(736d) Selective Hydrogenation With Ag-Doped Pt Nanoparticle Catalysts

Alloying has been a successful approach to modify the reactivity of metal nanoparticle surfaces; for example, dopant atoms on the nanoparticle surface can increase turnover, improve selectivity, enhance resistance to deactivation, and promote multiple reaction functionality. However, the reactivity of alloyed NP catalysts often evolves during reaction, particularly at high temperatures, where atoms can easily diffuse and alter surface composition and structure. Dynamic rearrangements, coupled with adsorption of strongly bound intermediates, can make “stable” surface compositions which promote high reactivity difficult to achieve over a large range of operating conditions. As such, gaining insight into improving the activity and selectivity of alloy NP catalysts is a challenging task.

    In this work, Ag-doped Pt nanoparticles (NP) were tested for continuous-flow C₂H₂ hydrogenation reactivity and CO adsorption−desorption from 100 to 300 °C to investigate how submonolayer levels of an insoluble metal dopant distribute on steps and terraces of a NP surface and modify reactivity [1,2]. Ag incorporation onto Pt step and terrace sites, and resultant temperature-dependent desorption behavior, were probed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO. At low coverage, Ag appeared to occupy Pt step sites; for higher coverage, Ag occupied both step and terrace sites, which was observed to lower the CO desorption temperature. The catalytic influence of sub-monolayer Ag incorporation into Pt NP surfaces was also probed in-situ during batch and continuous-flow C₂H₂ hydrogenation. NP catalysts were found to be highly active and selective for C₂H₂ to C₂H₄ hydrogenation. Ag-doped Pt NP catalysts with ~0.5 ML Ag showed increased selectivity with temperature up to 200 ^oC, in contrast to commercial hydrogenation catalysts (e.g., Pt, Pd, PdAg) that are known to have activity-selectivity tradeoffs. Highly correlated reactivity and DRIFTS data highlight the importance of step site blocking and desorption from Pt during selective C₂H₂ hydrogenation. Furthermore, measurements over multiple reaction cycles from 100 to 300 to 100 °C demonstrate how immiscible metal dopants can preferentially modify NP surface compositions and promote active, selective, and stable catalysts over large temperature operating windows.

[1] L.C. Jones, Z. Buras, M.J. Gordon, J. Phys. Chem. C 116, 12982 (2012).

[2] L.C. Jones, M.J. Gordon, J. Phys. Chem. C 116, 23472 (2012).