Tao Wang,Benlong Yu,Long Mu,Zhilin Chen,Chao Xiao,Shuming Peng
The palladium oxide phase (PdOx) plays a critical role in catalytic methane oxidation and CO oxidation reactions. Precise characterization of supported PdOx by CO-FTIR spectroscopy is often accompanied by the reduction of PdOx by CO. Lowering the CO partial pressure to 0.05% CO/N2 (0.5 mbar) and the adsorption temperature to −130 °C is a proper condition to inhibit the reduction completely. Then CO-FTIR spectroscopy was conducted on a series of supported PdOx catalysts. We find that CO adsorbed on coordinatively unsaturated Pd atoms (Pdcus) on the PdOx surface gives three adsorption bands at ∼2145 cm–1. Three types of surface Pdcus atoms with different degrees of coordination unsaturation (types I, II, and III) exist in supported PdOx catalysts; their νCO are located at 2133, 2145, and 2160 cm–1, respectively. The order of coordination unsaturation of surface Pdcus should be type III < type II < type I. Since νCO of surface Pdcus is between νCO of Pd2+(CO) and Pd+(CO), the effective charge of surface Pdcus is 1 < n < 2. For small PdOx clusters in zeolite-supported PdOx catalysts, surface Pdcus atoms with more coordination unsaturation (i.e., step, edge, and corner sites) are more abundant, leading to greater IR intensity of adsorbed CO.
Tao Wang,Qiao Yu,Zhilin Chen,Chao Xiao,Shuming Peng
The active phase for methane combustion is still in debate. Low-temperature CO-FTIR spectroscopy was used to probe the working surface state on a series of Pd/Al2O3 (PdAl) and Pd/silicalite-1 (PdSil) catalysts with different starting states: oxidized, reduced (R), reoxidized (Ro). The surface of prereduced PdAl (Pd0) is transformed to an oxidized state (PdOx) with a CO stretching vibration (νCO) at ∼2157 cm–1 when it is aged in the methane oxidation mixture. The aging-induced transformation from Pd0 to PdOx occurred at 250 °C or above. Although PdSil has multiple kinds of PdOx entities, prereduced PdSil also transformed into PdOx. For PdAl(4)-R-A250, XRD patterns suggest weak bulk oxidation of Pd0 at 250 °C, XPS spectra indicate 28% of surface Pd0 atoms are oxidized, but CO-FTIR spectra confirm all the “top surface” Pd0 atoms are oxidized. It demonstrates the advantage of CO-FTIR spectroscopy in probing the top surface state. Finally, a direct correlation between operando FTIR spectroscopy and low-temperature CO-FTIR spectroscopy was established. Operando FTIR spectra over PdSil during methane oxidation at 250 °C gives two bands at 2124 cm–1 (CO–Pd+) and 2161 cm–1 (CO–Pd2+). After cooling in nitrogen, CO-FTIR spectra give two similar bands at 2134 cm–1 and 2157 cm–1. The “working surface state” can be preserved in CO-FTIR. For PdAl(4), operando FTIR spectra at 250 and 300 °C give no bands of adsorbed CO. Adding CO into the methane oxidation mixture resulted in a single CO–Pd2+ band at 2163 cm–1, which is in accord with their CO-FTIR spectra. For operando FTIR spectroscopy, the observation of the bands of adsorbed CO depends on the accumulation of the CO intermediates, but their accumulation is limited to reaction temperatures below 300 °C and to PdSil (not PdAl). These limitations make low-temperature CO-FTIR spectroscopy more general and sensitive.
危险品化学品经营许可证（带存储）