Adsorption and Hydrogenation of Carbon Monoxide on Polycrystalline Rhodium at High Gas Pressures

• Published in: The journal of physical chemistry. B 1997-04, Vol.101 (15), p.2874-2883
• Main Authors: Williams, Christopher T, Black, Carol A, Weaver, Michael J, Takoudis, Christos G
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp963429e

Surface-enhanced Raman spectroscopy (SERS) in conjunction with mass spectroscopy (MS) has been utilized to investigate the adsorption and hydrogenation of carbon monoxide on polycrystalline rhodium surfaces. The SERS-active Rh substrates were prepared by electrodeposition of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400 °C) and pressures (here up to 1 atm). The SER spectra reveal that CO adsorbed primarily on atop sites (νRh - C = 470 cm-1) and desorbed by about 250−300 °C under all gas-phase conditions examined. Partial dissociation of the CO adlayer, however, was obtained at temperatures as low as 100 °C, most likely facilitated by the large number of steps and kinks present on these roughened surfaces. This was evidenced by a partial removal of CO at temperatures (ca 100 °C) well below those expected for thermal desorption (200−250 °C) and supported by the observed formation of surface carbonate (665 cm-1) under these conditions. The CO dissociation, however, is hampered at lower temperatures (250 °C) when a low percentage of CO (≤1%) in the feed reactant stream was used. The influence of the adsorbed species on the overall methanation rates is discussed in light of these findings. Utilizing the seconds time-scale resolution of SERS, the exchange between gas-phase and adsorbed CO was also studied. The results of such transient 13CO/12CO exchange experiments reveal that this desorption pathway is weakly activated (≈1 kcal mol-1), first order with respect to CO coverage, yet independent of CO partial pressure in the regime studied (8−760 Torr). This contrasts the first-order pressure dependence for much lower CO partial pressures (≤10-5 Torr) reported earlier in the literature. A rate law and mechanism are proposed which ac