Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense Department of Chemical Engineering The University of Michigan July 29, 2011 Hello all; And good morning. I would like to thank you all to coming to my talk this morning. I am especially grateful for the presence of my friends and family for support. Today I will talk about some of the work I did for the development of silver catalysts for the reduction of Nox.

Smog Consists of particulate matter and ground-level ozone Caused by reaction of NOx and hydrocarbons in the presence of sunlight Contributes to a number of health issues Emphysema, asthma, bronchitis, shortness of breath We all know of the perils of photochemical smog – we all are well aware of the reduction of air quality smog contributes to our cities (click). We all know of the of the respiratory distress such as asthma and choking that can be caused by smog. And it is still a problem in our communities, so naturally we should strive to reduce its production as much as possible. So what is smog? (click) It is the reaction of Nox, sunlight, and VOCs. All are key to the formation of smog, but the main component of focus is Nox http://upload.wikimedia.org/wikipedia/commons/9/96/SmogNY.jpg

Where is majority of NOx generated? Lean-burn engines produce more NOx Increasing number of vehicles are lean-burn Diesel, gasoline-powered Because more than half of the Nox emissions, the key component in smog, come from mobile sources, such as light and heavy duty vehicles. An increasing number of those vehicles come from (click) lean burn vehicles such as diesels. Lean burn engines are gaining popularity because they have better fuel economies than their gas counterparts due to the high air/fuel ratios, which improves combustion of the fuel. However, as a result, the (click) amount of NOx produced is much greater than in gas powered cars Other Activities include industrial process, oil/gas, waste incineration, agricultural burning, and solvent use/waste EPA 2011

NOx Emission Standards NOx Standard (g/mile) 72% In response, the EPA set the first standards in 1975, and those standards have gotten more stringent over the years. The latest standard the EPA has set was phased from 2004-2010 model year cars. Although the EPA has not set any new standards as of now, the California Air Resources Board has set a new standard of 0.02 g/mile for model year cars 2014-2022. The EPA will likely follow this standard in the next round. These large reductions require the research of new technologies. There are many technologies that have been studied, and I won’t go into all of them in the presentation, but I will focus on the one that I did my research on, termed (click) Vehicle Model Year Increasingly stringent emission standards require significant technological advances1,2 U.S. EPA Office of Mobile Sources California Air Resources Board LEV Level III

NOx Technologies Technology Benefits Challenges Authors NOx decomposition Most direct method High reaction temperatures (~900OC), thermodynamically difficult Iwamoto et al., App. Cat., 1991 NOx storage-reduction/Lean NOx traps Well established method, no additional reducing agent SO2 deactivation, thermal degradation Matsumoto, Cat. Today, 2004 Selective Catalytic Reduction (SCR)with Urea Well established for heavy duty vehicles, no fuel penalty Urea freezes at -10oC, hydrocarbon poisoning at low temps Koebel et al., Cat. Today, 2000

Hydrocarbon Selective Catalytic Reduction (HC-SCR) of NOx HC + NOx + O2 → N2 (desired) + CO2 + H2O HC + NOx + O2 → N2O (undesired) + CO2 + H2O Many supported metals shown to be active as catalysts Silver (Ag), Palladium (Pd), Platinum (Pt), Rhodium (Rh), Iron (Fe), Cobalt, (Co), Gold (Au) Zeolites, Metal Oxides Silver supported on alumina (Ag/Al2O3) has been widely investigated Activity is negligible below 400oC1 Hydrocarbon Selective Catalytic Reduction of NOx, or HC-SCR. In this method of NOx reduction, (click) unburned hydrocarbons from the exhaust react with NOx in the presence of oxygen over a catalyst to reduce the NOx. The reaction can proceed to N2 or N2O, with N2O being the undesired product as it is has 310 times the global warming potential of CO2. (Click) many supported metals have shown activity, such as the metals listed above with the supports but (click) Ag/Al2O3 has been one of the most studied because of its high selectivity to N2. However, between (click) 150-400 C activity and selectivity are minimal. Let’s take a look at the Ag/Al2O3 performance to explain the lack of activity (click) Burch et al., Topics in Catalysis, 2004

Ag/Al2O3 catalyst Conversion (%) Temperature(oC) NOx w/o H2 Hydrocarbon w/o H2 720 ppm NO 4340 ppm reductant (as C1) 4.3% O2 7.2% H2O Let’s look at Ag/Al2O3’s performance. The red and blue curve with the hollow shapes show the NOx and HC behavior without H2. The NOx does not light off (i.e. react) until 400oC . The HC lights off at a slightly lower temperature, indicating that the HC initiates the reaction. (click) Temperature(oC) Burch et al., Topics in Catalysis, 2004

Hydrogen Promotion of HC-SCR NOx with H2 0.72% H2 720 ppm NO 4340 ppm reductant (as C1) 4.3% O2 7.2% H2O Conversion (%) Hydrocarbon with H2 Temperature(oC) One way that has been shown to enhance the low temperature performance is to add hydrogen to the mixture. With the presence of H2 the light off temperature of both curves decreases by 200oC. Again, the light off of the HC precedes the light off of NOx, which shows that the H2 reduces the temperature at which the activation of the HC begins to occur It is proposed that the presence of H2 enhances the formation of key intermediates for the reaction and also cleans the Ag surface of strongly bound nitrates that can poison the catalyst surface It appears that just addition of H2 can improve the low temperature of Ag/Al2O3 by itself. Yet we always want to maximize conversion throughout the entire temperature range. We still don’t achieve complete activity until 300 C. In addition, the concentration of H2 is key. In this study, .72% of H2 was used. Since a reformer will be used to produce the H2, trying to use as little as possible would reduce the energy consumption of the reformer. when dropping to below 0.1% H2, the results change NOx light-off temperature decreased significantly with H2 addition NOx light-off coincides with hydrocarbon light-off Burch et al., Topics in Catalysis, 2004

Platinum Group Metal Addition Platinum Group Metals (such as Pd, Pt, and Rh) reduce NOx at lower temperatures compared to Ag1-2 Effect is significant on NOx conversion between 200-600oC3-5 200-400oC is the range of focus of my research NOx catalytic activity of C3H6-SCR3 NOx Conversion (%) Another technique of enhancing performance is to add a second metal onto an active monometallic catalyst. Doing this can physically or chemically affect the structure of the active site, with the desire to make it more active. Typically these changes can be small, but the desire to improve catalytic performance to the highest level warrants further study. Bimetallic catalysts have been applied to many reactions and NOx SCR is no exception. (Click) The use of noble metals particularly the 3 used in TWC (Pt, Pd, Rh) are suggested as the second metal because they (1) have good low temperature activity and (2) have been shown to utilize H2 and CO as reductants. (click) A number of studies exist with both Ag and a noble metal together for NOx reduction. He et al observed a small but significant improvement in NOx performance when adding small amounts of Pd to Ag catalyst (second reference). The contribution was explained through an enhancement in the formation of a key intermediate. Temperature(oC) 1. Obuchi et al., App. Cat. B, Env, 1993 2. Burch et al., App. Cat. B, Env, 2002 3. He et al., App. Cat. B, Env, 2003 4. Sato et al., Cat. Comm., 2003 5. Kotsifa et al., Cat. Letters, 2002

Develop active and selective bimetallic catalysts for NOx reduction Objectives: Develop active and selective bimetallic catalysts for NOx reduction Define effects of H2 and PGMs on the activity and selectivity of Ag/Al2O3 Examine the effect of impregnation order on NOx reduction performance So in light of the potential benefits of H2 and the second metal in improving Ag/Al2O3 activity for deNOx, the following hypothesis are proposed for the expected behavior of bimetallic catalysts First I focus on the issue of loading amount, and I hypothesized that (click) the loading would affect the conversion and selectivity (click) Secondly, the type of noble metal on top of the Ag/Al2O3 will affect the conversion and selectivity Lastly (click) the order of loading will affect the conversion and selectivity

Presentation Overview Introduction Catalyst Characterization and Screening Results: Effect of Loading Results: Effect of Loading Order Summary and Future Work Now that I have provided an introduction to my research, I will next proceed with the characterization and screening. But first I would like to talk about my experimental setup.

Experimental Setup - Celero Allows up to 8 catalysts to be tested in one experiment Fully automated Ar T Celero NO/Ar R1 R2 Vent C3H6/Ar R3 R4 R5 R6 CO/CO2 Chemiluminescent NOx Analyzer R7 R8 H2/Ar I ran all my catalysts in the Celero (Symyx Techonologies). The diagram is listed above. With the Celero I could measure up to 8 catalysts at a time. I would measure the flow in each reactor, and after, would calculate the amount of catalyst needed to keep the GHSV constant across the wells. Water was injected into the reactor through a vaporizer and caught in a water trap. Downstream is a chemiluminescent NOx analyzer that would measure NOx and NO, and a Varian micro GC that had a PoraPlot Q column (CO2, N2O, C3H6) and a Molesieve (H2, O2, CO, N2) O2 Varian 4900 micro GC Water Trap H2O

Experimental Conditions Preparation Method: Incipient Wetness Bimetallic catalysts made by sequential impregnation (Ag loaded first) Calcined in air at 600oC for 3 hours Noble metal loadings based on total amount of atomic Ag on a 2% Ag/Al2O3 catalyst Atomic count = 1.1*1020 Ag atoms/g catalyst 𝑁𝑂 𝑥 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛= 𝑁𝑂𝑥(𝑖𝑛𝑙𝑒𝑡) −𝑁𝑂𝑥(𝑜𝑢𝑡𝑙𝑒𝑡) 𝑁𝑂𝑥(𝑖𝑛𝑙𝑒𝑡) ×100 𝑁 2 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦= 𝑁𝑂𝑥 𝑖𝑛𝑙𝑒𝑡 − 𝑁𝑂𝑥 𝑜𝑢𝑡𝑙𝑒𝑡 − 𝑁2𝑂 𝑜𝑢𝑡𝑙𝑒𝑡 𝑁𝑂𝑥 𝑖𝑛𝑙𝑒𝑡 − 𝑁𝑂𝑥 𝑜𝑢𝑡𝑙𝑒𝑡 ×100 Component Concentration NO 600 ppm CO 800 ppm CO2 4% H2O O2 10% H2 3200 ppm C3H6 1800 ppm Ar balance The following gases and their concentrations are listed above. The NO, CO, CO2, H2O, and O2 concentrations were midpoints of the concentration ranges based on the diesel exhaust characteristics given in Supported Metals in Catalysis by Anderson and Garcia (2005). H2 and C3H6 concentrations were based on maintaining and 4:1 and 9:1 ratio of H2/CO and HC/NOx (by carbon atoms) which were the highest ratios I used in my DOE which I will talk about later. These are in the regions mentioned in literature:H2 concentrations in literature range from 200 ppm (Satakowa APB 2003) to 1% (Dimaggio, SAE 2009) and C3H6 ranged from 200 ppm (Satakowa APB 2003) to 6000 ppm (Shimizu APB 2007). The Monometallic catalysts were tested at 400C at a GHSV of 60000 1/hr. Incipient wetness was the preparation method (although there is literature suggesting other methods such as sol gel resulted in better dispersion of Ag). Bimetallics were made by sequential impregnation, with Ag first (as done by He and researchers from Eco-enviromental Research Center in China) The equations for conversions and selectivity are listed below. N2 could not be accurately measured on the Varian micro-GC at the low concentrations expected, so the N2O concentration was used to calculate the selectivity, assuming that no other nitrogen containing compounds were formed (the micro GC cannot detect NH3)

Characterization – Elemental Analysis Catalyst Actual Ag Metal Loading Target Noble Metal Loading Actual Noble Metal Loading Ag/Al2O3 1.55 ± 0.03% Ag - Ag-1% Pd/Al2O3 2.13 ± 0.01% Ag 0.019% Pd 0.009± 0.001% Pd Ag-1% Pt/Al2O3 1.27 ± 0.01% Ag 0.035% Pt 0.020 ± 0.003% Pt Ag-1% Rh/Al2O3 1.82 ± 0.02% Ag 0.02% Rh 0.008 ± 0.001% Rh Ag-10% Pd/Al2O3 2.10 ± 0.02% Ag 0.19% Pd 0.17 ± 0.01% Pd Ag-10% Pt/Al2O3 1.83 ± 0.02% Ag 0.35% Pt 0.24 ± 0.01% Pt Ag-10% Rh/Al2O3 1.47 ± 0.03% Ag 0.20% Rh 0.09 ± 0.01% Rh Surface area and elemental loading measurements are listed. As earlier stated, catalysts were prepared by incipient wetness. The precursor (all nitrate-based) was impregnated onto the support. The catalysts were dried at 110oC for 8-12 hours and calcined at 600C for 3 hours. For bimetallics, there was a calcination step in between each impregnation. In terms of elemental loading, the

NOx Conversion – Al2O3-Supported Bimetallic Catalysts Addition of noble metal does not seem to improve Ag/Al2O3 performance Interesting note is that at low loadings, Pd and Pt don’t results in detrimental effect. This corrabates Wang et al in 2004, where they corraborate small improvements in conversion between 200 and 400 for Pd and Pt. They did not test Rh. Sato and Kostifa (et al) tested Rh and saw improvements – they said this to be from formation of Ag clusters and Rh alloying (both have been disproven) For low-loading, Pd and Pt had no effect: Rh suppressed activity For high-loading, Pd, Pt, Rh affect activity, reaching maximum at 300oC

N2 Selectivity – Al2O3-Supported Bimetallic Catalysts For low-loading, selectivity increased with temperature For high-loading, selectivity decreased with temperature

Presentation Overview Introduction Catalyst Characterization and Screening Results: Effect of Loading Results: Effect of Loading Order Summary and Future Work Now that I have provided an introduction to my research, I will next proceed with the characterization and screening. But first I would like to talk about my experimental setup.

Design of Experiment (DOE) Setup Factors Levels HC/NOx ratio 3:1 6:1 9:1 H2/CO ratio 0:1 2:1 4:1 Second Metal Atomic Loading 0% 1% 10% Second Metal Type Pd Pt Rh Temperature (oC) 200 300 400 The matrix is listed here. 5 factors were looked at – HC/Nox, h2/CO, loading, type of metal, and temperature. For HC/Nox, many researchers have set a 6:1 ratio for study (assuming to be the best ratio) so I chose equidistant ratio below and above that ideal value. For H2/CO, as mentioned earlier, there have been many studied values. I set my midpoint at 1600 and chose equidistant points. I also did no H2 in the interest of seeing some promotion of Ag due to just the presence of the second metal The metals chosen are all in the TWC and have been shown to 1) reduce Nox and 2) do it at to a degree at low temperatures (200-400C). The temperatures were chosen as diesel exhaust typically resides between 150-400C. I conducted a five factor fractional experiment, and loadings were based off the total amount of silver atoms on the surface (i.e. 0.1 is equal the number of noble metal atoms that are 1/10 of the total siliver atoms. 3k full factorial design (k = 5) total of 243 independent observations Levels and responses normalized from 0 to 1

𝑋 3 = 𝑆𝑒𝑐𝑜𝑛𝑑 𝑀𝑒𝑡𝑎𝑙 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 10% Normalization 𝑋 1 = 𝐻𝐶: 𝑁𝑂 𝑥 𝑅𝑎𝑡𝑖𝑜 9 𝑋 2 = 𝐻 2 :𝐶𝑂 𝑅𝑎𝑡𝑖𝑜 4 𝑋 3 = 𝑆𝑒𝑐𝑜𝑛𝑑 𝑀𝑒𝑡𝑎𝑙 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 10% 𝑋 4 = 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 400 Factors were normalized by diving the level by the highest tested factor. Also the response were normalized from 0 to 1. In this way everything could be measured under the same scale. The total number of data points is 81 (3^5-1). The metal type factor could not be normalized, so in doing regression, it was done that three different equations were developed for each metal. 𝑁𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑜𝑟 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦= 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑂𝑅 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 100

DOE Procedure Develop Hypothesis Select Factors and Levels Calculate p values Run Experiment and Collect Data Plot Effects Determine Statistical Significance The analysis is done by testing a null hypothesis. The null hypothesis assumes a factor is of no effect until its p value (or probability of no effect) is low enough to accept in the model. A smaller p value means more evidence that the null hypothesis is not true. We can plot the effects (Main and Interactions) to visually show significance. If the resulting effect lines deviate significantly from the mean of the response, that indicates possible significance.

Main Effects: NOx Conversion All factors are significant Increasing loading caused decrease in conversion P value ≈ 0.00 P value ≈ 0.00 P value ≈ 0.00 Mean NOx Conversion Main effect of NOx conversion shows that every effect, statistically wise, is significant. The improvement in conversion due to HC/Nox, h2/CO and temperature is expected. The reaction is driven by availability of the HC for reduction, the presence of H2 for low temperature activity, and temperature. Interesting, the loading result in a negative performance, and the metal type is also significant (although the Pd drop accounts for that). P value ≈ 0.05 P value ≈ 0.00

Main Effects: N2 Selectivity H2/CO ratio, second metal loading, second metal type, temperature significant Presence of PGM causes N2O formation (increasing as more is added) P value ≈ 0.00 P value ≈ 0.06 P value ≈ 0.02 Mean N2 Selectivity Main effect of NOx conversion shows that every effect, statistically wise, is significant. The improvement in conversion due to HC/Nox, h2/CO and temperature is expected. The reaction is driven by availabilty of the HC for reduction, the presence of H2 for low temperature activity, and temperature. Interesting, the loading result in a negative performance, and the metal type is also significant (although the Pd drop accounts for that). P value ≈ 0.01 P value ≈ 0.00

Significant Interactions – Loading/Temperature 0% 1% 10% Mean NOx Conversion P value ≈ 0.00 Characteristics of detrimental effect of loading 10% PGM Loading > 300oC 200 300 400 Temperature (oC)

Effect of Loading Observation: Increasing amount of second metal onto Ag/Al2O3 is detrimental on NOx conversion and N2 selectivity (primarily at high loadings at 300oC) Possible Reasons: Unselective combustion of the hydrocarbon and increased formation of N2O by noble metals Site blocking of Ag by noble metals

Loading Effect – Bimetallic Catalysts Ag Ag-1% Pd Ag-1% Pt Ag-1% Rh Ag-10% Pd Ag-10% Pt Ag-10% Rh HC conversion increases to 100% at high loadings NOx conversion subsequently decreases

N2 Selectivity – Al2O3-supported Monometallic Catalysts Ag exhibits high selectivity throughout temperature range Selectivity not significantly affected at 1% loading At 10% loading, significant decreases in selectivity occur over PGM

Is Site Blocking an Issue? Catalyst O2 uptake (μmol/g) Ag/Al2O3 2 ± 1 Ag-10% Pd/Al2O3 1.0 ± 0.1 Ag-10% Pt/Al2O3 1.2 ± 0.3 Ag-10% Rh/Al2O3 3 ±1 O2 uptake virtually unchanged with addition of PGM Suggests site blocking is not an issue Oxidized in air at 600oC for 1 hour Degassed in He for 1 hour Reduced in 10% H2/Ar at 250oC for 2 hours Degassed in He at 260oC for 1 hour Pulsed 1% O2/He at 170oC

Effect of Loading Observation: Increasing amount of second metal onto Ag/Al2O3 is detrimental on NOx conversion and N2 selectivity (primarily at high loadings at 300oC) Possible Reasons: Unselective combustion of the hydrocarbon and increased formation of N2O by noble metals Site blocking of Ag by noble metals

Presentation Overview Introduction Catalyst Characterization and Screening Results: Effect of Loading Results: Effect of Loading Order Summary and Future Work Now that I have provided an introduction to my research, I will next proceed with the characterization and screening. But first I would like to talk about my experimental setup.

Loading Order – Prior Research NOx reduction improved when adding Ag after addition of small amount of Rh metal with decane1 HC-SCR with CH4 activity improved when Co was added after Zn on a Co-Zn/HZSM-5 catalyst2 1.Sato et al., Cat. Comm., 2003 2.Ren et al., App. Cat. B: Env., 2002

Significant Interactions – Loading Order with Metal Type – NOx Conversion Insignificant for Pt and Rh (within error) Addition of Ag after Pd results in ≈ 6% improvement in conversion 0.35 Error = 1.8% Mean NOx Conversion 0.3 P value ≈ 0.02 Loading Order

Significant Interactions – Loading Order with Metal Type – N2 selectivity Insignificant for Pt and Rh (within error) Addition of Ag after Pd results in ≈ 12% improvement in selectivity 0.8 Error = 3.2% Mean N2 Selectivity P value ≈ 0.04 0.7 Loading Order

Effect of Loading Order Observation: Switching the order improves performance only for Pd bimetallic catalysts Possible Reasons: Greater surface concentration of Ag on the surface when added after Pd Pd miscible with Ag

TPR: Pd-based catalysts H2 consumption peak from Ag significantly when Ag added second Suggests higher Ag surface concentration 10% Pd-Ag/Al2O3 Ag-10% Pd/Al2O3 10% Pd/Al2O3 1% Pd-Ag/Al2O3 Ag-1% Pd/Al2O3 1% Pd/Al2O3 Ag/Al2O3 Temperature programmed reduction experiments for the Pd catalysts are shown above. The procedure is listed above. The alumina showed a reduction peak at about 360 C owning to weakly bound O2 surface species on the support. The peak at 120oC corresponds to the reduction of PdO and the peak at 250C corresponds to the reduction of Ag2O to Ag metal. Typically a sharp negative H2 consumption peak is seen around 70oC on Pd-based catalysts, but it is not seen here. This may be due to the fast temperature ramp (done to sharpen peaks) Al2O3 Oxidized in air at 600oC for 1 hour Degassed in Ar for 1 hour Cooled to RT in Ar Ramped from RT to 500oC in 10% H2/Ar at 20oC/min

Ag-Pd – miscibility or surface interactions? 1600 Metal Surface Energies (J/m2) Ag 1.2 Pd 1.9 Pt 2.3 Rh 2.5 Temperature (oC) 900 Ag and Pd are not miscible until 900oC1 Surface energies suggest Pd migrates to surface more readily and could interact with Ag compared to Pt and Rh2 1.I. Karakaya, Journal of Phase Equlibria, 1986 2. Vitos et al., Surface Science, 1998

Ag metal dispersion - Ag-first vs. Ag-second loaded O2 uptake significantly increases when Ag added after Pd Negligible change for Pt bimetallics: no change for Rh catalysts Catalyst O2 uptake (μmol/g) Ag-10% Pd/Al2O3 1.0 ± 0.1 10% Pd-Ag/Al2O3 6 ± 1 Ag-10% Pt/Al2O3 1.2 ± 0.3 10% Pt-Ag/Al2O3 2 ± 1 Ag-10% Rh/Al2O3 3 ± 1 10% Rh-Ag/Al2O3

Effect of Loading Order Observation: Switching the order improves performance only for Pd bimetallic catalysts Possible Reasons: Greater surface concentration of Ag on the surface when added after Pd Pd miscible with Ag

Presentation Overview Introduction Catalyst Characterization and Screening Results: Effect of Loading Results: Effect of Loading Order Summary and Future Work Now that I have provided an introduction to my research, I will next proceed with the characterization and screening. But first I would like to talk about my experimental setup.

Summary and Conclusions Increasing loading of second metal was detrimental on the NOx conversion and N2 selectivity Due to increased unselective combustion of C3H6 and formation of N2O with increased noble metal concentration Loading order mattered only for Pd bimetallic catalysts Increased surface concentration of Ag when Ag added after Pd The combination of both H2 and noble metal addition did not result in a better performing catalyst compared to Ag/Al2O3 by itself

Suggestions for Future Work Reduce noble metal amounts even smaller EXAFS (Extended X-Ray Absorption Fine Structure) to identify electronic effects of loading order on Ag-Pd bimetallic catalysts Further investigate differences in metal type

Professor Levi Thompson My Committee Acknowledgements Professor Levi Thompson My Committee Professor Galen Fisher Professor Erdogan Gulari Professor Phillip Savage Professor Arvind Atreya Quantum Sciences Inc. Past and Present Members of Thompson Group Friends (especially in SMES-G, SCOR, and AGEP) My Family God

Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense Department of Chemical Engineering The University of Michigan July 29, 2011 Hello all; And good morning. I would like to thank you all to coming to my talk this morning. I am especially grateful for the presence of my friends and family for support. Today I will talk about some of the work I did for the development of silver catalysts for the reduction of Nox.

Engine Exhaust Characteristics (before treatment) Three way catalytic converter (TWC) can reduce Nox emissions from gas engines up to 90% TWC cannot do this in oxidizing environment Exhaust Component Diesel Engine Gasoline Engine Nitrogen Oxides 200-1000 ppm 100-4000 ppm Total Hydrocarbons 10-330 ppm 400-5000 ppm CO 150-1200 ppm 0.1-6% O2 5-15% 0.2-2% H2O 1-7% 10-12% CO2 3-13% 10-18% Sulfur Oxides 10-100 ppm 15-60 ppm Particulates 50-400 mg/m3 n/a Temperature RT-700oC RT-1100oC Diesels have advantages over internal combustion engines. They produce less HCs, CO, and CO2 as shown in the chart of engine exhaust characteristics. Specifically, they run at higher air-to-fuel ratios. The increased air intake allows fuel to be burned more efficiently, therefore, improving the fuel economy. With the added air, however, comes in cost in terms of NOx emissions. The three way catalysts in the typical car can achieve up to 90% conversion of pollutants, but cannot reduce NOx under oxidizing conditions. So begs the question, why is Nox bad? From Supported Metals in Catalysis, Anderson and Garcia, 2005.

Noble Metal HC-SCR Good low temperature performance (200-400oC) Characterized by volcano plot behavior Tend to produce significant amounts of N2O Activity of 1% loaded noble metal catalysts for NOx reduction by C3H61 Burch and Millington, Cat. Today, 1996

Proposed Mechanism – Base Metal Oxides NO or hydrocarbon will react with O2 to form adsorbed NOx or acetate species Decomposition could create isocyanate, cyanate, ammonia intermediates Reduce to N2 Burch et al., Topics in Catalysis, 2004

Proposed Mechanism – Base Metal Oxides – H2 Enhancement The positive effect of H2 is shown in the mechanism of HC-SCR over Ag/Al2O3. The HC reacts with the NOx and O2 to create adsorbed NOx and acetate species. The mechanism goes through a series of steps where adsorbed –ONO, -CN, and –NCO species are formed to create N2. Adsorbed NOx can also be activated (converted to NO2) and react with –CN. The formation of the R-NCO is generally the rate determining step, and the presence of H2 enhances the formation rate of R-NCO. It can also help in the formation other key intermediates such as NH3 and R-NH2

Mechanism over Noble Metals Dissociation – reduction (Burch et al., App. Cat. B. Env. 1994) Z is an adsorption site Reaction occurs on reduced noble metal

F Distribution Table Residual degrees of freedom (number of treatments - number of levels) Factor degrees of freedom (number of levels – 1) Use F distribution table to get specific Fobs values Compare with calculated F values from ANOVA

C3H6 Conversion – Monometallic Catalysts

Main Effects: N2 Selectivity H2/CO ratio, second metal loading, temperature significant presence of H2 and increasing temperature enhance NOx conversion over Ag/Al2O3 and therefore N2 formation Presence of noble metal causes N2O formation (increasing as more is added) P value ≈ 0.00 P value ≈0.21 P value ≈ 0.00 Main effect of NOx conversion shows that every effect, statistically wise, is significant. The improvement in conversion due to HC/Nox, h2/CO and temperature is expected. The reaction is driven by availabilty of the HC for reduction, the presence of H2 for low temperature activity, and temperature. Interesting, the loading result in a negative performance, and the metal type is also significant (although the Pd drop accounts for that). P value ≈ 0.07 P value ≈ 0.00

DRIFTS – Bimetallics at 400oC hydroxyls Trace CO2(g) C=C? formates T = 400oC H2/CO = 4 HC/NOx = 9 Presence of formates, hydroxyls and v(C=C) bond [He et al, APB: Env., 2003; Wichterlova et al., J. Cat., 2005] Surface species and intensities similar regardless of second metal type

TPR: Pt-based catalysts Reduction of PtO2 at 180oC 180oC 0.1 Pt-Ag/Al2O3 H2 consumption peak from Ag significantly larger on Ag-second loaded catalysts Ag-0.1 Pt/Al2O3 0.1 Pt/Al2O3 0.01 Pt-Ag/Al2O3 Ag-0.01 Pt/Al2O3 0.01 Pt/Al2O3 Observe shift in Ag reduction peak on high-loading Ag-Pt catalysts (from 230oC-200oC) Ag/Al2O3 Similarly for Pt catalysts, the reduction at 180C is due to the reduction of PtO2 to Pt metal. On Ag-Pt, there is a shoulder starting around 80C which then gives way to the reduction of Ag which is initiated at around 100C. The max temperature of the reduction peak is about 210oC indicating the presence of the Pt improved the reducibility of the Ag Al2O3

TPR: Rh-based catalysts Reduction of Rh2O3 at 120oC 120oC 0.1 Rh-Ag/Al2O3 Ag-0.1 Rh/Al2O3 0.1 Rh/Al2O3 Observe shift in Ag reduction peak on high-loading Ag-Rh catalysts (from 230oC-140oC) 0.01 Rh-Ag/Al2O3 Ag-0.01 Rh/Al2O3 0.01 Rh/Al2O3 Ag/Al2O3 For rhodium, a sharp peak at 120C is seen, corresponding the reduction of Rh2O3 to Rh metal. On Ag-0.1 Rh, there is a dominant reduction peak at about 160C. This suggests a promotional effect due to Rh. Al2O3

Surface Coverage Surface Coverage ~ 15% for Ag, .1% for 0.01 loaded catalysts and 1% for 0.1 loaded catalysts

Theoretical Ag-Pd Orientation Top view of the geometry of (1 1 1) orientation for 3 X 3 surface system: The white circles are the surface atoms (if not labeled, those atoms are Ag. Jaatinen et al., Vacuum, 2004

Competitiveness Factor