Tuning the surface reactivity of oxides by peroxide species

The Mars-van Krevelen mechanism is the foundation for oxide-catalyzed oxidation reactions and relies on spatiotemporally separated redox steps. Herein, we demonstrate the tunability of this separation with peroxide species formed by excessively adsorbed oxygen, thereby modifying the catalytic activity and selectivity of the oxide. Using CuO as an example, we show that a surface layer of peroxide species acts as a promotor to significantly enhance CuO reducibility in favor of H2 oxidation but conversely as an inhibitor to suppress CuO reduction against CO oxidation. Together with atomistic modeling, we identify that this opposite effect of the peroxide on the two oxidation reactions stems from its modification on coordinately unsaturated sites of the oxide surface. By differentiating the chemical functionality between lattice oxygen and peroxide, these results are closely relevant to a wide range of catalytic oxidation reactions using excessively adsorbed oxygen to activate lattice oxygen and tune the activity and selectivity of redox sites.

Identifying the Role of Dynamic Surface Hydroxides in the Dehydrogenation of Ti-Doped NaAlH4

Solid-state metal hydrides are prime candidates to replace compressed hydrogen for fuel cell vehicles due to their high volumetric capacities. Sodium aluminum hydride has long been studied as an archetype for higher-capacity metal hydrides, with improved reversibility demonstrated through the addition of titanium catalysts; however, atomistic mechanisms for surface processes, including hydrogen desorption, are still uncertain. Here, operando and ex situ measurements from a suite of diagnostic tools probing multiple length scales are combined with ab initio simulations to provide a detailed and unbiased view of the evolution of the surface chemistry during hydrogen release. In contrast to some previously proposed mechanisms, the titanium dopant does not directly facilitate desorption at the surface. Instead, oxidized surface species, even on well-protected NaAlH₄ samples, evolve during dehydrogenation to form surface hydroxides with differing levels of hydrogen saturation. Additionally, the presence of these oxidized species leads to considerably lower computed barriers for H₂ formation compared to pristine hydride surfaces, suggesting that oxygen may actively participate in hydrogen release, rather than merely inhibiting diffusion as is commonly presumed. These results demonstrate how close experiment-theory feedback can elucidate mechanistic understanding of complex metal hydride chemistry and potentially impactful roles of unavoidable surface impurities.

Synthesis and Structural Evolution of Nickel-Cobalt Nanoparticles Under H2 and CO2

Bimetallic nanoparticle (NP) catalysts are interesting for the development of selective catalysts in reactions such as the reduction of CO2 by H2 to form hydrocarbons. Here the synthesis of Ni-Co NPs is studied, and the morphological and structural changes resulting from their activation (via oxidation/reduction cycles), and from their operation under reaction conditions, are presented. Using ambient-pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and transmission electron microscopy, it is found that the initial core-shell structure evolves to form a surface alloy due to nickel migration from the core. Interestingly, the core consists of a Ni-rich single crystal and a void with sharp interfaces. Residual phosphorous species, coming from the ligands used for synthesis, are found initially concentrated in the NP core, which later diffuse to the surface.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).