Supporting Information (SI)

Transcription

1 1 Supporting Information (SI) Catalyst synthesis (Pt in ceria): (1) ME catalysts: For Pt supported on ceria catalysts (synthesized by micro-emulsion technique), the preparation procedures are as follows: g of cetyltrimethylammonium bromide (CTAB) was added into ml of dry toluene with vigorous stirring. A suspension of CTAB in toluene was formed. Then, the Pt precursor salt solution with ceria was prepared by dissolving appropriate of (NH 4 ) 2 PtCl 4 and g of cerium (III) nitrate hexahydrate salt into ml of DI water. The aqueous solution of Pt precursor salt with ceria was added dropwisely to the suspension of CTAB in toluene and was stirred overnight. A solution of g of NaOH dissolved in ml of DI water was added into the reaction mixture and stirred for another 2 hours. Then, the reaction mixture was allowed to age for 6 days with a constant stirring (the aging process is essential to form metal encapsulation in ceria). After the ageing step, the reaction mixture was centrifuged for 20 minutes (1000 rev/min) to collect the product. The product was washed with EtOH to remove surfactants. The solution was repeatedly centrifuged after each washing. Finally, the product was washed in reflux EtOH three times for overnight. The solid product obtained was dried in air for overnight. After that, the powder was calcined at 400 C (2 C/min) for 2 hours before use. (2) MEs catalysts: Further improvement in the encapsulation chemistry was achieved when NaOH was added to the metal precursor microemulsion, giving a precipitation within the micelle, followed by the subsequent addition of the cerium (III) nitrate. This gave a significant improvement in the degree of metal encapsulation (denoted hereafter as MEs samples). All the MEs samples, prepared in this manner, give CO chemisorption values < 2.0%, for example, the MEs 5%Pt/ceria gives metal dispersion < 0.8% and MEs 5%Pt/5%Au/ceria < 1.8%. This bottomup construction approach by first building up NM containing core, followed by adding ceria containing material inside the micelle to form a catalyst particle in a sequential manner, was used for subsequent catalyst production. The details of preparation, metal loading, support used and doper was shown in Table 1. The Pt precursor salt solution with ceria was prepared by dissolving appropriate of (NH 4 ) 2 PtCl 4 into the DI water. The aqueous solution of Pt precursor salt was then added dropwise to the suspension of CTAB in toluene and was stirred overnight. A solution of g of NaOH pre-dissolved in ml of DI water was added into the reaction mixture and stirred for 2 hours before adding a solution of g of cerium (III) nitrate hexahydrate. Notice that the total molar water to surfactant ratio of the resulting mixture was still maintained at 30 as the ME method. The reaction mixture was allowed to age for 6 days with a constant stirring. After the ageing step, the reaction mixture was centrifuged for 20 minutes (1000 rev/min) to collect the product. The product was washed with EtOH to remove surfactants. The solution was repeatedly centrifuged after each washing. The solid product obtained was dried in air for overnight. The catalysts were pre-treated with the reactant gas mixture (8% CO, 10% CO 2, 1% CH 4, 32.5% H 2, bal. N 2 ) at 400 o C before catalytic testing. (3) Co-precipitation (Co-ppt) sample: For 2% Pt supported on CeO x catalyst (SP04) (synthesized by the co-precipitation method), the preparation procedures are as follows g Ammonium tetrachloroplatinate (II) was dissolved into a ml aqueous solution of 0.2 M cerium (III) nitrate hexahydrate and sprayed into a ml ammonia solution (or

2 2 NaOH) under a constant stirring. The precipitate in ammonia solution was allowed to age at room temperature with stirring for another two hours. Then, it was collected by centrifugation (1000 Rev/min) and washed twice with water and once with ethanol to remove any remaining ammonia and reaction by-product. The solid was dried in 333 K vacuum oven for 2 hours. Then, it was dried in a flow of nitrogen at 100 ml/min with temperature programming (2 K/min) up to 623 K and stand for another 5 hours. After the drying procedure, it was prereduced with 50ml/min hydrogen at temperature programming (2 K/min) up to 523 K and stand for another 3 hours. The other catalysts synthesized by co-precipitation were also being synthesized following these procedures but with variations in precursor salt and amount of reactant used. (4) Incipient wetness samples: For 5% Pt supported on ceria catalysts (synthesized by incipient wetness impregnation method), the preparation procedures are as follows. Cerium (III) nitrate hexahydrate was calcined in static air with temperature programming from room temperature to 600 C (25 C/ min) and was held at 600 C for further 10 hours. Ammonium tetrachloro-platinate (II) was dissolved into D.I. water. The Pt-precursor solution was then added onto the newly formed ceria. The product was dried under air with temperature programming from room temperature to 100 C (5 C/min) and was held at 100 C for 10 hours. The catalyst was calcined under 30 ml/min N 2 with temperature programming from room temperature to 500 C (20 C/min) and was held for 2 hours. Catalyst prospects (Pt/ceria): There is renewed interest in the catalysed water-gas-shift reaction, WGS (CO + H 2 O CO 2 + H 2 ) for the purification of hydrogen-rich feeds obtained through hydrocarbon reforming. Hydrogen is seen as the key fuel for future power generation using fuel cells for both stationary and mobile applications. The use of WGS reactors with small-scale reformers imposes different requirements compared to large-scale H 2 production plants, which are based on commercial Cu/ZnO catalyst technology. These requirements involve stability towards start-up/shut down cycles and repeated purging with air, together with good lowtemperature activity and stability. Also, high selectivity to the WGS reaction is required over the competing reaction of methanation (CO + 2H 2 CH 4 ) as this lowers the H 2 content of the final feed. Noble metal (NM)/ceria based catalysts have been extensively investigated in recent years and have been shown to be more resistant to deactivation after start up/shut down cycles than standard Cu-based catalysts, which tend to be pyrophoric on exposure to air after reduction. Although simple catalyst formulations are intrinsically unselective regarding CH 4 formation they can be promoted to suppress some degree of methanation activity 1. On the other hand, these catalysts suffer from on-stream deactivation, which has been attributed to CO-promoted metal sintering 2, metal assisted over reduction of ceria 3 or carbon deposition on the metal 4 and very recently the carbonate formation 5. These render the Pt/CeO 2 as a WGS catalyst limited for fuel processors integrated to PEM cell but this type of catalyst with a strong Pt ceria interaction will act as a good example to demonstrate the nanoengineering concept. 1. Liu, X., Ruettinger, W., Xu, X., Farrauto, R. Applied Catalysis B: Environmental 56, 69, (2005). 2. Wang, X., Gorte, R. J., Wagner, J. P. Deactivation mechanisms for Pd/ceria during the water-gas shift reaction. J. Catal., 212 (2), (2002). 3. Zalc, J. M., Sokolovskii, V., Loffler, D. G. Are noble metal-based water-gas shift catalysts prqactical for automotive fuel processing. J. Catal., 206 (1), (2002). 4. Goguet, A., Meunier, F. C., Breen, J. P., Burch, R., Petch, M. I., Ghenciu, A. F. Study of the origin of the deactivation of a Pt/CeO 2 catalyst during reverse water gas shift (RWGS) reaction. J. Catal., 226, (2004). 5. Ghenciu, A. F. Review of fuel processing catalysts for hydrogen production in PEM fuel cell systems. Current Opinion in Solid State and Materials Science 6, (2002). and also US 6, and US 6,562,315.

3 3 Further Material Characterizations: (1) XRD of MEs 5%Pt/ceria 600 CeO 2 (111) Lin (Counts) CeO 2 (200) CeO 2 (220) CeO 2 (311) Pt (111) Theta S Fig. 1. A typical XRD spectrum of 5 wt% Pt /CeO 2 prepared by micro-emulsion. An estimation of ceria particle size via CeO 2 (111) line broadening from the spectrum with taking instrumental broadening into account shows the ceria particle size of about 3.7 ± 0.5 nm. Pt size is estimated to be ~2 nm but with a large degree of error because of the extremely broad Pt(111) peak. (2) XPS study of MEs 5%Pt/ceria: Name Centre BE FWHM ev At. % F Q SF Title C1s Y 1 1 C1s 20 C1s Y 1 1 C1s 20 O1s Y O1s 20 O1s Y O1s 20 O1s Y O1s 20 Ce3d N Ce3d 20 Na1s N Na1s 20 Pt4f7Pt(0) Y Pt4f 20 Pt4f7Pt(II) Y Pt4f 20 XPS can detect the electronic state of the catalyst. The analysis detected Ce as primarily Ce(IV), albeit with a very slightly lower value binding energy than the literature value( ev). The analysis of Pt4f spectrum shows that two states were detected, Pt(0) and Pt(II), the Pt4f7/2 peaks of which are illustrated. A partial formation of Pt o and Ce 4+ from Pt 2+ /Ce 3+ is clearly evident. A very low Pt/Ce ratio of 0.04 was detected (much lower than those of impregnated and coppt samples), which is in an excellent agreement of the Pt encapsulation in ceria.

4 4 (3) Methane formation using CO/H 2 CH4 production (%) Reactor Temperature ( C) coppt 5%Pt ME 5%Pt MEs 5%Pt/5%Au S Fig. 2. A plot of methane production versus reactor temperature under the flow of 8% CO, 32.49% H 2 and balancing with N 2 over the three different catalysts at the similar GHSV: The ME samples gave much less methane production than the co-precipitated sample clearly reflecting the small degree of metal exposure. (4) Band Transition Evaluation: 50 Reflectance pure CeO 2 MEs- 5%Pt nm S Fig. 3. UV-vis diffuse reflectance spectra of pure CeO 2 and MEs- 5%Pt/ceria. The band transition was evaluated by UV-vis diffuse reflectant from the equation: α(hυ) = A (hυ Eg) m/2, where α is the absorption coefficient, hυ is the photon energy and m = 1 for the direct band transition between bands. The energy of the band transition is calculated by extrapolating a straight line to the abscissa axis. Thus, α is zero, then Eg = hυ. This experiment in S Fig. 3 clearly shows the blue shift of the absorption edge upon encapsulating Pt metal in ceria.

5 5 (5) Elemental Mapping Elemental mapping of single isolated particle of the MEs- 5%Pt/ceria sample indicated the enrichment of Pt in the core region as shown below: Element App Intensity Weight% Weight% Atomic% Ce L Pt M S Fig. 4. Elemental analysis of the core region of MEs 5%Pt/ceria (6) TPR Use of Temperature Programmed Reduction (TPR) on the materials clearly supports the model of Pt in ceria. As seen from the S Fig. 5 below, a typical reduction profile of the conventionally co-precipitated Pt/CeO 2 sample gives a first reduction peak at a low temperature (82 C vs. 439 C in pure ceria), which is assigned mainly to the reduction of surface oxygen on ceria assisted by hydrogen spillover on exposed metal [Levy, P. J., Primet, M. Appl. Catal. 70, (1991)]. Interestingly, MEs 1-5% Pt/ceria samples show a first reduction peak of o C. Although still promoting the reduction of the CeO 2 overlayer compared to pure ceria (MEs 5% Pt/ceria shows a lower first reduction peak than MEs 1%Pt/ceria), the very low temperature peak is not observed suggesting that as no extensive Pt sites are exposed, spillover of H 2 is not favourable taken place. This is confirmed by the TPR profile of MEs 27% and 10% (not shown) Pt/CeO 2 which shows a clear low temperature peak and a relatively high CO chemisorption uptake value. Notice that exposure of metal does not necessary imply a decrease in WGS activity. In fact, the MEs 10%Pt/CeO 2 sample with some degree of metal exposure still gives good WGS activity which seems to depend on the effectiveness of the Pt-ceria interface created. S Fig. 5: Temperature programmed reduction profiles of MEs 5, 10 and 27%Pt/CeO 2 and coprecipitated 2%Pt/CeO 2.10 o C/min ramp rate in 100% H 2 from room temperature to 400 o C, reductions monitored gravimetrically using a IGA (Hiden) apparatus. Rate of weight change (mg/min) Co-ppt 2%Pt MEs-27% MEs-5% MEs-1% Temperature/ o C

6 6 The TPR spectrum of the co-precipitated 2%Pt/ceria shows typical oxygen loss: (mmol./g) with two reduction peaks (1.736 mmol/g surface oxygen loss by spillover hydrogen over exposed metal at 82 o C and mmol/g bulk oxygen at 354 o C); MEs 1 to 27% Pt/ceria samples prepared using a sequential microemulsion method, metal particle is totally embedded in ceria in the 1 and 5% Pt cases (see Fig. 1): TPR of MEs- 5%Pt/ceria shows a higher oxygen loss: (mmol/g) with two reduction peaks at higher temperatures (2.624 at 190 o C; at 410 o C) indicating absence of exposed metal for hydrogen spillover. For MEs-27%Pt/ceria, a clear phase segregation of metal from ceria is apparent (low temperature reduction spillover peak observed). Acknowledgements: This work is funded by EPSRC and Johnson Matthey at the UK. We thank the EPSRC- CARMAC Catalysis Consortium (Queen s University of Belfast; Cambridge Univ. and Reading Univ. ; Johnson Matthey; Grace Chemicals; Robertson Brothers, plc.) for valuable discussions and for supporting experiments. The support of this work with SuperSTEM microscopic facilities at National Daresbury laboratory, UK is also acknowledged.