9258
J. Phys. Chem. B 1998, 102, 9258-9265
Surface Characterization of Cu-M (M ) Ti, Zr, or Hf) Alloy Powder Catalysts A Ä rpa´ d Molna´ r* Department of Organic Chemistry, Jo´ zsef Attila UniVersity, Do´ m te´ r 8, H-6720, Szeged, Hungary
Imre Berto´ ti and Ja´ nos Sze´ pvo1 lgyi Research Laboratory of Materials and EnVironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525, Budapest, Hungary
Gabriele Mulas and Giorgio Cocco Dipartimento di Chimica, UniVersita´ di Sassari, Via Vienna 2, I-07100 Sassari, Italy ReceiVed: May 12, 1998
X-ray photoelectron spectroscopy was used for surface characterization of amorphous Cu-M alloy powders (Cu40Ti60, Cu50Zr50, and Cu65Hf35) produced by mechanical alloying of the constituents. When used as catalyst in the dehydrogenation of alcohols Cu-Zr and Cu-Hf were found to undergo structural changes resulting in surface copper enrichment with a substantial fraction of copper being in fully reduced, metallic (Cu0) state. This result accounts for the high and stable catalytic activity of Cu-Zr and Cu-Hf in the transformations of alcohols. In contrast, when Cu-Ti is used in inert atmosphere, surface enrichment of titanium is observed. Although segregation of copper occurs in the presence of hydrogen, only oxidized copper species are detected on the surface. In either case, the catalytic activity of Cu-Ti is inferior to the other two alloy samples. TiH2 is shown by XRD to be the characteristic species formed under hydrogen. This is stable in the presence of hydrogen and, therefore, Cu acts as an oxygen scavenger and forms oxidized surface species that do not show catalytic activity.
Introduction It has been found in a recent study1 that copper-containing bicomponent amorphous alloy powders produced by mechanical alloying undergo in situ activation when applied as catalyst in the transformation of various alcohols at elevated temperature (523-573 K). In the dehydrogenation of 2-propanol to acetone and in the transformation of allyl alcohol to form propanal and 1-propanol, active, stable, and selective catalysts are generated from Cu-Zr and Cu-Hf. DSC and XRD data indicated bulk crystallization. Measurement of Cu0 surface area and preliminary XPS studies showed copper segregation to the surface. CuTi, in contrast, exhibited negligible activity, and its surface features were different from those of the other two alloys. Here the results of a detailed study with respect to the surface characteristics of all three copper alloy catalysts are reported. An attempt is also made to rationalize their catalytic behavior on the basis of surface features. Experimental Section Materials. Copper (-150 mesh, 99.5%, impurities in ppm: Sn 130, Fe 100, Ca 40, Si 25, Ag 15, Mg 15, Ni 15, Zn 10, Co 4, Zr 3), titanium (-100 mesh, 99.9%, impurities in ppm: Zr 130, Cr 120, Al 55, Ni 40, Ca 15, Mn 12), and hafnium (-325 mesh, 99.5%, contains 3.5% zirconium, impurities in ppm: O 620, N 120, C
