Catalysts for the clean production of hydrogen. Chris Hardacre Queen s University, Belfast, UK

Transcription

1 Catalysts for the clean production of hydrogen Chris Hardacre Queen s University, Belfast, UK c.hardacre@qub.ac.uk

2 Acknowledgments Robbie Burch Alex Goguet Peijun Hu Richard Joyner John Breen Fred Meunier Simon Mun Daniele Tibiletti Ratcha Pilamsombat Helen Daly Ying Chen D. Thompsett J. Fisher A. Amiero Fonseca SPUR II DEL

3 The Hydrogen Economy

4 Source Storage and Conversion via Fuel Cells Usage

5 Comparison of Emissions 25 (Pounds of emissions per 1,000 kwh) NOx CO Non-methane hydrocarbons SOx Particulate Average U.S fossil fuel plant Combined cycle gas turbine Fuel Cell

6 Possible Chemical Routes to Hydrogen

7 Biomass Treatment Liquefaction/gasification Rotating Cone Reactor (300 rpm) Char + sand + sand Biomass fast pyrolysis ca. 1 s 550 C no O 2 Vapours + Char Condensation Bio-oil Liquids + Gases (H 2, CO, CH 4, C 2 H 2, C 2 H 4 )

8 Steam reforming of bio-oil CH 1.32 O H 2 O CO H 2 Feeding bio-oil is difficult! - Reactant = complex mixture, reactive > 80 C - Only partly soluble in water cannot co-feed bio-oil & water in single syringe

9 Bio-oil Steam Reforming 200 mg mg cordierite; T = 860 C ± 30 C liquid flowrate: bio-oil = 14 μl min -1 ; H 2 O = 96 μl min -1 ; N 2 flowrate = 50 sccm GeHSV = 3090 h -1 Best: Pt and Rh / Ce-ZrO 2 Up to 70 % (80 %) H 2 yield 1% Rh/Al 2 O 3 1% Pt/Al 2 O 3 1% Rh/CeZrO 2 1% Pt/CeZrO 2

10 Yield (%) Steam reforming over monolitic catalysts 2%Pt/CeZrO 2 catalyst 5 x 1cm ; m = 3.18g Liquid flow rate: bio-oil = 14.0 μl.min -1, water = 36.2 μl.min -1 N 2 flowrate: = 50 sccm GeHSV = H2 H2-CO Time (h) > 50 h

11 Overall process Fuel Cell CO + H 2 O CO 2 + H 2 ΔH = kj/mol (0.4% CO + 80% H 2 ) 45-70% H 2 + CO, CO 2, H 2 O, N 2, sulphur species <50 ppm CO (< 10 ppm preferably) High purity H 2 is required to prevent poisoning of the fuel cell anode

12 Low temperature WGS goal To achieve equilibrium conversion < 200 o C Non pyrophoric catalyst used, therefore conventional Cu/ZnO not possible Long term stable activity on stream Short term stable activity to changes in gas composition and temperature

13 Activity tests 2.0% CO, 2.5% CO 2, 7.5% H 2 O, 8.1% H 2 in N 2 GSV: 40 L g -1 h -1 What is the structure of the catalyst and the role of the support? 2% Au/CeZrO 4 ( ), 0.2% Au/CeZrO 4 ( ) 2% Au/CeO 2 (x), 2% Au/TiO 2 ( ), 2% Pt/CeO 2 ( )

14 In-situ EXAFS Au-O Au-Au Au Ce (i) (ii) 2% Au/CeZrO 4 catalyst as-received at room temperature under WGS reaction at 100 C (iii) (iv) Distance / Å after exposure to WGS at 450 C - data recorded under WGS at 100 C following oxidation in air at 150 o C after reaction under WGS at 100 o C. Similar for 0.2wt%Au/CeZrO 4 J. Phys. Chem. B 109 (2005) 22553

15 % 12 CO, 7% H 2 O, 13% H 2 in Ar CO adsorbed CO x on Au(0) 125 C In-situ Diffuse Reflectance IR Spectroscopy formates C Wavenumber cm -1 CO(ads)/Au is observable even at high T ΔH ads ~ 40 kj mol -1

16 Catalyst under reaction conditions Inactive gold? Possibly the CO at 2096 cm -1 Au δδ+ near neighbour Au δ+ in Ce 4+ site Consistent with DRIFTS, EXAFS and XPS, CO adsorbed on Au 0

17 WGS Mechanisms Au-Ceria CO + H 2 O = CO 2 + H 2 The formate mechanism O O H C H H C CO H 2 O Ce O O O O H O Ce Ce O O H H Shido, Iwasawa, J. Catal. 141 (1993) 71 Jacobs, Davis, Appl. Catal B, 284 (2005) 31 T. Tabakova et al., Appl. Catal B. 49 (2004) 73 CO 2 + H 2 Behm et al., J. Catal. 244 (2006) 137 The redox mechanism

18 DRIFTS + MS + SSITKA (Steady-State Isotopic Transient Kinetic Analysis) 12 CO + H 2 O DRIFTS Goguet et al., J Phys. Chem. B 108 (2004) Mass spectrometer 12 CO 2 13 CO 2 Vent reactor time 13 CO + H 2 O Abs S 1 * S 2 * S 2 S Wavenumber (cm -1 ) 500

19 Relative Intensity Evolution under 13 C-feed at 155 C 2% 12 CO + 7 % H 2 O 2% 13 CO + 7 % H 2 O CO 2 formate carbonate Time after isotopic switch (min)

20 Relative Intensity Evolution under 13 C-feed at 220 C 2% 12 CO + 7 % H 2 O 2% 13 CO + 7 % H 2 O CO C: formates are minor intermediate carbonate formate 2200 C: formates possibly main intermediate Time after isotopic switch (min) However it is dangerous to extrapolate results!

21 Is there another mechanism? DFT studies Relaxation Gold ion in cation site Activated oxygen

22 Possible Carboxylate Mechanism CO(g) + Au δδ+ = CO Au 2H 2 O(g) + 2V O = 2OH o + 2H Au CO Au + OH o = HOOC Au HOOC Au + OH o = CO 2 (g) + H 2 O(g) 2H Au = H 2 (g) CO(g) + H 2 O(g) = CO 2 (g) + H 2 (g) Faraday Discussions (2012) in press

23 CO + H 2 O = CO 2 + H 2 H H O CO

24 CO + H 2 O = CO 2 + H 2 H H H O CO CO 2 O

25 COOH+OH Numerical calculation of Turnover Frequencies for Au 3 /CeO 2 at 450 K Formate = 2.9x10-9 s -1 Redox = 1.5x10-11 s -1 (COOH+OH) = 2.1x10-2 s -1 Experimental value at 450 K = 7.5x10-2 s -1

26 Catalyst Stability Investigation of the on stream and thermal deactivation of Au/CeZrO 4 J. Phys. Chem. B 109 (2005) and 111 (2007) 16927

27 CO Conversion / % Effect of the exposure to N 2, H 2 or H 2 O Exp. Conditions: 2% CO, 2.4% CO 2, 8% H 2, 19% H 2 O Fresh catalyst 20 After 16 hrs in H 2 / N 2 flow at 200 C Refilling saturator 10 After 24 hrs in N 2 flow at 200 C 0 After 15 hrs in H 2 O / N 2 flow at 200 C Time / h on stream

28 Deactivation rate /% h Effect H 2 O concentration Water concentration / %

29 Reminder: CO(ads)/Au-ZrCeO 4 CO adsorption at interfacial gold sites 2096 cm -1 Can we use this as a probe for activity?

30 Au-CO bands under WGS feed Change in the nature of the Au-CO bands with time on stream Loss of: Au δ+ -CO species at 2125 cm -1 Au 0 -CO species at 2098 cm -1 Increase in intensity of bands due to Au δ- -CO" species between 2070 and 1950 cm -1 Log 1/R 0.02 Au 0 -CO Au δ+ -CO 2125 cm cm cm -1 Au δ- -CO Formation of Au δ- species while we observe a loss in activity suggests Au δ- species are not active for the WGS reaction Wavenumber (cm -1 ) Journal of Catalysis, 273 (2010) 257

31 Au(0) Au x-? Au x+ Correlates well with the activity data

32 DFT of Hydrolysis of Au-oxide interaction

33 CO Conversion / % Activity tests: Fresh vs. Aged Loss in activity Temperature / C Fresh 2% Au/CeZrO 4 ramped ( ), 2% Au/CeZrO 4 pre-exposed to WGS mixture at 350 C ( ), Fresh 0.2% Au/CeZrO 4 ramped ( ), 0.2% Au/CeZrO 4 pre-exposed to WGS mixture at 350 C ( )

34 %CO conversion Thermal deactivation: CO conversion % CO conversion has halved after initial treatment at 400 C; initial rapid thermal deactivation Reaction at 400 C Reaction at 150 C Time (mins) Slower deactivation observed at 150 C after subsequent treatments at 400 C

35 Thermal deactivation: Au species Intensity of the Au 0 -CO band follows the CO conversion Au 0 is the most stable and active Au species

36 Au-CO band area Catalysis Today 180 (2012) 131 Correlation between Au 0 -CO band area and %CO conversion as a function of preparation method Washed water Washed 0.1 M NH 4 OH Washed 0.1 M Na 2 CO % HAuCl 4 AuBr Au/CZS-II Au/CZS-I % CO conversion at 100 C Catalysts: 2% Au/CeZrO 4 (prepared from HAuCl 4 ) washed with water, 0.1 M NH 4 OH or 0.1 M Na 2 CO 3, 2% Au/CeZrO 4 prepared from AuBr 3 washed with water, 1% Au/CeZrO 4 prepared from HAuCl 4 and washed with water Sulphated 2%Au/CeZrO 4 catalysts (Au/CZS-I and Au/CZS-II)

37 Deactivation Model

38 Normalised CO conversion Enhanced stability: effect of CO No Au δ+ -CO observed under CO 2 free feed: catalyst is active and more stable with no Au δ+ present Au 0 - CO full feed pretreatment / CO 2 free feed no pretreatment / CO 2 free feed no pretreatment / full WGS feed Time (min) Au δ+ - CO Log 1/R Wavenumber (cm -1 ) Pretreatment: 2% CO, 8.1% H 2, 7.5% H 2 O and 2.1% CO 2 in He at 200 C Feed: 2% CO, 8.1% H 2, 7.5% H 2 O in He

39 Comparison of on stream stability for 0.2%Au/CeZrO 4 at 200 o C with and without pretreatment Pretreatment of catalyst enhances stability

40 Conclusions Predominant state of gold in WGS catalysts is Au 0. Gold has to be in good contact with the CeZrO 4 support nature of support is critical On thermal treatment the catalyst deactivates by loss of metal-support interaction. Water causes on stream deactivation by hydrolysing the metal-support interaction Stabilisation can be achieved by using a CO 2 free feed for 0.2% Au/CeZrO 4 CO 2 required to form active catalyst but then interaction of H 3 O + /HCO 3 - with basic support leads to enhanced hydrolysis