Active gold species on cerium oxide nanoshapes for methanol steam reforming and the water gas shift reactions

We report that a small amount (<1 at%) of gold on ceria single crystals prepared as nanorods (10 +/- 2.8 by 50-200 nm) of {110} and {100} crystal surfaces shows excellent catalytic activity in both the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions at low temperatures (<250 degrees C). The ceria nanorods bind and stabilize gold as atoms and clusters (<1 nm, TEM invisible). On the other hand, gold nanoparticles (similar to 3 nm) are found on the {100} surfaces of ceria nanocubes. Very low rates of SRM and WGS were measured on the Au-ceria {100} cubes, while the rates on Au-ceria {110} rods were at least an order of magnitude higher. However, the apparent activation energies did not depend on the shape of ceria. Strong bonded Au(n)-O-Ce species are the active sites and these are present only in negligible concentrations on the {100} surfaces. Thus, both reactions are structure-insensitive on Au-ceria. SRM proceeds through the methyl formate route. The Au-ceria {110} catalyst shows both high SRM activity and high selectivity to CO(2) at temperatures below 250 degrees C.