequence, the activity and selectivity might be observed at significantly lower
temperature, which was supported by the performance measurements in figure10. Figure11b shows that the reduction of Co was accompanied with partially reduction of Ru.
From all above, a conclusion might be drawn that the active phase is a bimetallic reduced Co-Ru and the presence of Ru facilities the reduction of Co.
These results showed the big advantage of NAP-XPS comparable with conventional XPS, where we would not have been able to observe the reduction of Co and Ru under UHV conditions. These results could be e supported by in-situ TEM study, where the dispersion of the reduced Co and Ru on the surface can be monitored.
18
4 Synchrotron- based in situ ambient pressure XPS
4.1 Advantages
Synchrotron can provide variable controlled incident X-ray beam energy to fit the experimental desire, as consequence, the surface sensitivity can be maximized or minimize accordingly. Additionally, the ionization cross-section of the substrate can be controlled. Furthermore, It provides a high resolution radiation with small spot size incident radiation, which is a great beneficial point to decrease significantly the radius of the first differential pumping station aperture, accordingly, the pressure in the high pressure reactor (analysis chamber), can be increased orders of magnitudes. Due to the high electron flux, the time needed for measurements is significantly decreased to domain of few seconds or even less, accordingly, that enables following surface dynamic changes process in-situ [9, 30].
One of the most relevant question in the heterogeneous catalyst is, where the adsorbates are adsorbed on the surface e.g. on the terraces or on the edges of the surface. In the third generation synchrotron, the soft X-ray source is applicable with energy up to 2KeV with extremely high resolution radiation. The resolution is down to below 50 meV compared to 900 meV with standard Mg Kα radiation and 300 meV with monochromatic Al Kα that allows the detection of the small binding energy difference of the adsorbed molecules. Accordingly, investigating the small species on the different sub-surface and even the vibrational of fine structure can be identified (e.g. CO adsorption on the transition metals surface and O2 on Ag surface [38, 39]).
Another big practical advantage is the access to other valuable and complementary analytical techniques such as near-edge x-ray adsorption spectroscopy (NEXAFS), which probes the orientation density of the unoccupied outermost electrons, Low energy electron diffraction (LEED), which gives complete information about the structure of the surface and extended X-Ray absorption spectroscopy which gives information about the distance between the neighbored atoms [20].
4.2 Disadvantages
The limited availability of the synchrotron is one of the disadvantages. Since the Synchrotron locations are mainly based in national facilities, a reservation for experiment time in advance should be made. Additionally, all the preparation for the experiments must be made and transported to the synchrotron location, which might bring additionally changes like contamination.
Furthermore, the high density beam used, it is an advantage as mentioned previously, however; it could damage the thin films of the soft materials like polymers [9, 19].
19
4.1 Applications
4.1.1 Water adsorption on polycrystalline vanadium from ultra-high vacuum to ambient relative humidity
The investigation of influence of water on the transition metal by using NAP-XPS can be performed directly under increasing the water pressure during the measurements. In this case a more realistic condition can be investigated and the on-line surface dynamic changes can be followed. A good example is the adsorption of water on polycrystalline vanadium[5].
This example was carried out at the Molecular Environmental Science beamline (11.0.2) at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory [40].
Vanadium has received a lot of attention due to the wide range of its applications e.g. steel production, as a coating material and as a very promising alternative candidate to replace the costly metal e.g. Pd in the hydrogen purification processes. Figure12 illustrates the effect of water pressure on the oxidation of the vanadium surface. At very low water pressure 2.9x10-9 Torr is mainly V0 present that was indicated with the metallic vanadium peak at 512.2 eV.
Increasing the water pressure up till 1.2x10-6 Torr did not show any changes in the oxidation state of the vanadium.
That is more or less the UHV conditions, which means more realistic conditions are needed. Figure12 shows that the vanadium oxide and vanadium hydroxide starts to be formed from water pressure 1.2x10-6 Torr due to the
dissociative adsorbed of water, which was confirmed by V-Ox peaks which can be seen at 515.2 eV and V-OH at 513 eV. Further increasing in the peak broadness was observed by increasing the water pressure, which indicates increasing the oxidation of the vanadium. Analysis of the thickness of the formed oxide coverage was carried out by using tunable incident radiation.
Figure11 V 2P1/2 and V2P3/2 NAP-XPS of V- polycrystalline foil at 310 K in water vapor. The water pressure is (a) 2.9x10-9 Torr, (b) 1.2x10-6 Torr and (c) 1.2X10-5 Torr. The oxidation starts from water pressure 1.2x10-6 torr re-produced from [5].
20 Figure13 shows the depth profile analysis of the formed vanadium oxide by using 2P peak intensity and variable incident photo energy. As the incident photo energy decreases the effective attenuation length of the ejected photoelectron decreases and becomes more surface sensitive, see appendix1 for more details. The used incident photo energies were 635 eV, 735 eV, 835 eV, 935 eV, 1135 eV and 1235 eV, which corresponding to a kinetic energies of ~ 120 eV, 220 eV, 320 eV, 420 eV, 520 eV and 620 eV. The peak intensity was normalized with respect to the metal peak intensity is better illustrating the changes in the metal/oxide ratio. Since the ratio V-O/V0
decreases with increasing the incident photo energy, figure13 confirmed that the formed oxide is on the surface. The calculated oxide coverage was 1.4 nm which corresponds to about 4 ML V2O3 under this condition. For more
details regarding the calculation the reader is referred to the reference [5]. This study showed how beneficial NAP-XPS was to investigate the influence of water at more realistic condition, where there was no influence observed at UHV conditions. The result was very surprising which supported an internal study on another transition metal, the study was carried out in-situ and the exposure to water was performed in a separate chamber then the surface after the exposure to water was transported to the XPS” conventional XPS” chamber [41]
.
Figure12 depth analysis of the formed V-oxide by using variable incident photo energy namely, 635 eV, 735 eV, 835 eV, 935 eV, 1135 eV and 1235 eV corresponding to a kinetic energy of the photoelectrons of ~ 120 eV, 220 eV, 320 eV, 420 eV, 520 eV and 620 eV[5]. The experiment was carried out at 0.5 Torr and 267 K
21
4.1.2 Study the importance of the Metal–Oxide Interface in the Water–Gas Shift Reaction by Near Ambient-Pressure X-ray Photoelectron Spectroscopy
This study was performed at the Advanced Light Source in Berkeley, CA (beamline 9.3.2). AVG Scienta R4000 HiPP analyzer was used [27]
.
Hydrogenation reaction is essential in the food production, fertilizers, chemicals and fuels. The major source of hydrogen is from reforming the hydrocarbons coming from the natural gas and the crude oil.
In the produced hydrogen there is CO impurities (1-10%) which lead to decrease the performance of many catalysts used in the industrial applications [42]. It is very beneficial to remove these CO impurities. One way to achieve this goal is the water-gas shift reaction (WGSR, CO + H2O CO2 + H2), where CO can be converted to H2 and
CO2.
Cu (111) is used as benchmark in studying the WGSR, however; CeOx /Cu (111) is very active. Theory predicts that
this WGSR is likely takes place though formation of carbonate-like species and dissociation of water was considered to be the rate determining step [43]. Water does not dissociate on Cu (111) surface, however; water dissociates on CeOx/Cu (111), which indicates that there must be a role of the oxide in this reaction. Previous infrared reflection
absorption spectroscopy (IRRAS) study under UHV conditions of OD adsorbed on the CeOx surface and
co-adsorbed of CO showed that CO is co-adsorbed on the top of Cu(111) and do not interact with the isolated OD on CeOx [6]. To figure out the role of the oxide and the mechanism of the reaction more realist condition is needed. In
this study NAP-XPS was used to investigate the WGSR under elevated pressure and temperature
.
Figure14 shows NAP-XPS spectra under WGSR condition (90 mTorr CO and 30 mTorr H2O). At 473 K in (a)
only Cu (111) was used, a peak observed at 533.3 eV in the O 1s spectra which was assigned to the water, a peak at 286.2 eV in the C 1s spectra which was assigned to chemisorbed CO and peak at 285 eV in the C 1s spectra was assigned to adventitious carbon (decomposed residual carbon, which indicates with C0) [30, 44]. From this
observation, a simple conclusion can be drawn; there was nothing happened on the surface of Cu (111) regarding
Figure13 shows NAP-XPS of Cu(111) and CeOx/Cu(111) under more realistic WGSR reproduced from[6]
22 WGSR, e.g. dissociation of water and formation of OH. When CeOx nanoparticles were deposited on Cu (111)
surface, a dramatically change in the O 1s spectra (b spectra) was observed. The peak at 530.0 eV was assigned to the oxide in the ceria, while the broad peak at 531.8 eV was assigned to OH group. It is well known from the UHV study that OH on Cu (111) observed at 531.5 eV, while on ceria was observed at 532.4 eV. That might indicate that the observed OH was on both the Cu (111) and CeOx (on the interfaces between Cu (111) and the ceria). Once the
temperature was increased (c in O 1s spectra), the binding energy of the O 1s peak corresponding to OH shifted to lower binding energy (mainly on Cu (111)). Furthermore, a decreasing in the intensity of the OH peak while a new peak was observed at 532.3 eV in the O 1s spectra was observed with further increasing the temperature, and that was accompanied with another peak in the C 1s spectra at 288.5 eV which was assigned to CO2- . for more details
how that was assigned, the reader is referred to read the reference [6]. From all above, one conclusion is most likely; the WGSR might take place through CO2- as intermediate which supports the IRRAS data, where CO-2 species was
detected , for more details [6].
This very good example illustrates how the so called pressure gap can be overcome, and give a worth insight mechanistic information while UHV conditions provided in fact no worthy information and was far away from the real conditions.
23
5 Conclusion
A huge achievement has been done in NAP-XPS to reach more realistic conditions to close the so called pressure gap. In the analyzer design point of view, a differential pumping/aperture systems can be placed before the electrostatic lens, which decreases the pressure significantly before the inlet of the analyzer. Other possibility is modifying the analyzer in such way that a pre- lens section is placed in the front of the analyzer which focuses the photoelectron trajectories beam and improves the detected signal quality.
In the analysis chamber (where the sample is placed), there were different possibilities were adapted. The sample can be placed in the XPS chamber, or separating the sample from the analyzer and the X-ray source by using microreactor. In case of using microreactor, an additional analytical technique can be used to combine the real catalytic performance with the dynamic surface changes by NAP-XPS. X-ray source can be differential pumped to reduce the pressure and protect the source.
Recently, many research groups are able to use lab-based NAP-XPS which has a great advantage of accessibilities and simple to modify, however, synchrotron based NAP-XPS provides high resolution XPS and higher pressure can be achieved owing to the small spot size of the photoelectron beam and accordingly, small aperture can be used. We belief that the operating pressure in NAP-XPS will be increased order of magnitude comparing with the current operating pressure in particularly synchrotron based NAP-XPS, and that enables following the reaction on the surfaces at more realistic conditions.
24
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27
7 Appendices
7.1 Appendix 1
28
7.2 Appendix 2
Near ambient pressure XPS timeline, illustrating the number of Publications and the number of new instruments. Red indicates laboratory based, blue indicates synchrotron-based instruments, reproduced from [10]
29
7.3 Appendix 3:
Restructuring of the surface under more realistic reaction conditions
STM images (50 nm×50 nm) of the Pt(557) surface under different CO pressure: clean surface in UHV (~1×10−10 Torr), surface under ~1×10−8 Torr of CO, and under high pressure CO (1 Torr), reproduced from [8]
