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Abstract: The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary and secondary () interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of , both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage. [Copyright &y& Elsevier]

Abstract: Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e., the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1wt.% Pd/Al2O3 and Density Functional Theory on Pd(111) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Brønsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3). [Copyright &y& Elsevier]

We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue are the groups of Prof. Javier Pérez-Ramírez at ETH Zurich, Switzerland, and Prof. Núria López at the Institute of Chemical Research of Catalonia (ICIQ), Spain. The cover illustrates major evolutionary changes in the design of palladium catalysts for alkyne semi-hydrogenation. Read the full text of the article at . [ABSTRACT FROM AUTHOR]