Zn on Pd (111)

<strong>Zn</strong> on Pd (111)Basic Investigations to understand the behaviour of <strong>Zn</strong> onmonocrystaline PalladiumJörg KillmannSeminar Festkörperphysik SS08Tu Graz

Points of interest• Motivation• Literature Data• Results• Quantification of themonolayer• Conclusion• Acknowledgement

MotivationPd/<strong>Zn</strong>O as a catalyst for steamreforming of methanol.Methanol can be used for H 2storageCH 3OH + H 2O CO 2+ 3H 2• It is important that few to no CO is formed• Need high selective catalyst• Cu + <strong>Zn</strong>O is already used, T Sintering Problem• Pd <strong>Zn</strong>O has similar electronic properties

Literature Data• Bayer (2006): XPS, UPS, LEED• Gabasch (2006): TDS, LEED• Jeroro (2007): TDS, LEED, HREELBayer et al: Pd 3D 5/2 XPS Spektra after deposition of 1 ML <strong>Zn</strong>(‏‎30s‏)‏ @ 105K and annealing

• Bayer (2006): XPS, UPS, LEEDBayer et al: Pd 3D5/2 XPS Spektra after deposition of 1 ML <strong>Zn</strong>(‏‎30s‏)‏ @ 105K and annealingBayer et al: Development of the Intensities ofPd3d 5/2 compenents

• Gabasch (2006)Gabasch et al: <strong>Zn</strong> Tds Spectra obtained after deposition of variousamounts of <strong>Zn</strong> at 300K

• Gabasch (2006)Gabasch et al:Proposed modell for the interaction of <strong>Zn</strong> with Pd

(‏‎2007‎‏)‏ • JeroroJeroro et al: Co Tds on Pd/<strong>Zn</strong> (annealedat 500K) as function of <strong>Zn</strong> coverage

(‏‎2007‎‏)‏ • JeroroJeroro et al: Co Tds on Pd/<strong>Zn</strong> (annealedat 500K) as function of <strong>Zn</strong> coverageJeroro et al: Pd/<strong>Zn</strong> annealed to550K. 0.5ML CO afterwards

ComparisonStructure :Bayer: 2x1 , surface alloy 300 -550K,bulk alloy >550K,Gabasch: 2x2 3 <strong>Zn</strong> adlayer <strong>Zn</strong> diffuses in Pd at T>650Kforming(4 *sqrt3 /3 x 6 )‏Jeroro:2x1,2x2,4x4

ComparisonCoverage quantification:Bayer: XPS, CO ditration Up to 3ML <strong>Zn</strong> formPd<strong>Zn</strong> surface alloyGabasch: CO, O, Microb.,TDS 1 ML <strong>Zn</strong> in high Temp. peakJeroro:CO, Microb.

Results

(‏Weirum STM (@GunterSTM, ½ ML <strong>Zn</strong> on Pd at 300K. Dendritic growth, 2x1 visible

(‏Weirum STM (@Gunter• 2x1 Alloy• Holes are 2.4 Å deep• 2x1 Alloy in HolesSTM, 2 ML <strong>Zn</strong> on Pd at elevated T

<strong>Zn</strong>/Pd(111) tds spectra<strong>Zn</strong> coverage series prepared at 300K

Auger Peak intensity vs T<strong>Zn</strong>/Pd deposited at 150K, annealed stepwise.<strong>Zn</strong> Signal stays constant till 400K, decreases slowly till750K and rapidly afterwards. With low starting coverage <strong>Zn</strong>Signal stays constant to 750K, dropping off at that point.

Monolayer quantificationA Monolayer is defined as 1 adsorbed atom per substrat atom. We used differentmethods to get knowledge of it.• Micro balance. Direct calculation of Hz corresponding to 1 ML.• Co titration• Change of slope in uptake curve• Decrease of Auger Pd Signal due to Inelastic mean free path of electrons in <strong>Zn</strong>

Monolayer quantificationMicro balance:• Sticking coef. of <strong>Zn</strong> is very sensibleto contaminations on surface• Micro balance stabilizes after severaldays of evaperation• Geometry of UHV system has impactDifferent amounts of <strong>Zn</strong> were prepared @ 300K onSulfur contaminated Pd. Pd was sputtered (cleaned)stepwiseML= 20Hz

Monolayer quantificationUptake curve:• Change of sticking coefficient of <strong>Zn</strong>on Pd inflicts a slope change of theuptake curve• Not observed• Sticking coef. also mostlyindependent from T<strong>Zn</strong>/Pd Coverage Seriesat 200 and 300K

Monolayer quantificationAuger Intensity decrease :• Auger is surface sensitive due toIMFP• By knowing the theretical, andmeasured, development of AugerPeak height of Pd during adsorptionof <strong>Zn</strong>, the ML can be found.• Not accuratePd <strong>Zn</strong> coverage series @ 160Klogarithmic plotML= 27Hz

Monolayer quantificationWe assume the ML toaccord to a frequencychange of 23 Hz.• β 2consists of 1.8 to 2.5 ML• These two ML form an Pd/<strong>Zn</strong> alloy(‏‎2x1‎‏)‏• Additional <strong>Zn</strong> forms LowTP• β 1: 1 st Order, 1 - 2 ML• α 1-α 3:0 Order, 1 – 3 ML• α 4: 0 Order, Multilayer peak<strong>Zn</strong>/Pd coverage Series, <strong>Zn</strong> deposited at @300K

Overview• 300K: Alloying starts• 430K: Desorption ofthick(>2.5) Layers of zn• -700K: <strong>Zn</strong> diffuses reversiblyinto the Pd bulk• 750K: Start of desorption• 750K: Evaporation at this Tinduces very thick Bulk alloywith 4 * √3 /3 x 6 superstructure100hz <strong>Zn</strong> @ 750K, cooled downafterwards, 59, 70 eV

AcknowledgementAdolf WinklerMarkus KratzerAnton TamtöglSpecial thanks go to:Gunther WeirumRobert SchennachHans Peter Koch

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