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#