PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite

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PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite Reduction in Water Sarah Seraj, Pranaw Kunal, Hao Li, Graeme Henkelman, Simon M. Humphrey, and Charles J. Werth ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03647 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 25, 2017

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PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite Reduction in Water Sarah Seraj,a Pranaw Kunal,b Hao Li,b Graeme Henkelman,*,b Simon M. Humphrey,*,b and Charles J. Werth*, a a

Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, 301 E. Dean Keeton St., Stop C1700, Austin, TX 78712 b

Department of Chemistry, The University of Texas at Austin, 100 E 24th St., Stop A1590, Austin, Texas 78712

KEYWORDS: nitrite hydrogenation · microwave synthesis · palladium-gold alloys · heterogeneous catalysis · density functional theory (DFT) · sulfide poisoning · water treatment

ABSTRACT: Well-defined palladium-gold nanoparticles (PdAuNPs) with randomly alloyed structures and broadly tunable compositions were studied in catalytic nitrite (NO2‒) reduction. The catalysts were synthesized using a microwave-assisted polyol co-reduction method. PdxAu(100‒x)NPs with systematically varied compositions (x = 18‒83) were supported on amorphous silica (SiO2) and studied as model catalysts for aqueous NO2‒ reduction in a batch reactor, using H2 as the electron donor. The reactions followed pseudo-first order kinetics for ≥ 80% NO2‒ conversion. The PdxAu(100‒x)NP-SiO2 catalysts showed a volcano-like correlation

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between NO2‒ reduction activity and x; the highest activity was observed for Pd53Au47, with an associated first-order rate constant of 5.12 L min‒1 gmetal‒1. Alloy NPs with greater proportions of Au were found to reduce the loss in catalytic activity due to sulfide fouling. Density functional theory calculations indicate that this is because Au weakens sulfur binding at PdAuNP surfaces due to atomic ensemble, electronic and strain effects, and thus reduces sulfur poisoning. The environmental relevance of the most active supported catalyst was evaluated by subjecting it to five cycles of catalytic NO2‒ reduction. Catalytic activity decreased over multiple cycles, but analysis of the post-reaction PdxAu(100‒x)NP-SiO2 materials using complementary techniques indicated that there were no significant structural changes. Most importantly, we show that PdxAu(100‒x)NP-SiO2 alloys are significantly more active NO2‒ reduction catalysts compared to pure Pd catalysts.

INTRODUCTION Nitrate (NO3‒) is one of the most ubiquitous groundwater contaminants in the world.1 It is regulated by the U.S. Environmental Protection Agency (EPA) at a maximum contaminant level (MCL) of 10 mg L-1 as N, due to its adverse health effects. It causes methemoglobinemia in infants (also known as blue baby syndrome) and forms carcinogenic N-nitroso compounds in the human body.2 Groundwater often has low levels of naturally-occurring NO3‒ anions, which can become elevated due to anthropogenic sources, such as nitrogen-containing fertilizers, livestock and septic systems.3,4

Shallow, rural, domestic wells are the most susceptible to NO3‒

contamination, particularly those located in agricultural areas.5 Domestic wells are a particular concern because they are not regulated by the Federal Safe Drinking Water Act, and are a source of drinking water for ~46 million Americans.3 A national analysis of groundwater from 1992 to


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2004 by the National Water-Quality Assessment (NAWQA) Program of the United States Geological Survey (USGS) found that NO3‒ concentrations exceeded the MCL in about 7% of 2,388 sampled domestic wells. A previous NAWQA study reported a NO3‒ MCL exceedance of 11% in domestic wells.6 The most common method for NO3‒ removal from drinking water is ion exchange. Ion exchange resins effectively remove NO3‒ from water. However, once the resin capacity is exhausted, regeneration of the resin using a concentrated brine solution is required. This results in a secondary waste stream that must be disposed, or otherwise treated.1,7 In fact, brine can account for ~77% of the total operational and maintenance costs for an ion-exchange systems.8 A drinking water plant with a flow of 9500 m3/day requires roughly eight metric tons of salt per day to maintain the system.9 The salt cost over a 20-year plant life can be more than twice that of initial equipment cost.1 Moreover, the brine waste stream raises additional environmental concerns related to its disposal into sewers or surface water, which is highly regulated.9 Overall, ion exchange processes for NO3‒ abatement in drinking water present significant environmental issues, as well as being a major component of the overall process cost. Biological and catalytic treatment of nitrate in drinking water are two emerging technologies for removing NO3‒ from drinking water. The main drawback of biological denitrification is that long startup times are required to achieve sufficient biomass growth; this is problematic if treatment is intermittent.10,11 A higher chlorine demand and the potential for pathogen growth are additional concerns. Catalytic treatment has emerged as a promising alternative because of its easy start-up and operation, and its potential for destroying NO3‒ without requiring a secondary waste stream. The proposed reaction mechanism for catalytic NO3‒ reduction is shown in Scheme 1.12 The first step of NO3‒ reduction is catalytic reduction to nitrite, NO2‒. This process


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requires a metal capable of H2 activation (e.g., Pd, Pt), as well as a promoter metal (e.g., In, Sn, Cu). While the promoter metal cannot dissociate H2 by itself, it facilitates reduction by using “spillover” adsorbed surface hydrogen from Pd (or Pt).12–14 All of the subsequent reaction steps

Scheme 1. Accepted pathways for the Pd-catalyzed reduction of nitrate and nitrite anions in aqueous solution.

do not require the promoter metal. The two end products of catalytic NO3‒ reduction are nitrogen gas (N2) and/or ammonia (NH3); inert N2 gas is the preferred product for potable water treatment. Solution conditions affect N2 versus NH3 production by influencing catalyst surface charge and/or competing anions for adsorption.15,16 For example, as pH increases, surface charge decreases and OH- concentrations increase. Both of these conditions decrease NO3- and NO2adsorption, decrease NO2- reduction rates,17 increase H:N species surface coverage, and increase NH3 production.15 The presence of elevated HCO3- can also complete with NO3- and NO2- for adsorption sites and increase NH3 production.18 It has been shown that bimetallic Pd-based catalysts with either Cu or In as the secondary promoter metal provide a highly active catalytic system for NO3‒ reduction, which is also highly selective for the formation of N2 over NH3.12 When Pt or Rh is used instead of Pd, the activity is retained but more NH3 is produced.

Among the promoter metals, In has emerged as the

preferred choice because it is more stable than Cu with respect to dissolution.19 Pd and In are typically co-loaded onto support materials to facilitate ease of processing, handling and recovery (i.e., for incorporation into packed-bed reactors) and, in some cases, to increase catalytic activity (e.g., due to dispersion effects). Carbon-based supports (e.g., activated carbon or graphite) and


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ceramic-based supports (e.g., Al2O3, SiO2) are commonly employed. Carbon-based supports are reported to result in a higher selectivity toward NH3 for NO2‒ reduction.20 Perhaps the most significant current obstacle to the implementation of an economically- and environmentally-sustainable NO3‒ treatment technology is the identification of a catalyst system that can achieve acceptable activity and longevity.9,21 Efforts to address these challenges have focused on tailoring catalyst design at the molecular level, for example: controlling the spatial dispersion of Pd and promoter metal on support media;22,23 the use of exotic support materials that can induce

unique electronic properties (SMSI);20,24 the use of well-defined Pd

nanoparticles (NPs);25 and, combining Pd at the nanoscale with catalytically inactive metals (e.g., Au) to enhance hydrogenation activity.26 Recent efforts in the last category have primarily focused on synthesizing highly active core-shell NPs,27 wherein Pd shells were deposited onto AuNP cores to enhance reduction of the intermediate NO2‒ to N2 through electronic effects of the underlying Au on the outer Pd atoms. Wong and co-workers first demonstrated enhanced reactivity of Au@PdNPs compared to pure PdNPs in aqueous-phase dechlorination reactions27,28 and for NO2‒ reduction.26 The presence of Au in PdAu alloys has also been shown to increase resistance to poisoning by sulfide,29 which is a natural water foulant, with the theory that Au inhibits formation of the sulfide phase, Pd4S.30 An attractive, but less studied alternative to core-shell NPs is to employ homogenous bimetallic alloy NPs that may also enhance the apparent Pd hydrogenation activity. While such materials have been examined for gas phase hydrogenation reactions (e.g., Chen et al.),31 they have not been extended to the evaluation of aqueous phase reactions, including NO3‒ and NO2‒ reduction. Alloyed or intermetallic structures take advantage of d-band intermixing and ensemble effects between two or more metals as a means to modulate reactivity at the NP surface. In


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instances where the relative amounts of each metal can be controllably adjusted across a wide miscibility gap, it is possible to continually tune surface reactivity and thus identify an optimum for a given catalytic reaction process.32–34 For example, Garcia et al.32 alloyed Rh with Ag or Au to produce NP catalysts that had up to fifteen times higher activity for cyclohexene hydrogenation compared to pure Rh, despite Au and Ag alone being catalytically inactive for hydrogenation. Theoretical modeling showed that the high activity in the Rh/Au and Rh/Ag alloys was a result of particular atomic ensembles that resulted in favorable hydrogen and alkene binding energies, which resulted in acceleration of the overall hydrogenation reaction. The overall goal of this work is to identify highly active PdAu alloy catalysts for aqueous phase NO2‒ reduction. To achieve this goal, NPs with Pd:Au ratios from 5:1 to 1:5 were synthesized using a novel microwave-assisted synthesis method that permits greater control over NP composition compared to conventional heating methods.35 The resulting PdAuNPs were loaded onto SiO2 supports and evaluated for their NO2‒ reduction activity in model batch reactions, using H2 as the electron donor. Amorphous SiO2 was chosen as a support for this work because it is relatively chemically unreactive. Therefore, while it permits easy dispersion and handling/recovery of the NP catalysts, the catalytic properties of the NPs should not be significantly perturbed by the support. Catalytic NO2‒ reduction was also studied as a function of sulfide exposure in order to fully evaluate the potential for fouling resistance induced by substitution of Au into the Pd lattice. Density functional theory (DFT) was used to interpret the results obtained from these sulfide-fouling studies, by calculating the sulfur binding energies at different atomic ensembles. The most active catalyst identified from our model studies was subjected to repeated cycles of NO2‒ treatment to evaluate its longevity for water treatment. Low- and high-resolution transmission electron microscopy (TEM), powder X-ray diffraction


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(PXRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES) and/or energy-dispersive X-ray spectroscopy (EDS) were used to characterize the catalysts before and after each set of experiments. The data obtained allowed us to probe the possible mechanisms responsible for NO2‒ reduction activity, catalyst fouling and catalyst durability.

RESULTS AND DISCUSSION 1. Catalyst Characterization and Performance The theoretical and measured values of x for all PdxAu(100-x)NPs synthesized in this work are shown in Table 1. The results show that the Pd:Au molar ratios from ICP-OES agreed well with the targeted values. The metal mass loading obtained on the SiO2 supports ranged from 0.7-1.8 wt% (Column 3, Table 1).


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Table 1. Composition of SiO2-supported PdAuNP. Target Molar %

Actual Molar %*

Mass %*

Pd

Au

Pd

Au

Pd

Au

100

0

100

0

1.56

0.00

84

16

83

17

1.20

0.46

75

25

73

27

0.68

0.46

67

33

68

32

0.79

0.70

50

50

53

47

0.30

0.50

45

55

44

56

0.19

0.44

40

60

39

61

0.20

0.58

33

67

35

65

0.37

1.25

16

84

18

82

0.06

0.51

0

100

0

100

0.00

1.83

*Based on ICP-OES of SiO2-supported PdAuNP The PdAuNPs had average sizes ranging from 2.2-2.7 nm; the average size was found to gradually increase for more Pd-rich NPs (Figure 1). As was observed in previous work, the alloy NPs were all smaller compared to the pure PdNP or AuNP metal counterparts synthesized by the same method (14.8 ± 4.7 nm & 4.4 ± 0.8 nm, respectively). TEM images of the PdAuNPs confirmed a majority of cuboctahedral-shaped entities with good monodispersity (Figure 1). The average NP size and monodispersity remained unaltered upon incipient wetness impregnation


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onto amorphous SiO2 (Figure S3a). PXRD analysis was used to confirm that the PdAuNPs were

Figure 1. Representative TEM images of PdxAu100-x NPs with their size histograms (inset). Scale bars are 50 nm. randomly alloyed in the bulk: as seen in Figure 2, the positions of the most intense Bragg reflections show a gradual shift as a function of Pd:Au composition. The observed maxima for

Figure 2. XRD patterns of Pd53Au47 and Pd NPs with reference peaks for Au (gold lines) and Pd (black lines).


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the (111) reflection was found to shift from 2θmax = 39.9° for pure PdNPs (FCC Pd, expected = 40.1°)36 to lower angles as the content of Au was increased, until reaching 38.2° for pure AuNPs. By comparison, NPs with composition Pd53Au47 had 2θmax = 39.2° due to the (111) reflection; this compares closely to the calculated value of 39.1° for Pd50Au50NPs (assuming an FCC lattice structure).35 2D-EDS mapping of individual NPs also supported random alloying between Pd and Au, with no evident metal segregation (Figure S1b). In bulk, Pd and Au are miscible for all compositions. At the nanoscale, Knecht et al. show that Pd and Au are also well-mixed as alloys in 1.5 nm particles under non-oxidation conditions.37 Though theoretical models have predicted moderate surface segregation of Au on a PdAu(111) at temperatures between 700-1000 K,38 significant surface segregation is not expected at the lower synthesis and operating temperature of our NPs. It was shown in previous work that the distribution of oxidation states of the metal sub-surface atoms changed as a function of PdAu alloy composition. Specifically, a larger proportion of Pd was found in the Pd(II) oxidation state relative to Pd(0) as the NPs became more Pd-rich.35 This is to be expected since a larger proportion of Pd atoms would be found at (or near) the NP surface and should be oxidized in their native state (air-exposed). The same trend was observed in this study; a clear Pd(II) signal was observed for Pd-rich NPs by XPS, whereas this signal was almost indistinguishable for Pd53Au47NPs and more Au-rich analogues (Figure S5, S6). Pseudo-first-order kinetics were observed for the first 80% of NO2‒ reduction using the SiO2supported PdAuNP catalysts. Apparent first-order rate constants were determined from NO2‒ reduction profiles, and normalized to the total PdAu loading of each catalyst obtained from ICPOES studies (Figure 3). The catalytic activity displayed volcano-like behavior, in which a peak in activity was observed for the Pd53Au47NP-SiO2 catalyst. It should also be noted that the


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monometallic AuNP-SiO2 catalyst showed no activity in the same reaction. The measured rate constant obtained for the Pd53Au47NP-SiO2 catalyst was 5.12 L min‒1 gmetal‒1. All PdAu alloys were significantly more active in NO2‒ reduction compared to pure Pd NPs, despite Au being

Figure 3. Total metal mass normalized first-order rate constants of SiO2-supported NP catalyst at varying Pd:Au ratios. catalytically inactive.

Interestingly, the most active catalyst composition identified here

(Pd53Au47) is similar in composition to the previously-determined optimal composition for vapor-phase cyclohexene hydrogenation (Pd59Au41).35 This may be because the rate-limiting step in each reaction is similar. In past studies of alkene hydrogenation, DFT suggested that while Pd-rich NPs were able to more rapidly undergo oxidative addition of H2, the resulting surface-bound H atoms were more strongly bound than for more Au-rich alloy surfaces. Hence, H migration (and product desorption) was slower for more Pd-rich surfaces. Pd59Au41NPs appeared to offer an optimal balance between reagent adsorption, migration and product desorption. It is not unreasonable to assume that similar arguments are important in the aqueous phase catalytic reduction of surface-bound NO2‒ by co-adsorbed H.


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The observed selectivity for NO2‒ reduction to N2 was greater than 98% for all of the PdAuNP catalyst compositions studied, as well as for pure PdNPs (Table S4). This is similar to previously reported values for NO2‒ reduction using other Pd-based catalysts.17,26 While NO2‒ reduction by unsupported PdAuNPs was found to result in NP agglomeration (Figure S2a), TEM images of the post-reaction catalysts showed that no agglomeration had occurred for the SiO2-supported catalysts (Figure S3b,c). An alternative plot of the trend in normalized turnover frequencies (TOFs) as a function of molar Pd:Au composition also reveals a TOF maximum around Pd53Au47, in agreement with the observed trend in maximum activity; the TOF values were normalized based on the total number of Pd and Au surface atoms present in each supported catalyst (calculated assuming regular cuboctahedral NP morphology, using average NP sizes measured by TEM and the total wt% Pd+Au loading obtained from ICP-OES) (Figure S10).

2. Sulfide Fouling Experiments Next, the PdAuNP-SiO2 catalysts were subjected to sulfide-fouling experiments.

The

normalized pseudo-first-order rate constants were found to be significantly lower than those obtained for the SiO2-supported catalysts in the absence of sulfide. Activities of 21% or less of the original activity were observed when Na2S was dosed into the reactions (Figure 4b). It is


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Figure 4. (a) Metal mass normalized first-order rate constants of sulfide-fouled SiO2-supported NP catalyst at varying Pd:Au ratios. (b) Activity of sulfide-fouled Pd53Au47/SiO2 compared to non-fouled catalyst. (c) XRD patterns of Pd53Au47 NPs fouled with sulfide at 10x, 50x and 250x concentrations used in sulfide-fouled catalysis experiments (0.16 mol S/mol surface metal was used in sulfide-fouling catalysis experiments). The inset is an expansion of the (111) peak. Reference peaks are for Au (gold lines) and Pd (black lines).

assumed that addition of Na2S results in sulfide (S2‒) becoming adsorbed to the NP surfaces, thus blocking adsorption sites for incoming NO2‒ and/or H2, resulting in lower overall reduction activity. When the best performing catalyst (Pd53Au47NPs-SiO2) was exposed to sulfide, a rate constant of 0.76 L min‒1 gmetal‒1 was measured (Figure 4a). Interestingly, the best performing catalyst in terms of relative activity after fouling was found to be Pd44Au56, which suffered a


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79% reduction in activity compared to the un-fouled value.

Moreover, all of the alloyed

PdAuNPs catalysts fared significantly better than the pure Pd catalyst upon S2‒ exposure, which became virtually inactive (96% reduction in activity).

These observations pointed us to a

potentially important relationship between Pd:Au content and the resulting ability of the alloyed NPs surfaces to resist poisoning by S2‒. It is known that metals which are more noble (e.g., Au) are less susceptible to poisoning by strongly interacting adsorbates such as O2‒ and S2‒. Accordingly, as the relative proportion of Au surface sites are increased in the alloy NPs, the extent of S2‒ fouling should be reduced. To further explore this supposition, the Pd53Au47NPSiO2 catalyst was exposed to much higher S2‒ levels (10x, 50x and 250x concentrations of S2‒) to probe chemisorption of S2‒ onto Pd and Au surface atoms. While there was no difference in the PXRD pattern at 10x S2‒ addition, the (111) reflection maximum shifted toward pure Au as the concentration of S2‒ was increased further (Figure 4c). The peak was located at 2θmax= 39.1° for 10x S, corresponding exactly to the theoretical value for Pd50Au50NPs, but became shifted to 38.5° for 50x S2‒, and 37.8° for 250x S2‒ addition. At the highest levels of S2‒-fouling, PdS peaks were also detected. It appears that as the extent of S2‒-fouling is increased, preferential adsorption of S2‒ to Pd diminishes the XRD peak signal from Pd atoms in the PdAu NPs. S2fouling also oxidizes Pd(0) sites on the PdAu NPs to Pd(+2), as observed with XPS (Table S3). However, XPS results indicated that the incorporation of Au inhibits the oxidation of Pd(0) to Pd(2+) during S2- fouling. Moreover, as the relative proportion of Au in the PdAu alloy system is increased, the extent of oxidation of Pd(0) to Pd(+2) is also reduced.


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3. Calculations of Sulfur Binding Energies To better understand the apparent tolerance of particular compositions of PdAuNP catalysts towards activity loss upon S2‒ addition, atomic ensemble and electronic effects were evaluated using DFT-calculated S binding energies. Pd atomic ensembles on Au(111) with varying numbers of Pd atoms (Figure 5a) were considered to determine trends in S binding energies (Figure 5b).

Figure 5. a) Top views of Pd/Au(111) surfaces with adsorbed sulfur atoms. The blue, gold and yellow spheres represent Pd, Au and S atoms, respectively. (b) Calculated sulfur binding energies at the Pd ensembles shown in (a). The calculations indicate that a decrease in the Pd ensemble size on Au(111) weakens S binding. Interestingly, this weakening of S binding can be divided into two linear regimes: first, the S binding energy increases linearly with the Pd ensemble size ranging from 3 to 16 Pd atoms (the latter corresponding to a Pd monolayer in our model). Second, the slope of the linear trend is significantly larger for Pd ensemble sizes from 0 to 3. Our explanation of these two separate trends is that the former is dominated by electronic and strain effects while the latter is dominated by atomic ensemble effect.


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Since the S atom favorably binds at the Pd-rich three-fold hollow site,39 reducing the Pd ensemble size from the Pd monolayer to Pd3 doesn’t change the three-fold atomic environment of the adsorbed S atom. Instead, changes in the S binding energy are dominated by the surface electronics. Specifically, our calculations show a systematic increase of charge transfer from the Pd binding site as the Pd ensemble size is reduced and that this charge transfer is linearly related to a reduction in the S binding energy (Figure S12). Perhaps more important for this system is the strain effect. Large Pd ensembles are strained to be commensurate with Au, resulting in stronger S binding whereas smaller Pd ensembles have smaller Pd-Pd bond lengths with weaker S binding (Figure S13). When the Pd ensemble size is smaller than 3, there are four different triatomic ensembles (Pd3, Pd2Au1, Pd1Au2 and Au3) that provide different three-fold environments to the adsorbed S atom. The binding properties of these four three-fold binding sites tune the S bindings more significantly since they consist of different triatomic arrangements. Calculations of S binding at Au ensembles with varying sizes on Pd(111) (Figure S14), show a similar conclusion. In summary, our DFT calculations indicate that alloying Au into Pd leads to weaker S binding and higher activities in the S-fouling experiments as compared to pure Pd, in line with what was observed experimentally.

4. Environmental Relevance and Catalyst Recyclability Studies Pd and Au are precious metals; they represent a significant fraction of catalytic treatment costs for NO3‒ or NO2‒ in water, and also have a significant environmental impact.21 The feasibility of using precious metal-based catalysts for water treatment relies on their longevity over repeated cycles of treatment, and over months to years of operation. The Pd53Au47NP-SiO2 catalyst was subject to repeated cycles of NO2‒ reduction in batch, and metal normalized pseudo-first-order


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rate constants were determined for each cycle (Figure 6a).

With each cycle, the activity

decreased, until the final activity after 5 cycles was 34% of the initial activity. The experiment was repeated

Figure 6. (a) Activity of Pd53Au47/SiO2 with multiple cycles of catalysis. (b) XRD patterns of Pd53Au47 NPs before catalytic nitrite reduction, after nine cycles of catalytic nitrite reduction, and after twenty cycles. The inset is an expansion of the (111) peak. Reference peaks are for Au (gold lines) and Pd (black lines). for 20 cycles using the Pd53Au47NPs, and the used NPs were characterized to probe deactivation mechanisms.

PXRD results showed no evidence of metal segregation (Figure 6b), and

importantly indicated that the majority of Pd and Au atoms remained in an alloyed form. XPS


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results showed no changes in either Pd or Au oxidation state (Figure S5). In agreement with these measurements, ICP-OES analysis of the used catalysts did not reveal measurable changes in % relative metal loading. Hence, it is unclear what mechanism leads to post-reaction catalyst deactivation as there was no clear evidence of metal de-alloying, segregation, dissolution or oxidation from PXRD, XPS and ICP-OES.

However, it is possible that Pd enrichment occurred

on the surface of metal NPs, and is responsible for the reduction in activity with repeated NO2reduction cycles. XPS peak integration, when compared to a bulk technique like ICP-OES, yielded a higher ratio of Pd:Au, which indicates possible surface enrichment of Pd. For example, XPS-derived Pd:Au ratio of Pd53Au47 NPs was 62:38. For SiO2-supported Pd53Au47 NPs, Pd:Au ratio after one and three NO2- reduction cycles were 61:39 and 67:33 respectively (Table S1). Sulfide-fouling further enriched Pd on the surface of Pd53Au47 NPs, with the Pd:Au ratio increasing to 83:17 (Table S2). While using PXRD, Pd surface enrichment might have been indiscernible since PXRD peak signals are averaged over the entire NP volume. Overall, the results indicate that PdAu alloy catalysts have markedly greater activity for NO2‒ reduction than Pd-only catalysts, and that the alloys offer a unique protection mechanism from sulfide fouling. However, the results also indicate that catalyst deactivation is possible over repeated treatment cycles, and further work is needed to probe deactivation mechanisms, and develop new synthesis strategies that maintain catalytic activity over long-term use.

EXPERIMENTAL SECTION Materials K2PdCl4 (99%; Strem Chemicals), HAuCl4 (99.9%; Strem Chemicals), poly(vinylpyrrolidone) (PVP; {C6H9NO}n, = 58 000; Alfa Aesar), ethylene glycol (EG; 99.8%; Fisher Scientific)


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and NaBH4 (98%; Alfa Aesar) were used for all microwave-assisted NP synthesis reactions. Pluronic P-123, polyethylene glycol ( = 5,800; Sigma Aldrich), HCl (12.1 M; Fisher Scientific), n-decane (99%; Acros Organics), NH4F (96%; Alfa Aesar), and tetraethyl orthosilicate (Si(OC2H5)4, 98%; Alfa Aesar) were used to prepare the amorphous SiO2 support. NaNO2 (99%; Sigma Aldrich) was used as the nitrite source in the kinetic experiments, while Na2S·9H2O (98%; Sigma Aldrich) was used as the sulfide source in the fouling experiments. KH2PO4 (99%; Sigma Aldrich) and K2HPO4 (98%; Sigma Aldrich) were used to control pH in the kinetic experiments. H2 gas (99.9%+; Praxair) was used as the elector donor in kinetic experiments. All solvents and reagents were analytical grade unless stated otherwise.

Catalyst Preparation and Characterization The catalysts used in this work were alloyed PdxAu(100-x)NPs supported on an amorphous SiO2; synthesis of the NPs was directly adapted from previous work.35 The variable ‘x’ represents the molar percentage of Pd in the alloy. The NPs were synthesized via a polyol reduction method with microwave-assisted heating, using previously published methods.32,35,40,41 Advantages of using microwave-assisted heating include the production of nanosized “hotspots” in solution that are considered to be much hotter than the temperature of the bulk solvent.41 These hotspots are favorable zones for NP nucleation, producing highly homogenous and crystalline nucleates.32 A MARS 5 (CEM Corp.) microwave system with a maximum power of 1600 W (2.45 GHz) was used for all NP syntheses. The reaction temperature was controlled via a fiber-optic temperature sensor (RTP300+). A solution of excess PVP (225 mg) in EG (15 cm3) was first heated to 150 °C via microwave-assisted heating (MwH) under magnetic-stirring at 450 rpm. EG served as both the reaction medium and reductant, while PVP served as a capping agent and


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Page 20 of 30

reductant.42 NaBH4; (5.6 equiv. per Au) was employed as an additional reductant for Au(III), and was added along with PVP to EG. The metal precursors (K2PdCl4 and HAuCl4) were dissolved by sonication in a separate EG solution (2.5 cm3) and added to the pre-heated PVP/NaBH4/EG solution with a syringe pump at a rate of 300 cm3 min-1. The molar amounts of metal precursors used was determined based on the target stoichiometric ratio of the NPs; and the total molar amount of metal added was always 0.10 mmol. After 5 min of microwave heating at 150°C, the reaction flask (50 cm3 round-bottom flask fitted with a water-fed reflux condenser) was submerged in an ice-water bath to quench the reaction. Next, the NPs were precipitated by the addition of acetone (72.5 cm3) and isolated by centrifugation (5500 rpm, 5 min). The supernatant was discarded, and the remaining solids were re-dispersed into ethanol (15 cm3) and sonicated (1 min) to ensure complete re-dispersion. Hexanes were then added (75 cm3) to effect re-precipitation and the NPs were isolated by centrifugation. After pouring off the supernatant, the NPs were allowed to air-dry overnight and stored in 50 mL polypropylene tubes as dry polymer films. Amorphous SiO2 was prepared following a previously reported method.43 Briefly, Pluronic P123 (2.40 g) was stirred into an HCl (85 cm3; 1.03 M) solution, followed by the addition of ndecane (25 cm3; 87.2 mmol). After stirring for 1-2 hours, NH4F (0.028 g; 0.76 mmol) was added as a hydrolysis catalyst, and tetraethyl orthosilicate (5.6 ml; 25.2 mmol) was added dropwise. Stirring was continued for a further 24 h at 40 °C. The resulting opaque slurry was decanted into a propylene bottle, placed in a static convection oven at 100 °C, heated for 48 h and then filtered. Excess surfactant was removed by washing with copious amounts of ethanol, followed by calcination in a box furnace at 550 °C for 6 hours. The PdxAu(100-x)NPs were finally loaded onto the SiO2 particles via incipient wetness impregnation in a 1:1 ethanol-water suspension followed


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by sonication, filtration and oven-drying at 70 °C for 12 h. When loading the NPs onto the SiO2, a SiO2:NP mass ratio of approximately 7.5:1 was employed in order to obtain total metal loadings in the 0.7-1.8% mass range.

Analytical Methods NO2‒ concentrations were analyzed by ion chromatography (Dionex ICS-2100; Dionex IonPac AS18 column; 32mM KOH eluent; 1 cm3 min-1 eluent flow rate). NH3 concentrations were measured with Hach colorimetric kits (Salicylate method; 0.02 to 2.50 mg L-1 NH3-N). Percent metal loading levels and elemental analyses of the supported catalysts were determined by ICPOES (Agilent; Varian 710). Samples were prepared for ICP-OES by digesting 5 mg of each catalyst in aqua regia (10 cm3; prepared with trace metal grade HCl and HNO3). To ensure complete digestion of metals, after 12 h the aqua regia volume was reduced to below 4 cm3 by heating. Solutions were filtered to remove all SiO2 and diluted with 2% HCl acid prior to analysis. The measurement of pH and temperature was performed with a temperature-adjusted pH electrode (Radiometer Analytical; PHC3081-8). Transmission Electron Microscope (TEM) images were obtained using a FEI Tecnai microscope, operated at 80kV. TEM grids (Formvar/carbon 200 mesh, copper; Ted Pella) were prepared by drop-casting ethanol suspensions of sample onto the grid, which were then air-dried. The average NP size was calculated by manually measuring at least 200 particles with the ImageJ software package. HR-TEM images were obtained using a JEOL 2010F transmission electron microscope, which was operated at 200 keV with a 0.19 nm point-to-point resolution.


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Kinetic Experiments NO2‒ reduction experiments were conducted at constant temperature (22 ±1 oC) and atmospheric pressure under magnetic stirring in a septum-sealed glass batch reactor (60 cm3). The reaction medium contained phosphate buffer (40 cm3) amended with the prepared catalyst (0.5 g L-1). The phosphate buffer maintained a pH of 6.4. The suspension was pre-sparged by bubbling with H2 gas for one hour to saturate the water and headspace, and then the reaction was initiated with a 100 µL aliquot addition of NaNO2 (0.87 M) to achieve an initial concentration of 100 mg L-1 of NO2‒. Samples were taken at regular time intervals for analysis; the total sample volume removed was ≤10% of the total solution volume. Since the catalyst suspension was homogenously dispersed in the reactor, it was assumed that the catalyst concentration in the reactor remained approximately constant. The kinetic experiments for sulfide fouling were performed using a similar method. However, Na2S·(9H2O) was added and allowed to mix with the catalyst for 20 min under H2 before amending the reactor with an aliquot of the conc. NO2‒ solution to initiate the reaction. A loading of 0.16 mol S per mol of surface metal was targeted. This amount of sulfide was chosen in order to cause partial (but not complete) catalyst fouling, as demonstrated in prior work for fouling of a monometallic Pd catalyst.44 Catalyst longevity was evaluated by conducting five sequential NO2‒ reduction experiments with the same catalyst. At the end of each reduction reaction experiment, the catalyst-amended solution was re-sparged with H2 for 1 h, and then amended with a new aliquot of NO2‒ solution (100 mg L‒1). As described above, it was assumed that the catalyst concentration in the reactor remained approximately constant because samples were removed from a well-mixed reactor (i.e., the catalyst was well-dispersed). Triplicate


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experiments were performed to derive rate constants for each unique catalyst composition. Sulfide fouling and longevity experiments were performed in duplicate.

Kinetic Modelling NO2‒ reduction was found to follow pseudo-first-order kinetics for consumption of greater than 80% of the initial NO2‒ concentration (Figure S15). The observed first-order rate constants were normalized based on the % total metal concentration (Pd+Au) for each catalyst, obtained from linear regression of the natural log of concentration versus time plots, using the following equation: ௗ஼

−

ష ಿೀమ

ௗ௧

ଵ

(஼ ) =݇௢௕௦ ‫ܥ‬ ಾ

ష

ேைమ

‒

where CNO2- represents the aqueous nitrite concentration (mg NO2 L‒1), CM represents the concentration of metal in the catalyst (gmetal L‒1), and kobs is the observed first-order rate constant normalized by metal concentration (L gmetal‒1 min‒1).

Density Functional Theory Sulfur binding energies at different atomic ensembles were calculated by DFT using the Vienna Ab initio Simulation Package.45

Electron correlation was evaluated within the

generalized gradient approximation (GGA) using the method of Perdew−Burke−Ernzerhof (PBE).46 For the valence electrons, Kohn−Sham wave functions were expanded in a plane wave basis set with an energy cut-off of 400 eV.47 Core electrons were described with the projector augmented-wave method.48 The Brillouin zone was sampled with a (3×3×1) Monkhorst−Pack kpoint mesh and integrated using the method of Methfessel and Paxton.49 Geometries were


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considered optimized when the force on each atom fell below 0.05 eV Å-1. Electron charge transfer was quantified with a Bader analysis.50 Spin polarization was tested and used when necessary. Sulfur binding energies were calculated on the (111) surface of a 4×4×4 slab, face centered cubic (FCC) surface. For each slab, the uppermost two layers were allowed to relax while the bottom two layers were kept fixed in bulk positions. The sulfur binding energies ‫ܧ‬ௌ௨௟௙௨௥ were calculated as ‫ܧ‬ௌ௨௟௙௨௥ = ‫ܧ‬ௌ௨௟௙௨௥ାௌ௟௔௕ − ‫ܧ‬ௌ௟௔௕ − ‫ܧ‬ௌ௨௟௙௨௥ where ‫ܧ‬ௌ௨௟௙௨௥ାௌ௟௔௕ is the total energy of the slab system with an adsorbed sulfur atom at three-fold hollow site, ‫ܧ‬ௌ௟௔௕ is the total energy of the bare slab, ‫ܧ‬ௌ௨௟௙௨௥ is the energy of a sulfur atom.

ASSOCIATED CONTENT Supporting Information. Additional TEM, PXRD, XPS, EDS, and HRTEM data for the catalysts; additional experimental information; additional sulfur binding configurations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Graeme Henkelman; Email: [email protected] *Simon Humphrey; Email: [email protected] *Charles Werth; Email: [email protected]


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Notes The authors declare no competing financial interest. Acknowledgements This project has been funded partially with funds from the State of Texas through the Texas Hazardous Waste Research Center. This work is also supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1610403 (S.S), the National Science Foundation Division of Chemistry under Grant No. CHE-1505135 (P.K., H.L., S.M.H. & G.H.) and the Welch Foundation (F-1738). The contents do not necessarily reflect the views and policies of the sponsors nor does the mention of trade names or commercial products constitute endorsement or recommendation for use. The authors wish to thank Dr. Dwight Romanovicz (U.T. Austin; TEM), Dr. Hugo Celio (U.T. Austin; XPS), and Dr. Karalee Jarvis (U.T. Austin; HR-TEM) for analytical assistance.

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PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite

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