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Structure and Catalytic Activity of Copper Nanoparticles

Catalysts are substances that promote and accelerate chemical reactions without being consumed during the process and are widely used in industrial processes to produce various chemicals.

Catalysts based on copper nanoparticles dispersed in an oxide support benefit various reactions, such as the synthesis of methanol, the alcohol dehydrogenation, or the water gas shift (WGS) reaction which is one of the main processes for hydrogen production on an industrial scale. In this reaction, carbon monoxide reacts with water to produce carbon dioxide $ \rm CO_2 $ and hydrogen gas $\rm H_2$.

The activity of catalysts in these reactions is sensitive both to the morphology of the metal nanoparticles and to their interactions with the support. Therefore, Paula C. P. Caldas et al.  [1] used the facilities of the Brazilian Synchrotron Light Laboratory (LNLS) to investigate how the catalytic activity and the properties of the active sites on the surface of copper nanoparticles can be modified by its size and its interaction with ceria ($ \rm CeO_2 $).

[caption id="attachment_48426" align="alignleft" width="375"] Figure 1: Correlation between the bond length of $\rm Cu\, --- O$ and the catalyst turnover frequency (TOF) for the catalysts analyzed under WGS conditions with different proportions of copper and ceria.[/caption]

In this study, the researchers used the WGS reaction as a model for the analysis of the activity of the copper catalysts, supported in $\rm \gamma-Al_2O_3$, with different proportions of the metal. The experiments included X-ray absorption analyses (XANES and EXAFS) performed on LNLS’ XAFS1 beamline.

The group observed that the larger the size of the copper nanoparticles, the longer the bond length between copper and oxygen atoms $\rm Cu\, --- O$ in the catalyst, and consequently the greater its catalytic activity. This happens because a longer bond length corresponds to a lower bonding energy between the atoms, which leads to a higher electron density for the copper on the surface of the catalyst and a higher reactivity for the oxygen. Thus, there is an increase in the rate of intermediate reactions of the WGS process.

Furthermore, ceria was added in different proportions to the copper catalyst, which also provided an increase in the $\rm Cu\, --- O$ bond length without increasing the size of the nanoparticles. For example, for the catalyst containing $10\, wt\, \%$ copper ($\rm 10Cu$), the bond length increased from $ 1.890 ± 0.02$ Å to $ 1.941 ± 0.012$ Å with the addition of $4\, wt\, \%$ ceria ($ \rm 4Ce 10Cu $). The correlation between the bond length $\rm Cu\, --- O$ and the turnover frequency (TOF) for the catalysts analyzed under WGS conditions with different copper and ceria proportions is shown in Figure 1.

The experimental results presented by the researchers indicate that the addition of ceria adjusts the $\rm Cu\, --- O$ bond length, which determines the reaction rate of the WGS process and demonstrates that the fine structure of the active sites at the interface of the nanoparticles must be determined for the understanding of the effects of nanoparticle size and support on the activity of the catalysts used in this type of reaction.   Source: [1] Paula C. P. Caldas, Jean Marcel R. Gallo, Alejandro Lopez-Castillo, Daniela Zanchet, and José Maria C. Bueno. The Structure of the Cu–CuO Sites Determines the Catalytic Activity of Cu Nanoparticles. ACS Catal., 2017, 7 (4), pp 2419–2424. DOI: 10.1021/acscatal.6b03642

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A new x-ray technique to unravel electronic properties of actinide compounds

Actinides are a series of chemical elements that form the basis of nuclear fission technology, finding applications in strategic areas such as power generation, space exploration, diagnostics and medical treatments, and also in some special glass. Thorium (Th) and Uranium (U) are the most abundant actinides in the Earth's crust.

A deeper understanding of the properties of uranium and other actinides is necessary not only for their more efficient use in existing applications but also for proposing new applications. Several open questions remain, progress in this area usually limited in part by the difficulty in handling these materials safely.

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LNLS’ Users get together at the 27th RAU

The Annual Users Meeting of the Brazilian Synchrotron Light Laboratory (LNLS) attracts Brazilian and foreign researchers every year to the debate, exchange of experiences and integration of the community of users of the Laboratory.

The 27th edition of the event was held from November 22nd to 24th, 2017, on the campus of the Brazilian Center for Research in Energy and Materials (CNPEM), in Campinas, Brazil. The meeting was attended by 140 participants from different countries.

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LNLS members are awarded

Two members of the LNLS were recently awarded at international conferences. The engineer Daniel Tavares received the first award granted to early-career professionals by the conference ICALEPCS, related to control systems for large scientific facilities. The researcher Francisco Carlos Barbosa Maia received the award for best poster during the WIRMS event, which brings together staff and users of infrared beamlines from laboratories around the world.

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NEW CATALYSTS FOR THE SYNTHESIS OF ORGANIC COMPOUNDS

The production of chemical compounds from simpler organic molecules is of great importance for various industrial processes. It is based on the bonding between carbons of the precursor organic compounds, aided by catalysts (typically transition metals). These reactions make it possible to obtain natural and synthetic substances for the development of new materials, such as polymers and pharmaceuticals.

In particular, the so-called carbon-carbon (CC) cross-coupling reactions, in which two different precursor molecules are bound to form the final chemical compound, are of such importance that their development granted the 2010 Nobel Prize in chemistry to researchers Richard F. Heck, Ei-ichi Negishi and Akira Suzuki.

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Oxygen and the Degradation of Black Phosphorus

Semiconductors are a class of materials essential for the electronics industry. They have intermediate properties between conductors and insulators, which can be modified by doping with different chemical elements or by the application of electric fields or light.

Black phosphorus is a stable form of phosphorus whose crystalline structure is composed by stacking two-dimensional, one atom thick, thin layers. This material has immense potential to be used in electronic devices at the nanoscale due to its semiconductor properties, which can be adjusted by the number of atomic layers according to the need.

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New Catalysts for Hydrogen Production

The hydrogen gas ($\rm H_2$) is one of the best alternatives to fossil fuels because its combustion has as final product only water vapor. However, several technological challenges still need to be overcome in order to make it economically viable.

One way to produce hydrogen is by breaking down water molecules $\rm H_2O$, with formation of $\rm H_2$ molecules. The main reaction in this process is the Hydrogen Evolution Reaction (HER) in which the protons in an acid medium are reduced and form hydrogen gas by electrons passed through catalysts.

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Improving the Treatment of Industrial Waste

Synthetic dyes are in constant use in a wide variety of industries, from textile to cosmetics. Both the production and use of these substances can lead to environmental problems if they are not properly degraded or removed from industrial effluents. Among the many physical, chemical or biological processes that can be used for the treatment of such wastes, the adsorption processes are noteworthy for combining low cost and high removal rates.

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Call for proposals for the LNLS beamlines

Submission period: September 1st to 30th, 2017. Beamtime: First Semester, 2018. Research proposals are submitted through the SAU Online portal.

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Improving Hydrogen Storage

The search for clean, renewable and cheap energy sources has intensified in recent years with the growing consensus that the rise in the planet's average temperature, and the consequent intensification of extreme weather events, is caused by human action.

Hydrogen ($\rm H_2$) is one of the best alternatives to fossil fuels, especially because its combustion has only water vapor as final product. However, the economic viability of the production, storage and distribution of hydrogen for power generation still requires solutions for several technological challenges.

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