Ambient Pressure Studies of Surface Reactions over Pd/Cu Single-Atom Alloys

Single-atom alloys (SAA) offer a promising route towards more selective catalysts that utilize lower amounts of rare and expensive transition metals. The SAA can be formed from the low coverage deposition of a catalytically active Pt-group metal onto a less active metal. These materials exhibit unique catalytic activity compared to their monometallic counterparts, often combining the activity of the Pt-group metal and the selectivity of the metal host. Characterizing these materials under reaction conditions is of considerable interest to heterogeneous catalysis research. Active sites of heterogeneous catalysts exhibit dynamics that make them difficult to characterize, especially under reaction conditions. It is therefore relevant to probe the surface structure of the alloys in the presence of gas-phase reactants. Polarization dependent reflection absorption infrared spectroscopy (PD-RAIRS) has been utilized as a major tool to study the Pd/Cu(111) SAA surface. The presence of Pd as isolated single atoms in the Cu(111) surface is established using adsorbed CO as a probe with PD-RAIRS. The properties of the SAA surface are further characterized with Auger spectroscopy (AES), and temperature programmed desorption (TPD) of H2. In the presence of 1×10−2 Torr of CO at 300 K, significant CO coverages are only achieved when Pd is present on the surface. The surface Pd coverage is determined from CO peak areas in the PD-RAIR spectra. PD-RAIRS has been used to study C2H2 hydrogenation between 180 and 500 K on clean Cu(111) and Pd/Cu(111). At a total pressure of 1 Torr and a C2H2:H2 ratio of 1:100, annealing the SAA with less than 1% Pd to 480 K leads to complete conversion of all gas phase C2H2 to gas phase ethylene (C2H4), without any gas phase ethane (C2H6) produced. At elevated pressures, adsorbates have the ability to form bonds to surface atoms with bonds stronger than those between surface atoms, causing surfaces to restructure. This is especially true for CO at elevated pressures on Cu(111). Peaks for adsorbed CO on the corners and edges of nanoclusters formed in-situ show the ability of PD-RAIRS to examine structural changes to surfaces. The PD-RAIRS technique has been extended to study the SAA Pd/Cu(111) system for ambient pressure CO2 hydrogenation. Formate, acetate, and methoxy formation are observed when the Cu(111) surface is annealed to 550 K in the presence of 50 Torr of CO2 and 150 Torr of H2. Surface restructuring of Pd atoms on Cu(111) to become more dispersed at high temperatures is evidenced by PD-RAIR spectra of CO.