Impact of the Local Environment of Amines on the Activity for CO2_{2} Hydrogenation over Bifunctional Basic – Metallic Catalysts

Bifunctional basic-metallic catalysts proved to be efficient for the selective hydrogenation of CO$_{2}$ to methanol. The activity of these catalysts depends on the cooperative interaction between the amine groups and metallic sites, which is a function of amine group density, Pd particle perimeter length and the geometric properties of support pores. The pore width has the highest effect on the activity, increasing the methanol yield by about half an order of magnitude. Confining the space leads to a three-dimensional utilization of the available metal surface sites compared to a two-dimensional distribution of the bifunctional sites in larger pores, where the metal particle diameter is the decisive factor for the catalytic properties

Development of photochemical and electrochemical cells for operando X-ray absorption spectroscopy during photocatalytic and electrocatalytic reactions

Photochemical and electrochemical reactions are highly relevant processes for (i) transforming chemicals (e.g. photoreduction of isopropanol to acetone, electrochemical hydrogenation of benzaldehyde to benzyl alcohol, etc.), and (ii) sustainable energy production (e.g. photoreduction of CO$_2$ to methanol, electrocatalytic H$_2$ evolution reaction). It is therefore of importance to monitor the structural changes and to understand the properties of active sites under photocatalytic and electrocatalytic reaction conditions. Operando X-ray absorption spectroscopy (XAS) provides the means to investigate the nature of active sites under realistic reaction conditions. In this contribution, we describe the successful development of photochemical and electrochemical cells for operando XAS measurements during photocatalytic and electrocatalytic reactions. We have used the operando photochemical cell to monitor the formation of Pt nanoparticles on graphitic carbon nitride nanosheets (g-C$_3$N$_4$-ns) via photodeposition under visible light illumination and observed the formation of highly dispersed Pt nanoparticles with an estimated size of ∼2.5 nm and >60% dispersion. We have also tested this cell to follow the oxidation state of Pt in Pt/TiO$_2$ and Pt/g-C$_3$N$_4$-ns during H$_2$ evolution reaction (HER). We observed that Pt predominantly existed as metallic (reduced) Pt$^0$ species under HER conditions, and that PtO$_x$ species were partially reduced from Pt$^{IV}$ to Pt$^0$ upon illumination with UV or visible light. The rates of H$_2$ evolution obtained in the photochemical cell (12.1 mmol g$^{−1}$ h$^{−1}$ on Pt/TiO$_2$ and 1.01 mmol g$^{−1}$ h$^{−1}$ on Pt/g-C$_3$N$_4$-ns) were comparable to that obtained in a standard top-irradiated photoreactor (16.6 mmol g$^{−1}$ h$^{−1}$ on Pt/TiO$_2$ and 1.76 mmol g$^{−1}$ h$^{−1}$ on Pt/g-C$_3$N$_4$-ns). The operando electrochemical cell was successfully tested to monitor the changes in the structure and oxidation state of Pd in Pd/C electrocatalyst during electrocatalytic hydrogenation (ECH) of benzaldehyde. It was demonstrated that Pd in Pd/C was present in a partially reduced state (∼80% Pd0 and ∼20% PdII) and Pd nanoparticles did not degrade upon the application of an external potential under ECH reaction conditions

Enhanced Activity in Methane Dry Reforming by Carbon Dioxide Induced Metal-Oxide Interface Restructuring of Nickel/Zirconia

The activity of the Ni/ZrO$_2$ catalyst for the dry reforming of methane was significantly enhanced by activation and regeneration in the presence of CO$_2$. Exposure to CO$_2$ maximized and dynamically restructured the Ni–ZrO$_2$ interface, which was critical to obtain a stable catalyst. The ZrO$_{2−x}$ species at this interface was found to act catalytically by accepting and transferring one oxygen atom of CO$_2$ to the Ni surface, which resulted in the release of CO in the decomposition of transiently formed carbonates. This was found to open a fast additional reaction pathway to convert CO$_2$ that eclipsed direct CO$_2$ dissociation on Ni. The kinetically controlled availability of atomic oxygen by this pathway reduced the carbon concentration on the surface, which led to less refractory carbon deposition and more facile regeneration

Structure sensitivity of the hydrogenation of crotonaldehyde over Pt/SiO2 and Pt/TiO2

Hydrogenation of crotonaldehyde has been studied over SiO2- and TiO2-supported Pt catalysts. Over Pt/SiO2, the selectivity to the primary products butyraldehyde and crotylalcohol depends critically on the Pt particle size; i.e., the selectivity to the unsaturated alcohol increases with increasing particle size. For large metal particles, the high fraction of Pt(111) surfaces is concluded to favor the adsorption of crotonaldehyde via the carbonyl bond. On small Pt particles, the high abundance of metal atoms in low coordination allows unconstrained adsorption of both double bonds. In this case, the hydrogenation of the C=C bond is kinetically favored. Activity and selectivity of Pt/TiO2catalysts after low-temperature reduction are similar to those of Pt/SiO2. After high-temperature reduction the selectivity to crotylalcohol is generally enhanced. The selectivity of Pt/TiO2catalysts is then determined by the metal particle size and the extent of decoration of Pt with TiOxparticles. The presence of coordinatively unsaturated Ti cations in these oxide particles enhances the sorption strength of the C=O bond resulting in an enhanced selectivity to crotylalcohol. The effects of metal particle size and promotion by TiOxare additive. TiOxpromotion of catalysts with small particles and the presence of unpromoted large particles allow reaching of selectivities to crotylalcohol of approximately 45%. Promotion of large Pt particles with TiOxyields 64% selectivity to crotylalcohol