$YNAMIC฀ OBSERVATIONS฀OF฀ !U฀CATALYSTS฀BY฀ ENVIRONMENTAL฀ ELECTRON฀MICROSCOPY 3฀'IORGIO฀-฀#ABIm฀#2฀(ENRY #).!-  ฀฀#.23฀#AMPUS฀DE฀,UMINY฀#ASE฀฀฀ -ARSEILLE฀#EDEX฀฀&RANCE  ฀!SSOCIATED฀TO฀THE฀5NIVERSITIES฀OF฀!IX฀-ARSEILLE฀฀AND฀ !BSTRACT !U฀CLUSTERS฀IN฀THE฀SIZE฀RANGE฀฀NM฀SUPPORTED฀ON฀ -G/฀AND฀4I/  ฀ARE฀STUDIED฀AT฀THE฀ATOMIC฀SCALE฀BY฀฀ (IGH฀2ESOLUTION฀4RANSMISSION฀%LECTRON฀-ICROSCOPY฀ (24%-฀IN฀STANDARD฀CONDITIONS฀AND฀DURING฀CYCLES฀฀ OF฀GAS฀TREATMENTS฀BY฀ENVIRONMENTAL฀(24%- 4HEIR฀MORPHOLOGY฀THE฀STRUCTURE฀OF฀THE฀INTERFACE฀฀ WITH฀THE฀SUBSTRATE฀AND฀THEIR฀ADHESION฀ENERGY฀ARE฀ DEDUCED฀FROM฀TOP฀AND฀PROｽLE฀VIEWS฀ACCORDING฀TO฀฀ THE฀PREPARATION฀TECHNIQUES฀BY฀ATOMIC฀DEPOSITION฀฀ IN฀ULTRA฀HIGH฀VACUUM฀OR฀BY฀DEPOSITION฀PRECIPITATION฀ IN฀WET฀CONDITIONS 5NDER฀HYDROGEN฀ATMOSPHERE฀THE฀GOLD฀CLUSTERS฀ EXHIBIT฀A฀TRUNCATED฀OCTAHEDRON฀SHAPE฀WHILE฀UNDER฀ OXYGEN฀THEY฀SHOW฀A฀ROUNDING฀OF฀THE฀SHAPE฀4HESE฀ EVOLUTION฀ARE฀REVERSIBLE฀IN฀HYDROGEN฀฀OXYGEN฀CYCLES฀ )N฀THE฀CASE฀OF฀THE฀#/฀OXIDATION฀BY฀!U฀CLUSTERS฀ SUPPORTED฀ON฀4I/  ฀THE฀FORMATION฀OF฀#/  ฀IN฀THE฀ MICROSCOPE฀SAMPLE฀HOLDER฀IS฀EVIDENCED฀BY฀A฀ DIFFERENTIALLY฀PUMPED฀MASS฀SPECTROMETER฀CONNECTED฀ TO฀THE฀ENVIRONMENTAL฀SAMPLE฀HOLDER฀AT฀THE฀SAME฀ TIME฀AS฀THE฀ROUNDING฀OF฀THE฀CLUSTERS฀4HE฀IMPLICATION฀ OF฀THE฀SHAPE฀EVOLUTION฀IN฀#/฀OXIDATION฀IS฀DISCUSSED )NTRODUCTION -฀ (ARUTA฀ AND฀ COWORKERS฀ ;=฀ HAVE฀ SHOWN฀ THAT฀ NANOMETER฀ SIZED฀ GOLD฀ CLUSTERS฀ ON฀ SEVERAL฀ OXIDE฀ SURFACES฀ WERE฀ VERY฀ ACTIVE฀ FOR฀ #/฀ OXIDATION฀ AT฀ LOW฀ TEMPERATURE฀ WHEREAS฀ GOLD฀ IN฀THE฀BULK฀STATE฀IS฀CATALYTICALLY฀INACTIVE฀;=฀!N฀EXPLOSION฀OF฀ PAPERS฀HAS฀APPEARED฀IN฀RECENT฀YEARS฀4HE฀CATALYTIC฀ASPECT฀OF฀ THESES฀STUDIES฀;=฀AS฀WELL฀THE฀SURFACE฀SCIENCE฀ASPECT฀฀;=฀ARE฀ WELL฀DESCRIBED฀IN฀SOME฀RECENT฀REVIEWS฀;=฀ )T฀TURNS฀OUT฀THAT฀!U฀PARTICLES฀SUPPORTED฀ON฀SEVERAL฀OXIDES฀ SUCH฀ AS฀ 4I/  ฀ -G/฀ &E  /  ฀ #O/X฀ .I/฀ IN฀ THE฀ SIZE฀ RANGE฀ ฀ NM฀ HAVE฀ IMPORTANT฀ CATALYTIC฀ ACTIVITIES฀ )T฀ IS฀ GENERALLY฀ OBSERVED฀ THAT฀ BY฀ DECREASING฀ PARTICLE฀ SIZE฀ BELOW฀ ฀ NM฀ THE฀ RATE฀ OF฀ THE฀ CATALYTIC฀ OXIDATION฀ OF฀ #/฀ INCREASES฀ AND฀ THAT฀ IF฀ THE฀!U฀CLUSTERS฀ARE฀SUPPORTED฀ON฀MORE฀REDUCIBLE฀OXIDES฀THE฀ ACTIVITY฀ IS฀ ALSO฀ HIGHER฀ (OWEVER฀ IT฀ IS฀ DIFｽCULT฀ TO฀ DISENTANGLE฀ BETWEEN฀ THE฀ SUPPORT฀ AND฀ SIZE฀ EFFECTS฀ BECAUSE฀ DEPENDING฀ ON฀THE฀PREPARATION฀METHOD฀AND฀ON฀THE฀NATURE฀OF฀THE฀SUPPORT฀ PARTICLES฀WITH฀DIFFERENT฀SIZES฀ARE฀OBTAINED฀4O฀OVERCOME฀THIS฀ PROBLEM฀ 3฀ !RII฀ ET฀ AL฀ ;=฀ HAVE฀ DEPOSITED฀ SIZE฀ SELECTED฀ GOLD฀ CLUSTERS฀PREPARED฀BY฀LASER฀VAPORIZATION฀WITH฀A฀MEAN฀SIZE฀OF฀ ฀TO฀฀NM฀DETERMINED฀BY฀4%-฀ON฀ALUMINA฀ZIRCONIA฀AND฀ TITANIA฀OXIDE฀POWDERS฀4HEY฀CLEARLY฀FOUND฀THAT฀GOLD฀PARTICLES฀ ARE฀A฀LITTLE฀MORE฀ACTIVE฀ON฀TITANIA฀THAN฀ON฀ZIRCONIA฀AND฀MUCH฀ MORE฀ACTIVE฀ON฀THESE฀TWO฀SUPPORTS฀THAN฀ON฀ALUMINA฀4HESE฀ RESULTS฀CLEARLY฀EVIDENCE฀A฀SUPPORT฀EFFECT฀ !N฀ INTIMATE฀ CONTACT฀ BETWEEN฀ THE฀ !U฀ DEPOSIT฀ AND฀ THE฀ OXIDE฀ SURFACE฀ INCREASES฀ THE฀ CATALYTIC฀ ACTIVITY฀ ;=฀ 4HE฀ ACTIVE฀ INTERFACIAL฀GOLD฀ATOMS฀COULD฀BE฀CATIONIC฀;฀= !S฀ CONCERNING฀ THE฀ INCREASE฀ OF฀ ACTIVITY฀ BY฀ DECREASING฀ CLUSTERS฀ SIZE฀ IT฀ HAS฀ BEEN฀ SHOWN฀ THAT฀ ฀ ATOMS฀ GOLD฀ CLUSTERS฀ DEPOSITED฀ON฀-G/฀ｽLMS฀ARE฀ACTIVE฀FOR฀#/฀OXIDATION฀AT฀ROOM฀ TEMPERATURE฀24฀;=฀(OWEVER฀IT฀NOT฀YET฀KNOWN฀IF฀THESE฀FEW฀ ATOMS฀CLUSTERS฀ARE฀MORE฀ACTIVE฀THAN฀฀NM฀CLUSTERS฀GENERALLY฀ USED฀IN฀CATALYSIS฀BY฀GOLD฀!NOTHER฀IMPORTANT฀FACTOR฀IN฀CATALYSIS฀ IS฀ THE฀ MORPHOLOGY฀ OF฀ THE฀ METAL฀ CLUSTERS฀ ;=฀ 4HE฀ SHAPE฀ OF฀ GOLD฀ PARTICLES฀ PREPARED฀ BY฀ VACUUM฀ EVAPORATION฀ ON฀ -G/฀ SUBSTRATES฀ AT฀ A฀ TEMPERATURE฀ OF฀ AT฀ LEAST฀ #฀ HAS฀ BEEN฀ STUDIED฀BY฀EX฀SITU฀(24%-฀฀;=฀AND฀BY฀IN฀SITU฀!&-฀;฀=฀ 4HEY฀PRESENT฀A฀TRUNCATED฀OCTAHEDRON฀SHAPE฀EXPOSING฀SMOOTH฀ ฀ AND฀ ฀ FACETS฀ !฀ MOLECULAR฀ BEAM฀ STUDY฀ OF฀ THE฀ #/฀ OXIDATION฀ON฀GOLD฀CLUSTERS฀GROWN฀UNDER฀5(6฀ON฀-G/฀฀ SINGLE฀CRYSTALS฀HAS฀SHOWN฀THAT฀NO฀ACTIVITY฀WAS฀DETECTABLE฀FOR฀ PARTICLES฀LARGER฀THAN฀฀NM฀AND฀THAT฀฀NM฀CLUSTERS฀WERE฀VERY฀ ACTIVE฀ AT฀ 24฀ REACTION฀ PROBABILITY฀ OF฀ ฀ ;=฀ 4HE฀ SAMPLE฀ WITH฀฀NM฀PARTICLES฀WERE฀PREPARED฀AT฀24฀AND฀THE฀PARTICLES฀ PRESENTED฀ FACETED฀ FCC฀ SHAPES฀ MULTIPLE฀ TWINS฀ WITH฀ ｽVEFOLD฀ SYMMETRY฀ AND฀ FOR฀ A฀ LARGE฀ NUMBER฀ UNIDENTIｽED฀ SHAPES฀ !S฀ CONCERNING฀GOLD฀CLUSTERS฀PREPARED฀BY฀WET฀CHEMICAL฀METHODS฀ IT฀ HAS฀ BEEN฀ SHOWN฀ BY฀ (24%-฀ OBSERVATION฀ THAT฀ FOR฀ GOLD฀ PARTICLES฀ PREPARED฀ BY฀ DEPOSITION฀ PRECIPITATION฀ ON฀ 4I/  ฀ POWDER฀ THE฀ PARTICLES฀ WITH฀ A฀ WELL฀ FACETED฀ TRUNCATED฀ OCTAHEDRON฀ SHAPE฀ WERE฀ LESS฀ ACTIVE฀ THAN฀ THOSE฀ WITH฀ A฀ MORE฀ ROUNDED฀SHAPE฀;= 4HE฀ ORIGIN฀ OF฀ THE฀ CATALYTIC฀ PROPERTIES฀ IN฀ THE฀ CASE฀ OF฀ NANOMETER฀SIZED฀CLUSTERS฀IS฀ATTRIBUTED฀TO฀DIFFERENT฀FACTORS฀AS฀ THE฀EXISTENCE฀OF฀LOW฀COORDINATION฀SITES฀EDGES฀CORNERS฀;=฀ OR฀INTERFACE฀SITES฀;=฀CHARGE฀TRANSFER฀BETWEEN฀THE฀OXIDE฀AND฀ 'OLD฀"ULLETIN฀฀฀ 