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Article

Example on the Use of Operando Spectroscopy for Developing Mechanistic Insights into Industrial Catalysts and Catalytic Processes

by
Claire Chunjuan Zhang
,
Shiang Sung
,
Sage Hartlaub
,
Ivan Petrovic
and
Bilge Yilmaz
*
BASF Corporation, 25 Middlesex-Essex Tpk, Iselin, NJ 08830, USA
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(2), 200; https://doi.org/10.3390/catal11020200
Submission received: 21 January 2021 / Revised: 26 January 2021 / Accepted: 29 January 2021 / Published: 3 February 2021
(This article belongs to the Special Issue 2020s Vision on Catalysts: Operando Characterization Methods)
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Abstract

:
In this contribution, we present an industrial example on how tailored operando spectroscopic methodologies provide the insights needed for the development of new catalytic technologies and support their global utilization. We describe the use of operando spectroscopic methods to investigate how the CO oxidation performance of catalysts is impacted by NOx, H2, temperature, and moisture, as well as the catalyst support. This operando spectroscopic analysis provides mechanistic insights into the current diesel oxidation catalyst (DOC) system and shines light on the material and process development efforts on future DOC catalysts for low-temperature emission control to meet the new regulations. This investigation has shown that at cold-start temperatures, the nitrate growth can occupy the precious metal–alumina support interfacial sites that are critical for O2 dissociation and/or oxygen transfer and hinder CO conversion. Introduction of hydrogen on the catalyst surface can inhibit the nitrate growth, which in turn keeps these critical interfacial sites open.

1. Introduction

The development of novel catalysts is necessary to support the sustainable progress of our industrialized society. Characterization efforts provide critical information on catalytic technologies and help develop a mechanistic understanding to support further advancements in their industrial application. Operando/in-situ spectroscopy is a powerful material characterization tool in the field of catalysis, which conducts spectroscopic characterization under catalytic working conditions. Consequently, the material structural information can be correlated with catalytic activity and selectivity. Ultimately, fundamental reaction mechanisms can be revealed based on operando spectroscopy analysis. The material characterization is usually carried out by one of the following spectroscopy methodologies: Fourier-transformed infrared (FT-IR) [1], ultraviolet–visible (UV-Vis) [2], Raman [3], nuclear magnetic resonance (NMR) [4], and X-ray spectroscopy methods involving diffraction, adsorption, and scattering [5,6,7]. The catalytic activities are measured by Mass Spectrometers or FT-IR Gas Analyzers. By utilizing these methods and others, operando spectroscopy plays a significant role in the electro-catalysis, heterogeneous catalysis, photocatalysis, and battery fields [8,9,10,11,12].
The structural identification of reactants, intermediates, and products is a critical aspect of the development of new and improved industrial catalytic processes. It is even more insightful when it includes the study of surface adsorbates at catalytic working conditions. The variation of surface adsorbates as conditions change (e.g., temperature and gas feed composition) can be correlated to the changes in material properties and catalytic performance under these reaction conditions. Another piece of structural identification is about the solid structures that constitute the catalyst architecture, as it dictates key performance attributes such as accessibility, stability, and regenerability. Information from both aspects can be identified by operando/in-situ spectroscopy, which shines light on the catalytic material development and identification, as well as the mechanistic understanding. In this contribution, we will present an example of using operando diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) for the structural characterization of catalysts and the development of mechanisms for catalytic applications in the field of environmental catalysis.
The diesel oxidation catalyst (DOC) is one of the key parts of the catalytic converter used in diesel engine after-treatment systems. The DOC works under lean conditions with an excess of air-to-fuel ratio compared with the stoichiometric mixture in gasoline engines. The major function of the DOC is the oxidation of CO and hydrocarbons (HC) in the diesel engine exhaust. The NO in the exhaust can also be oxidized to NO2 when passing through the DOC. The typical DOC materials include alumina-supported platinum group metals (PGMs) with hydrocarbon traps [13]. A common issue with supported PGM catalysts at low temperatures is the CO poisoning—the strongly bonded CO on the PGM surface limits the O2 access for oxidation, which can be usually overcome by raising the temperature [14]. In order to improve the DOC activity at low temperatures, researchers introduced hydrogen pulses on DOC catalysts during the cold start of diesel engines, which significantly increased the CO conversion at low temperatures [15,16,17,18,19,20,21].
Possible mechanisms for the improved CO conversion by hydrogen pulsing include, but are not limited to: (1) the competitive adsorption between CO molecules and hydrogen on PGM surfaces; (2) more active sites being generated by H2 reduction on PGM; (3) a more active O atom generated via NOx reduction by H2; and (4) release of active sites taken by nitrates due to nitrate decomposition by H2 reduction. In this study, a mechanistic investigation of hydrogen pulsing’s impact on DOC performance by operando spectroscopy was conducted on an alumina-supported Pt catalyst. Spectroscopic evidence on working catalysts provide insights into improved CO oxidation activities due to the release of PGM–support interfacial sites under H2 pulsing at the cold start of diesel engines.

2. Results and Discussion

In order to decouple various contributing factors, a simple system of CO oxidation without NOx in the gas feed was first examined for the hydrogen impact. In this set of experiments, interesting questions about molecular interaction processes to be considered include: (1) whether the hydride on the PGM surface can be formed and how CO adsorption will be impacted by H2 (competitive adsorption between hydride and CO); (2) how does the PGM surface change under H2 impact; (3) whether there is an impact on CO conversion from H2; and (4) what is the rate-limiting step, as well as other points that could support a deeper mechanistic understanding. To investigate the H2 and CO interaction on the PGM surface, CO DRIFTS experiments on a dehydrated and reduced fresh Pt/Alumina sample were conducted. Figure 1 shows the DRIFTS data for CO desorption (pre-adsorbed CO by flowing 70 mL/min of 1% CO/Ar for 30 min) at three temperatures in two environments—Argon vs 2.4% H2/Ar. In Figure 1, the solid lines represent the desorption in Argon, while the dashed lines represent the desorption in 2.4% H2/Ar. The peak at 2060–2070 cm−1 and the shoulder at 2082–2086 cm−1 can be assigned to the CO linearly adsorbed on the metallic Pt and Pd sites, respectively [22,23]. The wide and weak features at 1950–1990 cm−1 are due to the CO being bridge-adsorbed on two PGM atoms [24]. A broad peak at ~1820 cm−1 can be assigned to the CO adsorbed on three PGM atoms, which is an indication of a large PGM particle formation [25]. Figure 1 clearly shows that the pre-adsorbed CO on the linear adsorption sites can be completely desorbed in Argon within 20 min, while only a slight decrease was observed for the desorption in H2. The stronger CO adsorption on the reduced sample is likely due to the more available metallic Pt0 and Pd0 surface upon reduction. The peak intensity on the double- and triple-bridged CO adsorption sites was barely changed within 20 min for both desorption environments. Therefore, the introduction of H2 does not remove CO from the PGM surface, suggesting that the CO adsorption on the PGM surface is stronger than hydride bonding (if any). As a first step in this mechanistic investigation, this CO DRIFTS experiment confirms that there is no/negligible competitive adsorption between CO and H2 on the PGM surface.
To address questions 2–4 of this mechanistic investigation, CO oxidation reactions were conducted to examine the hydrogen impact. Figure 2 shows the CO adsorbed on the PGM surface during the oxidation reactions at different temperatures on an aged Pt/Alumina sample. Under the presence of hydrogen, the PGM surface is reduced with a significantly increased peak at 2067 cm−1 for CO-Pt0 at 90 and 120 °C. Correspondingly, about a 10–15% higher CO conversion was achieved at lower temperatures in the activity results shown in Figure 3a. This result suggests that at low temperatures, there is a PGM surface reduction upon hydrogen exposure, which slightly increases the CO oxidation efficiency. Additionally, analysis of the water content detected in the Mass Spectrometer in Figure 3b indicates that at 90 and 120 °C the reaction 2H2 + O2 = 2H2O is not activated. At 180 °C, the aged Pt/Alumina catalyst is so active that it reaches a 100% conversion with or without H2 present. In the meantime, the water content is significantly increased, suggesting that both reactions of 2CO + O2 = 2CO2 and 2H2 + O2 = 2H2O are activated. Generally, the O2 dissociation is believed to have a higher activation energy than the combination of CO or hydrogen with an activated O* atom on a catalytic surface. During the operando spectroscopy experiments, usually a small amount of CO2 (10–20% as shown in Figure 3) can be observed upon the CO’s introduction over the catalyst at low temperatures, which is due to the combination of CO with an activated O* atom on the catalytic surface. This process is a surface cleaning (reduction) process of catalysts by fuel (CO), similar to the hydrogen reduction, but to a lesser degree. If the temperature is held in this low range (<150 °C) for a stationary test, the decreasing of CO conversion over time would be expected since the active surface O* atom is depleted without a comparable rate of recovery. However, at higher temperatures, higher CO conversion is achieved upon the activated O2 dissociation. More direct evidence of a higher activation energy for the O2 dissociation was achieved in a similar study for a CO oxidation reaction under H2 (2000 ppm CO, 7200 ppm H2, and 10% O2). An Arrhenius-type plot of CO2 and water is shown in Figure 4 during a continuous temperature ramping process. A process with a higher activation energy is initiated at above 158 °C for both CO oxidation and H2 oxidation reactions, which confirms the O2 dissociation process with a higher activation energy.
As a summary for the CO oxidation without the interference of NOx, it can be concluded that the presence of hydrogen reduces the PGM surface, which in turn increases the CO conversion by 10–15% at low temperatures (90–120 °C). There is little competitive adsorption between CO and H2 on the PGM surface, i.e., introduction of hydrogen does not remove adsorbed CO from PGM surfaces. Additionally, it was demonstrated that the O2 dissociation has a higher activation energy than the combination of CO with active O*, which can be activated at temperatures above 158 °C.
The second system that was explored was NO oxidation under the hydrogen impact at low temperatures. In general, NO under lean conditions passing through a DOC would be oxidized to NO2, which can be either released into the gas phase for the downstream selective catalytic reduction (SCR) or stored on the catalyst support as a nitrate species [15,26]. NOx reduction is usually believed to be unfavorable because any reductant present would react first with the high concentration of O2 in the diesel exhaust gas. This statement holds true at temperatures higher than the activation temperature for O2 dissociation (i.e., 158 °C measured in our operando spectroscopy experiments as shown in Figure 4). In the following operando spectroscopy study, when the temperature is below the activated temperature for O2 dissociation, the exception for NO reduction can exist under lean conditions. When a small amount of hydrogen is present, NOx can undergo reduction reactions when the temperature is below 150 °C even in lean conditions with an excess of O2. Figure 5 shows the IR spectra in the range of 1200–1900 cm−1 with and without hydrogen for NO oxidation at 90–180 °C. A significantly larger amount of nitrate species (in the range of 1500–1700 cm−1) are observed when H2 is not present (Figure 5a vs. Figure 5c) since NO undergoes oxidation under this condition. The nitrate species are stable with similar IR peak intensities in the temperature range of 90–180 °C. However, when H2 is present, gas phase N2O as a NOx reduction product is observed as shown in Figure 6 with higher intensities when T is below 150 °C. In the meantime, a small amount of nitrate species can be found from Figure 5c. These processes can be illustrated with the schematic drawings in Figure 5b,d for without and with H2 impact, respectively. As a summary to NO oxidation under the hydrogen impact at low temperatures, the presence of hydrogen can inhibit the nitrate growth and promote NOx reduction before O2 dissociation is activated.
The third set of experiments was designed to investigate the hydrogen impact on CO oxidation under the interference of NO and/or NOx at 90–180 °C [27]. In this section of the study, the following questions were addressed via operando spectroscopy: (1) NO/NOx impact on CO conversion; (2) interaction between NO/NOx and CO; and (3) hydrogen impact on CO oxidation with NO/NOx present, which can be answered from the results shown in Figure 7, Figure 8, Figure 9 and Figure 10 during the oxidation reactions of the CO and NO/NOx mixture under hydrogen impact at 90–180 °C. Firstly, when H2 is not present, the interaction between CO and NO is negligible with little N2O (blue solid line in Figure 7c). Some N2O is observed in the CO-O2-NOx experiment (blue solid line in Figure 7d) at 90–180 °C, which may come from the NO2 self-decomposition and/or the CO and NO2 interaction. The corresponding IR spectra in the nitrate region (Figure 8) suggest, in both cases (CO-O2-NO and CO-O2-NOx), that a large amount of nitrate species grow on the catalyst, which indicates that the NO oxidation is dominating when H2 is not present at 90–180 °C. With similar formation mechanisms as for Lean NOx Trap materials, the nitrate, formed by NO oxidation at the PGM surface at 90–180 °C, is transferred to the support (illustrated in Figure 5b), which occupies the PGM–support interfacial sites (Figure 10). As a result, CO conversion (black solid lines in Figure 7a,b) is compromised at 180 °C after O2 dissociation is activated.
However, when H2 is present, almost no nitrate is built up in the case of CO-O2-NO-H2 reactions at 90–180 °C (Figure 8), and a much smaller amount of nitrate in the case of CO-O2-NOx-H2 reactions at 180 °C (Figure 8). The decrease of nitrate build-up brings the CO conversion back to 100% and 70% at 180 °C for CO-O2-NO-H2 and CO-O2-NOx-H2 reactions, respectively. In the meantime, more N2O is observed at 180 °C in the MKS results shown as light blue dashed lines in Figure 7.
The relationship between the CO conversion and the nitrate growth at 180 °C was systematically investigated in Figure 9 with a total of eight experiments, as labeled in the x-axis. A reverse correlation between the CO conversion and the nitrate growth is clearly established. This result suggests that the interfacial sites between the PGM and the support materials are one of the major sites for O2 dissociation and/or oxygen transfer. Once these sites are occupied by nitrate (stable at 90–180 °C), the CO conversion can be compromised by more than 50% (black solid lines in Figure 7 at 180 °C). The introduction of hydrogen inhibits the nitrate growth (Figure 10), which keeps the interfacial sites open for accessibility to active O* coming from O2 dissociation and/or oxygen transferred from the support.
Through different combinations of reactions, a clear set of operando spectroscopic data and insights were obtained to fundamentally understand the hydrogen impact on the CO oxidation with the influence of NOx over Pt/Alumina DOC catalysts. The results suggest that at the engine cold start temperature (90–180 °C), the nitrate growth can take away the PGM–support interfacial sites for O2 dissociation and/or oxygen transfer, which compromises CO conversion. The introduction of hydrogen inhibits the nitrate growth, which keeps the interfacial sites open.

3. Materials and Methods

Sample information: The supported Pt/Alumina catalyst was prepared by an incipient wetness impregnation method using tetraamine platinum solution as a precursor and then calcined before a slurry preparation. The catalyst slurry was prepared by dispersing solid catalyst powders with binders in water phase with media milling to reach the desired particle sizes. The ball milling process was done by roller-type or small-media mills. Coated catalysts on honeycomb cores were prepared by the dip coating method, and then calcined at 550 °C for 3 h. The aged catalyst was treated at 800 °C for 16 h in 10% H2O–10% O2 balanced in N2. Aged ~1% Pt/alumina catalysts were scraped off from the coated cores.
Infrared Spectroscopy: DRIFTS experiments were performed on a Varian 7000 FT-IR spectrometer (Santa Clara, CA, USA) equipped with an MCT detector and a Pike Technology high-temperature environmental chamber with a KBr window. Spectrum collection was performed under the diffuse reflection mode. The samples were ground into fine powders with a mortar and pestle, and then packed into the sample cup. The sample was dehydrated in Ar at 200 or 400 °C for 1 h with a ramp rate of 20 °C /min. IR spectra were collected after dehydration, once the chamber had been cooled to 30 °C.
Operando DRIFTS experiments were carried out on an Agilent CARY680 FT-IR spectrometer equipped with a mercury cadmium telluride (MCT-HgCdTe) detector (Santa Clara, CA, USA) and a Linkam high-temperature environmental chamber with a calcium fluoride (CaF2) window. The sample powder was dehydrated in flowing Ar or reduced in flowing 2.4% H2/Ar at 400 °C for 1 h at a flow rate of 70 mL/min. The DRIFTS were collected during the operando reactions carried out at various temperatures. The absorbance spectra from the DRIFTS were ratioed to the background spectrum (the initial spectrum in Ar before reaction) and then used for analysis. In order to compare the DRIFTS data between the samples with different treatments (dehydrated vs. reduced), a common background spectrum in Argon on a KBr sample was used. The steady-state reactions for operando spectroscopy analysis were carried out under simplified gas feed compositions with a similar gas feed content of CO (1000–2000 ppm), O2 (5–15%), NOx (100–500 ppm), and/or H2 (2000–3000 ppm) as for reactor testing conditions. The HC components are not considered in the operando reactions for simplicity. Additionally, due to the high sensitivity to moisture of the FT-IR spectrometer, the steam component was also removed in the operando reactions. A Hiden Analytical mass spectrometer (MS) (Livonia, MI, USA) and an FT-IR gas cell analyzer— MKS MultiGas (Andover, MA, USA) were used to monitor the gas phase components in the exhaust of the operando reactor.

4. Conclusions

A systematic operando DRIFTS Spectroscopy study was conducted to generate insights and develop a mechanistic understanding of the role of hydrogen on the CO oxidation over Pt/Alumina DOC catalysts as well as the influence of NOx on this catalytic system that is of critical importance for the automotive industry. Based on the observed results, it can be concluded that the nitrate growth can occupy the PGM–support interfacial sites that are critical for O2 dissociation and/or oxygen transfer at cold start temperatures for the engine (90–180 °C) and hinder CO conversion. The introduction of hydrogen on the catalyst surface inhibits the nitrate growth, which in turn keeps these critical interfacial sites open. The operando spectroscopic analysis and the identified mechanistic insights can guide the material and process developments for advancements in DOC catalyst design and application to meet the new regulations for low-temperature emission control.

5. Patents

A patent application was filed resulting from the work reported in this manuscript [27].

Author Contributions

Conceptualization, C.C.Z., B.Y.; Data curation, C.C.Z., S.H.; Investigation, C.C.Z. and S.S.; Methodology, C.C.Z. and B.Y.; Project administration, S.S., B.Y.; Resources, I.P., B.Y.; Writing—original draft, C.C.Z. and B.Y.; Writing–review & editing, S.S., I.P. and B.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. IR results of a diesel oxidation catalyst (DOC) catalyst sample for CO desorption in Ar compared with 2.4% H2/Ar at 90, 120, and 180 °C (pre-adsorbed with CO (1%) balanced in Ar for 30 min before each desorption).
Figure 1. IR results of a diesel oxidation catalyst (DOC) catalyst sample for CO desorption in Ar compared with 2.4% H2/Ar at 90, 120, and 180 °C (pre-adsorbed with CO (1%) balanced in Ar for 30 min before each desorption).
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Figure 2. CO diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) collected during CO oxidation reactions (CO 1000 ppm, O2 10%) (a) without and (b) with H2 present (H2 2400 ppm) at 90, 120, and 180 °C.
Figure 2. CO diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) collected during CO oxidation reactions (CO 1000 ppm, O2 10%) (a) without and (b) with H2 present (H2 2400 ppm) at 90, 120, and 180 °C.
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Figure 3. The activity results for (a) CO conversion and (b) water content collected during CO oxidation reactions (CO 1000 ppm, O2 10%) without (solid line) and with (dashed line) H2 present (H2 2400 ppm) at 90, 120, and 180 °C.
Figure 3. The activity results for (a) CO conversion and (b) water content collected during CO oxidation reactions (CO 1000 ppm, O2 10%) without (solid line) and with (dashed line) H2 present (H2 2400 ppm) at 90, 120, and 180 °C.
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Figure 4. MS water and CO2 signals as a function of 1/T for a DOC catalyst sample under a mixture of CO (2000 ppm), H2 (7200 ppm), and O2 (10%) balanced in Ar during cooling from 300 °C to 30 °C.
Figure 4. MS water and CO2 signals as a function of 1/T for a DOC catalyst sample under a mixture of CO (2000 ppm), H2 (7200 ppm), and O2 (10%) balanced in Ar during cooling from 300 °C to 30 °C.
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Figure 5. Nitrate spectra in Fourier-transformed infrared (FT-IR) collected during NO oxidation (NO 280 ppm, O2 10%) reactions without (a) and with (c) H2 present (H2 2300 ppm) at 90, 120, and 180 °C, with their schematic mechanisms shown in (b,d), respectively.
Figure 5. Nitrate spectra in Fourier-transformed infrared (FT-IR) collected during NO oxidation (NO 280 ppm, O2 10%) reactions without (a) and with (c) H2 present (H2 2300 ppm) at 90, 120, and 180 °C, with their schematic mechanisms shown in (b,d), respectively.
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Figure 6. MKS and MS results for (a) NO conversion and (b) N2O content collected during NO oxidation reactions (NO 280 ppm, O2 10%) without (solid line) and with (dashed line) H2 present (H2 2300 ppm) at 90, 120, and 180 °C.
Figure 6. MKS and MS results for (a) NO conversion and (b) N2O content collected during NO oxidation reactions (NO 280 ppm, O2 10%) without (solid line) and with (dashed line) H2 present (H2 2300 ppm) at 90, 120, and 180 °C.
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Figure 7. MKS results for (a,b) CO conversion and (c,d) N2O content collected during CO oxidation reactions (CO 1000 ppm, O2 10%) under the impact of NO (a,c) (NO 280 ppm) and NOx (b,d) (NO 280 ppm, NO2 190 ppm), without (solid line) and with (dashed line) H2 present (H2 2300 ppm) at 90, 120, and 180 °C.
Figure 7. MKS results for (a,b) CO conversion and (c,d) N2O content collected during CO oxidation reactions (CO 1000 ppm, O2 10%) under the impact of NO (a,c) (NO 280 ppm) and NOx (b,d) (NO 280 ppm, NO2 190 ppm), without (solid line) and with (dashed line) H2 present (H2 2300 ppm) at 90, 120, and 180 °C.
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Figure 8. Nitrate spectra in the FT-IR collected during CO oxidation reactions under the impact of NO and NOx, without and with H2 present at 90, 120, and 180 °C.
Figure 8. Nitrate spectra in the FT-IR collected during CO oxidation reactions under the impact of NO and NOx, without and with H2 present at 90, 120, and 180 °C.
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Figure 9. CO conversion and nitrate peak area in the FT-IR collected in different reactions at 180 °C, which indicates that the CO conversion is reversely impacted by the nitrate growth.
Figure 9. CO conversion and nitrate peak area in the FT-IR collected in different reactions at 180 °C, which indicates that the CO conversion is reversely impacted by the nitrate growth.
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Figure 10. Schematic mechanisms show the CO oxidation without (a) and with (b) the impact of nitrate. When nitrate is populated at the platinum group metal (PGM)–support interface, the oxygen dissociation sites are occupied by nitrate, which causes the compromised CO conversion.
Figure 10. Schematic mechanisms show the CO oxidation without (a) and with (b) the impact of nitrate. When nitrate is populated at the platinum group metal (PGM)–support interface, the oxygen dissociation sites are occupied by nitrate, which causes the compromised CO conversion.
Catalysts 11 00200 g010
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Zhang, C.C.; Sung, S.; Hartlaub, S.; Petrovic, I.; Yilmaz, B. Example on the Use of Operando Spectroscopy for Developing Mechanistic Insights into Industrial Catalysts and Catalytic Processes. Catalysts 2021, 11, 200. https://doi.org/10.3390/catal11020200

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Zhang CC, Sung S, Hartlaub S, Petrovic I, Yilmaz B. Example on the Use of Operando Spectroscopy for Developing Mechanistic Insights into Industrial Catalysts and Catalytic Processes. Catalysts. 2021; 11(2):200. https://doi.org/10.3390/catal11020200

Chicago/Turabian Style

Zhang, Claire Chunjuan, Shiang Sung, Sage Hartlaub, Ivan Petrovic, and Bilge Yilmaz. 2021. "Example on the Use of Operando Spectroscopy for Developing Mechanistic Insights into Industrial Catalysts and Catalytic Processes" Catalysts 11, no. 2: 200. https://doi.org/10.3390/catal11020200

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