Discovery of a Ni-Ga catalyst for carbon dioxide

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Mar 2, 2014 - catalysts and found that Ni5Ga3 is particularly active and selective. ..... J. B. & Nielsen, P. E. H. in Handbook of Heterogeneous Catalysis.
ARTICLES PUBLISHED ONLINE: 2 MARCH 2014 | DOI: 10.1038/NCHEM.1873

Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol Felix Studt1, Irek Sharafutdinov2, Frank Abild-Pedersen1, Christian F. Elkjær2, Jens S. Hummelshøj1, Søren Dahl2, Ib Chorkendorff2 and Jens K. Nørskov1,3 * The use of methanol as a fuel and chemical feedstock could become very important in the development of a more sustainable society if methanol could be efficiently obtained from the direct reduction of CO2 using solar-generated hydrogen. If hydrogen production is to be decentralized, small-scale CO2 reduction devices are required that operate at low pressures. Here, we report the discovery of a Ni-Ga catalyst that reduces CO2 to methanol at ambient pressure. The catalyst was identified through a descriptor-based analysis of the process and the use of computational methods to identify Ni-Ga intermetallic compounds as stable candidates with good activity. We synthesized and tested a series of catalysts and found that Ni5Ga3 is particularly active and selective. Comparison with conventional Cu/ZnO/Al2O3 catalysts revealed the same or better methanol synthesis activity, as well as considerably lower production of CO. We suggest that this is a first step towards the development of small-scale low-pressure devices for CO2 reduction to methanol.

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ature reduces CO2 photochemically to store energy, and devising an artificial process to replicate this remains one of the grand challenges in modern chemistry1–4. One possibility, which is currently the subject of very active research, is a photo-electrochemical process, but finding an electrocatalyst that is selective and has a low overpotential is challenging5–11. An alternative approach would be to first generate molecular hydrogen via a photo-electrochemical process or an electrochemical process using electrical power from photovoltaic cells or wind turbines12,13. If the hydrogen were then used in a heterogeneously catalysed process to reduce CO2 to methanol, a sustainable source of liquid fuel would be established. Today, methanol is produced in large facilities from CO, CO2 and H2 (derived from fossil resources) in a high-pressure (50–100 bar) process using a Cu/ZnO/Al2O3 catalyst14. If hydrogen production is to be distributed and produced in small-scale devices, it would be attractive if the subsequent conversion of H2 into a liquid fuel could also be performed in simpler, low-pressure decentralized units. This is not, however, simply a case of reengineering the technology currently optimized for high-pressure conversion of syngas into methanol, because a low-pressure CO2 reduction process may require a different catalyst. Another challenge arises with the use of CO-free CO2 , which will lead to CO as a by-product of methanol via the reverse water–gas shift (rWGS) reaction. The production of CO not only reduces the yield of methanol—it also has a negative effect when methanol is used in fuel cells because CO poisons the Pt catalyst used. Using the industrial Cu/ZnO/Al2O3 catalyst (which is optimized for different reaction conditions including a CO-rich feed) in low-pressure methanol synthesis leads to significant CO production, so new catalysts are needed to advance this field. In the present Article, we report the discovery of a new, nonprecious metal catalyst working at low pressure with similar or higher methanol yield than the current Cu/ZnO/Al2O3 methanol synthesis catalyst15–17. We use a computational descriptor-based approach to guide us towards a new class of Ni-Ga catalysts and

show experimentally that it has the unique property that it reduces CO2 to methanol without producing large amounts of CO via the rWGS reaction.

Results A large literature exists about the methanol synthesis reaction over supported copper catalysts18–28. Here, we consider the direct CO2 reduction to methanol. Grabow and Mavrikakis have considered many different reaction paths and suggested the following to be most likely29,30: H2 (g) + 2* ↔ 2H*

(1)

CO2 (g) + H* ↔ HCOO*

(2)

HCOO* + H* ↔ HCOOH* + *

(3)

HCOOH* + H* ↔ H2 COOH* + *

(4)

H2 COOH* + * ↔ H2 CO* + OH*

(5)

H2 CO* + H* ↔ H3 CO* + *

(6)

H3 CO* + H* ↔ CH3 OH(g) + 2*

(7)

OH* + H* ↔ H2 O(g) + 2*

(8)

The symbol * represents a surface site or an adsorbed species. A simple mean-field kinetic model is used to elucidate trends in reactivity. The model treats all reaction steps as being potentially rate-determining and solves the rate of methanol production under steady-state conditions, similar to those described for other reactions31,32. There are a total of eight activation energies for the forward elementary steps. Together with the eight elementary reaction energies, these define the complete energy-space of the

1

SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA, Centre for Individual Nanoparticle Functionality (CINF), Department of Physics, Building 307 Technical University of Denmark, DK-2800 Lyngby, Denmark, 3 SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA. * e-mail: [email protected] 2

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Figure 1 | Theoretical activity volcano for CO2 hydrogenation to methanol. Turnover frequency (TOF) is plotted as a function of DEO , relative to Cu(211). DEO for the stepped 211 surfaces of copper, nickel and palladium is depicted as open black circles, and Cu þ Zn is depicted in orange. DEO for Ni-Ga intermetallic compounds is depicted in red. Closed circles indicate nickel-rich sites, open circles gallium-rich sites and half-open circles mixed sites. Reaction conditions are 500 K, 1 bar, and a CO2:H2 ratio of 1:3.

reaction. These energies have been calculated with density functional theory (DFT) using the RPBE exchange-correlation energy functional33 for a selected set of metals (see Supplementary Tables 1 and 2 for a table of all the energies). In each case we chose a stepped face-centred cubic fcc(211) surface to represent the active site30,34. We have shown that van der Waals (vdW) interactions can be important for the energetics of CO2 reduction for Cu(211) using the BEEF–vdW functional35,36. In the following we include such effects by assuming that the extra effect of van der Waals interactions is the same as on copper for all the other metals considered. Given the non-specific nature of the dispersion interactions, and the fact that the catalytically interesting metals are close in bonding to copper, this is a very reasonable approximation. We will now describe the approach we have taken to reduce the number of energy parameters in the methanol synthesis from 16 to only 1. In doing so we lose some accuracy, but it is important to build such a model for at least two reasons. First, it allows us to understand the trends in catalytic activity among the metals. Second, it is a very efficient way of identifying new catalyst leads37,38. We find that scaling relations exist between the oxygen adsorption energy, DEO , and the adsorption energies and transition-state energies of all the hydrogenated forms of CO2 when we compare different metal surfaces (see Supplementary Figs. 1 and 2 for the complete set of scaling relations). The result is a complete mapping of all the relevant energies in the methanol kinetics onto only one parameter, DEO. To a first approximation this parameter characterizes the catalytic properties uniquely. Solving the steady-state microkinetic model with the input of these scaling relations yields the calculated rate of CO2 hydrogenation as a function of DEO at ambient pressure and 500 K, as shown in Fig. 1. Values of DEO for the elemental metals copper, palladium and nickel are included in this volcano plot. The optimum in reaction rate is a result of competition between having a too weak interaction with oxygen (resulting in too unstable intermediates and high reaction barriers) and a too strong coupling to oxygen (giving rise to surface poisoning by formate, and possibly other species bound through oxygen). At atmospheric pressure, elemental copper is closest to the top, while nickel and palladium bind oxygen too strongly and weakly,

Intensity (a.u.)

∆EO – ∆EO (eV)

δ-Ni5Ga3

β-NiGa

Cu/ZnO/Al2O3 40

50

70 2θ (deg)

80

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Figure 2 | Characterization of the catalysts studied. a, TEM images of Ni5Ga3 and NiGa. b, In situ XRD patterns of Ni3Ga, Ni5Ga3 and NiGa intermetallic compounds as well as Cu/ZnO/Al2O3.

respectively. In the figure we have also included zinc doping in a copper step to model the active site of the ZnO promoted commercial catalyst. This has been shown theoretically and experimentally to be a good description30, and the model captures the nearoptimal activity of such a site. Our one-descriptor model therefore provides a good starting point for discovering other potential catalysts. We note that even though the zinc promoted copper steps have close to optimal activity, the density of such sites is small in a doped system30, and a more homogeneous catalyst with the same activity per site but more active sites would be advantageous. In Fig. 1 we have included predictions of the simple models for the mixed-metal system Ni-Ga. We chose Ni-Ga because this comprises a series of stable intermetallic compounds with large ordering energies (for example, Ni-Ga is calculated to have a heat of formation of 20.64 eV/formula unit (two atoms)). This increases the chance that the surfaces exhibit a truncated bulk structure, making modelling simpler. Several of the Ni-Ga intermetallic compounds show active sites with oxygen adsorption energies close to the optimum. We synthesized a series of Ni-Ga catalysts with different Ni:Ga ratios supported on silica using incipient wetness co-impregnation followed by high-temperature reduction in H2. The Ni-Ga catalysts were characterized using X-ray diffraction (XRD) together with transmission electron microscopy (TEM) (Fig. 2). As can be seen from the XRD diffraction patterns, all three different Ni-Ga intermetallic compounds, Ni3Ga, NiGa and Ni5Ga3 , could be prepared rather phasepure, which can be attributed to the high formation energy of the different phases and the very sharp lines in the Ni-Ga phase diagram39. The TEM images presented in Fig. 2a reveal an average particle size of 5.1 nm for the Ni5Ga3 and 6.2 nm for the NiGa.

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important consideration for the present group of catalysts. A CO2 reduction process that mainly produces methanol and water is highly desirable, because the CO will need to be recycled or burned. The conventional Cu-Zn catalyst has a high rate of rWGS. The data in Fig. 3 show that this is not the case for the Ni-Ga catalysts.

MeOH

240

260

Figure 3 | The measured activity and selectivity towards methanol synthesis as a function of temperature for the studied catalysts. a, Yield of methanol of a series of NixGay catalysts compared with Cu/ZnO/Al2O3 as a function of temperature at atmospheric pressure. Gas composition: 75% H2 and 25% CO2. Gas hourly space velocity ¼ 6,000 h21. b, CO-free selectivity towards methanol and dimethyl ether in per cent. c, Comparison of the CO to MeOH ratio of Cu/ZnO/Al2O3 with Ni5Ga3.

We tested the Ni-Ga catalysts with CO2 hydrogenation at a pressure of 1 bar in a tubular fixed-bed reactor. For comparison, a conventional Cu/ZnO/Al2O3 catalyst was synthesized following the procedure described in ref. 40. This procedure has been shown to produce catalysts that are at least as good as the commercial catalysts for short-term testing17. The XRD pattern of the Cu/ZnO/Al2O3 catalyst is also shown in Fig. 2b. The measured Brunauer-Emmet-Teller surface area of this catalyst (92 m2 g21) is comparable to that reported in ref. 40. Figure 3 shows the measured activity and selectivity towards methanol synthesis as a function of temperature for the different SiO2 supported Ni-Ga catalysts. The amount of active metal (Ni þ Ga or Cu), in moles, was the same for all catalysts under investigation. The corresponding values of the active surface area, estimated from XRD and TEM analysis, can be found in Supplementary Table 3. Ni5Ga3/SiO2 stands out as being particularly active towards methanol synthesis. In fact, at temperatures above 220 8C, the yield of methanol is considerably higher than with Cu/ZnO/Al2O3. The selectivity, including all products except CO, is very high for Cu-Zn, Ni5Ga3 and NiGa. Only Ni3Ga produces significant amounts of methane. Notably, the CO-to-methanol ratio of Ni5Ga3 is significantly lower than that of Cu/ZnO/Al2O3 (Fig. 3c). The selectivity towards CO compared to methanol, that is, the rate of the rWGS versus the rate of methanol synthesis is an 322

The experimental data raise two interesting questions: (1) what is the relationship between the activity volcano in Fig. 1 and the ranking of the activity data in Fig. 3; (2) why is the rWGS activity of the Ni-Ga catalysts lower than for the copper-based catalyst. Part of the answer to the first question is related to the observation made earlier that there are two factors affecting the rate, the activity and number of active sites. The copper particles will be most active at the relatively few places where they are promoted by zinc, whereas the active sites on the intermetallic compounds do not need the presence of a promoter. The difference in activity between Cu-Zn and the intermetallic compounds is therefore probably masked by differences in the number of active sites. When it comes to the ranking of the different Ni-Ga catalysts, the main discrepancy between the volcano in Fig. 1 and the experimental data in Fig. 3 is the Ni3Ga mixed sites catalyst, which is predicted to be very active, but found not to be so. The reason for this, we suggest, is that the nickel sites become poisoned by adsorbed CO and eventually (through dissociation) by carbon (see the following and the Supplementary Fig. 3) and subsequent phase separation. In fact, if we distinguish between nickel-rich sites and gallium-rich sites (as done in Fig. 1 by using different symbols), it can be seen that the gallium-rich sites follow the ordering observed experimentally very well. The difference in rWGS and methanol synthesis activity can be understood with the same picture. The gallium-rich sites facilitate methanol synthesis and the nickel-rich sites do rWGS (and methanation) until they become self-poisoned by CO and carbon. It is different for Cu-Zn, where both rWGS and methanol synthesis proceed at the same surface site. Because CO does not bind strongly enough to copper, no poisoning effect will be observed, which translates into a higher rWGS activity and hence lower methanol selectivity. A more detailed analysis supporting this argument can be found in the Supplementary Section ‘Reverse water-gas-shift volcano’. We note that the high activity of the 0.20 After regeneration in H2 at 350 °C Product (g[product]/g[cat]*h)

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Ni5Ga3 200 °C 1 atm CO2:H2 1:3 DME

5 × 10–4 CH4

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Figure 4 | Deactivation of Ni5Ga3 with time on stream. The reaction was carried out at 200 8C, atmospheric pressure and with a CO2 to H2 ratio of 1:3. Regeneration of the catalyst in H2 at 350 8C is shown. Asterisks mark temperature crashes for several hours that have not been accounted for in the total time on stream. NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry

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Ni-Ga catalysts at high temperatures is related to the low rWGS activity. The amount of water in the gas is smaller, shifting the equilibrium concentration of methanol up, hence substantially reducing the backwards reaction. We performed stability tests for the best catalyst, Ni5Ga3/SiO2 , under reaction conditions. Figure 4 shows the production of methanol, CO, dimethyl ether and methane of Ni5Ga3/SiO2 as a function of time on stream, at 200 8C and atmospheric pressure. After initial deactivation of the catalyst, the activity with respect to all products remains quite constant, with CO and methane activity dropping most, supporting the notion developed above that there are two different sites, one for rWGS and methanation (nickel-rich site) and one for methanol formation (gallium-rich site). Ni5Ga3 was tested for a period of over 60 h, after which we tried to regenerate the catalyst through reduction with hydrogen at 350 8C for 2 h. As can be seen in Fig. 4, the catalyst could be successfully regenerated to its original activity. Reduction with hydrogen yields an amount of methane equivalent to poisoning of "10% of the catalyst surface area (see Supplementary Table 3 for details), again confirming our analysis above.

Concluding remarks The Ni-Ga catalysts are not optimized but already show interesting activity, selectivity and stability for ambient-pressure CO2 reduction. Importantly, they are superior to the existing Cu/ZnO/Al2O3 catalyst with respect to their ability to reduce the rWGS activity in favour of methanol production. A process producing mainly methanol and water would provide an excellent fuel for a fuel cell41,42 and could be interesting in connection with a decentralized use of solar- or wind-generated hydrogen. There are many challenges to be overcome to make such a process viable. A process to efficiently separate CO2 from air may be the largest43–46, followed by process design and, of course, optimization and test, including stability and resistance to poisoning of the catalysts. The Ni-Ga catalysts provide a good starting point for a new catalyst system based on non-precious metals showing new and interesting effects of suppressing the rWGS.

Methods Density functional theory (DFT) calculations for the intermediates and transition states were carried out on the (211) surfaces of copper, silver, palladium, platinum and rhodium using the Dacapo code (http://wiki.fysik.dtu.dk/dacapo). The computational set-up and model surfaces used are identical to those described in ref. 47. Determination of NiGa and Ni3Ga intermetallic compound stability was performed as described in ref. 47. DEC and DEO were retrieved from ref. 47, as found in CatApp48. Further information about calculations regarding Ni5Ga3 as well as CO adsorption can be found in the Supplementary Fig. 4. Gas-phase values obtained for CO2 and HCOOH were corrected as described in refs 35 and 49. Contributions from van der Waals interactions were included as estimated by comparison to calculations performed with the BEEF–vdW functional described elsewhere.35 Steady-state solutions to the microkinetic model were found as described in ref. 32. Ni-Ga catalysts were prepared using incipient wetness impregnation of a mixed aqueous solution of nickel and gallium nitrates (Sigma Aldrich) on silica (SaintGobain NorPro). The samples were directly reduced in H2 for 2 h at 700 8C. The conventional Cu/ZnO/Al2O3 catalyst was prepared following the procedure described in ref. 40. Activity measurements were carried out at a total flow rate of 100 Nml min21 in a tubular fixed-bed reactor with a CO2 to H2 ratio of 3:1 at atmospheric pressures. Catalyst loading was 0.472 g for Ni3Ga, 0.476 g for NiGa, 0.474 g for Ni5Ga3 and 0.167 g for Cu/ZnO/Al2O3 , ensuring that the total amount of nickel and gallium in moles matched the amount of copper in Cu/ZnO/Al2O3. The metal loading of the Ni-Ga and Cu/ZnO/Al2O3 catalysts was 17 wt% and 48 wt%, respectively. The outlet stream was sampled every 15 min using a gas chromatograph (Agilent 7890A). TEM measurements were performed using a FEI Technai TEM operating at 200 kV. XRD patterns were recorded with a PANalytical X’Pert PRO diffractometer equipped with an Anton Paar XRK in situ cell and a gas flow control system.

Received 12 November 2013; accepted 14 January 2014; published online 2 March 2014

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Acknowledgements F.S., F.A-P., J.S.H. and J.K.N. acknowledge support from the US Department of Energy. This work was partly supported by The Danish National Research Foundation’s Centre for Individual Nanoparticle Functionality (DNRF54) and partly by the Catalysis for Sustainable Energy initiative, which is funded by the Danish Ministry of Science, Technology, and Innovation. The authors also thank J. R. Rostrup-Nielsen for helpful discussions.

Author contributions F.S., F.A-P., J.S.H. and J.K.N. contributed to the computational work in this article. I.S., C.F.E., S.D. and I.C. contributed to the experimental work.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to J.K.N.

Competing financial interests The authors declare no competing financial interests.

NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry

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