Using Biomass Gasification Mineral Residue as Catalyst to Produce Light Olefins from CO, CO2, and H2 Mixtures

Abstract Gasification is a process to transform solids, such as agricultural and municipal waste, into gaseous feedstock for making transportation fuels. The so‐called coarse solid residue (CSR) that remains after this conversion process is currently discarded as a process solid residue. In the context of transitioning from a linear to a circular society, the feasibility of using the solid process residue from waste gasification as a solid catalyst for light olefin production from CO, CO2, and H2 mixtures was investigated. This CSR‐derived catalyst converted biomass‐derived syngas, a H2‐poor mixture of CO, CO2, H2, and N2, into methane (57 %) and C2–C4 olefins (43 %) at 450 °C and 20 bar. The main active ingredient of CSR was Fe, and it was discovered with operando X‐ray diffraction that metallic Fe, present after pre‐reduction in H2, transformed into an Fe carbide phase under reaction conditions. The increased formation of Fe carbides correlated with an increase in CO conversion and olefin selectivity. The presence of alkali elements, such as Na and K, in CSR‐derived catalyst increased olefin production as well.


Introduction
Ceaselessly increasing both global greenhouse gas emissions and energy demand while depleting fossil resources constitutes a major issue for today's society. To solve this, radical changes in awareness, mindset, and behavior of both the consumer and industry are inevitable. Currently, we are living in a mostly linear economy; resources are converted to products and disposed after usage. Efforts are being made to transition to a circular economy, where resources are recycled. [1] It is thus indispensable to explore potential solutions and set off in new directions. On the one hand, we need to find efficient ways to mitigate anthropogenic emissions of greenhouse gases, such as CO 2 , and reverse global warming effects. [2] On the other hand, alternative feedstocks are required to meet the needs of the further increasing energy demand. [3][4][5] During the last years, the awareness of proceeding climate change and the urgent need to act grew, which forced the implementation of several climate change mitigation policies. [2] Nevertheless, the global CO 2 emissions are predicted to continuously increase. To solve this issue, carbon capture and storage (CCS) and carbon capture and utilization (CCU) are promising options. Especially the efficient conversion of captured CO 2 into value-added products, including fuels and chemical building blocks, could be a significant breakthrough. [6,7] In this context, thermochemical CO 2 hydrogenation towards value-added products gained attraction, as the broad range of possible output comprises not only hydrocarbons, but also higher alcohols and oxygenates. [8][9][10][11] In the past decade, many research efforts were made to revive the more than 100 years old Sabatier reaction, which is the catalytic hydrogenation of CO 2 towards methane. [12] Ni-based catalysts are typically used in this process because of their high activity and selectivity, while being inexpensive compared to noble metal-based catalysts, such as Rh, Ru, Au, and Pt. [12][13][14][15][16][17][18][19][20] However, converting CO 2 towards value-added products other than methane, using relatively cheap, abundant, and non-toxic transition metal catalysts (i. e., Fe, Co, Cu) would be a major advance.
Moreover, using waste residues as solid catalysts for CO 2 conversion towards value-added products would be of particular interest for the industrial sector. Hereby, on the one hand high costs for waste handling could be minimized, while on the other hand, industrial CO 2 emissions could be directly converted into value-added products in a circular fashion. Ideally, gaseous industrial waste streams could directly be used as reactant over the solid process waste products, which then act as solid catalysts for producing value-added products. To this end, the "recycled process residue catalyst" should be able to convert the waste stream, generally consisting of a mixture of various gases. For example, waste streams from biomass char gasification consist of CO, CO 2 , H 2 , and inert gases. [21][22][23][24][25] The solid residue that remains after this process may contain many different elements in various phases and oxidation states. Typically the main components are Si, Al, Ca, Mg, Fe, K, and Na, [22,24] but, for example, Ti, S, and P have also been reported. [23] It has already been documented that iron-containing char from biomass gasification catalyzes hydrocarbon cracking reactions. [26] The ideal solid residue would possess both CO 2 hydrogenation and Fischer-Tropsch synthesis activity, as the gaseous stream from gasification consists mainly of CO, CO 2 , and H 2 . For Fischer-Tropsch synthesis (FTS), an industrial process to convert CO and H 2 into synthetic fuels, Co and Fe are the most widely used catalysts. [27][28][29] Cobalt operates at 200-250°C and mainly yields linear C 5 + paraffins. Iron, on the other hand, is able to operate in a broader temperature range (200-350°C) and typically produces more olefins and oxygenates, particularly at higher temperatures (320-350°C). [25,[30][31][32] Fischer-Tropsch-to-Olefins (FTO), a subclass of the FTS process, is particularly interesting for the direct conversion of alternative carbon resources to lower olefins. [33] For this sustainable process, iron-based catalysts are preferred over cobalt-based catalysts because of their high olefin selectivity, low cost, and high water gas shift (WGS) activity. The latter enables the catalyst to alter the H 2 /CO ratio of the syngas, [32,33] which is essential when, for example, biomass is used as feedstock and the H 2 /CO ratio of the resulting syngas is below 1. [25,34,35] Biomass-derived syngas may additionally contain CO 2 and N 2 since air is typically used as oxidation agent in the biomass gasification process. [36] The specifications of biomass-derived syngas may lead to low conversion efficiency and worse catalyst performance. [35] Apart from the traditional FTS process, CO 2 hydrogenation to fuels, also referred to as modified (M)FTS, gained attraction in terms of CO 2 mitigation strategies. [37][38][39][40] Febased FTO catalysts yielded a very stable product selectivity when changing the gas feed from traditional CO and H 2 mixtures towards CO 2 , CO, and H 2 . [11,41,42] Besides, the addition of alkali metals, such as K and Na, to Fe-based catalysts has been reported to improve long-chain hydrocarbon and olefin production from CO 2 . [11,43,44] Accordingly, solid residue containing Fe and alkali metals might represent a promising candidate for industrial waste stream conversion.
In this work, we have investigated the applicability of a coarse solid residue (CSR) in CO 2 /CO/H 2 conversion. The CSR catalyst material was generated during a solid waste gasification process at Enerkem (Westbury, Canada). Hereby, we aimed to employ the CSR sample without further modification, potentially enabling the direct usage of industrial solid waste as a suitable solid catalyst to convert gaseous industrial waste streams into valuable products. To evaluate the potential of this CSR sample for CO/CO 2 hydrogenation, an Fe/SiO 2 reference catalyst with comparable iron oxide nanoparticle sizes was used, hereby mimicking the main active ingredients of the CSR sample. We show that CSR catalyst materials make methane and olefins from a CO, CO 2 , H 2 , and N 2 gas feed, thereby mimicking the composition of biomass-derived syngas. Alkali promoter effects on the CO 2 and CO hydrogenation performances were investigated using a KÀ Fe/SiO 2 reference catalyst. The presence of K enhanced the (reverse) (R)WGS activity and led to an increase in olefin production. Besides catalytic testing, operando X-ray diffraction (XRD) and Raman spectroscopy studies were performed to gain insights into the catalytically active phase and deducing a fundamental understanding of structure-performance correlations in the CSR samples. It was found that the increased presence of the iron carbide phase in the catalyst materials coincided with an increase in olefin selectivity.

Chemical composition
The mineral composition of CSR obtained during solid waste gasification, as determined with X-ray fluorescence (XRF) and inductively coupled plasma -optical emission spectroscopy (ICP-OES), can be found in Table 1. The main components were the metal oxides SiO 2 , CaO, and Al 2 O 3 , which in heterogeneous catalysis generally function as support or binder material and stabilize the catalytically active metal nanoparticles. Another important component in the CSR sample was Fe 2 O 3 . Fe is believed to be the catalytically active component in solid residues from gasification processes [26] and also the active ingredient in FTO-based catalyst materials. [45,46] Then, the alkali metals Na and K in CSR can act as a promoter or a poison, depending on their concentration and interplay with the active metal phase. [46][47][48] As promoters, the alkali metals both increase the reducibility of iron oxides and the carbon deposition rate. The latter is beneficial for FTO, as the active phase is considered an Fe carbide phase. [45] In the CO 2 hydrogenation reaction, the addition of alkali metals to Fe-based catalysts has been reported to increase the selectivity towards high-valued olefins due to RWGS activity. [43,48] Moreover, Mg and Ca that are present in CSR have been reported to increase the deactivation rate and the methane formation rate compared to unpromoted and Kpromoted iron-based FTO catalysts. [50] Besides, CSR contained Cr and Cu, which are known to promote the (R)WGS reaction as well. [51]

Morphology of the coarse solid residue sample
The morphology of the CSR sample and spatial distribution of the elements were investigated with electron microscopy (EM) and energy-dispersive X-ray spectroscopy (EDX). The CSR morphology resembled a typical Fe/SiO 2 heterogeneous catalyst: Fe nanoparticles supported by a SiO 2 matrix ( Figure 1). However, CSR contained more elements than just Fe and Si. For example, Al appeared to be in the same location as Si, whereas Ca and Ti appeared to be in close vicinity of the Fe nanoparticles ( Figures S1 and S2). The average Fe 2 O 3 nanoparticle size was 64 � 16 nm from the high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images ( Figure S3).

Crystalline phases and Fe crystallite sizes
The crystalline phases and Fe crystallite sizes in the CSR sample and Fe/SiO 2 reference catalyst were analyzed with XRD ( Figures 2 and S4

C 2 + hydrocarbon production from CO/CO 2 /H 2 mixtures
The CSR sample and the Fe/SiO 2 reference catalyst were catalytically tested in CO/CO 2 /H 2 /N 2 = 4.5 : 2.5 : 3 : 1 and at 5 bar pressure ( Figure 3). This particular gas feed composition was chosen with renewable energy resources in mind, as biomassderived syngas typically has a gas composition with H 2 /CO < 1 and may contain CO 2 and N 2 . [25,34] Prior to the reaction, the sample was heated in H 2 at 450°C to transform Fe 2 O 3 into metallic Fe. The reduction profiles of CSR and Fe/SiO 2 can be found in Figure S5. First, we explored the influence of reaction temperature by performing multiple tests at 250, 350, and 450°C (Figure 3a, c). We found 450°C as the optimum operating temperature for the CSR sample (Figure 3a), as the total carbon conversion increased with temperature. Besides, the very exothermic methane formation reactions from CO (ΔH = À 220 kJ mol À 1 at 450°C) and CO 2 (ΔH = À 183 kJ mol À 1 at 450°C) are thermodynamically more favorable at relatively low temperatures (see also thermodynamic calculations in Figure S6) and in high H 2 concentrations. This was indeed reflected by the methane selectivity, which was the lowest at 450°C. From CO and H 2 the C 2 and C 3 olefin formation reactions are exothermic, while from CO 2 and H 2 these reactions are endothermic. [49] In our study, higher temperature appeared favorable for the lower olefin yield (Tables S1 and S2) from the CO/CO 2 /H 2 /N 2 = 4.5 : 2.5 : 3 : 1 gas feed; propene was only formed at 350 and 450°C and not at lower temperatures. Compared to the total carbon conversion of the Fe/SiO 2 reference catalyst (24.9 % at 450°C), CSR had a lower overall carbon conversion (16.0 % at  450°C) (Tables S1-S4). The lower carbon conversion of CSR compared to the Fe/SiO 2 reference catalyst is explained by the lower content of the active ingredient Fe in CSR (1.9 wt %) compared to Fe/SiO 2 (7.7 wt %), as displayed in Table 1. Besides, the high content of K (1.9 wt %) and Na (2.5 wt %) in CSR could have a detrimental effect on catalytic performance. [47] Moreover, Mg and Ca in CSR could increase the deactivation rate and the methane formation rate during the FTO reaction. [50] The stabilities of the CSR sample and Fe/SiO 2 were evaluated in CO/CO 2 /H 2 /N 2 = 4.5 : 2.5 : 3 : 1, P = 5 bar, and T = 450°C (see Figure 3b, d and Tables S2 and S4). For CSR, the methane (59.3 %), C 2 (28.6 %), and C 3 (11.6 %) selectivities remained stable over the course of 24 h. In some of the gas chromatography (GC) injections, C 4 products were detected as well (Figure 4b and Table S2; 0.5 % average selectivity). However, the total carbon conversion decreased over time. Considering the high CO/CO 2 ratio (CO/CO 2 = 1.8) in the gas feed, the occurrence of the WGS reaction (CO + H 2 O!CO 2 + H 2 ; ΔH = À 37.8 kJ mol À 1 at 450°C) could provide a potential explanation for the decrease in carbon conversion. Fe-based catalysts are known to promote this reaction at moderately high temperatures (350-500°C). [51] Besides, CSR contains Cr, Cu, K, and Na, which are known to promote the WGS reaction as well. [51] CO and the inevitably formed H 2 O then produce CO 2 , which is less reactive than CO and consequently lowers the total carbon conversion. Besides, Mg and Ca, also present in CSR, have been reported to increase the deactivation rate during the FTO reaction. [50] An alternative explanation for the lower carbon conversion could be the oxidation of iron phases on the catalyst surface. CO 2 and H 2 O are known to have an oxidizing effect and thereby deactivate conventional iron-based catalysts. [25,32,52]

Fe carbides in the spent catalyst materials
The spent CSR sample as well as the Fe/SiO 2 reference catalyst material contained Fe carbides as determined with XRD (Figure 2). The Hägg carbide (Fe 5 C 2 ) is known to be the most stable Fe carbide phase under FTO reaction conditions, [45] and this was the only Fe carbide phase observed in this study. Fe 5 C 2 has also been proposed as main active phase in the FTO reaction and postulated as responsible phase for hydrocarbon chain growth. [53] To further investigate the correlations between alkali promoter elements, (R)WGS activity, Fe carbide formation, and catalytic performance, CSR, Fe/SiO 2 , and KÀ Fe/SiO 2 were tested consecutively for CO 2 hydrogenation, FTO, and again CO 2 hydrogenation to assess whether iron carbide formation affected the RWGS activity.

Reverse water gas shift activity promoted by alkali elements
To examine the occurrence of the RWGS reaction, we tested the CSR sample, the Fe/SiO 2 reference catalyst material, and a K- promoted Fe/SiO 2 reference catalyst material [0.71 wt % K and 7.6 wt % Fe (ICP-OES)] for CO 2 hydrogenation. Alkali elements, like K, are known to promote the (R)WGS reaction, [51] leading to an increased CO selectivity during CO 2 hydrogenation, which could boost the overall catalyst performance. First, the samples were pre-reduced at 450°C in N 2 /H 2 = 2 for 1 h. Then, the CO 2 hydrogenation performance was tested for at T = 250°C, P = 5 bar, and H 2 /CO 2 = 3. Subsequently, the FTO performance was tested at T = 350°C, P = 5 bar, and H 2 /CO = 0.7. Thereafter, the samples were again tested for CO 2 hydrogenation performance to investigate whether iron carbide formation, known to occur in the presence of H 2 -poor syngas, [45,46] affected RWGS activity. As can be seen in Figure 4, relatively low CO 2 conversions were observed at 250°C for all samples ( � 2 %). Interestingly, CSR displayed high RWGS activity and produced 91.9 % CO during the first CO 2 hydrogenation step (Figure 4a, b). The Fe/SiO 2 catalyst did not produce any CO and appeared thus inactive for the RWGS reaction (Figure 4c, d). However, the presence of the alkali element K promoted the RWGS activity drastically, as the KÀ Fe/SiO 2 catalyst produced 30.4 % CO (Figure 4e, f). The RWGS activity induced by K was beneficial for C 2 + production, as the C 2 + selectivity increased from 12.9 % with Fe/SiO 2 to 28.2 % with KÀ Fe/SiO 2 (CO-free selectivities). Besides, the C 2 -C 4 hydrocarbons produced by KÀ Fe/SiO 2 contained more olefins compared to unpromoted Fe/SiO 2 . The complete product distribution as well as olefin/paraffin ratios can be found in Figure 4. Catalytic performance in (consecutively) CO 2 hydrogenation reaction, FTO reaction, and subsequent CO 2 hydrogenation reaction of (a,b) CSR, (c,d) Fe/SiO 2 , and (e,f) KÀ Fe/SiO 2 . The CO 2 hydrogenation steps were carried out at T = 250°C, P = 5 bar, H 2 /CO 2 = 3, GHSV = 3070 h À 1 , while the FTO step was carried out at T = 350°C, P = 5 bar, H 2 /CO = 0.7, GHSV = 2425 h À 1 . The hydrocarbon (CH 4 and C 2 + ) selectivities displayed are CO and/or CO 2 -free. Prior to the first CO 2 hydrogenation step, the samples were pre-reduced at 450°C in N 2 /H 2 = 2 for 1 h.
Tables S5 and S6. Increased C 2 + production and olefin selectivity by iron-based catalysts upon K promotion has been reported previously for CO 2 hydrogenation. [25,43] During the FTO step, the initial apparent CO conversions were high ( � 11-25 %) for all samples (Figure 4), likely due to CO consumption for iron carbide formation. Under FTO conditions, carbon diffusion into iron has a lower activation barrier than the FTO reaction. [31] Hence, CO will mostly be used for iron carbide formation until a saturated metal carbide is formed. For Fe/SiO 2 , the catalyst performance deteriorated slightly over time, as more methane and CO 2 were produced (Figure 4c, d). KÀ Fe/SiO 2 (Figure 4e, f), on the other hand, produced less methane and CO 2 , while more C 2 + was formed over time. For CSR (Figure 4a, b), the CO 2 selectivity went down, the methane selectivity became slightly higher, and the C 2 + selectivity slightly lower over time (Tables S5 and S6). Alkali elements thus appeared to limit catalyst deactivation during the FTO reaction. Besides, alkali promoters clearly enhanced the WGS activity, as average CO 2 selectivities of 14.6, 27.5, and 73.1 % were observed for Fe/SiO 2 , KÀ Fe/SiO 2 , and CSR, respectively. The C 2 + selectivity again seemed to benefit from K promotion: On average Fe/SiO 2 produced 34.5 % C 2 + (Figure 4d), while KÀ Fe/SiO 2 displayed 40.1 % C 2 + hydrocarbons (Figure 4f), and CSR 60.5 % (Figure 4b) CO 2 -free selectivities). The C 2 + hydrocarbons produced by KÀ Fe/SiO 2 and CSR contained more olefins compared to unpromoted Fe/SiO 2 .
In the second, consecutive, CO 2 hydrogenation step, higher methane and CO selectivities were observed compared to the first CO 2 hydrogenation step for all samples. For CSR, 99.4 % CO was observed (Figure 4b), while Fe/SiO 2 displayed 48.0 % (Figure 4d) and KÀ Fe/SiO 2 58.1 % (Figure 4f). The iron carbide phase, as formed under FTO conditions (Figures 2 and S7), thus appeared to have a higher RWGS activity compared to the metallic iron phase present after reduction. These consecutive CO 2 hydrogenation/FTO/CO 2 hydrogenation experiments were additionally conducted at 450°C for CSR and Fe/SiO 2 (Table S7). Although the CO 2 and CO conversions were higher at 450°C than at 250 or 350°C, the general conclusions as drawn above also applied at higher temperature.

Fe carbides evolved as active phase in the coarse solid residue sample under reaction conditions
To further investigate the formation of iron carbides, operando XRD was performed under FTO conditions. Operando XRD patterns of the CSR sample were recorded after reduction at 450°C in H 2 , and during CO hydrogenation (H 2 /CO = 0.7) for 70 h at 450°C and 5 bar (see Figure 5). After the reduction procedure, the CSR sample contained a mixture of Fe 3 O 4 and metallic Fe (Figure 5a, b). Under H 2 -poor FTO reaction conditions this slowly transformed into the Hägg carbide phase (Fe 5 C 2 ) (Figure 5b), which correlated with an increase in CO conversion and an increase in C 2 + selectivity (Figure 5c, d). C 5 products were only detected after around 30 h time-on-stream. Both the CO conversion and the C 2 + selectivity reached a stable level after about 55 h time-on-stream. The olefin/paraffin ratio in the C 2 -C 4 products increased with increasing reaction time, suggesting that the Fe 5 C 2 phase is more selective to olefins compared to metallic Fe. A complete overview of all the products detected, including all isomers, can be found in Table S8. At 40-60 h time-on-stream (green XRD pattern in Figure 5b), CSR solely contained the Hägg carbide phase (Fe 5 C 2 ), while Fe 3 O 4 and metallic Fe were not observed. We can thus conclude that Hägg carbide is the (most) active phase for C 2 + production, as its emergence was correlated with enhanced catalytic performance. This is in accordance with an earlier study that ascribed the increase in C 2 + selectivity to the transition of metallic iron to iron carbides. [54] Improving C 2 + selectivity with increased reaction pressure To improve the selectivity to lower olefins, we increased the pressure to 20 bar and tested the CSR sample at different temperatures (250-450°C) ( Figure 6). Again, 450°C appeared to be the optimum temperature for CO/CO 2 conversion (total C conversion = 45.9 %) and C 2 + production. At 20 bar the methane selectivity went down to 57.1 % and the C 2 -C 4 olefin selectivity increased up to 42.9 %. The olefin/paraffin ratio increased with pressure as well (Table S9). Even though the CSR sample may not outperform commercial Fe-based FTO catalysts (CH 4 selectivity = 3-42 % [30] ) or Fe-based catalysts developed for CO 2 to fuels (CH 4 selectivity = 3-16 % [11,43] ), our results present an attractive strategy in the field of renewables. [25,55] Fecontaining CSR catalyzes the conversion of CO/CO 2 to valuable olefins and would otherwise have been discarded as an industrial waste product. Repurposing the waste product as a CO/CO 2 conversion catalyst represents an example of a strategy to reuse and thus minimize industrial waste streams.

Carbonaceous deposits evolved under optimized reaction conditions
To investigate whether or not carbon deposits were formed under reaction conditions, we have performed operando Raman micro-spectroscopy studies (Figure 7a, b). While X-ray diffraction techniques, like XRD, are usually the method of choice for crystalline solid materials (i. e., long-range order), Raman spectroscopy is more promising to analyze carbon deposits with a highly disordered structure. [56,57] Raman spectroscopy probes molecular structures (short-range order) and is sensitive to the degree of structural disorder. We compared the fresh CSR sample with an Fe 2 O 3 (hematite) reference. CSR indeed displayed the characteristic Fe 2 O 3 peaks at 222, 242, 291, 408, 490, 608, 662, and 1309 cm À 1 (Figure 7a). [58] Structural changes were not yet observed at 150°C in H 2 . However, at 350°C line broadening was visible, indicating that Fe 2 O 3 was transformed  into Fe 3 O 4 (magnetite). At 450°C, the iron oxide peaks had disappeared, indicating the presence of the Raman-inactive metallic iron. Then, CO/CO 2 hydrogenation was carried out at 20 bar and 450°C. After 1-3 h of reaction, bands at 1340 and 1580 cm À 1 appeared, indicating the formation of carbonaceous species. [54,58] This is in line with earlier studies on Fe-based FTO catalysts, where carbonaceous species were observed during CO hydrogenation. [45] Inactive carbon species could play a role in catalyst deactivation, [32] but carbonaceous species have also been reported as intermediates in CO hydrogenation processes. [54,58] While carbonaceous species evolved during our experiments, the total carbon conversion, as measured (semiquantitatively) with mass spectrometry (MS) and compared to a blank measurement, decreased. Besides, the MS response of CH 4 increased, while the C 2 -C 4 olefins MS response (Figure 7c) decreased over time. Although the deposition of carbonaceous species coincided with deteriorating catalytic performance, the apparent catalyst deactivation could also be assigned to other causes. For example, the presence of Mg and Ca has previously been reported to increase catalyst deactivation and methane formation. [50] Moreover, the presence of K, Na, Cr, and Cu in CSR promote the WGS reaction, [51] which could increase the amount of CO 2 in the gas feed and consequently decrease the catalytic performance. Re-oxidation of iron (carbide) nanoparticles was, however, not observed in the Raman spectra.

Conclusions
A coarse solid residue (CSR) material, which is a waste product obtained from solid waste gasification, was tested as a potential solid catalyst for CO and CO 2 hydrogenation. Fe carbides were identified as active phase during CO/CO 2 conversion with operando X-ray diffraction and were linked to an increase in CO/CO 2 conversion and the desired light olefin selectivity. More specifically, the CSR material produced 57 % methane and 43 % C 2 -C 4 olefins from a CO/CO 2 /H 2 mixture with a total C conversion of 46 % at T = 450°C and P = 20 bar. The alkali elements in the CSR material appeared responsible for the (reverse) water gas shift activity and for an increased C 2 + olefin production. With these new insights, the gasification process conditions could be optimized to obtain a catalytically superior CSR material, for example, with a higher Fe content. Strategies to reuse waste streams, like the one presented in this work, should be widely employed to minimize industrial waste output. Besides, recycling waste streams will decrease the usage of valuable raw materials required for, for example, catalyst synthesis. Considering that the CSR material used catalyzes the conversion of CO/CO 2 mixtures into valuable olefins and would otherwise have been discarded as industrial waste, our findings offer a new perspective on how waste streams can be utilized. In this manner, it adds to the concept of materials circularity and related metal scarcity abatement.

Catalyst preparation
The industrial waste sample, a coarse solid residue (CSR) sample, also called "slag", was generated during solid waste residue gasification process at Enerkem (Westbury, Canada). The CSR sample was used without further treatment. The Fe/SiO 2 (7.7 wt % Fe) and the KÀ Fe/SiO 2 (0.71 wt % K and 7.6 wt % Fe) reference catalysts were prepared by the incipient wetness impregnation (IWI) technique. To this purpose, commercially available high-purity grade silica gel (Davisil Grade 643, pore size 150 Å, 200-425 mesh, Sigma-Aldrich) was used as support material. The synthesis procedure was conducted as follows. Initially, the required amount of FeCl 3 · 6 H 2 O (Sigma-Aldrich, � 99 %) was dissolved in water, whereby the volume of water was adjusted to that of the pore volume of the silica support. After impregnation and drying at 60°C for 24 h, calcination of the impregnated catalyst was conducted under flowing N 2 (100 mL min À 1 ) in a tubular furnace at 450°C for 5 h (5°C min À 1 ramp). The K-promoted Fe/SiO 2 catalyst was prepared by consecutive impregnation with an aqueous solution of K 2 CO 3 (Sigma-Aldrich, � 99 %), drying, and calcination as described above.

Catalyst characterization
Elemental composition: The metal concentrations were determined via ICP-OES with a PerkinElmer Avio ® 500 ICP Optical Emission Spectrometer. Here, the CSR and Fe/SiO 2 samples were prepared by iron extraction in aqua regia. Additionally, the chemical composition of the CSR sample was analyzed by XRF spectrometry (Panalytical, Axios Advanced).
Transmission electron microscopy: Electron microscopy investigations were performed in transmission mode and HAADF using a FEI Talos F200X microscope operating at 200 kV. Elemental mapping was performed using EDX. For the TEM measurements, the samples were suspended in ethanol under ultrasonic vibrations. Subsequently, a drop of the suspension was deposited onto a Carbontype B copper 200 mesh grid. For determining the particle size distribution of the supported nanoparticles from TEM images, the software ImageJ was used for manually fitting the particle diameters (> 100 particles).
Ex-situ X-ray diffraction: Ex-situ XRD patterns of the fresh and spent catalysts were recorded on a Bruker D2 Phaser X-ray diffractometer using Co K α12 radiation (λ = 1.790 Å) in the range of 2θ = 20-80°with a scan step size of 0.01°and scan time 1 s per step. The Fe 2 O 3 average crystallite sizes were estimated by applying the Scherrer equation (k-factor of 0.9) to the (012) diffraction of Fe 2 O 3 (2θ = 28.0°).
Operando X-ray diffraction: Operando XRD patterns were recorded on a Bruker D8 Discover X-ray diffractometer in Debye-Scherrer transmission (capillary) geometry with a Mo (K α1 = 0.709 Å) source was used. At the beginning of each operando reaction run, the capillary was moved to the focus of the X-ray beam (beam � 600 × 15 000 μm, height × width) for maximum diffraction. The XRD patterns were collected over a 2θ range of 7-25°with a scan step size of 0.015°. Data were collected of the fresh sample, during reduction at 450°C in pure H 2 (3 mL min À 1 ), after reduction, and during CO hydrogenation (CO/H 2 /He = 2.25 : 1.5 : 1). The products were analyzed with on-line GC (Thermo Fischer Scientific).
Operando Raman micro-spectroscopy: Raman spectra were recorded using a Horiba Xplora with a 532 nm laser and 1200 grating for 30 s with 5 accumulations. For operando experiments, Raman high temperature reaction chamber from Harrick Scientific, suitable for high pressures and temperatures, was used. To monitor the iron phase during the reduction procedure, the CSR sample was heated to 450°C with 10°C min À 1 in 3 mL min À 1 H 2 and 3 mL min À 1 Ar. Then, the sample was exposed to 2.2 mL min À 1 CO, 1.2 mL min À 1 CO 2 , 1.5 mL min H 2 , and 0.2 mL min À 1 Ar at P = 20 bar and T = 450°C. Meanwhile, Raman spectra were recorded to monitor the iron phase and carbonaceous species under reaction conditions. The gaseous products were analyzed with on-line MS (Pfeiffer Vacuum).

Catalytic performance
Catalyst performance was tested in a fixed-bed reactor. The steel reactor was typically filled with 200-250 mg of catalyst sample sieved to a grain size of 150-425 μm. The sample was plugged between two quartz wool plugs. The reactor was placed in an oven and was connected to the gas inlet and outlet. A back pressure controller (BPC) was incorporated in the gas line connected to the outlet to maintain a defined pressure. An on-line gas Thermo Fischer Trace 1300 GC was used for product analysis. The catalyst was reduced in 20 mL min À 1 H 2 and 40 mL min À 1 N 2 at 450°C for 1 h (10°C min À 1 ramp). Then, the reactor was cooled down to 250°C with a 10°C min À 1 ramp in the same atmosphere. At 250°C the gas flow was switched to 22.5 mL min À 1 CO, 12.5 mL min À 1 CO 2 , 15 mL min À 1 H 2 , 5 mL min À 1 N 2 , and 1.2 mL min À 1 Ar. The pressure was built up with 1 bar min À 1 to 5 bar or 20 bar. For 12-24 h the products of the reaction were analyzed with an on-line GC (injection every 23 min). This was repeated at 300-450°C. The conversion and selectivities were calculated from the obtained GC data. The amounts of converted CO or CO 2 were calculated using Equation (1): A CO and A Ar represent the thermal conductivity detector (TCD) peak area of CO and Ar during the reaction. A 0 CO and A 0 Ar are the TCD peak areas of CO and Ar recorded during a blank measurement. The selectivity was calculated using Equation (2): In this equation, A i corresponds to the peak area of product i, and F i represents the response factor of the analyte. [59]