chapter  11
32 Pages

Supported Metal Oxides and the Surface Density Metric

WithWilliam V. Knowles, Michael O. Nutt, Michael S. Wong

Abstract .................................................................................................................. 252 11.1 Introduction................................................................................................ 252 11.2 Synthesis of Supported Metal Oxide Catalysts ........................................ 253

11.2.1 Impregnation ............................................................................... 253 11.2.2 Equilibrium Adsorption .............................................................. 254 11.2.3 Deposition-Precipitation ............................................................. 254 11.2.4 Grafting ....................................................................................... 255 11.2.5 Coprecipitation............................................................................ 255 11.2.6 Role of Calcination..................................................................... 256

11.3 Molecular Surface Structures .................................................................... 257 11.3.1 Isolated Species .......................................................................... 257 11.3.2 Oligomeric and Polymerized Species ........................................ 258 11.3.3 Crystalline Species...................................................................... 258

11.4 Determination of Surface Saturation and Monolayer Coverage .............. 259 11.4.1 Idealized Models of SMOs......................................................... 261 11.4.2 X-ray Diffraction and Raman Spectroscopy .............................. 262 11.4.3 X-ray Photoelectron Spectroscopy and Ion

Scattering Spectroscopy.............................................................. 262 11.4.4 Infrared Spectroscopy and Selective Chemisorption ................. 263 11.4.5 UV-Vis Diffuse Reflectance Spectroscopy................................. 265

11.5 Characterization of Surface Density ......................................................... 266 11.5.1 Determination of Surface Oxide Content................................... 266 11.5.2 Considerations for Surface Area ................................................ 267 11.5.3 Calculation Methods ................................................................... 268 11.5.4 Most Appropriate Surface Density Calculation Method ........... 269

11.6 Comparing Published Surface Density Values for Various WOx/ZrO2 Materials..................................................................... 270 11.6.1 Wachs and Coworkers ................................................................ 271

11.6.2 Iglesia and Coworkers ................................................................ 271 11.6.3 Knözinger and Coworkers .......................................................... 273

11.7 Summary.................................................................................................... 275 11.8 Appendix.................................................................................................... 275

11.8.1 Preparation Method for ZrOx(OH)4-2x Support .......................... 275 11.8.2 General Incipient Wetness Impregnation Method...................... 275

Acknowledgments.................................................................................................. 276 References.............................................................................................................. 276

Supported metal oxides (SMOs) comprise a large class of catalytic materials used in numerous industrial processes. There are many conventional approaches to preparing these materials, ranging from impregnation and equilibrium adsorption to grafting and coprecipitation. Independent of preparation methods, one of the key metrics in characterizing SMOs is surface density, which quantifies the amount of the supported metal oxide relative to the underlying support surface area. Catalytic activity is correlated to the surface density-dependent structure of the supported species. There are different definitions for surface density and different methods for its determination, though, causing some difficulties in reconciling structure-activity results reported by different researchers. Here, a rigorous analysis of the different surface density calculation methods is presented, using tungstated zirconia as an example.