Grantee Research Project Results
Final Report: Electrocatalysis for Environmentally Friendly Energy Production Systems
EPA Grant Number: R831495Title: Electrocatalysis for Environmentally Friendly Energy Production Systems
Investigators: Pfefferle, Lisa , McEnally, Charles
Institution: Yale University
EPA Project Officer: Aja, Hayley
Project Period: December 1, 2003 through November 30, 2006 (Extended to November 30, 2007)
Project Amount: $375,000
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities , Nanotechnology
Objective:
We investigated the catalytic combustion of methane over Pd based catalysts, focusing on catalyst deactivation over time in a low temperature regime below 450°C. FTIR was used to follow the development of OH on the catalyst systems both during methane combustion and desorption of OH after the methane flow was terminated. Deactivation was most pronounced on supports where OH desorption was slow. We also investigated electrochemical promotion of the methane combustion reaction over Pd/YSZ. This report summarizes our current findings.
Summary/Accomplishments (Outputs/Outcomes):
Catalyst Deactivation
Methane oxidation over Pd-based catalysts is of interest due to its attractive potential for catalytic combustion. A drawback though, is its tendency to deactivate during the reaction process. In the low temperature regime, below 450°C, water poisoning has been proposed to play a primary role in catalytic deactivation by the formation of palladium hydroxide during the reaction process.[1-4]
In this study, we observe that catalytic deactivation is related to the accumulation of surface hydroxyls on the catalyst support, but that the effect is support dependent. At steady state conditions most of the hydroxyl accumulation is support related.
Figure 1 shows typical spectra for four Pd-based catalysts with different metal supports, illustrating that hydroxyl coverage is dependent on the support. Each of the spectra in figure 1 were obtained during a methane combustion reaction at 325°C, each spectra was taken at the same time frame - about 20 minutes into the methane combustion reaction, and each spectra is for conditions with similar methane conversions. The conversion amounts were adjusted by varying the amount of catalyst, while all other conditions, including reactant flow rates, were held constant.
Figure 1: FTIR spectra of Pd supported on MCM-41, titania, alumina & MgO.
The peaks in figure 1 represent the incremental concentration of methane, CO2, and hydroxyls resulting from methane combustion. The hydroxyl peak for Pd/MCM-41 is affected by underlying silinol groups here located at 3,735 cm-1 (3,742 cm-1 at room temperature) which were not removed in the heating process prior to start of the methane combustion reaction.
We note in figure 1 that the predominant hydroxyl peaks are different for each catalyst support system. For example, Pd/MCM-41 has a primary hydroxyl vibrational peak about 3,735 cm-1, while Pd/TiO2 has a primary vibrational hydroxyl peak at 3,660 cm-1. The largest fraction of the hydroxyls on the surface are related to the supports. This is important to the reaction mechanism, as re-oxidation of the palladium involves oxygen from the support. We and others have shown using oxygen isotopes that lattice oxygen is involved in the conversion of methane to water and carbon dioxide. Consequently, hydroxyl formation must be occurring on lattice oxygen sites.[5-8]. This provides a possible mechanism by which an OH loaded support causes deactivation of the Pd catalyst. The OH groups are formed on the Pd catalyst and spilled over to the support. Bare supports placed in a combustion environment do not demonstrate high OH coverage.
A conclusion as to which peaks are related to a Pd bound surface hydroxyl is not directly possible from this data. Although all of the supported catalyst systems show peaks distinguishable at 3,732 – 3737 cm-1, especially apparent in early reaction time data, further studies to isolate the Pd OH peaks are needed. Strong interference is experienced due to interference from broad OH peaks related to the support especially at high conversions.
Figure 2 shows time series of the normalized peak areas for CO2, CH4, and the hydroxyl regions for each of the tested catalysts. The peak areas shown in figure 2 tie to sample spectra shown in figure 1.
Figure 2: Normalized peak area vs time for carbon dioxide, methane, and hydroxyl peak regions. Each peak area is normalized to its maximum output. Illustrated catalysts are Pd loaded on MCM-41, titania, alumina, & MgO.
The CO2 peaks in figure 2 reflect catalytic activity of the samples. Catalytic deactivation is present on Pd/Al2O3 and Pd/MgO, and non-apparent for these reaction conditions on Pd/TiO2 and Pd/MCM-41 (although the deactivation is noted on Pd/TiO2 at lower temperatures. The reaction on Pd/TiO2 is virtually at steady state, while the reaction on Pd/MCM-41 shows increasing CO2 levels throughout the methane combustion period.
We observe in figure 2 that the CH4 peak areas show a steep decline to zero after the methane gas flow was stopped and the CO2 peak area follows, indicating that these gases easily desorb from all tested catalyst materials. The continuation of the hydroxyl peak after the methane oxidation reaction stopped indicates that hydroxyls do not desorb quickly, and their rate of desorption varies with catalyst support. The MgO supported catalyst showing the largest deactivation also had the slowest desorption rate.
The carbon dioxide “bump” in all of the figure 2 graphs, about the time that methane flow is stopped, is an artifact from the change in methane concentration when methane flow is stopped. The decreased methane flow causes the CO2 residence time to increase.
Figure 3 shows normalized conversion, as measured by CO2 formation, as hydroxyl coverage on each support increases. The conversions are normalized to maximum conversion in the reaction over Pd/Al2O3. Figure 3 illustrates that the for catalysts with significant deactivation, Pd/Al2O3 and Pd/MgO, deactivation occurs as the hydroxyl level increases. The relatively high hydroxyl coverage on Pd/Al2O3 and Pd/MgO is also illustrated in figure 1. An interesting feature of Pd/TiO2 is that deactivation was not noted, even at high hydroxyl coverage.
In figure 2, we observed that the Pd/Al2O3 and Pd/MgO catalysts showed similar conversion with time. Figure 3 reveals that OH uptake on Pd/MgO is significantly faster.
Figure 3: Normalized Conversion vs hydroxyl coverage. Conversion is measured by carbon dioxide FTIR peak area.
Oxygen and hydrogen surface mobility can be an important aspect in many catalytic processes.[9] Duprez found that the relative oxygen and hydrogen surface mobility on MgO > Al2O3 > SiO2.[9]. Additionally, Lin et al found that oxygen mobility for Pd/Al2O3 > Pd/TiO2.[10] Deactivation was noted at high hydroxyl coverage for the two catalysts with the broadest peaks in the OH region of the IR spectra, and both have relatively high oxygen mobility on the support. These catalyst systems also showed slower rates of OH desorption after methane flow was stopped. It should be noted that zirconia and ceria supports, which also have high oxygen mobility but significant oxygen exchange with the bulk, exhibit fast OH desorption in contrast with MgO or alumina.[11] Further studies are in progress to investigate causal relationships.
Electrochemical Promotion of Oxygen Transport through YSZ
Yttrium stabilized zirconia (YSZ) is of interest because of it’s properties as an oxygen ion conductor. Used as a membrane, YSZ may allow oxygen to pass from the atmosphere into a methane combustion chamber, while preventing while CO2 product inside the methane combustion chamber from entering the atmosphere. Our experiments with YSZ focus on applying a potential to electrochemically improve oxygen transport and methane conversion.
The initial experiments were configured with methane, oxygen, and helium flowing into a pyrex reactor vessel containing a YSZ pellet with a thin layer of Pd on one side of the pellet. Gold electrodes were also attached to the pellet in order to generate an electric field through the pellet. The pellet was held in place by gold wires attached to the gold electrodes, and the gold wires ex- tended to a potentiostat voltage source. Product gases flowed from the reactor to a gas chromatography instrument, which measured the concentration of methane and CO2.
An example of results from applying a potential on a Pd/YSZ catalyst pellet at stoichiometric and then fuel lean conditions are shown in Figure 4. The data in Figure 4 was obtained by first flowing a stoichiometric ratio of methane and oxygen in helium carrier gas overnight so that a steady state conversion level was achieved. A one volt potential was then applied through the Pd/YSZ sample until conversion reached a new steady state. The voltage was then discontinued. Subsequently, excess oxygen was introduced into the reactant flow and after conversion leveled off, a one volt potential was again applied until conversion stabilized. Finally, the potential was removed while excess oxygen continued to flow.
Figure 4: Gas chromatography analysis of methane conversion during methane oxidation reaction over a YSZ pellet coated with PdO.
Clearly the addition of excess oxygen to the reactant flow has a dramatic effect on conversion levels, while electro-chemical promotion produced a more moderate increase in methane conversion.
To test the effectiveness of electrochemical promotion under conditions which would sequester CO2, a YSZ tube was coated with PdO on its interior wall, with gold electrodes positioned on the interior and exterior walls. A 5% methane in helium mixture flowed into the YSZ tube in a closed system, such that air would not enter the YSZ tube. Oxygen was supplied the interior of the YSZ tube by oxygen ion conduction through the YSZ tube wall. A one volt potential was applied to promote oxygen ion transport. The YSZ tube was heated to 500°C, and products from the methane combustion reaction flowed from the YSZ tube to a gas chromatography apparatus.
The results of a typical test with the YSZ tube is shown in figure 5. We note that there is a small degree of methane conversion prior to supplying an electric potential. This is due to the oxygen concentration gradient between the exterior and interior of the tube, and the resulting oxygen oxygen transport through the YSZ wall. When a one volt potential is applied to the system, methane conversion increases from about 0.5% to 2.5%.
Figure 5: Gas chromatography analysis of a methane oxidation reaction on the interior wall of a YSZ tube coated with PdO. The YSZ tube acts as a membrane, allowing oxygen transport through the tube, while sequestering CO2.
Conclusions:
Deactivation of palladium based catalysts during methane combustion is dependent on the naturee of the catalytic support. While is it generally accepted that water poisoning is responsible for catalytic deactivation, the accumulation of hydroxyls on the catalyst support appears to play a role in catalytic reaction. In our tested catalysts, species with high oxygen surface mobility exhibited the greatest hydroxyl accumulation and catalytic deactivation. In earlier tests however high oxygen mobility supports that also had significant oxygen exchange with the bulk of the support showed the least deactivation.
Electrical promotion of methane combustion over PdO on YSZ moderately improves catalytic activity. However, oxygen concentration within the fuel mixture is observed to have a far greater impact on conversion levels. Electrochemical pumping of oxygen across a membrane, however, is likely a useful approach for the design of catalytic combustors for CO2 sequestration. Such reactors without electrochemical pumping have shown to be viable and are about a factor of 2 in cost to being economically feasible. The factor limiting the economic design is a membrane allowing a high oxygen transport rate. The use of electrochemical pumping of oxygen would allow a smaller volume reactor because oxygen transport rates are significantly increased at a modest energy penalty.
References:
[1] Cullis, C. F. & Willatt, B. M. The inhibition of hydrocarbon oxidation over supported precious metal-catalysts. J CATAL 86, 187-200 (1984).
[2] Burch, R., Urbano, F. J. & Loader, P. K. Methane Combustion over palladium catalysts - the effect of carbon-dioxide and water on Activity. APPL CATAL A-GEN 123, 173-184 (1995).
[3] Burch, R., Loader, P. K. & Urbano, F. J. Some aspects of hydrocarbon activation on platinum group metal combustion catalysts. CATAL TODAY 27, 243-248 (1996).
[4] Roth, D., Gelin, P., Primet, M. & Tena, E. Catalytic behaviour of Cl-free and Cl-containing Pd/Al2O3 catalysts in the total oxidation of methane APPL CATAL A-GEN 203, 37-45 (2000).
[5] Muller, C. A., Maciejewski, M., Koeppel, R. A., Tschan, R. & Baiker, A. Role of lattice oxygen in the combustion of methane over PdO/ZrO2: Combined pulse TG/DTA and MS study with O-18-labeled catalyst. J PHYS CHEM-US 100, 20006-20014 (1996).
[6] Muller, C. A., Maciejewski, M., Koeppel, R. A. & Baiker, A. Combustion of methane over palladium/zirconia: effect of Pd-particle size and role of lattice oxygen. CATAL TODAY 47, 245-252 (1999).
[7] Ciuparu, D., Altman, E. & Pfefferle, L. Contributions of lattice oxygen in methane combustion over PdO-based catalysts. J CATAL 203, 64-74 (2001).
[8] Ciuparu, D., Bozon-Verduraz, F. & Pfefferle, L. Oxygen exchange between palladium and oxide supports in combustion catalysts. J PHYS CHEM B 106, 3434-3442 (2002).
[9] Duprez, D. Study of surface mobility by isotopic exchange: recent developments and perspectives. STUD SURF SCI CATAL 112, 13-28 (1997).
[10] Lin, W., Zhu, Y. X., Wu, N. Z., Xie, Y. C., et al. Total oxidation of methane at low temperature over Pd/TiO2/Al2O3: effects of the support and residual chlorine ions. APPL CATAL B-ENVIRON 50, 59-66 (2004).
[11] Ciuparu, D., Perkins, E., Pfefferle, L. In situ DR-FTIR investigation of surface hydroxyls on γ-Al2O3 supported PdO catalysts during methane combustion, APPL CATAL A-GEN 263, 145-153 (2003).
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 10 publications | 2 publications in selected types | All 2 journal articles |
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Haider P, Haller GL, Pfefferle L, Ciuparu D. New approach to avoid erroneous interpretation of results derived from generalized two-dimensional correlation analysis for applications in catalysis. Applied Spectroscopy 2005;59(8):1060-1067. |
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Persson K, Pfefferle LD, Schwartz W, Ersson A, Jaras SG. Stability of palladium-based catalysts during catalytic combustion of methane: the influence of water. Applied Catalysis B: Environmental 2007;74(3-4):242-250. |
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Supplemental Keywords:
RFA, Scientific Discipline, Sustainable Industry/Business, Chemical Engineering, Sustainable Environment, Environmental Chemistry, Technology for Sustainable Environment, Economics and Business, energy conservation, oxidation reactions, catalysts, electrocatalysis, energy efficiency, environmentally benign catalysts, zero emissions combustors, catalysis, pollution preventionProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.