Synthesis, Structure Characterization and Study of a New Kind of Catalyst: A Monolith of Nickel Made by Additive Manufacturing Coated with Platinum

Abstract

The monitoring of environmental contamination is an important issue to protect human health and the atmospheric environment. In this study, the optical imaging of mesh structures not coated and coated with platinum was performed to analyze the optical characteristics of the lattices. A nickel monolith catalyst was manufactured via additive manufacturing and coated with platinum, and it was presented to characterize the catalyst properties. The analysis focused on the process of coating using hydrazine bath as a reducing agent. The results showed an increase in the thickness of the coating with baths with durations of 1.5 h, 2.0 h and 2.5 h. The coating thickness was strongly dependent on time duration. The SEM images and EDX were used to confirm the process of coating and analyze the presence of platinum on the catalyst. Coating layers were very thin, and others were not homogeneous over the surface. When the catalyst was exposed to platinum for 2.5 h, the catalyst showed an efficiency of 0.06% for NOx, 0.10%, for CO and 0.09% for HC reduction.

1. Introduction

The complex reactions that take place during fuel combustion are not yet fully understood [1,2]. The air/fuel (A/F) ratio is considered to be one of the main sources of HC emissions formation during combustion [3,4,5,6,7]. Furthermore, spark ignition (SI) and combustion such as gasoline direct injection (GDI), which are able to operate with stoichiometric or lean and rich mixture conditions, make operation for catalysts and emissions conversion difficult, meaning controlling the combustion pollutants becomes challenging. Treatment management to control exhaust emissions has been taking place for almost four decades, with hard work in the automotive industry to overcome the difficulties of controlling exhaust emissions and create highly effective technologies. However, there are still some challenges facing researchers, such as low-temperature engine combustion, which releases high amounts of unburned hydrocarbon. By the 1980s, the Three Way-Convertor (TWC) was presented to control NOx, CO and HC simultaneously [8]. However, the selection of catalysts supports for the synthesis of heterogeneous catalyst matters has been investigated earlier, such as aluminum oxide (Al₂O₃), calcium oxide, (CaO), magnesium oxide (MgO), etc., due to their high stability and porosity [9,10,11,12].

Despite the evolution of cleaner fuels and combustion technologies, diesel combustion engines release plenty of nitrogen oxides (NOx), particulate matter (PM) and soot particles compared to SI engine emissions. The solution to decrease NOx and PM emissions is using i: exhaust gas recirculation (EGR); ii: and selective catalytic reduction (SCR); iii: diesel particulate filters (DPFs); in addition, more options are available for diesel combustion such as alternative fuels such as biofuels and gas to liquid (GTL) fuels. Gases emitted from modern diesel engines still require a modern advanced after-treatment system to meet the required emission legislation [13,14,15,16,17,18]. Diesel engines have high performance, good fuel economy, durability, high torque capability and no throttling losses. Diesel vehicles release particulate matter which consists of different types of chemical components such as organic carbon, trace elements, elemental carbon, inorganic ions, etc. It is highly harmful to humans and the environment, as they cause severe respiration problems. Conditions of excess oxygen (HC-SCR) have received much attention as one of the most promising and straightforward methods for reducing NOx emissions. Extensive research has been undertaken using different types of hydrocarbons as reductants [19,20,21,22,23]. However, although a number of catalysts have been tested in the HC-SCR, no catalyst has been considered suitable for the diesel de-NOx conditions. In these conditions, the catalysts are required to be hydrothermally stable and active at relatively low temperatures in the presence of sulfur oxides (Sox) and water vapor. Many base oxides/metals (e.g., Al₂O₃, TiO₂, ZrO₂ and MgO with these oxides promoted by, Pt, Co, Ni, Cu, Fe, Sn, Ga, In and Ag compounds) are active catalysts for HC-SCR of NOx [24,25].

NOx reduction activity occurs at a narrow temperature window of 200–300 °C, which is also associated with the formation of N₂O. Moreover, zeolite-based catalysts, which are the majority of the de-NOx catalysts reported in the literature, are unlikely to be suitable as automotive catalysts due to their instability under hydrothermal conditions [26,27,28].

Variations in catalyst dosage, irradiation period, solution pH, and phenol content were used to investigate photo catalytic degradation. The results showed that, at pH 5, with 20 mg of TiO₂ NPs photo catalyst and 10 mg/L phenol in water, composite nanofibers had the highest activity; this high activity is due to the presence of more active binding sites exposed on the surface of TiO₂ NPs cross-linked on the nanofibers’ surface, as well as the separation of electron–hole pairs being more effective.

The use of photo catalytic composite nanofiber membranes to degrade malachite green and acid red 27 to non-toxic and innocuous compounds was investigated. Electro spinning was used to make the PAN/SiO₂-TiO₂-NH₂ nanofiber membrane, which was then chemically cross linked, and the effective fabrication of a nanofiber membrane was validated using scanning electron microscope (SEM), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and X-ray diffraction (XRD) findings.

The results demonstrate that, with an excellent distribution of SiO₂ and TiO₂ on the PAN nanofiber membrane surface, the PAN/SiO₂-TiO₂-NH₂ nanofiber membrane exhibited the maximum deterioration, and with artificial visible light (125 W irradiation), the 100% photo degradation of MG and AR 27 was obtained after 9 and 25 min, respectively. Furthermore, the nanofiber membrane displayed high photo degradation stability and reusability for MG and AR 27 and had the potential to be used in industrial applications. In [29,30], these studies showed that the porous coordination polymers of Mg (II) dihyroxyterephtalate (Mg-MOF-74) were synthesized, characterized and evaluated for the catalytic cyclo addition process between carbon dioxide CO₂ and propylene oxide at varied Ptx loadings to create propylene carbonate; the reaction was examined at temperatures ranging from 100 °C to 170 °C and pressures ranging from 9.1 bar to 19.5 bar, as well as reaction durations ranging from 4 h to 15 h. In the presence of dimethyl formamide (DMF) and dichloromethane (DCM), which serve as solvents and promoters for the reaction, the results reveal that increasing Pt loading on the catalyst surface enhances selectivity toward propylene carbonate, and the number of uncoordinated MgO sites in the Mg-MOF-74 framework structure grows. The number of uncoordinated MgO defect sites and PO conversion both increased progressively with Pt loading and PC selectivity during the reaction. When Ptx/Mg-MOF-74, DMF, and DCM are used together, they have a synergistic impact that outperforms using them separately.

Trash disposal which produced suitable biomass from date pits was explored for use in the manufacturing of green carbon catalysts and biodiesel. The Taguchi technique of Response Surface Methodology (RSM) was used to investigate the influence of many process factors on the yield of biodiesel generated, including the reaction temperature, time, catalyst type, and methanol to oil ratio. When the process temperature was set to 65 °C, the catalyst type C3 (6 percent KOH on carbon) was used and the methanol to oil ratio was set to 9:1, the optimal yield was 91.6 percent [31,32].

More studies have attempted to understand the combustions and emission characteristics of spark ignition engines fueled by gasoline, methane, ethane, propane and hydrogen; when using pure ethanol and methanol instead of gasoline, the power of the engine decreased and tailpipe emissions such as CO and NOx were reduced, while fuel consumption was increased [33,34]. Another study found that the toxicity of exhaust gases decreased with an increase in engine load. The average concentration of total-PAHs emitted using biofuel with percentages of B25 and B50 fuels were from 11.7% to 54.8% lower than those from B0 at all engine loads. Additionally, the concentration of aliphatic emissions was negligible compared to aromatic compounds [35,36]

The aim of this study was to build a new catalyst with a nickel monolith, manufactured via additive manufacturing and coated by platinum, which had very good chemical and physical properties (good resistance to corrosion and stable at a high temperature), and as a catalyst, black powder was used to catalyze a reaction at a high temperature. A coating process was carried out under different time durations, and catalytic tests were carried out under different engine conditions to observe the catalyst efficiency of eliminating HC, CO, CO₂ and NOx emissions.

2. Methodology

2.1. Design and Selective Laser Melting Additive Manufacturing of the Specimens

The selective laser melting additive manufacturing process is used to fabricate three-dimensional (3D) structures by adding layer upon layer at a time. According to the material chosen for the object, selective laser melting (SLM) was used. The lattice was melted and fused into powder layers on top of each other according to the designed model. The method required an absolutely inert atmosphere to avoid pollution, and the design was based on diamond unit cells, as shown in Figure 1. The strut diameter was set to 400 µm, and the strut length was 2 mm. The lattice was designed to have dimensions of 20 × 20 × 11 mm³.

2.2. Electro Less Deposition of Platinum

Platinum coating is the process of nickel coating with platinum using a hydrazine bath as a reducing agent.

The experiment was carried out according to the European patent EP0423005A1 [37]. The bath contained three key compounds: a platinum compound (H₂Pt(OH)₆), a reducing agent (N₂H₄, H₂O) and a stabilizing agent (As₂O₅). The compounds were prepared in two stages: i: a stock solution of platinum was prepared as shown in Figure 2. ii: the bath was reduced and stabilized. The platinum hydroxide was not stable enough to ensure stability towards the reducing agent; it had to form a complex with oxidation. For that, the next steps were followed:

Table 1. Average size of coating with time duration.

Duration of Deposition	Thickness of the Coating
1.5 h	0.73 µm
2.0 h	2.66 µm
2.5 h	4.43 µm

presents the average thickness of the coating according to the duration in the electro less bath. After being rinsed with deionized water and washed with ultrasonic cleaner, the samples were prepared for deposition in the bath, and after being deposed for a few minutes, the bath became darker and it was difficult to see the samples.

After one hour, hydrazine was added to reload the bath.

2.3. Catalyst Characterization

SEM was used to estimate the homogeneity and thickness of the sample coating, and three images were taken on each sample on different magnifications with different time durations of 1.5 h, 2.0 h and 2.5 h. The presences of element components of the final catalysts were detected using energy dispersive X-ray spectroscopy (EDX).

3. Experimental Setup

3.1. Engine

The new catalyst efficiency was tested using a Lister Peter TR1, naturally aspirated, air-cooled, single-cylinder, direct injection engine (Figure 3). Standard commercial diesel fuel was used in this test under different engine conditions (

Table 2. Engine conditions.

Exhaust Flow Rate	Engine Speed (rpm)	Exhaust Temperature (°C)	Oxygen (%)	Engine Load	GHSV (kh−1)
1 L/min	1500	380	0.02	8 Nm	25
1 L/min	2000	360	0.01	8 Nm	27
1 L/min	2500	345	0.01	8 Nm	24

). The injection timing was kept at 22° CA before top dead center (bTDC) according to manufacturing specifications. An adjustable valve was installed in the tailpipe in order to obtain a portion of the exhaust. The engine exhaust gas was controlled by an adjustable valve and fed to the mini-reactor through a heated flexible steel pipe kept at 200 °C with a heater line to avoid hydrocarbon condensation.

3.2. Emission Analyzer

The catalyst was fitted inside the reactor to be fed with exhaust gases. Fourier Transform Infrared Spectroscopy (FTIR) 2100 MKS (Figure 4) was used to measure the sample which was earlier pumped via a heated line maintained at 190 °C. The catalyst was placed after the exhaust, tightened from both sides to the exhaust pipes and connected via a heating line to the emission analyzer (FTIR).

3.3. Measurements

All measurements had uncertainty, as seen in

Table 3. Uncertainty.

S.N	Parameter Name	Instrument Uncertainty
1	Load (Nm)	±0.01
2	Temperature	±1 °C
3	Speed (rpm)	±10
4	CO (ppm))	±0.01
5	HC (ppm)	±0.01
6	NOx (ppm)	±0.01
7	CO2 (%)	±0.01
8	H2O (%)	±0.01
9	H2O condensation	±0.01
10	Thickness of coating (µm)	±0.01
11	Crank angel degree (° CA)	±0.1

.

4. Results and Discussion

4.1. Catalyst Characterization

The brightest surface in Figure 4 represents the platinum coating. As seen in the figures, the coating is not homogeneous, and some areas of the nickel specimen are not coated (dark grey). The average thickness was calculated by analyzing different areas in the obtained SEM images, and SEM images of the sample are the parameters of the quality and coating thickness (

Table A1. Catalyst coating duration.

Duration of Deposition
1.5 h		2.0 h		2.5 h
Measure ImageJ	Thickness (μm)	Measure ImageJ	Thickness (μm)	Measure ImageJ	Thickness (μm)
138.667	20	66.002	10	61	20
2	0.655738	9.394	3.08	13.509	4.42918
2.007	0.658033	6.364	2.086557	11.667	3.825246
2.713	0.889508	7.826	2.565902	17.67	5.793443
4.197	1.376066	10.259	3.363607	16.014	5.250492
2.5	0.819672	6.708	2.199344	8.673	2.843607
Average Thickness	0.73317 μm	Average Thickness	2.65908 μm	Average Thickness	4.42839 μm

(Appendix A)).

The EDX spectra on the sample without coating (a), coated for 1.5 h (b), coated for 2.0 h (c) and (d) coated for 2.5 h is shown in Figure 5. In each spectrum, there was platinum, nickel, chromium and cobalt; these impurities may have been due to the SLM chamber gases that might have contaminated the sample. The composition and the elementary proportions are given in Appendix A.

4.2. Catalyst Exposed for 2.0 h to Platinum

The results presented in Figure 6 show that there was not any significant difference in gaseous emissions. The water reduced slightly, and CO₂ seemed to be reduced by a small percentage. In terms of regulated gaseous emissions, NOx and CO were quite similar, and high hydrocarbons were slightly reduced.

In this case, the possible decrease in HC and CO achieved after catalysis is thought to be due to the space velocity of the new catalyst, since the HC and CO elimination imply an oxidation, which would produce higher concentrations of H₂O and CO₂. The NOx reduction was around 10 ppm, which is attributed to the fact that NOx reacted with the presence of hydrocarbon in the line due to the combination of hydrocarbon and high temperature. The reduction in hydrocarbons can be due to different circumstances. The reasons could be either that part of the hydrocarbon condensates along the reactor, leading to a decrease, or hydrocarbons could react with NOx presence in the exhaust.

4.3. Catalyst Exposed for 2.5 h to Platinum

The second test was carried out with a catalyst which was exposed for 2.5 h to platinum; the results showed there was not a significant effect on the emissions (Figure 7). Moreover, it was highlighted that the results changed due the precision of the equipment rather than experiment variables. The H₂O increased slightly after the catalysis; on the other hand, CO₂ reduced slightly and kept the same tendency that was in the previous experiment.

In terms of NOx, the experiment resulted in a slight increase in NOx, which is attributed to the variability of the equipment; the hydrocarbons did not show any relevant difference between before and after catalysis. Nevertheless, the second experiment showed a general reduction of 200 ppm of HC with respect to the previous test. This effect was attributed to a higher level of the hydrocarbon condensations along the lines.

Finally, CO reported a decrease of about 30 ppm. This could be due to an oxidation in the catalyst, but we considered that it was unlikely since no increase in CO₂ concentration was reported after the catalysis.

According to the results, the catalyst did not show a significant emissions reduction performance; the catalyst exposed for 1.5 h to platinum was not tested as the coated catalysts, which did not show any acceptable coating quality. Flow has an important influence on the catalyst performance, and due to the limited size of the manufactured catalyst, a trade-off between the flow necessary for these catalysts and the minimum flow to feed the equipment could not be found. The minimum achievable flow rate in this experiment was not sufficient to provide any significant insight into the catalytic activity of the novel platinum–nickel catalysts.

5. Conclusions

The results of this study, presented the following key findings:

1-

Coating process

i-

The platinum hydroxide showed a poor stability towards the reducing agent (hydrazine bath). It must form a complex with oxalate ion.

ii-

SEM images were used to estimate the homogeneity of the catalyst coating, and then a treatment with Image was used to measure the thickness of the layer.

Analyzing the sample spectrum, simplify recognizing the composition of the surface and quantifying the proportion of each coating element.

iii-

Coating duration has a great impact on samples’ thicknesses.

2-

Engine emissions

i-

The catalyst showed a weak performance in terms of eliminating HC, CO and NOx emissions.

ii-

Prototype catalyst efficiency could be affected by the poor homogeneity of the coated material.

iii-

The thickness of the coated material influenced the catalyst performance.

iv-

A comparison between the coated catalyst and an uncoated one could be conducted in future work.