Au-Cu Catalyst for Low Temperature CO Oxidation

Transcription

1 -Cu Catalyst for Low Temperature CO Oxidation J. Chris Bauer Todd J. Toops (co-principal Investigator) James E. Parks (co-principal Investigator) Oak Ridge National Laboratory Energy and Transportation Science Division

2 Objectives and Relevance Develop emission control technologies that perform at low temperatures (<15ºC) Enable Fuel-efficient engines with low exhaust temperatures to meet emission regulations Pt and Pd traditional oxidation catalysts - Oxidize CO and HC - Light-off temperature ~2 C Low temperature exhaust during start-up Advanced combustion engines: - Higher efficiency - Lower exhaust temperatures Exhaust temperature can remain near 2 C during FTP Goal Possible solution: catalyst TWC 2 Presentation name

3 Gold Catalysts Gold is well known for high catalytic activity at low temperatures - Bulk not active catalyst - /TiO 2 can catalyze CO oxidation -5 to -3 C Gold nanoparticles sinter easily - Low melting point (~163 C) versus Pt (177 C). - Weak interaction with metal oxides - Sintering causes deactivation Why bimetallic -Cu nanoparticle catalysts - Activity - Stability - Selectivity - Reduce Cost Structure of -Cu can be manipulated Oxidation CuO x Random Alloy 3 Presentation name Ordered Alloy Heterostructure

4 Synthesis of Cu/SiO 2 Catalyst Supported nanoparticles serve as templates to synthesize small and disperse intermetallic Cu nanoparticles Synthesized using aqueous/solution techniques SiO 2 (en) 2 Cl 3 ph=1 (1. M NaOH) SiO 2 Reduce in When oxidized, catalyst core-shell ACTIVE for CO oxidation H 2 ~15 C SiO 2 amine ligand coats the surface of gold Cu(C 2 H 3 O 2 ) 2 Organic solution at 3 C SiO 2 Cu Reduction H 2 ~15 C Metallic Cu addition Reduced catalyst = Cu Alloy (Inactive) SiO 2 Oxidation When reduced, catalyst forms a Cu alloy Inactive for CO oxidation Oxidized catalyst core + CuO x shell SiO 2 4 Presentation name H. Zhu et al. Applied Catalysis A: General 27, 326, Bauer et al. Phys. Chem. Chem. Phys., 211, 13,

5 Intensity (Arbitrary Units) XRD of Intermetallic Cu Count Cu Alloy has a face centered cubic structure nm Average = 4. nm Median = 3.7 nm Position (Degrees (2 )) XRD shifts to indicate Cu alloy formation Particle diameter (nm) Particle Diameter (nm) Particles remain small and disperse across the SiO 2 after Cu addition 5 Presentation name Bauer et al. Phys. Chem. Chem. Phys., 211, 13,

6 Activation of Cu Catalyst Oxidation 5 C, 75 sccm 1% O 2 + 1% H 2 O 3 C, 5% H 2 /Ar Reductio n θ Active Catalyst CuO core - amorphous CuO x shell Inactive Catalyst Cu 4.1 2θ Cu Alloy Cu 4.3 2θ θ 6 Presentation name Bauer et al. Phys. Chem. Chem. Phys., 211, 13,

7 Cu Alloying Inhibits Activity 4, O 2 3, C O C O 2, 4 H 2, 4 O 2 35 C O 2, 4 H 2 Reaction Temperature = 25 C 35 C O 2, 35 H 2 35 C O 2 Alloy as-synthesized 2, O 2 4, O 2, 3, H 2 3 C O 2 25 C O 2 Only active after oxidation above 3 C. Alloy formation causes loss of activity. Little CO absorption on reduced Cu nanoparticles. CO absorption occurs when transformed to -CuO x hybrid particles. 7 Presentation name Bauer et al. Phys. Chem. Chem. Phys., 211, 13,

8 CO Conversion 2 catalyst is excellent for low temperature CO oxidation Pt/Al 2 O 3 catalyst show little activity below 2 C T 5% = C Pt/Al 2 O 3 space velocity: W/F =.5 g h/mol is 27k h Cu/SiO 2 W/F =.5 g h/mol Cu/SiO 2 W/F =.25 g h/mol Similar loadings 2 shows high activity even at 5 C Reactivity as low as C Pt/Al 2 O 3 W/F =.25 g h/mol Pt/Al 2 O 3 W/F =.5 g h/mol Temperature ( C) Pretreatment: 1% O 2 + 1% H 2 O + Ar 55 C, 16 h, 75 sccm Catalyst = 5-1 mg 1% O 2 1% H 2 O Ar (balance) Flow Rate = 75 sccm 5 8 Presentation name

9 CO Conversion (%) (%) CO Conversion (%) Low temperature activity is limited in the presence of NO and hydrocarbons Strong inhibition by both NO and HC Pt/Al 2 O 3 displays less impact, but still shows inhibition Catalyst = 5 mg 1% O 2 1% H 2 O CO, NO, C 3 H 6 as listed Ar (balance) Flow Rate = 75 sccm % C 3 H 6 +.5% NO +.1% C 3 H 6 +.5% NO % C 3 H 6 +.5% NO +.1% C 3 H 6 +.5% NO Temperature ( C) Temperature ( C) NO and propylene compete with CO for active sites on catalyst surface. - CO oxidation is inhibited. What if the catalytic properties of both catalysts were combined? 9 Presentation name

10 CO Conversion Combination 2 and Pt/Al 2 O 3 studied to explore potential 2 and Pt/Al 2 O 3 were physically mixed together CO oxidation activity increases compared to Pt/Al 2 O 3 but not as high 2 alone Cu Cu + Pt Pt Temperature ( C) CO-only oxidation W/F =.5 g h mol -1 Catalyst = 1 mg 1% O 2 1% H 2 O Ar (balance) Flow Rate = 75 sccm Pretreatment conditions: 55 C for 16 in 1% O 2, 1% H 2 O, and Ar. 1 Presentation name

11 Cu/SiO 2 + Pt/Al 2 O 3 Physical Mixture: CO Oxidation Under C 3 H 6 CO Conversion (%) Propylene Oxidation (%) CO Oxidation C 3 H 6 Oxidation Pt Pt Flow Rate = 75 sccm.1% C 3 H 6 1% O 2 1% H 2 O Ar (balance) Temperature ( C) Flow Rate = 75 sccm.1% C 3 H 6 1% O 2 1% H 2 O Ar (balance) Temperature ( C) CO and propylene oxidation activities of the Cu/SiO 2 + Pt/Al 2 O 3 catalyst is comparable to Pt/Al 2 O 3 11 Presentation name

12 CO CO Conversion (%) (%) NO NO Conversion (%) (%) NO oxidation synergy observed 2 + Pt/Al 2 O 3 physical mixture Improved low temperature CO-oxidation in the presence of NO Better than either individual catalyst NO oxidation to NO 2 approaches equilibrium limit at 25 C Considerably more active than Pt/Al 2 O Pt/Al 2 O Equilibrium Cu/SiO Flow Rate = 75 sccm.5% NO 1% O 2 1% H 2 O Ar (balance) Temperature ( C) Pt/Al 2 O 3 Cu/SiO Temperature ( C) Theory: 1. NO oxidation inhibited by CO on Pt catalyst oxidizes CO, thus improving NO oxidation 12 Presentation name

13 Avg. Particle Diameter (nm) XRD TEM XRD TEM XRD TEM XRD TEM CO Conversion Durability a concern with SiO 2 2 aged in 1% O 2 + 1% H 2 O in Ar Catalyst relatively stable up to 7 C Only very low temperature activity (T< 15 C) diminishes with increasing aging temperature Particles grow up to ~25 nm in diameter after thermally aged at 8 C for 1h (8-9 nm avg.) Sulfur also shown to strongly deactivate Improved metal support interactions needed Fresh 6C 1h 7C 1h Aging Temperature ( C) 8C 1h 8 C, 1h 5nm C, 1h Temperature ( o C) CO-only oxidation W/F =.249 g h/mol 1% O 2 1% H 2 O Ar (balance) Flow Rate = 75 sccm 13 Presentation name

14 CO Conversion Supporting Cu catalyst on ceriazirconia shows improved stability Same synthesis procedure followed as described in slide 1 2 (75)-ZrO 2 (25) Even with low weight loading high activity shown with unaged sample W/F =.25 g*h/mol SV = ~95, h -1 ; denser than SiO 2 T 5% = 6 C T 9% = 98 C Activity drops after aging at 8 C, but is still very high T 5% = 125 C T 9% = 155 C Temperature ( C) Aging Temp. Cu-55C 16h 6C 16h 7C 16h 8C 16h CO-only oxidation W/F =.25 g*h/mol 1% O 2 1% H 2 O Ar (balance) Flow Rate = 75 sccm 14 Presentation name

15 CO-oxidation T-9 ( C) CO-oxidation T-9 ( C) Catalysts studied show promise, but challenges remain T-9 compared for each catalyst and condition studied T-9 = temperature where 9% conversion is achieved The lower the better 9% Oxidation of HCs and CO at 15 C will continue to be difficult, but exploiting synergies of catalysts show promise Goal 2 and Pt/Al 2 O 3 show impact from NO and HCs Mixing catalysts results in ~35 C drop in T-9 Matching active catalysts with the right support shows promise for overcoming durability challenges 9% conv. achieved w/ 8 C aging Cu/SiO2 2 Pt/Al2O3 2 O 3 Mixture CO-only CO+NO Cu/SiO2 Cu/CZ Fresh Aged at 8 C Goal Goal 15 Presentation name

16 Summary Relevance: Advanced combustion modes have greater efficiency and consequently lower exhaust temperatures Simultaneous increase in efficiency and decrease in allowable emissions necessitates improved emissions control system performance, especially at low temperatures Approach: Pursue innovative catalyst technologies to improve low temperature emissions control Evaluate performance, investigate durability, characterize materials, identify fundamental limitations Technical Accomplishments: Investigated activity, durability and material properties core-shell oxidation catalyst Identified synergistic effects of physical mixture and Pt catalysts that overcome some of the observed inhibitions Synthesized new catalysts with a range of supports, that significantly improve durability Future Work: Continue investigation on Cu with ceria-zirconia and other supports Move into NOx reduction catalysts and trap materials Move from powder catalysis to washcoated cores and further validation in engine exhaust 16 Presentation name

17 Acknowledgments Oak Ridge National Laboratory Energy and Transportation Science Division Todd J. Toops James. E. Parks Co-principle investigator Co-principle investigator Oak Ridge National Laboratory Chemical Sciences Division Sheng Dai Stephen H. Overbury Larry Allured TEM Zili Wu - FTIR Funding: Current funding provided through EERE Office of Vehicle Technologies, Gurpreet Singh and Ken Howden. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U. S. Department of Energy. Initial research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC5-OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. 17 Presentation name