Effects of trace elements in biodiesel on the performance of diesel oxidation catalysts in heavy-duty vehicles
Jonas Granestrand
Doctoral Thesis in Chemical Engineering KTH Royal Institute of Technology Department of Chemical Engineering Stockholm, Sweden 2020
Effects of trace elements in biodiesel on the performance of diesel oxidation catalysts in heavy-duty vehicles
Jonas Granestrand
TRITA-CBH-FOU-2020:49 ISBN: 978-91-7873-662-1
© Jonas Granestrand, Stockholm 2020 Tryck: US-AB, Stockholm 2020
Academic dissertation which, with permission of KTH Royal Institute of Technology, is submitted for public defence for the degree of PhD in Chemical Engineering, on October 23 2020, 1:00 pm in Ångdomen, Osquars Backe 31, Stockholm. The faculty opponent is Dr. Todd J. Toops, Oak Ridge National Laboratory, USA.
I Abstract
To reduce net greenhouse gas emissions, a shift towards adoption of biofuels is ongoing in the transport sector. Heavy-duty diesel vehicles are equipped with aftertreatment
equipment, comprising of catalysts and filters, and how this equipment is affected by the use of biofuels is not yet fully understood. Fatty Acid Methyl Ester (FAME) biodiesel may contain high loadings of different trace elements stemming either from the biomass used as raw material, or from the production process. These trace elements could act as poisons that deactivate the aftertreatment catalysts.
The objective of this work was to study how the diesel oxidation catalyst (DOC) is affected by the presence of trace elements in FAME biodiesel. The DOC reduces emissions of CO and hydrocarbons, but furthermore, it generates NO2 from NO present in the exhaust, which is essential for downstream aftertreatment components to operate optimally. Because the DOC is located at the inlet of the aftertreatment system, it is subjected to high
concentrations of trace elements, compared to downstream components.
An investigation of a DOC that had been used for an entire lifetime in a vehicle operating on FAME biodiesel revealed that phosphorus appeared to have the largest effect on DOC activity, causing considerable deactivation in the NO oxidation reaction. Na and Ca, on the other hand, appeared to have little effect. The largest cause of deactivation was thermal aging, rather than poisoning by trace components.
In-situ X-ray Absorption Spectroscopy studies of poisoned model catalysts showed a strong electronic interaction between P and catalytically active Pt/Pd particles during CO oxidation reaction conditions. In contrast, such effects were not observed for Na and K. Elemental mapping with Energy dispersive spectroscopy (EDS) of scanning transmission electron micrographs showed that phosphorus was co-located with Pt-Pd, whereas Na and K, on the other hand, were evenly distributed throughout the Al2O3 washcoat support.
Keywords: Diesel Oxidation catalyst, heavy-duty vehicles, catalyst deactivation, biodiesel, NAP-XPS, XANES, Phosphorus, alkali metals, Pt/Pd/Al2O3
II Sammanfattning
För att minska utsläppen av växthusgaser, har transportsektorn börjat övergå till användning av biobränslen. Tunga dieselfordon är utrustade med efterbehandlingssystem, bestående av katalysatorer och filter. Hur dessa komponenter påverkas av användandet av biobränslen är inte fullt utrett. Biodiesel i form av fettsyrametylestrar (FAME) kan innehålla höga halter av orenheter med ursprung antingen i biomassan som har använts som råvara, eller från produktionsprocessen. Dessa orenheter skulle kunna förgifta katalysatorerna i
efterbehandlingssystemet.
Arbetet som beskrivs i denna avhandling hade som mål att undersöka hur
dieseloxidationskatalysatorn (DOC) påverkas av orenheter i FAME-biodiesel. DOCn minskar utsläpp av CO och kolväten, men den genererar också NO2 från NO, vilket är nödvändigt för att övriga komponenter i efterbehandlingssystemet ska fungera optimalt. Då DOCn är först efter motorn i efterbehandlingssystemet utsätts den för högre halter av orenheter i bränslet än övriga komponenter.
En katalysator som hade använts en hel livstid i ett fordon som drevs av FAME-biodiesel undersöktes. Dessa studier visade att P hade störst effekt på den katalytiska aktiviteten i DOC och minskade aktiviteten för NO-oxidation betydligt. Däremot hade Na och Ca endast en marginell effekt på aktiviteten. Den största orsaken till katalysatordeaktivering var dock termisk åldring snarare än förgiftning.
In-situ-studier av förgiftade modellkatalysatorer med röntgenabsorptionsspektroskopi visade en stark elektronisk interaktion mellan P och de katalytiskt aktiva partiklarna av Pt- och Pd-metall under oxidation av CO. Däremot syntes inga sådana interaktioner för Na eller K. Enligt elektronspektroskopi band P till Pt-Pd-partiklar, medan Na och K var jämnt
utspridda över katalysatorns washcoat av Al2O3.
Nyckelord: Dieseloxidationskatalysator, tunga lastbilar, katalysatordeaktivering, biodiesel, NAP-XPS, XANES, fosfor, alkalimetaller, Pt/Pd/Al2O3
III List of publications
The following publications are appended to this doctoral thesis:
Paper I
Catalytic aftertreatment systems for trucks fueled by biofuels – aspects on the impact of fuel quality on catalyst deactivation, Jonas Granestrand*, Sandra Dahlin*, Oliver Immele,
Leonhard Schmalhorst, Cornelia Lantto, Marita Nilsson, Rodrigo Suárez París, Francesco Regali and Lars J. Pettersson, RSC Catalysis 30 (2018) 64–145
Paper II
Assessment of the impact of trace elements in FAME biodiesel on diesel oxidation catalyst activity after full lifetime of operation in a heavy-duty truck, Jonas Granestrand, Rodrigo Suárez París, Marita Nilsson, Francesco Regali, Lars J. Pettersson, submitted (2020) Paper III
In-situ characterization by Near-Ambient Pressure XPS of the catalytically active phase of Pt/Al2O3 during NO and CO oxidation, Susanna L. Bergman*, Jonas Granestrand*, Yu Tang, Rodrigo Suárez París, Marita Nilsson, Franklin Feng Tao, Chunhua Tang, Stephen J.
Pennycook, Lars J. Pettersson, Steven L. Bernasek, Applied Catalysis B: Environmental 220 (2018) 506–511
Paper IV
Probing the Oxidation/Reduction Dynamics of Fresh and P-, Na-, and K-Contaminated Pt/Pd/Al2O3 Diesel Oxidation Catalysts by STEM, TPR, and in Situ XANES, Susanna L.
Bergman*, Jonas Granestrand*, Shibo Xi, Yonghua Du, Yu Tang, Chunhua Tang, Liene Kienkas, Lars J. Pettersson, and Steven L. Bernasek, The Journal of Physical Chemistry C 124 (2020) 2945–2952
* These authors share principal authorship
IV My contributions to the Publications
Paper I: I and Sandra Dahlin wrote the major parts of the paper and share principal
authorship. We both contributed to planning of the paper, reviewing the literature and to all chapters of the paper. My main contribution to chapter 3 was the text about diesel oxidation catalysts.
Paper II: I am the principal author of the paper, and was responsible for writing the text, and for planning and carrying out the experiments, as well as for the analysis and discussion of results. The vehicle-aged sample and the cell-aged sample were provided by Scania.
Paper III: I and Susanna L. Bergman wrote the major part of the text and share principal authorship. We both contributed to planning the experiments, to carrying out the majority of the experiments, and to interpreting the results. I synthesized the catalyst and carried out chemisorption measurements. Dr. Yu Tang and Assoc. Prof. Franklin Feng Tao carried out complementary experiments with the XPS system designated XPS system 2 in the paper.
Chunhua Tang and Prof. Stephen J. Pennycook performed STEM imaging.
Paper IV: I and Susanna L. Bergman wrote the major part of the text and share principal authorship. We both contributed to planning the experiments and to interpreting the results. I was responsible for the preparation and poisoning of the model catalyst samples that were investigated. Shibo Xi and Dr. Yonghua Du performed the XANES measurements and provided input on analyzing the results. Dr. Yu Tang also provided input on analyzing the results. Chunhua Tang and Prof. Stephen J. Pennycook performed STEM imaging. Liene Kienkas contributed to synthesizing the catalysts and performing poison impregnation, and performed SEM/TEM-EDS experiments discussed in the supporting information. Prof. Steven L. Bernasek contributed to the text, to final editing and provided input to the discussion section.
V Conference contributions
Conference contributions related to this thesis are listed below. The presenting author is marked with boldface text.
Oral presentations
Oxidation state changes during catalytic oxidation of NO on Pt/Al2O3 observed by in-situ near ambient pressure XPS, Jonas Granestrand, Susanna L. Bergman, Marita Nilsson, Steven L.
Bernasek and Lars J. Pettersson, 9th International Conference on Environmental Catalysis (ICEC), Newcastle, Australia, July 10–13, 2016
In-situ characterization of the catalytically active phase of Pt/Al2O3 during NO and CO oxidation by ambient pressure XPS, Susanna L. Bergman, Jonas Granestrand, Yu Tang, Rodrigo Suarez Paris, Marita Nilsson, Feng Tao, Lars J. Pettersson, Steven L. Bernasek, 25th North American Catalysis Society Meeting (NAM), Denver, USA, June 4–9, 2017
Effects of individual FAME biodiesel impurities on diesel oxidation catalyst after full lifetime operation in a vehicle, Jonas Granestrand, Liene Kienkas, Rodrigo Suárez París, Francesco Regali, Ulf Nylén, Marita Nilsson and Lars J. Pettersson, 18th Nordic Symposium on Catalysis (NSC), Copenhagen, August 26–28, 2018
Poster presentations
Effect of biodiesel on the aftertreatment system in a heavy-duty truck: Influence of Na, K and P on diesel oxidation catalyst, Jonas Granestrand, Marita Nilsson and Lars J. Pettersson, 24th North American Catalysis Society Meeting (NAM), Pittsburgh, USA 14–19 June, 2015
In-situ Near Ambient Pressure XPS Characterization of the Catalytically Active Phase of Pt/Al2O3 during NO and CO Oxidation, Susanna L. Bergman, Jonas Granestrand, Yu Tang, Rodrigo Suarez Paris, Marita Nilsson, Feng Tao, Lars J. Pettersson, Steven L. Bernasek, Europacat 2017, August 27–31, 2017, Florence, Italy
Effect of FAME biodiesel impurities on a diesel oxidation catalyst after full useful life
operation in a heavy-duty truck, Jonas Granestrand, Rodrigo Suárez París, Ulf Nylén, Marita Nilsson and Lars J. Pettersson, The 8th Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT8), Yokohama, Japan, August 5–10, 2018
In-situ Probing of the Oxidation/Reduction Dynamics of Pure and Contaminated
Pt/Pd/Al2O3DOC Catalysts by XAFS and Flow Reactor Measurements, Susanna L. Bergman, Jonas Granestrand, Yonghua Du, Lars J. Pettersson, and Steven L. Bernasek, The 8th Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT8), Yokohama, Japan, August 5–10, 2018
In-situ Probing of the Oxidation/Reduction Dynamics of Pure and Na, P, K-contaminated Pt/Pd/Al2O3 DOC Catalysts by XAFS and flow reactor measurements, Susanna L. Bergman,
VI
Jonas Granestrand, Yonghua Du, Lars J. Pettersson, and Steven L. Bernasek, 18th Nordic Symposium on Catalysis (NSC), Copenhagen, August 26–28, 2018
Other contributions
Other contributions, not included in this thesis. The presenting author is marked with boldface text.
The role of alkali in heterogeneous catalysis for gas cleaning in stationary and mobile applications, P.H. Moud, J. Granestrand, S. Dahlin, M. Nilsson, K. J. Andersson, L.J.
Pettersson, K. Engvall,.249th American Chemical Society National Meeting, Denver, Colorado, 22–27 March, 2015 (Invited lecture)
The Use of Biofuels in Heavy-Duty Trucks – Fuel Production and Exhaust Treatment. L.J.
Pettersson, J. Granestrand and S. Dahlin, 10th International Conference on Environmental Catalysis, Tianjin, China, September 22–26, 2018 (Invited keynote lecture)
Deactivation of automotive catalysts for biofuel-powered heavy-duty vehicles, . L.J.
Pettersson, S. Dahlin and J. Granestrand, 11th International Conference on Environmental Catalysis, Manchester, UK, September 6–9, 2020 (Invited keynote lecture)
VII List of abbreviations
ASTM: American Society for Testing and Materials
BET: Brunauer–Emmett–Teller
BJH: Barrett-Joyner-Halenda
BX: X is the volume percentage of biodiesel in the blend CEM: Controlled evaporation and mixing
CO: Carbon monoxide
CO2: Carbon dioxide
DFT: Density Functional Theory
DOC: Diesel Oxidation Catalyst
DPF: Diesel Particulate Filter
EDS: Energy dispersive X-ray spectroscopy
EPA: Environmental Protection Agency
EXAFS: Extended X-ray Absorption Fine Structure
EU: European Union
FAME: Fatty Acid Methyl Ester
HVO: Hydrotreated Vegetable Oil
ICP-OES: Inductively Coupled Plasma – Optical Emission Spectroscopy
LHHW: Langmuir-Hinshelwood-Hougen-Watson
NAP-XPS: Near-Ambient X-ray Photoelectron Spectroscopy
NOx: Nitrogen oxides
NO: Nitric oxide
NO2: Nitrogen dioxide
RME: Rape methyl ester
RT: Room temperature
SCR: Selective catalytic reduction
SEM: Scanning Electron Microscopy
STEM: Scanning transmission electron microscopy TEM: Transmission electron microscopy
TPR: Temperature programmed reduction
US: United States
XANES: X-ray absorption near-edge spectroscopy
XAS: X-ray absorption spectroscopy
XRF: X-ray Fluorescence
VIII
Table of Contents
1. Introduction ... 1
1.1 Setting the scene ... 1
1.2 Objectives and thesis outline ... 9
2. Effects of FAME biodiesel impurities on diesel oxidation catalysts (Paper I) ... 11
2.1 FAME production and impurities ... 11
2.2 Diesel oxidation catalysts ... 13
2.3 Effects of fuel impurities on diesel oxidation catalysts ... 16
3 Experimental methods ... 19
3.1 Catalyst activity testing setup ... 19
3.2 Description of investigated catalysts ... 22
3.2.1 Vehicle aged and engine-cell aged catalyst samples ... 22
3.2.2 Protocol for sequential selective removal of poisons from the vehicle-aged sample ... 22
3.2.3 Synthesis of model catalysts ... 23
3.2.4 Poisoning of model catalysts by impregnation ... 24
3.3 Catalyst characterization ... 25
3.3.1 Nitrogen adsorption ... 25
3.3.2 CO chemisorption ... 25
3.3.3 Bulk elemental composition analysis ... 25
3.3.4 Electron microscopy ... 26
3.3.5. Temperature-Programmed Reduction ... 27
3.3.6. In-situ Near-Ambient X-ray Photoelectron Spectroscopy... 27
3.3.7. In-situ X-ray Absorption Spectroscopy ... 29
4. Investigation of a sample from a vehicle used for an entire regulatory lifetime (Paper II) . 31 5. Characterization studies of interactions between trace elements and catalytically active material ... 37
5.1 Oxidation state of Pt under NO oxidation reaction conditions (Paper III) ... 37
5.2 Characterization study of poisoned model catalysts (Paper IV) ... 40
6 Conclusions and future work... 47
6.1 Conclusions ... 47
6.2 Future work ... 49
IX
Acknowledgements ... 51 References ... 53
X
1
1. Introduction
1.1 Setting the sceneThe global mean surface temperature has increased considerably over pre-industrial levels as a consequence of greenhouse gas emissions. As illustrated in Figure 1, this increase in global temperature is correlated with global emissions of CO2. To limit the negative effects of global warming, the Paris agreement calls for limiting the temperature increase over pre- industrial levels to well below 2°C, and to pursue efforts to limit the increase to no more than 1.5 °C [1]. To reach this 1.5 °C target, the Intergovernmental Panel on Climate Change (IPCC) has reported that global CO2 emissions need to be reduced by 45 % by 2030
compared to 2010 levels, and net zero emissions of greenhouse gases need to be reached by 2050 [2].
Figure 1: Global annual industrial net CO2 emissions [3] and global surface mean temperature anomaly compared to the 20th century average [4].
The transport sector is responsible for a large share of CO2 emissions. As shown in Figure 2, the transport sector is responsible for over a quarter of total CO2 emissions in the European Union and the United States. Reducing CO2 emissions in the transport sector will be crucial to reach CO2 emission reductions targets.
2
Figure 2: Share of total CO2 emissions from fuel combustion originating from the transport sector for the European Union, United States, China, Japan, India and Brazil [5].
2017, the Swedish parliament committed to reaching net zero greenhouse gas emissions by 2045. For the transport sector specifically, a target was set for 2030 of a 70 % reduction (excepting domestic air traffic) compared to 2010 [6].
Over the last 50 years, transport of goods on roads has increased considerably in Sweden.
The vast majority of increases in overall goods transport came from road transport, while goods transport by sea and rail increased more slowly, see Figure 3. Road transport is responsible for an overwhelming majority of the energy usage in the Swedish transport sector (Figure 4).
3
Figure 3 Payload-distance in billions of tonkm of goods transport in the Swedish transportation sector (domestic and international) [7].
Figure 4: Yearly energy usage for domestic Swedish transports [8].
4
To lower the climate impact in the transport sector a multi-pronged approach is necessary. A combination of electrification, hybridization, engine efficiency improvements and biofuels are all needed. The European Union has a target of reducing CO2 emissions in heavy-duty vehicles by 15 % from 2025 onwards compared to the reference period of 2019–2020. From 2030 onwards, the reduction target is 30 % compared to the reference period [9].
Biofuels has been and will be an important part of reducing the climate impact of the climate sector. While long-term, electrification will be important, biofuel adoption has already seen rapid scale-up and will remain an important part of our CO2 emissions reduction strategy for the foreseeable future. The rise of biofuels in the Swedish transport sector during the 2000s is illustrated in Figure 5 and Figure 6. The majority of this increase is in the form of HVO and FAME biodiesel, rather than biogas and bioethanol, as shown in Figure 6. A large portion of the biodiesel usage is due to blending of biodiesel with petrodiesel, which can be used in any diesel engine vehicle. The Swedish government has set a target for 2030 which requires sufficient blending of biofuel in gasoline and diesel to achieve a reduction in greenhouse gas emissions by 28 % and 66 %, respectively [10].
Figure 5: Energy usage from Swedish domestic transport by fuel type [8].
5
Figure 6: Share of energy usage in Swedish domestic transport by different biofuels [11].
In addition to greenhouse gases, combustion engines emit local pollutants that are hazardous to health and the environment. Particulate matter can penetrate into human lungs and has been linked to increased mortality by respiratory [12] and cardiovascular disease [13]. NOx contributes to acid rain and smog formation [14], as well as to increased mortality by respiratory diseases [15]. To alleviate these harmful effects, policymakers have enacted emission standards which limit the amounts of pollutants that are allowed to be emitted from vehicles [16]. Vehicle manufacturers need to certify that emissions of pollutants stay below certain levels during dynamometer test cycles meant to mimic standard vehicle operation. Regulated pollutants include NOx, particulate matter,
hydrocarbons and CO. These standards are intermittently made stricter as better pollutant abatement technology becomes available. As shown in Figure 7, the allowed amounts of NOx
and particulate matter emissions have decreased by several orders of magnitude since emission standards were first enforced. This has led to a significant decrease of pollutant concentration in air in cities worldwide. For example, NO2 and particulate levels at Hornsgatan in Stockholm, one of the busiest and most polluted streets in the city, have gradually decreased over the last few decades, despite increases in traffic. See Figure 8.
6
Figure 7 Evolution of emission standards for heavy-duty diesel vehicles in the European Union [16] and United states [17].
7
Figure 8: Yearly average of measured concentrations of NO2 [18], PM10 [19] and PM2.5 [20] in air at
Hornsgatan in Stockholm. The horizontal lines represent the target values for NO2 and PM10 (black line) and PM2.5 (blue line).
Figure 9: Schematic representation of a Euro VI heavy-duty diesel exhaust treatment system.
8
To adhere to emission standards, a combination of engine modifications and catalytic aftertreatment is employed. A schematic view of a typical Euro VI heavy-duty vehicle exhaust treatment system is shown in Figure 9. Immediately after the engine a diesel oxidation catalyst (DOC) is installed. The DOC oxidizes CO and hydrocarbons into CO2 and H2O, as well as a fraction of NO into NO2 [21]. The DOC is followed by a diesel particulate filter where particles are trapped. To avoid clogging this filter, it is continuously regenerated by oxidizing particles trapped there. After the DPF, a selective catalytic reduction catalyst reduces NO and NO2 by NH3 derived from a urea solution, into N2. Finally, the system may include an ammonia slip catalyst that oxidizes excess NH3. The NO2 formed in the DOC allows downstream components of the aftertreatment system to work optimally. NO2 acts as an oxidant that helps remove combustible particles stuck in the filter, at a lower temperature than O2 does. However, the ash consisting of Inorganic particles cannot be removed this way. Furthermore, optimal SCR activity is achieved with a molar ratio of NO2 to NO of 1, which represents a much higher relative concentration of NO2 than what is emitted from the engine.
In summary, the DOC fulfills all of the following functions in an aftertreatment system
• Takes care of CO and gaseous hydrocarbons
• Oxidizes the soluble organic fraction of the particulate matter
• Oxidizes NO into NO2
– Facilitates particulate filter regeneration – Increases NOx removal activity
• Supplies heat to the aftertreatment system
9 1.2 Objectives and thesis outline
The objective of my work was to investigate the effect of FAME impurities on the performance of diesel oxidation catalysts. This relates to several of the United Nations Sustainable Development Goals [22]. Specifically, it relates to goals 3 (good health and well- being), 7 (affordable and clean energy), 11 (sustainable cities and communities), and 13 (climate action).
The following specific goals were formulated during my project:
1. Search the literature and summarize the elements and components commonly present in FAME biodiesel fuel and exhaust with the greatest potential to affect DOC activity.
2. Analyze a vehicle-aged catalyst to study poisoning of a DOC by trace elements in FAME biodiesel under realistic conditions.
3. Identify structural factors that may affect the catalytic activity of the DOC, through the use of in-situ characterization techniques.
4. Summarize the results of the project in a table mapping the effects of biodiesel trace elements on the DOC activity.
Goal 1 was achieved in Paper I, which is a literature review outlining which impurities are present in biofuels and summarizing previous research performed on their effects on the aftertreatment system. It is summarized in chapter 2 of this thesis, with a focus on the diesel oxidation catalyst.
Goal 2 was achieved in Paper II, which details an investigation of a DOC that had been in operation for an entire regulatory lifetime in a heavy-duty vehicle powered by FAME biodiesel. This paper is summarized in chapter 4 of this thesis.
Goal 3 was achieved in Papers III and IV which describe advanced material characterization of poisoned model catalysts. The results of these papers also helped explain the effects observed in Paper II. These papers are summarized in chapter 5 of this thesis.
Finally, through a synthesis of the results achieved in pursuit of the other goals, a table mapping the effects of biodiesel trace elements on DOC activity could be produced (goal 4).
This table is presented in the Conclusions section, Chapter 6.1.
10
11
2. Effects of FAME biodiesel impurities on diesel oxidation catalysts (Paper I)
2.1 FAME production and impurities
Fatty acid methyl ester (FAME) biodiesel is produced from an esterification reaction between methanol and triglycerides from biomass oils. A reaction scheme for this process is given in Figure 10. FAME biodiesel requirements and specifications are described by the 14214 standard in the European Union, and the ASTM 65751 standard in the United States [23].
FAME biodiesel can either be used blended with conventional fossil diesel or neat. Biodiesel blends are denoted BX, where X is the volume percentage of biodiesel in the blend, e.g. B100 is pure biodiesel, while B20 is a blend of 20 % biodiesel with conventional fossil diesel.
Figure 10: Reaction scheme for the transesterification reaction used in the production of FAME biodiesel.
An overview of the process for production of FAME is shown in Figure 11. As feedstocks, vegetable oils (e.g. rapeseed oil and soybean oil), animal fats or waste cooking oils are used [24]. Phospholipids in the biomass used as feedstock can give rise to phosphorus impurities in the final biodiesel product. After pretreatment, the transesterification reaction of
triglycerides with methanol is carried out. Most commonly, an alkaline homogeneous catalyst is used in this step, for example NaOH or KOH. Insufficient separation of catalyst salts from the biodiesel produced gives rise to the presence of Na or K impurities in the final product. Following transesterification, washing steps can introduce Ca and Mg impurities in the final product originating from dry wash adsorbents or hard water used in such clean-up steps [25].
Figure 11: Overview of the production process for FAME biodiesel, showing the origin of the different trace impurities present in the final product. The bolded impurities are regulated by fuel standards.
12
The allowed levels of trace element content according biodiesel fuel standards are summarized in Table 1.
Table 1: Maximum allowed content of trace elements according to biodiesel standards [23].
EN14214 ASTM D6751
Sulfur 10 mg/kg S15: 15 ppm
S500: 0.05 %
Group 1 metals (Na+K) 5.0 mg/kg 5 mg/kg
Group 2 metals (Ca+Mg) 5.0 mg/kg 5 mg/kg
Phosphorus 4.0 mg/kg 0.001 wt-% (i.e. 10 mg/kg)
13 2.2 Diesel oxidation catalysts
The material of choice for commercial diesel oxidation catalysts is some combination of platinum and palladium dispersed on porous γ-Al2O3. This catalytic washcoat is then coated onto honeycomb monoliths to ensure low pressure drop, a large geometric surface area and low resistance to mass transfer of reactants to the catalyst surface [26]. Pt tends to be more active for oxidation of hydrocarbons and NO, while Pd is more active for oxidation of CO [27]. As such, optimal reactivity for the complex mixture of gases that makes up diesel engine exhaust is achieved with a combination of Pt and Pd. One study showed optimal activity for a gas mixture, modeled after diesel exhaust and containing H2O vapor, with a mass ratio of Pt/Pd of 3:1 [28]. Furthermore, a mixture of Pt and Pd on Al2O3 is more
resistant to thermal deactivation by sintering than is Pt/Al2O3 [29]. Pd is much more active in CH4 oxidation than is Pt [30], so in vehicles powered by biogas or natural gas, Pd/Al2O3
catalysts are often used. To reduce costs, there is also some research on using base metal oxides as the active material in diesel oxidation catalysts. For example, perovskites [31, 32]
and Mn-mullite (Sm, Gd) Mn2O5 [33] have been proposed as candidates, though their adoption is prevented by insufficient catalytic activity at low temperatures.
The oxidation of CO by O2 is one of the most well-studied catalytic reactions in the literature.
Under many conditions it follows a Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism [21], as described by the reactions R1–R3 below:
O2(g) ⇌ 2O(ads) (R1)
CO(g) ⇌ CO(ads) (R2)
CO(ads) + O(ads) ⇌ CO2(g) (R3)
Furthermore, platinum oxides are continuously formed and decomposed on the catalyst surface. This can be described by reactions R4–R5 below. (The platinum oxides are written as PtO in the reaction formulae, but the exact stoichiometric ratio of O to Pt is not necessarily 1).
O(ads) + Pt → PtO (R4)
PtO + CO(ads) → Pt + CO2 (R5)
The formation and decomposition of platinum oxides affects the reaction rate. Specifically, this phenomenon has been used to kinetically model oscillations in the CO oxidation rate, which can be observed under certain conditions. One study investigating CO oxidation on polycrystalline Pt wire [34] postulated that when the catalytic surface mainly has O2
adsorbed, the reaction rate is high. However, in this state, platinum oxides are gradually formed, and when there is a critical proportion of platinum oxides on the surface, the reaction rate is lowered considerably as the platinum oxides inhibit adsorption of reactants.
In this new reaction rate regime, the surface is mainly covered by CO and little oxygen
14
adsorbs, leading to a low reaction rate. Over time, adsorbed CO reduces platinum oxides (R5) until the proportion of oxides on the surface is lowered to a critical level at which the reaction rate switches back to the initial faster state.
For catalysts coated onto a monolith, a temperature hysteresis for CO oxidation has been observed, wherein the catalyst is less active during ignition (heat-up) than during extinction (cool-down) [35]. During ignition, the surface tends to be saturated with CO which gives a slower reaction. When the catalyst has been heated, however, the surface contains more O2, which gives higher catalyst activity on extinction than on ignition. Furthermore, thermal inertia in the reactor, leading to ununiform temperature distributions, which are not
accounted for when measuring the temperature at the catalyst’s inlet, can also help explain this phenomenon [36].
For oxidation of hydrocarbons, an important effect is that oxidation of a mixture of various hydrocarbons behaves differently compared to oxidation of each hydrocarbon alone [27, 37]. Different hydrocarbons in a mixture compete for adsorption sites, and each component of the mixture inhibits the oxidation of the other components. As such, catalytic light-off for each component of the mixture occurs at a higher temperature than for the case where it is oxidized alone. Similar inhibition effects are observed between CO and hydrocarbons, where the presence of CO delays light-off of hydrocarbons to higher temperatures. This CO
inhibition is more severe on Pt/Al2O3 than on Pd/Al2O3 [27].
Similar to CO oxidation, oxidation of NO can be described using the LHHW scheme, as in reactions R6–R8 below [21]:
O2(g) ⇌ 2O(ads) (R6)
NO(g) ⇌ NO(ads) (R7)
NO(ads) + O(ads) ⇌ NO2(g) (R8)
Formation of platinum oxides on the surface, as described by R9 – R10 below, by either oxygen or NO2, can affect the catalyst activity. It is important to note that NO2 is a stronger oxidant than oxygen, and as such NO2 can more easily react to form platinum oxides.
Platinum oxide is catalytically less active for the NO oxidation reaction than metallic
platinum. In other words, NO2, formed in the NO oxidation reaction can inhibit the reaction [38] by contributing to formation of platinum oxides (here denoted as PtO, even though the exact stoichiometric ratio of O to Pt is not necessarily 1).
O(ads) + Pt → PtO (R9)
NO2(g) + Pt → PtO + NO (R10)
Formation of platinum oxides by NO2 leads to a unique phenomenon known as inverse temperature hysteresis. In contrast to CO oxidation, activity in the NO oxidation reaction is
15
higher during ignition than during extinction [36]. This has been connected to formation of less catalytically active platinum oxides, observed via X-ray Photoelectron Spectroscopy (XPS) [39, 40] and X-ray Absorption Spectroscopy (XAS) studies [41]. NO2 can oxidize Pt to a higher degree than O2 can, possibly because of NO2 having a higher coordinative flexibility than does O2, which facilitates dissociative adsorption of NO2 on the Pt surface at high O coverage [40]. At intermediate temperatures 200–300 °C during ignition, NO2 formed as a product in the NO oxidation reaction oxidizes Pt catalyst particle sites into (less catalytically active) platinum oxide sites, whereas at lower temperatures during extinction, NO reduces platinum oxides back to metallic Pt. Because small Pt particles are more easily oxidized than large Pt particles, overall NO oxidation activity can actually increase after thermally induced sintering [42]. Even though larger Pt particles will have fewer sites available for reactants to adsorb on, the lower tendency of larger Pt particles to form platinum oxides causes an increase in NO oxidation reaction rate per available Pt site (known as the turnover
frequency) sufficiently large to overcome the loss of active sites caused by particle sintering.
If the thermal deactivation is sufficiently severe, however, the overall reaction rate will still be decreased.
Mixture effects occur in co-oxidation of NO, CO and hydrocarbons. NO2 may act as an oxidant of yet not oxidized CO and hydrocarbons. In other words, CO and hydrocarbons act as reductants of NO2 formed by NO oxidation, and as such net NO2 production does not occur until at catalyst temperatures sufficiently high to achieve high conversion of CO and hydrocarbons [43, 44]. Finally, NO oxidation is considerably inhibited by the presence of H2O in the reactant mixture [45]. Diesel exhaust contains a considerable concentration of H2O vapor formed in the combustion process, so in order to accurately mimic the conditions in an aftertreatment system, experiments need to be performed with H2O vapor present in the reactant mixture.
16
2.3 Effects of fuel impurities on diesel oxidation catalysts
Paper I summarized several earlier studies concerning aging of DOCs during operation in vehicles on a variety of fuels [46-54]. Such vehicle-aged catalysts generally contained significant loadings of S, P, Ca, Zn and soot deposits. There tended to be a sharp axial gradient of trace elements, with higher content close to the inlet of the catalyst, compared to the outlet. S tended to have a more uniform distribution axially than the other elements.
P was often observed to be present in a penetration layer of up to 10 µm at the surface of the washcoat, while S was more evenly dispersed throughout the depth of the washcoat.
In a vehicle, several modes of catalyst deactivation can occur simultaneously, including sintering, fouling by soot, and poisoning by any of the trace elements present. As such, it is difficult to determine which of these mechanisms is responsible, and to what degree, for loss of catalyst activity after operation in a vehicle. The most common approach to attempt to deconvolute these different modes of deactivation is to test the catalyst activity after removal of some, or all, of the potential poisons. One study of decommissioned DOCs from buses observed significant amounts of P, S and soot in the DOC samples. After an oxidative treatment that selectively removed soot, the catalyst activity was almost completely
restored, meaning that soot alone likely was responsible for the loss in catalyst activity [51].
A method of sequential treatments, with the aim of selective removal of one impurity at a time was developed by Cummins [53]. The first step was a thermal treatment intended to remove soot, followed by a thermal treatment at higher temperature to remove S, and finally an acid treatment meant to remove P. Using this method, the authors were able to conclude that the inlet of the catalysts had been deactivated for the C3H6 oxidation reaction by P, while the outlet of the catalysts had been deactivated for the NO oxidation reaction by thermal sintering. This approach of sequentially removing poisoning elements, followed by catalyst activity tests after each removal step, was later further developed by other
researchers. Of particular note was a study of an aftertreatment system subjected to accelerated bench-aging, using B20 fuel doped by elevated levels of a sodium salt [55]. The sequential poison removal approach used in the Cummins study was modified by inclusion of a water washing step, after the S removal step, but before the acid treatment step, with the purpose of selectively removing Na and Ca. The largest improvement in activity was
observed after the acid treatment step, while the water treatment caused little
improvement. As such, it could be concluded that poisoning by P, rather than by Na, was the main cause of catalyst deactivation.
Poisoning by P has also been studied through investigation of lab-aged samples. One study [56] subjected Pt/Al2O3 and Pt/Pd/Al2O3 catalysts to P by exposure to a gas stream that had been passed through an aqueous solution of (NH4)2HPO4, while the catalyst samples were heated at 400 °C. With a high concentration of precursor salt in the aqueous solution, phosphorus loadings of 4.7 wt-% and 3.3 wt-% were achieved on the monometallic Pt and bimetallic Pt/Pd samples, respectively. At such high P loadings, the catalyst activity was severely decreased in all of the CO, C3H6 and NO oxidation reactions. The conversion of NO
17
dropped from 50 % to under 20 % in these samples. The samples were investigated by TEM in a later study [57], revealing that the morphology of the precious metal particles had been affected. In the Pt/Pd samples, the treatment by phosphorus exposure had led to a small increase in precious metal particle size, compared to samples treated at the same
temperature but without P exposure. In the monometallic Pt/Al2O3 samples, the shape of the catalyst particles had been affected as well, containing more smooth and spherical Pt particles, compared to a fresh catalyst which had more irregularly shaped particles. Such changes in precious metal particle morphology, due to the presence of P during treatments of the catalyst samples at 800 °C had been observed previously [58] and have been
postulated to be a cause of reduced catalyst activity in the NO oxidation reaction after operation in a vehicle [59]
DOC poisoning by sulfur has been rather extensively studied. The presence of 20–30 ppm of SO2 in the reactant feed has been observed to reduce the activity of Pt/Al2O3 catalysts in oxidation of CO, C3H6 [60] and NO [61] and of Pd/Al2O3 catalysts in oxidation of CH4 [62] as SO2 competes with reactants for adsorption sites on the catalyst. In contrast, activity in oxidation of C3H8 has been observed to improve in the presence of SO2 [60]. The choice of catalyst support affects the interaction with S. Sulfur poisoning has been observed to be more severe with SiO2 as the support rather than with Al2O3. On the other hand, catalyst activity can be restored by removal of S by treatment at a high temperature with SiO2 as the support, whereas with Al2O3 as the support, such treatments only partially restore the catalyst activity. The authors proposed that SO2 that adsorbs on the precious metal sites is oxidized to SO3, and on Al2O3 this SO3 spills over onto the support and is stored as Al2(SO4)3, whereas on SiO2 no such spillover occurs. For this reason, with Al2O3 as the support material, a smaller portion of the adsorbed S will be located on the precious metal particles,
explaining why the activity is not as severely affected as with SiO2 as the support material.
However, sulfur bound on an Al2O3 support also becomes more difficult to remove by thermal treatment [63]. This spillover of S to the support may in turn be affected by the presence of H2O in the reactant stream. When H2O is present, the Al2O3 support is hydrated, leading to less spillover of SO3 to the support. Therefore, the sulfating Al2O3 support cannot mitigate catalyst poisoning by S as effectively. Finally, S has been observed to affect sintering of Pt in Pt/Al2O3 catalysts. Treatment for 22 hours at 250 °C in an atmosphere containing 500 ppm NO, 8% O2 and 30 ppm SO2 led to a widened particle size distribution. It caused
formation of a small number of large Pt particles, while many of the small particles characteristic of a fresh catalyst still remained [64]. After removal of S, by a reductive treatment, the presence of large Pt particles was found to lead to improved activity in oxidation of both NO and C3H6[65].
18
19
3 Experimental methods
3.1 Catalyst activity testing setup
A catalyst activity testing rig was designed and constructed during the project. It is schematically shown in Figure 12. During testing, the catalyst is located in a quartz tube (outer diameter 25 mm, and inner diameter 23 mm) inside a Carbolite tubular furnace. The furnace is controlled by a Eurotherm 3508P1 microcontroller, which is connected to a thermocouple built into the furnace. The sample temperature during experiments is measured using a K-type thermocouple located 3 mm in front of the catalyst sample inlet, and logged using a National Instruments Data acquisition (DAQ) device. A bored-through uncoated monolith piece is placed in front of the sample to fix the radial position of the thermocouple so that it does not differ between different runs. This bored-through monolith piece also acts as a thermal mass that keeps the temperature distribution inside the furnace more even. Another such monolith piece is placed after the tested catalyst sample for that purpose. The gas composition going into the furnace is controlled by Bronkhorst thermal mass flow controllers. The water vapor concentration is determined by a Bronkhorst Controlled Evaporation Mixer (CEM) system, to which the flows of gas and water are controlled by thermal mass flow controllers. The testing rig can also vaporize liquid fuel, using a separate controlled evaporation mixer system. The liquid flow entering the
controlled evaporation mixer system is controlled by a Coriolis mass flow controller, which allows accurate flow control regardless of what liquid is used. The gas composition after the reactor is measured with a MKS 2030 Fourier-Transform Infra-red (FTIR) MultiGas Analyser, using Rowaco’s diesel exhaust method for spectrum analysis. The FTIR cell window material is BaF2.
Gas composition analysis by an FTIR instrument is based on the principle that molecules may absorb light in the infrared region, leading to rotational and vibrational transitions in the molecule [66]. Each molecule has a unique IR absorbance pattern. By performing a Fourier Transform of the absorption spectrum of a gas mixture, absorbance as a function of frequency is obtained. By analysis of frequencies in the aggregate spectrum where
absorbance is unique to individual components of the gas mixture, the composition of the gas mixture can be obtained.
20
Figure 12: Schematic view of the catalyst activity rig designed and constructed during the project.
21
The gas lines before and after the furnace are heated using 832 W heating tapes wrapped around the tubes. Before the reactor, the purpose is to ensure that the temperature in the tubes in between the CEMs and the reactor never falls below the dew point of the mixture, for which a temperature of 60–80 °C is sufficient. The lines after the furnace are heated because the FTIR requires an inlet temperature of 193 °C. The heating tapes are controlled by solid state relays, connected to JUMO iTRON temperature controllers, which in turn are connected to thermocouples at different locations in the gas lines. The electrical wiring of the gas line temperature control system is illustrated in Figure 13.
Figure 13: Schematic illustration of the gas line temperature control system.
22 3.2 Description of investigated catalysts
3.2.1 Vehicle aged and engine-cell aged catalyst samples
To study the long-term effects on DOC activity of FAME biodiesel contaminants during vehicle operation, samples were taken from a vehicle that had been in operation for an entire lifetime (700 000 km) using FAME biodiesel as fuel. The catalyst was a conventional Pt/Pd catalyst, supported on γ-Al2O3. As illustrated in Figure 14, cylindrical samples were taken from the inlet and the outlet of this vehicle-aged catalyst. The dimensions of the samples are illustrated in the figure. The results for these vehicle-aged samples were compared to a fresh catalyst sample with the same catalyst formulation and to a sample from the inlet of catalyst that had been aged in an engine cell for 1100 h with a total FAME consumption of 71 000 dm3.
Figure 14: Schematic illustration of sampling from the vehicle-aged catalyst.
3.2.2 Protocol for sequential selective removal of poisons from the vehicle-aged sample
In order to assess the effect on catalyst activity of each individual trace element, a top-down approach of sequential selective trace element removal procedures was used [55]. Each procedure was intended to selectively remove one potential poison at a time. After each removal step, the catalyst activity was evaluated, and any difference in activity compared to before the removal step can be attributed to the contaminants removed in that step. A summary of the conditions and purposes of the different removal treatments is given in Table 2.
23
Table 2Overview of the procedures used to selectively remove trace elements from the studied catalyst sample.
Treatment Conditions Purpose
4 h at 400 °C 10 % O2, 7 % H2O + balance N2 at a GHSV of 40 000 h-1
Removal of adsorbed hydrocarbons and H2O Desulfation at
600 °C
10 min reductive 10 min oxidative 10 min reductive 10 min oxidative, all at a GHSV of 20 000 h-1
Reductive: 1 % H2 + 7 % H2O + balance N2
Oxidative: 10 % O2 + 7 % H2O + balance N2
Removal of S
De-ionized water wash
8*15 minutes of deionized water
washing at 70 °C in ultrasound sonicator, followed by 2 h drying at 100 °C
Removal of water-soluble contaminants (primarily Na)
Acid wash 2*1 h treatment in aqueous solution of 5 wt-% acetic acid and 5 wt-% oxalic acid at 70 °C in ultrasound sonicator, followed by thorough washing with deionized water and 2 h drying at 100 °C.
Removal of all remaining contaminants
(including P)
Activity testing atmosphere
10 % O2, 5 % CO2, 5 % H2O, 1000 ppm NO, 200 ppm CO, 200 ppm C3H6 + balance N2 at a GHSV of 80 000 h-1
Testing the activity of the catalyst after each
removal step
3.2.3 Synthesis of model catalysts
Model catalysts were synthesized for the purpose of studying them with various
characterization methods. The model catalysts were synthesized using the incipient wetness method. The γ-Al2O3 support was impregnated drop-wise by an aqueous solution of catalyst precursor. See Figure 15. For Paper III, monometallic Pt/Al2O3 catalysts were made, while bimetallic Pt/Pd/Al2O3 catalysts were synthesized for Paper IV. In both cases, the target loading of total precious metal was 1.2 wt-%. The precursors used were Pt(NO3)4 and Pd(NO3)2.The Al2O3 support was impregnated in two steps. In each step, it was impregnated by the amount of precursor solution needed to completely fill its pore volume. The precursor solution was added one drop at a time. Each of the impregnation steps was followed by drying for 3 hours at 110 °C. After the second drying step, the samples were calcined at 500 °C for 4 hours, in air.
24
Figure 15: The catalyst synthesis procedure. Shown from left to right: Al2O3 powder, drop wise impregnation of catalyst precursor solution, impregnated powder and catalyst precursor solution.
3.2.4 Poisoning of model catalysts by impregnation
The bimetallic model catalysts studied in Paper IV were poisoned using the incipient wetness method, similar to the procedure used for catalyst preparation. The precursor salts used were NaNO3, KNO3, and (NH4)2HPO4. Unlike the catalyst synthesis, all precursor solution was added in a single dropwise impregnation step. This impregnation was followed by drying for 3 hours at 110 °C, and then by calcination at 500 °C for 4 hours, in air. The target loading of poison was 0.32 mmol of Na, K or P per gram of total catalyst. This molar loading
corresponds to a mass loading of 1 % of P in the catalyst washcoat.
25 3.3 Catalyst characterization
3.3.1 Nitrogen adsorption
To determine the pore volume of the Al2O3 support used for synthesis of model catalysts, nitrogen adsorption experiments in a Micromeritics ASAP 2020 instrument were carried out.
The results were analyzed using the Brunauer–Emmett–Teller (BET) [67] and Barrett-Joyner- Halenda (BJH) [68] models.
3.3.2 CO chemisorption
The average catalytic particle size can be estimated using CO chemisorption experiments. In Paper III, this was done using a Micromeritics ASAP 2020C instrument. A double isotherm method [69] was used to distinguish physisorbed CO from chemisorbed CO. Following evacuation, the sample was subjected to reduction by H2 for 2 hours at 400 °C, after which the analysis was performed. After acquiring a first isotherm, corresponding to both
physisorbed CO and chemisorbed CO, the sample was evacuated to remove the physisorbed CO, while more strongly bound chemisorbed CO remained. This was followed by acquisition of a second isotherm, which then corresponded only to physisorbed CO. To determine the amount of chemisorbed CO, the linear portion of both isotherms was extrapolated to zero pressure. The difference between these values for the two isotherms corresponds to
chemisorbed CO. The dispersion was calculated by assuming an adsorption stoichiometry of CO on platinum of 1:1, which corresponds to linearly bonded CO, and comparing the amount of chemisorbed CO to the total number of Pt atoms in the sample. Finally, the percentage dispersion (%D) was used to calculate the average Pt particle size (d) using the following equation:
𝑑 = 108/%𝐷 [70]
3.3.3 Bulk elemental composition analysis
X-ray fluorescence (XRF) was used to quickly probe the elemental composition of catalyst samples. Following emission of photoelectrons after X-ray absorption, a core hole is formed.
This state is unstable, and will be relaxed by replacement of the core holes by electrons from higher energy levels. This in turn, causes emission of X-rays, so called fluorescence. The wavelengths of emitted fluorescence radiation are characteristic of the specific element probed, and therefore analysis of the emitted fluorescence radiation can give quantitative information about the elemental composition of the studied sample [71]. In paper II, XRF analysis was performed using a PANalytical, epsilon3 XL XRF instrument. For each analysis, 3.2 g of catalyst sample, including monolith, was ground into a fine powder using ball milling, then mixed with 0.8 g of C38H76N2O2 binder, and finally pressed into briquettes.
In papers III and IV, inductively coupled plasma optical emission spectroscopy (ICP-OES) [72]
was used to verify the metal loading of synthesized model catalyst samples. With this method, samples, dissolved in water, are atomized into an aerosol, which in turn is led into
26
ultra-hot plasma. Atoms in the sample are excited by the plasma and emit electromagnetic radiation that is characteristic of each element in the sample.
3.3.4 Electron microscopy
The resolution of an optical microscope is limited by the wavelength of light. This can be described by the Rayleigh criterion, which states that the angular resolution is proportional to the ratio of the wavelength of the light source to the aperture diameter of the
instrument. In other words, the shorter the wavelength of the light, the finer the resolution achieved. Because the de Broglie wavelength [73] of an electron is orders of magnitudes shorter than that of visible light, an electron microscope can achieve finer resolutions than an optical microscope. State of the art electron microscopes are able to achieve atomic resolutions.
With scanning transmission electron microscopy (STEM), a beam of electrons is transmitted through a thin sample [74]. The electrons are scattered by the sample, and then detected. By scanning the electron beam in a raster pattern over the sample, an image of the sample can be generated. Differences in atomic number in the material can generate a difference in the contrast in the image, which when studying catalysts can be used to image the catalyst particles dispersed on the support material.
With a scanning electron microscope, a bulk sample can be imaged, giving information about the surface topography of the sample. Interaction of the incident electron beam with the electrical field of nuclei in the sample causes elastic scattering of the electrons, and
scattered electrons can be detected by a backscatter detector. The incident beam can also be inelastically scattered via interactions with electrons in the sample, which causes ionization of the sample and ejection of secondary electrons, which are detected by a secondary electron detector [75].
Beam electrons can cause excitation of electrons in the studied sample. When this excited state is relaxed, it gives rise to X-ray fluorescence. The wavelengths of this fluorescence emission are characteristic of different elements. The fluorescence can be detected with an energy dispersive spectrometer or wavelength dispersive spectrometer and analyzed to give information about the chemical composition of different spots or areas of the sample, or yield elemental mapping when combined with the electron micrographs [75].
In Papers III and IV, scanning transmission electron microscopy (STEM) was performed using a JEOL JEM-ARM200F instrument with a spherical aberration corrector at an acceleration voltage of 200 keV. An Oxford X-maxN 100TLE EDS detector was used for elemental mapping of the samples. The catalyst powder was dispersed in ethanol and deposited on a Cu mesh with lacey carbon film. In paper IV, scanning electron microscopy was performed using a JEOL JSM-7000F instrument at an acceleration voltage of 20 kV.
27 3.3.5. Temperature-Programmed Reduction
Temperature-programmed reduction (TPR) is a catalyst characterization method whereby a catalyst sample is exposed to a stream of reductant (typically H2), while heated [76]. By measuring the consumption of reductant as a function of sample temperature, information about the reducibility of a catalyst sample can be obtained. H2-TPR was used in paper IV to determine the effect of impregnation of potential DOC poisons on the reducibility of model Pt/Pd/Al2O3 catalyst samples.
The TPR analysis was carried out with a Micromeritics TPD/TPR 2900 analyzer. After drying in He for 90 min at 200 °C, catalyst samples were cooled to room temperature in He, and dwelled at room temperature while flushed with 10 % H2 in Ar until a stable baseline was obtained. Finally, analysis was performed by heating the sample in the 10 % H2/Ar mixture at 5 °C/min up to 800 °C, while monitoring the consumption of H2.
3.3.6. In-situ Near-Ambient X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS) is based on ejection of electrons following
absorption of X-ray radiation [77]. The electrons themselves are detected and their kinetic energy determined. The kinetic energies of photoelectrons can be related to the binding energies of the electrons, which are characteristic of the energy levels of different elements’
electronic shells. Furthermore, binding energies of core electrons can be shifted by a few eV, because of electron donation to or from neighboring atoms. Analysis of such chemical shifts in binding energies can provide chemical information about the sample, including
information about oxidation state. XPS is highly surface sensitive, because only electrons from the outermost surface can penetrate through the studied material to reach the detector.
Because photoelectrons are inelastically scattered by gas-phase molecules, XPS traditionally has to be performed in an ultra-high vacuum environment. However, recent advances in differential pumping systems allow for XPS analysis of samples in closed cells with gaseous atmospheres in the mbar range [78]. In paper III, we used such near-ambient pressure XPS systems to study the oxidation state of Pt in a Pt/Al2O3 model catalyst under atmospheres meant to mimic NO oxidation and CO oxidation atmospheres. Figure 16 shows a schematic representation of an AP-XPS system. Figure 17 shows photographs of the NAP-XPS system used for the bulk of the measurements performed.
28
Figure 16: Schematic view of an NAP-XPS system.
Figure 17: Photographs of an NAP-XPS system.
During the XPS experiments, the gas composition in the reaction cell was controlled using leak valves to control the flow of gas through the cell from two independent gas lines.
Before the experiments, the sample was evacuated at 700 K in ultra-high vacuum, followed by reduction in flowing hydrogen at 500 K. Al 2p and Pt 4d spectra were collected in vacuum and in O2, NO, CO, NO +O2 and CO + O2 atmosphere, with a partial pressure of each
component of 1 Torr. The temperature in the gas cell during spectra acquisition was 400 or 500 K. Sample charging was corrected for by referencing the spectra to the literature value of Al 2p. Differential charging was not observed.
29 3.3.7. In-situ X-ray Absorption Spectroscopy
Similar to XPS, in X-ray absorption spectroscopy (XAS), a sample is irradiated with X-rays.
Absorption of X-rays by an atom causes ejection or excitation of core electrons. The excited electrons are scattered by neighboring atoms, and analysis of the absorption peak may therefore provide chemical information about the studied sample. Specifically, the height of the so called absorption edge “white line” is often related to the oxidation state of the probed element. X-ray absorption spectroscopy requires tunable, monochromatic and highly focused X-ray radiation, and is therefore performed using synchrotron radiation. In Paper IV, X-ray absorption experiments were performed on samples subjected in-situ to a CO
oxidation atmosphere. The data was collected in the transmission mode, where the intensity of inelastically scattered X-rays is normalized to the incident radiation.
The experiments were carried out using the XAFCA beamline of the Singapore Synchrotron Light Source. A two-crystal monochromator with Si(111) crystals was used. At the sample position, the size of the beam was ∼ 2 mm X 1mm. FMB-OXFORD IC Spec ionization chambers were used as detectors for the incident and transmitted X-rays. During
experiments, the Pt L3 edge was monitored in XANES measurements, from 11.5 to 11.6 keV.
A spectrum for Pt foil was taken at the beginning of the data collection run and used to calibrate the energy scale. The data were analyzed and normalized using Athena XAS data processing software [79], and presented as Xμ(E) vs E plots, calculated by the following formula, from the intensities of the incident and transmitted beam, I0 and I1, respectively.
ln𝐼𝑂 𝐼1
XANES spectra were collected for the Pt/Pd/Al2O3 catalyst and the poisoned samples as prepared, after reduction, and after being subjected, in-situ, to a temperature ramp in a CO oxidation reaction atmosphere. The conditions inside the in-situ cell during sample
acquisition are summarized in Table 3. The spectra presented are merged spectra combining several scans collected at the conditions indicated in the table.
Table 3: Temperatures and gas composition in the in-situ cell during XANES spectra collection.
Gas composition Temperature X-ray absorption spectra collected
ambient Room temperature
(RT)
Spectra recorded at room temperature
H2 RT–600 K,
7.5 K/min
at 600 K in He after the reductive treatment
1000 ppm CO, 10 % O2, 5 % H2O, balance N2
RT–500 K, 2 K/min at 500 K in a continuous flow of reactant mixture
30
31
4. Investigation of a sample from a vehicle used for an entire regulatory lifetime (Paper II)
The trace element content, measured by XRF, of the samples taken from the vehicle-aged and cell-aged catalysts studied in Paper II is shown in Figure 18. The samples from the vehicle-aged catalyst contained high loadings of S, P, Na and Ca, but not much K. For Na and Ca, there was a strong axial gradient, with much more at the front section than at the rear section. P and S were, axially, more evenly spread. While the axial distribution of S, Na and Ca is similar to that observed in many other studies, the relatively even distribution of P differs from observations in many earlier studies [47, 49, 50, 52-54] of vehicle-aged catalysts.
These earlier studies had instead observed a rather strong axial gradient in P content after vehicle aging, with more P present at the front. However, the catalysts investigated in those studies had not been in use for a full lifetime. It is possible that during long-time operation of the vehicle, the front of the DOC becomes saturated with P, in which case more P would start accumulating further toward the rear of the catalyst. In the cell-aged sample, only S content was of similar magnitude to that at the front of the vehicle-aged sample. The content of P, Na and Ca was much lower in the cell-aged sample.
Figure 18: Trace element content of samples taken from the investigated vehicle-aged and cell-aged DOCs.
A comparison of the activity of the investigated samples in the CO, C3H6 and NO oxidation reactions is shown in Figure 19. The activity does not appear to differ much between the front and the rear of the vehicle-aged catalyst. They were both severely deactivated for the NO oxidation reaction, moderately deactivated for the C3H6 oxidation reaction and slightly deactivated for the CO oxidation reaction. The cell-aged sample is less severely deactivated overall, with CO and C3H6 oxidation activities close to the fresh sample and considerably
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
S Na K Ca P
%
Vehicle-aged DOC, Front
Vehicle-aged DOC, Rear
Cell-aged DOC, Front
32
lowered activity in the NO oxidation reaction, but less severely so than for the samples taken from the vehicle-aged catalyst. To determine whether the lower activity of the vehicle-aged catalyst was connected to its higher loadings of P and Na, the sample from the front of this catalyst was investigated further with the objective of determining the effect of each individual trace element on catalyst activity.
Figure 19: Catalytic activity of the samples taken from the vehicle-aged and cell-aged catalyst, compared to a fresh catalyst of the same type, in the CO, C3H6 and NO oxidation reactions.
For the sample taken from the front of the vehicle-aged catalyst, a protocol of sequential poison removal was used to assess the effect of the different potential poisons present on the catalyst. The details of the procedures is described in Section 3.2.2 To investigate
whether the removal procedures had the intended effect, elemental analysis was performed on a series of samples that had undergone the described there. As illustrated schematically in Figure 20, XRF analyses were performed for a sample taken directly from the catalyst, as received, for a sample submitted only to oxidative treatment at 400 °C, for a sample submitted to oxidative treatment at 400 °C, then desulfation at 600 °C, and so on, with the final sample for XRF analysis having undergone all of the sequential removal steps.
33
Figure 20: Schematic illustration of the approach used for validation of trace element removal procedures by XRF analyses.
Overall, the contaminant removal procedures did give the intended results. XRF results after the different removal procedures are shown in Figure 21. P was not removed until the acid treatment step, and Na plus Ca were not removed until after the water treatment step.
While the S removal step was not able to remove all the sulfur present, it was selective in only removing sulfur. The oxidative soot removal step did not remove any of the trace elements probed by XRF. However, visual inspection showed that it was successful in
removing a visible surface layer of soot that was present on the catalyst samples as received from the vehicle. This is shown in the photograph in Figure 22.
