Next Article in Journal / Special Issue
Preparation and Performance of Modified Red Mud-Based Catalysts for Selective Catalytic Reduction of NOx with NH3
Previous Article in Journal
Catalytic Degradation of Ortho-Chlorophenol Using Activated Carbon Modified by Different Methods
Previous Article in Special Issue
Sulfur and Water Resistance of Mn-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 8
Issue 1
10.3390/catal8010038
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium

by
Linda Ström
*,
Per-Anders Carlsson
,
Magnus Skoglundh
and
Hanna Härelind
Competence Centre for Catalysis, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(1), 38; https://doi.org/10.3390/catal8010038
Submission received: 1 January 2018 / Revised: 15 January 2018 / Accepted: 17 January 2018 / Published: 19 January 2018
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
Browse Figures
Versions Notes

Abstract

:
Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NOx with ammonia. Particularly, we focus on the active phase of the catalyst and the formation of surface species, as a function of the gas environment. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy was used to follow the oxidation state of the silver and indium phases, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to elucidate the formation of surface species during SCR conditions. In addition, the NOx reduction efficiency of the materials was evaluated using H2-assisted NH3-SCR. The DRIFTS results show that the Ag/Al2O3 sample forms NO-containing surface species during SCR conditions to a higher extent compared to the In/Al2O3 sample. The silver sample also appears to be more reduced by H2 than the indium sample, as revealed by UV-vis spectroscopic experiments. Addition of H2, however, may promote the formation of highly dispersed In2O3 clusters, which previously have been suggested to be important for the SCR reaction. The affinity to adsorb NH3 is confirmed by both temperature programmed desorption (NH3-TPD) and in situ DRIFTS to be higher for the In/Al2O3 sample compared to Ag/Al2O3. The strong adsorption of NH3 may inhibit (self-poison) the NH3 activation, thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity compared to Ag/Al2O3.

Graphical Abstract

1. Introduction

The development of fuel-efficient engines, operating under lean conditions, is motivated by fluctuating oil prices, more stringent emission legislations, and climate changes. Among the most attractive exhaust aftertreatment techniques for lean NOx reduction is the selective catalytic reduction with either ammonia (NH3-SCR) or hydrocarbons (HC-SCR). For example, Cu-based zeolites have recently been shown to exhibit high NOx removal activity in a wide temperature range [1,2,3]. In contrast to NH3-SCR, the HC-SCR catalysts need to be further improved as to be competitive. This puts pressure on building new understanding of the materials and mechanisms for HC-SCR. It seems, though, that NH3 is a key intermediate for both techniques [4,5]. Due to its excellent thermal and mechanical stability, alumina is the most widely used catalyst support material [6]. The silver-alumina system has been studied for HC-SCR applications and with the pioneering work by Satokawa [7] in 2000, it was shown that the catalytic activity for NOx reduction can be further improved by the addition of small amounts of hydrogen. This widely studied phenomenon [8,9,10,11] is denoted the ‘hydrogen effect’ and has previously been regarded as limited to silver-based catalysts only [12]. However, recently In/Al2O3, which also has been studied for SCR applications [13,14,15,16,17,18,19], was found to exhibit a hydrogen effect, albeit to a lower extent compared to Ag/Al2O3 [20]. The hydrogen effect over Ag/Al2O3 has been suggested to originate from the reduction of adsorbed nitrogen species [10,21,22], changes in the type of Ag species [23,24,25], and/or enhanced activation of the reductant [9,11]. However, in contrast to other precious metal catalysts, alumina-supported silver is not active for H2-SCR [26]. From a practical point of view, H2 can be provided to the vehicle’s exhaust after treatment by on-board reforming of for example solid amine salts or fuel [19,27].
Several studies have focused on the role of silver phases in the Ag/Al2O3 catalyst [28,29,30,31,32,33], suggesting the active phase for the selective reduction of NOx to be Agnδ+-clusters [32,33] and Ag+ ions [28,29], or a combination of these. Except for these species, metallic silver (Ag0) is recognized as responsible for complete combustion of the reductant [34]. In the In/Al2O3 catalyst, highly dispersed indium cluster sites (In3+) have been identified as the active component for hydrocarbon activation during HC-SCR [17].
A deeper understanding of the underlying mechanisms of the hydrogen effect is useful, not only to improve the SCR catalysts, but also for facilitating development of systems that reduce NOx efficiently without the addition of hydrogen. In this study, we compare Ag/Al2O3 and In/Al2O3 catalysts, for the SCR of NOx with ammonia as model reductant. In particular, we are focusing on how the catalytically active silver and indium phases are affected by the SCR environment, and how the formed surface species interact with these phases, using diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and ammonia-temperature programmed desorption (NH3-TPD).

2. Results

In this study, we compared H2-assisted NH3-SCR of NOx over Ag/Al2O3 and In/Al2O3. In Section 2.1, we show the NOx conversion and formed species during the SCR experiments. In addition, we compare the density of acidic sites by NH3-TPD (Section 2.2). We have also focused on the possible changes in the active phase and surface species as a function of gas phase components. These results are presented in Section 2.3 and Section 2.4

2.1. Catalytic Activity

The Ag/Al2O3 and In/Al2O3 samples were evaluated as NH3-SCR catalysts using a flow reactor with a feed composition of 500 ppm NO, 500 ppm NH3, 10% O2, 5% H2O, and Ar as carrier gas. The experiments were subsequently repeated with the addition of 1000 ppm H2.
The NOx reduction and NH3 conversion, as well as the formation of NO2 and N2O over Ag/Al2O3, are shown in Figure 1. Without the addition of H2, the catalyst is inactive for NOx reduction. However, H2-assisted NH3-SCR reduces NOx in a broad temperature window and with a maximum reduction just above 80%. The selectivity towards N2 is high, in accordance with previous reports [35,36], 80% when H2 is present in the feed. It can also be seen that the low N2O formation decreases even further when H2 is present, confirming the results of Kondratenko et al., that H2 suppresses the total N2O production [25]. This is assigned to H2-induced Ag0 formation, which likely is responsible for the N2O decomposition [37]. The negative values of NOx reduction above 350 °C observed when H2 is absent (black line in Figure 1a) is likely due to the oxidation of NH3 to NO, which also explains the significant amount of NH3 converted in Figure 1b [35]. Shimizu and Satsuma [32] suggest that the addition of H2 to NH3 + O2 enhances the oxidative activation of NH3 by decreasing the activation energy of the rate-determining step, (i.e., formation of NHx).
The In/Al2O3 catalyst is, like Ag/Al2O3, inactive for NOx reduction with solely NH3, as seen in Figure 2. With the addition of H2, the NOx reduction clearly increases, however, to a considerably lower level compared to Ag/Al2O3. In order to separate between the effect of the active phase and the Al2O3 support, the equivalent experiments were executed for γ-Al2O3, which is totally inactive for both the NOx reduction and NH3 conversion, even with addition of H2 (results not shown), in accordance with the findings of Doronkin et al. [19].

2.2. Surface Acidity

The surface of the alumina support consists of a combination of aluminum and oxygen ions, which may exhibit lower coordination numbers compared to ions of the bulk. The surface ions hold vacant sites, which, at ambient temperature, are always occupied by either dissociatively adsorbed water in the form of surface hydroxyl (OH) groups, or by coordinated water molecules [38]. Twelve different configurations of OH can be present at the surface, bearing slightly different net charges, consequently possessing different properties, such as variations in acidity [39,40].
Ammonia-TPD experiments were performed in order to investigate the surface acidity of the samples. The desorption profiles of NH3 during the NH3-TPD measurements over the γ-Al2O3, Ag/Al2O3 and In/Al2O3 samples, are shown in Figure 3. The total amount of desorbed ammonia, summarized in Table 1, shows that the highest concentration of acidic sites is found for the γ-Al2O3 sample, followed by In/Al2O3 and Ag/Al2O3, which possessed the lowest concentration of acidic sites. Comparing the NH3 desorption peaks for the samples; the weakest type of acidic site (i.e., the peak with lowest desorption temperature) is found at a higher temperature for the γ-Al2O3 sample, corresponding to 19% of the total desorbed amount of NH3, compared to the Ag/Al2O3 (49%) and In/Al2O3 (16%) samples, respectively. The peak representing the strongest type of acidic site, i.e., the peak at the highest temperature, is also found at a higher temperature for the γ-Al2O3 sample. In addition, the highest desorption-temperature peak of the impregnated samples corresponds to the middle-temperature desorption peak of γ-Al2O3, indicating that the impregnation with silver and indium, respectively, results in less strong acidity for these samples. For the γ-Al2O3 sample, 33% of the total desorbed amount of NH3 is adsorbed on this (strongest acidic) type of site, compared to 6% in the Ag/Al2O3 and 52% in the In/Al2O3 sample. However, the Gaussian peak representing the strongest acidic sites of the γ-Al2O3 sample is centered around almost 100 °C higher temperature compared to the peak holding the most acidic site of In/Al2O3. This implies that the impregnation procedure of γ-Al2O3 leads to an electronical modification and physical blockage of acidic sites at the catalyst support.

2.3. Characterization of the Oxidation State of the Active Phase

Silver and indium species present in the catalyst samples were characterized using diffuse reflectance UV-vis spectroscopy. In order to investigate the influence of the NH3-SCR reaction components, the samples were pretreated with NO, NH3 and H2, respectively. The UV-vis spectrum of the fresh (i.e., non-pretreated) Ag/Al2O3 sample (alumina subtracted) is shown in Figure 4, with absorbance peaks assigned according to Table 2. The spectrum shows that the sample contains a mixture of isolated Ag+-ions, Agnδ+-clusters and Ag0. Note that some isolated Ag+-ions, which exhibited peaks below 200 nm, may have been present in the samples without being detected, since the spectrum only contained signals above 200 nm due to instrument limitations [41].
Figure 5 shows, in the same scale as Figure 4, the UV-vis spectra of the Ag/Al2O3 sample after pretreatments in H2, NO and NH3 at 300 °C. The H2 pretreatment results in increased intensity of the bands at wavelengths corresponding to Agnδ+-clusters and metallic silver, as shown in Figure 5a. In contrast, the absorption spectrum recorded after pretreatment in NO (Figure 5b) shows decreased intensity, compared to the fresh sample, at wavelengths corresponding to Agnδ+-clusters and metallic silver, indicating that NO slightly oxidizes the catalyst surface at 300 °C. In Figure 5c, the spectrum recorded after NH3-pretreatment shows that the surface is reduced by NH3. Compared to the fresh sample, this pretreatment shifts peaks from clusters to more completely reduced Ag phases.
The UV-vis spectrum of the fresh (i.e., non-pretreated) In/Al2O3 sample is shown in Figure 6. Absorption peaks in the range 200–450 nm are assigned to In2O3 [49,50,51,52,53,54]. Lv et al. [49] experienced that increased concentration of In2O3 results in a broadening and a redshift of the absorbance edge. The spectrum recorded after the H2-pretreatment shows broadened absorption peaks with a slight redshift (see Figure 7a). However, peaks at wavelengths above 450 nm increases somewhat after the H2-pretreatment, indicating the presence of more reduced indium species. Increased and broadened absorption peaks in the range 200–450 nm are detected after pretreatments also with NO and NH3 at 300 °C (see Figure 7b,c), which could indicate increased In2O3 concentration in the sample.

2.4. Evaluation of Surface Species

The gas environment influences the surface of the catalyst, and in order to study the active phase and connect that to the reaction itself, DRIFTS was used to follow the formation of surface species at reaction conditions. Figure 8 shows the formation of surface species for the Ag/Al2O3, In/Al2O3 and γ-Al2O3 samples during exposure to NO, NH3, H2 and O2 at 300 °C. The measurements were performed after 10 min exposure to the gas mixture in the last measurement sequence (see Table 3). All observed peaks remained after the specific gas component that gave rise to the corresponding absorption band, was switched off. This indicates that the surface species are chemisorbed to the samples. Below, absorption bands assigned to the adsorption of NO and NH3 are presented separately. Also, absorption bands assigned to hydroxyl groups (i.e., bands at 3500–3800 cm−1 [55]) were observed (Figure 8). Comparing the relative peak intensities within the OH-band area of the infra-red (IR) patterns, it can be shown that the spectrum of the γ-Al2O3 sample resembled the one of In/Al2O3 more closely, compared to Ag/Al2O3.

2.4.1. Assignment of NO Absorption Bands

Exposing Al2O3-based catalysts to NO and O2 (or to NO2) leads to the formation of surface nitrate and nitrite species. Nitrites have been recognized as an intermediate state in the formation of nitrates over Ag/Al2O3 [21], which may be a reason for the nitrite band to appear at 1228 cm−1 in the γ-Al2O3 and In/Al2O3 spectra, but not in the spectrum of Ag/Al2O3. Absorption bands related to the symmetric N=O stretching vibrations are located in the region between 1650 and 1500 cm−1, while the asymmetrical stretching of the O–N–O group can be detected between 1200 and 1350 cm−1 [21,34,56]. The spectra of In/Al2O3 and γ-Al2O3 exhibit broad peaks centered around 1256 cm−1, as shown in Figure 8. This band is assigned to (bidentate) nitrate [57]. In the same region, at 1302 cm−1, Ag/Al2O3 exhibits a sharp peak, which is assigned to monodentate nitrate [21,57]. The three peaks located at 1575, 1595 and 1613 cm−1 for the Ag/Al2O3 sample are assigned to bridged-, bidentate- and mono-dentate nitrate, respectively [21,41]. A summary of all absorption band assignments upon NO exposure is found in Table 4.

2.4.2. Assignment of NH3 Absorption Bands

The symmetric and asymmetric vibration of surface coordinated NH3 results in absorption bands at 1275 and 1587 cm−1, respectively [61]. Moreover, the bands at 1397 and 1692 cm−1 are likely due to the adsorption of NH4+ ions at Brønstedt acidic sites [61,62]. The less pronounced absorption bands at 3380 and 3400 cm−1 are assigned to the symmetric and asymmetric N–H stretching vibrations of NH3 hydrogen bonded to surface OH [62,63]. A summary of the absorption bands associated with ammonia is found in Table 5.

3. Discussion

The NH3-SCR results (Figure 1 and Figure 2) show that the H2-assisted reduction of NO at 300 °C is high over the Ag/Al2O3 sample and low over the In/Al2O3 sample. In situ DRIFTS results (Figure 8) show that the formation of NO-containing species is higher over the Ag/Al2O3 sample compared to the In/Al2O3 and γ-Al2O3 samples at this temperature. An efficient adsorption of the gas phase species is crucial for achieving an effective catalytic conversion. However, all absorption bands assigned to NH3 adsorption are more pronounced for the γ-Al2O3 and In/Al2O3 samples, compared to the Ag/Al2O3 sample. This indicates that the adsorption of NH3-surface species is more efficient over the former two samples compared to the latter, which is supported by the NH3-TPD (Figure 3), showing that the γ-Al2O3 and In/Al2O3 samples provide higher density of acidic sites, compared to the Ag/Al2O3 sample. Furthermore, the DRIFTS spectra show differences at the wavelengths corresponding to OH-groups. The In/Al2O3 pattern resembles the one of γ-Al2O3 to a higher degree compared to Ag/Al2O3. Since alumina is impregnated with Ag and In in equivalent molar amounts, this implies that Ag affects the acidic properties of the OH-rich alumina surface to a higher degree compared to In, resulting in a lower concentration of acidic sites for Ag/Al2O3.
Silver clusters (Agnδ+) have previously been identified as the prime species for the activity in H2-assisted NH3-SCR [64], and it has been shown that the activity is linearly proportional to the relative amount of these clusters [32]. Shimizu and Satsuma [32] suggest the following reaction mechanism for H2-NH3-SCR over Ag/Al2O3: (i) dissociation of H2 on the Ag site, (ii) spillover of H+ to form a proton on Al2O3, (iii) aggregation of isolated Ag+ ions to Agnδ+-clusters (n ≤ 8), (iv) reduction of O2 promoted by Agnδ+-clusters and H+ to O2−, H2O and Agn(δ+x)+ or Ag+, (v) N–H activation by O2− to yield NHx (x ≤ 2) (vi) oxidation of NO by O2− forming NO2, (vii) reaction between NHx and NO to yield N2 and H2O. The study by Tamm et al. [65] confirms that silver is needed for the dissociation of H2, which directly participates in the reaction mechanism and also that the NO to NO2 oxidation is part of this mechanism.
The UV-vis spectra of the In/Al2O3 sample show increased peak intensity for bands assigned to In2O3 but also increased absorbance at higher wavelengths (>450 nm) after pretreatment in H2. The reaction mechanism of H2-assisted NH3 over In/Al2O3 could therefore resemble what Shimizu et al. suggested for Ag/Al2O3. However, the NH3-TPD profiles of the catalysts elucidate the difference in acidity between the catalysts, where the In/Al2O3 sample contains significantly stronger acidic sites compared to the Ag/Al2O3 sample. An important difference in the mechanisms between the two catalysts could therefore be the stronger affinity between NH3 and In/Al2O3, possibly hindering the NH3 activation and thereby hindering further reaction over this catalyst to a higher extent compared to Ag/Al2O3. Another possible issue that may restrain the NOx conversion over In/Al2O3 is the lower adsorption of NO species at the catalyst surface.

4. Materials and Methods

4.1. Catalyst Preparation and Basic Characterization

Two catalyst samples, 2.0 wt % Ag/Al2O3 and 2.1 wt % In/Al2O3 (which corresponds to equivalent molar amount of Ag and In, respectively), were prepared by incipient wetness impregnation of γ-Al2O3 (PURALOX®SBa 200, Sasol, Hamburg, Germany) using freeze-drying, according to the procedure described in detail previously [20], and briefly below. Silver nitrate (≥99.0% Sigma-Aldrich/Merck, Darmstadt, Germany) and indium nitrate hydrate (99.99% Sigma-Aldrich) were used as the active phase precursor for Ag and In, respectively. After impregnation, the samples were frozen in liquid nitrogen, subsequently freeze-dried, and finally calcined in air at 600 °C for four hours. The as prepared powder samples were characterized with respect to surface area by N2 sorption (BET) and crystal structure by X-ray diffraction (XRD), as described in details elsewhere [20].
For the evaluation of the catalytic activity for NH3-SCR, monolith samples with 188 channels (400 CPSI, Ø = 20 mm, L = 20 mm) were cut from a commercial cordierite honeycomb structure (Corning, Corning, NY, USA) and calcined in air at 600 °C for one hour. Binder agent (DISPERAL® P2, Sasol) and one of the powder catalysts (ratio 1:4) in 1:1-ratio ethanol-water solutions were mixed to form washcoat slurries. The monoliths were dipped into the slurries, gently shaken for removal of excess slurry, dried (90 °C in air), and subsequently calcined (500 °C, 3 min). The coating procedure was repeated until the washcoat mass corresponded to 20% of the coated monolith mass. Finally, the monolith samples were calcined in air (600 °C, 1 h).
The catalyst samples have previously been characterized with respect to surface area and crystal structure [20]. The specific surface areas are 197, 185, and 188 m2/g, for γ-Al2O3, Ag/Al2O3 and In/Al2O3, respectively. X-ray diffractograms indicate that the main crystalline phase in all samples is γ-Al2O3, and other crystalline phases (if any) are only present as particles smaller than 3–5 nm [66], in line with previous studies [17,18].

4.2. Lean NOx Reduction Experiments

The catalytic activity for NH3-SCR was evaluated in extinction experiments (500–100 °C, 10 °C/min) at a flow rate of 3500 mL/min (GHSV of 33,400 h−1) in a flow reactor setup previously described by Kannisto et al. [67]. Briefly, the reactor consisted of a horizontal quarts tube (L = 80 cm, Øi = 22 mm) heated by a metal coil. The catalyst sample was positioned close to the tube outlet, surrounded by bare monoliths for shielding of heat radiation to the thermocouples [68], which were placed inside and just before the coated monolith. Prior to each measurement, the sample was pretreated in 10% O2 (Ar-balance) at 500 °C for 30 min. During the experiments, the gas composition was 500 ppm NO, 500 ppm NH3, 10% O2, and 5% H2O, in the presence or absence of 1000 ppm H2 (Ar-balance). The outlet gas composition was analyzed by a gas phase Fourier transform infrared (FTIR) spectrometer (MKS 2030, MKS Instruments, Telford, UK). The reduction of NOx and conversion of NH3 were obtained from the ratios of the differences between the inlet and outlet concentrations to the corresponding inlet concentration.

4.3. UV-Vis Spectroscopy

The samples, pretreated in varying SCR gas components, were characterized by diffuse reflectance UV-vis spectroscopy. Reflectance spectra in the wavelength range 200–1200 nm were recorded using a Varian Cary 5000 UV-vis near-IR (NIR) spectrophotometer, equipped with an external DRA-2500 unit (Agilent, Santa Clara, CA, USA). The same flow reactor equipment as in the NOx reduction experiments was used for the pretreatments, where the samples were exposed to NO (1500 ppm, Ar balance), NH3 (1500 ppm, Ar balance), and H2 (3000 ppm, Ar balance), respectively. The pretreatments were carried out in a gas flow of 100 mL/min for 20 min at 300 °C. During data processing, the spectrum of the alumina support (which was pretreated in the same way as the Ag/Al2O3 and In/Al2O3 samples) was subtracted, and each spectrum was deconvoluted using Gaussian peaks for evaluation purposes.

4.4. In Situ DRIFTS

The surface species on the samples were characterized during SCR reaction conditions by in situ DRIFTS. The instrument used was a Bruker Vertex70 spectrometer equipped with a high-temperature reaction cell (Harrick Scientific, Pleasantville, NY, USA) with KBr windows. Prior to the measurements, the samples were pressed into tablets and then crushed by a mortar, in order to enlarge the powder particles to avoid channeling during the measurement. Subsequently, the powder fraction of the crushed tablets was sieved to a size range of 38 to 75 μm.
After placing the sample in the reaction cell, the Ag/Al2O3 and In/Al2O3 samples were pretreated at 500 °C in NO (2000 ppm, Ar-bal.) for 30 min, followed by O2 (10%, Ar bal.) for 45 min, and finally H2 (1000 ppm, Ar bal.) for 15 min. The γ-Al2O3 sample was pretreated in O2 (10%, Ar bal.) for 45 min and then H2 (1000 ppm, Ar bal.) for 15 min. The gas conditions during the measurement sequence is listed in Table 3, and corresponded to 500 ppm NO, 500 ppm NH3, 1000 ppm H2, and 10% O2 (Ar balance). Each sequence lasted for 10 min and data were collected a few seconds after starting the experiment, after 5 min, and finally after 10 min. A resolution of 1 cm−1 was adapted, and 128 scans were recorded for the background spectra (recorded in Ar at 300 °C), and 64 scans for the measurements. All experiments were carried out at 300 °C in a flow rate of 100 mL/min and the data are presented as absorbance (logI/R).

4.5. NH3-TPD

The concentration and strength of acidic sites of the coated monolith samples were measured by NH3-TPD. The flow reactor presented in Section 2.2 was used for this purpose, where the sample first was pretreated in 10% O2 (20 min) in order to remove carbonaceous matter, flushed with argon (5 min), and then exposed to 1000 ppm H2 (20 min) at 550 °C. During the following NH3-TPD experiment, the catalyst surface was exposed to NH3 (1000 ppm) at 100 °C until saturation, followed by Ar flushing until the NH3 signal vanished. The temperature was subsequently increased linearly to 550 °C (20 °C/min) while the desorbed NH3 was measured continuously. For peak identification purposes, the NH3-TPD profiles obtained are deconvoluted into Gaussian peaks.

5. Conclusions

This work shows that NO-species are formed to a higher extent over Ag/Al2O3 during SCR conditions, compared to In/Al2O3. The latter provide higher density of acidic sites, quantified by NH3-TPD, and also exhibit higher NH3 adsorption, as shown by DRIFTS. Moreover, the Ag/Al2O3 sample is more clearly reduced by H2, compared to In/Al2O3. However, H2 seems to promote the formation of highly dispersed In2O3 clusters, which previously have been suggested to be crucial for HC-SCR and could be important for the activation of the reducing agent also during H2-assisted NH3-SCR. Since adsorption of reactants in suitable proportions is crucial for high catalytic activity, an important difference between the two catalysts could be the stronger affinity for NH3 over In/Al2O3, compared to Ag/Al2O3. This can possibly inhibit the NH3 activation over the former, and thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity, compared to Ag/Al2O3.

Acknowledgments

This work has been financially supported by the Swedish Research Council and was performed within the Competence Centre for Catalysis, which is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies: AB Volvo, ECAPS AB, Haldor Topsøe A/S, Scania CV AB, Volvo Car Corporation AB and Wärtsilä Finland Oy.

Author Contributions

L.S., P.-A.C., M.S. and H.H. conceived and designed the experiments; L.S performed the experiments; L.S., P.-A.C., M.S. and H.H. analyzed the data; L.S., P.-A.C., M.S. and H.H. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

  1. Wang, J.H.; Zhao, H.W.; Haller, G.; Li, Y.D. Recent advances in the selective catalytic reduction of NOx with NH3 on Cu-Chabazite catalysts. Appl. Catal. B Environ. 2017, 202, 346–354. [Google Scholar] [CrossRef]
  2. Shishkin, A.; Shwan, S.; Pingel, T.N.; Olsson, E.; Clemens, A.; Carlsson, P.A.; Härelind, H.; Skoglundh, M. Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ion-exchange. Catalysts 2017, 7, 232. [Google Scholar] [CrossRef]
  3. Clemens, A.K.S.; Shishkin, A.; Carlsson, P.A.; Skoglundh, M.; Martinez-Casado, F.J.; Matej, Z.; Balmes, O.; Härelind, H. Reaction-driven ion exchange of copper into zeolite SSZ-13. ACS Catal. 2015, 5, 6209–6218. [Google Scholar] [CrossRef]
  4. Gunnarsson, F.; Pihl, J.A.; Toops, T.J.; Skoglundh, M.; Härelind, H. Lean NOx reduction over Ag/alumina catalysts via ethanol-SCR using ethanol/gasoline blends. Appl. Catal. B Environ. 2017, 202, 42–50. [Google Scholar] [CrossRef]
  5. Matarrese, R.; Ingelsten, H.H.; Skoglundh, M. Aspects of reducing agent and role of amine species in the reduction of NO over H-ZSM-5 in oxygen excess. J. Catal. 2008, 258, 386–392. [Google Scholar] [CrossRef]
  6. Chorkendorff, I.; Niemantsverdriet, J.W. Concepts of Modern Catalysis and Kinetics; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007. [Google Scholar]
  7. Satokawa, S. Enhancing the NO/C3H8/O2 reaction by using H2 over Ag/Al2O3 catalysts under lean-exhaust conditions. Chem. Lett. 2000, 29, 294–295. [Google Scholar] [CrossRef]
  8. Breen, J.P.; Burch, R.; Hardacre, C.; Hill, C.J. Structural investigation of the promotional effect of hydrogen during the selective catalytic reduction of NOx with hydrocarbons over Ag/Al2O3 catalysts. J. Phys. Chem. B 2005, 109, 4805–4807. [Google Scholar] [CrossRef] [PubMed]
  9. Satokawa, S.; Shibata, J.; Shimizu, K.; Atsushi, S.; Hattori, T. Promotion effect of H2 on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al2O3. Appl. Catal. B Environ. 2003, 42, 179–186. [Google Scholar] [CrossRef]
  10. Kannisto, H.; Ingelsten, H.H.; Skoglundh, M. Aspects of the role of hydrogen in H2-assisted HC-SCR over Ag-Al2O3. Top. Catal. 2009, 52, 1817. [Google Scholar] [CrossRef]
  11. Shibata, J.; Takada, Y.; Shichi, A.; Satokawa, S.; Satsuma, A.; Hattori, T. Ag cluster as active species for SCR of NO by propane in the presence of hydrogen over Ag-MFI. J. Catal. 2004, 222, 368–376. [Google Scholar] [CrossRef]
  12. Breen, J.P.; Burch, R. A review of the effect of the addition of hydrogen in the selective catalytic reduction of NOx with hydrocarbons on silver catalysts. Top. Catal. 2006, 39, 53–58. [Google Scholar] [CrossRef]
  13. Boutros, M.; Starck, J.; de Tymowski, B.; Trichard, J.-M.; Da Costa, P. On the effect of poor metals (Al, Ga, In) on the NOx conversion in ethanol selective catalytic reduction. Top. Catal. 2009, 52, 1786. [Google Scholar] [CrossRef]
  14. Li, J.H.; Hao, J.M.; Cui, X.Y.; Fu, L.X. Influence of preparation methods of In2O3/Al2O3 catalyst on selective catalytic reduction of NO by propene in the presence of oxygen. Catal. Lett. 2005, 103, 75–82. [Google Scholar] [CrossRef]
  15. Maunula, T.; Kintaichi, Y.; Inaba, M.; Haneda, M.; Sato, K.; Hamada, H. Enhanced activity of in and Ga-supported sol-gel alumina catalysts for NO reduction by hydrocarbons in lean conditions. Appl. Catal. B Environ. 1998, 15, 291–304. [Google Scholar] [CrossRef]
  16. Maunula, T.; Kintaichi, Y.; Haneda, M.; Hamada, H. Preparation and reaction mechanistic characterization of sol-gel indium/alumina catalysts developed for NOx reduction by propene in lean conditions. Catal. Lett. 1999, 61, 121–130. [Google Scholar] [CrossRef]
  17. Park, P.W.; Ragle, C.S.; Boyer, C.L.; Balmer, M.L.; Engelhard, M.; McCready, D. In2O3/Al2O3 catalysts for NOx reduction in lean condition. J. Catal. 2002, 210, 97–105. [Google Scholar] [CrossRef]
  18. Erkfeldt, S.; Petersson, M.; Palmqvist, A. Alumina-supported In2O3, Ga2O3 and B2O3 catalysts for lean NOx reduction with dimethyl ether. Appl. Catal. B Environ. 2012, 117, 369–383. [Google Scholar] [CrossRef]
  19. Doronkin, D.E.; Fogel, S.; Tamm, S.; Olsson, L.; Khan, T.S.; Bligaard, T.; Gabrielsson, P.; Dahl, S. Study of the “Fast SCR”-like mechanism of H2-assisted SCR of NOx with ammonia over Ag/Al2O3. Appl. Catal. B Environ. 2012, 113, 228–236. [Google Scholar] [CrossRef] [Green Version]
  20. Ström, L.; Carlsson, P.-A.; Skoglundh, M.; Härelind, H. Hydrogen-assisted SCR of NOx over alumina-supported silver and indium catalysts using C2-hydrocarbons and oxygenates. Appl. Catal. B Environ. 2016, 181, 403–412. [Google Scholar] [CrossRef]
  21. Tamm, S.; Vallim, N.; Skoglundh, M.; Olsson, L. The influence of hydrogen on the stability of nitrates during H2-assisted SCR over Ag/Al2O3 catalysts—A drift study. J. Catal. 2013, 307, 153–161. [Google Scholar] [CrossRef]
  22. Sadokhina, N.A.; Doronkin, D.E.; Baeva, G.N.; Dahl, S.; Stakheev, A.Y. Reactivity of surface nitrates in H2-assisted SCR of NOx over Ag/Al2O3 catalyst. Top. Catal. 2013, 56, 737–744. [Google Scholar] [CrossRef]
  23. Kim, P.S.; Kim, M.K.; Cho, B.K.; Nam, I.-S.; Oh, S.H. Effect of H2 on denox performance of HC-SCR over Ag/Al2O3: Morphological, chemical and kinetic changes. J. Catal. 2013, 301, 65–76. [Google Scholar] [CrossRef]
  24. Thomas, C. On an additional promoting role of hydrogen in the H2-assisted C3H6-SCR of NOx on Ag/Al2O3: A lowering of the temperature of formation-decomposition of the organo-NOx intermediates? Appl. Catal. B Environ. 2015, 162, 454–462. [Google Scholar] [CrossRef]
  25. Kondratenko, E.V.; Kondratenko, V.A.; Richter, M.; Fricke, R. Influence of O2 and H2 on NO reduction by NH3 over Ag/Al2O3: A transient isotopic approach. J. Catal. 2006, 239, 23–33. [Google Scholar] [CrossRef]
  26. Burch, R.; Breen, J.P.; Meunier, F.C. A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts. Appl. Catal. B Environ. 2002, 39, 283–303. [Google Scholar] [CrossRef]
  27. Gunnarsson, F.; Granlund, M.Z.; Englund, M.; Dawody, J.; Pettersson, L.J.; Härelind, H. Combining HC-SCR over Ag/Al2O3 and hydrogen generation over Rh/CeO2-ZrO2 using biofuels: An integrated system approach for real applications. Appl. Catal. B Environ. 2015, 162, 583–592. [Google Scholar] [CrossRef]
  28. Shimizu, K.; Shibata, J.; Yoshida, H.; Satsuma, A.; Hattori, T. Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons: Structure of active sites and reaction mechanism. Appl. Catal. B Environ. 2001, 30, 151–162. [Google Scholar] [CrossRef]
  29. Bogdanchikova, N.; Meunier, F.C.; Avalos-Borja, M.; Breen, J.P.; Pestryakov, A. On the nature of the silver phases of Ag/Al2O3 catalysts for reactions involving nitric oxide. Appl. Catal. B Environ. 2002, 36, 287–297. [Google Scholar] [CrossRef]
  30. Iglesias-Juez, A.; Hungria, A.B.; Martinez-Arias, A.; Fuerte, A.; Fernandez-Garcia, M.; Anderson, J.A.; Conesa, J.C.; Soria, J. Nature and catalytic role of active silver species in the lean NOx reduction with C3H6 in the presence of water. J. Catal. 2003, 217, 310–323. [Google Scholar] [CrossRef]
  31. Kannisto, H.; Ingelsten, H.H.; Skoglundh, M. Ag-Al2O3 catalysts for lean NOx reduction-influence of preparation method and reductant. J. Mol. Catal. A Chem. 2009, 302, 86–96. [Google Scholar] [CrossRef]
  32. Shimizu, K.-I.; Satsuma, A. Reaction mechanism of H2-promoted selective catalytic reduction of NO with NH3 over Ag/Al2O3. J. Phys. Chem. C 2007, 111, 2259–2264. [Google Scholar] [CrossRef]
  33. Shibata, J.; Shimizu, K.; Takada, Y.; Shichia, A.; Yoshida, H.; Satokawa, S.; Satsuma, A.; Hattori, T. Structure of active Ag clusters in Ag zeolites for SCR of NO by propane in the presence of hydrogen. J. Catal. 2004, 227, 367–374. [Google Scholar] [CrossRef]
  34. Meunier, F.C.; Breen, J.P.; Zuzaniuk, V.; Olsson, M.; Ross, J.R.H. Mechanistic aspects of the selective reduction of NO by propene over alumina and silver-alumina catalysts. J. Catal. 1999, 187, 493–505. [Google Scholar] [CrossRef]
  35. Tamm, S.; Fogel, S.; Gabrielsson, P.; Skoglundh, M.; Olsson, L. The effect of the gas composition on hydrogen-assisted NH3-SCR over Ag/Al2O3. Appl. Catal. B Environ. 2013, 136, 168–176. [Google Scholar] [CrossRef]
  36. Richter, M.; Fricke, R.; Eckelt, R. Unusual activity enhancement of NO conversion over Ag/Al2O3 by using a mixed NH3/H2 reductant under lean conditions. Catal. Lett. 2004, 94, 115–118. [Google Scholar] [CrossRef]
  37. Kondratenko, V.A.; Bentrup, U.; Richter, M.; Hansen, T.W.; Kondratenko, E.V. Mechanistic aspects of N2O and N2 formation in no reduction by NH3 over Ag/Al2O3: The effect of O2 and H2. Appl. Catal. B Environ. 2008, 84, 497–504. [Google Scholar] [CrossRef]
  38. Poisson, R.; Brunelle, J.-P.; Nortier, P. Catalyst Supports and Supported Catalysts. Theoretical and Applied Concepts; Stiles, A.B., Ed.; Stoneham: Butterworth, Malasia, 1987; pp. 44–47. [Google Scholar]
  39. Knozinger, H.; Ratnasamy, P. Catalytic aluminas—Surface models and characterization of surface sites. Catal. Rev. Sci. Eng. 1978, 17, 31–70. [Google Scholar] [CrossRef]
  40. Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. Use of DFT to achieve a rational understanding of acid-basic properties of gamma-alumina surfaces. J. Catal. 2004, 226, 54–68. [Google Scholar] [CrossRef]
  41. Sazama, P.; Capek, L.; Drobna, H.; Sobalik, Z.; Dedecek, J.; Arve, K.; Wichterlova, B. Enhancement of decane-SCR-NOx over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV-vis study. J. Catal. 2005, 232, 302–317. [Google Scholar] [CrossRef]
  42. Shi, C.; Cheng, M.J.; Qu, Z.P.; Bao, X.H. Investigation on the catalytic roles of silver species in the selective catalytic reduction of NOx with methane. Appl. Catal. B Environ. 2004, 51, 171–181. [Google Scholar] [CrossRef]
  43. Miao, S.J.; Wang, Y.; Ma, D.; Zhu, Q.J.; Zhou, S.T.; Su, L.L.; Tan, D.L.; Bao, X.H. Effect of Ag+ cations on nonoxidative activation of methane to C2-hydrocarbons. J. Phys. Chem. B 2004, 108, 17866–17871. [Google Scholar] [CrossRef]
  44. Musi, A.; Massiani, P.; Brouri, D.; Trichard, J.-M.; Da Costa, P. On the characterisation of silver species for SCR of NOx with ethanol. Catal. Lett. 2009, 128, 25–30. [Google Scholar] [CrossRef]
  45. Sato, K.; Yoshinari, T.; Kintaichi, Y.; Haneda, M.; Hamada, H. Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al2O3 for the selective reduction of NO with decane. Appl. Catal. B Environ. 2003, 44, 67–78. [Google Scholar] [CrossRef]
  46. Männikkö, M.; Skoglundh, M.; Ingelsten, H.H. Selective catalytic reduction of NOx with methanol over supported silver catalysts. Appl. Catal. B Environ. 2012, 119, 256–266. [Google Scholar] [CrossRef]
  47. Pestryakov, A.N.; Davydov, A.A. Study of supported silver states by the method of electron spectroscopy of diffuse reflectance. J. Electron Spectrosc. Relat. Phenom. 1995, 74, 195–199. [Google Scholar] [CrossRef]
  48. She, X.; Flytzani-Stephanopoulos, M. The role of AgOAl species in silver-alumina catalysts for the selective catalytic reduction of NOx with methane. J. Catal. 2006, 237, 79–93. [Google Scholar] [CrossRef]
  49. Lv, J.; Kako, T.; Li, Z.; Zou, Z.; Ye, J. Synthesis and photocatalytic activities of NaNbO3 rods modified by In2O3 nanoparticles. J. Phys. Chem. C 2010, 114, 6157–6162. [Google Scholar] [CrossRef]
  50. Yang, X.; Xu, J.; Wong, T.; Yang, Q.; Lee, C.-S. Synthesis of In2O3-In2S3 core-shell nanorods with inverted type-I structure for photocatalytic H2 generation. Phys. Chem. Chem. Phys. 2013, 15, 12688–12693. [Google Scholar] [CrossRef] [PubMed]
  51. Zhu, G.; Guo, L.; Shen, X.; Ji, Z.; Chen, K.; Zhou, H. Monodispersed In2O3 mesoporous nanospheres: One-step facile synthesis and the improved gas-sensing performance. Sens. Actuators B Chem. 2015, 220, 977–985. [Google Scholar] [CrossRef]
  52. Zhang, F.; Li, X.; Zhao, Q.; Zhang, Q.; Tade, M.; Liu, S. Fabrication of α-Fe2O3/In2O3 composite hollow microspheres: A novel hybrid photocatalyst for toluene degradation under visible light. J. Colloid Interface Sci. 2015, 457, 18–26. [Google Scholar] [CrossRef] [PubMed]
  53. Yin, J.Z.; Huang, S.B.; Jian, Z.C.; Pan, M.L.; Zhang, Y.Q.; Fei, Z.B.; Xu, X.R. Enhancement of the visible light photocatalytic activity of heterojunction In2O3/BiVO4 composites. Appl. Phys. A Mater. Sci. Process. 2015, 120, 1529–1535. [Google Scholar] [CrossRef]
  54. Liu, Q.; Zhang, W.; Liu, R.; Mao, G. Controlled synthesis of monodispersed sub-50 nm nanoporous In2O3 spheres and their photoelectrochemical performance. Eur. J. Inorg. Chem. 2015, 2015, 845–851. [Google Scholar] [CrossRef]
  55. Morterra, C.; Magnacca, G. A case study: Surface chemistry and surface structure of catalytic aluminas, as studied by vibrational spectroscopy of adsorbed species. Catal. Today 1996, 27, 497–532. [Google Scholar] [CrossRef]
  56. Hadjiivanov, K.I. Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. Sci. Eng. 2000, 42, 71–144. [Google Scholar] [CrossRef]
  57. Underwood, G.M.; Miller, T.M.; Grassian, V.H. Transmission FT-IR and knudsen cell study of the heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles. J. Phys. Chem. A 1999, 103, 6184–6190. [Google Scholar] [CrossRef]
  58. Satsuma, A.; Shimizu, K. In situ FT/IR study of selective catalytic reduction of NO over alumina-based catalysts. Prog. Energy Combust. Sci. 2003, 29, 71–84. [Google Scholar] [CrossRef]
  59. Adams, E.C.; Skoglundh, M.; Gabrielsson, P.; Carlsson, P.A. Passive SCR: The effect of H2 to NO ratio on the formation of NH3 over alumina supported platinum and palladium catalysts. Top. Catal. 2016, 59, 970–975. [Google Scholar] [CrossRef]
  60. Adams, E.C.; Skoglundh, M.; Folic, M.; Bendixen, E.C.; Gabrielsson, P.; Carlsson, P.-A. Ammonia formation over supported platinum and palladium catalysts. Appl. Catal. B Environ. 2015, 165, 10–19. [Google Scholar] [CrossRef]
  61. Acke, F.; Westerberg, B.; Skoglundh, M. Selective reduction of no by HNCO over PT promoted Al2O3. J. Catal. 1998, 179, 528–536. [Google Scholar] [CrossRef]
  62. Centeno, M.A.; Carrizosa, I.; Odriozola, J.A. In situ drifts study of the SCR reaction of NO with NH3 in the presence of O2 over lanthanide doped V2O5/Al2O3 catalysts. Appl. Catal. B Environ. 1998, 19, 67–73. [Google Scholar] [CrossRef]
  63. Wallin, M.; Grönbeck, H.; Spetz, A.L.; Skoglundh, M. Vibrational study of ammonia adsorption on Pt/SiO2. Appl. Surf. Sci. 2004, 235, 487–500. [Google Scholar] [CrossRef]
  64. Yu, L.; Zhong, Q.; Zhang, S. The enhancement for SCR of NO by NH3 over the H2 or CO pretreated Ag/gamma-Al2O3 catalyst. Phys. Chem. Chem. Phys. 2014, 16, 12560–12566. [Google Scholar] [CrossRef] [PubMed]
  65. Tamm, S. The role of silver for the H2-effect in H2-assisted selective catalytic reduction of NOx with NH3 over Ag/Al2O3. Catal. Lett. 2013, 143, 957–965. [Google Scholar] [CrossRef]
  66. Anderson, J.R.; Pratt, K.C. Introduction to Characterization and Testing of Catalysts; Academic Press Inc.: Melbourne, Australia, 1985. [Google Scholar]
  67. Kannisto, H.; Karatzas, X.; Edvardsson, J.; Pettersson, L.J.; Ingelsten, H.H. Efficient low temperature lean NOx reduction over Ag/Al2O3—A system approach. Appl. Catal. B Environ. 2011, 104, 74–83. [Google Scholar] [CrossRef]
  68. Wang-Hansen, C.; Kamp, C.J.; Skoglundh, M.; Andersson, B.; Carlsson, P.-A. Experimental method for kinetic studies of gas-solid reactions: Oxidation of carbonaceous matter. J. Phys. Chem. C 2011, 115, 16098–16108. [Google Scholar] [CrossRef]
Catalysts 08 00038 g001 550
Figure 1. NH3-SCR over Ag/Al2O3. (a) NOx reduction; (b) NH3 conversion; (c) NO2 formation and (d) N2O formation over the Ag/Al2O3 catalyst as a function of temperature. Inlet gas composition: 500 ppm NO, 500 ppm NH3, 10% O2 and 5% H2O, Ar-bal. (red lines represents the addition of 1000 ppm H2).
Figure 1. NH3-SCR over Ag/Al2O3. (a) NOx reduction; (b) NH3 conversion; (c) NO2 formation and (d) N2O formation over the Ag/Al2O3 catalyst as a function of temperature. Inlet gas composition: 500 ppm NO, 500 ppm NH3, 10% O2 and 5% H2O, Ar-bal. (red lines represents the addition of 1000 ppm H2).
Catalysts 08 00038 g001
Catalysts 08 00038 g002 550
Figure 2. NH3-SCR over In/Al2O3. (a) NOx reduction; (b) NH3 conversion; (c) NO2 formation and (d) N2O formation over the In/Al2O3 sample as a function of temperature. Inlet gas composition: 500 ppm NO, 500 ppm NH3, 10% O2 and 5% H2O, Ar-bal. (red lines represents the addition of 1000 ppm H2).
Figure 2. NH3-SCR over In/Al2O3. (a) NOx reduction; (b) NH3 conversion; (c) NO2 formation and (d) N2O formation over the In/Al2O3 sample as a function of temperature. Inlet gas composition: 500 ppm NO, 500 ppm NH3, 10% O2 and 5% H2O, Ar-bal. (red lines represents the addition of 1000 ppm H2).
Catalysts 08 00038 g002
Catalysts 08 00038 g003 550
Figure 3. NH3-temperature programmed desorption (TPD) profiles for (a) γ-Al2O3; (b) Ag/Al2O3 and (c) In/Al2O3, with the desorbed NH3 concentration as a function of the temperature. The desorbed amount of NH3 in the deconvoluted peaks are denoted in percentage of the measured total desorbed amount of NH3.
Figure 3. NH3-temperature programmed desorption (TPD) profiles for (a) γ-Al2O3; (b) Ag/Al2O3 and (c) In/Al2O3, with the desorbed NH3 concentration as a function of the temperature. The desorbed amount of NH3 in the deconvoluted peaks are denoted in percentage of the measured total desorbed amount of NH3.
Catalysts 08 00038 g003
Catalysts 08 00038 g004 550
Figure 4. Ultra-violet (UV)-vis spectrum of the fresh (i.e., non-pretreated) Ag/Al2O3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength. The peak ranges assigned to isolated Ag+-ions, Agnδ+-clusters and Ag0 are denoted.
Figure 4. Ultra-violet (UV)-vis spectrum of the fresh (i.e., non-pretreated) Ag/Al2O3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength. The peak ranges assigned to isolated Ag+-ions, Agnδ+-clusters and Ag0 are denoted.
Catalysts 08 00038 g004
Catalysts 08 00038 g005 550
Figure 5. UV-vis spectra for the Ag/Al2O3 sample, with the absorbance for the alumina sample subtracted, plotted as a function of the wavelength. The graphs represent the spectra after pretreatment in (a) H2, (b) NO, and (c) NH3 at 300 °C. The scale of these figures is the same as of Figure 4.
Figure 5. UV-vis spectra for the Ag/Al2O3 sample, with the absorbance for the alumina sample subtracted, plotted as a function of the wavelength. The graphs represent the spectra after pretreatment in (a) H2, (b) NO, and (c) NH3 at 300 °C. The scale of these figures is the same as of Figure 4.
Catalysts 08 00038 g005
Catalysts 08 00038 g006 550
Figure 6. UV-vis spectrum of the fresh (i.e., non-pretreated) In/Al2O3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength. The peak range assigned to In2O3 is denoted.
Figure 6. UV-vis spectrum of the fresh (i.e., non-pretreated) In/Al2O3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength. The peak range assigned to In2O3 is denoted.
Catalysts 08 00038 g006
Catalysts 08 00038 g007 550
Figure 7. UV-vis spectra for the In/Al2O3 sample, with the absorbance for the alumina sample subtracted, plotted as a function of the wavelength. The graphs represent the spectra after pretreatment in (a) H2, (b) NO, and (c) NH3 at 300 °C. The scale of these figures is the same as of Figure 6.
Figure 7. UV-vis spectra for the In/Al2O3 sample, with the absorbance for the alumina sample subtracted, plotted as a function of the wavelength. The graphs represent the spectra after pretreatment in (a) H2, (b) NO, and (c) NH3 at 300 °C. The scale of these figures is the same as of Figure 6.
Catalysts 08 00038 g007
Catalysts 08 00038 g008 550
Figure 8. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra showing formation of surface species for the Ag/Al2O3, In/Al2O3 and γ-Al2O3 samples during the exposure to selective catalytic reduction (SCR) reaction conditions (NO, NH3, H2, O2, Ar-bal.) at 300 °C.
Figure 8. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra showing formation of surface species for the Ag/Al2O3, In/Al2O3 and γ-Al2O3 samples during the exposure to selective catalytic reduction (SCR) reaction conditions (NO, NH3, H2, O2, Ar-bal.) at 300 °C.
Catalysts 08 00038 g008
Table
Table 1. Total desorbed amount of NH3 during the NH3-TPD experiments.
Table 1. Total desorbed amount of NH3 during the NH3-TPD experiments.
SampleDesorbed NH3 (mmol/cm2)
γ-Al2O311.2
Ag/Al2O37.4
In/Al2O39.5
Table
Table 2. Assignment of absorption peaks in the UV-vis spectra of Ag/Al2O3.
Table 2. Assignment of absorption peaks in the UV-vis spectra of Ag/Al2O3.
SpeciesWavelength [nm]Reference
Isolated Ag+-ions192–250[32]
196, 212, 224[42]
220[43]
215–240[44]
212, 260[44]
Agnδ+-clusters260–370[37]
238–272[45,46]
322[42]
350, 285[32]
Ag0>390[28,29,32,42,46,47,48]
Table
Table 3. The seven DRIFTS sequences. Gas condition: 500 ppm NO, 1000 ppm H2 and 500 ppm NH3. All sequences included 10% O2 and Ar as the carrier gas.
Table 3. The seven DRIFTS sequences. Gas condition: 500 ppm NO, 1000 ppm H2 and 500 ppm NH3. All sequences included 10% O2 and Ar as the carrier gas.
SequenceNOH2NH3
1NO
2NOH2
3NOH2NH3
4H2NH3
5NH3
6NONH3
7NOH2NH3
Table
Table 4. Assignments of infra-red (IR) peaks associated with nitrite and nitrate species.
Table 4. Assignments of infra-red (IR) peaks associated with nitrite and nitrate species.
Wavenumber (cm−1)Appears in SampleSurface SpeciesReferences
1228Al2O3, In/Al2O3Nitrite[21]
1256Al2O3, In/Al2O3Bidentate nitrate[21,58]
1302Ag/Al2O3Monodentate nitrate[21]
1534Ag/Al2O3Monodentate nitrate[21]
1545Al2O3, In/Al2O3Monodentate nitrate[21]
1575Ag/Al2O3Monodentate nitrate[21]
1595Ag/Al2O3Bidentate nitrate[21]
1613Ag/Al2O3Bridged nitrate[21,59,60]
Table
Table 5. Assignments of IR peaks associated with ammonia surface species.
Table 5. Assignments of IR peaks associated with ammonia surface species.
Wavenumber (cm−1)VibrationReference
1275Symmetric bending of surface coordinated NH3[61]
1397NH4+ ions at Brønstedt acidic site[62,63]
1587Asymmetric bending of surface coordinated NH3[61]
1692NH4+ ions at Brønstedt acidic site[62,63]
3380Symmetric NH stretching vibrations of NH3 hydrogen bonded to surface OH[62,63]
3400Asymmetric NH stretching vibrations of NH3 hydrogen bonded to surface OH[63]

Share and Cite

MDPI and ACS Style

Ström, L.; Carlsson, P.-A.; Skoglundh, M.; Härelind, H. Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium. Catalysts 2018, 8, 38. https://doi.org/10.3390/catal8010038

AMA Style

Ström L, Carlsson P-A, Skoglundh M, Härelind H. Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium. Catalysts. 2018; 8(1):38. https://doi.org/10.3390/catal8010038

Chicago/Turabian Style

Ström, Linda, Per-Anders Carlsson, Magnus Skoglundh, and Hanna Härelind. 2018. "Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium" Catalysts 8, no. 1: 38. https://doi.org/10.3390/catal8010038

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop