Synthesis, Characterization and Evaluation of Dilute Limit Alloy Bimetallic Catalysts for Bio-Oil Upgrading

Well-defined single-atom catalytic sites with unique geometric and electronic properties are at the forefront of catalyst research. One type of SAC is a so-called dilute limit alloy (DLA), where single metal atom sites are supported on (or in) the surface of a second metal. This category of well-dispersed atoms alloyed on a metal surface has shown to be effective in reactions like selective hydrogenation of alkynes and dienes to alkenes, ethanol dehydrogenation, and the Ullmann reaction of aryl chlorides. In biomass conversion, this type of catalyst may find its way to address and improve conversion and yield that are crucial for it to be economically feasible, given that the cost of production is greatly dependent on catalyst cost and catalyst life cycle. In this work, the generalizable, scalable, and facile synthesis method of strong electrostatic adsorption (SEA) is pushed to the limit of metal dilution to prepare single atom alloy catalysts. It is evaluated for the upgrading of a bio-oil model compound. First, silica-supported dilute limit alloy (DLA) bimetallic catalysts (Y1X/SiO2, Y=Pt, Pd and Ru; X=Cu, Co and Ni) were prepared using modified simultaneous strong electrostatic adsorption (co-SEA) with controllable metal ratio deposition to obtain ultrasmall nanoparticles of the abundant metal X alloyed with an isolated atom Y on the surface. Monometallic catalysts (Pd, Pt, Ru, Cu, Co, and Ni) were also prepared by SEA. H2-TPR of dried DLA catalysts suggest close interaction between the two metals involve even at high dilution and the diffraction patterns of as-synthesized reduced catalysts implies ultrasmall nanoparticle. Confirming the existence of this isolated atom was the next step with the application of CO probe Fourier transform infrared (CO-FTIR) spectroscopy on a two-metal surface where a convoluted spectra complicates the analysis. A summary of CO vibration frequencies adsorbed on these isolated atoms were made. Finally, these DLA catalysts were evaluated in aqueous phase furfural hydrogenation reaction in a batch reactor. Monometallic nickel was activated with the addition of single atom sites on the surface. Also, changes in product selectivity were observed for cobalt- and copper-based catalyst with the addition of single atom sites. It was observed that Pd-X DLA and Y-Co based DLA are resistant to sintering after 10 hours of aqueous-phase reaction (150 °C, 430 psig H2) . Minimal dissolution of the abundant metal was observed for Cu-based DL

The Rational Synthesis of Bimetallic Catalysts on Oxide Supports

Catalysts play an important role in many chemical reactions. However, simple impregnated monometallic catalysts are often limited in their function. One way to overcome this limitation is through the incorporation of a secondary metal to the catalysts. These bimetallic catalysts often have synergistic benefits not observed in the monometallic analogues. Here, we focused on the synthesis of bimetallic catalysts using rational methods in order to improve catalyst function; specifically by tuning the particle size, morphology, and composition. The two methods of interests were Strong Electrostatic Adsorption (SEA) and Electroless Deposition (ED). An adaptation of SEA to bimetallic catalysts (coSEA) was use to synthesize ultrasmall highly dispersed alloyed nanoparticles on high surface area oxide supports (alumina and silica). Bimetallic catalysts of Pt, Pd, Co, Cu, and Ni having ~1nm nanoparticles were synthesized over silica, and Pt-Pd bimetallics were synthesized over aluminosilicates. These co-SEA catalysts have improved bimetallic interactions due to the close proximity and well-mixing of atoms. The Pt-Pd catalysts were evaluated as diesel oxidation catalysts using a simulated diesel exhaust at ORNL. The coSEA catalysts were more active and stable compared to conventional co-impregnated catalysts. Moreover, these highly alloyed co-SEA catalysts remained more alloyed after high temperature treatments (\u3e700°C) when compared to typical co-impregnation catalysts. Core-shell catalyst stabilization using surface free energy (SFE) principles was investigated through annealing treatments followed by catalyst characterization. The principle of anchoring low SFE metals on high SFE cores was demonstrated through the coupling of SEA for the nanoparticle cores and ED for the nanoparticle shells. The Ag-Ir core-shell materials resisted sintering with particle size growing only twofold for the bimetallic catalysts compared to over a tenfold size increase in the monometallic catalysts. This work demonstrated the effectiveness of using rational synthesis methods for bimetallic catalysts over simple co-impregnations. Having precise control on particle morphology, whether core-shell or alloyed, and size are important in catalyst design where high metal utilization and intimate bimetallic interaction are desired to reduce the amount of expensive precious group metals. By utilizing ED and SEA, we demonstrated new possibilities in improved bimetallic catalyst design that were unachievable with current conventional methods

Chemically Ordered Pt–Co–Cu/C as Excellent Electrochemical Catalyst for Oxygen Reduction Reaction

This paper reveals the ordered structure and composition effect to electrochemical catalytic activity towards oxygen reduction reaction (ORR) of ternary metallic Pt–Co–Cu/C catalysts. Bimetallic Pt-Co alloy nanoparticles (NPs) represent an emerging class of electrocatalysts for ORR, but practical applications, e.g. in fuel cells, have been hindered by low catalytic performances owning to crystal phase and atomic composition. Cu is introduced into Pt-Co/C lattices to form PtCoxCu1−x/C (x = 0.25, 0.5 and 0.75) ternary-face-centered tetragonal (fct) ordered ternary metallic NPs. The chemically ordered Pt–Co–Cu/C catalysts exhibit excellent performance of 1.31 A mg−1 Pt in mass activity and 0.59 A cm−2 Pt in specific activity which are significantly higher than Pt-Co/C and commercial Johnson Matthey (JM) Pt/C catalysts, because of the ordered crystal phase and composition control modified the Pt-Pt atoms distance and the surface electronic properties. The presence of Cu improves the surface electronic structure, as well as enhances the stability of catalysts

이원 금속 시스템의 설계와 구현을 통한 전기화학적 이산화탄소 환원 반응 촉매 안정성의 이해에 대한 연구

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2022.2. 주영창.이산화탄소를 부가가치 원료 및 재생가능 연료로 변환하는 전기화학 이산화탄소 환원 반응은 차세대 에너지 및 환경 솔루션으로 점점 더 연구되고 있다. 이산화탄소 환원 반응에서 부가가치 연료와 화학 원료를 형성할 때, 가스 확산 전극(GDE)에서의 전극 촉매 설계는 낮은 과전압, 높은 전력 효율 및 산물 선택도를 실현하는 데 중요하다. 이 특징에 기초하여, 본 논문은 이원 금속의 배치를 달리하는 관점에서 이원 금속 촉매를 설계하고 입증하는 새로운 전략에 대해 연구했다. 첫째, 이산화탄소 환원 반응 중 촉매 표면의 이산화탄소가 고갈되어 촉매의 활성도가 감소하는 것이 기존에 보고되었다. 셀 구조가 개선됨에 따라 이산화탄소는 가스 확산층을 통해 전극에 공급되는 것이 가능해져 이전보다 높은 전류 밀도 또는 전압 조건에서 측정이 가능해졌다. 이러한 조건에서는 이전에 보고되던 것과 마찬가지로 촉매 표면의 이산화탄소 고갈 현상이 문제가 될 것이고 향후 산업화 단계에서는 더욱 가혹한 조건 하에서 반응이 일어날 것으로 예상되므로, 반응 중 촉매 주변의 국소 환경을 제어할 수 있는 연구의 필요성이 높아질 것으로 예상된다. 이러한 관점에서, 본 연구에서는 국소적인 이산화탄소 농도를 증가시키기 위해 효과적인 구조와 제조 공정을 제안하였다. 이를 통해, 아연이 분산된 탄소 나노섬유에 포함된 은 나노입자를 단순한 공정에 의해 합성되었다. 전하를 띈 아연 단일 원자는 보다 높은 전류 밀도에서도 국소적인 이산화탄소 농도를 증가시킴으로써 은 나노입자의 일산화탄소 선택도를 향상시켰다. 또한‘환원성의’ 금속 나노입자가 포함된 ‘산화성의’ 금속이 분산된 탄소 지지체의 열역학적 설계 및 합성 방법은 열역학적 깁스 자유 에너지를 고려하여 다양한 촉매의 제조에 확장이 가능하다. 둘째, 구리는 C2+를 생산할 수 있는 독특한 금속이기 때문에 여러 방법의 개선을 통해 구리의 성능을 향상시키기 위해 많은 연구가 이루어지고 있다. 그러나 이산화탄소 환원 반응 동안 발생하는 재구성 현상으로 인해 초기의 촉매 구조가 유지되지 않을 수 있다. 이를 방지하기 위해 첫 연구처럼 촉매를 단단히 잡아 구조를 유지할 수 있는 지지체를 사용하는 방법이 있지만, 장기적으로는 최적의 촉매를 설계하기 위해 재구성 현상의 심도 있는 이해와 고려가 설계 과정에 포함되어야 한다. 또한, 현재의 재구성 연구는 아직 연구의 초기 단계이기 때문에 순수한 구리에 머물러 있는 단계이다. 따라서 이원 금속 또는 다른 조건에서 구리의 재구성이 어떻게 변하는지를 연구해야 한다. 구리-은, 구리-아연 합금의 재구성 거동의 변화를 관찰함으로써 이산화탄소 환원 반응과 재구성 현상에 대한 해석이 이루어졌다. 구리-은 합금에서는 (111)면으로 구리의 재구성이 일어나면서 주 산물이 C2+에서 C1으로 전이되었다. 한편, 구리-아연 합금에서는 머랭과 같은 아연 화합물 형성에 의해 아연의 용해가 억제되는 것이 확인되었다. 이것은 증착된 구리 합금의 이산화탄소 환원 반응 경로 및 재구성 거동을 조작하기 위한 통찰력을 제공할 수 있다. 이원 금속에서 이산화탄소 환원 반응 성능 변화에 대한 연구는 국소 환경과 촉매의 재구성 거동을 고려한 촉매 구조의 복잡한 설계에 적용될 수 있고 본 논문에서 사용된 열역학적 접근법은 예측에 기반한 금속 촉매 후보를 증가시킬 수 있어 향후 더욱 향상된 촉매를 합성하는데 크게 기여할 것이다.The electrochemical CO2 reduction reaction (CO2RR), which converts CO2 into value-added feedstocks and renewable fuels, has been increasingly studied as a next-generation energy and environmental solution. For value-added fuel and chemical feedstock formation in CO2RR, design of electrocatalysts in gas diffusion electrode (GDE) is significant to achieve low overpotential, high FE and product selectivity. Based on this feature, this thesis investigated a new strategy of designing and demonstrating binary metal catalyst in metal arrangement perspective. Firstly, it has been reported that the catalyst surface is depleted of CO2 due to the vigorous CO2RR reaction, thereby reducing the activity of the catalyst. As the cell structure is changed from H-Cell to flow cell and membrane electrode assembly (MEA), CO2 can be supplied to the electrode through the gas diffusion layer (GDL) which enabled measurement at higher current density or voltage condition. In the future, the reaction is expected to occur under harsher conditions at industrialization stage, so the need for research that can control the local environment around the catalyst during the reaction is expected to increase. In this point of view, effective structure and fabrication process were designed to enhance local CO2 concentration. Ag nanoparticles embedded in Zn dispersed carbon nanofiber was synthesized by a simple one-pot, self-forming strategy. Charged Zn single atoms have improved CO selectivity of the Ag nanoparticles by enhancing the local CO2 concentration even in higher current density. This thermodynamic design and synthesis methodology of ‘oxidative’ metal single atoms-dispersed C matrix with embedded ‘reductive’ metal nanoparticles can be extended to the fabrication of various materials by considering the thermodynamic Gibbs free energy for oxidation of the elements and this can provide the tailored material synthesis for various applications. Secondly, Cu is a unique metal capable of producing C2+ and many studies are being conducted to improve the performance of Cu through various modifications. However, there is a possibility that the initially modified catalyst structure may not be maintained due to the reconstruction phenomenon that occurs during the CO2RR. To prevent this, there is a way to use a support that can hold the catalyst firmly, but in the long run, understanding and consideration of reconstruction should be included in order to design an optimal catalyst through profound understanding. Moreover, current reconstruction research is focused on pure Cu since it is still early stage of research. Therefore, it is necessary to study how Cu reconstruction changes in binary metals or other modified condition. Through the observation of reconstruction behavior change in Cu-Ag, Cu-Zn alloys, the unique interpretation of previously reported phenomenon into the CO2RR was conducted. Transition of main products from C2+ to C1 by (111) faceted Cu reconstruction in Cu-Ag alloy. On the other hand, Zn dissolution was suppressed by meringue formation in Cu-Zn alloy. This can provide an insight to steering the reaction pathway and reconstruction behavior of sputtered Cu alloy active materials for the CO2RR of MEA cell. This thesis of CO2RR performance change in binary metal could be applicable to the designing of complicated catalyst structure with the consideration of local environment and reconstruction behavior of catalyst. Moreover, the thermodynamic approach which used in this thesis could increase the metal catalyst candidates with prediction.Abstract i Table of Contents iv List of Tables vii List of Figures viii Chapter 1. Introduction 1.1. Self-intensifying global climate crisis 1 1.2. Electrochemical CO2 reduction reaction 6 1.3. Issues and challenges in electrochemical CO2RR 7 1.3.1. Surface depletion of CO2 8 1.3.2. Reconstruction of Cu 9 1.4. Thesis objectives 12 1.5. Organization of the thesis 13 Chapter 2. Theoretical Background 2.1. Classification of metal for electrochemical CO2 reduction 15 2.1.1. CO producing metals 17 2.1.2. Cu 20 2.2. Thermodynamic design of catalyst structure and fabrication method 27 Chapter 3. Improved Local CO2 Concentration of Ag Nanoparticles through Zn Single Atom Besiegement for Efficient CO2RR 3.1. Introduction 33 3.2. Experimental procedure 34 3.2.1. Structure design & thermodynamic prediction 39 3.2.2. Synthesis of target structure 42 3.3. Structure verification 43 3.4. Electrochemical analysis 58 3.5. Summary 63 Chapter 4. Reconstruction Behavior Change of DC Magnetron Sputtered Ag or Zn alloyed Cu under Electrochemical CO2RR 4.1. Introduction 64 4.2. Experimental procedure 67 4.2.1. Material selection 67 4.2.2. Fabrication of catalyst 70 4.3. Verification of catalyst 75 4.4. Electrochemical analysis 92 4.5. Reconstruction process in Cu-Ag and Cu-Zn alloys 102 4.6. Summary 111 Chapter 5. Conclusion 5.1. Summary of results 112 5.2. Future work and suggested research 114 References 115 Abstract (In Korean) 125 Curriculum Vitae 128박

Synergistic Computational–Experimental Discovery of Highly Selective PtCu Nanocluster Catalysts for Acetylene Semihydrogenation

Semihydrogenation of acetylene (SHA) in an ethylene-rich stream is an important process for polymer industries. Presently, Pd-based catalysts have demonstrated good acetylene conversion (XC2H2), however, at the expense of ethylene selectivity (SC2H4). In this study, we have employed a systematic approach using density functional theory (DFT) to identify the best catalyst in a Cu–Pt system. The DFT results showed that with a 55 atom system at ∼1.1 Pt/Cu ratio for Pt28Cu27/Al2O3, the d-band center shifted −2.2 eV relative to the Fermi level leading to electron-saturated Pt, which allows only adsorption of ethylene via a π-bond, resulting in theoretical 99.7% SC2H4 at nearly complete XC2H2. Based on the DFT results, Pt–Cu/Al2O3 (PtCu) and Pt/Al2O3 (Pt) nanocatalysts were synthesized via cluster beam deposition (CBD), and their properties and activities were correlated with the computational predictions. For bimetallic PtCu, the electron microscopy results show the formation of alloys. The bimetallic PtCu catalyst closely mimics the DFT predictions in terms of both electronic structure, as confirmed by X-ray photoelectron spectroscopy, and catalytic activity. The alloying of Pt with Cu was responsible for the high C2H4 specific yield resulting from electron transfer between Cu and Pt, thus making PtCu a promising catalyst for SHA

Controlling Hydrocarbon (De)Hydrogenation Pathways with Bifunctional PtCu Single-Atom Alloys

The conversions of surface-bound alkyl groups to alkanes and alkenes are important steps in many heterogeneously catalyzed reactions. On the one hand, while Pt is ubiquitous in industry because of its high activity toward C-H activation, many Pt-based catalysts tend to overbind reactive intermediates, which leads to deactivation by carbon deposition and coke formation. On the other hand, Cu binds intermediates more weakly than Pt, but activation barriers tend to be higher on Cu. We examine the reactivity of ethyl, the simplest alkyl group that can undergo hydrogenation and dehydrogenation via β-elimination, and show that isolated Pt atoms in Cu enable low-temperature hydrogenation of ethyl, unseen on Cu, while avoiding the decomposition pathways on pure Pt that lead to coking. Furthermore, we confirm the predictions of our theoretical model and experimentally demonstrate that the selectivity of ethyl (de)hydrogenation can be controlled by changing the surface coverage of hydrogen