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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
APPLICATION OF AMBIENT PRESSURE X-RAY PHOTOELECTRON SPECTROSCOPY TO CATALYSIS Authoritative and detailed reference on ambient-pressure x-ray photoelectron spectroscopy for practitioners and researchers starting in the field Application of Ambient Pressure X-ray Photoelectron Spectroscopy to Catalysis introduces a relatively new analytical method and its applications to chemistry, energy, environmental, and materials sciences, particularly the field of heterogeneous catalysis, covering its background and historical development, its principles, the instrumentation required to use it, analysis of data collected with it, and the challenges it faces. The features of this method are described early in the text; the starting chapters provide a base for understanding how AP-XPS tracks crucial information in terms of the surface of a catalyst during catalysis. The second half of this book delves into the specific applications of AP-XPS to fundamental studies of different catalytic reactions. In later chapters, the focus is on how AP-XPS could provide key information toward understanding catalytic mechanisms. To aid in reader comprehension, the takeaways of each chapter are underlined. In Application of Ambient Pressure X-ray Photoelectron Spectroscopy to Catalysis, readers can expect to find detailed information on specific topics such as: * Going from surface of model catalyst in UHV to surface of nanoparticle catalyst during catalysis * Application of XPS from surface in UHV to surface in gas or liquid phase and fundamentals of X-ray spectroscopy * Significance and challenges of studying surface of a catalyst in gaseous phase and instrumentation of ambient pressure X-ray photoelectron spectrometers * Experimental methods of AP-XPS studies and difference in data analysis between AP-XPS and high vacuum XPS Ambient Pressure X-Ray Photoelectron Spectroscopy is an ideal resource for entry level researchers and students involved in x-ray photoelectron spectroscopy. Additionally, the text will appeal to scientists in more senior roles in academic and government laboratory institutions in the fields of chemistry, chemical engineering, energy science, and materials science.
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright Page
Preface
1 From Surface of Model Catalyst in UHV to Surface of Nanoparticle Catalyst During Catalysis
References
2 Application of XPS: from Surface in UHV to Surface in Gas or Liquid Phase
2.1 Origin of X‐ray Photoelectron Spectroscopy
2.2 Applications of XPS to Study Surface in High Vacuum
2.3 Applications of XPS to Study Sample in Gas Phase
2.4 Applications of XPS to Study Sample in Liquid Phase
References
3 Fundamentals of X‐ray Photoelectron Spectroscopy
3.1 Principle of XPS
3.2 Generation of X‐ray
3.3 Excitation of Photoelectron and Chemical Shift
3.4 Measurements of Energy of Photoelectrons
3.5 Measurements of Intensity of Photoelectrons
References
4 Instrumentation of XPS
4.1 Regular X‐ray Source
4.2 X‐ray Source with a Monochromator
4.3 Energy Analyzer
4.4 Detector
References
5 Significance and Challenge of Studying Surface of a Catalyst in Gaseous Phase
5.1 Origin of Difference between Surface in UHV and Surface in Reactant Gas
5.2 Intrinsic Feature of Catalytic Sites on Surface: Environmental Sensitivity
5.3 Ex Situ, Semi‐in Situ, and In Situ/Operando Studies of Catalyst Surface at Ambient Pressure of Reactants
5.4 Ex Situ, Semi‐in Situ, and In Situ/Operando Studies of Catalyst Structure at High Pressure
5.5 Technical Challenges in Studying Surface of a Catalyst in Gas Phase
References
6 Instrumentation of Ambient Pressure X‐ray Photoelectron Spectrometer
6.1 X‐ray Source for AP‐XPS Studies
6.2 Reaction Cell with Capability of Flowing Gas
6.3 Differential Pumping Energy Analyzer with High Transmission
6.4 Mass Spectrometer with Capability of Measurement of Catalytic Performance
References
7 Experimental Methods of AP‐XPS Studies
7.1 Leak Test of Reaction Cell
7.2 Exclusion of Catalysis by Reaction Cell
7.3 Tunning and Control of Sample‐Aperture Distance
7.4 Sample Heating and Temperature Control
7.5 Online Measurement of Reactants and Products
7.6 Spectroscopic Titration of Surface Species
References
8 Difference in Data Analysis Between AP‐XPS and High Vacuum XPS
8.1 Potential Difference in Measuring Atomic Ratio of Two Elements on Catalyst Surface
8.2 Difference in Intensity of Photoelectrons Collected by Energy Analyzer
8.3 Difference in Resolution and Baseline of Spectrum
8.4 Difference in Spectrum between Free Molecules in Gas and Adsorbed Molecules on Surface
8.5 Calibration of Nominal Atomic Ratio A/Z of a Catalyst Surface in a Pure Gas
8.6 Calibration of Nominal Atomic Ratio A/Z of a Catalyst Surface in a Mixture of Reactants
8.7 Calibration of Nominal Atomic Ratio A/Z of a Catalyst Surface in a Pure Gas Obtained at Different Temperature for Fair Comparison
References
9 Significance of Using AP‐XPS in Studies of Catalysis
9.1 Fundamental of Catalyst Surface
9.2 Significance of Characterization of Surface of a Catalyst in Gas Phase
9.3 Significance of Using AP‐XPS in Fundamental Studies of Catalysis
References
10 CO Oxidation on Single Crystal Model Catalysts
10.1 Pt(557) and Pt(332) in CO
10.2 CO Oxidation on Pd(100), Pd(111), and Pd(110)
10.3 CO Oxidation on Pt(110) and Pt(111)
10.4 CO Oxidation on Rh(110)
10.5 CO Oxidation on Cu(111)
References
11 CO Oxidation on High Surface Area Catalysts
11.1 CO Oxidation on Rh Nanoparticles
11.2 CO Oxidation on Ru Nanoparticles
References
12 Hydrogenation of Carbon Dioxide
References
13 Water–Gas Shift
13.1 Co
3
O
4
and Pt/Co
3
O
4
13.2 Pt, Au, Pd, and Cu Supported on CeO
2
Nanorods
13.3 CuO−Cr
2
O
3
−Fe
2
O
3
References
14 Complete Oxidation of Methane
14.1 Complete Oxidation of Methane on NiCo
2
O
4
14.2 Complete Oxidation of Methane on NiFe
2
O
4
14.3 Complete Oxidation of Methane on NiO with Different Surface Structures
References
15 Partial Oxidation of Methanol
15.1 Partial Oxidation of Methanol on Pd
1
Zn
3
/ZnO
15.2 Partial Oxidation of Methanol on Ir
1
Zn
3
/ZnO
References
16 Partial Oxidation of Methane
16.1 Partial Oxidation of Methane on Pd/CeO
2
16.2 Partial Oxidation of Methane on Pt/CeO
2
16.3 Partial Oxidation of Methane on Rh/CeO
2
References
17 Oxidative Coupling of Methane
17.1 OCM on Supported Na
2
WO
4
and Hypothesized Active Phase Na
2
O
2
17.2 First Observation of Na
2
O
2
through AP‐XPS Studies at 800 °C
17.3 Formation of a Thin Layer of Na
2
O
2
Supported on Na
2
WO
4
References
18 Dry and Steam Reforming of Methane
18.1 Dry Reforming of CH
4
on CeO
2
Anchored with Ni
1
and Ru
1
Sites
18.2 Steam Reforming of CH
4
on CeO
2
Anchored with Ni
1
and Ru
1
Single‐atom Sites
References
19 Reduction of NO with CO
19.1 Reduction of NO with CO on Co
3
O
4
19.2 Reduction of NO with CO on Rh
1
Co
3
Clusters Supported on CoO
References
20 Tuning Catalyst Surfaces for Developing Catalysts
20.1 Capability of Compositional Restructuring Checkable with AP‐XPS
20.2 Tracking Restructuring of Bimetallic Surface under Reaction and Catalytic Conditions for Tuning Catalytic Performance of a Bimetallic Catalyst
References
21 Photocatalysis
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 List of symbols of energy levels of electrons in atoms.
Table 3.2 Distribution of intensity and energy of X‐rays generated from Al ...
Table 3.3 Energy and line width of X‐rays generated from elements at their ...
Chapter 8
Table 8.1 Cross sections of inelastic scattering of photoelectrons with kin...
Chapter 20
Table 20.1 List of measured activation barrier of restructured bimetallic ca...
List of Illustrations
Chapter 1
Figure 1.1 Surface structural model and scanning tunneling microscopy (STM) ...
Figure 1.2 Structural model of a stepped model catalyst Pt(557) and evolutio...
Figure 1.3 Schematics of the structure of a catalyst nanoparticle. (a) Catal...
Chapter 2
Figure 2.1 Schematics showing the potential impact of gas or liquid environm...
Figure 2.2 Schematic showing study of nanoparticles in a static liquid with ...
Figure 2.3 Schematics and scanning electron microscopy (SEM) images of the S...
Figure 2.4 XPS studies of Ag nanoparticles dispersed in a mixture of triprop...
Figure 2.5 Reactor system designed for studies of the surface of catalyst na...
Figure 2.7 SEM studies of the reaction cell filled with a solution of Ag nan...
Figure 2.8 XPS studies of a solution of Ag nanoparticles flowing through a w...
Figure 2.9 XPS studies of air flowing through the reaction cell. (a) Air flo...
Chapter 3
Figure 3.1 Abundances of elements on Earth shown in molar ratio. The value o...
Figure 3.2 Periodic table listing abundance of elements on Earth shown in ma...
Figure 3.3a Periodic table listing binding energy of the most intense or mos...
Figure 3.3b List of atomic subshell photoionization cross sections of the mo...
Figure 3.3c List of atomic subshell photoionization cross sections of the mo...
Figure 3.3d List of atomic subshell photoionization cross sections of the mo...
Figure 3.3e List of atomic subshell photoionization cross sections of the mo...
Figure 3.3f List of atomic subshell photoionization cross sections of the mo...
Figure 3.3g List of atomic subshell photoionization cross sections of the mo...
Figure 3.3h List of atomic subshell photoionization cross sections of the mo...
Figure 3.3i List of atomic subshell photoionization cross sections of the mo...
Figure 3.4 Schematic showing the processes of the generation of X‐ray (a1–a4...
Figure 3.5 Periodic table listing energies of frequently used or analyzed X‐...
Figure 3.6 Examples showing correlation between EN of the element of atom
j
...
Figure 3.7 XPS spectra of the 2p, 3s and valence regions of transition metal...
Figure 3.8 XPS spectra of Co 2p of Co metal and CoO. The spin‐orbital split ...
Figure 3.9 Schematics showing (a) shake‐up effect process and a spectrum con...
Figure 3.10 XPS peaks of Cu 2p of CuO (a) and metal Cu (b).
Figure 3.11 XPS peaks of Ti 2p and Cr 2p of Ti and Cr metals.
Figure 3.12 Examples of shake‐up peaks. (a) C 1s of polystyrene and its shak...
Figure 3.13 The observed satellite peaks due to surface (S) and bulk (B) pla...
Figure 3.14 Schematic showing the formation of an Auger electron along with ...
Chapter 4
Figure 4.1 Published and prepared drawings of X‐ray monochromator of lab‐bas...
Figure 4.2 Geometry of focusing spectrometers: (a) Johann mode; (b) Johansso...
Figure 4.3 Represented effective area within which the incidence angle lies ...
Figure 4.4 Schematic of HAS consisting of two concentric hemispheres with ra...
Figure 4.5 Schematic of three types of devices including (a) dynode, (b) cha...
Chapter 5
Figure 5.1 Schematic presentation of ex situ studies (empty green and blue c...
Figure 5.2 Surface structure of Pt(557) catalyst in UHV (a), 5 × 10
−8
...
Figure 5.3 AP‐XPS peaks of Pt 4f and O 1s of Pt(557) catalyst at 25 °C in UH...
Figure 5.4 Structure and stability of Rh active sites. (a, b) Theoretical mo...
Figure 5.5 Reversible restructuring of Rh‐based active sites for steam refor...
Figure 5.6 Operando studies of Ru catalyst for Fischer–Tropsch synthesis usi...
Figure 5.7 Schematic showing the difference in using a technique of photon‐i...
Figure 5.8 Attenuation of photoelectrons of 1000 eV traveling in the gas pha...
Chapter 6
Figure 6.1 Universal curve of metal elements and carbon.
Figure 6.2 Schematic of gas region around a catalyst and X‐ray absorption by...
Figure 6.3 Schematics (a and b) showing two modes of AP‐XPS studies and (c a...
Figure 6.4 Schematics showing the configuration of sample and aperture in a ...
Figure 6.5 Sealing mechanism for different interfaces with the UHV environme...
Figure 6.6 Important alignment of X‐ray beam and trajectory of photoelectron...
Figure 6.7 Drawing to show how a reaction cell installed in the AP‐XPS appro...
Figure 6.8 Sealing performance of the in situ reaction cell. (a) Location of...
Figure 6.9 Sample heating characteristic of the reaction cell. (a) Plot of t...
Figure 6.10 Mass spectrometer data from two types of reaction cells with dif...
Figure 6.11 Schematic showing the working principle of focusing lenses in di...
Figure 6.12 Schematics showing the difference between (a) the analysis of pr...
Figure 6.13 Analysis of composition of gas in the reaction cell during catal...
Figure 6.14 Evolution of products during (a) CH
4
complete oxidation in on
Chapter 7
Figure 7.1 Test results of Ag 3d
5/2
peak intensity as a function of sample‐a...
Figure 7.2 Schematic and photo of distance between the sample and the front ...
Figure 7.3 Schematic showing the process of approaching the sample to the fr...
Figure 7.4 (a) Diagram showing the setup for online gas products analysis. A...
Figure 7.5 The coverage of surface‐oxygen species [Θ(O)] while dosing CO, as...
Chapter 8
Figure 8.1 Schematic showing the intensity of photoelectrons entering the en...
Figure 8.2 Ag 3d
5/2
spectra of Ag foil under UHV environment (a) and 20 Torr...
Figure 8.3 O 1s spectra of 2 and 7 nm Rh nanoparticles in 500 mTorr O
2
(a an...
Figure 8.4 Energy diagram of the catalyst surface, gas molecules, and the en...
Figure 8.5 Demonstration of the calibration of the atomic ratio of the eleme...
Chapter 10
Figure 10.1 Photoemission spectra of the Pt 4f (a) and O 1s core levels (b) ...
Figure 10.2 Coverage of CO on Pt(557) in CO at different pressures as marked...
Figure 10.3 AP‐XPS studies of CO oxidation on Pd(100) facet. (a) Mass spectr...
Figure 10.4 AP‐XPS studies of Pd(111) surface in 0.2 Torr O
2
in the temperat...
Figure 10.5 AP‐XPS studies of Pd(111) surface CO oxidation reaction on Pd(11...
Figure 10.6 AP‐XPS studies of Pd(111) surface at 300 °C in the mixture of CO...
Figure 10.7 CO oxidation reaction at 400 °C. (a) Partial pressures of O
2
, CO...
Figure 10.8 AP‐XPS and mass spectrometry studies of CO oxidation Pd(110). (a...
Figure 10.9 Surface structure and chemistry of Pt(110) in UHV and O
2
at 0.05...
Figure 10.10 AP‐XPS studies of O 1s spectrum of Pt(110) surface exposed to 0...
Figure 10.11 Pt 4f XPS spectra of a 2.8 ML α‐PtO
2
film (red line) and of an ...
Figure 10.12 AP‐XPS titration of the evolution of the mixed PtO/α‐PtO
2
forme...
Figure 10.13 The coverage of surface‐oxygen species [Θ(O)] while dosing CO, ...
Figure 10.14 In situ observation of CO adsorption on Rh(110)‐(1 × 2)‐O at 25...
Figure 10.15 Time lapse in situ HP‐STM images of the same area at P
CO
= 0.08...
Figure 10.17 In situ AP‐XPS studies of CO oxidation on Rh(110)‐(1 × 2)‐O as ...
Figure 10.18 AP‐XPS studies of (a) O 1s and (b) C 1s spectra of a Cu(111) sa...
Figure 10.19 Coverage (±20%) of adsorbed species with respect to a flat Cu(1...
Chapter 11
Figure 11.1 Size‐dependent catalytic activity in CO oxidation. (a) Turnover ...
Figure 11.2 AP‐XPS spectra of Rh 3d taken under 0.5 Torr O
2
in the temperatu...
Figure 11.3 AP‐XPS spectra of the Rh 3d
5/2
peaks of 2 and 7 nm Rh nanopartic...
Figure 11.4 O 1s spectra of 2 and 7 nm Rh NPs in 500 mTorr O
2
(a and b) and ...
Figure 11.5 Ru 3d spectra showing reversibility of oxidation (under 0.2 Torr...
Figure 11.6 AP‐XPS spectra of O 1s of 6 nm Ru nanoparticles during CO oxidat...
Figure 11.7 AP‐XPS spectra of Ru3p of Ru nanoparticles under CO oxidation co...
Figure 11.8 AP‐XPS of surface of Ru nanoparticles and their catalytic activi...
Chapter 12
Figure 12.1 Co 2p and Ru 3p XPS peaks of Co
3
O
4
and (Co
0.95
Ru
0.05
)
3
O
4
under r...
Figure 12.2 Catalytic performances of 5% Ru/SiO
2
, Co
3
O
4
, and (Co
0.95
Ru
0.05
)
3
Figure 12.3 Significant promotion of Ru to hydrogenation of CO
2
on Co.
Chapter 13
Figure 13.1 AP‐XPS studies of pure Co
3
O
4
during pretreatment in H
2
(a1–a3 an...
Figure 13.2 Kinetic studies of WGS on (a) pure Co
3
O
4
nanorod catalyst in the...
Figure 13.3 Photoemission features of Co 2p and Pt 4d
5/2
of as‐synthesized P...
Figure 13.4 Schematics of the three identified catalysts CoO
1–
x
, Pt
1
Co
Figure 13.5 High‐resolution transmission electron microscope (TEM) images of...
Figure 13.6 AP‐XPS studies of (a) Ce 3d and (b) Au 4f of Au/CeO
2
nanorods un...
Figure 13.7 AP‐XPS studies of (a) Ce 3d and (b) Pt 4f of Pt/CeO
2
under react...
Figure 13.8 Evolution of atomic fractions of Ce
3+
of Au NPs/CeO
2
(a) and Pt ...
Figure 13.9 AP‐XPS studies of CuO–Cr
2
O
3
–Fe
2
O
3
surface and mass spectrometry ...
Figure 13.10 Time‐resolved AP‐XPS studies of Cu 2p region of CuO–Cr
2
O
3
–Fe
2
O
3
Figure 13.11 Schematics of the copper‐chromium‐iron oxide catalyst before an...
Chapter 14
Figure 14.1 Catalytic performance of catalysts for complete oxidation of met...
Figure 14.2 Photoemission features of NiCo
2
O
4
at different temperatures duri...
Figure 14.3 Evolution of surface concentration of carbon‐containing species ...
Figure 14.4 DFT calculations of subpathways for transforming CHO to product ...
Figure 14.5 Catalytic activity of NiFe
2
O
4
in terms of conversion of CH
4
as a...
Figure 14.6 XPS peaks of (a) Ni 2p, (b) Fe 2p, (c) O 1s, and (d) C 1s of NiF...
Figure 14.7 Nominal surface atomic ratio, (a), (b), and (c) at differe...
Figure 14.8 Quantitative analysis of surface atomic fraction of Fe or Ni amo...
Figure 14.9 High‐angle annular dark field scanning transmission electron mic...
Figure 14.10 Optimized surface structures of NiO(100), NiO(110), and NiO(111...
Figure 14.11 Catalysis on partially reduced cubic NiO and their catalytic ac...
Figure 14.12 Operando studies of (110) surface of planar sheet NiO and (100)...
Chapter 15
Figure 15.1 Catalytic performances of (a) pure ZnO, (b) 0.01 wt% Pd/ZnO−O
2
−H
Figure 15.2 Schematics showing (a) nanoscale alloy particle (
M
α
–M
Figure 15.3 Operando studies of the surface of 0.05 wt% Pd/ZnO during pretre...
Figure 15.4 HAADF‐STEM images of the used 0.01 wt% Ir/ZnO catalyst. (a) and ...
Figure 15.5 Catalytic performances of (a) 20 mg of ZnO, (b) 20 mg of 0.01 wt...
Figure 15.6 AP‐XPS studies of Ir 4f of used 0.05 wt% Ir/ZnO and during obser...
Chapter 16
Figure 16.1 Catalytic performance of Pd–CeO
2
–H
2
and Pd–CeO
2
–air for partial ...
Figure 16.2 XPS spectra of the in situ studies of Pd 3d of Pd–CeO
2
–air (a), ...
Figure 16.3 Catalytic performances of Pt–CeO
2
–H
2
and Pt–CeO
2
–air. Conversion...
Figure 16.4 AP‐XPS studies of Pt 4f of Pt–CeO
2
–air (a), Pt 4f of Pt–CeO
2
–H
2
...
Figure 16.5 Catalytic performance of Rh–CeO
2
–H
2
and Rh–CeO
2
–air. Conversion ...
Figure 16.6 XPS spectra of the in situ studies of Rh 3d of Rh–CeO
2
–air (a), ...
Chapter 17
Figure 17.1 Catalytic results for the CH
4
/O
2
/H
2
O reaction using the Na
2
WO
4
...
Figure 17.2 AP‐XPS studies of Na 1s, O 1s, and W 4f for the catalyst collect...
Figure 17.3 Area ratios of (a) Na 1s/O 1s and (b) Na 1s/ W 4f
7/2
of the cata...
Chapter 18
Figure 18.1 The schematic of a catalyst surface consists of two sets of sing...
Figure 18.2 Aberration‐corrected STEM images of Ce
0.95
Ni
0.025
Ru
0.025
O
2
catal...
Figure 18.3 Catalytic performances of Ce
0.95
Ni
0.05
O
2
, Ce
0.95
Ru
0.05
O
2
, and Ce
Figure 18.4 Arrhenius plots and measurement of turnover frequencies of the t...
Figure 18.5 Ce 3d, O 1s, Ru 3p, and Ni 2p photoemission features from AP‐XPS...
Figure 18.6 Structure of an intermediate‐temperature fuel cell. (a) Schemati...
Figure 18.7 Catalytic performances of steam reforming of CH
4
on Ce
0.90
Ni
0.05
Figure 18.8 AP‐XPS analysis of singly dispersed Ni atoms (Ni
1
) and Ru atoms ...
Chapter 19
Figure 19.1 TEM images of as‐synthesized Co
3
O
4
nanorods. (a) Large‐scale TEM...
Figure 19.2 Catalytic activity (the left Y‐axis, black line in figure) and s...
Figure 19.3 AP‐XPS studies of Co 2p (a) and Co 2p
3/2
(b) of Co
3
O
4
in the mix...
Figure 19.4 Schematic of Co
3
O
4
nanorods and the CoO formed through restructu...
Figure 19.5 Atomic ratio of oxygen to cobalt of Co
3
O
4
and CoO
1−
x
(form...
Figure 19.6 Studies of catalytic performances under two different catalytic ...
Figure 19.7 Schematic of synthesis of a catalyst of singly dispersed bimetal...
Figure 19.8 AP‐XPS studies of surface chemistry of a catalyst prosecutor und...
Figure 19.9 Characterizations in the preparation of isolated bimetallic site...
Chapter 20
Figure 20.1 Evolutions of surface compositions of Rh–Pd, Rh–Pt, and Pd–Pt bi...
Figure 20.2 Structural models of (a) the as‐synthesized Pt−Cu regular nanocu...
Figure 20.3 Procedures to form a new catalyst surface through a single or se...
Figure 20.4 Evolution of surface composition (Pt and Cu atomic fractions) of...
Chapter 21
Figure 21.1 STEM images of (Rh
2–
y
Cr
y
O
3
)/(Ga
1–
x
Zn
x
)(N
1–
x
O
x
)...
Figure 21.2 HS‐LEIS depth profile for photocatalysts using (a) He
+
ion gas a...
Figure 21.3 AP‐XPS studies of surface of (Rh
2–
y
Cr
y
O
3
)/(Ga
1–
x
Zn
x
)...
Guide
Cover Page
Title Page
Copyright Page
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Application of Ambient Pressure X‐ray Photoelectron Spectroscopy to Catalysis
Franklin Tao
Department of Chemical and Petroleum Engineering
University of Kansas
USA
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Library of Congress Cataloging‐in‐Publication Data9781119845447
Cover Design: WileyCover Image: Courtesy of Mark Biesinger; Courtesy of Franklin Tao
Preface
The technique of X‐ray photoelectron spectroscopy (XPS) originates from the major contributions of two Nobel Prize laureates: the theoretical explanation of the photoelectric effect by Albert Einstein and the experimental and instrumental development work of Kai Siegbahn. The pioneering work of these two physicists led to the development of XPS analysis of various types of materials. Work originating with Kai Siegbahn and his coworkers and benefiting from the instrumentation improvement and developments of scientists at the Lawrence Berkeley National Laboratory, the Fritz Haber Institute, and other research groups, has resulted in the availability of ambient pressure X‐ray photoelectron spectroscopy (AP‐XPS). This instrumental approach provides a significant surface analysis technique which can be used in numerous topics of real world including studies of heterogeneous catalysis, advanced materials and devices, energy science, and environmental science.
The application of AP‐XPS has been driven to a large extent by the interest in exploring the environment‐induced or environment‐maintained dynamic surface that differs from the static traditional surface often explored in high vacuum surface science. Studies of an environment‐induced or ‐maintained surface require a technique that has the capability of characterizing these surfaces in the ambient environment. Here an environment is broadly defined as a gaseous environment at a certain pressure or even a liquid environment where a sample such as a real catalyst might be located.
Catalysis is the basis for chemical production in various important applications. Although fundamental studies of catalysis do not directly produce commercial value, they provide open, scientific knowledge helpful for discovering and developing catalysts with potential to be used in the production of value‐added chemicals. A significant aspect of fundamental studies of heterogeneous catalysis is the accurate characterization of the catalyst, particularly the authentic surface of the heterogeneous catalyst in its working environment. In general, information about the surface includes both surface chemistry and surface structure. Here, surface chemistry refers to the constituting elements, the quantitative composition, the electronic state and chemical environment of the atoms of the catalyst surface, the bonding between catalyst atoms and adsorbates, and the interaction between catalyst surface atoms and gaseous molecules of the environment. AP‐XPS is an appropriate, powerful technique for uncovering the chemistry of an environment‐induced or ‐maintained surface of a catalyst under the reaction conditions of catalysis. AP‐XPS has been widely used in the field of heterogeneous catalysis for over a decade.
This is the first research monograph focused primarily on the method and application of AP‐XPS. Before the unique features of AP‐XPS are introduced, the principle and instrumentation of XPS and the interpretation of XPS data are described briefly in Chapters 1–4. The features of AP‐XPS are presented in Chapters 5–9. These chapters provide a basis for understanding how AP‐XPS observes and tracks the surface chemistry of a catalyst under real catalytic conditions. The second half of this book (Chapters 10–21) presents the application of AP‐XPS to fundamental studies of over ten different catalytic reactions. In these chapters, the focus is on how AP‐XPS can provide key information for understanding catalytic mechanisms. The takeaways in each chapter are in italics. There have been a great number of examples of AP‐XPS studies of catalysis published in the literature, but it is not possible to discuss all of them in this book. I do appreciate them. Numerous excellent works from the communities of catalysis and AP‐XPS have not been included. Many excellent studies could not be discussed in detail or even cited as examples here. The particular works discussed as examples provide an overview of the sorts of information and mechanistic understanding that have been obtained using AP‐XPS in the study of catalysis. The literature is filled with many more examples that could have been included.
Franklin Tao
University of Kansas
2022
2Application of XPS: from Surface in UHV to Surface in Gas or Liquid Phase
2.1 Origin of X‐ray Photoelectron Spectroscopy
X‐ray photoelectron spectroscopy (XPS) is one of most widely used spectroscopies in catalysis, surface science, materials science, and energy science. Together with Auger electron spectroscopy (AES), it is traditionally called electron spectroscopy for chemical analysis (ESCA) since it identifies constituting elements, measures the composition of a solid surface, a liquid phase, or even a gas phase, and analyzes chemical environment of a type of atoms of a sample.
This spectroscopy is closely related to or even is originated at the very early discovery of the photoionization effect through irradiating surface of a sample by Heinrich Hertz in 1887.1 At the beginning of twentieth century, Philippe Lenard defined the light‐induced generation of electrons from a solid surface.2,3 Lenard found the kinetic energy of electrons from a given emitter in terms of a sample does not exceed a maximum value related to the sample. In the nearly same period, Albert Einstein explained the generation of electrons from a metal surface through photoionization with a language of quantum theory.4 Ten years later, Rutherford, Robinson, and Rawlinson were aware of the fact that the kinetic energy of generated electrons from a metal surface is the difference between photon energy and binding energy of electrons in their atoms.5–7 Later Robinson and Young found that a different chemical bonding of atoms can shift kinetic energy of electrons generated from a solid surface, driving potential applications of the photoelectric phenomenon to understanding of chemical environment of a specific type of atoms on surface or in subsurface region of a solid.5,8 The documentation of the development of X‐ray photoelectron spectroscopy before 1960 can be found in literature.13 Since 1950s,9 pioneering efforts of Siegbahn et al. gave very early high‐resolution XPS spectra.10–13 Kai Siegbahn developed methods and instruments for achieving highly accurate measurements of kinetic energy of photoelectrons generated from atoms by irradiating them with X‐ray.10 Siegbahn published the first books of electron spectroscopy in 1960s.11,12 He was honored with Nobel Prize in physics in 1981 for his seminal contribution to the development of high‐resolution electron spectroscopy and spectrometer.10
2.2 Applications of XPS to Study Surface in High Vacuum
Numerous excellent books of surface science or surface characterization have described the applications of XPS in high vacuum.14–19 In brief, it can qualitatively identify constituting elements of a sample, interpret chemical environment of atoms, and quantitatively analyze its composition. It has been used in every corner of science, technology and engineering sectors of our society. Here it is not necessary to repeat its wide applications and significance.
When the analyte is deep layers away from the sample surface, a buried region, or even bulk of a sample, a sputtering‐assisted depth profile analysis is necessary. Alternatively, hard X‐ray XPS can provide important information of such an analyte. Hard X‐ray generates photoelectrons with a higher energy that have longer inelastic mean free path, λ, in contrast to soft X‐ray or tender X‐ray. Thus, the information volume of hard X‐ray XPS is much larger; in other words, much deeper region can also contribute to the intensity of an XPS peak if a hard X‐ray XPS is used. The hard X‐ray XPS has been used in studies of solid state science and functional devices such as development of battery materials, semiconducting devices, and superconducting materials.20,21
2.3 Applications of XPS to Study Sample in Gas Phase
Over three quarters of heterogeneous catalysis reactions are performed at solid–gas interface. For those reactions, a catalyst is retained in gas phase of reactants. As discussed in Chapter 1, surface of a catalyst likely restructures in gaseous environment of reactant(s) driven by both pressure and temperature factors. More importantly, in many cases these structures are dynamic, and they are maintained by the gaseous environment and temperature. When the gaseous environment or/and the temperature factor is withdrawn, the dynamics structure likely changes largely. To establish an intrinsic correlation between a catalyst surface and its corresponding catalytic performance, it is necessary to track the authentic surface formed at catalysis temperatures in gas phase and maintained under a catalytic condition. Ambient pressure X‐ray photoelectron spectroscopy (AP‐XPS) is the appropriate analytic technique for surface of a catalyst at catalysis temperature in gas phase. Chapters 3 and 4 present data analysis and instrumentation of XPS in general. Significance and challenges of studying surface of a catalyst in gaseous phase are discussed in Chapter 5. Instrumentation of ambient pressure X‐ray photoelectron spectrometer is introduced in Chapter 6. Chapter 7 describes experimental methods of AP‐XPS studies. Data analysis of AP‐XPS was introduced in Chapter 8. Chapter 9 briefs the significance of using AP‐XPS in studies of catalysis. Chapters 10–21 are AP‐XPS studies of different types of catalytic reactions.
2.4 Applications of XPS to Study Sample in Liquid Phase
A high vacuum XPS can be used in fundamental studies of surface of nanoparticles in liquid phase. This is driven by the fact that nanoparticles function in liquid environment such as catalysis at a liquid‐solid interface, electrocatalysis on anode or cathode in liquid, and molecular catalysis in liquid. Figure 2.1d schematically illustrates that the high density of molecules in liquid surrounding a nanoparticle could restructure surface of the nanoparticle.22 From fundamental science point of view, it is necessary to explore surface of these nanoparticles functioning in liquid. An easy approach to study these nanoparticles is to filter these nanoparticles after catalysis, dry them, and then place them in a high vacuum environment of an XPS system for analysis. However, the separation of functional nanoparticles from its working environment could make us incapable of knowing its authentic surface chemistry under working condition. To uncover surface of a functional material such as a nanoparticle catalyst in liquid environment, XPS studies of these nanoparticles in static liquid for flowing liquids were demonstrated recently.22–24
Figure 2.1 Schematics showing the potential impact of gas or liquid environment on surface of catalyst nanoparticles. (a) Surface of a catalyst nanoparticle with adsorbed molecules of reactants A and B in ultrahigh vacuum (UHV) environment. (b) Surface of a catalyst nanoparticle in a gas of reactants at a low pressure, p1. (c) Surface of a catalyst nanoparticle in a gas of reactants at a relatively high pressure, p2. (d) Surface of a catalyst nanoparticle surrounded by solvent and solute molecules of liquid.
Source: Reproduced with permission from Nguyen et al.22/American Chemical Society.
2.4.1 XPS Studies of Surface of Nanoparticle Catalyst in Static Liquid
Figures 2.2 and 2.3 present how XPS was used to study surface of nanoparticles dispersed in liquid phase.22,23 For this purpose, a specific cell is needed for retaining the sample consisting of nanoparticles and liquid. Simply speaking, the sample is a liquid containing well‐dispersed nanoparticles as schematically shown in Figure 2.2b. Generally, there is no limit for size of these nanoparticles to be studied with this method.22 The liquid can be a mixture of any solvent and solute. The solvent can be water or organics. In the following example, the solute and solvent are phenol and tripropylene glycol monomethyl ether, respectively. The mixture of phenol and tripropylene glycol monomethyl ether is the organic solution wherein Ag nanoparticles were dispersed. Figure 2.3a shows the cell structure. A two‐layer graphene membrane is the wall to isolate the liquid containing well‐dispersed nanoparticles from the high vacuum environment of XPS chamber (Figure 2.2b). Notably, the two‐layer graphene membrane is transparent for X‐ray photons and photoelectrons because they are single‐atom thick graphene layers and there are sub‐nanometer benzene‐like holes in each single‐atom layer. X‐ray can irradiate nanoparticles in liquid. More importantly, this cell allows a part of the photoelectrons generated from the nanoparticles to transmit through the liquid between catalyst surface and then the holes in the graphene layers to enter the high vacuum environment of XPS immediately before they are analyzed by an energy analyzer. In principle, a high vacuum XPS can perform an analysis for surface of nanoparticles dispersed in a liquid if a lab‐based X‐ray beam can be strong enough to generate enough photoelectrons for being collected by an energy analyzer and then detected by an detector although a synchrotron beam having high X‐ray flux density is preferred. As seen from the scanning electron microscopy (SEM) image in Figure 2.4c, the liquid containing well‐dispersed Ag nanoparticles was placed in the cavity of the cell. The cavity of the cell is for holding liquid and is schematically shown in Figure 2.3b. As the graphene window well separated the liquid environment of the catalyst from the high vacuum environment of XPS, both the liquid of the cell and the high vacuum environment of XPS survive during XPS studies of nanoparticles in the liquid in the cell. Figure 2.4g is the XPS spectrum of Ag 3d photoelectrons generated from Ag nanoparticles dispersed in liquid.
Figure 2.2 Schematic showing study of nanoparticles in a static liquid with XPS. (a) Schematic showing the integration of a liquid cell filled with a nanoparticle‐containing liquid. (b) Schematic showing the interactions between X‐ray photons and solvent and solute molecules and nanoparticles in the liquid, as well as the interactions between the generated photoelectrons and the solution between the nanoparticle surface and the two‐layer graphene.
Source: Reproduced with permission from Nguyen et al.22/American Chemical Society.
Figure 2.3 Schematics and scanning electron microscopy (SEM) images of the Si3N4 window for a liquid cell. (a) Schematic of a miniature silicon wafer with an ultrathin Si3N4 membrane at its center. (b) Cross‐sectional schematic of the Si3N4 membrane located at the center of the wafer. (c) SEM image of pores bored in the Si3N4 membrane by SEM ion milling. Cr/Au layers were deposited on the Si3N4 window near to the pore to enhance the adhesion of the graphene layers. (d) SEM image of a graphene window covering the pore of the Si3N4 window. (e) SEM image of the same graphene window as (e) but taken with the sample tilted 50° from normal.
Source: Reproduced with permission from Nguyen et al.22/American Chemical Society.
Figure 2.4 XPS studies of Ag nanoparticles dispersed in a mixture of tripropylene glycol monomethyl ether and phenol sealed in a cell of static liquid. (a) Schematic of the micro‐opening region of the cell covered by the graphene membrane, which separates the liquid from the UHV environment of XPS. (b, c) SEM images of a graphene liquid cell (b) without liquid and (c) filled with liquid containing tripropylene glycol methyl ether (TPM) (solvent), phenol (reactant), and Ag nanoparticles; the concentration of Ag nanoparticles in solution is 10 wt% or 1.08 vol%; the bright spots are Ag nanoparticles dispersed in the liquid. (d, e) EDX spectra acquired at spot A (off graphene window region) of (c) and spot B (graphene window region) of the liquid cell in (c). (f) Chemical mapping of the distribution of Ag 3d photoelectrons generated from a 20 μm × 20 μm region of the cell in (c); the bright region of the mapping is a Ag nanoparticle dispersed in the liquid sealed in the cell. (g) XPS spectra of Ag 3d of spot A (black) and spot B (green); spot B are Ag nanoparticles dispersed in the liquid sealed in the cell.
Source: Reproduced with permission from Nguyen et al.22/American Chemical Society.
Only these nanoparticles in liquid near the graphene window could contribute photoelectrons for analysis by the energy analyzer due to the limited mean free path of these generated photoelectrons in liquid. Notably, the nanoparticles studied in reference 22–23 were dispersed in liquid instead of being deposited on the graphene window. In general, nanoparticles in liquid with a distance of <10 nm from the graphene window can contribute to the intensity of XPS peak. This information depth is largely determined by (1) the original kinetic energy of photoelectrons of an interested element and the density of the interested elements in the liquid and (2) the molecular density and constituting elements of solute and solvent of the liquid. To increase the information depth, a hard X‐ray can be used to generate photoelectrons with higher kinetic energies. Then, surface of nanoparticles farther from the graphene window can also contribute to the XPS peak. Thus, such a liquid cell‐based XPS study of a nanoparticle catalyst in liquid using hard X‐ray can provide more information of nanoparticles in a deeper region of the liquid although photoionization cross section of a specific subshell such as Ag 3d decreases along increase of X‐ray energy. It is expected such a well‐designed reaction cell can be used in studies of homogeneous catalysts in terms of molecular catalysts in solution or even under a catalytic condition.
2.4.2 XPS Studies of Surface of Nanoparticle Catalyst in Flowing Liquid
The dispersion of nanoparticles in a static liquid at the region near the graphene window may not reflect the dispersion of nanoparticles in a flowing liquid due to gravitation or surface tension of liquid. The catalyst nanoparticles or even reactant molecules may not be distributed in the solution homogeneously. In addition, the accumulation of product molecules on or near the catalyst nanoparticle surface could prevent reactant molecules or intermediates from accessing catalytic sites on the nanoparticle surface. Driven by these concerns, a flowing reaction cell for XPS studies of a catalyst in liquid was designed and successfully fabricated by Tao group as shown in Figures 2.5–2.8. With this flowing cell, the nanoparticles in a flowing liquid can be characterized with a high vacuum XPS as well. In other words, the nanoparticles in liquid are flowing through the cell when they are being characterized by a high vacuum XPS.23
Figure 2.5 Reactor system designed for studies of the surface of catalyst nanoparticles in a flowing liquid. (a) Schematic of the whole system. (b) and (c) External and internal views of the Si3N4 cell; the brown ring shows epoxy instead of an O‐ring (epoxy was used to seal the two parts externally). (d) Photograph of the reaction cell. (e) Enlargement of the graphene membrane on a Si3N4 membrane.
Source: Reproduced with permission from Nguyen et al.23/Royal Society of Chemistry.
Figure 2.6
