SUPPLEMENTARY INFORMATION

ATR-FTIR Spectrokinetic Analysis of the CO Adsorption and Oxidation at Water/Platinum Interface

Alejo Aguirre, Claudio L. A. Berli, Sebastián E. Collins*

Instituto de Desarrollo Tecnológico para la Industria Química (INTEC), Universidad

Nacional del Litoral, CONICET. Güemes 3450, S3000GLN, Santa Fe, Argentina.

SI 1. Analysis of intra-particle mass transport

The particles in the bed may have an inner porous structure, thus concentration

gradients could take place inside the particles. This problem is well known in

heterogeneous catalysis. For a first order reaction and spherical particles, the

concentration of the adsorbate into the particle as a function of the r coordinate is given

by [1,2]:

Ci

Ci0 =( R

r ) senh [ϕ (r /R ) ]senh (ϕ) [ 1

Φp ] (SI.1)

where, R is the particle radius, ϕ=R3 √ ε p

−1av ka Γ 0

Deff

is the Thiele modulus and

Φ p=[ ϕtanh (ϕ )

−1] 1Shp

+1. Deff is the effective diffusion coefficient, εp is the porosity of

the particle and Shp is the Sherwood number for the particle; Shp=k lp R /Deff , where kl

p is

the mass-transfer coefficient outside the particle. To neglect the intra-particle gradient,

the Thieles modulus must be very small (ϕ→0). We can compare ϕ with ϕcl:

ϕcl

ϕ=3 b

R √ ε−1

ε p−1 ≈ b

R (SI.2)

To get good spectroscopy and kinetic results, the particle size must be much smaller

than the bed height (b>>R) [4], then, the Thieles modulus for the particle is smaller

than the Thiele modulus for the catalyst layer. Therefore, if the absence of concentration

gradients in the bed is guaranteed (ϕcl2 < 0,1); it can ensure that gradients inside the

particles are negligible.

SI 2. Parameters for Pt/Al2O3

The effective diffusion coefficient was estimated as [3]:

Deff =ε D i

τ=0.4 Di (SI.3)

The product avΓ0was calculated as:

av Γ 0=ρ (% Pt ) D

M Pt(SI.4)

Where ρis the catalyst density, D is the dispersion (61%), %Pt is the metal charge and

MPt is the platinum molar mass.

SI 3. Fitting of the CO oxidation with a one site model

(i) CO adsorption on Pt sites:

CO+Pt❑→CO−Pt

(ii) dissociative adsorption of O2 on Pt sites:

O2+2Pt❑→2O−Pt❑

(iii) CO oxidation on Pt sites:

CO−Pt❑+O−Pt❑→C O2+2 Pt❑

Figure S1.

SI 4. CO oxidation model

The differential equations described in section 3.3 in terms of the relative surface

coverage (θ) of each species, are obtained after normalizing by the total number of

platinum sites per unit area (ΓPt0). Then, the apparent kinetic constants for each step are:

k1(s−1)=k t Γ Pt

0 (SI.5)

k 2(s−1)=kr(1−f )Γ Pt0 (SI.6)

k3(s−1)=2 kaO2CO2

0 (1−f )Γ Pt0 (SI.7)

k 4(s−1)=2 kaO2 )¿CO2

0 (1−f )Γ Pt0 (SI.8)

where kt(mol cm-2 s-1) is the interconversion constant, kr (mol cm-2 s-1) is the oxidation

constant, CO2

0 is the oxygen concentration in the bulk solution (1.27x10-3 M); k aO 2 and

kaO2 )¿ (mol cm-2 M-1 s-1) are the dissociative adsorption constants of O2 on low-

coordination Pt sites and on defects Pt sites [5]. The total number of low-coordination Pt

sites is Γ PtI =(1−f )Γ Pt

0 ; and the total number of high-coordination Pt sites is Γ PtII =f Γ Pt

0 .

The error between the model and the experimental data was calculated by setting as the

normalized integrated IR signal ( A ¿ the sum of the relative surface coverage of CO on

each sites:

A=θCOI (1− f )+θCO

II f (SI.9)

Figure S.2: Evolution of the surface coverage of CO-PtI (black), CO-PtII (red) and O-PtI

(blue) as function of time as predicted by the model for (A) Pt/ZnSe and (B) Pt/Al2O3

SI 5. Mass transport limitations in CO oxidation.

The consumption of dissolved O2 is due the adsorption process. So, the mass balance for

O2 at the reaction interface in the non-porous film case is [6]:

k l (CO2

0 −CO2)=Γ 0 ,Pt r ads (SI.10)

where Γ0,Pt is the number of binding sites per surface area for the Pt film and rads is the

adsorption rate described by:

rads=12

CO2

¿ [k3 (1−θCOI −θO

I )2+k4 (1−θOI )2 ] (SI.11)

Then, the dimensionless concentration gradient is:

(1−CO 2

¿ )=Γ0 , Pt

kl CO 2

0 rads (SI.12)

The highest gradient concentration occurs when the adsorption rate reach a maximum (

radsmax¿. The rate of adsorption could be obtained from the solution of the differential

equations described in section 3.3. The radsmax was 0.0142 s-1 at 97s. Therefore, the Da and

Bi numbers can be obtained as:

Da=Γ0 , Pt

kl CO 2

0 radsmax

(SI.13)

Bi=hΓ0 , Pt

DO2CO 2

0 radsmax

(SI.14)

Where DO2 is the oxygen diffusion coefficient.

For the catalyst layer, the normalized and simplified oxygen mass balance, described by

eq 12 is:

d2C i¿

d b¿2 =ε−1av Γ 0b

2

Deff CO 2

0 rads (SI.15)

The dimensionless gradient at z=b, described by boundary condition 15 is:

(1−CO2

¿ b )= ε−1 av Γ0 bk lCO2

0 r ads (SI.16)

The radsmax was 0.0183 s-1 at 92 s for the catalyst layer. Therefore, the Thiele modulus (ϕcl)

and the ϕcl2 /Sh ratio can be obtained as:

ϕcl2 =

ε−1 av Γ0 b2

D eff CO2

0 radsmax (SI.17)

ϕcl2

S h=

ε−1 av Γ0 bk lCO2

0 r adsmax (SI.18)

References:

1. G.F. Froment, K.B. Bischoff, J. De Wilde, Chemical Reactor Analysis and

Design, third ed., J. Wiley, New York, 2011.

2. R. Bird, W.E. Stewart, E. N. Lightfoot, Transport phenomena, second ed., J.

Wiley, New York, 2002.

3. R.H. Perry, D.W. Green. Perry's Chemical Engineers' Handbook, Seventh

Edition, McGraw-Hills, New York, 1999.

4. T. Bürgi, A. Baiker, Adv. Catal. 50 (2006) 227-283.

5. V.P. Zhdanov, Surf. Sci. Rep. 45 (2002) 231-326.

6. A. Aguirre, P.A. Kler, C.L.A. Berli, S.E. Collins, Chem. Eng. J. 243 (2014) 197-

206.