Manganese and Cobalt oxides as highly active catalysts for CO oxidation

UNIVERSITE LIBRE DE BRUXELLES FACULTE DES SCIENCES

Chimie Physique des Matériaux (Catalyse - Tribologie)

Viacheslav IABLOKOV

Dissertation presented to obtain a PhD degree in Science

Supervisor: Prof. Norbert Kruse

October 2011

Manganese and Cobalt oxides as

highly active catalysts for CO oxidation

ii

1 Acknowledgments

First and foremost I want to thank my supervisor Prof. Norbert Kruse. I appreciate all his ideas, actions and funding to make my Ph.D. experience productive and stimulating. It would have been very difficult without his kindness and smartness.

Moreover, he is my role model.

I want to thank "past" and "new" CPMCT members. Of course, special thanks to Gerome for his practical and scientific advice, not least, for his friendship and fun at the lab. I would like to thank Jean-Marie and Thierry for their help and support. Thanks to Aline and Olivier for their friendship and constant support.

This thesis would not have been completed without the help and advice of Inga and Petrica. Thank you very much for your being in the lab, you are good friends.

I am especially grateful for scientific discussions and nice time spent together to Prof. Sergej Petrovich and Rafal. Dr. Krisztina Frey is our hungarian colleague. I wish her all the best and appreciate our collaboration. I also want to thank Lidija for our friendship and Thomas Doneux for his assistance in analytical analysis.

I am most grateful to Prof. Gabor A. Somorjai for giving me the opportunity to work in his group at the Lawrence Berkeley National Laboratory (Berkeley, USA). It was good experience and great time spent in California. I also thank Selim and my korean friends for supporting me and awesome memories.

I should not forget my good friends (Zhirkov's family, Yura and Pasha) in Brussels.

I acknowledge our philosophical discussions and seeking the sense of life to build my complete picture of the world.

Surely, I am the happiest guy in the world because I have my ukrainian friends. I appreciate our "Thermometer's group" and my best friends Syrotchuk and Anja.

I would like to acknowledge PhD financial support by ‘‘ARC’’ (Communauté Francaise de la Belgique). I also thank Fond David et Alice Van Buuren for the financial support to accomplish my thesis and BRIC grant at the ULB (Bureau des Relations Internationales et de la Coopération de l’ULB) for the journey (4 months) to Berkeley, USA. Brouckére-Solvay grant is also acknowledged (scientific congress on Catalysis, held in Salamanca, Spain).

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2 Résumé

Ce travail de thèse aborde la synthèse de matériaux catalytiques nanostructurés à base de manganèse et d’oxydes de cobalt. Les paramètres importants régissant cette synthèse ont été établis. La corrélation entre les propriétés structurales du catalyseur et l’activité catalytique, ainsi que le mécanisme d’oxydation du monoxyde de carbone (CO) ont été analysés au moyen d’une grande variété de méthodes expérimentales physico-chimiques.

De l’oxyde de manganèse non-stœchiométrique (MnOx) a été préparé par décomposition spinodale d’oxalate de manganèse trihydraté en ayant recours à la technique d’oxydation en variation programmée de la température (TPO). L’analyse quantitative de ces données TPO et les résultats obtenus par spectroscopie de structure au front d’absorption des rayons X (XANES), ainsi que par spectroscopie des photoélectrons X (XPS) ont permis d’estimer la stœchiométrie de l’oxyde avec un x situé entre 1.61 et 1.67. En accord avec une surface spécifique élevée et la combinaison d’isothermes d’adsorption/désorption de type I et IV, la microscopie électronique à transmission à haute résolution (HRTEM) démontre la présence de micro-bâtonnets caractéristiques et « imbriqués » les uns dans les autres, accompagnés de particules nanocristallines à l’extrémité de ces bâtonnets.

Les découvertes faites par spectroscopie infra-rouge à transformée Fourier par réflexion diffuse (DRIFTS) et par études isotopiques et cinétiques suggèrent que l’adsorption des deux molécules, CO et O2, est suivie par leur réaction en surface via des intermédiaires de type carbonate/formiate, pour finalement produire du CO2. Nous supposons un mécanisme de type Mars-van Krevelen où l’oxygène appartenant à la structure de type MnOx prend part à l’oxydation catalytique du CO à basse température.

Ce sont les atomes d’oxygène de l’oxyde qui sont présents à la surface qui participent à l’oxydation du CO préalablement adsorbé.

Une structure spinelle d’oxyde de cobalt Co3O4 dans laquelle le cobalt présente deux états de valence (+2 et +3) a été choisie pour élucider l’effet de la taille des particules sur l’activité lors de la réaction d’oxydation du CO. Tout d’abord, des nanoparticules monodisperses de cobalt métallique présentant une un écart-type en taille inférieur à 8% ont été synthétisées à partir de carbonyle de cobalt (Co2(CO)8) par une méthode optimisée d’injection à chaud. Un contrôle de la taille des nanoparticules dans la gamme 3 à 11 nm a pu être obtenu en variant la température d’injection du carbonyle de cobalt dans une solution de dichlorobenzène et d’acide oléique. La microscopie électronique à transmission (TEM) nous montre que ces particules de cobalt sont quasiment hémisphériques. Les nanoparticules de cobalt sont (ensuite) déposées dans de la silice poreuse de type MCF-17 et activées par TPO menant à des nanoparticules d’oxyde de cobalt. Des études par diffraction des rayons X (XRD) et spectroscopie des photoélectrons X (XPS) ont démontré la structure spinelle Co3O4. Finalement, l’activité des catalyseurs obtenus vis-à-vis de l’oxydation du monoxyde de carbone a été mesurée à 423 K en fonction de la taille des particules. Les particules de Co3O4 présentant une taille allant de 5 à 8 nm se sont révélé les plus actives. Ceci peut s’expliquer par une plus grande mobilité des atomes d’oxygène à la surface des nanoparticules d’oxyde de cobalt.

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3 Abstract

In this research important parameters for the synthesis of nanostructured manganese and cobalt oxides catalytic materials were established. The correlation between the structural properties and the catalytic activity as well as the mechanism of the low-temperature CO oxidation were analysed by means of a wide range of physico- chemical experimental methods.

Non-stoichiometric manganese oxide (MnO_x) was obtained by a spinodal transformation of manganese oxalate trihydrate using temperature-programmed oxidation (TPO). The quantitative evaluation of TPO combined with X-ray Absorption Near Edge Structure Spectroscopy (XANES) data and X-ray Photoelectron Spectroscopy (XPS) allowed estimating the x-values to 1.61…1.67. In accordance with the high specific surface area and mixed-type I/IV adsorption/desorption isotherms of MnO_x, high resolution TEM demonstrated the occurrence of “nested” micro-rod features along with nanocrystalline particles in the endings of the rods.

The findings of the Diffuse Reflectance Infra-Red Fourier Transform Spectroscopy (DRIFTS), isotopic and the kinetic studies suggested that the adsorption of both molecules, CO and O2, was followed by their surface reaction via carbonate/formate-like intermediates to produce CO₂. We assumed a Mars-van Krevelen type mechanism where oxygen belonging to the MnO_x structure took part in the low-temperature catalytic CO oxidation. However, these mobile oxygen species were not part of the bulk oxide lattice, and therefore they were able to “hop” on the surface and feed the oxygen species needed for the oxidation of previously adsorbed CO.

A spinel Co₃O₄ oxide in which cobalt is present in two valence states (+2 and +3) was chosen to elucidate a particle size effect on the activity in CO oxidation reaction.

First, monodisperse cobalt metal nanoparticles (NPs) with a standard deviation in size less than 8% have been synthesized by an optimized “hot injection” method from cobalt carbonyl (Co2(CO)8). The size control in the 3 to 11 nm range was achieved by varying the temperature of the injection of cobalt carbonyl into dichlorobenzene solution in the presence of oleic acid. TEM showed nearly spherical cobalt particles. Then, Co NPs were impregnated into silica support (MCF-17) followed by the TPO treatment to obtain cobalt oxide NPs. XPS and XRD highlighted the spinel structure of Co₃O₄. The activity in CO oxidation at 423 K of the Co₃O₄ particles was measured as a function of the size. The superior CO oxidation activity in the 5 to 8 nm size range was related to enhanced oxygen mobility on the surface of the cobalt oxide NPs.

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Table of contents

1 Chapter 1: Introduction 1

2 Chapter 2: Aim of the thesis 11

3 Chapter 3: State of the Art 15

3.1 Heterogeneous catalytic oxidation of CO over transition metal oxides 17

3.1.1 CO and O₂ adsorption 17

3.1.2 Catalytic activity of metal oxides in CO oxidation 21 3.1.3 Catalytic activity of mixed oxide catalysts 24 3.1.4 Kinetics and mechanism of CO oxidation over transition metal oxides 26

3.1.5 Particle size effects 31

3.2 Synthesis of well-defined metal oxides 35

3.2.1 Traditional methods 35

3.2.2 Nano-structured metal oxides 37

4 Chapter 4: Experimental part 47

4.1 Sample preparation 49

4.1.1 Manganese oxide catalysts 49

4.1.2 Co₃O₄/MCF-17 catalysts 55

4.2 Characterisation technique 59

4.2.1 Experimental set-up 59

4.2.2 Mass-spectrometer analysis 61

4.2.3 Temperature-programmed treatments 64

4.2.4 BET measurements 65

4.2.5 XRD 69

4.2.6 XPS 70

4.2.7 (HR)TEM 71

4.2.8 AAS 73

4.2.9 DRIFTS 73

4.2.10 XANES 76

4.3 Catalytic set-up 78

x

4.3.1 CO oxidation reaction 78

4.3.2 SSITKA 78

5 Chapter 5: Characterisation of manganese oxides 83

5.1 Manganese oxalates 85

5.2 Temperature-programmed activation 89

5.3 BET and TEM 95

5.4 XANES and XPS 99

5.5 Temperature-programmed studies (TPR and TPD) 105

5.6 DRIFTS 107

6 Chapter 6 : Mn-oxides in catalytic CO oxidation 121

6.1 Catalytic activity of manganese oxides 123

6.2 Deactivation of MnOx catalyst 127

6.3 Kinetics and mechanism 129

6.4 SSITKA 137

7 Chapter 7: Synthesis of Co₃O₄/MCF-17 catalysts 143 7.1 Synthesis development of monodisperse cobalt nanoparticles 145 7.1.1 Effect of the temperature and oversaturation on the size of cobalt

nanoparticles

146

7.1.2 Effect of the concentration and purity of oleic acid on the size of nanoparticles

152

7.2 Preparation of Co₃O₄/MCF-17 samples 155

8 Chapter 8: Size effect of Co₃O₄ NPs in CO oxidation 163

9 Chapter 9: Conclusions and perspectives 171

10 References 175

1 Chapter 1: Introduction

The term “catalysis

Heterogeneous catalysis is an integral part of the modern technology, since about 80% of all industrial chemicals are manufactured by catalytic reactions. Table 1.1 shows a brief list with some of the most important catalytic processes developed in the 20^th century¹.

” was introduced by Berzelius in 1836. Ostwald, one of the first Noble prize winners in chemistry (1905) suggested a definition (in 1895) which is valid even today: “a catalyst accelerates a chemical reaction without affecting the position of the equilibrium”. It was formerly assumed that the catalyst remained unchanged. For the present time, certainly, we know that the catalyst is involved to chemical bonding with the reactants during the catalytic process: the reactants are bound to one form of the catalyst, and the products are released from another, regenerating the initial state.

Catalyst Process Inventor, year

Pt/Rh nets Nitric acid by NH3 oxidation Ostwald, 1906

+2Fe Ammonia synthesis from N2, H2 Mittasch, Haber, Bosch, 1908; Production, 1913 (BASF)

Fe, Mo, Sn Hydrogenation of coal to

hydrocarbons Bergius, 1913; Pier, 1927

ZnO/Cr2O3 Methanol synthesis from CO/H2 Mittasch, 1923 Fe, Co, Ni Hydrocarbons from CO/H2 (motor

fuels) Fischer, Tropsch, 1925

Ag Oxidation of ethylene to ethylene

oxide Lefort, 1930

Al2O3/SiO2 Cracking of hydrocarbons Houdry, 1937 Co Hydroformylation of ethylene to

propanal Roelen, 1938 (Ruhrchemie)

aluminosilicates Cracking in a fluidized bed Lewis, Gilliland, 1939 (Standard Oil) Ti compounds Ethylene polymerization, low-

pressure Ziegler, Natta, 1954

Rh-, Ru

complexes Hydrogenation, isomerization,

hydroformylation Wilkinson, 1964

Rh/chiral

phosphine Asymmetric hydrogenation Knowles, 1974; L-Dopa (Monsanto)

Pt, Rh/monolith Three-way catalyst General Motors, Ford, 1974 Zeolites Methanol conversion to

hydrocarbons Mobil Chemical Co., 1975

Ni/chelate

phosphine α-olefines from ethylene Shell (SHOP process) 1977 zirconocene/MAO Polymerization of olefins Sinn, Kaminsky, 1985 V, W, Ti oxides/

monolith Selective catalytic reduction SCR

(power plants) ~1986

Table 1.1. Catalytic processes developed in 1900 - 2000¹

As it is seen from Table 1.1, the catalysis has traditionally been associated with the chemical and refinery industry and the production of chemicals with increased yields, decreased waste and decreased volumes of pollutants. However, it is only since the 1970’s that catalysis has become familiar to the general public, mainly because of developments in environmental protection being the well-known and widely used catalytic converter for vehicles. To date, almost all vehicles are equipped with internal combustion engines with either spark-ignited or compression-ignited devices².

The energy is released by flame combustion of fossil fuels in the reaction with oxygen present in air:

CmHn + (m + n/4) O2  m CO2 + n/2 H2O

Carbon dioxide and water are the main products of this reaction but incomplete combustion of the fuel causes the emission of other, more harmful, products (including hydrocarbons, and carbon monoxide). Most fossil fuels have some amount of sulphur-containing and nitrogen-containing constituents as well, which results in the production of sulphur oxide and nitrogen oxide emissions. The NOx is also formed due to the very high temperatures in the combustion process by direct reaction between nitrogen and oxygen.

Carbon monoxide is a colourless and odourless gas, and is slightly lighter than air. CO is toxic and harmful to human beings and animals. On average, an exposure at 50 ppm or greater is dangerous to human health. Levels above 300 ppm for more than 1-2 h can lead to death, and exposure to 800 ppm (0.08%) can be fatal after an hour³. Nitrogen oxides (NOx) are also highly poisonous and present a major environmental problem such as acid rain and smog.

Uncombusted hydrocarbons (VOC, volatile organic compounds) generally consist of cracking fragments of the original hydrocarbons. Some part of them are aromatic compounds which are toxic and potential carcinogens. Moreover, hydrocarbons contribute to the formation of photochemical smog, in combination with nitrogen oxides and sunlight.

The chart below in Figure 1.1 shows the summary of carbon monoxide (CO), nitrogen oxides (NOx) and uncombusted hydrocarbons (VOC) emissions by source sector based on data collected by the U. S. Environmental Protection Agency in 2005⁴. From the data in Figure 1.1, it is clear that on-road traffic is one of the major sources of CO, hydrocarbons and nitrogen oxides emissions. Demographic expansion (human population doubled in the last 50 years) accompanied by an economic boom (1980 - 2008) had an immense effect on the number of vehicles on the roads. Currently, the global vehicle park (i.e. total number of vehicles in operation) is around 700 million units with some research institutes projecting an extra 300 million vehicles being placed on the roads in the next 10 years⁵.

Legislation in the area of automotive exhaust control was first introduced in the Unites States in 1970. A few years later, many developed countries started to regulate pollutants emitted from both mobile and stationary sources as well. The development of environmental legislation, accompanied by an efficient mechanism of compliance supervision (i.e. penalties for non-compliance and tax exemptions for over-

compliance) led to the introduction of very effective exhaust after-treatment technologies with a direct result in the levels of pollutants emitted by vehicles⁶.

Figure 1.1. Total emissions (in tons) of carbon monoxide (CO), nitrogen oxides (NOx) and uncombusted hydrocarbons (VOC) in the USA, 2005

One of the best technological solutions in reducing engine-out emissions was the invention of the three-way catalyst (TWC). According to theory, the composition of the exhaust gas from gasoline engines depends to a great extent on the engine air to fuel ratio (A/F). The air to fuel ratio is defined as the ratio between the mass of air and the mass of fuel consumed by the engine:

For a gasoline engine, an A/F ratio of 14.7 is the stoichiometric ratio at which there is sufficient O₂ for complete combustion of all hydrocarbons in the fuel. If the A/F ratio is below this value, then the engine is operating under excess fuel conditions and there is insufficient O₂ to completely combust the fuel. Thus, the mixture in the engine is called fuel rich and the exhaust gas contains more reducing reactants (CO, HC) than oxidising reactants (NO, O₂). If the A/F ratio exceeds 14.7, then the engine is operating under excess air conditions and there is more than enough O₂ to fully combust the fuel. Now the mixture is called fuel lean and the exhaust gas contains more oxidizing reactants than reducing reactants^{7, 8}.

A common way to classify the exhaust gas composition independently of the exact fuel composition is the lambda (λ) factor, defined as:

At λ = 1, there is a stoichiometric mixture of air and fuel. If λ > 1, an excess amount of O2 is present in the exhaust gas and with increasing λ the CO and HC

- 50.000.000

On Road Vehicles Non Road Equipment Fires Residential Wood C ombustion Industrial Processes Waste Disposal Fossil Fuel C ombustion Electricity Generation

Tons

VOC NOx CO

emissions decrease because of better combustion with more O2 present. At λ < 1 there is not sufficient O2 available for the combustion of CO and HC. This is clearly seen in Figure 1.2.

Figure 1.2. Effect of air/fuel ratio on the operation of a catalytic converter (reprinted from ⁹)

As shown in Figure 1.2, there is a window centered at λ = 1 for the optimum in the simultaneous removal of CO, HCs and NOx emissions. At higher oxygen content (λ

> 1) the NO_x reduction is extremely difficult. This negative effect is reasonable, because the CO oxidation reaction consumes too much CO and hence the NO conversion fails. On the other hand, if the oxygen content is too low (λ < 1) all of the NO_x is converted, but hydrocarbons and CO are not completely oxidized.

A multitude of reactions can occur between the exhaust gas constituents. The main reactions that may occur are:

 Oxidation of HC, CO and H₂ (stoichiometric and lean exhaust gas conditions) CmHn + (m + ¼n) O2  m CO2 + ½n H2O

CO + ½ O2  CO2

H2 + ½ O2  H2O

 Oxidation/reduction reactions involving nitrogen oxide (usually under stoichiometric and rich exhaust gas conditions)

CO + NO  ½ N2 + CO2

CmHn + 2(m + 1/4n) NO  1(m + 1/4n) N2 + ½n H2O + m CO2

H2 + NO  ½ N2 + H2O

In addition, there are also some side-reactions which may occur CO + H₂O  CO₂ + H₂

CmHn + 2m H2O  m CO2 + (2m + ½n) H2

Therefore, a general idea of the three-way catalyst is to perform simultaneous oxidation/reduction reactions between the exhaust gas constituents producing non harmful substances like CO₂, N₂ and water over single catalyst. Adjusting the air to fuel value close to λ = 1 would thus provide a maximum level in reducing vehicle emissions. The overall reaction can be expressed as:

NO_x + HC + CO + O₂  CO₂ + H₂O + N₂

An yttrium stabilized zirconium-based (YSZ) electrochemical oxygen sensor in the exhaust gas controls the air to fuel ratio. The catalyst consists of Pt/Rh, Pd or Pd/Rh metals supported on La-stabilized alumina. The total precious metal loading is typically 0.9 to 2.2 g L^-1 catalyst volume. Platinum and palladium metals are used as an efficient oxidizing agent, while the rhodium is active in NOx reduction. Also some promoters are added, i.e. CeO₂, ZrO₂ which act primarily as an oxygen storage material and can stabilize the alumina carrier^{7, 10}. The TWC is currently the most widely used method of reducing vehicle emissions.

A drawback of the TWC is its incompatibility with lean burn technology (i.e. TWC cannot reduce NOx in excess oxygen)¹¹. Lean burn engines combust less fuel than stoichiometric engines and therefore emit less CO_2, a recognised greenhouse gas.

Nowadays, due to catalyst improvements the most significant part of the total emission during a trip, especially for short trips, takes place during the cold start of the engine (60-80 % of the total emitted)^{12, 13}. Obviously, the catalyst needs time to reach its normal operating temperatures (above 473 K), otherwise it remains largely inactive. Several methods have been developed and are continuously being investigated for dealing with this problem. Some of these approaches include electrically or chemically heated catalysts, exhaust gas ignition and pre-heat burners¹⁴. One of the most significant developments in this area has been the close- coupled catalyst which is mounted close to the exhaust ports of the engine so as to be rapidly heated by the hot exhaust and to reach light-off temperatures within 10s¹⁵.

Catalysts capable of oxidizing carbon monoxide at ambient temperature could be another solution of this issue. Gold-based catalysts are currently setting the standard for high activity performance in CO oxidation. As it was shown first by Haruta et al.^16,

17, nanometer-sized Au particles supported on various metal oxides of the first transition period (TiO₂, Fe₂O₃, Co₃O₄) demonstrated activity for the CO oxidation

reaction at temperatures as low as 195 K. Au/TiO₂ catalysts were reported to be particularly active. The size and the morphological shape of the Au nano-particles play a vital role in providing superior catalytic activity^{18, 19}. One of the major drawbacks of gold catalysts is the fact that they deactivate irreversibly both in storage and on-line use. The catalyst appears to deactivate through a combination of Au(III) reduction, Au nanoparticle agglomeration, loss of surface hydroxyl groups, loss of surface moisture, and accumulation of surface carbonates and formates²⁰.

Comparatively much less information is currently available on the performance of Ag-supported catalysts in the same reaction. Our group²¹ has developed a novel method to prepare active Ag/TiO₂ catalysts via co-precipitation of Ag- and TiO- oxalates. Catalysts containing between 4 and 10% (w/w) Ag demonstrated low- temperature CO oxidation activity, whereby T = 333 K was obtained for the 10%

Ag/TiO₂ sample. Studies of Ag-based catalysts with supports other than TiO₂ show a remarkably stable 100% CO conversion up to 523 K ^22-24. Unfortunately, no reliable information could so far be provided for the high-temperature stability of these catalysts.

Unsurprisingly, the price of noble metals is directly affected by changes in emission legislation which in turn force the introduction of certain automotive exhaust systems (most of them containing noble metals). For instance, over the last 10 years the price for platinum has grown up from 360 USD in 2000 to 1800 USD in 2011²⁵. Autocatalyst, industrial and investment demand for the noble metals in 2011 is expected to increase, raising the gross demand figure by 20%²⁶.

Based on this evidence, the catalytic oxidation of CO over transition metal oxide catalysts may gain a “second breath”, even if some oxides are well known to reveal catalytic activity since the beginning of the 20^th century (for instance, hopcalite Mn-Cu oxides)^27-29. Cobalt and manganese oxides demonstrate superior CO oxidation activity among transition metal oxides³⁰. In the case of cobalt, the most active oxide is spinel Co₃O₄ in which cobalt is present in two valence states (+2 and +3). Similarly high activity has also been reported for a number of manganese oxides^{28, 31-33}.

Figure 1.3 illustrates a rise in the number of publications compiled by questioning SciFinder for “metal oxide” AND “CO oxidation”. There seems to be an interest in continuing a “metal oxide” topic in the frame of the novel properties and numerous applications of well-defined nano-scale materials. This interest in CO oxidation reaction over metal oxides is, on the one hand, sparked by environmental concerns since gaseous CO is toxic and harmful to human beings and animals. On the other hand, there are still unanswered questions of a more fundamental nature, like the low-temperature activity of the above 3d metal oxides, the dependency of

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 50

100 150 200 250

num be r of publ ic a ti ons

year

catalytic activity on nanoparticle size, and preparation of stabile well-defined nano- sized metal oxides.

Figure 1.3. Increase of the number of publication over time (SciFinder, request as “metal oxide”

AND “CO oxidation”)

Over the last decade, an extensive experience in the procedures of catalyst preparation was accumulated by introducing concepts of nanotechnology to catalyst preparation. Heterogeneous catalysis is in many respects nanotechnology “per se”.

However, it has only been realized during these past ten years that the control and design of nano-sized systems will help bridge the gap to homogeneous catalysis and thus open completely new perspectives for the catalysis science. One of the major challenges is the development of a “synthetic toolbox” which would afford access to size and shape control of structures on the nano-scale. This would allow scientists to study the activity, selectivity, stability, etc. of catalysts influenced by the chemical and physical properties of the nanoparticles. The colloidal method appears at present to be the most versatile route for controlling the size and shape of nanoparticles^34-36.

For instance, monodisperse platinum nanoparticles of 1.7–7.1 nm were synthesized by alcohol reduction methods in the presence of polyvinylpyrrolidone (so- called “polyol” method) and were incorporated into mesoporous SBA-15 silica support³⁷. The as-prepared catalysts were tested in ethane hydrogenolysis to methane. It was found that the turnover rate declined with particle size, while the activation energy increased. These observations can be explained by coordinatively unsaturated surface atoms in small particles which have a higher reactivity and

subsequently a smaller barrier for hydrogenolysis than highly coordinated surface atoms of larger particles.

While in some cases of catalysis by metals the strong influence of the particle size and shape has been successfully demonstrated, this is much less so for metal oxide systems. To the best of our knowledge, no clear correlation between micro- and nano- structural properties and catalytic activity seems to be established over well- defined transition metal oxides, in particular CO oxidation reaction.

However, recent investigations have revealed that a number of surfactants typically used in preparation of nanoparticles such as containing amine- and phosphorous- groups could hinder the catalytic activity of the catalysts. Moreover, they were able to poison and to block the active surface irreversibly^{38, 39}. Thus, the challenge is to avoid using chemically active surfactants in the preparation route of nanoparticles. One way would be a synthesis of metal nanoparticles in the presence of the surfactant followed by its complete combustion in the presence of oxygen and simultaneous transformation of metal particles into the metal oxide without affecting structure of the surface.

2 Chapter 2: Aim of the thesis

The aim of this Ph.D. thesis

The main goal consists of the following objectives:

is the development of nanostructural transition metal oxide catalysts for the low-temperature CO oxidation reaction with the perspective of an application in automotive pollution control.

 preparation of the nano-sized metal oxides via oxalate and colloidal routes

 correlation of micro- and nanostructure of metal oxides with their activity in CO oxidation reaction

 to measure kinetics of the catalytic reaction at ambient temperature

 to establish nature of the intermediates on the surface

 to elucidate mechanism of the low-temperature CO oxidation activity

Manganese oxides in a shape of microrods have been prepared according to the oxalate route. Manganese oxalates have been decomposed in a temperature- programmed manner in the presence of oxygen to obtain manganese oxide samples and subsequent catalytic tests have been performed under steady-state conditions at appropriate temperatures. To reveal details of the catalytic reaction mechanism over the most active catalyst denoted as MnOx, a steady-state isotopic transient kinetic analysis (SSITKA) has been used. DRIFTS (Diffuse Reflectance Infra-Red Fourier Transform Spectroscopy) studies have been used to identify intermediate species present on the surface of the catalysts during CO oxidation reaction.

Methodology

In order to find structure-reactivity correlations a detailed characterisation work includes XRD (X-ray diffraction), TEM (Transmission Electron Microscopy), BET analysis, temperature-programmed studies, and XANES (X-ray Absorption Near-Edge Structure) spectroscopy as well as a surface specific technique like XPS (X-ray Photoelectron Spectroscopy). In particular, XANES and XPS are element-specific techniques and allow determining the chemical state of elements from the energy shifts of the absorption edge, as in XANES, or from the photoelectron lines, as in XPS.

Well-defined cobalt oxide catalysts with controlled size and shape have been prepared applying a colloidal route. Cobalt oxide was chosen because of its stabile (in contrary to MnO_x) chemical composition in form of spinel and invariant oxidation states (+2 and +3 only). “Hot injection” synthesis has been developed to produce monodisperse cobalt nanoparticles. Colloids of as-prepared cobalt nanoparticles have been impregnated into mesoporous silica support (MCF-17). Subsequently, they are transformed into cobalt oxide nanoparticles using the temperature-programmed oxidation to obtain model catalysts for studying a particle size effect.

CO oxidation activity has been measured under steady-state conditions as a function of the particle size of cobalt oxide. As in the case of manganese oxide catalysts, a number of techniques like TEM, XRD, and XPS have been used to characterize cobalt and cobalt oxide particles before and after thermal treatment, respectively. The oxygen mobility on the surface of cobalt oxide particles has been measured by ¹⁶O₂/¹⁸O₂ oxygen isotope exchange.

3 Chapter 3: State of the Art

3.1 Heterogeneous catalytic oxidation of CO over transition metal oxides

Reaction CO + ½ O₂CO₂ is known to be exothermic (ΔH°₂₉₈= -282.6 kJ mol^-1) and practically irreversible up to 1500 K (ΔG°298= -256.7 kJ mol^-1; ΔS°298= -86.5 J mol^-1 K^-1). Despite the significant decrease in free energy for the oxidation of CO, the reaction occurs via weakening the bonds in molecules of O₂ and CO, which is caused by activation of the reactants during their adsorption on the catalyst surface.

3.1.1 CO and O₂ adsorption

Carbon monoxide is one of the strongest diatomic molecules and a weak

electron donor.⁴⁰:

In the structure with three covalent bonds, the octet rule is satisfied, but the electropositive carbon has a negative bonds would be consistent with the very low dipole moment of the molecule if the bonds were nonpolar. The structure with one covalent bond expresses the greater electronegativity of oxygen and the calculated net atomic charges. None of them do exactly meet the real electronic structure. Calculations with that the structure with a other theoretical and experimental studies which show that despite the greater electronegativity of oxygen the dipole moment points from the more negative carbon end to the more positive oxygen end.

Carbon monoxide chemisorbed on various transition-metal surfaces is the most intensively studied of all adsorption systems^41-43. CO acts as an s electron donor through the 5s orbital mainly localized on C, and as a

π

π

For instance, vibrational spectroscopy studies indicate that the CO stretching frequency is sensitive to the local symmetry of CO adsorption site. Adsorption on a top site leaves CO with a higher frequency of vibration, although about 200 cm^-1 lower than in the gas phase mainly due to the back donation effect into the 2

π

Chemisorption in a threefold site lowers the CO vibration frequency the most, nearly to that of a C – O single bond in an alcohol or ether, for example.

Thermal-desorption studies clearly indicate that adsorbed CO have higher heats of adsorption at defect sites on the surface. On a stepped platinum surface Pt(533) at low coverages only the step sites are covered with CO. As the CO coverage is

increased, CO fills the terrace sites after all the step sites are covered; two thermal- desorption peaks appear, with the lower temperature peak indicating the weaker bonding⁴⁴.

Early experiments of the interaction of CO on the oxides of zinc, chromium and manganese established two different modes of adsorption⁴⁵. At room temperature CO was adsorbed reversibly on ZnO and the gas could be desorbed heating the oxide to about 373 K. The heat of adsorption, as measured by calorimetric experiments, was between 42 and 84 kJ mol^-1, so that chemisorption rather than physical adsorption was occurring.

On the oxides Mn₂O₃ and Mn₂O₃-Cr₂O₃, CO was irreversibly adsorbed in the sense that CO2 was desorbed on heating. The heat of adsorption was over 125 kJmol^-1 and a considerable amount of oxygen could be adsorbed after CO adsorption. This unsaturation towards oxygen was carefully measured and it was found that the quantity of oxygen taken up amounted to about one-half of the amount of preadsorbed CO. In agreement with the calorimetric data, it was possible to postulate a mechanism of adsorption and oxidation via the CO32- surface complex⁴⁶. A molecule of CO interacted with an oxygen ion of the metal oxide lattice, producing the surface carbonate ion. The unsaturation towards oxygen after CO adsorption occurred as a result of an anion vacancy produced by the CO adsorption. The process was shown by Garner⁴⁶ as

М²⁺О^2-М²⁺ М²⁺СО32- М²⁺ М²⁺СО32- М²⁺

→^СО ¹ →^/2^O2

О^2-М²⁺О^2- О^2-М²⁺ 2е О^2-М²⁺О^2-

More precise calorimetric measurements by Stone et al.,⁴⁷ revealed the formation of carbonate species. The essential difference between this mechanism and the theory proposed by Garner is that no lattice oxygen ions were invoked. The mechanism was represented as:

Two above described schemes are accepted to occur. Winter⁴⁸ showed that surface oxygen ions were completely exchanged following exposure of a thin film of nickel oxide sample which was previously outgassed at 813 K in an atmosphere containing ¹⁸O isotopes. The sample was then exposed to CO at 473 K (the surface of the oxide now contained ¹⁸O^2- ions), and no measurable exchange of oxygen was observed.

Similarly, CO₂ did not exchange its oxygen with the oxide ions of the solid as was assumed by Stone. On the contrary, in the study of the exchange of oxygen with outgassed cuprous oxide, it was found that both CO and CO₂ readily exchanged oxygen with the whole oxide surface at room temperature, and the exchange occurred at temperatures as low as 195 K.

To our knowledge, the site for CO chemisorption is a subject of debate in the literature^49-52. In case of spinel cobalt oxide Co3O4, two cations are available (Co³⁺ and Co²⁺)⁵³. Whereas most researches assume that CO adsorbs on Co³⁺, some authors have stated that CO would be adsorbed on Co²⁺ sites. The main argument of the former authors is that the surface of oxidized Co₃O₄ is almost exclusively composed of Co³⁺ cations and that adsorbing CO would require an electron donation at the solid surface. In other words, Co³⁺ would be reduced to Co²⁺ after CO adsorption, a process which would be observable by IR spectroscopy. However, DFT calculations lead to the conclusion that the preferred adsorption site of CO is Co^{3+ 54, 55}. CO adsorption occurring by the surface reaction between CO and Ow (oxygen bonded to both Co²⁺

and Co³⁺ cations) or O_s (oxygen bonded to three Co³⁺) is shown in Figure 3.1.

Figure 3.1. a) CO adsorption on a sublayer Co³⁺ ion. The adsorption becomes sterically hindered because there is little space between the surface oxygen ions where CO enters; b) CO adsorption at a surface exposed Co³⁺ ion. This is the only favourable site found for CO adsorption. Dark grey, oxygen; light grey, Co³⁺; black, Co²⁺; and white, carbon.

In the case of oxygen adsorption over oxides, the initial state of the adsorbent remains undetermined since the concentration of oxygen in the bulk and, particularly, in the sub-surface layers of oxides does not correspond in a one-to-one manner to the stoichiometric one, and depends on the temperature, pressure of oxygen, history of treatment, etc.

It is generally accepted that adsorbed O₂ on oxides undergoes a series of transformation steps during which the solid acts as an electron donor^{49, 56}:

O2(ads)  O2-

(ads)  O22-

(ads)  O^-(ads)  O^2-(lattice)

Oxygen species may differ not only in charge, but also in coordination, bond energy, etc. What species are really available under catalytic conditions and especially which one of them is the active oxidant, i.e., the species which can interact directly with the molecule to be oxidized – these questions are of great experimental difficulties and subject of many studies and discussions.

Formation of negative ions O2-, O22-, O^- and O^2- is found to be possible due to the high electrophilicity of oxygen. ESR spectroscopy observed molecular O₂^- ions over pre-reduced ZnO, TiO₂, SnO₂ and ZrO₂ during adsorption of oxygen. Along with the ESR signal from O2-, that from O^- is also observed⁵⁷. The species O^- alone is produced by adsorption of N₂O on oxides. With the temperature rise, a signal attributed to O^- disappears, apparently, as a result of its transformation to a more stable anion O^2-. Unfortunately, the ESR technique is inapplicable to paramagnetic oxides, most of which are transition metal oxides.

Surface oxygen species can be characterised by temperature programmed desorption (TPD). Typical TPD spectra may comprise 5 or 6 peaks of oxygen release ascribed to different surface oxygen species⁵⁸.

Isotopic exchange (¹⁶O2/¹⁸O2) is a sensitive method for identifying a uniformity or non-uniformity of exchangeable surface oxygen^{48, 56}. In general, those metal oxides active for catalytic oxidation are also active for the exchange. It should be noted that oxygen exchange between CO2 and O2 is known to take place over a series of metal oxides (MnO₂, ZnO, etc.) and is so rapid that the exchange of O₂ with the oxide surface is rate-determining.

Three types of exchange are distinguished depending on the number of atoms which O₂ molecule exchanges with the surface oxygen during its interaction with the

solid: zero, one and two⁵⁹.

exchange reactions of each type can be written as follows:

 ¹⁸O₂ + ¹⁶O₂ 2¹⁸O¹⁶O

 ¹⁸O2 + ¹⁶OS 18O¹⁶O + ¹⁸OS

 ¹⁸O2 + 2¹⁶OS 16O2 + 2¹⁸OS

2. It

usually proceeds at 600–800 K. Concerning the types of exchange, one should note a significant disagreement in the literature. According to Boreskov et al.,⁶⁰ the main

contribution of

oxides of transition metals. In disagreement with Boreskov et al., Winter et al.,⁶¹ suggests that the exchange over oxides, except those of Cu, V, Mo and W, primarily runs by the⁶² simultaneously observed all three types of exchange, their contributions varying with temperature. These disagreements may relate to the different origin of the samples, different experimental conditions as well as to insufficient accuracy as to the identification of exchange types, especially if the results are not corrected by separate measurements of the hetero- and homoexchange rates, which are quite frequent cases.

Experimental observations provide a concept, the essence of which can be presented in the simplified way⁶³:

 Selective oxidation is provided by strongly bonded lattice oxygen having nucleophilic nature.

 Complete oxidation is provided by weakly bonded reactive oxygen having electrophilic nature.

3.1.2 Catalytic activity of metal oxides in CO oxidation

The literature on CO oxidation over base metal oxide catalysts can be divided into two distinct periods⁴⁹:

 Until 1975, when noble metal catalysts definitively imposed their supremacy in automotive exhaust gas catalysis;

 After 2000, when these materials became an interesting alternative for the more and more expensive noble metals. Their main application was VOC abatement coming from stationary sources.

A number of transition metal oxides show high catalytic activity in CO oxidation and have found wide use in respirators and exhaust after-treatment²⁸. Nevertheless, they are inferior in specific catalytic activity compared to the platinum group metals.

Having analyzed numerous data, Krylov³⁰ has found the following order of the activity variations:

MnO₂>CoO>Co₃O₄>MnO>CdO>Ag₂O>CuO>NiO>SnO₂>

>Cu₂O>Co₂O₃>ZnO>TiO₂>Fe₂O₃>ZrO₂>Cr₂O₃>CeO₂>

>V₂O₅>>HgO>WO₃>ThO₂>BeO>MgO>Al₂O₃>SiO₂

This order is close to that found for activities of the same oxides with respect to hydrogen oxidation.

Metal oxides can be classified according to their electrical conductivity, which is related to their catalytic activity⁶⁴. The three groups are:

1) n-type semiconductors 2) p-type semiconductors 3) insulators

Electrical conductivity in n-type metal oxides arises from quasi-free electrons that exist due to an excess of electrons present in the lattice. N-type metal oxides are in general not active oxidation catalysts. P-type metal oxides are electron deficient in the lattice and conduct electrons by means of positive “holes”. These oxides are generally active oxidation catalysts. The insulators have very low electrical conductivities and are not active catalysts. As a result of these different electrical properties oxygen adsorption occurs much easier on p-type oxides, because electrons can be easily removed from the metal cations to form active species such as O^- and O2-. Such a mechanism is not present on the n-type metal oxides, where oxygen adsorption can only take place on prereduced surfaces, so as to replace the oxide ions, O^2-, that were removed in a reducing pre-treatment. Different kinds of oxygen species either adsorbed O^- on p-type metal oxides or lattice O^2- in n-type oxides lead to very different activities in CO oxidation reaction. Because, the adsorbed oxygen species are more active than the lattice oxide ions, p-type oxides are generally more active for CO oxidation reactions.

Cobalt and manganese oxides demonstrate superior CO oxidation activity among transition metal oxides³⁰. In the case of cobalt, the most active oxide is spinel Co3O4 in which cobalt is present in two valence states (+2 and +3). Above 1173 K, Co₃O₄ spontaneously loses an oxygen atom to give CoO. Co₃O₄ is an ideal spinel structure in which one-eighth of the tetrahedral sites are occupied by Co²⁺ cations while half of the octahedral sites are occupied by Co³⁺ cations⁵³. However, deviation from the structure of the spinel oxides is often observed^{65, 66}. Activity values confirm that the cobalt oxide is a very active catalyst for CO oxidation. The highest turnover frequencies over noble metals amount to 1 – 10 s^-1 at 523 K, that is, 22-220 µmol_CO m^-2 s^-1. The intrinsic activity of cobalt oxide is very close to that of noble metals⁴⁹. Lin et al.,⁶⁷ have found the following ranking of activity (mean oxidation state in parentheses):

CoO (⁺²) ≈ Co3O4 (^+8/3) » CoO(OH) (⁺³) > CoOx (^>+3)

The coexistence of Co²⁺-Co³⁺ ion pairs seems to be essential for the catalytic activity^{67, 68}. Xie et al.,⁵² investigated oxidation reactions over ComOn clusters (m=3–9, n=3–13) in a fast flow reactor. Significant changes of activity were observed

depending on the value of m. This was explained by the specific sites for O adsorption (Co²⁺-O-Co³⁺) and for CO adsorption (Co²⁺-CO), which requires a good balance between Co²⁺ and Co³⁺ species for an optimum activity in oxidation.

Manganese oxides have also been reported for a low-temperature CO oxidation activity^{28, 31-33}; however, no such site requirement has ever been claimed to exist for these oxides. This is probably due to the structural flexibility of Mn oxides which exist in a number of different stoichiometric (as MnO, Mn2O3, Mn3O4 Mn5O8 and MnO2 along with their polymorphs) and non-stoichiometric phases, with the Mn valence varying smoothly between +4, as in MnO2, to +2, as in MnO⁴⁰.

The transformation temperature for manganese oxides in air has been defined in the following manner^{69, 70}:

MnO2 808K→ Mn2O3 →1206K Mn3O4 →1433K MnO

Higher temperature and lower oxygen partial pressures favour manganese oxide phases with higher manganese to oxygen ratios. Amankwah et al.,⁷¹ have presented the equilibrium composition of the phases in the Manganese-Air system calculated as a function of temperature. This theoretical technique uses the Gibbs free energy minimization method. The results are shown in Figure 3.2.

Figure 3.2. Equilibrium composition in the Mn-Air system

It can be seen that under a given set of conditions there is an equilibrium mixture of the various manganese oxides. Starting at room temperature, the amount of MnO₂ continuously decreases; on the other hand, Mn₂O₃ begins to form at about 480 K and reaches a maximum value of about 60 mol% at about 930 K. The formation of Mn3O4 occurs at about 620 K.

It should be noted that in a real heating process, the equilibrium may not likely be achieved and therefore the actual phase composition is much depend on kinetic factors, such as a heating ramp, surface area, processing time, than thermodynamic considerations.

Most of the studies carried out involving CO oxidation were performed with stoichiometric Mn oxides. In an early work, Klier et al.,⁷² and Kanungo et al.,⁷³ found high catalytic activity over bulk MnO2 at 293 K and 353 K, respectively. Liang et al.,⁷⁴ synthesized MnO₂ catalysts with nanorod morphology and found their catalytic activity to change according to α- ≅ δ- > γ- > β-MnO2. The variation of the catalytic activity with the polymorphism of the sample at otherwise identical bulk composition can only be explained by assuming that the specific surface termination and, consequently, the Mn-O bond strength are the determining factors. For commercially available manganese oxides the following trend of CO oxidation activity (at 523 K) was reported by Ramesh et al.,⁷⁵ and Wang et al.,⁷⁶:

MnO ≤ MnO2 < Mn2O3

Cuprous oxide reacts with CO at temperatures which are substantially higher than those at which manganese dioxide is effective^{77, 78}. By itself, it is not an effective catalyst at ordinary temperatures. It may, however, greatly increase the activity of other oxides when used in mixtures of suitable proportion^{79, 80}.

Other oxides have been rarely used in CO oxidation. Copper oxides, iron oxides, ceria oxides, etc. are mainly used as additives to improve the activity of single metal oxides.

3.1.3 Catalytic activity of mixed oxide catalysts

Among the various mixed-oxide structures the most active ones are the mixtures of those oxides that exhibit a high activity in their pure state⁴⁹. For example, the so-called “hopcalites” prepared by mixing manganese oxides with copper oxide (60% MnO₂ and 40% CuO) or a larger number of oxides (50% MnO₂, 30% CuO, 15%

CoO₃ and 5% Ag₂O₃) have found wide application in practice^{28, 81}. Many other mixtures and compounds of spinel and perovskites have also been studied. Although some of the chromites and ferrites exhibit notable activities, the temperatures needed are usually higher than those required by reactions catalysed by manganese dioxide.

Alumina-supported copper oxide, catalysts based on copper chromite and on some manganites and ferrites have also found practical application⁵⁶.

Oxide perovskites (general structure ABO3, A and B being two cations and O an oxygen anion) represent probably the most studied mixed system for catalytic CO

oxidation⁸². The first studies notifying the properties of perovskite-based solids for CO oxidation were reported in the mid-1970s ^{49, 83-86}. Libby⁸⁷ suggested the replacement of noble metal by perovskite as automotive exhaust catalyst. One of the advantages of the perovskite structure is the possibility to prepare a wide number of different compositions of solids, by changing either the A cation or the B cation, or by partially substituting each cation by cations of the same or different valence to finely tune the redox and surface properties of the solids.

Voorhoeve et al.,⁸³ reported the activity of more than 8 different compositions of perovskite based either on manganese or cobalt, the A cation being a rare earth, substituted or not by lead [(La,Pr)CoO₃ or (La,Pr,Nd)_1-xPb_xMnO₃]. They proposed some of these structures (PrCoO₃ and Nd_1-xPb_xMnO₃) as substituents to Pt in exhaust after- treatment catalysts, especially due to the stability of these catalysts in reaction (0.5%

Pt/Al₂O₃, which is more active but suffers from significantly rapid deactivation).

Nevertheless, temperatures of 5%, 10%, and 20% CO conversion (T₅, T₁₀, T₂₀, respectively) measured for the perovskites remain low compared to those of Pt/Al2O3. A value of T₅ =353 K was reported for Pt/Al₂O₃ whereas T₅ = 433 K for PrCoO₃ (reaction conditions: CO/O₂ molar ratio=2:1; 30 mL min^-1; ca. 2 cm³ of catalyst)⁴⁹.

Conventional parameters such as surface area and nature of cations are generally not sufficient to explain the catalytic behavior of perovskites. Structural defects and oxygen mobility (in relation with the Mars–van Krevelen mechanism) are also important factors controlling the catalytic activity.

Other mixed oxides, such as spinels (AB₂O₄), were also studied for CO oxidation. However, they are generally less active than perovskites or single oxides.

For instance, CuCr2O4 material has a specific activity (1% O2 and 1% CO) of 5 molCO2

min^-1 m^-2, which is much higher than the specific activity obtained for the FeCr₂O₄ (only 0.33 molCO2 min^-1 m^-2)^{28, 49, 88}. The specific activity of the CuCr2O4 materials is of the same range of orders as over perovskites (LaCoO3, BaCoO3, LaMnO3), but remains lower than that obtained for the CuO (11 mol_CO2 min^-1 m^-2) and Co₃O₄ (25 – 5 mol_CO2 min^-1 m^-2) classical single oxides⁸⁹. Pirogova et al.,⁹⁰ reported the activity in CO oxidation (5–6% CO in air, GHSV=600 h^-1) of more than ten different spinel-type mixed oxides (AB₂O₄ where A=Cu, Ni, Mn, Zn, Mg, Co; B=Co, Cr, Al). It was clearly observed that the most active materials were those containing two transition metals (in A and in B position). The cobaltite series is then more active than the chromite series, which is also more active than the aluminate series.

The main drawback in using perovskites and spinels is related to the high temperatures needed to transform the precursors into the desired structures. The so- formed structures will in turn possess very low surface areas (due to sintering).

Fluorite-type oxides, commonly comprising Cu with CeO₂, and ZrO₂ are known for their high oxygen mobility and high numbers of oxygen vacancies^91-94. However, the fluorite oxides have traditionally not been considered as active oxidation catalysts at moderate reaction temperature. They are mainly used as catalyst additives.

Notably, CeO2 has been used as a thermal stabilizer and oxygen storage medium^{95, 96}.

3.1.4 Kinetics and mechanism of CO oxidation over transition metal oxides Heterogeneous oxidation of carbon monoxide in the presence of an oxide catalyst occurs via one of two general mechanisms⁵⁶:

1) The oxygen and carbon monoxide are chemisorbed on the oxide surface.

Reaction occurs in the chemisorbed layer and CO₂ is produced. The carbon dioxide is then desorbed and the process repeated.

2) Carbon monoxide is chemisorbed and reacts with the surface / lattice oxygen to generate CO2 which desorbs from the surface. A partially reduced surface is observed following CO₂ desorption. Finally, gas phase oxygen is used to re- oxidise the sample and a new cycle can restart

At temperatures of about 300 K, a mechanism involving a CO₃-like complex is stated⁴⁵. First, both CO and O2 are adsorbed over the oxide surface followed by surface reaction to produce carbonate species:

CO + * ¹

1

k k−

← →

O2 + 2 * ²

2

k k−

← →

CO* + O2* →^k3 CO3* + * CO₃* + CO* →^k4 2CO₂ + 2 *

According to this scheme, the rate expression according to a Langmuir- Hinshelwood mechanism can be developed. The rate expression can be defined as:

2 2 1

2 1 3

) 1

(

2

O CO

O CO

p K p

K

p p K K r k

+

= +

where * is a vacant site r is the reaction rate

k3 is the rate constant (here, limiting rate constant of reaction) K₁ is the equilibrium constant for CO adsorption/desorption