via a ruthenium-based decomposition catalyst and

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High-purity H produced from NH via a ruthenium-based decomposition catalyst and vanadium-based membrane Krystina Lamb, David Viano, Matthew Langley, San Shwe Hla, and Michael Dolan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01476 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Industrial & Engineering Chemistry Research

High-purity H2 produced from NH3 via a ruthenium-based decomposition catalyst and vanadium-based membrane Krystina E. Lamb, David Viano, Matthew Langley, San Shwe Hla, Michael D. Dolan* Commonwealth Scientific and Industrial Research Organisation, Low Emissions Technologies Group, Energy. Postal address; Queensland Centre for Advanced Technologies, 1 Technology Court, Pullenvale, QLD 4069, Australia. *Corresponding author. Email; [email protected]

Abstract Ammonia is a prospective hydrogen transport intermediate due to its high volumetric and gravimetric H2 densities, and existing production and distribution infrastructure. However, its ultimate use in mobile PEM fuel cells necessitates decomposition and purification at or near the point of use. In this study, the production of high purity H2 from NH3 using a two-stage process has been demonstrated by coupling separate decomposition (150g of 1wt.% Ru on Al2O3 catalyst) and purification (a single 150cm2, Pd-coated tubular vanadium membrane) stages. Equilibrium NH3 decomposition and >90% H2 recovery was demonstrated with a catalyst temperature of 450˚C and membrane temperature of 340˚C, with an overall H2 production rate of 0.75kg/day. Mass spectrometry showed that levels of N2 and NH3 impurities were below detection limits. This configuration is readily scalable by increasing the catalyst loading and membrane area (through use of multiple tubes in parallel), and could enable a pathway for distributed use of H2 from NH3 in mobile and stationary power generation.

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1 Introduction Recent reductions in the costs of renewable power generation, electrolysis and PEM fuel cell technologies will likely see continued growth in the use of hydrogen (H2) as a renewable energy vector. For a nation like Australia with abundant solar and wind energy resources but a relatively small domestic energy market, the opportunity exists for renewable energy exports, in the form of H2, to much larger markets in Asia and Europe. The low density of H2 gas imposes a significant barrier to establishment of this industry, and necessitates its conversion to liquid prior to long-distance transport and long-term storage. However, H2 liquefaction requires a temperature of -253˚C, giving a density of 70kg/m3,1 resulting in an energy penalty greater than 30%.2 An alternative approach is to use ammonia (NH3) as a H2 intermediate, due to its high volumetric (120 kg.H2/m3) and gravimetric (17.7wt.%) H2 density.3 Catalytically decomposing NH3 could supply the emerging H2 fuel cell 3, 4 market more cheaply due to increased storage time,5 and reduced pressure vessel and cryogenic costs.

1.1 Catalytic decomposition of NH3 Despite NH3 decomposition being thermodynamically favourable due to the low equilibrium NH3 concentration (< 2 %) at temperatures above 400 ˚C, demonstrated commercial processes employ temperatures approaching 1000 ˚C due to using cheaper, large volumes of catalyst. The widespread deployment of H2 production from NH3 decomposition for smaller consumers will necessitate reduced temperatures to increase efficiency, and reduce materials costs and start-up time. Ru-based catalysts allow equilibrium decomposition to be achieved at ~ 450 ˚C, and are already produced commercially.6 Previous studies have shown that alkali promotion can increase the activity of Ru-based catalysts for NH3 decomposition,7, 8 and LiOH is an effective promoter of Ru on γ-Al2O3.9 We recently determined the NH3 decomposition rate over a LiOH-promoted Ru-based catalyst to be as shown in Equation 1 (350-475˚C) and 2 (500-600˚C).10 These expressions allow the required amount of catalyst for specific operating conditions to be calculated. The reaction rate increases with increasing NH3 partial pressure (as expected), but is inhibited slightly by product H2. = 36.18

. . . . . 1

! . . . . . 1

= 8.73

−

−

(1)

(2)

Ultra-high purity hydrogen is required for proton exchange membrane (PEM) fuel cells currently on the market, as impurities can cause damage to the active components.11 The relevant standard (ISO14687, parts 2 and 3) for mobile and stationary PEM fuel cells, respectively, set the allowable NH3 level at just 100 µmol/mol.12 To achieve this with a catalyst alone would require 99.99998% decomposition, far exceeding equilibrium at a practical temperatures. NH3 can be scrubbed from a gas stream using several adsorption processes, such as water washing, or solid beds of MgCl2 or zeolite. Only removing NH3 is not enough for H2 for fuel cell electric vehicle (FCEV) applications due to the need for compression, and consequently N2 concentration should be < 100 ppm. Therefore, N2 must also be separated from the H2 product through suitably selective processes, such as pressure swing adsorption or using a H2-selective membrane.


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1.2 H2 separation and purification H2-selective metal membranes are a solid-state hydrogen purification technology which uses a solution-diffusion mechanism and a trans-membrane partial pressure difference to extract H2 from mixed gas streams. H2-selective metal membranes are particularly suited to this application as they are tolerant to NH3,13 and infinitely selective to H2 (i.e., producing a pure H2 product) when defect free.14 Suitable membrane types include thin, supported Pd-based membranes,14-18 or thicker Pdcoated vanadium (V) membranes, such as those reported by this group.19, 20 An NH3 decomposition catalyst and H2-selective membrane can be combined in two different configurations. The simplest approach is to use sequential decomposition and separation stages, as this allows the respective materials to be maintained at their optimum temperature, but also limits the achievable NH3 decomposition to equilibrium. The second approach is the catalytic membrane reactor (CMR), where the catalyst and membrane are housed in a single vessel, and the in situ H2 separation allows the equilibrium barrier to be exceeded.21, 22 CMRs require the catalyst and membrane to have compatible operating temperatures, and increases the complexity of the system in terms of physical design and managing the heat dynamics of the reaction without compromising the durability of the membrane. Given that equilibrium decomposition of NH3 at 450˚C and 5 bar is around 98 %, it is not clear whether the additional 2 % in NH3 decomposition offsets the added complexity of this configuration over separate decomposition and separation stages.

1.3 Two stage process for H2 production from NH3 An advantages of separate decomposition and separation stages is that the system can be operated more flexibly. In a CMR, the bed and membrane temperature must remain stable to prevent shock to the membrane. When separated, the catalyst temperature can be varied without affecting the membrane, which allows the system to be operated flexibly depending on the economic and location drivers. A system such as this could be operated in one of three ways: to maximise recovery of H2, to use combustion of the retentate to drive the endothermic decomposition reaction, or to co-locate the H2 production plant with a direct-NH3 power generation device. Each of these models of operation are outlined in Figure 1. The first mode is most suited to when there is a source of waste or renewable heat that can be used to drive the endothermic NH3 decomposition reaction, and H2 production is to be maximised. This has implications on the temperatures and flowrates that are achievable in the system to maintain high efficiency, especially when the residual NH3 will need to be scrubbed from the gas stream creating waste from the system. Mode 2 is suited to a stand-alone system, where the NH3 and the retentate can be combusted to produce heat for the endothermic reaction and to destroy residual NH3 prior to venting. Mode 3 allows for residual NH3 and unrecovered H2 to be returned to a fuel cell, turbine or engine to produce electricity, with waste heat from power generation used to drive the decomposition reactor. In this mode, the NH3 decomposition stage may be run with low NH3 decomposition and H2 recovery to produce electricity when demand is greater, then at high decomposition and recovery to produce H2 when demand for electricity is less.



Figure 1: Three possible modes of operation for an NH3-to-H2 system

Achieving high NH3 conversion, coupled with generation of high-purity H2, in a compact module, will potentially enable the use of NH3 as a hydrogen vector between points of production and points of use. Here, for the first time, we present the configuration and performance data for a two-stage ammonia-to-hydrogen system incorporating an ammonia decomposition 23 and a vanadium-based membrane.20 We show that ultra-high purity H2 can be generated directly from NH3 using a thermally-coupled, two-stage process which is readily scalable and able to meet the fuel purity standards for fuel cell electric vehicles. In doing so we demonstrate that ammonia can potentially be used as a feedstock for H2 production at or near FCEV refuelling stations. 20The performance of this system, with respect to NH3 decomposition, H2 production rate, H2 recovery and H2 product purity, is presented.

2 Materials and methods 2.1 Membrane A hydrogen-selective membrane was prepared by coating a 99.9 % vanadium tube with inner and outer palladium catalyst layers of 0.5 µm. Membrane dimensions were 500 mm long, 9.52 mm outer diameter and 0.26 mm wall thickness. The manufacturing and sealing process were reported previously.20 The membrane was activated under 8 standard litres per minute (slpm) H2 flow at 340 ˚C for 4 hours.

2.2 Catalyst The NH3 decomposition catalyst was procured from a commercial supplier and featured a 1 wt.% Ru active layer, and a 3 wt.% LiOH promoter, on the outside of a 1.5 mm spherical α-Al2O3 support. The morphology and performance characteristics of this catalyst were reported previously.23 The catalyst was reduced under 8 slpm H2 flow for 4 hours at 500 ˚C before switching to NH3.

2.3 System Figure 2 shows the piping and instrument diagram (P&ID) for the reactor and post reactor, with the mass flow controllers and input gases excluded for clarity. NH3, H2 and N2 flows were controlled using Bronkhorst analog mass flow controllers (MFC), calibrated using a 300-30000 slpm volume flow meter (Mesa Labs DryCal Defender 510). The permeate was under vacuum, resulting in a pressure of ~0.1 Bara, and the gas flow was measured using a 2-100 slpm Bronkhorst mass flow meter. The NH3 MFC was calibrated using N2 gas and then corrected using a K-factor of 0.731. The


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retentate, being a mixed gas, was measured using a DryCal Defender 510 after the NH3 was scrubbed from using a water wash, followed by SiO2 and MgCl2 absorption beds. A mass spectrometer (European Spectroscopy Systems GeneSys) was used to analyse the purity of the permeate gas, and atomic mass numbers of 14, 15, 16, 17 and 28 were monitored for N, NHx and N2 impurities by comparing the permeate from decomposed NH3 to permeate from a 99.99 % H2 feed.

Figure 2. P&ID of the catalyst and membrane system used for decomposing NH3 and separating H2

2.4 Testing procedure The NH3 decomposition catalyst was tested at several loadings (52 and 150 g), temperatures (450 and 500 ˚C) and flow rates 0.6 to 4.5 slpm. The maximum flow rate was limited by the system’s ability to maintain a constant catalyst bed temperature, as the high flow rate and endothermic reaction creates a significant temperature gradient across the catalyst bed. The catalyst temperature gradient did not exceed 20°C under any condition. The pressure in the feed/retentate was held at 5 bara, and the pressure in the permeate was 0.1 bara, resulting in a pressure difference of 4.9 bara.H2 flux through the membrane at 340 ˚C was also tested under a simulated 100% decomposed NH3 feed (75% H2 + 25% N2). Additionally, NH3 decomposition and H2 flux was measured over approximately 80 hours, using 150 g of catalyst at 450 ˚C and an NH3 flowrate of 4.2 slpm, with mass spectrometry measurements taken at various intervals for purity analysis.



The error bars for NH3 conversion and H2 flow, shown in Figures 3 to 6, are calculated from errors measured in the mass flow controllers and mass flow meters, calibrated against a certified volumetric flow meter.


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3 Results and Discussion 3.1 Ammonia decomposition performance Figure 3 presents the NH3 decomposition achieved by the two catalyst loadings at two temperatures. For the 52 g loading, equilibrium decomposition was not achieved, though by increasing the catalyst bed temperature from 450 to 500 ˚C, 90 % decomposition was achieved at a flowrate of 2.25 slpm. Increasing the catalyst bed loading by three times to 150 g resulting in higher total decomposition at lower temperatures, and at 450 ˚C, equilibrium decomposition was achieved for flowrates up to 5.75 slpm of NH3. Conversion at higher flow rates for the 150 g and 450 °C condition was not measured due to the bed temperature gradient far exceeding 20 °C.

Figure 3. NH3 decomposition (%) attained with two catalyst loadings and two catalyst temperatures, at a pressure of 5.0 bara

3.2 Hydrogen flux and recovery Figure 4 illustrates the total H2 recovery through the membrane (340 ˚C) under the same conditions as Figure 3. This shows that the maximum possible H2 recovery is significantly dependent on NH3 decomposition. However, once decomposition is at equilibrium, the recovery is limited by the membrane dynamics. Figure 5 shows the H2 flux and recovery (%) under a 75% H2 + 25% N2 feed, with varying feed rate at a total pressure of 5 bara and membrane temperature of 340 ˚C. As is characteristic of membrane separators, there is an inverse relationship between H2 flux and recovery. Removal of H2 through the membrane depletes H2 from the feed stream which decreases the trans-membrane pressure difference, and consequently, reduces the driving force for permeation. As the feed flow rate is decreased, the rate at which H2 permeates through the membrane approaches the rate at which H2 is fed to the reactor, the end result being a decrease in H2 flux (due to reduced driving force) but an increase in recovery (as more H2 exits the reactor via the membrane rather than via the retentate. The inverse is true at higher feed rates.



Figure 4. Total recovery of H2 through the membrane from catalytically decomposed NH3 gas streams at two catalyst loadings and two catalyst temperatures, with a membrane temperature of 340 °C and feed pressure of 5.0 bara

Figure 5. H2 flux and recovery under simulated 100 % decomposed NH3 feed at a membrane temperature of 340 °C and feed pressure of 5.0 bara


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3.3 Long term testing Figure 6 shows measured H2 flux and recovery over 80 hours using 150 g of catalyst at 450 ˚C, a membrane temperature of 340 ˚C, and NH3 flowrate of 4.2 slpm. H2 flux was initially 6.0 slpm, and reduced to 5.8 slpm over the course of the experiment. The magnitude of this decrease is in line with previous observations by this 20 and other groups, 24 and is attributed surface restructuring. As we have showed previously, exposure of the membrane surface to air for several minutes at 350°C is sufficient to restore the hydrogen permeability to its initial value24. The recovered H2 (or overall efficiency) scales directly with changes to H2 flux, which in this system is due to small fluctuations in total pressure and the gradual decrease in permeability. Over the duration of the test, this system produced approximately 2.5 kg of H2.

Figure 6. H2 flux and recovery over an extended period

3.4 Hydrogen purity The purity of the permeate H2 was tested using mass spectrometry by comparing it with high purity H2. Figure 7 shows mass spectrograms of the H2 permeate during the extended test under decomposed NH3, compared to a baseline of H2 permeate from a 99.99 % H2 feed. The figure also shows a plot of the ratio the two spectra. There is no significant change in the intensity of either the N, NH3 or H2O peaks, indicating that purity of the H2 product arising from decomposed NH3 is equivalent to or greater than that of 99.99 % H2. Spectra were taken at several intervals over the course of testing, after the membrane was air exposed and resealed, and no changes or leaks were detected.



Figure 7. Mass spectra of H2 permeate with i) decomposed NH3 feed and ii) 99.99% H2 feed, and iii) signal ratio of these two spectra

The high NH3 decomposition, H2 recovery and purity, and resilience to thermal cycling seen in this study show that PEM fuel cell-grade H2 can be produced using NH3 as a feedstock via the catalyst and membrane separation technologies demonstrated here. As discussed, there are several potential modes of operating this kind of system, depending on the conditions. When the only requirement of the system is to provide H2 for fuel cell systems and it can be heated using a renewable or waste heat source, then H2 recovery can be maximised, though at the expense of H2 production rate. Using low NH3 flowrates and high catalyst loadings at 450 ˚C, decomposition of NH3 near equilibrium and very high H2 recoveries can be achieved. In a stand-alone system, residual H2 in the retentate stream can be used as an accelerant to improve NH3 combustion characteristics to provide heat for the catalyst and membrane beds. If only 75 % of H2 is recovered, then the energy content of the retentate will approximately equal the endothermic requirement for NH3 decomposition. This can easily be achieved using this system with a high catalyst loading at 450 ˚C, and higher flowrates. Similarly, if the unit is coupled to a direct NH3 engine, turbine or fuel cell, the residual H2 can again be utilized. An advantage of this system is that it can be operated flexibly to maximise economic return, for example by favouring electrical or H2 output, depending on short-term demand.


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4 Conclusions In this study, the production of high purity H2 from NH3 using a two-stage process has been demonstrated, achieving an overall efficiency of ~ 90 %. This system produced 0.75 kg of H2 per day using one 150cm2 Pd/V/Pd tubular membrane at 340 ˚C, and 150 g of LiOH promoted 1 wt.% Ru on Al2O3 catalyst at 450 ˚C. Some degradation of membrane permeation was recorded over the course of a ~80 hr experiment that can be attributed to reversible surface restructuring. Mass spectrometry measurements of the H2 product showed that N2, NH3 and other contaminants were below detection limits, confirming that these membranes are capable of producing high-purity H2. These results show that the complexity of a catalytic membrane reactor, which would enable the higher decomposition of NH3 under operating similar conditions, is not required to achieve equilibrium NH3 decomposition at reasonable temperatures. This demonstration shows that the direct production of H2 from NH3 which meets ISO14687 is possible using this two-stage technology which is readily scalable by increasing the catalyst loadings and increasing membrane surface area. This could enable a pathway for distributed H2 from NH3 utilisation in mobile and stationary power generation.

Acknowledgement This work is supported by the Science and Industry Endowment Fund.



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23. Lamb, K. E.; Hla, S. S.; Dolan, M. D., Ammonia decomposition kinetics over LiOH-promoted, α-Al2O3-supported Ru catalyst, submitted. International Journal of Hydrogen Energy 2018. 24. Lundin, S.-T. B.; Patki, N. S.; Fuerst, T. F.; Wolden, C. A.; Way, J. D., Inhibition of hydrogen flux in palladium membranes by pressure–induced restructuring of the membrane surface. Journal of Membrane Science 2017, 535, (Supplement C), 70-78.



TOC graphic Two stage catalyst membrane reactor for producing high purity H2 from decomposed NH3


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via a ruthenium-based decomposition catalyst and

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