Advanced Search
RSS LinkedIn Twitter

Journal Archive

Johnson Matthey Technol. Rev., 2015, 59, (4), 303

doi:10.1595/205651315x689117

Interplay between Silver and Gold Nanoparticles in Production of Hydrogen from Methanol

Developing a highly stable bimetallic catalyst for fuel cell applications

SHARE THIS PAGE:

Article Synopsis

Hydrogen production from methanol oxidation over silver-gold/zinc oxide (AgAu/ZnO) catalysts was investigated. Bimetallic catalysts produced higher hydrogen yield and lower carbon monoxide and water yields than Ag/ZnO catalyst without deactivation during 72 h on stream at 250°C. In addition, the presence of Au in the bimetallic catalyst facilitated the preferential oxidation of CO to CO2. Structural analysis of bimetallic catalysts indicated that the strong interaction between Ag and Au particles in the nano-range (4.2 nm–7.2 nm) efficiently enhanced the reducibility of non-selective silver oxide (Ag2O) species. Furthermore dispersion of metal particles in bimetallic AgAu/ZnO catalysts did not significantly change after reaction; however, dispersion of Ag species in Ag/ZnO catalyst was remarkably decreased.

1. Introduction

Direct partial oxidation of methanol (POM) to hydrogen reduces the complexity of hydrogen-fuelled proton exchange membrane (PEM) fuel cells. Methanol can be easily oxidised into hydrogen at relatively low temperatures (<250°C) (1, 2). In addition, it contains no carbon-carbon bond and it has a high H:C ratio therefore reducing the risk of coke formation and catalyst fouling.

Bimetallic catalysts based on noble metals (gold, silver and platinum) and copper are known to be more active for hydrogen production from methanol than monometallic ones (318). Ag has the lowest price among noble metals, which makes it ideal for use as an industrial oxidation catalyst. The electrochemical and steam reforming activity of bimetallic catalysts based on Ag has been studied extensively (1822). However, there are few studies in the literature dealing with direct POM to hydrogen on Ag-based catalysts (9, 2325). Recently, Ag/ZnO catalyst was found to be active for POM to hydrogen; however, the catalyst produced high CO yield (~6%) and was rapidly deactivated (25). On the other hand, bimetallic combinations such as Au-Ag significantly improved the activity and the stability of Ag catalyst in CO oxidation to CO2 at low temperature (2632). Furthermore, much attention has been focused on using Ag-Au bimetallic catalysts for other important reactions such as oxidation of alcohols, dechlorination of organochlorides, hydrocarbon-selective catalytic reduction of nitrogen oxides (NOx), decomposition of organic pollutants, hydrogenation of esters and ethylene oxidation (3340). The activity in a desired application is determined by the oxidation state of the reactive species, interaction between Ag and Au particles, the particle size, shape and location on the support controlled by the preparation process and the nature of the support (41, 42).

The main objective of this work was to develop a highly stable bimetallic catalyst based on noble metals (Ag and Au) with high performance in the oxidation of methanol to hydrogen with low CO formation. The effect of adding Au on the physicochemical properties of Ag/ZnO was also studied.

2. Experimental

2.1 Catalysts Preparation

2.1.1 Preparation of Zinc Oxide Support

Nano-sized ZnO support was synthesised by direct precipitation (25). Analytical grade zinc nitrate (Zn(NO3)2) and ammonium carbonate ((NH4)2CO3) (Sigma-Aldrich, 99.5%) were first dissolved in deionised water to form solutions of 1.5 mol l–1 and 2.25 mol l–1, respectively. The Zn(NO3)2 solution was slowly poured into the (NH4)2CO3 solution with vigorous stirring and then the precipitate derived from the reaction was collected by filtration and rinsed three times with high-purity water and ethanol. The product was dried at 80°C to form the ZnO precursor. Finally, the precursor was calcined at 550°C for 2 h in a muffle furnace to obtain nanoscale ZnO particles. The average crystal size of ZnO was ca. 35.2 nm, calculated from X-ray diffraction (XRD) by the Debye-Scherrer formula (25).

2.1.2 Preparation of Monometallic (Silver or Gold) Catalysts

The preparation of either Au/ZnO2 or Ag/ZnO2 catalyst with 5.0 wt% as theoretical loading was performed by deposition-precipitation (DP) with sodium carbonate (Na2CO3) at pH 8.5. Gold(III) chloride trihydrate (HAuCl4·3H2O) and silver nitrate (AgNO3), both from Sigma-Aldrich, were used as Au and Ag precursors. ZnO support was suspended in an aqueous solution of metal precursor, then the pH was controlled by the addition of 0.5 M Na2CO3. After DP, all samples were centrifuged, washed with water four times, centrifuged again and dried under vacuum for 2 h at 80°C. After drying, the samples were stored at room temperature in desiccators under vacuum, away from light, in order to prevent any alteration (26, 43). Catalysts were calcined at 300°C for 3 h.

2.1.3 Preparation of Bimetallic Catalysts

Preparation of bimetallic Ag1yAuy (y = 0.1, 0.25 and 0.5, where y is the mass fraction of Au with respect to sum of weights of Au and Ag) catalysts supported on ZnO with 5.0 wt% as theoretical loading were also performed by DP with Na2CO3. The oxide support was suspended in an aqueous solution of HAuCl4·3H2O and AgNO3. The initial pH was ~3, which was then adjusted to 8.5 by drop wise addition of 0.5 M Na2CO3 to promote metal hydroxide precipitation on zinc peroxide (ZnO2).The obtained samples were washed, dried, stored and calcined as described above.

2.1.4 Preparation of Silver and Gold Mechanical Mixture

The mechanical mixture (Ag0.5Au0.5Zn)mix catalyst was prepared by dispersing equal amounts of both 2.5 wt% Ag/ZnO and 2.5 wt% Au/ZnO powders (prepared by DP) in 200 ml n-pentane to give a total of 5 wt% metals as theoretical loading. The suspension was stirred vigorously for 20 min and then ultrasonically for 5 min. The n-pentane was evaporated at 40°C and the obtained solid was dried at 100°C overnight without further calcination.

2.2 Catalysts Characterisation

The Ag and Au content in these catalysts was determined by atomic absorption spectroscopy (AAS) on a Perkin Elmer model 3100. XRD measurements were performed on a Philips X’Pert multipurpose X-ray diffractometer (MPD) using Cu Kα1,2 radiation (λ = 1.5405 Å) for 2θ angles varying from 10° to 80°. Hydrogen temperature-programmed reduction (H2-TPR) was performed using a ChemBET 300 Quantachrome. 100 mg sample of the freshly calcined catalyst was subjected to a heat treatment (20°C min–1 up to 1000°C) in a gas flow (85 ml min–1) composed of a mixture of 5 vol% hydrogen and 95 vol% nitrogen. Prior to the TPR experiments, the samples were heated for 3 h under an inert atmosphere (nitrogen) at 200°C. The surface areas (SBET) of the various samples were determined from the adsorption of nitrogen gas at liquid nitrogen temperature (–195.8°C) using a NOVA3200e (Quantachrome Instruments, USA). Before the measurements, all samples were perfectly degassed at 150°C and 10–4 Torr overnight. Transmission electron micrographs were obtained using a JEOL 1200 EX II transmission electron microscope (TEM) operated with an acceleration voltage of 50 kV. Nitrous oxide (N2O) pulse chemisorption was applied to determine the Ag degree of dispersion using ChemBET 3000 and the TPR-Win V. 1.50 software; further details can be found elsewhere (25). Energy dispersive X-ray analyses (EDX) were recorded using a Quanta FEG 250 microscope, equipped with EDX spectrometer (TexSEM Laboratories (TSL) EDAX, AMETEK, Inc, USA). Ultraviolet-visible (UV-Vis) reflection spectra were recorded on a JASCO V-570 spectrophotometer.

2.3 Catalytic Test

Catalytic tests were performed at atmospheric pressure in a tubular quartz reactor with 6 mm internal diameter. The reaction was carried out at 250°C and in a differential mode at conversion ca. 5% by varying the space velocity through changing the catalyst weight. The feed and product gas compositions were determined by online gas chromatography (GC), using a Bruker 450 GC equipped with three channels. The first is for hydrogen analysis using a thermal conductivity detector (TCD). The gas separation was performed by HayeSep Q and 5 Å molecular sieves. Channel two is for analysing non-flammable gases (O2, N2, CO and CO2) using TCD and separation was accomplished by HayeSep Q and MolSieve 13X columns connected in series. The third channel is for analysing oxygenates (methanol, formic acid and formaldehyde) and separation was accomplished by HayeSep Q and Varian SelectTM columns. The catalyst was diluted with silicon dioxide (SiO2) to 10 wt% to prevent hot-spot formation in the bed. The catalyst activation was performed in situ by exposing the catalyst to 100 ml min–1 of 10% H2/N2 and increasing the temperature to 250°C at 10°C min–1. This temperature was maintained for 1 h. Subsequently, the furnace temperature was lowered to ~100°C. The partial oxidation experiments were performed under a total flow rate of 220 ml min–1 with an O2/methanol molar ratio of 0.5, balanced with nitrogen and the weight hourly space velocity (WHSV) was from 8.8 × 104 ml h–1 g–1 to 13.2 × 104 ml h–1 g–1 (WHSV = flow rate of feed gas (ml h–1) per weight of catalyst (g)). The catalysts’ stability was examined followed by the change of the rate of hydrogen production at a higher temperature i.e. 350°C and at iso-conversion 5% by changing WHSV from 14.7 × 104 ml h–1 g–1 to 18.9 ×104 ml h–1 g–1. Furthermore, the external and internal mass transfer limitations of the catalytic system were tested. Catalytic tests were carried out using different particle sizes of AgZn and Ag0.5Au0.5Zn catalysts in the range 125 μm–1000 μm at the constant WHSV = 8.8 × 104 ml h–1 g–1 and 13.2 × 104 ml h–1 g–1, respectively. The results showed that catalyst of particle sizes 212 μm to 710 μm exhibited constant conversion without pressure drop at the constant WHSV. Thus the catalysts were ground and sieved to 355 μm−500 μm. In addition, the calculation of effectiveness factor (η) for AgZn sample with an average particle diameter of 356 μm showed that the values are close to 1.0; at iso-conversion 5%, T = 350°C and WHSV = 8.8 × 104 ml h–1 g–1, indicating no internal diffusion limitation in the catalytic system. The activity of the catalyst was expressed in terms of experimental rate (Rateexpt = mmole of products per gram catalyst). The following formula (Equation (i)) was used to calculate the theoretical rate (Ratetheort) of hydrogen production over AgAuZn catalysts:

Ratetheort = Rateexpt of AgZn (1–y) + Rateexpt of AuZn (y) (i)

3. Results and Discussion

In the case of bimetallic samples, the actual Ag and Au fractions are also very close to the nominal values (Table I). Table I shows the measured values of the BET surface areas of catalysts. Interestingly, the BET surface area of the ZnO support was not significantly altered after loading of Ag or Au or both. The results of N2O chemisorptions shows that the degree of dispersion of Ag in Au-containing catalysts (i.e. bimetallic samples) was less than in AgZn by ca. 4% (shown in Table I). This may suggest an interaction between Ag and Au particles in the bimetallic catalysts. The lower dispersion of Ag and Au particles on ZnO in bimetallic catalysts was clearly verified by an increase of crystallite size of metals measured by XRD (Table I).

Table I

Characterisation of the Catalysts

CatalystNominal fractionAg, %aAu, %aActual fractionSBET, m2 g–1bMetal dispersionc, %XRD crystal sizef
ZnO 38.8
AgZn 4.6 0 36.3 42.0 (28.1)e 5.8
AuZn 0 4.5 36.9 46.9 (32.0) 4.2
Ag0.9Au0.1Zn 0.1 4.3 0.4 0.09 36.4 41.5 (37.3) 6.1
Ag0.75Au0.25Zn 0.25 3.5 1.1 0.24 35.7 39.8 (38.2) 6.4
Ag0.5Au0.5Zn 0.5 2.3 2.2 0.49 37.0 38.3 (37.1) 7.2

 aWeight percentage from atomic absorption spectroscopy

b From N2 adsorption

c Ag and/or Au

e Value in parenthesis is metal dispersion after reaction

f Calculated from XRD peak at 2θ ∼ 38.1°

Figure 1 shows XRD patterns of AgZn, AuZn and Ag0.5Au0.5Zn catalysts. All of the diffraction peaks of ZnO could be indexed to the hexagonal phase reported in the Joint Committee on Powder Diffraction Standards (JCPDS) file (36-1451). For AgZn catalyst, the peaks characteristic of the cubic Ag2O phase were detected at 2θ = 38.08° and 44.29° (JCPDS file, 65-3289 and 42-0874). In the case of AuZn catalyst the peaks at 2θ = 38.17° and 44.38° are characteristic of the single pure metallic Au phase (JCPDS 04-0784). However, in the case of Ag0.5Au0.5Zn catalysts, Au-Ag alloy could not be distinguished from Ag2O and metallic Au based on the XRD patterns because the diffraction lines characteristic of Au and Ag are overlapped.

Fig. 1.

Wide-angle XRD patterns of the AgZn, AuZn and Ag0.5Au0.5Zn catalysts. Figure inset XRD patterns of the same catalysts in the range 37° to 47°. Peaks marked by the symbols “□”, “+” and “●” indicate the peaks assigned to ZnO, Ag2O and metallic Au, respectively

Wide-angle XRD patterns of the AgZn, AuZn and Ag0.5Au0.5Zn catalysts. Figure inset XRD patterns of the same catalysts in the range 37° to 47°. Peaks marked by the symbols “□”, “+” and “●” indicate the peaks assigned to ZnO, Ag2O and metallic Au, respectively

Based on both XRD and TEM results, both Ag particles and Au particles supported on ZnO in monometallic and bimetallic catalysts were in the nano-range 4.2 nm–7.2 nm (see Table I and Figure 2). The average size of the Ag particles in the bimetallic Ag0.5Au0.5Zn catalyst (Figure 2(b)) was larger than that in the monometallic AgZn catalyst (Figure 2(a)). Furthermore, as shown from the TEM micrograph of (Ag0.5Au0.5Zn)mix, the contact between Ag and Au particles was lower than that between these particles in Ag0.5Au0.5Zn (Figure 2(c)). This suggests that Ag-Au ensembles may be formed due to the interaction between Ag and Au. On the other hand, the elemental ratio of Au:Ag by EDX for Ag0.5Au0.5Zn catalyst was 0.86 as shown in the EDX pattern (Figure 3(a)), i.e. the Ag-Au composite composition was approaching 1:1. However, the Au-Ag ratio for the same catalyst prepared by mechanical mixing was found to be 1.4, as estimated from the EDX pattern (Figure 4(a)). It was evident that the outer layers of the bimetallic mixture in this case is enriched in Au. From EDX mapping of the Ag0.5Au0.5Zn catalyst (Figures 3(b) and 3(c)), the contrast between Ag and Au was fairly clear in the homogeneous distribution occupying the same location on the ZnO top surface i.e. in good contact (44). However the EDX mapping of the mechanically mixed composition (Figures 4(b) and 4(c)) shows remarkably aggregated Ag nanoparticles on the top surface of ZnO indicating improper contact between the composites.

Fig. 2.

TEM micrographs of the catalysts: (a) AgZn; (b) and (c) Ag0.5Au0.5Zn prepared by DP method at different magnifications from ×100,000 to ×350,000 and (d) (Ag0.5Au0.5Zn)mix prepared by mechanical mixing

TEM micrographs of the catalysts: (a) AgZn; (b) and (c) Ag0.5Au0.5Zn prepared by DP method at different magnifications from ×100,000 to ×350,000 and (d) (Ag0.5Au0.5Zn)mix prepared by mechanical mixing

Fig. 3.

Ag0.5Au0.5Zn: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

Ag0.5Au0.5Zn: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

Fig. 4.

(Ag0.5Au0.5Zn)mix: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

(Ag0.5Au0.5Zn)mix: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

Figure 5 shows the H2-TPR patterns of AgZn, AuZn and Ag1–yAuy/ZnO (y = 0.25 and 0.5) catalysts. The TPR pattern of AgZn catalyst showed only a major peak at 192.5°C and AuZn catalyst showed one main weak TPR peak at 132.4°C. These peaks indicated the presence of Ag2O and gold oxide in the AgZn and AuZn catalysts (45, 46). The fact that the XRD analysis of AuZn catalyst did not reveal the presence of any Au oxide species (Figure 1), this may suggest that gold oxide crystallites are highly dispersed on the surface and/or Au crystallite sizes are smaller than 5 nm (47). Although AgZn and AuZn have approximately the same amount (wt%) of Ag and Au (Table I), yet the hydrogen consumption for AuZn was markedly lower than that for AgZn sample (Figure 5). This indicates that the majority of Au nanoparticles exist in a metallic state. On the other hand, the TPR features of Ag1–yAuy/ZnO catalysts did not significantly change compared with AgZn. However, adding Au to AgZn catalyst promoted Ag2O reduction, namely the TPR peak characteristics of Ag2O reduction was shifted toward a lower temperature. As the Au content increased from 1.25 wt% to 2.5 wt% the main peak shifted from 179.8°C to 163.2°C.

Fig. 5.

Temperature-programmed reduction profiles of monometallic AgZn and AuZn catalysts and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts

Temperature-programmed reduction profiles of monometallic AgZn and AuZn catalysts and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts

The UV/Vis diffuse-reflectance spectra of the monometallic Ag/ZnO and Au/ZnO catalysts as well as three bimetallic catalysts (Ag0.75Au0.25Zn, Ag0.5Au0.5Zn and (Ag0.5Au0.5Zn)mix) are compared in Figure 6. As evident from Figure 6 curve A for Ag/Zn and curve E for Au/Zn, the obtained spectra of the monometallic catalysts reduced at 300°C show a broad absorption band due to the surface plasmon resonance (SPR) of Ag and Au nanoparticles at ca. 480 nm and 546 nm, respectively (26, 48). In addition, one plasmon band was observed for each bimetallic system and the plasmon maximum was red-shifted from 480 nm to 540 nm with increasing Au content as shown in Figure 6 curves B and C, suggesting the formation of Au-Ag alloy (49). However the plasmon band characteristic of (Ag0.5Au0.5Zn)mix (Figure 6 curve D) was wider than that of Ag0.5Au0.5Zn prepared by DP. It seems that this peak can decompose to two surface plasmon peaks corresponding to the monometallic counterparts. Furthermore, Figure 6 inset shows a typical band at 400 nm which is characteristic of ZnO (50).

Fig. 6.

UV-visible spectra of diffuse reflectance of the monometallic and bimetallic catalysts: A Ag/Zn; B Ag0.75Au0.25Zn; C Ag0.5Au0.5Zn; D (Ag0.5Au0.5Zn)mix; E Au/ZnO

UV-visible spectra of diffuse reflectance of the monometallic and bimetallic catalysts: A Ag/Zn; B Ag0.75Au0.25Zn; C Ag0.5Au0.5Zn; D (Ag0.5Au0.5Zn)mix; E Au/ZnO

Hydrogen, CO, CO2 and H2O production rates at 250°C and at ca. 5% methanol conversion over monometallic and bimetallic AgxAu1–xZn catalysts are presented in Figure 7. It is clear that adding Au increased the selectivity of AgZn catalyst towards hydrogen and CO2. The optimal performance in methanol oxidation to hydrogen was achieved by Ag0.5Au0.5Zn catalyst. However, as shown in Figure 7, selectivity toward CO and H2O decreased with increasing Au content and reached a minimum in the case of Ag0.5Au0.5Zn catalyst. Furthermore, a complementary investigation to confirm synergism between Ag and Au, including theoretical calculations, showed that the experimentally measured rates of hydrogen formation over AgAuZn catalysts are higher than the calculated ones (Figure 8). On the other hand, AgZn catalyst showed a decrease in the activity during 20 h time-on-stream (TOS) (Figure 9). In contrast, a stable activity was observed after ca. 7 h TOS for Ag0.5Au0.5Zn catalyst up to 72 h (Figure 9 inset).

Fig. 7.

Hydrogen and byproducts (CO2, H2O and CO) production rates over monometallic catalysts and bimetallic Ag1–xAuxZn catalysts at 250°C and iso-conversion 5%

Hydrogen and byproducts (CO2, H2O and CO) production rates over monometallic catalysts and bimetallic Ag1–xAuxZn catalysts at 250°C and iso-conversion 5%

Fig. 8.

Comparison between experimental and theoretical rates of hydrogen production over AgAuZn catalysts at 250°C and iso-conversion 5%

Comparison between experimental and theoretical rates of hydrogen production over AgAuZn catalysts at 250°C and iso-conversion 5%

Fig. 9.

Time course of the methanol conversion over AgZn catalyst and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts at 350°C and iso-conversion 5%

Time course of the methanol conversion over AgZn catalyst and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts at 350°C and iso-conversion 5%

A significant contribution of CO from methanol decomposition and/or reverse water gas shift (Equations (ii) and (iii)) was observed over Ag/ZnO catalyst; however, a decrease in CO formation over bimetallic AgAuZn catalysts was observed. This can be explained by the presence of Au particles, possibly by consuming oxygen with preferential oxidation of CO to CO2 (Equation (iv)) (2632). This suggestion runs in good harmony with the observed increase of the rate of CO2 formation over AgAuZn catalysts especially Ag0.5Au0.5Zn catalyst. Sasirekha et al. (28) discussed the effect of promoting Ag catalyst with Au for the preferential oxidation of CO in a hydrogen-rich stream. It could be proposed that the formation of bimetallic alloy in Au-Ag/cerium(IV) oxide (CeO2) catalyst with Au/Ag ratio of 5:5, which showed a lower reduction temperature, is the reason for its excellent performance toward CO to CO2 reaction. Herein, the probable interaction between Au and Ag (XRD, TPR, EDX mapping, UV/Vis reflectance and N2O chemisorptions) may be responsible for improving CO oxidation to CO2.

CH3OH → CO + 2H2 (ii)

CO2 + H2 → CO + H2O (iii)

CO + O2 → CO2 (iv)

Adjusting the valency of Ag species could lead to a variation in both hydrogen and CO selectivity. Ag on non-doped AgZn catalyst exists as Ag2O (as shown by XRD), namely it is in the Ag+ state. Ag+ is regarded as an inactive state for POM to hydrogen (24). On the other hand, it was reported that Ag+ is active in the conversion of methanol to CO. H2-TPR results showed that adding Au induced a change in the reduction profile of Ag+ species and may result in the more active species Agn+ (n <1.0) on the AgAuZn catalysts surface. These Ag species may speed up the rate of hydrogen formation and decrease the rate of CO formation. Similarly, Yang et al. have reported that the reduction of CuO was enhanced by the presence of Au in Cu/ZnO (6). The enhanced reducibility of CuO has been explained in terms of the tendency of Au to decrease the strength of the Cu–O bond located in the vicinity of Cu. Therefore, it can be suggested that the Ag–O bond was weakened by the presence of Au which seems to be due to a certain degree of interaction between Au and Ag oxides in these catalysts.

In order to test the idea that the reduction of Ag2O is enhanced by the presence of Au, the (Ag0.5Au0.5Zn)mix catalyst was also synthesised by mechanical mixing to decrease contact between Ag and Au particles. Interestingly, the (Ag0.5Au0.5Zn)mix catalyst showed a lower hydrogen production rate than that of Ag0.5Au0.5Zn catalyst prepared by DP (Figure 8). In particular, as shown from the TEM micrograph and EDX mapping of the (Ag0.5Au0.5Zn)mix, the contact between Ag and Au particles was lower than that between these particles in Ag0.5Au0.5Zn (Figure 2(c)). These findings may support the interpretation that the synergetic effects between Ag and Au were due to the strong interaction between Ag and Au nanoparticles (as show by UV-Vis). Furthermore, one cannot exclude that the synergetic effects between interacting Ag and Au in Ag0.5Au0.5Zn catalyst (prepared by DP) for production of hydrogen may also be due to the hydrogen spillover effect (24). Hydrogen adsorbed on Ag sites may be spilt over to neighbouring Au particles in high contact (51), which are known to have high affinity for adsorbing hydrogen (52). The spillover hydrogen on Au particles may be desorbed as hydrogen through ZnO rather than being oxidised to water.

Even though adding Au to AgZn catalyst decreased metal dispersion however, dispersion of metal particles in bimetallic AgAu/ZnO catalysts did not significantly change after reaction compared with AgZn catalyst (Table I). This can explain the observed higher stability of bimetallic Ag0.5Au0.5Zn catalyst with 72 h on stream (Figure 9 inset) during POM reaction at 350°C.

4. Conclusions

Bimetallic AgAuZn catalyst samples produced a lower amount of CO than AgZn catalyst which encourages the use of these catalysts in a hydrogen fuel cell to avoid any deactivation. The bimetallic catalysts containing 2.5 wt% Ag and 2.5 wt% Au exhibited the highest hydrogen production rate and had the lowest CO production rate. It was suggested that interaction between Ag and Au particles in the AgAuZn catalyst, detected from TPR and UV-Vis results, was responsible for enhancing the reducibility of Ag2O species in this catalyst. From TEM and EDX mapping investigations it was concluded that the contact between Ag and Au particles in AgAuZn catalyst prepared by DP is greater than that in a comparable catalyst prepared by mechanical mixing. The latter catalyst played an important role in the hydrogen spillover effect. The reaction pathway for POM over AgAuZn involved a preferential oxidation of CO to CO2 over Au sites.

Acknowledgement

The author would like to thank the supporters of the King Faisal University, Saudi Arabia, and the School of Catalysis at the Egyptian Petroleum Research Institute who generously made resources available to undertake this study.

BACK TO TOP

References

  1.  L. Mo, X. Zheng and C.-T. Yeh, ChemPhysChem, 2005, 6, (8), 1470 LINK http://dx.doi.org/10.1002/cphc.200500119
  2.  C.-T. Yeh, Y.-J. Chen and H.-S. Hung, ‘A Process of Producing Hydrogen under Low Temperature’, Taiwan Patent 226,308; 2005
  3.  T.-C. Ou, F.-W. Chang and L. S. Roselin, J. Mol. Catal. A: Chem., 2008, 293, (1–2), 8 LINK http://dx.doi.org/10.1016/j.molcata.2008.06.017
  4.  F.-W. Chang, T.-C. Ou, L. S. Roselin, W.-S. Chen, S.-C. Lai and H.-M. Wu, J. Mol. Catal. A: Chem., 2009, 313, (1–2), 55 LINK http://dx.doi.org/10.1016/j.molcata.2009.08.002
  5.  V. Dal Santo, A. Gallo, A. Naldoni, M. Guidotti and R. Psaro, Catal. Today, 2012, 197, (1), 190 LINK http://dx.doi.org/10.1016/j.cattod.2012.07.037
  6.  H.-C. Yang, F.-W. Chang and L. S. Roselin, J. Mol. Catal. A: Chem., 2007, 276, (1–2), 184 LINK http://dx.doi.org/10.1016/j.molcata.2007.07.002
  7.  Y.-C. Lin, K. L. Hohn and S. M. Stagg-Williams, Appl. Catal. A: Gen., 2007, 327, (2), 164 LINK http://dx.doi.org/10.1016/j.apcata.2007.05.009
  8.  Y.-J. Huang, K. L. Ng and H.-Y. Huang, Int. J. Hydrogen Energy, 2011, 36, (23), 15203 LINK http://dx.doi.org/10.1016/j.ijhydene.2011.08.101
  9.  E. Qayyum, V. A. Castillo, K. Warrington, M. A. Barakat and J. N. Kuhn, Catal. Commun., 2012, 28, 128 LINK http://dx.doi.org/10.1016/j.catcom.2012.08.026
  10.  H. Zhu, Z. Guo, X. Zhang, K. Han, Y. Guo, F. Wang, Z. Wang and Y. Wei, Int. J. Hydrogen Energy, 2012, 37, (1), 873 LINK http://dx.doi.org/10.1016/j.ijhydene.2011.04.032
  11.  C. Pojanavaraphan, A. Luengnaruemitchai and E. Gulari, Appl. Catal. A: Gen., 2013, 456, 135 LINK http://dx.doi.org/10.1016/j.apcata.2013.02.010
  12.  C. Pojanavaraphan, A. Luengnaruemitchai and E. Gulari, Int. J. Hydrogen. Energy, 2013, 38, (3), 1348 LINK http://dx.doi.org/10.1016/j.ijhydene.2012.10.117
  13.  S. Pongstabodee, S. Monyanon and A. Luengnaruemitchai, J. Ind. Eng. Chem., 2012, 18, (4), 1272 LINK http://dx.doi.org/10.1016/j.jiec.2012.01.021
  14.  F.-W. Chang, L. S. Roselin and T.-C. Ou, Appl. Catal. A: Gen., 2008, 334, (1–2), 147 LINK http://dx.doi.org/10.1016/j.apcata.2007.10.003
  15.  M.-L. Xu, X.-K. Yang, Y.-J. Zhang, S.-B. Xia, P. Dong and G.-T. Yang, Rare Metals, 2015, 34, (1), 12 LINK http://dx.doi.org/10.1007/s12598-014-0400-6
  16.  R. Feng, M. Li and J. Liu, Rare Metals, 2012, 31, (5), 451 LINK http://dx.doi.org/10.1007/s12598-012-0538-z
  17.  I. T. Schwartz, A. P. Jonke, M. Josowicz and J. Janata, Catal. Lett., 2013, 143, (7), 636 LINK http://dx.doi.org/10.1007/s10562-013-1029-z
  18.  M. Rashid, T.-S. Jun, Y. Jung and Y. S. Kim, Sens. Actuators B: Chem., 2015, 208, 7 LINK http://dx.doi.org/10.1016/j.snb.2014.11.005
  19.  J. Cao, M. Guo, J. Wu, J. Xu, W. Wang and Z. Chen, J. Power Sources, 2015, 277, (1), 155 LINK http://dx.doi.org/10.1016/j.jpowsour.2014.12.017
  20.  Y. Tong, J. Pu, H. Wang, S. Wang, C. Liu and Z. Wang, J. Electroanal. Chem., 2014, 728, 66 LINK http://dx.doi.org/10.1016/j.jelechem.2014.06.030
  21.  Y. Luo, Y. Xiao, G. Cai, Y. Zheng and K. Wei, Fuel, 2012, 93, 533 LINK http://dx.doi.org/10.1016/j.fuel.2011.10.027
  22.  S. H. Israni and M. P. Harold, J. Membrane Sci., 2011, 369, (1–2), 375 LINK http://dx.doi.org/10.1016/j.memsci.2010.12.029
  23.  L. Mo, X. Zheng and C.-T. Yeh, Chem. Commun., 2004, (12), 1426 LINK http://dx.doi.org/10.1039/B401463D
  24.  L. Mo, A. H. Wan, X. Zheng and C.-T. Yeh, Catal. Today, 2009, 148, (1–2), 124 LINK http://dx.doi.org/10.1016/j.cattod.2009.04.002
  25.  H. M. AbdelDayem, S. S. Al-Shihry and S. A. Hassan, Ind. Eng. Chem. Res., 2014, 53, (51), 19884 LINK http://dx.doi.org/10.1021/ie502609z
  26.  A. Sandoval, A. Aguilar, C. Louis, A. Traverse and R. Zanella, J. Catal., 2011, 281, (1), 40 LINK http://dx.doi.org/10.1016/j.jcat.2011.04.003
  27.  Z. Qu, G. Ke, Y. Wang, M. Liu, T. Jiang and J. Gao, Appl. Surf. Sci., 2013, 277, 293 LINK http://dx.doi.org/10.1016/j.apsusc.2013.04.051
  28.  N. Sasirekha, P. Sangeetha and Y.-W. Chen, J. Phys. Chem. C, 2014, 118, (28), 15226 LINK http://dx.doi.org/10.1021/jp500102g
  29.  G. Nagy, T. Benkó, L. Borkó, T. Csay, A. Horváth, K. Frey and A. Beck, React. Kinet. Mech. Catal., 2015, 115, (1), 45 LINK http://dx.doi.org/10.1007/s11144-015-0835-2
  30.  J. H. Byeon and J.-W. Kim, ACS Appl. Mater. Interfaces, 2014, 6, (5), 3105 LINK http://dx.doi.org/10.1021/am500047n
  31.  Y. Iizuka, R. Inoue, T. Miura, N. Morita, N. Toshima, T. Honma and H. Oji, Appl. Catal. A: Gen., 2014, 483, 63 LINK http://dx.doi.org/10.1016/j.apcata.2014.06.030
  32.  T. Déronzier, F. Morfin, M. Lomello and J.-L. Rousset, J. Catal., 2014, 311, 221 LINK http://dx.doi.org/10.1016/j.jcat.2013.12.001
  33.  W. Li, A. Wang, X. Liu and T. Zhang, Appl. Catal. A: Gen., 2012, 433–434, 146 LINK http://dx.doi.org/10.1016/j.apcata.2012.05.014
  34.  X. Huang, X. Wang, M. Tan, X. Zou, W. Ding and X. Lu, Appl. Catal. A: Gen., 2013, 467, 407 LINK http://dx.doi.org/10.1016/j.apcata.2013.07.062
  35.  X. Huang, X. Wang, X. Wang, X. Wang, M. Tan, W. Ding and X. Lu, J. Catal., 2013, 301, 217 LINK http://dx.doi.org/10.1016/j.jcat.2013.02.011
  36.  L. V. Romashov, L. L. Khemchyan, E. G. Gordeev, I. O. Koshevoy, S. P. Tunik and V. P. Ananikov, Organometallics, 2014, 33, (21), 6003 LINK http://dx.doi.org/10.1021/om500620u
  37.  P. M. More, D. L. Nguyen, P. Granger, C. Dujardin, M. K. Dongare and S. B. Umbarkar, Appl. Catal. B: Environ., 2015,174–175, 145 LINK http://dx.doi.org/10.1016/j.apcatb.2015.02.035
  38.  D. X. Huy, H.-J. Lee, Y. B. Lee and W. S. Choi, J. Colloid Interface Sci., 2014, 425, 178 LINK http://dx.doi.org/10.1016/j.jcis.2014.02.037
  39.  J. Zheng, H. Lin, Y.-n. Wang, X. Zheng, X. Duan and Y. Yuan, J. Catal., 2013, 297, 110 LINK http://dx.doi.org/10.1016/j.jcat.2012.09.023
  40.  S. Rojluechai, S. Chavadej, J. W. Schwank and V. Meeyoo, Catal. Commun., 2007, 8, (1), 57 LINK http://dx.doi.org/10.1016/j.catcom.2006.05.029
  41.  G. C. Bond, C. Louis and D. T. Thompson, “Catalysis by Gold”, Catalytic Science Series, Vol. 6, Imperial College Press, London, UK, 2006
  42.  T. Mallat and A. Baiker, Ann. Rev. Chem. Biomol. Eng., 2012, 3, 11 LINK http://dx.doi.org/10.1146/annurev-chembioeng-062011-081046
  43.  F. Zane, V. Trevisan, F. Pinna, M. Signoretto and F. Menegazzo, Appl. Catal. B: Environ., 2009, 89, (1–2), 303 LINK http://dx.doi.org/10.1016/j.apcatb.2009.02.004
  44.  J. Zhang, B. Lai, Z. Chen, S. Chu, G. Chu and R. Peng, Nanoscale Res. Lett., 2014, 9, 237 LINK http://dx.doi.org/10.1186/1556-276X-9-237
  45.  Y. Hu, W. Lü, D. Liu, J. Liu, L. Shi and Q. Sun, J. Nat. Gas Chem., 2009, 18, (4), 445 LINK http://dx.doi.org/10.1016/S1003-9953(08)60139-5
  46.  F. Yinga, S. Wang, C.-T. Au and S.-Y. Lai, Gold Bull., 2010, 43, (4), 241 LINK http://dx.doi.org/10.1007%2FBF03214994
  47.  C.-K. Chang, Y.-J. Chen and C.-t. Yeh, Appl. Catal. A: Gen., 1998, 174, (1–2), 13 LINK http://dx.doi.org/10.1016/S0926-860X(98)00188-4
  48.  A. N. Pestryakov, N. E. Bogdanchikova and A. Knop-Gericke, Catal. Today, 2004, 91–92, 49 LINK http://dx.doi.org/10.1016/j.cattod.2004.03.008
  49.  S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, (40), 8410 LINK http://dx.doi.org/10.1021/jp9917648
  50.  T. Xu, L. Zhang, H. Cheng and Y. Zhu, Appl. Catal. B: Environ., 2011, 101, (3–4), 382 LINK http://dx.doi.org/10.1016/j.apcatb.2010.10.007
  51.  L.-T. Weng and B. Delmon, Appl. Catal. A: Gen., 1992, 81, (2), 141 LINK http://dx.doi.org/10.1016/0926-860X(92)80093-R
  52.  C. Kartusch and J. A. van Bokhoven, Gold Bull., 2009, 42, (4), 343 LINK http://dx.doi.org/10.1007/BF03214957
 

The Author

Hany AbdelDayem holds a BSc degree in Chemistry and an MSc in catalysis from Ain Shams University, Egypt, as well as a PhD in Chemistry for research on chemical kinetics and catalysis from the Université Catholique de Louvain, Belgium. He subsequently held a Fulbright postdoctoral award at the University of Pittsburgh, USA, and a postdoctoral fellowship from the Agence Universitaire de la Francophonie (AUF) at the Université Catholique de Louvain. In 2001 he was appointed Assistant Professor in the Chemistry Department at Ain Shams University. He joined King Faisal University, Saudi Arabia, as Assistant Professor from 2005 up to 2014. In 2015 he is the appointing Associate Professor in physical chemistry at Ain Shams University. His main focus is the development of heterogeneous catalytic processes, including petrochemical and green energy. He is particularly interested in catalyst synthesis, ex- and in situ characterisation of catalysts and reaction kinetics.

Read more from this issue »

BACK TO TOP

SHARE THIS PAGE: