Adsorption of cyclohexane and benzene on ordered tin/platinum (111) surface alloys

Abstract

Cyclohexane conversion to benzene over bimetallic Pt catalysts is an important prototypical reaction for fundamental studies of selective catalytic dehydrogenation catalysis. We have studied the adsorption and dehydrogenation of cyclohexane and benzene on Pt( 1 1 1) and two ordered Sn/Pt( 1 1 1) surface alloys using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and sticking coefficient measurements. Vapor-depositing Sn on the Pt(ll1) surface and annealing gives a (2 X 2) or ( 4 3 X d3)R3Oo LEED pattern, producing well-defined surfaces identified as the (1 1 1) face of Pt3Sn and a substitutional alloy of composition Pt2Sn. Cyclohexane adsorbed onto all three surfaces at 100-1 55 K with an initial sticking coefficient of unity. Precursor mediated adsorption kinetics are indicated by the coverage independence of the initial sticking coefficient up to l/3-1/4 of saturation coverage of the monolayer (B:& = 0.13 ML). The desorption temperature of the chemisorbed cyclohexane monolayer is decreased upon alloying of the Pt( 1 1 1) surface with Sn, and cyclohexane desorbs in a narrow peak characteristic of each surface. The adsorption energy is reduced from 58 kJ/mol on Pt( 11 1) by 9 kJ/mol on the (2 X 2) Sn/Pt( 11 1) surface alloy and by 12 kJ/mol on the ( d 3 X d3)R3Oo S n / P t ( l l l ) surface alloy,indicating an electronic effect of Sn on cyclohexane adsorption on the Pt(ll1) surface. In supporting studies of benzene adsorption, Sn converts most of the strongly chemisorbed benzene to physisorbed benzene. In contrast to the reduction in chemisorption, the initial sticking coefficient shows little dependence on the Sn concentration. This phenomenon is attributed to the presence of a modifier precursor state in benzene adsorption. The decomposition of benzene is completely suppressed under UHV conditions due to the presence of Sn in the Sn/Pt( 11 1) surface alloys. These observations bring new information into discussions of the role of Sn in selective catalytic dehydrogenation reactions over supported Pt-Sn catalysts.

Calorimetric Measurement of the Heat of Adsorption of Benzene on Pt(111)

“…Here, 1.00 ML is defined as the coverage at which the sticking probability drops to zero at 300 K (2.3 × 10 14 benzenes/cm 2 as determined in ref 31). The initial sticking probability, S o , of benzene on Pt(111) has been measured previously to be 1.0 at 100 K 40 and 0.95 at 200 K, 43 but to our knowledge, has not been measured at room temperature. These values are consistent with the present result (0.97 at 300 K), suggesting a nearly temperature-independent sticking probability below 300 K. We have fitted the coverage dependence of the sticking probability with the Kisliuk 61 model, which is a model for precursor-mediated sticking.…”

“…Thus, its adsorption energy is of substantial fundamental importance. At 300 K, benzene adsorbs molecularly on Pt(111) 30,33,36,43 with its molecular plane parallel to the surface, interacting with the Pt through its aromatic π electron system. 28,29,33,37,51 Thus, its heat of adsorption is 6 times the C-Pt(111) bond energy for a carbon atom in an aromatic ring parallel to the surface.…”

Surface alloys and alloy surfaces: the platinum-tin system

“…Simple "probe" gases such as CO have been studied after adsorption on this system [45] as well as a variety of organic molecules such as acetylene [46], cyclohexane and benzene [47,48], butane and isobutane [49], methanol, ethanol and water [50]. Several surface reactions of the above gases were also studied.…”

Effect of Coadsorbed Water on Deep Oxidation Mechanisms: Temperature-Programmed Reactions of Benzene and Hydroxyl on the Pt(111) Surface

“…The interactions of benzene with the Pt(111) surface have been thoroughly studied using a variety of surface science techniques [18][19][20][21][22][23][24][25][26]. Based on these characterizations, benzene pi-bonds to Pt(111) with the ring parallel to the plane of the surface [18,20,22].…”

“…Based on these characterizations, benzene pi-bonds to Pt(111) with the ring parallel to the plane of the surface [18,20,22]. The chemisorbed benzene layer consists of two states, which are desorbed with activation energies of 82-88 kJ/mol and 117-129 kJ/mol over the 280-520 K temperature range [19,21,24]. More recently DFT calculations have determined the most preferred site for benzene adsorption on the Pt(111) surface is the bridge site, which is occupied first during adsorption, while at higher coverages a threefold hollow site is occupied [26].…”