Tetrazole-Stabilized Gold Nanoparticles for Catalytic Applications

1 Introduction

Tetrazoles are artificial, five-membered heterocycles consisting of four nitrogen and one carbon atom with different substitutions [1]. Due to the high nitrogen content (80% for parent tetrazoles) and large negative enthalpies, they are applicable in photography, pyrotechnics, explosives, and rocket fuel. Nonetheless, most tetrazoles are thermodynamically stable in the condensed, crystalline phase. Additionally, the tetrazole group shows isosteric behavior to the carboxylic group while it is resistant to the metabolism, resulting in several biological applications [2], [3].

Initially, tetrazoles are introduced to nanoscience in 2007 for the stabilization of nanoparticles (NPs) [4]. As prepared tetrazole-stabilized particles can potentially be used in field effect transistors, switches, sensors, solar cells, and exchange spring magnets. Usually, the presence of ligands negatively affects the electronic and catalytic properties of the devices built from NPs [4], [5], [6], [7], [8]. Fortunately, a thermal heat treatment can be used to remove the tetrazole ligands from the NP surface without a trace resulting in an increased performance. Mostly, the decomposition of the tetrazole starts above 200°C under the formation of a high proportion of gaseous products. It was demonstrated, that by decomposing under mild conditions the tetrazole stabilizing agent is released from the surface while a coagulation is prevented at the same time [4]. Additionally, the thermally induced decomposition has been used for the direct preparation of CdS NPs from tetrazole metal complexes [9].

To further develop this idea, we report on a proof-of-concept application of tetrazole-stabilized Au NPs in heterogeneous catalysis. By a thermally induced removal of the organic surfactant, the catalytic performance was improved dramatically.

First mentioned in the 1970s, the catalytic activity of Au NPs is well-known nowadays and is part of intensive research [10], [11], [12], [13]. For this material, the best-known benchmark application in catalysis is the CO oxidation, which drains in scenarios for air cleaning (gas mask, gas sensors) and hydrogen cleaning for fuel cells [12], [14].

Implementing this basic reaction, tetrazole-stabilized Au NPs are deposited on TiO₂ as support material. Using this combination, the CO adsorbs on the metallic gold surface while the reaction with the oxygen proceeds in the perimeter interface between the Au and TiO₂, as described in more detail in Ref. [13]. Among other aspects like the active facets, the support material and the contact angle between the metal and the support, the catalytic activity of Au NPs is influenced by the diameter of the NPs with a tremendous rise in the CO oxidation activity for NPs being less than 5 nm in diameter [13], [15].

To synthesize such small Au NPs, not many alternatives are known using weak-bounded ligands. Therefore, we adapted two different approaches using the tetrazoles 1 and 2 (Figure 1). The first follows a ligand exchange procedure with 5-(2-mercaptoethyl)-1H-tetrazole (1) (Figure 1, left) from aqueous-synthesized, citrate-stabilized Au NPs, which refers to the recipe of Lesnyak et al. [16]. The other adopts the route of Oh et al. in which Au NPs are reported with diameters down to cluster sizes synthesized in the presence of disulfides [17]. In our case, the tetrazole 2 (Figure 1, right), which is a precursor for ligand 1, was used as described in [18]. Of a sui generis design, this route consolidates the last reaction step in the preparation of the ligand and the synthesis of the Au NPs. Therefore, the disulfide 2 is cleaved to 5-(2-mercaptoethyl)-1H-tetrazole (1) while the reduction of AuCl₄⁻ is proceeded. In conclusion, both approaches in spite of their different strategies result in a stabilization of the required Au NPs (d<5 nm) stabilized with ligand 1. This NPs can be further validated for various catalytic applications.

Fig. 1:

Tetrazoles (1=5-(2-mercaptoethyl)-1H-tetrazole, 2=1,2-bis(2-(1H-tetrazol-5-yl)ethyl)disulfane) used for the preparation of Au NPs in the present work.

After immobilization of the NPs on the support material and a gentle, thermally induced decomposition of the tetrazole surfactant, the catalytic activity was exemplarily investigated in the CO oxidation (Scheme 1).

Scheme 1:

Loading of the support material TiO₂ with tetrazole-stabilized Au NPs and following thermal treatment results in ligand-free NPs with enhanced catalytic activity.

2 Experimental section

3 Results and discussion

For an increased catalytic activity, Au NPs with a diameter of less than 5 nm have been synthesized according to the two different approaches. For the ligand exchange procedure, the amount of ligand 1 is adjusted with respect to the stoichiometry as in Ref. [16]. It results in tetrazole-stabilized NPs with a diameter of ~5 nm (Figure 2a). In contrast, the direct synthesis with ligand 2 provides Au NPs in a range of 2.4±0.7 nm via in situ splitting of the disulfide 2 in the presence of NaBH₄ acting as reducing agent (Figure 2b) [18]. The absorption spectrum of the small Au NPs (Figure 2c) exhibits no plasmon band due to the strong size dependence of the interband transitions and additional charge-transfer effects between the NP and the capping ligands. This strong size dependence leads to a superposition of several bands, which cannot be identified as a significant plasmon band which we normally observe for bigger Au NPs [18]. In accordance to this, the Au NPs with a diameter of ~5 nm show a clearly developed plasmon resonance peak. The ligand exchange with 1 indicates no influence on the electronic structure of the metal reflected in no observable shift or damping of the plasmon band. It is concluded, that both approaches result in Au NPs stabilized with 5-(2-mercaptoethyl)-1H-tetrazole (1) and possessing different sizes not exceeding the desired size range.

Fig. 2:

TEM images of ~5 nm (a) and ~2–3 nm (b) Au colloids. Absorbance spectra (c) of the citrate-stabilized and ligand-exchanged (1) Au NPs as well as the absorbance spectra of the directly synthesized Au NPs with 2. The plasmon band of the bigger particles at ~520 nm is clearly developed.

To examine the decomposition pathway of the organic surfactant (1) under air, thermogravimetric analysis (TGA) has been used (Figure 3). Inferring from these investigations, the degradation of the tetrazole 1 on the Au NP surface proceeds in three steps. Typically, at low temperatures around 200°C the tetrazole ring with parts of the carbon side chain (-Tz, -TzMe, -TzEt) starts to decompose while the complete decomposition of the carbon rests (-Et, -Me) and the final release of the sulfur from the Au surface requires higher temperatures of ~300–400°C and over 500°C, respectively (see Supporting Information, Table S1). The sulfur shows a high affinity to the Au surface originating from the high binding energy [19]. Generally, relatively high temperatures are required for the complete removal of the organics from the NP surface.

Fig. 3:

TGA measurements coupled with differential scanning calorimetry (DSC) of tetrazole-stabilized Au NPs directly synthesized with 2 (-Tz= -CN₄; -Me=-CH₃; -Et=-C₂H₅).

To ensure an optimized dispersion of the catalytic active material in the reactor, the tetrazole-stabilized Au NPs are immobilized on TiO₂ by using the wet infiltration method. As a consequence, it is resulting in a reddish support material for the bigger particles (Sample 1) and in a brownish TiO₂ for the smaller particles (Sample 2). Just by eye inspection, we deduce that the bigger NPs did not agglomerate during this procedure due to their preserved nanoparticular properties, in this case their color (Figure 4).

Fig. 4:

Loading of the support material TiO₂ with ~5 nm tetrazole-stabilized Au NPs (Sample 1).

According to the TGA measurements, the Au/TiO₂ materials are thermally treated at 500°C under air, which corresponds to 75.5% of the removed organics, to induce the decomposition of the tetrazole stabilizer (1). Neither by scanning transmission electron microscopy (STEM) nor by scanning electron microscopy (SEM) measurements on the Au/TiO₂ sample, only a few agglomerates are observed during the impregnation of the support material and the subsequent heat treatment (Figure 5). Nevertheless, partial sintering of the NPs cannot be excluded at such high temperatures and the proportion of the sintered NPs is difficult to assess.

Fig. 5:

STEM (left) and SEM (right) measurements of Sample 1 after heat treatment at 500°C under air.

Finally, the heat treated Au/TiO₂ materials were used for CO oxidation. In comparison to the untreated samples which show no activity over the whole investigated temperature range, the heat treated samples exhibited an enhanced CO conversion (Figure 6). The thermal treatment of the catalyst allows to remove the tetrazole ligands and to uncover the catalytically active Au surface. This explains the increased activity from literally zero level for the nonannealed samples up to 43% and 59%, for the annealed Samples 2 and 1, respectively. With an increase of the reaction temperature the activity of the thermally treated catalyst samples raises, as expected assuming Arrhenius behavior. In general, the results show the expected effect to establish a catalyst which can initially be activated at modest temperatures (500°C). Before, at low temperatures (below 200°C) and under air, the NPs are protected by the ligand shell making them attractive for the use as long time storable catalysts on demand. We have to admit, however, that in comparison to the state of the art optimized catalysts (up to 100% CO conversion at room temperature) the observed values are still moderate [20].

Fig. 6:

Results of the catalytic measurements in the CO oxidation on tetrazole-stabilized Au NPs immobilized on the support material TiO₂. Comparative studies have been made for the heat treated and untreated Au/TiO₂ materials as well as for the influence of the size and the synthetic procedure used.

The tetrazole-stabilized Au NPs are in the desired size region for a proposed high catalytic activity as described by Haruta [13]. Although, as it is seen from our investigations, the different NP sizes show opposed activities: bigger NPs show higher conversion rates with rising temperature than the smaller ones. The divergent performance is based on two main influences. First, the two synthetic procedures for the production of the Au NPs can lead to different tetrazole quantities on the NP surface and, therefore, different amounts of remaining sulfur after the heat treatment. The sulfur itself blocks catalytically active facets which results in a diminished performance of the catalyst. This behavior influences the catalytic activity of the smaller particles (Sample 2) more drastically. According to our results, the sulfur residue on the smaller Au NPs seems to be higher than on the bigger NPs which were produced via the ligand exchange procedure. Second, it was shown, that the performance of the Au catalyst was generally influenced by the Au NP size. Investigations of Du et al. showed, that the highest catalytic activity in the CO oxidation was observed for NPs with a size of ~3.8 nm [21]. This size dependence was attributed to the contact boundary between the Au particles and the TiO₂ support for the adsorption of CO and is in good agreement with our results. In addition, sintering of the particles at the used treatment temperature of 500°C cannot be excluded. This can also influence the size of the Au colloids and results in a decreased catalytic activity.

Tetrazoles start to decompose at relatively low temperatures, but, as it was observed, the synthetic approach to Au NPs and their size seems to have a crucial influence on the catalytic activity of the particles in the CO oxidation. Additional studies should focus on the ligand exchange procedure using tetrazoles with higher nitrogen content, different chain lengths (Tz-, Tz-Me-, Tz-Et-, etc.) or different surface binding groups (NH₂, COOH instead of SH). This can eventually increase the catalytic activity while decomposing non-residue under milder conditions.

4 Conclusion

In summary, we have demonstrated a proof-of-concept application of tetrazole-stabilized Au NPs in the CO oxidation. Different synthetic strategies have been used for the synthesis of ~2–3 nm and ~5 nm 5-(2-mercaptoethyl)-1H-tetrazole (1) stabilized NPs, respectively. By using the wet infiltration technique, the particles were successfully immobilized on the support material TiO₂. A thermal heat treatment of the Au/TiO₂ material according to TGA investigations resulted in an increased catalytic activity in comparison to the untreated samples, demonstrating the possibility to remove the tetrazole surfactant residue-free from the NP surface. Our investigations show the possibility to use tetrazole-stabilized NPs as long-time storable catalysts on demand which can be thermally activated under modest conditions, while preserving their catalytically active size range of several nanometers.