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Hydrogen Production from Methanol over Gold Supported on ZnO and CeO2 Nanoshapes† Matthew B. Boucher,§ Nan Yi,§ Forrest Gittleson,| Branko Zugic, Howard Saltsburg, and Maria Flytzani-Stephanopoulos* Department of Chemical and Biological Engineering, Tufts UniVersity, 4 Colby Street, Medford, Massachusetts 02155, United States ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: NoVember 3, 2010
In the present study we have found that gold supported on CeO2 and ZnO with well-defined crystal structures or shapes is an excellent catalyst for the low-temperature (175-225 °C) steam reforming of methanol (SRM). We have compared the active nature of gold dispersed on {0001} surfaces of ZnO nanorods and polyhedra to our previous work, which demonstrates that TEM-invisible gold dispersed on the {110} surfaces of CeO2 catalyzes the SRM reaction in a cooperative mechanism with CeO2. Similar to Au-CeO2, we have found that Au-O bonds are essential species for SRM over Au-ZnO and the same apparent activation energy (110-120 kJ · mol-1) of the reaction was calculated for both catalysts. On the basis of temperature-programmed surface reaction/mass spectrometry analysis, we determined that the SRM reaction on both Au-ZnO and Au-CeO2 involves methanol dehydrogenation, methyl formate hydrolysis, and formic acid decomposition steps to produce CO2 and H2. Better than 95% catalyst selectivity to CO2 was found over the temperature range from 175 to 250 °C for both catalysts. In the presence of methanol, the water-gas shift reaction is suppressed and is not part of the mechanism at temperatures below 250 °C. The SRM stability of the Au-ZnO and Au-CeO2 systems is good for practical application of this type catalyst. 1. Introduction Global concerns regarding harmful emissions along with the growing energy demand have spurred the development of sustainable fuel sources. Hydrogen (H2) fuel cells (FCs) have garnered considerable attention due to their high efficiency and lack of harmful emissions. Alcohols have become very promising candidates as reforming substrates for on-demand H2 production. Methanol, for example, is easy to store and transport, and can be derived from a variety of sources ranging from coal to biomass. There are three main processes by which methanol can be used to produce H2: oxidative reforming, decomposition, and steam reforming of methanol (ORM, DOM, and SRM, respectively). SRM is endothermic, though sufficient H2 yields (75%) can still be obtained with high selectivity to CO2. As such, SRM can be used for on-demand H2 production in smallscale fuel cell applications.1,2 The development of heterogeneous catalysts for the lowtemperature conversion of methanol to H2 is widely studied; at low temperatures thermodynamics favors the formation of CO2 rather than CO, which is a criteria pollutant and a poison to PEMFC anode catalysts. Due to their availability as commercial methanol synthesis catalysts, copper containing materials such as Cu-ZnO and Cu-ZnO/Al2O3 were first considered for lowtemperature SRM.2-4 An important drawback of Cu-based catalysts is that temperature fluctuations and exposure to air can cause their rapid deactivation. More recently, group VIII metals, such as palladium, have been studied as a replacement for copper. Pd-ZnO has been identified as the most active of Pd-based catalysts for SRM, with high selectivity to CO2 due †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed,
[email protected]. § Authors made equal contributions. | Present address: Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, CT 06520.
to the formation of a Pd-Zn surface alloy.5,6 Furthermore, the addition of Au to Pd-ZnO has been shown to have a promotional effect on its activity and selectivity for ORM.7 The unique ability of Au to promote selective redox reactions at low temperatures separates it from the more popular group VIII metals and makes it a promising candidate for methanol conversion to H2.8,9 While the electronic properties of nanometer-sized gold may induce substrate dissociation, metal oxide supports can play an important role in supplying oxygen, and in anchoring and dispersing the active gold species.8-10 To date, studies involving H2 production from methanol over gold supported on metal oxides are rather limited. Au nanoparticles supported on TiO2 and ZnO have been shown to be active for ORM,7,12 SRM,11,13 and methanol synthesis.15-18 Driess, Muhler, and co-workers have studied methanol synthesis over ZnO and Au-ZnO prepared by colloidal gold deposition, and have shown that activity correlates with the presence of oxygen defects in the ZnO lattice.17,18 Work by our group has demonstrated a strong correlation between the structure of CeO2 and the activity of Au-CeO2 for the water-gas shift (WGS) and SRM reactions.19-23 Si et al.19 were able to explain this by focusing on the structural effect of CeO2, using shape-controlled nanostructures to disperse the active gold species. They found that the fully oxidized {110} surface of CeO2 nanorods (10 nm × 50 nm) can atomically disperse Au to form Au-O-Ce species; consequently, high activity was observed for the WGS reaction. Recently Yi et al.23 showed the same effect for the SRM reaction on Au-O-Ce species. This was not the case for gold supported on the {100} surfaces of CeO2 nanocubes, where gold nanoparticles (3 nm) were inactive for either reaction.19,23 Thus, it is possible that reactions like the WGS and other reactions involving small molecules take place on very small gold clusters (one-four atoms) properly stabilized by -O or -OH species on neighboring metal atoms. The structure/activity relationships of such
10.1021/jp106589n 2011 American Chemical Society Published on Web 11/29/2010
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complexes and the potential convergence of homogeneous and heterogeneous catalysis have been studied and also presented in a recent review by Thomas.24,25 There is still considerable debate as to the nature of Au-support interaction. In the present study, we have compared the activity of Au supported on ZnO and CeO2 nanoshapes for the SRM reaction. We show that Au supported on ZnO nanostructures with a high fraction of polar {0001} surfaces is active for SRM. We compare this activity and shape effect to what has been previously reported for Au on the {110} surfaces of CeO2.20,23 Temperature-programmed surface reaction/mass spectrometry (TPSR/MS) is used to follow the low-temperature (95% selectivity was observed below 250 °C (Figure 8). CO2 and H2 formation occurred at ∼175 °C for both Au-CeO2 and Au-ZnO (Figure 8, Temp. I), which is in good agreement with the steady-state reaction data presented above (Figures 4 and 5). CO formation was observed on both Au-CeO2 and Au-ZnO at 250 °C (Figure 8, Temp. II) The increase in CO formation observed in the temperature range III may be due to the decomposition of methyl formate37 or may have contributions from the reverse-WGS reaction when methanol conversion is complete. To further investigate methyl formate production at low temperature (30-80 °C), CH3OH+O2-TPSR was carried out under partial oxidation conditions (2:1, CH3OH:O2). Friend and co-workers38 have recently shown that the availability of surface oxygen is critical in the production of methyl formate by partial oxidation of methanol over Au catalysts. Specifically, they have shown that oxygen from Ag-O in leached Au-Ag foils activates the Au catalyst. In our tests, when O2 is introduced with methanol over Au-ZnO, an increase in methyl formate production between 30 and 80 °C is observed; however, this is not observed over Au-CeO2 (both prereduced and unreduced) or Au-free ZnO (Figure 9). The methyl formate concentration at 75 °C (prior to CO2 production over Au-ZnO) is ∼3%, from an original methanol concentration of ∼16%. While ZnO (0001j) surfaces have been shown to bind methyl formate weakly, the coupling of methanol does not take place at these temperatures on ZnO.30 On Au-CeO2 it is possible that highly reactive, adsorbed oxygen species29,39,40 activate the methanol combustion
Boucher et al.
Figure 8. CH3OH + H2O-TPSR: (a) 1 atom % Au-ZnO polyhedra; (b) 1 atom % Au-CeO2 rods.
Figure 9. CH3OH + O2-TPSR: (a) 1 atom % Au-ZnO polyhedra; (b) effluent methyl formate (black and red solid lines) and formic acid (black and red dash lines) for 1 atom % Au-ZnO and 1 atom % Au-CeO2 rods during CH3OH + O2-TPSR.
reaction and methyl formate is a short-lived intermediate. To check this hypothesis, methyl formate temperature-programmed desorption (HCOOCH3-TPD) (Figure 10) was conducted over prereduced Au-CeO2 and Au-ZnO as described in the
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Figure 10. HCOOCH3-TPD over 1 atom % Au-ZnO polyhedra and 1 atom % Au-CeO2 rods: (black and blue solid lines) CO2 and (black and blue dash lines) CH4 from decomposition of HCOOCH3.
Experimental Section. Both catalysts decomposed HCOOCH3 selectively to CO2 and CH4; no CO formation was detected. It was observed from HCOOCH3-TPD that Au-CeO2 was able to bind and decompose methyl formate at much higher yields than Au-ZnO. This is consistent with the higher number of active Au-O sites on the Au-CeO2 rods than on the Au-ZnO polyhedra, as discussed in the previous section. 3.4. Mechanistic Considerations. The mechanism for SRM is under debate even for the extensively studied Cu-based catalysts. Both the methanol decomposition followed by water-gas shift reaction pathway41,42 and the methyl formate route34,35 have been proposed for Cu-based SRM catalysts. The stabilization of methoxy on group IB metals has been shown to lead to formaldehyde formation.43,44 Additionally, Au(111) and (110) can facilitate the formation of methyl formate.30,31 Our group has demonstrated that addition of CO to the SRM reaction mixture does not affect the concentration of H2O or the overall methanol conversion over Au-CeO2; hence the WGS reaction is not involved in the SRM reaction pathway,20,23 even though these catalysts are excellent WGS catalysts in the absence of methanol.19,21,22,29 The CO2 selectivity measured between 175 and 225 °C for SRM over Au-CeO2 was higher than the equilibrium selectivity.20,23 This supports the idea that CO2 was produced via hydrolysis of methyl formate and subsequent decomposition of formic acid and that the WGS reaction was not involved in this sequence. The proposed reaction pathway for SRM over Au-CeO2 is shown below:
2CH3OH f HCOOCH3 + 2H2
(I)
HCOOCH3 + H2O f HCOOH + CH3OH
(II)
HCOOH f CO2 + H2
(III)
It is also worth mentioning the role of CeO2 in the SRM mechanism. For the methanol steam reforming reaction, it appears that gold clusters (
