Identification and Hydrogenation of C2 on Pt(111)

Selective formation of acetate intermediate prolongs robust ethylene removal at 0 °C for 15 days

Efficient ethylene (C₂H₄) removal below room temperatures, especially near 0  °C, is of great importance to suppress that the vegetables and fruits spoil during cold-chain transportation and storage. However, no catalysts have been developed to fulfill the longer-than-2-h C₂H₄ removal at this low temperature effectively. Here we prepare gold-platinum (Au-Pt) nanoalloy catalysts that show robust C₂H₄ (of 50 ppm) removal capacity at 0 °C for 15 days (360 h). We find, by virtue of operando Fourier transformed infrared spectroscopy and online temperature-programmed desorption equipped mass spectrometry, that the Au-Pt nanoalloys favor the formation of acetate from selective C₂H₄ oxidation. And this on-site-formed acetate intermediate would partially cover the catalyst surface at 0 °C, thus exposing active sites to prolong the continuous and effective C₂H₄ removal. We also demonstrate, by heat treatment, that the performance of the used catalysts will be fully recovered for at least two times.

Ethylene: Its adsorption, reaction, and coking on Pt/h-BN/Rh(111) nanocluster arrays

We present well-ordered Pt nanocluster arrays supported on the h-BN/Rh(111) Moiré as a model system for an ethylene dehydrogenation catalyst. Thereby, the h-BN nanomesh serves as a chemically inert eggbox-like template for clusters with a narrow size distribution. The thermal evolution of ethylene is investigated by synchrotron-based high-resolution in situ x-ray photoelectron spectroscopy on the Pt nanoclusters. We compare our results with data on Pt(111) and Pt(355). Interestingly, the Pt nanoclusters and Pt(355) behave very similarly. Both open a new reaction pathway via vinylidene in addition to the route via ethylidyne known for Pt(111). Due to the importance of coking in ethylene dehydrogenation on Pt catalysts, we also studied C₂H₄ adsorption and decomposition on carbon precovered Pt nanoclusters. While the amount of adsorbed ethylene decreases linearly with the carbon coverage, we found that edge sites are more affected than facet sites and that the vinylidene reaction pathway is effectively suppressed by carbon residues.

New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions

Infrared absorption spectroscopy has proven to be one of the most powerful spectroscopic techniques available for the characterization of catalytic systems. Although the history of IR absorption spectroscopy in catalysis is long, the technique continues to provide key fundamental information about a variety of catalysts and catalytic reactions, and to also offer novel options for the acquisition of new information on both reaction mechanisms and the nature of the solids used as catalysts. In this review, an overview is provided of the main contributions that have been derived from IR absorption spectroscopy studies of catalytic systems, and a discussion is included on new trends and new potential directions of research involving IR in catalysis. We start by briefly describing the power of Fourier-transform IR (FTIR) instruments and the main experimental IR setups available, namely, transmission (TIR), diffuse reflectance (DRIFTS), attenuated total reflection (ATR-IR), and reflection-absorption (RAIRS), for advancing research in catalysis. We then discuss the different environments under which IR characterization of catalysts is carried out, including in situ and operando studies of typical catalytic processes in gas-phase, research with model catalysts in ultrahigh vacuum (UHV) and so-called high-pressure cell instruments, and work involving liquid/solid interfaces. A presentation of the type of information extracted from IR data follows in terms of the identification of adsorbed intermediates, the characterization of the surfaces of the catalysts themselves, the quantitation of IR intensities to extract surface coverages, and the use of probe molecules to identify and titrate specific catalytic sites. Finally, the different options for carrying out kinetic studies with temporal resolution such as rapid-scan FTIR, step-scan FTIR, and the use of tunable lasers or synchrotron sources, and to obtain spatially resolved spectra, by sample rastering or by 2D imaging, are introduced.

Unusual Bonding in Platinum Carbido Clusters

Vibrational spectroscopy and density functional theory calculations are used to determine the structures of small gas-phase platinum carbido clusters PtnC(+), n = 3-5. The carbon atom is found to prefer three-coordinate binding sites near the center of the cluster, in contrast to most previously investigated adatoms on transition metal clusters. The Pt3C unit is particularly stable, and binding of the carbon atom also leads to significant rearrangement of the metal framework when compared to the bare clusters.

An effusive molecular beam technique for studies of polyatomic gas-surface reactivity and energy transfer

An effusive molecular beam technique is described to measure alkane dissociative sticking coefficients, S(T(g), T(s); ϑ), on metal surfaces for which the impinging gas temperature, T(g), and surface temperature, T(s), can be independently varied, along with the angle of incidence, ϑ, of the impinging gas. Effusive beam experiments with T(g) = T(s) = T allow for determination of angle-resolved dissociative sticking coefficients, S(T; ϑ), which when averaged over the cos (ϑ)/π angular distribution appropriate to the impinging flux from a thermal ambient gas yield the thermal dissociative sticking coefficient, S(T). Nonequilibrium S(T(g), T(s); ϑ) measurements for which T(g) ≠ T(s) provide additional opportunities to characterize the transition state and gas-surface energy transfer at reactive energies. A resistively heated effusive molecular beam doser controls the T(g) of the impinging gas striking the surface. The flux of molecules striking the surface from the effusive beam is determined from knowledge of the dosing geometry, chamber pressure, and pumping speed. Separate experiments with a calibrated leak serve to fix the chamber pumping speed. Postdosing Auger electron spectroscopy is used to measure the carbon of the alkyl radical reaction product that is deposited on the surface as a result of alkane dissociative sticking. As implemented in a typical ultrahigh vacuum chamber for surface analysis, the technique has provided access to a dynamic range of roughly 6 orders of magnitude in the initial dissociative sticking coefficient for small alkanes on Pt(111).