Document [original]

Very Important Paper

Controlling the Coke Formation in Dehydrogenation of

Propane by Adding Nickel to Supported Gallium Oxide

Robert Baumgarten,[a] Piyush Ingale,[a, b] Fabian Ebert,[a] Aliaksei Mazheika,[a] Esteban Gioria,[a]

Katharina Trapp,[a] Kevin D. Profita,[c] Raoul Naumann d’Alnoncourt,*[a] Matthias Driess,[a, c]

and Frank Rosowski[a, d]

Atomic layer deposition was applied on mesoporous silica to

synthesize a highly dispersed gallium oxide catalyst. This system

was used as starting material to investigate different loadings of

nickel in the dehydrogenation of propane under industrially

relevant, Oleflex-like conditions. The formation of NiGa alloys

was confirmed by X-ray diffraction analysis and electron micro-

scopy. Surprisingly, the nanoalloys enhanced the selectivity

towards C3H6while decreasing the tendency for coking. Herein,

in situ thermogravimetry, and measured mass fractions of

carbon revealed that the coking rate was reduced by over 50%

compared to the pristine gallium oxide. Generally, the increased

selectivity can be explained by the partial hydrogenation and

reduction of the gallium oxide surface. The optimum temper-

ature for the removal of deposited carbon was evaluated by a

temperature programmed oxidation. Finally, the best-perform-

ing NiGaOxcatalyst was employed in a cycled experiment with

periodic reaction and regeneration tests. After regeneration, the

selected NiGaOxcatalyst provided a higher yield of propylene

compared to the unmodified gallium oxide.

Introduction

Propylene is an essential intermediate in the chemical industry

and is mainly used for the production of polypropylene,

acrylonitrile, and propylene oxide. Most propylene is obtained

as a byproduct from steam cracking and fluid catalytic cracking

processes which in recent times are unable to satisfy the ever-

increasing demand alone.[1] Therefore, on-purpose technologies

emerged targeting the production of propylene from abundant

shale gas.[2] The non-oxidative propane dehydrogenation (PDH)

is of particular interest as it generates hydrogen as a valuable

byproduct.[3] The most prominent commercial techniques for

the dehydrogenation of light alkanes are the Catofin (CB&I, ABB

Lummus), Oleflex (UOP Honeywell), and STAR process (Thys-

senKrupp Uhde).[4]

C3H8gð Þ ÐC3H6gð Þ þH2gð Þ DH0

298:15 K¼ þ124:3kJ

mol (1)

The dehydrogenation of propane is a highly endothermic

reaction, limited by its thermodynamic equilibrium [Equa-

tion (1)]. Hence, elevated reaction temperatures are required

(up to 650°C) to enable industrially relevant propane

conversions.[1,4] However, high temperatures diminish the che-

mo-selectivity to propylene and favor unintended cracking or

deep dehydrogenation.[5,6] Both can eventually result in the

formation of coke which accelerates the deactivation of the

catalyst.[6,7]

As the deposition of coke cannot be fully avoided, industrial

catalysts are periodically regenerated by burning the coke in

air.[3,8] Moreover, H2or steam is often co-dosed to the reactant

stream to mitigate the coking rate.[9–12] Consequently, restrained

coke formation and stability under regeneration conditions are

important requirements for the catalyst. The Catofin process

employs supported CrOxcatalysts whereas the Oleflex and STAR

processes apply different bimetallic PtSn systems (Oleflex:

KPtSn/Al2O3, STAR: PtSn/ZnAl2O4/CaO-Al2O3).[4] The use of a

CrOxcatalyst requires frequent regeneration (every 15–25 min)

and is environmentally questionable due to its toxicity.[13]

Therefore, Pt-based catalysts are preferable, providing a

propylene selectivity of around 90% and reaching operation

periods of up to 10 days.[3,4] Particularly PtGa alloys have been

reported with high propylene selectivity (around 95%), propane

conversion (up to 40%), and inhibited coke formation.[14,15]

Nevertheless, noble metals should be replaced by more

abundant elements to reduce catalyst costs. In this regard,

gallium oxide alone was found to be active in the PDH

[a] R. Baumgarten, Dr. P. Ingale, F. Ebert, Dr. A. Mazheika, Dr. E. Gioria,

K. Trapp, Dr. R. Naumann d’Alnoncourt, Prof. Dr. M. Driess, Dr. F. Rosowski

BasCat – UniCat BASF JointLab

Technische Universität Berlin

Hardenberstraße 36, 10623 Berlin (Germany)

E-mail: [email protected]

[b] Dr. P. Ingale

hte GmbH

Kurpfalzring 104, 69123 Heidelberg (Germany)

[c] K. D. Profita, Prof. Dr. M. Driess

Institut für Chemie: Metallorganik und Anorganische Materialien

Technische Universität Berlin

Straße des 17. Juni 135, 10623 Berlin (Germany)

[d] Dr. F. Rosowski

Catalysis Research

BASF SE

Carl-Bosch-Straße 38, 67056 Ludwigshafen (Germany)

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/cctc.202301261

an open access article under the terms of the Creative Commons Attribution

License, which permits use, distribution and reproduction in any medium,

provided the original work is properly cited.

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www.chemcatchem.org

Research Article

doi.org/10.1002/cctc.202301261

reaction.[16,17] Herein, the Lewis acidic Ga(III) sites are accepted

as the catalytically relevant species.[18–20] Furthermore, two

patents have been filed claiming the usage of gallium oxide-

based catalysts, applying silica- and alumina-containing support

materials.[21,22]

Interestingly, the addition of preferably 50–200 ppm nickel

was shown to have a beneficial effect on the overall propylene

yield (36% at 580°C).[21] However, the actual role of nickel was

not discussed and analysis of possibly formed NiGa alloys was

not considered. Moreover, the catalysts were only tested for a

few minutes in which the initial activity could be preserved.[21]

In the end, this catalyst type has never found application on an

industrial scale, despite the high dehydrogenation activity.

Pure nickel catalysts are known to be active in the hydro-

genolysis and cracking of light alkanes.[23–26] Large Ni-ensembles

of above 12 adjacent Ni-atoms provide multi-site adsorption,

leading to undesired breaking of CC bonds instead of CH

activation.[27,28] Platinum is also a well-known hydrogenolysis

and cracking catalyst.[2,29–31] Hence, Pt is often alloyed with

metals like Sn,[11,32,33] Zn[34–36] or Ga[37–39] which function as an

atomic spacer to prevent larger Pt-ensembles, while providing a

beneficial electronic modification. Particularly in the case of

PtSn, extensive research was conducted on both the geo-

metric effect[32,40] and the electronic effect,[41] resulting from the

addition of Sn.

A similar principle was reported for nickel when being used

for the non-oxidative dehydrogenation of alkanes.[23] For

instance, Ni was alloyed with Cu,[42,43] Au,[44] Mo,[45] Sn[46–48] or

Zn[49,50] leading also to beneficial geometric and electronic

modifications. However, there are only a handful of studies

specifically targeting the PDH reaction using alloyed Ni.[46,51,52]

Only two previous studies are known to have tested the

combination of Ni and Ga in the dehydrogenation of light

alkanes.[51,52] Only one of them studied this system for PDH.[51] It

was stated that Ga-rich surface compositions on 1:1 NiGa alloys

provide the highest propylene selectivity (ca. 90%) and

conversion of propane (20–10%). Yet, the propane feed

concentration was only 10% which is far from industrially

relevant conditions.

Furthermore, results from one of the previous studies[51]

indicated that the use of SiO2as a support material is not

beneficial, whereas using Al2O3led to the highest propylene

activity. Although the intrinsic activity of Al2O3is well known in

PDH,[53–55] the mentioned report[51] lacked performance data of

the pure Al2O3used as support material. Hence, the individual

contributions of NiGa alloys and Al2O3to the catalyst’s perform-

ance were not transparent.

In this study, SiO2was chosen as the support material as it is

inert in the PDH reaction.[46] That way, the catalytic performance

derives exclusively from the deposited components. Addition-

ally, silica provides a weak metal-support interaction which

facilitates the mobility and formation of metal alloys.[56–58]

We report the synthesis of a gallium oxide catalyst, modified

with NiGa alloys, combining atomic layer deposition (ALD) and

incipient wetness impregnation. ALD is a well-established

technique used for the deposition of uniform, nanoscale films

in the semiconductor industry.[59] In the last decade, ALD also

emerged as a precise tool for the synthesis of heterogeneous

catalysts.[60] The method was applied to deposit protective

overcoats on nanoparticles,[61–63] promotors on bulk oxides,[64,65]

the synthesis of nanoparticles[66] or nanoalloys,[36] and the

distribution of active metal oxides.[67–69] So far, only one

publication is known which applied ALD for the synthesis of

gallium oxide catalysts used for propane dehydrogenation.[70]

However, alumina was used as support material and the

dehydrogenation reaction was CO2-assisted.

Here, ALD was applied to deposit a homogeneous interface

of gallium oxide on silica. The high dispersion of GaOxensured

intimate contact with the subsequently impregnated nickel

precursor. A schematic synthesis procedure is depicted in

Figure 1 and an investigation of the ALD method is reported in

detail elsewhere.[71] The modified gallium oxide catalysts were

tested for the PDH reaction under industrially relevant, Oleflex-

like conditions. Moreover, the resulting NiGa species and

respective tendencies for coke formation were determined.

Results and Discussion

Catalytic performance (PDH)

The objective of this study was to determine the influence of

nickel on the catalytic behavior of gallium oxide in the

dehydrogenation of propane. Mesoporous silica (SiO2) was

chosen as the support material on which gallium oxide was

deposited. The advantage of SiO2is that it does not contribute

to the catalyst’s performance which makes the activities of the

individual components more transparent.[72]

Since nickel should be added subsequently, it is reasonable

to disperse gallium oxide evenly on the support. This would

ensure intimate contact between nickel and gallium oxide,

inhibiting the segregation of nickel particles.

Previous studies have demonstrated that atomic layer

deposition (ALD) can be used to distribute gallium oxide (GaOx)

homogenously on SiO2powder.[71] Therefore, ALD was applied

to synthesize GaOx(ALD)/SiO2as reference material for this

study. One ALD cycle was conducted yielding a Ga loading of

Figure 1. Schematic synthesis procedure of the NiGa-modified gallium oxide

catalysts, supported on silica. The denotation ALD indicates atomic layer

deposition and IWI stands for incipient wetness impregnation.

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14 wt%. Furthermore, gallium oxide was applied in a compara-

ble amount to SiO2using incipient wetness impregnation (IWI)

as a reference.

The catalytic dehydrogenation performance of both starting

materials, along with unmodified SiO2, is shown in Figure 2. The

fresh catalysts were pre-treated in 10% H2and afterward

exposed to Oleflex-like conditions with high propane partial

pressure and H2co-dosing (59% C3H8and 29% H2, 600°C). After

a rapid deactivation within one hour, the unmodified SiO2

support material was found to be nearly inactive, with propane

conversions below 3%. Hence, the blind activity of SiO2and the

quartz reactor itself are neglectable after the first hour. The

initial higher activity of SiO2should also not affect the perform-

ance of the GaOx(ALD)/SiO2catalysts, as SiO2was homoge-

neously covered with GaOxafter ALD.[71] In the initial phase, the

GaOx(ALD)/SiO2catalyst exhibited a high propane conversion of

around 30%. Within 5 hours of time-on-stream (TOS), however,

the conversion decreased to below 20%. At the same time, the

selectivity towards propylene gradually increased and reached

a plateau at 81%. Afterward, the selectivity towards propylene

remained stable and the conversion of propane decreased only

by 5% over the last 7 hours.

The supported gallium oxide catalyst synthesized by IWI

(Ga2O3(IWI)/SiO2) showed a similar behavior. However, its

propane conversion remained around 5% below that of the

GaOx(ALD)/SiO2. Moreover, the selectivity of propylene was

similar for both catalysts which suggests that the same active

phase was formed. Therefore, the superior activity of the ALD

catalyst derived from its higher dispersion of gallium oxide. This

is in line with several reported studies comparing ALD and IWI-

prepared catalysts.[36,65,69]

Consequently, atomic layer deposition (ALD) was further

used to prepare the starting material for the subsequent

impregnation with nickel. In total, four different NiGaOx/SiO2

catalysts were synthesized containing 0.8 wt%, 1.6 wt%,

3.2 wt%, and 10 wt% Ni on GaOx(ALD)/SiO2. The resulting

nominal atomic ratios are listed in Table 1, together with the

specific surface areas (SSA) determined by N2physisorption.

Each NiGaOx/SiO2catalyst possessed an SSA of around 330 m2/

g due to the support material SiO2, which provided 505 m2/g.

The set of catalysts was tested in the dehydrogenation of

propane (PDH) as previously demonstrated on the unmodified

GaOxcatalyst. The achieved yields of each catalyst are depicted

in Figure 3(b). Initially, all NiGaOx/SiO2catalysts resulted in

lower production of propylene compared to GaOx/SiO2. The

reduction in yield can be directly correlated with an increase in

nickel content. Generally, this trend can be attributed to the

fact that the addition of nickel negatively impacted the overall

conversion of propane.

However, it is noteworthy that the NiGaOx/SiO2catalysts

consistently exhibited a higher propylene selectivity than GaOx/

SiO2. Moreover, low nickel loadings (0.8 and 1.6 wt%) led to

only a minor decrease in conversion (1% at 12 h TOS).

Therefore, the propylene yield achieved with the

Ni(1.6 wt%)GaOx/SiO2catalysts reached that of GaOx/SiO2

towards the middle of the experiment.

Still, the lower conversion (Figure 3(a)) of the NiGaOx/SiO2

catalysts indicate that the formed NiGa species is less active

than standalone GaOxor not active in the PDH reaction at all.

The lower activity could partially derive from a reduction of

available gallium oxide surface area. For instance, nickel might

consume gallium atoms to generate alloys or block active sites

Figure 2. Conversion of propane and selectivity towards propylene against

time-on-stream over two different gallium oxide catalysts. The selectivity of

SiO2was below 40 % and is shown in the SI (Figure S9) to maintain the

simplicity of the graph. The thermodynamic equilibrium conversion is

indicated by a dashed grey line at 37%. Conditions: T=600°C, 59% C3H8,

total flow: 17 mL/min (C3H8:H2:He 10:5:2 mL/min).

Table 1. Compositions and specific surface areas of the NiGaOxcatalysts.

Name Molar ratio [Ni:Ga] SA[a] [m2/g] XRD phase[b]

GaOx(ALD)/SiO214 wt% Ga 336 amorphous[71]

Ni(0.8%)GaOx/SiO21:18 – Ni2Ga3

Ni(1.6%)GaOx/SiO21:9 337 Ni2Ga3

Ni(3.2%)GaOx/SiO21:4 – Ni1Ga1

Ni(10%)GaOx/SiO22:3 317 Ni3Ga1+Ni1Ga1

Ga2O3(IWI)/SiO212 wt% Ga 326 –

Ni(1.6%)/SiO21.6 wt% Ni 489 Ni metal

[a] by N2physisorption and BET method. [b] Main reflections after 12 h PDH.

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on the oxidic framework. This phenomenon will be further

discussed below.

Unfortunately, the only published example which reports

on the interplay of nickel and gallium for PDH does not discuss

the activity of gallium oxide alone.[51] It was only stated that Ga-

rich alloys or Ga-terminated alloys are most active in PDH. Here,

however, the propane conversion decreased whenever Ni was

added to gallium oxide, regardless of the formed NiGa phase.

The actual benefit of adding nickel to gallium oxide

becomes apparent when considering the product selectivity, as

in Figure 3 (c). Each NiGaOx/SiO2catalyst, possessing 3.2 wt%

or less Ni, provided an over 5 % higher propylene selectivity

than GaOx/SiO2during the entire experimental run. At 12 h TOS,

the Ni(0.8 wt%)GaOx/SiO2catalyst reached a propylene selec-

tivity of 89%, which was 7% above that of the pure oxide. This

level of selectivity actually approaches the values found for the

PtZn[34–36] and PtGa[14,15] systems, with only the conversion rate

being lower on NiGaOx.

In contrast to the Ga-containing catalysts, the standalone

Ni/SiO2showed a much lower tendency to produce propylene.

The conversion of propane remained at 7% and the propylene

selectivity was 12% at 12 h TOS. At the same time the

selectivity towards methane (>40%) was significantly higher

compared to the NiGaOx/SiO2catalysts. This followed the

expectations, as nickel is known for hydrogenolysis and

cracking of propane which leads to methane.[27,28]

When Ni was added to GaOx, the methane selectivity

dropped to below 3% during the whole PDH tests. This is the

first indicator, that the Ni deposited on GaOxdid not form

segregated particles. Therefore, the Ni atoms must be incorpo-

rated into the GaOxframework which inhibits its typical

hydrogenolysis and cracking activity.

Only the Ni(10 wt%)GaOx/SiO2provided a slightly higher

tendency to produce methane than GaOx(ALD)/SiO2. At the

same time, the propane selectivity never reached that of the

other NiGaOx/SiO2catalysts. This suggests that despite the

high dispersion of GaOx, not all Ni was incorporated in the case

of the highest Ni loading. Consequently, to increase the

propylene selectivity of GaOxwhile maintaining the propane

conversion, only small amounts of Ni, below or equal to a molar

ratio of 1:9 (Ni:Ga) should be considered.

Regarding the gaseous side products like methane, ethane,

and ethylene, the other NiGaOx/SiO2catalysts showed the

same selectivity distribution as the unmodified GaOx. Therefore,

it appears that the observed selectivity advantage of nickel

addition does not pertain to the inhibition of CC cleavage

(hydrogenolysis or cracking).[5,6]

As mentioned before, the production of propylene can also

be inhibited by deep dehydrogenation of adsorbed intermedi-

ates (excessive removal of hydrogen).[5,6,10] Therefore, the

superior propylene selectivity observed for NiGaOx/SiO2cata-

lysts must derive from a decreased tendency for deep

dehydrogenation. This reaction pathway leads to multiply

dehydrogenated alkenes/alkynes acting as coke precursors and

formation of high boiling hydrocarbons like aromatics.[6,10,73]

Unwanted, heavy compounds which were not detected by

the GC are labeled as �C5Hxin Figure 3(c). In fact, the higher

propylene selectivity of NiGaOx/SiO2catalysts was accompa-

nied by a decreased tendency for carbon loss (production of

undetected �C5Hx). Herein, coke deposited directly on the

catalyst might only be a fraction of the unwanted compounds.

However, the quantity of formed coke should scale with the

selectivity to the non-detected, heavier molecules.

Coke formation

When heavy compounds are formed by undesired side

reactions during PDH, a fraction of it is deposited as coke on

the catalyst surface.[6,10,73] Therefore, the higher propane

selectivity of NiGaOx/SiO2catalysts should be accompanied by

a lower tendency to form coke. To confirm this, the coking rate

on selected catalysts during propane dehydrogenation was

investigated in situ using a thermogravimetric balance.

The respective sample was placed in a crucible on which

the same propane feed was applied as in the PDH experiments.

During the reaction, the change in mass of the crucible was

monitored with a balance and the chamber geometry allowed

some of the propane to pass alongside the crucible. The

resulting mass changes over time on stream are shown in

Figure 4(b).

Figure 3. (a) Conversion of propane at 12 h TOS and (b) yield of propylene over time on gallium oxide catalysts modified with different loadings of nickel.

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In the case of Ni(10%)GaOx/SiO2, the mass increased

significantly by over 10% within the first two hours. Subse-

quently, the rate of coke deposition reduced and the mass

increased by an additional 15% over the last 10 hours. This

suggests again, that not all nickel was sufficiently alloyed with

gallium at this loading, leading to a cracking behavior

characteristic of nickel. Moreover, the decreased rate in the final

phase might be due to the blockage of surface sites by carbon

and the deactivation of the gallium oxide. This is also reflected

in the significant drop in propylene yield as described above.

GaOx/SiO2led to a constant mass gain during the

thermogravimetric PDH process. After 12 h TOS, the mass

increased by over 4.3 wt % which is significantly lower com-

pared to the sample with Ni(10%)GaOx/SiO2. However,

Ni(1.6%)GaOx/SiO2resulted in an overall lower deposition rate

than the GaOx/SiO2, leading to a 2.7 wt% mass gain. This is the

first indicator, that the lower propylene selectivity of GaOx/SiO2

derived from an increased tendency to form coke.

Although the geometry of the balance chamber differed

from that of the fixed bed reactor, the mass gains are in line

with the carbon contents determined by combustion analysis.

Herein, the amount of deposited carbon was measured by ex-

situ oxidation of the catalyst bed retrieved from the reactor.

Calculated mass fractions of carbon formed after 12 h PDH are

displayed in Figure 4(a).

In fact, the catalyst with 10 wt % Ni led to the deposition of

24 wt% carbon, which is in the same range as the mass change

observed using thermogravimetry. This outcome closely

matches the coke content received when using pure nickel on

SiO2with a carbon fraction of 30 wt%. When only gallium oxide

is applied for PDH, the amount of carbon drops significantly to

4 wt%. Once more, this value aligns with the increase in mass

detected by thermogravimetric analysis.

The lowest mass fraction of carbon was obtained when

combining gallium oxide with the NiGa nanoalloys. The

NiGaOxcatalysts in the range of 0.8 wt% to 3.2 wt% Ni all led

to carbon fractions below 2 wt%. Especially the sample with

3.2 wt% Ni had a significantly decreased carbon content of only

1.1 wt%. Unfortunately, recent literature reports on the NiGa

system do not target the actual amounts of deposited carbon

or its removal in detail.[51,52]

Additionally, the produced amounts of propylene were

calculated considering the respective yields and inlet flows of

propane (see also Figure S10). The deposited carbon mass was

put in relation to the cumulative quantity of propylene to

determine a coke production rate (Figure 4(a) below).

The unmodified GaOx/SiO2provided a rate of 8.3 kg of coke

per ton of propylene over 12 hours. In other words, the amount

of coke produced equaled almost 1% of the total propylene

production in terms of mass. The NiGaOx/SiO2catalysts with

less than 10 wt% of nickel, however, resulted in only half the

coke production rate (<4 kg/t).

The best value was achieved by applying 0.8 wt% Ni, which

also showed the highest selectivity towards propylene. This

means, that by tuning the gallium oxide with small amounts of

NiGa alloys, the propylene could be synthesized with half the

coking rate. Therefore, the addition of NiGa is a trade-off

between a slightly reduced single-pass conversion of propane

and an increased selectivity to the desired product.

Especially the mitigation of coking is a crucial target for

dehydrogenation processes.[3,8] The greater the loss of propane

through conversion into coke, the higher the emission of CO2

during the oxidative regeneration. In this case, especially the

gallium oxide with 1.6 wt% Ni would be the most efficient

candidate as it delivers a comparable propylene yield as the

pure oxide while producing 50% less coke.

Generally, the coking tendency, and overall carbon loss due

to the formation of heavy compounds, can be reduced by co-

feeding hydrogen.[9–12] Figure 4(c) shows the performance of

the Ni(1.6%)GaOx/SiO2catalyst with and without H2co-feed, at

the same conditions as before. Upon removal of H2from the

stream, the selectivity towards propylene dropped to the value

achieved with the GaOx/SiO2sample (ca. 81%). Furthermore,

the fraction of heavier species (�C5Hx) increased to a similar

level as for the unmodified oxide. This suggests that hydrogen

directly influences the NiGaOxsystem and the reaction path-

Figure 4. (a) Mass fractions of deposited carbon, determined by elemental analysis, and mass of carbon deposited per total amount of propylene synthesized

(in kg/t) of different Ni-modified GaOx/SiO2catalysts (both after 12 h PDH). (b) in situ mass-change of the crucible filled with catalyst during PDH in a magnetic

suspension balance. (c) Product distribution of one NiGa catalyst with and without H2co-feed. Conditions: T =600 °C, 59% C3H8, total flow: 17 mL/min

(C3H8:H2:He 10:5:2 mL/min).

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way leading to propylene. This aspect will be the matter of

further discussion in the last section.

Regeneration of NiGaOx/SiO2

The Ni(1.6%)GaOx/SiO2catalyst resulted in the highest

propylene yield among the NiGaOxsamples. Therefore, this

system was further investigated in multiple consecutive PDH

cycles.

Generally, the coke being deposited on the surface of the

catalyst induces deactivation during the dehydrogenation

process. In order to regenerate the catalyst without removing it

from the reactor, the coke was burned off. Temperature-

programmed oxidation (TPO) experiments were conducted to

identify conditions at which the coke is removed. The results of

TPO are shown in Figure 5(a) and Figure S11. The spent catalyst

was assembled in a thermogravimetric scale and heated to

1100°C in syn-air (20% O2in N2, 10 K/min). The blue dashes

along the mass curve indicate the maximum slopes deriving

from the minima of the first derivatives.

At 148°C, physisorbed water is evaporated leading to the

first drop in mass. The second rapid decrease at 540°C accounts

for the removal of carbon content as CO2. This was also

observed in the online mass spectrometer. The two elevations

around 540°C point to an increase in mass due to the oxidation

of metal content. Therefore, re-oxidation of the NiGa species

might not be fully avoided when burning off the deposited

carbon. Despite the overlay of the two mass-changing effects,

the order of magnitude of the mass loss is in line with the

measured carbon content (below 2 wt%). Identified conditions

were applied for an oxidative treatment during the PDH process

to execute three consecutive reaction cycles, which is shown in

Figure 5(b). In each cycle, the Ni(1.6%)GaOx/SiO2catalyst was

exposed to the reaction conditions for 12 h and afterward

purged with 20% O2at 550°C for one hour. The next cycle

started with activation in diluted hydrogen at 600°C for one

hour, followed by applying the standard PDH conditions. As a

reference, the GaOx/SiO2sample was regenerated under the

same conditions and tested in two cycles.

In each of the three testing segments, the yield of

propylene progressively decreased over time as observed for

the first 12 hours. The initial yield level in the first cycle using

Ni(1.6%)GaOx/SiO2was restored by 85% in the second seg-

ment. Nevertheless, along all PDH cycles, the NiGaOxcatalyst

always provided a selectivity towards propylene of over 86%.

This rather points towards a maintained nature of the active

phases or its successful regeneration during the oxidative

treatment.

Moreover, the yield provided by the Ni(1.6%)GaOx/SiO2

sample exceeded the yield of the unmodified gallium oxide, in

the second cycle. Its conversion was similar to that of the oxide

while its propylene selectivity was up to 5% higher. The

conversion and selectivity of both catalysts at the end of the

second cycle is displayed in Figure 5 (c).

Generally, the decreased yield originated from a slightly

lowered conversion, which was the case for both,

Ni(1.6%)GaOx/SiO2and GaOx/SiO2. Therefore, the slight de-

crease in yield between the first two cycles might be

rationalized by the reduction of available oxide surface area. In

fact, STEM EDX mappings demonstrated that the gallium oxide

agglomerated after three cycles (Figure S12). Generally, areas,

where gallium was detected, appeared more concentrated

compared to the images after only one cycle. The structural

integrity will be further evaluated in the next section.

Nevertheless, in the third cycle, the initial propylene yield of

the second cycle was recovered by 97% which indicates full

restoration of the catalyst state. As this was not the case from

the first to the second cycle, the most active sites might be

altered rapidly at the beginning of the first segment. Moreover,

this indicates that the absolute reduction of the surface area of

gallium oxide and its mobility is limited. At this point, future

studies might elucidate the deactivation behavior of NiGaOx

catalysts in more detail. Generally, the demonstration of the

recyclability of the NiGaOxcatalyst is significant to promote

the usage of nickel-based systems for PDH.[74] The maintained

Figure 5. (a) Thermogravimetric analysis of the spent Ni(1.6 wt%)GaOx/SiO2sample after 12 h PDH. The temperature programmed oxidation was conducted

at a rate of 10 K/min in 20% O2 (in N2, 20 mL/min). The image at the bottom represents the online mass spectra of H2O and CO2. (b) Yield of propylene

against time-on-stream. (c) Performance comparison between 12 h and 27 TOS for both catalysts. Conditions: T=600°C, 59% C3H8, total flow: 17 mL/min

(C3H8:H2:He 10:5:2 mL/min). Regeneration: T=550°C, 20% O2(in He, 50 mL/min) for one hour.

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selectivity towards propylene and recovered conversion of

propane make the NiGaOxsystem attractive for future studies.

Structural investigation

Powder X-ray diffraction (XRD) and scanning transition electron

microscopy (STEM) were performed to investigate how Ni is

incorporated into GaOx. The diffractograms of the spent

samples (after PDH) are displayed in Figure 6(a) and the range

is adjusted to the main reflections of the relevant metals and

alloys. As a result, the addition of Ni to GaOx/SiO2led to the

appearance of distinct maxima matching different NiGa alloys.

None of the samples displayed clear reflections as it would

occur for crystalline Ni metal or NiO.[75,76] Furthermore, diffracto-

grams of Ni/SiO2contained a different pattern than the

NiGaOx/SiO2samples (Figure S3).

Only the Ni(10 wt%)GaOxcatalyst showed indications of

NiO at 43.2°(2θ) or Ni at 44.5°and 52°(Figure S2). This

rationalizes its higher selectivity to methane as the un-alloyed

and segregated nickel still catalyzed hydrogenolysis and/or

cracking reactions. Despite the presence of a molar excess of Ga

in all catalyst compositions, higher Ni loadings led to Ga-poor

alloys (Figure 6(a)). Moreover, each NiGaOx/SiO2material

displayed reflections for gallium oxide after PDH (Figure S2).

Hence, the reduction of GaOxmight primarily occur in the

vicinity of the deposited nickel, thereby restricting its mobility

to a certain extent.

In fact, the X-ray photoelectron spectroscopy (XPS) of the

1.6 wt% sample revealed, that nickel is metallic after PDH

(Figure S3). Herein, its signal is similar to the one acquired for

pure nickel supported on SiO2, with a maximum of 853.3 eV.

Additionally, NiO can be ruled out as it typically appears at

lower binding energies.[77]

The XRD of the NiGaOx/SiO2catalysts with fractions of

0.8 wt% and 1.6 wt% Ni showed twin maxima that are typical

for Ni2Ga3alloys. Moreover, the samples with increased Ni

loading resulted in Ni-richer alloys. Herein, the main reflection

of the 3.2 wt % Ni sample can be assigned to Ni1Ga1and for the

10 wt% Ni sample, a mixture of Ni1Ga1and Ni3Ga1alloys was

observed. These findings indicate that GaOxis partially reduced

to Ga metal which alloys with the impregnated Ni under the

highly reductive reaction conditions.[17] This also underlines the

hypothesis, that the observed decrease in activity of the

NiGaOx/SiO2catalysts might stem from the consumption of

available GaOxsurface sites through alloying with Ni.

Yet, the decreased activity might also derive from the

interplay between the NiGa alloys and GaOxphase, as the

selectivity and coke formation are also significantly altered. To

rationalize the performance differences between the NiGaOx/

SiO2and GaOx/SiO2catalysts, the influence of H2must be

considered. Especially the synergetic relationship between the

NiGa alloys and GaOxvia a potential H2spillover is conceivable.

Saerens etal.,[10] showed the positive impact of H2co-dosing

during propane dehydrogenation on Pt(111) surface. The

theoretical simulations, supported by experimental data, clearly

showed that by increasing the hydrogen surface coverage, the

formation of deeply dehydrogenated species, such as

ethylidene and methylidyne, can be reduced.

As evident from past studies on CO2hydrogenation,[78,79]

NiGa alloys are known to activate hydrogen and transfer it to

Ga2O3for the hydrogenation of intermediates to methanol. We

suggest that a similar H2spillover mechanism takes place on

the NiGaOx/SiO2surface during dehydrogenation reaction. The

formed NiGa alloys would activate H2from the co-feed or

generated during the dehydrogenation, and transfer it to

adjacent GaOxsites.

This would not only rationalize the increased propylene

selectivity but also the decreased conversion. The hydride

donation from HNiGa to GaOxpartially reduces Ga(III) to

Ga(�II) which diminishes the number of adsorption sites for

propane. Simultaneously, the higher abundance of hydro-

Figure 6. (a) X-ray diffractograms of the Ni-modified gallium oxide catalyst after 12 h TOS. The main reflections of crystalline NiGa alloys, Ni metal, and NiO are

indicated according to literature. (b) Scanning electron microscopy (STEM) images (HAADF), EDX mappings and EDX line scan of GaOx/SiO2modified with

1.6 wt% Ni. Atomic counts of Ni and Ga were collected and determined by the EDX detector. Ga is indicated in purple and Ni in turquoise color. (c) EDX

mappings of 0.8 wt % Ni on gallium oxide. (d) STEM images of 10 wt% Ni on gallium oxide. All measurements were conducted on spent catalysts, after 12 h

PDH.

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genated sites decreases the dehydrogenation potential of

already adsorbed propane. Consequently, coking is mitigated

because the hydrogen enrichment would inhibit the deep

dehydrogenation pathway to propylidene, ethylidene or meth-

ylidyne.[6,10,46,73] This hypothesis is in agreement with the results

shown in Figure 4(c). Herein, removing the H2cofeed decreased

the propylene selectivity and increased the carbon loss. Addi-

tionally, adding H-donating NiGa alloys to gallium oxide

significantly reduced the coke formation when H2was co-

dosed.

In a recent study by Zhang etal.,[80] Pt was shown to

promote the H2dissociation and increase the surface coverage

of hydrogen species on GaOx. In the case of PdGaOx, Collins

etal. demonstrated that the reduction effect was indeed

accomplished by hydrogen spillover from the noble metal to

gallium oxide.[81]

Generally, the hydrogen spill-over and partial reduction of

gallium oxide were validated using XPS analysis.[80,81]

Here, XPS analysis was applied to the selected NiGaOx/SiO2

catalyst to determine the reduction degree of GaOxafter PDH.

The spectra of the Ga 2p3/2 region of GaOx/SiO2and

Ni(1.6 wt%)GaOx/SiO2are displayed in Figure 7.

The spectrum of the unmodified GaOxcatalyst consisted

exclusively of the Ga(III) signal originating from Ga2O3. The

NiGaOxsamples could be deconvoluted into three peaks at

1118.9, 1117.6, and 1116.8 eV, which can be assigned to Ga(III),

Ga(II or I), and Ga(0).[18,80,82,83] Herein, the partially reduced GaOx,

in the oxidation state one or two, is summarized as Gaδ+. The

binding energy deviation from metallic and oxidic gallium

mainly derives from the difference in charge of Ga.[80,82,83]

Around 2.3 wt% of Ga should be captured as metallic

gallium in the alloys, considering that mainly Ni2Ga3was formed

and 1.6 wt% Ni was deposited on 14 wt% Ga. This would

correspond to a molar fraction of all gallium of 16.6 mol% Ga(0).

In fact, when the signal areas of the three gallium species

resulting from XPS are juxtaposed, an allocation of 13 % is

ascertained for metallic Ga(0). This value is in good agreement

with the estimated mole fraction which validates these XPS

results. Moreover, the Ga(III) makes up for a mole fraction of

70 mol% and the partially reduced Gaδ+accounts for 17 mol%.

Since the pure oxide did not show partially reduced gallium,

the reduction must be accomplished by the NiGa. This

essentially supports the hypothesis of a hydrogen spill-over

from NiGa to GaOxduring PDH.

STEM was conducted to reveal the structure of the

supported NiGa alloys after PDH. Results for the Ni-

(1.6 wt%)GaOx/SiO2catalyst are shown in Figure 6(b), while

images of the other samples are listed in the SI (Figure S4–6).

The HAADF image revealed the presence of individual nano-

particles on the SiO2surface with an average diameter of 19 nm

(�6).

Moreover, energy dispersive X-ray diffraction (EDX) scans

were conducted to determine the location of nickel and gallium

along the sample. The mappings disclosed that the nano-

particles consist of nickel and gallium, while the nickel counts

are always situated in close proximity to gallium. Applying EDX

also compliment the Ni:Ga compositions found by XRD. For

example, one area selective mapping quantified a signal count

ratio of 38 to 62 (Ni:Ga), which is in line with the X-ray

diffractions indicating Ni2Ga3alloys. Herein, a nanoparticle was

scanned exclusively situated at the edge or a shallow part of

the support material. That way, signal noise originating from

the support or other particles underneath is minimized, and

only the selected nanoparticle accounts for the EDX detections.

An EDX line scan along two particles of the

Ni(1.6 wt%)GaOxspecimen underlines these findings (Fig-

ure 6(b), right). The average counts of nickel were approx-

imately two-thirds that of gallium along both particles.

Consequently, nickel and gallium are always situated next to

each other in a constant composition. Additionally, the dark-

field image showed sharp edges of the nanoparticles which can

also be observed as a steep rise of counts in the line scan.

Hence, core-shell particles are ruled out.

The observed alloy formation between nickel and gallium

rationalizes again the respective selectivity towards propylene.

Once NiGa nanoalloys are formed, Ga atoms impart the

geometric modification of Ni, which inhibits its tendency for

CC cleavage.[27,28]

It is noticeable that nickel and gallium no longer form

uniform nanoparticles after the regeneration of

Ni(1.6 wt%)GaOx/SiO2(Figure S12). Therefore, the regeneration

process distorted the NiGa alloys by the harsh oxidation and

subsequent reduction. A temporary formation of oxides agrees

with the slight increase in mass observed during the TPO

(Figure 4(a)).

Furthermore, XRD analysis revealed that gallium oxide

crystals are present after three PDH cycles, whereas it was still

XRD-amorphous after only 12 h TOS (Figure S12). Additionally,

the reflection associated with Ni2Ga3alloys nearly vanished

while crystalline Ni3Ga1was formed. As the catalyst maintained

its propylene selectivity, the molar ratio of the nanoalloys is in

fact not decisive. This suggests that as long as all nickel is

alloyed with gallium, the hydrogen spillover and partial

reduction of the oxide takes place. These observations comple-

ment the STEM results (Figure S12), which suggested that the

integrity of the nanoalloys was disturbed by the regeneration

procedure. Ultimately, it is evident that gallium oxide is partially

reduced and becomes mobile under reaction conditions. This

Figure 7. X-ray photoelectron spectra (XPS) of the Ga 2p3/2 region of GaOx-

(ALD)/SiO2and NiGaGaOx/SiO2after 12 h PDH.

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suggests an irreversible change of active sites, which explains

the loss in activity after the first regeneration (Figure 5(b)).

Area selective EDX mappings were also performed to

determine whether the nickel is spread atomically on the

gallium oxide. Respective mappings along the 0.8 wt% NiGaOx

catalyst are shown in Figure 6(c). This sample was chosen as it

led to the highest propylene selectivity. However, the resulting

spectrum, received from an area where no nanoparticle is

located, shows no indication of nickel. Moreover, the scan of a

nanoalloy on the same sample provided clear signals at

energies assigned to diffracted radiation deriving from the

Ni(Kα) shell.[84] Hence, the EDX results show no indication of

smaller nickel clusters or even single atoms.

An exemplary image of the 10 wt% Ni sample is displayed

in Figure 6(d). It was observed, that not only the Ni-fraction of

the nanoalloys increased with higher Ni-loading, but also the

nanoparticle count. The STEM images suggested that the

1.6 wt% Ni catalyst held 350 nanoparticles per μm2, whereas

the 10 wt% sample reached 3300/μm2(Table S3). Therefore, a

higher density of nanoalloys translates to a lower propane

conversion, which again supports the idea, that gallium oxide is

the active species and not NiGa.

Since the present alloy compositions were found, final DFT

calculations could be conducted to complement the findings

for the coking behavior. In several DFT studies, the correlation

between the adsorption energy of a single carbon atom on a

metallic surface and the proneness for the formation of coke

was observed. In the case of Ni, this was reported for the water-

gas-shift reaction,[85] the dry reforming of methane,[86,87] and for

other transition metals and alloys in PDH.[88] These studies

demonstrated that the C-atom adsorption energy is a descriptor

for the formation of coke. Here, DFT calculations were carried

out to validate the coking ability of different NiGa surfaces. A

detailed description of the theoretical process is given in the

Supporting Information.

The data points in Figure 8 represent the statistical average

according to the Boltzmann distribution of different facets in

NixGayalloys. The adsorption energies on various facets and

surface terminations, used to determine this average, were

calculated and listed in Table S2. Upon initial observation, it is

evident that incorporating gallium into nickel leads to lower

absolute values of adsorption energies of carbon. Herein, the Ni

and Ni1Ga3surfaces provided the highest Boltzmann averages

of around 7 eV and 6.3 eV. To demonstrate the influence of

Ga on the carbon deposition on NiGa alloys, we also present

the values for pure metallic Ga. Thereby, the insertion of more

gallium leads to a gradual decrease in energy, ultimately

reaching 5.5 eV, as observed on the Ni2Ga3facets.

At the same time, the catalysts holding pure nickel or not

fully alloyed Ni led to the highest amount of coke. Samples

containing alloys like Ni1Ga1or Ni2Ga3, however, had a

significantly diminished tendency for coking. Therefore, the

NiGa nanoalloys might not only prevent the formation of coke

on gallium oxide but also on their own surfaces. Generally, this

is in agreement with reports in which, for example, Sn was

introduced to reduce the cracking ability of Nickel.[46–48]

Conclusions

This study brings attention to the synergetic benefits between

gallium oxide and nickel for propane dehydrogenation (PDH).

Silica was decorated with gallium oxide using atomic layer

deposition (ALD), resulting in a superior propane conversion

and gallium dispersion compared to the synthesis through

impregnation. Therefore, the ALD catalyst was used as the

starting material on which different loadings of nickel were

deposited and tested under Oleflex-like conditions.

XRD measurements indicated the formation of Ni3Ga1,

Ni1Ga1, and Ni2Ga3alloys while lower nickel loadings yielded

gallium-richer species. The presence of alloy nanoparticles with

an average diameter of 12–19 nm was confirmed by scanning

transition electron microscopy (STEM). Furthermore, EDX map-

pings complimented the atomic ratios found by XRD.

Surprisingly, the formed NiGa alloys did not directly affect

the conversion of propane. However, the selectivity to

propylene was significantly enhanced. Conducting in situ

thermogravimetric analysis under PDH conditions revealed that

the addition of Ni to GaOxreduced the coking rate substantially.

Compared to the unmodified gallium oxide, the formed NiGa

alloys decreased the coke deposition by over 50%.

Similar to previous reports, the decreased tendency for

coking and higher propylene selectivity could be rationalized

by a partial reduction of Ga(III) sites. In fact, XPS studies

suggested the formation of a partially reduced gallium species

only when NiGa alloys were present. As NiGa alloys are

recognized for their capability to activate hydrogen, the

reduction can be explained by a hydrogen spillover. The

hydrogen enrichment of the gallium oxide prevents the deep

dehydrogenation of adsorbed intermediates which inhibits

coking. Additionally, DFT calculations indicated that NiGa alloys

exhibit a low tendency for coke deposition compared to Ni.

Finally, the Ni(1.6 wt%)GaOx/SiO2system was regenerated

in a periodic PDH process by an oxidative treatment. The coke

was successfully burned off between the PDH cycles, yet XRD

Figure 8. Adsorption energies of carbon on specific NiGa alloy compositions

(calculated by DFT). Main points indicate the Boltzmann average of the

respective composition and the lines indicate the energy bandwidth for all

facets of the composition. Ni and Ga indicate a metallic, non-oxidic, surface.

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and STEM indicated that the structural integrity of the nano-

alloys and gallium oxide was disturbed. Nevertheless, the

propylene selectivity was maintained in all three PDH periods.

Generally, the NiGaOx/SiO2samples come close to the

selectivities reported for PtGa catalysts. In future studies, tuning

of the metal loadings might improve the NiGaOxsystem even

further. Moreover, the contribution of NiGa could be fully

unraveled by applying colloidal synthesis to prepare defined

NiGa nanoparticles. Thereby, their catalytic behavior can be

detangled from gallium oxide and hydrogen activation studies

might help to understand the hydrogen spillover.

Experimental Section

Catalyst synthesis

The used chemicals and resulting metal loadings are listed in the

supporting information. Atomic Layer Deposition (ALD) of gallium

oxide was carried out in a self-designed setup of which a detailed

description is given elsewhere.[89] Trimethylgallium (TMG) and

HPLC-grade water were used as a precursor-reactant combination

and the overall ALD process is described in detail elsewhere.[71]

For the catalyst synthesis, mesoporous silica (SiO2) powder (500 m2/

g) was filled into a tubular fixed bed reactor made of quartz glass

(20 mL). The ALD process was conducted under a constant gas flow

of 100 mL/min at atmospheric pressure. The powder substrate was

maintained at 150°C while the TMG and water dosing units were

kept at room temperature. Both reactants were sequentially fed

into the reactor using argon as the carrier and purge gas. The

applied ALD sequence (cycle) was TMG/Ar-purge/H2O/Ar-purge.

The point of precursor or reactant saturation was determined by

online mass spectrometry.

Both reactants were dosed into the reactor until the mass traces for

m/z=69 (TMG) or m/z=18 (H2O) broke through and reached a

plateau. For each sample, one ALD cycle was conducted providing

14 wt% Ga. The resulting material is denoted as GaOx(ALD)/SiO2.

The preparation of the NiGaOx/SiO2catalysts is schematically

depicted in Figure 1. First gallium oxide was distributed on

mesoporous silica powder by ALD. Subsequently, in order to enable

scalability of the nickel content, incipient wetness impregnation

(IWI) was selected as a convenient deposition technique. Nickel

nitrate hydrate was dissolved in HPLC-grade water which equaled

the maximum amount of water absorbed by the GaOx(ALD)/SiO2

powder. The solution was distributed on the support and dried in

air at 80°C for 12 h. Afterward, the precursor material was reduced

at 600°C (5 K/min rate), in 5 % H2(in N2, 200 mL/min), for 3 h,

yielding NiGaOx(ALD)/SiO2.

As a reference, gallium oxide was supported on the silica powder

by IWI. Gallium nitrate hydrate was dissolved in HPLC-grade water

which equaled the maximum water absorption of the SiO2powder.

The solution was deposited onto the SiO2support and dried in air

at 80°C for 12 h. Afterwards, the precursor was calcined at 500°C

(5 K/min), in 20% O2(in N2, 200 mL/min), for 3 h, yielding

Ga2O3(IWI)/SiO2with 12 wt% Ga.

Characterization methods

Synthesized materials were characterized using the following

analysis methods: Inductively coupled plasma atomic emission

spectroscopy (ICP-OES), combustion analysis (CHN), powder X-ray

diffraction (XRD), nitrogen physisorption measurements (applying

the B.E.T. method), X-ray photoelectron spectroscopy (XPS), scan-

ning transmission electron microscopy coupled with energy-

dispersive X-ray mapping (STEM-EDX), mass spectrometer-coupled

thermogravimetric analysis (TG-MS) and temperature programmed

oxidation (TPO). Detailed descriptions of the analysis procedures

are given in the supporting information.

DFT Calculations

Density functional theory (DFT) calculations were used to determine

the adsorption energy of carbon on the surface of different NiGa

alloys. The adsorption energies were calculated based on the

relaxation of a single carbon atom on different facets of NiGa alloys.

Average values were determined according to the Boltzmann

distribution of the formation energies.[90] A detailed description of

the procedure and resulting data is listed in the Supporting

Information (Table S2).

Propane dehydrogenation (PDH)

The pre-reduced catalysts were compared for their activity in the

dehydrogenation of propane. Catalytic experiments were con-

ducted in a continuous flow set-up equipped with a quartz tube as

a fixed bed reactor, designed by Integrated Lab Solutions. The

applied reactor had an inner diameter of 10 mm while the volume

of the catalysts resulted in a bed height of 5 mm. In all runs, the

amount of each catalyst was fixed to 500 mg. Prior to a catalytic

test, the samples were activated in situ under a continuous flow of

10% H2(50 mL/min, in He) at 600°C (10 K/min rate) for 1 h.

Subsequently, the reactor was purged with 50 mL/min He for 5 min

and the gas flow was switched to 17 mL/min, containing 59% C3H8

and 29% H2in He (1 bar). The resulting gas-hourly-space velocity

(GHSV) was 2040 mLg1h1and the temperature at the catalyst

bed was maintained at 600°C. During the regeneration experiment,

the catalyst was first exposed to 20% O2(in He, 50 mL/min) at

550°C and then re-activated as described above. The effluent gas

stream was monitored by an online gas chromatograph (Agilent

7890A) equipped with a flame ionization and thermal conductivity

detector. Propane conversion (X), product selectivity (S), and

propylene yield (Y) were calculated according to Equations (2)–(4).

XC3H8¼ ð1_

FC3H8;out

FC3H8;in Þ � 100 (2)

Si¼ni�_

Fi;out

nC3H8�ð_

FC3H8;in _

FC3H8;outÞ�100 (3)

YC3H6¼XC3H8�SC3H6�0:01 (4)

Where _

F represents the flow rates before entering the reactor (in)

or in the effluent gas (out) respectively. The product selectivities (3)

are calculated based on the respective product concentration and

the amount of propane converted. nirepresents the number of

carbon atoms of the respective compound. Compounds which

were not detected by the GC are labeled as �C5Hx. These

compounds are mostly heavy hydrocarbons and coke, which were

deposited on the catalyst bed or condensed at colder regions after

the reactor exhaust.

The in situ coke formation studies were conducted in a Rubotherm

magnetic suspension balance (DynTHERM HP-ST, 2010-01001-D).

100 mg of a catalyst was filled in a quartz glass crucible attached to

a quartz glass holder. The samples were heated (10 K/min) to

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600°C in 50 ml/min N2and afterwards activated in 10% H2in N2.

Afterwards, the gas feed was switched to the testing composition

(C3H8/H2/N2=1/0.5/0.33) at a pressure of 1.1 bar. The mass-change

was tracked for 12 hours and the first measuring point, after

switching to propane-rich conditions, was taken as a reference

point. The influence of buoyancy was determined through meas-

urement with an empty crucible and subtracted from each mass

curve.

Acknowledgements

The authors gratefully acknowledge financial support from the

Deutsche Forschungsgemeinschaft (DFG, German Research

Foundation) under Germany’s ExcellenceStrategy – EXC 2008-

390540038-UniSysCat. The authors appreciate the valuable

support of the following colleagues: Sophie Hund, Christina

Eichenauer (TU Berlin) and Christian Rohner, Franz Schmidt

(FHI). The work was conducted in the frame-work of the BasCat

JointLab between BASF SE and the TU Berlin. Open access

funding enabled and organized by Projekt DEAL. Open Access

funding enabled and organized by Projekt DEAL.

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available

from the corresponding author upon reasonable request.

Keywords: alloy ·coke formation ·gallium oxide ·nickel ·

propane dehydrogenation

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Manuscript received: October 10, 2023

Revised manuscript received: November 7, 2023

Accepted manuscript online: November 16, 2023

Version of record online: ■■■,■■■■

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Research Article

doi.org/10.1002/cctc.202301261

RESEARCH ARTICLE

A supported gallium oxide catalyst,

synthesized by atomic layer deposi-

tion, was modified by the addition of

nickel, and used in the dehydrogena-

tion of propane. Resulting NiGa nano-

particles helped to reduce the coke

formation on gallium oxide under in-

dustrially relevant conditions. Finally,

the catalyst system could be regener-

ated by an oxidative treatment.

R. Baumgarten, Dr. P. Ingale, F. Ebert,

Dr. A. Mazheika, Dr. E. Gioria, K. Trapp,

K. D. Profita, Dr. R. Naumann d’Alnon-

court*, Prof. Dr. M. Driess, Dr. F.

Rosowski

1 – 13

Controlling the Coke Formation in

Dehydrogenation of Propane by

Adding Nickel to Supported Gallium

Oxide

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