Francisco J. Pérez-Alonso

Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

The Pt(111)/Electrolyte Interface under Oxygen Reduction Reaction Conditions: An Electrochemical Impedance Spectroscopy Study

The Pt(111)/electrolyte interface has been characterized during the oxygen reduction reaction (ORR) in 0.1 M HClO(4) using electrochemical impedance spectroscopy. The surface was studied within the potential region where adsorption of OH* and O* species occur without significant place exchange between the adsorbate and Pt surface atoms (0.45-1.15 V vs RHE). An equivalent electric circuit is proposed to model the Pt(111)/electrolyte interface under ORR conditions within the selected potential window. This equivalent circuit reflects three processes with different time constants, which occur simultaneously during the ORR at Pt(111). Density functional theory (DFT) calculations were used to correlate and interpret the results of the measurements. The calculations indicate that the coadsorption of ClO(4)* and Cl* with OH* is unlikely. Our analysis suggests that the two-dimensional (2D) structures formed in O(2)-free solution are also formed under ORR conditions.

Design of an Active Site towards Optimal Electrocatalysis: Overlayers, Surface Alloys and Near-Surface Alloys of Cu/Pt(111)**

Therefore,thedesign of the appropriate active site is crucial to obtain highcatalytic activity, especially where multi-functionality isneeded. However, the control of a given surface on anatom-by-atom basis is particularly challenging.The electrochemical oxidation of CO is the prototypicalbifunctional reaction.

Evolution of the bulk structure and surface species on Fe–Ce catalysts during the Fischer–Tropsch synthesis

Two Fe–Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (α-Fe2O3), respectively. Their performance in the Fischer–Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free α-Fe2O3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts were tested for different periods of time, carefully passivated, recovered from the reactor and characterized by different techniques. The FT activity of the Ce-loaded and Ce-free catalysts decreased initially, but at a certain point the catalytic activity started to increase. The time needed to reach this inflection point depended on the catalyst composition, being shorter for the Ce-promoted catalysts. The catalytic activity of the Ce-free catalyst increased when the Fe3C species were transformed into χ-Fe2.5C, which are suggested to be the carbide phase present when polymerized carbon species (Cβ) are formed. The addition of Ce to the iron oxyhydroxide developed solids with a higher BET surface area. Besides, these samples displayed a higher FT activity at long time-on-stream (TOS). Moreover, Ce addition also facilitated the formation of the Cβ species previous to the evolution of Fe3C into χ-Fe2.5C, and therefore, promoted the FT synthesis reaction.

Effect of the N content of Fe/N/graphene catalysts for the oxygen reduction reaction in alkaline media

In this work a series of N–modified graphene composites with different N/C ratios have been synthesised. The incorporation of Fe atoms into the N–modified graphene composites leads to the formation of Fe/N/C ensembles on the outer graphene layers along with Fe3C and metallic Fe phases in the bulk of the graphite nanoplates as revealed by X-ray absorption and XPS analyses. The adequate choice of the N/C atomic ratio of precursors to prepare Fe/N/graphene based materials is crucial to obtain electrocatalysts with an optimal performance for the ORR. The activity for the oxygen reduction reaction (ORR) of the Fe/N–graphene based electrocatalysts increases with increasing amount of accessible nitrogen, that is, with the amount of nitrogen by surface area.

Evidences of Two‐Regimes in the Measurement of Ru Particle Size Effect for CO Dissociation during Fischer–Tropsch Synthesis

This work assesses the effect of the particle size for the Fischer–Tropsch synthesis with catalysts based on Ru particles between 4 and 71 nm. CO dissociation is a structure‐sensitive reaction, that is, the turnover frequency (TOF) is affected by Ru particle size. Herein it is demonstrated that two regimes exist for the effect of Ru particle size. On the one hand, the expected relationship of TOF with particle size, that is, the TOF increases with Ru particles <10 nm, is only observed if measured at steady‐state conditions. On the contrary, the TOF increases constantly with Ru particle size if measured at the initial state. However, the TOF measured for catalysts with Ru particles >10 nm declines during time on stream. These observations suggest that two regimes for the measurement of CO dissociation exist during Fischer–Tropsch synthesis. The reason for this is that two sites for CO dissociation on the Ru particles >10 nm exist at the initial state, terraces and step‐edges, but the latter ones deactivate during time on stream.

Identifying the time-dependent predominance regimes of step and terrace sites for the Fischer–Tropsch synthesis on ruthenium based catalysts

The Fischer–Tropsch synthesis (FTS) is an appealing process to generate liquid fuels from syngas. The elusive nature of FTS active sites is clarified here through a comprehensive theoretical-experimental study of CO dissociation on supported Ru catalysts. Our theoretical calculations show that boron (B) adsorption is stronger than that of CO on step-edge sites. In view of this, we deposited 2D-like Ru islands on the rutile phase of TiO2, and blocked step-edge sites by addition of B. Our results show that the initial reaction rate for the FTS of boron-dosed Ru/TiO2 is lower than that of clean Ru/TiO2 but the steady-state rates are identical. This indicates that CO dissociation can take place on Ru catalysts on steps and on terraces, but the contribution of each type of site to the overall CO dissociation rate varies during the reaction course: both types of reaction sites are simultaneously active on fresh catalysts under realistic FTS conditions, while only terraces sites are active under steady-state operation conditions.

Fischer-Tropsch Synthesis. Reduction Behavior and Catalytic Activity of Fe-Ce Systems

Several Fe‐Ce catalysts for FT synthesis were prepared following two different methods: coprecipitation from Fe and Ce nitrate solutions and a physical mixture of pure Fe and Ce precursors. The iron phases present in the activated catalysts were identified by XRD and Mossbauer spectroscopy. A good correlation between both techniques was found. The results revealed that the cerium oxide in the samples prepared by coprecipitation produces two effects: (i), stabilization of metastable species (Fe1−xO), and (ii), a decrease in the crystallite size of the iron species upon increasing Ce‐contents, as inferred from an increase in superparamagnetic species. The catalysts were tested in CO hydrogenation in a flow reactor. It was found that selectivity towards light olefins increases for the coprecipitated Ce‐containing catalysts, whereas CO conversion followed the opposite trend. Since the Fe1−xO phase was detected in these catalysts, it is suggested that the formation of the Fe1−xO phase would be responsible for ...