CAMPINAS 2020
UNIVERSIDADE ESTADUAL DE CAMPINAS INSTITUTO DE QUÍMICA
TANNA ELYN RODRIGUES FIUZA
THE ROLE OF THE SUPPORT IN THE STABILIZATION OF THE METALLIC PHASE IN THE PREFERENTIAL OXIDATION OF CO (CO-PROX) REACTION
O PAPEL DO SUPORTE NA ESTABILIZAÇÃO DA FASE METÁLICA NA REAÇÃO DE OXIDAÇÃO PREFERENCIAL DE CO (CO-PROX)
CAMPINAS 2020
THE ROLE OF THE SUPPORT IN THE STABILIZATION OF THE METALLIC PHASE IN THE PREFERENTIAL OXIDATION OF CO (CO-PROX) REACTION
O PAPEL DO SUPORTE NA ESTABILIZAÇÃO DA FASE METÁLICA NA REAÇÃO DE OXIDAÇÃO PREFERENCIAL DE CO (CO-PROX)
Tese de Doutorado apresentada ao Instituto de Química da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Doutora em Ciências.
Doctor’s Thesis presented to the Institute of Chemistry of the University of Campinas as part of the requirements to obtain the title of Doctor in Sciences.
Supervisor: Dr. Daniela Zanchet
O arquivo digital corresponde à versão final da Tese defendida pela aluna Tanna Elyn Rodrigues Fiuza e orientada pela Profa. Dra. Daniela Zanchet.
Simone Luiz Alves - CRB 8/9094
Fiuza, Tanna Elyn Rodrigues,
F586r FiuThe role of the support in the stabilization of the metallic phase in the preferential oxidation of CO (CO-PROX) reaction / Tanna Elyn Rodrigues Fiuza. – Campinas, SP : [s.n.], 2020.
FiuOrientador: Daniela Zanchet.
FiuTese (doutorado) – Universidade Estadual de Campinas, Instituto de Química.
Fiu1. Catálise heterogênea. 2. Oxidação preferencial de CO. 3. Nanopartículas coloidais. 4. Caracterização in situ. 5. Óxidos. I. Zanchet, Daniela, 1972-. II. Universidade Estadual de Campinas. Instituto de Química. III. Título.
Informações para Biblioteca Digital
Título em outro idioma: O papel do suporte na estabilização da fase metálica na reação de
oxidação preferencial de CO (CO-PROX)
Palavras-chave em inglês: Heterogeneous catalysis Preferential CO oxidation Colloidal nanoparticles In situ characterization Oxides
Área de concentração: Química Inorgânica Titulação: Doutora em Ciências
Banca examinadora:
Daniela Zanchet [Orientador] Fábio Barboza Passos Liane Márcia Rossi Diego Pereira dos Santos Paulo Cesar de Sousa Filho
Data de defesa: 22-04-2020
Programa de Pós-Graduação: Química
Identificação e informações acadêmicas do(a) aluno(a)
- ORCID do autor: https://orcid.org/0000-0001-7841-2521 - Currículo Lattes do autor: http://lattes.cnpq.br/2707402142796627
Profa. Dra. Daniela Zanchet (Orientadora)
Prof. Dr. Fábio Barboza Passos (Universidade Federal Fluminense – UFF)
Profa. Dra. Liane Márcia Rossi (Universidade de São Paulo - USP)
Prof. Dr. Diego Pereira dos Santos (Universidade Estadual de Campinas – Unicamp)
Prof. Dr. Paulo Cesar de Sousa Filho (Universidade Estadual de Campinas – Unicamp)
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 Tese de Doutorado defendida pela aluna Tanna Elyn Rodrigues Fiuza, aprovada pela Comissão Julgadora em 22 de abril de 2020.
Esta tese representa o fim de uma etapa de aprendizado, muito esforço e amor pela ciência. Embora a autoria possua apenas um nome, sem o auxílio de muitas outras mãos (e cérebros), seria inviável chegar ao final desta jornada. Assim, seguem os meus agradecimentos para todos que contribuíram de alguma forma para a execução deste trabalho.
Aos meus pais, Jorge e Rosana, que não mediram esforços ao longo de toda a minha vida para que eu tivesse condições de me dedicar aos estudos da melhor forma possível. Eles sempre foram o meu referencial de dedicação, comprometimento, caráter e educação. À Verônica, minha irmã, agradeço todas as palavras de apoio desde quando eu estava na graduação, me dizendo que era capaz e que tudo daria certo. Aos demais membros da minha família, que sempre me apoiaram, mostraram interesse pelo que eu faço e, cada um na sua maneira, me disseram que tudo daria certo no final.
Ao Rodrigo, meu namorado, eu poderia escrever várias páginas agradecendo todo o apoio, amor e paciência, principalmente quando as coisas se tornaram mais difíceis. Mesmo à distância, em outro continente, recebi apoio como se estivesse ao meu lado. Os pequenos detalhes do dia a dia, as sugestões, os sábados e domingos que ele me acompanhou ao laboratório para que eu pudesse, aos 45 do segundo tempo finalizar as amostras para as medidas no LNLS, os lanches que ele levava ao CNPEM quando eu precisei ficar até mais tarde na linha de luz ou no microscópio... São muitas coisas que fizeram a diferença nestes quatro anos e me permitiram total dedicação ao desenvolvimento da pesquisa.
À família do Rodrigo, especial seus pais, Regina e Martin, por todo o carinho e apoio, e aos primos Sérgio e Carol, de Campinas, que ajudaram muito, deram muitas sugestões e dicas importantes, além claro da amizade.
À Daniela, minha orientadora, que aceitou me orientar mesmo sem me conhecer pessoalmente, acreditando que eu poderia desenvolver um bom trabalho, mesmo eu nunca tendo trabalhado com catálise e nanomateriais antes. Sua dedicação, ética e paixão pela pesquisa me ensinaram muito nestes quatro anos, e com certeza serão ensinamentos que levarei não só para a carreira, mas para todos os aspectos da minha vida.
Aos professores da banca examinadora, que aceitaram prontamente participar da avaliação deste trabalho e com certeza trarão contribuições valiosas.
Aos amigos do Grupo de Catálise e Nanomateriais (GCN). Agradeço à Priscila, Luelc, Felipe, Arthur e Diego pela acolhida na minha chegada ao grupo e toda a ajuda no início da caminhada. Agradeço também aos amigos do grupo com quem interagi por mais tempo, e foram fundamentais ao longo desta caminhada: Danielle, Tathiana, Isaías, Karen, Igor, Leonardo e Eduarda. Foram muitas sínteses, testes catalíticos, cafés e discussões que foram cruciais para que eu aprendesse algo novo todo dia, me tornando não só uma pesquisadora, mas uma pessoa melhor. Não posso deixar de destacar a participação de todos nas várias medidas no LNLS, em que eles
Ao pessoal dos times de basquete da LCN (Liga das Ciências Naturais) e LAU (Liga das Atléticas da Unicamp), com o qual pude aproveitar muitos momentos divertidos, que me ajudaram a manter a saúde física e mental durante o doutorado. Não cito nomes para não ser injusta com ninguém, visto que foram muitas as pessoas que passaram por estes times, mas cada um(a) de alguma forma contribuiu e sou grata por isso. “1,2,3... Au, Au, Au!”
À minha segunda família paranaense de Campinas, da “vila do Chaves”, Sara, Itamar, Gregório e Butiá (o labrador que acha que é gente). Foram vários churrascos de domingo, várias conversas, almoços, jantares, sorvetes e travessuras do Gregório e do Butiá que me divertiram e tornaram minha vida melhor em Campinas, principalmente na ausência do Rodrigo. Eu não poderia querer vizinhos melhores. Aos amigos de graduação Rafael, Dhésmon, Ana e Douglas, que proporcionaram diversos momentos de diversão sobre os momentos difíceis do doutorado, tornando tudo muito mais leve e animado.
Aos técnicos dos laboratórios multiusuários do IQ-Unicamp Renata Magueta, Claudia Martelli, Sonia Fanelli, Karen Goraieb, Déborah Simoni, Raquel Miller, Ricardo Pereira, pelas análises utilizadas neste trabalho. Ao pessoal do LNLS por toda ajuda ao longo das várias medidas realizadas: Júnior Maurício (XAFS2), Cristiane Rodella e Amanda Iglesias (XPD), Simone Betim e Natalia Moreno (LQU), e Fabio Zambello (GAA). Aos especialistas do LNNano Carlos Ospina, Vishnu Mogili e Gisele Dalmônico pelo auxílio e atenção durante as medidas de TEM. Em especial, agradeço ao Jefferson Bettini, não só pelas valiosas dicas que facilitaram muito a operação do microscópio, mas pela companhia e conversa durante as várias noites de medida. À Jô, da recepção, por me ajudar várias vezes quando precisei.
Agradeço aos demais funcionários da Unicamp, de diversos setores (zeladoria, segurança, limpeza, restaurante universitário etc.), pois, apesar de não trabalhar diretamente com estas pessoas, elas são fundamentais no dia a dia e muitas vezes a importância delas no bom funcionamento não é lembrado ou reconhecido.
Ao CNPq pela bolsa (Processo CNPq 140414/2016-9). À FAPESP pelos auxílios financeiros.
O presente trabalho foi realizado com apoio da Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Código de Financiamento 001.
Na catálise heterogênea, o suporte possui papel chave na estabilização dos sítios ativos na superfície do catalisador. No caso da reação de oxidação de CO (CO-OX, ou quando em atmosfera rica em H2, CO-PROX), a natureza do suporte pode também impactar na conversão do CO devido à sua participação no mecanismo reacional. Visando explorar em detalhe estes efeitos, nanopartículas (NPs) coloidais de AuCu foram suportadas em SiO2 e CeO2 comerciais, bem como no suporte CeO2/SiO2 preparado por impregnação, com diferentes teores de CeO2. Para a OX-CO, a presença do Cu na liga aumentou a atividade catalítica e a resistência à sinterização, em comparação com as NPs de Au. Além disto, a importância da liga pré-formada na estabilização de sítios mais ativos ficou clara. Se tratando da CO-PROX, a liga AuCu se mostrou mais ativa que as espécies Au-CuOx. Se tratando dos suportes, o CeO2 levou à maior conversão de CO em comparação ao SiO2. No caso do suporte CeO2/SiO2, o teor de CeO2 impactou de maneira significativa a mobilidade das espécies CuOx e a reincorporação do Cu em atmosfera redutora, o que modificou a performance catalítica dos catalisadores. Com base nas informações obtidas a partir dos sistemas catalíticos preparados a partir das NPs de AuCu suportadas, um novo suporte foi proposto: LaCo1-xFexO3 (x = 0.0, 0.1, 0.5, 1.0). Estas perovskitas foram aplicadas como catalisadores e como como suportes para baixo teor de Pt (0.5 % m/m), na reação de CO-PROX e os resultados obtidos foram promissores. Sem adição de Pt, as composições com x ≠ 1.0 atingiram em torno de 80 % de conversão de CO. Na presença de Pt a performance catalítica das perovskitas foi modificada, em especial a perovskita LaFeO3, que apresentou não só aumento na conversão de CO, mas também diminuição da temperatura de máxima conversão. Assim, os resultados obtidos com as perovskitas se mostraram promissores e indicaram a importância da utilização dos conhecimentos adquiridos em sistemas mais bem conhecidos na busca por novos sistemas catalíticos.
In heterogeneous catalysis, the support has a key role in the stabilization of the active sites on the surface of the catalysts. For the CO oxidation reaction (CO-OX, or under H2-rich atmosphere. the preferential oxidation of CO, CO-PROX), the nature of the support may also impact the CO conversion due to its role in the reactional mechanism. Aiming to deeply explore the support’s effects, colloidal AuCu NPs were supported on commercial SiO2 and CeO2, as well as on CeO2/SiO2 prepared by impregnation of different CeO2 loadings. Under the CO-OX reaction, the presence of Cu/CuOx improved the catalytic performance and the sintering-resistance of the NPs. Also, the importance of the pre-made alloy in the stabilization of unique active sites was clear. In the CO-PROX reaction, the AuCu alloy was more active than the Au-CuOx species. With regards to the supports, the CeO2 led to a higher CO conversion in comparison with the SiO2. In the case of CeO2/SiO2, the CeO2 amount impacted on the mobility of the CuOx species and the Cu reincorporation under reducing atmosphere, modifying the catalytic performance of the catalysts. Based on the information obtained by the AuCu-based catalysts, a new support was proposed to be employed in the CO-PROX reaction: LaCo1-xFexO3 (x = 0.0, 0.1, 0.5, 1.0) perovskites . These perovskites were used in CO-PROX as catalysts and as support for Pt (0.5 % wt.) and the results were promising. Without Pt, the compositions with x ≠ 1.0 achieve around 80 % of CO conversion. In the presence of Pt, the catalytic performance of the perovskite was changed, especially for the x = 1.0. In this case, the CO conversion was increased, and the temperature of the maximum conversion was decreased. Thus, the results obtained for the La-based perovskites clearly showed the promising performance of this system and showed the importance of the use of the knowledge obtained for a well-known system to prepare new catalytic systems.
AC-TEM: Aberration-corrected transmission electron microscopy
AuCuXCe: Catalysts prepared by impregnation of AuCu NPs on the XCe supports BF: Bright Field
EDS: Energy dispersive spectroscopy ER: Eley-Rideal
EXAFS: Extended X-ray absorption fine structure FCC: face-centered cubic
HAADF: High Angle Annular Dark Field
ICP-OES: Inductively Coupled Plasma - Optical Emission Spectrometry LH: Langmuir-Hinshelwood
LNLS: Brazilian Synchrotron Light Laboratory
LNNano: Brazilian Nanotechnology National Laboratory MC: Maximum CO conversion (%)
MvK: Mars-van Krevelen NPs: Nanoparticles CO-OX: CO oxidation
PEMFC: Proton exchange membrane fuel cell PROX-CO: Preferential oxidation of CO
OER: oxygen evolution reaction ORR: oxygen reduction reaction SR: Steam reforming
STEM: Scanning transmission electron microscopy TEM: Transmission electron microscopy
TMC: Temperature of maximum CO conversion (°C) UV-Vis: Ultraviolet-visible
XAFS: X-ray absorption fine structure
XANES: X-ray absorption near-edge structure
XCe: Supports prepared with different loadings (% wt.) of CeO2 on SiO2 (X = 4, 12 and 20 %)
XRD: X-ray diffraction XRF: X-ray fluorescence
SUMMARY OF EACH CHAPTER………...11
CHAPTER 1 – General aspects of catalysis: reactions and catalysts…………..……12
CHAPTER 2 – The impact of the oxidizing and reducing pretreatments and chlorine on the catalytic performance of AuCu_CeO2 catalysts in CO oxidation (OX-CO)………...34
CHAPTER 3 – Colloidal AuCu NPs supported on commercial SiO2 and CeO2 as catalysts for CO-PROX reaction……….…….53
CHAPTER 4 – Probing the effect of CeO2 loading on the Cu reincorporation in the AuCu-based catalysts by in situ XRD and XAFS………..…….……74
CHAPTER 5 – Impact of the Pt in the reducibility of the LaCo1-xFexO3 perovskites and catalytic activity in the preferential oxidation of CO……….…..……91
CHAPTER 6 – General conclusion and perspectives………..………..…101
REFERENCES……….…103
APPENDIX A – Supporting information – Chapter 2………..…………142
APPENDIX B – Supporting information – Chapter 3………..…………146
SUMMARY OF EACH CHAPTER
This thesis is organized in 6 Chapters. Chapter 1 describes the general aspects of catalysis, introducing the catalytic systems and the reactions that were focused on this thesis. Chapter 2 highlights the impact of the activation protocol on the stabilization of the active sites for CO-OX reaction and the sintering-resistance of CeO2-supported AuCu NPs. Additionally, in this Chapter, we discussed how chlorine negatively affects the AuCu/CeO2 catalyst. Based on the meaningful information discussed in Chapter 2, Chapter 3 points out the role of the support nature (non-reducible vs reducible) on the mobility of the CuOx species, impacting the stabilization of the AuCu alloy under H2-rich atmosphere of CO-PROX reaction. In particular, the in situ ray diffraction and X-ray absorption techniques were deeply explored to better understand the catalysts. Considering the impact of the SiO2 and CeO2 on the dealloy and realloy processes shown in Chapter 3, Chapter 4 shows the use of a new set of supports, CeO2/SiO2 with different amounts of CeO2. In this Chapter, we also employed in situ synchrotron light techniques aiming to probe the impact of the metal-support interaction on the Cu reincorporation under reducing atmosphere and how the extension of the realloy process impacts on the catalytic performance of the catalysis in CO-PROX reaction. In Chapter 5 we provided insights about the catalytic performance of La-Based perovskites (LaCo1-xFexO3) as catalysts and support for low loadings of Pt in CO-PROX reaction, showing a promising catalytic system. Finally, Chapter 6 covers the general conclusion based on the results discussed in Chapters 2-5, pointing out the perspectives for the catalytic systems.
CHAPTER 1 – General aspects of catalysis: reactions and catalysts.
This chapter discusses the general aspects of Catalysis, introducing the reactions of CO oxidation (CO-OX) and preferential oxidation of CO (CO-PROX), that were extensively explored in this work. In this chapter, an overview about metallic nanoparticles and oxides as supports is presented, and the way that they interact producing unique active sites for the catalysis is discussed.
1.1 General aspects of CO oxidation OX) and preferential oxidation of CO (CO-PROX) reactions
Several essential products to the society are obtained by chemical transformation of different compounds, and the population growth has led to the need for improving the process implies the use of catalysis. In industry, around 90 % of chemical processes that involve the product manufacture are carried out using catalysts.1 The catalyst provides an alternative path to the non-catalyzed one, with a lower energy barrier (Figure 1.1a). Although the alternative path is usually more complex, the lower energy barrier leads to a faster reaction. It is important to remark that the catalyst only changes the kinetics of the reaction. Thus, if the global reaction is not thermodynamically favored, the catalyst will not change it.1 Catalysts can be classified into three major groups: homogeneous, heterogeneous, and enzymatic. Only heterogeneous catalysts will be discussed in this thesis.
Figure 1.1. Generic energy diagram of the catalyzed and uncatalyzed reaction. (b) General representation of a heterogeneous catalyst based in metallic nanoparticles (NPs) (purple spheres) supported on oxides (rough gray rectangle).
Heterogeneous catalysts are widely used in industrial processes, such as fuel production2, energy conversion3, and reducing toxic emissions.4 The heterogeneous catalysis is carried out using a catalyst in a different state of reactants; commonly the catalysts are solids and the reactants are in gas and/or liquid phase.1,5 There are many classes of catalysts and the choice depends on the type of reaction (liquid or gas phase), reactional conditions (pressure, temperature, and so on) and the reactants involved. This thesis focuses on supported catalysts, based on a metallic phase highly dispersed on oxides surface (Figure 1.1b). The chosen reactions, CO oxidation (CO-OX) and preferential oxidation of CO (CO-PROX) take place on the gas
phase.
Noble metals such as Pd, Pt, Au, and Rh are widely used as catalysts for CO-OX and CO-PROX reaction due to its high catalytic activity.6–8 The relationship between catalytic activity and particle size is reported since around 1930, which was associated with the external grain surface.9 In 1980-90, the seminal work by Haruta reported the high catalytic activity of Au nanoparticles (NPs) in the CO oxidation reaction, in which Au bulk was inactive.10–12 These reports were a milestone in the study of metallic NPs in catalysis. More recently, many studies about the relation of the particle size were reported, indicating the smaller particles are more active due to the higher ratio between surface and bulk atoms (Figure 1.2a). Since many reactions are sensitive to the different sites on the surface of the catalysts, and the atoms of lower coordination are commonly the most active13, the increase of catalytic activity as a function of the decrease of the particle size is expected. It is important to mention, however, the occurrence of the optimal size, in agreement with the Sabatier principle. The Sabatier principle14 states a qualitative concept about the interaction between the catalyst and the reactants, in which the bond strength should be optimal to allow the activation of the reactant and the desorption of the product leading to a volcano-like plot (Fig 1.2 b).15,16 Neumann et al.17 reported the CO-OX reaction on Pt NPs with 1, 2, 3 and 4 nm supported on Al2O3. The authors demonstrated that the high activity is related to a larger number of low-coordinated surface atoms, favoring the catalytic performance of Pt NPs smaller than 3 nm; the maximum activity was achieved for Pt NPs with 2 nm and it agrees with theoretical reports.13,18–20
Figure 1.2. (a) Scheme of the fraction of the corner, edge, and surface atoms as a function of the particle diameter. Adapted from 21–23. (b) Volcano plot of the Sabatier principle. When the binding of the reactants on the catalyst's surface is weak (gray
region), the reaction rate is limited by the activation of the reactant; when the binding is stronger (orange region), the reaction rate is limited by the desorption of the products. The top of the volcano plot is the optimal condition, with a higher reaction rate.
The relevance of catalysis research to address the challenges related to the environment has substantially increased in the last decades, particularly in the reduction/mitigation of pollutant emissions and in the development of sustainable processes and renewable energy sources.24 In this context, the CO-OX reaction has stood out in the last years,25–31 despite being used for several decades.11 The CO-OX reaction (Eq. 1) consists of CO oxidation by O2 to form CO2 and is used to mitigate the emission of CO,24,32 mainly originated in the incomplete combustion of fuels.33 The CO oxidation is also a key step in the application of H2 produced by steam reforming (SR)34–39 as an energy carrier for fuel cells, particularly the proton exchange membrane fuel cells (PEMFCs).40 This class of fuel cells uses Pt-based catalysts that can be poisoned by CO, even in small amounts, and therefore, it is crucial to reduce the CO content from the H2 stream generated by SR reactions below 10 ppm.40–42 Under these conditions, the CO oxidation takes place in an H2-rich atmosphere (CO-PROX), and H2 oxidation and CO hydrogenation can occur as undesired parallel reactions (Eq. 2,3). To minimize this drawback, the catalysts have to be active, selective to CO2 production, and stable under highly reductive conditions.
CO(g) + ½ O2(g) → CO2(g) (Hr298K = -280 kJ mol-1) (1) H2(g) + ½ O2(g) → H2O(g) (Hr298 = -241,8 kJ mol-1) (2) CO(g) + 3 H2(g) → CH4(g) + H2O(g) (Hr298K = -206 kJ mol-1) (3)
Both CO-OX and CO-PROX reaction are used as model reactions, to address fundamental aspects of catalytic systems, when carried out under ideal conditions.43 Considering the real condition, particularly for the CO-PROX the H2O and CO2 contents in the H2 stream are not negligible, impacting on the catalytic activity. The main impact is the blocking of the active sites, which is more pronounced in the presence of the CO2, due to the formation of carbonates.44–48 It is possible to found in the literature some reports which mention the beneficial impact of the presence of the CO2 and H2O in the reactional mixture, but they are the minority.47,49,50 As an example, Davó-Quiñonero et al.49 found evidence that the particular improvement of the catalytic
performance of the Cu/Cryptomelane in presence of H2O + CO2 relies on the stabilization of the support, avoiding the reduction of the cryptomelane (KxMn8O16) to the hausmannite (Mn3O4). Mozer et al.51 reported that the presence of H2O impacted positively in the CO-PROX reaction, while the CO2 decreases the CO conversion. Interestingly, in the case of a realistic reformate, the simultaneous presence of H2O and CO2 causes the compensation of both effects depending on the temperature.
1.2 Catalysts for CO-OX and CO-PROX reactions
The CO-OX and CO-PROX reactions require the activation of both CO and O2 to produce CO2; in particular, the CO-PROX reaction requires the use of a catalyst in which the CO adsorption is favored against H2, to reduce the undesired H2 oxidation.8 Pt NPs have been extensively explored for CO-OX and CO-PROX reactions due to the strong binding of CO. On the other hand, the high coverage of Pt surface by CO reduces the adsorption of O2,52 and the reaction only takes place with the increase of the temperature.53 Kandoi et al.54 reported theoretical calculations in which the Au(111) and Cu(111) are more selective than Pt at lower temperatures for CO-PROX reaction. Although the Pt has higher coverage, the energy barrier for the OH formation (and consequently to form H2O) is higher in comparison with the values obtained for Au and Cu. At higher temperatures, the Cu is the most selective metal to the CO2 production among the studied ones. It agrees with the fact of the Au and Cu (or CuO) have been successfully explored in CO-OX and CO-PROX reactions. It is clear, therefore, that both the CO and O2 activations are crucial steps for both CO-OX and CO-PROX reactions and three mechanisms have been underlining, according to the catalyst: Eley-Rideal (ER), Langmuir-Hinshelwood (LH) and Mars-van Krevelen (MvK).
In the ER mechanism, it is proposed that the O2 is preadsorbed and its exposition to CO leads to the CO2 formation. There are theoretical and experimental reports in the literature supporting this mechanism for some catalytic systems, such as Fe anchored on graphene oxide.55–57 However, the LH mechanism is the most common one associated with catalysts based on metals (i.e. Au, Pt, Pd) supported in inert supports, such as active carbon, SiO2 or Al2O3.55,58–61 The LH mechanism is based on the adsorption of CO and O2 on the catalytically active sites and the temperature determines the surface coverage of the catalyst surface (Figure 1.3a).62,63 In the ER mechanism, the CO must interact directly with the adsorbed oxygen, that is
covering only a small fraction of the metallic surface. The CO that is adsorbed on the metal surface will react via the LH mechanism.64,65
Due to the high dependence of the surface coverage in the LH mechanism, bimetallic NPs have attracted much attention in this context due to the improvement of its properties caused by the presence of a second metal. The bimetallic NPs can form heterostructures, in which the two metals form an interface (i.e. core-shell, onion-like and hollow structure), or can form alloys. The chemical ordering is defined by thermodynamics, forming the structure with the lower Gibbs free energy.66 Concerning the nanoalloys, the second metal may acts as another active site, allowing the non-competitive LH mechanism. The more oxyphilic metal provides a new site to activate the O2,67 improving the CO conversion in CO-OX and CO-PROX reactions (Figure 1.3b).
The formation of the nanoalloys is also associated with electronic perturbations that modify the reactivity of the surface of the NPs, impacting on the adsorption of the reactants;68,69 this topic will be more extensively discussed in the next section. Among the nanoalloys already reported on the literature for OX and CO-PROX reactions, the AuCu alloy represents an interesting system, since the Cu sites can be active sites to the O2 activation, allowing the non-competitive LH mechanism. Also, the AuCu alloy has shown an enhancement in CO2 tolerance in the CO-PROX reaction.70 It is important to note that the AuCu alloy is impacted by the reactional atmosphere. Under the CO-OX reaction, the highly oxidizing atmosphere leads to the segregation of the alloy, forming Au-CuOx species on the support, while under CO-PROX, the highly reducing atmosphere induces the realloy, that is not complete, forming Au1-xCux species.27–29,31,71–73. Interestingly, the presence of the Cu, in particular, CuOx, is reported as anchoring sites to Au, improving the sintering-resistance of the catalysts under the stream.71,72 It will be discussed in detail in Chapters 2 and 3.
Figure 1.3. Schematic representation of the (a) competitive and (b) non-competitive LH mechanism. The CO and O indicated in red are those which are adsorbed on the metallic nanoparticle and react to produce the CO2. In (a), the CO and O2 are adsorbed (and the latter dissociated) on the monometallic nanoparticle (pink spheres). Due to the higher coverage of the CO, just a few O2 are dissociated and the CO conversion is low. In (b), the second metal (green spheres) mainly adsorb and dissociate the O2, while the CO remains adsorbed on the host metal, increasing the CO conversion.
Another mechanism related to the CO-OX and CO-PROX reactions is the MvK mechanism (Figure 1.4a). Both MvK and LH mechanisms can take place simultaneously,74 and the catalyst nature determines which one will dominate. The MvK mechanism is commonly associated with catalysts produced with bimetallic NPs in which the second metal is prone to form MeOx (Me = Ni, Co, Fe, Cu) species under the oxidant atmosphere (particularly for the CO-OX reaction).75,76 It may also be the main mechanism when mono and bimetallic NPs are supported on reducible oxides, such as CeO2 and TiO2, due to the presence of reactive lattice oxygen.44,77–79 Two cases have been proposed; in the first one, CO is oxidized by the oxygen from the metal-support perimeter forming an oxygen vacancy that activates the molecular O2 from the stream.80 This is the mechanism proposed for CO-OX reaction on Au/TiO2 catalysts, both experimentally and theoretically.79,81–83 This mechanism was also reported by theoretical calculation for Au supported on ZrO2. Although the ZrO2 is not a reducible oxide, the presence of the Au decreases significantly the energy required to form oxygen vacancies at the perimeter sites.84 The second case that has been discussed is based on the labile oxygen from the surface of the support, which will react with the CO adsorbed on the metallic surface, producing the CO2. It leads to the formation of an oxygen vacancy and promoting the reduction of the cation from the support (Figure 1.4b); the O2 from the reactional stream refills the oxygen vacancy. Scirè et al.85 reported the role of the iron oxide as support for Au NPs for CO-PROX reaction; the authors proposed that the CO and H2 are adsorbed on the Au surface,
and are oxidized by this mechanism. The oxidation of CO by the oxygen from the iron oxide surface implies the reduction of Fe3+ to Fe2+.86 It was also demonstrated by Aragão et al.44 for Pt clusters supported on maghemite (-Fe2O3) as catalysts for CO-PROX reaction, in which the increase of the catalytic performance was associated with the partial reduction of Fe3+ to Fe2+. The high catalytic performance of catalysts prepared with -Fe2O3 is related to the higher reducibility of this oxide in comparison with the -Fe2O3.87 Differences in the CO oxidation as a function of the oxidation state of the support were also reported for Au/MnOx catalysts.88
Figure 1.4. (a) Schematic representation of the MvK mechanism. The CO and O indicated in red are those adsorbed on the metallic nanoparticle, while the blue O indicates the lattice oxygen. When the adsorbed CO reacts with the lattice oxygen, an oxygen vacancy is formed (green circle), that is replenished by the O2 from the stream, being available to oxidize another CO molecule. (b) CeO2 unit cell (fluorite structure). Green spheres indicate the Ce4+ and blue spheres indicate the lattice oxygen. The release of the lattice oxygen leads to the formation of an oxygen vacancy and two Ce3+ (brown sphere); when the oxygen vacancy is refilled, the Ce3+ cations are re-oxidized to Ce4+.
Among the reducible oxides, the CeO2 is highlighted as support for different metals and employed in several reactions. Concerning the CO-OX and CO-PROX reactions, the CeO2 is one of the most studied reducible oxides as supports for catalysis for many reasons. The CeO2 enhances the catalytic performance by the participation in reactional mechanism,50,89–91 also, is known by the improvement of the CO-resistance and sintering-resistance of the metallic phase.78,92–96 Finally, this oxide is commonly employed due to the suppression of the H2 oxidation in CO-PROX, in comparison with other oxides;97 these beneficial properties are closely related to the metal-support interaction, that is highly affected by the size and morphology of the
CeO2.94,98,99 Ha et al.100 reported that the catalytic activity of catalysts based on Au supported on CeO2 is highly dependent of the morphology of the CeO2 NPs. The Au supported on nano-octahedra presented higher activity for CO-OX reaction than the Au supported on nano-cubes. The results were explained by theoretical calculations that showed that the formation of oxygen vacancies on the (100) CeO2 surface in the presence of Au is energetically favored. The impact of CeO2 morphology was also reported for other metal NPs and CeO2 morphologies.101–109 CeO2 has also been used as a promoter, commonly supported on SiO2 and Al2O3, for many reactions including CO-OX,110–113 particularly for those carried out under harsh conditions (i.e. high temperatures), in which the CeO2 is prone to sinter, decreasing the surface area and inducing the sintering of the metallic particles.110
Generally, the presence of the CeO2 improves the catalytic activity due to the increase of the reducibility of the support,114 as well as due to the enhancement of the metal-support interaction.115,116 However, the role of the CeO2 amount as a promoter is little discussed for bimetallic NPs, particularly for the AuCu alloy. Considering the importance of the Cu-CeO2 interface, and the differences in the mobility of the CuOx species depending on the support nature,28,71 the AuCu NPs supported on CeO2-SiO2 or CeO2-Al2O3 seems to be a system that should be further studied. Doping of the CeO2, producing solid solutions, have also been addressed in the literature;117 the vast majority reported doping as beneficial for catalytic activity. The dopants decrease the energy to form the oxygen vacancies and facilitate the reduction of Ce4+ to Ce3+ to compensate the charges,118–121 improving the catalytic activity.122–124 In particular, the Cu-doped CeO2 (as well as Cu on CeO2 surface) have attracted attention due to the redox pairs formed by their interaction (Ce4+/Ce3+ and Cu2+/Cu+ or Cu0/Cu+).125 Not surprisingly, the CuO/CeO2 catalysts have been extensively explored for CO oxidation, mainly for CO-PROX, due to its high selectivity to CO2.126,127
Contrasting with the enhancement of catalytic performance, as aforementioned, the CeO2 is easily poisoned by chlorine. Commonly, the preparation of CeO2-supported catalysts employs metal chlorides, i.e. HAuCl4.3H2O, among others. The residual chlorine blocks the metallic surface,27 and decreases the CeO2 reducibility due to the formation of the CeOCl species, as described for Pd- and Ir-based catalysts applied to CO-PROX reaction.128–133 For the Au/CeO2 catalyst, the residual chlorine was also associated with the sintering of the Au NPs after
calcination.134,135 There are successful protocols described in the literature to decrease the presence of residual chlorine, such as reductive pretreatment,27 which cleans the metallic surface, but not detailed protocols to clean up the CeO2 surface could be found.
The extensive study of the CeO2 as support provided meaningful information about the role of the reducible oxides in the CO-OX and CO-PROX reactional mechanisms, particularly under ideal conditions. With the increase of the demand for more active and selective catalytic systems for reactions such as H2 production and purification and other reactions involved in CO2 conversions, such as reverse water-gas shift (RWGS) and CO2 reduction, the knowledge provided by the research on CeO2 can be extended for other oxides. In this context, the La-based oxides with perovskite crystal structure appear as promising candidates to be employed as support for many reactions, and the use of CO-OX and CO-PROX reactions under ideal conditions can shed light on the properties of these oxides in catalysis.
Perovskite oxides have been attracting attention in several fields,136–139 including catalysis. Oxides with perovskite structure have a general ABO3 structure, where the A-sites are commonly occupied by alkaline, earth-alkaline, or lanthanides metals cations that are 12-fold coordinated to oxygen. The B-sites are mostly occupied by transition metal cations such as Mn, Co, Ni, and Fe, coordinated to 6 oxygen atoms, forming octahedral units (Figure 1.5a).140,141 Around 90 % of the metals of the periodic table can form perovskite oxides,140, and the interest in this kind of material relies on its capability to tune the properties by changing the metals that occupy the A and B sites. By changing the B-site, it is possible to modulate the reducibility of the perovskite surface;142–144 the partial substitution of La3+ by Sr2+ in the A site has been associated with the increasing of the number of oxygen vacancies.
Figure 1.5. (a) Perovskite structure. Schematic representation of the Co 3d and O 2p overlapping in (a) LaCoO3, in which the Co3+ has the low spin state, and in (b) LaCo0.9Fe0.1O3, with Co3+ presenting high spin state induced by the Fe3+. Adapted from 145.
In catalysis, the La-based perovskites stand out due to their activity in some reactions, being considered a promising substitute to the catalysts based on noble-metals.137,146–148 It is possible to highlight the use of La-based perovskites in water splitting, air purification, reduction of CO2 and N2, as well as electrodes for fuel cells.149 Thalinger et al.150 reported that the La0.6Sr0.4FeO3- perovskite is active to RWGS and methane conversion. The reduced form of the perovskite promoted the RWGS, while the oxidized perovskite led to the total methane oxidation to CO2, clearly showing the impact of the pretreatments on the activation of the reactants and consequently on the catalytic performance. The exchange of Co3+ by Fe3+ increases the catalytic activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).145,151 In LaCoO3, the Co3+ ions are in the low spin state, and the hybridization of the Co 3d states takes place via the occupied Co (t2g), just above the Fermi level (Figure 1.5b). The presence of 10 % Fe3+ induces the spin-state transition of the Co3+ ions to the high spin state, and the hybridization involves both Co (t2g) and Co (eg). Additionally, there is a narrowing in the gap between the Co (t2g) and Co (eg) (Figure 1.5c).145,151 Stoerzinger et al.152 reported that the proximity of the center of the valence band to the Fermi level is directly related to the reactivity of the surface of La1-xSrxMnO3 perovskite for ORR; closer to the Fermi level, more reactive toward water, forming more hydroxyl species. Chen et al.153 reported the changes in the crystalline structure of the La1-xSrx-CoO3 as a function of the Sr2+ content. With the exchange of the La3+ by Sr2+ions, there is a phase transition from rhombohedral to cubic structure, and the alignment of the Co-O-Co bonds improves the overlap between the occupied O 2p and unoccupied Co 3d state, enhancing the OER.
The most common synthetic routes to produce La-based perovskites lead to the production of the oxides with low surface area, that has to be overcome to increase the catalytic performance. As for the CeO2, the La-based perovskites have been supported on oxides such as SiO2 and Al2O3, producing smaller particles.154,155 Other strategies have been the use of templates,156,157 microemulsion method,158 hydrothermal synthesis,159 supercritical CO2160, and others.161 Regardless of the composition and the synthesis method to produce La-based perovskites, it is known
that the cation segregation to the surface takes place, leading to surface enrichment and, depending on the conditions, to the formation of a separate single metal oxide phases.162,163 The composition of the surface dominates the catalyst-reactant interaction, highly impacting the catalytic activity.164,165 Wang et al.166 associated the Sr-enriched surface with the improvement of the overlapping between the Co 3d with O 2p states, improving the O2 activation for the CO-OX reaction. On the other hand, Rupp et al.167 found that the segregated SrO phase blocked the Co terminations, decreasing the catalytic activity of the La1-xSrxCoO3- for the ORR. It is important to mention that the La-based perovskites are highly active in some reactions, but have limited performance in others, demanding the concomitant use of metals to improve the catalytic activity. In this context, the perovskite oxides have been explored as precursors to produce supported mono e bimetallic NPs for different reactions, aiming to obtain homogeneous particles characterized by a strong metal-support interaction (Figure 1.6).143,168–170
Figure 1.6. Schematic representation of the segregation of the AB’1-xB’’xO3 perovskite (red rectangle), which contains different cations in B-site, under reductive pretreatment. Under these conditions, it is possible to produce (a) core-shell structure, (b) monometallic B’, and B’’ NPs in contact with each other and (c) B’-B’’ nanoalloys.
The extension of the segregation of the perovskite in the respective oxides and the sequential reduction to the metallic phase will depend on the treatment protocol. There are reports in the literature in which the reversible segregation is explored. Nishihata et al.171 reported in 2002 their “intelligent catalyst” to control automotive emissions. Under reducing atmosphere, the catalyst LaFe0.57Co0.38Pd0.05O3 presented partial segregation forming PdCo alloy NPs with a random face-centered cubic (fcc) structure on the surface. Under the oxidative condition, the oxidation of the metallic NPs and the migration of species recovered the initial perovskite, indicating that this process was reversible. Tanaka et al.172 showed that the strong metal-support
interaction stabilizes the Pt NPs formed by segregation of the CaTi0.98Pt0.02O3 perovskite under a reducing atmosphere. The catalyst was exposed to automotive exhaustion at 900 °C during 100 h leading to stable Pt NPs of 3 nm in size. Under similar conditions, a Pt/-Al2O3 catalyst evolved to Pt NPs larger than 100 nm. More recently, reported evidence of the processes described by Nishihata and Tanaka using aberration-corrected transmission electron microscopy (AC-TEM). Although Katz et al.173 had questioned the extension of the previously reported effects, they evidenced the Pd diffusion into the LaFeO3 perovskite structure.
Another approach is the impregnation of the metallic phase in low loadings on the La-based perovskites, aiming to avoid the sintering of the metallic particles; additionally, it may promote the reducibility of the oxides and the oxygen activation.157,174 There are in the literature conflicting reports about the effect of the noble metal as part of the perovskite structure, in comparison with the species generated by the impregnation method.175 In particular, for the CO-PROX, in which the reducibility of the support and the oxygen vacancies play a crucial role, there are few reports in the literature about the use of the bare and noble-metal containing La-based perovskites.176,177 Considering the H2-rich atmosphere, that may affect the surface-enrichment and the interaction of the metal with the surface of the perovskite, there is plenty of opportunities that could be explored, to rationalize the results that may be extended to other reactions that are carried out under reductive conditions.
1.3 Nanoalloys and colloidal synthesis
In the nanoalloys, a second metal can substitute atoms in the crystal lattice of the major component (random alloy) or can substitute atoms at specific positions, leading to changes in the crystal structure (chemically ordered alloy or intermetallic).178,179 Ferrando and Johnston (2008) pointed out a few factors that can affect the atomic distribution in bimetallic NPs.179 Considering an AB bimetallic nanoparticle, the A-B bond must be stronger than A-A and B-B bonds to form the alloy, and the element with the lowest surface energy will segregate to the surface. It is worth to mention that in the case of the presence of organic ligands on the surface, the metal that will segregate is the one that binds more strongly to the ligand. Ruban et al.180 reported in the ’90s a database of surface segregation energies of single transition metal impurities in transition-metal host structure. The formation of an alloy can also
be discussed based on the empirical rules of Hume-Rothery, which indicates the alloy that will be formed preferentially when the atomic radii, crystal structure, valence, and electronegativity are similar.181 In the case of AuCu nanoalloy, it can be formed in all Au:Cu ratios, assuming random chemical ordering or forming intermetallic compounds. The intermetallics are Au3Cu1 (fcc), Au1Cu1 (tetragonal) and Au1Cu3 (fcc). The AuCu alloy follows the Hume-Rothery rules since both metals present fcc crystal structure, the difference between RAu (144 pm) and RCu (128 pm) is less than 15 % and they have same valences.181 The difference of the Mulliken electronegativity between Au and Cu is around 29 %; however, when the nanoscale is considered, it is possible to replace the electronegativity Hume-Rothery rule by the molar heat of vaporization. In this context, the relative difference between the molar heat of Au (HvapAu = 334 kJ.mol-1) and Cu (HvapCu = 300 kJ.mol-1) is around 11 %, indicating the Au-Cu system occurs preferentially as alloy, even at nanoscale.181
In metallic nanoalloys, the enrichment of one of the metals on the surface has been described for several systems.182,183 There are some conflicts in the literature about the elements that segregated to the surface; it is associated with the theoretical methods used in the studies.181,184 However, there is a consensus that the decrease of the surface energy is the driving force for the segregation, considering that the NPs are isolated.183 The surface-enrichment of one of the metals from the alloy can be associated with the ensemble and strain effects that impact their catalytic performance. The ensemble effect is related to the arrangement of the surface atoms, impacting not only in the neighborhood of the active sites but also in the number of surface atoms of each metal, controlling the adsorption of reactants.185,186 In the case of CO oxidation, there is a dependence of the strength of CO adsorption and the O2 dissociation with the activity, and the disposition of the atoms on the surface of the alloy will highly impact the CO2 production.187,188 The strain effect is caused by a heteroatom in the particle, as a dopant, or when there is a difference in the lattice parameters of the overlayer and host metals; the former can be associated with a layer of the surface-enriched metal from the alloy. When a layer of smaller atoms is on the top of atomic layers of larger atoms (Figure 1.7a), a tensile strain is generated, decreasing the orbital superposition. It causes the narrowing of the d-band, and the d-band center (Ed) is upper-shifted closer to the Fermi level (EF) to preserve the d-band filling. The opposite condition causes the compression strain, increasing the orbitals superposition, leading to the broadening of the d-band, and down-shift of the Ed, away from the EF (Figure 1.7b).
The modifications in the position of the d-band center allow the tuning of the reactivity of the metallic surface. Exploring the modification of the d-band center in terms of the Sabatier principle, an alloy formation is an interesting approach to tune the strength of surface-adsorbate, achieving a condition closer to the optimal one. For example, it is very common to decrease the strength of metal-CO bond, to decrease the poisoning of the metallic surface.67,189,190 However, it is important to choose the correct metals, based on the trend of the segregation. Considering a generic M-N nanoalloy, in which the metal M is larger than N, the preferential segregation of the N to the surface will cause the tensile strain, and the surface will strongly bind the CO, increasing the CO-poisoning (Figure 1.7c). The electronic perturbations by strain can be also found in heterostructures, such as core-shell,191 however, only the nanoalloys have been discussed in this thesis.
Figure 1.7. Schematic representation of the shift of the d-band center due to the strain. In (a), the tensile strain caused by the presence of a smaller atom in the surface atomic layer induces a narrowing of the band occurs, followed by an upper-shift of the d-band center to preserve the d-band filling. In (b), the opposite is shown; the larger atom in the atomic layer causes the broadening of the d-band, followed by a down-shift of the center of the band to preserve the band filling. The purple and green spheres indicate the larger and smaller atoms, the dashed blue line indicates the d-band center and the red arrow indicates the direction of the shift (up or down); EF indicates the Fermi level. Adapted from 192,193. (c) The shift of the d-band center as a function of the underlying atomic layer and the overlayer. The shades of blue indicate the down-shift of the d-band center, while the shades of red indicate the upper-shift of the d-band center. Adapted from 194.
The calculations showed in Figure 1.7c are related to thin films; for NPs, the scenario concerning the strain effect is more complex, since the strain may be associated with the size, shape, twinning, as well as due to the presence of a second metal forming core-shell or alloys (Figure 1.8a).195 Wu et al.196 reported that the Pt3Ni
catalysts presented higher catalytic activity in the oxygen reduction reaction (ORR) than PtPd and PtAu alloys. Also, they found a direct relationship between the strain induced by the morphology in the Pt3Ni NPs, with the icosahedral NPs presenting higher catalytic activity than the octahedra.196 A deep comprehension of the strain in NPs have been achieved recently with the use of AC-TEM, in which it is possible to visualize the modification in the position of the atoms.197 Wan et al.198 nicely reported how the strain in the LiCoO2 substrate used in the Li-ion battery electrode affects the strain of Pt NPs. Depending on the spacing between the layers of Co-O octahedra, there is a compression or a tension in the Pt NPs, which can be seen in HR-TEM images (Figure 1.8b).
Figure 1.8. (a) Schematic representation of the strain in NPs depending on the (i) size, (ii) shape, (iii) twinning, (iv) formation of core-shell, and (v) alloy. Adapted from 195. In (b), the decrease in the spacing of the Co octahedra induces the compression strain in the Pt NPs, with reduction of the spacing of Pt (111) facets from 2.29 to 2.18 Å; the opposite behavior induces the increase of the (111) spacing to 2.42 Å The dashed red line indicates the position of the Pt atoms of the Pt NPs in the pristine LCO. The yellow dashed line indicates the position of the Pt atoms of the Pt NPs in contact with the strained LCO. Adapted from 198.
As mentioned before, the preferential segregation of metal of a nanoalloy will depend on the surface energy of each metal; however, in presence of the supports, capping ligands and under reactional conditions, as well as due to the synthesis method (kinetic effect), the trend of the segregation could be the opposite.199,200 Theoretically, AuCu alloy should present the Au-enrichment on the surface;201
however, the Cu-enrichment of the surface of the AuCu NPs usually takes place under realistic experimental conditions.71 This behavior has been discussed for different systems, in which the more oxyphilic metals are more prone to segregate to the surface and be oxidized, forming MeOx species.76,202 The metal-support interaction can also act as a driving force to favor the segregation of the nanoalloy. By theoretical calculations, it was found that the CeO2 induces the segregation of the second metal in some Au-based alloys toward the metal-CeO2 interface (Figure 1.9a,b).203 In particular for the AuCu alloy, the charge transfer from the Cu0 at the interface to the CeO2 leads to the reduction of the surface Ce4+ to Ce3+, and the reverse oxygen spillover induces the formation of CuOx species (Figure 1.9c).204,205
Figure 1.9. Schematic representation of the proposed segregation of the AuCu alloy supported on CeO2, based on the theoretical calculations reported in the literature.203– 205. (a) AuCu NP with Au (purple spheres) and Cu (green spheres) supported on CeO2 (yellow). (b) Due to the interaction between the particle with the support, the Cu atoms migrate to the interface between the NP and the CeO2, mainly exposing the Au atoms on the surface. (c) Charge transfer from Cu0 to CeO2 induces the reduction of Ce4+ to Ce3+ at the perimeter (red spheres), and the reverse oxygen spillover leads to the formation of CuOx species (blue spheres).
There are in the literature many synthesis protocols to produce nanoalloys.206 In this context, the colloidal synthesis has been widely used, due to the control of the size, shape, and composition. In the colloidal synthesis, the metal precursor, commonly a salt with the element in its cationic state (Mx+), is used in association with and reducing agent (alcohols, hydrazine, sodium borohydride, among others) to reduce the Mx+ to M0. The use of appropriate capping ligands is also crucial to control the growth of the NPs, producing homogeneous particles, with narrow size distribution (Figure 1.10a).207,208 Due to this fact, the colloidal synthesis is also used to synthesize bimetallic NPs, which can be performed using the one-pot or two-pot approach. In the former, both metal precursors are solubilized in the solvents/capping ligands and are reduced during the heating in the presence of a reducing agent. Destro
et al.209,210 extensively followed the synthesis of the AuCu alloy by the one-pot approach and found by in situ XAFS and UV-Vis absorption spectroscopy that the Au is firstly reduced at low temperature; the Cu2+ is reduced at higher temperatures, forming a temporary Cu2O-type phase. The Cu-rich phase is only incorporated in the NPs after the soak time at the final temperature of the synthesis, indicating that there is a digestive-ripening-like process involved in the formation of the alloy.209,210 In the case of the two-pot approach, the synthesis of the alloy is performed in two steps. The first one is the synthesis of NPs from one metal of the alloy are synthesized with controlled size and shape, followed by the purification step. Then, these pre-synthesized NPs act as seeds for the nucleation and reduction, followed by the diffusion of the second metal.211,212
In Figure 1.10b-e, the TEM images of the colloidal AuCu NPs synthesized by the two-pot approach are compared; (b,c) are the particles synthesized from Au-seeds with average size around 6 nm, while (d,e) were produced from Au-Au-seeds with 3 nm. Both (c) and (e) show that regardless of the size of the seeds, details in the synthesis protocol can dramatically change the size and shape of the colloidal NPs, decreasing the yield, increasing the average particle size, and broadening the size distribution. The improvement of the homogeneity was achieved only increasing the time of the solubilization of the Cu precursor in oleylamine and oleic acid before the injection of the Au-seeds. Another important point to highlight in the colloidal synthesis is the nature of the organic ligands. The capping ligands also plays a role in the control of the shape of the NPs, since they are adsorbed preferentially in specific facets (Figure 1.10f).207,213,214
Figure 1.10. (a) Schematic representation of the colloidal synthesis, in which the metal Mx+ precursor (red spheres) are reduced in presence of capping agents, forming the colloidal metallic NPs (yellow spheres) covered by organic ligands to avoid the agglomeration. (b-e) show the TEM images of the colloidal AuCu NPs synthesized by the two-pot approach,71 using Au-seeds with an average size around 6 (b,c) and 3 (d,e) nm. The NPs in (c) and (e) present homogeneous size and shape, in comparison with (b) and (d). The optimization of the synthesis parameters (solubilization, reductor injection, and control of the temperature) is crucial to produce monodisperse NPs. Scale bar: 20 nm. (Images collected by the author). (f) Schematic representation of the evolution of the particle shape as a function of the nature of the capping ligands. The colored facets indicate the presence of capping ligands; green indicates the capping ligands adsorbed on (100) facets and pink indicates the capping ligands adsorbed on (111) facets. The arrows indicate the evolution of the growth process, in which the preferential binding of capping agents on (100) facets leads to the synthesis of nano-cubes while the adsorption of the capping agent preferentially on (111) facets leads to the formation of nano-octahedron.
The mechanism of the formation of the colloidal NPs was described by LaMer and detailed by Ress in the mid-twentieth century, in which the colloidal synthesis takes place in three main steps (Figure 1.11a). Initially, the cationic precursors in the presence of the capping agents form the monomers. The concentration of monomers increases in the solution until the supersaturation, starting the nucleation; that is the second and the most important step in the colloidal process. In this step different nuclei can be formed, depending on the concentration of the
monomer throughout the reaction, changing the shape of the particle (Figure 1.11b). The third step is the growth of the particles, and the reaction time will define the final size of the particle. The nucleation and growth steps are affected by the temperature, allowing the tuning of the particle size by favoring some of these steps. Peng et al.215 described the colloidal synthesis of Au NPs, injecting the reducing agent at different temperatures, in which the particle size of the Au NPs decreased with the increase of the temperature at the injection time.215 The increasing of the temperature favored the nucleation step, which quickly consumes the monomers, leading to fast nucleation, forming smaller Au NPs. Therefore, the colloidal synthesis is a powerful approach to synthesize NPs since several parameters can be modulated, such as the ratios of metal:capping agent and metal:reducing agent, reaction time, and temperature, allowing the fine tune of the particle size and shape.
Figure 1.11. (a) Schematic illustration of the mechanism proposed by LaMer for the particle formation, plotting atomic concentration against time, showing the different steps: initial stage (gray), nucleation (red), and growth (yellow). The blue curve shows the evolution of the concentration of the monomers as a function of time. Examples of different seeds and subsequent particles that can be obtained are shown. Yellow, green, and purple indicate, respectively, (111), (100) and (110) facets. Twin planes are delineated in the figure with red lines. Adapted from 216–219. (b) Effect of Ag+ concentration on the morphology of Ag particles. Red, green, and purple indicate the plot of high, medium, and low concentration of Ag+ as a function of time. Below, the different morphologies as a function of the concentration of Ag+ are shown. Adapted from 220.
The high level of control in size, shape, and composition of metallic NPs is, clearly, a great advantage of the colloidal method; however, it is important to mention the disadvantages of this method. The capping ligands are highly effective to protect
the metallic surface, avoiding the sintering of the NPs also may block the adsorption of the reactants.
There are in the literature several reports about different methods to remove the capping ligands,221–225 and the thermal treatment is the most reported pretreatment. It is necessary the right balance between the temperature, atmosphere (i.e. oxidative or reductive), and the soak time since the metallic NPs are prone to sinter under harsh conditions, and soft conditions may not lead to the complete removal of the capping ligands.226 Cargnello et al.222 reported an interesting approach, in which the supported Pd NPs are exposed to high temperatures (~700 °C) just for few seconds and no significant change in the average size and size distribution of the NPs was found. It is important to highlight that the NPs studied by Cargnello222 had an average size of 7.8 nm, thus, the effectivity of this procedure to smaller NPs (< 3 nm) remains unclear. The lack of detailed information about the impact of the treatments to remove the capping ligands can be extended for the supported nanoalloys. It is consensus that the improvement of the sintering-resistance of the metallic NPs depends on the anchoring sites.93–96
In the case of the AuCu nanoalloy, the thermal treatment under the oxidative atmosphere is unanimous in the activation of the catalysts.27,28,71,72,76,227 As aforementioned, the AuCu alloy under oxidizing is prone to form the Au-CuOx heterostructure.27–29,31,71,72,228. On SiO2, these species which is related to the increase in the sintering-resistance of the Au NPs.72,228 For CeO2-supported catalysts, it has been described in the literature that the surface defects improve the stabilization of the Au NPs, avoiding the extensive sintering and deactivation of the catalysts. However, the role of the CuOx species on the enhancing of the sintering-resistance of the Au NPs on CeO2 was not discussed in detail in the literature. Also, there is a lack in the reports about other activation protocols for the SiO2- and CeO2-supported AuCu NPs, i.e. reductive pretreatments and may represent a path with the potential to provide meaningful information about the AuCu-based catalysts.
1.4 Motivation and goals
AuCu nanoalloy is an interesting system for catalytic application and there is a lack of detailed information about the behavior of this alloy in contact with CeO2, one of the most relevant reducible support. The synthesis methodology highly affects
the species stabilized on the CeO2 surface, hindering the consensus about the role of the Cu in the AuCu/CeO2 catalyst. Besides, the use of the CeO2 as promotor (CeO2/SiO2) is reported for many systems and reactions; however, the impact of the CeO2 amount on the mobility of the CuOx species is a crucial point that should be discussed in detail, since it may change de interfaces and consequently the catalytic activity of the AuCu/CeO2/SiO2 system. Therefore, our general goal was a better understanding of the role of the CeO2-based supports on the mobility of the CuOx species and the catalytic activity of the supported AuCu nanoalloy for OX and CO-PROX reaction. In this part of the work, our specific goals were:
- To understand the impact of the Cu, the activation protocols, and chlorine in the sintering-resistance of AuCu NPs in contact with oxides.
- A better comprehension of the role of CeO2, a reducible oxide, in comparison with SiO2 (non-reducible) on the CO-PROX reaction.
- The impact of the CeO2 loading in the dealloy-realloy process of AuCu NPs on CeO2-SiO2 promoted supports and the effect of the alloy composition in the CO-PROX reaction.
Fundamental studies are crucial to better understand the structure-activity relationship and catalyst properties as well as to identify potential new catalytic systems. In this part of the work, we explored LaCo1-xFexO3 perovskites as new support for the CO-PROX reaction. Our specific goals related to this system were:
- To understand the impact of the exchange of the Co3+ by Fe3+ in the catalytic activity and selectivity of the bare LaCo1-xFexO3 perovskites.
- To use the LaCo1-xFexO3 perovskites as supports for Pt, aiming to find evidence of the modification of the properties of the LaCo1-xFexO3 perovskite due to the metal-support interaction.
CHAPTER 2 – The impact of the oxidizing and reducing pretreatments and chlorine on the catalytic performance of AuCu_CeO2 catalysts in CO oxidation (CO-OX)
Graphical abstract:
This chapter discusses the impact of reductive and oxidative activation protocols on the catalytic activity of CeO2-supported Au and AuCu nanoparticles. Additionally, we evaluate the negative impact of the presence of chlorine on the AuCu_CeO2 catalyst.
The content of this chapter is an adapted version of the following submitted paper: Tanna Elyn Rodrigues Fiuza, Danielle Santos Gonçalves, Igor Ferreira Gomesand Daniela Zanchet, CeO2-supported Au and AuCu Catalysts for CO Oxidation:
Impact of activation protocol and residual chlorine on the active sites, Catalysis Today, submitted.
2.1 INTRODUCTION
Cerium (IV) oxide (CeO2) has been widely employed as support for noble (Au, Pt, Pd) and non-noble (Ni, Co, Cu) metals to produce catalysts for a broad set of reactions, such as water-gas shift (WGS), aqueous-phase and steam reforming of oxygenates, CO oxidation (CO-OX), CO preferential oxidation (CO-PROX; CO oxidation under H2-rich atmosphere), among others.71,229–238 The interest of this oxide lies in its low energy of oxygen vacancy formation and high oxygen storage capacity (OSC) that may favor bifunctional reaction mechanisms.90,239 A well-known example is Au/CeO2 catalysts applied to CO-OX in which the presence of CeO2 increases dramatically the CO conversion at lower temperatures in comparison with non-reducible oxides, such as SiO2 and Al2O3.71,91 The enhancement of the CO conversion has been associated with the Mars van Krevelen mechanism in which the CO is oxidized to CO2 by the CeO2 lattice oxygen, leaving an oxygen vacancy which is refilled by the activation of, closing the catalytic cycle.240 This bifunctional pathway may be dominant even under the reducing conditions of CO-PROX. However, one of the drawbacks of the Au/CeO2 system and other Au-based catalysts is the low sintering resistance of Au under reaction conditions and many efforts have been employed to maximize the metal-support interaction, increasing the anchoring effect.93–96,241 A successful strategy has been the formation of Au alloys, such as AuCu. The formation of an Au-CuOx (AuCu-CuOx) interface under the CO-OX atmosphere enhances both the Au stability against sintering and the catalytic activity in comparison with the monometallic counterpart.29,70,72,86,227 Najafishirtari et al.29 also showed that pretreatments, reductive or oxidative, affected the catalytic activity of AuCu_Al2O3 catalysts by influencing the formation of the Au-CuOx interface. Destro et al.28 reported that the nature of the support played a crucial role in the mobility of the CuOx species and the formation of a stable Au-CuOx interface, indirectly affecting the CO-OX catalytic performance. More specifically, although SiO2 and Al2O3 are both considered “inert” supports in this reaction, the SiO2-supported catalyst presented higher catalytic activity because the CuOx species remained in the vicinity of the Au NPs whereas on Al2O3 they spread on the support surface.
The combination of two metals and alloy formation is also an efficient way to modulate the catalytic properties by electronic and structural modifications. Nonetheless, the stability of alloy NPs is strongly dependent on several parameters
