Understanding the Reactivity of Oxide-Supported Bimetallic Clusters:  Reaction of NO with CO on TiO₂(110)-Supported Pt−Rh Clusters

Abstract

The reactions of CO, NO, and NO with CO have been studied on Pt, Rh, and bimetallic Pt-Rh clusters deposited on TiO 2 (110). The following four cluster surfaces were investigated: 4 ML of Rh, 4 ML of Pt, 2 ML of Rh + 2 ML of Pt (Rh + Pt), and 2 ML of Pt + 2 ML of Rh (Pt + Rh). Scanning tunneling microscopy studies demonstrated that the surfaces exhibited similar cluster sizes and densities, and low-energy ion scattering experiments showed that the surfaces of the bimetallic clusters were Pt-rich (20-30% Rh) regardless of the order of metal deposition; therefore, both Pt and Rh atoms are capable of diffusing to the cluster surface at room temperature. Notably, heating the surface caused substantial encapsulation of the metal clusters by titania at 700 K and complete encapsulation at 800 K. In temperature programmed desorption experiments, the activities of the Pt and Rh clusters for CO and NO dissociation were found to be higher than those of the (111) surfaces of the corresponding single crystals. For both reactions, the activities of the Rh + Pt and Pt + Rh clusters were identical to each other and intermediate between that of pure Rh and pure Pt. For the reaction of NO with CO, the bimetallic clusters exhibited the greatest production of CO 2 and the highest fraction of NO dissociation. On pure Rh clusters, CO 2 production is inhibited by the preferential adsorption of NO over CO, whereas on the pure Pt clusters, CO adsorption is favored over NO. Only the Pt-Rh surfaces can provide sites for both NO dissociation and CO adsorption that are necessary for facilitating CO 2 formation.

Characterization of Pt–Au and Ni–Au Clusters on TiO2(110)

“…Experiments were conducted on a rutile TiO 2 (110) crystal (1 cm 9 1 cm 9 0.1 cm, Princeton Scientific Corp.) in two ultrahigh vacuum (UHV) chambers, which have been previously described in detail [10,23,29,30,[32][33][34][35]. LEIS and STM experiments were carried out in the first chamber, which has a base pressure of B5 9 10 -11 Torr, and is equipped with a fine focus ion gun (Omicron, ISE100) and hemispherical analyzer (EA125) for LEIS experiments, as well as a variable-temperature STM (Omicron, [10,23,30,32].…”

“…LEIS and STM experiments were carried out in the first chamber, which has a base pressure of B5 9 10 -11 Torr, and is equipped with a fine focus ion gun (Omicron, ISE100) and hemispherical analyzer (EA125) for LEIS experiments, as well as a variable-temperature STM (Omicron, [10,23,30,32]. In the second chamber, soft X-ray photoelectron spectroscopy (sXPS) experiments were conducted on the U12a beamline at the National Synchrotron Light Source at a chamber pressure of B2 9 10 -10 Torr [29,[33][34][35].…”

“…bombardment for *20 min, followed by annealing at 900-1050 K. The cleanliness of the surface was confirmed with a combination of STM, XPS, LEIS and LEED experiments. The crystal was attached to a Ta backplate using Ta foil straps, and the temperature of the sample was measured with a Type C or Type K thermocouple spot-welded to the backplate; the thermocouple readings were independently calibrated with an infrared pyrometer [10].…”

“…In general, it has been shown that the bimetallic surfaces have properties that are different from the pure metal constituents [1][2][3][4][5]. In some cases, each metal facilitates different steps in a reaction mechanism [6][7][8] or provides binding sites for different adsorbates [9,10]. In other cases, electronic interactions between the metals or the creation of new mixed-metal sites give rise to different chemical activity [3,5,[11][12][13][14][15][16][17].…”

“…During this treatment, the crystal becomes an n-type semiconductor through the loss of oxygen from the crystal. Furthermore, titania is a support that is known to participate in catalytic reactions by providing a source of oxygen for reactions on the metal particles [10,[23][24][25]. Because metals like Pt [26][27][28][29][30] and Ni [23,25] interact strongly with titania while Au [31] does not, it may be possible to tune the extent of cluster-support interactions by changing the bimetallic compositions.…”