Ethanol Dehydrogenation: a Reaction Path Study by Means of Temporal Analysis of Products

catalysts

Article Ethanol Dehydrogenation: A Reaction Path Study by Means of Temporal Analysis of Products

Joachim Pasel 1,*, Johannes Häusler 1 , Dirk Schmitt 1, Helen Valencia 2,3, Maria Meledina 2,3, Joachim Mayer 2,3 and Ralf Peters 1

1 Institute of Energy and Climate Research, IEK-14: Electrochemical Process Engineering, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; [email protected] (J.H.); [email protected] (D.S.); [email protected] (R.P.) 2 Central Facility for Electron Microscopy (GFE), RWTH Aachen University, 52074 Aachen, Germany; [email protected] (H.V.); [email protected] (M.M.); [email protected] (J.M.) 3 Ernst Ruska-Centre (ER-C) for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany * Correspondence: [email protected]; Tel.: +49-2461-61-5140

Received: 4 September 2020; Accepted: 4 October 2020; Published: 6 October 2020

Abstract: Conventional fossil fuels such as gasoline or diesel should be substituted in the future by environmentally-friendly alternatives in order to reduce emissions in the transport sector and thus mitigate global warming. In this regard, iso-butanol is very promising as its chemical and physical properties are very similar to those of gasoline. Therefore, ongoing research deals with the development of catalytically-supported synthesis routes to iso-butanol, starting from renewably-generated methanol. This research has already revealed that the dehydrogenation of ethanol plays an important role in the reaction sequence from methanol to iso-butanol. To improve the fundamental understanding of the ethanol dehydrogenation step, the Temporal Analysis of Products (TAP) methodology was applied to illuminate that the catalysts used, Pt/C, Ir/C and Cu/C, are very active in ethanol adsorption. H2 and acetaldehyde are formed on the catalyst surfaces, with the latter quickly decomposing into CO and CH4 under the given reaction conditions. Based on the TAP results, this paper proposes a reaction scheme for ethanol dehydrogenation and acetaldehyde decomposition on the respective catalysts. The samples are characterized by means of N2 sorption and Scanning Transmission Electron Microscopy (STEM).

Keywords: renewable fuels; ethanol dehydrogenation; acetaldehyde decomposition; C-supported precious metal catalysts; Temporal Analysis of Products; Scanning Transmission Electron Microscopy

1. Introduction Recently, basic research into new catalysts for the synthesis of higher alcohols on a renewable basis via aldolic condensation has been gaining increasing attention [1–6]. Amongst these alcohols, iso-butanol in particular has been identiﬁed as very promising from various perspectives. It has been found that its physical and chemical properties (i.e., density, viscosity, solubility in water, lower heating value, boiling point, ﬂash point, self-ignition temperature, motor octane number, etc.) are similar to those of conventional gasoline and conform to relevant standards for that substance [7]. Additionally, experiments and simulations have demonstrated that the combustion properties of pure iso-butanol and iso-butanol/gasoline blends, respectively, widely resemble the combustion characteristics of conventional gasoline. It has been demonstrated that using pure iso-butanol or iso-butanol/gasoline blends might even enhance the performance of internal combustion engines by reducing the quantities

Catalysts 2020, 10, 1151; doi:10.3390/catal10101151 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1151 2 of 20

of NOx, CO and hydrocarbon emissions [2,8]. Singh and Sung [9] found that butanol isomers are advantageous with respect to soot formation in comparison to the isomers of butane. Thus, iso-butanol clearly offers the potential to become a “green” future fuel for the transport sector if it is synthesized from renewable feedstocks. Against this background, the Forschungszentrum Jülich has been a partner in the project “C3-Mobility”, funded by the German Federal Ministry for Economic Affairs and Energy, since 2018. The fundamental assumption of this project is that in future mobility concepts for the transport sector, renewable methanol will be an important raw material to be refined in different processes, yielding viable transport fuels. It is also assumed in these scenarios that renewable methanol will be produced locally from H2 and CO2 in different venues worldwide, where wind- or solar-driven water electrolysis facilities for H2 production can be economically operated and where CO2 being separated from the exhausts of, e.g., steelworks, cement facilities or even ambient air will be available 3 in suCatalystsfficient 2020 quantities., 10, x FOR PEER In “CREVIEW-Mobility”, Jülich’s specific refining path that starts from renewable3 of 20 methanol, deals with the development of suitable and experimentally-proven synthesis routes to producecomputational iso-butanol. methods This includes such as dens the evaluationity functional of theory proper or reaction a combination conditions of experimental (pressure, methods temperature, etc.) and modelling active catalysts. tools to investigate correlations between catalyst activity and selectivity on the one Figurehand and1 shows material a characteristics reaction scheme such as that crystal constitutes faces, monolayer a possible coverages, solution etc., for on the other synthesis [37– of iso-butanol,41]. starting with methanol and ethanol. It is an adaption of the “hydrogen-borrowing Autthanit et al. [42,43] investigated AgLi/TiO2 and VOx/SBA-15 catalysts under non-oxidative mechanism” proposed by Siddiki et al. [10] that uses a Pt/C catalyst for H uptake and subsequent and oxidative reaction conditions and found significantly higher conversions2 and acetaldehyde release. The original scope of the work of Sidikki et al. was the catalytic methylation of C–H bonds in yields in the case of oxidative dehydrogenation. Ag nanoparticles on SiO2 and SiO2/CeO2 were tested alcohols,by Mamontov ketones andet al. indoles [44], who with observed methanol. increased For thecatalytic synthesis activity of when iso-butanol Ag/SiO displayed2 was modified in Figure by 1, one moleCeO2of nanoparticles. ethanol and Dutov methanol, et al.respectively, [45] tested Ag/OMS-2/SiO react in the presence2 catalysts of for NaOH, the selective and exemplarily, oxidation of a Pt/C catalystethanol at 120 to◦ acetaldehydeC to form acetaldehyde and observed and a positive formaldehyde, effect of whichAg doping in turn on furtherthe oxidation reacts rate to form but also acrolein, and afterundesired hydrogenation, total oxidation, propanol. leading Anotherto CO2. Giannakis comparable et al. C-methylation[46] investigated step Au and on theNiAu Pt /singleC-catalyst atom then yieldsalloys iso-butanol. and found However, that the addition the third of C-methylation Ni increased the step catalytic in Figure activity1 from and iso-butanolreduced the tosintering undesired 2,2-dimethyl-1-propanolprocesses. The best results has were only obtained been demonstrated at 280 °C. The same in a homogeneouseffect of increased catalyst activity thus and stability far [11] and was observed by Shan et al. [47,48] when they doped their Cu nanoparticles with Ni to form a NiCu is not assumed to take place on a Pt/C catalyst. The first step of this reaction sequence, i.e., the alloy. Li et al. [49] used a metal-free catalyst consisting of nitrogen-doped graphenes and found dehydrogenation of ethanol to acetaldehyde, is the focus of this paper. To better comprehend it from acetaldehyde to be the only product of ethanol dehydrogenation at reaction temperatures of between a mechanistic200 °C and point350 °C. of Ob-Eye view (residenceet al. [50–52] time used distributionactivated carbons of products as support and and residual doped them educts, with reactionCo, intermediates,Ce, Cu and undesired Ni in the temperature side reactions, range subsequent of 250 °C to aldol 400 condensation°C. The Cu-containing reactions), catalyst a Temporal showed Analysisthe of Productshighest (TAP)activity investigation and most promising of precious selectivity metal regarding (Pt, Ir, supported acetaldehyde on active (more carbon) than 96%). and Wang base metalet al. (Cu on active[53] tested carbon) Au catalysts supported was on ZnZrO performed.x at temperatures These three between samples 30 were°C and chosen 400 °Cas and it wasobserved demonstrated that the in a recentaddition experimental of Au favors investigation the dehydrogenation by Häusler etroute al. [12over] that the theydehydration are also suitablepath. Mitran for the et synthesisal. [54] of desiredinvestigated iso-butanol mixtures according of ethanol to the and reaction methanol scheme with their from H Figure1−xTi2(PO1.4)3−x(SO4)x-catalyst and observed higher conversions of ethanol (> 95%) compared to those of methanol (55%).

FigureFigure 1. Reaction 1. Reaction pathway pathway for for the the synthesissynthesis of iso-butanol iso-butanol from from ethanol ethanol and and methanol. methanol.

Only2. Results a few and studies Discussion in the literature focus on the mechanistic aspects of ethanol dehydrogenation. Ashok et al. [13] used Co nanoparticles in a mechanistic study and found aldehyde and acetate 2.1. Catalyst Characterization components, together with H2,H2O, CO2 and CH4. Kim and Ryu [14] worked with V2O5/TiO2 catalysts

2.1.1. N2 Sorption Table 1 summarizes the specific surface areas of the different catalysts from this study. Cu/C shows the largest surface of 1271 m2 g-1, while that of the Pt/C sample was the smallest, with 805 m2 g-1. Figure 2 shows that the average pore diameter of all three catalysts was in the range of approximately 20 nm.

Table 1. Specific surface areas of the different C-supported catalysts.

Catalysts 2020, 10, 1151 3 of 20 and detected both ethanol dehydration to ethene and ethanol dehydrogenation to acetaldehyde. The latter was favored by higher V2O5 loadings. In the case of the work of Krutpijit et al. [15], the acetaldehyde route was preferred over the ethene path, when the AgLi catalyst was modified by Al2O3. The differentiation between dehydrogenation and dehydration will be an important point of the experiments in this paper, as the dehydration route does not lead to desired iso-butanol. Many research groups have found Cu catalysts to be highly active for ethanol dehydrogenation, revealing selectivities towards acetaldehyde of almost 100% in some cases [16–25]. A positive effect is reported when the Cu content increased in the range of 1–5 wt % [26]. Hanukovich et al. [27] reported that the rate-determining step of ethanol dehydrogenation on Cu can be the fission of the O–H bond or that of the Cα–H bond. An important topic with respect to catalysts for ethanol dehydrogenation is the significant deactivation that occurs due to coke formation on the catalyst surface and the sintering of the active catalyst particles. This was found in the case of PdZn nanoparticles [28], monometallic Cu [29,30], bi- or tri-metallic Cu alloys [29,31–33], SiO2− supported Cu catalysts [34] and V/Mg− Al catalysts [35,36]. There is a considerable number of papers that use computational methods such as density functional theory or a combination of experimental methods and modelling tools to investigate correlations between catalyst activity and selectivity on the one hand and material characteristics such as crystal faces, monolayer coverages, etc., on the other [37–41]. Autthanit et al. [42,43] investigated AgLi/TiO2 and VOx/SBA-15 catalysts under non-oxidative and oxidative reaction conditions and found significantly higher conversions and acetaldehyde yields in the case of oxidative dehydrogenation. Ag nanoparticles on SiO2 and SiO2/CeO2 were tested by Mamontov et al. [44], who observed increased catalytic activity when Ag/SiO2 was modified by CeO2 nanoparticles. Dutov et al. [45] tested Ag/OMS-2/SiO2 catalysts for the selective oxidation of ethanol to acetaldehyde and observed a positive effect of Ag doping on the oxidation rate but also undesired total oxidation, leading to CO2. Giannakis et al. [46] investigated Au and NiAu single atom alloys and found that the addition of Ni increased the catalytic activity and reduced the sintering processes. The best results were obtained at 280 ◦C. The same effect of increased activity and stability was observed by Shan et al. [47,48] when they doped their Cu nanoparticles with Ni to form a NiCu alloy. Li et al. [49] used a metal-free catalyst consisting of nitrogen-doped graphenes and found acetaldehyde to be the only product of ethanol dehydrogenation at reaction temperatures of between 200 ◦C and 350 ◦C. Ob-Eye et al. [50–52] used activated carbons as support and doped them with Co, Ce, Cu and Ni in the temperature range of 250 ◦C to 400 ◦C. The Cu-containing catalyst showed the highest activity and most promising selectivity regarding acetaldehyde (more than 96%). Wang et al. [53] tested Au supported on ZnZrOx at temperatures between 30 ◦C and 400 ◦C and observed that the addition of Au favors the dehydrogenation route over the dehydration path. Mitran et al. [54] investigated mixtures of ethanol and methanol with their H1 xTi2(PO4)3 x(SO4)x-catalyst and observed higher conversions − − of ethanol (> 95%) compared to those of methanol (55%).

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. N2 Sorption Table1 summarizes the speciﬁc surface areas of the di ﬀerent catalysts from this study. Cu/C shows 2 1 2 1 the largest surface of 1271 m g− , while that of the Pt/C sample was the smallest, with 805 m g− . Figure2 shows that the average pore diameter of all three catalysts was in the range of approximately 20 nm. Catalysts 2020, 10, 1151 4 of 20

Catalysts 2020, 10, x FOR PEER REVIEW 4 of 20 Table 1. Speciﬁc surface areas of the diﬀerent C-supported catalysts.

Catalyst Specific Surface Area 2 1 Catalyst Speciﬁc Surface Area m g − 2 -1 5 wt % Pt/Cm g 805 1 wt5 wt % Ir %/C Pt/C 805 1013 3 wt1 %wt Cu %/C Ir/C 1013 1271 3 wt % Cu/C 1271

Figure 2. Pore size distributions of the different diﬀerent C-supported catalysts.

2.1.2. Scanning Transmission Electron MicroscopyMicroscopy Z-contrast, high-angle,high-angle, annularannular dark dark field field (HAADF) (HAADF) STEM STEM combined combined with with energy-dispersive energy-dispersive X-ray X- (EDX)ray (EDX) elemental elemental mapping mapping was applied was applied to investigate to investigate the morphology the morphology and distribution and distribution of deposited of catalyticdeposited nanoparticles catalytic nanoparticles on the carbon on support the carbon (see Figure support3). Di (seefferences Figure in 3). chemical Differences compositions in chemical were visualized,compositions as thewere image visualized, contrast as was the proportional image contra tost the was atomic proportional number of to scattering the atomic atoms. number Hence, of thescattering metals atoms. on the Hence, C support the metals appear on as the bright C support contrast appear features as bright in the contrast HAADF features images; in seethe FigureHAADF3. Theimages; fact see that Figure these were3. The the fact corresponding that these were elements the corresponding was confirmed elements by EDX was mapping. confirmed The by top-left EDX imagemapping. in row The a)top-left shows image an overview in row a) of shows Pt nanoparticles an overview in of the Pt fresh nanoparticles catalyst of in ~2 the nm fresh in sizecatalyst on the of carbon~2 nm in support. size on Somethe carbon agglomeration support. Some was also agglomeration observed. Ir was nanoparticles also observed. in the Ir freshnanoparticles material in had the a similarfresh material distribution had a within similar the distribution carbon support within (see the rowcarbon b), andsupport particles (see ofrow ~2.5 b), nmand in particles size. Based of ~2.5 on variousnm in size. HAADF-STEM Based on various images acquiredHAADF-STEM for the Irimages containing acquired material for the (not Ir shown containing here), material a trend of (not the Irshown nanoparticles here), a trend to form of the agglomerates Ir nanoparticles was noticed.to form agglomerates In the meantime, was Ptnoticed. nanoparticles In the meantime, had a lower Pt tendencynanoparticles to agglomerate. had a lower Smaller tendency agglomerates to agglomerate. (compared Smaller to theagglomerates case of Ir) were (compared observed to inthe the case fresh of material.Ir) were observed High-Resolution in the fresh (HR) material. HAADF-STEM High-Resolution images of Pt (HR) and HAADF-STEM Ir nanoparticles images were also of displayedPt and Ir onnanoparticles the right side were of Figurealso displayed3 with corresponding on the right Fast-Fourier-Transformside of Figure 3 with corresponding (FFT) patterns Fast-Fourier- in the insets. PtTransform and Ir nanoparticles (FFT) patterns most in the likely insets. matched Pt and theIr na metallicnoparticles face-centered most likely cubic matched crystal the structure metallic face- (fcc). Thecentered images cubic were crystal taken structure along the (fcc). [011] The and images [001] zone were axis taken [55 ].along The freshthe [011] Cu-containing and [001] zone nanoparticles axis [55]. ofThe ~6 fresh nm in Cu-containing size on the carbon nanoparticles support were of ~6 investigated nm in size on by the a similar carbon set-up support and were the results investigated were placed by a insimilar row c).set-up In the and images, the results the clearwere nanoparticulateplaced in row c). nature In the of images, the investigated the clear nanoparticulate brighter features nature was visible.of the investigated The EDX map brighter shows features a clear Cuwas and visibl C enrichmente. The EDX inmap the shows expected a clear regions Cu and used C for enrichment mapping. Thein the TEM expected grid gave regions a relatively used for uniform mapping. level The of signal,TEM grid as the gave carbon a relatively coating uniform in the grid level was of a relativelysignal, as homogeneousthe carbon coating amorphous in the grid thin was layer a relatively (e.g., see homogeneous the upper left amorphous corner of thethin map—the layer (e.g., carbon see the gridupper is visibleleft corner in that of the position). map—the These carbon qualitative grid is data visible are in shown that position). to evidence These the presence qualitative ofCu data rich are particles shown onto evidence the carbon the support. presence of Cu rich particles on the carbon support.

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FigureFigure 3. HAADF-STEM 3. HAADF-STEM overviews overviews alongside alongside survey survey imag imageses of the of corresponding the corresponding EDX element EDX maps element mapsof: of: (a) ( afresh) fresh Pt/C Pt catalyst/C catalyst (top part); (top part);(b) fresh (b )Ir/C fresh catalyst Ir/C catalyst(middle part); (middle and part);(c) fresh and Cu/C (c) catalyst fresh Cu /C catalyst(bottom (bottom part). part). The metals The metalsare represented are represented in green, inwith green, the C-support with the in C-support orange within in orange the EDX within the EDXmaps. maps. HR HAADF-STEM HR HAADF-STEM images imagesfor Pt/C for and Pt /Ir/CC and show Ir/ Ca showclose up a closeof nanoparticles up of nanoparticles with Fast- with Fast-Fourier-TransformFourier-Transform (FFT) (FFT) patterns patterns in the in theinsets. insets.

2.2. Temporal2.2. Temporal Analysis Analysis of Products of Products

2.2.1.2.2.1. Experiments Experiments with with Pt/ Pt/CC

CH2OH → CH3CHO + H2 endothermic (1) CH2OH CH3CHO + H2 endothermic (1) →

CH3CHO → CO + CH4 exothermic (2) CH CHO CO + CH exothermic (2) 3 → 4 CH3CHO → CO + C + 2 H2 endothermic (3) CH CHO CO + C + 2 H endothermic (3) Figure 4a shows the height-normalized3 → transient responses2 to pulses of ethanol (34.6 vol %) in FigureAr at m/z-values4a shows theof two height-normalized for H2, 16 for CH4 transient, 28 for CO, responses 29 for acetaldehyde to pulses ofand ethanol 40 for (34.6Ar at volan %) exemplary reaction temperature of 200 °C. An H2 pre-reduced Pt/C catalyst was used. In addition to in Ar at m/z-values of two for H2, 16 for CH4, 28 for CO, 29 for acetaldehyde and 40 for Ar at an the above-mentioned m/z-values, that at 31, which is significant for ethanol, was also recorded during exemplary reaction temperature of 200 ◦C. An H2 pre-reduced Pt/C catalyst was used. In addition to this experiment. However, under these reaction conditions, ethanol was completely consumed by the the above-mentioned m/z-values, that at 31, which is signiﬁcant for ethanol, was also recorded during catalyst and the corresponding response pulse at an m/z-value of 31 did not show any measurable this experiment.peak. This fact However, considerably under simplifies these reaction the mass-spectrometric conditions, ethanol analysis, was completely as the mass consumed fractions of by the catalystethanol, and theespecially corresponding at an m/z-value response of 29, pulsewould at un anfavorably m/z-value perturb of31 the did analys notis show of acetaldehyde. any measurable It peak. This fact considerably simpliﬁes the mass-spectrometric analysis, as the mass fractions of ethanol, especially at an m/z-value of 29, would unfavorably perturb the analysis of acetaldehyde. It can be concluded that the pre-reduced Pt/C catalyst was very active for the adsorption of ethanol and its further reactions. Catalysts 2020, 10, x FOR PEER REVIEW 6 of 20

can be concluded that the pre-reduced Pt/C catalyst was very active for the adsorption of ethanol and its further reactions. In this respect, Figure 4a shows an interesting number of detected reaction products, and additionally, in which order (residence time distribution) and in which way (desorption behavior) they exit the micro-reactor. The figure illustrates that acetaldehyde and H2 are produced and then desorb from the catalyst surface, indicating that ethanol dehydrogenation, according to reaction Equation (1), takes place under the reaction conditions of the TAP experiments. In addition to acetaldehyde and H2, CO and CH4 also desorb from the catalyst surface. The formation of CO and CH4 can be explained by the decomposition of acetaldehyde according to Equation (2). Ouyang et al. [56] also reported on the formation of CO and CH4 on Pd clusters being deposited on ZnO. In order to reveal if there was also a reaction to Equation (3), which additionally reflects C-formation on the catalyst surface as relevant over the course of ethanol dehydrogenation, 10,000 pulses of ethanol (34.6 vol % in Ar) were pulsed over the H2 pre-reduced Pt/C catalyst at 200 °C. Before and after this very long series of ethanol pulses, mixtures of 50% O2 in Ar were injected into the TAP micro-reactor at a temperature of 200 °C, while recording the transient responses of CO2 and CO, respectively. The results of this experiment are shown in Figure 5a and 5b. While CO2 and CO desorption show quite low intensities prior to the 10,000 ethanol pulses as a result of the reaction between the carbon support and injected O2 molecules, their levels significantly increase after the 10,000 ethanol pulses, clearly indicating that C deposits are formed when ethanol is pulsed over the catalyst surface. Fortunately, these deposits can be fully removed by injecting a large number of O2 pulses. Thus, reaction Equation (3) must be taken into consideration when proposing a reaction scheme. Looking more closely at the transient responses of CO and acetaldehyde in Figure 4a, it becomes obvious that they coincide in their rising sections with lower residence times up to approximately 0.3–0.4 s. This means that both molecules desorb on a comparable time scale from the catalyst surface in this time-span and permits the conclusion that CO formation from acetaldehyde, which requires many steps on the catalyst surface, like C–C and C–H bond cleavages, is a very rapid process. It is also fast in relation to the desorption of acetaldehyde itself and in comparison to other, slower steps Catalysts 2020, 10, 1151 6 of 20 on the catalyst surface that control the profound tailings of the response pulses of CH4 and H2 in Figure 4a and 4b in particular.

FigureFigure 4. 4.Height-normalized Height-normalized transient responses responses to topuls pulseses of 34.6 of 34.6 vol vol% ethanol % ethanol in Ar inon Ar the on H2 thepre- H2 pre-reducedreduced Pt/C Pt/ Ccatalyst catalyst (20(20 mg), mg), T = T200= °C,200 at◦C, m/z-values at m/z-values of: (a)of: 2 for (a )H 22, for 16 for H2 ,CH 164 for, 28 CHfor 4CO,, 28 29 for for CO, 29acetaldehyde for acetaldehyde and and40 for 40 Ar for up Ar to up2.0 tos; 2.0(b) s;2 for (b) H 22for and H 162 and for CH 16 for4 over CH the4 over entire the time-span. entire time-span.

In this respect, Figure4a shows an interesting number of detected reaction products, and additionally, in which order (residence time distribution) and in which way (desorption behavior) they exit the micro-reactor. The figure illustrates that acetaldehyde and H2 are produced and then desorb from the catalyst surface, indicating that ethanol dehydrogenation, according to reaction Equation (1), takes place under the reaction conditions of the TAP experiments. In addition to acetaldehyde and H2, CO and CH4 also desorb from the catalyst surface. The formation of CO and CH4 can be explained by the decomposition of acetaldehyde according to Equation (2). Ouyang et al. [56] also reported on the formation of CO and CH4 on Pd clusters being deposited on ZnO. In order to reveal if there was also a reaction to Equation (3), which additionally reflects C-formation on the catalyst surface as relevant over the course of ethanol dehydrogenation, 10,000 pulses of ethanol (34.6 vol % in Ar) were pulsed over the H2 pre-reduced Pt/C catalyst at 200 ◦C. Before and after this very long series of ethanol pulses, mixtures of 50% O2 in Ar were injected into the TAP micro-reactor at a temperature of 200 ◦C, while recording the transient responses of CO2 and CO, respectively. The results of this experiment are shown in Figure5a,b. While CO 2 and CO desorption show quite low intensities prior to the 10,000 ethanol pulses as a result of the reaction between the carbon support and injected O2 molecules, their levels significantly increase after the 10,000 ethanol pulses, clearly indicating that C deposits are formed when ethanol is pulsed over the catalyst surface. Fortunately, these deposits can be fully removed by injecting a large number of O2 pulses. Thus, reaction Equation (3) must be taken Catalystsinto consideration 2020, 10, x FOR when PEER proposingREVIEW a reaction scheme. 7 of 20

Figure 5. Transient responses of: ( a) CO2 andand ( b)) CO to pulses of 50% O2 inin Ar at 200 ◦°CC before and after 10,000 10,000 pulses pulses of of ethanol ethanol (34.6 (34.6 vo voll % % in in Ar) Ar) were were pulsed pulsed over over the the H2 Hpre-reduced2 pre-reduced Pt/C Pt catalyst/C catalyst (20 (20mg) mg) at 200 at 200°C. ◦C.

Looking more closely at the transient responses of CO and acetaldehyde in Figure4a, it becomes The very broad shapes of the strongly delayed transient responses of CH4 and H2 in Figure 4b raiseobvious the thatquestion they as coincide to whether in their they rising are caused sections by withreversible lower adsorption residence timesand spillover up to approximately processes on the Pt/C catalyst surface or if they are due to the slow surface steps of further acetaldehyde decomposition, as per reaction Equations (2) and (3). To elucidate this question, pulses of 38.6 vol % acetaldehyde in Ar and of single mixtures of 50 vol % CH4 and H2, respectively, in Ar were pulsed over the H2 pre-reduced Pt/C catalyst at temperatures between 100 °C and 250 °C. Additionally, a carbon black sample was applied. As a result, Figure 6a presents the height-normalized transient responses at an m/z-value of 16 for CH4 for three different cases: (i) pulsing of acetaldehyde (38.6 vol %) in Ar over Pt/C; (ii) pulsing of a mixture of 50 vol % CH4 in Ar over the Pt/C catalyst; and (iii) pulsing of the same mixture over carbon black. As a means of comparison, the averaged response pulse for Ar is also depicted. The exemplary reaction temperature is again 200 °C. It can be seen from Figure 6a that CH4 from single pulsing over the Pt/C catalyst and over carbon black shows relatively narrow and almost congruent transient responses with a residence time that is only slightly longer than that of Ar, representing the diffusion-only case. This finding proves that reversible CH4 adsorption on Pt and CH4 spillover from Pt to the C support can be excluded as an explanation for the observed broad response pulses of CH4, as observed in case (i).

Figure 6. Height-normalized transient responses of CH4 and H2, respectively, at 200 °C to: (a) pulses

of 38.6 vol % acetaldehyde in Ar and 50 vol % CH4 in Ar over H2 pre-reduced Pt/C and over carbon black; (b) pulses of 38.6 vo l% acetaldehyde in Ar and 50 vol % H2 in Ar over H2 pre-reduced Pt/C and over carbon black.

A slightly different picture emerges for H2 in Figure 6b, which shows the height-normalized transient responses at an m/z-value of 2 for H2 for the same three cases given above. Of course, H2 is used instead of CH4 in the Ar mixture, with the resulting transient responses again being compared to that of the averaged Ar curves. The transient H2 response from acetaldehyde pulsing over Pt/C is

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Catalysts 2020, 10, 1151 7 of 20

0.3–0.4 s. This means that both molecules desorb on a comparable time scale from the catalyst surface in this time-span and permits the conclusion that CO formation from acetaldehyde, which requires manyFigure steps 5. on Transient the catalyst responses surface, of: like(a) CO C–C2 and and (b C–H) CO bondto pulses cleavages, of 50% O is2 ain very Ar at rapid 200 °C process. before and It is also fast inafter relation 10,000 to pulses the desorption of ethanol (34.6 of acetaldehyde vol % in Ar) were itself pulsed and in over comparison the H2 pre-reduced to other, Pt/C slower catalyst steps (20 on the catalystmg) surface at 200 °C. that control the profound tailings of the response pulses of CH4 and H2 in Figure4a,b in particular. The very broad shapes of the strongly delayed transient responses of CH4 and H2 in Figure 4b The very broad shapes of the strongly delayed transient responses of CH4 and H2 in Figure4b raise thethe questionquestion asas to to whether whether they they are are caused caused by by reversible reversible adsorption adsorption and and spillover spillover processes processes on theon thePt/C Pt/C catalyst catalyst surface surface or if they or areif they due toare the due slow to surface the slow steps surface of further steps acetaldehyde of further decomposition, acetaldehyde decomposition,as per reaction Equationsas per reaction (2) and Equations (3). To elucidate(2) and (3). this To question,elucidate pulsesthis question, of 38.6 volpulses % acetaldehydeof 38.6 vol % acetaldehyde in Ar and of single mixtures of 50 vol % CH4 and H2, respectively, in Ar were pulsed in Ar and of single mixtures of 50 vol % CH4 and H2, respectively, in Ar were pulsed over the H2 over the H2 pre-reduced Pt/C catalyst at temperatures between 100 °C and 250 °C. Additionally, a pre-reduced Pt/C catalyst at temperatures between 100 ◦C and 250 ◦C. Additionally, a carbon black carbonsample black was applied. sample Aswas a result,applied. Figure As a6 aresult, presents Figure the height-normalized6a presents the height-normalized transient responses transient at an responses at an m/z-value of 16 for CH4 for three different cases: (i) pulsing of acetaldehyde (38.6 vol m/z-value of 16 for CH4 for three different cases: (i) pulsing of acetaldehyde (38.6 vol %) in Ar over %) in Ar over Pt/C; (ii) pulsing of a mixture of 50 vol % CH4 in Ar over the Pt/C catalyst; and (iii) Pt/C; (ii) pulsing of a mixture of 50 vol % CH4 in Ar over the Pt/C catalyst; and (iii) pulsing of the pulsingsame mixture of the oversame carbon mixture black. over Ascarbon a means black. of As comparison, a means of the comparison, averaged response the averaged pulse response for Ar is pulse for Ar is also depicted. The exemplary reaction temperature is again 200 °C. It can be seen from also depicted. The exemplary reaction temperature is again 200 ◦C. It can be seen from Figure6a that Figure 6a that CH4 from single pulsing over the Pt/C catalyst and over carbon black shows relatively CH4 from single pulsing over the Pt/C catalyst and over carbon black shows relatively narrow and narrowalmost congruentand almost transient congruent responses transient with responses a residence with timea residence that is onlytime slightlythat is only longer slightly than thatlonger of than that of Ar, representing the diffusion-only case. This finding proves that reversible CH4 Ar, representing the diffusion-only case. This finding proves that reversible CH4 adsorption on Pt adsorption on Pt and CH4 spillover from Pt to the C support can be excluded as an explanation for and CH4 spillover from Pt to the C support can be excluded as an explanation for the observed broad the observed broad response pulses of CH4, as observed in case (i). response pulses of CH4, as observed in case (i).

Figure 6. Height-normalized transient responses of CH4 and H2, respectively, at 200 ◦C to: (a) pulses Figure 6. Height-normalized transient responses of CH4 and H2, respectively, at 200 °C to: (a) pulses of 38.6 vol % acetaldehyde in Ar and 50 vol % CH4 in Ar over H2 pre-reduced Pt/C and over carbon of 38.6 vol % acetaldehyde in Ar and 50 vol % CH4 in Ar over H2 pre-reduced Pt/C and over carbon black; (b) pulses of 38.6 vo l% acetaldehyde in Ar and 50 vol % H2 in Ar over H2 pre-reduced Pt/C and black; (b) pulses of 38.6 vo l% acetaldehyde in Ar and 50 vol % H2 in Ar over H2 pre-reduced Pt/C and over carbon black. over carbon black.

A slightly different picture emerges for H2 in Figure6b, which shows the height-normalized A slightly different picture emerges for H2 in Figure 6b, which shows the height-normalized transient responses at an m/z-value of 2 for H2 for the same three cases given above. Of course, H2 is transient responses at an m/z-value of 2 for H2 for the same three cases given above. Of course, H2 is used instead of CH4 in the Ar mixture, with the resulting transient responses again being compared used instead of CH4 in the Ar mixture, with the resulting transient responses again being compared to that of the averaged Ar curves. The transient H2 response from acetaldehyde pulsing over Pt/C is to that of the averaged Ar curves. The transient H2 response from acetaldehyde pulsing over Pt/C is by far the broadest curve in this figure, as is the case for CH4, but that from pulsing the mixture of H2/Ar over the Pt/C catalyst, is much broader than the Ar response pulse. Moreover, it is considerably broader than the signal for H2 evolution from carbon black. In the case of H2 and unlike CH4, reversible adsorption on Pt and spillover processes from the Pt to the C support play a significant role. In the literature, spillover on Pt has been investigated and positively highlighted for catalysts for H2 storage and hydrogenation reactions, respectively [57–60]. Both reaction types are essential for the proper utilization of the Siddiki reaction scheme from Figure1. However, Figure6b also makes it very clear Catalysts 2020, 10, 1151 8 of 20 that reversible adsorption and spillover alone are not sufficient to explain the very broad and delayed response pulse for H2 when acetaldehyde is pulsed over the Pt/C catalyst. It can be concluded from the data in Figures4–6 that the following steps for the decomposition of acetaldehyde as a product of preceding ethanol dehydrogenation, including hydrogen spillover, can be proposed, whereby the ethoxy intermediate in reaction step (4) has already been proposed in the literature [13,17,34,38,48]. Step (4) must be considered irreversible for the Pt/C catalyst, as no ethanol molecules are observed in the outlet of the TAP micro-reactor in the entire temperature range between 100 ◦C and 250 ◦C. In reaction step (5), the ethoxy intermediate further decomposes into acetaldehyde, partly desorbing from the catalyst surface and adsorbed H species (H*) and, of course, adsorbed acetaldehyde species (CH3CHO*). During reaction step (6), acetaldehyde quickly decomposes into CO, carbonaceous deposits C*, adsorbed methyl species (CH3*) and H*.

CH CH OH CH CH O* + H* (4) 3 2 → 3 2 CH CH O* CH CHO + H* + CH CHO* (5) 3 2 ↔ 3 3 CH CHO * CO + C* + CH * + H* (6) 3 ↔ 3 CH * + H* spillover CH (7) 3 → → 4 H* + H* spillover H (8) → → 2 In this scheme, the CH3* and H* adsorbates have considerably long residence times on the catalyst surface including some spillover processes on the catalyst surface before they recombine as CH4 or H2. The recombination step e) is practically irreversible under these reaction conditions. This scheme is consistent with:

The total consumption of ethanol by the catalyst surface; • The slow CH and H evolution curves; • 4 2 The relatively fast evolution of CO and acetaldehyde on a comparable time scale; • The strongly increased formation of CO and CO due to carbon burn-off on the catalyst surface • 2 after the 10,000 pulses of ethanol; The transient responses from single CH /H pulsing over the Pt/C sample. • 4 2 Other reaction products, such as acetic acid (m/z = 60), butanol isomers (m/z = 56), diethyl ether (m/z = 59) and ethyl acetate (m/z = 70) or CO2 (m/z = 44) are not detected in the outlet flow of the micro-reactor under the TAP reaction conditions. The formation of acetic acid on a CuCr catalyst, starting from ethanol dehydrogenation, is mechanistically described by Xiang et al. [61], though under oxidative reaction conditions in the presence of O2. It was concluded from Figure4a that acetaldehyde decomposition, yielding CO, C*, CH3* and H* adsorbates as per Equation (6), is very fast so that a condensation reaction of two acetaldehyde molecules possibly yielding acetic acid, butanol isomers or ethyl acetate is not likely to proceed under these reaction conditions. It is known from the literature [62] that strongly acidic catalysts like H-ZSM-5 must be applied for the dehydration of ethanol to form ethene and diethyl ether. The above-described trends and phenomena at 200 ◦C for the transient responses of the different components of the ethanol dehydrogenation/acetaldehyde decomposition reaction network are qualitatively highly comparable to those at the other investigated reaction temperatures of 100 ◦C, 150 ◦C and 250 ◦C, and so these will not be presented here. In the following, the Ar-normalized transient responses to pulses of ethanol in Ar will be depicted and discussed in relation to the four reaction products of acetaldehyde, CO, CH4 and H2 at different reaction temperatures (100 ◦C–250 ◦C) so as to gain a more quantitative picture and verify the reaction scheme from Equations (4) to (8). The Ar-normalization mathematically reflects that the chosen experimental parameters such as the pulse width or the actual pressure of the reaction mixture in the blend tank might influence the height and shape of the respective transient responses. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 20

at 200 °C for the transient responses of the different components of the ethanol dehydrogenation/acetaldehyde decomposition reaction network are qualitatively highly comparable to those at the other investigated reaction temperatures of 100 °C, 150 °C and 250 °C, and so these will not be presented here. In the following, the Ar-normalized transient responses to pulses of ethanol in Ar will be depicted and discussed in relation to the four reaction products of acetaldehyde, CO, CH4 and H2 at different reaction temperatures (100 °C–250 °C) so as to gain a more quantitative picture and verify the reaction scheme from Equations (4) to (8). The Ar-normalization mathematically reflects that the Catalysts 2020, 10, 1151 9 of 20 chosen experimental parameters such as the pulse width or the actual pressure of the reaction mixture in the blend tank might influence the height and shape of the respective transient responses.

FigureFigure 7. 7.Ar-normalized Ar-normalized transienttransient responses to to pulses pulses of of 34.6 34.6 vol vol % % ethanol ethanol inin Ar Ar at attemperatures temperatures between 100 °C and 250 °C on the H2 pre-reduced Pt/C catalyst (20 mg), (a) the m/z-value of 29 for between 100 ◦C and 250 ◦C on the H2 pre-reduced Pt/C catalyst (20 mg), (a) the m/z-value of 29 for acetaldehyde; (b) the m/z-value of 28 for CO; (c) the m/z-value of 16 for CH4; (d) the magnification of acetaldehyde; (b) the m/z-value of 28 for CO; (c) the m/z-value of 16 for CH4;(d) the magniﬁcation of (c); (e) the m/z-value of 2 for H2. (c); (e) the m/z-value of 2 for H2.

FigureFigure7a 7a shows shows that that acetaldehyde acetaldehyde desorbsdesorbs fromfrom thethe catalyst surface surface at at very very low low intensities, intensities, which slightly increase with rising reaction temperature (increasing the transient response height and which slightly increase with rising reaction temperature (increasing the transient response height and zeroth moments of the pulses). The rising sections of all four response pulses coincide at short zeroth moments of the pulses). The rising sections of all four response pulses coincide at short residence residence times but their tailing becomes significantly broader with increasing temperature. This times but their tailing becomes signiﬁcantly broader with increasing temperature. This indicates an indicates an increased interaction of the acetaldehyde molecules with the catalyst at higher reaction increased interaction of the acetaldehyde molecules with the catalyst at higher reaction temperatures. temperatures. In general, it can be concluded from the very low intensities at all temperatures that In general, it can be concluded from the very low intensities at all temperatures that acetaldehyde is a short-lived intermediate of the reaction network, and only very few molecules traverse through the catalyst bed without reaction. In Figure7b, a comparable dependency of the transient response height (and the zeroth moments of the pulses) on the reaction temperature can be observed for CO. It is the lowest at 100 ◦C and continuously increases when the temperature rises to 250 ◦C. Again, the four transient responses coincide in their rising sections and the tailings become broader at higher temperatures, especially at 250 ◦C. Higher temperatures favor the desorption of CO from the catalyst surface, which runs a more complex course at 250 ◦C than at 100 ◦C. However, the intensities of the CO response pulses are much higher than those of acetaldehyde. CO is not short-lived and intermediate but a ﬁnal product of acetaldehyde decomposition (cf. Equation (6)). Catalysts 2020, 10, x FOR PEER REVIEW 10 of 20

acetaldehyde is a short-lived intermediate of the reaction network, and only very few molecules traverse through the catalyst bed without reaction. In Figure 7b, a comparable dependency of the transient response height (and the zeroth moments of the pulses) on the reaction temperature can be observed for CO. It is the lowest at 100 °C and continuously increases when the temperature rises to 250 °C. Again, the four transient responses coincide in their rising sections and the tailings become Catalystsbroader2020 at, 10 higher, 1151 temperatures, especially at 250 °C. Higher temperatures favor the desorption of 10CO of 20 from the catalyst surface, which runs a more complex course at 250 °C than at 100 °C. However, the intensities of the CO response pulses are much higher than those of acetaldehyde. CO is not short- Significantly, different pictures with respect to the shapes of the transient responses and their lived and intermediate but a final product of acetaldehyde decomposition (cf. Equation (6)). temperature dependencies emerge for CH in Figure7c and H in Figure7e. In the case of CH , Significantly, different pictures with respect4 to the shapes of2 the transient responses and their 4 at 100 ◦C the intensity is approximately zero, as can be seen in the magnification in Figure7d; only very temperature dependencies emerge for CH4 in Figure 7c and H2 in Figure 7e. In the case of CH4, at 100 few°C CH the4 intensitymolecules is approximately desorb from the zero, catalyst as can surface. be seen in The the magnification magnificationalso in Figure illustrates 7d; only that very at 150 few◦ C, a slightly increased intensity of the CH transient response is detected which is, however, much lower CH4 molecules desorb from the catalyst4 surface. The magnification also illustrates that at 150 °C, a thanslightly that ofincreased the CO intensity response of pulse the CH at4 150transient◦C. In response this respect, is detected Figure which8a,b indicateis, however, that much CH 4 lowerand H 2 desorbthan atthat large of the quantities CO response from pulse the surface at 150 of°C. Pt In/C, this when respect, only Figure CH4 (50 8a and vol %)8b inindicate Ar and that H 2CH(504 and vol %) inH Ar,2 desorb respectively, at large are quantities pulsed. from In both the cases,surface the of fractionPt/C, when of moles only CH desorbing4 (50 vol from%) in PtAr/C and is independentH2 (50 vol of%) the in temperature Ar, respectively, and amounts are pulsed. to approximately In both cases, 65% the tofraction 68% for of H moles2 and 100%desorbing for CH from4. On Pt/C carbon is black,independent the corresponding of the temperature set of experiments and amounts with to approximately mixtures of 65% CH4 toand 68% H for2, respectively,H2 and 100% for in ArCH (not4. shownOn carbon in the black, figures) the indicatescorresponding that theset of fraction experiments for H 2withslightly mixtures increases of CH4 with and H rising2, respectively, temperatures, in fromAr approximately(not shown in 80%the atfigures) 100 ◦C indicates to 95% at that 250 ◦theC, whilefraction in thefor caseH2 slightly of CH4 ,increases it is 100%. with rising temperatures, from approximately 80% at 100 °C to 95% at 250 °C, while in the case of CH4, it is 100%.

FigureFigure 8. 8.Ar-normalized Ar-normalized transient transient responses responses of of CH CH4 and4 and H H2,2 respectively,, respectively, at at temperatures temperatures between between 100 and100 250 and◦C 250 and °C averaged and averaged response response pulses puls ofes Ar of onAr theon the Pt/ CPt/C catalyst catalyst to: to: (a ()a pulses) pulses of of 50 50 vol vol % % CHCH44 in Ar;in ( bAr;) pulses (b) pulses of 50 of vol 50 %vol H %2 inH2 Ar. in Ar.

Thus,Thus, the the low low intensities intensities of of the the CHCH44 evolutionevolution curves curves at at 100 100 °C◦C and and 150 150 °C◦ Cin in Figure Figure 7d7 dcannot cannot bebe explained explained by by the the very very strong strong adsorption adsorption ofof CHCH4 on the catalyst catalyst surface. surface. Instead, Instead, they they are are due due to to thethe fact fact that that Equation Equation (7), (7), i.e., i.e., the the recombination recombination ofof CH3** and and H* H* being being formed formed in in the the acetaldehyde acetaldehyde decomposition,decomposition, is is kinetically kinetically hindered hindered atat lowerlower reactionreaction temperatures between between 100 100 °C◦C and and 150 150 °C.◦C. ThereThere is is a verya very small small peak peak at at the the beginningbeginning of the CH 44 responseresponse in in Figure Figure 7d,7d, which which is isconsistent consistent with the fast production of CH4. A small fraction of CH4 rapidly leaves the micro-reactor, which might with the fast production of CH4. A small fraction of CH4 rapidly leaves the micro-reactor, which might havehave been been formed formed in in reaction reaction steps steps otherother thanthan thosethose proposed in in Equations Equations (4)–(8). (4)–(8). The The same same very very small signal is also observed in the rising part of the CH4 transient response at 200 °C. However, CH4 small signal is also observed in the rising part of the CH4 transient response at 200 ◦C. However, desorption from the catalyst surface in Figure 7c is much more pronounced, with a very broad tailing CH4 desorption from the catalyst surface in Figure7c is much more pronounced, with a very broad at 200 °C, and becomes even stronger and broader at 250 °C. It can be concluded that at higher tailing at 200 ◦C, and becomes even stronger and broader at 250 ◦C. It can be concluded that at higher reaction temperatures, the activation barrier for the irreversible step of CH3* and H* recombination reaction temperatures, the activation barrier for the irreversible step of CH3* and H* recombination can can be much more readily surmounted. In addition, CH3* and H* adsorbates, having also been be much more readily surmounted. In addition, CH * and H* adsorbates, having also been previously previously formed on the catalyst surface at 100 °C3 and 150 °C, can now recombine and desorb, formed on the catalyst surface at 100 ◦C and 150 ◦C, can now recombine and desorb, leading to these leading to these large amounts of CH4, especially at 250 °C. The transient responses for H2 in the largetemperature amounts range of CH from4, especially 100 °C to at 250 250 °C◦ C.in TheFigure transient 7e show responses very similar for trends H2 in compared the temperature to those range of from 100 C to 250 C in Figure7e show very similar trends compared to those of CH . At 100 C and CH4. At◦ 100 °C and◦ 150 °C, the amounts of H2 molecules desorbing from the catalyst surface4 are ◦very 150 ◦C, the amounts of H2 molecules desorbing from the catalyst surface are very low. As in the case of CH 4,H2 desorption strongly increases with rising temperatures at 200 ◦C, and especially, at 250 ◦C. Analogous to the case of CH4, here the recombination of H* species (Equation (8)) is strongly favored by higher reaction temperatures and is therefore responsible for the immense desorption of H2 from the catalyst surface at 200 ◦C and 250 ◦C. Catalysts 2020, 10, 1151 11 of 20

2.2.2. Experiments with Ir/C This subsection describes and discusses the results that were obtained when a mixture of 34.6 vol % ethanol in Ar was pulsed over an H2 pre-reduced Ir/C catalyst in the temperature range between 100–250 ◦C. Ir/C was chosen for the experimental evaluation, as it was also very active for the synthesis of iso-butanol according to the reaction scheme from Figure1 in recent investigations by Häusler et al. [12]. Liang et al. [63] used Ir for their AuPtIr core-shell catalyst and state that Ir enhances ethanol dehydrogenation. Li et al. [64] modeled a catalyst ensemble for ethanol dehydrogenation. This ensemble is based on a triatomic inert substrate consisting of, e.g., Au, Ag and Cu, onto which a strong-binding metal such as Ir is doped to activate the C–H and O–H bonds in ethanol. The function of the triatomic substrate is to enable the fast desorption of H2 and to suppress coke formation. Sheng et al. [65] investigated Pt doped with Ir, Ru, Rh, Pd and Os as the catalyst for ethanol electro-oxidation and found a promoting effect of Ir with respect to the β-dehydrogenation of ethanol. In this respect, Figure9a shows that, in contrast to the experiments with the Pt /C catalyst, ethanol is not completely consumed in the investigated temperature range between 100–250 ◦C, but measurable amounts desorb from the Ir/C catalyst surface. These amounts increase with increasing reaction temperatures and are the highest by far at 250 ◦C. A quantitative analysis reveals that ethanol conversion is close to 100% at 100–150 ◦C and decreases to the range of approximately 90% at 250 ◦C. Thus, the Ir/C catalyst is also highly active for the ethanol dehydrogenation reaction. Boualouche et al. [66] report based on DFT calculations that for their CuIr alloy, scission of the O–H bond is the activation step during ethanol dehydrogenation. It is common to all four ethanol transient responses in Figure9a that, at the beginning, there is a peak with low intensity being almost as narrow as the Ar response pulse, which indicates that a few ethanol molecules are able to move very quickly through the Ir/C catalyst zone without adsorption or reaction. However, at 100 ◦C and 150 ◦C, for the majority of the ethanol molecules, the consumption process is irreversible, as in the case of the Pt/C catalyst. At 200 ◦C and 250 ◦C, in contrast, very slow desorption of ethanol molecules from the catalyst surface also occurs, yielding very broad response pulses for ethanol. Considering these findings for the reaction scheme derived above for the Pt/C catalyst, it can be concluded that in the case of the Ir/C catalyst, step (4), i.e., the adsorption and dissociation of ethanol, is not fully irreversible. To further illuminate the residence time distribution of the products of ethanol dehydrogenation on Ir/C, Figure9b shows the height-normalized transient responses from the same set of experiments with the Ir/C sample at m/z-values of 2 for H2, 16 for CH4, 28 for CO, 44 for acetaldehyde and 40 for Ar at a reaction temperature of 200 ◦C. This time, an m/z-value of 44 was chosen for acetaldehyde, as it was only slightly affected by the respective ethanol fragment (in contrast to 29), and so qualitative conclusions can be drawn from the shape and residence time of the response pulse. For the sake of clarity, the transient response of ethanol was not included in this figure. On the one hand, there are many parallels to the findings obtained with the Pt/C catalyst from Figure4a):

H and acetaldehyde as products of ethanol dehydrogenation are formed, together with CO and • 2 CH4 from acetaldehyde decomposition. Zhang et al. [67] and Cai et al. [68] investigated steam reforming of ethanol using an Ir/CeO2 catalyst. They first observed ethanol dehydrogenation to acetaldehyde followed by decomposition of acetaldehyde to CH4 and CO. These findings are in good agreement with those presented here. Other reaction products, such as acetic acid (m/z = 60), butanol isomers (m/z = 56), diethyl ether • (m/z = 59) or ethyl acetate (m/z = 70), are not detected. Boualouche et al. [66], however, found CuIr alloys being active for the formation of ethyl acetate. The evolution curve of CO and that of acetaldehyde at lower residence times are very narrow and • only slightly broader than that of Ar, representing the diffusion-only case. The transient responses of CO and acetaldehyde coincide in their rising sections at lower residence • times. This again means that they desorb on a comparable time scale from the catalyst surface Catalysts 2020, 10, x FOR PEER REVIEW 12 of 20

• The transient responses of CO and acetaldehyde coincide in their rising sections at lower residence times. This again means that they desorb on a comparable time scale from the catalyst surface within this time-span and confirms reaction step (6) for the Ir/C catalyst, i.e., the fast CO formation from acetaldehyde. • The response pulses of H2 and those of CH4 at higher residence times are very broad, again raising the question of whether the recombination steps of long-lived CH3* and H* adsorbates, as per Equations (7) and (8), are responsible or reversible adsorption and spillover processes. However, on the other hand, there are also some differences in the results from the experiment Catalystswith 2020the ,Pt/C10, 1151 catalyst in Figure 4a): 12 of 20 • The acetaldehyde response pulse is much broader at longer residence times, indicating that reaction step (5), i.e., the decomposition of the ethoxy adsorbate into rapidly desorbing withinacetaldehyde this time-span and H* and species, conﬁrms should reaction be ex steptended (6) forby thethe Irformation/C catalyst, of i.e.,a long-lived the fast CO CH formation3CHO* fromadsorbate. acetaldehyde. • 4 TheThe response evolution pulses curve of for H2 CHand thosereveals of a CH fairly4 at narrow higher residencesection at shorter times are residence very broad, times. again On Ir/C, raising • the decomposition of acetaldehyde, according to reaction step (6), directly yields a small fraction the question of whether the recombination steps of long-lived CH3* and H* adsorbates, as per Equationsof rapidly (7) desorbing and (8), areCH responsible4 molecules. or reversible adsorption and spillover processes.

FigureFigure 9. 9.(a )(a Ar-normalized) Ar-normalized transient transient responsesresponses to pulses of of 34.6 34.6 vol vol % % ethanol ethanol in in Ar Ar at attemperatures temperatures

betweenbetween 100 100◦C °C and and 250 250◦C on°C theon Hthe2 pre-reducedH2 pre-reduced Ir/C Ir/C catalyst catalyst (20 mg)(20 mg) at an at m /anz-value m/z-value of 31 forof 31 ethanol; for responseethanol; pulse response of Ar pulse (m/ z-valueof Ar (m/z-value of 40) at 200of 40)◦C at for 200 comparison. °C for comparison. (b) Height-normalized (b) Height-normalized transient transient responses to pulses of 34.6 vol % ethanol in Ar at m/z-values of 2 for H2, 16 for CH4, 28 for responses to pulses of 34.6 vol % ethanol in Ar at m/z-values of 2 for H2, 16 for CH4, 28 for CO, 44 for CO, 44 for acetaldehyde and 40 for Ar on the H2 pre-reduced Ir/C catalyst (20 mg), T = 200 °C. acetaldehyde and 40 for Ar on the H2 pre-reduced Ir/C catalyst (20 mg), T = 200 ◦C.

2.2.3.However, Experiments on the with other Cu/C hand, there are also some differences in the results from the experiment with theThe Pt /lastC catalyst set of TAP in Figure experiments4a): concerning ethanol dehydrogenation presented in this paper dealsThe with acetaldehyde the catalytic response behavior pulse of a is Cu/C much catalyst broader. Based at longer on literature residence findings times,indicating that Cu catalysts that reaction are • distinguishedstep (5), i.e., by the high decomposition selectivities oftowards the ethoxy acetal adsorbatedehyde [16,18,20,22,24,26] into rapidly desorbing, it was acetaldehyde assumed that and using Cu might lead to a divergent reaction scheme. In this respect, Figure 10a presents the Ar- H* species, should be extended by the formation of a long-lived CH3CHO* adsorbate. normalized transient responses to pulses of ethanol in Ar at temperatures between 100 °C and 250 °C The evolution curve for CH reveals a fairly narrow section at shorter residence times. On Ir/C, • on the Cu/C catalyst at an m/z-value4 of 31, representing ethanol. As in the case of the Ir/C sample and the decomposition of acetaldehyde, according to reaction step (6), directly yields a small fraction in contrast to the Pt/C catalyst, ethanol is not fully consumed within the entire temperature range. Theof quantities rapidly desorbing of ethanol CH molecules4 molecules. desorbing from the catalyst surface are very low, at 100 °C and 150 °C, which means that, at these temperatures, the majority of the ethanol molecules are consumed 2.2.3. Experiments with Cu/C by the catalyst surface and only a few can very quickly traverse the catalyst bed. At 200 °C and 250 °C,The however, last set very of TAPslow experimentsdesorption of concerningmuch larger ethanolquantities dehydrogenation of ethanol occurs, presented which results in this in very paper dealsbroad with response the catalytic pulses. behavior A quantitative of a Cu calculation/C catalyst. shows Based that on literatureethanol conversion findings thatis close Cu to catalysts 100% at are distinguished100-150 °C and by highdecreases selectivities to the range towards of approximately acetaldehyde [ 1685-90%,18,20 at,22 200,24, 26°C], and it was 250 assumed °C, respectively. that using CuThus, might the lead Cu/C to catalyst a divergent is still reaction highly active scheme. for Inethanol this respect, adsorption Figure and 10 possiblea presents further the reactions, Ar-normalized but slightly less suitable than Ir/C, and especially, Pt/C. Therefore, it can be concluded that also on the transient responses to pulses of ethanol in Ar at temperatures between 100 ◦C and 250 ◦C on the Cu/C catalystCu/C atcatalyst, an m/ z-valuestep (4) of the 31, representingreaction scheme, ethanol. i.e., the As adsorption in the case and ofthe dissociation Ir/C sample of ethanol, and incontrast is again to thenot Pt /fullyC catalyst, irreversible. ethanol To is gain not fullya better consumed understanding within of the the entire way temperaturein which ethanol range. dehydrogenation The quantities of ethanol molecules desorbing from the catalyst surface are very low, at 100 ◦C and 150 ◦C, which means that, at these temperatures, the majority of the ethanol molecules are consumed by the catalyst surface and only a few can very quickly traverse the catalyst bed. At 200 ◦C and 250 ◦C, however, very slow desorption of much larger quantities of ethanol occurs, which results in very broad response pulses. A quantitative calculation shows that ethanol conversion is close to 100% at 100–150 ◦C and decreases to the range of approximately 85–90% at 200 ◦C and 250 ◦C, respectively. Thus, the Cu/C catalyst is still highly active for ethanol adsorption and possible further reactions, but slightly less suitable than Ir/C, and especially, Pt/C. Therefore, it can be concluded that also on the Cu/C catalyst, step (4) of the reaction scheme, i.e., the adsorption and dissociation of ethanol, is again not fully irreversible. To gain a better understanding of the way in which ethanol dehydrogenation proceeds on the Cu/C sample, Figure 10b depicts the height-normalized transient responses at m/z-values of 2 for H2, 16 for CH4, Catalysts 2020, 10, x FOR PEER REVIEW 13 of 20 Catalysts 2020, 10, 1151 13 of 20 proceeds on the Cu/C sample, Figure 10b depicts the height-normalized transient responses at m/z- values28 for CO,of 2 44for for H2 acetaldehyde, 16 for CH4, 28 and for40 CO, for 44 Ar for at acetaldehyde a reaction temperature and 40 for of Ar 200 at a◦C. reaction The figure temperature shows a ofvery 200 similar °C. The residence figure shows time distribution,a very similar as residence in the case time of thedistribution, Ir/C sample. as in The the only case apparentof the Ir/C di sample.fference Theis that only the apparent transient difference response foris that acetaldehyde the transient has response a much shorterfor acetaldehyde narrow section, has a wheremuch itshorter again narrowcoincides section, with the where evolution it again curve coincides of CO with and a the longer evolution broad curve tailing. of This CO indicatesand a longer that broad the formation tailing. Thisof a long-livedindicates that CH 3theCHO* formation adsorbate of a playslong-lived a more CH important3CHO* adsorbate role on the plays Cu /aC more catalyst important than it doesrole on thethe otherCu/C twocatalyst samples, than Ptit/ Cdoes and on Ir/ C.the Reaction other two products, samples, such Pt/C as aceticand Ir/C. acid Reaction (m/z = 60), products, butanol isomerssuch as acetic(m/z = acid56), (m/z diethyl = 60), ether butanol (m/z = isomers59) or ethyl (m/z acetate= 56), diethyl (m/z = ether70), are (m/z not = detected 59) or ethyl with acetate the Cu (m/z/C catalyst. = 70), areThe not decomposition detected with of the acetaldehyde Cu/C catalyst. to form The CO, decomposition H2 and CH4 ofshown acetaldehyde in Figure to 10 formb, is inCO, contrast H2 and to CH the4 shownexperimental in Figure results 10b, of is many in contrast papers into the the literature experimental working results with Cuof many catalysts, papers which in reportthe literature on high workingselectivities with towards Cu catalysts, acetaldehyde which report of in parts on high 100% selectivities [17,19,21,23 towards,25]. This acetaldehyde discrepancy of might in parts be due 100% to [17,19,21,23,25].the ultra-high vacuum This discrepancy reaction conditions might be of due the to TAP the experiments ultra-high vacuum in this work. reaction conditions of the TAP experiments in this work.

Figure 10. ((aa)) Ar-normalized Ar-normalized transient transient responses responses to to pulses pulses of 34.6 34.6 vol vol % % ethanol ethanol in in Ar Ar at at temperatures temperatures between 100 °C◦C and and 250 °C◦C on on the H 22 pre-reducedpre-reduced Cu/C Cu/C catalyst catalyst (20 (20 mg) mg) at at an an m/z-value m/z-value of of 31 for ethanol; response response pulse of Ar (m(m/z-value/z-value of 40)40) atat 200200 ◦°CC forfor comparison.comparison. (b) Height-normalized

transienttransient responses responses to to pulses of of 34.6 vol % ethanol in Ar at mm/z-values/z-values of 2 for HH22, 16 for CH4,, 28 28 for for

CO, 44 for acetaldehyde and 40 for Ar on the H2 pre-reducedpre-reduced Cu/C Cu/C catalyst (20 mg), T = 200200 °C.◦C.

InIn Table Table2 2,, thethe datadata fromfrom TableTable1 1is is expanded expanded to to include include the the above-mentioned above-mentioned calculated calculated ethanol ethanol conversions of of the the different diﬀerent C-suppo C-supportedrted catalysts at at 200 °C.◦C. It becomes obvious that there is no correlation betweenbetween thethe speciﬁcspecific surface surface areas areas and and the the ethanol ethanol conversions. conversions. The The same same holds holds true true for thefor thepore pore size size distributions distributions from from Figure Figure2. 2.

Table 2. Specific surface areas and ethanol conversions of the different C-supported catalysts. Table 2. Specific surface areas and ethanol conversions of the different C-supported catalysts. Catalyst 2 1 Calculated Ethanol Conversions at 200 C% SpecificSpecific Surface AreaArea m g− Calculated Ethanol Conversions at 200 °C◦ Catalyst 5 wt % Pt/Cm2 g− 8051 % ~100 15 wt wt % % Ir /Pt/CC 805 1013 ~ 100 ~95 3 wt % Cu/C 1271 ~85 1 wt % Ir/C 1013 ~ 95 3 wt % Cu/C 1271 ~ 85 Figure 11 graphically summarizes the experimental results from the TAP experiments described in this section. Please note that, for the sake of simplicity, the representation of the molecules is only a simple approximation of the chemical reality. The same holds true for the way the adsorption of the species on Pt is depicted. The TAP experiments cannot shed light on this point.

Catalysts 2020, 10, 1151 14 of 20 Catalysts 2020, 10, x FOR PEER REVIEW 14 of 20

FigureFigure 11. 11. ReactionReaction scheme scheme for for ethanol ethanol dehy dehydrogenationdrogenation on on a a Pt/C Pt/C catalyst. catalyst. 3. Materials and Methods Figure 11 graphically summarizes the experimental results from the TAP experiments described in3.1. this Catalysts section. Please note that, for the sake of simplicity, the representation of the molecules is only a simple approximation of the chemical reality. The same holds true for the way the adsorption of The 5 wt % Pt/C, 1 wt % Ir/C and 3 wt % Cu/C catalysts were purchased from Alfa Aesar (Karlsruhe, the species on Pt is depicted. The TAP experiments cannot shed light on this point. Germany), while Cabot Corporation (Boston, USA) supplied the carbon black sample (Vulcan XC 72). 3. Materials and Methods 3.2. N2 Sorption 3.1. CatalystsThe specific surface areas of the different catalysts from Table1 were determined by the adsorption of N at a temperature of 196 C (liquid N ). Approximately 0.1–0.4 g of the respective catalysts The2 5 wt % Pt/C, 1 wt− % Ir/C◦ and 3 wt2 % Cu/C catalysts were purchased from Alfa Aesar were filled into a BET cannula and dried under vacuum conditions at 50 C in the Autosorb iQ (Karlsruhe, Germany), while Cabot Corporation (Boston, USA) supplied the◦ carbon black sample (version 5.20.17081) apparatus of the company Quantachrome (Odelzhausen, Germany). The samples (Vulcan XC 72). were degassed at 150 ◦C until their masses remained constant. The values for the surface area were then investigated by applying the Brunauer-Emmet-Teller (BET) methodology [69]. The pore size 3.2. N2 Sorption distributions, as depicted in Figure2, were calculated from the adsorption isotherms in the range of The specific surface areas of the different catalysts from Table 1 were determined by the p/p0 = 0.01–0.95 by means of the Cranston-Inclay method [70]. adsorption of N2 at a temperature of −196 °C (liquid N2). Approximately 0.1–0.4 g of the respective catalysts3.3. Scanning were Electronfilled into Microscopy a BET cannula and dried under vacuum conditions at 50 °C in the Autosorb iQ (version 5.20.17081) apparatus of the company Quantachrome (Odelzhausen, Germany). The The size and distribution of the catalyst within the support was analyzed by means of Scanning samples were degassed at 150 °C until their masses remained constant. The values for the surface Transmission Electron Microscopy (STEM). The samples were deposited on a Cu C-film grid. High Angle area were then investigated by applying the Brunauer-Emmet-Teller (BET) methodology [69]. The Annular Dark Field (HAADF) imaging, in combination with Energy-Dispersive X-ray (EDX) elemental pore size distributions, as depicted in Figure 2, were calculated from the adsorption isotherms in the mapping, were acquired using an aberration corrected FEI Titan G2 80-200 STEM field emission range of p/ p 0 = 0.01–0.95 by means of the Cranston-Inclay method [70]. electron microscope at 200 eV and a Super-X EDX system [71].

3.3. Scanning Electron Microscopy The size and distribution of the catalyst within the support was analyzed by means of Scanning Transmission Electron Microscopy (STEM). The samples were deposited on a Cu C-film grid. High Angle Annular Dark Field (HAADF) imaging, in combination with Energy-Dispersive X-ray (EDX)

Catalysts 2020, 10, 1151 15 of 20

3.4. Temporal Analysis of Products The commercial TAP-KPC reactor system (Mithra Technologies, Inc., Foley, USA), which was used for the transient experiments in this paper and also the way the “thin-zone reactor” concept was established, is described in detail in Pasel et al. [72]. For the TAP experiments, the micro-reactor contained 20 mg of the catalysts and the carbon black sample, respectively, with particle sizes between 200 µm and 400 µm. The number of moles of ethanol, acetaldehyde, CO, CH4 and H2 in the respective 8 inlet pulses during the different kinds of experiments was in the range of 1–5 *10− . All catalysts were reduced at 400 ◦C prior to the experimental runs by a flow of H2 (2.5 vol % in Ar) at 400 ◦C for approximately 2 h. For these reductive experiments, the so-called “flow-mode” of the TAP system was used, which operates at atmospheric pressure, in contrast to the transient pulse experiments running at ultra-high vacuum conditions. The carbon black sample was reduced in a flow of pure H2 at 400 ◦C. Whenever ethanol was pulsed over the surfaces of the different catalysts, its inlet concentration was approximately 34.6 vol % in Ar. This value was set by injecting liquid ethanol at room temperature into the evacuated blend tank and then pressurizing it to 23 PSI with Ar. Afterwards, the blend tank with the ethanol/Ar mixture was heated from 20 ◦C to a temperature of 80 ◦C. According to the Antoine equation and the corresponding constants for ethanol in the suitable temperature range [73], the resulting additional partial pressure of ethanol in the blend tank at 80 ◦C can be calculated to 15.7 PSI. Reflecting the pressure increase in the blend tank due to its heating from 20 ◦C to 80 ◦C, an ethanol concentration of 34.6 vol % arises as a result. The lines from the blend tank to the manifold of the TAP system containing the pulse valves and manifold itself were heated to 110 ◦C to prevent the ethanol molecules from condensing on the metal parts of the experimental set-up prior to being pulsed into the micro-reactor. For the experiments with ethanol pulsing, the reaction temperature was varied between 100 ◦C and 250 ◦C. In a similar way, the concentration of acetaldehyde in the inlet pulses during the experiments with respect to acetaldehyde adsorption and decomposition on the Pt/C catalyst was set to 38.6 vol %, whereby the temperature of the tank with the acetaldehyde/Ar mixture amounted to 20 ◦C and that of the lines from the blend tank to the manifold was 80 ◦C. Additionally, for the acetaldehyde decomposition experiments, the reaction temperature was modified between 100 ◦C and 250 ◦C. The experiments for the single adsorption and desorption behavior of the reaction products CO, CH4 and H2 on the different catalysts and on carbon black were run with mixtures of 50 vol % of the respective gases in Ar in the temperature range between 100 ◦C and 250 ◦C. The transient responses displayed in the figures of this paper were recorded using a mass spectrometer (Stanford Research Systems RGA 200) at m/z-values of 2 for H2, 16 for CH4, 28 for CO, 29 and 44 for acetaldehyde, 31 for ethanol and 40 for Ar. These were either height-normalized or Ar-normalized. The way in which these normalizations and mass correction of the recorded time values used at the X-axis in all figures was performed is described in detail in a former publication by Pasel et al. [72]. In addition, possible condensation products of acetaldehyde such as acetic acid (m/z = 60), butanol isomers (m/z = 56), diethyl ether (m/z = 59) or ethyl acetate (m/z = 70) were analyzed. By also investigating the m/z-values of 12 and 25 when necessary, it was ensured that the observed transient responses at an m/z-value of 28 could be definitely ascribed to CO rather than the ethene.

4. Conclusions The results of the transient TAP experiments with the Pt/C, Ir/C and Cu/C catalysts enable the development of a reaction scheme for ethanol dehydrogenation on these samples. They prove that in each case, ethanol fully adsorbs (Pt/C) or at least to a large extent (Ir/C and Cu/C) on the catalysts. This slight difference in calculated ethanol conversions (cf. Table2) correlates with the findings from the STEM investigations that Pt shows the weakest tendency towards particle agglomeration. Thus, all three catalysts are very well-suited to activating the O–H bond in ethanol. Literature data gained on different catalysts allow the assumption that first ethoxy species bound to the catalyst surface might be Catalysts 2020, 10, 1151 16 of 20 formed. The TAP experiments presented here offer experimental evidence that these further decompose into acetaldehyde, being a short-lived reaction intermediate that quickly decomposes into CO and some very long-lived CH3* and H* adsorbates on the catalyst surface. These may in turn undergo spillover and then recombine, yielding CH4 and H2. CH4 and H2 formation are strongly favored by higher reaction temperatures of 200–250 ◦C when the needed activation barrier for the recombination process is surmounted. The experimental finding that carbonaceous deposits are formed on the Pt/C catalyst constitutes a substantial risk of catalyst deactivation by blocking the accessible active catalyst sites. By burning-off these C deposits, catalyst regeneration has been proved to be successful. From the more general perspective of iso-butanol synthesis, the observed H2 spillover on the Pt/C catalyst can be considered advantageous, as it might improve the catalyst’s capability for H2 uptake and release in the course of the hydrogen-borrowing mechanism. In fact, in parallel experiments at Jülich aimed at iso-butanol synthesis in an autoclave reactor at elevated pressures, Pt/C is found to be more promising than Ir/C and Cu/C, respectively. It is apparent from the scheme in Figure1 that the formation of acetaldehyde and its condensation with formaldehyde is essential for the synthesis of iso-butanol. The TAP experiments reported in this paper, however, enable the conclusion to be drawn that the observed fast decomposition of acetaldehyde on Pt/C, Ir/C and Cu/C will hinder the condensation of acetaldehyde and formaldehyde and thus disrupt the reaction pathway leading to iso-butanol. Nevertheless, at elevated pressures in an autoclave reactor, a substantially different reaction mechanism might prevail on the Pt/C, Ir/C and Cu/C catalysts.

Author Contributions: Conceptualization, J.P. and J.H.; Funding acquisition, J.M. and R.P.; Investigation, J.P., D.S., M.M. and H.V.; Methodology, J.P.; Supervision, J.M. and R.P.; Writing – original draft, J.P., M.M. and H.V.; Writing – review & editing, J.P., J.H., M.M. and H.V. All authors have read and agreed to the published version of the manuscript. Funding: Parts of this work have been funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), funding number: 19I18006P. Furthermore, H.V. wants to thank the German Federal Ministry of Education and Research, project MEET-HiEnD III – Materials and Components to Meet High Energy Density Batteries with the funding number 03XP0258C. M.M. wants to acknowledge the Verbundvorhaben iNEW: Inkubator Nachhaltige Elektrochemische Wertschöpfungsketten with the funding number 03SF0589A (German Federal Ministry for Economic Affairs and Energy BMWi). Conflicts of Interest: The authors have no conflicts of interest to declare.

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