Water-Gas-Shift reaction over Pt/CeO 2 : spectators & reaction intermediates by an operando spectrokinetic analysis F.C. Meunier CenTACat, Queen’s University Belfast, UK Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

The H 2 -economy

Today’s Typical PEM Fuel Cell Note: PEMFC proton-exchanged membrane fuel cell polymer electrolyte membrane fuel cell SPEFC solid polymer electrolyte fuel cell

microns H2H2 O2O2 Today’s Typical PEM Fuel Cell

Steam reforming (SR) of Hydrocarbons Steam reforming of a hydrocarbon is essentially the reduction of water with this reductant to form a mixture of carbon oxides and hydrogen: a H 2 O + C x H y  b CO + c CO 2 + d H 2 The overall process requires 6 major steps:

Methane steam reforming equilibrium as a function of Temperature P = 1 bar H2O = 3 kmol CH4 = 1 kmol

 max yield of H 2 from CH 4 at high T  The proportion of CO/H 2 can be further modified by the water- gas-shift reaction CO + H 2 O  CO 2 + H 2  max yield of H 2 from CO at low T If only CO-free H 2 is required, need to use additional low T WGS reactors H2OH2O H2H2 CO 2 H2H2 CO WGS

Process diagram for H 2 production: reformer condition: PH 2 O/PCH 4 = , T exit = , P exit = atm

WGS Mechanisms over Ceria-based catalysts ATYPICAL FEED 2 H 2 CO O 2 H +  + The formate mechanism Shido, Iwasawa, J. Catal. 141 (1993) 71 Jacobs, Williams, Graham, Sparks, Davis, J. Phys. Chem. B 107 (2003) Jacobs, Davis, Appl. Catal B, 284 (2005) 31 OO HH Ce CO O C O H Ce O H O C OO H2OH2O OH H +H2+H2 CO 2 The Redox mechanism Bunluesin, Gorte, Graham, Appl. Catal. B, 15 (1998) 107 Li, Fu, Flytzani-Stephanopoulos, Appl. Catal. B, 27 (2000) H 2CeO OCe OH CO 2 (g) + Ce 2 O 3 2CeO 2 CO(ads) CO(g) + Pt + → + → + →

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

SSITKA: Steady-State Isotopic Transient Kinetic Analysis Combined IR (transmission) + MS / SSITKA Balakos, Chuang, Srinivas, J. Catal. 1993, 140, 281. For C isotopes: Conversion of syngas (CO / H 2 mixtures) CO 2 -reforming of methane Typically using either PFR or IR cell Combined DRIFTS + MS /SSITKA Single reactor

R R* Vent Tubular plug flow reactor Mass spectrometer R P R* P*P* SSITKA: Steady-State Isotopic Transient Kinetic Analysis time P

The “Classical” Analysis gives limited information   r = N Mean surface residence time (s) Rate of production (mol g -1 s -1 ) Concentration of active sites (mol g -1 ) Related to the “activity” of the sites Measured at steady state

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

Diffuse Reflectance Infrared Fourier Transform Spectroscopy What is it ? (DRIFTS) is a technique that collects and analyzes scattered IR energy. DRIFTS TRANSMISSION

R-WGS DRIFTS Setup O 2 H CO 2 H 2 +  + 2% Pt/CeO 2 (J.M.) Pre-reduced in 50% H 2 at 300°C T=225°C 1% 12 CO 2 + 4% H 2 Pre-reduced in 50% H 2 at 300°C T=225°C 1% 12 CO 2 + 4% H 2

Modified DRIFT cell: Residence Time Distribution Step Ar/Kr  Ar, 100 sccm total flow Kr normalised signal RTD /s

Non-modified DRIFT cell purge time ca. 300 s Flow rate: 150 sccm Jacobs and Davis, Appl. Catal. A 284 (2005) 31

Non-modified DRIFT cell purge time Formates Carbonyls Carbonates Dead zone? Jacobs and Davis, Appl. Catal. A 284 (2005) 31

Quantitative DRIFTS IoIo IoIo I Reflectance R oo = I / I 0 I Strongly absorbing matrix Olinger and Griffiths, Anal. Chem. 60 (1988) 2427, = “absorbance” Kubelka-Munk

Quantitative DRIFTS Wavenumber /cm Detector signal /a.u. Matyshak and Krylov, Catal. Today, 25, 1 (1995) I o : incident beam I : over catalyst I ads : for feed + catalyst I I ads R oo = I / I 0 R = I ads / I 0 IoIo

Quantitative DRIFTS If I ~ Io Kubelka-Munk ‘Absorbance` Matyshak-Krylov

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

2. Reactivity of surface species in different feeds ? Steady-state R-WGS feed ( 12 CO 2 +H 2 ) Pure Ar labelled R-WGS feed ( 13 CO 2 +H 2 ) Removal of 12 C-surface species Removal of 12 C-surface species

Assignment of the IR bands R-WGS: 1% 12 CO 2 + 4% H 2 T=225°C FORMATES C H OO CARBONATES C O OO 2059 C CARBONYLS O

Integration Method FORMATESCARBONATESCARBONYLS Formates: cm -1, baseline at 3050 cm -1 ; Carbonyls: cm -1, baseline at 2150 cm -1 ; Carbonates: cm -1, baseline at 795 cm -1. Needs calibration, see P. Hollins

Validation of the integration method

Reactivity in Ar at 225°C Wavenumber (cm -1 ) FormatesCarbonatesCarbonyls 12 CO 2 +H 2 then Ar

Reactivity in Ar at 225°C Reactivity: Formates >> Carbonates Steady state 1% CO 2 + 4% H 2 Pure Ar

Reactivity during SSITKA at 225°C Wavenumber (cm -1 ) FormatesCarbonatesCarbonyls 12 CO 2 +H 2 then 13 CO 2 +H 2

Reactivity: Carbonates >> Formates Steady-state 1% 12 CO 2 + 4% H 2 Steady-state 1% 13 CO 2 + 4% H 2 Reactivity during SSITKA at 225°C

Conclusion DRIFT experiments Reactivity under Ar NOT representative of true reactivity of the surface species Tibiletti et al., Chem. Comm. (2004) 1636 Isotopic exchangeDesorption / Reaction in Ar O 2 H CO 2 H 2 +  +

Why does the feed composition matter? feed oxidising/reducing nature modify ceria surface oxidation state Meunier et al., Appl. Catal. A (2005) in press modify bonding strength of chemisorbed species

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

SSITKA (Single reactor) MS Determination of the Rate of RDS DRIFT Investigation Surface Species 3. Spectators & Reaction Intermediates ?

R-WGS: SSITKA / DRIFT + MS Setup Bruker FTIR Hiden MS PE 8700 GC DRIFTS cell MFC H 2 Ar 12 CO 2 V MFC H 2 Ar He H 2 Ar H 2 V 13 CO 2 MFC H 2 Ar 12 CO 2 V Vent MFC H 2 Ar H 2 H 2 V 13 CO 2

SSITKA / DRIFT+MS experiments 1% 12 CO 2 + 4% H 2 then 1% 13 CO 2 + 4% H 2 O 2 H CO 2 H 2 +  +

Formates Carbonyls Carbonates R-WGS: SSITKA / DRIFT+MS Results  = time for 50% exchange 55 ±6 secCarbonate 54 ±6 secCarbonyl 660 ±30 secFormate 57 ±2 sec 13 CO(g) O 2 H CO 2 H 2 +  +

Number of CO(g) precursors Area = f (CO precursors at the surface) 2.2 x mol g cat -1

44 28 RWGS studied by TGA-MS at 225 °C 1% CO 2 + 4% H 2 in Ar Ar Moles of CO(g) (mol g cat -1 ) Formate conc. (mol g cat -1 ) 2.2 x < 1.4 x h mm H2H2 Minority of “fast” formates cannot account for CO formation

Moles of CO(g) (mol g cat -1 ) CO(g) /Pt surface 2.2 x times more CO(g) precursors than surface Pt atoms CeO 2 CO Pt X Carbonyls cannot be the ONLY reaction intermediate Number of CO(g) precursors

Reaction Mechanism R-WGS (Minor route) (Main route) (Minor route) A. Goguet et al., J Phys. Chem. B 108 (2004) 20240

Experimental conditions WGS feed: 100 sccm of 1% 12 CO+ 10% H 2 O in Ar T=200°C O 2 H CO 2 H 2 +  + WGS: SSITKA / DRIFT+MS

WGS: SSITKA / DRIFT+MS Setup MFC 12 CO V MFC V CO MFC 12 CO V Vent Bruker FTIR Hiden MS Vent MFC Ar Kr Ar V PE 8700 GC 13 Water saturator DRIFTS cell Ar

Assignment of the IR bands RWGS 1% 12 CO 2 + 4% H 2 T=225°C C H OO C O OO 2059 C O Wavenumber /cm -1 WGS 1% 12 CO+ 10 % H 2 O T=200°C Abs. (a.u.)

DRIFTS band evolution during 12 CO- 13 CO exchange during WGS

WGS: DRIFT + MS / SSITKA 12 C-Formates 13 CO 2 WGS: Formates also minor intermediate O 2 H CO 2 H 2 +  + CO(ads), carbonates ?

WGS: Comparison of IR and MS 13 CO 2 (g) signals during 12 CO- 13 CO switch MS not needed!

Conclusions Use steady-state or operando conditions to get true reactivity. SSITKA / (DRIFT + MS) using a single reactor CARBONATES: main reaction intermediate for R-WGS FORMATES: spectator /“minor” reaction intermediate for R-WGS & WGS Mechanistic studies: WGS – Pt/CeO 2

Outline 1. Introduction - WGS Mechanisms over Pt/CeO 2 - SSITKA - DRIFTS cell design 2.Reactivity of surface species in 2  feeds R-WGS: 12 CO 2 /H 2 DRIFT: 13 CO 2 /H 2 (SSITKA) versus Ar purge 3. Spectators & Reaction intermediates R-WGS and WGS DRIFT + MS / SSITKA using a single reactor

Acknowledgements Prof. Robbie Burch Prof. Chris Hardacre Dr John Breen Dr Daniele Tibiletti Dr Alex Goguet Dr Sergyi Shekhtman Mr David Reid

Acknowledgments