TPD-ClassPresentation-2 · 2012. 9. 21. · 0.000# 0.002# 0.004# 0.006# 0.008# 0.010# 0.012# 500#...
Transcript of TPD-ClassPresentation-2 · 2012. 9. 21. · 0.000# 0.002# 0.004# 0.006# 0.008# 0.010# 0.012# 500#...
0.000#
0.002#
0.004#
0.006#
0.008#
0.010#
0.012#
500# 520# 540# 560# 580# 600# 620# 640# 660# 680# 700#
β=5#s%1#
β=10#s%1#
β=30#s%1#
β=50#s%1#
Rate#of#desorption#vs.#temperature#(K)#
0#
0.002#
0.004#
0.006#
0.008#
0.01#
0.012#
0.014#
0.016#
200# 300# 400# 500# 600# 700# 800# 900# 1000#
25#kcal/mol#
35#kcal/mol#
50#kcal/mol#
Rate#of#desorption#
Temperature#(K)#
1st#Order#
2nd#Order#
(From#King,#1975)#
!#CO#on#Pt(111):#Activation#energy#of#~100kJ/mol#can#be#estimated##(from##http://www.sci.wsu.edu/idea/TPD/)#
β" Tp# ln(tp2/b)# 1/tp#5" 563" 11.0" 0.00177"10" 573" 10.4" 0.00174"20" 588" 9.8" 0.00170"
y#=#17012x#%#19.207#R²#=#0.98788#
9.6#
9.8#
10#
10.2#
10.4#
10.6#
10.8#
11#
11.2#
0.00168# 0.0017# 0.00172# 0.00174# 0.00176# 0.00178#
E/R# 17012" ""E# 34024" kcal/mol"
With#data#at#three#heating#rates,#the#activation#energy#for#this#simulated#data#is#estimated#at#34#kcal/mol#
Redhead’s#plot#method#=>#E~150kJ/mol#
y#=#12603x#%#11.642#R²#=#0.97718#
8#
9#
10#
11#
12#
13#
0.0017# 0.00175# 0.0018# 0.00185# 0.0019# 0.00195#
ln(Tp^2/b)+vs.+1/Tp++
Peak#temperature#method#=>#E~105#kJ/mol#
Why#are#the#predicted#activation#energies#from#the#two#methods#so#different?#
(From#Masuda#et#al.,#1997)#
0#
50#
100#
150#
200#
250#
100# 200# 300# 400# 500# 600# 700# 800# 900#
Eact(kJ/mol)#vs#Tp#(K)#
E~100kJ/mol#
Can’t#use#Redhead#plot!#(why?)#
Don’t#have#data#at#different#heating#rates,#can’t#use#Peak#Temperature#method#
• The#spectrum#has#three#distinct#peaks.#• These#correspond#to#three#states#of#H2#
adsorption#on#the#catalyst#• In#this#exercise,#lets#focus#on#the#first#(low#
temperature)#state#
A#single#heating#rate#~10#K/min#
Let#us#just#guess#at#the#Eact#based#on#the#following#‘Redheadlike’#plot#
(From#Smeds#et#al.,#1996)#
(Han#et#al.,#2006)#
• The#oxygen#TPD#spectrum#demonstrates#one#peak#whereas#the#NO2#demonstrates#several#peaks#
#• Comparing#with#previous#slide,#the#
800K#peak#is#indicative#of#Edes~50kcal/mol#
• The#complex#shapes#of#TPD#curves#indicate#effects#such#as#coverage%dependent#kinetic#parameters,#multiple#adsorbed#states,#etc.##
• Unsymmetric#shape!#• Two#distinct#(but#overlapping)#
peaks#are#visible#• Tp#for#first#peak#is#not#changing#as#
the#initial#coverage#changes#• However,#it#changes#a#fair#bit#for#
the#second#one#• The#Pd#foil#spectrum#is#simpler;#
occurs#at#higher#temperatures#
• First#order#reaction;#followed#by#higher#order#reaction#
• Molecular#adsorption,#followed#by#dissociation#
• Weakening#of#the#adsorption#bond#because#of#the#alloying#
(From#Johanek#t#al.,#2001)#
! The#TPD#spectra#exhibit#several#interesting#features#including#heating#rate#&#initial#coverage#dependence#
! Analysis#of#TPD#spectra#allows#us#to#logically#derive#several#bits#of#information#about#adsorptions#and#desorptions…#
! If#you#look#at#enough#number#of#TPD#spectra,#however#complex#they#are,#you’ll#become#good#at#identifying#important#features#(like#in#quizzes…#)#
! D.#A.#King,#“Thermal#desorption#from#metal#surfaces:#A#review,”#Surface#Sciences#47#(1975)#384#
! T.#Masuda,#Y.#Fujikata,#H.#Ikeda,#S.#Matsushita,#K.#Hashimoto,#“A#method#for#calculating#the#activation#energy#distribution#for#desorption#of#ammonia#using#a#TPD#spectrum#obtained#under#desorption#control#conditions,”#Applied#Catalysis#A:#General#162#(1997)#2940#
! P.#A.#Redhead,#“Thermal#desorption#of#gases,”#Vacuum#12#(1962)#203#
! S.#Smeds,#T.#Salmi,#L.#P.#Lindfors,#0.Krause,#“Chemisorption#and#TPD#studies#of#hydrogen#on#Ni/AI203,”#Applied#Catalysis#A:#General#144#(1996)#177e194#
! V.#Johanek,#N.#Tsud,#V.#Matolin,#I.#Stara,"“TPD#and#XPS#study#of#the#CO#adsorption#on#transitioneSP#metal#systems:#Pd#and#Al,”#Vacuum#63#(2001)#15e22#