Cu and Ru Catalysts: Rational Design and Structural Evolution in CO Oxidation

The research for surface active sites with outstanding catalytic activity and selectivity is continuing apace. The rationally designed catalyst with optimised energy level and geometric configuration is key to achieving novel reaction pathways with superior performance. Compared to the conventional supported nanomaterials as catalysts, single atomic site catalysts (SASCs) not only inherit the excellent recyclability but are also featured with ultimate atom efficiency, high structural uniformity and tunable coordination environment. These great advantages of SASCs are due to their unique atomic dispersion nature that preserves features of both heterogeneous and homogeneous catalysts. Especially the homogeneity of SASC enables convincing identification and characterisation of real active sites, making it an ideal platform to establish the definitive structure-activity relationship and to validate reaction mechanisms. Therefore, rational designed SASC has become the most prominent material to fabricate desired active sites with outstanding catalytic activity and selectivity. In this thesis, chemical synthesis strategies and characterisation techniques for SASCs are carefully reviewed. The limitations and future perspectives from a subjective view of the current methodology are discussed in detail as well. As inspired by pioneers’ work, rational designed Ru and Cu SASCs are prepared to investigate their distinct relationships between catalyst structures and reaction behaviours during CO oxidation reaction. With the help of combined in situ characterisation techniques, the structural evolution of active sites for both Ru and Cu catalysts were carefully studied. In the first project, the ultimate rational design of Ru active centre is realised by building surface single-sites to mimic molecular Ru catalysts. Inspired by a homogeneous Ru(II) complex, an air-stable surface -[bipy-Ru(II)(CO)2Cl2] single-site is designed through precise engineering of geometric and electronic structures from -[bipy-Ru(III)Cl4]- site. Such Ru(II) single-site enable oxidation of CO while the Ru(III) site is completely inert, providing an excellent prototype of the synthetic strategy which is generally applicable to transition metals. The second project focuses on the electronic metal-support interactions (EMSI) which describe electron flow between metal sites and a metal oxide support. For CuO-CeO2 catalysts, the electron withdrawing effect on Cu species introduced by electrophilic Ce4+ is maximised for atomically dispersed Cu sites over CeO2 surface. Experiment evidence shows the energy levels of 3d orbitals of isolated Cu(I)/(II) sites are decreased by Ce4+ cations in the support framework. It is demonstrated by in situ study that a [Cu(I)O2]3- site on CeO2 could selectively adsorb molecular O2 and form a rarely reported electrophilic 2-O2 species, leading to ten times higher activity than CuO clusters in CO oxidation. The third project is derived from the previous study of CuO-CeO2 catalysts, in which an unbalanced electron transfer between Cu and Ce is observed for CuO clusters dominant samples. To explain the reaction pathway of CeO2 supported CuO clusters in CO oxidation, an electronic metal-support-carbon interaction (EMSCI) based on EMSI is proposed. In the CuO-CeO2 redox, an additional flow of electron from metallic Cu to surface carbon species is observed by combined in situ studies, providing a complete picture of the mass and electron flow in the catalytic redox cycles

Self-assembled carbon nanoribbons with the heteroatom doping used as ultrafast charging cathodes in zinc-ion hybrid supercapacitors

Zinc-ion hybrid supercapacitors (ZHSs) are highly desirable for large-scale energy storage applications owing to the merits of high safety, low cost and ultra-long cycle life. The poor rate performance of cathodes, however, severely hinders their application. Herein, aqueous ZHSs with superior performance were fabricated by employing a series of ultrathin carbon nanobelts modified with B, N, O (CPTHB-Bx). The heteroatom doping can significantly modify the chemical behaviors of carbon frameworks, which could generate numerous active sites and accelerate the charge transport. The systematic investigation reveals that the B–N groups are active species for fast Zn-ion adsorption and desorption. As a result, the best-performed CPTHB-B2 exhibits an excellent electrochemical performance as cathodes in ZHSs, delivering a high specific capacitance of 415.3 F g−1 at 0.5 A g−1, a record high capacitance retention of 81% when increasing the current densities from 0.5 to 100 A g−1, an outstanding energy density of 131.9 W h kg−1 and an exceptionally high power density of 42.1 kW kg−1. Our work provides a new cathode design for ultrafast charging Zn-ion storage devices

Metal-Specific Reactivity in Single-Atom Catalysts: CO Oxidation on 4d and 5d Transition Metals Atomically Dispersed on MgO

[EN] Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones toward a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and Sd (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O-2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial deconfinement of the monatomic metal centers driven by CO at precatalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96 +/- 19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.XAS experiments were performed at B18 beamline, Diamond Light Source, United Kingdom (proposals Nr. SP17377 and SP19072) and BL22 beamline, ALBA Light Source, Spain (experiment 2019023278). Beamline scientists D. Gianolio (Diamond) and L. Simonelli and C. Marini (ALBA) are acknowledged for their assistance with the beamline setup during XAS experiments. The authors are grateful to M. Garcia, E. Andres, M. E. Martinez, and I. Lopez (ITQ) for assistance during the XAS experiments. J. Ternieden (MPIKOFO) is acknowledged for the performance of XRD experiments. J.M. Salas (ITQ) is acknowledged for his experimental contribution to the CO-FTIR studies, and M.D. Soriano and A. Munoz for the recording of XP spectra. P.N.P. and F.S. acknowledge support by the state of BademWurttemberg through bwHPC (bwUnicluster and JUSTUS, RV bw17D01) and support from the Helmholtz Association is also gratefully acknowledged. This research received funding from the Alexander von Humboldt Foundation (postdoctoral grant to B.B.S.), the Max Planck Society and the Fonds der Chemischen Industry (FCI, Germany). The authors are grateful to Prof. Ferdi Schuth for the provision of lab facilities and support throughout the project. Funding from the Spanish Ministry of Science, Innovation and Universities (projects SEV 2016-0683 and RTI2018-096399-A-100) is also acknowledged.Sarma, BB.; Plessow, PN.; Agostini, G.; Concepción Heydorn, P.; Pfänder, N.; Kang, L.; Wang, FR.... (2020). 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Improving the ORR Performance by Enhancing the Pt Oxidation Resistance

Proton exchange membrane fuel cells require oxygen reduction catalysts with high activity and stability. Pt based alloy materials are most widely applied ORR catalyst due to its high intrinsic activity, but usually suffer from rapid deactivation as a result of particle agglomeration, detachment, Ostwald ripening and/or Pt dissolution. Here we investigate the degradation of the PdPt alloys via in situ X-ray absorption fine structure, Δμ analysis, identical location-electron microscopy and DFT calculations. We conclude that the origin of high activity and stability of the PdPt catalyst stems from the oxidation resistance of metallic Pt, forming mainly surface adsorbed O species at high potentials. Two stage degradation process are observed, showing an evolution of dynamic surface dependent ORR performance along with the deactivation process. The careful design of Pt alloy structure leads to controlled surface oxygen behaviours. This opens a new way to increase the lifespan of fuel cells and improve the Pt utilization efficiency

Precisely visit the performance modulation of functionalized separator in Li-S batteries via consecutive multiscale analysis

Despite progress of functionalized separator in preventing the shuttle effect and promoting the sulfur utilization, the precise and non-destructive investigation of structure-function-performance associativity remains limited so far in Li-S batteries. Here, we build consecutive multiscale analysis via combining X-ray absorption fine structure (XAFS) and X-ray computational tomography (CT) techniques to precisely visit the structure-function-performance relationship. XAFS measurement offers the atomic scale changes in the chemical structure and environment. Moreover, a non-destructive technique of X-ray CT proves the functionalized separator role for microscopic scale, which is powerful chaining to bridge the chemical structures of the materials with the overall performance modulation of cells. Benefiting from this consecutive multiscale analysis, we report that the uniform doping of Sr2+ into the perovskite LaMnO3-δ material changes the Mn oxidation states and conductivity (chemical structure), leading to effective lithium polysulfide trapping and accelerated sulfur redox (separator function), and resulting in outstanding cell performance

Direct Anonymous Attestation with Optimal TPM Signing Efficiency

Direct Anonymous Attestation (DAA) is an anonymous signature scheme, which allows the Trusted Platform Module (TPM), a small chip embedded in a host computer, to attest to the state of the host system, while preserving the privacy of the user. DAA provides two signature modes: fully anonymous signatures and pseudonymous signatures. One main goal of designing DAA schemes is to reduce the TPM signing workload as much as possible, as the TPM has only limited resources. In an optimal DAA scheme, the signing workload on the TPM will be no more than that required for a normal signature like ECSchnorr. To date, no scheme has achieved the optimal signing efficiency for both signature modes. In this paper, we propose the first DAA scheme which achieves the optimal TPM signing efficiency for both signature modes. In this scheme, the TPM takes only a single exponentiation to generate a signature, and this single exponentiation can be pre-computed. Our scheme can be implemented using the existing TPM 2.0 commands, and thus is compatible with the TPM 2.0 specification. We benchmarked the TPM 2.0 commands needed for three DAA use cases on an Infineon TPM 2.0 chip, and also implemented the host signing and verification algorithm for our scheme on a laptop with 1.80GHz Intel Core i7-8550U CPU. Our experimental results show that our DAA scheme obtains a total signing time of about 144 ms for either of two signature modes (compared to an online signing time of about 65 ms). Based on our benchmark results for the pseudonymous signature mode, our scheme is roughly 2x (resp., 5x) faster than the existing DAA schemes supported by TPM 2.0 in terms of total (resp., online) signing efficiency. In addition, our DAA scheme supports selective attribute disclosure, which can satisfy more application require- ments. We also extend our DAA scheme to support signature-based revocation and to guarantee privacy against subverted TPMs. The two extended DAA schemes keep the TPM signing efficiency optimal for both of two signa- ture modes, and outperform existing related schemes in terms of signing performance

Cathode–Electrolyte Interface Modification by Binder Engineering for High-Performance Aqueous Zinc-Ion Batteries

A stable cathode-electrolyte interface (CEI) is crucial for aqueous zinc-ion batteries (AZIBs), but it is less investigated. Commercial binder poly(vinylidene fluoride) (PVDF) is widely used without scrutinizing its suitability and cathode-electrolyte interface (CEI) in AZIBs. A water-soluble binder is developed that facilitated the in situ formation of a CEI protecting layer tuning the interfacial morphology. By combining a polysaccharide sodium alginate (SA) with a hydrophobic polytetrafluoroethylene (PTFE), the surface morphology, and charge storage kinetics can be confined from diffusion-dominated to capacitance-controlled processes. The underpinning mechanism investigates experimentally in both kinetic and thermodynamic perspectives demonstrate that the COO- from SA acts as an anionic polyelectrolyte facilitating the adsorption of Zn2+ ; meanwhile fluoride atoms on PTFE backbone provide hydrophobicity to break desolvation penalty. The hybrid binder is beneficial in providing a higher areal flux of Zn2+ at the CEI, where the Zn-Birnessite MnO2 battery with the hybrid binder exhibits an average specific capacity 45.6% higher than that with conventional PVDF binders; moreover, a reduced interface activation energy attained fosters a superior rate capability and a capacity retention of 99.1% in 1000 cycles. The hybrid binder also reduces the cost compared to the PVDF/NMP, which is a universal strategy to modify interface morphology

Identification and manipulation of dynamic active site deficiency-induced competing reactions in electrocatalytic oxidation processes

A detrimental competition between the urea oxidation reaction (UOR) and oxygen evolution reaction is identified. Strategies are proposed to alleviate such competition and boost the performance of the UOR and other organic compound oxidation reactions