Luận văn thạc sĩ nghiên cứu quá trình oxy hóa hiếu khí của rượu trên cấu trúc nano kim loại Được hỗ trợ bằng oxit vai trò của hạt nano so với các nguyên tử Đơn lẻ cô lập

The catalytic performance of metal nanoparticles in the oxidation of alcohol reactions can be influcnced by many factors, including the size and shape of melal clusters and nanoparticles

DAI HOC BACH KHOA HA NOI

LUAN VAN THAC Si

Nghiên cứu quá trình oxy hoá hiếu khí của rượu

trên cầu trúc nano kim loại được hỗ trợ bằng oxit

— Vai trò của hạt nano so với các nguyễn tử đơn lẻ

CONG HOA XA HOT CHU NGHIA VIRT NAM

Độc lập — Ty do— Hanh phic

BAN XAC NHAN CHINH SUA LUAN VAN THAC SI

Hg va tén tac gid lua ăn: Mạc Văn Hưng

Tử tài luận văn: Nghiên cứu quá trinh oxy hoá hiểu khí của rượu trên câu

trúc nano kim loại được hề trợ bằng oxit — Vai trò của hạt nano so với cáo nguyễn

tử đơn lẻ cô lập

Chuyên ngành: Hoá học

Mã số học viên: 20211200ML

Tác giả, Người hướng dẫn khoa học và Hội đẳng cham hain văn xác nhận

ta chữa, bỗ sung luận văn theo biên bản hop Hội đồng ngày 26/10/2023

với oác nội dung sau:

~_ Chính sửa lỗi chỉnh tả, trình bảy,

~ Đã bổ sung một sỏ chỉ tiết thực nghiệm

~_ Đã bổ sung, chỉnh sửa một số giải thích

Ngày tháng năm

CHỦ TỊCH HỘI ĐỒNG

HANOI UNIVERSITY OF SCLENCE AND TECHOLOGY

MASTER THESIS

Aerobic oxidation of alcohols on the oxide-

supported metal nanostructures - role of

nanoparticle versus to the single atoms

MAC VAN HUNG

Hung MV211200M@sis.hust.edu.vn

Master of Scicence in Chemistry

Supervisor: Assoc Prof Dr Vu Anh Tuan

Signature Institute: Chemical Engineering Institute

Tanoi, 10/2023

ACKNOWLEDGMENTS

First of all, I would like to like to give my gratitude to my co-supervisors

Dr Ali Mohamed Abdel-Mageed (Surface Chemistry in Applied Catalysis, LIKAT) and Assoc Prof Dr Vu Anh Tuan (Department of Analytical Chemis HUST), for their help and support throughout imy thesis work ‘there are no words that can express my appreciation for everything that they have done for me

Tam also extremely grateful lo my colleagues in the group, Dr Jawaher Mosrati, Dr Katja Neubauer, M.Sc Evaristo Salaya, M.Sc Sebastian Lobner, Dr Mohammed Al-yusufi, Mrs Julia Schroeder They have tanght me many things in

a year

I would like to thank to Mrs Anja Simmula for ICP measurements, Mr Reinhard Rekell for nibegen adsorplion, and Dr Hanan Ata for Hz-TPR micasuroments Thank should also go to Service Depariment Analyties for XRD, and XPS analysis

1 would like to thank the RoHan project for giving me the excellent opportunity to study in Germany with full financial support

Lastly, it is impossible not to mention my family and friends, especially my parents, my brother, who gave me encouragement and emotional support along the research path

ABTRACT

‘the oxidation of alcohols to the corresponding carbonyl compounds, such

as aldehydes, ketones, and carboxylic acids, has received the most attention These

compounds are versatile and valuable intermediates in manufacturing pharmaccuticals, perfumes, and flavorings [1] The catalytic performance of metal nanoparticles in the oxidation of alcohol reactions can be influcnced by many factors, including the size and shape of melal clusters and nanoparticles, promoters, and interactions between supports and active sites Ru has attracted considerable interest in catalysis not only because of its relatively low cost compared to commonly studied and industrially applied noble metals such as Au,

Tt, Pd, but also due to its tunability by different support effects, especially in

combination with T1O: Here, we aim to at identification of the role of Ru

janoparticle versus to the single atoms for catalytic activity and product selectrvity

in the aerobic oxidation of benzyl alechol

Ru/TiOy catalysis were prepared using a wetness impregnation method The

Ru particle size was controled by varing the Ru loading from 0.2 to 3.0 wi% The materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (MPS), high-angle annular dark field scanning transmission electron

amieroscopy (HAADF-STHM) and energy dispersive spectroscopy (EDS), Hạ-

temperature-programmed-reduction (IPR) The state oxidation of Ru on ‘TiO: support is metallic Ru and Ru* for the fresh catalyst and Ru‘* for the spent catalysl The HAAD-STEM resulls show the highly disparsed Ru on the supporl,

1

and [his is preserved aller 20h oxidalion reaction al 150 °C Furlhermore, the meal-

support interaction between Ru and TiO, support becomes stronger with a decrease

in particle sive We demonstrated thal the conversion af beruzyl alcohol and product selectivity significantly depended on the particle size Single atoms Ru on TiOz show the highest 83.62% selectivity of benzaldehyde, and it decreased when the

Ru size increased

tạ

COTENTS

COTENTS

LIST OF FIGURES

LIST OF TABLES LIST OF ABBREVIATIONS CHAPTER 1 INTRODUCTIO® 1.1 Background - - 9

1.2 The objectives of the thesis ca TÔ CHAPTER 2 THEORTCAT BASTS

2.1 Titantim đioxide wll 2.1.1 Proper and siructure of titanium dioxide - 11

2.1.2, TiOa as catalyst guppOIf, co " 2 2.1.3 Synthesis of TiO, - 14 2.2 Ruflienruim cà ciinereteerrirerirerrirree ¬ )

3.2.1 Physicochemical properties and application of ruthenium 15

2.2.2 Ru-based catalysts for oxidation of alcohol LÕ 2.3 Isolated single-alom-site calalysLs 18 2.3.1 Concept and properties of isolated single-atom-site catalysts 18 2.3.2, Preparation of isolated single-alom-sile calalysls 19

2.4, Analytical methods .cccscssctsoisnnvtnsenessenseee ¬

3.4.1 X-ray điữaction (XRD) - - 21

2.3.2 Niưogen adsorption at 77 K

23.2 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) 24 3.3.3 X-ray photoelectron spegtrOsGOpy suasa.24 2.3.4, High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy (EDS) 25 2.5 Aerobic oxidation of aleohol CHAPTER 3 EXPERIMENT SECTION

3.1 Preparation of catalyst

3.2 Catalyst characterization - 30

43 Structure and morphology of catalyst aller reaction 46

CILAPTER § GENERAL CONCLUSIONS AND OUTLOOK

REFERENCES

LIST OF FIGURES

Figure 2 1 Crystal bulk structure of (a) anatase, (b) rutile, and (c) brookite

11

Figure 2 2 ‘the SMSI effect occwzing during the reduction treatment by

‘Hh The gray circles represent metallic nanoparticles, while the blue parts represent

Vigure 2 3 Biomimetic aerobie oxidation of aloohols 7

Figure 2 4 Schematic illustrate the changes of surface itve energy and specific activily per mela! alom with metal particle size and the support el

13 OM

Figure 2 5, Methods for preparmg ISAS catalyst: mass-selected soft-

landing (A) and wet chemistry (B) ào cac — ˆ

Figure 2 6 Two-dimensional representation for the derivation of the Bragg

Figure 2 7 Clasaificntin o[physisorption isotherms 23 Figure 2 8 Schematic of image formation in a STEM, showing the on-axis small brighi-field detector and the larger annular dark-field detector, pink 26

Figure 2 9, Dehydrogenation mechanism of alcchol oxidation over Pt

Figure 2 10 Evolution of geometric and electronic structures of single

atom, metal clusters, and nanoparticles -csss2tsseerseersersee.e.- 28

Figure 2 11, Reaction scheme for the benzyl alechol oxidalion 29 Figue 3 1 Schematic illustration of the preparation of RwTiO2z

igure 4 2 (@) No adsorption-desorption isotherms and (b) pore size

distribution curves 3 of different loading, Ru/li0: catalysts —-

Tipure 4 3 HAAD-STEM/RDX elememal mapping of (ai-a4) Ru/TiOs-l,

Cy -ba) RuTiOa-2, (cr-ca) Rev TiO23, and (died) Ru/TIO2-4 36

Figure 4 4 HAAD-STEM images of (a-b) RwTi0:-1, (c-d) RWTi02-2, (c- f) Ru/liOe-3, and (g-h) Ru/TiO2-4, and (i) the Ru particle size distribution of

Figure 4 5 XPS spectra of Ru 3d/C Is region of the Ru catalysts 38 Figure 4 6 XPS spectra of O Is and Ti 2p of the RWTiO2 catalysts 39

Figure 4 7 H2-TPR profiles of the four Ru/TiO2 catalysis 40 Figure 4 8 Conversion and selectivity of benzaldehyde over RWTiO2-4

Figure 4, 10, Selectivity of benzaldehyde and tumn-over frequency (TOK) of

RTA and leaching Ru/TiOz-4 catalysts - - 43

Figure 4 11, XRD spectrum of Ru/TiOz-4 anđ Ru/TiO-4 leaching catalyst

aM Figure 4 12 Effect of reaction temporature on the conversion and seleptivily [or aerobic oxidation of benzyl alechol 44

Figure 4 13 Effect of solvent on conversion and selectivity for aerobic oxidation of benzy] alcolioL -: sàn 211102 1e AS

Figure 4 14 XRD spectrum of four spent Ru/TiQ) catalysis 46

LIST OF TABLES Table 1 Properties of anatase, rutile, and brookite 12

‘fable 2 Ru loading and textural properties of the prepared catalysts 36

Table 3 The TiO, crystallite and H, uptake and FHWM value of four

Tablc 4 Conveision and selectivity of fowr Ru/TIO¿ catalysts 42

Table $ Conversion and selectivity of RWTiO2-4 catalyst with different

X-ay diffraction Brunauer Emmett ‘Teller

Induetively Coupled Plasma Optical Emission

Spectroscopy

‘X-ray photoelectron spectroscopy

Jligh-angle annular dark field scanning transmission

electron microscopy

Energy dispersive spectroscopy

Oxygen

‘Tarn-over frequency Dimethyl sulfoxide

N.N-dimethy! formamide

CIIAPTER 1 INTRODUCTION 1.1, Background

‘the oxidation of alcohol is a major step in producing a wide range of chemicals These compounds are versatile and valuable intermediates in organic

synthesis, for fundamental research and industrial manufacturing [1] The world-

wide anual production of carbonyl compounds is over 107 tones and many of

these compounds are produced from the oxidation of alcchols Especially, the

selective oxidation of benzyl alcchol to higher-value-added products such as benzaldehyde has received increasing attention due to its tremendous application

in perfumery, pharmaceutical, dyestuff, and agrochemical industries [2] This reaction is commonly performed with relatively expensive stoichiometric oxidants such as activated dimethyl sulfoxide reagemts, permanganates, copper(II), and chromium(L¥) salts [3] However, such methods produce a large amount of heavy anetal waste and are also usually carried out in organic solvents, and are therefore not environmentally benign Therefore, extensive methods have been developed

under mild conditions with high selectivity and efficiency using

Molecular oxygen (Q2) is known an cnviroumentally friendly oxidant that is

natwally available and does not generate harmful products Based on this

perspective, many homogeneous and heterogeneous catalytic pathways using Oz

as a begin oxidant have been studied

cancr oxidants

During the past decade, the acrobie oxidation of alochol has long been studied using different systems starting from transilion metals reaching metal

oxide supported precious metals, including Aul4], Pt [5], Pa [6, 7], Co [8], Cu |9],

Ru [10,11] The Pd, Au, and Pt-based catalysts consistently exhibited high activity

even under mild conditions [lowever, for the liquid phase oxidation of alcohol,

they deactivate quickly because of the aggregation of nanoparticles and the

leaching metal species [12, 13] On the other hand, non-precious metals are rarely

used for the aerobic oxidation of alcohot in the liquid plase because of their low

activity

Rulhenum-based-catalysis have altracted considerable interest in the oxidation of alechols not only because of its relatively lower cost compared to

commonly studied and industrially applied noble metals (Au, Pt, and Pd) but also

due to its high-tunability of its electronic and structural characteristics by different support effects [14] However, the activity performance of the Ru catalyst is significantly dependent on many factors, such as the size and valence state of Ru,

the interaction between Ru and support, solvents, and additives |15, 16] For instance, Zhao demonstrated that small-size Ru of 2 nm exhibits excellent

sclechvily (90%) [17] Mori used Ru/C for acrobic oxidation of 1-pharyl-

pentanol in toluene, showing higher conversion than other solvents [18] Titanium and graphile supporied catalysts were introduced by Graham J Hutchings as promising catalysts for the aerobic oxidation of glycerol [19] The catalysts showed interestingly 100 % selectivity

Size reduction of ruthenium parlivles is a great slralegy lo optimize eatalytic

activity and reduce the price of ruthenium-based catalysts The reduction in metal sive benefits the performance of calalysis in olher aspects: (1) low-coordination

environment of metal centers, which is ascribed to the unsaturated metal atoms

exposed on the small-sized particles; (2) quantum size effects, where confinement

of electrons leads to a discrete energy level distributions and a distinctive highest

gecupied molecular orbital (HOMO) lowest occupied molecular orbital (LUMO)

gap; (3) metal support interactions, which originate from the chemical bonding, effect between metal and supports and the associated interface, as well as the charge transter between metal species and supports |20], leading the nanoclusters

of molals usually show a distine! size effect on Iheir reacivitics Revenlly, some theoretical and experimental studies have demonstrated that subnanometer-sized metal clusters sourelinies lave belter performance activily or seleclivily than their nanometer-sized counterparts [21, 22]

Based on the above discussion, in this thesis, we prepared a high dispersion

of ruthenium on a high-surface area TiQz using the wel-ness impregnation method The size of rutherman parlicles was vortrolled by varying the concentration of nuthenium,

1.2 The objectives of the thesis

Tn short, we aim al the rational design and decp characterization of highly active, selective, Ru metallic nano-catalysts supported by thermally stable and

tmable high-surface area TiO: for the aerobic oxidation of benzyl alcohols The

core of the thesis aims primarily at establishing a structure-reactivity relationship

of this reaction in the liquid phase This will be carried out using TiOz-supparted

Ru nanostructures ‘[he work program includes the following objectives

4) Synthesis und characterization of size-selected Ru parlicles loading into the high surface area of TiO

ii) Catalytic activity characterization of the prepared materials for aerobic oxidation of bemyl alcoliol in the liquid phase using O2 withoul any solvent

iii) Characterization of electronic and structural properties of the catalyst after reaction

10

CHAPTER 2 THEORICAL BASIS

2.1, Titanium dioxide

2.1.1 Properties and structure of titanium dioxide

Since its commercial production in the early twentieth century, titanium

dioxide (TiO2) has been widely used as a pigment in sunscreens, paints, toothpaste,

ete [23-26] Pure TiO; is either colorless or white, but it also tends to exist in other

colors due to the presence of impurities All three polymorphs of TiO have a very high refractive index and are temperately hard and dense In 1972, Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2

electrode under ultraviolet light Since then, enormous efforts have been made to

research TiO2 material, leading to many promising applications in various fields

TiO has three main crystal polymorphs: anatase, rutile, and brookite, and

each polymorph exhibits different physical properties Among the three phases,

rutile is the most stable under ambient conditions, while anatase and brookite are

metastable at all temperatures In addition, the anatase and brookite phases can

transform to rutile when heated The stability of the three TiO2 phases also depends

on the particle size Anatase is the most thermodynamically stable phase when the nanoparticles are smaller than 11 nm Brookite is the most stable phase for

nanoparticles between 11-35 nm, while rutile is shown to be the most stable for

nanoparticles more significant than 35 nm in size [27]

Figure 2 1 Crystal bulk structure of (a) anatase, (b) rutile, and (c)

brookite [28]

The structure of TiO: is formed by chains of distorted TiO¢ octahedra, where

each Ti atom is surrounded by six oxygen atoms The three-dimensional stacking

of the octahedra in anatase, rutile, and brookite is exhibited in figure 2.1 It can be seen that anatase has a smaller cell volume than rutile and brookite The unit cell

of tetragonal anatase, rutile, and brookite is four, two, and eight TiO2 units, respectively The other properties of the three phases, including lattice parameters, unit cell volume, density, band gap, and hardness, are summarized in table 1

Brookite is the least stable phase among the phases of TiO2 due to an orthorhombic

structure; thus, it is challenging to synthesize Therefore, most practical

applications have been carried out with anatase and rutile Moreover, the existence

of either or both of these phases leads to different activities for chemical reactions

1I

Table 1 Propartics of anatase, rutile, and brookile

The TÌO; phase is the critical parameter determining the properties of the

materials, therefore, numerous researchers have been devoted to investigating the

phase transformation between anatase, rutile, and brookite ‘The transformation of

the anatase to the rutile phase is a reconstructive and irreversible process due to the breaking and reforming of bonds |29] The transformation temperature of the anatase to the rutile significantly depends on raw materials, synthesis methods, and

heat Now conditions Tl is a wide range [rom 400 1a 1200°C [30] The brookie

phase was transformed into the rutile phase at 800°C TiO2 is always difficult to dissolve or reacl when the sample is healed above 1000°C for a long period of

time Phase transformation is of considerable interest as the transformation is

capable of altering the properties and performance of the catalyst

The electronic structure of TiOz has becn explored by using a variety of theoretical approaches and experimental techriques It is well known that TiQ2 is classified as an n-lype semiconductor thal bas a conducion band with an edge of low energy formed by the vacanL Tit d bands, and a valence band where the upper edge is formed of the 0? filled pm bands The band-gap energy of indirect electron

transition is 3.0 and 3.2 eV for rutile and anatase, respectively [31]

2.1.2 TiO? as catalyst support

During the decades, combining metallic nanoparticles with catalyst support has been extensively investigated ta enhance the catalytic performance and optimize selectivity in chemical synthesis Catalyst support plays a key role in improving stability and reducing the amount of costly metal used dus to its umque properties, including high surface area, tunable shape, and high dispersity [32] Besides, it exerts a structural and active phasc-linked effect, which can increase the dispersion of metal species and inhibit the aggregation of active metal particles Therefore, the selected heterogeneous catalyst support can relain ils specific properties [33] Support catalysts can be classified into four groups, including () 12

solid acid supports like zeolite, (ii) reducible oxide supports such as titania, ceria, zine, and zirconia, (iii) refractory oxide supports like alumina and silica, and (iv) carbon-based supports like mesoporous carbon, carbon nanotubes [30] Each support category exhibits different catalytic performance and selectivity towards the chemical reaction

Among the reducible oxide supports, TiO, is famous and proven to contribute to better catalytic activities as it acts as a promoter, a carrier for metal and metal oxides, and an additive or a catalyst [34] The better catalytic activity of TiO2 was strongly attributed to the strong metal-support interaction (SMSI)

Tauster and co-workers were the first to discover the SMSI and defined the concept

of SMSlas the drastic changes in the chemisorption properties of group 8-1 0 noble metals when supported on TiO2 [35] Prototypical examples of SMSI phenomena were only observed for metallic particles supported on reducible oxides such as

TiO2 [36] and CeO, [37] TiOz has exhibited SMSI after reduction at high

temperatures under a hydrogen environment SMSI is an important and unique

effect that occurs due to an electronic or geometrical interaction and charge transfer

between metallic nanoparticles and a support (see figure 2.2) [38] The metallic

nanoparticles are capped with functional groups of support that migrate to the

surface of the metallic nanoparticles during the reduction process This capping

layer can produce new active sites and improve the adsorptive and efficiency of the catalyst The parameters can affect the strength of the metal-support

interaction, including metal particle dispersion on the support, the geometry of

adsorption, metal size, and electronic structures

\J \J

Reduction Time

Metal TiO, Reduction Capping Layer

Figure 2 2 The SMSI effect occurring during the reduction treatment by

Hi The gray circles represent metallic nanoparticles, while the blue parts

represent the TiO2 support [30]

Besides the interaction of the metal and support, the size of the active sites

is another factor that is pertinent to the efficiency of metal-supported catalysts The metal particle size after heat treatment is highly dependent on the nature of the support Fang and co-workers reported that gold particle size was significantly increased on carbon and SiO2 while slightly increasing on TiO», after calcination

13

This indicates that TiOz cificiency prevents the growth and sintering of metal

particles [39] TiO, is well-known as a weak acid support, mainly with Lewis acid properties, in which Bronsted acidily is nol observed Lewis acid sites tend to attract the oxygen ion pair of the target molecules, leading to higher selectivity for the hydrogenated product [40]

2.1.3 Synthesis of TiO2

Generally, top-down and bottom-up approaches can be used to synthesize different metallic and metallic oxide nanoparticles The top-down methods involve the breakdown of large or bulk molecules for generating required nanomaterials Meanwhile, the bottom-up methods involve assembling single atoms and molecules into larger nanomaterials ‘the physical and chemical methods are widely used (o synthesize different nanoparticles

% Hlydrothermal and solvo-thermal method Both the hydrothermal and the solvo-thermal methods for creating

nanoparticles involve an autoclave, am isolated system operating al high pressures and lemperalures Nolably, the pressure:

considered carefully to employ an autoclave with a proper design The operating temperatures are selected around 200 °C and the pressure is usually less than 100

‘bars These critical conditions enhance the solubility and reactivity of metal salts

and complexes that are difficult to dissolve at ordinary conditions (temperature <

100 °C, and pressure < 1 bar) Nurthermore, the high - temperature and - pressure cause in a dramatical change of solvent properties such as the dielectric constant

or viscosity that leads to reduce the solubility of species present ‘This results in the supersaturation of the solution, then allowing nucleation and growth of the cryslals The solvothermal method employs von-aqueous solvents, while the hydrothermal method uses aqueous solvenls

as well as powders, there are drawbacks, including cost-effectiveness because it

aecessitates sustaining low tcmperatures, which is an cnoray-intensive and

expensive procedure Additionally, this has been identified as a constraint for controlling the characteristics of powders and results in limitations [or producing

TiO, on a wide scale utilizing spray pyrolysis Similarly, it is acknowledged that electrophoretic syrthesis of TiQ2 is quick and simple, bul it tacks Nexibility in

terms of selecting, water as a solvent It requires such solvents, which are harmful

to the environment as a result of their ability to eradicate wildlife, aquatic

14

ecosystems, and aquatic life Tn addition, evaporating these sulvents contributes to

air contamination, which is detrimental to human health

% Sol-gel method Sol-gel is known as a wel chemical lechrrique used broadly in material science TL is a conversion of precursor solution to an inorganic solid via polymerization driven by water A sol (dispersed colloidal particles in liquid) formed in a result of hydrolysis is condensed to a gel The sol-gel process occurs

in four steps: (i) hydrolysis to generate a sol, (ii) polycondensation to generate a gel, (iii) drying and (iv) thermal decomposition to obtain materials ‘This technique

is very suitable for the synthesis and preparation of hybrid materials as it operates

at low temperatures and offers homogeneous molecular level compositions In

addiiion, i also resulis in defined shape, controlled and uniform size nana

particles, TiO2 has been synthesized via the thoroughly suudied sol-gel process, which involves the creation of a sol, gelation, and solvent removal to produce extremely crystalline NPs However, the sol-gel method is notorious for being pricey because it calls for the use of pricey raw ingredients, and the drying process

“%& Microwave assisted method

The feature of microwave heating is fast heating during a short time and high reaction rate, yield, and selectivity in comparison to commonly used heating methods In this method, one commonly accepted strategy involves the homogeneous and rapid heating of the reacting mixture to the appropriate temperature ‘Ihe experimental conditions seem promising and controllable to carry out the synthesis using this technique However, the methods that make use

of microwave assistance do not offer good value for the money because they call fur energy intensive syslems

2.2 Ruthenium

2.2.1 Physicochemical properties and application of ruthenium

Ruthenium, the symbol Ru, and the “Ith position in the periodic table of elements are part of the transition metals group, Ru was identified and isolated by Karl Karlovich in 1844 [41], Ru is the 74th most abundant metal, a rare element, and is part of the precious metals, being the first of thơ series besides rhodium, palladium, osmium, iridiwn, and platinum However, ru is still the least expensive

precious metal

Ru is a hard, silvery-white metal that is unalterable and does not tarnish in ambient conditions ‘The electronic configuration of Ru is [Kr]4d’5s! for the

isolated atom in the ground state Ru has a wide oxidation state range from LI to

VL, the most common oxidation states are LU, II, and LV These different oxidation

states provide a large number of stable Ru catalysts Ru is not casily oxidized al

alnospheriv conditions to [urm RuCs, a stable oxide thal can be formed under

oxygen pressure Anolber nithenium oxide is rulhenium tetroxide, a volatile and very loxic compound with powerlul oxidizing The dissolution of rulhenium is nol

15

easy and requires the use of aqua regia in beating conditions The erystalline structure of bulk ruthenium is hexagonal closed-packed (hep), but at the nanoscale, ruthenium has a face-centered cubic (foc) structure [42]

‘the main application of ruthenium concems technological devices and catalyst sectors In 2018, ruthenium consumption was 42 tons for industrial applications, including electronics (33%), electrochemistry (17%), and chemistry (37%) [43] Th catalysis, rutheniun is # polyvalent metal thal proved to be aclive

in both homogeneous and heterogeneous conditions For homogeneous catalysis, nuthenium complexes are able to activate unique and multiple bonds and make possible selective CC, C H, or C heteroatom bond formation and cleavage [441] Ruthenjum-based catalysts have been efficient catalysts for many organic reactions such as alkylation, allylation, arylation, cyclization, cyclopropanetrione, hydrogenation, hydroformylation, hydrosilylation, hydroxylation, isomerization, olefin metathesis, oxidation, transfer hydrogenation, tandem reactions [43] or heterogeneous catalysis, ruthenium is reputed to be active in the hydrogenation of nitrogen for ammonia synthesis, hydrogenation of diverse substrates Hke olefins and earbonylated molecules but also of aromaties for which moleeular ruthenium

is nol known, as well as for dehydrogenation of amine boranes and hydrogen evolulion reactions Tnlerestingly, i is nol well known for the hydrogenation of CO; and dehydrogenation of formic acid RuQ2 was an excellent oxidation catalyst

in heterogeneous catalysis (mainly oxidation of CO) and electrocatalysis (oxidation of water)

2.2.2, Ru-based catalysis for oxidation of alcuhol

Ruthenium compounds have been extensively studied as catalysts for the aerobic oxidation of aloohols in both homogeneous and heterogeneous catalysts

For homogeneous catalysts, ruthenium gives the broadest range of oxidation states

from +2 to +8 herefore, a large variety of oxidative transformations has been

developed Masakatsu ard Sator have used RuCk and RuCh(PhjP)s as catalysts

for the aerobic oxidation of allylic and benzylic alcohols under mild conditions

with a high yield of corresponding carbony! compounds afler 48b reaction [45] Tn addition, the catalytic activity of RuCh(PPhs); complex can be increased by using ionic liquids as solvents [46] Ruthenium compounds are widely used as catalysts

for hydrogen-transfer reactions These systems can be readily adapted to the aerobic oxidation of alcohols by employing oxygen molecules, and a hydrogen acceptor, as a co-catalyst Mor example, Backvall and co-workers reported the fast catalytic systems for the oxidation of secondary alcohols using benzoquinone and cobalt Schiff's base complex ‘fhe proposed mechanism in a multistep process is

exhibited in figure 2.3 47] The dehydrogenation of lhe alcohol with the rulhemum

complex to afford the ketone product and a ruthenium dihydride The latter undagous hydrogen transfer to the benzoquimone to give hydroquinone with concomitant regeneration of the ruthenium catalyst The cobalt Schiff’s base complex catalyzes the subsequent aerobic oxidation of the hydroquinone to benzoquinone to complete the catalytic cycle Moreover, some studies have

16

replaced hydroquinone and eoball complex with a stable nitroxyl radical, 2,2,6,6- Tetramethylpiperidinyloxy! (TEMPO), to establish a remarkably effective catalyst for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, respectively, with high conversion and selectivity

sĩ NADTINADH +H 13 Ubrquinone of Cytochrome &

Figure 2 3 Biomimetic acrobic oxidation of alcohols [47 |,

A high+valent perruthenale catalyst, tetra-tepropylarmmonium perruthenate

(TPAP), is ar excellent air-stable catalyst that docs not evaporate and is soluble m

a wide range of organic solvers Ti has long been known that ruthenium tetroxide, generated by the reaclion of ruthenium dioxide with poriodale, smoothly oxidizes various alechols to the corresponding carbonyl compounds Ley and coworkers

subsequently showed that TPAF is an excellent catalyst for the selective oxidation

of a wide variety of alcohols using N-Methylmorpholine N-oxide (NMO) as the

stoichiometric oxidant [43] More recently, Katsuki and co-workers have

published several papers on Ru-salen-based catalysts ‘They designed an efficient

catalyst for the photo-induced chemoselective oxidation of primary alcohols in the

presence of sccondary ones, and upon further derivatization of chiral ligands, they

could accomplish eflicienl kinetic resolutions af secondary alcohols and

desymmetrization of meso-dicls [49]

Supported ruthenium has discovered highly high catalytic activity in the heterogeneous oxidation of alcohols In 1977, Ley and co-workers pioneered a report on using polymer-supported perruthenate (PSP) in the aerobic oxidation of

alcohols However, (his catalyst, sulfered from oxidalive degradation of the

polymer support Soon later, they found a mesoporous silicate (MCM-41) as an efficient alternative support for TPAP and showed the recyctability of this catalyst

up to 12 times [50] Zeolites-supported catalysts were introduced by Bi-Zeng Zhan

as a promising catalyst for the aerobic oxidation of alcohols [51] The zeolite-

confined nano-Ruw, exhibited extraordinarily high activity and selectivity in the

oxidation of both activated and inactivated alcohols to the corresponding

aldehydes and ketones under aerobic conditions without using either a cocatalyst

or a sacrificial reducing agent Kaneda and co-workers developed a monomeric ruthenium cation on the surface of hydroxyapatite for the oxidation of various

alcohols [52] The catalysts showed ilercsiingly 100% conversion and 99%

17

selevtivily However, the main disadvantage of (his process was lhe need for a high catalyst loading (17 mol) Ruthenium supperted on alumina was developed by Yamaguchi and Mizuno ard demonstrated the ability to oxidize both primary, secondary, and non-activated alcohols in PhCT; as solvent or even in solvent-free conditions [53] Moreover, the use of superparamagnetic nanoparticles as a supporting material for immobilized metal catalysts was reported For example, the same group showed that a ruthenium hydroxide species on magnetite performed very well, and catalyst/product separation was straightforward Indeed, after the completion of the oxidation reaction, a permanent magnet was attached

to the outside wall of the glass reactor to hold the catalyst magnetically, and the reaclion solution, including the produci(s), was separaled by simple decantation

[54]

2.3 Isolated single-atom-site catalysts

2.3.1, Concept and properties of isolated single-atom-site calalysts

Despite the tremendous achievements of nanocatalysts in the past decades, the pursuit of catalysts with improved activity-enhanced selectivity and clear

structure activity relationships has never stopped Ihe proportion of active sites

on the surface of a catalyst increases sharply as its particle sive decreases This increase reaches a limit whon the catalyst size shrinks to a single atom However, single melal aioms are themsclyes cxiremely unstable and require a proper substrate to stabilize and isolate them Metal atoms dispersed on support comprise

a new type of catalyst, namely isolated single-atom-site catalysts (ISAS) ISAS catalysts not only have 100% utilization of metal atoms, but they also maximize the metal-support interface Based on these two structural characteristics, the catalytic behavior of ISAS catalysts is quite different from that of nanoparticle

catalysts The support of an 1SAS catalyst is likely to have a great influence on the

properties of the metal centers Some of the earliest records of ISAS were based

on investigating the unusual catalytic behaviors of supportsd metal catalysis For

example, in 2003, ceria-supporled Au and PL calalysls were found tp be able lo preserve their calalyhe aelivily loward the waler—gas sluft reaclion even afler the remaval of metal nanoparticles from the support by highly corrosive reagents, implying that the unseen metal ISAS species may be the actual active entities [55]

Likewise, Zhang and coworkers revealed in 2005 that supported metal catalysts with low metal loadings were more active than those with high metal loadings, entirely unexpected according to conventional pearls of wisdom [56]

The metal-isolated single aloms, combined with the supports, constitule a new family of catalysts characterized by maximized atom utilization and defined active centers The metal sites in these catalysts are anchored by chemical bonding with coordinating atoms on the solid-state support The isolation of metal sites gives rise to an atom utilization approaching 100%, which is a considerable advantage, particularly for noble-metal-based catalysts Apart from the significantly improved atom economy, another significant featwe of ISAS cafalysls is thal Ihe aclive conlers are spalially isolated on the supports, and the

18

supports have unprecedented influence in determining their overall catalytic property These features endow the ISAS catalysts with new catalytic behaviors,

such as high resistance to detrimental side reactions that typically occur on

multiple-metal sites

The size reduction generates an increase in the unsaturated coordination environment of the metal species (see figure 2.4), Accordingly, the surface free energy of the metal components increases, and the metal sites become more and

more active for chemical interactions with the support and adsorbates, which accounts for the size effects of metal nanocatalysts In the extreme case of ISAS,

because of the highly active valence electrons, the quantum confinement of

electrons, and the sparse quantum level of metal atoms, the surface free energy of metal species reaches a maximum, which then leads to promoted chemical interactions with the support and unique chemical [20] Single-atom

©

Minimizing Metal Sizes

Figure 2 4 Schematic illustrate the changes of surface free energy and

specific activity per metal atom with metal particle size and the support effects

on stabilizing single atoms [20]

In addition, ISAS catalysts are more well-defined, uniform, and suitable for

structure-property investigation than nanoparticles (which comprise sites at

corners, edges, and terraces) The achievement of well-defined active centers and

their tunability would empower us to understand the catalytic processes at the

atomic and molecular levels

2.3.2 Preparation of isolated single-atom-site catalysts

A prerequisite for the application of an ISAS catalyst is to prepare highly dispersed single atoms of a defined species on appropriate supports However, the fabrication of such an ISAS catalyst is a significant challenge because of the aggregation tendency of single metal atoms Previously, using mass-selected soft-

19

landing techniques or improved wet chemistry methods, atomic dispersion of the metal species on supports had been achieved, as exhibited in figure 2.5

Metal complex

Figure 2 5 Methods for preparing ISAS catalyst: mass-selected soft-

landing (A) and wet chemistry (B) [20]

The mass-selected soft-landing technique is powerful in preparing

supported metal clusters or even ISAS because of its exact control of the size of

metal species by using mass-selected molecular or atom beams and precise

regulation of the surface structure of the support by combining these with ultrahigh

vacuum surface science procedures This technique provides excellent model

catalysts for fundamental studies, on the atomic level, of metal-support

interactions and cluster size effects Several experimental and theoretical studies

based on the soft-landing method have addressed the catalytic properties of metal

species on supports [57-59] However, such an expensive and low-yield fabrication method limits its wide application and is clearly unsuitable for practical industrial

applications of heterogeneous catalysis Other alternatives for preparing ISAS catalysts are urgently needed

In the wet-chemistry method, due to the precursor materials already

containing single-atom metal species, the objective is to anchor metal species on the high surface area support through a chemical reaction and to avoid their

aggregation during the post-treatment processes Experimentally, anchoring

mononuclear organometallic complexes on supports can be achieved by utilization

of the coordination between the ligands of the complex and the surface groups of

the support materials In many heterogeneously catalytic processes, more

accessible metal active sites are needed for the activation of reactants, which

requires the pretreatment of catalysts to remove useless or even poisonous ligands

As a result, it runs the risk of the single-atom metal species aggregating to larger particles Therefore, a strong metal-support interaction is the key to preventing

aggregation of single atoms on the surface In this case, the anchoring sites with surface species of the support as ligands, such as surface uncapped sites or other

aggregation inhibitors on supports like alkali-ion contaminants or residual organic ligands during chemical preparations, will play an essential role in stabilizing single-atom metal species Since the anchoring sites on the support are not always abundant, low metal loadings with high-surface-area supports are generally

20

required lo achieve ISAS catalysis Metal-organic frameworks, vaolite, and polymer carbon nitride are usually used to synthesize ISAS catalysts

2.4, Analytical methods

2.4.1 X-ray diffraction (XRD)

Jn 1985, Wilhelm Conrad Roentgen discovered that X-rays are high-energy

electromagnetic waves with a wavelength between 10 and 10! nm |601, and then Laue, Hriedrich, and Knipping used X-rays to investigate the crystalline of

materials [61 | Sinee then, these methods lave been further developed to become

powerful matcrials science and cngincering tools The generation of X-rays is generally achieved by the use of sealed lubes, rolating anodes, or syrichrotron radiation sources Sealed tubes and rotating anodes, which are used in laboratory equipment, both produce X-rays by the same principle Electrons generated by heating a tungsten filament in a vacuum are accelerated through a high potential field and then directed to a target, which then emits X-rays The incident electrons induce two effects leading to the generation of X-rays: the first is the deceleration

of the electrons leading to the emission of X-ray photons with a broad continuous distribution of wavelength The second is the ionization of the impinged atoms by

gjecting clectrons (rom the inmer shells In order to gel amore stable state, electrons

from outer shells “jump” into these gaps The difference between the electron energies of the itmer shell and the incoming electron is emiited in the form of photons, with characteristic energy depending on the initial and final shell position

of the electrons and on the material The characteristic radiation requires minimum excitation potential of the electrons ta be emitted, which depends on the target material In X-ray diffraction, the Ka radiation of the characteristic radiation is often used since this is usually the most intense ‘Ihe capital letter indicates the shell from which the electron was knocked out and the index from which of the higher shells the electron relaxes At o, for example, the electron jumps from the

next higher TL shell into the Ko shell For diffraction experiments, however,

monochromatic X-rays are almost always required This is generated with the help

of metal Glters Metals absorb X-rays particularly well when the energy is sufficient to knock out internal electrons This effect allows the K radiation to be separated from the Ky radiation in the case of a copper anode with nickel filters Tlowever, the metal filter only suppresses the unwanted part of the characteristic radiation However, it does not eliminate it so that a monochromator, a single crystal with a precisely adjusted positioning in the beam path, scatters all unwanted wavelengths out of the primary beam

interfere with each other However, only one reflection can appear in the recorded diffractogram if the rays interfere constructively This is only the case if the diffracted beams are phase-shifted relative to one another by an integer multiple n

of the wavelength 4 Thus, one of the interfering rays has to cover a n -A longer path after diffraction Due to the spatially periodic arrangement of the atoms in the crystal lattice, the interatomic distances are fixed Thus, at an angle of incidence

6, the atoms can form parallel lattice planes whose distance is d Figure 6 shows

that the path difference must correspond to twice the distance in order to fulfill the

interference condition

However, the distance a is not known, but since it is a right-angled triangle,

it can be calculated with the angle @ and the grid plane distance d

22

is well known, the Brurauer—Finmell—Teller (BET) is the most common theory

used to calculate the specific surface area [62] For this, it is assumed that the adsorplive behaves like an ideal gas, the molecules in the gas phase do nol interac with each other, and the adsorptive can adsorb in several layers The model can be used to determine the so-called monolayer capacity from the measured isotherm, which, together with the cross-section of the adsorbed molecule, enables the specific surface area to be caloulated Barrett, Joyner, and Halenda (3/H) method

in 1951 is the most popular way of deriving the pore size distribution from an appropriate nitrogen isotlierm [63]

—, ta)

small extemal surfaces Type II isotherms are assigned to the physisorption of most gases on nonporous or macroporous adsorbents In the case of a type Lil isotherm,

23

the adsorbent-adsorbale interaclions arc now relalively weak, and the adsorbed

molecules are clustered around the most favorable sites on the surface of a

nouporous or macroporous solid Type TV isotherrns are aliribuled Lo mesoporous

materials The adsorbent-adsorptive interactions and the interactions between the molecules in the condensed state determine the adsorption behavior in mesopores

In the low relative pressure range, the type V isotherm shape is very similar to that

of type LI, indicating the relatively weak adsorbent adsorbate interactions At higher relative pressure, molecular clustering is followed by pore filling The reversible stepwise type VI iscthorm is representative of layor-by-layer adsorption ona highly uniform nonporous surface [64]

2.3.2, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES}

TCP-ORS is an instrumental snalylieat techmque thal allows thế

quantilication of elamentals in both solid and Liquid This technique is capable of analysing upward of 70 elements simultaneously and detecting conventralions in the parts per million With multiple emission wavelengths per element,

interferences from spectral overlaps can generally be overcome [65] In ICP-OES,

the sample is thermally excited by a plasma Plasmas are gases in which ions and electrons exist tagether The plasma is ignited using a Tesla coil that emits a high voltage spark that ionizes the argon A high-frequency coil then inductively introduces energy into the plasma The electrons in the plasma move so fast that

their kinetic energy corresponds to a temperature of about 10000 K Due to the

high temperature, electrons in the sample are exciled into cnergetivally higher lying orbitals Aller a shorl time, the cxoited state relaxes and the cleetrons "jump"

curve

2.3.3 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is one of the most widely used methods that provides the chemical composition of material surfaces within 10 um,

along with clemental information and detection sensitivity of 0.1-1 at.% for all the

elements except Hz and He, which makes it a unique lool for surface characterivation, specilically corrosion evaluation [66] Due lo the external photo

effect, electrons are knocked out of the shell of the atoms when irradiated by X-

ray The work function is applied, and the remaining energy of the X-ray photon is converted into kinetic energy of the electron (see formula 4) Since monochromatic X-rays are used, the work function can be determined exactly from the energy difference between the irradiated photon and the kinetic energy of the electron

‘The work function is different for each orbital of each element, so the elemental

cumposition of the sample can be determined However, chemical bonding slightly

24

shilis the energy level of an orbilal As a result, Ihe binding silualion and the

binding partner can be identified by peak unfolding (deconvolution)

KE = hv - BE (4)

Where KD is the electron kinetic energy

BE is the clectron binding cnergy

his the Planck constant

vis the frequency

2.3.4 High-angle annular dark field scanning transmission electron microscopy (IIAADF-STEM) and energy dispersive spectroscopy (EDS)

Scanning transmission electron microscope (STEM) has been used to

deicnming the local aiomic arrangements of interfaces, surfaces, planar, line, poinl

defects, previpitates, and clemenis ina wide range of materials STEM is a process where pre-specimen lenses focus the beam into a small probe that is scanned in a raster pattern across the sample (see figure 2.8) This is similar to scanning electron microscopy (SEM) except that here, the transmission signal is collected, and it is

customary to use an electron transparent transmission electron microscopy (TM)

thin foil, producing significantly improved resolution because beam-spreading ot particle scattering events are reduced in thin samples A small on-axis detector, will an ouler collection angle (ypically less than 5 mrad, produces a brighl-lield (BF) STEM image The origin of the image contrast is the interference between overlapping Bragg disks (these disks are groon in figure 2.8) Duc to the theory of reciprocity, BF-STEM, with a convergent iHlumination and a small oreaxis detector, is analogous to BF high-resolution TEM, with a small point source and a

large detector collecting the resultant scattered beams Using an annular detector

to only collect the electrons scattered to higher angles, typically larger than 80 amrad, often referred to as high angle annular dark-field (LAADIN, produces a coherency loss in the signal detected

Bright field detector

Figure 2 8 Schematic of image formation in a STEM, showing the on-

axis small bright-field detector and the larger annular dark-field detector, pink

2.5 Aerobic oxidation of alcohol

Oxidation catalysis, an indispensable transformation reaction in synthetic

chemistry and industrial manufacturing, has contributed to over 30% of the total

production in the current chemical industry [67] Among the oxidation processes,

the oxidation of alcohols to the corresponding carbonyl compounds, such as

aldehydes, ketones, and carboxylic acids, has received the most attention These

compounds are versatile and valuable intermediates in manufacturing

pharmaceuticals, perfumes, and flavorings [1] The oxidation of alcohols is commonly performed in homogeneous catalysts using relatively expensive stoichiometric oxidants such as activated dimethyl sulfoxide reagents,

permanganates, copper(II), and chromium(IV) salts This approach exhibits high

selectivity due to its steric and electronic effects but faces the problems of

separation and reuse By contrast, heterogeneous catalysts show absolute

advantages for practical applications regarding catalyst separation and recovery

To date, the application of solid catalysts for vapor and liquid-phase oxidation alcohols using molecular oxygen (O:) is well established The use of O2 is an environmentally friendly and safe oxidant Therefore, significant efforts have been made to design and prepare efficient heterogeneous catalysts for the thermal-,

photo-, electro-, and photoelectron-catalytic oxidation of alcohol [68], and various noble and transition metals have been investigated in this process

26

Acoording lo the classical dehydrogenation mevhaniam of alcohol oxidation over Pt-group metal catalysts, the adsorbed alcohol dehydrogenates in two elementary steps [3, 69, 70] Figure 2.9 shows the dehydrogenation mectuanism of

alcohol oxidation aver Pt-group metal catalysts The (-II band of alcohol breaks

‘upon adsorption on the surface sites, affording an adsorbed alkoxide and hydrogen

[71] In the adsorbed alkoxide, the 2-C-II bond is weaker than the other C-II bonds

due to the electron-withdrawing effect of the oxygen atom, leading to the preferential breaking of the f-C-H bond, which is the rate-determining step Another class of mechanistic models assumes that the rate-detormining step

involves direct interaction of the adsorbed oxidizing species with the adsorbed

reactant or us partially dehydrogenated intermedialc [72-74] However, Tamas

‘Mallat and Alfons Baiker have presumed that the key role of oxygen is to suppress calalysL deactivation due to the strong adsorption of byproducts [75] Following this approach, the oxidative cleaning of the surface sites, or even prevention of the decomposition pathway is the primary role of oxygen and not the oxidation of the coproduct hydrogen [76] Partial regeneration of the catalysts by oxidativeremoval

of strongly adsorbed poisoning species can lead to a dramatic rate acceleration compared to anaerobic dehydrogenation and thus to the (false) conclusion that oxygen is dircetly involved in the sate-detormining step of the reaction [77] According to this model, some reactions run only on a partially oxygen-covered

RCH;OH¿¿ — RCHjO,, <—> RCHOog

Figure 2 9, Dehydrogenation mechanism of alechol oxidation over Pt-

group metal catalyst [71]

¢ because chemisorbed oxygen is necessary to eliminate (oxidize)

The catalytic performance of metal nanoparticles in the aerobic oxidation

of alcohol reactions can be influenced by many factors including the size and

shape of metal clusters and nanoparticles, promoters, and interactions between supports and active sites It is well known that the electronic properties of metal particles should strongly change when going below 1 nm (see figure 2.10) So, it could be expected that the sub-nanometer sized metal paiticles would interact differently with reactants, showing distinct reactivity concerning larger ramoparticles For cxample, hydrotalcite-supporled Au nanoparticles with differen

sives (ranging from 2.1 to 21 nm) were prepared for benzyl alcohol oxidation [78] The resulls show thal both TOF and initial rate merease with decr

of Aunanoparticles In particular, the TOF increases dramatically when the particle size of Au decreases from ~/ nm, indicating that these coordinatively unsaturated

sing the sive