Mechanism of nitrite hydrogenation over Pd/γ-Al2O3 according a rigorous kinetic study

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Mechanism of nitrite hydrogenation over Pd/

c

-Al

2 O

3 according a

rigorous kinetic study

Pengyu Xu

a

_{, Shilpa Agarwal}

b

_{, Leon Lefferts}

a a

Catalytic Processes and Materials Group, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, PO Box 217, 7500 AE Enschede, the Netherlands

b

Infineum UK Ltd, Milton Hill Business & Technology Centre, PO Box 1, Abingdon, Oxfordshire OX13 6BB, UK

a r t i c l e i n f o

Article history:

Received 26 September 2019 Revised 4 December 2019 Accepted 3 January 2020 Available online 1 February 2020 Keywords:

Kinetic study Pd/c-Al2O3

Mass transfer limitation Nitrite hydrogenation Mechanism

a b s t r a c t

The kinetics of nitrite hydrogenation over a Pd/c-Al2O3catalyst was studied in a semi-batch slurry reactor

at atmospheric pressure, in absence of any mass transfer effects. The hydrogen concentration and pH were kept constant during an experiment by continuously flowing a gas mixture containing hydrogen and 10% v/v CO2. The kinetic experiments were performed in an unprecedented wide concentration

win-dow of nitrite and hydrogen, revealing extreme variation in the apparent orders in hydrogen and nitrite, including reaction orders in hydrogen between 2 and 0.3, whereas the order in nitrite varied between 0.4 and0.9. The rate of reaction is almost exclusively determined by the rate of formation of N2as the

selec-tivity to ammonia is very low. A Langmuir-Hinshelwood mechanism with competitive adsorption is in operation. Several mechanistic pathways, as well as possible rate determining steps in those pathways, are discussed based on these observations in combination with prior knowledge on the mechanism in lit-erature, resulting in a revised mechanistic scheme. It is concluded that formation of NH via dissociative hydrogenation of HNOH is the rate determining step, whereas molecular N2forms via reaction of NH with

either NO, NOH or HNOH. N-N bond formation via dimerization of adsorbed NO or adsorbed N can be excluded.

Ó 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Nitrate pollution in water is becoming a severe problem all over the world caused by emissions from agricultural and industrial activities[1], threatening human health, including blue baby syn-drome, high blood pressure, diabetes, liver damage, and various cancers [2–5]. The World Health Organization (WHO) the maxi-mum allowable levels of nitrate and nitrite concentration in drink-ing water are 50 mg/L as nitrate ion for nitrate, 3 mg/L as nitrite ion for nitrite, and 1.5 mg/L for ammonia, respectively[6]. Various pro-cesses have been developed to remove nitrate from water, includ-ing ion exchange, reverse osmosis, electro dialysis, photocatalytic reduction, catalytic reduction, and biological methods[2,4,7–12]. Among these techniques, catalytic reduction of nitrate with reduc-ing agents is attractive because it converts nitrate to harmless nitrogen gas in the absence of any nutrients and without producing a highly concentrated brine[2,4,10,12–17].

Since the first successful demonstration of catalytic reduction of nitrate by Vorlop and Tacke [3], extensive research[18–30]has been performed, mostly using hydrogen gas as the reducing agent.

It is well known that hydrogenation of nitrate proceeds in two steps. First, nitrate is reduced to nitrite, requiring a non-noble pro-motor such as e.g. Cu, which is generally rate determining. Further conversion of nitrite is much faster and determines the selectivity according to the following reaction equations:

2NO2þ 3H2þ 2Hþ! Catalyst N2þ 4H2O ð1Þ NO2þ 3H2þ 2Hþ! Catalyst NHþ4þ 2H2O ð2Þ

Catalytic hydrogenation of nitrite in water has been studied using different noble metal catalysts and different support materi-als[3,10,31–38]. Among them, Hörold et al. [3] tested different active metal catalyst including Pd, Pt, Ir, Ru and Rh, reporting that Pd based catalyst have good activity and the lowest selectivity to NH4+, which has been confirmed in several other studies[12,39–

46]. Maximizing the selectivity to N2 is the key issue because NH4+is at least equally undesired in drinking water with a maximal acceptable concentration of 1.5 mg/L[6].

Further suppressing of ammonia formation requires good knowledge of reaction kinetics and the catalytic mechanism. Few studies on kinetic and mechanism have been published. Wärn et al.[47]presented detailed kinetic data on nitrate hydrogenation

https://doi.org/10.1016/j.jcat.2020.01.003

0021-9517/Ó 2020 The Authors. Published by Elsevier Inc.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). E-mail address:l.lefferts@utwente.nl(L. Lefferts)

Contents lists available atScienceDirect

Journal of Catalysis

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over Pd-Cu/

c

-Al2O3in a monolith reactor and proposed a mecha-nistic reaction pathway. In this mechanism, adsorbed NO (NOads) is proposed as a key intermediate species in the conversion of nitrite to N2and NH3. NOadsis also proposed as the key intermedi-ate species in nitrite hydrogenation[19,48–51]. Ebbesen et al.[49]

were the first to actually observe NOadsas the intermediate species during nitrite hydrogenation based on ATR-IR spectroscopy. On the other hand, NOadscan also dissociate into Nadsand Oadsatom, as observed by Zhao et al. work [51]. To summarize, the pathway from NOadsto N2and NH4+is still unclear and under debate. Most hypotheses in literature rely on knowledge based on NO hydro-genation on Pd at relatively high temperature and ultra-high-vacuum conditions[13,19,48,52,53]. Clearly, these conditions are very different compared to operation in aqueous solution and the mechanism in operation might be quite different.

The first study on intrinsic kinetics of nitrite hydrogenation by Pintar et al. [11]reported an overall rate expression based on a Langmuir-Hinshelwood mechanism. Table 1shows an overview of the results in later studies on the kinetics of nitrite hydrogena-tion. The concentration range used in these studies is relatively narrow; for nitrite between 0.1 and 1 mM and hydrogen pressure between 0.1 and 1 bar [11,12,48,54–56]. The apparent reaction order in hydrogen and nitrite varies in the range between 0 and 0.5 and 0 and 1, respectively.

We observed an apparent negative reaction order in hydrogen in previous work by Postma et al.[57]and Espinosa et al.[54]using membrane reactors, which was rationalized based on extreme low concentration ratio of the nitrite/hydrogen achieved in the mem-brane reactor. These results were qualitative though because of the complex concentration gradients in these studies. To confirm this quantitatively, intrinsic kinetic experiments in a broad win-dow of nitrite and hydrogen concentrations are required.

The goal of this work is to determine intrinsic kinetics in an extremely wide window of concentrations, in order to test the hypothesis that reaction orders can become negative. The kinetic data are obtained in an isothermal semi-batch slurry reactor. The consequence of this rigorous kinetic data for hypotheses on the reaction mechanism will be discussed as well.

2. Experimental section 2.1. Materials

Commercial

c

-Al2O3 powder used as catalyst support in this study was obtained from BASF. Palladium precursor tetra-ammine-palladium (II) nitrate solution (10 wt% in H2O, 99.99%), sodium nitrite (99.99%) and ammonium (50% v/v water) were pur-chased from Sigma-Aldrich. All the aqueous solutions were pre-pared using ultra purified water obtained from a water purification system (Millipore, Synergy).

2.2. Catalyst preparation

The 1 wt% Pd/

c

-Al2O3catalyst was prepared by wet impregna-tion method. Typically 10 g of the sieved alumina support (particle

smaller than 20

l

m) was calcined at 600°C for 4 h to remove any organic contaminants. Then the calcined support was suspended in 100 mL millQ water. The pH of the solution was adjusted by adding 2 mL ammonia solution to maintain the pH around 9, in order to ensure electrostatic interaction of Pd(NH3)42+with the negatively charged alumina surface. Subsequently, 3 g of the palladium pre-cursor solution (10 wt%) was slowly added in the suspended solu-tion. The final solution was stirred at room temperature for at least 1 h and then transferred to a rotary evaporator to remove water. Finally the catalyst was calcined in air (flow rate 30 mL/min) at 400°C for 3 h (heating rate 5 °C/min), and subsequently reduced in 50% H2diluted in N2(total flow rate 60 mL/min) at the same temperature for 3 h.

2.3. Catalyst characterization

The BET surface area of the prepared catalyst was determined with N2physisorption at 77 K (Micromeritics Tristar). For BET anal-ysis, all the samples were degassed in vacuum at 300°C for 24 h. Pd loading on the alumina support was determined with X-ray fluo-rescence spectroscopy (XRF, Philips PW 1480). Pd particle size was determined using TEM (FEI Tecnai F30), measuring at least 300 particles at ten different spots in the sample. CO chemisorption at room temperature was used to determine the accessible metal surface area in gas phase (Chemisorb 2750, Micromeritics). Typi-cally, the sample was reduced at room temperature in hydrogen for 1 h and then flushed with He at the same temperature for 0.5 h. Then CO was introduced as pulses and the responses were recorded using a TCD detector. We assumed that the stoichiomet-ric ratio of number of adsorbed CO molecules and number of acces-sible Pd surface atoms is one.

2.4. Catalytic tests

Activity and selectivity of the catalysts were measured in a 1 L batch reactor at 20°C, atmospheric pressure and a pH value of 5.5 maintained by buffering continuously with CO2(0.1 bar). The glass reactor (DURANÒBAFFLED, WIDE MOUTH BOTTLE GLS 80Ò) with diameter of 10.1 cm and height 22.2 cm is used for the catalytic testing (Fig. S1). The reactor has four connections on the reactor lid for gas-in, gas-out, sampling and stirring shaft equipped with 4 stirring blades.

Typically for a standard experiment, 0.05 g catalyst was sus-pended in 0.3 L millQ water and stirred at 700 rpm under 0.8 bar hydrogen (0.1 bar CO2, 0.1 bar He) for at least 1 h, removing dis-solved oxygen and reducing the catalyst. After that, the hydrogen pressure is changed to the value of choice. Reaction is started on introduction of 3 mL NaNO2solution (100 mmol/L) in the glass reactor. Hydrogen pressure was varied between 0.01 and 0.8 bar, and the nitrite concentration was varied between 0.3 and 10 mmol/L. Experiments with higher catalyst load were performed under the same reaction conditions to check the absence of mass transfer limitations.

During the catalytic test, samples were collected using a 2.5 mL syringe (BD Plastipak) and filtered using a syringe filter (PTFE,

Table 1

Kinetics of nitrite hydrogenation at room temperature reported in literature; papers labelled a report qualitative apparent data.

Catalyst Reactor Hydrogen pressure (bar) Nitrite concentration (mM) Hydrogen order Nitrite order pH

Pd/c-Al2O3[11] Slurry 0.11–1 0.11–0.65 0–0.5 0–1 4.7

Pd/ACC[55] Slurry 1.8–6.4 1.63 0 1 4.5–8

Pd_Cu/AC[48] Tubular 0.1–1 0.65–1.08 0.3 1 5.4

Pd/AC[56] Fixed bed 0.3–0.7 0.27–0.45 0.4 0.7 4.5–9

Pd/Al2O3[12]a Membrane 0.01–1 0.24–2.4 0 N/A 7

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0.2

l

m, Whatman) to remove catalyst particles. Nitrite and ammo-nium concentrations were measured using ion-chromatography (DIONEX, ICS 3000) equipped with an UltiMate autosampler. Nitrite conversion and ammonium selectivity were calculated according to Eqs.(3) and (4), respectively. Since it is well known that ammonia and nitrogen are the only products formed during hydrogenation of nitrite reaction[47–50,54], nitrogen was calcu-lated based on the mass balance.

NO2 con

v

ersiont1¼ ½NO 2t0 ½NO 2t1 ½NO 2t0 100 ð3Þ NHþ4 selecti

v

ityt1¼ ½NHþ 4t1 ½NO 2t0 ½NO2t1 100 ð4Þ where½NO

2t0is the initial nitrite concentration,½NO2t1is the con-centration of nitrite at t1,½NHþ

4t1is the concentration of ammonium at t1.

The initial activity is reported as a TOF in mole nitrite per mole surface Pd per minute.

3. Results

3.1. Characterization of the catalyst

Table 2summarizes the characterization results of the prepared catalysts, showing that the metal loading is close to the targeted 1 wt%. The BET surface area of the support and Pd/

c

-Al2O3catalyst are equal, indicating the structure and porosity of the support remain unchanged after metal loading. The Pd dispersion is 58% according CO chemisorption, equivalent to a averaged particle size of 2 nm, assuming Pd is hemisphere shape and the size of uniform. This is in good agreement with estimated Pd particle size based on TEM. Typical HRTEM images of the catalyst are shown in the sup-plementary information(Fig. S2). The particle size distribution of

the Pd particles is shown inFig. S3, showing that, although the majority of the particles is sized 2 nm, also some larger Pd particles are present.

3.2. Nitrite hydrogenation reaction

Fig. 1shows a typical experimental result, showing the change in concentration of nitrite and ammonium with time during nitrite hydrogenation. The initial rate is calculated from the slope in the nitrite concentration profile, considering exclusively data at con-version lower than 10%, as shown in the inset ofFig. 1a.Fig. 1b shows that the ammonium concentration increases gradually with time, which is usually assigned to the decreasing nitrite/hydrogen ratio[3,51,56,58]. Similar experiments were performed in a wide window of reactant concentrations as presented inTable 3, includ-ing variation of the nitrite concentration between 0.3 and 10 mmol/L and H2 pressure between 0.01 and 0.8 bar. Conse-quently, the H2concentration in water varied between 0.01 and 0.6 mmol/L, calculated based on the Henry coefficient (1282.05 L * atm/mol at 25°C)[59]. It should be noted that determining kinetics based on batch experiments is possible only when assum-ing that both deactivation as well as dynamic changes in the cata-lyst structure during the experiment do not influence the performance. The catalyst is stable in continuous steady-state experiments for days (not shown) and any deactivation would not influence the results anyhow because we asses exclusively ini-tial activity. This assumption that any dynamic changes during the initial stage of the batch experiment do not influence the results, cannot be tested with steady state experiments and is usually implicitly made in kinetic studies with batch experiments, which is generally accepted.

Table 2

Characterization results of the Pd/c-Al2O3catalyst.

Catalyst particle size,lm <20 BET surface area, m2

/g 195

XRF metal loading, wt% 0.9

Pd dispersion, CO-chemisorption, % 58 Pd particle size, CO-chemisorption, nm 2 Mean Pd particle size based on TEM, nm 2.2

Fig. 1. (a) nitrite concentration as a function of time obtained using slurry reactor with 10 mM initial nitrite concentration and 0.8 bar hydrogen pressure, with a zoomed-in initial points that are used to obtain initial rate, (b) ammonium concentration as a function of time.

Table 3

Range of operating conditions of the nitrite hydrogenation in a slurry reactor.

Reaction temperature,°C 20

Reaction volume, L 0.3

pH of the solution 5.5

Stirring speed, rpm 700

Average catalyst particles size,lm <20

Amount of catalyst, g 0.05

Total gas flow rate, mL/min 100 Total operating pressure, bar 1 Carbon dioxide partial pressure, bar 0.1 Hydrogen partial pressure, bar 0.01–0.8 Helium partial pressure (balance), bar 0.1–0.89 Initial nitrite concentration, mmol/L 0.3–10

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3.3. The effect of the hydrogen pressure and initial nitrite concentration

Fig. 2a shows that the initial activity varies with hydrogen pres-sure in a similar manner for both 1 mM and 10 mM nitrite concen-trations, except that higher rates are observed with low nitrite concentration (1 mM) in combination with low hydrogen pressure. The selectivity to ammonium increases with decreasing nitrite/hy-drogen ratio, in agreement with literature[3,51]. However,Fig. 2b shows that the initial selectivity to ammonium remains constant with increasing hydrogen pressure for both 1 and 10 mM nitrite concentrations.

Fig. 3a presents the effect of the initial nitrite concentration on the catalytic activity. Surprisingly, this effect is strongly influenced by the hydrogen pressure. At 0.8 bar hydrogen pressure, the activ-ity first increases with nitrite concentration and then stabilizes. In contrast, at 0.05 bar hydrogen pressure, the activity decreases sig-nificantly with increasing nitrite concentration. Fig. 3b presents the trend in the selectivity to ammonium varying the nitrite con-centration, resulting in very similar trends at different hydrogen pressures. Selectivity to ammonium decreases with increasing nitrite concentration. Clearly, the selectivity to ammonium is much

more strongly affected by the nitrite concentration (Fig. 3b) than by hydrogen pressure (Fig. 2b). Experimental data on ammonia selectivity have a significant error margin, especially in the case of low nitrite concentration. This is caused by the fact that the ammonia concentration in the initial part of the experiment are so low that ammonia analysis is possible only with significant experimental scatter.

3.4. Mass transfer

In order to study intrinsic kinetics of nitrite hydrogenation, absence of any mass transfer limitation must be ensured. We per-formed several experiments to rule out both internal and external mass transfer limitations.

3.4.1. Internal mass transfer

The catalyst particle size has no effect on the initial rate when the particles are 45

l

m or smaller, as shown in Table S1 in

supplementary information(Section 3.1), whereas larger catalyst particles clearly show evidence that internal mass transfer is limit-ing because the activity decreases with increaslimit-ing particle size. Therefore, the data presented are not influenced by internal mass

Fig. 2. (a) Initial activity and (b) initial selectivity to ammonium as a function of hydrogen pressure for 10 (red curve) and 1 mM (black) nitrite concentration.

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transport limitations, as all experiments were performed with cat-alyst with particles of averaged 20

l

m (<38

l

m). This is in good agreement with values of the Weisz-Prater criterion (Cwp) smaller than 1 (Table S2), for all nitrite and hydrogen concentrations applied. The calculation are presented in thesupporting informa-tion, Section 3.1.

3.4.2. External mass transfer

Increasing the amount of catalyst does not influence the activity per gram catalyst (Table S3, Section 3.2), demonstrating experi-mentally that transport at the gas-liquid interface (G-L) is not lim-iting. In addition, the rate-constant for transport at the external surface of the catalyst (L-S) (kls as) is estimated to be 3.3 * 103 s1, one order of magnitude larger than the maximum observed rate constant (kobs = 1.92 * 104s1, seesupporting information for details in Section 3.2). Also Mears criterion shows that external transport is not limiting. In short, we can exclude mass transfer limitation in and around the catalyst particles.

4. Discussion

The activity as well as the trends with concentrations of the reactants vary significantly in the broad window of hydrogen and nitrite concentrations, as can be seen inFigs. 2a and 3a. The first part of the discussion will provide the resulting reaction orders. The influence of concentration on reaction orders will then be dis-cussed qualitatively in terms of a Langmuir-Hinshelwood mecha-nism, as full modelling of the relatively complex micro-kinetic scheme is impossible with the information available. The reaction rate is strongly dominated by the rate of formation of N2as the selectivity to ammonia is typically a few percent. After discussion of reaction rates, the reaction schemes will be elaborated based on the selectivity data.

4.1. Apparent reaction orders

Fig. 4 shows the reaction orders in nitrite and hydrogen as obtained from log-log plots of activity and reactant concentrations. The slopes in the plot indicate the reaction order in hydrogen (Fig. 4a) and nitrite (Fig. 4b). As shown in Fig. 4a, the order in hydrogen is about 2 at low hydrogen pressure and almost indepen-dent of the nitrite concentration. Increasing the hydrogen pressure causes the order in hydrogen to decrease significantly to around 0.3 and 0.4 for nitrite concentration of 1 mM and 10 mM, respectively.

As can be seen in Fig. 4b (black line), the order in nitrite is always negative at 0.05 bar hydrogen pressure, independent of the nitrite concentration. In contrast, at high hydrogen pressure (0.8 bar) the order in nitrite varies between 0.5 at low nitrite con-centration (below 1 mM) and 0 at higher nitrite concon-centration (above 1 mM).

Table 4summarizes the observed apparent reaction orders in nitrite and hydrogen. The results obtained at high hydrogen pres-sure and low nitrite concentration (0.3 order in H2 and 0.4 in nitrite) are in good agreement with literature (Table 1). The high reaction order of 2 in hydrogen as well as the negative order in nitrite at low hydrogen pressures (0.05 bar) have never reported before to the best of our knowledge, which can be understood based on the fact that the window of concentrations in this study is much broader than in previous studies.

However, recent work from our group with membrane contact reactors[54,57] provided qualitative proof for negative reaction order in hydrogen, which was tentatively explained with zones in the contact membrane operating at extreme low nitrite concen-tration. Negative orders in hydrogen are not observed in this work (Table 2), which seems reasonable when considering that the nitrite concentration in the experiments with the membrane

Fig. 4. (a) Effect of hydrogen pressure on reaction rate for 1 and 10 mM nitrite concentration, (b) effect of nitrite concentration on reaction rate for 0.05 and 0.8 bar hydrogen pressure.

Table 4

Overview of the apparent reaction orders in nitrite and hydrogen information in all ranges of the nitrite and hydrogen concentrations.

Low hydrogen pressure High hydrogen pressure

Hydrogen order Nitrite order Hydrogen order Nitrite order

Low nitrite concentration 1.9 ± 0.1 0.9 ± 0.1 0.3 ± 0.1 0.4 ± 0.1

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contact reactor could be decreased to much lower values (0.044 mM) compared to batch experiments (0.3 mM) without compromising accuracy of the experiment. The concentrations gra-dients in the membrane contactor discussed above induce even lower concentrations.

Summarizing, negative reaction orders in nitrite and hydrogen are observed, here and in previous work, only when the windows of concentrations are sufficiently broad, pointing to competitive adsorption in a Langmuir-Hinshelwood mechanism. We will dis-cuss the mechanism further below.

4.2. Reaction mechanism

Several reaction mechanisms have been proposed for the reduc-tion of nitrite in aqueous solureduc-tions using Pd based catalysts

[50,51,60–64]. There is general consensus about three elementary steps, i.e. nitrite adsorption, dissociative adsorption of hydrogen and conversion of nitrite to adsorbed NOads. Furthermore, dissoci-ation of NOadsto Nadsis often assumed[47,48,50,65], implying that NOadsand Nadsare key intermediate species that are further con-verted to N2and NH4+, via pathways that are under debate.

To date three reaction pathways have been proposed for the conversion of adsorbed NOads.

I. NOadsreduction to NHadsspecies[19,47,66] II. NOadsreduction to NOHadsspecies[62]

III. NOadscoupling without Hads, e.g. NOadsdimerization (as also proposed in electrochemical studies[67–70]) or coupling of Nadsand NOadsto N2Oads[48–50,65];

We will now discuss the fit of these models to the experimental observations, assuming in all three cases:

(1) Fast and equilibrated adsorption of H2and NO2;

(2) Langmuir-Hinshelwood mechanism with one type of active site, implying competitive adsorption. This is obviously nec-essary to account for negative reaction orders. The fact that the orders change strongly in still a relatively narrow

win-dow of conditions is probably caused by competition of mul-tiple species at the Pd surface;

(3) All reactions after the rate determining step (RDS) are fast and the equilibrium is at the side of desorbed products, resulting in low surface coverages of the species involved on Pd.

Please note that we do not discuss separate pathways to N2and NH4+at this stage because the reaction rate is almost exclusively determined by the formation of N2, as explained above. The inter-pretation is qualitative because full micro-kinetic modelling for this complex reaction with the data available is simply not possi-ble. Therefore, models I, II and III will be simplified to schemes with the same type of intermediates but limited to the proposed RDS and the accompanying pre-equilibria. The fact that the reaction order in hydrogen is 2 at low hydrogen pressure (Table 1) implies that Hadsis not only involved in the RDS, but must also be involved in three pre-equilibria elementary steps, determining the

concentration of Hadsin the RDS. According to this principle, possi-ble RDSs are selected.

4.2.1. Model I: NOadsreduction to NHads

This scheme, proposed by Wärn et al.[47], suggests that NOads dissociates first to atomic Nadsbefore the N-H bond is formed. The proposition was rejected in several studies[48–50,65]based on the fact that NHads species has never been detected. Unfortunately, these arguments are based on NOadsconversion on Pd at relatively high temperature and ultra-vacuum conditions. So we cannot exclude this scheme for reaction in aqueous phase (seeScheme 1). The overall rate equation is derived based on the elementary steps as shown below. The reaction (6) is selected as the RDS according to the argument described above: four adsorbed hydro-gen atoms need to be involved, respectively in three pre-equilibria and in the RDS. All steps after the RDS are not relevant because of assumption 3.

H2þ 2 2H ðequilibrium reaction 1; K1Þ

NO2þ NO2 ðequilibrium reaction 2; K2Þ

NO2 þ Hþ HþNOþ H2Oþ ðequilibrium reaction 3; K3Þ

NOþ N_{þ O} _{ðequilibrium reaction 4; K} 4Þ Oþ 2H_{3 þH} 2O ðequilibrium reaction 5; K5Þ N_{þ H}_{! NH}_þ _{reaction 6; k} 6 ð Þ RDS

where * represents an empty site on the Pd surface and O* repre-sents an oxygen atom adsorbed on the Pd surface, as an example for all surface species.

It should be noted that reaction(5)can also be replaced by step-wise hydrogenation of Oadsvia OHadsto H2O, resulting in the same mathematical description as all these steps are in equilibrium. The details and derivation of the rate expression are shown in support-ing informationSection 4.1, leading to the rate Eq.(5).

In case the hydrogen pressure is low, the equation simplifies to Eq.(6):

Rate¼K12K2K3K4K5k6½Hþ½NO2 H½ 2 2

ð1 þ K2½NO2Þ

2 ð6Þ

Clearly, the reaction order in hydrogen is 2 at low hydrogen pressure, whereas the reaction order in nitrite can vary between 1 and 1, agreeing well with the experimental results (Table 4). It should be noted that the RDS cannot be a reaction before reac-tion(6), as it would lead to a lower maximal order in hydrogen. Equilibrium step 4 enhances the concentration of Nadsvia NOads, whereas equilibrium 5 prevents high coverage with Oads. In case of high hydrogen pressure, the reaction orders in hydrogen can vary between 1 and 2, whereas the order in nitrite may vary between1 and 1, which is also in agreement with experimental results (details insupporting informationSection 4.1). In conclu-sion, model I is one candidate for the reaction mechanism. Rate¼ k6hHhN¼ K12K2K3K4K5k6Hþ ½NO 2½ H22 ð1 þ K10:5½ H2 0:5_{þ K} 2½NO2 þ K10:5K2K3½Hþ½NO2 H½ 2 0:5_{þ K} 11:5K2K3K4K5½Hþ½NO2 H½ 2 1:5_Þ2 ð5Þ

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4.2.2. Model II: NOadsreduction to NOHads

The second option is that NOadsreacts with Hadsto form NOHads, without first dissociation to atomic Nads. This type of mechanism was proposed based on DFT calculations by Shin et al.[62]. We consider three possible pathways meeting the condition that three hydrogen atoms are involved in the pre-equilibria steps and one hydrogen atom is involved in the RDS. The essential difference in

these schemes is the level of hydrogenation of the NOHx,ads under-going cleavage of the N_{AO bond, i.e. NOH}ads, HNOHads and H2NOHads: Model IIa SeeScheme 2a. Model IIb SeeScheme 2b Model IIc SeeScheme 2c

Here we illustrate the evaluation of the kinetic scheme for the first pathway (Model IIa) as an example, whereas the same detailed description for Models IIb and IIc are shown insupporting informa-tionSection 4.2.

For model IIa the elementary steps are:

H2þ 2H ðequilibrium reaction 1; K1Þ

NOþ H_NOH_þ _{equilibrium reaction 4; K}

4 ð Þ NOHþ H_N_{þ H} 2Oþ ðequilibrium reaction 5; K5Þ N_{þ H}_{! NH}_þ _{reaction 6; k} 6 ð ÞRDS

As in the previous section, it is assumed that the formation of NHadsis the RDS as otherwise second order in hydrogen cannot be obtained. The resulting kinetic equation is derived in the sup-porting informationSection 4.2:

At low hydrogen pressure, the equation simplifies to: Rate¼K12K2K3K4K5k6½Hþ½NO2 H½ 2

2

ð1 þ K2½NO2Þ

2 ð8Þ

This confirms that also in this case the reaction order in hydro-gen is 2, whereas the reaction orders in nitrite may vary between 1 and 1, agreeing well with the experimental results at low hydrogen pressure (Table 4). The resulting reaction orders in hydrogen and in nitrite are also in good agreement with our exper-imental results at high hydrogen pressure (details insupporting informationSection 4.2).

Summarizing, Model I and the three variations on model II agree with the kinetic data and cannot be rejected based on that. The difference between these models in the pathway for NO dissociation. In model I NOads dissociates directly, whereas NOads dissociation is H-assisted via NOHads, HNOHads or H2 -NOHads in respectively model IIa, IIb and IIc. Therefore, the intermediate surface species available for N-N bond formation in Model I are NOads and Nads. More intermediate species are involved in Model II; not only NOadsand Nads, but also HNOads or HNOHads, as all these species are involved in pre-equilibria before the RDS.

Rate¼ k6hHhN¼

K12K2K3K4K5k6½Hþ½NO2 H½ 22

ð1 þ K10:5½ H20:5þ K2½NO2 þ K10:5K2K3½Hþ½NO2 H½ 20:5þ K1K2K3K4½Hþ½NO2½H2 þ K11:5K2K3K4K5½Hþ½NO2 H½ 21:5Þ 2 ð7Þ

Scheme 2a. NOadsreduction to NOHads, first pathway via dissociative hydrogenation of NOHadswith N hydrogenation as RDS.

Scheme 2b. NOadsreduction to NOHads, second pathway dissociative hydrogenation of HNOHadsas RDS.

Scheme 2c. NOadsreduction to NOHads, third pathway via associative hydrogenation of HNOHads.

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4.2.3. NAN Bond formation

The mechanisms discussed so far do not consider formation of the N-N bond, implicitly assuming that NHadsconverts somehow to N2, proceeding relatively fast after the RDS in the mechanistic schemes. The formation of the NAN bond is thus supposedly not the RDS. We will now show that any scheme assuming formation of the N_{AN bond to be rate determining, cannot comply with the} observed kinetic data.

Let us first consider the following example, assuming that the N_{AN bond formation via NO}ads and Nads to form N2Oads, is the RDS according the following scheme, as proposed in several studies

[48,50,71](seeScheme 3):

The elementary steps for this case are: H2þ 2 2H ðequilibrium reaction 1; K1Þ

NOþ N_{þ O} _{ðequilibrium reaction 4; K} 4Þ Oþ 2H_{3 þH} 2O ðequilibrium reaction 5; K5Þ N_{þ NO}_{! N} 2Oþ ðreaction6 ; K6Þ RDS

It should be noted that reaction(5)can again be replaced by stepwise hydrogenation of Oads via OHads to H2O, like in

Section 4.2.1.

Following rate equation is derived based on this mechanism.

When hydrogen pressure is low, the equation simplifies to: Rate¼ k6hNOhN¼ K12K2K3K4K5k6NO2 2 H2 ½ 2 Hþ 2 ð1 þ K2½NO2Þ 2 ð10Þ

Thus, the reaction order in hydrogen is 2, but the reaction order in nitrite cannot turn negative, conflicting with our experimental data. Alternative schemes for NAN bond formation by coupling of combinations Nads, NOads, NOHads, NO2,adsand NHadsas RDS are evaluated in thesupporting informationSection 4.3. Some of these scenarios agree with the observed orders in hydrogen, but none of them can account for the observed negative reaction order in nitrite. Therefore, all options assuming the formation of the N-N bond as RDS are rejected, including the NOads dimerization pathway which is generally accepted for electrochemical reduction of nitrite

[67–70]. Nevertheless, at least one of these pathways must be in

operation to explain formation of N2, but the N-N bond formation is apparently fast, proceeding after the RDS. Actually, two pathways need consideration, i.e. to N2and NH4+respectively. Both pathways are slow compared to the formation on NHads, whereas the pathway to ammonia is even slower than the pathway to N2, as the selectivity to N2is always high.

4.2.4. Selectivity to N2and NH4+

Previous work in our group by Zhao et al.[51]showed that at the very end of a nitrite hydrogenation batch experiment, the Pd surface is significantly covered with N atoms. Continued exposure to hydrogen causes formation of ammonia at an extremely low rate and this reaction is therefore clearly not kinetically relevant. Apparently, hydrogenation of Nadsis too slow to contribute to the formation of ammonia. Therefore, we reject model I as well as model IIa. This conclusion disagrees with most of the propositions in literature[19,48,50,62,66,71]although it should be considered that formation of N2 via associative desorption of absorbed N-atoms is proposed as a logical option without real experimental evidence. Also Obuchi et al.[13]and Tanaka et al.[72]suggested N2 forms via dimerization of Nads; however, these studies were performed in the gas phase instead of in water. The result dis-cussed above [51], i.e. very slow formation of ammonia during exposure of a N covered Pd surface, also implies that associative adsorption of adsorbed N is apparently even slower and therefore not relevant.

Thus, the models IIb and IIc are the remaining options. The essential difference between IIb and IIc is that in IIb HNOH hydro-genates and dissociates immediately according:

HNOHþ H_{! NH}_{þ H}

2Oþ ðreaction 1Þ

Whereas in IIc the dissociation is not immediate:

HNOHþ H_{! H}

2NOHþ ðreaction 2Þ

However, considering assumption 3 in Section 4.2, all reactions after the RDS are fast and in model IIc this implies fast conversion according e.g.:

H2NOHþ H! NH2þ H2Oþ ðreaction 3Þ

ATR-IR experiments in aqueous phase by Ebbesen et al. [49]

showed that NH2,adsconverts to ammonium when exposed to H2. In contrast, NOadson the same Pd surface reacts significantly faster with H2, forming N2. Therefore, model IIc can be ruled out because NH2, ads cannot be an intermediate in the formation of N2. Thus

Rate¼ k6hNOhN¼ K12K2K3K4K5k6 NO2 2 H2 ½ 2 Hþ 2 ð1 þ K10:5½ H2 0:5_{þ K} 2½NO2 þ K10:5K2K3½Hþ½NO2 H½ 2 0:5_{þ K} 11:5K2K3K4K5½Hþ½NO2 H½ 2 1:5_Þ2 ð9Þ

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model IIb is the only option left, in which N2forms via NHadsas an intermediate, in agreement with the results of Wärna et al.[47]

and Pintar et al.[24]based on kinetic studies.

Six different N-containing species, i.e. NHads, Nads, NOads, NO2ads, NOHads and HNOHadscould be involved in the pathway to N2by reacting with NHads, according to model IIb. All possible reactions are listed below. The reactions suggested below are com-pletely associative, i.e. only one species is formed in addition to an open site. This is not certain though and also not important for the argument. The essence is that a product forms with a N-N bond as part of the pathway to form molecular N2.

NHþ NH_{! N} 2H2þ ðreaction 1Þ NHþ N_{! N} 2Hþ ðreaction 2Þ NHþ NO_{! N} 2OHþ ðreaction 3Þ NHþ NO 2 ! N2O2Hþ ðreaction 4Þ NHþ NOH_{! N} 2OH2þ ðreaction 5Þ NHþ HNOH_{! HN} 2OH2þ ðreaction 6Þ

The result Zhao et al.[51], as discussed above, showed that Nads converts extremely slowly into ammonium exclusively. As this pathway necessarily goes via NHads, both reactions (1) and (2) can be ruled out as part of the pathway to N2. The ATR-IR study of Ebbesen et al.[49]showed formation of NOadsand NH2,adsas a result of titrating adsorbed hydrogen on Pd with nitrite, causing exhaustion of hydrogen; however, no NO2adswas observed, exclud-ing reaction(4). So in conclusion, the remaining reactions(3),(5) and (6)are the possible pathways to form N2.

Scheme 4presents a revised mechanism for nitrite hydrogena-tion over Pd catalysts. The dotted lines to N2indicated that NHads reacts with NOads, NOHadsor HNOHads, whereas the equilibrium between these three species is clearly on the side of NO when hydrogen is exhausted, as NOads is detected with ATR-IR under such conditions. The reaction of NHads with NOads, NOHads or HNOHadsis fast compared to the overall RDS, but also fast com-pared to reaction of NH2,ads to NH3. This explains why NH2,ads hydrogenation proceeds only after exhaustion of NOads in the ATR-IR titration experiment[49]and that the selectivity to N2is very high is steady-state operation. The route via Nadsis basically a dead-end as the reaction of Nads to NHads extremely slow, whereas Nads dimerization is apparently even slower (see

Scheme 4).

5. Conclusions

Intrinsic kinetic data in an extreme wide concentration window of nitrite and hydrogen reveals extreme variation in the orders in hydrogen and nitrite, varying between 2 and 0.3 for hydrogen and between 0.4 and0.9 for nitrite. This clearly indicates that a Langmuir-Hinshelwood mechanism with competitive adsorption is in operation. The rate of conversion is determined by the rate of formation of N2as the selectivity to ammonia is always very low. Several mechanistic pathways as well as possible rate deter-mining steps in those pathways are discussed based on these observations in combination with prior knowledge on the mecha-nism in literature, resulting in a revised mechanistic scheme. It is concluded that formation of NH via dissociative hydrogenation of HNOH is the rate determining step, whereas molecular N2forms via reaction of NH with either NO, NOH or HNOH. N-N bond forma-tion via dimerizaforma-tion of adsorbed NO or adsorbed N can be excluded. Formation of NH4+ proceeds via hydrogenation of NH, which is significantly slower than reaction of NH with NO, NOH or HNOH. Atomic N on Pd is a spectator with extreme low reactivity.

Declaration of Competing Interest

The authors declared that there is no conflict of interest. Acknowledgement

The authors gratefully acknowledge financial support from China Scholarship Council. We are grateful to Rodrigo Fernández-Pachecofor from Zaragoza University for the TEM analysis, K. Altena–Schildkamp and T.M.L Velthuizen for chemical analysis. We acknowledge B. Geerdink for technical support.

Appendix A. Supplementary material

Supplementary data to this article can be found online at

https://doi.org/10.1016/j.jcat.2020.01.003. References

[1] G. Gulis, M. Czompolyova, J.R. Cerhan, An ecologic study of nitrate in municipal drinking water and cancer incidence in Trnava District, Slovakia, Environ. Res. 88 (2002) 182–187,https://doi.org/10.1006/enrs.2002.4331.

[2] C.S. Bruning-Fann, J.B. Kaneene, The effects of nitrate, nitrite and N-nitroso compounds on human health: a review, Vet. Hum. Toxicol. 35 (1993) 521–538,

http://www.ncbi.nlm.nih.gov/pubmed/8303822 (accessed September 20, 2017).

Scheme 4. Proposed mechanism of nitrite hydrogenation over Pd catalyst, dashed line indicate three possible reaction steps contributing to formation of N2, as well as two

(10)

[3] S. Hörold, K.-D. Vorlop, T. Tacke, M. Sell, Development of catalysts for a selective nitrate and nitrite removal from drinking water, Catal. Today 17 (1993) 21–30,https://doi.org/10.1016/0920-5861(93)80004-K.

[4] V. Mateˇju˚, S. Cˇ izˇinská, J. Krejcˇí, T. Janoch, Biological water denitrification-a review, Enzyme Microb. Technol. 14 (1992) 170–183,https://doi.org/10.1016/ 0141-0229(92)90062-S.

[5] J. Schullehner, B. Hansen, M. Thygesen, C.B. Pedersen, T. Sigsgaard, Nitrate in drinking water and colorectal cancer risk: a nationwide population-based cohort study, Int. J. Cancer 143 (2018) 73–79, https://doi.org/10.1002/ ijc.31306.

[6] W.H. Organization, Guidelines for Drinking-water Quality FOURTH EDITION, n.d.

[7] J.J.F. Scholten, Metal surface area and metal dispersion in catalysts, Stud. Surf. Sci. Catal. (1979) 685–714,https://doi.org/10.1016/S0167-2991(09)60244-5. [8] A. Bothner-By, L. Friedman, The reaction of nitrous acid with hydroxylamine, J.

Chem. Phys. 20 (1952) 459–462,https://doi.org/10.1063/1.1700442. [9] M. Boudart, Turnover rates in heterogeneous catalysis, Chem. Rev. 95 (1995)

661–666,https://doi.org/10.1021/cr00035a009.

[10] K.T. Ranjit, B. Viswanathan, Photocatalytic reduction of nitrite and nitrate ions to ammonia on M/TiO2 catalysts, J. Photochem. Photobiol. A Chem. 108 (1997) 73–78,https://doi.org/10.1016/S1010-6030(96)04505-4.

[11] A. Pintar, G. Bercˇicˇ, J. Levec, Catalytic liquid-phase nitrite reduction: kinetics and catalyst deactivation, AIChE J. 44 (1998) 2280–2292, https://doi.org/ 10.1002/aic.690441017.

[12] H.C. Aran, J.K. Chinthaginjala, R. Groote, T. Roelofs, L. Lefferts, M. Wessling, R.G. H.H. Lammertink, Porous ceramic mesoreactors: a new approach for gas– liquid contacting in multiphase microreaction technology, Chem. Eng. J. 169 (2011) 239–246,https://doi.org/10.1016/j.cej.2010.11.005.

[13] A. Obuchi, S. Naito, T. Onishi, K. Tamaru, Mechanism of catalytic reduction of NO by H2 or CO on a Pd foil; role of chemisorbed nitrogen on Pd, Surf. Sci. 122 (1982) 235–255,https://doi.org/10.1016/0039-6028(82)90076-0.

[14] V. Höller, I. Yuranov, L. Kiwi-Minsker, A. Renken, Structured multiphase reactors based on fibrous catalysts: nitrite hydrogenation as a case study, Catal. Today 69 (2001) 175–181, https://doi.org/10.1016/S0920-5861(01) 00367-4.

[15] S.D. Ebbesen, B.L. Mojet, L. Lefferts, In Situ attenuated total reflection infrared (ATR-IR) study of the adsorption of NO2-, NH2OH, and NH 4+ on Pd/Al2O3 and Pt/Al 2O3, Langmuir 24 (2008) 869–879,https://doi.org/10.1021/la7027725. [16] A.J. Lecloux, Chemical, biological and physical constrains in catalytic reduction

processes for purification of drinking water, Catal. Today. 53 (1999) 23–34,

https://doi.org/10.1016/S0920-5861(99)00100-5.

[17] M. Hu, Y. Liu, Z. Yao, L. Ma, X. Wang, Catalytic reduction for water treatment, Front. Environ. Sci. Eng. 12 (2018) 1–18, https://doi.org/10.1007/s11783-017-0972-0.

[18] G. Strukul, R. Gavagnin, F. Pinna, E. Modaferri, S. Perathoner, G. Centi, M. Marella, M. Tomaselli, Use of palladium based catalysts in the hydrogenation of nitrates in drinking water: from powders to membranes, Catal. Today 55 (2000) 139–149,https://doi.org/10.1016/S0920-5861(99)00233-3.

[19] O.M. Ilinitch, L.V. Nosova, V.V. Gorodetskii, V.P. Ivanov, S.N. Trukhan, E.N. Gribov, S.V. Bogdanov, F.P. Cuperus, Catalytic reduction of nitrate and nitrite ions by hydrogen: investigation of the reaction mechanism over Pd and Pd-Cu catalysts, J. Mol. Catal. A Chem. 158 (2000) 237–249,https://doi.org/10.1016/ S1381-1169(00)00070-4.

[20] Y. Sakamoto, K. Nakamura, R. Kushibiki, Y. Kamiya, T. Okuhara, A Two-stage catalytic process with Cu–Pd cluster/active carbon and Pd/b-zeolite for removal of nitrate in water, Chem. Lett. 34 (2005) 1510–1511,https://doi. org/10.1246/cl.2005.1510.

[21] D. Gašparovicˇová, M. Králik, M. Hronec, Z. Vallušová, H. Vinek, B. Corain, Supported Pd-Cu catalysts in the water phase reduction of nitrates: functional resin versus alumina, J. Mol. Catal. A Chem. 264 (2007) 93–102,https://doi. org/10.1016/j.molcata.2006.08.081.

[22] U. Prüsse, M. Hähnlein, J. Daum, K.D. Vorlop, Improving the catalytic nitrate reduction, Catal. Today 55 (2000) 79–90,https://doi.org/10.1016/S0920-5861 (99)00228-X.

[23] A. Kapoor, T. Viraraghavan, Nitrate removal from drinking water—review, J. Environ. Eng. 123 (1997) 371–380,https://doi.org/10.1061/(ASCE)0733-9372 (1997) 123:4(371).

[24] A. Pintar, J. Batista, J. Levec, T. Kajiuchi, Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions, Appl. Catal. B Environ. 11 (1996) 81–98,https://doi.org/10.1016/S0926-3373(96)00036-7.

[25] J. Sá, J. Montero, E. Duncan, J.A. Anderson, Bi modified Pd/SnO2catalysts for water denitration, Appl. Catal. B Environ. 73 (2007) 98–105,https://doi.org/ 10.1016/j.apcatb.2006.06.012.

[26] H.O.N. Tugaoen, S. Garcia-Segura, K. Hristovski, P. Westerhoff, Challenges in photocatalytic reduction of nitrate as a water treatment technology, Sci. Total Environ. 599–600 (2017) 1524–1551, https://doi.org/10.1016/j. scitotenv.2017.04.238.

[27] S. Tyagi, D. Rawtani, N. Khatri, M. Tharmavaram, Strategies for Nitrate removal from aqueous environment using nanotechnology: a review, J. Water Process Eng. 21 (2018) 84–95,https://doi.org/10.1016/j.jwpe.2017.12.005.

[28] F. Ruiz-Beviá, M.J. Fernández-Torres, Effective catalytic removal of nitrates from drinking water: an unresolved problem?, J. Clean. Prod. 217 (2019) 398– 408,https://doi.org/10.1016/j.jclepro.2019.01.261.

[29] G. Mendow, N.S. Veizaga, C.A. Querini, B.S. Sánchez, A continuous process for the catalytic reduction of water nitrate, J. Environ. Chem. Eng. 7 (2019) 102808,https://doi.org/10.1016/j.jece.2018.11.052.

[30] K.N. Heck, S. Garcia-Segura, P. Westerhoff, M.S. Wong, Catalytic converters for water treatment, Acc. Chem. Res. 52 (2019) 906–915,https://doi.org/10.1021/ acs.accounts.8b00642.

[31] F. Deganello, L.F. Liotta, A. Macaluso, A.M. Venezia, G. Deganello, Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd-Cu catalysts, Appl. Catal. B Environ. 24 (2000) 265–273, https://doi.org/ 10.1016/S0926-3373(99)00109-5.

[32] M. D’Arino, F. Pinna, G. Strukul, Nitrate and nitrite hydrogenation with Pd and Pt/SnO2 catalysts: the effect of the support porosity and the role of carbon dioxide in the control of selectivity, Appl. Catal. B Environ. 53 (2004) 161–168,

https://doi.org/10.1016/j.apcatb.2004.05.015.

[33] A. Miyazaki, T. Asakawa, Y. Nakano, I. Balint, Nitrite reduction on morphologically controlled Pt nanoparticles, Chem. Commun. (2005) 3730– 3732,https://doi.org/10.1039/B505537G.

[34] F. Liang, J. Fan, Y. Guo, M. Fan, J. Wang, H. Yang, Reduction of nitrite by ultrasound-dispersed nanoscale zero-valent iron particles, Ind. Eng. Chem. Res. 47 (2008) 8550–8554,https://doi.org/10.1021/ie8003946.

[35] K. Wada, T. Hirata, S. Hosokawa, S. Iwamoto, M. Inoue, Effect of supports on Pd–Cu bimetallic catalysts for nitrate and nitrite reduction in water, Catal. Today 185 (2012) 81–87,https://doi.org/10.1016/j.cattod.2011.07.021. [36] H. Qian, Z. Zhao, J.C. Velazquez, L.A. Pretzer, K.N. Heck, M.S. Wong,

Supporting palladium metal on gold nanoparticles improves its catalysis for nitrite reduction, Nanoscale 6 (2014) 358–364,https://doi.org/10.1039/ c3nr04540d.

[37] J. Lee, Y.G. Hur, M.-S. Kim, K.-Y. Lee, Catalytic reduction of nitrite in water over ceria- and ceria–zirconia-supported Pd catalysts, J. Mol. Catal. A Chem. 399 (2015) 48–52,https://doi.org/10.1016/j.molcata.2015.01.012.

[38] R. Brunet Espinosa, L. Lefferts, Ni in CNFs: highly active for nitrite hydrogenation, ACS Catal. 6 (2016) 5432–5440, https://doi.org/10.1021/ acscatal.6b01375.

[39] Y. Matatov-Meytal, V. Barelko, I. Yuranov, M. Sheintuch, Cloth catalysts in water denitrification. I. Pd on glass fibers, Appl. Catal. B Environ. 27 (2000) 127–135,https://doi.org/10.1016/S0926-3373(00)00141-7.

[40] H. Hayashi, S. Sugiyama, Y. Nomura, T. Yamazaki, Reduction of nitrite on Pd/C in fixed-bed operation with recycling of aqueous pollutant, J-Stage (2004) 1–7,

https://doi.org/10.11491/apcche.2004.0.613.0.

[41] J.K. Chinthaginjala, J.H. Bitter, L. Lefferts, Thin layer of carbon-nano-fibers (CNFs) as catalyst support for fast mass transfer in hydrogenation of nitrite, Appl. Catal. A Gen. 383 (2010) 24–32, https://doi.org/10.1016/j. apcata.2010.05.013.

[42] J.K. Chinthaginjala, A. Villa, D.S. Su, B.L. Mojet, L. Lefferts, Nitrite reduction over Pd supported CNFs: metal particle size effect on selectivity, Catal. Today 183 (2012) 119–123,https://doi.org/10.1016/j.cattod.2011.11.003.

[43] D.M. Shuai, J.K. Choe, J.R. Shapley, C.J. Werth, J. Charles, C.J. Werth, Enhanced activity and selectivity of carbon nanofiber supported Pd catalysts for nitrite reduction, Environ. Sci. Technol. 46 (2012) 2847–2855, https://doi.org/ 10.1021/es203200d.

[44] A.H. Pizarro, I. Torija, V.M. Monsalvo, Enhancement of Pd-based catalysts for the removal of nitrite and nitrate from water, J. Water Supply Res. Technol. (2018) 615–625,https://doi.org/10.2166/aqua.2018.024.

[45] Z. Zhang, W. Shi, W. Wang, Y. Xu, X. Bao, R. Zhang, B. Zhang, Y. Guo, F. Cui, Interfacial electronic effects of palladium nanocatalysts on the by-product ammonia selectivity during nitrite catalytic reduction, Environ. Sci. Nano 5 (2018) 338–349,https://doi.org/10.1039/c7en00909g.

[46] Y. Zhao, W. Liang, Y. Li, L. Lefferts, Effect of chlorine on performance of Pd catalysts prepared via colloidal immobilization, Catal. Today 297 (2017) 308– 315,https://doi.org/10.1016/j.cattod.2017.01.028.

[47] J. Wärn, I. Turunen, T. Salmi, T. Maunula, Kinetics of nitrate reduction in monolith reactor, Chem. Eng. Sci. 49 (1994) 5763–5773, https://doi.org/ 10.1016/0009-2509(94)00331-9.

[48] I. Mikami, Y. Sakamoto, Y. Yoshinaga, T. Okuhara, Kinetic and adsorption studies on the hydrogenation of nitrate and nitrite in water using Pd-Cu on active carbon support, Appl. Catal. B Environ. 44 (2003) 79–86,https://doi.org/ 10.1016/S0926-3373(03)00021-3.

[49] S.D. Ebbesen, B.L. Mojet, L. Lefferts, In situ ATR-IR study of nitrite hydrogenation over Pd/Al2O3, J. Catal. 256 (2008) 15–23,https://doi.org/ 10.1016/j.jcat.2008.02.013.

[50] R. Zhang, D. Shuai, K.A. Guy, J.R. Shapley, T.J. Strathmann, C.J. Werth, Elucidation of nitrate reduction mechanisms on a Pd-in bimetallic catalyst using isotope labeled nitrogen species, ChemCatChem. 5 (2013) 313–321,

https://doi.org/10.1002/cctc.201200457.

[51] Y. Zhao, N. Koteswara Rao, L. Lefferts, Adsorbed species on Pd catalyst during nitrite hydrogenation approaching complete conversion, J. Catal. 337 (2016) 102–110,https://doi.org/10.1016/j.jcat.2016.02.007.

[52] C.A. De Wolf, B.E. Nieuwenhuys, NO-H2 reaction over Pd(111), Surf. Sci. 469 (2000) 196–203,https://doi.org/10.1016/S0039-6028(00)00825-6.

[53] K. Tanaka, M. Ikai, Adsorbed atoms and molecules destined for a reaction, Top. Catal. 20 (2002) 25–33,https://doi.org/10.1023/A:1016587713694. [54] R. Brunet Espinosa, D. Rafieian, R.S. Postma, R.G.H. Lammertink, L. Lefferts,

Egg-shell membrane reactors for nitrite hydrogenation: manipulating kinetics and selectivity, Appl. Catal. B Environ. 224 (2018) 276–282,https://doi.org/ 10.1016/j.apcatb.2017.10.058.

[55] Y. Matatov-Meytal, Y. Shindler, M. Sheintuch, Cloth catalysts in water denitrification: III. pH inhibition of nitrite hydrogenation over Pd/ACC, Appl. Catal. B Environ. 45 (2003) 127–134,https://doi.org/10.1016/S0926-3373(03) 00126-7.

(11)

[56] J.K. Chinthaginjala, L. Lefferts, Support effect on selectivity of nitrite reduction in water, Appl. Catal. B Environ. 101 (2010) 144–149,https://doi.org/10.1016/j. apcatb.2010.09.023.

[57] R.S. Postma, R. Brunet Espinosa, L. Lefferts, Competitive adsorption of nitrite and hydrogen on palladium during nitrite hydrogenation, ChemCatChem. 10 (2018) 3770–3776, https://doi.org/10.1002/cctc. 201800523.

[58] C. Franch, R.G.H. Lammertink, L. Lefferts, Partially hydrophobized catalyst particles for aqueous nitrite hydrogenation, Appl. Catal. B Environ. 156–157 (2014) 166–172,https://doi.org/10.1016/j.apcatb.2014.03.020.

[59] Molecular Hydrogen Institute, n.d.http://www.molecularhydrogeninstitute. com/concentration-and-solubility-of-h2.

[60] V. Höller, K. Rådevik, I. Yuranov, L. Kiwi-Minsker, A. Renken, Reduction of nitrite-ions in water over Pd-supported on structured fibrous materials, Appl. Catal. B Environ. 32 (2001) 143–150,https://doi.org/10.1016/S0926-3373(01) 00139-4.

[61] S.D. Ebbesen, B.L. Mojet, L. Lefferts, Mechanistic investigation of the heterogeneous hydrogenation of nitrite over Pt/Al2O3 by attenuated total reflection infrared spectroscopy, J. Phys. Chem. C 113 (2009) 2503–2511,

https://doi.org/10.1021/jp8081886.

[62] H. Shin, S. Jung, S. Bae, W. Lee, H. Kim, Nitrite reduction mechanism on a Pd surface, Environ. Sci. Technol. 48 (2014) 12768–12774, https://doi.org/ 10.1021/es503772x.

[63] C.P. Theologides, G.G. Olympiou, P.G. Savva, K. Kapnisis, A. Anayiotos, C.N. Costa, A. KonstantinosKapnisis, C.N. Costa Anayiotos, Mechanistic aspects (SSITKA-DRIFTS) of the catalytic denitrification of water with hydrogen on Pd-Cu supported catalysts, Appl. Catal. B Environ. 205 (2016) 443–454,https:// doi.org/10.1016/j.apcatb.2016.12.055.

[64] X. Huo, D.J. Van Hoomissen, J. Liu, S. Vyas, T.J. Strathmann, Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts, Appl. Catal. B Environ. 211 (2017) 188–198,https://doi.org/10.1016/j.apcatb.2017.04.045. [65] X. Fan, C. Franch, E. Palomares, A.A. Lapkin, Simulation of catalytic reduction of

nitrates based on a mechanistic model, Chem. Eng. J. 175 (2011) 458–467,

https://doi.org/10.1016/j.cej.2011.09.069.

[66] H. Liu, Y. Zhang, Dynamic model of catalytic denitrification of nitrate over Pd-Cu catalysts from groundwater, Adv. Mater. Res. 322 (2011) 195–200,https:// doi.org/10.4028/www.scientific.net/AMR.322.195.

[67] M. Duca, B. Van Der Klugt, M.A. Hasnat, M. MacHida, M.T.M. Koper, Electrocatalytic reduction of nitrite on a polycrystalline rhodium electrode, J. Catal. 275 (2010) 61–69,https://doi.org/10.1016/j.jcat.2010.07.013. [68] M. Duca, B. Van Der Klugt, M.T.M. Koper, Electrocatalytic reduction of nitrite

on transition and coinage metals, Electrochim. Acta 68 (2012) 32–43,https:// doi.org/10.1016/j.electacta.2012.02.037.

[69] M.T. De Groot, M. Merkx, M.T.M. Koper, Heme release in myoglobin-DDAB films and its role in electrochemical NO reduction, J. Am. Chem. Soc. 127 (2005) 16224–16232,https://doi.org/10.1021/ja0546572.

[70] M.T. De Groot, M.T.M. Koper, The influence of nitrate concentration and acidity on the electrocatalytic reduction of nitrate on platinum, J. Electroanal. Chem. 562 (2004) 81–94,https://doi.org/10.1016/j.jelechem.2003.08.011. [71] J. Martínez, A. Ortiz, I. Ortiz, State-of-the-art and perspectives of the catalytic

and electrocatalytic reduction of aqueous nitrates, Appl. Catal. B Environ. 207 (2017) 42–59,https://doi.org/10.1016/j.apcatb.2017.02.016.

[72] Ken-ichi Tanaka, Taro Yamada, B.E. Nieuwenhuys, Synthesis of metastable surface complexes by chemical reactions N and NHx complexes on Pd(100), Rh (100) and Pt-Rh(100), Surf. Sci. 242 (1991) 503–507,https://doi.org/10.1016/ 0039-6028(91)90317-L.