Zn on Pd (111)
Zn on Pd (111)
Zn on Pd (111)
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<str<strong>on</strong>g>Zn</str<strong>on</strong>g> <strong>on</strong> <strong>Pd</strong> (<strong>111</strong>)Basic Investigati<strong>on</strong>s to understand the behaviour of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> <strong>on</strong>m<strong>on</strong>ocrystaline PalladiumJörg KillmannSeminar Festkörperphysik SS08Tu Graz
Points of interest• Motivati<strong>on</strong>• Literature Data• Results• Quantificati<strong>on</strong> of them<strong>on</strong>olayer• C<strong>on</strong>clusi<strong>on</strong>• Acknowledgement
Motivati<strong>on</strong><strong>Pd</strong>/<str<strong>on</strong>g>Zn</str<strong>on</strong>g>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 + <str<strong>on</strong>g>Zn</str<strong>on</strong>g>O is already used, T Sintering Problem• <strong>Pd</strong> <str<strong>on</strong>g>Zn</str<strong>on</strong>g>O has similar electr<strong>on</strong>ic properties
Literature Data• Bayer (2006): XPS, UPS, LEED• Gabasch (2006): TDS, LEED• Jeroro (2007): TDS, LEED, HREELBayer et al: <strong>Pd</strong> 3D 5/2 XPS Spektra after depositi<strong>on</strong> of 1 ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g>(30s) @ 105K and annealing
• Bayer (2006): XPS, UPS, LEEDBayer et al: <strong>Pd</strong> 3D5/2 XPS Spektra after depositi<strong>on</strong> of 1 ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g>(30s) @ 105K and annealingBayer et al: Development of the Intensities of<strong>Pd</strong>3d 5/2 compenents
• Gabasch (2006)Gabasch et al: <str<strong>on</strong>g>Zn</str<strong>on</strong>g> Tds Spectra obtained after depositi<strong>on</strong> of variousamounts of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> at 300K
• Gabasch (2006)Gabasch et al:Proposed modell for the interacti<strong>on</strong> of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> with <strong>Pd</strong>
(2007) • JeroroJeroro et al: Co Tds <strong>on</strong> <strong>Pd</strong>/<str<strong>on</strong>g>Zn</str<strong>on</strong>g> (annealedat 500K) as functi<strong>on</strong> of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> coverage
(2007) • JeroroJeroro et al: Co Tds <strong>on</strong> <strong>Pd</strong>/<str<strong>on</strong>g>Zn</str<strong>on</strong>g> (annealedat 500K) as functi<strong>on</strong> of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> coverageJeroro et al: <strong>Pd</strong>/<str<strong>on</strong>g>Zn</str<strong>on</strong>g> annealed to550K. 0.5ML CO afterwards
Comparis<strong>on</strong>Structure :Bayer: 2x1 , surface alloy 300 -550K,bulk alloy >550K,Gabasch: 2x2 3 <str<strong>on</strong>g>Zn</str<strong>on</strong>g> adlayer <str<strong>on</strong>g>Zn</str<strong>on</strong>g> diffuses in <strong>Pd</strong> at T>650Kforming(4 *sqrt3 /3 x 6 )Jeroro:2x1,2x2,4x4
Comparis<strong>on</strong>Coverage quantificati<strong>on</strong>:Bayer: XPS, CO ditrati<strong>on</strong> Up to 3ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g> form<strong>Pd</strong><str<strong>on</strong>g>Zn</str<strong>on</strong>g> surface alloyGabasch: CO, O, Microb.,TDS 1 ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g> in high Temp. peakJeroro:CO, Microb.
Results
(Weirum STM (@GunterSTM, ½ ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g> <strong>on</strong> <strong>Pd</strong> at 300K. Dendritic growth, 2x1 visible
(Weirum STM (@Gunter• 2x1 Alloy• Holes are 2.4 Å deep• 2x1 Alloy in HolesSTM, 2 ML <str<strong>on</strong>g>Zn</str<strong>on</strong>g> <strong>on</strong> <strong>Pd</strong> at elevated T
<str<strong>on</strong>g>Zn</str<strong>on</strong>g>/<strong>Pd</strong>(<strong>111</strong>) tds spectra<str<strong>on</strong>g>Zn</str<strong>on</strong>g> coverage series prepared at 300K
Auger Peak intensity vs T<str<strong>on</strong>g>Zn</str<strong>on</strong>g>/<strong>Pd</strong> deposited at 150K, annealed stepwise.<str<strong>on</strong>g>Zn</str<strong>on</strong>g> Signal stays c<strong>on</strong>stant till 400K, decreases slowly till750K and rapidly afterwards. With low starting coverage <str<strong>on</strong>g>Zn</str<strong>on</strong>g>Signal stays c<strong>on</strong>stant to 750K, dropping off at that point.
M<strong>on</strong>olayer quantificati<strong>on</strong>A M<strong>on</strong>olayer is defined as 1 adsorbed atom per substrat atom. We used differentmethods to get knowledge of it.• Micro balance. Direct calculati<strong>on</strong> of Hz corresp<strong>on</strong>ding to 1 ML.• Co titrati<strong>on</strong>• Change of slope in uptake curve• Decrease of Auger <strong>Pd</strong> Signal due to Inelastic mean free path of electr<strong>on</strong>s in <str<strong>on</strong>g>Zn</str<strong>on</strong>g>
M<strong>on</strong>olayer quantificati<strong>on</strong>Micro balance:• Sticking coef. of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> is very sensibleto c<strong>on</strong>taminati<strong>on</strong>s <strong>on</strong> surface• Micro balance stabilizes after severaldays of evaperati<strong>on</strong>• Geometry of UHV system has impactDifferent amounts of <str<strong>on</strong>g>Zn</str<strong>on</strong>g> were prepared @ 300K <strong>on</strong>Sulfur c<strong>on</strong>taminated <strong>Pd</strong>. <strong>Pd</strong> was sputtered (cleaned)stepwiseML= 20Hz
M<strong>on</strong>olayer quantificati<strong>on</strong>Uptake curve:• Change of sticking coefficient of <str<strong>on</strong>g>Zn</str<strong>on</strong>g><strong>on</strong> <strong>Pd</strong> inflicts a slope change of theuptake curve• Not observed• Sticking coef. also mostlyindependent from T<str<strong>on</strong>g>Zn</str<strong>on</strong>g>/<strong>Pd</strong> Coverage Seriesat 200 and 300K
M<strong>on</strong>olayer quantificati<strong>on</strong>Auger Intensity decrease :• Auger is surface sensitive due toIMFP• By knowing the theretical, andmeasured, development of AugerPeak height of <strong>Pd</strong> during adsorpti<strong>on</strong>of <str<strong>on</strong>g>Zn</str<strong>on</strong>g>, the ML can be found.• Not accurate<strong>Pd</strong> <str<strong>on</strong>g>Zn</str<strong>on</strong>g> coverage series @ 160Klogarithmic plotML= 27Hz
M<strong>on</strong>olayer quantificati<strong>on</strong>We assume the ML toaccord to a frequencychange of 23 Hz.• β 2c<strong>on</strong>sists of 1.8 to 2.5 ML• These two ML form an <strong>Pd</strong>/<str<strong>on</strong>g>Zn</str<strong>on</strong>g> alloy(2x1)• Additi<strong>on</strong>al <str<strong>on</strong>g>Zn</str<strong>on</strong>g> 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<str<strong>on</strong>g>Zn</str<strong>on</strong>g>/<strong>Pd</strong> coverage Series, <str<strong>on</strong>g>Zn</str<strong>on</strong>g> deposited at @300K
Overview• 300K: Alloying starts• 430K: Desorpti<strong>on</strong> ofthick(>2.5) Layers of zn• -700K: <str<strong>on</strong>g>Zn</str<strong>on</strong>g> diffuses reversiblyinto the <strong>Pd</strong> bulk• 750K: Start of desorpti<strong>on</strong>• 750K: Evaporati<strong>on</strong> at this Tinduces very thick Bulk alloywith 4 * √3 /3 x 6 superstructure100hz <str<strong>on</strong>g>Zn</str<strong>on</strong>g> @ 750K, cooled downafterwards, 59, 70 eV
AcknowledgementAdolf WinklerMarkus KratzerAnt<strong>on</strong> TamtöglSpecial thanks go to:Gunther WeirumRobert SchennachHans Peter Koch
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