145,750 research outputs found
CO Oxidation Catalysed by Pd-based Bimetallic Nanoalloys
Density functional theory based global geometry optimization has been used to
demonstrate the crucial influence of the geometry of the catalytic cluster on
the energy barriers for the CO oxidation reaction over Pd-based bimetallic
nanoalloys. We show that dramatic geometry change between the reaction
intermediates can lead to very high energy barriers and thus be prohibitive for
the whole process. This introduces challenges for both the design of new
catalysts, and theoretical methods employed. On the theory side, a careful
choice of geometric configurations of all reaction intermediates is crucial for
an adequate description of a possible reaction path. From the point of view of
the catalyst design, the cluster geometry can be controlled by adjusting the
level of interaction between the cluster and the dopant metal, as well as
between the adsorbate molecules and the catalyst cluster by mixing different
metals in a single nanoalloy particle. We show that substitution of a Pd atom
in the Pd cluster with a single Ag atom to form PdAg leads to
a potential improvement of the catalytic properties of the cluster for the CO
oxidation reaction. On the other hand, a single Au atom does not enhance the
properties of the catalyst, which is attributed to a weaker hybridization
between the cluster's constituent metals and the adsorbate molecules. Such
flexibility of properties of bimetallic nanoalloy clusters illustrates the
possibility of fine-tuning, which might be used for design of novel efficient
catalytic materials.Comment: 12 pages, 8 figure
Dissociative adsorption of methane on surface oxide structures of Pd-Pt alloys
The dissociative adsorption of methane on variously oxidized Pd, Pt and Pd-Pt
surfaces is investigated using density-functional theory, as a step towards
understanding the combustion of methane on these materials. For Pd-Pt alloys,
models of surface oxide structures are built on the basis of known oxides on Pd
and Pt. The methane adsorption energy presents large variations depending on
the oxide structure and composition. Adsorption is endothermic on the bare
Pd(111) metal surface as well as on stable thin layer oxide structures such as
the () surface oxide on Pd(100) and the PtO-like
oxide on Pt(111). Instead, large adsorption energies are obtained for the (100)
surface of bulk PdO, for metastable mixed PdPtO oxide
layers on Pt(100), and for Pd-Pt(111) surfaces covered with one oxygen
monolayer. In the latter case, we find a net thermodynamic preference for a
direct conversion of methane to methanol, which remains adsorbed on the
oxidized metal substrates via weak hydrogen-bond interactions
Cause of Chirality Consensus
Biological macromolecules, proteins and nucleic acids are composed
exclusively of chirally pure monomers. The chirality consensus appears vital
for life and it has even been considered as a prerequisite of life. However the
primary cause for the ubiquitous handedness has remained obscure. We propose
that the chirality consensus is a kinetic consequence that follows from the
principle of increasing entropy, i.e. the 2nd law of thermodynamics. Entropy
increases when an open system evolves by decreasing gradients in free energy
with more and more efficient mechanisms of energy transduction. The rate of
entropy increase is the universal fitness criterion of natural selection that
favors diverse functional molecules and drives the system to the chirality
consensus to attain and maintain high-entropy non-equilibrium states.Comment: 8 pages, 2 figure
Policies and Motivations for the CO2 Valorization through the Sabatier Reaction Using Structured Catalysts. A Review of the Most Recent Advances
The current scenario where the effects of global warming are more and more evident, has motivated different initiatives for facing this, such as the creation of global policies with a clear environmental guideline. Within these policies, the control of Greenhouse Gase (GHG) emissions has been defined as mandatory, but for carrying out this, a smart strategy is proposed. This is the application of a circular economy model, which seeks to minimize the generation of waste and maximize the efficient use of resources. From this point of view, CO2 recycling is an alternative to reduce emissions to the atmosphere, and we need to look for new business models which valorization this compound which now must be considered as a renewable carbon source. This has renewed the interest in known processes for the chemical transformation of CO2 but that have not been applied at industrial level because they do not offer evident profitability. For example, the methane produced in the Sabatier reaction has a great potential for application, but this depends on the existence of a sustainable supply of hydrogen and a greater efficiency during the process that allows maximizing energy efficiency and thermal control to maximize the methane yield. Regarding energy efficiency and thermal control of the process, the use of structured reactors is an appropriate strategy. The evolution of new technologies, such as 3D printing, and the consolidation of knowledge in the structing of catalysts has enabled the use of these reactors to develop a wide range of possibilities in the field. In this sense, the present review presents a brief description of the main policies that have motivated the transition to a circular economy model and within this, to CO2 recycling. This allows understanding, why efforts are being focused on the development of different reactions for CO2 valorization. Special attention to the case of the Sabatier reaction and in the application of structured reactors for such process is paid
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Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst.
Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system's catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts
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