Journal of Catalysis 307 (2013) 153–161

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Journal of Catalysis

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The influence of hydrogen on the stability of nitrates during H2-assistedSCR over Ag/Al2O3 catalysts – A DRIFT study

0021-9517/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcat.2013.07.003

⇑ Corresponding author. Address: Chemical Reaction Engineering, CompetenceCentre for Catalysis, Department of Chemical and Biological Engineering, ChalmersUniversity of Technology, SE-412 96 Göteborg, Sweden.

E-mail address: louise.olsson@chalmers.se (L. Olsson).

Stefanie Tamm a,b, Negar Vallim a,b, Magnus Skoglundh a,c, Louise Olsson a,b,⇑a Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Swedenb Chemical Reaction Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Swedenc Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

a r t i c l e i n f o

Article history:Received 25 January 2013Revised 18 June 2013Accepted 3 July 2013

Keywords:DRIFTSAg/Al2O3

AgOAgNO3

H2-effectNitrate reductionNitriteLean NOx reduction

a b s t r a c t

Silver/alumina is a promising catalyst for the selective catalytic reduction of NOx by hydrocarbons (HC-SCR) in excess of oxygen, especially when adding small amounts of hydrogen. The same material showsexcellent low-temperature activity for NH3-SCR in the presence of hydrogen. Since NH3 has been pro-posed to be an intermediate in HC-SCR, it is possible that the role of hydrogen is the same for both sys-tems. Here, we study the effect of H2 in the presence of excess O2 on the differently adsorbed nitrates andnitrites on the Al2O3 support and on nitrates on silver by diffuse reflectance infrared Fourier transform(DRIFT) spectroscopy and show that nitrites are an intermediate in the formation of monodentate nitrate.Moreover, hydrogen promotes the conversion of nitrites and some bidentate nitrates to mainly monoden-tate and some bridge-bound nitrates. Although monodentate nitrates are stable, they do not poison thecatalyst surface, since the NOx reduction remained constant during 10 h of experiment at 250 �C. Contraryto the effect on nitrates bound to Al2O3, no effect of hydrogen on nitrates formed on AgO was observed.These nitrates were stable in gas mixtures containing hydrogen but decompose easily in the presence ofNH3 at 100 �C. This indicates that the silver particles of an Ag/Al2O3 catalyst will not be poisoned bynitrates in H2-assisted NH3-SCR. Moreover, nitrates on silver migrate to the Al2O3 support already at100 �C.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

The reduction of CO2 emissions from the transportation sectoris crucial in view of global warming. This can be achieved by theimplementation of modern engines which operate more fuel effi-cient owing to excess oxygen in the combustion. However, excessoxygen in the exhaust hinders the conventional three-way catalystfrom reducing the nitrogen oxides (NOx) formed in the engine.Moreover, the exhaust gas temperature of modern engines has de-creased during the last years, and now, activity for NOx reduction isneeded already below 200 �C. One promising concept to meetthese requirements is the selective catalytic reduction of NOx byhydrocarbons (HC-SCR) over silver/alumina (Ag/Al2O3) catalysts.Addition of small amounts of hydrogen as co-reductant to differenthydrocarbons gives good activity for NOx reduction at low temper-atures [1,2]. The same catalyst is also active for hydrogen-assistedNH3-SCR reaching up to 90% conversion already at 200 �C with highselectivity to N2 [3–5]. In the absence of hydrogen, Ag/Al2O3

catalysts cannot reduce NOx with NH3, and in the presence ofhydrogen as single reducing agent, oxidation of NO to NO2 is pro-moted, but practically, no NOx reduction is observed [3,6,7].

Since the addition of hydrogen as co-reductant can increase NOx

conversion from 0 to 100% over an Ag/Al2O3 catalyst at a fixed tem-perature [6], it was concluded that hydrogen must have an effecton the catalyst morphology or change the reaction mechanism.An increase in the number of small silver clusters, which are re-garded to be the active species, has been observed with UV–visbut disproved as an effect of the addition of hydrogen [7–9]. More-over, reduction of silver species, is discussed but cannot be clearlyattributed to the presence of hydrogen [7,10,11]. With an O2-TPDand H2-TPR, it was shown that molecular and atomic adsorbedoxygen leave the surface of silver particles below 300 �C [12], whileoxygen in the bulk of silver can be observed to 700 �C [12]. The sil-ver species seem, thus, to be partially oxidized under reaction con-ditions [7]. This is in accordance with observations from XPSmeasurements of pre-oxidized and pre-reduced Ag/Al2O3 samples[11]. The observation of larger amounts of carbon containing sur-face species in the presence of hydrogen has been attributed to amodified reaction mechanism in the presence of hydrogen[6,13,14]. However, the increased formation of carbon containingintermediates cannot be the only effect, since Ag/Al2O3 catalysts

154 S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161

are also active for NH3-SCR, as discussed before. Other observed ef-fects are an increased formation of reactive gas-phase species,which have been shown to be important over Ag/Al2O3 catalysts[15,16], and the formation of highly reactive oxygen ions on thecatalyst surface [17]. A further suggested role of H2 is the removalof poisoning surface species [6]. Meunier et al. suggested that Ag/Al2O3 catalysts can be poisoned by nitrates [18]. Apparent contra-dictory observations of the addition of hydrogen on these surfacenitrates have been reported. On the one hand, an increase in theamount of surface nitrates is reported [6,7], and on the other hand,it is suggested from kinetic modeling, flow reactor tests and com-parison of DRIFT results in the presence and absence of H2 thathydrogen reduces or removes nitrates from the catalyst [11,19–21]. However, all of these publications use indirect methods, anda decrease in nitrate bands in DRIFT experiments in a H2 and O2

containing flow is not shown. Brosius et al. [10] proposed thathydrogen removes nitrates from the silver but increases theamount of nitrates on the Al2O3 support as concluded from UV–vis experiments. In the present study, we examine more detailsof the effect of hydrogen on nitrates in the presence of excess oxy-gen. With diffuse reflectance infrared Fourier transform (DRIFT)spectroscopy, three differently bound nitrates can be discriminatedon the Al2O3 support. The effect of hydrogen on these nitrates isstudied, and the stability of nitrates on silver is examinedseparately.

2. Materials and methods

A 2 wt% silver alumina sample was prepared by a sol–gel meth-od as previously described in Ref. [22], c-alumina powder (SASOLPuralox SBa-200) was calcined for 2 h at 550 �C in static air, andAgO (Riedel de Haën AG) was used as received.

2.1. DRIFT experiments

In situ DRIFT spectroscopy experiments were performed using aBioRad FTS 6000 FTIR spectrometer equipped with a high-temper-ature reaction cell (Harrick Scientific, Praying Mantis) with KBrwindows. The temperature of the reaction cell was controlled witha K-type thermocouple connected to a Eurotherm 2416 tempera-ture controller. Gases were introduced into the reaction cell viaindividual mass flow controllers (Bronkhorst Hi-Tech). The gascomposition at the outlet of the DRIFTS cell was analyzed by massspectrometry (Balzers QuadStar 420). For the background spectra,60 scans were recorded with a resolution of 1 cm�1, and the

100

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40

20

0

Con

cent

ratio

n [p

pm] a

nd

700600500400300200100

Time [min]

NO

x con

vers

ion

[%]

NO

NH3NO2N2O

NOx conversion

Fig. 1. NOx conversion and gas concentrations at 250 �C as a function of time in agas mixture of 250 ppm NO, 250 ppm NH3, 750 ppm H2, and 10% O2.

evolution of absorption bands in the spectra was followed usingthe kinetic mode (9 scans/spectrum, 6 spectra/min). The data arepresented as absorbance, which is defined as the logarithm of theinverse reflectance (logI/R). All experiments were carried out usinga total flow rate of 100 mL/min.

Experiments with the Ag/Al2O3 and the Al2O3 sample startedwith a pretreatment in a flow of 10% O2 in Ar at 550 �C for30 min. The sample was cooled to 250 �C in Ar where a backgroundwas recorded and the DRIFT measurement was started 30 s beforegases were allowed to adsorb on the catalyst. In a first step-re-sponse experiment, the influence of the gases present in H2-as-sisted NH3-SCR (NO, NH3, and H2) on the surface species wastested at 250 �C on the Ag/Al2O3 sample. In this experiment, thesample was alternating exposed to SCR reaction conditions(500 ppm NO, 500 ppm NH3, 500 ppm H2, and 8% O2) and to gasmixtures where one of the gases was removed. The length of eachstep was 1 h. Details of the sequence of gases are presented in thelegend and the top part of Fig. 2.

The stability of nitrates adsorbed on the alumina of both an Ag/Al2O3 and a pure Al2O3 sample was tested. Following a pretreat-ment as described for the experiment before, nitrates were allowedto form on the catalyst surface from gas mixtures of 500 ppm NOx

(NO or NO2), 10% O2, and 1250 ppm H2. After this initial step of ni-trate formation, the stability of the nitrates was tested in inert gas(Ar), 10% O2, and a mixture of 1250 ppm H2 and 10% O2 in step-re-sponse experiments. The step length varied but was normally keptat or below 5 min to avoid major accumulation of spectator spe-cies. Details of the gas mixtures are indicated in the respective fig-ure and the figure caption.

The stability of nitrates on silver oxide was tested. For this pur-pose, the AgO sample was heated at 5 �C/min in Ar in to 100 �C,where a background was taken. This lower temperature was cho-sen to stay well below of the melting point of AgO and formedAgNO3 at 280 and 212 �C, respectively. The AgO sample was ex-posed to 500 ppm NO2 and 10% O2 in Ar for 15 min to form nitrates,and the DRIFT measurement was started 30 s before the exposure.Subsequently, the stability of the nitrates was tested in 500 ppmNH3, 500 ppm NO, 1250 ppm H2, and 10% O2 in Ar or mixtures ofthese gases for 15 min.

2.2. Assignments of nitrate and nitrite species

Nitrate and nitrite species are typically observed over Al2O3-based catalysts in two different regions: in the N@O stretching re-gion between 1650 and 1500 cm�1 [23] and between 1350 and1200 cm�1 where the asymmetrical stretching of the OANAOgroup can be seen [18,24]. In the following experiments, four dif-ferent bands can be discerned in the N@O stretching region, i.e.,at 1611, 1586/1570, 1558, and 1546/1537 cm�1. Due to the overlapof the bands, only the evolution of the bands at 1611, 1586/1570,and 1546/1537 cm�1 is followed. The band at 1558 cm�1 showsthe same behavior as the band at 1546/1537 cm�1 and will there-fore not be treated separately in the following discussion. In addi-tion, three bands of the OANAO group can be discriminated at1304, 1250, and 1228 cm�1 which can be assigned to nitratesand/or nitrites. The evolution of the bands with time in Fig. 3shows that the evolution of the band at 1228 cm�1 does not haveany similarities with any other observed bands and is assigned toa nitrite species [7,16,18,24,25]. Moreover, the bands at 1546/1537 and 1304 cm�1 and the bands at 1585/1570 and 1250 cm�1

change simultaneously and are caused by the same species. Inthe literature, many authors agree on the assignment of bands inthe N@O stretching region to monodentate (1550 cm�1), bidentate(1585 cm�1), and bridge-bound nitrates (1611 cm�1) [7,15,18,20,24,26–29]. However, most of these papers report a differentcovariation in bands in the N@O stretching region and bands below

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0.1

1: NH32: NH3 + H23: NH3 + NO + H24: NO + H25: NH3 + NO + H26: NH3 + H27: NH3 + NO + H28: NH3 + NO9: NH3 + NO + H2

monodentatenitrate (1304)

bridge-boundnitrate (1611)

b

1 2 3 4 5 6 7 8 9

Fig. 2. (a) Evolution of bands in a step-response experiment over the Ag/Al2O3 sample at 250 �C. (b) Evolution of the height of the monodentate nitrate band at 1304 cm�1 andthe bridge-bound nitrate band at 1611 cm�1 during the experiment. The baseline level was defined at 1800 cm�1. Step 1: 500 ppm NH3 and 10% O2, step 2: 500 ppm NH3,1250 ppm H2 and 10% O2, step 3: 500 ppm NO, 500 ppm NH3, 1250 ppm H2, and 10% O2, step 4: 500 ppm NO, 1250 ppm H2, and 10% O2, step 5: 500 ppm NO, 500 ppm NH3,1250 ppm H2, and 10% O2, step 6: 500 ppm NH3, 1250 ppm H2, and 10% O2, step 7: 500 ppm NO, 500 ppm NH3, 1250 ppm H2, and 10% O2, step 8: 500 ppm NO, 500 ppm NH3,and 10% O2, step 9: 500 ppm NO, 500 ppm NH3, 1250 ppm H2, and 10% O2. The duration of each step was 1 h.

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Ag/Al2O3: NO2 + O2

Al2O3: NO2 + O2

Ag/Al2O3: NO + O2 + H2

NO

x + O

2 (+H

2)

Ar

NO

x + O

2 (+H

2)

O2 i

Ar

H2 +

O2 i

Ar

H2 +

O2 i

Ar

O2 i

Ar

1611

b

bid. nitrates (1586)

monod. nitrates (1304)

monod. nitrates (1546)

bridge-b. nitrates (1611)

bid. nitrates (1250)

nitrites (1228)

nitrites (1228)bridge-b. nitrates (1611)bid. nitrates (1250)

bid. nitrates (1586)monod. nitrates (1304)

monod. nitrates (1546)

nitrites (1228)

bid. nitrates (1250)bridge-b. nitrates (1611)

bid. nitrates (1586)monod. nitrates (1546)

monod. nitrates (1304)

1 2 3 4 5 6 7

Fig. 3. Formation and stability of nitrates over Ag/Al2O3 (top and bottom panel) and c-Al2O3 (middle panel) formed from gas mixtures of NO2 and O2 (top and middle panels)or NO, O2, and H2 (bottom panel) and subsequent flushing in different gas mixtures at 250 �C. (a) DRIFT spectra, (b) height of nitrate absorption bands. The baseline level wasdefined at 1800 cm�1. Step 1: 500 ppm NOx, (1250 ppm H2), and 10% O2 in Ar, step 2: Ar, step 3: 10% O2 in Ar, step 4: 1250 ppm H2 and 10% O2 in Ar, step 5: 500 ppm NOx,(1250 ppm H2), and 10% O2 in Ar, step 6: 10% O2 in Ar, step 7: 1250 ppm H2 and 10% O2 in Ar. Step length: 5 min.

S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161 155

Table 1Assignment of nitrates and nitrites bands.

Wavenumber (cm�1) Surface species References

1228 Nitrites [7,15,17,24,25]1250–1262 Bidentate nitrates [19,29]1304 Monodentate nitrate [19,29]1530–1546 Monodentate nitrate [19,29]1564–1558 Monodentate nitrate [19,29]1570–1590 Bidentate nitrates [19,29]1611 Bridge-bound nitrates [27,28]

156 S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161

1350 cm�1 than observed in the present study. Nevertheless, weassign the differently bound species according to the vibration inthe N@O stretching region and focus the discussion on these bands.Accordingly, bands at 1546/1537 and 1304 cm�1 are here assignedto monodentate nitrate, and bands at 1585/1570 and 1250 cm�1

are assigned to bidentate nitrates, which is in accordance withRefs. [20,29]. The assignments of the bands are summarized inTable 1.

3. Results

It has been shown in the literature that Ag/Al2O3 catalysts areactive for H2-assisted NH3-SCR from 150 �C with a maximum con-version of close to 100% between 250 and 300 �C [3–5]. However,in the absence of H2, the Ag/Al2O3 catalysts do not show any signif-icant activity for NH3-SCR [3,4,30]. Therefore, we here chose 250 �Cas reaction temperature for our experiments. Fig. 1 shows the NOx

conversion and the concentrations of NO, NH3, N2O, and NO2 dur-ing H2-assisted NH3-SCR of our catalyst in a flow reactor at 250 �Cas a function of time. The conversion increased in the beginningand stabilized at 90% NOx conversion after about 3 h.

3.1. Step-response experiment with long exposure times

Fig. 2a shows the evolution of absorption bands on Ag/Al2O3

during a step-response experiment at 250 �C. The spectra were re-corded after exposure of the sample for 1 h just before switchingthe gas composition to the next step. In an atmosphere of NH3

and O2 in the first step, several positive bands appear, which allpreviously have been assigned to differently bound NHx species,mainly NH3 and NH4

+ [31–36]. Adding H2 to the feed, the bandsat 1530 and 1262 cm�1 increase further and a new band at1304 cm�1 starts to form. In the following steps, the band at1304 cm�1 clearly increases, the band at 1530 cm�1 shifts to1540 cm�1, and new bands form at 1611, 1590, and 1564 cm�1.All these bands can be attributed to differently bound nitrates[31,37–39] as already discussed in Section 2.2.

From step 3, the first one at which nitrates clearly can be distin-guished, the nitrates increase throughout the entire experiment asexemplified by the height of the nitrate bands at 1611 and1304 cm�1 in Fig. 2b. These bands were chosen because a clear dis-tinction to the NHx bands observed in steps 1 and 2 is possible.Since nitrates are pointed out to be important intermediates inHC-SCR in the literature [20,27,28], a decrease in the nitrate bandsis expected in step 6 when NO in the feed is switched off. Oneinterpretation is that this buildup of nitrates is due to poisoningof the catalyst surface by nitrates. However, the flow reactor testspresented in Fig. 1 showed increasing activity which stabilizedafter about 3 h, indicating that the observed nitrates are not a poi-son of the catalyst, but are rather stable spectator species. On thecontrary, the increase in NOx conversion during the first 3 h inthe flow reactor might indicate that a certain surface coverage withnitrates and nitrites is beneficial for high NOx conversion. More-over, the constant increase in the amount of nitrates indicate that

the observed nitrate species are mainly located on the Al2O3 sup-port, while nitrates on silver cannot be distinguished due to similarwavenumbers for nitrates adsorbed on silver or on alumina. This isin accordance with Refs. [28,40,41], who concluded that the major-ity of the nitrates on an Ag/Al2O3 catalyst is located on the alumina.In step 8 in Fig. 2, the supply of H2 was switched off after a stepwith NO, NH3, H2, and O2 present in order to see the influence ofhydrogen on the surface species. In the absence of H2, the nitratescontinue to increase. In step 9, H2 was switched in again, and theheight of the nitrate bands also continues to increase. This behav-ior can be explained with the long exposure times and conse-quently the large amount of surface species which make isdifficult to observe smaller changes. Consequently, we tested thestability of nitrates on the catalyst surface with shorter steps last-ing 2 or 5 min to avoid the buildup of spectator species whichmight obstruct the detection of the changes in the nitrate bands.

3.2. Stability of nitrate species on Al2O3

Fig. 3a shows the formation of nitrate species from NO2 and O2

(top panel) and NO, O2, and H2 (bottom panel) on Ag/Al2O3, andfrom NO2 and O2 on c-Al2O3 (middle panel). In each panel, the bot-tom black line shows the spectrum recorded after 2 min exposureto the NO or NO2 containing feed. The following spectra were re-corded at the end of each further step. Bands at 1611, 1586,1558, 1546, 1304, and 1250 cm�1 are observed in all three experi-ments but in different proportions to each other. The appearance ofthe same band on Ag/Al2O3 as on c-Al2O3 confirms the conclusionfrom Section 3.1, that the nitrates observed on the Ag/Al2O3 sampleare mainly located on the Al2O3 support. The different proportionsof the bands in the experiments indicate that the presence of silverand the nature of nitrogen oxide (NO or NO2) in the feed gas influ-ence how the nitrates preferably are bound on the catalyst. All thesame, it is important to keep in mind that the wavenumbers of ni-trates and nitrites on silver sites are very close to those on alumina,which impedes a clear distinction and minor amounts of nitratesand nitrites on silver will be masked by the larger amounts of thesespecies on the alumina.

In all three experiments, the nitrate bands clearly increase instep 1, when NO or NO2 is included in the feed. In the followingsteps (2–4) without NO or NO2 present, all the nitrates appear tobe stable after being formed from NO2 and O2 exposure, indepen-dently of the sample. This is stressed by the evaluation of the peakheight of monodentate (1546 cm�1), bidentate (1586 cm�1), andbridged nitrates (1611 cm�1) as a function of time as shown inFig. 3b. However, the bands of the bidentate and bridged nitratespecies (1586 and 1611 cm�1) decrease in the last step of theexperiment in the presence of H2. The decrease in bidentate bandsthrough addition of hydrogen is in accordance with Ref. [20]. Incontrast to the bidentate and bridged nitrates (1586 and1611 cm�1), the monodentate nitrate species (1546 cm�1) do notdecrease throughout the entire experiment. The higher stabilityof the monodentate nitrate (1546 cm�1) is in accordance withthe results presented by Kameoka et al. who observed that thesenitrates are stable at 400 �C in inert gas, while all other nitratebands decreased [28]. We can, thus, conclude that in accordancewith the literature [10,11], some nitrates can be reduced by addi-tion of small amounts of hydrogen. This is seemingly in contradic-tion to the results presented in Fig. 2. However, due to the largeamount of nitrates adsorbed on the catalyst after 8 h of experi-ments, a minor decrease in the amount of nitrate species is notclearly observable. In addition, the height of the band of the mono-dentate species is stable or possibly decreases somewhat in thebeginning of step 9.

In addition to the nitrate bands, the evolution of a nitrite bandat 1228 cm�1 is studied. This band is hard to distinguish in the

S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161 157

spectra shown in Fig. 3a for three reasons: (i) The band is overlap-ping with the bidentate nitrate band at 1250 cm�1, (ii) the band issmall at the times, when the spectra are taken, and (iii) it is partlyhidden in the noise from the Al2O3 framework in the case of the c-Al2O3 spectra. However, the position of the band was identified inan experiment with exposure only to NO and O2 (Fig. 5), and theevolution of this band is in accordance to other experiments (seebelow). The nitrite band increases fast when the NO2 feed isswitched on and starts to decrease already after a few minutes(top and middle panel of Fig. 3b). The rapid increase followed byan early decrease in the nitrite band is a typical behavior of anintermediate species. When nitrate and nitrite species are formedupon exposure to a NO, O2, and H2 containing feed, no clear bandof nitrite species is observed at 1228 cm�1 (bottom panel inFig. 3b). The absence of this nitrite band will be discussed in moredetail below.

In contrast to the nitrate species formed from a gas mixture ofNO2 and O2, the amount of all nitrates formed from a gas mixtureof NO, O2, and H2 increases during exposure to a gas stream con-sisting of only H2 and O2 in Ar (steps 4 and 7). This effect is unex-pected but occurs independently of the preceding step as shown inthe bottom panel of Fig. 3b. Moreover, the increase in the nitratespecies is consistent with the observation of Fig. 2a that the nitratebands increase in a gas mixture of NH3, H2, and O2 at 250 �C, a tem-perature where according to Ref. [3], no oxidation of NH3 takesplace. The explanation of the increase in the nitrate band by addi-tion of H2 is not trivial. Since the peak height of the nitrate band isstable in the steps before (only Ar, step 2; O2 in Ar, steps 3 and 6), itis unlikely that any spare NO is remaining in the gas phase, whichcan adsorb on the surface in the step containing H2. Another pos-sible explanation is the existence of loosely bound NO, whichforms nitrates in the presence of H2. Even that explanation is unli-kely, since no decrease in any band is observed simultaneously inthe wavenumber region between 4000 and 1200 cm�1. An increasein adsorption bands without decrease in any other bands can beachieved by transformation of nitrates or nitrites to differently

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0.1

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15581546

1570 1304

1304

1250

1228

0

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26 min

a

NO

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2 (+

H2)

m

b

Fig. 4. Formation and stability of nitrates over Ag/Al2O3 formed from gas mixtures of NOdifferent gas mixtures at 250 �C. (a) DRIFT spectra, (b) height of nitrate absorption bands.spectra are taken. Step sequence: 500 ppm NO2, (1250 ppm H2) and 10% O2 in Ar for 2 minrepeated for 5 times.

bound nitrates or nitrites with adsorption bands close to eachother but higher adsorption coefficient. Another possibility is theoxidation of nitrites to nitrates. For some nitrites, adsorption bandsbelow 1200 cm�1 are reported on other oxides [23]. These nitritescannot be detected with DRIFTS over Al2O3 due to the absorption ofthe Al2O3 framework below 1200 cm�1. In addition, the nitriteband observed at 1228 cm�1 overlaps with the bidentate nitrateband at 1250 cm�1 hindering detection of small amounts. A trans-formation of nitrites with wavenumbers around 1228 cm�1 to ni-trates explains the observed increase in all nitrate bands exceptthe nitrate band at 1250 cm�1, which stays stable in a flow ofhydrogen. A possible interpretation is that nitrites on the silverare transformed to nitrates on alumina in the presence of hydrogenand hereby clean the silver. In summary, the experiments confirmagain that the presence of H2 affects the nitrate species adsorbedon Al2O3, but these changes differ depending on the type and theamount of the nitrate and nitrite species. In order to study the sta-bility of the nitrates in more detail, new experiments were per-formed. Although diffuse reflectance FTIR spectroscopy is not aquantitative tool, the clearly lower intensity of the bands in thebottom panel of Fig. 3a compared to the middle and top panelsindicates that lower amounts of nitrates are adsorbed on the sur-face when using NO, O2, and H2 compared to NO2 and O2. To probeif hydrogen has a different effect on the stability of nitrates at lowcoverage than at high coverage, the exposure time of the sample toa gas mixture of NO2 and O2 was shortened to 2 min in the newexperiments. The top panel of Fig. 4a shows the spectra obtainedand Fig. 4b the evolution of the most important bands as a functionof time. The stepwise increasing amount of nitrates formed duringeach NOx adsorption step allows distinguishing between the effectof the gas mixture which had been studied in Fig. 3 and the amountof nitrates on the surface. After 2 min of nitrate formation, thebidentate nitrates (1570 cm�1) decrease in a flow of H2 and O2 inAr, whereas the bridged (1611 cm�1) and monodentate(1535 cm�1) nitrates increase slightly. In contrast, after 10 min ofnitrate formation (20-min total experiment time), the bridged

252015105

NO

2 + O

2 (+

H2)

NO

2 + O

2 (+

H2)

NO

2 + O

2 (+

H2)

NO

2 + O

2 (+

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O2 i

Ar

O2 i

Ar

O2 i

Ar

O2 i

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O2 +

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O2 +

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Ar

bid. nitrates(1570)

monod. nitrates (1535)

bridge-b. nitrates (1611)

bridge-b. nitrates (1611)

nitrites (1228)

onod. nitrates(1546)

bid. nitrates (1586)

nitrites (1228)

2 and O2 (top panel) and NO2, H2, and O2 (bottom panel) and subsequent flushing inThe baseline level was defined at 1800 cm�1. Vertical lines indicate the times when, 10% O2 in Ar for 1 min, 1250 ppm H2 and 10% O2 in Ar for 2 min. This sequence was

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+ O

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+ O

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O2 +

H2

O2 +

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O2 +

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O2

O2

O2

O2 +

H2

O2

nitrites(1228)

monod. nitrates(1535)

bid. nitrates(1570)

bridge-b.nitrates (1611)

Fig. 5. Formation and stability of nitrates over Ag/Al2O3 formed from gas mixtures of NO and O2 and subsequent flushing in different gas mixtures at 250 �C. (a) DRIFT spectra,(b) height of nitrate absorption bands. The baseline level was defined at 1800 cm�1. Vertical lines indicate the times when spectra are taken. Step sequence: 500 ppm NO and10% O2 in Ar for 5 min, 10% O2 in Ar for 1 min, 1250 ppm H2 and 10% O2 in Ar for 2 min. This sequence was repeated 5 times, with a longer exposure to NO and O2 during the4th sequence.

158 S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161

nitrates (1611 cm�1) are stable. This experiment shows that the ef-fect of H2 on the nitrates depends on the amount of nitrates on thecatalyst surface: At low surface concentrations, H2 assists the for-mation of nitrates, especially monodentate (1535 cm�1) andbridged nitrates (1611 cm�1). On the other hand, at a higher sur-face coverage, the formation of bridge-bound nitrates(1611 cm�1) is less favorable, and thus, more monodentate nitrates(1535 cm�1) are formed. Since the results agree, moreover, withthe results of Fig. 3, we can conclude that H2 changes the relativeamounts of differently bound nitrates. This effect is observed inde-pendently if the sample was exposed to NO2 or NO and H2.

The bottom panel of Fig. 4 shows the spectra and evolution ofthe same nitrate and nitrite bands during stepwise exposure ofan Ag/Al2O3 sample to a gas mixture of NO2, H2 and O2. Comparingthe top and the bottom panel of Fig. 4, it is obvious that in the pres-ence of H2, the formation of monodentate nitrates (1546 cm�1) in-creases. Moreover, the nitrates monotonously increase throughoutthe experiment, and no decrease in any peak height is observed,indicating that the presence of hydrogen during the exposure ofthe catalyst to NO or NO2 already changes the ratio of the type ofnitrates formed on the sample. An exposure of additional H2 doesnot have any further effect.

Abso

rban

ce [-

]

1700 1600 1500 1400 1300 1200

Wavenumber [cm-1]

Hei

ght o

f pea

ks

100

0.05

1611

1570

1535

1304

1228

0

49 min

a

NO

2 + O

2

NO

2 + O

2

NO

+ O

2

NO

+ O

O2 O

2

O2H2 +

O2

b

Fig. 6. Formation and stability of nitrates over Ag/Al2O3 formed from gas mixtures of NObands. The baseline level was defined at 1800 cm�1. Vertical lines indicate the times whein Ar for 1 min, 500 ppm NO and 10% O2 in Ar for 2 min, 10% O2 in Ar for 1 min, 1250 ppexposure to NO and O2 during the 3rd sequence.

Fig. 5 shows the evolution of nitrate species on an Ag/Al2O3 cat-alyst during alternating exposure of the catalyst to NO and O2 in Ar,O2 in Ar, and H2 and O2 in Ar. In contrast to the previous experi-ments, a nitrite band at 1228 cm�1 is evident in the DRIFT spectraof Fig. 5a. The nitrite band and the bidentate nitrate band(1570 cm�1) clearly increase during each exposure of NO, whilethe bands of monodentate nitrates (1535 cm�1) and bridge-boundnitrates (1611 cm�1) increase much slower. Removal of the NOfrom the feed stops the growth of the nitrite and bidentate nitrates.During the exposure of H2, nitrites and bidentate nitrates are rap-idly consumed, while mainly monodentate and some bridge-bound nitrates are formed, showing that H2 assists the conversionof nitrites and bidentate bound nitrates to monodentate andbridge-bound nitrates.

Finally, we tested whether NO can reduce nitrate species to ni-trite species in the presence of oxygen on Ag/Al2O3. This reductionof nitrates to nitrites has been proposed as part of the reactionmechanism for NOx adsorption and desorption over Cu/ZSM-5[42] and has been predicted by DFT calculations over Al2O3 [43].Fig. 6 shows the evolution of bands during exposure of the catalystto NO2 or NO and O2, subsequent flushing in O2 in Ar and exposureto H2 and O2 in Ar. Similar to the previous experiments, the amount

403020

NO

2 + O

2

NO

2 + O

22

NO

+ O

2

NO

+ O

2

O2

O2

O2

O2H2 +

O2

H2 +

O2

H2 +

O2

O2

bid. nitrates(1570)

monod. nitrates (1535)

bridge-b. nitrates (1611)

nitrites (1228)

2 and O2, and NO and O2 at 250 �C. (a) DRIFT spectra, (b) height of nitrate absorptionn spectra are taken. Step sequence: 500 ppm NO2 and 10% O2 in Ar for 2 min, 10% O2

m H2 and 10% O2 in Ar for 1 min. This sequence was repeated 4 times, with a longer

bitra

ry u

nits

]

Ar O2 i Ar H2 + O2 i Ar

S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161 159

of nitrites (1228 cm�1) and bidentate nitrates (1570 cm�1) in-creases rapidly upon exposure to NO2 and decreases upon expo-sure to H2. During short exposures of 1 min to NO and O2, allnitrate and nitrite species are stable; but after exposure of the cat-alyst to NO and O2 for 5 min, the amount of all nitrates and nitritesincreases, especially the bidentate and bridge-bound nitrates.However, no reduction of nitrates is observed.

peak

are

a [a

r

30252015105

Time [min]

Fig. 8. Evolution of the band area at 1535 cm�1 integrated between 1622 and1440 cm�1 as a function of time in different gas mixtures at 100 �C.

3.3. Stability of nitrate species on silver oxide

From the paragraphs above, we concluded that H2 influenceshow nitrates preferably are bound and promotes the oxidation ofnitrites to nitrates. Moreover, we showed that nitrates located onsilver cannot be distinguished from those adsorbed to aluminaover a 2 wt% Ag/Al2O3 catalyst in DRIFT experiments. In order toclearly detect nitrates on silver, we adsorbed NO2 on silver oxide(AgO), which has been discussed in the literature as the active sil-ver species, and tested the stability of the formed nitrates in differ-ent gas mixtures. Spectra similar to the uppermost spectrum inFig. 7 were obtained after exposing a powder sample of AgO toNO2 and O2 in Ar. The band formed at 1302 cm�1 can be assignedto nitrates or possibly nitrites in analogy to assignment on Ag/Al2O3 catalysts. The shoulder at higher wavenumbers indicatesthe existence of several differently bound nitrates. A small bandat 1755 cm�1 can be caused by adsorbed N2O4 species which havebeen reported to cause bands in that region over Ag/Al2O3 [23,28].These species are stable in inert atmosphere and in the presence ofH2 and O2 in Ar as shown by the difference spectra of Fig. 7b. In thepresence of NO (NO and O2, or NO, O2, and H2), the nitrate bandscontinue to increase, while in parallel, negative bands form at1785 cm�1 and between 1510 and 1290 cm�1. Since the back-ground spectra of AgO (not shown) show absorption bands at thesewavenumbers, the observed negative bands are probably causedby the consumption of AgO. This is reasonable, since NO needs atleast an additional oxygen atom to form nitrites and nitrates.Moreover, a double band centered at 1070 cm�1 grows, which wetentatively assign to monodentate nitrite species, since accordingto Hadjiivanov, monodentate nitrites on several different oxidescause bands around 1070 cm�1 [23]. Upon exposure to NH3, largenegative bands appear which we previously assigned to the con-sumption of AgO.

In summary, nitrate and nitrite species formed on AgO are sta-ble in inert atmosphere, in oxygen and a mixture of H2 and O2 in Ar.Exposure to NO and O2 in Ar results in increased intensity of thenitrate bands, while they disappear rapidly when NH3 is presentin the feed.

Abso

rban

ce [-

]

2000 1500 1000wavenunmber [cm-1]

1302

10701755

1785

ArO2 +

NO + O

NO +

NH3 +

NH3 + O

1620

0.2

Fig. 7. (a) Spectra obtained after exposure of AgO to a gas mixture of NO2 and O2 in Aspectrum at the end of NOx adsorption and after flushing.

3.4. Migration of nitrate species from silver to alumina

In addition to the stability of the nitrates, we tested the migra-tion of the nitrates from the silver to the Al2O3 support. For thispurpose, 20 wt% AgNO3 was mechanically mixed with c-Al2O3.When heating the sample to 100 �C in Ar, we observed tiny nitratebands grow at 1535, 1300, and 1257 cm�1. Since the backgroundcontains the information of Al2O3 and AgNO3, the appearance ofnew bands indicates migration of the nitrates from AgNO3 to thesupport. Fig. 8 shows the evolution of the band area at1535 cm�1 as a function of time. The nitrate bands increase withthe same rate independently whether the sample is exposed toAr or to 10% O2 in Ar. Adding 1250 ppm H2 to the feed results ina higher mobility of the nitrates. However, it needs to be stressedthat the amount of migrated nitrates is very small.

4. Discussion

The role of nitrates and the effect of hydrogen on nitrate speciesduring SCR over Ag/Al2O3 have been discussed for a long time inliterature. From flow reactor experiments, it is known that thepresence of hydrogen increases the oxidation of NO to NO2 [30].Moreover, higher amounts of nitrates are adsorbed on Ag/Al2O3

with NO2 than with NO in the gas phase [39]. Fig. 1 shows an in-crease in the activity for NOx reduction during the first 3 h on

2000 1500 1000

1620H2

2 + H2

O2

O2 + H2

2

1070

1295

1785

0.2

wavenunmber [cm-1]

r and subsequent exposure to different gas mixtures. (b) Difference between the

160 S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161

stream. One possible explanation for the increase of NOx conver-sion is the increase of the available nitrates on the surface, as ob-served in Fig. 2. However, the NOx conversion levels out afterabout 3 h, while the amount of nitrates continues to increase after10 h of experiment, indicating that only a certain level may be ben-eficial. In contrast, it has been proposed that nitrates poison thecatalyst, while H2 reduces the nitrates, possibly to nitrites, andthereby increases the reaction rate [6,19,20]. In Fig. 2b, it is shownthat the nitrates adsorbed on the Ag/Al2O3 catalyst after 10 h ofexperiment still do not have reached true steady-state conditions.In the gas phase, however, steady state is reached much faster indi-cating that the observed increase in surface species is due to spec-tator species. This conclusion is in accordance with Ref. [41].Moreover, we show in Figs. 3–6 that nitrites initially are formedupon adsorption of NO or NO2. These nitrites transform to nitratesduring the adsorption of NO2. When the nitrites were formed fromadsorption of NO, the transformation of nitrites to nitrates isslower and considerably accelerated by the addition of H2 asshown in Fig. 5. The same effect is observed for NO2, but to less ex-tent as shown in Fig. 4. When H2 is present during adsorption ofNO2 in the feed, less nitrites and more monodentate and bridge-bound nitrate species are observed, which previously have been re-ported to be the most stable type of nitrate species [28]. In sum-mary, the presence of hydrogen promotes the transformation ofthe less stable nitrites and bidentate nitrates to the more stablebridge-bound and monodentate nitrates. Brosius et al. found thatthe storage capacity of NOx is reduced in the presence of hydrogenand correlates this to the absence of nitrates on silver [10]. The ob-served conversion of nitrites to nitrates by the addition of hydro-gen in the present experiments may, thus, be due to migration ofnitrites on silver to nitrates on alumina, which is in accordancewith the data presented in Fig. 8. In the bottom panel of Fig. 3,the conversion of nitrites to nitrates seems like nitrate formationwithout consumption of any NOx species at low surface coverage.This phenomenon is probably caused by the fact that some nitritespecies cause bands which overlap with the absorbance of the cat-alyst material or with the nitrate band at 1250 cm�1 and are there-fore not detected. The role of the differently bound nitrate andnitrite species to the alumina in the reaction mechanism duringNH3-SCR is still not clear from the presented data.

In addition to Al2O3, silver is another crucial part of an Ag/Al2O3

catalyst. Bogdanchikova et al. [44] proposed that small clusters ofsilver oxides are the active silver phase for NOx reduction, whilemetallic silver particles are mainly active for NO oxidation toNO2. As well as on the Al2O3 support, nitrates are also formed onthe silver species and might poison the catalyst. This view is sup-ported by first-principle calculations of small silver clusters onAl2O3, which gave high adsorption energies for nitrates [45]. Inaddition, silver oxide (AgO) was found as silver species on Ag/Al2O3 catalysts [46], and Furusawa et al. [47] proposed that theactivity for NO oxidation is suppressed due to nitrates poisoningAgO species. However, the results presented in Fig. 7 show that ni-trates on AgO are removed at 100 �C by NH3, while they are stableat the same temperature in the presence of 1250 ppm H2. There-fore, it is likely that the nitrates formed on silver particles on Ag/Al2O3 will behave in the same way, indicating that nitrates areno poison during NH3-SCR but rather spectators.

Finally, in Fig. 8, it is shown that spillover of nitrates from silverto the Al2O3 support occurs and increases in the presence of H2.According to first-principle calculations, adsorption energies for ni-trates are high, especially at the interface between small silverclusters and the Al2O3 support [45]. However, even with highadsorption energies, the nitrates move on the surface as shownby ab initio molecular dynamics simulations [48]. This is in linewith the spillover of nitrates which has been proposed for NOx

storage catalysts [49,50].

5. Conclusions

The addition of small amounts of hydrogen during HC-SCR overAg/Al2O3 catalysts markedly increases the activity for NOx reduc-tion at low temperatures. For NH3-SCR over Ag/Al2O3, hydrogenis a necessary co-reductant for the reaction. Since NH3 has beenproposed as an intermediate in HC-SCR, it is likely that the roleof hydrogen is the same in both systems. From DRIFT experiments,where NOx is adsorbed on Ag/Al2O3 or c-Al2O3 samples and the for-mation and consumption of nitrate and nitrite species are fol-lowed, nitrites appear to be an intermediate in the formation ofmonodentate nitrate. Moreover, we can show that hydrogen pro-motes the conversion of nitrites and bidentate nitrate species tomainly monodentate and some bridge-bound nitrates. Althoughmonodentate nitrates are stable, they do not poison the catalystsurface, since NOx reduction remained constant during 10 h ofexperiment at 250 �C. On the contrary to the effect of hydrogenon nitrates on Al2O3, we could not observe any effect of hydrogenat 100 �C on nitrates formed on AgO. These were stable in gas com-positions containing hydrogen. However, the previously men-tioned nitrate species decompose easily in the presence of NH3 at100 �C, indicating that the silver particles of an Ag/Al2O3 catalystwill not be poisoned by nitrates. Moreover, nitrates on silver wereshown to migrate to the Al2O3 support already at 100 �C.

Acknowledgments

The work is financially supported by the Danish Council forStrategic research and was performed at the Competence Centerfor Catalysis, which is hosted by Chalmers University of Technol-ogy and financially supported by the Swedish Energy Agency andthe member companies AB Volvo, ECAPS AB, Haldor Topsøe A/S,Scania CV AB and Volvo Car Corporation AB. The collaboration withHaldor Topsøe, Amminex and DTU is gratefully acknowledged.

References

[1] K. Arve, H. Backman, F. Klingstedt, K. Eränen, D.Y. Murzin, Hydrogen as aremedy for the detrimental effect of aromatic and cyclic compounds on theHC-SCR over Ag/alumina, Appl. Catal., B: Environ. 70 (2007) 65–72.

[2] K. Shimizu, M. Tsuzuki, A. Satsuma, Effects of hydrogen and oxygenatedhydrocarbons on the activity and SO2-tolerance of Ag/Al2O3 for selectivereduction of NO, Appl. Catal., B: Environ. 71 (2007) 80–84.

[3] D.E. Doronkin, S. Fogel, S. Tamm, L. Olsson, T. Khan, T. Bligaard, P. Gabrielsson,S. Dahl, Study of the ‘‘Fast SCR’’-like mechanism of H2-assisted SCR of NOx withammonia over Ag/Al2O3, Appl. Catal., B: Environ. 113–114 (2012) 228–236.

[4] M. Richter, R. Fricke, R. Eckelt, Unusual activity enhancement of NO conversionover Ag/Al2O3 by using a mixed NH3/H2 reductant under lean conditions, Catal.Lett. 94 (2004) 115–118.

[5] K.I. Shimizu, A. Satsuma, Hydrogen assisted urea-SCR and NH3-SCR withsilver–alumina as highly active and SO2-tolerant de-NOx catalysis, Appl. Catal.,B: Environ. 77 (2007) 202–205.

[6] R. Burch, J.P. Breen, C.J. Hill, B. Krutzsch, B. Konrad, E. Jobson, L. Cider, K.Eränen, F. Klingstedt, L.E. Lindfors, Exceptional activity for NOx reduction atlow temperatures using combinations of hydrogen and higher hydrocarbonson Ag/Al2O3 catalysts, Top. Catal. 30–31 (2004) 19–25.

[7] P. Sazama, L. Capek, H. Drobna, Z. Sobalik, J. Dedecek, K. Arve, B. Wichterlová,Enhancement of decane-SCR-NOx over Ag/alumina by hydrogen. Reactionkinetics and in situ FTIR and UV–vis study, J. Catal. 232 (2005) 302–317.

[8] J. Shibata, Y. Takada, A. Shichi, S. Satokawa, A. Satsuma, T. Hattori, Ag cluster asactive species for SCR of NO by propane in the presence of hydrogen over Ag-MFI, J. Catal. 222 (2004) 368–376.

[9] J.P. Breen, R. Burch, C. Hardacre, C.J. Hill, Structural investigation of thepromotional effect of hydrogen during the selective catalytic reduction of NOxwith hydrocarbons over Ag/Al2O3 catalysts, J. Phys. Chem. B 109 (2005) 4805–4807.

[10] R. Brosius, K. Arve, M.H. Groothaert, J.A. Martens, Adsorption chemistry of NOx

on Ag/Al2O3 catalyst for selective catalytic reduction of NOx usinghydrocarbons, J. Catal. 231 (2005) 344–353.

[11] H. Kannisto, H.H. Ingelsten, M. Skoglundh, Aspects of the role of hydrogen inH2-assisted HC-SCR over Ag–Al2O3, Top. Catal. 52 (2009) 1817–1820.

[12] L. Gang, B.G. Anderson, J. van Grondelle, R.A. van Santen, Low temperatureselective oxidation of ammonia to nitrogen on silver-based catalysts, Appl.Catal., B: Environ. 40 (2003) 101–110.

S. Tamm et al. / Journal of Catalysis 307 (2013) 153–161 161

[13] J. Shibata, K. Shimizu, S. Satokawa, A. Satsuma, T. Hattori, Promotion effect ofhydrogen on surface steps in SCR of NO by propane over alumina-based silvercatalyst as examined by transient FT-IR, Phys. Chem. Chem. Phys. 5 (2003)2154–2160.

[14] X.L. Zhang, Y.B. Yu, H. He, Effect of hydrogen on reaction intermediates in theselective catalytic reduction of NOx by C3H6, Appl. Catal., B: Environ. 76 (2007)241–247.

[15] K. Eränen, F. Klingstedt, K. Arve, L.E. Lindfors, D.Y. Murzin, On the mechanismof the selective catalytic reduction of NO with higher hydrocarbons over asilver/alumina catalyst, J. Catal. 227 (2004) 328–343.

[16] K. Eränen, L.E. Lindfors, F. Klingstedt, D.Y. Murzin, Continuous reduction of NOwith octane over a silver/alumina catalyst in oxygen-rich exhaust gases:combined heterogeneous and surface-mediated homogeneous reactions, J.Catal. 219 (2003) 25–40.

[17] K. Shimizu, A. Satsuma, Reaction mechanism of H2-promoted selectivecatalytic reduction of NO with NH3 over Ag/Al2O3, J. Phys. Chem. C 111(2007) 2259–2264.

[18] F.C. Meunier, J.P. Breen, V. Zuzaniuk, M. Olsson, J.R.H. Ross, Mechanisticaspects of the selective reduction of NO by propene over alumina and silver–alumina catalysts, J. Catal. 187 (1999) 493–505.

[19] D. Creaser, H. Kannisto, J. Sjöblom, H.H. Ingelsten, Kinetic modeling of selectivecatalytic reduction of NOx with octane over Ag–Al2O3, Appl. Catal., B: Environ.90 (2009) 18–28.

[20] K. Shimizu, J. Shibata, A. Satsuma, Kinetic and in situ infrared studies on SCR ofNO with propane by silver/alumina catalyst: role of H2 on O2 activation andretardation of nitrate poisoning, J. Catal. 239 (2006) 402–409.

[21] Y. Guo, J. Chen, H. Kameyama, Promoted activity of the selective catalyticreduction of NOx with propene by H2 addition over a metal-monolithic anodicalumina-supported Ag catalyst, Appl. Catal., A: Gen. 397 (2011) 163–170.

[22] H. Kannisto, H.H. Ingelsten, M. Skoglundh, Ag–Al2O3 catalysts for lean NOx

reduction-Influence of preparation method and reductant, J. Mol. Catal. A 302(2009) 86–96.

[23] K.I. Hadjiivanov, Identification of neutral and charged NxOy surface species byIR spectroscopy, Catal. Rev. – Sci. Eng. 42 (2000) 71–144.

[24] B. Wichterlová, P. Sazama, J.P. Breen, R. Burch, C.J. Hill, L. Capek, Z. Sobalík, Anin situ UV–vis and FTIR spectroscopy study of the effect of H2 and CO duringthe selective catalytic reduction of nitrogen oxides over a silver aluminacatalyst, J. Catal. 235 (2005) 195–200.

[25] M. Richter, U. Bentrup, R. Eckelt, M. Schneider, M.M. Pohl, R. Fricke, The effectof hydrogen on the selective catalytic reduction of NO in excess oxygen overAg/Al2O3, Appl. Catal., B: Environ. 51 (2004) 261–274.

[26] A. Iglesias-Juez, A.B. Hungria, A. Martinez-Arias, A. Fuerte, M. Fernandez-Garcia, J.A. Anderson, J.C. Conesa, J. Soria, Nature and catalytic role of activesilver species in the lean NOx reduction with C3H6 in the presence of water, J.Catal. 217 (2003) 310–323.

[27] H. He, Y.B. Yu, Selective catalytic reduction of NOx over Ag/Al2O3 catalyst: fromreaction mechanism to diesel engine test, Catal. Today 100 (2005) 37–47.

[28] S. Kameoka, Y. Ukisu, T. Miyadera, Selective catalytic reduction of NOx withCH3OH, C2H5OH and C3H6 in the presence of O2 over Ag/Al2O3 catalyst: role ofsurface nitrate species, Phys. Chem. Chem. Phys. 2 (2000) 367–372.

[29] A. Satsuma, K. Shimizu, In situ FT/IR study of selective catalytic reduction ofNO over alumina-based catalysts, Prog. Energy Combust. Sci. 29 (2003) 71–84.

[30] S. Tamm, S. Fogel, P. Gabrielsson, M. Skoglundh, L. Olsson, The effect of the gascomposition on hydrogen assisted NH3-SCR over Ag/Al2O3, Appl. Catal., B:Environ. 136–137 (2013) 168–176.

[31] J.B. Peri, Infrared study of adsorption of ammonia on dry c-alumina, J. Phys.Chem. 69 (1965) 231–239.

[32] A.A. Tsyganenko, D.V. Pozdnyakov, V.N. Filimonov, Infrared study of surfacespecies arising from ammonia adsorption on oxide surfaces, J. Mol. Struct. 29(1975) 299–318.

[33] Z.M. Wang, M. Yamaguchi, I. Goto, M. Kumagai, Characterization of Ag/Al2O3

de-NOx catalysts by probing surface acidity and basicity of the supportingsubstrate, Phys. Chem. Chem. Phys. 2 (2000) 3007–3015.

[34] L. Zhang, H. He, Mechanism of selective catalytic oxidation of ammonia tonitrogen over Ag/Al2O3, J. Catal. 268 (2009) 18–25.

[35] R.B. Jin, Y. Liu, Z.B. Wu, H.Q. Wang, T.T. Gu, Low-temperature selective catalyticreduction of NO with NH3 over Mn–Ce oxides supported on TiO2 and Al2O3: acomparative study, Chemosphere 78 (2010) 1160–1166.

[36] G. Busca, H. Saussey, O. Saur, J.C. Lavalley, V. Lorenzelli, FTIR characterizationof the surface-acidity of different titanium-dioxide anatase preparations, Appl.Catal. 14 (1985) 245–260.

[37] A.A. Davydov, Infrared Spectroscopy of Adsorbed Species on the Surface ofTransition Metal Oxides, John Wiley & Sons, Chichester, New York, Bisbane,Toronto, Singapore, 1984.

[38] S. Tamm, H.H. Ingelsten, A.E.C. Palmqvist, On the different roles of isocyanateand cyanide species in propene-SCR over silver/alumina, J. Catal. 255 (2008)304–312.

[39] S. Tamm, H.H. Ingelsten, M. Skoglundh, A.E.C. Palmqvist, Mechanistic aspectsof the selective catalytic reduction of NOx by dimethyl ether and methanolover c-Al2O3, J. Catal. 276 (2010) 402–411.

[40] X. She, M. Flytzani-Stephanopoulos, The role of AgOAl species in silver–alumina catalysts for the selective catalytic reduction of NOx with methane, J.Catal. 237 (2006) 79–93.

[41] S. Chansai, R. Burch, C. Hardacre, J. Breen, F. Meunier, The use of short time-on-stream in situ spectroscopic transient kinetic isotope techniques to investigatethe mechanism of hydrocarbon selective catalytic reduction (HC-SCR) of NOx

at low temperatures, J. Catal. 281 (2011) 98–105.[42] L. Olsson, H. Sjövall, R.J. Blint, Detailed kinetic modeling of NOx adsorption and

NO oxidation over Cu-ZSM-5, Appl. Catal., B: Environ. 87 (2009) 200–210.[43] H.H. Ingelsten, A. Hellman, H. Kannisto, H. Grönbeck, Experimental and

theoretical characterization of NOx species on Ag/a-Al2O3, J. Mol. Catal. A:Chem. 314 (2009) 102–109.

[44] N. Bogdanchikova, F.C. Meunier, M. Avalos-Borja, J.P. Breen, A. Pestryakov, Onthe nature of the silver phases of Ag/Al2O3 catalysts for reactions involvingnitric oxide, Appl. Catal., B: Environ. 36 (2002) 287–297.

[45] A. Hellman, H. Grönbeck, First-principles studies of NOx chemistry on Agn/a-Al2O3, J. Phys. Chem. C 113 (2009) 3674–3682.

[46] K. Arve, K. Svennerberg, F. Klingstedt, K. Eränen, L. Wallenberg, J. Bovin, L.Capek, D. Murzin, Structure–activity relationship in HC-SCR of NOx, by TEM,O2-chemisorption, and EDXS study of Ag/Al2O3, J. Phys. Chem. B 110 (2006)420–427.

[47] T. Furusawa, L. Lefferts, K. Seshan, K. Aika, Comparison of Ag/Al2O3 and Ag-ZSM5 catalysts for the selective reduction of NO with propylene in thepresence of oxygen, Appl. Catal., B: Environ. 42 (2003) 25–34.

[48] P. Broqvist, I. Panas, H. Gronbeck, The nature of NOx species on BaO(100): anab initio molecular dynamics study, J. Phys. Chem. B 109 (2005) 15410–15416.

[49] A. Lindholm, N.W. Currier, J.H. Li, A. Yezerets, L. Olsson, Detailed kineticmodeling of NOx storage and reduction with hydrogen as the reducing agentand in the presence of CO2 and H2O over a Pt/Ba/Al catalyst, J. Catal. 258 (2008)273–288.

[50] L. Olsson, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, Kinetic study ofNO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/Al2O3, J. Phys. Chem. B105 (2001) 6895–6906.