CAMPINAS 2020
UNIVERSIDADE ESTADUAL DE CAMPINAS INSTITUTO DE QUÍMICA
RAQUEL CRISTINA AFFONSO
STUDY OF THE ELECTROOXIDATION OF 2-BUTANOL IN PLATINUM SURFACES
ESTUDO DA ELETRO-OXIDAÇÃO DE 2-BUTANOL EM SUPERFÍCIES DE PLATINA
RAQUEL CRISTINA AFFONSO
STUDY OF THE ELECTROOXIDATION OF 2-BUTANOL IN PLATINUM SURFACES
ESTUDO DA ELETRO-OXIDAÇÃO DE 2-BUTANOL EM SUPERFÍCIES DE PLATINA
Dissertação de Mestrado apresentada ao Instituto de Química da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Mestra em Química na área de Físico-Química.
Master's Dissertation presented to the Institute of Chemistry of the State University of Campinas as part of the requirements to obtain the title of Master in Chemistry in the area of Physical Chemistry.
Supervisor: Dr. Pablo Sebastian Fernandez
O arquivo digital corresponde à versão final da Dissertação defendida pela aluna Raquel Cristina Affonso e orientada pelo Prof. Dr. Pablo Sebastian Fernandez.
Ficha catalográfica
Universidade Estadual de Campinas Biblioteca do Instituto de Química
Informações para Biblioteca Digital
Título em outro idioma: Estudo da eletro-oxidação de 2-butanol em catalisadores de platina
Palavras-chave em inglês: Electrooxidation
Platinum catalysts Butanol
Área de concentração: Físico-Química
Titulação: Mestra em Química na área de Físico-Química Banca examinadora:
Pablo Sebastian Fernandez [Orientador] Emilio Carlos de Lucca Júnior
Giuseppe Abíola Câmara da Silva Data de defesa: 28-02-2020
Programa de Pós-Graduação: Química Identificação e informações acadêmicas do(a) aluno(a) - ORCID do autor: https://orcid.org/0000-0002-1581-6649
- Currículo Lattes do autor: http://lattes.cnpq.br/1745321814426163
Simone Luiz Alves - CRB 8/9094
Affonso, Raquel Cristina, 1989-
Af28s Aff Study of the electrooxidation of 2-butanol in platinum surfaces / Raquel Cristina Affonso. – Campinas, SP : [s.n.], 2020.
Aff Orientador: Pablo Sebastian Fernandez.
Aff Dissertação (mestrado) – Universidade Estadual de Campinas, Instituto de Química.
Aff 1 . Eletro-oxidação. 2. Catalisadores de platina. 3. Butanol. I. Fernandez, Pablo Sebastian, 1983-. II. Universidade Estadual de Campinas. Instituto de Química. III. Título.
BANCA EXAMINADORA
Prof. Dr. Pablo Sebastian Fernandez (Orientador)
Prof. Dr. Emilio Carlos de Lucca Júnior (Universidade Estadual de Campinas)
Prof. Dr. Giuseppe Abíola Câmara da Silva (Universidade Federal de Mato Grosso do Sul)
A Ata da defesa assinada pelos membros da Comissão Examinadora, consta no SIGA/Sistema de Fluxo de Dissertação/Tese e na Secretaria do Programa da Unidade.
Este exemplar corresponde à redação final da Dissertação de Mestrado
defendida pela aluna RAQUEL
CRISTINA AFFONSO, aprovada pela
Comissão Julgadora em 28 de fevereiro de 2020.
Epigrapher
“How would you like to live in Looking glass House, Kitty? I wonder if they’d give
you milk in there? Perhaps looking-glass milk isn’t good to drink…”
Lewis Carroll, Through the Looking-Glass and What Alice Found There.
Agradecimentos
Antes mesmo de iniciar o mestrado, contei com o apoio de pessoas muito queridas. Aos meus pais, Vera e Dirceu, obrigada pelo apoio incondicional, por acreditarem tanto em mim, por todo cuidado e carinho que se intensificaram nesses dois últimos anos. Com vocês eu aprendo todos os dias a ser mais perseverante e a nunca deixar de sonhar. Aos meus irmãos, Rodrigo e Aline, por estarem sempre ao meu lado, pelas novas ideias e planos.
Aos meus amigos (em ordem alfabética, para não causar discórdia) Ana T., Bruna T., João S., Magda e Nahyan por, apesar da falta de tempo, estarem presentes no meu dia a dia. Vocês me fazem sempre lembrar o quão gratificante e único é ensinar. À Marília, que parece ainda morar pertinho de mim, apesar da distância e do passar dos anos. Obrigada pelas conversas, por sempre me ouvir e, mais importante do que isso, por me deixar ser titia da Cora.
Ao professor Pablo pela oportunidade em participar de seu grupo de pesquisa e, tão valoroso quanto isso, por todo ensinamento, ajuda e paciência. Obrigada por realmente se importar com os alunos – tenha certeza que isso faz muita diferença na vida de cada um de nós.
Ao Victor, por me acompanhar nos primeiros dias (e nos intermediários, e nos finais) e me ensinar muito do que sei sobre a parte experimental. Ao Matheus, por sempre me ajudar quando eu não sabia o que estava acontecendo. À Marta, pelos nossos longos dias de estudo e conversas sobre a vida. À Hamsa, que em tão pouco tempo me fez conhecer um pouquinho de um universo tão incrível. A Helen e Isabela, por todo auxílio nesses últimos meses. Aos demais amigos e colegas do CampEG, por toda vivência. Que a colaboração e empatia façam sempre parte do nosso convívio.
Aos funcionários do Instituto de Química cujo suporte, a cada dia, é indispensável.
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.
Resumo
O modo no qual os átomos estão organizados na superfície de um catalisador exerce influência direta na cinética e no mecanismo de reações químicas. Isso ocorre devido às diferentes energias de adsorção de uma mesma molécula nessas faces catalíticas. Em um fio metálico comum, os átomos estão organizados de variadas formas por toda sua extensão, sendo o resultado obtido para uma reação através dessa superfície, dita policristalina, equivalente à contribuição de todos os diferentes domínios presentes. O uso de monocristais ajuda a entender a relação entre os arranjos dos átomos na superfície de um catalisador e sua atividade, pois sua ordenação atômica é repetida periodicamente por toda a superfície.
Neste trabalho, catalisadores poli e monocristalinos de platina foram utilizados na reação de eletro-oxidação do 2-butanol em soluções de ácido sulfúrico e ácido perclórico. A platina foi escolhida em consequência de sua reatividade frente à oxidação de álcoois. Os monocristais usados possuem índices de Miller (100), (110), (111), (211), (431) e (531). Para realizar um estudo fundamental e manter o sistema o mais simples possível, o 2-butanol foi utilizado como reagente, já que é o menor álcool quiral comercialmente disponível. A compreensão dos aspectos fundamentais da interação entre esse álcool, os íons presentes na solução eletrolítica e os diferentes catalisadores de platina, é o primeiro passo para o desenvolvimento de sistemas mais favoráveis à eletrocatálise e eletrossíntese assimétrica.
Os experimentos mostraram que Pt(111) é a superfície mais ativa para a oxidação do álcool. Experimentos em H2SO4 e HClO4 mostraram forte influência do
ânion, sendo a reação mais rápida na presença de HClO4. Medidas de FTIR in situ
mostraram que butanona e CO2 são os produtos da reação, sendo o primeiro, o produto formado em maior quantidade, independentemente do catalisador utilizado. Finalmente, a reação em monocristais não quirais mostrou que os enantiômeros mais puros de álcoois disponíveis no mercado não possuem a pureza necessária para este estudo.
Abstract
The way that atoms are organized on the surface of a catalyst has a direct influence on the kinetics and mechanism of chemical reactions. This influence is due to the differences in adsorption energy of the same molecule on the different catalytic surfaces. In a common metallic wire, the atoms are organized in different ways throughout its surface (polycrystalline surface), and the result obtained for a reaction, is due to the contribution of all the different atomic orientations present. The use of single crystals helps understanding the relation between the arrangements of the atoms at the surface of a catalyst and its activity, as its atomic ordering is repeated periodically throughout the surface.
In this work, poly and monocrystalline platinum catalysts were used for the electrooxidation of 2-butanol, in solutions of sulfuric acid and perchloric acid. Platinum was chosen due to its reactivity for the oxidation of alcohols. Single crystals with Miller indices (100), (110), (111), (211), (431) and (531) were employed in the present work. In order to perform a fundamental study and keep the systems as simple as possible, we use 2-butanol as reactant, because it is the smallest chiral alcohol commercially available. The understanding of fundamental aspects of the interaction between this alcohol, the ions present in the electrolyte solution and the different platinum catalysts, is the first step towards the development of systems more favorable to electrocatalysis and asymmetric electro-synthesis.
We found Pt(111) is the most active surface for the oxidation of the alcohol. Experiments in H2SO4 and HClO4 showed a strong influence of the anion, being the reaction faster in the presence of HClO4. FTIR in situ measurements showed that bu-tanone and CO2 are the products of the reaction, the first being the product formed in greater quantity, regardless of the catalyst used. Finally, the reaction in non-chiral sin-gle crystals showed that the purest alcohols enantiomers available in the market do not have a required purity for this study.
Summary
1. Introduction ... 10
1.1. The choice of molecule for this work ... 10
1.2. Chirality and its importance ... 13
1.3. Single crystals ... 17
2. Objectives ... 22
3. Experimental Procedure ... 23
3.1. The electrochemical cell ... 23
3.2. The electrochemical preparation and characterization of electrodes ... 25
3.3. In situ Fourier Transform Infrared Spectroscopy experiments ... 27
4. Chemicals ... 29
5. Voltammetric profile of platinum electrodes ... 30
6. Electrooxidation of 2-butanol in non-chiral single crystals ... 34
6.1. Electrooxidation of racemic 2-butanol ... 34
6.2. In situ Fourier Transform Infrared Spectroscopy Analysis ... 36
7. Electrooxidation of 2-butanol in chiral single crystals ... 41
8. Conclusions and Perspectives ... 44
9. Support Information ... 45
9.1. Electrooxidation of 2-butanol enantiomers ... 45
1. Introduction
1.1. The choice of molecule for this work
The electrooxidation of alcohols is widely studied due to the possibility of producing compounds with higher added value and even their use in fuel cells. Even for simple alcohols such as methanol, the reaction mechanism is complex, and may involve different intermediate species, products and by-products[1]. Overall, the electrooxidation of alcohols involve the alcohol adsorption, breaking of the inter-atomic bonds, electronic charge transfer, reaction between oxygenated species and fragments from the alcohol, and reaction products desorption[2]. In 2-butanol structure, the carbon linked to the hydroxyl is bonded to three other different groups, i.e., a hydrogen atom, an ethyl, and a methyl group (Figure 1), making this molecule the smallest commercial alcohol that presents chirality.
C H3
CH3 OH
*
Figure 1. Structure of 2-butanol. It is the smallest alcohol that have a stereogenic
center (identified with an asterisk) and presents chirality.
The electrooxidation of 2-butanol (EO2B), a secondary alcohol, can generate as a product a ketone (in this case, butanone), an aldehyde, a carboxylic acid, a polyol, CO or CO2 (Figure 2). To our knowledge, there are few studies in the literature regarding specifically the 2-butanol electrooxidation reaction (EO2B)[1-6] but none of them with the same catalysts used in this study.
Raicheva et. al.[1] analyzed the influence of the carbon chain and the hydroxyl position in different primary, secondary and tertiary alcohols (this last one can be oxidized only by breaking the carbon chain). In addition, they investigated the effect of temperature in the EO2B in sulfuric acid using a polycrystalline platinum electrode (concept that will be discussed in more detail in section 1.3 - Single Crystals). They showed that the increase in the temperature favored the reaction, which is expected,
but in complex reactions there are examples of reactions with apparent negative activation energies[3-5].
Takky et. al.[3-5] extensively explored the reaction in an alkaline media, comparing the responses obtained for 2-butanol in different platinum catalysts (differing only in the organization of atoms at the surface of the catalyst, as will be explained in more detail in subtopic 1.3) and experimental conditions (temperature, electrolyte, alcohol concentration, scan rate and potential limit). They showed that in NaOH solution the catalyst is tolerant to poisoning. More interestingly, at temperatures above 45°C, they measured a negative activation energy.
OH CH3 C H3 C H3 CH3 O OH CO C O O C H3 CH3 O C H3 CH3 OH OH H3C CH3 O O 2-butanol
Figure 2. Some products that can be produced by the EO2B. The choice is rather
arbitrary, as the only aim of the figure is to show that the molecule can generate products with diverse combinations of carbonyl, hydroxyl and acidic groups. There are many other C3 and C2 products that could have be drawn here.
The interaction of organic molecules with Pt can generate chemisorbed intermediates with different structures. It has been proposed that the key intermediate for the equilibrium alcohol-ketone (Equation 1) is formed by binding the carbon linked to the hydroxyl group (through the dehydrogenation) to the Pt surface (Figure 3)[7].
𝐶4𝐻10𝑂 ⇆ 𝐶4𝐻8𝑂 + 2𝐻++ 2𝑒− (1)
These kinds of molecules are also interesting because their high energy density, thus, the complete oxidation of 2-butanol can generate 24𝑒− for every molecule of the alcohol oxidized (Equation 2). However, it is an extremely difficult reaction involving a myriad of unknown intermediates.
𝐶4𝐻10𝑂 ⇆ 𝐶𝑂2+ 24𝐻++ 24𝑒− (2)
Figure 3. The scheme shows the mechanism for 2-butanone and carbon dioxide,
products that can be obtained in the EO2B. 2-butanol interacts by the carbon linked to the hydroxyl with the platinum surface.
1.2. Chirality and its importance
The concept of chirality was stated by Louis Pasteur, in 1847, in his crystal-lography and optical activity studies of natural tartaric acid and its derivatives, the tar-trates[8]. Besides being a remarkable scientist, Pasteur is also a gifted artist. To do his artworks, he used the lithograph technique, a method in which firstly an image is drawn in a surface and, then transferred to a paper, producing an image that is equivalent to the mirror image of the original drawing. Probably, the familiarity with the effects of mirror inversion helped him with this important finding. While studying the crystals of sodium ammonium paratartrate he recognized that there were two different sorts, one being the mirror image of the other.
Any structure which does not have symmetry elements type 2 i.e., a plane σ=S1, an inversion center i=S2 or a rotation-reflection axis, S2n, is chiral and can exist as two forms related as mirror-image. These forms are called enantiomers and cannot be superimposed[9]. A didactic example is the relationship between the hands (Figure 4). Although anatomically similar, there is no operation that makes the right hand to overlap the left hand. One is the mirror image of the other, then they are enantiomers.
Figure 4. The hands are mirror images of each other and cannot be superimposed, so
they can call enantiomers.
Molecules containing one carbon atom connected with four different groups will not have a plane of symmetry and must therefore be chiral. This atom is labeled as stereogenic or chiral center, such as the alpha carbon of alanine (Figure 5), one of the twenty natural amino acids (the building blocks of proteins). From all the natural
amino acids, which are synthesized by the body, only one is not chiral, which shows the importance of this property in the reactions that occur in living beings[10].
Figure 5. Alanine molecular structure. The chiral carbon is bounded to a N, a methyl
group, a carboxylic acid and a hydrogen (above). The image below shows the structure of the (R)-enantiomer (left) and of the (S)-enantiomer. The carboxylic acid and the methyl group are behind the plane of the sheet in both molecules. If we rotate one of the molecules to align the 3 carbon atoms of the chain, the rotated molecule will end with the methyl and the carboxylic acid in front of the sheet not matching the corresponding groups of its enantiomer as expected.
Enantiomers are stereoisomers, in other words, they are isomers that have the same parts linked together, but in a different way. Stereoisomers are different molecules, because it is not possible to interconvert them without breaking a bond. The letters R and S are used to describe the absolute configuration of groups at chiral center and, for this, it is needed to follow the Cahn-Ingold-Prelog rule. First, it is necessary to assign a priority number to each group at the stereogenic center: atoms with higher atomic mass receive higher priority. After this, the molecule is arranged in order to put the lowest priority substituent pointing to the paper. If the groups are arranged in a clockwise manner, the chiral center get the label R; if they are in the anticlockwise, they are called S.
Almost all the chiral molecules produced by the organisms are found as a single enantiomer[10]. For instance, while plants generate only (S)-alanine, the (R)-ala-nine can be found only in bacterial cells. The enantiomers have the same physical and chemical properties, such as NMR and IR spectra, boiling point and density [10], but
they differ only in the rotation of a plane-polarized light. While an enantiomer rotates the plane-polarized light in one direction, the other does it in the opposite. Another way to classify an enantiomer, is looking at the direction that each one rotates the plane-polarized light, which has not correlation with the molecule being R or S, the molecule that rotates to the right is the (+)-enantiomer (or dextrorotatory). On the other hand, if the light is rotated to the left, the isomer is labeled (-)-enantiomer (or levorotatory).
Enzymes present in living organisms are sensitive to chirality, i.e., they in-teract in different ways with chiral molecules, whether they are synthesized by the body or ingested, for example, in the form of medicines [10]. Some drugs are chiral and usu-ally only one specular form provides the desired effect, the other being inactive or even toxic, as it is the case with thalidomide (Figure 6) enantiomers. While the (R)-enantio-mer has analgesic and sedative properties, the (S)-enantio(R)-enantio-mer causes mutations in the fetus[9]. N O O N H O O *
Figure 6. Thalidomide has one chiral carbon (identified with an asterisk) and occur in
two enantiomeric forms, which react by different ways with the human body. In this case, these molecules can interconvert in the organism, thus is not effective to make the separation of the enantiomers.
It is important for the pharmaceutical and medical fields[11,12] to study the chiral specificity. Compared to traditional methods for the preparation of organic mole-cules, more advanced techniques to promote asymmetric catalysis (a process in which the formation of one of the stereoisomers is favored) are needed. In this context, chiral catalysts are used for the selective preparation of a desired enantiomer both, in homo-geneous and heterohomo-geneous catalysis[13]. These catalysts can be obtained by modify-ing a surface by bindmodify-ing a chiral molecule[14,15] or simply using an intrinsically chiral surface[6,16,17] (which will be explained in more detail in the next topic). Künle et. al.[14], through the adsorption of the cysteine enantiomers on a non-chiral gold surface (Miller index (110)) showed, using scanning tunneling microscopy (STM), that this process
transforms the surface in a chiral surface. Overall, the adsorption of a chiral molecule in an achiral surface, turn it chiral.
1.3. Single crystals
There are different ways to increase the rate of a reaction. Several times, it can be done by raising the temperature of the system (causing an increase in the ki-netic energy of the molecules), increasing the concentration of reactants (which in-creases the number of effective collisions) or adding a catalyst, a substance or a ma-terial that participates in the reaction, but it is not consumed by it[18]
All chemical reactions have a specific activation energy (Ea), which is the minimum energy necessary for the reaction to occur. A catalyst accelerates the reac-tion by providing an alternative pathway between the reactants and products, with a smaller Ea (Figure 7). Therefore, a greater amount of reactant molecules can be con-verted into products with the same energy in presence of a catalyst.
Figure 7. A catalyst promotes an increase in the activation energy of the reaction.
Electrochemical reactions occur at the electrified interphase between the electrode and the solution. As a result, its properties dictate the kinetics of the electro-catalytic reactions[18]. The electronic distribution at the surface of the catalyst depends on its nature, on the adsorbates (electrolyte, reactants, intermediates and products) and on the atom’s symmetry at its surface. Thus, the same material (for instance, Pt) with different atoms ordering at its surface, will interact in different way with the mole-cules present in the electric double layer. Going more into details, the structure of the
double layer will change with the atom’s symmetry of the catalyst[19-21]. In any real ma-terial, the atoms at the surface are randomly arranged, and different symmetries can be found. Thus, the result for any process occurring on the material contains responses coming from all sites with different symmetries, with distinct kinetics and selectivity. For a fundamental study of the relationship between reactants, intermediates and catalyst surface, a common approach is to use single crystals surfaces (Figure 8).
Figure 8. Flats and stepped surfaces with Miller index (100), (110), (111) and (211).
Highlighted triangles show adatoms with (111) symmetry, the black rectangles those with (100) configuration and the red rectangles the (110).
In a single crystal, the surface is formed by a single crystallographic orientation, with a well-known atomic ordering, allowing to establish connections between the experimental results with the geometry of the active sites. Figure 8 shows the periodic arrangement of the atoms at the surface of different single crystals of a face-centered cubic material. Each orientation is ascribed a set of three numbers, called Miller indices[22], which are represented between parenthesis. The (111) surface is flat, and it is the orientation which maximizes the coordination of each atom, as each atom is surrounded by 6 other atoms on the plane and by 3 in the next layer. The (100) surface is also flat, and each atom is in contact with other 4 atoms on the same plane.
The (110) surface, however, has atoms in two different levels, and they have 2 neighboring atoms on the same plane. In contrast, the (211) surface is formed by a regular arrangement of terraces with (111) symmetry and steps with (100) symmetry. All these surfaces are non-chiral, as the mirror image of any of these surfaces will render a superimposable image.
Surfaces with (431) and (531) orientations meet the necessary conditions to be chiral, whereas one side of the kink site is greater than the other. Figure 9 shows that both have terraces and steps with kinks and that the surfaces are not superimpos-able to the corresponding mirror images, characterizing them as enantiomers. As for an organic molecule, there is also a rule for classifying the single crystals as R or S, based on the priority criterion (111) > (100) > (110)[16]. Thus, a surface in which this order is clockwise is classified as R and anticlockwise as S. The relationship of the two surfaces and their classification (R or S) was manifested by the direction of the splitting of the low-energy electron diffraction (LEED) spots[17].
Figure 9. Surfaces with (431) and (531) orientations, and their corresponding mirror
images. If one of the structures is turned to align the steps, the kinks will point in opposite directions, showing the impossibility to superimpose the chiral structures, as expected. The highlighted regions show the structures with different symmetries in each kink (which are repeated periodically), and the curved arrows follow the priority order that allows determination of the configuration of each surface.
In 1996, the first article about the interaction of chiral molecules in chiral single crystals was published by Gellman's group[6], the work that inspired the present
study. Knowing the chirality of kinked single crystals, they investigated the enantio-meric differentiation of (R)-2-butanol and (S)-2-butanol in Ag(643)R and Ag(643)S. Fig-ure 10 shows a representative scheme of the interaction of these molecules in a chiral surface. The interaction of the 2-butanol enantiomers with each surface was studied employing temperature-programmed desorption (TPD) measurements in order to de-termine the effect of surface chirality on the heats of desorption. The desorption of both alcohol enantiomers from the clean surfaces and the decomposition of (R)-2-butanox-ide and (S)-2-butanox(R)-2-butanox-ide by β-hydride elimination on the pre-oxidized surfaces were investigated. A thorough analysis of the peak desorption temperature and its shape, showed no difference, indicating that the chirality effect on the activation barrier of these reactions was less than 0.1kcal.mol-1.
Figure 10. Enantiomers of 2-butanol interacting in a kink site of a chiral surface.
(R)-2-butanol is interacting with the methyl group with the terrace, the oxygen is in front of the step with symmetry (100) (yellow and orange atom) and the ethyl group with the (110) steps. On the other hand, when the S-enantiomer is positioned in front of a kink with the same symmetry, the methyl group interact again with the terrace, but, in this case, the oxygen is pointing towards the step with symmetry (110) and the CH3 towards the (100). (H=white, C=light blue and O=red).
2. Objectives
- To electrochemically characterize chiral single crystals.
- To study the influence of the crystallographic orientation of different platinum catalysts (chiral and non-chiral) on the electrooxidation reaction of 2-butanol.
3. Experimental Procedure
3.1. The electrochemical cell
The electrochemical experiments were performed in conventional three-electrode cells (Figure 11). A hydrogen three-electrode was used as a reference three-electrode (RHE) and a high area polycrystalline platinum wire as a counter electrode (CE). Platinum single crystals were used as working electrodes (WE), with surfaces with Miller indices (100), (110), (111), (211), (431) and (531), the latter two being chiral (R and S configurations). The cells were purged with N2 before and throughout the process, as the presence of atmospheric air may disrupt and/or react with the surfaces of the single crystals or contaminate the solution.
Figure 11. (A) Conventional three-electrode cell, where are the platinum CE on the left
and the WE on the right and (B) reversible hydrogen electrode (RHE).
Two electrochemical cells were used, one for checking the ordering of atoms on the catalytic surface before the electrooxidation reaction and the other for EO2B. Both procedures were performed at room temperature (25 ± 1°C), using the cyclic voltammetry technique, in which a linear triangular potential scan is applied to the working electrode (catalyst) and the generated electric current is measured[23]. The potential between WE and RE was controlled by an Autolab potentiostat, model
PGSTAT101. The same device measures the current flowing through the electrical circuit as a function of electrochemical potential. EO2B was performed in triplicate in order to guarantee the reproducibility of the data.
3.2. The electrochemical preparation and characterization of electrodes
Chiral single crystals have never been commercially available and, at this moment (at least to our knowledge), any single crystal (both chiral and non-chiral) bead can be bought. All the electrodes used in this work were prepared by our collaborator, Prof. Juan Feliu by the method described elsewhere[24]
. To work with well-ordered single crystals surfaces, it is important to follow a strict procedure for its cleaning/ordering and to the subsequent electrochemical characterization.
Prior to any electrochemical experiment, the cell was let overnight in a KMnO4 solution to remove possible organic compounds present in the cells, which directly interferes in the electrochemical measurements. Then, it was washed with hydrogen peroxide and sulfuric acid solution, in order to remove the residual KMnO4. After that, the cell was washed and boiled with Milli-Q water three times, to completely remove organic substances and the chemicals used for the cleaning.
To organize the atoms on the surface of the single crystal, the metal was heated with a flame generated by the butane gas combustion for 5 seconds after it became incandescent. It was then placed in the atmosphere of a cooling system (Figure 12) containing a reducing atmosphere of H2 and Ar in a 1: 2 ratio. After 25 seconds, the electrode was immersed in the cooling liquid consisting of Milli-Q water (18.2 mΩ.cm) in equilibrium with the gases. A drop of this liquid was left on the catalytic surface to protect it from the contact with the atmosphere of the laboratory and thus enable the transfer to the electrochemical cell[25].
Figure 12. (A) The Platinum single crystal electrode is heated using a butane flame,
and then placed in the atmosphere of a cooling system (B). Subsequently, it is put in contact with the ultrapure water of the cooling system and then retired with a drop in its surface to be transported to the electrochemical cell, as shown in figure (C).
The calculation of the current density (j) recorded with the electrodes (211), (431) and (531) was obtained by dividing the current obtained from the cyclic voltammetry by the geometric area of each electrode. The areas were calculated using the software ImageJ®, an image analysis program, in which from an electrode surface image it is possible to calculate its surface geometric area. For polycrystalline (Ptp), (100), (110) and (111), the surface area was measured by the hydrogen underpotential desorption method[26]. Table 1 shows the areas of each electrode.
Table 1. Surface areas of the working electrodes.
Working electrode Surface area (cm2) Working electrode Surface area (cm2)
Ptp 0,070 Pt(431) R 0,040
Pt(100) 0,040 Pt(431) S 0,037
Pt(110) 0,044 Pt(531) R 0,036
Pt(111) 0,042 Pt(531) S 0,032
3.3. In situ Fourier Transform Infrared Spectroscopy experiments
In situ Fourier transform infrared (FTIR) experiments were performed using
a Shimadzu IR Prestige-21 spectrometer with MCT detector and a electrochemical cell equipped with a CaF2 window placed on top of a specular reflection accessory (Veemax II, Pike Technologies). The spectra were collected with an average of 256 interferograms and 4cm−1 resolution and are presented in absorbance units (a.u.) as –log(R/Ro), where R and Ro represent, respectively, the reflectivity at the sample and reference potentials. Positive bands correspond to gain at the sample potential with respect to the reference potential, and the negative bands, to the loss of it.
In the electrochemical cell (Figure 13), a RHE was used as a reference electrode, a Ptp as a CE and a single crystal as WE. The measurements were performed in the transmission mode. Thus, the single crystal electrode was gently pressed onto the CaF2 windows in order to form a thin layer of electrolyte between the electrode and the windows in order to minimize the absorption of the IR beam by the solution. More technical information about FTIR in situ can be found else were[27]..The EO2B was performed using Pt(100), Pt(110) and Pt(111) as catalysts. The catalysts were prepared as described in 3.2. The electrochemical preparation and
characterization of electrodes. In situ FTIR spectra were obtained during linear sweep
voltammetry, from 0.20V to 0.95V, in solution of 0.1M 2-butanol (racemic) + 0.1M HClO4. EO2B was performed in duplicate.
Figure 13. Conventional three-electrode cell used in FTIR experiments, reference
4. Chemicals
The experiments were performed using the following chemicals: sulfuric acid (ISO, Emsure®), perchloric acid (Merck), (R)-(-)-2-butanol ( ≥ 99%, Sigma-Aldrich), (S)-(+)-2-butanol ( ≥ 99%, Sigma-Sigma-Aldrich), 2-butanol (≥ 99,5%, Sigma-Aldrich) and deuterium oxide 99.9% (Sigma-Aldrich).
5. Voltammetric profile of platinum electrodes
In order to verify the cleanliness and ordering of atoms on the working electrode surface and thus ensure the quality of the data, before each EO2B, cyclic voltammetry experiments were performed in H2SO4 or HClO4 solution, according to the medium in which the electrooxidation of the alcohol will be performed, with a scan rate of 50mV.s-1 (Figures14-17). The profiles obtained were compared with the literature[28] in order to confirm the ordering and cleaning of the working electrodes. The voltammograms in sulfuric acid differ from those obtained in perchloric acid in the position and intensity of the peaks, because the sulfate and perchlorate ions present in the electrolyte interact differentlywith the surface of the catalysts[29]., i.e., sulfate and bisulfate anions adsorb stronger to Pt than perchlorate.
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 -200 -150 -100 -50 0 50 100 150 hydrogen adsorption j / A.cm -2 E / V vs. RHE H2SO4 HClO4 Ptp double layer hydrogen dessorption
Figure 14. Voltammetric profile of Ptp in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) with a scan rate of 50mV.s-1.
Voltammograms shows the presence of the electric double layer and the hydrogen regions[28]. The electric double layer is the region in which the current density is constant, since this phenomenon is characterized by the absence of electron transfer between the WE and the species in solution, i.e., it is a non-faradaic phenomenon. The hydrogen regions obtained from reversible proton adsorption/desorption constitute the
peaks of the voltammogram. Positives currents are related to the desorption of H from the electrode and negative currents to the opposite process (Equation 3).
Pt + H++ e−⇆ Pt − H (3)
As shown in Figures 8 and 9, the single crystals (211), (431) and (531) have regions with symmetries (100), (110) and (111). The potential at which each peak in the voltammogram occurs depends on the organization of the atoms on the surface. Thus, the profile obtained for an unknown surface can be compared with the response of single crystals to figure out the distribution of the different atoms arrangements at the surface or, as in our case, to check the correct preparation of our single crystals. Besides, our results, show the well-known influence of the anion in the energy of the H adsorption/desorption on Pt. Thus, in order to compare the response of a surface with that well-stablished for a given single crystal, it is important to perform the experiments in the same conditions (temperature, scan rate, etc.) and mandatory to do it in the same electrolyte.
Figure 15 shows the voltammetric profiles of Pt(100) and Pt(110) in H2SO4 (black line) and HClO4 (red line) solutions. Pt(100) has two H adsorption/desorption peaks, the first at 0.27V and the second at 0.38V. In HClO4, the peaks are located at 0.30V and 0.35V. Pt(110) has only one peak in H2SO4, at 0.14V. In HClO4, two peaks are obtained, one at 0.13V and the other at 0.30V. The symmetry of the hydrogen peaks indicates that this process is reversible.
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 -125 -100 -75 -50 -25 0 25 50 75 100 125 0,0 0,2 0,4 0,6 0,8 -125 -100 -75 -50 -25 0 25 50 75 100 125 j / A.cm -2 E / V vs. RHE H2SO4 HClO4 Pt (100) Pt (110) H2SO4 HClO4 j / A.cm -2 E / V vs. RHE
Figure 15. Voltammetric profile of Pt(100) and Pt(110) in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) with a scan rate of 50mV.s-1.
In H2SO4 solution, Pt(111) have peaks at 0.35V and 0.44V, and only one at 0.80V in HClO4. As seen in topic 1.3 - Single crystals, Pt(211) has steps with Miller indices (100) and terrace (111). In H2SO4, the peak at 0.27V coincides with the position of one of the Pt(100) peaks (Figure 16). In HClO4, the peak at approximately 0.30V corresponds to one of the Pt(100) peaks.
0,0 0,2 0,4 0,6 0,8 -80 -60 -40 -20 0 20 40 60 80 Pt (111) H2SO4 HClO4 j / A.cm -2 E / V vs. RHE 0,0 0,2 0,4 0,6 0,8 -100 -75 -50 -25 0 25 50 75 100 Pt (211) j / A.cm -2 E / V vs. RHE H2SO4 HClO4
Figure 16. Voltammetric profile of Pt(111) and (211) in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) with a scan rate of 50 mV.s-1.
Both Pt(431) and Pt(531) have terraces (111) and steps (100) and (110). The difference between these two configurations is the length of the steps (Figure 9). In Pt(431), the step with configuration (100) has two atoms and that with configuration (110), has three atoms. Thus, it is expected that in the voltammetric profile of Pt(431),
the peak corresponding to the (110) sites be more intense than that coming from (100) sites. In H2SO4 the peaks are approximately at 0.12V (more intense and matching that for (111) sites) and 0.26V (less intense and matching that for (100) sites) giving the expected result It is worth to mention that the characterization of these specific chiral electrodes is presented for the first time in this work (at least to our knowledge). Pt(531) have (100) and (110) step sites with two atoms. As expected the result is similar to that obtained with Pt(431), but in this case the signal coming from the (110) sites is less intense. 0,0 0,2 0,4 0,6 0,8 -100 -75 -50 -25 0 25 50 75 100 H2SO4 HClO4 j / A.cm -2 E / V vs. RHE Pt (431) 0,0 0,2 0,4 0,6 0,8 -60 -40 -20 0 20 40 j / A.cm -2 E / V vs. RHE Pt (531) H2SO4 HClO4
Figure 17. Voltammetric profile of working electrodes in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) with a scan rate of 50mV.s-1. Only one voltammetric profile is presented for each pair of chiral single crystals (i.e. (431)R, (431)S and (531)R, (531)S) because they are similar.
6. Electrooxidation of 2-butanol in non-chiral single crystals
6.1. Electrooxidation of racemic 2-butanol
Figure 18(A) shows the voltammogram obtained in the EO2B of the racemic sample, in a concentration of 0.1M alcohol + 0.1M HClO4. The current densities for Pt(100) and Pt(110) were multiplied by 2 to facilitate analysis of the results. Higher activity was obtained in Pt(111). The reaction starts at 0.28V and have the maximum current density (12mA.cm-2) at 0.61V. For Pt(110), the EO2B starts at 0.43V and reaches a maximum value (2.3mA.cm-2) at 0.71V. The electrode (100) is the least active. The oxidation profile has two peaks, the first being 0.37V corresponding to the hydrogen desorption process. EO2B starts at approximately 0.61V and provides a current density (j = 1.3mA.cm-2) at about 0.8V, almost 10 times lower than that obtained in Pt(111).
The EO2B of racemic alcohol was also performed in a 0.1M solution of sulfuric acid on non-chiral platinum surfaces. The voltammogram obtained is shown in Figure 18(B). As in perchloric acid solution Pt(111) has higher catalytic activity. The reaction starts at 0.27V and attain the highest current density (0.6mA.cm-2) at 0.42V. Inverse behavior was obtained for the two other single crystals, being that in Pt(100) 2-butanol oxidizes more easily than in Pt(110). For Pt(100), the peaks at approximately 0.27V and 0.39V correspond to the hydrogen desorption. Oxidation starts at 0.65V and
the maximum current density (0.17mA.cm-2) is attained at 0,78V. For Pt(110), EO2B
0,2 0,4 0,6 0,8 0 2 4 6 8 10 12 x2 0.1 M 2-butanol (racemic) 0.1 M HClO4 j / m A.cm -2 E / V vs. RHE Pt(100)x2 Pt(110)x2 Pt(111) x2 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0.1 M 2-butanol (racemic) 0.1 M H2SO4 j / m A.cm -2 Pt(100) Pt(110) Pt(111) (B) E / V vs. RHE
Figure 18. EO2B voltammetric profile of 10mM racemic 2-butanol on Pt(100), (110)
and (111), in (A) 0.1M HClO4 solution (the current densities for Pt(100) and Pt(110) were multiplied by 2) and (B) 0.1M H2SO4 solution, with a scan rate of 50mV.s-1.
These results show the huge influence of the electrolyte on the catalyst activity. It is well-known that sulfate (SO42-) adsorbs more strongly to Pt surfaces that perchlorate ions (ClO4-)[29]. The important differences in current densities can be tentatively ascribed to the blockage of some key Pt sites by the ion, thus inhibiting or lowering the rate of formation of one or more key intermediates of the reaction. Even if the anion effect is well-known in electrocatalysis, we have never seen such a strong inhibitory effect of sulphate for an oxidation of a small organic molecule on Pt. In addition, as initially expected, there are different electrooxidation responses of the enantiomers when catalyzed by the same chiral single crystal.
6.2. In situ Fourier Transform Infrared Spectroscopy Analysis
In order to understand the influence of the symmetry of the basal planes of Pt on the EO2B, we performed in situ FTIR measurements in solution of 0.1M 2-butanol (racemic) + 0.1M HClO4, using as WE Pt(100), Pt(110) and Pt(111). Electrooxidations were performed in H2O and D2O. Using H2O, it is possible to observe the CO2 for-mation, as this band is located in a region where H2O do not absorb the radiation. Deuterated water, on the other hand, allowed a clearer view of the formation of
car-bonyl group (C=O), whose band overlaps an intense band of H2O. The CO2 band is
not observed due to the strong absorption of D2O in this region. The obtained spectra are shown in Figures 19-21.
Figure 19 shows the spectra obtained for EO2B catalyzed by Pt(100). In H2O (left), it is possible to observe the beginning of the formation of CO2 (2340cm-1) at 0.70V. The band for butanone (at 1695cm-1) is difficult to follow due to the interference caused for H2O as stated above. The peak of CO2 remains practically the same between 0.75V and 0.95V. The spectra obtained in D2O, permit to follow the formation of 2-butanone, which appear at 0.55V and increases along the potential scanning. The band around 1100cm-1 is due to the migration of ClO4- ions towards the thin layer attracted by the increase in the positive charge of the electrode and also by the excess of protons which are released during the oxidation, as shown in Equations 1 and 2.
2500 2000 1500 1000 2500 2000 1500 1000 D2O 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0,90 Tr an smita nce / % Wavelenght / cm-1 0,95 E / V vs. RHE H2O Pt (100) R/R 0= 1%
Figure 19. FTIR spectra for Pt(100) in H2O (left) and D2O (right). Electrolyte composition was 0.1M 2-butanol (racemic) + 0.1M HClO4. Highlighted in red is the peak in 2340cm-1, referring to the presence of CO
2 and, in blue, we have the peak in 1695cm-1, referring to the formation of butanone.
In Pt(110) it is possible to observe the formation of 2-butanone from 0.50V and CO2 from 0.70V when the measurement is performed in water (left spectra). In D2O (right), the formation of 2-butanone starts at approximately 0.40V (Figure 20). Even if it is distorted by the band corresponding to H2O at 1620cm-1, the peaks for 2-butanone (at 1695cm-1) are more intense than the CO
2 (2340cm-1) in all potentials. When EO2B is catalyzed by Pt(111) (Figure 21), the reaction starts at 0.30 V, with the formation of 2-butanone. CO2 starts to be produced at 0.60V, and the intensity of the corresponding band in much less intense than that for 2-butanone.
2500 2000 1500 1000 2500 2000 1500 1000 Tr an smita nce / % Wavelenght / cm-1 D2O 0,90 0,95 E / V vs. RHE H2O Pt (110) R/R0= 1% 0,85 0,80 0,75 0,70 0,65 0,60 0,55 0,50 0,45 0,40 0,35 0,30 0,25 0,20
Figure 20. FTIR spectra for Pt(110) in H2O (left) and D2O (right). Electrolyte composition was 0.1M 2-butanol (racemic) + 0.1M HClO4. Highlighted in red is the peak in in 2340cm-1 referring to the presence of CO
2 and, in blue, we have the peak in 1695cm-1, referring to the formation of carbonyl group.
2500 2000 1500 1000 2500 2000 1500 1000 0,90 0,95 0,85 0,80 0,75 0,70 0,65 0,60 0,55 0,50 0,45 0,40 0,35 0,30 0,25 0,20 E / V vs. RHE D2O H2O Pt (111) R/R0= 1% Tr an smita nce / % Wavelenght / cm-1
Figure 21. FTIR spectra for Pt(111) in H2O (left) and D2O (right). Electrolyte composition was 0.1M 2-butanol (racemic) + 0.1M HClO4. Highlighted in red is the peak in in 2340cm-1, referring to the presence of CO2 and, in blue, we have the peak in 1695cm-1, referring to the formation of carbonyl group.
The analysis of the results for the three electrodes allows to conclude that Pt(111) is the catalyst with the greatest difficulty to completely oxidize the molecule, since the intensity ratio (2-butanone band/CO2 band) is the lower among the three catalysts, followed by Pt(110) and Pt(100), i.e., the intensity ratio trend follow the same trend of the catalyst´s activity.
Even if the formation of CO2 generates 24 electrons and the oxidation to 2-butanone only 2, while there must be several intermediates in the reaction mechanism to go from 2-butanol to CO2, the oxidation to 2-butanone could in principle, to occur
through one intermediate (Figure 3)[7] and, in consequence under lower
thermodynamic constrains[30].
The relative higher quantity of CO2 formed by Pt(100) and Pt(110) suggests that these surfaces are probably more prone to break the C-C bonds and, in consequence, to generate partially oxidized intermediates. The breaking of organic molecules by Pt is generally accompanied by the poisoning of the surface[31]. One of
the most common poison formed in reactions of electrooxidation of alcohols on Pt is CO[16], which is generally proposed as one of the intermediates of the complete oxidation of the organic molecules. Unfortunately, we were not able to detect any intermediate of the reaction.
Even if we based our discussion in the signal intensity ratio, the fact that the CO2 band at 2340cm-1 have a much higher absorption coefficient that virtually all the aldehyde, ketones and acids generally found in studies of the oxidation of small organic molecules and the results found by Camara et. al.[32] strongly suggest that the reaction is highly selective to butanone in the three surfaces.
Figure 22. Scheme of the products that be produced by the EO2B. The size of the
7. Electrooxidation of 2-butanol in chiral single crystals
In order to verify the different electrochemical behavior of the 2-butanol enantiomers on the chiral surfaces, the electrooxidation of each enantiomer was performed in 0.1M H2SO4 and 0.1M HClO4 solutions. Figure 23 shows the results obtained for the electrooxidation of the (R) and (S)-2-butanol in Pt(431)S.
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 j / A.cm -2 E / V vs. RHE 0.1 M H2SO4 0.1 M HClO4 (R)-2-butanol Pt (431)S j / A.cm -2 E / V vs. RHE 0.1 M H2SO4 0.1 M HClO4 (S)-2-butanol Pt(431)S
Figure 23. Voltammetric profiles obtained in EO2B catalyzed by Pt(431)S , for 10mM of each alcohol enantiomer, in ,0.1M H2SO4 solution (black line) and 0.1M HClO4 solution(red line), with a scan rate of 50mV.s-1.
As expected, different responses for the 2-butanol enantiomers were obtained in the same chiral catalyst, suggesting the occurrence of enantiomeric differentiation. Nevertheless, the EO2B was performed using as a catalyst Pt(110), a non-chiral surface, in order to confirm that the different behavior of the enantiomers on the chiral catalysts was due to the different interaction between them (or some intermediate) and the surface. The response of (R)-2-butanol and (S)-2-butanol must be the same on any non-chiral electrode, since both molecules will interact with the surface in the same way. The results are shown in Figure 24.
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 (R)-2-butanol Pt(110) j / A.cm -2 E / V vs RHE H2SO4 HClO4 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(110)
Figure 24. Voltammetric profile of 10mM (R)-2-butanol and 10mM (S)-2-butanol
electrooxidation in Pt(431)S, a non-chiral surface, in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) with a scan rate of 50mV.s-1.
Unfortunately, these results show that different currents densities and different dependence with the electrochemical potential are obtained for the electrooxidation of the enantiomers, even when the reaction occurs on a non-chiral surface, indicating the influence of some non-controlled influence in the experiment. The EO2B was performed with both alcohols and using all the single crystals presented in Table 1. We decided to show all these results as Supporting Information in order to avoid inserting several pages of information in the text being that it has the problems commented before.
These inconsistencies make us to strongly suspect that even if the alcohols have a nominal purity ≥ 99%, they do not have the same amount or type of impurities, which are certainly impacting the electrochemical results. To verify this hypothesis and which of the two enantiomers contain the impurities that interfere less in the results, we performed experiments with a racemic 2-butanol sample, with higher nominal purity (≥ 99,5%) with a distilled sample of this alcohol and with the pure enantiomers
Results obtained with the as received sample and the distilled are very similar, suggesting that it is indeed the response of a pure 2-butanol sample. Results obtained with the (R) and (S)-2-butanol under the same experimental conditions are different between them and when compared to the racemic mixture, confirming our hypothesis that the electrooxidation response of the enantiomers is not entirely due to the electrooxidation of the alcohol, but also have contribution of the impurities (Figure
25). Besides, these results confirm that the impurities act poisoning the electrode and that the nature and/or quantity of them are different in the samples of the enantiomers, which explain the different currents observed for the EO2B with the (R) and (S) enantiomer. 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 0 200 400 600 800 1000 1200 1400 j / A.cm -2 E / V vs. RHE 2-butanol (racemic) 2-butanol distilled (racemic) (R)-2-butanol
(S)-2-butanol
Ptp
Figure 25. Voltammetric profile of as-received racemic 2-butanol (black line), distilled
racemic 2-butanol (red line), (R)-2-butanol (blue line) and (S)-2-butanol (pink line) electrooxidation with Ptp, in 10mM alcohols + 0.1M H2SO4, with a scan rate of 50mV.s-1.
8. Conclusions and Perspectives
Pt(111) is the most active catalyst for the EO2B independently of the elec-trolyte used. Comparing Pt(100) and Pt(110), the former is more active in H2SO4 and the latter in HClO4.
The reaction is highly poisoned by the presence of sulphate, thus the EO2B is more than an order of magnitude faster for all the electrodes in HClO4.
FTIR in situ measurements showed that the alcohol is mainly oxidized to 2-butanone and that some CO2 is produced, regardless of the catalyst used.
We characterize two chiral single crystals electrodes for the first time. Un-fortunately, the EO2B in non-chiral single crystals showed that the alcohols enantio-mers do not have the required purity to study the enantiomeric differentiation of 2-butanol on chiral single crystals. Thus, we will proceed purifying by distillation the R-enantiomer (that showing more similar results to the racemic mixture) until attaining the desired purity.
Another important future task is to develop a HPLC protocol to separate the 2-butanol enantiomers in order to perform electrolysis experiments using chiral crystals with the racemic mixture and followed the kinetic of the consumption of the R and S enantiomer.
9. Support Information
9.1. Electrooxidation of 2-butanol enantiomers
Figures 26 and 27 shows the voltammograms obtained for EO2B of (R)-2-butanol for all single crystal electrodes used in this work. The reactions were performed with 10mM alcohol + 0.1M H2SO4 and 10mM alcohol + 0.1M HClO4 solutions, with a scan rate of 50mV.s-1. 0,3 0,4 0,5 0,6 0,7 0,8 0 200 400 600 800 1000 1200 0,3 0,4 0,5 0,6 0,7 0,8 0 200 400 600 800 1000 1200 (R)-2-butanol Pt(111) j / A.cm -2 E / V vs RHE H2SO4 HClO4 (R)-2-butanol Pt(211) j / A.cm -2 E / V vs RHE H2SO4 HClO4
Figure 26. EO2B voltammetric profiles obtained to (R)-2-butanol in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) solutions, with a scan rate of 50mV.s-1.
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 (R)-2-butanol Pt(100) j / A.cm -2 E / V vs RHE H2SO4 HClO4 (R)-2-butanol Pt(110) j / A.cm -2 E / V vs RHE H2SO4 HClO4
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 (R)-2-butanol Pt(431)S j / A.cm -2 E / V vs RHE H2SO4 HClO4 (R)-2-butanol Pt(431)R j / A.cm -2 E / V vs RHE H2SO4 HClO4 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 (R)-2-butanol Pt(531)S j / A.cm -2 E / V vs RHE H2SO4 HClO4 (R)-2-butanol Pt(531)R j / A.cm -2 E / V vs RHE H2SO4 HClO4
Figure 27. EO2B voltammetric profiles obtained to (R)-2-butanol in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) solutions, with a scan rate of 50mV.s-1.
Figures 28 and 29 presents the voltammograms of the (S)-2-butanol
electrooxidation. The reaction was performed with 10mM alcohol and in 0.1M H2SO4
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 H2SO4 HClO4 j / A.cm -2 E / V vs RHE (S)-2-butanol Pt(100) j / A.cm -2 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(110) 0,3 0,4 0,5 0,6 0,7 0,8 0 200 400 600 800 1000 1200 1400 0,3 0,4 0,5 0,6 0,7 0,8 0 200 400 600 800 1000 1200 1400 j / A.cm -2 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(111) j / A.cm -2 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(211)
Figure 28. EO2B voltammetric profiles obtained to (S)-2-butanol in 0.1M H2SO4 (black line) and 0.1 MHClO4(red line) solutions, with a scan rate of 50mV.s-1.
0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 600 700 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 600 700 j / A.cm -2 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(431)S (S)-2-butanol Pt(431)R j / A.cm -2 E / V vs RHE H2SO4 HClO4 0,3 0,4 0,5 0,6 0,7 0,8 0 50 100 150 200 250 0,3 0,4 0,5 0,6 0,7 0,8 0 50 100 150 200 250 (S)-2-butanol Pt(531)S j / A.cm -2 E / V vs RHE H2SO4 HClO4 (S)-2-butanol Pt(531)R j / A.cm -2 E / V vs RHE H2SO4 HClO4
Figure 29. EO2B voltammetric profiles obtained to (S)-2-butanol in 0.1M H2SO4 (black line) and 0.1M HClO4(red line) solutions, with a scan rate of 50mV.s-1.
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