Applied research on methane steam reforming properties of porous structural catalyst fabricated by selective laser melting technology

In this paper, structured catalysts with different specific surface areas were fabricated by selective laser melting (SLM), and their catalytic properties were tested by methane steam reforming experiments. The results show that the structured catalyst prepared by SLM shows high structural stability and catalytic activity with H2 yield of 51.44% and CH4 conversion of 71.94%. The structured catalyst prepared by SLM can be impregnated with the traditional catalyst, which can improve the catalytic efficiency. After impregnation, the H2 yield and CH4 conversion rate of structured catalyst can reach 71.98%, and 99.53%, respectively. Compared with the traditional catalyst, the methane conversion rate is significantly improved. This work provides a simple and efficient manufacturing method for the functional integration of catalyst and reactor, which will promote the development of chemical synthesis and SLM.


Introduction
Steam Reforming of Methane (SRM) is the most widely used hydrogen production technology in industry, which has the characteristics of a simple process with low cost, clean and high efficiency.Its large hydrogen production can meet the needs of industrialization [1][2][3][4][5][6][7].SRM is greatly affected by the catalyst, and the characteristics of the catalyst directly determine the effect of methane steam reforming.Therefore, the choice of catalyst is very important for the practical application of reforming reaction [8].Catalytic system is the core of modern energy, and it is generally composed of active components, structural carrier and reactor [9][10][11][12][13][14].In recent years, the integrated design concept of catalyst active component, structural carrier and reactor has become a c a new trend in the field of catalysis.Structured catalyst is considered to be a promising research direction in the field of heterogeneous catalysis.
With the ongoing advancement in 3D-printing technology, the scope of materials encompassed by this technology is continuously expanding.The extensive utilization of polymers, metals, metal oxides, and other materials in the field of 3D-printing presents novel possibilities and approaches for the design of controllable structure catalysts.Selective laser melting (SLM) technology exhibits exceptional processing flexibility and precision in forming, enabling the creation of intricate and high-performance porous catalyst structures [15][16][17].Through the design and optimization of structural models, highly complex and accurate 3D geometries can be flexibly prepared, which improves the heat transfer and mass transfer performance of the catalyst in fluid flow [18][19][20].Therefore, SLM can be applied in the development of high-performance catalysts.The porous structure catalysts prepared by SLM can improve the mass and heat transfer efficiency of gas in the reaction process, so as to increase the yield of the final product.
In the SRM, due to the inconsistent exothermic reaction, the uneven mixture of fluid into the catalytic reforming zone is easy to form hot spots, local overheating and carbon accumulation, and even clog the pipeline reactor in serious cases, resulting in reduced catalytic efficiency [21][22][23][24].Therefore, the optimal design of the reactor structure is imperative for ensuring the sustained stability and performance of the entire catalytic system.The high packing density of the traditional granular catalyst can easily lead to excessive pressure drop in the reactor and poor heat and mass transfer performance of the fluid during the reaction, thus affecting the activity of the catalyst [25][26][27][28][29][30].In recent years, the extensive utilization of static mixers in industrial mixing, heat transfer, and reaction units has presented a novel thinking for exploring structural catalysts in methane steam reforming [31,32].Qinhong Wei et al [14] utilized Selective Laser Melting (SLM) technology to design and fabricate autocatalytic reactors of Ni, Co, and Fe materials (denoted as FE-SCR, Co-SCR, and Ni-SCR, respectively).These three SCR catalytic reactors were employed for Fischer-Tropsch synthesis, carbon dioxide hydrogenation, and dry reforming of methane (DRM).In particular, Fe-SCR and Co-SCR successfully demonstrated the synthesis of liquid fuels through Fischer-Tropsch synthesis and CO 2 hydrogenation, respectively.Notably, Ni-SCR exhibited remarkable performance in terms of the conversion of CO 2 and CH 4 in high-temperature methane dry reforming.The obtained results manifest that the autocatalytic reactor developed via SLM exhibits desirable characteristics, including high pressure and high-temperature resistance, alongside exceptional catalytic capability.Milan Kundra et al [33] used Electron Beam Melting (EBM) technology to prepare a catalytic static mixer of nickel-based alloys, which was treated with etched or leachate solution to activate its surface for hydrogenation of alkenes, aldehydes and nitrates, and used a tubular continuous flow reactor.Their performance was compared with that of industry standard alumina catalysts.Catalytic static mixers prepared by EBM are treated chemically resulting in a significant increase in the surface area and catalytic activity of these static mixers.In particular, the monel alloy static mixer using a persulfate solution combination has a significant enrichment of nickel on the surface, and has improved catalytic activity and surprising selectivity for non-substituted double bonds.The application of SLM technology to methane reforming catalytic reactor can effectively improve the flexibility of reactor structure design and space utilization.At present, SLM technology has made preliminary progress in the field of catalysis, and some studies have also been carried out in methane reforming.The structural design of methane reforming reactor has already achieved initial results, but the reaction structure still needs further research.In particular, the effect of the porous structure of the reactor, the catalyst load and the integrated design of the catalyst reactor on the catalytic reforming effect of methane.
The catalyst structural design in this paper draws inspiration from the structure of static mixers and utilizes catalyst structures prepared using SLM technique.The specific surface area of catalyst markedly affects the methane steam catalytic reaction.Increasing the catalyst surface area by suitable structural design can lead to a higher number of loaded active sites, thus improving the catalytic efficiency.Additionally, a well-designed structure can enhance fluid mixing and heat and mass transfer effects.After comparing various types of static mixer structures, the SMX structure is selected as the structured catalyst of choice.Static mixers of the SMX structure type have good mixing efficiency and diffusion rate performance at low reynolds number, and compared with other types of static mixers, SMX static mixers can provide a larger surface area.Thus, more catalytic components adhere to the surface of the mixer to improve catalytic efficiency.The innovation of the paper is to elucidate the structure-activity relationship between catalytic efficiency and design parameters of SLM SMX porous structure.Four kinds of catalysts with different surface area structures were prepared by SLM, and the experiments of methane steam reforming were carried out.The geometric study of the structural catalyst shows that the structural catalyst prepared by SLM has good structural stability and excellent catalytic performance, and SLM itself can enhance synergies between catalyst and reactor.We wish that this study will facilitate the further development of selective laser melting in the field of catalysis.

Experimental procedure 2.1. Structural design
In this study, Plate type static mixer (SMX) is selected.Through consulting the data, it is found that with the increase of the arrangement Angle of adjacent mixing units, the lateral velocity of the SMX static mixer increases and the mixing effect increases gradually when the fluid flows through the mixing unit.The SMX static mixer has a compact structure and a larger specific surface area, enabling it to load more catalytic active components.As shown in figure 1(s), the SMX static mixer consists of a number of plates crossed at 90°, with rectangular holes traversed by 45°in an axial Angle and square holes traversed by a radial Angle, with each unit rotated by 90°r elative to the previous unit.The length of each SMX mixer unit is equal to its diameter (D = 10 mm), as shown in figure 1.The surface area of the catalyst was controlled by adjusting the size of the square hole (The dimensions of the square holes are shown in table 1), which was verified by the methane steam reforming experiment.

Catalyst preparation
In this study, structured catalysts were prepared using selective laser melting (SLM).The base powder used in the experiment was Inconel 625 spherical powder.The matrix powder used in the experiment was Inconel 625 spherical powder, and the microscopic morphology and particle size of the powder were shown in the figure 2. It can be seen that the raw material powder was mainly spherical powder and near-spherical powder.The agglomeration of shaped powder, irregular powder and less ellipsoid powder was not obvious, and the particle sizes of D 10 , D 50 and D 90 were 23.1 μm, 34.5 μm and 51.3 μm, respectively.Powders with a particle size of 35.3 μm accounted for most of the proportion.The process parameters were determined experimentally: laser power of 140 W, scanning speed of 1000 mm s −1 , and scanning pitch of 0.21 mm, and the catalyst with the desired structure and morphology were successfully prepared, as shown in figure 3. It has been demonstrated that the static mixer changed the flow rate  and direction of the fluid through the collision of the fluid with the mixer surface, thereby producing a mixing effect.Therefore, surface roughness held effects on the mixing effect of fluids in the reactor.Moreover, the surface of the structured catalyst prepared by SLM was uneven, and the surface roughness helped to improve the distribution of the active components of the catalyst and improve the catalytic efficiency of the catalyst.Thus, the roughness of the mixer surface had a certain impact on the sensitivity of SMX.
The catalyst preparation components for impregnation were prepared as shown in table 2. 3.5% of the binder polyvinyl alcohol (PVA) was dissolved in 120 ml of deionized water at 35 °C and stirred at 300 r min −1  Table 2. Catalyst formulation ingredients.

Composition
Dosage Condition until the PVA was fully dissolved and then 19% nickel nitrate (Ni(NO 3 ) 2 −6H 2 O) and 16% cerium nitrate (Ce(NO 3 ) 3 −6H 2 O) were added.After stirring until completely dissolved, 2.5% alumina (γ-Al 2 O 3 ) powder was added and stirred for 10 min, after which 0.5 g of KOS110 dispersant was added to the well-stirred solution and stirred for 10 ∼ 12 min to obtain the catalyst solution ready for impregnation.The 3D printed structured catalyst was then impregnated with the prepared catalyst solution for 10 min by ultrasonic impregnation, removed and dried at 100 °C for 2 h to observe the impregnation.The impregnation step was repeated until the printed structure was fully covered by the catalyst.The obtained impregnated catalyst was calcined at 600 °C for 4 h and then reduced by passing H 2 at 600 °C for 6 h to obtain a 3D printed porous structure impregnated with catalyst.The catalyst impregnation experiments were conducted on the whole SMX with 10 units.And the loading amount of catalyst was measured according to the weight differences of SMX before and after the catalyst impregnation.The impregnation amounts of the four structures of SMX-A, B, C, and D were calculated to be 2.324 ± 0.100 g, 1.774 ± 0.094 g, 1.422 ± 0.210 g, and 1.100 ± 0.160 g respectively through multiple weighing.

Catalyst characterization
The N 2 adsorption-desorption isotherm curves of the catalysts were collected using a Micromeritics ASAP 2020 physisorption apparatus.The nitrogen adsorption-desorption isotherm curves were obtained by adsorption and desorption of nitrogen at a liquid nitrogen temperature of 77 K.The specific surface area of the catalysts was calculated according to the Brunner−Emmet−Teller (BET) measurements equation, and the pore size distribution was calculated by the Barret-Joyner-Halenda (BJH) method using the nitrogen adsorptiondesorption isotherm.The structured catalyst surface morphology was obtained by scanning electron microscopy (SEM; Nova-Nano-450) and energy dispersive x-ray spectrometry (EDS).The phase composition of the Inconel 625 specimens was analyzed before and after catalysis or impregnation with catalyst, respectively.

Catalyst activity evaluation
In this experiment, a self-developed methane steam reforming experimental platform was built to evaluate the catalytic activity of the catalyst.The apparatus consists of a three-way gas phase feed, a steam generator, a heating furnace, and a gas detector, as shown in figure 4.
The SRM catalyst activity evaluation was carried out in a fixed bed stainless steel tube reactor (ID = 14 mm).The reaction feedstock was pure methane (99.99%) and deionised water.The deionised water was first injected into a steam generator at 150 °C through a syringe pump to gasify at a feed rate of 0.081 ml min −1 , with a methane gas flow rate of 50 ml min −1 , and an inert gas N 2 as the carrier gas flow rate of 5 ml min −1 , and a waterto-carbon ratio of 2.4:1, which was mixed in the steam generator and then fed into the reactor.The reaction temperature was set to 500 °C, 600 °C, 700 °C, 800 °C and 900 °C.The reaction gas was cooled and dried in a water bath and then fed into a gas detector to detect the ratio of CH 4 , H 2 , CO and CO 2 in the exhaust gas.
In this series of experiments, the percentage content of the four gases H 2 , CO 2 , CO and CH 4 in the tail gas was measured by a gas detector at the outlet, and the conversion, yield and selectivity were calculated based on the data collected from the experiments, and the results were calculated as follows.

Catalyst characterization
SEM and EDS were used to characterize the surface morphology of both the SLM fabricated porous samples and the impregnated samples, figure 5 shows the porous catalyst sample prepared by SLM.The results demonstrated that the SLM-fabricated structured catalyst exhibited a micro-porous network structure on the surface, with a large amount of adsorbed unmelted particles and a small amount of agglomerated particle powder.Such a micro-porous structure augmented the attachment sites of the active components on the catalyst surface, thereby effectively improving the catalytic performance.EDS element analysis results are displayed in figure 5(f).Ni and Cr elements were uniformly distributed on the catalyst surface, and the homogeneous dispersion of the Ni element contributed to enhancing the catalyst activity, while the distribution of Cr element helped to strengthen the catalyst's structural stability.
The surface characterization of the impregnated catalyst is shown in figure 6.After impregnation, the catalyst was successfully loaded onto the surface of the structure, and some unmelted particles still existed on the structure surface, which were not covered by the impregnated catalyst.The impregnated catalyst and the SLMfabricated structured catalyst were coordinated to improve the distribution of catalytic active components on the catalyst surface, thus enhancing its catalytic performance.In addition, EDS detection was performed on two points on the surface of the catalyst structure after immersion (figures 6(d), (e)).The immersed catalyst can effectively adhere to the surface of the SLM-prepared structure catalyst and cover the mesh pores on the surface.The particle firmly fixes the unmelted particles to the structure, and the combination of the impregnated catalyst  and the catalytic active components of the structural catalyst prepared by SLM effectively improves the catalytic efficiency.
To investigate the impact of catalyst structure on catalytic performance, the structured catalyst and impregnated structured catalyst were characterized using the BET testing instrument.Changes in the specific surface area and pore size distribution of the 3D printed structured catalyst before and after impregnation were analyzed.The results of the specific surface area and pore size distribution of the samples are presented in table 3.
There were pores on the surface of the structural catalyst prepared by SLM, and the pore volume was measured to be 4.67 × 10 −4 cm 3 /g by SEM observations.After the NiO-CeO 2 /γ-Al 2 O 3 catalyst was impregnated, the pore volume increased to 1.15 × 10 −3 cm 3 /g.There were new pores formed on the surface during the impregnation process, resulting in the increase of pore volume of the catalyst, and the increase of pore volume contributed to the improvement of catalytic efficiency.
According to the above table, the specific surface area of the 3D printed part was 4.9 × 10 −1 m 2 /g and the average pore size was 3.81 nm.After impregnation with the catalyst, the specific surface area of the surface area structure increased to 5.7 × 10 −1 m 2 /g and the pore volume and average pore size also increased significantly.It is assumed that the catalyst covered the mesh pore channels on the surface of the 3D printed part, but did not completely block the pore channels, resulting in a moderate increase in the specific surface area of the printed part.The high specific surface area, pore volume and pore size provide more reaction space for the catalytic reaction, thus ensuring a higher catalytic activity of the catalyst.
Figure 7 displays the N 2 adsorption-desorption isotherms and pore size distributions of structured catalysts fabricated by 3D printing and impregnation.The N 2 adsorption-desorption isotherms of the structured catalysts fabricated by SLM showed a combination of type I and type IV isotherms in figure 7(a), indicating the presence of both microporous and mesoporous structures.The corresponding pore size distribution curve obtained from nitrogen desorption data further confirmed that the material possessed a hierarchical micro/ mesoporous structure.After impregnation, the isotherms of the structured catalysts mainly exhibited a combination of type I and type IV isotherms, and the pore size distribution plot showed a hierarchical porous structure with micropores, mesopores, and macropores.The results indicated that the pore size of the 3D printed catalysts predominantly existed in the form of micro/mesopores.In contrast, the catalysts after impregnation had larger pore volumes and pore sizes, and the pore sizes were mainly distributed in a hierarchical porous structure.It is speculated that during the impregnation process, the catalyst adhered to the surface of the 3D printed part, covering the surface pores, and resulting in an increased pore volume.Notably, the structure of the 3D printed part did not collapse, and the pore structure maintained high structural strength during the impregnation treatment.
The structural catalyst prepared by SLM and the XRD pattern after immersion are shown in figure 8.It can be seen from the figure that the structured catalyst shows diffraction peaks for γ-Ni species at 2θ of 43.49°, 50.67°a nd 74.53°, which are applied to the diffraction peaks at the γ-Ni (111), ( 200) and (220) sites, respectively.It can be seen that the intensity of the diffraction peaks in the (111) plane of the structured catalyst is slightly reduced after impregnation of the catalyst, indicating that the loading of the catalyst covers the Ni element on the surface of the original 3D printed part and thereby reducing the intensity of the γ-Ni diffraction peaks.

Catalyst performance evaluation
In order to investigate the catalytic efficiency of the printed catalysts at different temperatures, four kinds of printed samples with different structures were inserted in the reaction tube, and the experimental equipment was assembled and connected according to a flow chart.The inlet gas flow rate of CH 4 was 50 ml min −1 , the flow rate of deionized water into the steam generator was 0.081 ml min −1 , and the water-carbon ratio was 2.4:1 by calculation.Meanwhile, 5 ml min −1 of inert gas N 2 was passed as the carrier gas.Prior to the gas injection, 20 ml min −1 of N 2 was introduced, the temperature was maintained up to 300 °C and held for 30 min.Then, by adjusting the temperature of the reactor, the catalyst in different temperatures (reaction temperature of 500 °C,  In the experiment, three evaluation indicators of four different structured catalysts gradually increased with the increasing temperature.However, in the low-temperature zone (500 °C, 600 °C, and 700 °C), the three performances of H 2 yield, CH 4 conversion rate, and CO selectivity were at a low level.According to the process of methane steam reforming reaction and thermodynamic parameters, methane steam reforming reaction did not fully occur at this time.The CH 4 conversion rate was at a low level.With the gradual increase of temperature, the reaction began, and the CH 4 conversion rate gradually increased.Therefore, within the temperature range of 700 °C −900 °C, the three indicators of the catalyst gradually increased, and the yields of CO and H 2 were significantly improved.When the temperature was above 700 °C, the promotion of methane steam reforming reaction was facilitated by the temperature rise, but the side reactions of water-gas shift(CO + H 2 O → CO 2 + H 2 ), methane decomposition reaction (CH 4 → C + 2H 2 ), and CO disproportionation reaction (2CO →C + CO 2 ) were also occurred simultaneously.Therefore, while the yields of CO and H 2 were increased, they were also consumed by the side reactions.
As demonstrated in the graph displaying the variation of structured catalyst performance with temperature, SMX-A structure has a larger specific surface area, a higher content of active ingredient Ni, a higher density of catalytic active sites, and a larger reaction space, which accounts for its better CH 4 conversion rate of 71.9%.Meanwhile, as the porosity increases and the specific surface area decreases, the CH 4 conversion rate and the H 2 yield gradually decrease.Both the activity graph and the graphs of H 2 yield, CH 4 conversion rate, and CO selectivity variation with temperature reveal that the increase of porosity leads to a gradual decrease in catalyst activity.SMX-A structure shows the highest catalytic activity, with a CH 4 conversion rate of 71.9% and an H 2 yield of 60%.However, when the structural porosity reaches 93.95%, the CH 4 conversion rate and the H 2 yield decrease to 56.8% and 53.6%, respectively.This slight decrease in catalytic activity might be attributed to the reduction in Ni content, which results in less-loading of active molecules Ni on the catalyst surface.As shown in figure 9(b), the conversion rates of SMX-D and CH 4 decreased at 900 degrees.In the reforming reaction, Cr element would react with C and water to form corresponding compounds, which would adhere to the surface of the catalyst.The surface area of SMX-D was small, causing the catalytically active molecules on its surface to be easily covered, thereby reducing the catalytic efficiency and CH 4 conversion rate.
To investigate the influence of structure on catalytic performance, NiO-CeO 2 /γ-Al 2 O 3 catalysts with A, B, and C structures were impregnated using the immersion method and methane steam reforming experiments were conducted at the same conditions.As shown in figure 10, the three performance indicators gradually increased with increasing temperature and there was a small amount of CH 4 reaction at low temperatures after the catalyst was adsorbed.When the temperature reached 900 °C, the performance indicators tended to be stable.With increasing surface area, H 2 yield and CH 4 conversion rate increased to a certain extent.The tail gas produced by SMX-B structure after impregnation of catalyst at 900 °C is 141.92 ml H 2 and 0.98 ml CH 4 .Specifically, the H 2 yield of SMX-B structure reached 71.98%, the CH 4 conversion rate was 99.53%, and the CO selectivity was 76.94%, which performed best among the three structures with different surface areas.However, it was also found that excessive surface area may lead to poor fluidity in the structure, affecting the catalytic efficiency.Meanwhile, higher surface area and smaller pores can cause coke accumulation and gradually block pores, which can hinder the catalyst's long-term usage and affect reactor fluidity.
As shown in figure 9, for SMX-B, when the temperature is lower than 700 °C, the catalytic reaction didn't occur.And catalytic reactions began to occur when the temperature rose above 700 °C.When the temperature reached 800 °C, the CO generated by the catalytic reaction reacted with O ions to generate CO 2 .At this time, the catalytic reaction efficiency is not high, resulting in a low CO content and low CO selectivity.When the temperature reached 900 °C, the catalytic reaction efficiency was greatly improved and the CO content was significantly increased, so the selectivity of CO was also improved.After impregnation with NiO-CeO 2 /γ-Al 2 O 3 catalyst, SMX-B began to undergo catalytic reaction at 500 °C.When the temperature was higher than 600 °C, the reaction rate accelerated and the CO content increased.So, the CO selectivity increased, resulting in fluctuations in gas detection.
In order to investigate the surface morphology of the catalyst before and after the reaction and the carbon build-up before and after the catalyst loading, the catalyst was characterized by SEM. Figure 11 shows the surface morphologies of 3D printed parts directly after the reforming reaction.The results showed that there are fine particles on the surface of catalyst, which are distributed on the surface of spherical particles.At the same time, no significant carbon accumulation was observed.presence of aggregation of Ni on the surface of small granular material which suggests that Ni is accumulated on the surface of the structured catalyst after catalysis.
Figure 12 shows the surface morphology of the impregnated catalyst prints after the catalytic reaction.It was found that there was a small amount of block accumulation on the surface.In order to determine whether the block was carbon deposition, EDS elemental analysis was performed on the block (figures 12(d), (e)).The results indicate that the main element in the block is C, indicating the formation of carbon deposition on the catalyst surface.The structural catalyst has a certain inhibitory effect on the formation of carbon deposition.However, due to the short catalytic reforming time, there was no significant carbon deposition phenomenon.

Conclusion
In this article, four different catalysts with distinct surface areas were prepared by using SLM technique, and characterized using BET, XRD, SEM, EDS and other methods.The catalyst performance of these structured catalysts was then evaluated using a catalytic evaluation device, and the catalytic performance of different surface areas was observed by changing the surface area of the structured catalyst.To compare the performance of structured catalysts prepared by SLM, NiO-CeO 2 /γ-Al 2 O 3 catalysts were impregnated on three structured catalysts with different surface areas using the traditional impregnation method, and their catalytic performance was investigated.The specific results are as follows: (1) SLM technology was successfully used to prepare functional integrated structured catalysts with catalytic effects, and the catalytic performance increased gradually with the increase in surface area.The catalytic performance of the structured catalysts prepared by SLM was evaluated using the developed catalytic evaluation device.The results showed that the best performance was achieved by the SLM-prepared SMX-A structured catalyst, with the highest CH 4 conversion rate of 71.94%, H 2 yield of 51.44%, and CO selectivity of 67.38% under the working condition of 900 °C.Although the structured catalysts prepared by SLM showed catalytic activity and structural stability compared with the traditional Ni/Al 2 O 3 catalysts, their catalytic performance still needs to be further improved.
(2) NiO-CeO 2 /γ-Al 2 O 3 catalysts were impregnated on structured catalysts prepared by SLM using the traditional impregnation method, and their catalytic performance was evaluated using the developed catalyst evaluation device.The results showed that the catalytic efficiency of the structured catalysts impregnated using the traditional impregnation method was significantly increased, and the catalytic performance increased first and then decreased with the increase in surface area.The SMX-B structured catalyst after impregnation showed the best performance, with the highest catalytic performance under the working condition of 900 °C, with a CH 4 conversion rate of 99.53%, H 2 yield of 71.98%, and CO selectivity of 76.94%.
(3) The catalytic efficiency of structural catalysts with different pore sizes prepared by SLM before and after impregnation was different.During the reaction process, the impregnated NiO-CeO 2 /γ-Al 2 O 3 catalyst was deactivated at high temperatures, resulting in clogging of the structural catalyst.In addition, as the experiment continued, side reactions continued to occur during the methane-water vapor reaction, causing the catalytically active components to be covered by the carbon produced by the reaction.This further caused catalyst clogging and reduced catalytic efficiency.Therefore, the SMX-A structure catalyst with the smallest pore size was more likely to be clogged, thereby reducing the catalytic efficiency.As the pore size increased, the coking and deactivation phenomena of the impregnated catalyst were improved, so the catalytic effect of SMX-B after impregnating the NiO-CeO 2 /γ-Al 2 O 3 catalyst was higher than that of SMX-A.However, the active components of the structural catalyst surface decreased and the catalytic effect gradually decreased with the increasing of pore size.
Compared with the traditional powder or granular nickel catalysts, our catalysts were manufactured by 3D printing technology and can be a cost-effective alternative to nickel or other transition metal catalysts that are structurally complex and difficult to prepare.This work provides a simple, rapid and feasible technique, which is conducive to the functional integration of the active component, the structural carrier and the reactor, and provides a useful reference for the subsequent design of the catalytic system.We expect that this autocatalytic design will open up new possibilities for 3D printing technology in the field of chemistry and catalysis.

Figure 3 .
Figure 3. Structured catalysts samples fabricated via SLM process: (a) Structural catalyst multi unit combination; (b) catalyst unit structures.

Figure 4 ./
Figure 4. Schematic diagram of the methane water reforming reactivity evaluation unit.

Figure 9 .
Figure 9. Performance of catalyst without treatment structure: (a) yield of H 2 with temperature; (b) conversion of CH 4 rate with temperature; (c) selectivity of CO as a function of temperature; (d) catalytic reactivity.

Figure 10 .
Figure 10.Impregnation catalyst performance diagram (a) yield of H 2 with temperature; (b) conversion of CH 4 rate with temperature; (c) selectivity of CO as a function of temperature; (d) catalytic reactivity.
To further analyze the reaction products on the catalyst surface, EDS tests were carried out.The display of different colored pixels in figures 11(d) and (e) indicates the

Table 3 .
Sample specific surface area pores and average pore size.