Metallic Pt as active sites for the water-gas shift reaction on alkali-promoted supported catalysts

The promotional effect of alkali additives (Na, Li, and K) in the low-temperature (200-250 degrees C) water-gas shift reaction was studied for Pt/Al2O3 and Pt/TiO2 catalysts. Sodium showed the highest promotion of the turnover frequency (TOF), normalized by Pt on the surface. The TOF at 250 degrees C and 6.8% CO, 22% H2O, 37% H-2, and 8.5% CO2 was enhanced by Na up to 107 times (0.7 s(-1)) that of Pt/Al2O3 (7 x 10(-3) s(-1)) and up to four times (0.7 s(-1)) the TOF of Pt/P25 TiO2 (0.2 s(-1)). The addition of Na on Pt/Al2O3 and Pt/TiO2 changed the reaction orders, with H2O order increasing by similar to 0.3, H-2 order increasing by 0.1-0.3, CO2 order decreasing by 0.2, and CO order decreasing by 0.1-0.2. Together with an approximately 20 kJ mole(-1) increase in E-a, these data suggest Na creates the same type of active sites on both supports. X-ray absorption spectroscopy (XAS) experiments under WGS conditions indicate that the formation of Pt oxides is dependent on the catalyst preparation method, the type, and the loading of alkali. On catalysts where Pt oxides were observed, no correlation was found between the TOF and the fraction of PtO. The series of Pt/Na/Al2O3 catalysts with the highest TOF showed fully reduced Pt under WGS conditions. Our results show that the promotion by alkali is caused by the modification of the properties of the support and that the active Pt remains in the metallic state. Thus, for metallic catalysts, alkali is added to the growing list of promoters that modify the support to improve water activation and the WGS reaction rate. (C) 2011 Elsevier Inc. All rights reserved.