Surface species and metal oxidation state during H2‐assisted NH3‐SCR of NOx over alumina‐supported silver and indium

: Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NO x 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 NO x reduction efﬁciency of the materials was evaluated using H 2 -assisted NH 3 -SCR. The DRIFTS results show that the Ag/Al 2 O 3 sample forms NO-containing surface species during SCR conditions to a higher extent compared to the In/Al 2 O 3 sample. The silver sample also appears to be more reduced by H 2 than the indium sample, as revealed by UV-vis spectroscopic experiments. Addition of H 2 , however, may promote the formation of highly dispersed In 2 O 3 clusters, which previously have been suggested to be important for the SCR reaction. The afﬁnity to adsorb NH 3 is conﬁrmed by both temperature programmed desorption (NH 3 -TPD) and in situ DRIFTS to be higher for the In/Al 2 O 3 sample compared to Ag/Al 2 O 3 . The strong adsorption of NH 3 may inhibit (self-poison) the NH 3 activation, thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity compared to Ag/Al 2 O 3 .


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 NO x reduction is the selective catalytic reduction with either ammonia (NH 3 -SCR) or hydrocarbons (HC-SCR).For example, Cu-based zeolites have recently been shown to exhibit high NO x removal activity in a wide temperature range [1][2][3].In contrast to NH 3 -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 NH 3 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 NO x 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/Al 2 O 3 , 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/Al 2 O 3 [20].The hydrogen effect over Ag/Al 2 O 3 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 H 2 -SCR [26].From a practical point of view, H 2 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/Al 2 O 3 catalyst [28][29][30][31][32][33], suggesting the active phase for the selective reduction of NO x to be Ag n δ+ -clusters [32,33] and Ag + ions [28,29], or a combination of these.Except for these species, metallic silver (Ag 0 ) is recognized as responsible for complete combustion of the reductant [34].In the In/Al 2 O 3 catalyst, highly dispersed indium cluster sites (In 3+ ) 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 NO x efficiently without the addition of hydrogen.In this study, we compare Ag/Al 2 O 3 and In/Al 2 O 3 catalysts, for the SCR of NO x 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 (NH 3 -TPD).

Results
In this study, we compared H 2 -assisted NH 3 -SCR of NO x over Ag/Al 2 O 3 and In/Al 2 O 3 .In Section 2.1, we show the NO x conversion and formed species during the SCR experiments.In addition, we compare the density of acidic sites by NH 3 -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 Sections 2.3 and 2.4

Catalytic Activity
The Ag/Al 2 O 3 and In/Al 2 O 3 samples were evaluated as NH 3 -SCR catalysts using a flow reactor with a feed composition of 500 ppm NO, 500 ppm NH 3 , 10% O 2 , 5% H 2 O, and Ar as carrier gas.The experiments were subsequently repeated with the addition of 1000 ppm H 2 .
The NO x reduction and NH 3 conversion, as well as the formation of NO 2 and N 2 O over Ag/Al 2 O 3 , are shown in Figure 1.Without the addition of H 2 , the catalyst is inactive for NO x reduction.However, H 2 -assisted NH 3 -SCR reduces NO x in a broad temperature window and with a maximum reduction just above 80%.The selectivity towards N 2 is high, in accordance with previous reports [35,36], 80% when H 2 is present in the feed.It can also be seen that the low N 2 O formation decreases even further when H 2 is present, confirming the results of Kondratenko et al., that H 2 suppresses the total N 2 O production [25].This is assigned to H 2 -induced Ag 0 formation, which likely is responsible for the N 2 O decomposition [37].The negative values of NO x reduction above 350 • C observed when H 2 is absent (black line in Figure 1a) is likely due to the oxidation of NH 3 to NO, which also explains the significant amount of NH 3 converted in Figure 1b [35].Shimizu and Satsuma [32] suggest that the addition of H 2 to NH 3 + O 2 enhances the oxidative activation of NH 3 by decreasing the activation energy of the rate-determining step, (i.e., formation of NH x ).
The In/Al 2 O 3 catalyst is, like Ag/Al 2 O 3 , inactive for NO x reduction with solely NH 3 , as seen in Figure 2.With the addition of H 2 , the NO x reduction clearly increases, however, to a considerably lower level compared to Ag/Al 2 O 3 .In order to separate between the effect of the active phase and the Al 2 O 3 support, the equivalent experiments were executed for γ-Al 2 O 3 , which is totally inactive for both the NO x reduction and NH 3 conversion, even with addition of H 2 (results not shown), in accordance with the findings of Doronkin et al. [19].

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 NH 3 during the NH 3 -TPD measurements over the γ-Al 2 O 3 , Ag/Al 2 O 3 and In/Al 2 O 3 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-Al 2 O 3 sample, followed by In/Al 2 O 3 and Ag/Al 2 O 3 , which possessed the lowest concentration of acidic sites.Comparing the NH 3 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-Al 2 O 3 sample, corresponding to 19% of the total desorbed amount of NH 3 , compared to the Ag/Al 2 O 3 (49%) and In/Al 2 O 3 (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-Al 2 O 3 sample.In addition, the highest desorption-temperature peak of the impregnated samples corresponds to the middle-temperature desorption peak of γ-Al 2 O 3 , indicating that the impregnation with silver and indium, respectively, results in less strong acidity for these samples.For the γ-Al   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 NH 3 -SCR reaction components, the samples were pretreated with NO, NH 3 and H 2 , respectively.The UV-vis spectrum of the fresh (i.e., non-pretreated) Ag/Al 2 O 3 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, Ag n δ+ -clusters and Ag 0 .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/Al 2 O 3 sample after pretreatments in H 2 , NO and NH 3 at 300 • C. The H 2 pretreatment results in increased intensity of the bands at wavelengths corresponding to Ag n δ+ -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 Ag n δ+ -clusters and metallic silver, indicating that NO slightly oxidizes the catalyst surface at 300 • C. In Figure 5c, the spectrum recorded after NH 3 -pretreatment shows that the surface is reduced by NH 3 .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/Al 2 O 3 sample is shown in Figure 6.Absorption peaks in the range 200-450 nm are assigned to In 2 O 3 [49][50][51][52][53][54].Lv et al. [49] experienced that increased concentration of In 2 O 3 results in a broadening and a redshift of the absorbance edge.The spectrum recorded after the H 2 -pretreatment shows broadened absorption peaks with a slight redshift (see Figure 7a).However, peaks at wavelengths above 450 nm increases somewhat after the

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/Al 2 O 3 , In/Al 2 O 3 and γ-Al 2 O 3 samples during exposure to NO, NH 3 , H 2 and O 2 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 NH 3 are presented separately.Also, absorption bands assigned to hydroxyl groups (i.e., bands at 3500-3800 cm −1 [55]) were observed (Figure 8).[21], which may be a reason for the nitrite band to appear at 1228 cm −1 in the γ-Al 2 O 3 and In/Al 2 O 3 spectra, but not in the spectrum of Ag/Al 2 O 3 .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/Al 2 O 3 and γ-Al 2 O 3 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/Al 2 O 3 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/Al 2 O 3 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.

Assignment of NH 3 Absorption Bands
The symmetric and asymmetric vibration of surface coordinated NH 3 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 NH 4 + 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 NH 3 hydrogen bonded to surface OH [62,63].A summary of the absorption bands associated with ammonia is found in Table 5.

Discussion
The NH 3 -SCR results (Figures 1 and 2) show that the H 2 -assisted reduction of NO at 300 • C is high over the Ag/Al 2 O 3 sample and low over the In/Al 2 O 3 sample.In situ DRIFTS results (Figure 8) show that the formation of NO-containing species is higher over the Ag/Al 2 O 3 sample compared to the In/Al 2 O 3 and γ-Al 2 O 3 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 NH 3 adsorption are more pronounced for the γ-Al 2 O 3 and In/Al 2 O 3 samples, compared to the Ag/Al 2 O 3 sample.This indicates that the adsorption of NH 3 -surface species is more efficient over the former two samples compared to the latter, which is supported by the NH 3 -TPD (Figure 3), showing that the γ-Al 2 O 3 and In/Al 2 O 3 samples provide higher density of acidic sites, compared to the Ag/Al 2 O 3 sample.Furthermore, the DRIFTS spectra show differences at the wavelengths corresponding to OH-groups.The In/Al 2 O 3 pattern resembles the one of γ-Al 2 O 3 to a higher degree compared to Ag/Al 2 O 3 .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/Al 2 O 3 .
Silver clusters (Ag n δ+ ) have previously been identified as the prime species for the activity in H 2 -assisted NH 3 -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]  .However, the NH 3 -TPD profiles of the catalysts elucidate the difference in acidity between the catalysts, where the In/Al 2 O 3 sample contains significantly stronger acidic sites compared to the Ag/Al 2 O 3 sample.An important difference in the mechanisms between the two catalysts could therefore be the stronger affinity between NH 3 and In/Al 2 O 3 , possibly hindering the NH 3 activation and thereby hindering further reaction over this catalyst to a higher extent compared to Ag/Al 2 O 3 .Another possible issue that may restrain the NO x conversion over In/Al 2 O 3 is the lower adsorption of NO species at the catalyst surface.

Catalyst Preparation and Basic Characterization
Two catalyst samples, 2.0 wt % Ag/Al 2 O 3 and 2.1 wt % In/Al 2 O 3 (which corresponds to equivalent molar amount of Ag and In, respectively), were prepared by incipient wetness impregnation of γ-Al 2 O 3 (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 N 2 sorption (BET) and crystal structure by X-ray diffraction (XRD), as described in details elsewhere [20].
For the evaluation of the catalytic activity for NH 3 -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 m 2 /g, for γ-Al 2 O 3 , Ag/Al 2 O 3 and In/Al 2 O 3 , respectively.X-ray diffractograms indicate that the main crystalline phase in all samples is γ-Al 2 O 3 , and other crystalline phases (if any) are only present as particles smaller than 3-5 nm [66], in line with previous studies [17,18].

Lean NO x Reduction Experiments
The catalytic activity for NH 3 -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, Ø i = 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% O 2 (Ar-balance) at 500 • C for 30 min.During the experiments, the gas composition was 500 ppm NO, 500 ppm NH 3 , 10% O 2 , and 5% H 2 O, in the presence or absence of 1000 ppm H 2 (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 NO x and conversion of NH 3 were obtained from the ratios of the differences between the inlet and outlet concentrations to the corresponding inlet concentration.

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 NO x reduction experiments was used for the pretreatments, where the samples were exposed to NO (1500 ppm, Ar balance), NH 3 (1500 ppm, Ar balance), and H 2 (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/Al 2 O 3 and In/Al 2 O 3 samples) was subtracted, and each spectrum was deconvoluted using Gaussian peaks for evaluation purposes.

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/Al 2 O 3 and In/Al 2 O 3 samples were pretreated at 500 • C in NO (2000 ppm, Ar-bal.) for 30 min, followed by O 2 (10%, Ar bal.) for 45 min, and finally H 2 (1000 ppm, Ar bal.) for 15 min.The γ-Al 2 O 3 sample was pretreated in O 2 (10%, Ar bal.) for 45 min and then H 2 (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 NH 3 , 1000 ppm H 2 , and 10% O 2 (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).

NH 3 -TPD
The concentration and strength of acidic sites of the coated monolith samples were measured by NH 3 -TPD.The flow reactor presented in Section 2.2 was used for this purpose, where the sample first was pretreated in 10% O 2 (20 min) in order to remove carbonaceous matter, flushed with argon (5 min), and then exposed to 1000 ppm H 2 (20 min) at 550 • C.During the following NH 3 -TPD experiment, the catalyst surface was exposed to NH 3 (1000 ppm) at 100 • C until saturation, followed by Ar flushing until the NH 3 signal vanished.The temperature was subsequently increased linearly to 550 • C (20 • C/min) while the desorbed NH 3 was measured continuously.For peak identification purposes, the NH 3 -TPD profiles obtained are deconvoluted into Gaussian peaks.

Conclusions
This work shows that NO-species are formed to a higher extent over Ag/Al 2 O 3 during SCR conditions, compared to In/Al 2 O 3 .The latter provide higher density of acidic sites, quantified by NH 3 -TPD, and also exhibit higher NH 3 adsorption, as shown by DRIFTS.Moreover, the Ag/Al 2 O 3 sample is more clearly reduced by H 2 , compared to In/Al 2 O 3 .However, H 2 seems to promote the formation of highly dispersed In 2 O 3 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 H 2 -assisted NH 3 -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 NH 3 over In/Al 2 O 3 , compared to Ag/Al 2 O 3 .This can possibly inhibit the NH 3 activation over the former, and thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity, compared to Ag/Al 2 O 3 .

Figure 2 .
Figure 2. NH 3 -SCR over In/Al 2 O 3 .(a) NO x reduction; (b) NH 3 conversion; (c) NO 2 formation and (d) N 2 O formation over the In/Al 2 O 3 sample as a function of temperature.Inlet gas composition: 500 ppm NO, 500 ppm NH 3 , 10% O 2 and 5% H 2 O, Ar-bal.(red lines represents the addition of 1000 ppm H 2 ).
2 O 3 sample, 33% of the total desorbed amount of NH 3 is adsorbed on this (strongest acidic) type of site, compared to 6% in the Ag/Al 2 O 3 and 52% in the In/Al 2 O 3 sample.However, the Gaussian peak representing the strongest acidic sites of the γ-Al 2 O 3 sample is centered around almost 100 • C higher temperature compared to the peak holding the most acidic site of In/Al 2 O 3 .This implies that the impregnation procedure of γ-Al 2 O 3 leads to an electronical modification and physical blockage of acidic sites at the catalyst support.

Figure 3 .
Figure 3. NH 3 -temperature programmed desorption (TPD) profiles for (a) γ-Al 2 O 3 ; (b) Ag/Al 2 O 3 and (c) In/Al 2 O 3 , with the desorbed NH 3 concentration as a function of the temperature.The desorbed amount of NH 3 in the deconvoluted peaks are denoted in percentage of the measured total desorbed amount of NH 3 .

Figure 4 .
Figure 4. Ultra-violet (UV)-vis spectrum of the fresh (i.e., non-pretreated) Ag/Al 2 O 3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength.The peak ranges assigned to isolated Ag + -ions, Ag n δ+ -clusters and Ag 0 are denoted.

Figure 5 .
Figure 5. UV-vis spectra for the Ag/Al 2 O 3 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) H 2 , (b) NO, and (c) NH 3 at 300 • C. The scale of these figures is the same as of Figure 4.
H 2 -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 NH 3 at 300 • C (see Figure 7b,c), which could indicate increased In 2 O 3 concentration in the sample.

Figure 6 .
Figure 6.UV-vis spectrum of the fresh (i.e., non-pretreated) In/Al 2 O 3 sample, with the absorbance for the alumina sample subtracted, as a function of the wavelength.The peak range assigned to In 2 O 3 is denoted.

Figure 7 .
Figure 7. UV-vis spectra for the In/Al 2 O 3 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) H 2 , (b) NO, and (c) NH 3 at 300 • C. The scale of these figures is the same as of Figure 6.
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-Al 2 O 3 sample resembled the one of In/Al 2 O 3 more closely, compared to Ag/Al 2 O 3 .

Table 1 .
Total desorbed amount of NH 3 during the NH 3 -TPD experiments.

Table 2 .
Assignment of absorption peaks in the UV-vis spectra of Ag/Al 2 O 3 .

Table 3 .
The seven DRIFTS sequences.Gas condition: 500 ppm NO, 1000 ppm H 2 and 500 ppm NH 3 .All sequences included 10% O 2 and Ar as the carrier gas.O 3 -based catalysts to NO and O 2 (or to NO 2 ) 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/Al 2 O 3

Table 4 .
Assignments of infra-red (IR) peaks associated with nitrite and nitrate species.

Table 5 .
Assignments of IR peaks associated with ammonia surface species.
suggest the following reaction mechanism for H 2 -NH 3 -SCR over Ag/Al 2 O 3 : (i) dissociation of H 2 on the Ag site, (ii) spillover of H + to form a proton on Al 2 O 3 , (iii) aggregation of isolated Ag + ions to Ag n δ+ -clusters (n ≤ 8), (iv) reduction of O 2 − forming NO 2 , (vii) reaction between NH x and NO to yield N 2 and H 2 O.The study by Tamm et al. [65] confirms that silver is needed for the dissociation of H 2 , which directly participates in the reaction mechanism and also that the NO to NO 2 oxidation is part of this mechanism.The UV-vis spectra of the In/Al 2 O 3 sample show increased peak intensity for bands assigned to In 2 O 3 but also increased absorbance at higher wavelengths (>450 nm) after pretreatment in H 2 .The reaction mechanism of H 2 -assisted NH 3 over In/Al 2 O 3 could therefore resemble what Shimizu et al. suggested for Ag/Al 2 O 3